Three kinds of hierarchical Mordenite Framework Inverted (MFI)-type nanozeolites including Si-MFI, Al-MFI and Ti-MFI were successfully synthesized by two-stage varying temperature hydrothermal treatment. Taking toluene as the probe molecule, a series of methods were used to evaluate the adsorption properties of as-synthesized hierarchical zeolites samples including the adsorption breakthrough curves, simulation of adsorption isotherms as well as kinetics models and toluene-TPD (the temperature programmed desorption of toluene).
Microporous and Mesoporous Materials 302 (2020) 110204 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso Adsorptive properties in toluene removal over hierarchical zeolites Shushu Huang a, b, Wei Deng b, Long Zhang b, Dengyao Yang b, c, d, Qiang Gao a, **, Zhengfang Tian e, ***, Limin Guo b, *, Tatsumi Ishihara c, d a Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan, 430074, China School of Environmental Science and Technology, Huazhong University of Science and Technology, Wuhan, 430074, China c Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 8190395, Japan d International Institute for Carbon-Neutral Energy Research, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 8190395, Japan e Hubei Key Laboratory of Processing and Application of Catalytic Materials, Huanggang Normal University, Huanggang, 438000, China b A R T I C L E I N F O A B S T R A C T Keywords: Adsorptive properties Toluene removal Hierarchical zeolites Three kinds of hierarchical Mordenite Framework Inverted (MFI)-type nanozeolites including Si-MFI, Al-MFI and Ti-MFI were successfully synthesized by two-stage varying temperature hydrothermal treatment Taking toluene as the probe molecule, a series of methods were used to evaluate the adsorption properties of as-synthesized hierarchical zeolites samples including the adsorption breakthrough curves, simulation of adsorption iso therms as well as kinetics models and toluene-TPD (the temperature programmed desorption of toluene) The dynamic adsorption results showed that Al-MFI exhibited the highest adsorption capacity for toluene (58 mg/ gads) under dry gas condition while Ti-MFI had the optimal toluene adsorption performance (45 mg/gads) under the wet gas condition (the relative humidity was 50%) The simulation results indicated that the adsorption behavior of toluene on hierarchical MFI nanozeolites conformed to the Freundlich principle, and the pseudo-firstorder adsorption model was suitable to elucidate the adsorption process Moreover, the fitting result of intra particle diffusion model indicated that the adsorption process was affected by multiple adsorption steps and the intraparticle diffusion was not the control step Introduction usually produces some toxic byproducts [13] Among them, adsorption has been still considered as one of the simplest and most effective technologies for VOCs removal due to its low cost and easy operation [12,13] Additionally, the adsorbents are favored to be reused by adsorption/desorption recycling Up to now, different kinds of porous materials (such as carbon-based materials, zeolites, organic polymers and composites) are practically used [14–19] As its non-flammable property, zeolites with large surface area and peculiar microporous channels have been considered as the most potential complementary adsorbents for activated carbons [7,20–23] For the conventional zeo lites, the diffusion of adsorbate molecules is usually restricted due to its small pore size [24,25] Then, the synthesis of hierarchical zeolites has been used to overcome drawbacks of the conventional zeolites [26–29] And the construction of nanosized zeolites is another good method to facilitate the mass diffusion [30,31] For example, the nanosized zeolite Y was used for various protein adsorption [32], and nanozeolites with increased external surface area showed the higher uptake of protein Volatile Organic Compounds (VOCs), which are widely used in in dustries such as petrochemicals, printing, pharmaceuticals and painting, is regarded as one of the major resources to air pollution including the photochemical smog and particulate matter In addition, VOCs itself has harmful effects on human health [1–3] Nowadays, VOCs removal on the end-of-pipe control is thus of great importance for protecting the envi ronment as well as public health Various technologies, such as adsorption, absorption, catalytic combustion and photocatalytic degra dation and