Luận văn thạc sĩ Kỹ thuật hóa học: Production of value-added chemicals from rice straw

PREFACE

Study Background

In recent years, the utilization of lignocellulosic biomass as a renewable source for energy and chemical platforms has been investigated by scientists all over the world [1] Lignin is one of the most potential renewable and sustainable energy resources which is present in a huge amount of agricultural waste such as maize, rice straw, corn stover, sugarcane bagasse, etc [2, 3] Amongst them, rice straw accounts for the highest proportion of nearly 50 million tons generated annually in Vietnam, especially in the Mekong delta However, most of the rice straw is burned resulting in huge emissions of harmful gasses such as NOx, CO, COô Therefore, lignin recovery from rice straw not only prepares a high calorific bio-fuel but also reduces their negative impacts on the environment [4±6]

Rice straw contains a significant amount of silica, which originates from the soil and enters the roots of the rice plant as mono-silicic acid, Si(OH)4 Evaporation and transpiration of water in the plant condense the monomeric Si(OH)4 species to their saturation point, thus leading to the polymerization into insoluble polysilicon acid [7, 8] Furthermore, the appearance of linkages among components was also confirmed: Lignin associates with polysaccharides, especially hemicellulose, via covalent bonds to form lignin-carbohydrate complexes Likewise, silica is hypothesized to have interaction with cellulose and lignin [9, 10] The recalcitrant structure of lignocellulosic biomass inhibits bio-refineries such as the fermentation of cellulose for bioethanol, conversion of lignin into value-added chemicals A lot of silica reduction methods were released but some require harsh conditions or special equipment and others are not efficient [9, 11, 12] Moreover, there are several desilication methods conducted in mill conditions giving good results, however, this method was not reported about lignin recovery or removing both lignin and silica for gaining cellulose and hemicellulose [13±15]

Keywords: lignin, bioethanol, circular bioeconomy, biooil, conversion.

Research aims and Objectives

The main objectives of this research were to study the valorization of the rice straw following the biorefinery concept with the main idea shown in Figure 1.1 The specific objectives were:

- Recovery of lignin from rice straw after pre-treatment

- Integration of delignification process, and bioethanol production toward a zero-waste biorefinery

- Conversion of obtained lignin into biocrude

- Evaluation of biorefinery approach and the life cycle of rice straw

Figure 1.1 The scientific idea of this study.

Outline of thesis

This work is split into two sections: a theoretical section (which includes a literature review) and an experimental section In Chapter 2 Literature Review, the theoretical portion is discussed This section contains a literature analysis of the raw material as well as relevant chemical techniques for understanding the new biorefinery approach and this study The experimental part of the thesis is described in chapter 3 Material and Method, and discusses planning of experiments The results from the experiments, and finally, conclusions and future work are presented in chapter 4 Results and Discussion, and chapter 5 Conclusion and Future Work.

LITERATURE REVIEW

Rice straw ± a potential lignocellulose biomass overview

Lignocellulose biomass, also known as lignocellulose, is the most abundant biorenewable substance on the planet [16], created by the photosynthesis process from atmospheric CO2 and water It is a complex matrix consisting primarily of polysaccharides, phenolic polymers, and proteins that is an integral component of plant woody cell walls As seen in Figure 2.1, LCB has a complex spatial structure in which cellulose (a carbohydrate polymer) is wrapped by a thick structure made up of hemicellulose (another carbohydrate polymer) and lignin (an aromatic polymer) Cellulose molecules are arranged in crystalline regions in regular coils, or in amorphous regions in random geometry Hemicellulose and lignin shield the microfibrils of cellulose polymers, which are connected by hydrogen and van der Waals bonds Carbohydrates, the main ingredient of cellulose and hemicellulose, account for about 70% of the dry weight of lignocellulose biomass and are the source of virtually all of the most promising bio-based building blocks and chemical intermediates, regardless of the conversion technologies used (biological or thermochemical) Lignin, which accounts for around 25% of the weight of lignocellulose biomass, is by far the most essential natural source of aromatics, apart from being a strong solid biofuel Because of this structure, lignocellulose can play an important role in the energy industry, as it can be used to produce a variety of energy products, including solid (briquettes), liquid (bioethanol and biodiesel), and gas (biogas and bio-H2) [17]

Figure 2.1 The structure of lignocellulose biomass [18]

Cellulose is a compound composed of polysaccharides, that consists of an open chain of D-JOXFRVH PROHFXOHV FRXSOHG WKURXJK ȕ-(1-4) glycosidic bonds with the formula (C6H10O5)n (Fig 2.1) Cellulose is that the commonest organic compound material on the market Cotton fiber is containing 90% of cellulose content, wood is 40%-50%, and dried hemp is containing close to 57% [19, 20] High amounts of cellulose contained in pulp and cotton for economic use cellulose is usually accustomed to yield composition boards and paper-type materials A prospective characteristic of cellulose is crystallinity Cellulose is converted into an amorphous solid at conditions of 25 MPa pressure and temperature of 320°C Many environment- friendly biofuels may be derived from conversions of cellulosic materials, such as agricultural residues and energy crops

Hemicellulose is a branched heteropolymer containing close to 500-3000 sugar units [21] It consists of several sugar units, with a prevalence of monosaccharide parts (xylose and arabinose) beside hexoses (mannose, glucose, galactose, and rhamnose) and acetylated sugars Hemicellulose cross-links with either cellulose or lignin, strengthening the semipermeable membrane (Figure 2.1) Though hemicelluloses are widely available, their utilization is harder compared to cellulose, due to their structural diversity, and the complex mechanism of the enzymatic hydrolysis of pentose sugar units However, hemicelluloses supply a lot of prospects for regioselective chemical and accelerator modifications compared to cellulose, thanks to the variability in sugar constituents, glycosidic linkages, and structure of glycosyl aspect chains still as 2 reactive hydroxyl group teams at the carbohydrate continuance unit

Lignin is that the third major element of LCB, having a polymeric complicated structure, which is responsible for some of the structural materials in the particular types of tissues of vascular plants and some algae [22] It is an inevitable a part of plant semipermeable membrane, particularly in bark and wood Lignin shows rigidity and hard quality because of the cross-linked synthetic polymers in its structure It's primarily amorphous (noncrystalline) It is a branched long-chain compound created of

3 varieties of monomers (Figure 2.2.), like primarily three-dimensional compound of 4-propenyl phenol (hydroxyphenyl unit or p-coumaryl alcohol), 4-propenyl-2-methoxy phenol (guaiacyl units or coniferyl alcohol), and 4-propenyl-2.5-dimethoxyl phenol (sinapyl alcohol or syringyl unit) [20]

Figure 2.2 Three type of lignin monomers: (A) p-coumaryl alcohol (hydroxyphenyl unit), (B) coniferyl alcohol (guaiacyl units), and (C) sinapyl alcohol (syringyl unit)

Rice straw, a residual byproduct of rice production at harvest, is one of abundant lignocellulosic biomass in the world Annual rice straw production is in the ranges of 100±140, 330±470, and 370±520 million t/year in Southeast Asia (SEA), the whole of Asia, and over the world, respectively [23] In Vietnam, rice straw accounts for the highest proportion of nearly 50 million tons generated annually, especially in the Mekong delta However, currently, most of the rice straw is burned (Figure 2.3) resulting in huge emissions of harmful gasses such as NOx, CO, COô Therefore, utilizing this material for production of bio-based products is an eco-friendly solution in context of climate changes

Figure 2.3 Burning rice straw in field (a) and Using rice straw for animal feeds

The biomass component in rice straw is mainly composed of 38% cellulose, 25% hemicellulose, and 12% lignin which are associated together to form a highly rigid network More recently, energy crops are raw materials used for the production of second-generation biofuels as they offer high biomass productivity Straw has a long history as an energy source: for many centuries, straw has been the most widely used raw material to burn fire From the middle of the 20th century, problems rose from pollution and the exhaustion of fossil fuels has increased the demand of biomass for the production of energy [24] More recently, second-generation biofuels were developed, based on the conversion of LCB components to liquid fuels Second- generation biofuels allow the utilization of the entire plants, such as woody crops, agricultural residues, or waste, as well as dedicated non-food energy crops grown on marginal land, thus allowing a dramatic increase of the productivity With a million tons of rice straw being wasted into the environment, rice straw is promising as a new feedstock for value-added chemicals such as renewable energy or material production [25].

