Epitaxial Growth of Layered-Bulky ZSM-5 Hybrid Catalysts for the

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Materials and Interfaces

Epitaxial Growth of Layered-Bulky ZSM-5 Hybrid Catalysts for the Methanol-to-Propylene Process Huiyong Chen, Wenjin Shang, Chenbiao Yang, Baoyu Liu, Chengyi Dai, Jianbo Zhang, Qingqing Hao, Ming Sun, and Xiaoxun Ma Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05472 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

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Industrial & Engineering Chemistry Research

Epitaxial Growth of Layered-Bulky ZSM-5 Hybrid Catalysts for the Methanol-to-Propylene Process Huiyong Chen* † ‡ §, Wenjin Shang†, Chenbiao Yang†, Baoyu Liu* ∥, Chengyi Dai † ‡ §, Jianbo Zhang † ‡ §, Qingqing Hao † ‡ §, Ming Sun † ‡ §, and Xiaoxun Ma † ‡ § † School

‡

of Chemical Engineering, Northwest University, Xi'an, Shaanxi 710069, China

Chemical Engineering Research Center of the Ministry of Education for Advanced Use

Technology of Shanbei Energy, Shaanxi Research Center of Engineering Technology for Clean Coal Conversion, Northwest University, Xi'an, Shaanxi 710069, China §

International Science & Technology Cooperation Base of MOST for Clean Utilization of

Hydrocarbon Resources, Collaborative Innovation Center for Development of Energy and Chemical Industry in Northern Shaanxi, Xi'an, Shaanxi 710069, China ∥

School of Chemical Engineering and Light Industry, Guangdong University of Technology,

Guangzhou, Guangdong 510006, China

_____________ Corresponding Author * E-mail: [email protected] (H. Chen) [email protected] (B. Liu) ACS Paragon Plus Environment

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ABSTRACT.

The hierarchical layered-bulky ZSM-5 (LBZ5) hybrid composites with a core-shell structure were synthesized by epitaxial growth of layered ZSM-5 nanosheets over bulky ZSM-5 crystals. The systematic balance of zeolitic microporosity and interlayered mesoporosity was achieved by elaborate regulation of the ratio between layered phase and bulky phase, resulting in a series of layered-bulky ZSM-5 hybrid composites with the controllable thickness of the layered shell ranged from 98 to 307 nm. The evaluation of catalytic performance in the methanol-to-propylene (MTP) reaction indicated the LBZ5 materials to be candidated catalysts for the MTP process with prolonged catalytic lifetime, superior coke toleration and enhanced propylene selectivity comparing with the parent bulky ZSM-5 (BZ5) and layered ZSM-5 (LZ5), due to their wellretained zeolitic framework, hierarchical meso-/microporosity by the layered-bulky composite structure and appropriate strong acidity.

KEYWORDS. Hierarchical zeolite, ZSM-5, Epitaxial growth, Core-shell structure, Methanol-topropylene

ACS Paragon Plus Environment

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1. INTRODUCTION The methanol-to-propylene (MTP) process has attracted extensive attention in both fundamental study and industrial application due to the upsurge in demand of propylene and high pressure of petroleum exhaustion, and the production of propylene from methanol conversion has been considered as one of the efficient solutions to the manufacture of propylene on a large scale with alternative of petroleum route since the methanol can be produced from coal or natural gas.

1-4

Theoretically, most of the acidic zeolites can be used as catalysts in the process of

methanol conversion to hydrocarbons, ZSM-11,

10

ZSM-22

11

and Beta,

12

5

such as SAPO-5,

6

SAPO-34,

7

SSZ-13,

8

ZSM-5,

9

and SAPO-34 and ZSM-5 are the most successful attempts

and have been successfully used in the large-scale manufacture. Normally, the porosity and acidity of the zeolite catalysts can be considered as the two main factors leading to the product selectivity, lifetime and stability. The SAPO-34 catalyst, which possesses the 8-membered ring window (0.38 nm × 0.38 nm) of CHA framework, shows a high selectivity (> 80 %) of light olefins (ethylene and propylene) in the reaction of methanol conversion as the pore size is close to the kinetic diameters of ethylene and propylene, leading to a methanol-to-olefins (MTO) process.

13

Meanwhile, the selectivity of ethylene in such MTO process is normally higher than

that of propylene. On the contrary, once apply the ZSM-5 catalyst, which has a threedimensional 10-membered ring micropore system (0.53 nm × 0.56 nm straight channels and 0.51 nm × 0.55 nm intersecting zigzag channels) of MFI framework, into the methanol conversion process, propylene will become the predominant product, leading to a high propylene-toethylene (P / E) ratio, due to the pore size selectivity. 14-17 However, restricted by its inherent microporosity of mass transport resistance, premature deactivation usually happens to the conversional zeolite catalyst with sole micropores due to the


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pore block by formed coke during the reaction, which strongly limited the practical application of zeolites in catalysis. 18-21 One of the effective solutions is to introduce larger pores (mesopore or macropore) within or between zeolite crystals, and the fabricated mesoporous or hierarchical porous zeolites will not only retain their native micropores to realize shape selectivity, but also provide an additional pore system to relieve the mass transport resistance. 22-24 Numerous efforts have been achieve to synthesize zeolites with hierarchical multi-porosity, such as post treatment, 25

recrystallization,

template method,

26

31.

assembly of zeolite nanocrystals,

27

hard template method

28-30

and soft

Among all these synthesis strategies, the soft template method by using

well-designed bifunctional ammonium surfactants composed of micropore directing multiammonium heads and mesopore directing alkyl tails as the hierarchical structure-direct agent has been significantly developed by Ryoo’s group,

