Preparation of mesoporous ZSM-5 catalysts using green templates and their performance in biomass catalytic pyrolysis

Bioresource Technology 289 (2019) 121729

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Preparation of mesoporous ZSM-5 catalysts using green templates and their performance in biomass catalytic pyrolysis

T

Qingfeng Chea, Minjiao Yangb,a, Xianhua Wanga, , Qing Yangc, Yingquan Chena, Xu Chena, Wei Chena, Junhao Hua, Kuo Zengc, Haiping Yanga,c, Hanping Chena,c ⁎

a

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, PR China China-EU Institute for Clean and Renewable Energy, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, PR China c Department of New Energy Science and Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, PR China b

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: Mesoporous ZSM-5 Green template Biomass Catalytic pyrolysis Aromatic production

The micropores present in ZSM-5 are beneficial to the production of aromatic compounds in biomass catalytic pyrolysis, although the small pore size leads to severe coke deposition on the catalyst. In this study, a micromesoporous structured ZSM-5 zeolite catalyst was synthesized and modified with green templates (sucrose, cellulose, and starch) to introduce additional mesopores. It was found that the catalysts modified using the sucrose and cellulose templates only exhibited a slight increase in their micropore volumes, while the mesopore volume of ZSM-ST (modified with the starch template) reached up to 0.359 cm3/g. This increase promoted the cracking of bulky oxygenates and suppressed the polymerization reaction on the ZSM-5 surface, thereby producing a greater number of aromatic products. Moreover, the benzene, toluene, and xylene (BTX) yields exhibited a positive correlation with the catalyst mesopore volume, with the highest BTX yield of 91.84 mg/g being obtained with 10% starch addition.

1. Introduction

content, render its application difficult in the existing petroleum-based infrastructure (Perkins et al., 2018). Thus, the upgrading of bio-oil to high-grade fuels through a one-step catalytic pyrolysis is considered to be a promising route to biomass utilization. ZSM-5 is a widely employed catalyst in the petrochemical and petroleum refining industries for the cracking of heavy hydrocarbons into small molecules. Indeed, the high thermal and hydrothermal stability of ZSM-5 render it a

Biomass is the only carbon-containing renewable energy source, and as such, the bio-oil produced from the fast pyrolysis of biomass is expected to be an important source for fuels and chemicals in the future (Wang et al., 2017). However, the complexity of the bio-oil composition in addition to its corrosiveness, low calorific value, and high oxygen

⁎

Corresponding author. E-mail address: [email protected] (X. Wang).

https://doi.org/10.1016/j.biortech.2019.121729 Received 26 May 2019; Received in revised form 27 June 2019; Accepted 28 June 2019 Available online 29 June 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.

