Synthesis and Consequence of Aggregated Nanosized ZSM-5 Zeolite

Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 10737−10749

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Synthesis and Consequence of Aggregated Nanosized ZSM‑5 Zeolite Crystals for Methanol to Propylene Reaction Zhijie Wu,*,† Kaiqiang Zhao,† Yan Zhang,‡ Tao Pan,† Shaohui Ge,§ Yana Ju,§ Tianshu Li,§ and Tao Dou†

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†

State Key Laboratory of Heavy Oil Processing and the Key Laboratory of Catalysis of CNPC, China University of Petroleum, Beijing, 102249, China ‡ Petroleum Refining and Chemical Branch, PetroChina Company Limited, Dongcheng, Beijing, 100011, China § Petrochemical Research Institute, PetroChina Company Limited, China Petroleum Innovation Base, Changping, Beijing, 100195, China ABSTRACT: Aggregated nanosized ZSM-5 crystals with a different size from 30−50 nm to 1000−2000 nm were prepared by a hydrothermal synthesis method using a two-step crystallization procedure for methanol to propylene (MTP) reaction. The decrease of crystal size leads to the growth of the ratio of weak acid sites to strong acid sites, and the number of Brønsted acid sites (BAS) at the external surface, but also to the decrease of Bronsted acid sites to Lewis acid sites (B/L) ratio and the number of BAS within zeolite. The study on the effect of crystal size and BAS location on the product distribution and lifetime of zeolite shows that internal BAS within the micropore is key to activation of the methanol to hydrocarbon, and the reduction of crystal size and the generation of mesopores of zeolite promote the selectivity of propylene as well as the lifetime of zeolite.

1. INTRODUCTION The methanol to propylene (MTP) process has been regarded as an alternative effective route to produce propylene from coal or natural gas, earning significant attention since the successful industrial MTP process by Shenhua Ningxia Coal Industry Group Co., Ltd. in China.1,2 ZSM-5 zeolite (MFI type) has been proven to be the effective industrial active component for MTP catalyst because of its high selectivity for propylene.3−37 However, a rapid deactivation due to the coke deposition occurs on conventional microsized ZSM-5 zeolites with large crystal size (1−5 μm) because of the intracrystalline diffusion limitation over microporous space of zeolite with straight channels (0.53 × 0.56 nm) and intersecting zigzag channels (0.51 × 0.55 nm).21,37 The catalytic lifetime of ZSM-5 zeolite in the MTP reaction could be remarkably enhanced by the introduction of secondary intracrystalline mesoporosity by post-synthetic modifications or mesoporous templating strategies.8,12,25−27 The post-synthetic modification with acidic or basic media usually produces uncontrollable mesoporous structure as well as the pollution of acidic or basic effluent. The mesoporous templating strategies via the direct hydrothermal synthesis can change the structure, morphology, and porosity of zeolites, but the use and removal of mesoporous templates before and after hydrothermal synthesis led to enhanced cost and a serious pollution problem for zeolite synthesis. Thus, further engineering the mesoporous ZSM-5 © 2019 American Chemical Society

crystals into nanoscale has been regarded as a promising direction to enhance the lifetime of zeolites by reducing the diffusion path length.38 In recent years, nanosized ZSM-5 zeolites with shorter diffusion paths have been reported to show superior catalytic stability in catalytic reactions involving Brønsted acid.31,39 However, nanosized zeolite crystal becomes thermodynamically unstable because of the increase of high surface energy and numerous surface defects by reducing the crystal size.25,31 Meanwhile, the practical use of these nanosized crystals ( 5) over a large ZSM-5 zeolite crystal.42 It should be noted that these conclusions are mostly obtained on a low silica ZSM-5 zeolite (i.e., Si/Al ratio < 80) at a low reaction temperature (i.e., 200) at the high reaction temperature (i.e., >723 K) suitable for MTP reaction.9,26 Recently, Yarulina et al. demonstrated that the formation of propylene is dependent on the isolation of BAS for the MTP reaction at 773 K.36 They confirmed that propylene selectivity is controlled by the BAS density, and the isolation is the key to maximize propylene selectivity by preventing secondary reactions leading to the formation of aromatics. Thus, a high silica zeolite crystal should be characteristic of high propylene selectivity,10,46,47 and enhanced selectivity can be obtained on nanosized ZSM-5 zeolite because of the shorter diffusion path.9,26 Here, we attempted to clarify the size effect of ZSM-5 zeolite crystal with a high Si/Al ratio (i.e., >100) in the MTP reaction. In fact, the hydrocarbon product distribution in the MTH reaction is varied with the probability of the aromatic-based and olefin-based catalytic cycles.22 Illias et al. co-fed a small amount of propylene with dimethyl ether at 548 K, and they found the propagation of olefin-based catalytic cycle increases relative to the aromatic-based catalytic cycle.48 However, Sun et al. reported the co-feeding of C3−6 olefins with methanol over a ZSM-5 zeolite (Si/Al = 90) do not suppress the aromatic-based catalytic cycle relative to the olefin-based catalytic cycle, when a higher methanol conversion (i.e., ∼100%) was obtained by increasing MTP reaction temperature to 723 K relevant for the industrial MTP process.20 Wu et al. pointed that the product distribution is determined by the olefin-based catalytic cycle over ZSM-5 zeolite at 733 K with complete methanol conversion.16 These results seem to show that the propylene should be predominantly formed from the olefin-based catalytic cycle via alkylation cracking using ZSM-5 zeolite at the reaction conditions applicable for the MTP process. Interestingly, Kim and Ryoo attempted to clarify the position of acid sites of ZSM-5 zeolite nanosheets in the MTH reaction, and they found that the BAS at the external surface of zeolite did enroll in the MTH reaction at 523−653 K, even they were proven to catalyze hydrocarbon cracking and Friedel−Crafts alkylation reaction.49 However, some work found that the selectivity of propylene is suppressed by the external acid sites of ZSM-5 zeolite in MTP reaction (>553 K).30,33 These reported results together suggest an obvious role of the catalytic function of external acid sites on ZSM-5 zeolite. Therefore, the role of the external acidity of the

