Postsynthesis, Characterization, and Catalytic Properties of

Oct 5, 2009 - Yong Wang, Yueming Liu,* Lingling Wang, Haihong Wu, Xiaohong Li, Mingyuan He, and. Peng Wu*. Shanghai Key Laboratory of Green ...
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J. Phys. Chem. C 2009, 113, 18753–18760

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Postsynthesis, Characterization, and Catalytic Properties of Aluminosilicates Analogous to MCM-56 Yong Wang, Yueming Liu,* Lingling Wang, Haihong Wu, Xiaohong Li, Mingyuan He, and Peng Wu* Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal UniVersity, North Zhongshan Rd. 3663, Shanghai 200062, P. R. China ReceiVed: May 13, 2009; ReVised Manuscript ReceiVed: August 30, 2009

Aluminosilicates with different Si/Al ratios and showing a structure similar to MCM-56 were postsynthesized from fully crystallized lamellar MWW-type precursors under controlled acid treatment. The effects of pretreatment conditions on the construction of MCM-56 analogue structure were investigated, and the physicochemical properties of the resulting materials were characterized by means of various techniques. When the precursors were treated in HNO3 solution with a concentration e2 M at temperatures lower than 353 K, a structure analogous to MCM-56 was formed favorably even after further calcination, whereas the treatment at higher temperatures or with 5 M HNO3 resulted in conventional three-dimensional MWW topology. The formation of MCM-56-like structure was closely related to the reorientation of the interlayer H-bond moieties as a result of a partial removal of occluded structure directing agent. Compared with MCM-22, the MCM-56 analogues possessed a structural disorder along the layer stacking direction but had a larger external surface, which mitigated effectively the steric restrictions imposed by the intracrystal micropores to bulky molecules. The MCM-56 zeolites, maintaining the basic structure units of the MWW zeolite, turned out to serve as promising solid acid catalysts for processing larger molecules. Introduction Crystalline aluminosilicate zeolites are important materials widely used in a number of petrochemical and chemical processes, including shape selective heterogeneous catalysts, separation, ion exchange, and sorption applications. From the viewpoint of serving as solid acid catalyst, the zeolite with the MWW topology, well-known as MCM-22,1,2 has attracted particular research attention in past decades because of its peculiar structure and industrial applications already found for ethylbenzene and cumene synthesis. The MWW topology is also known for PSH-3,3 SSZ-25,4 ERB-1,5 ITQ-1,6 and MCM-497 which are obtained from different synthesis procedures. The three-dimensional (3D) MWW, formed from a layered precursor, consists of two independent pore systems accessible via 10-membered ring (MR) windows, one of which is defined by 2D sinusoidal channels with elliptical ring cross sections of 0.41 × 0.51 nm2, and the other contains 12-MR supercages of 0.71 × 0.71 × 1.82 nm3 restricted by 10-MR openings of 0.45 × 0.55 nm2.1 In addition, half supercages form surface pockets on the outer crystal surfaces. These three types of pore systems have been found to play different roles in catalytic reactions. The 2D sinusoidal 10-MR channels embedded within the MWW sheets give rise to unique shape selectivity for reactant and product, while the 12-MR supercages, which are formed between stacked MWW sheets when the layers fuse with the creation of T-O-T linkages upon calcination of lamellar precursor, allow bimolecular reactions involving bulky intermediates.8,9 More attention has been paid to the role of surface pockets on the external surface due to their easy accessibility to large molecules, offering opportunities for processing bulky molecules. It is now well recognized that the acid sites located * Corresponding author. Tel/Fax: +86-21-6223-2292. E-mail: pwu@ chem.ecnu.edu.cn (P.W.); [email protected] (Y.L.).

