Transformation of LPG on HZSM-5 catalyst: Effects of tuned pores and acidity on product distribution

Fuel 254 (2019) 115615

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Transformation of LPG on HZSM-5 catalyst: Effects of tuned pores and acidity on product distribution Wen Zhang, Subing Fan, Jianli Zhang, Qingxiang Ma, Kangzhou Wang, Tian-Sheng Zhao

T

⁎

State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, PR China

G R A P H I C A L A B S T R A C T

HZ-5-140

acid: 0.757 ȝmol/m2

LPG

connected mesopore + short micropore

C3H8

acid: 1.078 ȝmol/m2

HZ-5-100

n-C4H10 i-C4H10

stacked mesopore + short micropore

CH4

n-C4H8 i-C4H8

C2= + C3=

HZ-5-M

acid: 2.686 ȝmol/m2

C5+ micropore

0

10

20

30

40

50

60

70

Selectivity/%

A R T I C LE I N FO

A B S T R A C T

Keywords: Nano HZSM-5 Connected meso-micropores Weak acidity LPG transformation Propylene and ethylene

On microporous micron HZSM-5 catalyst liquefied petroleum gas (LPG) tends to produce high heavier hydrocarbons (HCs) and methane. Here nano HZSM-5 with regular pores by recrystallization procedure was investigated for the selective transformation of LPG. HZSM-5 with different crystallite aggregation, mesopore property and acidity was synthesized. Mesopore-connected, micropore-shortened and weakly acidic HZSM-5 catalyst enhances remarkably the formation of light olefins from LPG. At reaction temperature of 600 °C and contact time (W/F) of 15 g·h/mol, the selectivity of propylene and ethylene reached 68.3% at the total conversion of LPG of 58.8%. Products of C5+ HCs and methane were reduced. Due to less diffusion resistance of synthetic HZSM-5 and its low Brönsted acid amount as well as low total acid amount, adverse side reactions are suppressed.

1. Introduction Liquefied petroleum gas (LPG), a mixture of propane, butane and butylene, commonly used as fuel, comes mainly from natural gas extraction or petroleum refining. In recent years industrial-scale coalbased methanol-to-propylene and Fisher-Tropsh synthesis processes near the coal-rich areas of China have side-produced considerable amount of LPG, which contains about 20% propane and 80% C4

⁎

hydrocarbons (HCs). How to deal with them locally has become the enterprises’ concern. Light olefins such as propylene and ethylene are basic raw materials for a variety of industrial chemicals [1], which come traditionally from cracking of hydrocarbons [2,3], in particular, of petroleum-based hydrocarbons. As the global demand for light olefins has been ever-increasing, their production processes or raw materials tend to diversity, for example, cracking of C4 HCs to propylene and ethylene [4–6],

Corresponding author. E-mail address: [email protected] (T.-S. Zhao).

https://doi.org/10.1016/j.fuel.2019.06.023 Received 12 February 2019; Received in revised form 24 April 2019; Accepted 5 June 2019 0016-2361/ © 2019 Published by Elsevier Ltd.

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Inst.) was added into Al-SBA-15 of 2 g with ultrasonic treatment for 10 min. The mixture was kept at 50 °C for 1 h. The same TPAOH treatment for Al-SBA-15 was repeated three times. Then the mixture was transferred to a beaker of 25 mL, placed into a Teflon sleeve of 200 mL with 2.5 mL pre-added deionized water. The mixture was first heated at 100 °C for 4 h and then at 120 °C for 12 h. The sample was heated to 650 °C at 10 °C/min and calcined in air for 5 h to remove the residual TPAOH. The obtained white powder sample was pelletized into 20–40 mesh for use and denoted as HZ-5-T, T was the crystallization temperature of Al-SBA-15. For comparison, commercial HZSM-5 (NKF-5-60FX, SiO2/ Al2O3 = 60) was merchandized from Nankai University Catalyst Factory and denoted as HZ-5-M.

