Nghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOxNghiên cứu xử lý khí thải chứa hợp chất thơm dễ bay hơi (benzen và toluen) sử dụng xúc tác trên cơ sở Cu (Co)-MnOx
LITERATURE REVIEW
VOLATILE ORGANIC COMPOUNDS (VOC s )
Volatile organic compounds (VOCs) are common organic compounds in the environment Some VOCs pollutants can adversely affect the environment and human health Among them, BTEX is particularly concerning due to its toxicity Benzene, in particular, is considered the most toxic and falls into the A1 group (the International Agency for Research on Cancer (IARC) designated benzene as a Group
1 carcinogen, which means it is a known carcinogen for humans Furthermore, based on solid evidence from human and animal studies, the Environmental Protection Agency (EPA) classified benzene as a Category A substance), indicating that it is a confirmed human carcinogen [3,12] Therefore, it is crucial to strictly regulate the release of VOCs to ensure that they are emitted into the atmosphere within safe concentration limits
This study focuses on toluene and benzene, the substances represented by aromantic VOCs, seriously influencing human health and the environment Especially removing VOCs such as benzene to protect human health is an urgent task that needs to be done for sustainable development and environmental protection
Volatile organic compounds (VOCs) are carbon-based compounds that react in the atmosphere, excluding carbon monoxide, dioxide, and acid According to the US-EPA, VOCs are classified based on their evaporation temperature.
The World Health Organization (WHO) classifies VOCs based on their boiling temperature
• Very volatile organic compounds (VVOCs)
• Semi-volatile organic compounds (SVOCs)
1 VVOCs < 0 o C to 50 o C – 100 o C propane, butane, methyl chlorine
240 o C – 260 o C formaldehyde, d-limonene, toluene, acetone, ethanol (ethyl alcohol) 2-propanol (isopropyl alcohol), hexanal
VOCs can be classified into natural and artificial sources Natural sources of VOCs include emissions from trees, wildlife, natural forest fires, and anaerobic processes On the other hand, artificial sources of VOCs include emissions from industrial production processes such as oil refining, chemical and pharmaceutical industries, pesticide production, printing ink, electronics industry, transportation, and human activities [15]
From an environmental perspective, the main sources of volatile organic compound (VOC) emissions are fuel and solvents, which account for over 80% of total VOC emissions Approximately 40% of these emissions come from traffic; using solvents such as paint and printing ink contributes to about 34% The remaining 7% comes from the transportation and storage of VOCs [16] Specifically, aromatic VOCs make up 16.7% of VOCs emitted from industrial processes and 22.2% released from solvent use [17]
For industry emissions, some industrial production processes, especially refinery and petrochemical, notably contribute to VOC emissions According to a study, VOC emissions ranged from 670 to 2770 ppm [18] In addition, coating manufacturing processes often contain numerous VOCs, such as octane, nonane, decane, undecane, dodecane, ethylbenzene, ethyl toluene, and trimethylbenzene [19] For example, in the coating industry in Taiwan, VOC concentrations in the primer coat booth reached 149 ± 99 ppm and 607 ± 322 ppm in the top coat oven [20]
Furthermore, VOCs are emitted from mobile sources as transportation sources Some roadside volatile organic compounds were found from transportation, with the contribution of motorbikes being BTEX [21] The Deng study highlights that VOCs from automobile exhaust mixtures are the primary source of atmospheric VOC pollution in urban areas of China Among these compounds, branched alkanes, primarily alkanes, accounted for 52%, with an average emission factor of 41.39 ±
12.81 mg km -1 veh -1 Following closely are benzene homologs, while alkenes represent the tiniest fraction Dimethylbutane showed the highest average emission factor (13.3 ± 3.19 mg km -1 veh -1 ), followed by toluene (7.06 ± 3.14 mg km -1 veh - ) and 2-methylpentane (6.44 ± 2.10 mg km -1 veh -1 ) [22]
1.1.4 Effects of VOCs on the environment and human health a Impact on the environment
The emission of VOCs into the atmosphere is closely related to environmental problems such as:
Participate in the process of forming a photochemical haze
The cycle of NOx in the troposphere begins with the decomposition of NO2 into
NO and atomic oxygen Atomic oxygen continues to react with O2 to form O3 After
O3 is formed, it reacts with NO to regenerate NO3, and O3 converts to O2 In this way, a closed cycle is created and does not form O3 to accumulate in the troposphere of the atmosphere (Figure 1.2)
If there is a presence of VOCs in the atmosphere, the VOCs will create strong oxidizing radicals These radicals interact with NO, and an increase of O3 in the troposphere leads to a breakdown in the balance of O3 In addition, the combination of ozone, nitrate compounds, and VOCs in the troposphere can create peroxyacetylnitrate compounds (PANs, RC(O)OONO3), which are the main components of photochemical smog (Figure 1.2)
Fig 1.1 The NO x cycle without and with the presence of VOC s [23]
The secondary organic aerosol formation and the increase of the greenhouse effect
VOCs are emitted to the environment in the lower troposphere, and then VOCs are oxidized However, some substances are not oxidized and come to the higher troposphere via convection phenomena, and they will accumulate in the troposphere They can absorb solar or infrared radiation from the ground, increasing the greenhouse effect
Most VOCs play an important role in atmospheric chemistry and participate in photochemical reactions to form ozone in the lower troposphere VOCs such as toluene, m-xylene, ethylbenzene, and long-chain alkanes generate secondary products or secondary organic aerosols [24] Ozone appears in the lower tropospheric, and organic aerosols are greenhouse gases Methane is the lightest carbon compound and is a significant greenhouse gas because of its short residence time [25]
Some VOCs compounds have a long retention time, do not react in the troposphere, can move to the stratosphere, and form strong oxidizing radicals that participate in ozone decomposition reactions, depleting the ozone layer in the stratosphere The cycle even creates a hole in the ozone layer b Effects on human health
Volatile organic compounds (VOCs) can be toxic substances that can directly affect living organisms, especially humans Depending on the concentration and the specific nature of the VOC, its effect on humans can vary When humans come into contact with VOCs, they can experience acute symptoms such as irritation of the eyes, nose, and throat, headaches, nausea, dizziness, and skin allergies VOCs can also cause harm to internal organs such as the liver and kidneys
AROMATIC VOCs (BENZENE, TOLUENE)
Benzene is of particular concern due to its significant properties Research on workplace exposure has revealed that being exposed to benzene can have toxic effects through both oral and inhalation routes [30] Because of its toxic nature and widespread occurrence, controlling and removing the levels of benzene in the ambient air is crucial a Benzene physico-chemical properties
Benzene exists in the vapor phase in the air and can remain there for a few hours to days, depending on environmental conditions and other pollutants
Table 1.3 Benzene physico-chemical properties [30]
6 Vapor pressure at 20 °C 75 mm Hg
9 Conversion factors in gaseous form
1 ppm=3.26 mg/m 3 at 20 °C and 1 atm pressure;
1 mg/m 3 =0.31 ppm b Source of benzene
Benzene can originate from both natural and human activities Natural sources encompass emissions from volcanoes and forest fires, while human activities such as crude oil, gasoline, and industrial processes also release benzene Benzene is utilized as a lubricant to produce plastics, resins, synthetic fibers, rubbers, dyes, detergents, drugs, and pesticides Other sources of benzene include vehicle exhaust, evaporation from motor vehicles, and petroleum retail outlets during petrol storage and distribution [30]
Benzene is released into the atmosphere during its production According to Merchant et al., 2024, benzene is in high demand for ethylbenzene, cumene, cyclohexane, and aniline production Benzene is a by-product of p-xylene production, and recent increases in this aromatic hydrocarbon output led to an extra supply of benzene Primary benzene consumers are China, the United States, and Western Europe [32]
1.2.2 Toluene a Toluene physico-chemical properties
Table 1.4 Toluene physico-chemical properties [31]
6 Vapor pressure at 25 °C 28.4 mm Hg
9 Conversion factors in gaseous form
1 ppm=3.76 mg/m 3 at 20 °C and 1 atm pressure b Source of toluene
Toluene is produced to serve as an intermediary for various purposes, including the production of benzene (50%) and toluene diisocyanate (9%), gasoline blending (34%), solvents (5%), and manufacturing of miscellaneous chemicals (2%)
Toluene serves as a crucial solvent in numerous industries, including paints, coatings, inks, adhesives, resins, and pharmaceuticals Due to its ability to dissolve substances, toluene is essential for the production of these materials Moreover, toluene's cleaning properties make it valuable in surface coating, printing, and the leather industry, where it effectively removes contaminants and aids in surface preparation.
AROMATIC VOCs (BENZENE, TOLUENE) EMISSION IN VIETNAM 12 1 Emission from traffic
In Ho Chi Minh City (HCMC), motorcycles were the majority emission source, accounting for over 90% of road traffic emissions Toluene accounted for the highest percentage of BTEX for traffic activities, while benzene was the lowest [34]
N o Emission sources Benzene (tons/year) Toluene (tons/year)
Traffic emissions in Ho Chi Minh City (HCMC) are predominantly generated by on-road vehicles, with diesel and gasoline serving as the exclusive fuels Motorcycles, in particular, contribute almost 99% of benzene and toluene pollutants Airports and railways have a negligible impact on benzene and toluene emissions, accounting for less than 0.1% This highlights the significant role of on-road vehicles, especially motorcycles, in air pollution, necessitating targeted measures to address their emissions.
Emissions from each manufacturing facility were determined based on procedure process emissions and emissions from combustion activities within the industry Primary pollution sectors included some main sectors such as textiles, metal production, food, and plastic The industry primarily utilized coal as the primary fuel, followed by various oil types (DO and FO), gasoline, wood, and wood products (Table1.6) [34] Thus, regarding the primary source of benzene, toluene in HCMC was consistent with previous research in the world For instance, Rad et al (2014) identified transportation and industry as the primary sources of BTEX pollution in Iran's metropolitan areas [35] Similarly, Cui et al (2012) emphasized fuel use and diesel exhaust as significant contributors to air pollution in Beijing, China [36]
Table 1.6 Emission from production and combustion activities of five primary industrial sectors in HCMC [34]
(tons/year) Benzene Toluene Benzene Toluene
1.3.3 Emission from waste recycling process
Numerous waste recycling processes, such as the waste tire pyrolysis process, release pollutants into the air Waste tyres are one of the essential sources of municipal solid waste [37] Vehicle tires contribute to pollution throughout their lifespan They become End-of-Life Tyres (ELTs) or waste tyres when they are no longer helpful The global generation of ELTs is increasing dynamically, with almost one billion ELTs produced annually [38,39] in Europe, more than 300 million passenger and truck tyres are replaced yearly Similarly, Vietnam has seen a rise in cars and motorcycles However, According to T A Pham, only 10% of these tires are reused or recycled, 43-46 % are used for thermal energy, and 44 - 47% are discharged into the environment or landfills [40]
Pyrolysis is a method that can be used to treat waste tires effectively because the process produces three main products: gas, liquid (pyrolysis oil), and solid (char), which have a high-quality value Pyrolysis gas has stable physical properties and low sulfur content, with a calorific value equivalent to natural gas However, the exhaust gas emissions from this process contain gases that contribute to environmental pollution, such as SO2, H2S, PAH, and VOCs (benzene, toluene,…) [41] The waste tyre pyrolysis plant at Minh Tan-Hai Phong waste treatment facility produces an exhaust gas mixture with around 69.59 ppm of benzene and 123.42 ppm of toluene Although toluene meets the Vietnamese technical standard QCVN 20:2009/BTNMT, the benzene is above 44 times the allowance concentration In addition, the exhaust gas flow in the plants is often diluted, leading to significantly higher levels of aromatic VOCs in the surrounding air that seriously impacts human health and the environment As a result, it is crucial to treat the exhaust gas mixture containing aromatic VOCs to remove these harmful compounds before they are released into the environment.
