Templating route for synthesizing mesoporous zeolites with improved catalytic properties

Nano Today (2009) 4, 292—301

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REVIEW

Templating route for synthesizing mesoporous zeolites with improved catalytic properties Xiangju Meng, Faisal Nawaz, Feng-Shou Xiao ∗ State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Qianjin Street 2699, Changchun 130012, China Received 26 March 2009; received in revised form 30 May 2009; accepted 8 June 2009 Available online 30 June 2009

KEYWORDS Mesoporous zeolites; Templating route; Excellent catalytic properties

Summary Zeolites with intracrystal mesopores have been paid much attention recently due to the combination of the advantages of mesoporous materials and zeolite crystals. This review describes the synthetic routes for preparation of mesoporous zeolites by various templates including nanostructured carbons, mesoscale cationic polymers, mesoscale organosilanes, and nanosized inorganic materials. The interaction between these mesoscale templates with silica or aluminosilicate species in the starting gels has been discussed. Furthermore, the excellent catalytic properties over these mesoporous zeolites are systemically summarized. © 2009 Published by Elsevier Ltd.

Introduction Crystalline zeolites with pore sizes typically at 0.4—1.2 nm are one of the most useful catalysts in industrial processes such as oil refining and organic synthesis, due to their large surface area, high adsorption capacity, uniform and intricate channels, high thermal and hydrothermal stabilities, and well-defined micropores with excellent shape-selectivity in catalysis [1—3]. However, relatively small and sole micropores in zeolites such as Beta, ZSM-5 and Y strongly influence the mass transfer in catalysis, which severely limit their catalytic performance [3—8]. Particularly, the bulky reactants are difficult to contact with active sites in such small micropores of zeolites. To overcome such drawbacks which are

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imposed by the micropores, numerous efforts have been made such as synthesis of zeolite nanocrystals [9], ultralarge pore zeolites and zeolite analogues [1,3,10—25], and ordered mesoporous materials [26—36]. Zeolite nanocrystals have much larger external surface area, which could expose more active sites than conventional zeolite crystals. However, due to this approach it is difficult to separate zeolite nanocrystals from a slurry mixture [9]. A series of successful synthesis of ultra-large pore zeolites and zeolite analogues are considered as an alternative strategy for solving mass transfer and catalytic conversion of bulky molecules, but high cost of organotemplates and relatively low stability hinder their wide applications in industry. The successful synthesis of ordered mesoporous materials [26—36], Fig. 1a such as MCM-41 [26,27] and SBA-15 [34,35] offers a totally new route to solve the limitation of micropores due to their uniform and adjustable mesopores, high surface area, large pore volume and versatile mesoporous walls. Unfortunately,

Templating route for synthesizing mesoporous zeolites

Figure 1

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Categorization of mesoporous materials.

these ordered mesoporous materials exhibit low hydrothermal stability and acidity compared with zeolites crystals, which is mainly attributed to the amorphous nature of the mesoporous walls [1,3,37]. Therefore, the strategies for improving hydrothermal stability of ordered mesoporous materials have been developed [3,24,25,38—58]. For example, hydrothermally stable ordered mesoporous aluminosilicates with strong acidity are successfully synthesized from the assembly of preformed zeolite nanoclusters with surfactant micelle, by combining advantages of mesoporous materials and zeolites crystals instead of upgrading their individual performances, reported almost simultaneously by two independent research groups of Xiao [38—45] and Pinnavaia [46—50]. Ultra-stable ordered mesoporous materials are successfully fabricated by ‘‘High-temperature synthesis’’ route [54,55]. Notably, although the hydrothermal stability of ordered mesoporous materials has been improved significantly, it is still insufficient for many industrial applications such as FCC catalysts [24,58]. The desirable materials are ordered mesoporous materials with crystalline zeolite walls (ordered mesoporous zeolites), which should have advantages of zeolite crystals (good hydrothermal stability and strong acidity) and mesoporous materials (good mass transfer). Unfortunately, it is not successful [1] and one of the reasons is that the mesoporous walls are too thin to accommodate stable zeolite structure (Fig. 1b). On the other hand, creation of disordered mesopores (5—50 nm) in zeolite crystals by post-treatments such as steaming and chemical treatments (acid, alkaline, EDTA, etc.) have been reported for long time (Fig. 1c), and these mesoporous zeolites with much thicker walls than ordered mesoporous materials are efficient for mass transfer and catalytic conversion of bulky molecules [1,19—25,59—69]. In the case of zeolites Y, mesoporosity ranging at 10—20 nm was formed during the dealumination of the zeolite by steaming [19]. Such mesoporous Y zeolite gave a higher catalytic activity in oil refining than conventional Y zeolite [1]. However, the post-treatments result in a decrease of zeolite crystallinity, which could influence catalytic properties in many reactions. In addition, these treatments also lead to the formation of amorphous aluminosilicate fragments existing in mesopores, which has negative effect for mass transfer [20]. Therefore, it is strongly desirable to obtain mesoporous zeolites with good crystallinity. Recently, a number of successful examples, for the preparation of mesoporous zeolites with good crystallinity by addition of mesoscale templates in the synthesis of zeolites,

