Silicalite-1 Derivational Desilication-Recrystallization to Prepare

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Kinetics, Catalysis, and Reaction Engineering

Silicalite-1 Derivational Desilication-Recrystallization to Prepare Hollow Nano-ZSM-5 and Highly Mesoporous Micro-ZSM-5 Catalyst for Methanol to Hydrocarbons Zhe Ma, Tingjun Fu, Yujie Wang, Juan Shao, Qian Ma, Chunmei Zhang, Liping Cui, and Zhong Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03858 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019

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Industrial & Engineering Chemistry Research

Silicalite-1 Derivational Desilication-Recrystallization to Prepare Hollow Nano-ZSM-5 and Highly Mesoporous Micro-ZSM-5 Catalyst for Methanol to Hydrocarbons Zhe Ma, Tingjun Fu, Yujie Wang, Juan Shao, Qian Ma, Chunmei Zhang, Liping Cui, Zhong Li*

Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China

 Corresponding author: Prof. Zhong Li and Dr. Tingjun Fu

Tel/Fax: +86 03516018526 E-mail: [email protected]

[email protected] 1

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT:

Mesoporous

ZSM-5

zeolites

were

directly

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synthesized

by

desilication-recrystallization of silicalite-1 with NaAlO2 as Al source and TPAOH as template. The fabrication mechanism of mesopores was studied based on different crystal size of silicalite-1 and liquid-to-solid ratio. This revealed that TPA+ played protective role in the controllable desilication while Na+ induced the production of mesopores during the recrystallization. When silicalite-1 with crystal size of 200 nm was treated under liquid-to-solid of 10 mL/g, hollow structured nano-ZSM-5 with many mesopores (23 nm) in shell was produced and its external surface area reached 121 m2 g-1. In comparison, homogeneous mesopores (15 nm) were introduced into bulk of micro-ZSM-5 under a higher liquid-to-solid of 30 mL/g, providing a maximal external surface area of 171 m2 g-1. The catalytic lifetime reached 192 h in MTH reaction at a WHSV of 4.7 h-1, 4 times greater than that of the micro-ZSM-5 synthesized via traditional hydrothermal synthesis. KEYWORDS: silicalite-1, desilication-recrystallization, ZSM-5 zeolite, high mesoporosity, methanol to hydrocarbons 1. INTRODUCTION As methanol can be abundantly produced from coal, the conversion of methanol to hydrocarbons (MTH) over acidic zeolite has become a significant alternative route for production of hydrocarbons independent of crude oil.1-3 It is well known that the pore structure and acidity of acidic zeolites, like beta, SAPO-34 and ZSM-5, can largely control the composition and distribution of MTH products.

4-7

Among these

diverse catalysts employed in MTH, ZSM-5 zeolite is widely used for its well-defined microporous structure, high hydrothermal stability and strong acidity, and could present high selectivity to gasoline, olefin and aromatics.8-10 However, the microporous structure of ZSM-5 zeolite results in mass transfer limitations, which seriously restricts its catalytic application.11,

12

The narrow micropores of ZSM-5

zeolites usually leads to slow diffusion of bulky reactants and products. In MTH reaction, the mass transfer limitations would reduce the access of the methanol to the acid sites, resulting in a low catalyst utilization and formation of carbon precursors. 2 ACS Paragon Plus Environment

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The formed carbon precursors are difficult to be diffused outside and can turn to coke to block the micropore channels or cover the active sites in the catalysts, leading to the rapid deactivation of ZSM-5 zeolite and a loss in selectivity. The rapid deactivation of ZSM-5 zeolite is a crucial issue during the methanol to hydrocarbons process.13-15 In this context, many researches introduced mesopores into the bulk phase of crystal to improve the diffusion properties of reactant molecules in micropores. The introduction of mesopores into micropores system could shorten the diffusion path length, allowing the reactants to reach the active sites more easily and also facilitating the products to diffuse out of the catalyst through the introduced mesopores.15-17 Usually, the ―bottom-up‖ or primary synthesis and ―top-down‖ or post-synthesis treatment are two synthetic strategies of hierarchical structure zeolites.

12 18-21

The

―bottom-up‖ approach is a direct syntheses method using hard templates such as carbon black or carbon fibers, as well as soft templates of surfactants or silanes. However, both hard and soft templates are expensive and the materials are difficult to be handled, limiting their commercial application. In contrast, the ―top-down‖ route, a post-synthetic strategy including desilication with alkali and dealumination with acid, is more popular in application. In addition, when compared to the dealumination with acid, the desilication using alkali is easier to form mesopores, and hence is widely used to prepare hierarchical ZSM-5 zeolites.21-24 Nevertheless, there are some problems associated with alkali treatment desilication approach. Firstly, during the alkali treatment, part of the ZSM-5 zeolite crystallinity is sacrificed due to the silica leaching from zeolite crystals. Ogura et al. 25 found that after treatment of 0.2 M NaOH solution, the resulting ZSM-5 lost a great majority of crystal (40%) when compared to the parent ZSM-5. Secondly, the fabricated mesopores are easily destroyed due to the variation of alkali treatment condition. More importantly, the fabricated mesopores are irregular and uneven due to the non-uniform desilication of ZSM-5 zeolite. Groen et al.

26

showed that Al is rich

in exterior while poor in interior part of the ZSM-5. The existence of Al distribution 3 ACS Paragon Plus Environment


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gradient caused a better tolerant to alkaline treatment for exterior than the interior part of ZSM-5. In another study, Groen et al. 27 demonstrated that the presence of high Al content (Si/Al50) caused excessive and unselective silicon dissolution, and eventually resulted in relatively large pores. Although a lot of researches have adjusted alkali treatment condition or SiO2/Al2O3 ratio of zeolite during the alkaline treatment to optimize mesoporous structure of ZSM-5, it is still a challenge to prepare the ZSM-5 catalysts with high mesoporosity. Many studies tried to find an effective method to increase the mesoporosity and crystallinity of ZSM-5 zeolites after their alkaline treatment. Dai et al.

28

synthesized

hollow zeolite nanocubes by desilication and recrystallization of silicalite-1 using tetrapropylammonium hydroxide (TPAOH) solution. They demonstrated that controlled silicon leaching from OH- could help introduce large voids to silicalite-1 crystals and the recrystallization of silicon with TPA+ assisted formation of intact thin shells. Following this work, we aspired to add NaAlO2 as aluminum source and TPAOH solution as organic alkaline structure-directing agent during the desilication and recrystallization process of parent silicalite-1, in which the inner silicon species were leached out with OH- and then the leached silicon species were recrystallized with aluminum source under the template of TPA+. This silicalite-1 derivational desilication-recrystallization was designed for the introduction of mesopores and the formation of silica-aluminum framework, aiming to produce highly mesoporous ZSM-5 zeolites. Here, we report a simple and efficient route to prepare ZSM-5 catalysts with strong

acidity

and

high

mesoporosity

through

silicalite-1

derivational

desilication-recrystallization. The prepared zeolites were characterized by XRD, TEM, N2 physisorption, NH3-TPD, ICP-AES，29Si and

27

Al MAS NMR. The process of

recrystallization, especially the mechanism of mesopores fabrication based on different particle sizes of silicalite (S-1) and diverse liquid-to-solid ratio, was studied. 4 ACS Paragon Plus Environment

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For S-1 with crystal size of 200 nm, after the desilication-recrystallization, a clearly hollow structured nano-ZSM-5 with rich mesopores in its shell was obtained. This was due to the co-effect of preferential desilication from Na+ and suppressive desilication from TPA+ on the surface.