so on, have been developed to remove VOCs [4–11] The VOCs absorption/adsorption can be achieved by dissolving VOCs in chemical solvents or adsorbing on the adsorbents, which is simple for practical application but sometimes not favorable due to its limited absorption and followed additional separation process [12] The incin eration method consumes large amount of energy to generate high temperature for the reaction and photocatalytic degradation method * Corresponding author ** Corresponding author *** Corresponding author E-mail addresses: gaoqiang@cug.edu.cn (Q Gao), tzf7801@163.com (Z Tian), lmguo@hust.edu.cn (L Guo) https://doi.org/10.1016/j.micromeso.2020.110204 Received 24 January 2020; Received in revised form 17 March 2020; Accepted 19 March 2020 Available online April 2020 1387-1811/© 2020 Elsevier Inc All rights reserved S Huang et al Microporous and Mesoporous Materials 302 (2020) 110204 Voung et al reported the nanozeolites with small particle size and high surface area demonstrated the improved activity in the standard gas oil cracking [33] Li and the co-workers also found nanosized zeolite Y exhibited the higher reaction rate in the selective catalytic reduction of NO2 with urea due to its more silicon hydroxyl groups resulting from its nanosized morphology and larger external surface area [34] Up to now, the hierarchical zeolites consisted of nanosized zeolites have been rarely reported for adsorptive removal of VOCs [30,31,35] Herein, we firstly synthesized three kinds of hierarchical zeolites consisted of nanosized MFI zeolites Then toluene was used as the probe molecule to evaluate the adsorptive properties of the as-synthesized hierarchical zeolites by the dynamic breakthrough experiments Furthermore, the adsorption simulation combined with toluene-TPD were adopted to elucidate the adsorption mechanism According to our knowledge, this is the first time to report hierarchical zeolites con sisted of nanosized MFI zeolites for the toluene adsorption Experimental section 2.1 Chemicals and regents All chemicals were used without further treatments: tetraethyl orthosilicate (TEOS, >99%), aluminum isopropoxide (AIP, >98%), tet rabutyl titanate (TBOT, >99%) and sodium hydroxide (NaOH, analyt ical reagent) were from Sinopharm Chemical Reagent Co., Ltd Tetrapropylammonium hydroxide solution (TPAOH, 25% in H2O) was from Shanghai Cainorise Chemical Co., Ltd Deionized water was used in the experiment 2.2 Materials synthesis Al and Ti-containing MFI nanozeolite aggregates were synthesized using two-stage varying temperature hydrothermal treatment [36] Typically, 0.31 g aluminum isopropoxide or 0.51 g tetrabutyl titanate was mixed with 15.62 g tetraethyl orthosilicate and then dissolved in 25.92 g deionized water The water solution of TPAOH (10.98 g, 25 wt% TPAOH diluted with 30 g deionized water) with 0.45 g NaOH was dropwise added into the above-mentioned solution After stirring for 3–5 h at 313 K, the mixed solution was further stirred for days at 373 K to obtain the precursor solution After cooling down, 60 g deionized water was added into the precursor solution and then the solution was treated at 423 K in Teflon-lined stainless-steel autoclaves for a second hydrothermal treatment After crystallization for 24 h, the samples were centrifuged, washed by deionized water three times, overnight dried at 373 K and finally calcined at 823 K for h The as-prepared samples were donated as Al-MFI or Ti-MFI The preparation of Si-MFI was similar with the above-mentioned procedure and without the addition of aluminum isopropoxide or tet rabutyl titanate 2.3 Sample characterization The powder XRD (X-Ray Diffraction) patterns of as-prepared samples were recorded on a Shimadzu XRD-7000 diffractometer using Cu Kα radiation (30 mV and 40 mA) from to 50� at a rate of 5� /min The actual Si/M (M ¼ Al or Ti) ratios were determined by X-ray Fluorescence (XRF) analysis on Shimadzu XRF-1800XRF Fourier transform infrared spectroscopy (FTIR) was conducted on a Bruker Tensor II spectrometer Sample disks were made by mixing dried samples with KBr and then pressed them into tablet form All infrared spectra were recorded over 400 to 4000 cmÀ region with a resolution of cmÀ The crystallinity of as-prepared hierarchical zeolite samples was estimated from (I550/I450) � 100%/0.72 where I550 and I450 were the intensities of the infrared bands at 550 and 450 cmÀ [37] The Ultraviolet–visible diffuse reflection spectroscopy (UV–Vis) was conducted on a Shimadzu UV-3600 spectrometer from 200 to 800 nm and BaSO4 was employed as Fig XRD patterns (A), FTIR spectra (B) of the as-prepared hierarchical zeolite samples and UV–vis diffuse reflectance spectra (C) of Ti-MFI S Huang et al Microporous and Mesoporous Materials 302 (2020) 110204 Table Textural properties of the as-prepared adsorbents Samples Si-MFI Al-MFI Ti-MFI a b c d e f Average lattice constantsa a b c 18.4672 20.0588 18.6905 19.3771 28.5813 19.7030 13.1867 13.4596 13.3906 SBETb (m2/g) Vtotalc (cm3/g) Vmicrod (cm3/g) RCe (%) Si/Mf 294 362 312 0.21 0.26 0.21 0.10 0.07 0.04 67 69 61 / 24.3 25.7 Calculated from the results of XRD Surface area are calculated by Brunauer-Emmett-Teller (BET) method Total pore volume is obtained from the single point adsorption volume at P/P0 ¼ 0.985 Micropore volume is obtained by the t-plot method Calculated by [(peak I550/peak I450) � 100%/0.72] from FTIR Measured by XRF Fig N2 sorption isotherms (A) and pore-size distributions (B) of the asprepared hierarchical zeolite samples internal standard The porosity of as-prepared samples was analyzed by N2 adsorption/desorption measurements using Micromeritics Tristar 3020 at 77 K Before measurement, all samples were degassed under N2 at 423 K for h The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method And the pore size distribution was calculated from the adsorption branch of N2 sorption isotherms Fig SEM images of as-prepared hierarchical zeolite samples Si-MFI (A), AlMFI (B) and Ti-MFI (C) S Huang et al Microporous and Mesoporous Materials 302 (2020) 110204 Fig The breakthrough curves of as-prepared hierarchical zeolite samples for toluene adsorption under dry condition (A) and humid condition (B, RH ¼ 50%) at 298 K using Barrett-Joyner-Halenda (BJH) methods Field emission scanning electron microscopic (FE-SEM) images were obtained with a ZEISS SIGMA 300 field emission scanning electron microscopy and the sample power was put onto the carbon tape for direct observation Transmission electron microscope (TEM) images were obtained on Hitachi HF5000 at an accelerating voltage of 200 kV 2.4 Dynamic adsorption measurements of toluene The toluene dynamic breakthrough curves were measured in a continuous flow fixed-bed quartz reactor at atmospheric pressure All the feed gases used in this work were of high-purity grade (99.99%) Gaseous toluene was generated by bubbling N2 through liquid toluene at a thermostatic water bath (the concentration of toluene was controlled by adjusting the flow rate of N2) The simulated exhaust gas consisted of 100 ppm toluene, 20% O2 and N2 (as balance gas) Vapor was introduced by bubbling through water at a thermostatic water bath (the vapor was controlled by adjusting the flow rate of N2) to maintain the atmosphere with the relative humidity of 50% Before adsorption experiments, the adsorbent was firstly degassed in 573 K under N2 atmosphere After being cooled to room temperature, the simulated exhaust gas was Fig TEM images of as-prepared hierarchical zeolite samples Si-MFI (A), AlMFI (B) and Ti-MFI (C) S Huang et al Microporous and Mesoporous Materials 302 (2020) 110204 Table Toluene dynamic adsorption capacity under different relative humidity and the fitting parameters of different isotherm models for various adsorbents at 298 K Samples Si-MFI Al-MFI Ti-MFI Dynamic adsorption capacity Q (mg/gads) Langmuir isotherm model RH ¼ RH ¼ 50% Qwet/Qdry qmax KL R 44 58 53 44 42 45 1.00 0.72 0.85 0.09 0.08 0.11 0.008 0.026 0.007 0.83 0.59 0.58 introduced to flow through the adsorbent bed (75 mg, 40–60 mesh) at a rate of 100 mL/min giving the GHSV of 80,000 mL/(g⋅h) The dynamic adsorption capacity (Q (mg/gads)) was calculated by the following equation: Q¼ Freundlich isotherm model Kf 1/n R2 0.007 0.025 0.007 0.38 0.17 0.42 0.99 0.99 0.96 shown in Fig 1C and Fig S1 Compared with the spectra of as-prepared sample Si-MFI and Al-MFI (Fig S1), the obvious absorption peak appeared at 210 nm for as-prepared sample Ti-MFI (Fig 1C), which was ascribed to the charge transfer phenomenon of O2À (2p) to tetrahedral coordination Ti4ỵ (3d) in Ti(OSi)4 structure and characteristic peak for isolated, tetrahedral coordinated Ti(OSi)4 species in hydrophobic zeolite frameworks [41] Therefore, the Ti species were successfully incorpo rated into the tetrahedral coordination framework of the as-prepared sample Ti-MFI In addition, the absence of the broad absorption bands at 260–280 nm indicated