Chemical Crosslinking Assembly of ZSM-5 Nanozeolites into Uniform

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Functional Inorganic Materials and Devices

Chemical Crosslinking Assembly of ZSM-5 Nanozeolites into Uniform and Hierarchically Porous Microparticles for High-Performance Acid Catalysis Chao Shang, Zhangxiong Wu, Winston Duo Wu, and Xiao Dong Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01681 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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ACS Applied Materials & Interfaces

Chemical Crosslinking Assembly of ZSM-5 Nanozeolites into Uniform and Hierarchically Porous Microparticles for High-Performance Acid Catalysis Chao Shang a, Zhangxiong Wu a*, Winston Duo Wu a, and Xiao Dong Chen a* a

School of Chemical and Environmental Engineering, College of Chemistry, Chemical

Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123, P. R. China. E-mail address *: [email protected] E-mail address *: [email protected]

KEYWORDS: ZSM-5, hierarchical zeolites, microparticle assembly, spray drying, acid catalysis

ABSTRACT: Hierarchically porous zeolites combining the advantages of desirable mass transport of nanozeolites and easy separation and handling of micro-zeolites are ideal candidates in catalytic applications. Facile routes for the assembly of zeolite microparticles with hierarchical

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porosity and high mechanical strength are much expected. Herein, based on a microfluidic jet spray drying technology, we report a facile and scalable chemical crosslinking assembly strategy for the synthesis of hierarchical zeolite microparticles by directly using the conventional as-synthesized nanozeolites suspension as a precursor. This route not only avoids the energy-intensive centrifugal separation process of nanozeolites, but also significantly increases the uniformity and mechanical strength of the microparticles. The soluble aluminosilicate species act as a stabilizer to improve the droplet stability during the drying process, and then as a “cross linker” to chemically bind and interconnect zeolite nanoparticles to form robust bodies after drying and calcination. Zeolite microparticles with variable morphologies (spherical, bowl-like and dimpled) and uniform and controllable sizes (from 70 ~ 108 μm) can be obtained by adjusting the experimental parameters. The particle formation mechanism is discussed with the zeolite microparticles obtained from the purified nanozeolite suspension as a control. The zeolite microparticles possess emerged uniform mesopores (~ 6 nm) and well-maintained high surface area, large pore volume, high microporosity and strong acidity of the original nanozeolites. As a result, they exhibit excellent acid catalytic performances in acetolysis of epichlorohydrin and catalytic cracking of low-density polyethylene, far better than those of the commercial ZSM-5.

INTRODUCTION Zeolites are crystalline microporous aluminosilicates that have been widely used in adsorption,1,2 separation,3,4 and catalysis5,6 because of their high surface areas, ordered micropores, high thermal and hydrothermal stabilities and strong acidities. However, the wide potential of conventional zeolites is limited by the slow mass transfer rates for bulky reactants or products, because the most active sites are confined in small micropores (< 0.8 nm in diameter).7,8 To make the active sites


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more accessible to bulky or large molecules, synthesis of zeolites with larger pores,9,10 hierarchical pore structures,7,11,12 and nanoscale particle sizes13 have been reported. Among them, reduction of the particle size of zeolites to the nanometer scale (< 100 nm) is particularly attractive because of the largely enhanced external surface areas, shortened molecular diffusion lengths and more exposed active sites,14,15 which can lead to improved catalytic performance. However, as compared with conventional micro-sized counterparts, nanozeolites are unlikely to be used in most fixed bed and continuous flow reactors because of the high pressure drops and the difficulty in separating them from reaction streams. It is highly desirable to develop a simple route to assemble nanozeolites into robust micro-sized bodies with sustained nanozeolite accessibility. Bulky self-assembled zeolites are considered to have great industrial potential because of the perfect combination of the advantages of both nano- and micro-sized zeolites,16-18 namely, high active site accessibility and easy separation from reaction mixtures. Xiao and co-workers have reported the synthesis of stable bulky beta zeolite particles via self-assembly of beta nanocrystals with cationic polymers under hydrothermal conditions. These zeolite particles show a similar catalytic property in the alkylation of benzene with isopropanol as that of beta nanocrystals.17 Zhao and co-workers have developed an in situ crystallization route to synthesize uniform functional microspheres composed of aggregated ZSM-5 nanorods and well-dispersed uniform Fe3O4 nanoparticles, showing a potential application in Fischer-Tropsch synthesis.19 Kang and coworkers have prepared various zeolite microspheres from nanozeolites by a polymerizationinduced colloidal aggregation method. The obtained zeolite microspheres well retain the properties of the original colloidal nanozeolites and exhibit similar performance as that of the nanozeolites in protein adsorption.20 Jia and co-workers have prepared self-assembled hierarchical ZSM-5 microspheres

via

a

simple

hydrothermal

procedure


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in

the

presence

of

3-


glycidoxypropyltrimethoxysilane.