dehydrogenation of propane to propylene [7], methanol to olefins [8], methanol to propylene [9], syngas to olefins [10] and so on. Obtaining propylene and ethylene from the coal-based by-produced LPG has attracted interest of industrial circles although few have been studied on the transformation of LPG (mixed HCs) to propylene and ethylene. Activation of light alkanes over HMFI-based zeolite catalysts has been paid attention in order to produce light olefins besides alkylation products and aromatics [11]. On the one hand, the size of traditional HZSM-5 polycrystalline is much larger than its microspore size, which limits diffusion and access of HC molecules to active sites, bringing about low reaction effectiveness and secondary reactions of desired products. On the other hand, in the case of using strong acidic HZSM-5, the side reactions exacerbate. During the cracking of C4 alkenes on strongly acidic HZSM-5, coking occurs, covering acid sites or blocking pore channels [4,12]. Various modified ZSM-5 zeolites have been studied for catalytic cracking of HCs to produce light olefins [13]. Modified HZSM-5 by weakening acidity and constructing mesopores raised light olefins in catalytic cracking of LPG but products methane or C5+ were higher [14–16]. Various efforts have been devoted to prepare mesoporous zeolites with enhanced diffusion/mass transport properties [17] and micromesoporous materials by zeolite recrystallization [18]. Nano HZSM-5 crystallite with shorter micropore channels and certain mesopores is conducive to mass transfer, increases selectivity of light olefins and alleviates deactivation in catalytic reactions of HCs [19–23]. However, randomly stacked mesopores of HZSM-5 crystalline are unordered. The diffusion path length of molecules in hierarchically porous materials might be idealized and even not be shortened [24]. Endowing interconnected mesopores to microporous materials improves diffusion notably whereas discrete or discontinuous ones has almost no such effect [25]. Thus both the diminution of HZSM-5 crystalline size for short path length through microporous channels and ordered mesopores for mass transfer are vital. Recently with low solvent conditions Al-SBA-15 was recrystallized into ZSM-5 aggregates of nanocrystal particles [26], where the ZSM-5 crystal growth is physically confined in a nano-scale domain as the pore wall is gradually depleted. Once the residual template of polyethylene oxide-polypropylene oxide-polyethylene oxide (P123) and impregnated template of tetrapropylammonium hydroxide (TPAOH) are removed, ordered mesopores as well as micropores are formed. In this work the catalytic performance of nano HZSM-5 with regular pores for the transformation of LPG was studied. Promotion of improved diffusion via ordered pores and effect of weak acidity on product distribution were discussed.

2.2. Sample characterization Phase analysis was carried out on a Rigaku D/MAX 2200PC X-ray diffractometer using Ni-filtered Cu Kα radiation at 40 kV and 30 mA. N2 physisorption was measured on a JW-BK132F surface analysis instrument. The sample was degassed at 350 °C for 3 h prior to measurement. The surface area was calculated using the Brunauer-Emmett-Teller (BET) model. Microspore volume was calculated using the t-plot method. Mesoporous volume was derived by subtracting microspore volume from total pore volume (p/p0 = 0.98). Meso- and microspore distribution were calculated using the Barrett-Joyner-Halenda (BJH) model and the Horvath-Kawazoe (HK) model, respectively. Morphology analysis was completed on a ZEISS EVO18 scanning electron microscope (SEM) operating at 30 kV and on a Hitachi HT7700 transmission electron microscope (TEM) at 100 kV. Ammonia temperature programmed desorption (NH3-TPD) was performed on a TP-5080 adsorption instrument. After pretreatment at 550 °C the sample adsorbed NH3 at ambient temperature until saturation, followed by He purge at 120 °C for 1 h. The desorbed NH3 amount was analyzed by a thermal conductivity detector (TCD) as the temperature was elevated at 20 °C/min from 120 °C to 650 °C. Acid type analysis by pyridine adsorption was completed on a Bruker TENSOR-27 Fourier-transform infrared (FT-IR) spectrometer. The self-supporting wafer of the sample was placed into an in situ cell and degassed at 500 °C for 1 h and then at 150 °C adsorbed the pyridine vapor for 0.5 h. The spectra were recorded after the sample was evacuated for 30 min at 150 °C, 250 °C and 350 °C, respectively. The adsorptions at near 1450 cm−1 and 1540 cm−1 are ascribed to Lewis (L) acid and Brönsted (B) acid sites, respectively. Acid amount was calculated from IR adsorption peak area multiplied by adsorption coefficient, referring to the literature method [27]. The data at 350 °C treatment was for strong acid amount; the data at 250 °C treatment subtracting the 350 °C data for medium-strong acid amount; the data at 150 °C treatment subtracting the 250 °C data for weak acid amount.