have been reported [25,70]. These mesoscale templates include nanostructured carbons [71—92], mesoscale cationic polymers [93—97], mesoscale organosilanes [98—103], and nanosized inorganic materials [104,105]. These mesoporous zeolites exhibited superior catalytic properties as compared with conventional zeolites. This article will systemically review the synthesis and catalysis of zeolites with intracrystal mesopores by templating routes, and the zeolites with intercrystal pores formed by crystal aggregation are not discussed.

Nanostructured carbon templating The first example for synthesizing mesoporous zeolites by templating route was reported by Jacobsen et al. [71]. They used nanosized carbon particles (about 12 nm) as mesoscale templates to disperse into the starting aluminosilicate gels. These carbon particles are encapsulated by growing zeolite crystals, producing ZSM-5 crystals embedded with carbon after zeolite crystallization. Removal of the embedded carbon matrix by calcination results in mesoporous ZSM-5 zeolite (Fig. 2). Similarly, mesoporous zeolites with structures of BEA [72], MEL [73] and MTW [74] are also templated by nanosized carbon particles. It is worth noting that nanosized carbon particles are generally hydrophobic, and it is not easy to disperse them homogeneously into the gels during the synthesis of zeolites. Therefore, it is better to pretreat these carbon particles in acidic or alkaline media, forming surface oxygen species with hydrophilic feature. Generally, the morphology of nanosized carbons is sphere-like, therefore the mesopores in zeolite crystals are mainly cave-like. In this case, these intracrystal mesopores are not open to the external surface of zeolite crystals. This shortcoming would influence catalytic conversion of bulky molecules. Schmidt et al. [75] and Boisen et al. [76] have used carbon nanotubes as templates to produce zeolites with mesoporous channels. In these samples, the mesopores of 12—30 nm in widths are uniform and straight, in accordance with the diameters of the carbon nanotubes (Fig. 3). If these carbon nanotubes are long enough, the straight mesopores could pass through zeolite crystals, which is favorable for catalytic conversion of bulky molecules. Similar mesoporous zeolites were also obtained by using carbon nanofibers as templates which are much cheaper than nanotubes [77].

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Figure 2 Schematic illustration of mesoporous zeolites templating from carbon nanoparticles. Reproduced by permission of Ref. [71]. Copyright 2000 American Chemical Society.

The catalytic properties of these mesoporous zeolites have been carefully investigated in alkylation of benzene with ethylene [78]. Interestingly, the mesoporous zeolites are much more catalytically active than the conventional zeolites, which can be ascribed to improved mass transportation in the mesoporous zeolite. Moreover, the mesoporous zeolites also show higher selectivity for ethylbenzene than conventional zeolite, which is assigned that the diffusion path in mesoporous zeolite is significantly shorter than that in conventional zeolite, and by-reactions such as ethylation of ethylbezene are suppressed remarkably. These results are in good agreement with the diffusive

Figure 3 Schematic illustration of mesoporous zeolite single crystals templated from carbon nanotubes. Reproduced by permission of Ref. [75]. Copyright 2001 American Chemical Society.