Rice straw- based biorefinery

Biorefineries could be the most promising means of creating a sustainable bio- based economy because of their undeniable benefits [26] Biorefinery cleanly produces various fuels, power, or heat that contribute to human energy needs; also generates various chemical commodities and bioproducts in an environmentally sustainable manner by utilizing local agricultural residues and municipal wastes feedstocks, thus reducing disposal problems [27] Local agricultural residues used as biorefinery feedstocks are derived from lignocellulose-rich biomass resources, including wood, straw, grasses, Biorefineries have been significantly advanced significantly towards fractionating lignocellulose to its major constituents in the past decade [27±29] These constituents have been studied to process continuously into a range of products and energy using different process configurations with zero-waste generation to create more economically feasible biorefineries [30] Recently, biorefineries have been significantly studied towards fractionating lignocellulose to its major constituents [29], then being processed continuously into a range of products and energy using different process configurations with zero-waste generation to create more economically feasible biorefineries [30] Paddy field-based biorefineries have gained much attention in recent years [31] Large rice-producing countries have adopted research strategies to develop small-scale biorefineries utilizing crop residues integrated with sustainable management of local agriculture [32] Several pilot-scale plants have been made available, followed by R&D activities to develop fully-fledged systems as [33±36] Paddy straw-based biorefinery systems could be based on a biochemical operation mode, including pretreatment/delignification, hydrolysis, and fermentation of cellulosic fractions into ethanol or other value-added products [30] Pretreatment is the most crucial challenge of the biorefinery development to enrich cellulosic components for hydrolysis and fermentation into ethanol because of stability and structural toughness to the cell walls of the rice straw This robustness is attributable to the cross- linking between the polysaccharides and the lignin via ether and ester linkages [5] Additionally, rice straw contains a significant amount of silica, which originates from the soil and enters the roots of the rice plant as mono silicic acid, Si(OH)4 Evaporation and transpiration of water in the plant condense the monomeric Si(OH)4 species to their saturation point, thus leading to the polymerization into insoluble polysilicon acid [7, 8] Thus, A number of rice straw pretreatment technologies have been developed during the last century via physical, chemical, and biological pathways to achieve a high content of separated lignin without losing cellulosic fractions as [37±39] Khaleghian et al [15] studied the effects of silica and lignin on hydrolysis of the bioethanol production process from rice straw They reported using organosolv pretreatment and sodium carbonate pretreatment to remove silica and lignin from rice straw, respectively The method removed 91% silica in rice straw and eliminated the effect of lignin on enzymatic hydrolysis However, this study did not address the pretreatment waste (the liquid after organosolv pretreatment and sodium carbonate pretreatment) Also, the organosolv pretreatment required harsh conditions and equipment (180 o C and high-pressure stainless-steel reactor) The protocol caused many difficulties for scaling-up into mass production

Figure 2.4 The biorefinery in Vietnam

In Viet Nam, crop residue-based biorefinery at a small scale was demonstrated by a pilot plant installation (Figure 2.4) It belongs to a "Biomass Town" project built up to meet biomass-orientated regional material and energy circulations [40] From this perspective, Tran et al., [35] published a report on bioethanol production from rice straw calculated by engineering data gained from system operation; therein, the process's energy supply was partly from rice husk carbonization In this work, 18.4kg ethanol was obtained from 100kg of alkali-pretreated rice straw Steam generated from the rice hush's carbonization covered more than 90% of the process's total energy However, this process still has remained some drawbacks related to system design, such as mass and heat loss or unused energy Also, silica and lignin-rich liquid drained from alkaline pretreatment were not recovered and fully valorized Some research activities were boosted to improve system design [41], and utilize sustainable feedstocks [42] In an approach to improve the process more economically, Tran's group found out that the bottom waste of ethanol distillation might contain a significant nitrogen source In contrast, corn-steep liquor (CSL), a primary nitrogen source for the yeast of ethanol fermentation, spent highly operational expense and was not commercially available in some countries Following this work, Le et al., [43] utilized ethanol distillation's bottom residue to replace CSL's role for SSF operation The study confirmed the potential of distillation residue with an equivalent weight of nitrogen to CSL The reuse of that residue was expected to reduce 98% of the demand for CSL and recycle 26% of water The research series of Tran et al [35], and Le et al [43] are efforts in biorefinery development and in particular the possibility to produce biofuels and biomaterials from paddy residual biomass Despite of achieving some positive outcomes, the performance of biorefinery plant in Viet Nam still has not synchronously improved and fully re-evaluated.

Production of rice straw lignin

The pretreatment process is crucial to disrupting the compact structure in biomass, which enhances the yield and success of potential valorization processes, especially in biorefinery In the case of rice straw, alkaline pretreatment is the outstanding selection for lignin recovery in rice straw since this method is more effective for herbaceous plants than woody plants [44] During the alkaline pretreatment, lignocellulose is subjected directly to an alkaline solution, which makes the linkages of silica and another component were broken down, and silica was released into an alkaline medium Moreover, lignin in rice straw and other grass plants possesses a high degree of ester bonds to hemicellulose that is easy to be cleaved by alkaline medium [45, 46] The resulting liquid after separating the pretreated biomass

LVFDOOHG³EODFNOLTXRU´ZKLFKLVRULJLQDOO\XVHGWRGHILQHWKHZDVWHOLTXRUIURPWKH kraft pulping process in paper industries The alkaline black liquor contains mainly dissolved lignin, silica, and a minor proportion of hemicellulose [47] That is ideal for effective fractionation of lignin and carbohydrate components as well as silica recovery [7, 8, 45] Among available alkaline agents, sodium hydroxide provides the highest yield of delignification [48] Moreover, the alkaline pretreatment process is a popular method in the bioethanol production industry, so this method would be suitable for upgrading the bioethanol production technique, while considering about economy, sodium hydroxide is a popular chemical and cheaper than potassium hydroxide or another alkaline In order to recover lignin and silica from black liquor, Kihlman et al listed three main methods: acidification, ultrafiltration, and electrolysis [49] Among these, the usage of acid to precipitate lignin have been dominated the others [50] Minu et al have analyzed the effects of mineral acids on lignin from rice straw, while Domínguez-Robles et al proceeded with these studies on wheat straw [45] Phosphoric acid offers the highest yield of lignin precipitation but requires a high concentration to reach pH values lower than 4 [50] Hydrochloric acid (HCl) is frequently used in a lab-scale experiment with black liquor obtained from grass plants [51±54] However, HCl causes corrosion to stainless steel, thus limiting its application in large-scale operation [50] Under the consideration of economic aspects and feasibility to apply to industrial production, sulfuric is the acid of choice with a reasonable high yield of lignin recovery [55, 56] Likewise, HCl and nitric acid (HNO3) had lower efficiency compared to sulfuric acid (H2SO4) [45]

Only few research groups Minu et al and Kaur et al [45, 57] published the recovery methods of lignin and silica from the waste stream of rice straw pretreatment to our best knowledge Minu et al [45] developed the efficient recovery of lignin and silica from black liquor by two-step pretreatment of rice straw by acid and alkaline peroxide Lignin and silica were isolated from precipitation using dilute H2SO4 for reducing the pH of the black liquor Chemical characterization of isolated lignin was done by FTIR and compared with commercial lignin to evaluate its potential industrial applications The filtrate quality of precipitate separation was also monitored in each step by COD and TDS analyses Kaur et al [57] reported a simple acid-base hydrolytic method to recover pure silica (SiO2) in a 17-nm size and lignin from straw residues The physio-chemical characteristics of lignin and nano-silica were determined using FTIR, TGA, XRD, TEM, and SEM studies, along with energy-dispersive X-ray spectroscopy (EDS) analysis Highly pure nano-silica and lignin were derived by acid precipitation at an overall yield of 9.26% and 2.30%, respectively The elemental composition of SiO2 and lignin was authenticated by FTIR pattern and EDS analysis Amorphous structures of both SiO2 and lignin were confirmed by XRD investigation This process worked perfectly at kilogram scale of paddy waste with a similar yield.