38

The amount of total acid sites for

nano-ZSN-5 was found to be 0.70 mmol g-1. Whereas, when S-1 with crystal size of 900 nm was used as parent, the hollow structure was not observed under the same treating conditions and the amount of acid sites was only 0.11 mmol g-1. Interestingly, when the liquid-to-solid ratio was increased, abundant mesopores were found to distribute homogeneously in the bulk phase and the acidity of prepared catalysts gradually increased. These highly mesoporous ZSM-5 catalysts were applied to MTH reaction. They exhibited higher activity and longer lifetime than ZSM-5 catalysts synthesized via traditional hydrothermal synthesis, because of the larger external surface, stronger acid strength and greater coke content. Particularly, for micro-ZSM-5 catalyst derived with a liquid-to-solid ratio of 30 mL/g, it possessed a maximal external surface area of 171 m2 g-1 and a proper acid amount of 0.65 mmol g-1, presenting the greatest coke content of 0.30 g g -1cat. Its lifetime reached 192 h which was 4 times greater than that of the micro-ZSM-5 prepared via conventional method. 2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation Firstly, two silicalite-1 (S-1) zeolites with different particle sizes were prepared by a previously reported method.28 Typically, the silicalite-1 with average crystal size of 200 nm (named as S200) was synthesized with a molar composition of 1 SiO2:0.27 TPAOH:8 EtOH:23 H2O using tetraethyl orthosilicate (TEOS, Kermal, 98.0%), tetrapropylammonium hydroxide (TPAOH, 25% aqueous solution) and ethyl alcohol (EtOH, 99.7%) as raw materials. The prepared mother gel was aged at 308 K for 5 h followed by hydrothermal treatment in an autoclave at 443 K for 72 h under stirring conditions. Then, the product was collected by centrifugation and washed thoroughly using deionized water. After drying overnight at 373 K, the S200 was obtained by 5 ACS Paragon Plus Environment


calcination in air at 823 K for 6 h. For comparison, silicalite-1 with average crystal size of 900 nm (named as S900) was prepared using the same method and the molar composition of gel was 1SiO2:0.17TPAOH:4EtOH: 46H2O. To prepare nano-ZSM-5 zeolite, 5 g nano-sized S200 was added into the 50 ml, 0.3M TPAOH solution with a liquid-to-solid ratio of 10mL/g. After that, 0.345 g NaAlO2 was added to the solution. The molar composition of final gel was 60 SiO2: 1 Al2O3: 11 TPAOH: 1 Na2O: 23 H2O. After being mixed adequately, the gel was transferred to Teflon container and hydrothermally treated under stirring conditions at 443 K for 72 h. The solid product was separated by filtration and dried overnight at 373 K. Finally, the TPA+ was removed by calcination in air at 823K for 6 h and Na-form sample was obtained. To obtain H-form zeolite, the ion exchange of Na-form sample was performed with 0.8 M NH4NO3 solution at 353 K for three times. Then, the sample was again recovered by centrifugation, dried overnight at 373 K and calcined at 823 K for 6 h to produce H-ZSM-5, abbreviated as HS200-10. Micro-ZSM-5 zeolite named HS900-10 was also synthesized based on the desilication-recrystallization of S-900 with same raw material ratio and preparation process as HS200-10. In order to optimize the structure of micro-ZSM-5, the other two micro-ZSM-5 zeolites were obtained by changing the liquid-to-solid ratio from 10 mL/g to 30 and 50 mL/g and were abbreviated as HS900-30 and HS900-50, respectively. For comparison, two ZSM-5 zeolites possessing same SiO2/Al2O3 ratio (SiO2/Al2O3=60) and similar crystal size with HS200-10 and HS900-30, were synthesized via traditional hydrothermal synthesis as reported previously.29 The crystal size of ZSM-5 was controlled by adjusting the H2O/Si ratio in the gel mixture. The H2O/Si ratio of 40 and 110 in crystallization solution were chosen to prepare nano-ZSM5 and micro-ZSM-5 zeolites and were denoted as HZ200 and HZ900, respectively. The detailed procedure can be found in supporting information. The above synthesis conditions are summarized in Table S1.

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2.2. Catalyst Characterization Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max 2500 diffractometer using Cu K radiation (40 kV, 40 mA, =1.54439 Å) in the 2θ range of 5-50°with a step size of 0.02 and at a scanning speed of 8(°)/min. Transmission electron microscopy (TEM) was used to measure the morphology and crystal size of the samples, through a JEM-2100F instrument operating at 200 kV. Before the measurements, the samples were dipped on the porous copper mesh coated with carbon film after dispersed into the ethanol solutions. The nitrogen physisorption isotherms of samples at 77 K were obtained on a Beishide 3H-2000PS2 instrument. Before each measurement, the sample was degassed under vacuum conditions at 523 K for 4 h. The total surface area was calculated by Brunauer-Emmett-Teller (BET) equation, while the t-plot method was applied to the calculation of external surface area, micropore area and micropore volume. The total pore volume was obtained from the amount of N2 adsorbed at p/po=0.99. The Barrett–Joyner–Halenda (BJH) method was used to calculate the pore size distribution. Temperature programmed desorption of ammonia (NH3-TPD) was performed to measure the acid amount on a Micromeritics Autochem II 2920 instrument. Prior to the measurement, 100 mg of sample was heated to 823 K for 90 min in He stream. In order to achieve the sufficient NH3 adsorption, the sample was put in a mixed gas flow of 85% He and 15 mol% NH3 after cooling to 393 K. After the adsorption, the sample was flushed in He flow for 1 h at 393 K to remove NH3 adsorbed weakly. Subsequently, the temperature was increased at a rate of 10 K min-1 to 823 K and the NH3-TPD profile was obtained. Inductively coupled plasma-atomic emission spectrometer (ICP-AES) was adopted to determine the Si and Al contents of the samples using an Autoscan16 TJA instrument. Typically, 100 mg of sample was suspended in the mixed solutions of hydrofluoric acid (40 wt%) and concentrated nitric acid (65 wt%) at 333 K, whose dosage were 0.5 mL and 2 mL, respectively. 7 ACS Paragon Plus Environment