data in such carbon-templating mesoporous zeolite [79]. The elution of iso-butane from packed beds of conventional and mesoporous zeolite catalysts shows that the diffusion of iso-butane is significantly faster in mesoporous zeolite than that in conventional zeolite. It provides independent support to the proposal that the beneficial effect of mesopores in catalysis is attributed to improved mass transport [79]. Furthermore, mesoporous zeolites have another advantage of supporting catalytically active particles such as a metal (Pt), an alloy (PtSn), and a metal carbide (␣-Mo2 C) [80]. In case of conventional zeolites, the supported active particles are aggregated on the outer surface of the zeolite crystals, particularly after thermal treatment. While using mesoporous zeolites, these active particles were evenly distributed throughout the mesopore system of the zeolitic support, even after calcination, leading to nanocrystals within mesoporous zeolite crystals. In this case, introduction of the nanocrystals is conveniently performed even at high concentrations, since the extensive mesopore system present in each crystal allows to disperse nanocrystals efficiently. With the development of synthetic techniques, a series of ordered mesoporous carbons, mainly CMKs materials, have been prepared. Very soon, these ordered mesoporous carbons are used as templates to synthesize mesoporous zeolites, and obtained results in independent groups are not well consistent with the order of mesopores in zeolite crystals [81—85]. Cho et al. [81] described procedures for the preparation of mesoporous zeolites, which involved carbonization of a phenol—formaldehyde resin in the pores of MCM-41, MCM-48, and SBA-15 and the subsequent application of these carbon—silicate mixtures as starting materials for zeolite crystallization. Although starting materials have ordered mesostructure, the mesopores in zeolite crystals with good crystallinity are disordered. Similarly, Mokaya et al. have prepared a highly crystallized ZSM-5 zeolite with disordered mesopores using CMK-3 as a template [82]. On the contrary, by careful control of crystallization time, Liu et al. have replicated mesoporous aluminosilicates of RMM-1 and RMM-3 from the use of CMK-1 and CMK-3 as templates [83]. Characterization showed that both samples have good

Templating route for synthesizing mesoporous zeolites order of mesostructures but no obvious reflection peaks in wide-angle XRD patterns, suggesting that these materials appear to be composite materials consisting of mesoporous MCM-48-type and SBA-15-type with nanosized zeolite structural units embedded in the mesopore walls [24]. These results suggest that it is difficult to synthesize ordered mesoporous zeolitic materials with obvious XRD peaks in both small and wide angles from the use of CMKs templates because the pore sizes (2—10 nm) of CMKs are too small to accommodate the stable zeolite nanocrystals. Recently, large pore mesoporous carbons were prepared by carbonization of sucrose as carbon source by using colloidal silica powder as template [85]. If an excess of a sufficiently concentrated zeolite synthesis gel is mixed with the porous carbon, the individual zeolite crystals partially encapsulate the porous carbon template due to the fragmentation of the porous carbon during the zeolite growth. After calcination for removal of carbon templates, zeolites with cave-like mesopores have been obtained, as shown in Fig. 4. These results suggest that the walls of the mesoporous carbons were not able to act as a real template for the formation of mesopores opened to the surface of zeolite crystals [84]. Compared with ordered mesoporous carbons of CMKs, the large pore mesoporous carbons have relatively low cost, which is potentially important for the production of the mesoporous zeolites with a large scale in industry. More recently, Hu et al. reported an ordered mesoporous aluminosilicate zeolite (OMZ-1, ordered mesoporous zeolite) with completely crystalline zeolitic pore walls by recrystallization of SBA-15 using in situ formed CMK-5 as the hard template [85]. In this work, authors only showed electron diffraction (ED) pattern of MFI zeolite in the sample,

295 but another possibility is a composite of ordered mesoporous aluminosilicate and MFI zeolite, which needs further detailed characterizations. Catalytic properties of these mesoporous zeolites which are templated from the porous carbons are also tested [86]. For example, in alkylation of benzene with ethylene, RMM1 showed nearly 3-fold higher activity than its mesoporous analogue Al-MCM-48 with similar Si/Al ratio [86]. While the conversion of RMM-1 is less than that of conventional ZSM-5, notable increases in 1,4-diethylbezene selectivity and substantial reductions in by-product yield were found for the two novel mesoporous aluminosilicate materials.