Conversion of straw lignin into bio-oil

Lignin isolated from black liquor of rice straw pretreatment obtained after has promising applications in bioplastics, composites, carbon fibers, adsorbents, and dispersants [58] Lignin then is chemically modified to a range of platform chemicals, followed by functionalizing or defunctionalizing into emerging structures and bulk chemicals [29] In context of the growing global population leads to an increasing demand for fuels and chemicals, resulting in many societal problems, including energy security and environmental concerns To reach a sustainable development of society, biorefinery operations that employ inedible biomass for the production of valuable (fuel, chemicals and materials) must be advanced In this regard, lignocellulose composed of cellulose, hemicellulose and lignin is a promising feedstock for biorefinery to produce biofuels and biochemical

Zeolite catalysts are mainly used as catalysts in refining and petrochemical settings due to their shape selectivity features and remarkable active site properties A significant number of studies verified the activity of ZSM-5 in the production of aromatic compounds from lignin [59] Based on the structure-activity relation in previous work, the bi-functional system's catalysts have been recognized as a preferred catalyst system for the hydrodeoxygenation process [60, 61] Nickel is one of the most preferred catalysts for lignin hydrogenolysis because of its low cost and moderate activity [62] The supported nickel catalyst exhibit promises for the selective cleavage of C-O linkages representing those dominant bonds in lignin [63] Besides, under the effects of the global energy crisis and global warming issues, attention is paid to the production of liquid fuels and chemicals like aromatics and phenols by thermal treatment of lignin via depolymerization and conversion in the hydrogen environment is more and more increased Most investigations on lignin degradation and conversion have been conducted on utilizing lignin model compounds, as monomeric, dimeric, and trimeric Kasakov et al.[64] reported the deconstruction and subsequent hydrodeoxygenation of organosolv of lignin using 39 wt.% liquid products yield with 83.0% monocyclic alkanes at 220 o C at 2MPa H2 Therefore, the development approach for converting the rice straw lignin is important for improving the rice industry's value.

Circular bioeconomy

Sustainable development Goals (SDGs) nominated by United Nations with 17 goals play a crucial role in leading many recent studies to improve world welfare Designing a circular economy model is effective for adopting these SDGs [65] Circular economy (CE) is a business concept that involves utilizing materials in a sustainable approach, thus extending its utility and values [66, 67] CE reduces raw material consumption and creates more valuable products from waste The CE concept is like a closed supply chain in which the waste materials are recycled and reused [68] 7KH (XURSHDQ 8QLRQ (8 ZDV GHILQHG &( DV ³$Q HFRQRP\ WKDW LV UHVWRUDWLYH DQG regenerative by design It aims to maintain the utility of products, components, and PDWHULDOV DQG UHWDLQ WKHLU YDOXH´[69] Therefore, CE practice is essential for environmental protection, sustainable production, and implementation of the SDGs In the context of the transformation of fossil resources into biomaterials, the bioeconomy was introduced for addressing the circularity aspects of bio-based materials and UHQHZDEOH UHVRXUFHV ³7KH ELRHFRQRP\ HQFRPSDVVHVthe production of renewable biological resources and their conversion into food, feed, bio-based products, and bioenergy It includes agriculture, forestry, fisheries, food, and pulp and paper production, as well as parts of the chemical, biotechnological, and energy industries Its sectors have a strong innovation potential because of their use of a wide range of sciences (life sciences, agronomy, ecology, food science, and social sciences), enabling industrial technologies (biotechnology, nanotechnology, information and FRPPXQLFDWLRQWHFKQRORJLHV,&7DQGHQJLQHHULQJDQGORFDODQGWDFLWNQRZOHGJH´[70] The bioeconomy promises to provide more bioenergy and biofuel for replacing fossil one; improve bioprocessing and biorefinery concepts and many different advantages related to biological resources [71] However, some reports highlight the negative impacts of the bioeconomy on water bodies and natural ecosystems [72] 7KHUHIRUH WKH PHUJLQJ RI WZR FRQFHSWV &( DQG %( OHG WR IRUP RI ³&LUFXODU

%LRHFRQRP\´ &%( $FFRUGLQJ WR[71], The circular bioeconomy is defined as an intersection of circular economy and bioeconomy with common topics including bio- based products; share, reuse, remanufacture, recycling; cascading use; utilization of organic waste streams; resources-efficient value chains; and organic recycling, nutrient cycling Therefore, CBE is an appropriate strategy for sustainable development of agro-products, especially agro-based-biorefinery Furthermore, cascading, a key strategy of CBE, used to increase the utilized efficiency of the product and was GHILQHG DV ³&DVFDGLQJ LV D VWUDWHJ\ IRU XVLQJ ELRPDVV LQ D PRUH HIILFLHQW ZD\ E\ reusing residues and recycled materials in sequential steps for as long as possible, before turning them into energy Cascading extends the total biomass resource base wiWKLQDJLYHQV\VWHP´ [73]

In Vietnam case study, rice is an essential food crop contributing to a massive biomass residue globally Therein, rice straw are the primary residues of rice production with high potential in creating value-added products [74] However, as mentioned, the value of these wastes has not been fully utilized; for instance, in Vietnam, rice straw is mainly treated by burning to cause vast carbon dioxide emissions (CO2) These solutions not only seriously affect the environment but also reduce the value of this waste Besides, the CBE of rice production, especially the appropriated solution in Vietnam, is not entirely introduced Therefore, research into the comprehensive treatment of these wastes toward CBE is necessary to add more value to rice production and protect the environment.

MATERIALS AND METHODS

Chemicals and Materials

Rice straw was collected from Cu Chi District, Ho Chi Minh City, Vietnam The paddy straw was thoroughly rinsed and air-dried under the sunlight until the moisture content of below 15% before being pulverized into pieces of 0.5-2 mm in length and stored in closed bags The amount of dry matter in rice straw was determined using Sartorius moisture analyzer MA37

The lignin, hemicellulose, and cellulose content of the dry matter were characterized using the method of Nation Renewable Energy Laboratory (NREL) with a report number of TP-510-42618[75]

All reagents were purchased from commercial suppliers with the pure grade, including sodium hydroxide (NaOH) and sulfuric acid (H2SO4), alumina sulphate (Al2(SO4)3), Nickel (II) nitrate hexahydrate (Ni(NO3)2.6H2O), Urea (CO(NH2)2), Tetrahydrofuran (THF, C4H8O), Tetra-propylammonium bromide (TPA-Br, (CH3CH2CH2)4N(Br)), Ethyl acetate (EtOAc, C4H8O2).

Experiments

This pretreatment process was carried out at atmospheric pressure [76] 300g rice straw was mixed in a 10L boiler with 4.5L NaOH 1w/v%, which was heated to 60qC in advance The mixture was simultaneously mixed by agitator (150rpm) and heated to 90qC in 15 minutes and maintained in 2 hours at 90 o C After pretreatment, the mixture was cooled down to 40qC, followed by vacuum filtration to remove residues The volume of obtained liquid, i.e., the black liquor, was about 4.3L with a pH value of 12.4 The volume loss of the obtained liquid is mainly due to the efficiency of the process of filtration of rice straws residue to gain black liquor and water evaporation during the mixing step

3.2.2 Insight the precipitation behavior of black liquor in different pH values

The single-stage acidification was conducted to demonstrate the precipitation behavior of the black liquor Ten samples containing 200mL black liquor were adjusted with diluted H2SO4 20w/v% to reach the pH value ranging from 10 to 1 After the acidification finishes, these samples were left 24 hours for aggregation and sedimentation Each settled mixture was then filtered, and the precipitate was thoroughly washed with deionized water before being dried at 90qC until a constant mass of solid was obtained The obtained precipitate was then ground by an agate mortar and pestle Those treated samples were analyzed to evaluate the effect of pH conditions on the precipitation and select an appropriate condition for two-stage acidification

3.2.3 Lignin recovery by 2-step acidification

Figure 3.1 Schematic representation of the process of recovery of lignin and silica from rice straw [77]

The black liquor was acidified to a pH value of 3 by H2SO4 20w/v% with two- step adjustments at pH 9 and 3 First, the black liquor had a pH value adjusted to a pH value of 9 and was then left 36 hours for silica precipitation The silica gel was then separated from the liquor by vacuum filtration Finally, the filtrate was diluted with the low concentrated H2SO4 20w/v% to recover lignin at pH 3

3.2.4 Synthesis Ni/ZSM-5 from rice husk silica

The synthesized Zeolite ZSM-5 was prepared following the hydrothermal method; therein, silica from rice husk ash was used as a silica source for ZSM-5 The following molar compositions were prepared for the ZSM-5 synthesis of 9.64 TPA-Br: 8.0 Na2O: 1.0 Al2O3: 90 SiO2: 3206.3 H2O