2. EXPERIMENTAL SECTION 2.1. Chemicals. Tetrapropyl ammonium hydroxide (TPAOH, 25 wt % in water), sodium aluminate (NaAlO2, 50−56 wt % Al2O3%), aluminum isopropoxide (AIP, 24 wt % Al2O3), 2,6-di-tert-butylpyridine (DTBP, 98%), and triphenylphosphine (TPP, 99%) were purchased from Aldrich. Silica gel (0.075−0.15 mm, 99 wt % SiO2, and >0.4 g cm−3 bulk density) was obtained from Qingdao Haiyang Chemical Co., Ltd. Sodium bromide (99 wt % NaBr), ammonium chloride (99 wt %, NH4Cl), methanol (CH 3OH, 99.9%), and sodium hydroxide (99 wt % NaOH) were obtained from Sinopharm Chemical Reagent Co., Ltd. 2.2. Zeolite Preparation. In our previous work,40 we developed a direct hydrothermal synthesis method to prepare aggregated nanosized ZSM-5 zeolite crystals with intercrystalline mesopores by the one-step hydrothermal crystallization of silicate-aluminate gels with a high solid/liquid ratio without the use of mesoporous templates or zeolite seeds,40 here a twostage crystallization procedure was developed to replace the one-step process. For the nucleation period proceeding at the first low temperature, the homogeneous zeolite crystal nuclei are generated, while fast zeolite crystal growth rate is realized and the structural defects of zeolite crystal are also healed.41−43 Here, the use of the silicate−aluminate gel precursor with a high solid/liquid ratio and the introduction of the nucleation process at low temperature lead to the formation of large amount of uniform nuclei for zeolite crystallization, and the intergrowth of zeolite crystal can be easily achieved in the high solid concentration system,25,39,40 resulting in the aggregation of nanosized zeolite crystals. For the synthesis of zeolite, 0.32 g of AIP was mixed with 28.0 g of TPAOH in 6.0 g of deionized water to form homogeneous aqueous solution by stirring at room temperature for 20 min. After that, 1.16 g of sodium bromide was dissolved in the above solution by vigorous stirring at room temperature for 20 min. Subsequently, 10.0 g of silica gel was directly transferred into the above solution by vigorous stirring for 1 h to compose a homogeneous silicate− aluminate gel with a Si/Al ratio at ∼110. The as-prepared gels were transferred into a 100 mL stainless steel autoclave with Teflon liners for the two-stage crystallization. First, a low temperature crystallization at 373 K under static conditions was carried out for 36 h, and then the crystallization temperature was increased to 443 K with 15 K min−1 ramping rate and held for 24 h. After the crystallization, the autoclave was quenched by flowing tap water, and the solid product was separated from solution by vacuum filtration and washed with deionized water. When the pH value of the filtrate was around 7, the solid product was transferred in a 373 K oven for ∼12 h. The sodium type ZSM-5 zeolite was obtained by the subsequent 6 h calcination in air at 823 K (2 K min−1 ramping rate) to remove the organic templates. To change the crystal size of zeolite, the amount of sodium bromide was varied from 0.5 to 2.2 g. For comparison, aggregated ZSM-5 zeolite crystals with a low Si/Al ratio at 50 and 25 (denoted as ZSM-5(A) and ZSM-5(B), respectively) in silicate−aluminate gel was also prepared. To obtain acidic ZSM-5 (denoted as HZSM-5) zeolite for catalytic reaction, the sodium form zeolite was first ionexchanged in 1 mol L−1 NH4Cl solution at 360 K for 2 h. The ion-exchanged zeolite (denoted as NH4-ZSM-5) samples were 10738

DOI: 10.1021/acs.iecr.9b00502 Ind. Eng. Chem. Res. 2019, 58, 10737−10749

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Industrial & Engineering Chemistry Research collected by vacuum filtration and washed with deionized water. The as-prepared NH4-ZSM-5 was then put in a 373 K over for ∼12 h, and calcined in air at 823 K (2 K min−1 ramping rate) for 6 h to prepare HZSM-5. The ion exchange was carried out twice. 2.3. Characterizations. Powder X-ray diffraction (XRD) patterns of zeolite samples were measured by a Bruker D8 Advance diffractometer, with Cu Kα radiation (40 kV, 40 mA). The scanning electron microscope (SEM) and the transmission electron microscope (TEM) were carried to measure the morphology and crystal size of the zeolite samples on a FEI Quanta 200 F and a JEM 2100 at 200 kV, respectively. For the TEM measurement, the zeolite sample was dropped onto the copper grid with a Lacey carbon support carbon after dispersing zeolite samples in ethanol by 10 min ultrasonic treatment. The Si/Al ratio in zeolite sample was analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) using an IRIS Advantage spectrometer. The textural properties of the zeolites were measured by the N2-sorption at 77 K. Before sorption measurements, all samples were pretreated by vacuum degassing at 573 K for 12 h to remove water. The measurement was carried out on a Micromeritics ASAP 2020. The Brunauer−Emmett−Teller (BET) equation was used to calculate the specific surface area (denoted as SBET) of the zeolite sample at the relative pressure P/P0 values below 0.2. It has been reported that the BJH method significantly underestimates the pore size for narrow mesopores (for pore diameter < ∼10 nm, the pore size will be underestimated by ∼20−30%),50,51 here the pore size distribution was also evaluated by nonlocal density functional theory (NLDFT) method.52 Temperature-programmed desorption of ammonia (NH3TPD) was measured on a Huasi DAS-7000 chemisorption unit. For a typical measurement, 0.10 g of zeolite samples (0.250−0.425 mm) was first treated at 773 K with flowing N2 gas stream (30 mL min−1) for 1 h to remove water and then cooled to 373 K under flowing N2 gas. Subsequently, anhydrous NH3 stream with 30 mL min−1 flow rate was introduced to replace N2 gas and kept for 30 min at 373 K. After the adsorption of the NH3 gas stream, the N2 gas with 20 mL min−1 flow rate was induced to remove excess NH3. Finally, the NH3-TPD was carried out by increasing the desorption temperature from 373 to 873 K with a ramping rate of 10 K min−1. The desorption amount of NH3 was determined simultaneously by the thermal conductivity detector (TCD). The acid type (B or L acidic site) and their corresponding concentration on zeolite samples were determined by the pyridine-IR spectra using Bruker Tensor II FTIR spectrometer. Prior to the measurement, the samples were mixed with KBr and were pressed into wafers. These wafers were degassed at 573 K for 2 h in vacuum, and then cooled to 373 K. After that, pyridine was introduced and kept for 30 min, then pyridine-IR spectra were recorded by evacuation at 623 K. 2.4. Determination of External Surface, Pore Mouth, and Internal BAS of Zeolite. The dehydration of methanol to dimethyl ether with the 2,6-di-tert-butylpyridine (DTBP) and triphenylphosphine (TPP) at 433 K was used to distinguish external surface, pore mouth, and internal BAS sites of ZSM-5 zeolites, respectively.53 Typically, DTBP or TPP dissolved in methanol was fed into a flowing N2 gas stream at 348 K using a syringe pump (KDS 100, KD Scientific). After the dehydration, the reaction product was