Solid state

29

Si and

Page 8 of 49

27

Al MAS NMR were recorded on a 600 MHz Bruker

Avance III NMR spectrometer equipped with two radio frequency channels to investigate the coordination of Si and Al in the framework for samples. The thermal gravimetric (TG) curves of deactivated samples were measured with a Perkin-Elmer Pyris 6 instrument under an air flow with a rate of 10 K/min from 303 to 1073 K. 2.3. Catalytic Performance The performance of the ZSM-5 catalysts in MTH reaction was evaluated using a high pressure fixed-bed reactor. Typically, 0.5 g of sieved catalyst was mixed with 2.5 g of silica sand and then loaded into the middle of a stainless steel tubular reactor. Before the MTH reaction, the catalyst was activated at 450 oC for 100 min under flowing nitrogen (35 mL min-1). Then, liquid methanol was gasified at 240 oC in a preheater at a flow rate of 0.05 mL min-1, and meanwhile introduced N2 at a flow rate of 35 mL min-1 into the reactor. The MTH reaction was performed under a set of constant conditions of temperature 400 oC, pressure 1 MPa and weight hourly speed velocity (WHSV) 4.7 h-1. The liquid hydrocarbons were obtained after the condensation of reaction products while the un-condensable vapor and gas were gathered with a gas sampling bag. These two kinds of products were analyzed through a SHIMADZU GC-2014C connected to FID detector and ZHONGKEHUIFEN GC-7820 equipped with TCD and FID detectors, respectively. From the GC results, we could calculate and obtain the methanol conversion and product selectivity. 3. RESULTS AND DISCUSSION 3.1. Pore Architectures and Morphology of ZSM-5 Zeolites. 3.1.1. Crystallinity analysis The powder XRD patterns of synthesized samples are shown in Fig. 1. Although all

ZSM-5

zeolites

presented

typical

MFI

structures

upon

desilication-recrystallization,30 the peak intensity of their XRD patterns decreased in different degrees compared to that of parent S-1 samples. In order to obtain the 8 ACS Paragon Plus Environment

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relative crystallinity of ZSM-5 zeolites, we selected sample S200 and S900 as the references and regarded their crystallinity as 100%. By calculating the ratio of the three peak areas in ranges of 22.5-25o of ZSM-5 samples relative to those of S-1 samples, the relative crystallinity of ZSM-5 was obtained, as listed in Table 1.31 Obviously, the relative crystallinity of prepared ZSM-5 zeolites were less than 100%, revealing a desilication process, during which the inner silicon species were etched by OH- and were leached out of crystal. Under the liquid-to-solid ratio of 10 mL/g, the relative crystallinity of obtained HS200-10 was 95.3% while the relative crystallinity of HS900-10 was only 73.6%. With the liquid-to-solid ratio increased to 30 and 50 mL/g, the obtained HS900-30 and HS900-50 exhibited an improved relative crystallinity of 95.2 and 96.2%, respectively. This illustrated a more difficult desilication-recrystallization process for S900 than S200 under the same liquid-to-solid ratio of 10 ml/g, and by increasing liquid-to-solid ratio S900 could achieve sufficient desilication-recrystallization. In addition, HZ200 and HZ900, prepared via traditional hydrothermal synthesis, possessed the same crystallinity as S200 and S900.

Figure 1. Powder XRD patterns of parent S-1 and prepared ZSM-5 zeolites. Table 1. Relative crystallinity of parent S-1 and prepared ZSM-5 zeolites. Samples a

Crystallinity (%)

S200

HS200-10

S900

HS900-10

HS900-30

HS900-50

100

95.3

100

73.6

95.2

96.2



3.1.2. Morphology analysis Representative TEM images of these samples are shown in Fig. 2. The S200 had regular coffin-like crystal morphology and the crystal size was about 200 nm (Fig. 2a). After the desilication-recrystallization of S200, a hollow structure with rich mesopores (23 nm) in its shell could be observed in HS200-10 (Fig. 2b). During the desilication-recrystallization process, TPA+ adsorbed on the outside surface of S-1 and partly protected it from being etching32 while the inner silicon species were leached out constantly by OH-. The leached silicon species quickly diffused out of the nano-sized S200 and then recrystallized with Al source under the template of TPA+ on the external surface of S200 to form silica-aluminum skeleton structure of ZSM-5 zeolite. During the recrystallization process, there were some Na+ absorbed on the outer surface of S200 and the new formed ZSM-5, where the silicon species cannot recrystallize due to the lack of TPA+. Finally, mesopores were successfully created in the shell of hollow structured ZSM-5 due to the skeletal-growth inhibition role of Na+ that adsorbed on the zeolite surface.33 Comparatively, for the desilication-recrystallization of S-1 with a crystal size of 900 nm (S900, Fig. 2c), many uneven mesopores ranging from several to tens of nanometers were observed in the bulk phase of HS900-10 (Fig. 2d, d’) when the liquid-to-solid ratio was also 10 mL/g. The large crystal size of S900 made it difficult for the leached silicon species to diffuse out of the crystal, resulting that the leached silicon species were recrystallized inside the crystals with the TPA+ and Al source entering through the channel. With the liquid-to-solid ratio increased to 30 mL/g, more OH- and TPA+ were available for S900 and the etching of silicon species and recrystallization were more likely to happen. This can be evidenced by the increased crystallinity of HS900-30 compared with HS900-10. Particularly, the increased OHpromoted the desilication process and created more mesopores in the bulk phase of HS900-30 (Fig. 2e) when compared to HS900-10. Moreover, the increased TPA+ provided more protection for near silicon species and promoted the formation of uniform mesopores. Finally, mesopores with pore size of about 15 nm were evenly 10 ACS Paragon Plus Environment

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distributed in the bulk phase of HS900-30 (Fig. 2e, e’). However, with the liquid-to-solid ratio further increased to 50 mL/g, excessive desilication was observed inside the crystal and obvious macroporous structure was formed in HS900-50 (Fig. 2f, f’), although the recrystallization was still proceeded. In addition, the TEM images of HZ200 and HZ900 synthesized by traditional hydrothermal synthesis are presented in Fig. S1. Compared with the conventional HZ200 and HZ900 zeolites, HS200-10 and HZ900-30 showed higher mesoporosity.