11,7g of NaOH dissolved in 260mL of distilled water was added to the mixture of 13.75g of TPA-Br in 100ml of distilled water and 55g of SiO2 from rice husk ash The mixture was stirred vigorously at ambient temperature until SiO2 was dispersed into the solution to obtain a liquid gel mixture with a pH of 14 (solution A) The sodium aluminate solution was then formed by dissolving 0.848g of NaOH in 80ml of distilled water with 7.6312g AlSO4, 3.18H2O in 100ml of distilled water; the mixture was vigorously agitated until a transparent solution (Solution B) was obtained.Both solutions (adding B±A) are stirred for 30 min, and then added H2SO4 20w/v% to lower the mixture's pH to 11 for gelation The gel ages at room temperature for 60 minutes before being hydrothermally treated at 175 o C using stainless steel autoclaves for 48h The autoclave is rapidly cooled after the reaction The product was filtered and washed with distilled water until the filtrate's pH dropped to 8 The final product was prepared by drying at 80 o C for 24h before calcination at 550 o C for 4 hours to obtain

The Ni/HZSM-5 was synthesized using the DP method Firstly, 2g of obtained zeolite from the above experiment was dispersed in 210 ml of Ni(NO3) 0.14M before the mixture was heated to 70 o C to gain solution A Simultaneously, 40 ml of Ni(NO3)2 solution was used to dissolve 10,2 g of urea before adding it into the solution A This mixture is heated to 90 °C to initiate the DP process The DP process was conducted for 3 h The suspension is cooled to ambient temperature and filtered to get products The obtained solid was washed 3 times with distilled water and dried at 110oC After that, dry solids are kilned at 400 o C for 2.5 hours at a heating rate of 1K.min -1

3.2.5 Conversion of rice straw lignin into bio-oil

The hydroprocessing reaction was performed in high-pressure reactors (Parr Instruments, 300 ml) In the experiment, the reactor was loaded with 0.5 g catalytic (Ni/ZSM-5, ZSM-5), 2 g lignin, 80 ml n-hexane, sealed and purged with H2 three times The reactor was pressurized with 60 bar of H2 and heated to 325 o C while vigorously stirring at 700rpm The temperature was reached within 15 min, which was used as the zero-reaction time After the reaction, the reactor is cooled to room temperature by an ice-water mixture The reaction product is centrifuged, filtered, and analyzed Each experiment involving a catalytic hydroprocessing reaction was carried out three times Data is an average of three runs' results

The content of ash and non-ash of the obtained precipitations were determined by treating samples at 900 ± 25 °C for 6 hours in Nabertherm muffle furnace model LT3/11, therein, the non-ash content was calculated based on the weight difference after calcining Fourier-transform infrared spectroscopy (FT-IR) spectra of the samples, ranging from 400 to 4000cm -1 with a 4cm -1 resolution, were acquired on KBr pellets using a PerkinElmer Frontier IR instrument X-ray diffraction (XRD) analysis was performed to demonstrate the structure of samples by using Bruker-D8 Model HTXLSPHQW WR UHFRUG WKH VFDWWHULQJ DQJOH ș DQG LWV LQWHQVLW\ 2SHUDWLQJ FRQGLWLRQV were from 10 to 80 o (2T) with a step size of 0.019 o and a step time of 43.00s at ambienW FRQGLWLRQ $ &X.Į 1L-ILOWHUHG UDGLDWLRQ Ȝ c ZDV DSSOLHG ZLWK D working voltage of 40kV The TGA results were investigated using Linseis TGA PT

1600 The sample was heated from room temperature to 800 o C with a heating rate of

The pH of the black liquors was determined by Thermo Scientific Expert pH meter The composition of the obtained precipitates, i.e., ash and lignin content were determined by using the NREL/TP-510-42618 method, i.e., by precipitation via two- step hydrolysis using sulfuric acid solution [75]

The yield of recovered lignin from rice straw was calculated by the equation: Ψܻ݈݅݁݀ ൌ ݉ ௥௔௪௟௜௚௡௜௡ ൈ ݌ ݉௧௢௧௔௟௟௜௚௡௜௡

Where: ݉ ௥௔௪௟௜௚௡௜௡ (g) is the mass of obtained lignin at pH 3 p (%) is the purity of obtained lignin at pH 3 ݉௧௢௧௔௟௟௜௚௡௜௡ (g) is the total lignin content in rice straw

Powder X-ray diffraction (XRD) patterns of catalysts were obtained on Bruker-D8 Model equipment to record scattering angle (2T) from 10 to 80º with a step size of

0.019º Surface area (SBET) was measured using the Brunauer Emmett and Teller (BET) method The pore size was calculated using the desorption branch of the isotherm according to the Barrett-Joyner Halenda (BJH) method The morphology of as-synthesized catalysts was recorded on scanning electron microscopy (SEM) NH3- Temperature Programmed Desorption (NH3-TPD) of the catalyst is carried out in a quartz tube reactor with a thermal conductivity detector (TCD)

For the catalytic hydroprocessing reaction of lignin, quantitative analysis of liquid products is performed by gas chromatography (GC) with a flame ionization detector (FID) and a DB-5 column (30 m x 0.25 mm x 0.1 àm) The composition in the liquid products is identified by GC/MS equipped with a DB-5MS column (30 m x 0.25 mm x 0.1 àm) Nitrogen is used as a carrier gas, with commercial standard reagents as external standards The oven temperature program included 3 phases: initially, the oven temperature is held at 60 o C in 4 min, followed by increasing temperature to

140 o C and held in 5 min with the heating rate of 10 o C/min Finally, the temperature is raised to 280 o C and hold for 9 min with the heating rate of 20 o C/min

The chemical structure of bio-oils obtained from the catalytic hydroprocessing of lignin was determined by FTIR using PerkinElmer Frontier IR

Conversion of rice straw lignin and production yield was calculated based on the formulas: ܥΨ ൌ ௠ ಽ೔ష ௠ ಽೝ ௠ ಽ೔ ൈ ͳͲͲΨ (1) ܪ ௟௤ ൌ ௠ ೗೜ ௠ ಽ೔ ൈ ͳͲͲΨ (2) ܪ ௦ ൌ ௠ ೞ ௠ ಽ೔ൈ ͳͲͲΨ (3) ܪ ௚ ൌ ͳͲͲΨ െ ܪ ௟௤ െ ܪ ௦ (4)

C%: Lignin conversion, % mLi: Mass of initial lignin, g mLr: Mass of lignin residue, g

Hlq: Liquid product yield, % mlq: Mass of liquid product, g

Hs: Solid product yield, % ms: Mass of solid product, g

RESULTS AND DISCUSSIONS

Lignin recovery with lower silica effects

Table 4.1 The composition of rice straw and black liquor Cellulose Hemicellulose Lignin Ash Dry matter

(wt.%) (wt.%) (wt.%) (wt.%) (wt.%)

The composition of dry matter in rice straw is determined and the result is shown in Table 4.1 The rice straw in Vietnam has 50 wt.% of cellulose, 22.45wt.% of hemicellulose, and 19.6wt.% of lignin Meanwhile, the ash percentage of 12.25wt.% represents the silica content in rice straw because the silica content of Vietnam rice straw ash was as high as 80wt% [74, 78] The pretreatment process successfully dissolves silica and lignin in an alkaline solution with the percentage of lignin and ash (silica) in the black liquor increases up to 51.81% and 25.14%, respectively, thus lignin and silica components can be isolated from the liquor

4.1.2 Insight the precipitation behavior of black liquor in different pH values

4.1.2.1 The physical changes in black liquor

The precipitation behavior in acidified black liquors from pH 10 to 1 was described through the physical aspect, weight, and ash (silica) content of the precipitates The analytical results at each stage of acidification are shown in Figure 4.1 indicated the trend of precipitation in black liquor

Figure 4.1 Mass of total precipitate, ash, and non-ash in the black liquors

As can be seen, the total mass of the precipitate witnesses a gradual increase with decreasing pH value from 10 to 6 and reaches a peak of 2.89g at pH 5 When the pH value of the black liquor decreases from 10 to 8, the amount of ash climbs marginally to a peak of approximately 80% at pH 8 and keeps stable until pH 5 According to Aujla et al and Inglesby et al., silica in rice straw dissolved in the alkaline medium is in the form of sodium silicate at pH 10 and becomes silicic acid when decreasing pH to lower than 10, explaining the gel formation and the appearance of precipitation as seen from Figure 2a [79] Zaky et al proposed two chemical equations to clarify the dependence of silica dissolution (Eq 1) and precipitation (Eq 2) on pH value [80]: SiO2 1D2+ĺ1D2SiO3 + H2O (Eq.1)

Na2SiO3 + H2SO4 ĺ6L22.H2O + Na2SO4 (Eq.2)