transferred into a six-way valve by a heated transfer line (at ∼373 K) and then was quantitatively injected into an online gas chromatography (GC 7890) equipped with a flame ionization detector (FID). The methanol and dimethyl ether in the product were separated by a capillary column (dimethylpolysiloxane J&W HP-1, 50 m × 0.32 mm × 0.52 μm) The decrease of the dehydration rate with the presence of DTBP or TPP reflects the accessibility of BAS to DTBP or TPP molecules in zeolites. Due to the inaccessible contact of TPP titrants with BAS at the pore mouth or in ZSM-5 zeolites, the fraction of BAS on external surface (denoted as fext, H+) was determined by the degree of decrease of the dehydration rate after inducing TPP. Unlike that, DTBP molecules can titrate the BAS at the pore mouth and the external surface of ZSM-5 zeolite, the sum of fractions of external surface and pore mouth BAS (denoted as fsum, H+) was calculated by the degree of loss in dehydration rate with the addition of DTBP. The fractions of pore mouth BAS (denoted as f pm, H+) were the difference between fsum, H+ and fext, H+. Based on the total acid (B and L acid sites) from NH3-TPD spectra and total BAS by pyridineIR spectra, the number of BAS at the external surface and pore mouth can then be evaluated, respectively. 2.5. Catalytic Reactions. The MTP reaction was performed in a fixed-bed reactor equipped with a quartz tube reactor with an inner diameter of 8 mm at 101 kPa. Typically, 0.1−0.5 g of HZSM-5 was diluted with fumed silica to 1.0 g and granulated into 0.425−0.850 mm, and then mixed with 1.0 g 0.425−0.850 mm quartz. Prior to the reaction, the zeolite sample was activated in a flowing N2 gas stream (100 mL gzeolite−1 min−1) at 823 K (10 K min−1 ramping rate) for 2 h. Then, the reactor was cooled naturally to 743 K, and the methanol aqueous solution with 1:1 molar ratio was fed. The total flow rate of methanol solution was systematically changed to achieve different weight hourly space velocity (WHSV) of methanol (0.5−16 h−1). The hydrocarbon selectivity was analyzed by an online gas chromatograph (GC 7890) equipped with a capillary column (HP-PLOT Q, 30 m × 0.53 mm × 0.40 μm) and a flame ionization detector (FID). Dimethyl ether (DME) was regarded as the unconverted reactant.

3. RESULTS AND DISCUSSION 3.1. Structural Properties of Zeolite. Here, a series of ZSM-5 zeolites with different molar compositions of silicatealuminate gel (1AIP: 110SiO2: 900H2O: 5TPAOH: xNaBr; x = 0, 1, 2, 4, 6, and 8, respectively) were prepared and indicated as ZSM-5xNa (x = 0, 1, 2, 4, 6, and 8, respectively). Figure 1 shows all ZSM-5 zeolites prepared with different amounts of NaBr in hydrothermal synthesis possess typical diffraction peaks at 2θ of 7.9°, 8.8°, 23.2°, 23.9°, and 24.4°, corresponding to the MFI topology structure (JCPDS No.: 49-0657). No other diffraction peaks can be observed in these patterns, and ZSM-5 samples with high crystallinity are found because of the high intensity of diffraction peaks. The intensity of the diffraction peaks around 2θ of 23° to 25° decreases with reducing NaBr in silicate-aluminate gel, indicating a bit decrease of crystallinity due to the reduce of crystal size of zeolite confirmed by the SEM and TEM results (see the following section). Figure 2 shows the SEM image of these ZSM-5 samples synthesized with different amounts of NaBr in hydrothermal synthesis. Obviously, all ZSM-5 samples show the aggregates of zeolite crystal with different crystal sizes. The morphology of 10739


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Figure 1. XRD patterns of ZSM-5xNa (x = 0, 1, 2, 4, 6, and 8, respectively) zeolite crystals.

Figure 3. TEM images of aggregated ZSM-5 zeolite crystals with different crystal sizes.

zeolite becomes difference by varying the NaBr concentration during hydrothermal synthesis. First, the primary crystal size of the ZSM-5 zeolite increases with increasing NaBr concentration. Second, ZSM-50Na without the presence of NaBr displays sphere-shaped aggregates of zeolite crystal with a size at 30−50 nm, while the cuboid-shaped aggregates could be observed with the addition of NaBr. All ZSM-5xNa (x = 0, 1, 2, and 4, respectively) samples possess rough surfaces of aggregates because small crystals (100 nm) is observed on the ZSM54Na sample, leading to fewer intercrystalline mesopores via the aggregation, which is further confirmed by the nitrogen sorption results. 3.2. Textural Properties of Zeolite. Figure 4 and Table 1 show the textural properties of ZSM-5 samples. All the N2sorption curves are ascribed to the type IV isotherms with type H3 loops at P/P0 = 0.4−0.99. This suggests the formation of mesopore structure of the as-prepared ZSM-5 zeolites. NLDFT method is adopted to calculate the pore size 10741


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Industrial & Engineering Chemistry Research Table 2. Acid Properties of Aggregated ZSM-5 Zeolite Crystals from NH3-TPD and Pyridine FT-IR Spectra Weak acidb

Strong acidb

Samples

Si/ Ala

T (K)

Acid amount (mmol g−1)

T (K)

Acid amount (mmol g−1)

Total acid amountb (mmol g−1)

Weak/Strong acid site ratiob

B/L acid site ratioc

ZSM-50Na ZSM-51Na ZSM-52Na ZSM-54Na ZSM-56Na ZSM-58Na ZSM-5(A) ZSM-5(B)

115 106 110 102 101 105 65 31

483 490 480 487 487 485 500 497

0.116 0.120 0.103 0.111 0.076 0.066 0.102 0.211

773 750 745 730 667 660 692 701

0.022 0.024 0.036 0.046 0.080 0.085 0.112 0.244

0.138 0.144 0.139 0.147 0.156 0.151 0.224 0.455

5.3 5.0 2.9 2.4 1.0 0.8 0.9 0.9

4.2 4.5 5.8 6.5 7.9 8.6 8.1 7.5

a

Determined from elemental analysis (ICP-OES). bAcid amounts and ratios of weak acid site/strong acid site (Weak/Strong ratio) were obtained from NH3-TPD spectra. cB/L acid site ratios were obtained from the pyridine FT-IR spectra.