32

and a series of hierarchical zeolites with

nanosheet-like morphology (2D zeolites) including ZSM-5 have been successfully synthesized. 33

The ZSM-5 nanosheets show the crystal size of unit cell thickness (~2 nm) towards the b-

direction and interlayered mesopores in size of several nanometers after structure pillaring. 34 The hierarchical zeolites have been widely used as catalysts in various reactions including the MTP process. Junjie Li et al. 35 reported the used of hierarchical ZSM-5 with tunable acidity by a simple base leaching method as a catalyst in the MTP reaction, and the hierarchical ZSM-5 catalyst showed significant improvement in catalytic lifetime and propylene selectivity comparing with the conventional sole microporous ZSM-5. Tian-Lu Cui et al.

16

reported the

facile synthesis of mesoporous ZSM-5 without the addition of mesoporogen, following with the catalytic application in the MTP reaction with long lifetime stability. Quanyi Wang et al.

36

reported the use of mesoporous ZSM-5 by soft template method in the reaction of methanol reaction, and the improved catalytic performance was ascribed to the good mesopore-micropore


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interconnectivity. Zhang's group reported a series of modified hierarchical ZSM-5/SiC catalysts and their catalytic application in the MTP reaction in recent years, demonstrating not only the superior performance but also industrial application advantage

37-40.

Considerable efforts in

catalytic application have demonstrated the superiority of 2D hierarchical zeolites.

41-44

Meanwhile, the lamellar ZSM-5 catalysts have shown significant improvements reflected in prolonged catalytic lifetime and enhanced selectivity of predominant products in the reaction of methanol conversion to hydrocarbons.

45, 46

However, the zeolite nanosheets generally have

limited crystallinity but overabundant silanol groups distributed on their larger external surfaces comparing with three-dimensional bulky crystals (3D zeolites).

32

Moreover, the structural

fragility coming from the thin layered crystal domains normally reduce rigidity and stability. Therefore, the development of 2D-3D hybrid zeolites with tunable balance of zeolitic microporosity and additional mesoporosity is highly desired. One synthesis strategy to obtain the 2D-3D hybrid zeolites has been achieved via a dualtemplate synthesis method, in which the traditional small molecule quaternary ammonium was used to direct the formation of the zeolitic framework, assisted with the large molecule bifunctional ammonium surfactant to lead the coherent assembly of zeolite nanosheets, resulting in the concurrent existence of layered structure and bulky phase in single particle of zeolites. 47-50 It should be noted that the 2D-3D hybrid zeolites obtained by the dual-template synthesis method generally show a composite structure with unclear boundary between layered phase and bulky phase, and nubilous competition between the two templates exists during the crystallization process. Another strategy to make 2D-3D hybrid zeolites is realized by epitaxial growth or overgrown of lamellar zeolite nansheets on the zeolite bulky crystals, resulting in unique coreshell structures constructed by bulk zeolite cores and lamellar zeolite shells. 51 And several kinds


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of combinations, such as lamellar silicalite-1 / bulky silicalite-1, lamellar silicalite-1 / bulky ZSM-5, lamellar silicalite-1 / bulky Beta, have been reported about synthesis design, structure characterization and application in both membrane separation and catalytic reaction.

51, 52

However, there is no catalytic application of such 2D-3D hybrid zeolites in the reaction of methanol conversion to our knowledge. In the present study, we demonstrated not only the epitaxial growth of layered-bulky ZSM-5 hybrid composites with the core-shell structure, but also elaborate regulation of 2D / 3D ratios to balance of hierarchical meso-/microporosity, aimed at the structure design of new-type catalysts for the methanol-to-propylene (MTP) process.

2. EXPERIMENTAL SECTION 2.1 Materials Tetraethyl orthosilicate (TEOS, 99 wt%) was purchased from Aladdin. Aluminum isopropoxide (Al(OPri)3, 98 wt%) was purchased from Alfa Aesar. Sodium hydroxide (96 wt%) was purchased from Zhengzhou Nepal chemical reagent plant. Sulfuric acid (H2SO4, 95 wt% ~ 98 wt%) was purchased from Sichuan Xilong chemical reagent plant. All reagents were invoked as purchased without any further purification. Deionized (DI) water was produced by MasterS15UV (Shanghai Hitech Instruments Co., Ltd) ultrapure water system in the lab. The conventional bulky ZSM-5 was supplied by Nankai University Catalyst Co., Ltd. The diquaternary ammonium surfactant [C22H45-N+(CH3)2-C6H12-N+(CH3)2-C6H13)Br2, C22-6-6Br2] was synthesized based on the method reported by Ryoo et al., 32 following with the operation of ion exchange to convert Br- to OH- by using anion exchange resin (Amberlite IRN-78, Alfa Aesar). The content of the obtained C22-6-6(OH)2 surfactant solution after ion exchange was 0.14 M in water.