no six-coordinate titanium species among the structure of Ti-MFI No absorption peak was observed at 330 nm indi cating the absence of isolated TiO2 within as-prepared sample Ti-MFI [42] XRF results indicated that the Si/M (M ¼ Al or Ti) ratios of the Al-MFI and Ti-MFI were about 24.3 and 25.7, respectively (Table 1) The N2 adsorption-desorption technique was used to understand the porous and nanostructure of as-prepared hierarchical zeolite samples and the results were shown in Fig The nitrogen sorption isotherms of the as-prepared samples (Fig 2A) exhibited two hysteresis loops The hysteresis loop corresponding to the relative pressure P/P0 > 0.4 was H4 type, which implied the narrow lit-like pores existing in the as-prepared samples [43] And the hysteresis loop existed at lower relative pressure (P/P0 was around 0.1–0.3) was ascribed to the crystalline-like phase transition of N2, which was also observed for MFI topology with high Si/Al ratios [44] The phase transition of the adsorbed N2 predominantly occurred in a non-reversible manner and led to low-pressure hysteresis [45] The corresponding pore size distribution curves were shown in Fig 2B and demonstrated the presence of mesopores between and nm In addition, the wide pore size distribution more than 10 nm was also observed for the as-prepared samples The detailed textual prop erties of as-prepared samples were summarized in Table The BET surface area of the as-synthesized zeolites distributed in 294–362 m2/g The total volume obtained from the single point adsorption volume at P/P0 ¼ 0.985 for Si-MFI was 0.21 cm3/g, for Al-MFI was 0.26 cm3/g and for Ti-MFI was 0.21 cm3/g, respectively In order to understand the morphology, the as-prepared samples were characterized by electron microscopy and the corresponding SEM images were shown in Fig The spherical nanozeolite crystal aggre gation with smooth surface could be well observed and the mesoporous structure resulted from the interparticle of nanozeolite crystal aggre gation, which was also well reflected by the results of N2 sorption iso therms The size of aggregated spherical nanozeolite of Si-MFI, Al-MFI and Ti-MFI were around 500–700, 200–450 and 350–550 nm, respec tively The TEM images of the as-prepared samples were shown in Fig and Fig S2 And the observation revealed that all the as-prepared SiMFI, Al-MFI and Ti-MFI possessed the mesoporous structure originated from the aggregation of nanosized zeolite particles In addition, the samples had a spherical morphology with a diameter of about 300–500 nm, which was also consistent with the SEM observations Cin FMt W where Cin is the concentration of toluene in the feed gas, F (mL/min) is the total flow rate, M is the relative molecular mass of toluene (92 g/ mol), W (g) is the weight of the loaded adsorbent, t (min) is the adsorption time 2.5 Temperature programmed desorption of toluene The temperature programmed desorption of toluene (Toluene-TPD) experiments were conducted at different ramping rates of temperature Before each experiment, 75 mg adsorbent was degassed at 573 K for h under high-purity Ar flow at 100 mL/min After being cooled to room temperature under flowing Ar, the sample was exposed to a mixture gas containing 1000 ppm toluene in balance with Ar for 30 Then, the gas flow was switched to pure Ar to remove the residual toluene within the measurement system Finally, the TPD-toluene profiles were measured as a function of temperature from 303 to 600 K with various heating rate from 4.5 to 18 K/min under Ar flow (100 mL/min) The signal of toluene was monitored by an online MS (mass spectrum, Hiden HPR 20) at the m/z ¼ 92 Results and discussions 3.1 Textual properties of samples The crystal structure of as-prepared samples was characterized by XRD and the results were shown in Fig 1A, which demonstrated the three samples had the typical MFI microporous zeolite structure (JCPDS, PDF#44–0003) without any additional diffraction peaks And the cell parameters, which were calculated based on the XRD results, of Al-MFI and Ti-MFI slightly increased (the detailed data were shown in Table 1) compared with those of Si-MFI, implying that both Al and Ti were suc cessfully doped into the lattice of zeolite In order to further estimate the framework crystallinity, the as-prepared samples were characterized by FTIR and the results were shown in Fig 1B It was reported that the peak at near 450 cmÀ was attributed to the stretching of Si–O bonds and