Carbon and polymer aerogel templating To obtain the zeolites with open mesopores, Tao et al. [87—91] have used mesoporous carbon aerogel (CA) and mesoporous resorcinol—formaldehyde (RF) aerogel to template mesoporous zeolites such as ZSM-5, Y and A, as shown in Fig. 5. Both aerogels have much larger mesopores and thicker mesoporous walls than those of CMKs, which could be basically stable during the synthesis of zeolites and the fragmentation of the templates such as porous carbon [84] might not happen. Mesoporous RF aerogel was prepared from polymerization of resorcinol and formaldehyde, followed by dryness under a supercritical condition with CO2 [106]. The treatment by supercritical CO2 could form uniform and open mesopores. After pyrolysis under N2 flow at 1050 ◦ C, mesoporous RF was successfully transformed into mesoporous CA. By using this route, the sample monoliths were successfully obtained because these aerogels are favorable to form the

Figure 4 Schematic illustration of mesoporous silicalite-I zeolite templated from mesoporous carbon with large pores. Reproduced by permission of Ref. [84]. Copyright 2007 Elsevier.

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Figure 5 Schematic illustration of mesoporous zeolites templated from carbon aerogel consisting of interconnected uniform carbon particles. Reproduced by permission of Ref. [87]. Copyright 2003 American Chemical Society.

monoliths. It is worth noting that the mesopore volume is small and the pores are large with a wider distribution via RF aerogel templating as compared to the pore structure obtained from CA aerogel templating, which is attributed to that CAs have relatively larger pore walls and less mesopore volume than RF aerogels. Furthermore, the slightly thicker and non-uniform walls of RF aerogels result in larger mesopores with a broader distribution in the mesopore-modified ZSM-5. Recently, Schüth et al. [92] also reproted a successful synthesis of silicalite-I zeolites with hierarchically porous structures. Interestingly, the monolithic zeolites show high selectivity typically above 80% to ␧-caprolactam combined with a high rate of reaction in the Beckmann rearrangement of cyclohexanone oxime [92]. To investigate if there is a diffusion limitation of such monoliths, the samples were also measured after grinding and sieving to particle sizes between 250 and 125 ␮m. The obtained results show that the reactivity is nearly identical to that of the monolith, confirming the absence of diffusion limitations on the particle level. Additionally, selectivity, which is the more sensitive factor with respect to diffusion limitations, is also almost unchanged. These results have strongly demonstrated that the presence of mesoporosity in zeolite crystals is very important for improving activity and selectivity in catalysis.

Cationic polymer templating Mesoporous zeolites synthesized from solid-templates of nanostructued carbons have shown excellent diffusion and catalytic performances, but their industrial applications are still limited by the complexity of the synthetic procedures and the hydrophobicity of the carbon templates. To simplify the procedures for synthesizing mesoporous zeolites, it is preferable to use mesoscale soft-templates because the soft-templates are easy to self-assemble with aluminosilicates in the synthesis of zeolites. However, the choice of the soft-templates is not easy and the following factors must be carefully considered: (1) Stability: the soft-templates should be stable in alkaline media even at relatively high temperatures of 140—180 ◦ C. Otherwise, the templates will be decomposed during the synthesis of zeolites; (2) Interaction: the soft-templates should interact with silica species

easily and strongly. It is well known that Coulomb force at molecular level is very strong and silica species under alkaline conditions for synthesizing zeolites have negative charges, thus, soft-templates with positive charges are preferred; (3) Morphology: the soft-templates should have suitable mesoscale sizes and their morphology in aqueous solution should be fiber-like. (4) Cost: the soft-templates should be low cost, which is very important for the largescale production of mesoporous zeolites in industry. In the earlier 2006, Xiao et al. reported a facile, controllable, and universal route for the synthesis of hierarchical mesoporous zeolites (Beta-H) from the use of soft-template, mesoscale cationic polymer (polydiallyldimethylammonium chloride, PDADMAC), for the first time [93]. The cationic polymer with positive charges and low cost is very stable up to 200 ◦ C. After self-assembling with aluminosilicate species, the cationic polymer is homogeneously dispersed into the synthetic gel. With the crystallization of zeolites in hydrothermal synthesis, the cationic polymers were entirely embedded in zeolite crystals. After calcination at 550 ◦ C for removal of the organo-templates, hierarchical mesoporous zeolites are successfully obtained (Fig. 6). SEM image in high magnification clearly show the hierarchical mesoporosity in the range of 5—40 nm, and partial connections between these hierarchical pores could be obviously observed. TEM image in high magnification (Fig. 7) shows both hierarchical mesopores (5—20 nm) and ordered micropores (near 0.8 nm) in Beta-H sample. Notably, hierarchical mesopores are partially continuous and opened to external surface of the sample, and crystal walls are partially connected to each other. Particularly, the direction of micropores in the sample is the same, indicating that the hierarchical mesopores are formed in one zeolite crystal. All of these results confirm that the mesopores are formed in intracrystal rather than intercrystal. It is worthy to mention that cationic polymers with low cost are easily changed by their composition, molecular weight, and architecture. Obviously, the synthesis of hierarchical mesoporous zeolites is not limited to Beta zeolite by the mixture of TEAOH with PDADMAC, and many mixtures of small organic ammonium and mesoscale cationic polymer templates can be used if they could interact with aluminosilicate species in alkaline media under crystallization conditions of zeolites. This method really opens the door for the synthesis of a series of hierarchical mesoporous