Figure 2. Typical TEM images of parent S-1 and prepared ZSM-5 zeolites. a) S200; b) HS200-10; c) S900; d, d’) HS900-10; e, e’) HS900-30; f, f’) HS900-50 3.1.3. Textural properties analysis The nitrogen physisorption isotherms of S-1 and ZSM-5 samples are shown in Fig. 3. Both S-1 (S200, S900) and conventional ZSM-5 (HZ200, HZ900) samples showed type I isotherms with a steep N2 uptake at low relative pressures, indicating their microporous structure according to the IUPAC classifications.34 After desilication-recrystallization of S200 and S900, the synthesized ZSM-5 zeolites 11 ACS Paragon Plus Environment


presented type IV isotherms with a remarkable H4-shaped hysteresis loop, showing the existence of mesopore structures.34 It is noteworthy that under the same liquid-to-solid ratio of 10mL/g, the hysteresis loop of HS200-10 was larger than that of HS900-10. This indicated that the internal silicon of S200 crystal was easier to be leached out. It also presented the formation of highly mesoporous structures, which was consistent with the TEM results. During the desilication-recrystallization process of S900, when the liquid-to-solid ratio was increased to 30mL/g, the hysteresis loop of the obtained HS900-30 enlarged obviously. However, with the liquid-to-solid ratio further increased to 50 mL/g, the hysteresis loop of the prepared HS900-50 was contracted. This demonstrated that the amount of OH- and TPA+ had a great influence on the pore forming process of S900 and moderate amount of OH- and TPA+ could promote the formation of highly mesoporous ZSM-5 zeolite (Fig. 2e).

Figure 3. N2 physisorption isotherms of parent S-1 and prepared ZSM-5 zeolites. The textural properties of these samples obtained from the isotherms are listed in Table 2. In general, both external surface area (Sext) and mesoporous volume (Vmeso) for ZSM-5 zeolites synthesized by desilication-recrystallization of S-1 were greater than those for S-1. This further demonstrated the introduction of mesopores into the bulk phase of zeolites during the desilication-recrystallization, which was consistent with the TEM results. The Sext and Vmeso of S200 were only 54 m2 g-1 and 0.26 cm3 g-1, respectively. After desilication-recrystallization of S200, the obtained HS200-10 12 ACS Paragon Plus Environment

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possessed a hollow structure with rich mesopores in shell and the Sext and Vmeso were increased to 121 m2 g-1 and 0.46 cm3 g-1, respectively. Whereas, the Sext and Vmeso of HS900-10 were only 89 m2 g-1 and 0.30 cm3 g-1, respectively. With the liquid-to-solid ratio increased to 30 mL/g, more mesopores were created in the bulk phase of crystal (Figure. 2e) and the Sext and Vmeso of HS900-30 reached 171 m2 g-1 and 0.39 cm3 g-1, respectively. However, with the liquid-to-solid ratio further increased to 50 mL/g, the Sext and Vmeso of the obtained HS900-50 were obviously reduced to 93 m2 g-1 and 0.23 cm3 g-1, respectively. This was due to the excessive desilication and structure damage of S900 caused by the large amount of OH-. In addition, the Sext and Vmeso of HZ200 and HZ900 were only 60 m2 g-1 and 0.32 cm3 g-1, 42 m2 g-1 and 0.08 cm3 g-1, respectively. All these supported the formation of highly mesoporous ZSM-5 zeolites by desilication-recrystallization of silicalite-1. Table 2. The textural properties of parent S-1 and fresh and coked ZSM-5 zeolites. Surface area (m2·g-1) Crystal Samples size SBET Smicro Sext (nm) fresh coked fresh coked fresh coked

Pore volume (cm3·g-1） Vtotal fresh coked

Vmeso fresh

coked

200

435

—

381

—

54

—

0.45

—

0.26

—

HS200-10 200

426

136

305

83

121

53

0.61

0.34

0.46

0.30

S200 HZ200

200

378

79

318

42

60

37

0.48

0.16

0.32

0.14

S900

900

412

—

369

—

43

—

0.27

—

0.09

—

HS900-10 900

293

200

204

131

89

69

0.41

0.33

0.30

0.27

HS900-30 900

442

132

271

85

171

47

0.52

0.17

0.39

0.13

HS900-50 900

419

116

326

66

93

50

0.38

0.22

0.23

0.19

393

105

351

69

42

36

0.24

0.11

0.08

0.08

HZ900

900

.

Note: SBET = BET surface area, calculated by the BET model; Smicro = micropore area, determined by the t-plot method; Sext = external surface area, determined by the t-plot method; Vtotal = total pore volume, determined from the adsorbed amount at p/p0 = 0.99; Vmicro = micropore volume, calculated by the t-plot method; Vmeso = Vtotal - Vmicro

The pore size distribution of S-1 and ZSM-5 samples is shown in Fig. 4. Clearly, for S200 and S900, low pore size distribution peak at ~2 nm can be observed, indicating some small mesopores (~2 nm) existed in S200 and S900. After desilication-recrystallization of S200, an obvious pore size distribution within the scope of 20-30 nm was observed for HS200-10, confirming that the mesopores were indeed introduced into the shell, combined with TEM results. The HS900-10 13 ACS Paragon Plus Environment


exhibited similar pore size distribution with HS200-10. However, compared with HS200-10, the peak intensity of pore size distribution (centered at 20-30 nm) was reduced, indicating a lower mesoporosity for HS900-10. Besides, HS900-10 also showed a sharp pore size distribution peak centered at ~2 nm, demonstrating the generation of many small mesopores under corresponding treating conditions. With the liquid-to-solid ratio increased to 30 mL/g, the increased amount of OH- and TPA+ caused the fusion of small mesopores and the sufficient recrystallization. This promoted the fabrication of obvious mesopores within the scope of 3-20 nm, and a pore size distribution centered at 10-20 nm was observed. Whereas, when the liquid-to-solid ratio was increased to 50 mL/g, the formed mesopores in bulk phase were destroyed and connected with each other, leading to obviously decreased distribution peak intensity at 10-20 nm for obtained HS900-50 sample. These variations of the pore size distribution were consistent with the TEM results. In addition, HZ200 and HZ900 both showed a pore size distribution centered at ~2 nm, which was similar with S200 and S900.

Figure 4. BJH pore size distribution of parent S-1 and prepared ZSM-5 zeolites. 3.2. Mechanism of Controlling the position of Desilication-Recrystallization. Based on the above analysis of pore structure, the fabrication mechanism of mesopore during the desilication-recrystallization of nano-sized S200 and micro-sized S900 are illustrated in Scheme 1. When nano-sized S200 (Scheme 1a) is used as 14 ACS Paragon Plus Environment