Therefore, the presence of dissolved silica (as silicic acid) in black liquor and the formation of sodium silicate precipitation upon acidification is the main reason for the rising and stability of ash content The precipitate due to gel formation can be ascribed to the formation of the silicic acid hydrate In the range of pH 5-7, the non-ash content witnesses a significant increase from 20% to 63% However, the ash content plummets 0

Weight (g) pH Mass of precipitate (g)

Ash (g)Volatile compounds (g) between pH 4 and 3 from 35% to 16 which is fit to the considerable drop of total precipitate weight According to Minu et al the acidification of black liquor to pH lower than 4 leads to the re-dissolution of silica [45] Hence, at pH 3 and lower, the weight of precipitate remains unchanged due to lignin content Thus, lignin can be recovered at pH 3 or lower

Figure 4.2 The color variation of the precipitate (a) and the filtrates of black liquors (b) at different pH values from 1 to 10

The appearance and color change of the precipitate is shown in Figure4.2a Black liquors treated in a pH range from 10 to 8 are in a state of a dense gel, which could be easily separated by filtration Substantial precipitation takes place when the pH of black liquor reaches a value of 10 due to the presence of silicates [55] The color of the precipitate at pH 10 to 8 is light golden brown while at pH 7 and lower, it noticeably changes to dark brown, which is proportional to the decolorization of the black liquors

[79] The appearance of brown shade in the precipitates is evident from the presence of lignin chromophore [81] Therefore, the co-precipitation of lignin and silica occurs at pH 7 and lower The sedimentation of treated liquors occurs instantaneously after the pH reached 3 or lower values The brown sediments settle to the bottom of the liquid phase, which takes at least 5 hours to accomplish and the obtained precipitate at this pH is in a slurry state This can be explained by the precipitation of lignin at low pH when lignin acts as a hydrocolloid due to the impacts of protonation of acid groups in lignin structure [54]

The color alteration of processed liquors at different pH values is illustrated in Figure 4.2b The filtrates are remaining dark brown until pH down to 5 before change into opaque reddish-brown Mussatto et al reported the color change of the black OLTXRUZKLFKRULJLQDWHGIURPEUHZHUảVVSHQWJUDLQZDVREVHUYHGIURPS+WR[81] Garcia et al demonstrated the transformation of black liquor from the treatment of

Miscanthus Sinensis with decreasing pH from 12 to 1 [55] Filtered liquors in both studies, which were collected from the single-step precipitation, turned from dark brown to light brown Alkaline-soluble derivatives, which are generated during lignin degradation such as quinones, carbonyl groups, carboxylic acids, hydroperoxyl radicals, phenolic hydroxyl groups, are responsible for the dark color of the black liquor [82]

4.1.2.2 The chemical changes in black liquor

Figure 4.3 FTIR and XRD spectrums of the precipitate from pH 10 to pH

6 (a, c) and from pH 5 to pH 1 (b, d)

The precipitates from pH 10 to 1 were analyzed using FTIR in the 4000-400 cm -1 region and shown in Figure 3a and 3b Based on the FTIR spectroscopy band assignments of the sample in Table 4.2, the spectra of all precipitates obtained at pH values ranging from 10 to 1 exhibit most of the lignin and silica bands In particular, the intensive bands between 3000 and 3500 cm -1 are assigned to OH stretching vibrations The lignin bands are present around 1510 cm -1 and 1605 cm -1 for aromatic skeletal vibration (C=C) of lignin (guaiacyl or syringyl) whilst the absorption bands around 1604 cm -1 and 1735 cm -1 can be assigned to the C=O stretching of lignin [83] The aromatic ring group is also found in the region of 800 cm -1 and 833 cm -1 [84] The presence of silica is indicated by the Si-O-Si bending region (458-561 cm -1 ) and the bands from 950-1000 cm -1 of Si-O-Si asymmetric stretching [85±87]

Table 4 2 FTIR frequency range and functional groups present in the sample

Frequency range (cm -1 ) Functional group

2800-3750 asymmetric stretching and bending vibrations of silanol OH groups (SiO±H)

1604-1735 C=O stretching of carbonyl, carboxyl and acetyl group and of xylans 1510-1605 Aromatic skeletal vibration (C=C) of lignin

800-833 -CH bonds in associated to aromatic rings

Figure 4.3a and 4.3b also illustrate that except for the domination of the absorption bands from 3000-3500 cm -1 , the spectra of the precipitates from pH 10 to 8 are significantly affected by silica bands However, the peak of silica decreases from pH 7 steadily (Figure 4.3a) and from pH 3 to 1 (Figure 4.3b), the effect of silica bands is negligible, whereas the lignin bands are more pronounced in the spectra of precipitates from pH 3-1, which also supports the assumption about the recovery of lignin at pH 3 with less impurity

Figure 4.3 c, d show an X-ray powder diffraction pattern of the precipitate from black liquors at several pH values [88] In general, the absence of peaks of likely impurities such as sodium sulfate and other salts or metals confirms the purity of recovered products [89, 90] The peak between 17q and 30q recorded in the precipitate from pH 10 to pH 6 indicates amorphous silica according to Liu et al and Tinio et al [91, 92] Together with the featureless diffractograms, the appearance of a diffuse maximum at 22.5q indicates the amorphous nature of silica existing in the recovered precipitates in this range of pH [93, 94]

The width of the peak of products at pH 5 and pH 4 is broader from precipitates obtained at higher pH values indicating a higher amorphous degree (Figure 4.3 c, d) This might be relevant to the strong alternation in the ash and organic contents in the precipitate at two pH levels The similar XRD patterns of products precipitating at pH lower than 3 are illustrated by a broadening of the peaks between 10q to 45q ș which indicates the domination of amorphous structure in precipitates at low pH Therefore, XRD diffractograms in this study prove the correlation in the distribution of silica and organic compounds in recovered products Kauldhar et al claimed that the XRD pattern of standard pure lignin showed a major diffraction peak between 23q to 32q ș[8]

Figure 4.4 TG analysis of the precipitates from pH 10 to 5 (a) and from pH 4 to 1 (b)

The thermogram of recovered products precipitated at basic pH from 10 to 1 is shown in Figure 4.4 We obtained four temperature zones, as interpreted in the following The precipitated products at pH 10, pH 9, pH 8, and pH 7 witnessed a thermal degradation by about 25wt.% The second group including profiles of pH 6, pH 5, and pH 4, had the total mass loss ranging from 41±53% The last group, which contains TGA curves of precipitate obtaining at pH 3, pH 2, and pH 1, experienced about 80wt.% loss due to thermal degradation

The first stage occurred below 100qC due to the evaporation of physically adsorbed water The thermogram of precipitates at pH 10 to pH 4 reported the fluctuated mass reduction in the range of 10±14wt.%, while the weight loss of products precipitated at pH 3 and lower decreased to 7% The adsorption of water on the surface of OH groups of silica is responsible for the higher moisture content of recovered products at pH 10±4 [8]

Hydroprocessing of lignin

ZSM-5, a solid acid catalyst with the critical feature of shape selectivity, is the most investigated and effective catalyst for a higher yield of aromatic and widely used in the petrochemistry industry In this study, the ZSM-5 and Ni/ZSM-5 were synthesized using silica from rice husk ash The chemical structure of ZSM-5 and Ni/ZSM-5 was determined by XRD, as shown in Figure 4.8

Figure 4.8 The XRD diffractograms of ZSM-5 and Ni/ZSM-5 (a), SEM micrographs of ZSM-5 (b) and Ni/ZSM-5 (d) and the TPD-Br of ZSM-5 and

$VFDQEHVHHQWKHPDLQSHDNVRIFDWDO\VWVDUHSUHVHQWHGDWWKHșRI7,9 o ; 9,04 o ; 13,59 o ; 14,19 o ; 15,14 o ; 15,91 o ; 23,38 o ; 24,16 o ; 25,63 o ; 30,18 o , and 45,54 o , which are in the range of typical specific peaks of ZSM-5[98] The diffractogram of ZSM-5 also corresponds to the MFI (Mordenite Framework Inverted) structure The appearance of new peaks at 2T is 37,17 o ; 43,27 o ; 62,88 o and 75,10 o correspond to the peak of NiO(111), NiO (200), NiO(220), and NiO(311) in the XRD spectra of Ni/ZSM-5 demonstrating the presence of the following NiO Nickel impregnation on the carrier

In the case of reduced Ni/ZSM-5 appear only three new peaks at 44.49 o , 51.84 o, and 76.39 o for the presence of nickel metal[99] The presence of Ni indicated that NiO was reduced to Ni during the catalytic reduction with hydrogen The XRF analysis results showed that Ni/ZSM-5 catalyst system's Ni content reached 23.68 wt%