2. Experimental 2.1. Synthesis of samples

2.3. Catalytic activity tests 2.1.1. Al-SBA-15 At a molar ratio of Al2O3:SiO2:P123:H2O = 1:10:0.146:1320, P123 of 6.6 g (EO20PO70EO20, Aldrich) was dissolved in 185.7 mL of deionized water under vigorous stirring for 0.5 h at 40 °C. Al2(SO4)3‧18H2O of 5.216 g (Shanghai Macklin Biochemical Co., Ltd.) was added and stirred for 3 h. Next, TEOS of 16 g (tetraethyl orthosilicate, Tianjin Jiangtian Chem. Technol. Co., Ltd.) was added and stirred at 40 °C for 39 h. Then the mixture was transferred to an autoclave, heated to the set temperatures at 2 °C/min and crystallized for 48 h. The product was filtrated, washed and dried at 60 °C overnight. Finally, the product was heated to 450 °C at 3 °C/min and calcined in air for 4 h. Samples were denoted as Al-SBA-15-T, T was crystallization temperature.

The LPG feedstock (from Beijing Huayuan Gas Co. Ltd), an analogue of the LPG by-product of industrial MTP unit, contained propane 18.1 wt%, n-butane 13.1 wt%, i-butane 40.9 wt%, n-butylene 13.0 wt% and i-butylene 14.9 wt%. A flow of the gaseous mixture of the preevaporated LPG and Ar (1:2.5, molar ratio) was fed into a minitype fixed-bed reactor (length 400 mm, o.d. 12 mm and i.d. 8 mm) operating at 600 °C and 0.1 MPa. The flow rate of the gas mixture was controlled using a mass flow controller (MFC). The reactor was loaded 1 g catalyst (20–40 mesh) and the contact time W/F was kept constant at 15 g·h·mol−1. The reactor was heated up to 600 °C at a rate of 20 °C/min in an Ar flow (20 mL/min) and then the gaseous mixture of the LPG and Ar was switched in. The reaction was proceeded time-on-stream (TOS) of 20 h for each test. The effluent gas was analyzed at an interval of 1.5 h on an online gas chromatograph (GC-9560) equipped with a HPAl2O3/KCl capillary column (50 m × 0.535 mm × 15 μm) and a flame ionization detector (FID). H2 was analyzed via TDX column (1 m) on a

2.1.2. HZSM-5 It was synthesized using recrystallization of Al-SBA-15 referring to literature [26]. An aqueous solution of TPAOH of 2 g (tetrapropylammonium hydroxide, 20 wt%, Beijing Xingfu Fine Chem. Res. 2

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Fig. 1. Pore distribution of synthetic Al-SBA-15.

GC TCD. The sampling was kept at constant temperature of 180 °C. Results were calculated as: Component conversion: C %−C % Conv. (Ci )% = in C %out × 100 , where Cin% and Cout% were carbon in molar fraction of the component before and after the reaction, respectively. Total feed conversion: Total conv.% = Σ [Conv. i×C % (Ci) × Ci%(feed)]. Product selectivity: Sel. (Ci )% = ∑ i×Ci % × 100 ; i

Ci % =

Ai × f Mi ∑Ai ×f Mi

× 100 , where i was carbon number; Ci%, carbon molar

fraction of the component after the reaction; Ai, GC peak area of the component; fMi, relative molar calibration factor of the component. 3. Results and discussion Al-SBA-15 was firstly synthesized via crystallization at 100 °C and 140 °C, respectively, which were calcined at 500 °C in air for 4 h. Three well-resolved XRD peaks (Fig. S1) by the (1 0 0), (1 1 0) and (2 0 0) crystal planes and the hysteresis loops of type IV (Fig. S2) indicate highly ordered 2D hexagonal mesostructure [28]. The peaks of Al-SBA15-140 slightly shifted toward low diffraction angles and the intensities were increased, implying its higher crystallinity. Furthermore, compared with Al-SBA-15-100, Al-SBA-15-140 had lower surface area (Table S1) due to its larger mesopore and mesopore volume (Fig. 1). The mesopore size and the most probable mesopore size for both were ca. 5–16 nm, 6–11 nm and 10.478 nm, 8.136 nm, respectively. Al-SBA15 synthesized at variable crystallization temperatures shows distrinct mesoporous sizes and framework stability [28]. At higher temperature, the hydrophilicity of block PEO (polyethylene oxide) of P123 decreases while its hydrophobicity increases, which enlarges the mesopores; besides, the framework density and the degree of the pore wall polymerization tend to increase [29]. That is, higher temperature enhances the crystallization of Al-SBA-15. Al-SBA-15-140 has more mesopores with the pore wall of high stability.