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parent, the added TPA+ and Na+ are adsorbed competitively on the S200 surface via electrostatic interactions (Scheme 1b). During the desilication process, TPA+ with large molecular size cannot get into the interior of S200 through micropores and partly protect the surface from being etching. Therefore, the silicon species near Na+ are removed preferentially and then mesopores are formed on the surface.32 The generated mesopores provide channels for the quick leaching of the inner silicon species out of the S200 crystal (Scheme 1c). This was followed by recrystallization of leached silicon species with Al source on the outer surface under the template of TPA+ to form the skeleton structure of ZSM-5. This continuous desilication-recrystallization leads to the production of hollow structured nano-ZSM-5. During the recrystallization process, there are some Na+ adsorbed on the outer surface of S200 and the new formed ZSM-5, where the silicon species cannot recrystallize due to the lack of TPA+.28 Finally, mesopores are created in the shell of hollow structured nano-ZSM-5 due to the inhibition role of Na+ in skeletal growth (Scheme 1c, d). Comparatively, when micro-sized S900 is used as parent (Scheme 1a’), the competitive adsorption also occurs on the surface (Scheme 1b’). Because of the protective effect from TPA+, the silicon species near Na+ will be removed preferentially, and thus formed mesopores can be used as channels on the surface. Compared with S200, the larger crystal size of S900 made it difficult for the leached silicon species to diffuse out of the crystal. On the contrary, the TPA+ and Al source can easily enter inside the crystal through the formed channels mentioned above, and then recrystallize with the silicate oligomers to form the skeleton structure of ZSM-5 zeolite (Scheme 1c’). After the sufficient desilication-recrystallization of S900 with liquid-to-solid of 30 mL/g, abundant mesopores are found to distribute homogeneously in the bulk phase of micro-ZSM-5 (Scheme 1d’). In conclusion, it was found that the co-effect of preferential desilication effect from Na+ and protected desilication effect from TPA+ determines the location of desilication. Whereas the position of recrystallization is determined by the diffusion path of silicalite-1, which ultimately affects the mesopore structure of prepared 15 ACS Paragon Plus Environment


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ZSM-5.

Scheme 1. Mechanism of controlling the position of desilication-recrystallization. 3.3. Acid Property of ZSM-5 Zeolites. 3.3.1. NH3-TPD and ICP-AES analysis The acid property of the prepared nano-ZSM-5 and micro-ZSM-5 were influenced by the desilication-recrystallization degrees of S200 and S900, respectively. The NH3-TPD profiles of the prepared ZSM-5 zeolites are presented in Fig. 5. As can be seen, all samples exhibited two well resolved desorption peaks, which is typical for ZSM-5 zeolite. The peak at 150-320 oC is known as the low temperature desorption peaks corresponding to weak acid sites, while the high temperature desorption peaks at 320-550 oC corresponding to strong acid sites.35, 36 By Gaussian fitting of two desorption peaks, we could obtain three peaks and assign them to weak, medium and strong acid sites, respectively. The acid strength could be reflected by the peak temperature while the peak areas would be associated with the acid amount.

16

As

shown in Table 3, HS200-10 possessed a total acid amount of 0.70 mmol g-1, much higher than 0.11 mmol g-1 for HS900-10, indicating that the Al source was not adequately embedded in the framework structure for S900 during the recrystallization process. This suggested that under the same liquid-to-solid of 10 mL/g, desilication-recrystallization process was easier to be proceeded for nano-sized S200 than micro-sized S900. When the liquid-to-solid ratio was increased to 30mL/g, the 16 ACS Paragon Plus Environment

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total acid amount of the prepared HS900-30 substantially increased to 0.65 mmol g-1, which was close to 0.70 mmol g-1 of HS200-10. Interestingly, with the liquid-to-solid ratio further increased to 50 mL/g, the total acid amount of the prepared HS900-50 reached 0.86 mmol g-1, which was larger than that of HS900-30 and HS200-10. In addition, the strength of acid sites of HS900-50 was also higher than that of HS900-30 and HS200-10. The above changes of acid amount could be reflected by the variation of SiO2/Al2O3 ratios determined by the ICP-AES results. As shown in Table 3, the SiO2/Al2O3 ratios of the three obtained micro-ZSM-5 decreased from 39.9 to 33.4 and 31.4, with the liquid-to-solid ratio increased from 10 to 30 and 50 mL/g, respectively. This indicated that the desilication-recrystallization process of micro-sized S900 requested more amount of OH- and TPA+ than that of nano-sized S200, so that enough Al source would be embedded in the framework structure and the strong acidity was obtained. Compared with HZ200 and HZ900 synthesized via traditional hydrothermal synthesis,

HS200-10,

HS900-30

and

HS900-50

prepared

by

sufficient

desilication-recrystallization exhibited larger amount of acid sites. This demonstrated that the silicalite-1 derivational desilication-recrystallization method is more favorable for the formation of silicon-aluminum skeleton structure of ZSM-5, namely the generation of acidic sites. This silicalite-1 derivational disilication-recrystallization method has a distinct advantage over the traditional hydrothermal strategy on the production of ZSM-5 with large acid amount and high acid strength, and thus can be applied to some reactions demanding for a strong acid catalyst. This can be reflected by the difference of SiO2/Al2O3 ratio between these two kinds of ZSM-5. The SiO2/Al2O3

ratios

of

HS200-10

and

HS900-30

obtained

by

sufficient

desilication-recrystallization were 41.7 and 33.4, respectively, much less than that of HZ200 and HZ900 (56.9 and 63.0, respectively).



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Figure 5. NH3-TPD profile of prepared ZSM-5 zeolites. Table 3. The acid site distributions and the SiO2/Al2O3 ratios of ZSM-5 zeolites. Samples

SiO2/Al2O3 a

HS200-10 HZ200 HS900-10 HS900-30 HS900-50 HZ900

41.7 56.9 39.9 33.4 31.4 63.0

Acid Amount(mmol g-1) Total b

Weak c

Moderate c

Strong c

0.70 0.41 0.11 0.65 0.86 0.41

0.28 0.16 0.06 0.29 0.45 0.13

0.18 0.13 0.03 0.12 0.16 0.06

0.24 0.12 0.02 0.24 0.25 0.22

Note: a Calculated by ICP-AES results; b Obtained via NH3-TPD analysis; c Obtained by Gaussian fitting analysis.

3.3.2. 29Si MAS NMR analysis As shown in Fig. 6, the deconvolution of

29

Si spectrum may lead to four peaks

centered at -102 ppm (SiOH), -106 ppm (Si(3Si,1Al)), and -113 and -116 (Si(4Si,0Al)).37 For S200，the content of Si(4Si,0Al) and SiOH sites were 91.84% and 8.16% respectively (Fig. 6a). After desilication-recrystallization of S200, these two values for HS200-10 sharply decreased to 83.67% and 1.09%, with an observation of Si(3Si,1Al) sites around -106 ppm whose content was 15.24% (Fig. 6b). This indicated that the Si species in Si(4Si,0Al) and SiOH were easily leached out and recrystallized with Al source under the template of TPA+ to form Si(3Si,1Al) sites of ZSM-5 zeolite during the desilication-recrystallization process of S200. Compared 18 ACS Paragon Plus Environment

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with S900, the content of SiOH sites for micro-HS900-10 decreased from 6.62 to 2.73%，with an observation of Si(3Si,1Al) sites around -106 ppm whose content was only 1.64% (Fig. 6d). When the liquid-to-solid ration was increased to 30 mL/g, the content of Si(3Si,1Al) sites for obtained HS900-30 rapidly increased to 14.32%, with a decrease in SiOH and Si(4Si,0Al) amount (Fig. 6e). With the liquid-to-solid ration further increasing to 50 mL/g, the prepared HS900-50 showed a higher Si(3Si,1Al) sites amount of 16.84% and a lower Si(4Si,0Al) sites amount of 82.23% (Fig. 6f), illustrating that more Si species in Si(4Si,0Al) and SiOH were leached out and formed more Si(3Si,1Al) sites with increasing the liquid-to-solid ratio. This demonstrated that the strong acidity and enough TPA+ played a decisive role in the production of ZSM-5 by desilication-recrystallization, which was consistent with NH3-TPD and ICP-AES results.