These

uniform

zeolite

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microspheres

possess

higher

crystallinity, smaller crystal sizes, higher BET surface areas and larger pore volumes as compared with the commercial ZSM-5 zeolites, thus demonstrating better catalytic performance in the methanol to aromatics reaction.21 In spite of these successes in development of assembled zeolites with comparative or better performance as compared with their nano-sized (or commercial) counterparts, the synthesis procedures often involve the use of either complicated experimental steps or large amounts of costly reagents. Moreover, the particles sizes of these reported ZSM-5 microparticles are either relatively small (in the range of sub- to a few micrometers) with broad particle size distributions or irregular morphologies.16,17,19-24 Besides, the lack of chemical binding among the nanozeolite building blocks in some zeolite assemblies renders their unsatisfactory mechanical stability. Therefore, it is attractive to assemble nanozeolites into mechanically stable and accessible zeolite microparticles with large and uniform particles sizes. Spray drying is an effective method to assemble molecules or nanoparticles into spherical agglomerates. The process is rapid, continuous, cost-effective and scalable.25,26 Starting from molecular precursors, with the assistance of evaporation induced self-assembly in spray drying, mesoporous silica,27-32 aluminasilicates,33 metal oxides,34,35 and carbons,36-39 have been reported. These microparticles are mechanically stable because they are made of covalently bonded frameworks. On the other hand, the spray drying method has also been used in assembly of nanoobjects into microparticles with different morphologies and sizes, such as assembling MCM-41 and MCM-48 nanospheres into spherical agglomerates,40 silica nanoparticles into hollow silica microcapsules,41,42 zeolite nanocrystals into hierarchical assemblies,43 and MIL-101 nanocrystal blocks into uniform microparticles.44 In these cases, however, due to the lack of chemical bonding among nanoparticles, they could be re-dispersed or crushed to fragments easily in reaction


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environments. In order to overcome this shortcoming, “chemical crosslinking” among individual nanoparticles should be rationally designed during assembly of nanoparticles into bulky superstructures. Herein in this work, we demonstrate the chemical crosslinking assembly of zeolite nanoparticles into uniform and robust microparticles with controllable morphologies, hierarchical porosities and high catalytic performance by using the microfluidic jet spray drying technology. The assynthesized nanozeolite suspension is directly used as the precursor for the spray drying process. The soluble aluminosilicate species in the precursor can not only help to stabilize the droplets to form uniform microparticles, but also form amorphous aluminosilicates which act as the chemical “binder” to “crosslink” the nanozeolites after the drying and calcination processes, and thus to enhance the mechanical stability of the obtained microparticles. The resultant zeolite microparticles maintain the crystal structure and porosity of the nanozeolites and possess a high mesoporosity. They are excellent candidates for acid catalytic applications due to the combination of the intrinsically short diffusion of nanozeolites and the advantages of bulky microparticles. In the catalytic studies of acetolysis of epichlorohydrin and thermal cracking of low-density polyethylene, the assembled zeolite microparticles show excellent catalytic performances.

EXPERIMENTAL SECTION Synthesis of the ZSM-5 nanozeolites The information of all the chemicals for the synthesis can be found in supporting information. The ZSM-5 nanozeolites with a Si/Al ratio of 100 was prepared by using a hydrothermal method.45 In a typical synthesis process, NaAlO2 (1.2 g) was added into a 1.0 M tetrapropylammonium hydroxide (TPAOH) aqueous solution (96.0 g), and stirred at room temperature for 30 min. Then,


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tetraethyl orthosilicate (TEOS, 76.8 g) was added drop-wise. The resulting gel was continuously stirred for 30 min, and then heated up to 80 oC in an oil bath for another 60 min under stirring. After removal the oil bath, the mixture was stirred at room temperature for 20 h and then transferred into a Teflon-lined autoclave and hydrothermally treated at 180 oC for 48 h. The solid product was collected by centrifugation (10000 rpm for 20 min) and washed with deionized water till the pH was neutral. This sample was denoted as N-ZSM-5.

Preparation of the precursors for spray drying Two types of precursors were used to synthesize zeolite microparticles. The first was the assynthesized nanozeolite suspension obtained after the hydrothermal treatment at 180 oC for 48 h. The cooled suspension was diluted and used as the precursor without any purification treatment. The second was the purified zeolite suspension obtained by repeated centrifugation and washing with deionized water, and finally redispersed in deionized water. The solid concentration of the two precursors were controlled from 6.0 to 12.0 wt%. The solid concentrations were measured by weighing a volume of 5.0 mL of the suspension before and after the drying in an oven at 80 oC for 24 h.

Spray drying assembly of uniform robust ZSM-5 microparticles The set-up for the spray drying experiments is a novel microfluidic jet spray dryer (MFJSD, Nantong Dong Concept New Material Technology Company, Nantong, China) equipped with a microfluidic aerosol nozzle (MFAN) based on our reported procedure.44,46,47 For a typical spray drying experiment, a dehumidified air (0.4 kg/cm2) was used to force a specific suspension precursor to jet through the MFAN. The nozzle has an orifice of 75 μm in diameter for all the


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synthesis unless otherwise stated. The liquid jet was then broken up into monodisperse and continuous liquid droplets by pulse disturbance with the aid of periodic piezo-ceramic vibrations. Hot air (~ 250 mL/min) was introduced into the drying chamber and the inlet drying temperature (150 ~ 190 oC) was maintained constantly at each experiment. The droplets were dried rapidly (within 2 seconds) in the drying chamber and collected at the outlet. All the collected samples were calcined in air at 550 oC for 6 h at a heating rate of 2 oC/min, and then used for characterizations. The details for the material characterization and measurement can be found in the supporting information.