Fig. 2. XRD patterns of HZSM-5 samples.

framework density and stability. The transformation of Al-SBA-15 to HZSM-5 via in-situ recrystallization is a gradual process of self-assembly. During recrystallization, the transformation of the dense, stable Al-SBA-15-140 is slow and the pore wall is gradually consumed, which control throughout the formation of HZSM-5. The formed HZSM-5 composes the wall of the final mesopores. The regular mesopores of AlSBA-15-140 maintain more durable, resulting in the aggregation of nano HZSM-5. By contrast, due to loose pore wall and relatively fast transformation, Al-SBA-15-100 is destructed easily, the domain-confinement to the HZSM-5 growth is weak. The resulting HZSM-5 likely collapses and does not aggregate. The transformation of Al-SBA-15 pore walls to HZSM-5 referring to literature [26] is schemed in Fig. 4. In other word, the quality of Al-SBA-15 affects the pore property of synthetic HZ-5. By contrast, merchandise HZ-5-M shaped in smooth hexagonal and twins crystallite of 1–2 μm. Three HZ-5 samples show hysteresis loops in their N2 adsorptiondesorption isotherms (Fig. S3). The hysteresis loop of type IV in P/P0 of 0.80–0.98 and adsorption volume of HZ-5-M were relatively small, ascribed to small amount of stacked mesopores among the polycrystalline particles. The type IV hysteresis loops (in P/P0 of 0.80–0.98 and 0.40–0.98, respectively) and adsorption volumes of HZ-5-100 and HZ-5-140 were obvious, implying that both have mesopores and HZ-5140 has smaller mesopores. As shown in Fig. 5, three samples had micropore of 0.55–0.60 nm. HZ-5 samples contained a large number of mesopores whereas HZ-5-M had macropores of 35–500 nm by the crystalline accumulation. The mesopores of HZ-5-100 were 6–50 nm, also by the crystalline accumulation. HZ-5-140 had mesopores in two ranges: 2.5–10 nm and 10–50 nm, by the intercrystalline pore of the aggregates and the crystalline accumulation, respectively. During the recrystallization, the framework of Al-SBA-15 possibly collapses, the formed nano HZSM-5 monomer would accumulate, resulting in stacked

3.1. Property of synthetic HZSM-5 HZSM-5 was synthesized from recrystallization of Al-SBA-15. The crystallization products were calcined at 450 °C in air for 4 h, leaving the template remove uncomplete, in order to confine the crystalline particle size of ZSM-5 growth. Five characteristic diffraction peaks at 2θ of 7.8°, 8.8°, 23.1°, 23.9°, and 24.3°, 45.6° (Fig. 2) appeared for synthetic and merchandise HZ-5 samples, ascribed to the MFI structure. HZ-5-140 had higher peak intensities than HZ-5-100, although the peak intensities of both were lower than that of HZ-5-M. HZ-5-140 showed relatively high crystallinity because Al-SBA-15-140 with more mesopores favors the approaching of TPAOH during recrystallization. Synthetic HZ-5-140 and HZ-5-100 showed crystallite size of ca.100 nm and ca.200 nm, respectively, but different morphology (Fig. 3). HZ-5-140 was crystallite aggregates whereas HZ-5-100 separate crystallite particles. As aforementioned, Al-SBA-15-140 has high 3

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Fig. 3. SEM (a) and TEM (b) images of HZSM-5 samples.

mesopores below 10 nm in nano HZSM-5 aggregates are mainly from the residual template inside Al-SBA-15. As shown in Table 1, synthetic HZSM-5 had slightly high micropore volume, much higher mesopore volume and the total pore volume than merchandise HZSM-5. The mesoporosity was increased by nearly 80%. Compared with HZ-5-100, the surface area of HZ-5-140 increased slightly, the most probable mesopore size was obviously decreased. These are attributed to the confinement of stable Al-SBA-15-140 mesostructure and the transformation of Al-SBA-15-140 into nano HZSM-5 aggregates. Two NH3 desorption peaks were observed for three samples, corresponding to weak acid and strong acid sites, respectively (Fig. S4). The former is assigned to terminal SiOH and SiOH in the channel, the later to the stronger B acid and L acid sites (bridged hydroxyl groups) [30–32]. Moreover, these two peaks of synthetic HZSM-5 samples shifted toward low temperatures because of their distinctions in acidity with merchandise HZ-5-M. The weak acid amount, strong acid amount and acid density decreased in order: merchandise HZ-5-M > synthetic