Table 4.3 The BET analysis of ZSM-5 and Ni/ZSM-5

Besides, the BET results (Table 4.3) showed that the specific surface area and pore volume of ZSM-5 decreased significantly from 313,946 m 2 /g and 0.233 cm 3 /g to 270,583 m 2 /g and 0.215 cm 3 /g when nickel metal was impregnated on the carrier The reason is that the accumulation of Ni on the molecular sieve orifice or deep into the channel interior, which blocked the molecular sieve channel interior, resulting in a smaller pore volume

The change from microvolume to meso volume is consistent with the observed tendency for adsorption-desorption isotherms (BET) in Figure 4.9 shows the BET isotherms of ZSM-5 and Ni/ZSM-5 According to IUPAC classification, both the isothermal adsorption lines of ZSM-5 and Ni/ZSM-5 have the form of isothermal adsorption of type I This type of isothermal adsorption line is typical for porous materials with a micro size The isotherms of the ZSM-5 and the Ni/ZSM-5 exhibit Hysteresis H4 rings, which characterize the micro-hole structure with several meso holes However, when nickel is impregnated with ZSM-5, the hysteresis ring becomes narrower Barton et al have explained this phenomenon to be due to the presence of sets of structures with meso size[100]

Figure 4 9 The BET isotherms of ZSM-5 and Ni/ZSM-5

The homogeneous and porous character of the ZSM-5 and Ni/ZSM-5 catalysts can be observed by the SEM micrograph, as shown in Figure 4.8b, d and Figure 4.10 ZSM-5 zeolite crystals have a spherical form with a relatively clean and bright surface After the nickel impregnation, the surface of the zeolite crystal becomes coarser and darker The particle size with a large surface and the catalyst's power appears in the agglutination due to the bonding together, covering the catalyst's surface Thus, the catalyst surface was more compact, the gap was smaller, and most crystalline grains tended to grow into cubic shapes (with sizes ranging from 2um± 8um) That may be due to the smaller crystal particle size and uneven size distribution; therefore, the catalyst surface did not have a fixed shape

Figure 4 10 The SEM of ZSM-5 (a,c) and Ni/ZSM-5 (b,d) in different magnification

The temperature-programmed desorption (TPD) was used to explore the acid properties of Ni/ZSM-5 and ZSM-5 catalysts, and the results were shown in Figure.4.8c In two cases, the two characteristic peak contributions centereGDWႏ DQG ႏ UHVSHFWLYHO\ are observed The peak of the high-temperature area is the largest, proving that its acidity is strongest The desorption peak of the higher temperature zone is reduced after Ni impregnation However, no significant change in the desorption peak at the lower temperature, suggesting that the introduction of nickel can effectively change the surface acidity of the molecular sieve catalyst and reduce the number of surface acid sites, especially the number of strong acid sites Also, Ni 2+ ions react with the Brứnsted acid centre, resulting in decreased Brứnsted acid site center In the Ni/ZSM-5 catalyst system, the total acids had a slight decrease in acid concentration than the ZSM-5, indicating that Ni 2+ may be poured into the inner hole of the ZSM-5 molecular sieve

The conversion of lignin was conducted with ZSM-5 and Ni/ZSM-5 catalyst and no catalyst The results, shown in Table 4.4, indicated there is almost no significant change in the yield of liquid products (LP), solid products (SP), and gaseous products (GP) in the case of no-catalytic and ZSM-5 catalyst ZSM-5 did not improve the yield for this reaction In contrast, the reaction using Ni/ZSM-5 catalysts showed a high LP and GP yield of 24.42 wt.% and 43.20 wt.%, respectively The reason is that the hydrogenolysis reaction of lignin on the nickel center forms phenolic compounds These compounds will continue to be hydrogenated and hydrogenolysis Thus, LP and GP's yield are higher than SP's yield with the Ni/ZSM-5 catalyst It can be concluded that the Ni/ZSM-5 is appropriate for catalytic hydroprocessing to synthesis bio-oil After the reaction, ethyl acetate was used for the separation of liquid production Almost liquid products dissolved into the EtOAc phase The solid products, including lignin residues and catalyst and char, were soluble in THF to determine the lignin conversion because the catalyst and char were insoluble by THF

Table 4.4 The product yield of catalytic hydroprocessing of lignin at reaction condition of 2g lignin, n-hexane (80 mL), solid catalyst (0.5 g), temperature (325 o C), reaction time (2h), 60 bar H 2

Catalyst Conversion yield Liquid products Solid products Gaseous products

Figure 4.11 FTIR spectra of bio-oil obtained from catalytic hydroprocessing of rice straw lignin with a various catalyst (a) Ni/ZSM-5, (b) ZSM-5, (c) no catalyst, (d) raw lignin Reaction condition of lignin (2g), n-hexane (80 mL), solid catalyst (0.5 g), temperature (325 o C), reaction time (2h), 60 bar H 2

FTIR analysis of three liquid products obtained from hydroprocessing reaction with different catalysts also showed in Figure 4.9 The interpretation of peaks in the spectrum was carried out using the information given in Table 4.2 Overall, the strong absorption of 3 bio-oils of 3418 cm -1 assigned to the stretching of the OH group However, there is a decreased intensity of the -OH group of bio-oils compared with the raw lignin The peak strength's gradual weakness at 3418 cm -1 indicated lignin's hydroprocessing to phenolic monomers and oligomers In the reaction with Ni/ZSM-5 catalyst, the OH group's intensity decreased compared with the others That proves that the Ni/ZSM-5 catalyst in hydroprocessing lignin effectively contributed to the -

OH reduction In three cases, the intensity of 2900cm -1 increases as the intensity of signals at 1515cm -1 , 840cm -1 decreases, and the disappearance of 1423cm -1 compared with the lignin curve shows hydrogenation of the aryl ring In particular, the Ni/ZSM-5 catalyst is more useful for the hydrogenation process by the disappearance of the peak of 840 cm -1 compared to the other two cases The C = O bonds (1700 cm -1 ) also decreased significantly compared to Ni/ZSM-5 catalyst A sharp decrease in peaks at

1220 cm -1 , 1122 cm -1 , and 1030 cm -1 of Ni/ZSM-5 catalyst compared to the other two cases indicated that cleavage of ether bonds in the lignin structure is more robust under the presence of nickel The elimination of the methoxy groups (O-CH3) was also demonstrated by a significant reduction in peak intensity of 1376 cm -1 , that results were consistent with the study of Hernandez et al [101] The FTIR results indicated that hydrogenation of aromatic structures, higher cleavage of ether linkages strongly occurred in the reactions performed with Ni/ZSM-5 [102]

Figure 4.12 The composition of bio-oil obtained from catalytic hydroprocessing of rice straw lignin with Ni/ZSM-5 and ZSM-5 Reaction condition of lignin (2g), n-hexane (80 mL), solid catalyst (0.5 g), temperature (325 o C), reaction time (2h),

The obtained liquid productions of hydroprocessing lignin were analyzed by gas chromatography (GC) with ionization of the flame detector (FID) and mass spectrometry detector (MS) As shown in Figure 4.12, phenols were the main products of both bio-oils because they are a basic lignin structure Ni/ZSM-5 favoured a higher yield of phenolic compounds (41.2 wt.% compared to 32.7 wt.% with ZSM-5) and lesser ketones compounds (5.2 wt.%) Both Ni/ZSM-5 and ZSM-5 catalyzed the formation of alkyl-substituted phenols as major products: phenol, 2-vinyl mural, ethoxy benzene, 3-methyl phenol, as shown in Table 4.5 Ni/ZSM-5 catalyzed the formation of products with significantly lower average molecular weight due to enhanced hydrogenation/hydrogenolysis effect by nickel supported over ZSM-5 Comparing to research of Zou et al [103], there are some compounds which are also the products of lignin depolymerization such as 3-ethylphenol, 4-ethylphenol, so on Besides, 2,4,6-trimethyphenol is the syringyl species Thus, it can demonstrate that the lignin structure contains aromatic compounds and the lignin depolymerization can produce the aromatic compounds

Table 4 5 The Compounds identified in liquid products of depolymerization of rice straw lignin with ZSM-5 and Ni/ZSM-5

10.682 Propanedioic acid, 2-propenyl-, diethyl ester 1.8 5.2

1 Total area was obtained based on the integration of 17 major peaks, without including the small peaks with area % < 1.5