in the hemicages play a very important role in catalytic reactions. For instance, the selective production of ethylbenzene and cumene through benzene alkylation with the corresponding olefins would take place essentially on these acid sites,2 whereas the contribution of the sinusoidal channels and supercages is almost negligible.10 With the purpose to apply MWW structure-based zeolites to petrochemical and fine chemical reactions involving bulky molecules, it is desirable to make full use of its interlayer supercages and surface pockets.11 Taking advantage of the flexibility and structural diversity of layered zeolites, MCM-22 is readily converted into the micromesoporous hybrid material MCM-36 by first swelling the lamellar precursor with surfactant and then interlayer pillaring the sheets with silica species.12 The swollen aluminosilicates and titanosilicates are both readily delaminated to give rise to fully phase delaminated materials, ITQ-213 and Del-Ti-MWW14 characterized by high external surface area and open reaction space. The delaminated materials show greatly enhanced activity in acid-catalyzed reactions and liquid-phase epoxidation reactions involving larger molecules. Nevertheless, the phase delamination requires multiple treatment steps, consumes a large quantity of chemicals and energy, and inevitably induces a partial degradation of the basic framework structure. Recently, we proposed a new strategy to expand the interlayer space of various layered zeolites through interlayer alkoxysilylation of the precursors, which leads to the formation of a 12-MR pore entrance between the MWW sheets.15 This is, however, considered not to eliminate totally the mass transfer limitation imposed still by an enlarged pore entrance to inner supercages. As a distinct member of the MWW family, MCM-5616 is first synthesized as the intermediate from the same synthetic gel as MCM-49 but within a shorter crystallization time. Although its true structure is still waiting for reliable structure

10.1021/jp904436c CCC: $40.75  2009 American Chemical Society Published on Web 10/05/2009

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resolution, MCM-56 is presumed to be a partly delaminated material composed of a disordered collection of MWW sheets and possesses a large portion of exposed external surface.11 Sharing the same primary structure with MCM-22, MCM-56 has a relatively low amount of supercages, but more 12-MR surface cups and acid sites exposed to the crystal exterior, and an easier accessibility to large organic molecules.17-19 The unprecedented structure properties imply that MCM-56 may have potential application to the alkylation of aromatics.16 Direct hydrothermal synthesis once has been the only method for the preparation of MCM-56, in which the crystallization rate and extent are needed to be controlled carefully. Furthermore, the variation of the Si/Al ratio of MCM-56 is limited in direct hydrothermal synthesis, as many studies dealing with synthesis of MCM-22 reported that a pure phase can only be obtained within a narrow range of Si/Al ratio of about 10-25.20,21 In addition to direct synthesis, we recently succeeded in preparing various metallosilicates structurally analogous to MCM-56 by using a universal postsynthesis method.22,23 This is based on the fact that 2D lamellar precursors are structurally soft and changeable materials which are possibly subjected to structural improvement, although their corresponding 3D zeolite structures formed after calcination are short of structural flexibility because of relatively rigid frameworks. A controlled acid treatment of Ti-MWW precursor is presumed to disturb regular stacking of MWW sheets, resulting in a disordered collection of MWW sheets along the c direction, which leads to a novel titanosilicate structurally analogous to MCM-56 even after further calcination. In this paper, we have carried out the preparation of aluminosilicate MCM-56 analogues with a wide range of Si/Al ratio through a postsynthesis method. The effects of posttreatment conditions on the formation of MCM-56 analogues were studied in detail. The resulting materials were well characterized for their physicochemical properties, structural features, as well as potential applications to the processes involving the reactants with different molecular sizes. Experimental Section Preparation of Materials. The MCM-22 lamellar precursors, designated as MCM-22-P, were hydrothermally synthesized using hexamethyleneimine (HMI) as a structure-directing agent (SDA) following the procedures reported previously.24 Fumed silica (Cab-o-sil M5) and sodium aluminate (49.9% Al2O3, 40.5% Na2O) were used as Si and Al sources, respectively, while sodium hydroxide was used to adjust the gel pH. The precursors were crystallized from the gels with molar compositions of 1SiO2:0.05Na2O:(0.02-0.033)Al2O3:0.35HMI:15H2O at 423 K for 5 days. The MWW-type precursors with lower Al contents were synthesized from the system for the crystallization of ERB-1 using boric acid as the supporting agent and piperidine (PI) as an SDA.25 The ERB-1 lamellar precursors were crystallized from the gels with molar compositions of 1SiO2: 0.15Na2O:(0-0.01)Al2O3:0.9PI:0.125B2O3:20H2O at 443 K for 4 days. The crystallization was carried out in rotated (100 rpm) Teflon-lined stainless autoclaves under autogenous pressure. The samples were filtered, washed, and dried at 373 K to obtain lamellar precursors with a wide range of Si/Al ratios from 15 to ∞. A direct calcination of the samples at 823 K for 10 h resulted in the products with 3D MWW structure. The Na-form 3D Al-MWW was transformed into H-form by ion exchange with 1 M HNO3 at 353 K for 24 h and then calcination in air at 773 K for 5 h.