VOCs CONTROL METHODS AND ENGINEERING
Substituting refers to replacing one material/solution with another that is more environmentally friendly, feasible, and meets the same or better technical requirements For instance, low-VOC or no-VOC water-based degreasing baths, paints, inks, glues, or adhesives can be used as alternatives for the old ones [42] b Process modification
Achieving good practice in process control requires an effective and efficient planning and regulating process This process ensures optimal resource consumption, production, and waste generation conditions To maintain these conditions, temperature, pH, pressure, water level, time, etc., should be continuously monitored and maintained at optimum levels By doing so, energy, water, and raw materials can be saved, and the efficiency of the industrial process can be increased while avoiding excessive waste and emissions Hence, reasonable control of the operating parameters is essential for better process control [42] c Leakage control
Optimizing processes and eliminating waste, spills, leaks, or any fault resulting in unnecessary losses is crucial for good housekeeping and conserving natural resources [42] For example, refineries may emit VOCs due to equipment leaks, such as pumps, valves, and connectors Two methods are available to control VOC refinery equipment leaks: 1) implementing a leak detection and repair program and 2) installing specific controls or leakless equipment [43]
1.4.2 Control by concentration and recovery a Condensation
The exhausted gas mixture is cooled below the boiling point of the pollutants The pollutants will condense and separate from the gas mixture in liquid form Advantage
• Indirect condensation can effectively separate gases at a high level of cleanliness;
• Suitable for low boiling point exhaust gas
• Not suitable for very low boiling point exhaust gas;
• Wastewater-containing pollutants can be created in the direct condensation process
• Using for high boiling points gases such as benzene, toluene, and NH3;
• Condensation of exhaust gases occurs during petroleum distillation and chemical processes at room temperature;
• Strong smell gases and gases with high water content are used in food technology b Absorption
Gas-phase pollutants can be absorbed into liquids to transfer them This process involves mass transfer, where the liquid/solution is the absorbent, and the pollutant is the absorbed substance
• High efficient with a good soluble gas;
• Suitable for low boiling point exhaust gases, high flow rate;
• Simple operation, and it is easy to maintain and repair;
• An absorbent solution is easy to find and can be reconstituted;
• Several products resulting from the absorption process can be used;
• combination gas treatment, dust separation, and condensation
• Expensive to reconstitute absorbent solution;
• If not reconstituted, wastewater will be generated;
• The device is bulky and takes up a lot of space;
• Removing the pollutant gases such as NOx, SO2, H2S, CO, CO2, Cl, HCl, HF, etc c Adsorption
Gas-phase pollutants can be adsorbed into solids to transfer them This process involves mass transfer, where the solids are the adsorbent, and the pollutant is the adsorbed substance
• Use for low concentration and high flow rate;
• Low-cost and easily accessible adsorbents;
• Not require energy, low operating costs;
• Not suitable for high-concentration exhaust gas cleaning;
• The bulky device takes up much space;
• Dust-collecting equipment is required before the adsorption system
The adsorption process is utilized for gases that are not flammable or difficult to burn but are valuable and require recovery It is also used for gases with low concentrations in the exhaust gas where other degassing processes are not applicable
Oxidation is a process that converts pollutants to less harmful or non-toxic forms, either through high temperature (incineration) or low temperature (biological processes)
This method can be used when pollutants in the exhaust stream are too high to be released directly into the air but too low to be effectively enriched or recovered Additionally, there may be certain situations where recovery is not feasible
It can be used to control VOCs and offshore oil exploitation processes a Combustion (Incineration)
These systems use thermal oxidation technology to remove VOCs with an efficiency of up to 95-99% in a temperature range from 704 to 982 o C [44]
As can be seen, VOCs are completely oxidized at higher temperatures than theoretical temperatures required(Table 1.7)
Table 1.7 Theoretical temperatures that decompose 99.99% of VOCs by thermal oxidation method in one-second residence time [45]
• Higher temperature than catalytic oxidation;
• High cost b Biological oxidation (Biofitration)
Biofiltration is a pollution control technique that employs biologically active media to remove biodegradable volatile organic compounds (VOCs), odors, and hazardous chemicals from polluted air This multi-step process involves passing untreated air through beds of biologically active media, where microorganisms oxidize pollutants and convert them into harmless carbon dioxide and water Biofiltration is an effective method for treating various air pollution problems, particularly in industries such as chemical manufacturing, petrochemical processing, and wastewater treatment facilities.
• Influenced by temperature and humidity c Catalytic oxidation
Catalytic oxidation works similarly to thermal oxidation but at a lower temperature, typically around 371 to 482 °C Using catalysts reduces the energy requirements for combustion [47]
Catalysts are substances that facilitate certain reactions without being altered themselves, thus affecting the reaction rate Catalysts used in the gas purification processes involve heterogeneous catalysis using solid catalysts that exhibit positive catalysis [48]
The catalytic oxidation process comprises multiple stages, including transferring reactants from the fluid to the catalytic surface and products from the catalytic surface back to the fluid Additionally, reactants and products diffuse into and out of the catalyst's pores Subsequently, reactants undergo activated adsorption, while products undergo desorption at the interface Finally, surface reactions of adsorbed reactants take place, culminating in the formation of chemically adsorbed products [48] Advantages:
• Influenced by some combustion products
Thus, various techniques are available for treating VOCs, each with advantages and disadvantages For the process of thermal oxidation, benzene concentration must fall within the range of 12000-78000 ppm, while the concentration of toluene should be within the range of 11000-71000 ppm for the combustion process to take place effectively Consequently, if either benzene or toluene concentration falls below the specified range, adding fuel becomes necessary to ensure optimal conditions for the process Thus, catalytic oxidation is highly effective for removing volatile organic compounds (VOCs) because it operates at lower temperatures than thermal oxidation This method provides several advantages, such as mitigating the impact of climate change and reducing energy consumption for environmental protection.
OVERVIEW OF CATALYTIC OXIDATION OF VOC s
Much research in the world has been conducted on catalysts for VOC oxidation, consisting of the noble-based catalysts such as Pt, Pd, Ag, and Au, and the non–noble metal-based catalysts such as Cr, Cu, Mn, Al, Ce, Co, and Fe
In the world a Noble metal-based catalyst
Noble-based catalysts have very high activity for oxidizing VOCs at low temperatures (< 200 o C) The catalytic activity of noble-based catalysts depends on the metal's nature, the support's nature, and the kind of compound to be oxidized [18] According to previous studies, Pt was the highest activity of the noble-based catalyst for VOCs oxidation at low temperatures In the study of Y Guo et al [49], the Pt/eggshell-Ar catalyst had high low-temperature activity 90% benzene can be converted at a temperature of 178 °C (T90= 178 o C), and catalytic activity remained stable over a long time Results showed that the catalyst remains active with water vapor and CO2 in the feed stream Z Rui et.al studied the Pt/HWT18 catalyst The results show that toluene conversion is 95% at 200 o C (T95= 200 o C) [50] According to H-J Joung et al., benzene and toluene were completely oxidized at temperatures as low as 112 o C and 109 o C on a 30 wt% Pt/CNT catalyst (T100 (Benzene) = 112 o C; T100
(Toluene) = 109 o C) [51] In addition, Pd is the best catalyst for oxidizing benzene and toluene Barakat and co-workers [52] studied the Pd5CeTi catalyst for toluene oxidation, showing that T100 = 250 o C H Deng et al [53] proved that the catalytic activity of the 0.8 wt% Pd-Ceramic-S catalyst (Pd supported on the ceramic fiber leached with H2SO4 before supporting) was very high, T90 = 225 o C According to K Bendahou et al., the 0.5 wt% Pd/SBA-15 and 1 wt% Pt/SBA-15 samples are highly active for toluene oxidation The palladium catalyst is more active than the platinum catalyst Thus, it can catalyze the complete toluene oxidation at 200 o C [54]
Noble-based catalyst for toluene oxidation was recently published by A Aboukaı¨s et al [55] with the M/CeO2 catalyst with M: Au, Ag, Cu The catalytic activity results showed that the Au/CeO2 catalyst prepared by deposition- precipitation methods could convert 100% toluene and propylene at 300 o C Han and colleagues researched the Au/Fe2O3 catalyst, the toluene conversion efficiency are
G Long et al studied the Ag/eggshell catalysts prepared by the impregnation method for benzene oxidation and compared them with the Ag/com-CaCO3 catalyst The Ag/eggshell showed higher activity results T90 = 257 o C The presence of water vapor or CO2 in the feed steam decreased catalytic activity but did not influence the structure of catalysts [57] Table 1.8 summarizes several studies on noble-base catalysts worldwide for VOCs oxidation These studies indicated that noble-based catalysts exhibited high activity in the VOC oxidation process However, they have high manufacturing costs, limited source materials, and are easily influenced by many substances in the feed stream
Table 1.8 The noble metal-based catalyst for VOCs oxidation in the world
N o Catalyst The preparation method Conditions Temperature
2 Pt/HWT18 simple ultrasonic- aided incipient wetness impregnation
3 Pt/carbon nanotube molecular-level mixing
Ceramic-S ultrasonic-assisted impregnation method on ceramic support
Pd/SBA-15 incipient wetness - 1000 toluene ppm
- GHSV = 100000 mL/(g h) 257/90 [57] b The non - noble metal– based catalyst
Recently, many studies have shown that non-noble metal oxides are potential catalysts for replacing noble metals in the VOC oxidation process They have various sources, low prices, and high activity These oxides have high electron activity and various positive oxidation states Although their activity is less than noble oxides, they are less poisonous and have good resistance to catalytic poisoning They are oxides of some transition metals such as Mn, Ce, Cu, and Co [59-60]
Researchers have extensively studied non-noble metal catalysts in recent times Huang et al [61] fabricated manganese oxide catalysts via a hydrothermal synthesis involving a 1:1 molar ratio mixture of -MnO2 and β-MnO2 Their experiments utilized an inlet toluene concentration of 500 ppm, a gas hourly space velocity (GHSV) of 30,000 mL/(g.h), and a temperature of 205 °C to achieve T100 combustion.
J Du et al.[62] demonstrated the effectiveness of the Mn0.6Ce0.4O2 catalyst for the toluene oxidation process, T100 = 210 o C Furthermore, Y Wang and the group [63] showed the effectiveness of the morphological forms of Mn2CeOx for the toluene oxidation process with T90 = 215 o C Results of this study show that morphology affects the activity and durability of Ce-Mn oxide catalysts when oxidizing toluene Catalysts in nanosheet form have the highest activity In addition, CuMnOx with different Cu/Mn ratios have been studied in powder form, but no research on these catalysts on cordierite substrate has been published Hu and colleagues [64] published the results of evaluating the catalytic activity of the CuMnO catalyst For the
Cu1Mn1O catalyst, T90= 214 o C, whereas for the Cu1Mn2O catalyst, T90= 224.4 o C, and for the Cu2Mn1O catalyst, T90= 234.3 o C, respectively In addition, Z.Ye et al [65] demonstrated that the CuMnOx catalyst prepared by the redox precipitation method has higher activity than the catalyst prepared by the co-precipitation method The CuMnOx with the molar Cu/Mn ratio of 0.52 prepared by co-precipitation showed T90= 200 o C Meanwhile, CuMnOx synthesized by the redox precipitation has T90= 190 o C According to Liu and colleagues [66], the CuMnOx-HS with a hollow spherical morphology (with the molar Cu/Mn ratio of 0.57) showed the highest catalytic activity for the toluene oxidation process (90 % toluene conversion at 212 o C)
Furthermore, Li et al [67] studied the catalytic activity of the CoMnOx catalyst The result showed that the benzene is oxidized completely at 210 o C over the Co1Mn1Ox catalyst Y Wang and colleagues [68] showed the activity of the Mn-Co catalysts with a flower-like morphology The CoMnOx catalyst (with the molar Co/Mn ratio of 4) exhibited the best catalytic activity (100 % toluene conversion at 239 o C) The excellent activity and stability of toluene oxidation over Mn-Co catalyst comes from the large surface area and porous morphology
Table 1.9 summarizes several studies on non – noble metal-base catalysts used for VOC oxidation
Table 1.9 Non - noble metal – based catalyst for the VOCs oxidation process in the world
MnO2 and β-MnO2 with ratio 1:1 hydrothermal
2 Mn0.6Ce0.4O2 combining redox precipitation and hydrothermal
(the molar Cu/Mn ratio of 1) hydrothermal
(the molar Cu/Mn ratio of 0.5)
(the molar Cu/Mn ratio of 2)
(the molar Cu/Mn ratio of 0.515)
(the molar Cu/Mn ratio of 1.2)
(the molar Cu/Mn ratio of 0.57)
(the molar Cu/Mn ratio of 1) annealing Co−Mn−1,3- propanediol precursors at
(the molar Co/Mn ratio of 4) simple template-free autoclave strategy
In summary, the catalyst exhibited the highest catalytic activity for the VOC oxidation process, consisting of noble and non–noble metal oxides such as Pd, Pt, Au,
Mn, Cu, Ce, and Co
In Vietnam a The noble metal–based catalyst in Vietnam
Following research trends in the world, in Vietnam, some authors had the research results on noble-based and non-noble catalysts for VOCs oxidation The T.H Pham et al.[69] synthesized the 1% Pd/γ-Al2O3 catalyst for the toluene oxidation process The results showed that T98= 250 o C In addition, Pt–CuO supported on γ
Al2O3, TiO2, CeO2, and γAl2O3 + CeO2 catalysts have been prepared and studied The catalyst containing CeO2 (PtCu/Ce and PtCu/CeAl) exhibited the best activity in oxidizing CO, xylene, and their mixture This catalyst can oxidize p-xylene completely individually at 300 0 C, and oxidize for the mixture at 200−225 o C (PtCu/Ce ) and 225−275 o C (PtCu/CeAl), respectively Catalyst has high activity at low temperatures and is unaffected by water vapor The catalyst system is suitable for oxidation at low temperatures [70].Recently, Prof Le Minh Thang and colleagues [71] studied the catalytic activity of the Au/MnCoCe oxide catalyst for toluene treatment Results show that 1% Au/MnCoCe is a potential catalyst for the complete oxidation of toluene with a toluene conversion of 100% at 300 o C Moreover, the combined adsorption-oxidation process decreases the reaction temperature over the catalyst system (the MnCoCe/AC and the Au/MnCoCe catalyst) At first, toluene was adsorbed on the catalyst MnCoCe/AC After the saturated adsorption at room temperature, the reactor temperature increased to 180 o C for the toluene desorption process and oxidation process with oxygen over the Au/MnCoCe catalyst Thus, combining the adsorption-oxidation processes improves the activity and decreases the temperature for complete toluene oxidation (T100= 250 o C)
Table 1.10 summarizes several studies on noble-base catalysts in Vietnam used for VOCs oxidation
Table 1.10 Noble metal-based catalyst in the world for VOCs oxidation in
250/100 b The non–noble catalyst in Vietnam
According to T.M Nguyen et al., Copper-doped manganese oxide catalyst exhibited high catalytic activity in the m-xylene oxidation; m-xylene is oxidized completely at 200 o C [72] In addition, Prof Le Minh Thang's group prepared a 10%
CoxCuyOz/AC catalysts with a Co/Cu molar ratio of 1:1 exhibit complete toluene oxidation upon its desorption from the catalyst at 180°C This finding was demonstrated by researchers who also experimented with different catalyst supports such as MCM41 and silica gel.