Fig. 4. Scheme for transformation of Al-SBA-15 to HZSM-5.

mesopores. Due to the framework of Al-SBA-15 becomes more stable as the crystallization temperature rises, the interconnected mesopores of 10 nm or less form in HZ-5-140 sample, as shown in Fig. 5. The

Fig. 5. Pore distribution of HZSM-5 samples. 4

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Table 1 Pore parameters of HZSM-5. Samples

SBET (m2/g)

Vmicro (cm3/g)

Vmeso (cm3/g)

Vtotal (cm3/g)

Dmicro (nm)

Dmeso (nm)

Mesoporosity (%)

HZ-5-140 HZ-5-100 HZ-5-M

342 333 239

0.142 0.142 0.113

0.321 0.374 0.070

0.463 0.516 0.183

0.576 0.567 0.592

8.161 23.901 2.174

69.33 72.48 38.25

Table 2 Pyridine-IR acidity of HZSM-5. Samples

HZ-5-140 HZ-5-100 HZ-5-M

Total acid (mmol/g)

Weak acid (mmol/g)

Med-strong acid (mmol/g)

Strong acid (mmol/g)

B

L

B/L

B

L

B/L

B

L

B/L

B

L

B/L

0.075 0.167 0.321

0.071 0.093 0.137

1.06 1.80 2.34

0.036 0.062 0.036

0.027 0.059 0.102

1.33 1.05 0.35

0.020 0.039 0.116

0.020 0.005 0.009

1.00 7.80 12.88

0.019 0.066 0.169

0.024 0.029 0.026

0.79 2.27 6.50

HZ-5-100 > synthetic HZ-5-140 (Table S2). It is inferred that the ratios of SiO2/Al2O3 of synthetic HZ-5-T samples are higher than that (60) of merchandise HZ-5-M. The pyridine adsorption IR spectra of HZSM-5 samples are shown in Fig. S5. The acid amounts of the three types of HZSM-5 samples are shown in Table 2. The total B acid and L acid amounts of synthetic samples were less than those of merchandize HZ-5-M. From HZ-5-140 to HZ-5-M, the B/L ratio of the weak acid was decreased whereas the B/ L ratios of the medium-strong acid and the strong acid were gradually increased. For HZ-5-140, the B/L ratio decreased in order: weak acid > medium-strong acid > strong acid. For HZ-5-100 and HZ-5-M, the B/L ratios of the medium-strong acid were higher than those of both their weak acid and strong acid. HZ-5-140 had the lowest total acid and B acid amounts, with the highest B/L ratio (1.33) of weak acid and relatively more weak L acid sites. HZ-5-M had the highest total acid, B acid amounts, and the B/L ratio (12.88) of the medium-strong acid. HZ5-100 had in-between B acid and L acid amounts. This is consistent in the NH3-TPD results.

Table 4 Product distribution for LPG transformation.* Samples

HZ-5-140 HZ-5-100 HZ-5-M

Selectivity in hydrocarbons (%) CH4

C2H6

C2H4

C3H6

2-C4H8

C5+

C2]+C3]

P/E

9.20 12.50 16.97

2.88 3.79 6.57

24.23 23.44 24.29

44.11 24.89 14.93

6.23 2.81 1.27

13.35 32.57 35.97

68.34 48.33 39.22

1.82 1.06 0.61

* < 13% × H2 was detected; C5+: C5-C9 hydrocarbons.