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suitable catalyst for the catalytic fast pyrolysis (CFP) of biomass (Jae et al., 2011), where lignocellulosic biomass, holocellulose, lignin, or biomass derivatives can be effectively converted into aromatic hydrocarbons. In a typical upgrading process, the pyrolysis oxygenates undergo a series of deoxygenation and aromatization reactions within the catalyst channels, with the released products including benzene, toluene, xylene, naphthalene, ethylene, and propylene, where oxygen is removed in the form of CO2, CO, and H2O (Yang et al., 2017a). In addition, a solid carbonaceous residue known as coke is generated during this process, and is deposited either on the catalyst surface or in the pores, resulting in catalyst deactivation with long-term use (Paasikallio et al., 2014). It has been reported that the physicochemical properties of ZSM-5 affect the conversion efficiency of the pyrolysis products, with the acidity and pore structure being considered the two most important factors. Thus, numerous studies have focused on adjusting the acidity through optimization of the SiO2/Al2O3 ratio (Engtrakul et al., 2016), the loading of metals or inorganic elements (Lin et al., 2015; Vichaphund et al., 2015), steam or acid washing-assisted dealumination (Mante et al., 2012; Zhang et al., 2017), and chemical deposition (Bo et al., 2016). These treatments allow optimization of the acidity and acid site distribution on ZSM-5 to ultimately improve the catalytic performance. However, adjusting the acidity only affects the reactants entering the catalyst pores, and has little effect on the oxygenates whose molecular sizes exceed the pore size of the catalyst. Although ZSM-5 consists of two perpendicularly intersecting channels of 10membered rings, i.e., straight channels (5.5 Å × 5.1 Å) and zigzag channels (5.6 Å × 5.3 Å) (Li et al., 2014), biomass pyrolysis vapors are a mixture of oxygenates with different molecular sizes, some of which have larger dimensions than the ZSM-5 pores, such as levoglucosan and phenols (Jae et al., 2011; Yu et al., 2012). In addition, Wang et al. compared the conversion efficiencies of glycolaldehyde, acetic acid, furfural, levoglucosan, and 5-hydroxymethyl furfural (HMF) over HZSM-5, and found that oxygenates with relatively large diameters exhibited a strong tendency to produce lower hydrocarbon yields and higher coke yields. This high coke yield was attributed to the geometric size barriers, especially in the case of HMF (6.2 Å) and levoglucosan (6.7 Å), as their diameters are larger than the pore size of HZSM-5 (Wang et al., 2014). Indeed, the bulky oxygenates cannot enter the pores of ZSM-5, and can only be converted at the external surface containing limited acidic sites. The introduction of mesopores into ZSM-5 is therefore considered an effective method to improve the transportation of reactants. The most common method is to treat ZSM-5 with an alkaline solution, such as NaOH, since the corrosion of silicon by the hydroxide ions will cause the collapse of the catalyst skeleton, resulting in the formation of mesopores (Ding et al., 2017; Shao et al., 2016). However, this method causes reductions in both the catalyst weight and acidity. In this respect, the non-destructive “bottom-up” method appears potentially advantageous, as it involves only the addition of a mesoporous template during catalyst synthesis (Gamliel et al., 2016). More specifically, Kelkar et al. prepared mesoporous ZSM-5 using silane-modified polymers as mesopore-generating agents. Compared with the conventional ZSM-5, the selectivity towards C8 and C9 monoaromatics from the CFP was promoted by 14.2% (Kelkar et al., 2014). In addition, Chen et al. found that different templating agents affected the morphology, crystallinity, acidity, and pore structure of the catalyst, where the hexadecyltrimethoxysilane (HTS)-treated ZSM-5 was found to exhibit a superior morphology to other organosilane-treated catalysts, giving a higher aromatic yield and a lower coke yield (Chen et al., 2018). Furthermore, hexadecyl trimethyl ammonium bromide (CTAB) has been employed as a template to modify ZSM-5, and it was found that only a small amount of CTAB addition was required to promote the formation of aromatic hydrocarbons, while excess addition greatly reduces the acidity of the catalyst and produces large quantities of coke (Zhang et al., 2018). These studies clearly demonstrate that template-based