Table 3. Acid Properties of Aggregated ZSM-5 Zeolite Crystals from NH3-TPD and Pyridine FT-IR Spectra, and DTBP and TPP Titration in Methanol Dehydration Samples

Si/Ala

Total Al sitesa

Total BAS sitesb

External and pore mouth BAS sitesc (mmol g−1)

fsum, H+d (%)

fext, H+e (%)

f pm, H+f (%)

ZSM-50Na ZSM-51Na ZSM-52Na ZSM-54Na ZSM-56Na ZSM-58Na

115 106 110 102 101 105

0.144 0.157 0.149 0.164 0.164 0.157

0.111 0.118 0.119 0.125 0.127 0.135

0.074 0.071 0.042 0.024 0.020 0.016

66.7 60.1 35.3 19.2 15.7 11.8

44.5 40.6 28.7 16.1 13.9 10.5

22.2 19.5 6.6 3.1 2.8 1.3

a

Determined from elemental analysis (ICP-OES). bCalculated from the total acid amount via NH3-TPD spectra and B/L acid site ratio via pyridine FT-IR spectra. cDetermined by DTBP titration in methanol dehydration. dFraction of external and pore mouth BAS calculated by (number of BAS by DTBP titration/number of BAS by NH3-TPD and pyridine FT-IR results). eFraction of external BAS calculated by (number of BAS by TPP titration/number of BAS by NH3-TPD and pyridine FT-IR results). fCalculated by f pm, H+= fsum, H+ − fext, H+.

Figure 6. SEM images of ZSM-5(A) and ZSM-5(B) samples.

773 K, respectively. Table 2 shows the calculation results by assuming 1 stoichiometric ratio of NH3 molecule to acid site during chemisorption. The total number of acid sites increases with a decrease in Si/Al ratio in ZSM-5 samples. In addition, ZSM-5xNa (x = 0, 1, 2, and 4, respectively) samples possess less moderate acid sites than that of ZSM-56Na and ZSM-58Na, but the strength of these acid sites is stronger due to the shift of desorption temperature from 660−667 K to 730−773 K. On the other hand, the ratio of weak acid site to strong acid site increases by decreasing zeolite crystal size. The B/L acid site ratio of BAS and Lewis acid site (LAS) is decreased by reducing zeolite crystal size, in agreement with the result that more defects occur on nanosized zeolite compared to conventional microsized zeolite crystal.31 These data together show that nanosized ZSM-5 zeolite crystal possesses relatively more weak acid sites, LAS, as well as stronger moderate acid sites, in comparison with microsized ZSM-5 zeolite crystal. Table 3 lists the Si/Al ratios and the number of BAS of ZSM-5 zeolite samples determined by ICP-OES measurement

that the fraction of the BAS at the external surface or in the mesopore of ZSM-5 zeolite is increased with the ratio of external area/micropore area via reducing the crystal size for aggregation. 3.3. Acidic Properties of Zeolite. The acid properties of all zeolite samples were investigated by NH3-TPD and pyridine FT-IR spectra. Figure 5 and Tables 2 and 3 show the acid properties of aggregated ZSM-5 zeolites with different crystal size and Si/Al ratios. Figure 6 exhibits the SEM images of ZSM-5(A) and ZSM-5(B) synthesized at the similar condition as ZSM-56Na, except a smaller Si/Al ratio in silicate−aluminate gel from 110 to 50 and 25, respectively. The aggregates of zeolite crystals are formed by the cuboid crystal with 200−500 nm, suggesting less impact of the Si/Al ratio of silicate− aluminate gel on the aggregation in the hydrothermal synthesis via our developed synthesis strategy. The NH3-TPD profiles in Figure 5 are characteristic of two Gassian distributions. The number of weak and moderate acid sites can be calculated based on the desorption peak areas at 373−523 K and 523− 10742


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Figure 7. B/L acid site ratio and weak acid/strong acid ratio versus relative external surface area and fraction of external surface and pore mouth BAS of ZSM-5 zeolites.

and NH3-TPD characterization as well as pyridine FT-IR spectra, respectively. Table 3 also provides the number of BAS at the external surface and pore mouth of each zeolite based on the DTBP titration in methanol dehydration. The fractions of BAS at the external and pore mouth (fsum, H+) were determined from DTBP titration to the number of BAS determined from pyridine IR spectra. It should be noted that bulky DTBP molecules are not accessible to the BAS within the micropores of ZSM-5 zeolites.53 The increase of fsum, H+ by decreasing zeolite crystal size reflects that the BAS originated from the zeolite framework Al are spatially modulated by the intergrowth of nanosize zeolite crystals. To identify the fraction of BAS at the external surface (fext, H+) or at the pore mouth (f pm, H+) of ZSM-5 zeolites, TPP titrant as organic base was induced in the methanol dehydration. TPP molecules prefer to anchor on the BAS at the external surface of zeolite because of its larger size than zeolite aperture size and its moderate base strength. The difference between the number of BAS determined by DTBP and TPP titrations indicates the number at the pore mouth of zeolite. Clearly, the number BAS at the pore mouth region is much lower than that at the external surface of ZSM-5 zeolites, especially for the ZSM-5 zeolites aggregated by crystal larger than 100 nm (ZSM-54Na, ZSM56Na, and ZSM-58Na, respectively). A correlation between B/L acid site ratio (shown in Table 2) and the relative external surface area (Sext/SBET, calculated from data in Table 1) of ZSM-5 zeolite samples is shown in Figure 7. It is obvious that the ratio of B/L acid site increases with the growth of relative surface area of ZSM-5 zeolite. This confirms the influence of spatially controlled crystal size on the acidity of zeolite. The increase in Lewis acidity from ZSM-58Na to ZSM-50Na is a result of increasing the external surface area via reducing crystal size of zeolites. It should be due to the surface defects of nanosized zeolite crystal, in which more weak acid sites are generated on the external surface and pore mouth of zeolites. Thus, acid amounts, types, and strength of the

Figure 8. (a) Conversion and (b) concentration profiles as a function of contact time for feeding the mixture of methanol and water (1:1 molar ratio) and (c) selectivity as a function of conversion over ZSM52Na at 743 K.