the bands around 550 cmÀ were the characteristic bands of double fivemembered rings of MFI zeolites [38] Meanwhile, the IR bands near 1100 cmÀ and 1230 cmÀ belonged to the asymmetric stretching vi bration of Si–O–Si bonds [39] An obvious peak at 550 cmÀ could be detected on as-prepared samples, indicating that all samples were MFI structure zeolite, which was consistent with the results of XRD charac terization The broadened peaks near 3500 cmÀ were attributed to hydroxyl groups of water molecules adsorbed on as-prepared samples [40] Among the prepared zeolites, the Si-MFI sample exhibited lower intensity of the adsorption peak, indicating the Si-MFI possessed better hydrophobicity The crystallinity of as-prepared Si-MFI, Al-MFI, and Ti-MFI were 67%, 69%, and 61%, respectively (Table 1) In order to further understand framework, the as-prepared samples were charac terized by UV–vis diffuse reflectance spectrum and the results were 3.2 Dynamic adsorption capacities The dynamic adsorption behaviors of toluene on Si-MFI, Al-MFI and Ti-MFI under dry and humid condition were measured and the break through curves were presented in Fig The breakthrough time of toluene adsorption over as-synthesized samples under dry gas (Fig 5A) was 30, 54 and 51 and the corresponding saturated adsorption ca pacity was 44, 58 and 53 mg/gads for Si-MFI, Al-MFI and Ti-MFI, S Huang et al Microporous and Mesoporous Materials 302 (2020) 110204 Fig Nonlinear fits of Langmuir and Freundlich isotherm model for toluene adsorption on as-prepared hierarchical zeolite samples Si-MFI (A), Al-MFI (B) and Ti-MFI (C) Fig Nonlinear fits of pseudo-first-order, pseudo-second-order kinetics model and Elovich model for toluene adsorption by as-prepared hierarchical zeolite samples Si-MFI (A), Al-MFI (B) and Ti-MFI (C) S Huang et al Microporous and Mesoporous Materials 302 (2020) 110204 Si-MFI and Ti-MFI well maintained the adsorption capacity during the reusing process and the values were 42 and 41 mg/gads, which corre sponded to 95.5% and 91.1% of the adsorption capacity during the first process for Si-MFI and Ti-MFI, respectively Table Kinetics parameters for the adsorption of toluene on various adsorbents at 298 K Model Si-MFI Al-MFI Ti-MFI Pseundo-firstorder Qe (mg/gads) k1 (minÀ 1) R2 62 0.012 0.9826 104 0.007 0.9890 100 0.007 0.9911 Pseundo-secondorder Qe (mg/gads) k2 (minÀ 1) R2 102 7.4 � 10À 0.9803 182 2.2 � 10À 0.9881 178 2.1 � 10À 0.9905 Elovich a (mg/gads∙minÀ 1) b (gads/mg) R2 2.07 0.062 0.926 2.24 0.049 0.902 2.00 0.057 0.898 Weber and Morris K1 (mg/ gads∙minÀ 0.5) C1 (mg/gads) R21 K2 (mg/ gads∙minÀ 0.5) C2 (mg/gads) R22 K3 (mg/ gads∙minÀ 0.5) C3 (mg/gads) R23 4.347 4.514 4.180 À 7.66 0.986 7.005 À 7.95 0.986 8.502 À 7.36 0.986 7.542 À 21.12 0.992 À 0.398 À 29.78 0.996 1.965 À 25.40 0.997 1.714 45.00 0.613 33.81 0.831 28.68 0.640 3.3 Adsorption isotherm model In order to understand the interactions between toluene molecules and the as-prepared samples under dry condition, the adsorption models of Langmuir and Freundlich [46] were applied to fit the experimental data and to study the toluene adsorption behavior over Si-MFI, Al-MFI and Ti-MFI The detailed information on the adsorption equilibrium models was enclosed in Supplementary Materials The nonlinear fits of the mentioned isotherm models were shown in Fig 6, and the fitted parameters were listed in Table It was observed that the correlation coefficients (R2 ¼ 0.99, 0.99 and 0.96 on Si-MFI, Al-MFI and Ti-MFI, respectively) of the Freundlich model were much higher than those obtained from the Langmuir model (R2 ¼ 0.83, 0.59 and 0.58 on Si-MFI, Al-MFI and Ti-MFI, respectively) The results indicated that the Freundlich model could well describe the toluene adsorption in the as-prepared three adsorbents, implying that the distribution of adsorp tion sites on the surface of MFI zeolite molecular sieves was uneven and toluene were adsorbed by the means of multi-layer adsorption Ac cording to the Freundlich theory, the value of Kf reflects the adsorption ability of the adsorbent The larger the value of Kf, the stronger the adsorption capacity [47] The calculated Kf values of Si-MFI, Al-MFI, and Ti-MFI were 0.007, 0.025, and 0.007 respectively, indicating that Al-MFI possessed a better adsorption performance for toluene than Si-MFI and Ti-MFI for toluene, which was consistent with the experi mental results The value of 1/n is correlated with the adsorption strength and the corresponding