Figure 6. 29Si MAS NMR spectra of parent S-1 and prepared ZSM-5 zeolites. 3.3.3. 27Al MAS NMR analysis The 27Al NMR spectra of all prepared ZSM-5 zeolites showed two peaks as can be seen in Fig. 7. The one at 53 ppm is categorized as tetrahedral aluminum, corresponding to framework Al (FAl). The other one at around -0.5 ppm is known as octahedral aluminum, corresponding to extra-framework Al (EFAl).38 For HS200-10, the content of framework Al was 87% and the content of extra-framework Al was 13%, demonstrating that most of the aluminum was embedded in the skeleton structure while a small portion existed in the form of non-skeleton aluminum during the desilication-recrystallization process of S200. Interestingly, for micro-ZSM-5 20 ACS Paragon Plus Environment

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catalysts, the content of framework Al increased from 72 to 85% while the content of extra-framework Al decreased from 28 to 15% with the liquid-to-solid ratio increasing from 10 to 50 mL/g, indicating that more aluminum was embedded in the skeleton structure. As seen in Fig. 7, the content of framework Al and extra-framework Al for HZ200 were the same as those for HS200-10 and the contents of two kind Al for HZ900 were similar with those for HS900-50. This further illustrated the feasibility of the desilication-recrystallization method for preparing ZSM-5. More importantly, compared with nano-sized S200, the desilication-recrystallization process of micro-sized S900 required more OH- and TPA+ to ensure that enough Al source would be embedded in the framework structure and similar content of framework Al would be obtained.

Figure 7. 27Al NAS NMR spectra of prepared ZSM-5 catalysts. 3.4. Catalytic Performance of ZSM-5 Catalysts for the MTH Reaction. The catalytic performance of prepared ZSM-5 catalysts in MTH was evaluated in a fixed-bed reactor under the conditions of temperature 400 oC, pressure 1MPa and WHSV 4.7 h-1. The methanol conversion with time on stream for these ZSM-5 samples are shown in Fig. 8a. The HS200-10 possessed a good stability and exhibited complete conversion of methanol (100%) within the 110 h of reaction and declined slowly later. In contrast, the methanol conversion of HS900-10 obtained under the 21 ACS Paragon Plus Environment


same conditions decreased from 97 to 84% within only 25 h. Larger amount of acid sites of HS200-10 (0.70 mmol g-1) than HS900-10 (0.11 mmol g-1) was responsible for the higher methanol conversion for HS200-10 than that of HS900-10. Interestingly, with the liquid-to-solid ratio increased to 30mL/g, the acid amount of obtained HS900-30 enhanced to 0.65 mmol g-1, methanol conversion improved obviously and maintained at 100% during the first 110 h reaction period. However, with the liquid-to-solid ratio further increased to 50 mL/g, the methanol conversion of prepared HS900-50 was still declined after 110 h and didn’t show better stability even if its acid amount increased to 0.86 mmol g-1, which was attributed to its reduced Vmeso (0.23 cm3 g-1) and Sext (93 m2 g-1). As seen in Fig. 8a, the methanol conversion of HS200-10, HS900-30 and HS900-50 synthesized by desilication-recrystallization were apparently higher than those of HZ200 and HZ900 produced by traditional hydrothermal synthesis. This confirmed that the increased acidity and improved mesoporosity promoted the efficient conversion of methanol. The liquid hydrocarbons yield versus time on stream for prepared catalysts are shown in Fig. 8b. As can be seen, the liquid hydrocarbons yield of all samples went up at the beginning, which was different from the methanol conversion that rapidly reached 100% (Fig. 8a). This demonstrated that in the conversion process of methanol, the growth of carbon chain needed some time to produce multi-carbon number hydrocarbons. After the yield had maintained at a stable stage, the coke covered the active sites and blocked the channel, resulting in the reduction of liquid hydrocarbons yield. By contrast, the catalytic performances of these catalysts were quite different and more precise data are summarized in Table 4. For HS200-10, the total liquid hydrocarbons production and the lifetime were 122 g g-1cat and 180 h, respectively. However, due to the low methanol conversion (Fig. 8a), no liquid hydrocarbon was received for HS900-10. For micro-ZSM-5, when the liquid-to-solid ratio was increased to 30 mL/g, the catalytic performance of the obtained HS900-30 improved significantly. Particularly, its total liquid hydrocarbons production and lifetime reached 141 g g-1cat and 192 h, respectively, even higher than those of HS200-10, 22 ACS Paragon Plus Environment

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which was due to the improved acidity (0.65 mmol g-1) and external surface area (171 m2 g-1). However, when the liquid-to-solid ratio was further increased to 50 mL/g, although the acidity of the derived HS900-50 increased to 0.86 mmol g-1, its low external surface area (93 m2 g-1) caused by the damage of mesoporous structure was adverse to product diffusion, its total production of liquid hydrocarbons and the lifetime reduced to 71 g g-1cat and 156 h, respectively, lower than those of HS200-10. On the whole, the ZSM-5 catalysts prepared by desilication-recrystallization of S-1 exhibited better catalytic performance than those ZSM-5 catalysts synthesized via traditional hydrothermal synthesis, which was due to the obviously increased acidity and improved mesoporosity. Especially for HS900-30, its total production of liquid hydrocarbons and the lifetime reached 141 g g-1cat and 192 h, respectively, which is 4 times greater than those of HZ900.

Figure 8. (a) Methanol conversion versus the time on stream. (b) The liquid hydrocarbons yield versus time on stream.



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Table 4. Catalytic performances of prepared ZSM-5 zeolites for MTH reaction. Liquid hydrocarbons

Lifetime (h)

Sample

Total production a (g g-1cat)

Yield of the stable stage (%)

HS200-10

122

19.6

180

HZ200

87

16.3

168

HS900-10

—

—

—

HS900-30

141

23.5

192

HS900-50

71

13.7

156

HZ900

25

17.6

48

a

Note: The total liquid hydrocarbons production per gram sample.