Wang et al. The postsynthesis of aluminosilicate MCM-56 analogues involved the acid treatment of as-synthesized Al-MWW precursors. The treatment was carried out at a solid-to-liquid weight ratio of 1:50 in 0.1-5 M HNO3 solution at a desirable temperature (298-353 K) for different periods of time (0.5-18 h). A subsequent calcination was carried out on the acid treated samples at 823 K for 10 h to remove residual organic species of SDA. As a control, the precursors were also refluxed in HNO3 solution. Characterization Methods. The crystalline structures of the samples were characterized by X-ray powder diffraction (XRD) patterns measured on a Bruker D8 ADVANCE diffractometer using Cu KR radiation. Scanning electron microscopy (SEM) was performed on a Hitachi-4800 microscope after suspending the sample in ethanol. N2 and Ar adsorption/desorption isotherms were measured at 77 and 87.3 K on an Autosorb Quantachrome 02108-KR-1 and BELSORP MAX analyzer, respectively, after evacuation at 573 K for 5 h. Inductively coupled plasma (ICP) was performed on a Thermo IRIS Intrepid II XSP atomic emission spectrometer to quantify the amount of aluminum and boron. TGA-DTG and CHN element analyses were carried out with a METTLER TOLEDO TGA/SDTA851 instrument and an Elementar Vario EL analyzer, respectively. The 29Si and 27Al MAS NMR spectra were recorded on a Bruker DSX 300 multinuclear solid-state magnetic resonance spectrometer. IR spectra were collected at room temperature on a Nicolet NEXUS-FTIR-670 spectrometer with a spectral resolution of 2 cm-1 after evacuating the self-supported wafer (30 mg of 20 mm Ø) at 773 K for 3 h in a quartz IR cell sealed with CaF2 windows. IR spectra of adsorbed pyridine were recorded after the wafer was first exposed to a pyridine vapor (1.3 kPa) in the cell at 323 K for 0.5 h and then evacuated at 423 K for 1 h to remove physisorbed pyridine. All of the spectra were collected at room temperature. Catalytic Reactions. 1,3,5-Triisopropylbenzene (TIPB) cracking and esterification of ethanol with acetic acid were carried out to evaluate the ability of the zeolite catalysts for processing bulky molecules and small substrates, respectively. The reactions were carried out in a fixed-bed quartz reactor (i.d. 10 mm) with a continuous flow system under atmospheric pressure. In a typical run, 0.2 g of catalyst was activated at 673 K for 1.5 h under a flow of dry N2 before the reaction. The reactor temperature was then decreased to 573 K, where the feed was switched to the reactants (2 mL min-1) and N2 carrier gas (30 mL min-1) to start the reaction. The liquid products were collected periodically with a cold trap at 273 K and analyzed on a gas chromatograph (Shimadzu 14B, FID detector) equipped with a 30 m DB-1 capillary column. For the TIPB cracking performed at various space velocities, the amount of catalyst (0.1-0.3 g) and the flow rate of N2 (10-100 mL min-1) were varied. In a poisoning experiment, 2,4-dimethylquinoline (DMQ) was cofed with the reactant continuously into the reactor at a rate of 100 µL h-1. Results and Discussion Postsynthesis of Aluminosilicate MCM-56 Analogues with Various Al Contents. A series of Al-MWW lamellar precursors with a wide range of Si/Al ratios (15-∞) were synthesized using HMI or PI as SDA. The XRD patterns of these precursors before and after the posttreatment with 2 M HNO3 are shown in Figure 1. All of the as-synthesized samples showed characteristic 001 and 002 diffraction peaks in the 2θ region of 3-7° due to a layered structure along the c-direction (Figure 1A). In addition, the samples also showed two well resolved diffractions due to 101 and 102 planes, which matched well with the MWW

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Figure 2. XRD patterns of the samples obtained by the acid treatment of MCM-22-P (Si/Al ) 15) in 0.1 M HNO3 for 18 h at (a) 298, (b) 323, (c) 353, and (d) 423 K and by further calcination at 823 K for 10 h.