Catalytic tests Before running the catalytic reactions, all the Na+-form samples were ion-exchanged with a 1.0 M aqueous solution of NH4Cl at 80 oC for 4 h and repeated for three times, followed by calcination at 550 oC for 6 h to convert them to H-form zeolites. The spray-drying-derived samples were labeled as SD-ZSM-5-x-y-M or SD-ZSM-5-x-y-W, where “x” stands for the inlet drying temperature, “y” for the solid concentration of the precursor, and “M” or “W” for the assynthesized mother suspension or the washing-centrifugation purified nanozeolite suspension, respectively. The commercial micro-sized H-type ZSM-5 (Denoted as C-ZSM-5) was used as a reference for a comparison study. For acetolysis of epichlorohydrin (ECH) with acetic acid, the reaction was carried out in a magnetically stirred 20 mL glass reactor fitted with a reflux condenser. Typically, 0.54 g of ECH was charged into the reactor containing 5.45 g of acetic acid. Then 200 mg of a certain zeolite catalyst was introduced. The mixture was stirred, heated and kept isothermal at a constant temperature (80 oC) in an oil bath. During the reaction process under stirring, 0.5 mL of the


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resultant mixture was taken out at different time intervals. For each withdrawn mixture, after filtration, the clear liquid sample was analyzed by gas chromatographic analysis (Shimadzu, GC2010 Plus) equipped with a flame ionization detector (FID). To evaluate the reusability of the catalysts, by using the sample SD-ZSM-5-190-12-M as a representative catalyst, the used catalyst was recovered from the reaction mixture by natural settling down for 20 min, separated by filtration and washing with ethanol, dried at oven of 80 ºC for 1 h. Then, the sample was re-dispersed in a fresh reactant solution to conduct the reaction under the same conditions. For cracking of low-density polyethylene (LDPE), a certain catalyst was well mixed with the LDPE powder at room temperature with the catalyst/LDPE weight ratio fixed at 1:9 and grinded for about 10 min. Then ~ 20 mg of the mixture was put into a corundum crucible, which was heated from 25 to 600 oC with a ramp rate of 10 oC/min in N2 flow (50 mL/min) by using a thermogravimetric analyzer (TGA/DGC 3+, Mettler Toledo). The conversion of LDPE was recorded by the weight loss during the reaction process.

RESULTS AND DISCUSSION Particle morphology and structure The ZSM-5 zeolite nanoparticles have been successfully assembled into bulky robust uniform microparticles via the chemical crosslinking assembly route with the assistance of the simple and scalable microfluidic jet spray drying technology. Representatively, scanning electron microscopy (SEM) images of the sample SD-ZSM-5-190-12-M, obtained by using the as-synthesized nanozeolite mother suspension as the precursor, at a solid concentration of 12.0 wt% and a drying temperature of 190 oC, show a spherical morphology with buckled holes within each particle


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(Figure 1a and b). The microparticles are monodispersed with no agglomeration. The particle size distribution is quite narrow, showing a sharp peak between 90 ~100 μm with a mean particle size of 98.12±5.85 μm (Figure 1c). With the other conditions fixed, decrease of the solid concentration from 12.0 to 6.0 wt% results in a gradual morphological transition. A low solid concentration of 6.0 wt% results in a bowl-like morphology with single large hole within each particle (Figure 1g and h), while a medium solid concentration of 9.0 wt% results in a morphology between spherical and bowl-like (Figure 1d and e). Besides, the decrease of the solid concentration results in increased particle sizes and broadened particle size distributions (Figure 1f and i). The mean particle sizes of the samples obtained at 9.0 and 6.0 wt% solid concentrations are 99.24±11.67 and 107.89±20.46 μm, respectively. The sizes of the holes of the microparticles also increase with the decrease of the solid concentrations, and they are estimated to be 34.94±6.82, 49.68±9.67 and 68.91±12.66 μm for the samples obtained at 12.0, 9.0, and 6.0 wt% solid concentrations, respectively. Distinctly, with the purified nanozeolite suspension as the starting precursor, under the same experimental conditions as those for the synthesis of the sample SD-ZSM-5-190-12-M, the resulted SD-ZSM-5-190-12-W microparticles show a bowl-like morphology, and a very broad particle size distribution between 60 ~160 μm (Figure 1j-l). This result indicates that the presence of the amorphous silicates in the mother suspension has a large influence on particle formation, which will be discussed in detail in follow section. Comparatively, with the other conditions fixed, the decrease of the drying temperature from 190 to 150 oC results in zeolite microparticles with similar shapes and sizes (Figure 1m and n). Slightly differently, a lower drying temperature results in a better degree of sphericility, smaller sizes of 32.19±7.92 μm for the buckled holes, and relatively smaller mean particle sizes of 96.95±6.79 μm (Figure 1o). Drying temperatures lower than 150 oC are not appropriate due to the inadequate