catalyst modification is a feasible method for the introduction of mesopores into ZSM-5, avoiding the structural damage and acidity loss caused by alkali treatment methods. However, the type of template employed ultimately determines the characteristics of the catalyst, and some templates made little contribution to mesopore generation, such as 3-(phenylamino)propyltrimethoxysilane (PAPTS) and CTAB. Moreover, the results presented by Chen et al. showed that the molecular size of the template is an important factor in determining the catalyst properties. More specifically, ZSM-5 catalysts modified using smaller templates (e.g., trimethoxymethylsilane, trimethoxy(propyl)silane, or trimethoxy(octyl)silane) maintained a similar structure to the conventional ZSM-5, while larger templates (e.g., hexadecyltrimethoxysilane) promoted the growth of new crystal particles on the catalyst surface (Chen et al., 2018). However, since the use of organosilane modifiers is uneconomical and unsustainable, further studies are required into the preparation of mesoporous catalysts while also considering the required catalyst characteristics and the type of templating agent. Due to the renewability and easy accessibility of biomaterials, three different materials (i.e., sucrose, starch, and cellulose) with different molecular sizes were selected for the purpose of this study to investigate whether they exhibit potential as a template. A commercial microporous ZSM-5 catalyst is also tested for comparison. The crystallinity, morphology, porosity, and acidity of the catalyst are characterized by X-ray diffraction (XRD) measurements, scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area measurements, and ammonia temperature-programmed desorption (NH3-TPD). The catalyst performance for biomass conversion is tested using pyrolysis-gas chromatography-mass spectrometry (PY-GC/MS). Overall, it is hope to find a suitable green template for the preparation of a mesoporous ZSM5 catalyst for application in the catalytic pyrolysis of biomass to produce benzene, toluene, and xylene (BTX). 2. Materials and methods 2.1. Materials Tetrapropylammonium hydroxide (TPAOH, 25 wt% in water, Sinopharm Chemical Reagent Co., Ltd), Sodium aluminate (NaAlO2, AR, Aladdin), Tetraethyl orthosilicate (TEOS, GC, > 99 wt%, Aladdin), sucrose (AR, Sinopharm Chemical Reagent Co., Ltd), cellulose microcrystalline (AR, Sinopharm Chemical Reagent Co., Ltd), soluble starch (AR, Sinopharm Chemical Reagent Co., Ltd). NH4Cl (AR, Sinopharm Chemical Reagent Co., Ltd). Wood sawdust was obtained locally. The raw materials were grounded and sieved to a uniform diameter of less than 150 μm, then the samples were dried at 105 °C in the oven for 24 h. The fiber analysis, proximate analysis, and ultimate analysis of the samples are shown in previous study (Che et al., 2019a). Commercial power ZSM-5 catalyst was purchased from the catalyst plant of Nankai University. The catalyst was calcined in a muffle furnace at 550 °C for 4 h, and then dried at 105 °C for 24 h before the experiment. 2.2. Catalysts preparation A series of ZSM-5 samples were synthesized using tradition hydrothermal method. Sucrose, cellulose, and starch were used as the mesopore templates for modifying the catalysts. A typical synthesis was carried out as follows. 0.227 g of NaAlO2 was dissolved in 6.767 g TPAOH solution under vigorous stirring for 2 h to obtain a clear aluminate solution. 8.667 g TEOS was added dropwise into 15 mL deionized water, after stirring for 30 min, aluminate solution was added dropwise. The mixture was stirred for 5 h to obtain an uniform sol-gel, then the template was added and stirred for another 5 h at 10 °C. The weight percentage of templates to SiO2 were 5, 10, 15, and 20%. Finally, The mixture was crystallized in a Teflon-lined stainless steel reactor at 180 °C for 48 h. 2

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The solid product was separated by centrifugation, washed to neutral with deionized water, dried overnight at 105 °C and calcined in air at 550 °C for 4 h. The H-type ZSM-5 sample was obtained as follows. Calcined ZSM-5 sample was put in 1 mol/L NH4Cl aqueous solution (with solid and liquid ratio of 1:40) and stirred for 4 h at 80 °C. Then the NH4+-type ZSM-5 was separated and washed after three times of ionexchange. Finally, the solid was dried overnight at 105 °C and calcined in air at 550 °C for 4 h. The synthesized ZSM-5 catalysts modified with sucrose, cellulose, starch with templates to SiO2 ratio of 10% were denoted as ZSM-SU, ZSM-CE, and ZSM-ST, while starch modified ZSM-5 catalysts with templates to SiO2 ratio of 5%, 15%, and 20% were denoted as ZSM-5%ST, ZSM-15%ST, ZSM-20%ST. The catalyst without template modification was denoted as ZSM-5, and the purchased commercial microporous ZSM-5 was denoted as ZSM-5 (con.)

(290 °C). The temperature of GC injector port was set at 280 °C, and the spilt rate was 80:1. The detected samples were separated using a capillary column (DB-5MS; 30 m × 0.25 mm × 0.25 um, Agilent). The oven was programmed to maintain at 40 °C for 2 min, then increase to 200 °C at a rate of 5 °C/min, then increase to 280 °C at 10 °C/min, and maintain temperature for 8 min. Product species were identified according to the NIST library and the quantitative analysis method of the products has been reported in previous study (Che et al., 2019b). The each experiment was repeated 3 times at the same conditions to ensure repeatability.