CoxCuyOz/MCM41 catalyst has better activity than that supported on silica gel, and toluene can be completely oxidized at a temperature of 450 o C [74]
Table 1.11 summarizes several studies on non-noble catalysts used for VOC oxidation in Vietnam
Table 1.11 Non-noble metal-based catalyst for VOCs oxidation process in
18000 mL/(g h) for the combined adsorption – oxidation process
Tables 1.9 and 1.11 demonstrate the potential of non-noble catalysts for oxidizing VOCs The results showed that the manganese-based catalyst displayed high catalytic activity for VOCs oxidation at low temperatures
OVERVIEW OF THE SYNTHESIS OF THE CATALYST
Sol-gel refers to a process wherein a gel is produced from the particles of a sol due to attractive forces causing the particles to bind together, creating a network In simpler terms, the sol-gel process involves the formation of a gel through the aggregation of particles in a sol This process is characterized by transforming a homogeneous solution of soluble monomer precursors into a gel, or sometimes a slurry, regardless of the underlying physicochemical mechanisms [104]
The coprecipitation method involves mixing cations in a solution and inducing precipitation to form desired catalysts This technique allows for rapid synthesis of homogeneous nanomaterials with varying size distributions The precipitation reaction is initiated by adding a primary solution (e.g., NaOH), and the resulting precipitates are then filtered, washed, and dried for further processing It is crucial to note that pH adjustments significantly influence the final product obtained using the coprecipitation method.
Hydrothermal synthesis involves the growth of crystalline material from an aqueous solution at elevated temperatures and vapor pressure This process typically utilizes a specially designed autoclave for the reactions It is known for its controllable synthesis strategy, where the crystalline phase, size, and morphology of the synthesized nanoparticles are influenced by reactant concentration, reaction temperature, time, pH value, and the types of solvent/surfactant used [106]
The thermal evaporation method involves creating a catalyst by evaporating a metal salt solution The resulting product is then dried and subjected to calcination Impregnation method
In impregnation methods, the support is impregnated in a solution containing the precursor, followed by drying and calcination at elevated temperatures to create a covalent bond between the support and the precursor This process is repeated multiple times until the catalyst achieves the desired percentage of the activated phase.
THE SUMMARY OF LITERATURE REVIEW
Various kinds of VOCs in the atmosphere should be treated to protect human health and the environment if their concentration exceeds a certain level BETX, such as benzene and toluene, was considered due to its impact on the environment and human health Several methods have been devised to tackle VOCs, and catalytic oxidation and adsorption are the most effective techniques
In the realm of VOC oxidation, pure metallic oxides, like manganese and copper, hold significant potential due to their robust catalytic performance However, the true potential of bimetallic oxides, such as manganese-copper, manganese-cobalt, and nickel-cobalt, remains largely untapped, with research yet to explore their optimal compositions for maximum catalytic activity Moreover, the emphasis on powder catalysts in previous studies fails to address the practical industry need for catalysts supported on stable substrates Furthermore, the impact of toxic industrial gases, especially sulfur compounds, warrants further investigation Notably, the combination of complete oxidation and adsorption techniques as a means to lower treatment temperatures has not been fully explored in the existing literature, offering avenues for innovative research.
Therefore, this study seeks to synthesize and investigate the bimetallic oxides of Cu-Mn, Co-Mn, and Ni-Co for completely oxidizing benzene and toluene The ratios of metals are optimized to enhance catalytic activity and reduce reaction temperature The optimal compositions are applied to produce catalysts impregnated on cordierite substrate to make the catalysts applicable in industry The optimal bimetallic catalysts are also impregnated on activated carbon to use as adsorbent for the adsorption process In the pilot system, complete oxidation and adsorption are applied for the first time to reduce the treatment temperature and increase the treatment efficiency.
EXPERIMENT
THE SYNTHESIS OF THE CATALYST
Chemical substances and substrates used to prepare catalysts in this study are listed in Table 2.1
Table 2.1 Chemical substances and substrates utilized for catalyst synthesis
Chemical substances and substrates Origin, M (g/mol), purity (%)
Ce(NO3)3.6H2O Xilong- China; MC4,23; ≥ 99%
Mn(NO3)2 solution 50% Xilong - China; M= 178,95; C = 49-51%
HNO3 solution 65% Merck – Germany; 1,37 g/ml; C ≥ 65%
Activated carbon Tra Bac JSC
Cordierite Hanoi University of Science and Technology
The NiCoOx catalyst was synthesized via a citric acid-assisted sol-gel method Samples were prepared with Ni/Co molar ratio of 1.5/1.5, denoted as NCO-1.5; ẵ, denoted as NCO-1.0 and 2/1, denoted as NCO-0.5 The desired amounts of Ni(NO3)2ã6H2O and Co(NO3)2ã6H2O were dissolved into 20 mL distilled water, and the equimolar citric acid of total metal precursors was added The solution was then transferred to an oven at 120 o C overnight to form a foam-like gel, then ground and calcined in air at 450 o C for 4 hours at a heating rate of 5 o C/min [107]
Fig 2.2 The acid-assisted sol-gel synthesis of the NCO catalyst
2.1.3 Synthesis of the manganese oxide a The hydrothermal synthesis of the manganese oxide
The hydrothermal method was successfully implemented in the synthesis of single metal oxides of manganese, including α-MnO2, α-MnO2, and β-MnO2, as demonstrated by N Huang et al [61] The preparation of α-MnO2 catalyst involved dissolving KMnO4 and H2C2O4·2H2O in distilled water, adding the H2C2O4 solution dropwise to the KMnO4 solution, and heating the mixture in a Teflon-lined autoclave at 120 °C for 12 hours The α-MnO2 catalyst was likewise synthesized but subjected to an elevated temperature of 150 °C.
9 mL ethanol + 1 mL PEG 2 mL deionized water
Calcinated 550 o C (1 o C/min)/5h Ni(NO3)2.6H2O Cu(NO3)2.6H2O 0.7 g Lysine 0.4 g citric acid
The NCO catalyst oC; the obtained catalysts were denoted as the α-MnO2120 and the α-MnO2150, respectively
Fig 2.3 The hydrothermal synthesis of α-MnO 2 120 and α-MnO 2 150 catalyst
Fig 2.4 The hydrothermal synthesis of β-MnO 2 catalyst
For the synthesis of the β-MnO2 catalyst, manganese sulfate monohydrate (MnSO4·H2O, 1.69 g) and ammonium persulfate ((NH4)2S2O8, 2.28 g) were dissolved in deionized water (80 mL) and stirred magnetically for approximately 30 minutes to achieve a uniform solution This solution was then transferred to a Teflon-lined stainless steel autoclave (200 mL) and subjected to hydrothermal treatment at 160 °C.
12 hours in an oven The product was collected, washed, filtered, dried at 80 o C, and then calcined at 300 o C for 5 hours b The sol-gel synthesis of the manganese oxide
The MnO2 catalyst was prepared using the sol-gel method 4.62 mL Mn(NO3)2 solution 50% was dissolved with 60 mL distilled water, and 4.16 g citric acid was dissolved with 16.64 mL distilled water; then, two solutions were mixed to form a homogeneous solution The solution was heated to 60 o C and evaporated at this temperature until a sticky gel was obtained The product was dried at 120 o C for 12 hours and calcined at 500 o C in the air for 3 hours at a 1 min/°C heating rate [108]
Fig 2.5 The sol-gel synthesis of the MnO 2 catalyst
The following table summarizes the NiCoOx and MnO2 catalysts
Table 2.2 Summarizes the synthesized non-noble metal-based catalysts
No Sample name The preparation method The molar ratio
1 NCO-1.5 a citric acid-assisted sol-gel Ni/Co= 1.5/1.5
2 NCO-1.0 a citric acid-assisted sol-gel Ni/Co= 1/2
3 NCO-0.5 a citric acid-assisted sol-gel Ni/Co= 2/1
2.1.4 The synthesis of CuMnO x catalyst a The hydrothermal synthesis of CuMnO x catalyst
Following the studies of J.Hu [109], CuMnOx catalyst was prepared using the hydrothermal method Firstly, 0.59 g KMnO4, 1.23 g Cu(NO3)2, and 1.04 g MnSO4ãH2O were dissolved with 75 mL distilled water, then stirred magnetically for about 30 min (300 rpm) to form a homogeneous solution before it was moved into a Teflon lined stainless steel autoclave (200 mL) After that, the solution in the autoclave was heated to 160 °C for 24 hours in an oven with a heating rate of 3 min/°C The product was collected, washed, filtered, dried at 120 °C, and then calcined at 500 °C in the air for 3 hours with a heating rate of 1 min/°C The molar ratio of Cu/Mn is 0.52 The catalyst was assigned as CuMnOx HT
Fig 2.6 The hydrothermal synthesis of the CuMnO x HT catalyst b The co-precipitation synthesis of the CuMnO x catalyst
The CuMnOx catalyst was prepared using the co-precipitation method 2.31 mL Mn(NO3)2 solution 50% was added with distilled water to form 30 mL Mn(NO3)2 solution, 1.23 g Cu(NO3)2.3H2O was dissolved with distilled water to form 30 mL Cu(NO3)2 solution, then two solutions were mixed to form a homogeneous solution Afterward, 20% K2CO3 solution (4,16g a K2CO3 and 16.68 mL H2O) was added to the solution to form the precipitation The product was washed, filtered, dried at 120 oC for 12 hours, and calcined at 500 o C in the air for 3 hours with a 1 min/°C heating rate The catalyst was assigned as CuMnOx CP1 [89] CuMnOx was synthesized in the same condition but calcined at 500 o C in the air for 3 hours twice at a heating rate of 1 min/°C to evaluate the influence of calcination conditions on catalytic activity
The catalyst was assigned as the CuMnOx CP 2 catalyst The molar ratio of Cu/Mn is 0.52
Fig 2.7 The co-precipitation synthesis of the CuMnO x CP catalyst c The thermal evaporation synthesis of the CuMnO x catalyst
Fig 2.8 The thermal evaporation synthesis of the CuMnO x TE catalyst
The CuMnOx catalyst was prepared using the thermal evaporation method with the same molar Cu/Mn ratio of the CuMnOxCP1 2.31 mL Mn(NO3)2 solution Mn(NO3)2 50% solution
The CuMnOx CP1 (CuMnOx CP2) catalyst catalyst (
50% was dissolved with distilled water (form 30 mL Mn(NO3)2 solution 50%), then 1.23 g Cu(NO3)2.3H2O was dissolved with distilled water (form 30 mL Cu(NO3)2 solution), then two solutions were mixed to form a homogeneous solution Afterward, the homogenous solution was evaporated at 80 o C, then dried at 120 o C for 12 hours Finally, the product was calcined at 500 o C in the air for 3 hours with a 1 min/°C heating rate The molar ratio of Cu/Mn is 0.52 The catalyst was assigned as CuMnOx
TE d The sol-gel synthesis of the CuMnO x catalyst
The CuMnOx catalyst was prepared using the sol-gel method Mn(NO3)2 0.33
M solution was mixed with Cu(NO3)2 solution 0.17 M to form a homogeneous solution Afterward, a 20% citric acid solution was added The solution was heated to 60 o C and evaporated at this temperature until a sticky gel was obtained The product was dried at 120 o C for 12 hours and calcined at 500 o C in air for 3 hours with a 1 min/°C heating rate The catalyst was assigned as CuMnOx [110]
The CuMnOx series of catalysts were prepared with varying molar ratios of Cu to Mn (0.33-3) using the same synthesis conditions The catalysts were designated according to their Cu/Mn molar ratios: CuMnOx13 (0.33), CuMnOx12 (0.52), CuMnOx11 (1), CuMnOx21 (2), and CuMnOx31 (3).