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drying resulting in sticky issues. Besides, the synthesis of hierarchical zeolite microparticles with smaller particle sizes is possible by decreasing the orifice diameter of the nozzle and the solid concentration. As an example, by using a nozzle with a smaller orifice diameter of 50 μm with all the other experimental parameters kept the same as those for the synthesis of SD-ZSM-5-190-12M, zeolite microparticles with uniform and smaller particle sizes of ~ 70.6 μm can be obtained (Figure S1). The surface of and internal fine structures of the zeolite microparticles were further investigated. Figure 2a shows the typical SEM image of the surface of a SD-ZSM-5-190-12-W microparticle obtained with the purified zeolite suspension, numerous and random packed nanozeolites can be observed. The cross section image (Figure 2b) shows that the microparticle is hollow with a shell thickness about 8 ~14 μm. Figure 3c shows an interface zone of external and internal areas of the microparticle. It clearly shows a quite similar structural architecture inside the microparticle as that of the surface. The nanozeolites, both inside and on the surface of the microparticle, are identical as compared with the original nanozeolites (Figure 2g), indicating that the structure of the nanozeolites can be well preserved during the spray drying process. Notably, the zeolite microparticles obtained from the as-synthesized nanozeolite mother suspension shows a significantly different fine structure. Numberless small nanoparticles of 20 ~ 30 nm in diameter can be clearly observed on its surface (Figure 2d). These nanoparticles are tightly packed to form a relatively dense surface. It is reasonable to infer that these nanoparticles are originated from the soluble silica species in the as-synthesized nanozeolite suspension, which would migrate to the droplet surface during solvent evaporation and be converted to nanoparticles after drying and calcination. After crushing, the cross section of an individual microparticle shows the solid structure (Figure 2e). The SEM image of an interface zone shows the zeolite nanozeolites in the


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SD-ZSM-5-190-12-M sample are less resolved and more tightly packed and connected (Figure 2f). The SEM image of an interface zone of the microparticle further reveals that the microparticle is composed of a dense surface layer composed of small nanoparticles and a mesoporous interior composed of interconnected nanozeolites (Figure 2f). These observations indicate that the nanozeolite in the SD-ZSM-5-190-12-M sample are “cross-linked” by amorphous silicates. Transmission electron microscope (TEM) images further reveal that the microparticle are composed of interconnected nanozeolites (Figure 2h), and plenty of interparticle mesopore voids of about 4 ~ 7 nm can be observed among the interconnected zeolite nanoparticles (Figure 2i).

Framework properties The powder X-ray diffraction (XRD) patterns and the calculated relative crystallinity of these zeolite samples are shown in Figure 3 and Table 1. All the zeolite samples exhibit the characteristic peaks assigned to the pure MFI structure.48-50 The sample SD-ZSM-5-190-12-W presents the identical diffraction peaks and crystallinity to those of N-ZSM-5, due to the complete removal of the soluble silicate species in the purified precursor. The SD-ZSM-5-190-12-M microparticles obtained from the as-synthesized nanozeolite mother suspension shows 7.8 % reduction in the intensity of diffraction peaks (Figure 3, curve c-f), and ~3 % reduction in crystallinity (Table 1). This is due to the presence of amorphous silica originating from the soluble silicates (~ 7.9 %) in the as-synthesized nanozeolite mother suspension. Some supernatant during the centrifugationwashing process for purifying the mother suspension was collected, dried at room temperature and calcined. The XRD pattern of the obtained solid shows a totally amorphous structure (Figure 3, curve h).


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Fourier transform infrared spectroscopy (FT-IR) spectra show identical absorption bands for the N-ZSM-5 and SD-ZSM-5-190-12-W samples (Figure 4A, curves a and b), indicating that the chemical properties of the zeolite nanoparticles can be preserved over the spray drying process. Specifically, the peak at ~ 454 cm-1 is ascribed to the vibration of the internal T-O bonds of TO4 tetrahedra (T = Si, Al). The band observed at 550 cm-1 is associated with double five member rings.51,52 The bands at 800 cm-1, 1106 cm-1, and 1225 cm-1 are assigned to the external symmetric stretching, internal asymmetric stretch, and external asymmetric stretch of Si-O-T,53,54 respectively. Distinctly, the FT-IR spectrum of the SD-ZSM-5-190-12-M sample obtained from the as-synthesized zeolite mother suspension shows a significantly weakened band of 973 cm-1 (Figure 4A, curve c). This weak absorption is mostly attributed to the vibration of the external surface silanol groups on the zeolite frameworks.55 A significant intensity loss of this absorption band is most likely caused by the chemical crosslink of these silanol groups with those from the amorphous silica during drying and calcination. This result further verifies that the zeolite nanoparticles in the SD-ZSM-5-190-12-M sample are interconnected, in accordance with SEM and TEM observations. The solid 29Si nuclear magnetic resonance (NMR) spectra of the samples N-ZSM-5, SD-ZSM5-190-12-W and SD-ZSM-5-190-12-M are shown in Figure 4B. The sample SD-ZSM-5-190-12W exhibits the same spectral features as those of the sample N-ZSM-5 (Figure 4B, curves a, b). The observed dominating peak at -113 ppm overlaps partly with the shoulder peak at ~ -116 ppm. These resonances are assigned to Si*(OSi)4 sites in the framework of the ZSM-5 structure. The 29Si

MAS NMR spectra also exhibit weak and broad resonances in the range from -99 to -108 ppm,

which are ascribed to AlOSi*(OSi)3 and/or HOSi*(OSi)3 sites in the ZSM-5 crystals.56 As compared with the samples N-ZSM-5 and SD-ZSM-5-190-12-W, the peak intensity at -106 ppm