2.3. Catalysts characterization

The crystal structures of the catalysts were characterized by XRD (Esupplementary data for this work can be found in e-version of this paper online). All samples were found to exhibit similar characteristic peaks of the MFI zeolite structure in the ranges of 7–9° and 22.5–25.0°, thereby indicating the successful preparation of a series of ZSM-5 catalysts. As the catalyst crystallinity is an important factor in determining the catalytic conversion of biomass to aromatics (Hoff et al., 2016), the relative crystallinity of each catalyst was calculated according to the total peak areas at 7–9° and 22.5–25.0° (Chen et al., 2015). Thus, ZSMSU exhibited the highest area and so was defined as 100%, while ZSMCE had the lowest relative crystallinity of 80.6%, and the conventional ZSM-5, synthesized ZSM-5, and ZSM-ST catalysts had high relative crystallinities of 94.15, 95.73, and 94.53%, respectively. It can be inferred from these results that the addition of a small template is beneficial to catalyst crystallization, while larger templates have the opposite effect. The morphologies of ZSM-5(con.), ZSM-5, and ZSM-ST can be observed in the SEM results (E-supplementary data for this work can be found in e-version of this paper online). More specifically, crystals of the conventional ZSM-5 catalyst adopt a brick-like structure measuring 1–2 μm, while the synthesized ZSM-5 catalyst forms a spherical or spindle-like assembly from small crystals measuring ∼50 nm. In addition, it can be seen from the comparison that the ZSM-ST catalyst assembly was smaller than that of the ZSM-5 catalyst. Fig. 1a shows the N2 adsorption‐desorption isotherms of the various ZSM-5 catalysts. More specifically, ZSM-5(con.) exhibited the characteristics of a typical type I isotherm, indicating that the conventional ZSM-5 is a microporous material. In contrast, the isotherms of the synthesized ZSM-5 and the template-modified ZSM-5 catalysts are type IV isotherms with hysteresis loops, thereby indicating the presence of numerous catalyst mesopores. The appearance of type IV hysteresis loops for the synthesized ZSM-5 was caused by the presence of both mesopores and macropores at highly relative pressures (p/ p0 = 0.8–1.0), due to the presence of inter-crystalline voids produced by stacking of the nano-sized crystals (Yang et al., 2017b). Moreover, upon the addition of starch during synthesis, additional mesopores were generated following template calcination between the crystals. The catalyst pore distributions are shown in Fig. 1b, where it is apparent that the conventional ZSM-5 contained only a small number of mesopores measuring 2–5 nm. In contrast, the mesopores of the synthesized ZSM-5 and the modified ZSM-5 catalysts were mainly distributed in two intervals, namely 2–5 nm and 16–45 nm, respectively. Compared with the conventional ZSM-5, the synthesized ZSM-5 catalyst exhibited a higher pore volume and a pore diameter of 2–5 nm, while the starch-modified ZSM-5 displayed the highest pore volume and a pore size of 16–45 nm. It can be inferred that the 16–45 nm mesopores were derived mainly from the inter-crystalline voids (Yang et al., 2017b), while the 2–5 nm mesopores were derived from the internal defects of the crystal (Gamliel et al., 2016; Lee et al., 2017). The catalysts composed of two intervals were considered hierarchical structured materials, and so could effectively promote the transport of pyrolysis vapors and inhibit coke deposition (Li et al., 2014).