Figure 1. XRD patterns of (A) Al-MWW lamellar precursors, (B) same as A but directly calcined at 823 K, (C) MCM-56 analogues asmade from the above precursors by treatment in 2 M HNO3 for 0.5 h at 298 K, and (D) calcined MCM-56 analogues. The precursors were synthesized at a Si/Al ratio of (a) 15, (b) 25, (c) 30, (d) 50, and (e) 100, respectively.

lamellar precursor reported in the literature.21 Thus, a pure MWW phase was obtained at different Si/Al ratios. Figure 1B showed the XRD patterns of directly calcined samples. They were significantly different from that of corresponding precursors. Upon calcination, one noticeable phenomenon was that the 00l peaks (e.g., 001 and 002) shifted to a higher angle region with reduced intensities, while the peaks due to the indexes of h00 and hk0 (e.g., 100 and 310) remained practically unchanged, indicating that the structural change essentially took place with the c-axis. It means that 2D MWW lamellar precursors were transformed to 3D MWW structure due to dehydration/ condensation of hydroxyl groups between the layers upon direct calcination. However, the acid treatment of MWW precursors in 2 M HNO3 at room temperature led to the products which showed MCM-56-like XRD patterns. The existent MCM-56, usually obtained through a direct synthesis procedure, is reported to be a disordered structure not well-known by definition.11,16 To distinguish the existent MCM-56, we denote the present materials as MCM-56 analogues. As shown in Figure 1C, the 101 and 102 diffractions overlapped seriously to become a broadband in the 2θ region of 7.5-11°; on the other hand, the 100 and 310 peaks remained intensive. It seems that the acid treatment kept the MWW sheets almost intact but caused a structural change only in their arrangement format. The broadband in the 2θ region of 7.5-11° can be taken as evidence for the formation of a structure analogue to MCM-56.16,22 After the as-synthesized MCM-56 analogues were calcined at 823 K for 10 h, the XRD patterns remained the same (Figure 1D), which means the MCM-56 structure constructed by postsynthesis was thermally stable. As a result, the aluminosilicate MCM-56 analogues containing a crystalline phase of MWW sheets were prepared successfully from the Al-MWW lamellar precursors with various Al contents. Parameter Affecting the Postsynthesis of Aluminosilicate MCM-56 Analogues. Using the MCM-22-P synthesized at Si/ Al ) 15 as a parent sample, we have investigated in detail the

Figure 3. XRD patterns of the samples obtained by the acid treatment of MCM-22-P (Si/Al ) 15) at a HNO3 concentration of (a) 0.1, (b) 0.5, (c) 1, (d) 2, and (e) 5 M at 298 K for 18 h and by further calcination at 823 K for 10 h.

effect of posttreatment conditions on the structural transfer from lamellar precursor to MCM-56 analogue. Figure 2 shows the influence of treatment temperature on the formation of MCM-56. When the concentration of HNO3 and the treatment time were fixed at 0.1 M and 18 h, respectively, the MCM-56 structure was constructed to have a pure phase and a good crystallinity at temperatures lower than 353 K (Figure 2a-c). The acid treatment at 423 K led to a pure 3D MWW structure which showed well separated 101 and 102 diffractions (Figure 2d). Therefore, the postsynthesis of MCM56 was preferable by the acid treatment operated at relatively low temperatures. Figure 3 shows the influence of acid concentration on the formation of MCM-56. When the precursor was washed with aqueous HNO3 solution with various concentrations at room temperature for 18 h, the MCM-56 structure was obtained in a wide HNO3 concentration of 0.1-2 M (Figure 3a-d), whereas the 3D MWW structure was obtained after the treatment in 5 M HNO3 (Figure 3e). Thus, the postsynthesis of MCM-56 was preferable by acid treatment at relatively low acid concentration. Figure 4 shows the influence of acid treatment time on the formation of MCM-56. When the acid treatment temperature and the concentration of HNO3 were fixed at 298 K and 0.1 M, respectively, the MCM-56 structure was already formed after the treatment for 0.5 h (Figure 4a), and was maintained the same when further prolonging the treatment time to 18 h (Figure 4b-e). These MCM-56 samples postsynthesized for a different treatment time showed almost the same XRD patterns with comparably intensive diffractions. In the case of direct hydrothermal synthesis, the optimal time window for the formation of MCM-56 is presumed to last for about 2-3 h.19 The present