The deactivation of ZSM-5 catalyst is mainly due to the formation of coke and its deposition on the catalyst, which subsequently covers the acid sites and blocks the channels.39, 40 To some extent, the difference of coke content among various catalysts could explain the difference of the catalytic performance discussed above. Coke content of the spent catalysts were studied by TG analysis (Fig. 9a). The relationship between lifetime and coke content is shown in Fig. 9b, in which we can see that the lifetime is proportional to the coke content for various samples. For HS200-10, the production of hollow structure with rich mesopores in its shell (Fig. 2b) accelerates the outward diffusion of interior coke precursor, resulting in a mass of coke formed on the outer surface. Therefore, the coke content of HS200-10 reached 0.30 g g -1cat and the lifetime was 180 h, higher than those of HZ200. However, for HS900-10 with low mesoporosity and poor acidity, liquid hydrocarbon products cannot be collected because of its low catalytic activity and its coke content of 0.03 g g-1cat. Interestingly, when the liquid-to-solid ratio was increased to 30 mL/g, the coke content of HS900-30 reached the highest value of 0.30 g g -1cat and the lifetime was the longest (192 h), even higher than those of HS200-10. This suggested its outstanding diffusivity because of the formation of abundant homogeneous mesopores in bulk phase. However, when the liquid-to-solid ratio was further increased to 50 mL/g, the damaged mesoporous structure of HS900-50 impeded the outward diffusion of internal coke precursor, leading to a lower coke content of 0.19 g g-1cat and a shorter lifetime of 156 h. It was noteworthy that the coke content and lifetime of HS900-30 24 ACS Paragon Plus Environment

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and HS900-50 were much higher than those of HZ900, which was due to the obviously improved mesoporosity.

Figure 9. (a) TG curves of the spent ZSM-5 catalysts. (b) The relationship between catalytic longevity and external surface area. The different coke contents discussed above could be reflected by the pore structure change after reaction to some extent. The N2 physisorption isotherms of the deactivated catalysts are shown in Fig. S2. During the MTH reaction, the formed coke deposited inside the channels and partly blocked the channels, resulting in the remarkable decrease of SBET and SMicro. In order to study the amount of coke deposition in mesopores and micropores of different ZSM-5 zeolites, ―Rmeso‖ and ―Rmicro‖ were defined as the decrement of mesopores and micropores, respectively and the data are summarized in Table 5. As can be seen, all samples exhibited higher value of Rmicro than that of Rmeso, indicating that most of coke deposited in micropores and a few in mesopores for all zeolites. On the whole, the value of Rmeso was mainly determined by the diffusivity and acidity of ZSM-5 catalysts. For HS200-10, although the produced abundant mesopores would accelerate the outward diffusion of interior coke precursor, its strong acidity could easily cause the coke deposition before they diffused to external, resulting in a small value of Rmeso (0.35) for HS200-10. However, for HS900-10, the acidity was poor (0.11 mmol g-1) and mesopores were seldom used and the value of Rmeso was only 0.10. Interestingly, although the acidity of HS900-30 increased obviously, the coke formation was mainly affected by its abundant bulk mesoporous. After introducing abundant mesopores into the bulk of HS900-30, more coke was deposited on the mesopore surface due to the remarkably improved 25 ACS Paragon Plus Environment


Page 26 of 49

diffusion property that promoted the diffusion of coke precursors, and a higher Rmeso value of 0.67 was presented. However, for HS900-50, the strongest acidity (0.86 mmol g-1) caused the rapid coke formation before they diffused to external. Besides, the diffusion of coke precursors was slowed down due to its decreased external surface area (93 m2 g-1). Hence, the coke formation on HS900-50 was not easy as HS900-30 and mainly happened in micropores, resulting in a small Rmeso value of 0.17. Table 5. The comparison between fresh and deactivated ZSM-5 catalysts on the textural properties. Samples

∆Vmeso (cm3 g-1) a

Rmeso=∆Vmeso /Vmeso

∆Vmicro (cm3 g-1) a

Rmicro =∆Vmicro /Vmicro

HS200-10 HZ200 HS900-10 HS900-30 HS900-50 HZ900

0.16 0.18 0.03 0.26 0.04 0

0.35 0.56 0.10 0.67 0.17 0

0.11 0.14 0.05 0.09 0.12 0.13

0.73 0.85 0.45 0.69 0.80 0.81

Note: a Calculated through the data in Table 2.

In MTH reaction, liquid hydrocarbons was important products, which mainly distributed in C4-C10 hydrocarbons. In this work, the liquid hydrocarbon products collected at the reaction time of 24 h was analyzed and the obtained liquid hydrocarbons product distribution are shown in Table S2 and Fig. 10. The differences of pore structures and acidity between various ZSM-5 led to the significant differences in carbon number distribution.41,

42

The light hydrocarbons (C4-C6)

selectivity for HZ200 was 21%. Compared with HZ200, HS200-10 possessed larger external surface (121 m2 g-1) which could provide shorter diffusion paths for molecules to diffuse out. This was unfavorable for the carbon chain growth but beneficial to the generation of light hydrocarbon. However, the selectivity of light hydrocarbons for HS200-10 was only 22%, similar to that of HZ200. This was attributed to its strong acidity (Table 3) that was favorable for the dehydrogenation and cyclization reaction and thus formed large amount of heavy products. For HS900-30, which possessed similar acid amount and acid strength with HS200-10, its selectivity of light hydrocarbons reached 36%, much higher than that of HS200-10, 26 ACS Paragon Plus Environment

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which was due to the larger external surface (171 m2 g-1) than that of HS200-10 (121 m2 g-1). Whereas, the small external surface area (93 m2 g-1) of HS900-50 caused by the damage of mesoporous structure reduced the diffusion of molecule, which was beneficial to the growth of carbon chain. Moreover, the increased acid amount and acid strength of HS900-50 (Table 3, Fig. 5) also facilitated the production of heavy hydrocarbons. As a consequence, the light hydrocarbons selectivity for HS900-50 reduced to 24%. Interestingly, the 28% selectivity of light hydrocarbons for HZ900 was between those of HS900-30 and HS900-50, which was due to the least amount and lowest strength of acid sites for HZ900 among these micro-ZSM-5 samples. In addition, compared with HZ900, the larger external surface of HZ200 was beneficial to the generation of light hydrocarbons, but the stronger acid strength of HZ200 caused the low light hydrocarbons selectivity. Based on the above discussion, it can be concluded that in MTH reaction, the excellent acid property of ZSM-5 including large acid amount and high acid strength was beneficial to the production of heavy hydrocarbons (C7-C11). Furthermore, the diffusion paths of ZSM-5 also limited the growth of carbon chain, and the improved diffusion properties could promote the production of light products (C4-C6).