Scale-up into pilot scale and integrated with bioethanol production process

4.3.1 Production of lignin in pilot scale

4.3.1.1 Lignin and silica isolation from black liquor in different scales

The methods used to extract lignin and silica from black liquor are developed and shown in Section 3.2.3 The lignin and silica recovery technique result in the bench-, and pilot-scale are shown in Table 4.6 Each precipitation's physical changes (e.g., color, state) were recorded Like Do et al (2020), the liquor's color changed from dark brown to olive-brown with decreasing pH and contrasted with the precipitate color The precipitate state also shifts from a dense gel at a basic pH of 9 to an acidic pH of

3 This phenomenon demonstrated the decolorization of black liquor again due to lignin precipitation, as reported by Mussatto et al [81] and García et al [55] Simultaneously, the recovery efficiency of lignin and silica at lab and pilot scales was above 90%, of which the immaculateness of lignin was over 70% The total volume of HCl, used in 2 different volumetric scales, is directly proportional to black liquor volume This result shows that the laboratory-scale process is suitable for growing and expanding industrial scale Moreover, with simplicity, safety, and green chemicals, this process can be considered a potential process for the comprehensive development of lignocellulosic biomass to convert into high-value products and bioenergy

Table 4.6 Results of lignin and silica recovery on different scales

Yield Bench-scale Pilot-scale

The total volume of HCl (mL) 100 5067

The FTIR spectra of silica and lignin obtained from the pilot-scale were shown in Figure 4.11 Overall, both spectra of lignin and silica are similar to each other A broad peak between 3000-3500cm -1 mainly formed two spectrums that indicated the stretching of H-bonded OH The peaks at 1510 cm -1 and 1605 cm -1 are attributed to aromatic skeletal vibration (C=C) of lignin (guaiacyl or syringyl) The shape peaks at

1604 cm -1 and 1735 cm -1 are assigned to the C=O stretching of lignin and presences on the silica bands [83] These findings proved an amount of lignin and other organic compounds re-bonding with silica and creating coprecipitation after pretreatment The interaction between Si and OH groups of water, lignin, and other compounds by hydrogen bonds was also reported by (Xia et al 2018) In contrast with silica bands, lignin bands are insignificantly influenced by silica because of the absence of characteristic peaks of silica at 458-561 cm -1 (Si-O-Si bending), 950-1000 cm -1 (Si-O-

Si asymmetric stretching) [85] Therefore, the lignin, recovered in this condition, is purer

Figure 4.13 The FTIR spectrum of obtained lignin and silica on a pilot scale

To verify the complicated linkage of silica and other chemicals in black liquor, thermogravimetric analysis (TGA) is used to explain and illustrate silica's thermal stability Additionally, lignin's thermal properties varied due to the biomass origin difference [8] The plots of the thermogravimetric analysis for lignin and silica, shown in Figure 3a

The first stage occurred at under 100ºC due to the evaporation of physically adsorbed water Accordingly, the values of weight loss within 100°C are 9.3 wt.%, 11.9 wt.% for lignin and silica, respectively The second stage was observed around 100ºC±250ºC, which indicated the decomposition of polysaccharides, aliphatic alcohols, and acids [95] The mass loss of silica was approximately 2.4 wt%, which confirms the negligible presence of non-lignin compounds co-precipitating with silica, while the weight loss of lignin was around 10 wt.% This noticeable loss implies the coprecipitation of lignin and other organic compounds, which is evident in the intricate linkage of lignin and another component [96] The increase in the thermal degradation of lignin occurred mainly in temperatures interval of 250ºC±500ºC, similar to the results of [56] In this stage, the decomposition of lignin happened strongly due to its aromatic structure

On the other hand, the silica curve is gradually stabilized When the temperature was over 480ºC, there is a steady-state on the silica thermogram with a negligible mass loss of less than 5% The thermal degradation of silica rose to 15 wt.% and have a stable trend when increasing temperature The XRD analysis results indicated that the obtained lignin and silica have an amorphous structure with a broad peak at 22.6 o and

Figure 4.14 The thermogram of silica and lignin (a) and the photograph of silica (b) and lignin (c) on a pilot scale

The obtained lignin is a dark-brown solid (Figure 3b), whereas the silica is a white powder (Figure 3c) The scanning electron microscopy of silica and lignin, as shown in Figure 4, illustrated the microstructure of silica and lignin Both lignin and silica appeared with granules of various dimensions Additionally, the porous structure of silica exhibited a large specific surface area These grains have an assembly trend to make a more prominent grain The acquired lignin powder is a slightly smooth surface and no fiber existence, which re-confirmed lignin's purity

Figure 4.15 The SEM of silica (a,b) and lignin (c,d) at smaller (a,c) and higher (b,d) magnification

4.3.2 Integrated with the bioethanol production process

The new sustainable process was designed by integrating the current bioethanol production system and the new lignin and silica recovery process and reusing the waste stream for cost reduction In the new system, besides the main product is bioethanol, the high-value co-products were also produced as lignin, silica, and charcoal The new system's mass and energy balance were evaluated and synchronized with the whole biorefinery process to demonstrate the new design's sustainability and efficiency compared to the old one

The material flow of the sustainable process using 203.5 kg dry rice straw was depicted in Figure 4.16 The material flow of the process was described by focusing on the changing of rice straw composition, the conversion of rice husk into charcoal, and the recovery of lignin, silica from the waste liquid In this process, the 203.5kg rice straw pretreatment was conducted at ambient temperature in 6-8h and the solid to liquid ratio of 1:5 (kg/kg) by NaOH solution of 1.0 wt.% An amount of lignin and silica was solubilized in an alkaline solution that led to changes in pretreated straw composition with 58.3 wt.% of cellulose, 18.6 wt.% of hemicellulose, and others (lignin, silica, and other) In the next step, the pretreated rice straw was fermented in SSF, and the fermentation broth then went through two distillation columns to get bioethanol 96 vol% The decreasing percentage of cellulose indicated the high yield of the SSF process The sewage after the lignin and silica recovery system has pH 3 The acidic sewage could be reused for acidification as the final liquor after this process contained almost no lignin, silica, cellulose, and hemicellulose At the same time, the high ash content was attributed to inorganic content Moreover, according to the high cellulose content, the distillation residue is potential for nano cellulose synthesis [104] Besides upgrading the new process's sustainability and reusing the distillation residue as a nitrogen source for SSF, nano-cellulose recovery from this residue is a potential route and needs more investigation for larger-scale applications

In the new system, the energy flow, as shown in Figure 6, was gained by various sources, including rice husk, rice straw, kerosene, LPG, and electricity Human labor was not involved in this energy calculation Rice husk conversion into charcoal and energy will be reported in the next publication The reuse waste stream process was also not calculated in this energy balance because of some weather issues (e.g., drying distillation residue) Although more than 10383,8MJ of energy was required to obtain ethanol, lignin, silica, and charcoal, the electricity consumption only covered 12.5% of this process's total energy The carbonization process of rice husk supplied more than 80% of the total energy consumption

Figure 4.16 The energy balance of the new sustainable process

The energy balance of the process was calculated based on the law of conservation of energy The calculation of energy balance was described in Equation 1:

Input energy [ Erice straw (2791MJ) + Erice husk (6081MJ) + Ekerosene (8.5MJ) + ELPG

(204.3MJ) + Eelectricity (1299MJ)] = Output energy [ Eethanol (638.4MJ)+ Echarcoal

(1866MJ) + Estream (1728.3MJ) + Eheatloss+unused (2327MJ) + Eresidue and unrecovered products

(1701.1MJ) + Elignin (1066MJ)+ Esilica (192MJ) + Esewage (865MJ)] = 10383.8MJ (1)

Wherein: Erice straw and Erice husk are energy in rice straw and husk, Ekerosene and ELPG are energy input of kerosene and LPG disbursed in the ignition process, Eelectricity is electricity supply for operating the system, Eethanol and Echarcoal are energy in ethanol produced and charcoal discharged, Estream is the energy of steam, Eheatloss+unused is heat loss and unused energy from rice husk carbonization process, Eresidue and unrecovered products are unrecovered energy of product and residues and unused energy from bioethanol production process boundary, Elignin and Esilica are energy in lignin and silica produced, respectively, and Esewage is the energy of liquid residue from black liquor treatment process

The energy required for the entire process's operation is about 10383.8MJ (including the energy of rice husk and straw and another supplier), higher than the traditional process (8661MJ) The electricity consumption for bioethanol production combined with the black liquor treatment process is 1299 MJ, increased by 554,4MJ compared to the traditional process (744.6MJ) In contrast, products and residues' unrecovered energy is expressively reduced nearly 1.5 times compared to the old process (2532.6MJ) The energy efficiency of biorefinery was recognized by Equation 2:

= 0.529 > 0.449* (the energy efficiency of current process)

The energy efficiency of 0.529 demonstrated that the new bioethanol production process was upgraded successfully, and utilizing energy for producing high value-add compounds is appropriate Moreover, the new bioethanol process reduced energy loss from unrecovered and unused material and upgrades this process's value chain in energy efficiency

Following this process, many products are produced with high applicability, including silica, lignin, charcoal, and bioethanol, while the wastes stream mainly included sodium chloride Producing more products increase the value of the process and reduce the wastes flux Additionally, by a simple method and using less energy, producing by-products can be used to pay part of the operation's cost The biorefinery is a promising technology for mass production to turn biomass into biofuel and biomaterial with interesting achievement However, there were still some drawbacks as the residue, heat loss, and unused energy, so this approach needs to improve some energy efficiency, such as optimizing the energy consumption of each unit process, to lower unexpected effects.