ZSM-5 zeolite crystal as well as the diffusion limitation of ZSM-5 zeolite varied with the crystal size, suggesting an important role of the crystal size on the activity and selectivity of MTP reaction as well as the stability of zeolite. 3.4. Catalytic Performance of Zeolite. A relation of the MTP reaction and the product selectivity as a function of contact time over ZSM-52Na at 743 K is shown in Figure 8. A wide variety of products, including light paraffins (C1−4), ethylene (C2=), propylene (C3=), butylene (C4=), C5 hydrocarbons, C6+ aliphatics, and aromatics have been observed in the MTP reactions. Hereby, the C5 hydrocarbons designates all 10743


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Table 4. Product Distribution of MTP Reaction over Aggregated ZSM-5 Zeolites with Different Crystal Sizes at Moderate and Complete Methanol Conversion Initially Reached Selectivity (%) Sample

WHSV of methanol (h−1)

Conversiona (%)

C1−C4

C2=

C3=

C4=

C5

C6+

Aromatics

Propylene/ethylene ratio

ZSM-50Na ZSM-51Na ZSM-52Na ZSM-54Na ZSM-56Na ZSM-58Na ZSM-5(A) ZSM-5(B) ZSM-50Na ZSM-51Na ZSM-52Na ZSM-54Na ZSM-56Na ZSM-58Na ZSM-5(A) ZSM-5(B)

∼8 ∼8 ∼10 ∼10 ∼11 ∼11 ∼13 ∼15 ∼4.5 ∼4.5 ∼5 ∼5 ∼6 ∼6.5 ∼8 ∼9

48.7 50.1 45.7 52.1 48.5 55.2 55.1 53.2 99.2 99.1 99.5 99.8 99.7 99.5 99.8 99.7

1.1 1.1 1.2 1.4 2.7 2.8 4.0 5.4 1.0 0.9 0.9 1.1 1.6 2.2 5.4 8.9

2.8 3.5 4.2 4.4 4.9 4.7 5.1 7.1 4.3 4.8 5.9 6.5 7.6 7.9 17.2 22.6

28.7 29.8 28.5 27.1 25.4 25.1 24.6 21.0 47.2 47.1 46.4 45.7 45.1 44.8 41.1 35.0

24.8 22.6 21.8 20.9 18.6 17.9 14.7 12.9 26.7 26.5 26.8 27.3 26.5 25.9 18.9 13.5

12.9 13.1 14.8 15.3 14.9 15.9 16.5 17.4 12.3 12.4 12.1 11.7 11.3 10.3 6.5 4.5

28.4 28.5 29.1 29.5 30.9 30.4 31.3 31.7 7.5 7.3 6.9 6.5 6.3 6.8 5.1 4.7

1.3 1.4 1.4 1.4 2.6 3.2 3.8 4.5 1.0 0.9 0.9 1.1 1.6 2.1 5.8 10.8

10.2 8.5 6.8 6.2 5.2 5.3 4.8 3.0 11.0 9.8 7.9 7.0 5.9 5.7 2.4 1.5

a

The MTP reaction was carried out at 743 K with 50.5 kPa methanol and 50.5 kPa H2O.

hydrocarbons with five carbon atoms, and the C6+ aliphatics encompass other heavier hydrocarbons other than aromatics.20 Aromatics and C2−C4 light paraffins are regarded as hydrogen transfer (HT) products. Clearly, the effect of contact time on methanol conversion shows an induction period for the onset of methanol conversion, in agreement with the autocatalytic phenomenon reported in MTH reactions. 19,20 A selfacceleration of methanol conversion is observed and a complete conversion can be achieved within a narrow range of contact times. Figure 8b shows the concentration of propylene and butylene reached a maximum at a complete conversion of methanol, while the C5 and C6+ aliphatic concentrations reached a maximum more or less at the contact time to achieve first 100% conversion of methanol, and then decreased by further increasing the contact times. These data suggest the formation routes for the hydrocarbons are changed by altering methanol conversion. For the MTH reaction with a low methanol conversion, the selectivity of hydrocarbon products is reactively low because the interaction between the remaining methanol and the BAS form frameworkbounded surface methoxy species, and then further react with olefins or aromatics.54 Bhan et al. proposed two reaction stages for the contact time with complete conversion of methanol:23 the first stage involves the “hydrocarbon pool” mechanism in the MTH reaction, in which the hydrocarbons form via both the propagation of olefin-based catalytic cycle and aromatic-based catalytic cycle, and the second stage involves the olefin interconversion. Figure 8c shows that the methanol conversion decreases from 100% to ∼20%, and the selectivities of aromatic and C1−4 alkane increases a bit from 0.9% to 1.5% and from 0.6 to 1.0%, respectively, indicating that the methanol concentration shows few influences on the hydrogen transfer reaction. Unlike the aromatics, when the methanol conversion reduced from 100% to ∼20%, the selectivities of propylene and butylene decrease from ∼49% to ∼30% and from ∼26% to ∼21%, the selectivities of C5 hydrocarbon and C6+ aliphatics increase from ∼12% to ∼15% and from ∼6% to ∼30%, respectively. These data together show that the light olefin should be mainly formed via the olefin-based catalytic cycle rather than aromatic-based catalytic