3. Results and discussion 3.1. Catalyst characterization

2.3.1. N2 adsorption-desorption isotherms The porous characteristics of catalysts were measured by N2 adsorption and desorption at −196 °C on a Quantachrome Autosorb-iQ instrument (USA). Prior to the adsorption measurement, the zeolites were degassed under high vacuum at 300 °C for 6 h. The surface area was calculated from the adsorption branch in the range of relative pressure from 0.05 to 0.35 by Brunauer–Emmett–Teller (BET) method. The total pore volume was estimated at a relative pressure of 0.99. The microporous volume was calculated using the t-plot method, and the mesoporous volume was obtained by subtracting the micropore volume from the total pore volume. The pore size distribution was calculated from the adsorption branch by non-local density functional theory (NLDFT) method using a model specifically designed for N2 adsorption on H-form zeolites (Gamliel et al., 2016). 2.3.2. Scanning electron microscope (SEM) Scanning electron microscope (SEM) images of the catalysts were obtained using a field emission scanning electron microscope (Sigma300) with a voltage of 2.00 kV. 2.3.3. X-ray diffraction (XRD) The phase compositions of the catalysts were analyzed using X-ray diffraction (X’Pert PRO, PANalytical B.V., Almelo, The Netherlands). The experimental conditions are: Cu/K. Ray, tube voltage = 40 kV, tube current = 30 mA, scanning range from 5 to 40°. 2.3.4. Temperature programmed desorption of ammonia (NH3-TPD) The acidity of zeolites were analyzed using temperature programmed desorption (TPD) experiments (BELCAT-M, MicrotracBEL Corporation, Japan). The steps of each experiment are as follows. First, the 100.0 mg sample was purged with He and heated from room temperature to 550 °C, and maintained for 120 min. Then, the sample was cooled to 100 °C and purged with NH3 at 40 mL/min for 60 min. The gas was then switched to He with a flow rate of 40 mL/min, and maintained for 60 min. Finally, the sample was heated to 700 °C at 10 °C/min to desorb NH3, and the signal was recorded from 100 °C. This was for eliminated the influence of physical adsorption of ammonia with TCD detector. 2.4. Biomass catalytic fast pyrolysis The catalytic fast pyrolysis experiments of sawdust were conducted using a CDS Pyroprobe 5250 pyrolyser. The sawdust and catalysts were loaded at the middle of the quartz tube using the loose quartz wool (CDS Analytical Inc.) as carrier platform. In a typical CFP experiment, 0.3 mg sawdust, and 1.2 mg zeolite were used. The samples were heated to 600 °C at a rate of 20 °C/ms and held for 15 s to completely pyrolyze. After CFP, the vapors were promptly swept to GC/MS (7890B/5977A, Agilent) with He (purity ≥ 99.999%.) via a heated transmission pipe 3

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Fig. 1. (a) N2 adsorption-desorption isotherms, and (b) pore size distributions of the different template-modified ZSM-5 catalysts. (c) N2 adsorption-desorption isotherms, and (d) pore size distributions of the various starch-modified ZSM-5 catalysts.

significantly higher than that of the synthesized catalyst. Each curve was divided into three peaks according to a previously reported method (Che et al., 2019b). Table 2 details the parameters of each catalyst, including the acidity, acid site distribution, and the temperature corresponding to the center of each peak. As shown, the acidity of ZSM5(con.) reached 887.1 µmol/g, which is significantly higher than that of the synthesized ZSM-5 catalyst, i.e., 594.2 µmol/g. In addition, the acidities of the sucrose- and starch-modified ZSM-5 catalysts were 623.3 and 613.8 µmol/g, respectively, while that of the cellulosemodified catalyst was slightly lower, at 574.6 µmol/g. The acid site distributions of the various catalysts also differed greatly, with the highest contents of strong, medium, and weak acid sites being obtained for ZSM-5(con.) (52.74%), ZSM-CE (26.87%), and ZSM-20% ST (51.41%), respectively. The corresponding temperatures of the strong, medium, and weak acids for the conventional catalyst were lower than those of the synthesized and modified catalysts, thereby indicating that the products produced using ZSM-5(con.) were more easily desorbed from the acid sites. Furthermore, the acidity properties of the ZSM-5 catalysts modified with different amounts of starch (Fig. 2b) indicate that excess starch addition will reduce the catalyst acidity, with the 10% starch-modified ZSM-5 catalyst exhibiting the highest strong acid content.

Table 1 Textural properties of ZSM-5 and templates modified ZSM-5 catalysts. Catalyst

SBET (m2/g)

VTotal (cm3/ g)

VMicro (cm3/ g)

VMeso (cm3/ g)

D (nm)