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obviously decreases for the sample SD-ZSM-5-190-12-M (Figure 4B, curve c), reflecting a decreased amount of silanol groups. This result further reveals that the zeolite nanoparticles in the sample SD-ZSM-5-190-12-M are “cross-linked” by the amorphous silicates via condensation of silanol groups. On the other hand, the 27Al MAS NMR spectra of the samples NZ-ZSM-5 and SDZSM-5-190-12-W both show only a single signal centered at ~ 54 ppm (Figure 4C, curves a and b), which is attributed to the tetrahedral framework Al species. For the sample SD-ZSM-5-19012-M, an additional weak peak at ~ 0 ppm can be observed (Figure 4C, curve c), which is attributed to the octahedral Al species. Roughly, in the sample SD-ZSM-5-190-12-M, ~ 95 % of Al exists in the zeolite framework, while the rest is located on the external surfaces because of the presence of amorphous aluminosilicates. Therefore, the acidity of the sample SD-ZSM-5-190-12-M is majorly originated from the framework Al with a minor and weak contribution from the amorphous aluminosilicates.

Textural properties Figure 5A illustrates the N2 adsorption/desorption isotherms of all the zeolite samples. The steep N2 uptake observed for all samples in the low relative pressure region (P/P0 0.1 and no obvious hysteresis observed (Figure 5A, curve g), indicating the absence of mesoporosity. The sample SD-ZSM-5-


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190-12-W exhibits very similar isotherms as compared with N-ZSM-5 (Figure 5A, curve b), indicating that the zeolite microparticles obtained from the purified nanozeolite suspension precursor resemble a pack of zeolite nanoparticles. The sample SD-ZSM-5-190-12-M obtained from the as-synthesized zeolite mother suspension precursor shows combinative type I and IV isotherms with wide H4 hysteresis loops (Figure 5A, curves c-f). This type of hysteresis is usually found in solids consisting of aggregates or agglomerates of particles forming slit shaped pores,59 in accordance with the structural features (tightly packed and interconnected zeolite nanoparticles) of these microparticles. Figure 5B shows the BJH pore size distribution of all the zeolite samples. From geometrical considerations, the smallest pores that can be found in a random close packing of nanosphere are those among three nanospheres. Therefore, a pore diameter of 19.6 nm can be calculated in the packed nanozeolite of 87 nm in size, which well matches the pore size distribution (16-28 nm) of the sample N-ZSM-5 (Figure 5B, curve a). The pore size distribution is slightly broadened to 18 ~ 51 nm for the sample SD-ZSM-5-190-12-W (Figure 5B, curve b), which can be attributed to a relatively loose packing inside the zeolite microparticles. This result indicates that zeolite microparticles obtained from the purified nanozeolite suspension resemble a pack of zeolite nanoparticles. For the samples SD-ZSM-5-190-y-M obtained from the as-synthesized zeolite mother suspension, weak and broad pore size distributions in the range of 16 ~ 50 nm and an emerged sharp peak at ~ 6 nm can be observed (Figure 5B, curves c-e). This new peak reveals the filling and crosslinking of amorphous silicates among the interparticle voids of the zeolite nanoparticles. It is worth mentioning that the emerged new peak shift to ~ 10 nm for the sample SD-ZSM-5-150-12-M obtained at a lower drying temperature (Figure 5B, curve f), due to the less tightly packed structures at a lower drying temperature.


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Table 1 summarizes the pore parameters of all the zeolite samples. The sample N-ZSM-5 exhibits the highest micropore surface area of 286 m2/g and a high external surface area of 189 m2/g, while the commercial sample C-ZSM-5 shows the lowest external surface area of 37 m2/g. The sample SD-ZSM-5-190-12-W shows similar micropore and external surface areas as compared with those of N-ZSM-5, but the total pore volume (0.55 cm3/g) is higher due to the loose packing. The samples obtained from the as-synthesized zeolite mother suspension show relatively larger BET surface areas and external surface areas than those of N-ZSM-5 due to the emerged mesopores. The surface areas of these samples decrease with the decrease of solid concentration and drying temperature. On the contrary, the micropore surface areas of these samples decreases to some extent (2.8 ~ 7.3 %) as compared with the N-ZSM-5, which can be attributed to the presence of ~ 7.9 wt% of amorphous silicates that may block some micropores. The above results suggest that hierarchically micro/mesoporous zeolite microparticles have been successfully obtained and their porosity can be adjusted to some extent by changing the drying temperature and solid concentration.

Acidity properties Figure 6 shows the NH3-TPD (temperature programmed desorption) profiles of the zeolite samples. It can be observed that all the samples exhibit two desorption peaks. The low-temperature peak (located at ~ 200 oC) can be ascribed to the ammonia desorption from the weak acid sites while the high-temperature peak (located at ~ 375 oC) can be attributed to the desorption from the strong acid sites. The NH3-TPD profiles of all the zeolite microparticles are similar, indicating that the intrinsic acidic properties of the N-ZSM-5 can be well sustained. Evaluation of mechanical stability