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2.2 Catalyst Preparation. The layered-bulky ZSM-5 (LBZ5) hybrid composites were synthesized by adding certain amount of the conversional bulky ZSM-5 (BZ5) crystals into the synthesis solutions of lamellar ZSM-5 (LZ5), in which the long-chain diquaternary ammonium cations [C22H45-N+(CH3)2C6H12-N+(CH3)2-C6H13)(OH)2, C22-6-6(OH)2] were used as the structure-directing agent. Typically, 0.20 g NaOH was dissolved in 3 g DI water. 0.017 g Al(OPri)3 was dissolved in 0.15 g H2SO4 and 3 g DI water with stirring. After dropwise adding the alkaline solution to acidic solution with vigorous stirring, 1.74 g TEOS and 6.5 g C22-6-6(OH)2 solution (0.14 M) was slowly added into the mixture and continuously stirred for 20 h in room temperature. Then, certain amount of the parent bulky ZSM-5 (BZ5) crystals was added into the obtained solution and stirred for 2 h. The molar composition of the final synthesis gel was 100 SiO2 : 10 C22-6-6(OH)2 :30 Na2O : 0.5 Al2O3 : 18 H2SO4 : 8000 H2O : x BZ5, where x was the additional amount of the parent bulky ZSM-5 (BZ5) crystals and set as 10 %、30 %、50 %、70 % and 90 % of the added silicon source of TEOS, and the obtained materials was named as LBZ5_10, LBZ5_30, LBZ5_50, LBZ5_70 and LBZ5_90, respectively. The synthesis gel was transferred into the Teflon-lined autoclave and heated at 423k for 5 days under tumbling condition at 40 rpm. The products were successively washed by DI water and dried at 343 K overnight, then calcined at 823 K in air for 6 h to remove the organic surfactants. The lamellar ZSM-5 (LZ5) was synthesized under the same procedure but without the addition of bulky ZSM-5 crystals, and the as-synthesized product was treated by structure pillaring before calcinations, following the method reported by Tsapatsis et al. 53


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All the ZSM-5 samples were ion-exchanged into the proton-formed catalysts by repeating the treatments three times with 1 M NH4NO3 at 353 K for 2 h, then dried at 373 K overnight and calcined at 823 K for 6 h, successively. 2.3 Catalyst Characterization Powder X-ray diffraction (XRD) patterns were obtained using a Bruker D8 Advance diffractometer equipped

with Cu Ka radiation range of 2θ = 1-40°. Scanning electron

microscopy (SEM) images were taken with a Zeiss Sigma instrument operated at 5.0 kV, and all samples were coated with Pt (approximately 10 Å) before the measurement. Transmission electron microscopy (TEM) images were taken with a FEI Tecnai G2 with an accelerating voltage of 200 kV. Ar adsorption-desorption isotherms were measured at 87 K with an AutosorbiQ (Quantachrome Instruments) analyzer. Before measurements, all samples were degassed under vacuum for 12 h at 623 K. Thermogravimetric (TG) analysis was performed on a TG 209 F1 Libra® from room temperature to 1073 K with a heating rate of 10 K min-1 in air. The Si/Al ratios were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) on an Optima 7000DV (PerkinElmer) spectrometer. The temperature-programmed desorption of ammonia (NH3-TPD) experiments were carried out on a Tianjin Xianquan TP-5080 analyzer equipped with a thermal conductivity detector (TCD) with a continuous temperature increase (10 K/min) under N2 flow (30 mL.min-1). 2.3 Catalyst Evaluation The methanol-to-propylene (MTP) reaction was performed in a quartz tubular fixed-bed reactor (6 mm inner diameter) at atmospheric pressure. For each measurement, 70 mg ZSM-5 catalyst was loaded in the center of reactor and in-situ activated at 823 k for 1 h in nitrogen flow of 25 mL.min-1, then cooled down to the reaction temperature of 723 K. The methanol was fed


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by passing the carrier gas of nitrogen (25 mL.min-1) through saturator containing methanol at 303 K, giving a weight hourly space velocity (WHSV) of 1.7 h-1. The products were analyzed by an online gas chromatograph (Fuli GC-9790Ⅱ) equipped with a flame ionization detector (FID) and Plot-Q column (30 m × 0.32 mm × 10 μm). The conversion and selectivity were calculated on CH2 basis, and dimethyl ether (DME) was considered as the reactant.

3. RESULTS AND DISCUSSION 3.1 Crystallographic Feature, Textural Property and Morphology Analysis The synthesis of layered-bulky ZSM-5 (LBZ5) hybrid composites were achieved by adding certain amount of the conversional bulky ZSM-5 (BZ5) crystals into the synthesis gels of the lamellar ZSM-5 (LZ5) assisted with the long-chain diquaternary ammonium cations [C22H45N+(CH3)2-C6H12-N+(CH3)2-C6H13)(OH)2, C22-6-6(OH)2] as the structure-directing agent. The additional amount of the bulky ZSM-5 crystals was precisely calculated as 10 %、30 %、50 %、 70 % and 90 % of the silicon source adding into the synthesis gels, respectively, in order to investigate the hybrid structure and morphology evolution of the composites, and the lamellar (2D) ZSM-5 was synthesized without the addition of bulky crystals as a comparison, which was further pillared to prevent interlayer condensation during the removal of the C22-6-6(OH)2 structure-directing agents. Figure 1 shows the XRD patterns of the conversional bulky ZSM-5 (BZ5), lamellar ZSM-5 (LZ5) and layered-bulky ZSM-5 (LBZ5) hybrid composites after the removal of organic structure-directing agents by calcination. The XRD patterns in the low angle (1-5°) (Figure 1A) reveal the layered structure of the LZ5 and various LBZ5 samples. The low-angle XRD pattern of LZ5 revealed an obvious broad peak, indicating its multi-layered structure was partially