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Figure 4. XRD patterns of the samples obtained by the acid treatment of MCM-22-P (Si/Al ) 15) in 0.1 M HNO3 at 298 K for (a) 0.5, (b) 2, (c) 6, (d) 10, and (e) 18 h and by further calcination at 823 K for 10 h.

Figure 5. TGA (A) and DTG (B) curves of the MCM-22-P (Si/Al ) 15) (a) and the samples obtained by the treatment of the precursor in 2 M HNO3 for 18 h at 298 K (b) and 423 K (c). The samples were dried at 373 K but without calcination.

postsynthesis methodology is insensitive to time duration and thus shows the advantage of easy control for the synthesis process. Possible Mechanism for Postsynthesis of MCM-56 Analogue Structure. The above results implied that a mild acid treatment (low temperature or low acid concentration) of the MWW precursors was favorable for the formation of MCM-56 analogue. However, the stronger treatment at high temperature or with concentrated acid solution led to the construction of conventional 3D MWW structure. The XRD patterns demonstrate that the MCM-56 analogues have disrupted MWW sheets apparently along the c-direction in comparison to the well ordered array in both the precursor and 3D MWW structure. Since intercalating, swelling, or interlayer expanding of MCM22-P with amines and quaternary ions always requires severe and special conditions, such as strong basic media and high temperature, the ordered layer stacking in the precursors is deduced to be held by the extensive hydrogen bonding, via pairing of silanols from opposing layers.11 The intercalated organic molecules of SDA, on the other hand, are considered to act as a filler of the space created by interlayer silanol bridges, and their amino groups may have chemical interaction with the H-bonded hydroxyls on the layer surface. These removable organic species in the interlayer space of MWW precursors are easily extracted in acid treatment. An unequal removal of SDA molecules would alter the orientation of interlayer H-bonding moieties, which leads to an irregular array of the sheets, forming a structure analogous to MCM-56. In order to make clear the role of interlayer H-bonding as well as organic species in structural change, the precursors and samples obtained by different posttreatments were characterized by thermal and elemental analyses. Figure 5 shows TGA and DTG curves of the MCM-22-P (Si/Al ) 15) and acid treated samples dried at 373 K but without calcination. The DTG curves implied three kinds of desorption temperature-dependent weight loss (Figure 5B). The weight loss below 473 K is attributed to adsorbed water, whereas the weight loss in the range 473-1073 K is mainly due to the removal of HMI molecules as well as the dehydroxylation. The oxidative decomposition of HMI molecules occluded in interlayer void spaces occurs in a lower temperature region of 473-643 K, while the decomposition of HMI molecules from the intralayers of 10-MR sinusoidal channels occurs in a higher temperature region of 643-1073 K.26,27 The CHN elemental analysis indicated that the organic species had a C/N atomic ratio of 5.8-6.2 for both the precursor and corresponding acid treated samples, suggesting that they existed as HMI molecules within the crystals. The content of the organic species, however, depended on the