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The mechanical stability of the zeolite microparticles upon exposure to ultrasound and vigorous magnetic stirring were evaluated. Two typical samples, SD-ZSM-5-190-12-W and SD-ZSM-5190-12-M, were dispersed in water and then subjected to ultrasonic treatment (600 W, 40 kHz) for 15 min. The particle size distributions before and after treatment are shown in Figure S2. For the untreated sample, their mean particle sizes are 101.46 and 88.58 μm, respectively, in good agreement with the values evaluated from SEM observations. After the ultrasonic treatment, the mean particle size of SD-ZSM-5-190-12-W dramatically decreases from 101.46 to 67.52 μm and becomes broader with the emergence of an obvious minor peak centered at 11.57 μm (Figure S2a). On the contrary, the mean particle size of SD-ZSM-5-190-12-M remains stable (Figure S2b). This result indicates that the sample obtained from the as-synthesized zeolite mother suspension is mechanically much more stable than the sample obtained from the purified zeolite suspension because of the chemical binding by amorphous silicates in the former sample. To further investigate the mechanically stability, a vigorous magnetic stirring (1000 rpm for 15 min) were conducted before ultrasonic treatment. Figure S3a shows the optical images of treated samples. The dispersion of the sample SD-ZSM-5-190-12-W after the treatment becomes a stable suspension, and cannot be completely settled down even after 24 h. This indicates that a considerable fraction of the microparticles can be fragmented or even fully disassembled into nanozeolites after the treatment. SEM image further confirms that the microparticles are cracked into small irregular particles (Figure S3b), and the disassemble packed nanozeolites can be clearly observed (Figure S3c). By contrast, the dispersion of sample SD-ZSM-5-190-12-M after the treatment can be settled down easily with very clear upper solution (Figure S3a). SEM image shows that most of the microparticles are still intact with the zeolite nanoparticles still connected together (Figure S3d).


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Particle formation mechanism Based on the above results, we propose the assembly mechanism of the uniform zeolite microparticles with hierarchical structures (Scheme 1). In both cases of using the purified and mother nanozeolite suspensions as the precursors, the atomized liquid droplets with uniformly distributed solutes are warmed up instantaneously in the initial stage of drying (Scheme 1a, e). Then, water is fast evaporated while the droplet shrinks isotropically. With the air-liquid interface temperature approaching that of the drying environment (150 ~ 190 oC), surface water is fully evaporated and the surface is piled up with aggregated solute particles, forming surface shells (Scheme 1b, f). In the case of using the purified nanozeolites as the precursor, the surface shell is composed of only aggregated zeolite nanoparticles (Scheme 1b, c). In the case of using the assynthesized nanozeolite mother suspension as the precursor, the shell is composed of aggregated zeolite nanoparticles filled with the zeolite template TPA+ and nano-sized amorphous silicates (Scheme 1f, g). With continuous drying and water evaporation, intensive inward capillary forces act on the surface shells. In the case of using the purified nanozeolites as the precursor, due the weak interactions (mainly weak Wan der Waals forces) among the zeolite nanoparticles, the surface shell is driven inward by the capillary strains and then buckled and further crumpled, leading to the formation of crumpled bowl-like microparticles (Scheme 1d, and Figure 1b). Inside the microparticles, loosely packed zeolite nanoparticles form numerous inter-particle mesopore voids with a broad range of 16 ~ 28 nm. The loose packing is because the diffusion of the zeolite nanoparticles is insufficient over the short drying period. On the other hand, in the case of using the as-synthesized nanozeolite mother suspension as the precursor, much strong interactions exist among the solutes of the surface shell. The electrostatic interactions among the template TPA+


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ions, the zeolite nanoparticles and amorphous silicates can bind them together. The hydrogen bonding and crosslinking of the silanol groups among the amorphous silicates and the zeolite nanoparticles can further make the shell rigid (Scheme 1i). The TPA+/amorphous silicates can form a network to support the zeolite nanoparticles. As a result, the shell is buckled only in small areas imposed by the capillary forces, leading to the formation of spherical microparticles with small buckled holes and relatively smaller particles sizes as compared with those obtained from the purified suspension (Scheme 1h, and Figure 1a). After calcination, the particle morphologies are sustained. For the microparticles obtained from the purified zeolite nanoparticles, the crosslinking among the zeolite nanoparticles is limited because of the low concentration of surface silanol groups and small contact areas among the zeolite nano particles. On the contrary, the chemical crosslink among the zeolite nanoparticles and the amorphous silicates are much more intensive. The formed amorphous silicate network can also support the zeolite nanoparticles. As a result, the microparticles obtained from the mother suspension possess a much high mechanical stability. On the surfaces of these microparticles, the nano-sized amorphous silicates make the particle surface relatively dense. Inside the microparticles, the zeolite nanoparticles are chemically linked and filled and supported by the amorphous silica network, leading to the formation of small interconnected mesopores of 6 ~10 nm.