cycle over high silica ZSM-5 zeolite in our reaction condition, in which the catalytic cracking activity is promoted by increasing temperature, and then a high light olefin selectivity while a lower selectivity of the hydrogen transfer products can be obtained. Moreover, the change of the product distribution versus conversion confirm that the propagation of the methylation route is dependent on methanol in the olefinbased catalytic cycle, in which the presence of methanol promotes the olefin methylation and inhibits olefin cracking. The product distribution of MTP reaction at high temperature over high silica ZSM-5 zeolite (Figure 8) suggests that light olefins should be achieved from the olefin-based catalytic cycle, in agreement with the other work.3−40,44−48 Moreover, the hydrocarbon product distribution on methanol conversion is varied by changing the dominant reaction pathway from olefin methylation, oligomerization, and cracking, respectively.16,54 Thus, to clarify the effect of zeolite crystal size on the product distribution of MTP reaction. The product distribution of ZSM-5 zeolite with different crystal size was compared with a moderate (i.e., ∼50%) methanol conversion, and a complete methanol conversion initially reached with a high weight hourly space velocity (WHSV) of methanol (i.e., 5−15 h−1), as shown in Table 4. Table 4 shows the product distributions of MTP reaction by controlling WHSV when ∼50% and ∼100% methanol conversion initially reached. For ZSM-5xNa (x = 0, 1, 2, 4, 6, and 8, respectively) samples, with the decrease of the size of zeolite crystal, BAS amount (proven in Figure 7, and Tables 2 and 3) decreases, leading to a promotion in the selectivity of propylene at moderate and complete methanol conversion. ZSM-5xNa (x = 0, 1, and 2, respectively) samples possess a higher selectivity of propylene and a larger ratio of propylene to ethylene. Moreover, ZSM-5(A) and ZSM-5(B) with a much lower Si/Al ratio and much more BAS amount shows much lower selectivities of light olefins (C2= + C3= + C4=) and propylene, but exhibit higher selectivities of ethylene and hydrogen transfer product (C1−4 alkanes and aromatics). This indicates that light olefins obtained over ZSM-5 zeolite with a high BAS density is formed from both olefin-based catalytic cycle and aromatic-based catalytic cycle routes at the MTP 10744


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Figure 9. Product distribution of MTP reaction over aggregated ZSM-5 zeolites with different crystal sizes at (a) moderate and (b) complete methanol conversion initially reached (743 K with 50.5 kPa methanol and 50.5 kPa H2O shown in Table 4).

reaction condition, in agreement with the work of Bhan et al.23 However, a higher selectivity of propylene and a larger ratio of propylene to ethylene over ZSM-5xNa (x = 0, 1, 2, 4, 6, and 8, respectively) suggest high silica ZSM-5 zeolite (Si/Al > 100) favors the olefin-based catalytic cycle routes for forming propylene and ethylene. Our results on the difference of product distribution among ZSM-5xNa (x = 0, 1, 2, 4, 6, and 8, respectively), ZSM-5(A) and ZSM-5(B) confirm the fact that the reduce of the density of BAS is the key to the selective formation of propylene.24,26,36,54 Bhan et al. proposed that light olefin (C2= + C3=) selectivity can be promoted by increasing crystal size of zeolites with a Si/ Al ratio varied from 38 to 88 and with a crystal size varied from 2 nm to 17 μm at 623 K for the MTH reaction.22 As the olefin catalytic cracking via acid site follows β cleavage rules of carbonium ion, C3= and C4= are the main cracking products. Thus, the selectivity of light olefins (C2= + C3=) and cracking

products (C2= + C3= + C4=) are compared among all zeolite samples shown in Figure 9. Among ZSM-56Na, ZSM-5(A), and ZSM-5(B) with similar crystal size but with different Si/Al ratios, the highest selectivity of light olefins (C2= + C3=) or cracking products (C2= + C3= + C4=) is observed on the high silica ZSM-56Na, as well as a lowest selectivity of hydrogen transfer product (C1−4 + aromatics) at moderate methanol conversion (Figure 9a). The trend is similar when the methanol conversion was promoted to complete conversion, except for the selectivity of light olefins (C2= + C3=), in which ZSM-5(A) and ZSM-5(B) possess a higher value (Figure 9b). Associated with a high selectivity of ethylene and a low ratio of propylene to ethylene over ZSM-5(A) and ZSM-5(B) than ZSM-56Na, clearly the aromatic-based catalytic cycle shows a distinct important role in the ZSM-5 zeolite with a high BAS density, and the extension of crystal size favors the aromaticbased catalytic cycle to generate light olefins (C2= + C3=). 10745


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work of Zhang et al.54 It should be noted that external BAS of zeolite has been proven to possess the same alkylation activity as internal BAS within the zeolite micropore.49,55,56 Here, we proposed that the reduce of diffusion barrier via decreasing crystal size results in a promotion of cracking product yields, as retaining light olefins in zeolite micropore over large zeolite crystal leads to olefin oligomerization for the formation of C5 and C6 product and to olefin aromatization via hydrogen transfer reaction in Figure 9a. On the other hand, Figure 9b shows that the selectivity of light olefins (C2= + C3=) and cracking products (C2= + C3= + C4=) is similar among ZSM5xNa (x = 0, 1, 2, 4, 6, and 8, respectively) samples, indicating that light olefins should be formed via olefin cracking and the methanol methylation is suppressed at a complete methanol conversion. Figure 7 together with Tables 1−3 shows that the number of BAS at the external surface and pore mouth of zeolite increases with decreasing zeolite crystal size. For ZSM-5 zeolite samples with a Si/Al ratio at ∼50, Kim et al. proposed that external surface BAS do not enroll in the activation of MTH reaction,46 while Losch et al. found that external surface passivation of nanosized ZSM-5 zeolites promotes the selectivity toward C2−4 light olefins in the MTO reaction.57 Both of them proposed that MTH reaction occurs solely within micropores of ZSM-5 zeolite, or even in the pore mouth of zeolites, as the appropriate zeolite aperture size is required for the formation of hydrocarbons based on the hydrocarbon pool mechanism. On the other hand, external surface BAS without space restriction can carry out the catalytic cracking and alkylation reaction.49,55,56 Clearly, zeolite with smaller crystal size should be characteristic of a high ratio of propylene to ethylene, as the generation of propylene and butylene via cracking over external surface BAS comes before ethylene. This is in agreement with our results shown in Table 4 and Figure 9. The amount of hydrogen transfer product (selectivity of C1−4 and aromatics in Figure 9) increases a bit from ZSM-50Na to ZSM-58Na at moderate and complete methanol conversion. Thus, a higher selectivity of ethylene (shown in Table 4) is observed via the aromatic-based catalytic cycle. For the moderate methanol conversion, the growth of carbon chain is relatively dominant by methylation or aromatization, ZSM-5 zeolite with large crystal (i.e., ZSM-56Na and ZSM-58Na in Figure 9a) favors C5, C6+, and aromatics, leading to promoting aromatic-based catalytic cycle for forming ethylene and propylene. For the complete methanol conversion, the growth of carbon chain is terminated with the absence of enough methanol, and light olefins are mainly obtained via catalytic cracking, and all ZSM-5xNa zeolite shows similar selectivity of light olefins (C2= + C3=). Considering the work of Kim49 and Losch,57 we believe the activity of zeolites with similar amounts of BAS in MTP reaction should be reduced by decreasing zeolite crystal size, because the fraction of external surface BAS greatly increases by changing large zeolite crystal to small zeolite crystal shown in Table 3. Indeed, to get a moderate methanol conversion in Table 4, a low weight hourly space velocity (WHSV) of methanol is required for the ZSM-50Na (∼8 h−1) than that of ZSM-58Na (∼15 h−1), as the amount of BAS distributed inside and pore mouth over ZSM-50Na calculated from Table 3 is 0.062 mmol g−1, much smaller than the amount obtained in ZSM-58Na with 0.121 mmol g−1. Thus, zeolite with smaller crystal size suffers fewer fractions of internal BAS for the MTH reaction, and the stability of zeolite in methanol conversion

Figure 10. (a) Conversion, (b) ethylene selectivity, and (c) propylene selectivity as a function of time on stream over the ZSM-5 zeolites with different crystal sizes (743 K with 50.5 kPa methanol and 50.5 kPa H2O, WHSV of 4 h−1).