treatment condition (Figure 5A and Table 1). As shown in Figure 5A, compared with the MCM-22-P, the weight loss of the acid treated samples decreased, which means a part of the HMI molecules were removed by acid treatment. Furthermore, when the treatment was intensified, the weight loss became less as more HMI molecules were removed. The TGA analysis showed that the weight loss of the MCM-22-P above 473 K was 17.1%, which was equal to the amount determined by element analysis (Table 1, no. 1). According to the above assignments for the weight loss in TGA curves, the amount of HMI molecules occluded in the interlayer void spaces and in the 10-MR sinusoidal intralayer channels of the precursor was about 6.7 and 10.4%, respectively. The acid treatment almost did not affect the weight loss in the temperature region 643-1073 K regardless of the different conditions, implying the HMI molecules were stuck tightly in the intralayer 10-MR channels. Nevertheless, the weight loss at 473-643 K varied with the treatment conditions, as the HMI molecules in interlayer void spaces have a higher mobility than those confined in the intralayer 10-MR channels. The mild acid treatments removed partially (about 50%) the organic species mainly existing between the MWW sheets (Table 1, nos. 2-5). The remaining organic molecules may change the orientation of H-bonded silanols from opposing layers in the neighborhood. This disturbs the parallel arrangement of MWW sheets, leading to a disordered collection along the c direction, even after further calcination. In the case of intensified treatments at higher temperature or higher acid concentration (Table 1, nos. 6 and 7), the organic species existing between the MWW sheets were removed to a greater extent. The influence of amine on the H-bonding of paired silanols is presumed to be weakened. An equal interlayer dehydroxylation then takes place to form a regularly arrayed 3D MWW structure after calcination. To further verify the correlation of the orientation of interlayer H-bonded silanols and organic species to the formation of MCM-56-like disordered structure, we carried out hydrothermal treatment of as-synthesized MCM-56 sample with cyclic amines. The treatment with HMI or PI at 353 K converted the MCM56 structure back to the MWW lamellar precursor which showed the 001 and 002 diffractions due to layered structure and also well resolved 101 and 102 diffractions due to ordered collection of MWW sheets along the c-direction (Figure 6b and c). Further calcination on the HMI or PI treated samples reasonably resulted in the 3D MWW structure (Figure 6d and 6e). Thus, similar to the MWW precursor, postsynthesized MCM-56 analogue is also amenable to structural modifications, and the structural interchange between the precursor and MCM-56 is reversible. Nevertheless, the XRD patterns showed that the structural

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TABLE 1: The Content of Organic Species and Weight Loss of the MCM-22-P (Si/Al ) 15) before and after Acid Treatment treatment condition no.

HNO3

temp

time

product structure

1 2 3 4 5 6 7

2M 2M 0.1 M 0.1 M 2M 5M

no RT 353 K RT RT reflux RT

18 h 18 h 18 h 0.5 h 18 h 18 h

precursor MCM-56 MCM-56 MCM-56 MCM-56 3D MWW 3D MWW

a

a

weight lossc (wt %)

HMI amountb (wt %)

90 nm) is a convenient model reaction widely used to evaluate the contribution of the external surface area of microporous materials.35 The main liquid products were diisopropylbenzene (DIPB) isomers, isopropylbenzene (IPB), and benzene (BZ) (Scheme 2). Figure 11A compares the results of MCM-56 analogues with various Al contents in cracking of TIPB with those of MCM-22. The TIPB conversion increased monotonously with increasing Al content for both MCM-22 and MCM56 analogue catalysts, but MCM-56 analogues showed much higher TIPB conversion than MCM-22. In addition, a significant change in the distribution of products was also observed.