ZSM-5(Con.) ZSM-5 ZSM-SU ZSM-CE ZSM-ST 5%ST 15%ST 20%ST

404.57 423.11 452.96 441.21 423.07 447.19 428.22 432.94

0.185 0.418 0.394 0.402 0.522 0.541 0.506 0.412

0.164 0.166 0.177 0.171 0.163 0.171 0.166 0.168

0.021 0.252 0.217 0.231 0.359 0.370 0.340 0.254

1.832 3.983 3.519 3.694 4.508 4.901 4.461 3.872

The textural properties of the various ZSM-5 catalysts are listed in Table 1, where it is apparent that the synthesized ZSM-5 has a higher BET surface area, mesoporous pore volume, and average pore size compared to ZSM-5(con.), with increases from 404.57 to 423.11 m2/g, 0.021 to 0.252 cm3/g, and 1.832 to 3.983 nm, respectively. In terms of the template-modified ZSM-5, ZSM-SU, and ZSM-CE catalysts, higher BET surface areas and microporous pore volumes were found in addition to lower mesoporous pore volumes and average pore sizes. More specifically, ZSM-ST exhibited the highest mesoporous pore volume and the largest average pore size (i.e., 0.359 cm3/g and 4.508 nm, respectively). The results obtained for the various starch-modified ZSM-5 catalysts (see Fig. 1c and d) indicate that the mesoporous volume and average pore size of the modified catalyst decreased gradually upon increasing the amount of added starch. Fig. 2 shows the NH3-TPD results for the various catalysts. As indicated in Fig. 2a, the difference between ZSM-5(con.) and the prepared ZSM-5 is significant, with the acidity of the conventional catalyst being

3.2. Biomass catalytic pyrolysis performances of the various catalysts The products obtained from the catalytic pyrolysis of biomass using the various catalysts were then analyzed both quantitatively and semiquantitatively using the peak areas of the GC/MS spectra. The total peak areas were used to indicate the organic content in each liquid product, and the relative content was used to indicate the distribution of the various components. In addition, the BTX yield was used to 4

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Fig. 2. NH3-TPD profiles of the various catalysts. (a) The template-modified ZSM-5 catalysts, and (b) the starch-modified ZSM-5 catalysts. Table 2 Acidity of ZSM-5 and templates modified ZSM-5 catalysts. Catalysts

ZSM-5(Con.) ZSM-5 ZSM-SU ZSM-CE ZSM-ST 5%ST 15%ST 20%ST

Acid amount (µmol/g)

Acid sites content (%)

Peak position (°C)

Weak

Medium

Strong

Weak

Medium

Strong

887.1 594.2 623.3 574.6 613.8 602.5 593.4 572.7

28.17 40.90 40.21 43.18 48.17 45.60 47.57 51.41

19.08 14.60 16.15 26.87 4.05 10.64 9.92 2.31

52.74 44.48 43.62 29.93 47.76 43.74 42.50 46.26

204.6 229.8 234.9 233.8 237.3 233.3 237.7 234.6

254.7 304.7 322.1 367.0 338.8 338.6 348.1 342.7

415.7 444.1 447.3 456.8 444.3 444.8 450.8 439.4

evaluate the production of valuable aromatic compounds from CFP. Table 3 summarizes the product distributions and yields achieved in the biomass catalytic pyrolysis process using the various types of ZSM-5 catalysts. Analysis of the total peak areas shows that the synthesized ZSM-5 promoted the formation of liquid organic products to a greater extent than the conventional ZSM-5 catalyst, with an increase in the peak area from 2.388 × 109 (for conventional ZSM-5) to 2.741 × 109 (for ZSM-5). Upon the addition of a template, the peak areas further increased by 0.76, 0.65, and 9.70% (for the sucrose-, cellulose-, and starch-modified ZSM-5 catalysts, respectively) when compared with the synthesized ZSM-5. Upon increasing the quantity of added starch, the peak area initially increased prior to decreasing, with the highest yield being obtained upon the addition of 10% starch. The relative contents of the liquid products also changed based on the ZSM-5 catalyst employed. For example, the content of oxygenated compounds (E-supplementary data for this work can be found in e-version of this paper online) decreased significantly from 29.63% (for conventional ZSM-5) to 13.15% (for ZSM-5). Among the template-modified ZSM-5 species, ZSM-ST produced the lowest oxygenate content. Fig. 3 shows the peak areas of the unconverted phenols in the organic products, where it can

Fig. 3. Peak areas of the phenol compounds obtained from biomass catalytic pyrolysis using the various catalysts.