Fig 2.9 The sol-gel synthesis of the CuMnO x catalyst
Sticky gel Cu(NO3)2 solution
2.1.5 The sol-gel synthesis of CoMnO x catalyst
The CoMnOx catalyst was synthesized using a process similar to the CuMnOx catalyst (Fig 2.9)
The series of the CoMnOx catalyst was synthesized under the same conditions but with a different molar Cu/Mn ratio (in the region of 1 – 9) The catalyst with a Co/Mn molar ratio of 1 is assigned as CoMnOx33 catalyst The catalyst with a Co/Mn molar ratio of 1.54 is assigned as CoMnOx32 catalyst The catalyst with a Co/Mn molar ratio of 3 is assigned as a CoMnOx31 catalyst The catalyst with a Co/Mn molar ratio of 6 is assigned as CoMnOx 61 catalyst The catalyst with a Co/Mn molar ratio of 9 is assigned as CoMnOx91 catalyst
The following table displays the details of mixed manganese and copper and mixed manganese and cobalt oxide catalysts
Table 2.3 Summary of the synthesized manganese-based catalyst in powder form
No Catalyst Preparation method Molar Cu/Mn ratio
CuMnOx catalyst in powder form
CoMnOx catalyst in powder form
2.1.6 The impregnation synthesis of CuMnO x 12/cordierite catalysts
A mixed copper manganese oxide catalyst supported on cordierite was synthesized using the impregnation method with citric acid
- 10g Cordierite was dried at 80 o C for 1 hour
Step 2: Synthesis of the catalyst
- A Mn(NO3)2 3.3 M solution was mixed with Cu(NO3)2 solution 1.7 M with a molar ratio of Cu/Mn of 0.52 to form a homogeneous solution Afterward, a 20% citric acid solution was added The solution was heated to 60 o C, and was evaporated at this temperature until a sticky gel was obtained Cordierite was added to the solution for
1 hour The impregnated cordierite was dried at room temperature for 30 minutes, then dried at 80 o C for 1 hour Afterward, the product was calcinated at 250 o C for 30 minutes with a heating rate of 5 o C/min The impregnation process was repeated, but the product was calcinated at 250 o C for 3 hours with a heating rate of 3 o C/min to get more of the CuMnOx12 catalyst on the cordierite surface Finally, after the third impregnation time, the solid was calcined at 500 o C at 3 o C/min for 3 hours [87] The obtained catalyst consisted of 23 wt% of the activated phase
Fig 2.10 The impregnation synthesis with citric acid of CuMnO x 12/cordierite
The remaining solution after the process was evaporated at 80 o C until the water evaporated completely, then was dried at 120 o C for 24 hours and was calcined at 500
Drying at room temperature for 30 minutes
Sticky gel Cu(NO3)2 solution
Calcination at 250 o C/30 minutes with rate at 5 o C/ minutes
CuMnOx12/cordierite oC for 3 hours with a heating rate of 3 o C/min The catalyst was assigned as CuMnOx
2.1.7 The impregnation synthesis of CuMnO x 12/AC catalysts
The following is a description of the impregnation synthesis of CuMnOx12/AC catalysts that studied for a long time on a laboratory scale:
Stage 1: Preparation of activated carbon (AC)
10g of AC was dried at 80 °C for 1 hour
Step 1: 6.93mL Mn(NO3)2 50% solution was mixed with 3.69 g Cu(NO3)2.3H2O for a Cu/Mn molar ratio of 0.52 Then, 60 mL of water was added to form a homogeneous solution
Step 2: 3-4 drops of nitric acid were added and mixed to prevent the hydrolysis of the metallic salt during the synthesis
Step 3: AC was added to the solution and impregnated for 8 hours
Step 4: After impregnation, the impregnated AC was dried at 100°C for 8 hours to evaporate water
Step 5: The obtained activated carbon was calcinated in N2 flow at 350 °C for 3 hours with a heating rate of 2 °C/min The resulting catalyst was identified as the CuMnOx12/AC catalyst
Fig 2.11 The impregnation synthesis of CuMnO x 12/AC catalylst
Impregnating for 8hours Stirring, 60 o C a homogeneous solution
Calcination 350 o C/3h (2 o C/ min) in N2 flow CuMnOx12/AC
The remaining solution after the procedure was evaporated at 120 °C for 2 hours until all the water had evaporated Then, it was calcined at 350 °C for 3 hours with a heating rate of 3°C/min The catalyst was identified as CuMnOx12 powder (AC)
2.1.8 The impregnation synthesis of CoMnO x 31/cordierite catalysts
The CoMnOx91/cordierite catalyst was synthesized using a process similar to the CuMnOx12/cordierite catalyst (Fig 2.11)
Table 2.4 Summary of the synthesized manganese-based catalyst supported on substrate/support
The molar Cu/Mn ratio
The percentag e of the activated phase (%)
CuMnOx12/cordie rite and the
1 5% CuMnOx12/ cordierite Impregnation with citric acid 0.52
The remaining solution after the impregnation process with citric acid
The remaining solution after the impregnation process
4 23% CoMnOx91/ cordierite Impregnation with citric acid 9 23
The remaining solution after the impregnation process with citric acid
CATALYST CHARACTERIZATION
2.2.1 N 2 Adsorption/ Desorption isotherm method (BET)
The nitrogen adsorption-desorption isotherm method is widely used to determine the material's surface area and pore size distribution
The Brunauer-Emmett-Teller (BET) method [111] is applied to derive the surface area from physisorption isotherm data For this purpose, applying the BET equation in the form is convenient
𝑝 0 (Eq 2.1) where na: the amount adsorbed at the relative pressure p/po; nm: the monolayer capacity;
C: a constant that is dependent on the isotherm shape
Then, the BET method is applied to the calculation of the surface area
SBET = nm L am (Eq 2.2) Where L: is the Avogadro constant am: is the average area
In addition, the BET measurement experiments are usually conducted at 77 o K, and the am is 0.162 nm 2 After the adsorption process reaches saturation, the N2 multilayer adsorption process occurs, and capillary size can be determined through the equilibrium pressure by the Barrett, Joyner, and Halenda (B.J.H) method or density functional theory DFT
In this thesis, the specific surface area of samples was measured at 77 K by the BET method using the N2 adsorption and desorption isotherm method on an ASAP 2010- Micromeritic (LIKAT, Rostock University ) and a Micromeritics Gemini VII 2390 device (GeViCat center, Hanoi University of Science and Technology)
X-ray diffraction (XRD) is an effective method to determine material structure, such as phase composition and crystalline size
X-rays are a form of electromagnetic radiation with a wavelength of about 0.01 to 10 nm (0.1 – 100 Å), discovered by W.C Rửntgen in 1895 X-ray energy is high so that it can penetrate solid objects
Based on the theory of crystal structure, the crystal lattice is built from atoms or ions or is evenly distributed in space in a particular order Atom in the crystal acts as a special diffraction grating when X-ray beams are irradiated into the material Atoms, ions, or molecules excited by the X-ray beam will become centers that emit reflected rays [112]
When passing through the material, X-ray radiation is diffracted by the atomic faces, corresponding to the distance d between the faces of the material If the angle between the incident ray and the diffracted ray is 2θ, according to Bragg's law:
2𝑑𝑠𝑖𝑛θ = nλ (Eq 2.3) Where: d: crystalline size; λ: the wavelength of X-rays; θ: the angle of diffraction; n: The natural number that represents the level of reflection
A diffraction pattern characterized for the material can be obtained by changing the incident angle Location, intensity, and peak width will provide information about the material's structure Therefore, the crystalline size is determined according to the Scherrer formula:
𝛽𝑐𝑜𝑠θ (Eq 2.4) Where: d: crystal size (nm); β: the width at the half-height of the peak; θ: the angle of diffraction (radians); λ: the wavelength of X-rays (0.154 nm);
Within the thesis's research scope, The D8 Advance Bruker device at the Faculty of Chemistry, Hanoi University of Science, Vietnam (with a Kα wavelength of 0.15406 nm, a scanning speed of 0.03 degrees/second, and a scanning angle 2θ varying from
X-ray diffraction (XRD) patterns were obtained using two diffractometers: the Stoe STADI P diffractometer (with a CuKα wavelength of 0.15406 nm, a scanning speed of 0.03 degrees/second, and a scanning angle 2θ varying from
2.2.3 Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX)
SEM is used to observe the surface phenomena of the materials A high-energy electron and the outcoming electrons/X-rays are used to capture the image of a sample SEM can give a detailed visual image of a particle with a high-quality and spatial resolution of 1 nm Although SEM only gives surface information, it is still considered a powerful instrument that can be used in characterizing the sample's crystallographic, magnetic, and electrical features and determining if any morphological changes of the particle have occurred after modifying the sample surface with other molecules Moreover, SEM can provide qualitative information, including topography, morphology, composition, and crystallographic information [113]
Additionally, it provides information about the surface features and texture, shape, size, and arrangement of the particles on the sample's surface The composition of elements and compounds in the sample and the percentage of elements in the compounds can also be provided Therefore, SEM is a multipurpose instrument that examines and analyzes high-resolution materials [113]
SEM/EDS was measured on a JCM-7000 NeoScope™ Benchtop SEM device, a JEOL brand magnified 35,000 times at GeViCat center, Hanoi University of Science and Technology
2.2.4 Hydrogen temperature – programmed reduction (TPR-H 2 )
Hydrogen temperature-programmed reduction (TPR-H2) measures a material's surface reducibility by heating it in a reducing environment.* The technique employs a thermal conductivity detector (TCD) to monitor changes in gas flow thermal conductivity, creating a TCD concentration curve.* This curve indicates the amount of hydrogen consumed during the reduction process, providing insights into the material's catalytic properties.
H2 consumed during the reduction process
The TPR-H2 profiles of the catalysts were measured using an AutoChem 2920 II– Micromeritics device at the GeVicat Center, Hanoi University of Science and Technology A quartz reactor with a U-shape configuration was used to load a sample weighing 100 mg to perform this test The reactor was heated at a rate of 10 °C/min in a He gas flow (50 mL/min) The temperature was raised to 300 °C and remained for an hour to remove any adsorbed substances in the sample After that, the system was cooled to 50 °C by the He gas flow The TPR-H2 experiment was conducted using a 5% H2/Ar gas mixture (30 mL/min) The temperature was increased from 25 °C to 900 °C at a heating rate of 10 °C/min The consumption of H2 was continuously monitored by a thermal conductivity detector
2.2.5 Fourier transform infrared spectroscopy (FTIR)
Fourier transform infrared spectroscopy (FTIR) employs infrared radiation to determine the functional groups within materials This technique falls within the infrared (IR) region of the electromagnetic spectrum, which spans from the visible to microwave ranges The IR region is further classified into three sections: near IR (14000– 4000 cm-1), mid-IR, and far IR.