values obtained from the fitting were less than 0.5 for the three as-synthesized samples, indicating that toluene could be well adsorbed on the as-synthesized the MFI nano zeolites [47] 3.4 Adsorption kinetics model To further understand the adsorption mechanism of toluene on assynthesized hierarchical zeolites samples under dry condition, three common models including pseudo-first-order adsorption, pseudosecond-order adsorption and Elovich adsorption models were used to fit the experimental data The detailed information was described in Supplementary Materials The fitting profiles of the experimental data by these three models were shown in Fig and the corresponding ki netic parameters were listed in Table The equilibrium adsorption capacities of toluene on Si-MFI, Al-MFI and Ti-MFI obtained from the fitting of pseudo-first-order adsorption model were 62, 104 and 100 mg/ gads, respectively The corresponding first-order adsorption rate con stants were 0.012, 0.007 and 0.007 minÀ with the correlation co efficients were 0.9826, 0.9890 and 0.9911 The fitting equilibrium adsorption capacities of toluene on the as-prepared zeolite samples (SiMFI, Al-MFI and Ti-MFI) by the pseudo-second-order adsorption were 102, 182 and 178 mg/gads, accompanied by the adsorption rate con stants of 7.4 � 10À 5, 2.2 � 10À and 2.1 � 10À minÀ The correlation coefficients of this model were 0.9803, 0.9881 and 0.9905, respectively The initial sorption rates of toluene on as-synthesized Si-MFI, Al-MFI and Ti-MFI obtained by Elovich adsorption model were 2.07, 2.24 and 2.00 mg/gads∙minÀ 1, the Elovich constants were 0.062, 0.049 and 0.057 gads/mg and the corresponding correlation coefficients were 0.926, 0.902 and 0.898, respectively According to the fitting results, the Elo vich model over the experimental data of as-prepared samples had the lowest correlation coefficients, which indicated that the Elovich model was unsuitable to describe the adsorption of toluene on Si-MFI, Al-MFI and Ti-MFI Compared with the fitting results from pseudo-second-order adsorption model, the equilibrium adsorption capacities obtained from pseudo-first-order model matched better with the experimental results Fig Linear fits of intraparticle diffusion for toluene adsorption on asprepared hierarchical zeolite samples respectively The as-prepared Al-MFI and Ti-MFI exhibited better per formance than Si-MFI, which indicated the doping of Al and Ti within the framework of the hierarchical zeolite could increase the toluene adsorption capacity When the relative humidity was fixed at 50%, the corresponding saturated adsorption capacity was 44, 42 and 45 mg/gads for Si-MFI, Al-MFI and Ti-MFI, respectively The ratios of saturated toluene adsorption capacity under humid and dry condition (Qwet/Qdry) among the above three as-prepared samples were 1.00, 0.72 and 0,85, as shown in Table Compared the adsorption performances for toluene of these three zeolites under dry and humid conditions, the toluene adsorption performance of Si-MFI remained when the relative humidity was fixed at 50% which could be ascribed to its high hydrophobicity In contrast, the H2O molecules in moisture gas were adsorbed on the adsorption sites and thus hindered toluene adsorption to some extent on the Al-MFI and Ti-MFI Furthermore, the reusability of Si-MFI, Al-MFI and Ti-MFI was also evaluated and the results were shown in Fig S3 The S Huang et al Microporous and Mesoporous Materials 302 (2020) 110204 Fig Toluene-TPD under different βH from as-prepared hierarchical zeolites samples Si-MFI (A), Al-MFI (B) and Ti-MFI (C) and the linear dependence between In (RT2p/βH) and 1/Tp for the desorption of toluene on the three samples (D) samples were shown in Fig and the fitting kinetic parameters were listed in Table There were three sections shown in the fitting plot, indicating that the adsorption was affected by multiple independent steps besides intraparticle diffusion [48] The three sections could be ascribed to external diffusion, intraparticle diffusion and the final adsorption equilibrium stage in turn [40,42] The slopes of the curve obtained in the second stage were the largest, indicating that the adsorption of toluene on as-prepared samples was not limited by the intraparticle diffusion In other words, the porous structure resulted from nanosized zeolite aggregation facilitated the mass diffusion during the toluene adsorption procedure Table Desorption activation energy, Ed and peak