be seen that the phenolic yield from the synthesized ZSM-5 was approximately half that of the conventional ZSM-5. In the context of other aromatic compounds, the alkylbenzene and alkylnaphthalene contents changed significantly, with the highest values of 13.87 and 21.02% being obtained for the ZSM-SU and ZSM-5%ST catalysts. The observed variations in the BTX yield were similar to those of the total organic peak areas; a high yield of 91.84 mg/g was obtained in the case of ZSMST, which represented an increase compared with the conventional ZSM-5. 3.3. Reaction mechanism of the improved CFP performance obtained using the template-modified ZSM-5 catalysts The physicochemical properties of the ZSM-5 catalyst are known to

Table 3 Products from biomass catalytic pyrolysis with various catalysts. Catalyst

ZSM-5 (Con.) ZSM-5 ZSM-SU ZSM-CE ZSM-ST ZSM-5%ST ZSM-15%ST ZSM-20%ST

Total peak area (×109)

2.388 2.741 2.762 2.759 3.007 2.942 2.844 2.768

Relative content of products (area %)

BTX yield (mg/g)

Benzene

Toluene

Xylene

Alkylbenzene

Naphthalene

Alkylnaphthalene

Oxygenates

5.28 5.79 6.03 5.71 6.29 5.97 5.92 5.47

16.26 17.56 17.57 16.41 19.28 17.22 17.59 17.51

22.01 24.99 25.22 24.51 25.36 24.59 24.84 23.09

8.85 12.60 13.87 11.66 12.31 12.65 13.07 11.04

5.50 6.40 6.51 6.29 6.91 5.94 6.48 6.81

12.47 19.51 18.21 19.51 18.41 21.02 19.29 19.91

29.63 13.15 12.60 15.90 11.44 12.61 12.81 16.17

5

62.32 79.31 80.37 76.93 91.84 84.69 82.33 76.47

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Fig. 4. Correlation of the product yields and catalyst mesoporous volume. (a) The total organic peak area, and (b) the BTX yield.

affect its catalytic performance, especially in the context of the pore structure and acidity. Indeed, previous literature has shown that the acidity of the ZSM-5 catalyst greatly influences the deoxidation efficiency of volatiles and the formation of aromatics, since oxygenates are transformed at the acid sites, and the pore structure is primarily responsible for product shape selectivity (Che et al., 2019b; Engtrakul et al., 2016). This means that a higher acidity, and in particular strong acid sites, is favorable for aromatic production. However, the NH3-TPD results obtained herein suggested that the conventional ZSM-5 catalyst exhibits a higher acidity and a stronger acid content than the other catalysts, but a poorer catalytic performance. Similarly, ZSM-20%ST displayed a strong acid content compared to ZSM-ST, but the obtained BTX yield was 16.7% lower than that of ZSM-ST. This may be due to the superior properties of the synthesized and modified ZSM-5 catalysts in terms of their pore structures. Fig. 4 shows the influence of the catalyst mesoporous content on the total peak areas of the products and the BTX yields. These results indicate that the total peak areas of the CFP products and the BTX yields are positively correlated with the mesoporous volume of the catalysts, with R values of 0.91 and 0.78 being obtained, respectively. Similarly, the catalysts containing a lower mesopore content (i.e., ZSM-5, ZSM-CE, and ZSM-20%ST) gave lower total peak areas and BTX yields. In contrast, the inclusion of an optimal amount of starch (10%) gave a higher mesoporous pore volume, thereby resulting in a higher total peak area and BTX yield. The relationship between the CFP products and the