For ZSM-5xNa (x = 0, 1, 2, 4, 6, and 8, respectively) samples, the light olefins (C2= + C3=) are mainly formed from the olefinbased catalytic cycle. Thus, we found that the increase of selectivity of propylene (shown in Table 4) and the cracking products (C2= + C3= + C4=) shown in Figure 9a, and a decrease of selectivity of hydrogen transfer product at moderate methanol conversion with decreasing crystal size. This shows that light olefin distribution is depended on the competitive reaction between olefin methylation and cracking with the presence of methanol in reaction stream, in agreement with the 10746


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Figure 11. Lifetime and BAS amount versus crystal size of ZSM-5 zeolites.

54Na (92 h) > and ZSM-51Na (90 h) > ZSM-50Na (80 h) > ZSM-56Na (76 h) > ZSM-58Na (60 h). Although the presence of mesopore structure could improve the coking resistance of zeolite,59 here ZSM-50Na and ZSM-51Na with a larger mesopore volume still exhibits much shorter lifetime than that of ZSM52Na. This should be due to the smaller amount of internal BAS in micropore accounting for the methanol to hydrocarbon reaction (Figure 11), leading to overloading of each internal BAS. On the other hand, for ZSM-5xNa (x = 2, 4, 6, and 8, respectively) zeolites with relatively close amount of internal BAS in micropore but with different crystal size from 80−150 nm to 1000−2000 nm (Figure 11), ZSM-52Na with the largest mesopore volume shown in Table 1 exhibits the longest lifetime, in agreement with the promotion of stability by inducing mesoporosity.59 These data together confirm the best stability of ZSM-52Na. As we proven above, light olefins are mainly formed via olefin-based catalytic cycle when a complete methanol conversion is achieved, and the reduction of crystal size promotes the diffusion of light olefins avoiding methylation or aromatization within micropores. Here, a relatively low WHSV at 4 h−1 was used, suggesting the occurrence of olefin interconversion with a large contact time.23 Figure 10b,c display the selectivity of propylene and ethylene at a complete methanol conversion, respectively. The selectivity of propylene increases a bit with the time on stream during 50 h MTP reaction, and the ethylene selectivity decreases a bit. Clearly, the smaller size of zeolite, the higher selectivity of propylene as well as the lower ethylene selectivity, because of the shorter diffusion path length avoiding olefin interconversion. Among all ZSM-5 zeolites, ZSM-5 2Na possesses the highest selectivity of propylene and largest lifetime, suggesting the potential choice as active component for developing industrial MTP catalyst.

becomes challenged. Kunkeler et al. found that the coke formation of ZSM-5 zeolite should be dependent on the acid sites at the external surface of zeolite, and surface passivation is required to improve the lifetime of zeolite in cracking.58 On the other hand, Kim et al. investigate the effect of mesoporosity on the improving coking deactivation of ZSM-5 zeolite in the MTH reaction.59 They proved that the catalytic lifetime of zeolite is mostly promoted by the improvement in the diffusion of coke precursors due to the reduced diffusion path lengths. Here, the intercrystalline mesopores originate from the aggregation of nanosized zeolite crystal shown in Figure 4 and Table 1. Moreover, Yarulina et al. confirmed that the formation of aromatics is suppressed because of the preferential methylation and destabilization of carbonium ion over LAS for the aromatic-based catalytic cycle, and hence reduce the generation of coke precursors.36 We noted that the B/L acid site ratio decreases by decreasing crystal size (Figure 7), while the weak acid/strong acid ratio increases (Figure 7) as well as the fraction of BAS at the external surface and pore mouth of zeolite (Table 3 and Figure 7). Clearly, it is necessary to screen suitable crystal size for zeolite with excellent stability as well as a high selectivity of propylene affected by the efficient amount of BAS inside and at the pore mouth of zeolite, the B/L acid site ratio, and weak acid/strong acid ratio. Thereby, we studied the change of conversion, and selectivity of propylene and ethylene as a function of time on stream shown in Figure 10. To screen the highly selective and stable zeolite for MTP reaction, the catalytic performance of the ZSM-5xNa (x = 0, 1, 2, 4, 6, and 8, respectively) zeolite with different crystal size (30−50 to 1000−2000 nm, Table 1) and BAS amount (shown in Table 3 and Figure7) were evaluated at 743 K with a fixed WHSV of 4 h−1. The methanol conversion and selectivity of propylene and ethylene are displayed in Figure 10. Obviously, the lifetime (maintaining methanol conversion above 95%) for ZSM-5 zeolite follows the order: ZSM-52Na (112 h) > ZSM10747