(4000–400 cm -1 ), and far IR (400–40 cm -1 ) IR spectroscopy is an advanced and extensively used analytical tool that explores the sample's chemical structure by irradiating it with IR radiations [113]
The basic principle of FTIR can be presented as follows: When infrared radiation is bombarded on a sample, it absorbs the light and creates many different vibration modes This absorption relates to the nature of bonds in the molecule The frequency ranges are typically used in region 4000–600 cm -1 Before the sample analysis, the background is recorded to avoid the influence of air and water vapor contamination peaks The absorption spectrum indicates various vibrations of the bonds in the sample molecule Thus, the functional group can quickly be identified [113]
FTIR spectra were measured using a Nicolet IS50 FT-IR spectrometer in the region
400 – 4000 cm -1 at the GeViCat center at Hanoi University of Science and Technology
Electron Paramagnetic Resonance (EPR) or Electron Spin Resonance (ESR) is a method applied in heterogeneous catalysis research to study the nature of materials containing free and unpaired electrons The technique's high sensitivity allows the investigation of low concentrations of active sites Additionally, it has another advantage: it detects the highly reactive paramagnetic intermediates during the reaction by in situ EPR spectroscopy without any spectroscopic interference
In terms of application, EPR is a selective method providing information about the oxidation states Combined with the studies using spectroscopy methods such as NMR and FT-IR, it is best to gather more information from interactions between the electron spin and proton
EVALUATION OF THE OXIDATION OF VOCs OVER THE CATALYSTS
The oxidation of VOCs is carried out in the reaction system presented in Fig 1.12 The powder samples (0,1 g) were placed in a micro-reactor An N2 flow of 9.5 mL/min went through the VOCs tank to bring VOCs vapor to the reactor The inlet VOCs flow was kept constant, and the outlet flow was detected by an online gas chromatograph (GC) Thermo Focus (Italia) with a Thermal Conductivity Detector (TCD) The oxidation reaction occurred in a stainless steel reactor with a flow of 100% O2 (20mL/min) added to the VOCs flow (Fig 2.12)
1 VOCs tank; 2 the micro reactor; 3 the furnace; V1 – V7 Valse
4 The temperature controller of the furnace; 5 GC with TCD; 6 Flow measuring device MFC1.Mass flow controller for the flow of O2 ; MFC2.Mass flow controller for the flow of
N2 ; MFC3 : Mass flow controller for the flow of SO2 (H2S)
Fig 2.12 Schematic diagram of the VOCs oxidation process in the laboratory a The VOCs oxidation process
The direct VOCs oxidation process was conducted through the following steps :
Step 1 Operate the GC system
Step 2: Take VOC into the VOC tank
Step 3 0.1 g catalyst was placed into microreactor
Step 4 Operate and set parameters of the flow on the MFC
Step 5: Once the inlet VOC concentration had stabilized, the temperature was gradually ramped up from room temperature to 400 °C throughout the experiment The reaction occurred between 150 °C and 400 °C, with a 50 °C temperature gradient, to utilize the catalyst effectively over a wide temperature range, as in real-world scenarios
GC parameters in the oxidation of benzene and toluene were presented in Table 2.5
Table 2.5 Some parameters of GC in the complete oxidation of VOCs process
Block temperature ( o C) 180 Oven run time (min) 8,5
The N2 flow (ml/min) 9.5 Initial Time (min) 1
The He flow (ml/min) 20 Ramp ( o C/min) 60
Final temperature ( o C) 210 Final Time (min) 5 b A study for the durability of catalysts
The experiment was conducted at temperatures between 150 °C and 350 °C (representing the actual temperature range in exhaust gas mixtures) for 4 hours (minimum time for an experiment interval) to assess the catalytic stability The catalytic activity process was monitored and recorded during the experiment every 12.5 minutes c Influence of sulfur compounds on the catalytic activity of the catalyst
The CuMnOx12 catalyst's catalytic activity was evaluated at 250 °C for 10 hours in the presence of sulfur compounds A 0.1 g powdered catalyst was loaded into a micro-reactor, and a 9.5 mL/min nitrogen flow transported VOC vapor mixed with 1 mL/min of SO2 or H2S to the reactor The VOCs flow rate remained constant, while the outlet flow was analyzed using an online GC-TCD Oxidation reactions took place in a stainless steel reactor where a flow of pure O2 (17 mL/min) was combined with the VOCs flow mixed with 1 mL/min of SO2 or H2S (2% SO2, 98% N2).
N2) or (5% H2S, 95% N2) d Calculation of the catalytic activity
Toluene/benzene conversion was determined by the following equation:
𝑪 𝑻 𝒊 : toluene concentration of inlet flow at a temperature T (ppm) ;
𝑪 𝑻 𝒐 : toluene concentration of outlet flow at a temperature T (ppm)
The conversion of toluene to CO2 was calculated as the equation:
𝟐(𝑻): the percentage of toluene converted into CO2 (%)
𝑪 𝑪𝑶 𝟎 𝟐 ,𝑻 : the CO2 concentration of outlet flow at a temperature T (ppm).
EVALUATION OF THE VOCs ADSORPTION, DESORPTION -
For this experiment, benzene was selected as a representative for aromatic VOCs research in the adsorption and desorption-oxidation process conducted in the system shown in Figure 2.13 The process consists of two steps:
1 Adsorption of benzene on the catalyst
2 Desorption and oxidation of the adsorbed benzene from the catalyst
1g of catalyst was added to a microreactor When the flow of 2000 ppm benzene was stable, the benzene adsorption process took place at room temperature while the concentration of benzene was continuously detected by the GC After the saturated adsorption, the reactor’s temperature was increased to 150 o C - 250 o C and a flow of
O2 (17 ml/min) was injected to the reactor while the benzene flow was closed to perform the desorption process The outlet flow released from the reactor was continuously examined The inlet flow and the outlet flow were detected by an online Gas chromatograph (GC) with a Thermal Conductivity Detector (TCD)
Adsorption capacity was determined [73] as follows as
SAD: The area formed by the vertical and horizontal axes and the graph of the adsorption process (ppm.min)
𝐶 𝐵 𝑖 : Benzene concentration of inlet flow (ppm)
𝐶 𝐵 𝑡 : Benzene concentration of flow at exact time (t) (ppm)
∆t: Time interval (min) m: Mass of the catalyst (g) Desorption capacity was determined [73] as follows as
SDE: The area formed by the vertical and horizontal axes and the graph of the adsorption process (ppm.min)
Benzene conversion was determined by the folllowing equation
𝜂 𝐵 𝑎𝑑𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 : Benzene conversion in the adsorption-desorption and oxidation process (%);
𝐴 𝐵 ⬚ : The benzene amount of inlet flow
𝐴 𝐵 𝑂2 : The benzene desorption amount was oxidized at a temperature of T The conversion of benzene into CO2 was calculated as equation:
6(𝐴 𝐵 𝑖 −𝐴 𝐵 𝑂2 ) 𝑋 100 (𝐸𝑞 2.13) Where, 𝛾 𝐶𝑂 2 𝐵 𝑎𝑑𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 : the conversion of benzene into CO2 in the desorption
𝐴 𝐶𝑂 𝑂2 2 ,𝐵 : the CO2 amount from the benzene desorption amount was oxidized.
EVALUATION OF THE COMBINATION OF THE VOCs OXIDATION –
The experiment about treating the exhausted gas mixture from the waste tire pyrolysis process was illustrated in Fig 2.14 The process included several steps
1 The waste tyres were placed in a pyrolysis reactor measuring 1300 mm in length and 1000 mm in width, where the pyrolysis process was undergone at 550 °C
2 As the reaction occurred, the exhaust gas mixture passed through a separator device measuring 320 mm in diameter and 800 mm in height Next, the mixture was separated into oil and gas by a condenser measuring 250 mm in diameter and 1048 mm in height to condense pyrolysis oil The pyrolysis gas was then mixed with air and fed back to the outer part of the pyrolysis reactor to provide heat for the pyrolysis process
3 The exhaust gas mixture from the outer part of the pyrolysis reactor then went through an oxidation tower measuring 360 mm in diameter and 1096 mm in height, where oxidation reactions occurred at 250 °C over the 15% CoMnOx31/cordierite catalyst Afterward, the exhausted gas mixture passed through an adsorption tower measuring 360 mm in diameter and 1096 mm in height, containing the 7% CuMnOx12/AC catalyst, before being discharged into the environment
Waste tyre pyrolysis produces three primary products: char, pyrolysis oil, and pyrolysis gas Pyrolysis gas left the pyrolysis reactor while char stayed inside the reactor The exhaust gas was then cooled to separate pyrolysis oil, oxidized to provide heat for the pyrolysis reactor, and then went through to the oxidation tower, which was oxidized and passed through an adsorption tower before being released into the environment The GC/MS analysis determined the trend of principal gases present in the exhaust gas Concentrations of benzene and toluene were measured at three locations where gas samplings were taken: before and after the oxidation tower, after the adsorption tower, and before release into the environment, using exhaust gas equipment monitoring
Fig 2.14 Schematic of the exhaust gas treatment system for waste tyre pyrolysis process
RESULTS AND DISCUSSION
CATALYST CHARACTERIZATION AND SELECTION OF CATALYSTS
It is crucial to have an appropriate catalyst for the oxidation of volatile organic compounds (VOCs) The catalyst must demonstrate high catalytic activity, stability, and long-term effectiveness Recently, researchers have been investigating non-noble metal-based catalysts for VOC oxidation These studies focus on studying and developing catalysts that exhibit high catalytic activity in removing aromatic VOCs, such as toluene For this study, toluene was selected as a representative aromatic VOCs to evaluate the catalytic activity of non-noble metal catalysts
Previous research has focused on studying non-noble metal catalysts such as
Mn, Co, Cu, Ni, and Ce for the oxidation of VOCs Among these catalysts, manganese-based ones have demonstrated effectiveness, affordability, availability from various sources, and high activity in VOCs oxidation The catalytic activity of Mn-based catalysts is attributed to their capacity to form different oxidation states and their oxygen storage capacity (OSC) [116] a Catalyst characterization
BET surface areas of α-MnO2 150, β-MnO2, and MnO2 catalysts are shown in Table 3.1 The α-MnO2150 catalyst possessed surface areas around 83.20 m 2 /g, which is six times greater than that of β-MnO2 catalyst (12.91 m 2 /g) and the MnO2 catalyst (14.44 m 2 /g ) and approximately the surface area of α-MnO2120 catalyst Based on this data, it is reasonable to assume that the α-MnO2150 catalyst may exhibit a higher catalytic activity level than β-MnO2 and MnO2 catalysts
Table 3.1 BET surface areas, H 2 consumption, and composition of manganese oxide catalysts
4 MnO2 14.40 2.33 39.91 60.09 - a The data were determined by quantitatively analyzing H 2 – TPR profiles b Calculated based on the EDX results of the samples
The XRD technique determined the crystal structure of the manganese oxide catalysts Fig 3.1 showed the diffraction peaks at 2θ: 12θ: 12.6 o ; 18.2 o ; 28.6 o ; 37.3 o ; 42.8 o ; 59.3 o respectively, which corresponds to the lattice (110), (200), (310), (211), (301), (521) [117] The diffraction peaks indicate that the crystal phase of α-MnO2 belongs to the tetragonal crystal system with JCPDS Card No: 44-0141 The average crystalline sizes of the catalyst are determined by the Scherer equation [118] is 12.3 nm For the β-MnO2 catalyst, the diffraction peaks at 28.7°, 37.3°, 40.9 o , 56.7°, 59.3°, and 72.1° are ascribed to the lattice (110), (101), (111), (211), (210), (220) and (301) belong to the JCPDS No 240735 of β-MnO2 [132] The XRD pattern of MnO2 indicated the diffraction peaks at 2θ: 32.9 o (222); 38.28 o (400); 45.06 o (332); 49.16 o (134); 55.16 o (440) of bixbyite Mn2O3 (JCPDS: 089-4836)[119] Besides, an amount of Mn5O8 [120] was detected in the catalyst In addition, The XRD analysis result also showed the amorphous materials for the α-MnO2120 sample Perhaps the manganese oxide catalyst synthesized by the hydrothermal has amorphous material at low hydrothermal temperatures
Fig 3.1 XRD patterns of the manganese oxide catalysts
TPR-H2 profiles of manganese oxides are presented in Fig 3.1 According to previous studies, the reduction of MnO2 can occur as MnO2 →Mn2O3→ Mn3O4 → MnO [121] The reduction temperatures for each stage are different The reduction of MnO2→Mn2O3 can happen at low temperatures (about < 250 o C); the reduction of
Mn2O3 reduces to Mn3O4 at 250-320 °C, and Mn3O4 further reduces to MnO at 400-430 °C MnO reduction to Mn requires temperatures over 750 °C The α-MnO2150 and α-MnO2120 catalysts reduce at 231 °C The α-MnO2 150 catalyst reduces in two stages: MnO2 → Mn2O3 at 231 °C and Mn2O3 → Mn3O4 at 285 °C The β- MnO2 catalyst reduces in two stages: MnO2→ Mn3O4 at 264 °C and Mn3O4 → MnO at 382 °C The MnO2 catalyst reduces in three stages: MnO2→Mn2O3 at 271 °C, Mn2O3→ Mn3O4 at 363 °C, and Mn3O4 → MnO at 403 °C.
The reduction behavior of Mn3O4 and MnO2 catalysts was investigated at various temperatures using hydrogen temperature-programmed reduction (H2-TPR) The results showed that Mn3O4 transformed into MnO at 363 o C and 403 o C, respectively Among the catalysts tested, α-MnO2 150 and b -MnO2 exhibited higher total H2 consumption compared to others This suggests that α-MnO2 150 is more easily reduced, a property consistent with its BET surface area and catalytic activity.