conversions of propane and n-butane on synthetic HZSM-5 dropped by about 50%. As shown in Table 4, on synthetic HZSM-5 higher selectivity toward C3H6, 2-C4H8, C2] + C3] and the ratio of propylene to ethylene (P/E) whereas lower selectivity for C5+ and CH4 were achieved than those on merchandise HZ-5-M. The selectivity of C2H4 and C2H6 on three HZSM-5 samples was comparative. Except for HCs products, H2 was produced during the reaction. The selectivity of C2] + C3] remained stable during 20 h reaction (Fig. 6) and HZ-5-140 functioned best in terms of the formation of C3H6. As the sample characterization indicated, merchandise HZSM-5 is microporous micron crystallite, containing less mesopores (mesoporosity 38%). It displays the highest acid density as well as total acid amount, which is mainly from medium-strong and strong B acid sites. On HZ-5-M the conversion of LPG is the highest because generally B acid sites of zeolites promote the activation of HCs. Meanwhile, C5+ HCs and CH4 are highly produced. The production of propylene and

3.2. Catalytic activity The activation and conversion of light HCs is complex processes. Alkanes can be activated on B acid and L acid sites [11]. The transformation of LPG (mixed C3, C4 HCs) on HZSM-5 toward propylene and ethylene may occur through carbocation reaction mechanism, mainly involving dehydrogenation/cracking of alkanes and dimerization-βcracking of C4 alkenes. Weaker B acid sites favor the dimerizationcleavage of C4 alkenes [23]. But adjacent B acid sites enhance dehydrogenation-cyclization-aromatization side reactions of olefin dimerization products rather than olefin selectivity [33]. Furthermore, long residence time of reaction molecules augments secondary reactions of primary olefin products [20,23]. The total conversions of LPG on three HZSM-5 samples all reached above 58% (Table 3). The five C3 and C4 components were simultaneously transformed. The conversion increased in order: on merchandise HZ-5-M > on synthetic HZ-5-100 > on HZ-5-140. The Table 3 Transformation of LPG on HZSM-5. Samples

HZ-5-140 HZ-5-100 HZ-5-M

Conversion (%) Total

C3H8

i-C4H10

n-C4H10

n-C4H8

i-C4H8

58.78 78.78 85.66

21.61 25.45 47.01

59.64 96.91 94.35

52.39 69.02 95.57

86.43 92.15 96.22

83.05 90.71 95.12

Fig. 6. Selectivity of C2] + C3] with time-on-stream. 5

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ethylene is low. Large crystal size, high diffusion resistance and strong acidity cause adverse side reactions of the primary products, such as hydride transfer, aromatization and so on [19,21]. By contrast, synthetic HZ-5-140 is nanosize crystallite with more interconnected and unimpeded mesopores (mesoporosity 69%), as well as shortened micropore channels. Its total acid amount, acid density and B acid amount are the lowest. Also, the microporous channels in HZ-5-140 are shortened through fabricating the uniform, interconnected mesopores. Secondary reactions of primary olefins products would be reduced during the LPG transformation as a result of faster diffusion of the product molecules. The formation of C3H6 and 2-C4H8 is dramatically improved whereas the C5+ HCs and methane products fall significantly despite at the expense of the total conversion of LPG. Nano HZ-5-100 crystallite has high but stacked mesopores and also short micropore channels. It has in-between total acid, B acid and L acid amounts, and exhibits better catalytic activity toward LPG transformation than HZ-5M in terms of the formation of C3H6.

[9]

[10]

[11]

[12]

[13]

[14]

[15]

4. Conclusions [16]

Nano HZSM-5 with regular meso-micropores and appropriate acidity promotes the catalytic transformation of LPG toward light olefins. Synthetic sample of HZSM-5-140 showed 100 nm crystallite aggregates, mesoporosity of 69%, acid density of 0.757 μmol/m2, and low Brönsted acid amount as well as low total acid amount, on which 58% LPG was converted at 600 °C with 68.3% selectivity for propylene and ethylene. The ratio of propylene to ethylene was 1.8. The formation of C5+ hydrocarbons and methane significantly dropped off. Side reactions of the primary products are suppressed owing to the improved diffusion by well-connected mesopore and shortened micropore, as well as weak acidity of synthetic HZSM-5. The acidity control for this mesomicroporous HZSM-5 preparation needs to further establish.

[17] [18]

[19]

[20]

[21]

Acknowledgments [22]

Financial support from the National Natural Science Fund of China (21563024), the East-West cooperation project of Ningxia Key R & D Plan (2017BY063) and project for Chemical Engineering & Technology Discipline (NXYLXK2017A04) are acknowledged.

[23]

Appendix A. Supplementary data

[24]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.06.023.

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