Acid catalytic performance For the catalytic acetolysis of ECH with acetic acid, although both the two small reactants can diffuse smoothly in the micropores and mesopores, the product of 1-acetoxy-3-choloro-2-propanol (Figure 7A) and the possible intermediates are much larger molecules. Their diffusion can be limited in the micropores. As a result, the conversion of ECH over the sample N-ZSM-5 is about


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75.24 %, which is 7.2 times higher than that (10.41 %) of C-ZSM-5 (Figure 7B), indicating that the presence of inter-particle pores can dramatically improve mass transport and thereby lead to a remarkable catalytic activity. As expected, the sample SD-ZSM-5-190-12-W shows a slightly enhanced catalytic activity of 77.75 % as compared with N-ZSM-5 (Figure 7B), due to the enlarged external surface area and total pore volume whereas the surface area and acid strength are close. The zeolite microparticles obtained from the as-synthesized zeolite mother suspension also show much better catalytic performance as compared with the commercial sample (Figure 7B), and only slightly inferior to that of the N-ZSM-5. The slightly lower catalytic activity is mostly caused by the presence of amorphous silica (~ 7.9 wt%), which can slightly deteriorate the accessibility of the zeolite nanoparticles. Among the zeolite microparticles obtained from the mother suspension, the sample SD-ZSM-5-150-12-M retained the highest catalytic activity (~ 96 %) of that of NZSM-5 (Figure 7B). In order to gain a better understanding of the catalytic reaction, the reaction kinetics were explored. The ECH conversion versus time at same reaction condition are shown in Figure 7C. The kinetic rate constants (ka), which are obtained by fitting of the experimental data according to the pseudo first order kinetic equation, -ln(1-x) = kat, are 0.0160 and 0.0141 min-1 for the samples N-ZSM-5 and SD-ZSM-5-150-12-M, respectively. The similar rate constants indicate that the hierarchical pore structures of the zeolite microparticles render as good as mass transport properties as those of the nanozeolites. Besides, after five catalytic cycles, only a slight decrease in catalytic activity can be observed (Figure 8a), which may be attributed to the mass loss of the catalyst over the sample recycling processes. Moreover, the morphology of the sample after five cycles of catalytic test generally remains intact with only a very small fraction of fragmented microparticles due to the shear force over the reaction process (Figure 8b). This result indicates that the sample has a good catalytic stability.


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The catalytic behavior of obtained zeolite samples in LDPE degradation reaction were studied. As shown in Figure 9A, the T50 (50 wt% conversion of LDPE) of pure LDPE is ~ 472 oC, comparable to reported results.16, 60, 61 This temperature is slightly reduced to 433 oC with a T50 reduction of 39 oC with the commercial sample C-ZSM-5 as the catalyst. The relatively poor catalytic activity can be assigned to its very small external surface area of 37 m2/g and inaccessibility of active sites inside the microporous system. The T50 temperature over the sample N-ZSM-5 is further reduced to 375.5 oC, and the T50 reduction is up to 96.3 oC as compared with that without catalyst. The higher catalytic performance of N-ZSM-5 results from the larger external surface providing more accessible external surface acid sites than that of C-ZSM-5.60,62 As expected, the sample SD-ZSM-5-190-12-W exhibits the identical catalytic performance as that of N-ZSM-5, due to their similar acid and textural properties. The cracking performance of the zeolite microparticles obtained from the as-synthesized zeolite mother suspension at different solid concentration or drying temperatures are slightly inferior to that of N-ZSM-5, and far better than that of the commercial sample C-ZSM-5 (Figure 9B). Normally, the cracking of LDPE can only proceed on the external surface acid sites due to the larger molecular size of LDPE, which cannot diffuse into the micropores of zeolites. Therefore, the much better performance of the synthesized zeolite samples as compared with the commercial one is mainly due to their much higher external surface areas (Table 1). The slight inferior performance of the samples obtained from the zeolite mother suspension as compared with the sample N-ZSM-5 can be ascribed to the presence of ~ 7.9 wt% of amorphous silica in the microparticles which reduce the total amount of acid sites and can retard or partially block the acid site accessibility.


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CONCLUSIONS In summary, uniform robust ZSM-5 microparticles with hierarchical porosities have been obtained by using a spray-drying-assisted chemical crosslinking assembly method with the as-synthesized nanozeolite mother suspension as the precursor. They show variable morphologies (spherical, bowl-like and dimpled) and controllable particle sizes (70.6 ~ 108 μm). They possess uniform mesopores (~ 6 nm), as well as well-maintained high surface area, large pore volume, high microporosity and strong acidity of the original nanozeolites. Assembly of zeolite microparticles with the purified zeolite suspension as the precursor has been comparatively studied with the particle evolution process discussed. The presence of the soluble aluminosilicate species is essential for forming uniform morphology and sizes and enhancing mechanical stability. The soluble aluminosilicate species act as the stabilizer for droplet drying and the “cross-linking” agent for nanozeolite binding over the drying and calcination processes. The obtained zeolite microparticles show a remarkable catalytic activity (~ 7.0 times higher than that of commercial ZSM-5) and a high stability in ECH acetolysis reaction. They can be also applied for the catalytic cracking of LDPE with performance much better than that of the commercial ZSM-5 and parallel to that of nanozeolites. The improved catalytic performances are attributed to the well-preserved high external surface area, large microporosity and strong acidity of the original ZSM-5 nanozeolites, as well as the emerged mesopores. The synthesis method can potentially provide a route for large-scale production of hierarchical zeolites. It is also expected to be feasible for the synthesis of other uniform hierarchical materials for various applications.

ASSOCIATED CONTENT Supporting Information.