Table 3.2 Total H 2 consumption of manganese oxide catalysts
Total H2 consumption (mmol/g) α-MnO2 120 231 7.34 7.34 α-MnO2150
MnO 2 Fit Peak 1 Fit Peak 2 Fit Peak 3 Cumulative Fit Peak
Fig 3.2 TPR-H 2 profile of manganese oxides catalysts (a TPR-H 2 profile of manganese oxides and b The cumulative fitting peak of MnO 2 catalyst)
Figure 3.3 shows the SEM images of manganese oxide catalysts For α-MnO2150 and α-MnO2120 catalysts, as can be seen from the pictures, the flowerlike whiskers of MnO2 were mixed with a few rods, the same as the study of Subramanian and Haoran [125,126] For the β-MnO2 catalyst, the catalyst showed a long, thin fiber structure (Fig 3.3), in accordance with a study by Du and co-workers [127] Whereas the MnO2 catalyst has a more porous structure (Fig 3.5) a α-MnO 2 120 b α-MnO 2 150 c β-MnO 2 catalyst d MnO 2 catalyst
Fig 3.3 SEM images of manganese oxide catalysts
Table 3.1 shows the atomic percentage of elements in the catalyst The results show that the molar Mn/O ratio is similar and approximately about 0.5 in accordance with the theoretical calculation Additionally, the results show the presence of K in the composition of α-MnO2120 and α-MnO2150 catalysts, which are in agreement with the reported results (Wang et al.) This might be because K + from the KMnO4 solution used during the synthesis is an exchange cation in the α-MnO2 structure to compensate for the negative charge generated when the cations Mn 2+ and Mn 3+ replace Mn 4+ in the MnO2 structure [128] b Catalytic activity of manganese oxide catalysts for toluene oxidation
The experiment was conducted with an inlet toluene concentration of 5000 ppm, GHSV of 5700 mL/(g.h), at temperature ranges from 150 o C to 400 o C A change in toluene conversion was presented, corresponding to the reaction time The catalytic activities of the manganese oxide catalysts are shown in Figure 3.4 a) At
150 o C, toluene conversion of manganese oxide catalysts is still low, but as temperature increases to 350 o C; the catalysts achieve a complete conversion, except for the MnO2 catalyst Experiment data also indicated that the α-MnO2150 catalyst possesses the highest toluene conversion at 300 o C
To lu en e c on ve rsion (%)
Fig 3.4 Evaluation of catalytic activity of manganese oxide catalysts (a Toluene conversion and b Yield of CO 2 )
Yield of CO2 was shown in Fig 3.4 b) Toluene is entirely converted into CO2 at 300 oC Oppositely, the yield of CO2 over manganese oxide catalysts was lower in regional temperature (150 o C – 250 o C), and α -MnO2 150 catalyst exhibited the highest yield of CO2 Thus, the results of the evaluation of catalytic activity clarified the influence of the preparation method on the catalytic activity of the manganese oxide catalyst The catalytic activity is also in good agreement with TPR H2 results α- MnO2 150 catalyst was reduced at the lowest temperature with the high H2 consumption Therefore, the α-MnO2 150 catalyst shows the highest activity in the toluene oxidation process
Fig 3.5 XRD patterns of catalysts
The XRD patterns (Fig 3.5) reveal sharp diffraction peaks indicative of well-crystallized NCO samples The structure resembles a mixture of NiCo2O4 and NiO, as determined by comparing standard PDF cards in the ICDD database The presence of NiO suggests the possible existence of a CoOx phase, despite its absence in the XRD data Notably, the main XRD peak for Co3O4 is very close to that of NiO Additionally, CoOx could be present in an amorphous, low-crystallinity, or fine-grained form undetectable by XRD, as is commonly observed with this material.
Table 3.3 presents BET surface areas, H2 consumption, and Co/Ni molar ratio of NCO samples
Table 3.3 BET surface areas, H 2 consumption, and Co/Ni molar ratio of NCO samples
3 NCO – 0.5 10 14.85 - a The data were determined by quantitatively analyzing the H 2 – TPR profiles b Calculated based on the EDX results of the samples
By comparing the total H2 consumption of NCO samples, as summarized in Table 3.3, it can be seen that total H2 consumption of the NCO-1.5 (20.63 mmol/g) is higher than that of NCO-1.0 (14.55 mmol/g), the latter is similar to NCO-0.5 (14.85 mmol/g) The total H2 consumption of NCO-1.5 sample is much higher than that of NCO-0.5 and NCO-1.0 This is why the toluene conversion of NCO-1.5 sample is higher than that of NCO-0.5 and NCO-1.0 This trend in high-temperature reducibility is in good agreement with the trend in catalytic performance TPR H2 experiment was performed to study the reducibility of NCO samples, and the results are shown in Fig 3.6 NCO-0.5 and NCO-1.0 samples show two main stepwise reductions at two different temperatures The reduction peak of NiO usually appears in the temperature range of 240–350 °C, which is relevant to the synthesis method [130] The reduction of Co3O4 results in two reduction peaks, corresponding to the reduction of Co3O4 to CoO (at 300 °C) and CoO to Co o (at 430 °C), respectively It also reported a broad peak at 317 °C for the reduction of Co3O4, which is broader than the reduction peak of CoO, indicating that two reduction steps (Co 3+ →Co 2+
→Co o ) occurred [129] In the present work, the reduction peak in the lower temperature range of 278–281 °C may be due to the reduction of NiO to Ni, whereas the reduction peak in the higher temperature range of 338–389 °C is attributed to the reduction of Co3O4 The reduction profile of NCO-1.5 differs from that of NCO-0.5 and NCO-1.0, showing two characteristic reduction peaks at higher temperatures (391 and 573 °C) The interaction between Co and Ni in the oxide structure may be the reason for the better oxide stability upon reduction, leading to an increase in the reduction temperature
Fig 3.6.TPR-H 2 profile of NCO catalysts b Catalytic activity of NCO catalysts
DEVELOPMENT OF MIXED MANGANESE AND COPPER OXIDE
(%) α- MnO 2 150 5000 toluene ppm GHSV = 5700 mL/(g.h) 300 o C/100
NCO – 1.5 5000 toluene ppm GHSV = 5700 mL/(g.h) 300 o C/100
CuMnO x 12 5000 toluene ppm GHSV = 5700 mL/(g.h) 250 o C/100
CoMnOx91 5000 toluene ppm GHSV = 5700 mL/(g.h) 250 o C/100
3.2 DEVELOPMENT OF MIXED MANGANESE AND COPPER OXIDE CATALYST
The study investigated the impact of various reaction conditions on catalytic activity of CuMnOx catalyst in the VOCs oxidation process
3.2.1 Influence of different inlet concentrations on catalytic activity of CuMnOx catalyst
In some industrial processes with a high potential for emitting aromatic VOCs, such as the refinery and petrochemical process, the concentration of VOCs released into the environment ranges from 670 to 2770 ppm In addition, for the coating industry in Taiwan, the VOC concentrations emit from 149 ± 99 to 607 ± 322 ppm (Section 1.1.2) Therefore, inlet VOC concentration is an influencing factor that must be considered when designing the laboratory pilot to be applied in the pilot and the exhaust gas treatment system
The thesis focuses on studying and developing catalysts that exhibit catalytic activity at low temperatures and have high activity at high concentrations for extended periods Therefore, in this study, the concentration of toluene, a reactant, significantly impacts a catalyst's catalytic activity An effective catalyst should demonstrate high activity across a range of toluene concentrations Results of evaluating the influence of inlet toluene concentration on catalytic activity was conducted in reaction conditions: a series of concentrations (1000 – 18000 toluene ppm), GHSV of 5700 mL/g.h, CuMnOx12 catalyst As shown in Figure 3.17, at an inlet concentration of 1000 ppm, toluene is completely oxidized at temperatures between 150 o C and 400 o C However, increasing the inlet concentration from 5000 toluene ppm to 18000 toluene ppm, conversion of toluene rises with an increase in reaction temperature For Fig 3.17, with an inlet concentration of 5000 ppm, at 250 oC, 100% toluene is oxidized completely In addition, the percentage of benzene converted into CO2 reaches 100% at all inlet toluene concentrations at the same temperature Therefore, inlet 5000 ppm toluene was chosen for study because this concentration is approximately the VOC concentration in the exhaust gas mixture from industrial processes such as refineries In addition, results of the selection catalyst for VOC oxidation show that the CuMnOx12 catalyst exhibited the highest catalytic activity at 250 o C
To lu en e c on ve rsion (%)
Fig 3.17 Influence of inlet toluene concentration on catalytic activity of
CuMnO x 12 catalyst (a Toluene conversion and b Yield of CO 2 )
Similarly, inlet benzene concentration significantly affects catalytic activity of catalyst A catalyst must show high activity across different benzene concentrations
Ben zen e c on ve rsion (%)
Fig 3.18 Results of evaluating influence of inlet benzene concentration on catalytic activity of CuMnO x 12 catalyst (a Benzene conversion and b Yield of
Result of evaluating the influence of inlet benzene concentration on catalytic activity was conducted in reaction conditions: a series of concentrations (1000 – 18000 benzene ppm), GHSV of 5700 mL/g.h Based on Figure 3.18 a and Fig 3.18 b., the highest benzene conversion is achieved at a lower temperature (150-200 °C) with an inlet concentration of 1000 ppm In contrast, at 250 °C, benzene conversion is completely oxidized at all concentrations The percentage of benzene converted into
CO2 is similar for all concentrations at this temperature Compared to the VOC concentration from the exhaust gas mixture from the waste tire pyrolysis process, an inlet benzene concentration of 2000 ppm was chosen for the following studies
3.2.2 Influence of different preparation methods on catalytic activity of
Characterization of catalysts a BET surface areas, H 2 consumption, and Cu/Mn molar ratio of CuMnO x samples
The results of the composition of catalysts can be seen in Table 3.7 The analysis revealed that the weight percentage of the catalyst is nearly the calculated weight percentage, except for the CuMnOx HT catalyst (the percentage of Cu, which is lower than the percentage of Cu calculated)
BET surface areas of catalysts are presented in Table 3.7 Results indicate that
CuMnOx 12 catalyst prepared using the sol-gel method had the largest surface area
CuMnOx catalyst was synthesized through co-precipitation in two conditions: calcination once and calcination twice It is important to note that the number of calcination processes did not affect the catalyst's surface area
Table 3.7 BET surface areas, total H 2 consumption, and Cu/Mn molar ratio of the CuMnO x samples
7 CuMnOx HT 5.38 - 0.26 a The data were estimated by quantitatively analyzing the H 2 – TPR profiles b Calculated based on the ICP-OES of the samples b XRD
CuMnO x HT ¨: Cu 1.5 Mn 1.5 O 4 §: Mn 2 O 3
*: CuO tenorite â: MnO 2 Pyrolusite ê: MnO 2 Hexagonal
Fig 3.19 XRD profile of CuMnO x catalysts synthesized by different methods
Fig 3.19 shows that presence of sub-crystalline phases depends on the synthesis methods It has been observed that presence of spinel phase Cu1.5Mn1.5O4
(ICDDPDF No 35-1172) [143] can be indicated by its diffraction peaks located at 2θ of 18.45, 30.45, 35.90, 37.57, 40.73, 54.20, 57.80, and 63.40, which correspond to the (111), (220), (311), (222), (400), (422), (333), and (440) crystal planes in all catalyst In addition, exits of the other phase are also identifiable through its diffraction peak For instance, Mn2O3, tenorite CuO, and pyrolusite MnO2 are present in the CuMnOx TE catalyst For CuMnOxHT catalyst, besides main crystalline phase, the hexagonal MnO, Mn2O3, tetragonal MnO2, and pyrolusite MnO2 also appear
Therefore, main phase Cu1.5Mn1.5O4 exits in catalyst are synthesized using different methods, whereas the presence of sub-crystalline phases depends on the synthesis methods. c EPR
EP R sig na l in te ns ity (a u )
Fig 3.20 EPR spectra of CuMnO x catalysts synthesized by different methods
EPR spectra of all studied catalysts are shown in Figure 3.20 EPR spectroscopy was used to explain the information on the nature of the active phase species The spectrum of CuMnOx catalyst (except for CuMnOx HT and CuMnOx
CP2 catalyst) displays an axial symmetry with a calculated g = 2.39 This signal was attributed to isolated Cu 2+ ions in octahedral coordination with tetragonal distortion The lack of EPR signal characteristics for Mn species revealed that they most likely exist as Mn 4 [137,144] EPR spectra of CuMnOx HT and CuMnOx CP1 catalysts showed different spectra than others This is caused by the difference of Cu 2+ in a catalyst d TPR H 2 profile
In Figure 3.21, TPR H2 profiles of catalysts are shown Reduction peaks at temperatures below 200°C, specifically at 155°C, 188°C, and 196°C for the CuMnOxCP2, CuMnOxTE, and CuMnOx12 catalysts, respectively, corresponds to the reduction of CuO → Cu2O or the reduction process of Cu-Mn spinel phase [145] This indicates that the interaction between Cu and Mn species could enhance the reducibility Furthermore, peaks at higher temperatures (200 – 250°C) are likely attributed to the step reduction of MnO2 to Mn2O3 Additionally, the peak at 369°C for the CuMnOxCP1 was identified as the step reduction of Mn2O3 to Mn3O4
According to A Mirzaei et al [145] A significant change in CuMnOx catalyst's TPR profile was observed, a mixture of various oxidation states of copper and manganese with multiple formation phases However, the XRD technique could not determine the amorphous phase of the catalyst The complexity of the mixed compounds makes it challenging to determine their TPR profiles accurately
Fig 3.21 TPR H 2 profile of catalysts
It has been discovered that the high total H2 consumption is likely a result of the interactions between Mn-Cu oxides Results obtained from the profile of H2-TPR indicate that these interactions in the spinel catalyst may reduce the reduction temperature, leading to a faster reaction When the preparation method was changed, Mn-Cu spinel and sub-crystalline phases appeared Therefore, the higher performance of the CuMnOx12 catalyst can be attributed to sub-crystalline phases
Table 3.8 Consumed hydrogen amount (mol/g) of catalysts
Temperature at maximum (C) Consumed hydrogen (mmol/g) Total H 2 consumption (mmol/g)
Catalytic activity of catalysts for toluene oxidation
The preparation method of CuMnOx catalysts significantly influences their catalytic activity for toluene oxidation Among various synthesis methods, the sol-gel method yielded the most active catalyst (CuMnOx 12), with toluene conversion exceeding 70% at 150 °C and complete oxidation to CO2 at 250 °C The CuMnOx 12 catalyst also achieved the highest yield of CO2 (66%) at 200 °C, compared to lower yields (34-50%) for other catalysts These results highlight the importance of synthesis method in optimizing the catalytic performance of CuMnOx catalysts for toluene oxidation, warranting further exploration of the CuMnOx 12 catalyst.