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retained by the structure pillaring even after the removal of the C22-6-6(OH)2 template. The LBZ5 samples were obtained without structure pillaring operation, and presented much weaker broad peaks in their low-angle XRD patterns comparing with that of LZ5, suggesting their multilayered assembly structures were not predominant. The XRD patterns in the wide angle (5 - 40°) (Figure 1B) indicated that all the ZSM-5 samples were with pure phase of MFI topology. The characteristic peaks of BZ5 in the 2θ range of 7 - 9° and 22 - 25° presented the highest intensity in the XRD pattern, suggesting its highest crystallinity, comparing with the other ZSM-5 samples. The relative crystallinity (RC) value of BZ5 was calculated according to the diffraction peaks at 7.9°, 8.8°, 23.2°, 23.9° and 24.4°, and further used as the reference 100% crystallinity. The lamellar ZSM-5 (LZ5) revealed not only the lowest crystallinity (RC value of 69.5) among all the ZSM-5 samples, but also missing some diffraction peaks in the XRD pattern, due to its 2-D layered structure. The RC values of LBZ5 synthesized with the increased additional amounts (10 - 90 % of the silicon source addition) of ZSM-5 bulky crystals gradually increased from 83.4 to 98.0. The relative crystallinity (RC) values of all the ZSM-5 samples are summarized in Table 1. Figure S1 shows the SEM image of the as-synthesized lamellar ZSM-5, which presents a highaspect-ratio platy structure and aggregation morphology as reported previously. The multilayered structure of LZ5 can be well retained after pillaring to prevent interlayer condensation during the removal of the C22-6-6(OH)2 structure-directing agents. 34 The SEM observation of LZ5 after structure pillaring (Figure 2a) indicates that the platy structure and aggregation morphology have been inherited after the pillaring operation. Figure 2b-f show the surface morphologies of the layered-bulky ZSM-5 (LBZ5) hybrid composites obtained by epitaxially grown the lamellar ZSM-5 over the crystals of the conventional bulky ZSM-5 (BZ5) with certain additional amount, and the LBZ5 particles reveal similar in particle size and shape but much higher roughness of


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surfaces comparing with the parent BZ5, which are the typical coffin-like particles with smooth surfaces (Figure 2g). As shown in Figure b-f, the surface morphology of LBZ5 can be controlled via the additional amount of parent BZ5, and the coverage degree, thickness and roughness of the epitaxially grown layered ZSM-5 will decrease with the additional amount of parent BZ5. Moreover, it is worth mention that sufficient amount of the parent BZ5 particles is necessary for the epitaxial growth process. With extremely limited amount of the parent BZ5, the excessive nutrition will not only induce the epitaxial growth of lamellar ZSM-5 over the parent BZ5 particles but also promote the nucleation of lamellar ZSM-5 individually in the synthesis solution. Figure S2 shows the SEM image of the as-synthesized product with 1 % additional amount of bulky ZSM-5 crystals, and it can be clearly found that individual particles of lamellar ZSM-5 mixed with the layered-bulky ZSM-5 hybrid composites by epitaxial growth. Figure 3a shows the Ar physisorption isotherms of the BZ5, LZ5 and BLZ5 samples. All the samples presented a similar trend of sharp increase in volume adsorbed at P/P0 < 0.05, suggesting them all possessed highly crystalline micropore structures. Significant differences happened to the Ar physisorption at P/P0 ranged from 0.35 to 0.95. Unlike the typical type I isotherm of BZ5, LZ5 and BLZ5 presented the type IV isotherm with the hysteresis loop, suggesting the existence of mesopores. As expected from the rough surface and epitaxially grown morphology, the LBZ5 composites showed broad hysteresis loops, indicating their considerable amount of mesopores in a wide pore size distribution, and the NLDFT and BJH analysis further revealed the mesopore size of the LBZ5 composites widely distributed from 2 to 15 nm (Figure 3b and Figure S3). The relatively concentrated shape of hysteresis loop of LZ5 represented its interlayer mesopores after pillaring were in a narrow distribution as reported previously.


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Table 2 summarizes the textural properties of the BZ5, LZ5 and LBZ5 samples. It can be found that the LBZ5 composites possess not only larger micropore volume than that of LZ5 due to their relatively higher crystallinity in Figure 1, but also considerably larger external surface area than that of BZ5 coming from their highly roughened surfaces and layer-bulk epitaxially grown morphology, indicating the LBZ5 composites to be a kind of hierarchical zeolites with meso-/microporosity. Furthermore, the micropore volume of LBZ5 gradually increased but the external surface area decreased with the increase of the additional amount of the parent ZSM-5 bulky crystals. The hierarchical factor (HF), defined as (Sext/SBET) × (Vmic/Vtotal), was used to classify the hierarchical porosity.

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LZ5 revealed the highest HF value of 0.14, which was

mainly contributed by the large external surface area of layered structures. LBZ5 also presented a high HF value (0.9 - 0.13) in similar magnitude with that of LZ5, and what expected was such a high HF value was contributed from both high micropore volume and considerably larger external surface. The sample of LBZ5_30 (30 % additional amount of parent bulky crystals of ZSM-5) displayed the highest HF value of 0.13 among all the LBZ5 samples, suggesting its hierarchical structure satisfies with both zeolitic microporosity and interlayered mesoporosity. The TEM observation (Figure 4) was applied to further compare the morphology and crystallographic structure of BZ5, LZ5 and LBZ5 materials. The sample of BZ5 presented regular-shaped particles in micron size and with smooth surface (Figure 4a), and a closer observation and selected area electron diffraction (SAED) pattern indicated the particles were highly crystalline single crystals (Figure 4b and insert in Figure 4a). The TEM images of the LZ5 sample after structure pillaring clearly showed the muilti-lamellar structure composed of ZSM-5 nanosheets with a thickness of 2 nm (Figure 4c and d). The LBZ5 composites were synthesized by adding certain amount of BZ5 particles into the synthesis solution of LZ5. The TEM images