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Additional information about the chemicals, characterization and measurement, SEM image of zeolite microparticles obtained with a nozzle with an orifice diameter of 50 μm, the particle size distribution before and after sonication, the optical images of the dispersions before and after magnetic stirring and sonication, and the SEM images of zeolite microparticles after magnetic stirring and sonication were included in the supporting information. This material is available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors acknowledge the financial supports form National Key Research and Development Program of China (International S&T cooperation program, ISTCP, 2016YFE0101200) and the National Natural Science Foundation of China (21676172, 21875153, 21501125). We also thank the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions


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and the Project of Scientific and Technologic Infrastructure of Suzhou (SZS201708) for supports. X. D. Chen and Z. Wu acknowledges the start-up funds from Soochow University.

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Figure 1. SEM images and the corresponding particle size distributions of the samples SD-ZSM5-190-12-M (a-c), SD-ZSM-5-190-9-M (d-f), SD-ZSM-5-190-6-M (g-i), SD-ZSM-5-190-12-W (j-l), and SD-ZSM-5-150-12-M (m-o), respectively.


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Figure 2. SEM images of the surface, internal, and boundary areas of SD-ZSM-5-190-12-W (a-c) and SD-ZSM-5-190-12-M (d-f). SEM image of N-ZSM-5 (g), and TEM images of SD-ZSM-5190-12-M (h, i). The red dotted lines in image c and f are the boundary areas of the external and internal of the zeolite microparticles.


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Figure 3. XRD patterns of the samples N-ZSM-5 (a), SD-ZSM-5-190-12-W (b), SD-ZSM-5-19012-M (c), SD-ZSM-5-190-9-M (d), SD-ZSM-D-190-6-M (e), SD-ZSM-5-150-12-M (f), C-ZSM5 (g), and the amorphous silica obtained from the as-synthesized zeolite mother suspension (h), respectively.


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Figure 4. FT-IR spectra (A), solid 29Si MAS NMR spectra (B), and solid 27Al MAS NMR spectra of the samples NZSM-5 (a), SD-ZSM-5-190-12-W (b), and SD-ZSM-5-190-12-M (c), respectively.


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Figure 5. N2 adsorption/desorption isotherms (A) and the corresponding pore size distribution curves (B) of the samples N-ZSM-5 (a), SD-ZSM-5-190-12-W (b), SD-ZSM-5-190-12-M (c), SDZSM-5-190-9-M (d), SD-ZSM-D-190-6-M (e), SD-ZSM-5-150-12-M (f), and C-ZSM-5 (g), respectively.


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Figure 6. NH3-TPD profiles of the samples N-ZSM-5 (a), SD-ZSM-5-190-12-W (b), SD-ZSM-5190-12-M (c), SD-ZSM-5-190-9-M (d), SD-ZSM-D-190-6-M (e), SD-ZSM-5-150-12-M (f), and C-ZSM-5 (g), respectively.


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Figure 7. Reaction diagram of epichlorohydrin with acetic acid (A), the ECH conversion (B) and reaction kinetics (C) over N-ZSM-5 (a), SD-ZSM-5-190-12-W (b), SD-ZSM-5-190-12-M (c), SDZSM-5-190-9-M (d), SD-ZSM-D-190-6-M (e), SD-ZSM-5-150-12-M (f), and C-ZSM-5 (g), respectively.


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Figure 8. The catalytic stability performance of the recycled sample SD-ZSM-5-190-12-M for five cycles (a), and the typical SEM image (b) of the sample collected after the five-cycle catalytic test. The red dotted circles indicate the broken microparticles.


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Figure 9: Residual weight curves of LDPE catalytic cracking (A), and the corresponding T50 reduction compared with LDPE catalytic cracking without catalyst (B) of N-ZSM-5 (a), SD-ZSM5-190-12-W (b), SD-ZSM-5-190-12-M (c), SD-ZSM-5-190-9-M (d), SD-ZSM-D-190-6-M (e), SD-ZSM-5-150-12-M (f), C-ZSM-5 (g) and pure LDPE (h).


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Scheme 1: Proposed formation mechanism of the uniform zeolite microparticles obtained from the purified nanozeolite suspension (a-d), and as-synthesized nanozeolite mother suspension (e-i) via spray drying.


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Table 1 Textural parameters of the various zeolite samples synthesized under different conditions. SBET a

SMicro b

SExterb

VTotal c

VMicrob

(m2/g)

(m2/g)

(m2/g)

(cm3/g)

(cm3/g)

N-ZSM-5

476

286

189

0.50

0.12

100

C-ZSM-5

267

230

37

0.18

0.11

85.1

SD-ZSM-5-190-12-W

480

288

193

0.55

0.12

99.7

SD-ZSM-5-190-12-M

509

265

245

0.51

0.11

97.0

SD-ZSM-5-190-9-M

498

266

232

0.49

0.11

97.6

SD-ZSM-5-190-6-M

491

265

226

0.49

0.11

97.5

SD-ZSM-5-150-12-M

492

278

214

0.47

0.11

97.6

Sample

RC d (%)

a

The surface area calculated by using the Brunauer-Emmett-Teller (BET) method, b The micropore and external surface area and micropore volume calculated by using t-plot method, c The total pore volume calculated from the volume adsorbed of at a P/P0 of 0.99, SExtra = SBETSMicro, and d The relative crystallinity estimated from the XRD patterns with the purified nanozeolites as a reference.


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TOC figure


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Chemical Crosslinking Assembly of ZSM-5 Nanozeolites into Uniform

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