Figure 10. The distribution of carbon number for liquid hydrocarbon over various ZSM-5 zeolites, n refers to carbon number. The liquid hydrocarbons product composition of above catalysts was also analyzed and the results are shown in Table S3 and Fig. 11. For all catalysts, liquid 27 ACS Paragon Plus Environment


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hydrocarbons product consists of aromatics (A), isoparaffins (iP), olefins (O), normal paraffins (nP) and naphthenes (N), among which isoparaffins and aromatics were the primary products. The liquid hydrocarbons products distribution was greatly affected by the variation of acidity and textural properties in different ZSM-5 catalysts. Large space and appropriate acidity could facilitate the hydrogen transfer reactions which was beneficial to the production of isoparaffins.43,

44

Whereas，the production of

aromatics was mainly determined by the strength of acid sites. High acid strength was favorable for the dehydrogenation and cyclization.44 For HS200-10, the selectivity of aromatics was 64.90%, higher than that of HZ200, which was mainly ascribed to its larger acid amount and higher acid strength than HZ200. Compared with HS200-10, the selectivity of aromatics for HS900-30 decreased to 49.11%, but the selectivity of isoparaffins increased to 33.13%. This was because the weak acidity of HS900-30 (Fig. 5 and Table 3) was not conductive to the production of aromatics and the higher mesoporosity than HS200-10 (Table 2) facilitated the hydrogen transfer reactions. Interestingly, for HS900-50 obtained with the liquid-to-solid ratio of 50 mL/g, the isoparaffins selectivity was reduced to 22.49% and the aromatics selectivity was increased to 63.82%, which was due to its lower diffusion property caused by excessive

desilication

and

enhanced

acid

property

caused

by

sufficient

recrystallization.. The weaker acidity of HZ900 compared with HS900-30 and HS900-50 led to a low aromatics selectivity of 41.37%. In addition, when compared HZ200 with HZ900, it could be found that although they had the same acid amount, the stronger acid strength of HZ200 than HZ900 (Fig. 5) led to the higher aromatics selectivity.


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Figure 11. The liquid hydrocarbons product distribution over various ZSM-5 zeolites, A, iP, O, nP and N refers to aromatics, isoparaffins, olefins, normal paraffins and naphthenes, respectively. 4. CONCLUSIONS In this study, two types of silicalite-1 with different crystal size were used as parent for desilication-recrystallization adding organic alkaline structure-directing agent and Al source to prepare mesoporous ZSM-5 catalysts. We found that during the desilication-recrystallization of nano-sized S200, TPA+ were partly adsorbed on the surface of S200 and protected the nearby silicon species from being etching by OH-, while the internal silicon species were easily dissolved and leached out. The leached silicon species were then recrystallized with Al source on the external surface of S200 under the template of TPA+ to form silicon-aluminum skeleton structure of ZSM-5 zeolite even under a low liquid-to-solid ratio of 10 mL/g. The existence of Na+ in the synthesis system impeded the recrystallization of leached silicon species and in turn led to the introduction of mesopores inside the shell. Finally, a hollow structured nano-ZSM-5 (HS200-10) with rich mesopores (23 nm) inside its shell was produced. It possessed a strong acidity of 0.70 mmol g-1 and a large external surface area of 121 m2 g-1. However, for micro-sized S900, its large crystal size of 900 nm led to the difficulty for the leached silicon species to diffuse out of the crystals and recrystallized inside the crystals with the TPA+ and Al source entering through the channel. Compared with S200, increasing the liquid-to-solid ratio to 30 mL/g could 29 ACS Paragon Plus Environment


promote the sufficient desilication-recrystallization of S900. Abundant mesopores (15 nm) were found to distribute homogeneously in the bulk phase of obtained micro-ZSM-5 (HS900-30). Its acid amount and external surface area were 0.65 mmol g-1 and 171 m2 g-1, respectively. This indicated that compared with S200, more TPA+ and OH- were required for S900 to ensure that enough Al source would be embedded in the framework structure to obtain strong acidity and form abundant mesopores. The large external surface area of above obtained ZSM-5 zeolites endowed them with excellent MTH catalytic activity, especially the stability. This is due to that the large external surface promotes the diffusion of coke precursors from micropores to external surface and increases the coke capacity of ZSM-5. Among all of the catalysts, HS900-30 with homogeneous mesoporous in bulk phase presented the greatest coke content of 0.30 g g -1cat and its lifetime reached 192 h which was 4 times greater than that of the micro-ZSM-5 prepared via conventional method. Besides, the comparative study of the effects of acid sites on product selectivity revealed that high strength of acid sites was favorable for cyclization and dehydrogenation reactions and increased the selectivity of aromatics. SUPPORTING INFORMATION Catalyst preparation procedure for HZ200 and HZ900, Notation of the samples and synthesis conditions, Typical TEM images of HZ200 and HZ900 zeolites, N2 physisorption isotherms of coked ZSM-5 zeolites, The distribution of carbon number for liquid hydrocarbon over various ZSM-5 zeolites, and The liquid hydrocarbon product distribution over various ZSM-5 zeolites. AUTHOR INFORMATION Corresponding Author * Prof. Zhong Li. E-mail: [email protected] * Dr. Tingjun Fu. E-mail: [email protected] ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (No.21606160) and the Natural Science Foundation for Shanxi Province 30 ACS Paragon Plus Environment

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(No.201701D221039, No.201701D121025).



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on-stream stability of fluid catalytic cracking (FCC) gasoline hydro-upgrading catalyst: Post-treatment of HZSM-5 zeolite by combined steaming and citric acid leaching. Catal. Today 2007, 125, 185-191.



For Table of Contents Only


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Figure captions: Figure 1. Powder XRD patterns of parent S-1 and prepared ZSM-5 zeolites. Figure 2. Typical TEM images of parent S-1 and prepared ZSM-5 zeolites: a) S200; b) HS200-10; c) S900; d, d’) S900-10; e, e’) HS900-30; f, f’) HS900-50. Figure 3. N2 physisorption isotherms of parent S-1 and prepared ZSM-5 zeolites. Figure 4. BJH pore size distributions of parent S-1 and prepared ZSM-5 zeolites. Scheme 1. Mechanism of controlling the position of desilication-recrystallization. Figure 5. NH3-TPD profile of prepared ZSM-5 zeolites. Figure 6. 29Si NAS NMR spectra of parent S-1 and prepared ZSM-5 zeolites. Figure 7. 27Al NAS NMR spectra of prepared ZSM-5 zeolites. Figure 8. (a) Conversion of methanol versus the time on stream. (b) The liquid hydrocarbons yield versus time on stream. Figure 9. (a) TG curves of the spent ZSM-5 catalysts. (b) The relationship between catalytic longevity and external surface area. Figure 10. The distribution of carbon number for liquid hydrocarbon over various ZSM-5 zeolites, n refers to carbon number. Figure 11. The liquid hydrocarbon product distribution over various ZSM-5 zeolites, A, iP, O, nP and N refers to aromatics, isoparaffins, olefins, normal paraffins and naphthenes, respectively.



Figure 1.


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Figure 2.



Figure 3.


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Figure 4.



Scheme 1.


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Figure 5.



Figure 6.


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Figure 7.



Figure 8.


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Figure 9.



Figure 10.


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Figure 11.


Silicalite-1 Derivational Desilication-Recrystallization to Prepare

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