mesopore volume of the catalysts thereby confirms that the mesopore content is a key parameter in determining the catalytic efficiency. Fig. 5 shows the pathway of the biomass catalytic pyrolysis process using the ZSM-5(con.), ZSM-5, and ZSM-ST catalysts. The volatiles generated from biomass fast pyrolysis give an extremely complex mixture containing acids, ketones, furans, sugars, and phenols (Chen et al., 2019). As larger molecules (such as sugars and branched phenolic substances) cannot match the pore size of the ZSM-5 catalyst, they polymerize on the external surface of the catalyst. Indeed, Du et al. found that the coke deposit on the outside surface of the catalyst is actually a type of “char” consisting of oxygenates and hydrocarbons (Du et al., 2013). This char is mainly derived from the polymerization of lignin derivatives, as they have a large molecular size and low reactivity. This solid carbonaceous residue covering the outer surface or accumulating in the mesopores of the catalyst will further prevent reactants from entering the pores. However, small molecules, such as acids, aldehydes, and ketones, also result in coke formation during the conversion process, and the generated coke deposits on the catalyst surface reduce the conversion efficiency of the reactants (Wang et al., 2014). In terms of the bulky oxygenates, their catalysis by the acid sites on the surfaces of mesopores results in their gradual decomposition into small molecules that can match the microporous channel size to allow them to undergo further conversion (Lazaridis et al., 2018). In addition, the presence of mesoporous channels provides additional inlets for the oxygenates, which reduce the negative effect of coke deposition on the

Fig. 5. The pathway of biomass catalytic pyrolysis process using the ZSM-5(con.), ZSM-5, and ZSM-ST catalysts. 6

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transport efficiency of the reactants. The results presented herein therefore show that the ZSM-5(con.) catalyst contains a limited number of mesopores, which results in the polymerization of bulky oxygenates to form coke on the surface, thereby preventing the reactants from entering the micropores. In contrast, the synthesized ZSM-5 is composed of small particle crystals rich in mesopores, thereby facilitating the conversion of pyrolysis volatiles. Upon modification by starch, the space between the crystal particles is further increased to form additional mesopores, which results in the highest yields of organics and aromatics. In contrast, the addition of sucrose and cellulose only caused a small increase in the catalyst micropore volume, with no obvious promoting effect being observed for aromatic formation. Similarly, when the amount of added starch was gradually increased from 10 to 20%, the mesoporous volume of the catalyst decreased from 0.359 to 0.254 cm3/g, resulting in a sharp decrease in the catalytic efficiency. It is worth noting that ZSM5%ST exhibited the highest mesopore volume of 0.370 cm3/g, but lower BTX and organic yields than ZSM-ST. Indeed, the NH3-TPD results show that the strong acid content of ZSM-5%ST is 43.74%, which is lower than that of ZSM-ST (47.76%), thereby indicating that the poor catalytic efficiency of ZSM-5%ST may be attributed to its lower strong acid content, as strong acid sites are effective sites for biomass catalytic pyrolysis conversion. These results indicate that both the acidity and pore structure of the catalyst play an important role in the catalytic pyrolysis of biomass, but the pore structure, and in particular the mesoporous volume, seems to be a the key parameter. It can therefore be concluded that increasing the mesopore volume of the catalyst can effectively increase the activity of the catalyst when the amount of acid is sufficient. However, increasing the amount of acid is a better choice when the amount of mesopores is increased to its limiting value (i.e., 0.359 cm3/g). The results reported herein therefore indicate that the introduction of additional mesopores in the ZSM-5 structure could be successfully achieved using starch, a green template, during the preparation process. This results in an effective promotion of the conversion of biomass to aromatics. In addition, It is found that the BTX yield exhibits positive correlations with the mesopore volume and the strong acid content of the modified ZSM-5 catalyst, although the mesopore volume has a more predominant effect on the performance and is easier to alter in an environmentally friendly manner. Therefore, the discovery and development of a more appropriate template that can optimize both the acidity and pore structure will simplify the catalyst modification process overall.

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4. Conclusion A micro-mesoporous structured ZSM-5 catalyst was prepared and modified with green templates to enhance the production of aromatic compounds in biomass catalytic pyrolysis. It was found that the benzene, toluene, and xylene (BTX) products were positively correlated with the mesopore volume of the catalysts. Furthermore, the addition of starch as a modifier during the synthesis of ZSM-5 effectively improved the mesoporous volume of the catalyst, with an optimal 10% starch loading producing a superior porosity and acidity to the other catalysts examined herein, thereby greatly enhancing the catalytic activity of ZSM-5 and producing a high BTX yield of 91.84 mg/g. Acknowledgement The authors gratefully acknowledge the support from the National Key R&D Program of China (2017YFE0124600), the National Natural Science Foundation of China (No. 51576087, 51622604), and the technical support from Analytical and Testing Center in Huazhong University of Science & Technology. 7

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