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of various LBZ5 samples showed that the particle size of the core components of LBZ5 was similar to that of parent BZ5, but the particle surface turned into roughness (Figure S4). The sample of LBZ5_30 was chosen as an example to further demonstrate the composite structure of LBZ5 materials by TEM observation (Figure 4e-g). It was clear to see that the lamellar ZSM-5 nanosheets growed over the surfaces of the ZSM-5 bulky crystals and obvious interface could be found between the layer section and the bulk phase (Figure 4f), indicating the LBZ5 to be an epitaxially grown material. 51 Furthermore, the outer-layer ZSM-5 nanosheets were inclined to be single layer distribution and more randomly grown on the surface of the parent ZSM-5 bulky crystals (Figure 4g), which constructed more opened interlayer mesopores than those of LZ5, and the results were corresponding to the Ar physisorption results. Figure 5 shows the thermogravimetric curves of as-synthesized LBZ5 and LZ5, and a certain extent of weight loss happens to all the as-synthesized samples. The weight loss at the temperatures below 200 oC can be ascribed to the removal of the physically adsorbed water, and the weight loss between 200 and 800 oC is due to the decomposion of the organic SDAs (C22-66(OH)2).

LZ5 showed much larger total weight loss (43.0 wt%) than LBZ5 by not only water

desorption (6.5 wt%) but also organic SDAs decomposion (36.5 wt%). The total weight losses of LBZ5 materials revealed an obviously decreasing trend with the increase of the additional amount of parent BZ5, which were 34.0 wt%, 27.5 wt%, 23.4 wt%, 20.4 wt% and 20.1 wt% for the samples of LBZ_10, LBZ_30, LBZ_50, LBZ_70 and LBZ_90, respectively, whereas the organic SDAs contents were 30.5 wt%, 24.73 wt%, 20.8 wt%, 18.1 wt% and 18.0 wt%. BZ5, which was the commercial ZSM-5 after calcinations, showed considerably limited weight loss of 1.8 wt% by water desorption (Figure S5). Based on the contents of adsorbed water and organic SDAs in BZ5, LZ5 and LBZ5, the mass fraction of BZ5 crystals in LBZ_10, LBZ_30, LBZ_50,


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LBZ_70 and LBZ_90 particles are 21.95%, 37.80%, 47.80%, 55.12% and 55.85%, respectively. The density of BZ5 is 1.80 g.cm-3 provided by the supplier, and the density of as-synthesized LZ5 is 1.54 g.cm-3 based on the previous report.

32, 51

The volume fraction of BZ5 crystals in

LBZ_10, LBZ_30, LBZ_50, LBZ_70 and LBZ_90 particles can be calculated to be 20.06%, 35.17%, 44.97%, 52.29% and 52.34%, respectively. The approximate particle volume of BZ5 is 1.12μm3 (2.8μm × 1μm × 0.4μm) based on the size measurement of ~100 particles of various BZ5 samples in SEM (Figure 2g). The average thickness of the lamellar ZSM-5 nanosheets epitaxially grown over the bulky ZSM-5 particles of the LBZ_10, LBZ_30, LBZ_50, LBZ_70 and LBZ_90 samples were calculated to be 307nm, 175nm, 115nm, 98nm and 98nm, respectively. 3.2 Si/Al Ratio and Acidity Analysis The acidity of aluminosilicate zeolites originates from the content and distribution of aluminium. The Si/Al ratio and the composition of the BZ5, LZ5 and LBZ5 catalysts were determined by the ICP analysis, and the results were summarized in Table 2. The conversional bulky ZSM-5 (BZ5), which was also used as the parent material to make LBZ5, presented the highest Si/Al ratio (149) among all the ZSM-5 catalysts. The lamellar ZSM-5 (LZ5) showed a higher Si/Al ratio (140) than the designed value (100), due to the addition of extra silicon source in the structure pillaring process. The layered-bulky ZSM-5 (LBZ5) composite catalysts, which were obtained by epitaxially grown the lamellar ZSM-5 over bulky ZSM-5 crystals, gave the Si/Al ratios ranged from 137 to 141, similar with that of LZ5. NH3-TPD technique was used to evaluate the acidic property of various ZSM-5 catalysts. Figure 6 shows the NH3-TPD profiles of BZ5, LZ5 and LBZ5. Similar curves with two obvious NH3 desorption peaks happened to the NH3-TPD profiles all the ZSM-5 catalysts. The peak


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locates in the temperature range of 160-200 oC (low temperature, LT) represents the weak acid sites resulting from the weakly acidic silanol groups or non-framework aluminum in zeolite catalysts, and the peak locates in the temperature range of 385-430 oC (high temperature, HT) can be assigned to the strong acid sites corresponding to the framework aluminum in zeolite catalysts.