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Schematic illustration of mesoporous zeolites templated from mesoscale cationic polymers.

zeolites such as ZSM-5 [93], X [94] and Y used widely in industry. As a model catalytic reaction, catalytic tests in alkylation of benzene with isopropanol shows that mesoporous Beta-H exhibits very high activity and selectivity as well as much longer catalytic life as compared with conventional Beta zeolite [93]. Considering the similarities of Beta-H to

Figure 7 HR TEM images of mesoporous Beta zeolite templated from mesoscale cationic polymer of PDADMAC. Reproduced by permission of Ref. [93]. Copyright 2006 WileyVCH.

conventional Beta sample for Si/Al ratios, aluminum distribution, and acidic strength as well as larger particle size of Beta-H than that of conventional Beta, the higher catalytic activity in alkylation of benzene with isopropanol over Beta-H should be directly assigned to the contribution of mesoporosity in the Beta-H sample. These results confirm that the presence of hierarchical mesopores in Beta-H sample is very important for the mass transport of reactants and products in catalysis. Furthermore, mesoporous zeolites have been used as acidic support to load noble metal particles and as a result obtaining supported noble metal catalysts with superior catalytic properties [95,96]. For example, palladium particles supported on mesoporous Beta zeolite (Pd/Beta-H) show much higher activity in deep hydrogenation of bulky aromatic pyrene than Pd/Beta, Pd/Al-MCM-41 and Pd/␥-Al2 O3 catalysts [95]. The high aromatic content in diesel fuel lowers the fuel quality and contributes significantly to the formation of undesirable emissions in exhaust gases and deep saturation of aromatic hydrocarbons over supported noble-metal catalyst such as Pd and Pt. And Pd—Pt is one of the ways to solve this problem. Catalytic hydrogenation of naphthalene and pyrene over Pd/Beta-H exhibits good sulfur tolerance in the presence of 200-ppm sulfur as compared with Pd/Al-MCM-41 [96]. The good sulfur tolerance in deep hydrogenation is very important for oil refining industry because the presence of sulfur strongly influences catalytic activity due to the poisoning of active sites. Additionally, the hydrodesulfurization (HDS) of 4,6dimethyldibenzothiophene (4,6-DMDBT), a typical sulfur molecule remaining in the fuels, shows that Pd/Beta-H has a higher activity than Pd/Al-MCM-41 and Pd/␥-Al2 O3 catalysts, which offers potentially novel catalysts for production of the ultra-clean fuels. Since the amount of Pd loading over various catalysts is the same, the differences in sulfur tolerance,

298 hydrogenation activity, and HDS ability of these catalysts are attributed to the difference in support acidity and porous structure. MCM-41 and ␥-Al2 O3 have large mesopore volume, but their acidity is low. Beta zeolite has high acidity, but it is short of mesostructure. Beta-H combines advantages of both mesoporous materials and zeolites, giving a large mesopore volume and high acidity [95,96]. More recently, Xie et al. [97] reported a successful synthesis of mesoporous zeolites of Beta and ZSM-11 using commercial polymers (polyvinyl butyral, PVB) as template for the formation of mesoporosity. Although PVB has no charges, excellent compatibility with inorganic materials ensures the good interaction between PVB with aluminosilicate species in the synthesis of zeolites. Upon the direct hydrothermal crystallization of PVB/aluminosilicate composite, the aluminosilicate was transformed into zeolites. Calcination of the obtained PVB/zeolite sample yielded the opening of micropores and mesopores in zeolites, respectively. TEM images of mesoporous Beta clearly show both ordered micropores and disordered mesopores in a single crystal. Very importantly, the mesoporous Beta shows stable activity (about 66%) in catalytic conversion of 1,2,4trimethylbenzene from time on stream of 14—42 h. As for the conventional Beta, the conversion drops gradually from 70% to 24%, indicating that this catalyst deactivates fast with the time on stream [97]. These results suggest that the presence of mesoporosity in zeolite crystals can suppress deactivation, which is attributed to the good mass transfer with blocking the coke formation as compared with conventional zeolites.

Organosilane templating It is well known that silanol species are easily polymerized in alkaline media, therefore it is reasonable thinking that there is a strong interaction between alkyloxylfunctionalized organosilane with aluminosilicate species under alkaline condition for synthesizing zeolites. Ryoo et al. have creatively used the amphiphilic organosilane ([(CH3 O)3 SiC3 H6 N(CH3 )2 Cn H2n+1 ]Cl) as a mesopore-directing

X. Meng et al. agent to synthesize mesoporous aluminosilicate and aluminophosphate zeolites [98—101], and the first work for preparation of mesoporous aluminosilicate zeolites with tunable mesoporosity was reported in the middle of 2006 [98]. In alkaline aqueous solution, the amphiphilic organosilane has both positive charge and silanols, which offers enough active sites to strongly interact with aluminosilicate species. In a typical synthesis, the amphiphilic organosilane was added to the initial synthesis composition of MFI zeolite containing the tetrapropylammonium ion (TPA+ ) as a structure directing agent for the MFI zeolite. Mesoporous MFI zeolite was obtained after calcination to remove these organic templates. Interestingly, the mesopore diameters can be finely tuned, typically in the range of 2—20 nm, depending on the molecular structure of the mesoporedirecting silanes and the hydrothermal synthesis conditions. Furthermore, this route is also extended to synthesize a series of aluminosilicate zeolites (e.g. LTA) [98] (Fig. 8) and aluminophosphates zeolites [99]. Ryoo et al. have carefully compared catalytic properties of mesoporous zeolites with conventional zeolites and ordered mesoporous materials such as MCM-41 [98,100,101]. In catalytic conversion of bulky molecules such as protection of benzaldehyde with pentaerythritol, condensation of benzaldehyde with 2-hydroxyacetophenone, esterification of benzylalcohol with hexanoic acid, and cracking of branched polyethylene [101] catalytic activities of mesoporous MFI zeolite were very high as compared with those of a conventional MFI zeolite. The mesoporous MFI almost completely lost its activities after selective dealumination of the surface aluminum species on the mesoporous walls using tartaric acid, confirming the contribution of mesopores in the catalyst. In catalytic conversion of small molecules [101], however, the mesoporous zeolites still exhibited high catalytic activities even after the selective dealumination on the surface of mesoporous walls. These results indicate that catalysis occurred in micropores and the presence of mesopores is favorable for mass transfer. By using this templating route, Prins et al. have prepared mesoporous MFI zeolites supported Pd catalyst, and investigated catalytic activities in HDS of 4,6-DMDBT over

Figure 8 SEM images of mesoporous MFI and LTA using mesoscale template of amphiphilic organosilane. Reproduced by permission of Ref. [98]. Copyright 2006 Nature Publishing Group.

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Figure 9 TEM images of mesoporous silicalite-I templated from nanocrystals of CaCO3 . Reproduced by permission of Ref. [105]. Copyright 2008 American Chemical Society.

noble-metal catalysts (Pd, Pt and Pd—Pt) on mesoporous MFI, conventional MFI and ␥-Al2 O3 supports [102]. Catalytic data show that sulfur removal by mesoporous zeolite supported metal catalysts was much more efficient than that by microporous MFI or ␥-Al2 O3 supported metal catalysts, suggesting that the mesoporous zeolites are ideal candidates as supports for deep HDS. In the later part of 2006, Pinnavaia et al. have synthesized a silylated polyethylenimine polymers, which was used as a porogen for the formation of intracrystal mesopores [103]. The presence of —SiO3 units on the polymer allows it to strongly interact with aluminosilicate species in a sol—gel. With the growth of zeolite crystals, the incorporated polymer becomes phase-segregated from the zeolite matrix, forming an intracrystal polymer network that is covalently linked to the zeolite framework. Calcination of the sample removes the polymer porogen and forms a zeolite with uniform intracrystal mesopores [103].

Inorganic material templating Very recently, Xie et al. [105] reported the synthesis of mesoporous MFI zeolites templated from inorganic material of CaCO3 which is cheap and easily available. After introduction of nanosized CaCO3 (50—100 nm) in the synthesis of silicalite-I zeolite, the nanosized CaCO3 can be trapped into the silicalite-I crystals. The encapsulated nanoparticles were removed by means of acid dissolution, giving rise to the intracrystal pores within the zeolite crystals (Fig. 9). In this work, it is notable that the hydroxyl groups on the surface of CaCO3 are essential to take the hard template effect. Inorganic material templating has also significance for preparing zeolites with additional function. For example, if a composite of nanosized CaCO3 trapped in aluminosilicate zeolites is calcined at high temperature (650 ◦ C), CaCO3 in zeolite crystals will be converted into CaO nanoparticles, which exhibit strong basicity. In this case, CaO/zeolite samples will have both strong acidity and strong basicity, which would be potentially useful for catalysis required for both acidic and basic sites.

Concluding remarks During the last decade, hierarchical mesoporous zeolites have emerged as an important class of materials in zeolite science and technology, and they have been paid much attention because of advantages such as good mass transfer and accessible bulky molecules. For the syntheses of mesoporous zeolites, the choice of mesoscale templates is very important. Although a series of mesoscale templates have been used, all of them are easy to disperse homogeneously into silica and aluminosilicate gels, indicating that the interaction between these mesoscale templates with silicate or aluminosilicate species play an important role for the formation of intracrystal mesoporosity in zeolites. Characterization of mesoporosity in zeolite crystals by high-resolution TEM technique is more difficult than that of ordered mesoporous materials such as MCM-41 and conventional zeolites because it is necessary to clearly observe two-level pores (ordered micropores at 0.4—1.2 nm and disordered mesopores at 2—50 nm). Otherwise, the mesoporous zeolites are not easy to distinguish from a composite of zeolites with mesoporous materials. Compared with conventional zeolites, these mesoporous zeolites show improved catalytic performances in a series of reactions, which are reasonably attributed to the enhanced transport for reactants and products as well as the access of bulky molecules. Furthermore, the mesoporous zeolites have advantages of both zeolites (strong acidity and good hydrothermal stability) and ordered mesoporous materials (good mass transfer), which are favorable for loading additionally active phases such as noble metal particles. For example, mesoporous MFI supported Pd catalyst is much more active than industrial catalyst of ␥-Al2 O3 supported Pd catalyst. Undoubtedly, the use of mesoporous zeolites as catalysts is expected to gain importance in the coming years and it will be interesting to see if the possibilities for templating mesoporous zeolites will lead to a significant improvement in catalytic properties, even obtaining completely new heterogeneous catalysts.

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Acknowledgements This work is supported by the State Basic Research Project of China (2009CB623507) and National Natural Science Foundation of China (20773049).

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Prof. Feng-Shou Xiao received his B.S. and M.S. degrees in the Department of Chemistry, Jilin University, China. From there he moved to the Catalysis Research Center, Hokkaido University, Japan, where he was involved in collaborative research between China and Japan. He was a Ph.D. student there for two years, and was awarded his Ph.D. degree at Jilin University in 1990. After postdoctoral work at the University of California at Davis, USA, he joined the faculty at Jilin University in 1994, where he is now a full and distinguished professor of Chemistry. For his research in nanoporous materials, Prof. Xiao has been recognized with the National Outstanding Award of Young Scientists in National Science Foundation of China in 1998 and Thomson Scientific Research Fronts Award in 2008. Currently he is the secretary of Asia-Pacific Association of Catalysis Societies.