Circular bioeconomy analysis

Nowadays, the "circular economy" is gaining popularity as a means of cost- effectively integrating the whole process In a linear economy, resources are generated, used, and disposed away, but in a circular economy, resources are exploited throughout their shelf life in order to maximize profit [105] Thus, recycling biowaste is a critical technique for maximizing the usage of the available biomass resource base in a circular bioeconomy, while reusing waste would be preferable to transporting waste to a new site to be used [106] The small cycle of paddy residues (Figure 4.17) was developed in this study by incorporating bioethanol processing into our life cycle design With the creation of different bio-products, we also create different usage phases and product lifespans In the first cycle, paddy straw is used for bioethanol production while rice husk is a feedstock of carbonization process to supply energy for bioethanol process [107] Steam generated from the rice husk's carbonization covered a part of the Bioethanol process's total energy During the production of bioethanol, the waste of this plant is utilized in a new cycle (second cycle) The recovered lignin and silica are used to produce bio-oil Conversion of lignin into biocrude is also created more valuable products range from liquid oils, phenols, aldehydes, alcohols which have profound application as chemicals, energy and fuels [108] Moreover, in this design, we also mention the utilization of charcoal from the carbonization process Charcoal from this process can be used as fertilizer reported [109±111] Besides, the existing literature allowed to creating the using path ways of silica and biocrude which related to agro-biorefinery concept In particular, biocrude from depolymerization of lignin should be upgrade for continuing use as an energy supply for biorefinery or a source of phenolics chemicals, while silica can be use as many industries as pharmacy, biosensor and especially in agriculture as a type of fertilizers [112]

Figure 4.17 Paddy residues-biorefinery based circular bioeconomy

Moreover, the high initial capital investment, high transport costs in comparison to biomass, and significant variations in biomass composition and supply throughout the year are all major problems and impediments for biorefineries High levels of capital investment can be justified by maximizing the added value of the resultant goods, such as by generating at least one high-value chemical/material product as well as low-grade and high-volume items like animal feed, fertilizers, and heat As a result, for cost-effective bio-based product recovery, the material should have features such as being readily available, non-edible, portable, low-cost, and toxic-free With all of this in mind, rice straw can be regarded as a suitable material for the creation of energy-efficient biofuels According to our suggestion small loop, a mass estimation of bioconversion of rice straw and rice husk was shown in Figure 4.18

Figure 4.18 The mass estimation of bioconversion of rice straw and rice husk in biorefinery concept

As can be seen, 203.5 kg of rice straw can be produced 19.8kg of lignin and

175 kg of pretreatment residue This residue with a significantly reduced lignin content can be used to produce bioethanol as reported by Tran et al.[76] or nano-cellulose as the study of Thakur et al [113] Meanwhile, 400kg of rice husk can release energy supplied to the boiler to maintain the temperature while releasing 115kg husk ash rice This ash is used to produce 80.5Kg of silica and 36.4kg biochar The obtained lignin and silica can produce 84kg ZSM-5 and 5.6kg bio-oil with enriching aromatic compounds With this method, more products from rice waste will be created to increase their economic value Considering present market value of lignin (approx

$0.65-1.0/kg) and silica ($0.8-1.2/kg), the amount of silica that could be recovered has industrial importance However, Martinez-Hernandez et al (2019) [114] observed a reduction in ethanol production costs ($2.02 gal) by conversion of the lignin to phenolics, which have a high market value Ramirez and Gursel [115] estimated a FDSLWDOFRVWRIẳPLOOLRQIRUWKHK\GURWKHUPDOXSJUDGLQJSURFHGXUHZKLFKLVPRUH than the benchmark with the highest return on investment (12 percent) and the shortest payback period of 5 years Therefore, conversion of rice straw lignin into biocrude is absolutely appropriate in this scenario, which is also a prime for development of biorefinery.

CONCLUSION AND FUTURE WORKS

In this study, the biorefinery based on residues and waste from the rice production chain (straw) represents an opportunity both to minimize waste production and to add more value in to those components which are associated with thea disposal cost The following findings were obtained as a result of the research:

Firstly, the effects of pH value on the behavior of the precipitates of lignin and silica from the black liquor of rice straw have been comprehensively investigated by step-by-step acidification with dilute H2SO4 The color of the precipitate changes from golden brown to dark brown with decreasing pH value, whereas the opposite trend is true for the filtrate The pH value of 3 is demonstrated to recover lignin most effectively based on chemical structure analysis, while a pH range of 10-8 was demonstrated to remove silica from the black liquor A two-step acidification process was developed with prior acidification of diluted sulfuric acid to pH 9 and removal of silica, followed by acidification to pH 3 for optimal lignin recovery These findings will be useful in mass production to identify lignin recovery during the acidification of black liquor Moreover, in the view of industrialization, by utilizing low concentration basic chemicals such as NaOH 1% and H2SO4 20%, this method not only produces value-add products but also saves energy and lowers waste in the environment because of its simplicity and efficiency By using diluted sodium hydroxide at a low concentration of 1w/v%, the precipitate obtained at pH 3 has the highest lignin purity of 65.74%, a recovery yield of 66.75%, and a silica reduction of 94.38%

Also, in this study, the biorefinery process with the zero-waste discharge concept was established thanks to the reuse and recovery of the waste stream with more valuable products produced The new system's energy consumption is more effective than the previous one, with an energy efficiency of 0.529 Lignin and silica were successfully recovered from black liquor generated from the bioethanol plant on a pilot scale with a high recovery yield Reusing the distillation residue as a Nitrogen source for the SSF step could reduce the purchase cost for CSL This biorefinery process, which is more sustainable than bioenergy production alone, promises a potential solution to sustainable development goals by providing higher-value creation

Finally, the rice residue's potential was demonstrated by the close life cycle, built by the chain of a simple method to improve its value The cycle could be an excellent way to lay the groundwork for developing a biorefinery model that adds more value to the rice production chain while adhering to circular bioeconomic principles Besides, the depolymerization of rice straw lignin was performed efficiently by Ni/ZSM-5 and ZSM-5 catalysts derived from rice husk The highest yield of liquid product (24.42 wt.%) and phenolic compounds (41.2 wt.%) was obtained from the Ni/ZSM-5 catalyzed reaction

Besides the achievements, there is some work that needs to be conducted to make the research better These following works were suggested by the author based on circular bioeconomy principles and biorefinery concepts:

- Statistics from the economic database for biorefineries in Vietnam potential biomass, price of bio-fuel in the Vietnam market, cost of building biorefinery, and so on

- Analyzing the new process using Life Cycle assessment tools, Techno-economy, Socio-economy for sustainability

- Investigating the use of biocrude in the production of biofuels, bio-based chemicals, etc

- Studying on utilizing bio-char for producing bio-materials

1 T M Le et al., ³Sustainable bioethanol and value-added chemicals production from paddy residues at pilot scale´ Clean Technologies and Environmental policy, vol 20, 2021

2 N H Do et al., ³7KH QRYHO PHWKRG WR UHGXFH WKH VLOLFD FRQWHQW LQ OLJQLQ UHFRYHUHGIURPEODFNOLTXRURULJLQDWLQJIURPULFHVWUDZ´Scientific Reports, vol 10, pp 21-26, 2020

3 N H Do et al., ³Lignin and sodium lignosulfonate production from the black liquor generated during the production of bioethanol from rice straw´ Vietnam Journal of Science and Technology, vol 58, pp 63-72, 2021

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