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Table 2 shows the acidity amounts of various ZSM-5 catalysts based on NH3-TPD

profiles. The sample of BZ5 revealed the total acidity of 148 μmol.g-1, based on the highest Si/Al ratio (149) among all the samples. The sample of LZ possessed a lower Si/Al ratio (140) than that of BZ5, but presented a much lower total acidity of 111 μmol.g-1, which was possibly due to its lower crystallinity. The various LBZ5 samples presented higher total acidity (162-177 μmol.g1)

as well as the strong acidity (106-115 μmol.g-1) than BZ5 and LZ5, meaning more aluminum

atoms participated and transformed as tetrahedral coordination framework sites. 3.3 Catalytic Performance for MTP Reaction The methanol-to-propylene (MTP) reaction was used to evaluate the catalytic performance of the layered-bulky ZSM-5 (LBZ5) hybrid composite catalysts in contrast to the lamellar ZSM-5 (LZ5) nanosheets after interlayer pillaring and the parent bulky ZSM-5 (BZ5). Figure 7 comparatively shows the methanol conversion and propylene selectivity as the function of time on stream for the MTP reaction over the BZ5, LZ5 and LBZ5 catalysts at 450 oC in a continuous flow fixed-bed reactor. It can be found that all three kinds of ZSM-5 catalysts presented similar high conversion (> 99.9 %) of methanol in the initial stage of the reaction, following with certain degree of attenuation happened with the increasing of the reaction time. Here, the catalyst lifetime was defined as the reaction time for the catalyst with methanol conversion higher than 80 %. The BZ5 catalyst showed the shortest catalyst lifetime (30 h) among all the ZSM-5 catalyst, suggesting the bulky phase of microporous crystals strongly inhibited the reaction due


15


Page 16 of 35

to the large mass transfer resistance by the long micropore diffusion length. The LZ5 catalyst revealed a significant improvement in catalyst lifetime (123 h) than BZ5, especially after structure pillaring, which can be ascribed to the remarkably reduced micropore diffusion length (from micrometer to nanometer) and the well-retained interlayer mesoporosity even after the removal of mesopore structure-directing agents by the structure pillaring operation. 9 Figure S6 shows the methanol conversion versus time on stream over the LZ5 catalyst without structure pillaring (LZ5_NP), and the catalyst lifetime is 93 h. As expected, the synthesized LBZ5 catalysts presented desirable catalytic performance with catalyst lifetime of 90, 171, 142, 125 and 110 h for the samples of LBZ5_10, LBZ5_30, LBZ5_50, LBZ5_70 and LBZ5_90. Especially, the sample of LBZ5_30 showed the longest catalyst lifetime of 171 h, which was 570 % and 139 % longer than those of BZ5 and LZ5, respectively. The prolonged catalytic lifetime of LZ5 and LBZ5 could be ascribed to their hierarchical structures, especially the introduction of mesopores and macropores, which could remarkably reduced micropore diffusion length. The hierarchical factor (HF) has been used to classify the hierarchical porosity of various ZSM-5 catalysts. BZ5 presented the lowest HF value of 0.04, leading to its shortest catalytic lifetime. LZ5 possessed the highest HF value of 0.14 and revealed significantly improvement in catalyst lifetime than BZ5. However, such high HF value of LZ5 was mainly contributed by the large external surface area, and the loss of microporosity to a certain extent due to its relatively lower crystallinity still restricted the activity of LZ5. With effective combination of microporsity and mesoporsity by epitaxial growth of lamellar ZSM-5 over bulky ZSM-5, resulting in a hybrid core-shell structure, the LBZ5 catalysts exhibited higher catalyst lifetimes than BZ5. And it was worth mention that the sample of LBZ5_30 revealed a relatively lower HF value of 0.13 than LZ5, but the longest catalyst lifetime among all the ZSM-


16

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5 catalysts including the other LBZ5 catalysts, due to its hierarchical structure simultaneously satisfied microporosity and mesoporosity. To further demonstrate the stability of LBZ5 catalysts in the MTP process, the used sample of LBZ5_30 after the MTP reaction was chosen as an example to detect the coke content in contrast with BZ5 and LZ5 by thermogravimetric analysis (Figure S7 and Table S1). Subtraction of the adsorbed water, the coke contents in the used BZ5, LZ5 and LBZ5_30 catalysts were 5.7 %, 12.0 % and 11.7 %, respectively. Based on the different catalytic lifetime of the three catalysts, the rate of coke formation (Rcoke), defined as eq 1, was used to evaluate the coke toleration during the reaction, and LBZ5_30 showed a much lower Rcoke (0.68 mg·g-1·h-1) than BZ5 (1.9 mg·g-1·h-1) and LZ5 (0.98 mg·g-1·h-1). It has been generally accepted that the introduction of hierarchical porosity will effectively relieve the mass transport resistance of the ZSM-5 catalyst and further improve its anti-deactivation ability in the MTP reactions based on the previous study in literatures. Comparing with the microporous phase in micron size of BZ5, the LZ5 catalyst benefited from its 2D nanosheet structure and presented a significant improvement in the coke content during the MTP reaction. However, the relatively lower crystallinity limited the activity and catalytic lifetime of the LZ5 catalyst, and gave a medium Rcoke. As expected, the LBZ5_30 showed not only the highest activity but also the best anti-deactivation ability among all three catalysts, reflecting as the longest catalyst lifetime and the lowest Rcoke in the MTP reaction, due to its superior structure properties of high crystallinity and 2D-3D hierarchical porosity, further demonstrated the superior catalytic performance of LBZ5 catalysts in coke toleration and stability during the MTP process.