To lu en e c on ve rsion (%)
CuO MnO2 CuMnOx 12 CuMnOx TE CuMnOx CP 1 CuMnOx CP 2 CuMnOx HT b
Fig 3.22 Results of evaluating influence of different preparation methods on catalytic activity of catalyst (a Toluene conversion and b Yield of CO 2 )
The catalytic activity of catalysts for benzene oxidation
Comparing the toluene oxidation process and the benzene oxidation process, there are similarities between the two processes Overall, catalytic activity of the CuMnOx synthesized by different methods exhibited higher activity in benzene oxidation process than toluene oxidation process (Fig 3.23)
Ben zen e c on ve rsion (%)
CuO MnO2 CuMnOx12 CuMnOxTE CuMnOxCP1 CuMnOxCP2 CuMnOxHT b
Fig 3.23 Results of evaluating influence of different preparation methods on catalytic activiy of catalysts (a Benzene conversion and b Yield of CO 2 )
Results of the evaluation catalytic activity of catalysts for benzene and toluene oxidation processes agreed with the research results of Tang et al [123]
Study by Tang and colleagues [123] describes the mechanisms of BTX oxidation process using a manganese-based catalyst These mechanisms include the Eley-
Rideal (1), Langmuir-Hinshelwood (2), and Mar-van Krevelen (3) mechanisms Mechanisms (1) and (2) involve adsorbed oxygen and oxygen vacancy and are typically observed at low temperatures Conversely, at high temperatures, concentration of BTX on the catalyst surface increases, leading to more lattice oxygen participating in the oxidation process, which is in line with mechanism (3) This results in a significant increase in activity from 200°C, and the catalyst demonstrates its best performance when lattice oxygen density on the surface is at its highest
Table 3.9 Comparison of catalytic performance of completely toluene oxidation
(the molar Cu/Mn ratio of 0.515) Sol-gel
-5000 toluene ppm (2000 benzen ppm), -GHSV = 5700 mL/(g h)
(the molar Cu/Mn ratio of 0.515)
(the molar Cu/Mn ratio of 0.515)
(the molar Cu/Mn ratio of 0.515)
(the molar Cu/Mn ratio of 0.515) hydrothermal 300/100
(the molar Cu/Mn ratio of 0.5) hydrothermal
THE DEVELOPMENT OF MIXED MANGANESE AND COBALT OXIDE
3.3.1 Influence of different molar Co/Mn ratios on catalytic activity of CoMnO x catalyst
A study determined the optimal Co/Mn ratio for synthesizing mixed manganese cobalt oxide catalysts for VOCs oxidation process
BET surface areas, H2 consumption, and Co/Mn molar ratio of samples are presented in Table 3.21
Table 3.21 BET surface areas, total H 2 consumption, and Co/Mn molar ratio of the samples
7 CoMnOx 91 60.61 13.27 8.8 23.49 a The data were determined by quantitatively analyzing the H 2 – TPR profiles b Calculated based on the ICP-OES of the samples c Calculated based on the XRD results of the samples
The X-ray diffraction (XRD) technique was employed to analyze the phase identification of both the pure oxide and the CoMnOx catalyst The analysis of the catalyst revealed diffraction peaks corresponding to the spinel crystal structure of
Co3O4, Co2.9Mn0.1O4, Co2.8Mn0.2O4, and Co2.6Mn0.4O4 CoMnOx31 and CoMnOx61 share the crystalline structure of Co2.8Mn0.2O4, whereas CoMnOx32 catalyst has the crystalline structure of Co2.6Mn0.4O4 and CoMnOx33 catalyst has the crystalline structure of Co2.9Mn0.1O4 Additionally, the CoMnOx91 catalyst exhibits the main crystalline phase of Co2.9Mn0.1O4 and a sub-crystal structure of Co3O4, potentially accounting for its heightened activity in the benzene and toluene oxidation process
Fig 3.50 XRD pattern of CoMnO x catalysts
Figure 3.51 shows FT-IR spectra of the pure oxide and CoMnOx catalysts The absorption spectra peak in the characteristic region (1000 - 400 cm -1 ) indicates the presence of metal oxides in both pure oxides and CoMnOx catalysts The adsorbed bands from 400 cm -1 to 800 cm -1 were in good agreement with Mn–O lattice vibration with the medium strong IR bands about 427 cm -1 , 482 cm -1 , and 514 cm -1 (mentioned in the previous sections) The absorbed spectra at (480 and 563 cm -1 ) were assigned to the Mn-O group for the pure MnO2 catalyst In addition, Fig 3.51 shows the spectrum of Co3O4 The OB3 and ABO vibrations in the spinel lattice are associated with sharp peaks at 554 and 656 cm -1 , respectively [165] The characteristic peaks of the cobalt and manganese oxide were indicated in the FTIR spectra of CoMnOx catalyst The CoMnOx31, CoMnOx61, and CoMnOx91 catalyst have a similar FT-IR spectrum of Co3O4 catalyst due to the large cobalt concentration Therefore, the analysis above indicates that cobalt and manganese are present in the catalyst
Tran smitt an ce (%) wavenumber (cm -1 )
Co 3 O 4 MnO 2 CoMnOx32 CoMnOx33 CoMnOx31 CoMnOx61 CoMnOx91
Mn-O Co-O Co-O Mn-O Co-O Co-O
Fig 3.51 FT – IR spectra of CoMnO x and pure oxide catalysts
MnO 2 CoMnO x 33 CoMnO x 32 CoMnO x 31 CoMnO x 61 CoMnO x 91
Fig 3.52 TPR - H 2 profile of CoMnO x and pure oxide catalysts
TPR-H2 results for the CoMnOx catalyst indicated a reduction process occurring at a lower temperature than the pure oxide catalyst Moreover, TPR-H2 profiles of catalysts were similar, with shoulder peaks observed at lower temperatures This peak was associated with reducing Co 3+ to Co 2+ and Co 2+ to Co o [129] Furthermore, peaks at higher temperatures suggested a reduction process involving CoO to Co or Mn3O4 to MnO in the mixed oxide catalyst
Catalytic activity of CoMnO x catalyst for toluene oxidation process
To lu en e c on ve rsion ( %)
Fig 3.53 Results of evaluating influence of different molar Co/Mn ratios on catalytic activity of CoMnO x catalyst (a toluene conversion and b Yield of CO 2 )
In Fig 3.53 a and Fig 3.53 b show Results of toluene conversion with an initial concentration of 5000 ppm The percentage of toluene conversion increased as reaction temperature rose from 150 °C to 400 °C The CuMnOx91 catalyst achieved maximum toluene conversion at 250 °C, while other catalysts accomplished complete toluene conversion from 300 °C This significant difference in performance indicates the potential of CuMnOx91 catalyst, which demonstrated better catalytic performance than other catalysts (T100 = 250 o C) In Fig 3.53 b, CoMnOx91 catalysts converted toluene completely to CO2 from 150 °C This is due to the synergistic effect of the combination of manganese and cobalt, which helps increase the activity compared to pure oxide catalysts and other CoMnOx catalysts with different molar Co/Mn ratios
Catalytic activity of CoMnO x catalyst for benzene oxidation process
Ben zen e c on ve rsion ( %)
Fig 3.54 The result of evaluating the influence of different molar Co/Mn ratios to the catalytic activity of the catalyst (a Benzene conversion and b Yield of CO 2 )
In Fig 3.54, CoMnOx 91 catalyst, synthesized using the sol-gel method and having a molar Co/Mn ratio of 9, displayed the highest catalytic activity in the benzene oxidation process The catalytic activity of CoMnOx catalyst origin, CoMnOx, features a porous structure and a substantial accessible surface area, enabling the diffusion of benzene and oxygen This structure may offer additional active sites for benzene oxidation, contributing to the high activity observed in CoMnOx catalyst The exceptional performance can be attributed to the synergistic impact of the large surface area, low-temperature reducibility, abundant high-valence non-noble metal oxide sites, and active surface oxygen species present in CoMnOx catalyst [67]
In this study, the effectiveness of CoMnOx catalyst was compared to another catalyst for the complete benzene oxidation process
Table 3.22 Comparison of catalytic performance of completely benzene oxidation
(the molar Co/Mn ratio of 1)
(the molar Co/Mn ratio of 1.545) 300/100
(the molar Co/Mn ratio of 3)
(the molar Co/Mn ratio of 6) 250/100
(the molar Co/Mn ratio of 9)
Co1Mn 1Ox catalyst (the molar Cu/Mn ratio of 1) annealing Co−Mn−1,3- propanediol precursors at
3.3.2 Investigation of the catalytic activity of the CoMnO x 91/cordierite catalyst in the direct VOCs oxidation process
Determine the percentage of activated phase supported on cordierite
The study aimed to assess how the percentage of activated phase affected the performance of cordierite-supported catalyst with a 2000 ppm benzene inlet flow.In this section, benzene, which is difficult to oxidize, was chosen to represent aromatic VOCs for investigation Figure 3.48 illustrates that as the percentage of the activated phase increased, so did the benzene conversion rate It was found that the 15% CoMnOx/cordierite catalyst could completely oxidize benzene at 350 °C
Consequently, the 15% activated phase was selected for further detailed investigation in the following section
Ben zen e c on ve rsion (%)
Fig 3.55 The result of evaluating the influence of different percentages of activated phase to the catalytic activiy of the catalyst (a Benzene conversion and b Yield of CO 2 )
The 15% CoMnO x 91/cordierite catalyst a Characterization of the CoMnO x 91/cordierite 15% catalyst
Figure 3.49 presents SEM images of cordierite and the 15% CoMnOx91/cordierite catalyst synthesized using the impregnation method with citric acid The contrast in the cordierite surface before and after the impregnation process indicates that the activated phase is uniformly distributed on the cordierite surface, confirming the successful support of the CoMnOx91 catalyst on the cordierite surface The composition analysis results can be found in Table 3.23, showing the composition of cordierite and the 15% CoMnOx/cordierite catalyst a b
Fig 3.56 SEM images of cordierite and 15% CoMnO x 31/cordierite catalyst
(a Cordierite b 15% CoMnO x 31/cordierite catalyst) Table 3.23 Composition of Cordierite and 15% CoMnO x 31/cordierite catalyst
Furthermore, results of the molar Co/Mn ratio of catalyst can be seen in Table 3.24
No Catalysts Co/Mn calculated Cu/Mn ICP-OES Cu/Mn EDX
Based on the EDX analysis, a change in cordierite composition was observed when an activated phase was impregnated on it This indicates that the catalyst's active phase is supported on cordierite, combining Co and Mn oxide on the cordierite's surface Additionally, the molar Cu/Mn ratio determined from the EDX or ICP-OES results is approximately 9, which aligns with the calculated molar Co/Mn ratio In addition, Figures 3.57 and 3.57 b show the EDX mapping profile, indicating the activated phase elements in the CoMnOx/cordierite composition and the uniform distribution of manganese and cobalt on the cordierite surface a b
Fig 3.57 a EDX mapping images of cordierite and b EDX mapping images of
The specific surface area of catalysts is displayed in Table 3.25 The CoMnOx31 catalyst has a specific surface area of 60.16 m 2 /g, significantly higher than the surface area of cordierite of 6.25 m 2 /g However, SBET of the 15% CoMnOx31/cordierite catalyst is 8.44 m 2 /g, lower than nine times the CoMnOx31 catalyst This decrease in specific surface area could be related to the percentage of active phases on the catalyst's surface The sol-gel process was repeated three times to enhance the catalyst's performance, which increased the amount of activate phase on the cordierite surface
Table 3.25 Specific surface area of catalysts
Fig 3.58 XRD pattern of 15% CoMnO x 31/cordierite catalyst
Fig 3.58 shows the main crystal phase as Co2.9Mn0.1O4 and the sub-crystalline phase as Co3O4 in the XRD pattern of the 15% CoMnOx91/cordierite catalyst, indicating successful support of the CoMnOx91 catalyst on the surface of cordierite
The TG/DSC curve of the 15% CuMnOx91/Cordierite catalyst that was not calcinated to study the calcination process exhibits three distinct stages in the temperature range of 0-500 °C:
- Between 0-100 °C, the catalyst undergoes a weight loss of approximately 3.46% due to the adsorbed substances on its surface, primarily attributed to water vapor after the sample drying process at 80°C before thermal analysis
- During the 200-300 °C range, the decomposition of cobalt hydroxide leads to the formation of Co3O4, resulting in a weight loss of up to 9.46%
- In addition, the temperature increase to 500 °C initiates a carbon combustion process [166].