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(6) Gayubo, A. G.; Aguayo, A. T.; Olazar, M.; Vivanco, R.; Bilbao, J. Kinetics of the irreversible deactivation of the HZSM-5 catalyst in the MTO process. Chem. Eng. Sci. 2003, 58, 5239−5249. (7) Kaarsholm, M.; Joensen, F.; Nerlov, J.; Cenni, R.; Chaouki, J.; Patience, G. S. Phosphorous modified ZSM-5: Deactivation and product distribution for MTO. Chem. Eng. Sci. 2007, 62, 5527−5532. (8) Mei, C. S.; Wen, P. Y.; Liu, Z. C.; Liu, H. X.; Wang, Y. D.; Yang, W. M.; Xie, Z. K.; Hua, W. M.; Gao, Z. Selective production of propylene from methanol: Mesoporosity development in high silica HZSM-5. J. Catal. 2008, 258, 243−249. (9) Firoozi, M.; Baghalha, M.; Asadi, M. The effect of micro and nano particle sizes of H-ZSM-5 on the selectivity of MTP reaction. Catal. Commun. 2009, 10, 1582−1585. (10) Liu, J.; Zhang, C. X.; Shen, Z. H.; Hua, W. M.; Tang, Y.; Shen, W.; Yue, Y. H.; Xu, H. L. Methanol to propylene: Effect of phosphorus on a high silica HZSM-5 catalyst. Catal. Commun. 2009, 10, 1506−1509. (11) Lee, K. Y.; Lee, H. K.; Ihm, S. K. Influence of Catalyst Binders on the Acidity and Catalytic Performance of HZSM-5 Zeolites for Methanol-to-Propylene (MTP) Process: Single and Binary Binder System. Top. Catal. 2010, 53, 247−253. (12) Sun, C.; Du, J. M.; Liu, J.; Yang, Y. S.; Ren, N.; Shen, W.; Xu, H. L.; Tang, Y. A facile route to synthesize endurable mesopore containing ZSM-5 catalyst for methanol to propylene reaction. Chem. Commun. 2010, 46, 2671−2673. (13) Vu, D. V.; Hirota, Y.; Nishiyama, N.; Egashira, Y.; Ueyama, K. High Propylene Selectivity in Methanol-to-olefin Reaction over HZSM-5 Catalyst Treated with Phosphoric Acid. J. Jpn. Pet. Inst. 2010, 53, 232−238. (14) Wei, R. C.; Li, C. Y.; Yang, C. H.; Shan, H. H. Effects of ammonium exchange and Si/Al ratio on the conversion of methanol to propylene over a novel and large particle size ZSM-5. J. Nat. Gas Chem. 2011, 20, 261−265. (15) Jiao, Y. L.; Jiang, C. H.; Yang, Z. M.; Zhang, J. S. Controllable synthesis of ZSM-5 coatings on SiC foam support for MTP application. Microporous Mesoporous Mater. 2012, 162, 152−158. (16) Wu, W. Z.; Guo, W. Y.; Xiao, W. D.; Luo, M. Methanol conversion to olefins (MTO) over H-ZSM-5: Evidence of product distribution governed by methanol conversion. Fuel Process. Technol. 2013, 108, 19−24. (17) Almutairi, S. M. T.; Mezari, B.; Pidko, E. A.; Magusin, P. C. M. M.; Hensen, E. J. M. Influence of steaming on the acidity and the methanol conversion reaction of HZSM-5 zeolite. J. Catal. 2013, 307, 194−203. (18) Hu, Z. J.; Zhang, H. B.; Wang, L.; Zhang, H. X.; Zhang, Y. H.; Xu, H. L.; Shen, W.; Tang, Y. Highly stable boron-modified hierarchical nanocrystalline ZSM-5 zeolite for the methanol to propylene reaction. Catal. Sci. Technol. 2014, 4, 2891−2895. (19) Sun, X. Y.; Mueller, S.; Liu, Y.; Shi, H.; Haller, G. L.; SanchezSanchez, M.; van Veen, A. C.; Lecher, J. A. On reaction pathways in the conversion of methanol to hydrocarbons on HZSM-5. J. Catal. 2014, 317, 185−197. (20) Sun, X. Y.; Mueller, S.; Shi, H.; Haller, G. L.; Sanchez-Sanchez, M.; van Veen, A. C.; Lecher, J. A. On the impact of co-feeding aromatics and olefins for the methanol-to-olefins reaction on HZSM5. J. Catal. 2014, 314, 21−31. (21) Mueller, S.; Liu, Y.; Vishnuvarthan, M.; Sun, X. Y.; van Veen, A. C.; Haller, G. L.; Sanchez-Sanchez, M.; Lecher, J. A. Coke formation and deactivation pathways on H-ZSM-5 in the conversion of methanol to olefins. J. Catal. 2015, 325, 48−59. (22) Khare, R.; Millar, D.; Bhan, A. A mechanistic basis for the effects of crystallite size on light olefin selectivity in methanol-tohydrocarbons conversion on MFI. J. Catal. 2015, 321, 23−31. (23) Khare, R.; Millar, D.; Bhan, A. Mechanistic studies of methanolto-hydrocarbons conversion on diffusion-free MFI samples. J. Catal. 2015, 329, 218−228. (24) Rostamizadeh, M.; Taeb, A. Highly selective Me-ZSM-5 catalyst for methanol to propylene (MTP). J. Ind. Eng. Chem. 2015, 27, 297−306.

4. CONCLUSIONS Here, a modified hydrothermal synthesis strategy has been developed to synthesize intercrystalline mesoporous aggregated ZSM-5 zeolite via intergrowth of nanosized zeolite crystal without the addition of mesopore templates in hydrothermal synthesis. The catalytic performance in the MTP reaction suitable for industrial MTP process over asprepared aggregated ZSM-5 zeolite crystals with a high Si/Al ratio (>100) has been elucidated based on the indirect “hydrocarbon-pool” mechanism in the methanol to hydrocarbon reaction. Olefin-based catalytic cycle accounts for the light olefin formation over high silica zeolite, and the selectivity of light olefin is dependent on the competitive reaction between olefin methylation and cracking reaction, in which a high ratio of propylene to ethylene is characteristic of zeolite. The reduction of crystal size of zeolite reduces the secondary reaction of light olefins, leading to a promotion of their selectivities. A systematic comparison of the ZSM-5 zeolite with different crystal size shows that external surface BAS, ratio of weak acid to strong acid, are enhanced by decreasing the crystal size, while a ratio of B/L acid sites decreases, which should be due to the increase of surface defects observed on nanosized zeolites. Therefore, the amount of internal BAS within micropore accounting for the methanol to hydrocarbon reduces with decreasing crystal size, which is detrimental to the stability of zeolite. Thus, the effect of crystal size on the stability and the propylene selectivity over ZSM-5 zeolite should be synchronized with the mesopore structure, the availability BAS within micropore, B/L acid site ratio, as well as the length of diffusion path.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86 10 89733235. Fax: +86 10 89734979. ORCID

Zhijie Wu: 0000-0002-8160-6615 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was supported by the Natural Science of Foundation China (U1662131, 21206192), the Science Foundation of China University of Petroleum-Beijing (C201603).

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REFERENCES

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Synthesis and Consequence of Aggregated Nanosized ZSM-5 Zeolite

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