Rcoke

M coke content (mg.g -1 ) (mg.g .h ) = Tcatalyst lifetime (h) -1

-1


(1)

17


Page 18 of 35

The selectivities of main products including propylene versus time on stream over BZ5, LZ5 and LBZ5 catalysts are shown in Figure 7 and Figure S8. All three kinds of ZSM-5 catalysts showed similar dynamic changes of products during the MTP reaction with a steady formation of C1-4 hydrocarbons and butylenes, propylene, gradual decrease of ethylene and propylene, and gradual increase of C5+ compounds. The LBZ5 catalysts presented high selectivity of propylene not only in the initial stage of the reaction but also during the lifetime, suggesting them to be considerably superior catalysts for the MTP reaction. The average selectivity of each product measured at the steady-state condition in the reaction over BZ5, LZ5 and LBZ5 catalysts is contrastively shown in Figure 8 and Table S2. Based on the “ dual cycle” mechanism of MTO/MTP reaction, propylene and butylenes are generated through alkene methylation/cracking pathways whereas ethylene is mainly formed from the lower methylbenzenes in the aromatic/ethylene cycle,

14, 55, 56

and previous reports have confirmed that efficiency diffusivity

and appropriate acidity can promote the alkylation-dealkylation reaction, resulting in high propylene selectivity and propylene to ethylene (P / E) ratio.

57

Benefitted from the 2-D layered

structure and the interlayered mesoporosity by pillaring, the LZ5 sample showed higher selectivity of propylene (40.0 %) and butylenes (23.8 %) and P / E ratio (8.2) than those of BZ5 (38.7 % , 23.3 % and 7.2). As expected, the LBZ5 catalysts presented further enhancement in selectivity of propylene (40.3-43.0 %) and butylenes (23.3-25.1 %) and P / E ratio (8.8-9.0), due to their feasible structure properties with high crystallinity, hybrid composite structure and hierarchical meso-/microporosity and suitably strong acidity. Among all the LBZ5 catalysts, LBZ5_70 revealed the highest propylene selectivity (43.0 %), and LBZ5_30 sample possessed not only the high propylene selectivity (41.4 %) but also the lowest ethylene selectivity (4.4 %), providing the largest P / E ratio of 9.4. Considering its long-term catalytic lifetime (171 h),


18

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LBZ5_30 is suggested to be an effective catalyst with high activity and propylene selectivity and superior stability for the MTP reaction.

4. CONCLUSIONS The ZSM-5 hybrid composites with high crystallinity, a core-shell structure composed by lamellar ZSM-5 as the shell and bulky ZSM-5 as the core and the hierarchical meso/microporosity coming from the zeolitic micropores and interlayered mesopores were synthesized through the epitaxial growth method. By adjusting the additional amount of the parent bulky ZSM-5 crystals into the synthesis solution of lamellar ZSM-5, the hybrid core-shell structure of the obtained layered-bulky ZSM-5 (LBZ5) composites can be controlled with the thickness of the layered shell ranged from 98 to 307 nm. The crystallinity as well as the micropore volume of LBZ5 increased while the mesopore volume decreased with the increasing of the additional amount of the parent bulky ZSM-5 crystals, and the sample of LBZ5_30 showed the largest hierarchical factor (HF) among various LBZ5 samples, suggesting its hierarchical structure simultaneously satisfied microporosity and mesoporosity. The catalytic performance of LBZ5 was evaluated by the methanol-to-propylene (MTP) reaction comparing with the parent bulky ZSM-5 (BZ5) and the lamellar ZSM-5 (LZ5), and significant improvements with prolonged catalytic lifetime, superior coke toleration and enhanced propylene selectivity suggested LBZ5 to be candidated catalysts for the MTP process due to their well-retained zeolitic framework, hierarchical meso-/microporosity by the layered-bulky composite structure and appropriate strong acidity. In particularly, the sample of LBZ5_70 revealed the highest propylene selectivity (43.0 %) among all the ZSM-5 catalysts, and the


19


Page 20 of 35

sample of LBZ5_30 presented not only the considerably high propylene selectivity (41.4 %) but also the longest catalytic lifetime (171 h) and the largest P / E ratio (9.4).


20

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FIGURES

A

B

g

g Intensity (a.u)

Intensity (a.u)

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


f e d

f e d

c

c

b

b

a 1

a

2 3 4 5 2 Theta (degree)

5

10

15 20 25 30 2 Theta (degree)

35

40

Figure 1. [A] Low-angle and [B] wide-angle XRD diffraction patterns of (a) LZ5, (b) LBZ5_10, (c) LBZ5_30, (d) LBZ5_50, (e) LBZ5_70, (f) LBZ5_90 and (g) BZ5.

a

b

e

c

f

d

g

Figure 2. SEM images of (a) LZ5 after interlayer pillaring, (b) LBZ5_10, (c) LBZ5_30, (d) LBZ5_50, (e) LBZ5_70, (f) LBZ5_90 and (g) BZ5.


21


600

300

3

200 100 0 0.0

0.2

0.4

b

-1

Incremental Pore Volume (cm .g )

3

400

3

a

LZ5 BLZ5_10 BLZ5_30 BLZ5_50 BLZ5_70 BLZ5_90 BZ5

500

-1

Ar Volume Absorbed (cm .g )

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

Page 22 of 35

p/p0

0.6

0.8

1

0

1.0

LZ5 BLZ5_10 BLZ5_30 BLZ5_50 BLZ5_70 BLZ5_90 BZ5

2

0

5

10 15 Pore Width (nm)

20

Figure 3. (a) Ar physisorption isotherms and (b) NLDFT pore size distributions of BZ5, LZ5 and BLZ5 samples.

a

b

e

c

f

d

g

Figure 4. Low magnification TEM images of (a) BZ5, (c) LZ5 and (e-f) LBZ5_30; (b), (d) and (g) high magnification TEM image of the selected area in (a), (c) and (f), respectively.(inset: electron diffraction patterns from the selected areas)