Fig 3.59 TG – DSC curve of 15% CoMnO x 91/ cordierite b Catalytic activity of 15% CoMnOx31/cordierite catalyst for benzene oxidation process
Investigating benzene oxidation at variable temperatures revealed a positive correlation between temperature, benzene conversion, and CO2 yield The CoMnOx91 powder catalyst exhibited superior performance at 250 °C, while the 15% CoMnOx 91/cordierite catalyst achieved complete benzene conversion at 350 °C Notably, the CoMnOx 91 powder catalyst consistently outperformed the 15% CoMnOx 91/cordierite catalyst in both benzene conversion and CO2 yield across the temperature range.
91/cordierite catalyst It is worth noting that the bare cordierite catalyst exhibited no benzene conversion and CO2 yield at lower temperatures and showed minimal activity at 350 °C This suggests that the activity of the CoMnOx 91 powder and the 15% CoMnOx 91/cordierite catalyst is linked to the activated phase Additionally, the fact that the CoMnOx 91/cordierite catalyst only contains 15 percent of the activated phase implies that the benzene conversion and CO2 yield are directly related to the catalyst's activated phase concentration
Ben zen e c on ve rsion (%)
Fig 3.60 Results of evaluating catalytic activity of 15% CoMnO x 91/cordierite, CoMnO x 91 powder, and cordierite for benzene oxidation process (a Benzene conversion and b Yield of CO 2 ) c Catalytic activity of 15% CoMnO x 91/cordierite catalyst for the toluene oxidation process
To lu en e c on ve rsion (%)
Fig 3.61 Results of evaluating catalytic activity of 15% CoMnO x 31/cordierite, CoMnO x 31 powder, and cordierite for toluene oxidation process (a Toluene conversion and b Yield of CO 2 )
The 15% CoMnOx91/cordierite catalyst showed high catalytic activity in the toluene oxidation process (reaction condition: 5000 toluene ppm, GHSV: 5700 mL/g.h) at 350 o C, similar to benzene oxidation process
Cordierite c Evaluation of durability of the 15% CoMnO x 31/cordierite at different temperatures in benzene oxidation process
Ben zen e c on ve rsion (%)
Fig 3.62 Evaluation of durability of 15% CoMnO x 91/cordierite at different temperatures
APPLICATION OF CoMn x 91/CORDIERITE FOR OXIDATION AND
3.4.1 Waste tyre a The composition of waste tyre
Rubber waste is a significant part of municipal solid waste In its structure, strong bonds form between sulfur and cross-links in natural rubber during the vulcanization process In addition, antioxidants are added to improve rubber production's durability, making decomposition challenging Tables 3.26 and 3.27 demonstrate that waste tyres possess volatile compounds and a low ash content with a higher calorific value than coal and biomass Based on these properties, they can be used as materials for processes such as pyrolysis, gasification, and liquefaction [167- 169]
Table 3.26 Composition of waste tyre [152]
N o Substances Mass (%) of passenger tyre Mass (%) of truck tyre
Table 3.27 The element composition of waste tyre [152]
Pyrolysis, a thermal decomposition process conducted in the absence of oxygen, offers an effective solution for treating waste tires Achieving an efficiency rate of approximately 70%, this method becomes even more efficient (up to 90%) when the resulting products are utilized as fuel.
The TGA/DSC curve shows that waste tyres start to decompose at 382 °C and complete the process around 550 °C The mass of waste tyre reduces by 10% at 320 °C, by 20% at 367 °C, by 30% at 386 °C, by 40% at 405 °C, and by 50% at 459 °C Januszewicz's research validates these findings, indicating that selecting a pyrolysis process temperature of 550 °C is appropriate [170]
Fig 3.63 The TG-DSC curve of waste tyre
Wavenumber (cm -1 ) waste tyre waste tyre after TGA measurement
Fig 3.64 The FT-IR spectra of a waste tyre and waste tyre after TGA measurement
There is a change in the waste tyre structure after TGA measurement (Fig 3.64) The functional groups can be determined in waste tyre structure based on Table 3.28
Table 3.28 The functional groups and compounds can be expected from the waste tyre as determined by analyzing its Fourier-transform infrared (FT-IR)
Alcohols, phenol, or carboxylic acid
810 – 693 810 – 693 810 – 660 900 - 675 C- H Aromatic hydrocarbon The FT-IR spectra (Fig 3.64) show that waste tyre is a complex mixture of various substances, with the primary functional groups being C-H and C=C These groups, crucial in understanding the tyre's composition, are found in organic compounds that are either straight or cyclic Most cyclic compounds have been decomposed after the thermal decomposition at 650°C, leaving only the C-H compounds, such as alkenes and compounds with O-H functional groups, characterized by their higher peak intensity
3.4.2 The exhausted gas treatment system in the pilot system
1-pyrolysis reactor, 2-separator, 3-condenser, 4-cooling tower, 5-oil receiver, 6-oxidation tower, 7- adsorption tower, 8-pump, 9- water tank
Fig 3.65 The technical drawings of the exhaust gas treatment system
The process for treating the exhausted gas mixture containing aromatic VOCs (toluene, benzene) from the waste tire pyrolysis process with a capacity of 3-5 m 3 /h is shown in Figure 3.65 The waste tyres were placed in a pyrolysis reactor, and the pyrolysis process was conducted at 550 °C As the reaction occurred, the gas-liquid mixture was directed through a separator device acting as a dust-settling chamber Subsequently, a condenser separated the mixture into oil and gas to recover the pyrolysis oil The pyrolysis gas, which has a high calorific value, was used as fuel for heating the pyrolysis reactor and then mixed with air before being reintroduced to the outer part of the pyrolysis reactor to provide heat for the pyrolysis process The exhausted gas mixture from the process was then directed through an oxidation tower, where oxidation reactions occurred at 250 °C using a 15% CoMnOx91/cordierite catalyst Aromatic VOCs (benzene, toluene,…) from the gas mixture were oxidized into CO2 and H2O Following this, the gas mixture passed through a cooler to cool the exhaust gas to 40 °C before going to an adsorption tower containing the 7% CuMnOx12/AC catalyst, which was finally released into the environment The role of the adsorption tower is to adsorb any remaining volatile organic compounds (VOCs) from the oxidation tower, ensuring that no VOCs are emitted into the environment Furthermore, the adsorption tower can be used if the oxidation tower encounters issues or does not operate effectively
To evaluate the efficacy of the catalyst, benzene and toluene concentrations were determined at three critical points during the gas sampling process These points included the state of the exhaust gas before and after the oxidation tower, as well as its condition following the adsorption tower The measurements were conducted using specialized exhaust gas monitoring equipment, ensuring accurate quantification of the target compounds.
3.4.3 Gas monitoring results from the exhausted gas treatment system in the waste tyre pyrolysis process
The catalysts' catalytic activity was assessed using a system for treating exhausted gas (Fig 3.65) and determined by GC-MS Figures 3.66 and 3.67 display data indicating that the 15% CoMnOx91/cordierite catalyst and the 7% CuMnOx12/AC catalyst exhibited high catalytic activity in oxidizing benzene and toluene This suggests that the catalyst is well-suited for removing benzene and toluene in this system and can be utilized in industrial plants
Ben zen e c on ve rsion (%)
After the oxidation tower After the adsorption tower
Fig 3.66 Benzene conversion from the exhausted gas treatment system
To lu en e c on ve rsion (%)
Date After oxidation tower After adsorption tower
Fig 3.67 Toluene conversion from the exhausted gas treatment system
In addition, the air quality monitoring was conducted, and the result is shown in Table 3.29
Table 3.29 Gas monitoring results from the exhausted gas treatment system in the waste tyre pyrolysis process
N o Substances Before the oxidation tower
In Table 3.29, it is shown that benzene and toluene were converted with impressive efficiency, with benzene achieving over 99% and toluene reaching more than 97% after passing through the oxidation tower However, benzene and toluene removal efficiency at the adsorption tower is relatively low This can be attributed to the proximity of the oxidation and adsorption towers, leading to inefficient heat exchange Compared to the laboratory results, the conversions of benzene and toluene on a pilot scale are notably higher ( more than 99% benzene conversion, 97% toluene conversion) due to significantly lower inlet concentrations of both compounds (87 ppm toluene and 588 ppm benzene) This outcome aligns with the laboratory results and indicates that the catalyst system can effectively eliminate aromatic VOCs, such as benzene and toluene, even at high concentrations for typical treatment systems
Thus, following the assessment of the catalytic activity of the 15% CoMnOx91/cordierite and the 7% CuMnOx12/AC catalyst in eliminating VOCs from the waste tyre pyrolysis treatment process, it is evident that the catalyst system can effectively be utilized for this purpose due to its high catalytic activity and thermal stability Furthermore, the system can sustain its catalytic activity in an environment containing sulfur and other impurities, such as dust These findings lay the groundwork for further research to mitigate the impact of sulfur substances on the catalyst's catalytic activity and explore the treatment process in other industrial applications
GENERAL CONCLUSIONS AND OUTLOOK General conclusions
• The catalyst characterization indicates that the Cu 2+ ions in the CuMnOx catalyst are in octahedral coordination with tetragonal distortion, while Mn species exist as Mn 4+ The structure of the CuMnOx catalyst contains the Cu-
Mn spinel phase; combining with Cu and Mn oxides results in smaller particle sizes The bimetallic CuMnOx possesses higher surface areas, lower reduction temperatures, and a higher consumption of H2, which is the primary factor in improving oxidation activity;
• The CuMnOx12 catalyst, synthesized using the sol-gel method with a molar Cu/Mn ratio of 0.5, exhibited the highest catalytic activity The 23% CuMnOx12/ cordierite catalyst was prepared using the impregnation method with citric acid to apply this catalyst in an environmental treatment system Moreover, this catalyst could convert toluene and benzene completely at 350 °C Furthermore, the effectiveness of the CuMnOx12 catalyst in the presence and absence of sulfur-containing compounds was studied It was observed that the introduction of SO2 resulted in a 20% decrease in benzene conversion, while H2S led to a 30% reduction It was found that the sulfur species adsorbed onto the catalyst's surface and reacted with the catalyst's composition, thereby impacting the catalytic activity and modifying the catalyst's structure Notably, the catalytic activity remained consistent even after a 10-hour testing period These findings have significant implications for applying catalytic systems to treat exhaust gases containing aromatic VOCs in the presence of sulfur- containing compounds
• The catalytic activity of the CuMnOx12/AC catalyst was evaluated in the adsorption-oxidation process of toluene and benzene The catalyst demonstrates a high adsorption capacity for both toluene and benzene, comparable to the original AC The 7% CuMnOx12/AC catalyst achieves a 46.57% benzene conversion, with 13.47% of adsorbed benzene being converted into CO2 at 150 °C On the other hand, the 25% CuMnOx12/AC catalyst shows a 65.79% benzene conversion, with 89.4% of adsorbed benzene being converted into CO2 at 150°C and reaching 94.41% benzene conversion, with completely adsorbed benzene being converted into CO2 at 250 °C These findings have significant implications for the use of adsorption-oxidation techniques in the elimination of aromatic VOCs
• The characterization of the catalyst indicates that the structure of the CoMnOx catalyst contains the Co-Mn spinel phase The bimetallic CoMnOx possesses higher surface areas, lower reduction temperatures, and a higher consumption of H2, which is the primary factor in improving oxidation activity;
• The CoMnOx 91 catalyst with a Co/Mn ratio of 9 synthesized using the sol- gel method resulted in the highest catalytic activity Its primary crystalline phase is Co2.9Mn0.1O4, and the sub-crystalline phase is Co3O4 The 15% CoMnOx91/cordierite catalyst exhibited higher catalytic activity than the 23% CuMnOx12/cordierite catalyst
The exhaust gas treatment system: