Stable, Porous, and bulky Particles with High External Surface and Large Pore Volumefrom Self-assembly of Zeolite Nanocrystals with Cationic Polymer

Jiangwei Song, Limin Ren, Chengyang Yin, Yanyan Ji, Zhifeng Wu, Jixue Li, andFeng-Shou Xiao*State Key Laboratory of Inorganic Synthesis and PreparatiVe Chemistry, Jilin UniVersity,Changchun 130012, P.R. China

ReceiVed: January 21, 2008; ReVised Manuscript ReceiVed: March 24, 2008

Stable, porous, and bulky beta zeolite particles (bulky-beta) with high external surface area and large porevolume were fabricated from the self-assembly of beta nanocrystals with cationic polymers under hydrothermalcondition, followed by filtration and calcination. The samples were characterized using powder X-ray diffraction,scanning electron microscopy, transmission electron microscopy, N2 isotherm, and thermogravimetricanalysis-differential thermal analysis. The results show that bulky-beta contains disordered mesopores, andits mesoporous walls are connected by beta nanocrystals with each other. The formation of mesoporosity inbulky-beta is proposed by the use of mesoscale organic template of cationic polymer in the synthesis. Thebulky particles of bulky-beta are mechanically stable even if subject to an ultrasonic treatment for 15 min.This is very useful for the separation of zeolite particles from a slurry system via a filtration route. Veryinterestingly, these particles show almost the same catalytic activity and a very high yield of solid product,compared with nanocrystals of beta zeolite. This method may provide a novel route for the synthesis ofcatalytically active zeolites, enabling good mass transport as stable and active catalysts in potential industrialapplications.

1. Introduction

Currently, great interest exists in the preparation of zeolitenanocrystals due to the decrease in the crystal size that resultsin high external surface area, fast diffusive rate, and manyexposed active sites.1–4 The preparation of zeolite nanocrystals,however, is very complex, compared with micrometer-sizedzeolites. Usually, zeolite nanocrystals are obtained from high-speed centrifugation of a slurry system, making it difficult forindustrial mass production.1–5 In particular, very small nano-crystals cannot be separated via the centrifugal route, leadingto a decrease in nanocrystal yield.1 In contrast, a large amountof industrially used zeolites with micrometer-sizes are normallyobtained from a filtration route that facilitates the ease ofseparation of zeolite crystals from the slurry system, offeringthe advantage of having a high yield of solid product.

To overcome the problems of zeolite nanocrystal preparation,several novel routes have been reported for the synthesis ofzeolites with fast mass transfer in catalytic reactions.6–14 Forexample, mesoporous zeolites with high external surface areaand mesopore volume have been successfully templated fromnanosized carbons,6,7 polystyrene sphere,8 silane-functionalizedpolymer porogen,9 organic-inorganic hybrid surfactant,10 cat-ionic polymers,11 and polymer resin;12 closely packed zeolitenanocrystals with mesopores are effectively transformed fromporous amorphous silica;13 nanozeolite microspheres withmesopores are formed by adding urea and formaldehyde duringsynthesis.14 Notably, the industrial applications of these meso-porous zeolites are still limited by the complexity of theirsynthetic procedures.

On the other hand, it is very successful to prepare orderedmesoporous materials from the self-assembly of small silica

species with various surfactants.15–19 For instance, MCM-41,with highly ordered hexagonal mesopores, is prepared from theself-assembly of silica species with cetryltrimethylammoniumbromide (CTAB) surfactant under alkaline condition;15 SBA-15, with larger ordered mesopores, is synthesized from the self-assembly of silica species with triblock polymer (P123) understrongly acidic media;16 MSU17 and MAS samples18 withordered mesostructures are obtained from the self-assembly ofpreformed zeolite nanoclusters with surfactants. In these ex-amples, the suitable Coulombic interaction between small silicaspecies with surfactants is a key factor for the self-assembly.15–19

In these cases, the sizes of silica species and preformed zeolitenanoclusters are relatively small (less than several nanometers),which could self-assemble with surfactant micelle due to thesuitable Coulombic interaction.20 However, if the crystal sizesof silica species are large enough (e.g., larger than 20 nm), itis not easy to take a self-assembly between bulky silica specieswith the surfactants. One possible explanation is that there isnot enough interaction between the bulky silica species and thecurrent surfactants that have a limited charge density.

Herein we report an alternative and simple route for one-potsynthesis of stable, porous, and bulky particles with high externalsurface area and large pore volume from self-assembly of zeolitenanocrystals with mesoscale cationic polymers that possess ahigh charge density. It is important to note that, because theseparticles exhibit a high solid yield, they can be easily separatedvia a filtration process. Catalytic alkylation of benzene withisopropanol shows that these bulky particles exhibit similaractivity and selectivity to zeolite nanocrystals. Obviously, thesebulky zeolite particles combine advantages of both the nano-crystals (high external surface area, large mesopore volume, andfast diffusive rate) and the micrometer-sized crystals (easyfiltration and high solid yield).

* To whom correspondence should be addressed; e-mail: fsxiao@mail.jlu.edu.cn; fax: +86-431-85168624; phone: +86-431-85168590.

J. Phys. Chem. C 2008, 112, 8609–8613 8609

10.1021/jp800598p CCC: $40.75 2008 American Chemical SocietyPublished on Web 05/20/2008

2. Experimental Methods

All reagents were used without any further purification.Tetraethylammonium hydroxide (TEAOH, 20-25 wt%) wassupplied by Changling Catalyst Company (China). Polydial-lyldimethylammonim chloride (PDADMAC, 40 wt%) waspurchased from Hangzhou Yinhu Chemical Company Ltd.(China). NaOH, NaAlO2, and fumed silica were obtained fromShanghai Chemical Reagent Company (China).

In a typical synthesis of stable, porous, and bulky betaparticles (bulky-beta), NaOH (0.16 g) and NaAlO2 (0.30 g) weremixed with TEAOH (31 mL), followed by the addition of fumedsilica (4.8 g). After the mixture was stirred for 5 h at roomtemperature, a homogeneous aluminosilicate gel was obtained.Then, the gel was crystallized at 140 °C for 3 d in a Teflon-lined stainless steel autoclave, and a nanosized beta zeolite slurrywas achieved, followed by the addition of cationic polymer(PDADMAC, 1.2 g) and stirring at room temperature for 10min. After the hydrothermal treatment at 140 °C for 3 d, thesolid product was collected by filtrating, dried in air, andcalcined at 550 °C for 5 h. In comparison, beta nanocrystals(nano-beta) were separated from the beta nanocrystal slurryusing high-speed centrifugation.

The morphology of the samples was observed with a field-emmission scanning electron microscopy (FESEM) on a JSM-6700F electron microscope (JEOL, Japan). Transmissionelectron microscopy (TEM) experiments were performed on aJEM-3010 electron microscope (JEOL, Japan) with an accelera-tion voltage of 300 kV. The powder X-ray diffraction (XRD)data were collected on a Rigaku D/MAX 2550 diffractometerwith Cu KR radiation (λ ) 1.5418 Å). Nitrogen adsorption anddesorption isotherms at 77 K were measured using a Micromer-itics ASAP 2020 M system. The samples were degassed for10 h at 300 °C before the measurements. Differential thermalanalysis (DTA) and thermogravimetric analysis (TG) wereperformed with Perkin-Elmer TGA7 and DTA-1700 apparatus,respectively.

The alkylation of benzene with isopropanol was carried outin a microreactor (inner diameter of 10 mm, weight of catalystof 0.5 g, sizes of 20-30 mesh) at a weight hourly space velocity(WHSV) of 10.0 mL/h and with a benzene/isopropanol ratio of4/1, at 200 °C, under a reaction pressure of 2.0 MPa. Thebenzene and isopropanol were pumped into the reactor usingthe pump (NP-KX-105). The products were analyzed online bya gas chromatograph (GC, Agilent 6890) equipped with a HP-5column and a flame ionization detector (FID).

3. Results and Discussion

Figure 1 shows X-ray diffraction (XRD) patterns of nano-beta and bulky-beta, revealing a series of peaks at 7.8, 13.5,14.6, 21.5 and 22.5° associated with beta zeolite structure (BEA)structure.21 Notably, compared with micrometer-sized betazeolite previously reported,22 the samples (Figure 1, panels aand b) show very broad peaks, suggesting that the sample sizesare very small.23 Figure 2a shows scanning electron micrograph(SEM) images of nano-beta, confirming the nanocrystals of betazeolite. The nano-beta sample has very uniform nanocrystalsin the range of about 30-80 nm. However, after hydrothermaltreatment of these beta nanocrystals with cationic polymer at140 °C for 3 days, bulky zeolite particles in the range of 2-20µm (bulky-beta, Figure 2b and Supporting Information FigureS1) were formed. These bulky particles (bulky-beta) are

Figure 1. XRD patterns of (a) beta nanocrystals (nano-beta) and (b)the sample self-assembled from beta nanocrystals in the presence ofcationic polymers (bulky-beta).

Figure 2. SEM images of (a) nano-beta and (b) bulky-beta underultrasonic treatment for 15 min at room temperature.

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mechanically stable, even if subject to ultrasonic treatment for15 min (Figure 2b). Clearly, the formation of bulky zeoliteparticles is favorable for the separation of zeolite solid productsfrom a slurry system via a filtration route.

Figure 3 shows transmission electron micrograph (TEM)images of bulky-beta. Notably, there is mesoporosity in thebulky-beta sample (Figure 3a), and the mesoporous walls areconnected by beta nanocrystals with each other. Particularly,TEM image in high magnification (Figure 3b) shows that thesample contains very small sizes of beta nanocrystals (less than20 nm). Possibly, the successful self-assembly of very smallsizes of beta nanocrystals in bulky-beta results in the high yieldof zeolite products. In contrast, it is difficult to observe suchsmall nanocrystals in the nano-beta sample collected by thecentrifugation (Figure 2a). Supporting Information Figure S1

shows the irregular morphology of bulky-beta with various sizes,indicating that it is not easy to control the particle sizes of bulky-beta.

Figure 4 shows nitrogen isotherms of nano-beta and bulky-beta samples, and their textural parameters are presented inTable 1. Notably, both samples show a steep increase occurringin the curve at a relative pressure of 10-6 < P/P0 < 0.01, whichis due to the filling of micropores.21 Another step can beidentified in the adsorption curve at a relative pressure of 0.85< P/P0 < 1.0, which is probably due to the presence of largemesoporous structures,15–19 in good agreement with thoseobserved in TEM images (Figure 3b). Furthermore, it isobserved that bulky-beta and nano-beta samples have verysimilar BET micropore surface area (417 and 423 m2/g), externalsurface area (185 and 184 m2/g), and micropore volume (0.19and 0.20 cm3/g), but bulky-beta has larger mesopore volume(0.84 cm3/g) than nano-beta (0.65 cm3/g, Table 1). The similarmicropore surface area and micropore volume suggest thatbulky-beta and nano-beta have similar zeolite crystallinity; thesimilar external surface area suggests that bulky-beta and nano-beta have similar crystal sizes; the difference in mesoporevolume between bulky-beta and nano-beta suggests the contri-bution of the cationic polymer in the preparation of the bulky-beta sample.

Figure 5a shows the TGA curve of as-synthesized nano-beta,exhibiting a total weight loss of nano-beta of about 22%, whichoccurs in two steps: about 3% weight loss at 50-200 °C dueto water desorption and about 19% weight loss at 200-650 °Cdue to TEA+ decomposition. Simultaneously, on the DTA curve(Figure 5a) there are obvious endothermic and exothermic peaksat 50-200 °C and 200-700 °C, which are reasonably assignedto the desorption and decomposition of water and TEA+,respectively. Compared with as-synthesized nano-beta, as-synthesized bulky-beta shows relatively high weight loss,reaching about 28%. Correspondingly, on the DTA curve thereis an additional exothermic peak at 200-400 °C, which isattributed to the decomposition of mesoscale cationic polymer.These results confirm that the bulky-beta sample contains both

Figure 3. TEM images of bulky-beta in (a) low and (b) highmagnification (The marked cycles present typical images of betananocrystals with very small sizes less than 20 nm).

Figure 4. N2 adsorption/desorption isotherms of calcined (a) nano-beta and (b) bulky-beta.

Stable, Porous, and Bulky Particles J. Phys. Chem. C, Vol. 112, No. 23, 2008 8611

a small organic template of TEA+ and a mesoscale template ofcationic polymer.

Additionally, we have also measured the inorganic composi-tion of the mother liquor, after the separation of the bulky-beta,and the solid samples of nano-beta and bulky-beta. The obtainedresults show that their ratios of Na/Si/Al were about 1/1/0, 2.3/15.8/1, and 2.1/16.2/1, respectively. Obviously, the introductionof cationic polymers will reduce the concentration of sodiumions, which also suggests that cationic polymers might have aninteraction with nanocrystals of beta zeolite.

The formation of stable, porous, and bulky particles of betazeolite (bulky-beta) should be directly attributed to the use ofcationic polymer rather than other reasons such as crystallizationtime. If the cationic polymer is absent during synthesis, the bulkyparticles cannot be obtained. Even if crystallization at 140 °Cis prolonged to 6 days, obtained products are still beta nano-crystals. Possibly, in alkaline media, there should be aninteraction between mesoscale cationic polymers with negativelycharged beta nanocrystals through the S+I- route, and thecationic polymers direct the assembly of beta zeolite nano-crystals into bulky-beta zeolite.20 After calcination at 550 °Cfor 5 h to remove the cationic polymers, mesoporosity in bulky-beta is formed, as proposed in Scheme 1. Because both largeand small nanocrystals of beta zeolite in the slurry system canassemble with cationic polymers to form bulky particles, arelatively high yield of solid product (66%) can be obtainedfrom a filtration process. In contrast, during the preparation ofbeta nanocrystals from a centrifugation, beta nanocrystals with

very small sizes cannot be separated from the slurry system,resulting in relatively low yield of solid product (42%, Ta-ble 1).

Particularly, if the cationic polymer used during synthesis issubstituted with normal surfactants such as CTAB, then thebulky particles cannot still be obtained. This phenomenonsuggests that the normal surfactants are difficult to direct theself-assembly of zeolite nanocrystals. The comparison of cationicpolymers with normal surfactants shows obvious differences;(1) cationic polymers have a much higher charge density thannormal surfactants due to their distinguishable molecularstructure; and (2) cationic polymers are mesoscale (5-40 nm),11

whose sizes are comparable with beta nanocrystals. On thecontrary, normal surfactants have relatively small sizes and arenot matching to beta nanocrystals. Possibly, the mesoscalecationic polymers with high charge density are easily assembledwith beta nanocrystals, forming bulky beta particles. Normalsurfactants, on the other hand, have relatively low charge densityand small sizes, as previously mentioned, thus making it difficultto create a strong interaction with beta nanocrystals.

It is worth noting that a large amount of examples for self-assembly of surfactant micelle with small silica species havebeen reported,15–19 but there are few works focused on the self-assembly of mesoscale organic templates with zeolite nano-crystals. Here, we show a novel route for the self-assembly ofbeta nanocrystals with mesoscale cationic polymer, formingbulky beta particles containing mesoporosity. Notably, themesopores in the bulky-beta sample are disordered (Figure 3),which may be related to the use of irregular cationic polymers.11

In comparison, the use of surfactant micelles usually results inthe formation of ordered mesostructures.15–19 However, thereis no obvious difference for mass transfer in catalytic reactionsover disordered and ordered mesoporous catalysts.

As a model catalytic reaction, Figure 6 shows the depend-encies of catalytic activity and selectivity on reaction time in

TABLE 1: Textural Parameters of Nano-beta and Bulky-beta Samples

samplemicropore surface

area,a m2 g-1external surface

area,b m2 g-1micropore volume,

cm3 g-1mesopores volume,c

cm3 g-1 sample solid yieldd %

nano-beta 423 184 0.20 0.65 42bulky-beta 417 185 0.19 0.84 66

a t-plot micropore surface area. b The difference between BET surface area and t-plot micropore surface area. c The difference between totalpore volume and micropore volume. The cumulative total pore volume of pores is between 1 and 500 Å diameter. d Sample solid yield wasestimated by weight ratio of calcined product yield with total of sodium aluminate and silica added in the starting gel.

Figure 5. Thermogravimetric (TG) and differential thermal analysis(DTA) curves of as-synthesized (a) nano-beta and (b) bulky-beta.

SCHEME 1: Proposed route for the synthesis of stable,porous, and bulky beta particles from a self-assembly ofbeta nanocrystals with cationic polymers underhydrothermal condition

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the alkylation of benzene with isopropanol over nano-beta andbulky-beta samples. Very interestingly, although their particlesare obviously distinguishable, bulky-beta shows very similarcatalytic properties to nano-beta, even a little higher than thoseof nano-beta. For example, after reaction occurs for 4.5 h, bulky-beta and nano-beta exhibit activities at near 100 and 98% andselectivities for cumene at 85 and 78%, respectively. Consider-ing the similarities of bulky-beta to nano-beta for Si/Al ratio,aluminum distribution, acidic strength, and BET surface area,it is suggested that similar catalytic properties over two samplesmight result from similar diffusive rate for reactants and productsin the alkylation. Obviously, the use of bulky beta particles withhigh activity in catalysis has an advantage for the preparationof large-scale industrial catalysts via a filtration route.

4. Conclusions

In summary, stable, porous, and bulky particles with highexternal surface area, large pore volume, and high solid yieldhave been fabricated from the self-assembly of beta nanocrystalswith mesoscale cationic polymers under hydrothermal condition.Very interestingly, these zeolite particles show similar catalyticproperties in the alkylation of benzene with isopropanol to betananocrystals. This work, we believe, may provide a new routefor the syntheses of zeolites with good mass transport as stableand active catalysts in potential industrial applications.

Acknowledgment. This work is supported by the State BasicResearch Project of China (2004CB217804) and NationalNatural Science Foundation of China (20573044 and 20773409).

Supporting Information Available: An enlarged SEMimage of bulky-beta is additionally provided. This material isavailable free of charge via the Internet at http://pubs.acs.org.

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Figure 6. The dependence of catalytic activities and selectivities onreaction time in alkylation of benzene with isopropanol over nano-beta (conv. 9, selec. 0) and bulky-beta (conv. 1, selec. 3).

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