22

Page 23 of 35

100 95 90 85 Mass Loss (%)

f e d c

80 75 70

b

65 60

a

55 50

100

200

300 400 500 600 Temperature (C)

20.1 wt% 20.4 wt% 23.4 wt% 27.5 wt%

34.0 wt%

43.0 wt%

700

800

Figure 5. TGA curves of as-synthesized (a) LZ5, (b) LBZ5_10, (c) LBZ5_30, (d) LBZ5_50, (e) LBZ5_70 and (f) LBZ5_90 before the removal of the organic SDAs.

g f

TCD Signal (a.u)

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


e d c b a

150 200 250 300 350 400 450 500 550 Temperature (C)

Figure 6. NH3-TPD curves obtained on (a) LZ5, (b) LBZ5_10, (c) LBZ5_30, (d) LBZ5_50, (e) LBZ5_70, (f) LBZ5_90 and (g) BZ5.


23


80

80

60

60

40

40

0

BLZ5_70 BLZ5_90 BZ5

LZ5 BLZ5_10 BLZ5_30 BLZ5_50

0

30

60 90 120 150 Time on Stream (h)

20

Selectivity of Propylene (%)

100

Conversion (%)

100

20

0 180

Figure 7. Methanol conversion and propylene selectivity as function of time on stream for the MTP reaction over BZ5, LZ5 and LBZ5 catalysts. Reaction conditions: T = 723 K, P = 1 atm, WHSV = 1.7 h-1.

50

C=2

C1-C4

C=3

C=4

C5+

40

Yield of Products (%)

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

Page 24 of 35

30 20 10 0

a

b

c

d

e

f

g

Figure 8. Comparison of product distribution measured at the steady-state condition in MTP Reaction over (a) LZ5, (b) LBZ5_10, (c) LBZ5_30, (d) LBZ5_50, (e) LBZ5_70, (f) LBZ5_90 and (g) BZ5.


24

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TABLES Table1 Relative crystallinity (RC) values and textural properties of the BZ5, LZ5 and LBZ5 samples.

Samples

Textural property b

RC value a

HF c

SBET [m2.g-1]

Sext [m2.g-1]

Vmic [cm3.g-1]

Vmeso [cm3.g-1]

Vtotal [cm3.g-1]

LZ5

69.5

496

301

0.12

0.38

0.50

0.14

BLZ5_10

83.4

553

227

0.16

0.45

0.61

0.11

BLZ5_30

93.8

442

139

0.18

0.28

0.46

0.13

BLZ5_50

97.5

452

118

0.19

0.24

0.43

0.11

BLZ5_70

97.3

438

92

0.20

0.20

0.39

0.11

BLZ5_90

98.0

496

83

0.20

0.20

0.39

0.09

BZ5

100

447

21

0.21

0.05

0.22

0.04

a

The relative crystallinity (RC) value calculated according to the diffraction peaks based on standard test

method ASTM D5758 - 01(2015). b

Si (surface area of micropores (i = mic), mesopores (i = ext) by NLDFT method), SBET (surface area by

Brunnauer−Emmet−Teller method), Vmic (micropore volume by t-plot method), Vtotal (total pore volume), Vmeso (mesopore volume, defined as Vtotal - Vmic). c

Hierarchical factor (HF) defined as (Sext/SBET) × (Vmic/Vtotal).


25


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Table 2. Si/Al ratio and acidity of the BZ5, LZ5 and LBZ5 catalysts.

Samples

Number of acid sites [μmol of NH3/g, TPD] b

Si/Al a Weak

Strong

Total

LZ5

140

29

82

111

BLZ5_10

139

64

112

176

BLZ5_30

141

57

106

163

BLZ5_50

141

55

107

162

BLZ5_70

137

62

115

177

BLZ5_90

140

64

109

173

BZ5

149

58

90

148

a

The Si/Al ratios were determined by ICP-OES analysis.

b

The quantities of acidities determined by NH3-TPD were measured by the amounts of ammonia desorbed at

160-200 and 385-430 °C, respectively.


26

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ASSOCIATED CONTENT Supporting Information SEM image of as-synthesized LZ5 before pillaring; SEM image of LBZ-5 synthesized with 1 % additional amount of bulky ZSM-5 crystals, BJH pore size distribution calculated from the adsorption isotherms; TEM images of LBZ5 catalysts; TGA curve of BZ5; methanol conversion versus time on stream over LBZ5_30, LZ5 with and without interlayer pillaring and hierarchical ZSM-5 directed by organosilane surfactant TPOAC; TGA curves of the used catalysts after the MTP reaction; product distributions versus time on stream; catalytic lifetime, coke content and coke formation rate of BZ5, LZ5 and LBZ5_30 catalysts during the MTP reaction; product distributions measured at a steady-state condition in the MTP reaction over BZ5, LZ5 and LBZ5 catalysts. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (H. Chen), [email protected] (B. Liu) ORCID Huiyong Chen: 0000-0003-0045-3141 Jianbo Zhang: 0000-0003-1302-4143 Ming Sun: 0000-0003-1005-9738 Xiaoxun Ma: 0000-0002-1603-5641 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


27


Page 28 of 35

This work was supported by the Natural Science Foundation of China (Nos. 21576221 and 21536009) and the Nova program supported by Natural Science Foundation of Shaanxi (No. 2018KJXX-014).


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Epitaxial Growth of Layered-Bulky ZSM-5 Hybrid Catalysts for the

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