temperature Tp of toluene on asprepared hierarchical zeolite samples Samples Si-MFI Al-MFI Ti-MFI Value of Tp (K) for peaks of TPD curve obtained at heating rate, βH 4.5 (K/ min) (K/ min) 13.5 (K/ min) 18 (K/ min) 346 345 344 353 355 354 363 359 362 369 366 366 Ed (kJ/ mol) R2 55.2 65.4 58.7 0.93 0.97 0.99 3.5 Toluene-TPD relatively Moreover, the correlation coefficients of pseudo-first-order adsorption model were larger than those from pseudo-second-order adsorption model, indicating that the adsorption of toluene on asprepared samples could be better illustrated by pseudo-first-order adsorption model In order to evaluate the intraparticle diffusion effect on adsorption kinetics, Weber and Morris theory was proposed The detailed infor mation was enclosed in Supplementary Materials The linear fits of intraparticle diffusion for toluene adsorption on as-synthesized zeolite On the purpose of understanding the interaction strength between toluene and the as-synthesized hierarchical zeolite samples, the tem perature programmed desorption of toluene (toluene-TPD) over assynthesized Si-MFI, Al-MFI and Ti-MFI was conducted at different ramping rates The detailed information on the procedure was enclosed in Supplementary Materials and the corresponding toluene-TPD curves of as-synthesized Si-MFI, Al-MFI and Ti-MFI at different heating rates were shown in Fig The peaks related to toluene desorption could be S Huang et al Microporous and Mesoporous Materials 302 (2020) 110204 observed as the temperature increasing In addition to the desorption peaks of physically-adsorbed toluene at 340–370 K, the toluene desorption peaks in the range of 450–500 K were also observed for AlMFI, indicating the stronger interaction between toluene and Al-MFI The desorption temperatures of toluene at different heating rates were showed in Fig and listed in Table and the gradual increase in the heating rates led to the temperature increase of toluene desorption The desorption energy of toluene on Si-MFI, Al-MFI and Ti-MFI were calculated according to toluene-TPD results from different heating rates and corresponding desorption energy was 55.2, 65.4 and 58.7 kJ/mol, respectively Normally, the higher energy required for desorption demonstrates the stronger the interaction between the adsorbate and the adsorbent since desorption process is endothermic Therefore, the 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programmed desorption of toluene The dynamic adsorption results showed that Al-MFI exhibited the highest adsorption capacity to toluene under dry gas conditions while Ti-MFI had the optimal toluene adsorption performance under the wet condition (the relative humidity was 50%) The fitting of adsorption isotherm results showed that the adsorption behavior of toluene on MFI nanozeolites conformed to the Freundlich principle The kinetic simu lation results showed that the pseudo-first-order adsorption model was suitable to elucidate the adsorption process And the toluene adsorption process over as-prepared samples was affected by multiple steps and the intraparticle diffusion was not the limited step In addition, the Al and Ti doping resulted in the increasing interaction strength between toluene and the as-prepared samples and the interaction strength followed the order: Al-MFI > Ti-MFI > Si-MFI The findings herein will offer valuable information for VOCs removal research and application by hierarchical zeolites Declaration of competing interest The authors declare no conflict of interest Acknowledgements The work was financially supported by National Key R&D program of China (2017YFE0127400), Natural Science Foundation of Hubei Prov ince (2019CFA070), the Opening Project of Hubei Key Laboratory of Processing and Application of Catalytic Materials (201829103) and Program for Huazhong University of Science and Technology (HUST) Academic Frontier Youth Team (2018QYTD03) The authors thank the HUST Analysis and Testing Center of for analytical support We really appreciate Miss Linlin Zhang from Shanghai Institute of Ceramics Chi nese Academy of Sciences for her kindness and support on the TEM measurements and suggestions during the special period of coronavirus prevalence Appendix A Supplementary data Supplementary data to this article can be found online at https://doi org/10.1016/j.micromeso.2020.110204 S Huang et al Microporous and Mesoporous Materials 302 (2020) 110204 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