Because cracking of TIPB is a successive reaction which yields DIPB, IPB, and BZ in turn, the product distribution may also reflect the extent of cracking. Figure 11B shows the product distribution in TIPB cracking over MCM-22 and MCM-56 analogue catalysts. The Al-free catalyst, that is, B-MWW, was almost inactive in the cracking of TIPB due to an extremely weak acidity of framework boron. With Al content increasing, the BZ and IPB selectivities increased, while the DIPB selectivity decreased for both MCM-22 and MCM-56. This is as expected because the MWW catalysts with a higher Al content contained more acid sites which made the reaction proceed deeply. However, the selectivity for deep cracking products such as IPB and BZ over MCM-56 analogue catalysts was higher than that of MCM-22 at identical Al content. This indicates that the MCM-56 analogues also showed a better cracking ability than MCM-22. To further confirm the superior ability of MCM-56 to 3D MWW, the TIPB cracking has been carried out by varying the contact time (expressed as W/F) in a wide range (Figure S1 in the Supporting Information). The catalysts compared were prepared from the same precursor (Si/Al ) 15). The reactions were carried out at 573 K with a feed rate of 5 mL h-1. The conversion of TIPB increased with increasing W/F for both MCM-22 and MCM-56 (Figure S1A in the Supporting Information). MCM-56 was more active than MCM-22 catalyst at the same W/F. With increasing W/F, the selectivity of DIPB decreased while IPB and BZ increased for these two catalysts (Figure S1B in the Supporting Information). It is clear that the cracking of TIPB is a consecutive reaction with DIPB as the primary product which was produced immediately after the TIPB molecules contacted the catalyst bed. When the contact time was prolonged, IPB and BZ were formed consecutively as a result of secondary cracking of DIPB (Figure S1B in the Supporting Information). Similarly, at the same contact time, MCM-56 showed higher IPB and BZ selectivities but a lower DIPB selectivity than 3D MWW. This further proved the MCM-56 analogues are promising catalysts for processing large molecules. To clarify the role of external surface in the cracking of TIPB, 2,4-dimethylquinoline (2,4-DMQ) with too large molecular

Figure 11. Dependence of TIPB conversion (A) and product selectivity (B) on the Al content of Al-MWW (b) and Al-MCM-56 (O) catalysts. Reaction conditions: cat., 0.2 g; feed rate, 2 mL h-1; N2, 30 mL h-1; temp, 573 K; time, 2 h.

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Wang et al. ogy Commission of Shanghai Municipality (08JC1408700, 09XD1401500, 07QA14017), 973 Program (2006CB202508), 863 Program (2007AA03Z34, 2008AA030801), and Shanghai Leading Academic Discipline Project (B409). Y.W. thanks PhD Program Scholarship Fund of ECNU 2008. Supporting Information Available: Dependence of TIPB conversion and product selectivity on the contact time over AlMWW and Al-MCM-56 and a comparison of acetic acid conversion between MCM-22 and MCM-56 in the absence or in the presence of 2,4-DMQ. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 12. Comparison of TIPB conversion between MCM-22 and MCM-56 in the absence or in the presence of 2,4-DMQ. For the reaction conditions, see Figure 11. 2,4-DMQ feed, 100 µL h-1.

dimensions to enter the pores of 10-MR windows was adopted to poison selectively the acid sites on the external surface.36 The TIPB conversion was 33 and 53% for MCM-22 and MCM56 prepared from the same precursor (Si/Al ) 15), respectively, when the reaction was carried out in the absence of 2,4-DMQ (Figure 12). When 2,4-DMQ was cofed into the reactor, the TIPB conversion decreased to a very low level for both catalysts as a result of extinguishing the acid sites on the external surface. Therefore, MCM-56 is characteristic of open reaction spaces which are favorable for the reactions involving large molecules. The MCM-56 analogue catalysts not only showed enhanced catalytic activity in the reactions of bulky molecules but also were still active for the reactions that need inner crystal active sites. As shown in Figure S2 in the Supporting Information, MCM-56 showed a significant activity for the esterification of ethanol and acetic acid, a reaction believed to take place mainly inside the intracrystal channels, since the conversion of acetic acid was not affected by the presence of 2,4-DMQ. Nevertheless, the catalytic activity of MCM-56 for small reactants was somewhat lower than that of MCM-22. This is because the disordered stacking of the MWW sheets in MCM-56 may make a part of framework Al wrapped as shown by IR spectra (Figure 9A), and then not easily accessible to the reactant molecules. Conclusions MCM-56-like aluminosilicate with various Al contents have been postsynthesized from fully crystallized MWW lamellar precursors by a mild acid treatment. The formation of MCM56 analogue structure depends on controlled removal of interlayer organic species and reorientation of interlayer H-bonded silanols in the precursors. A mild acid treatment removes partially the intercalating organic species and alters the Hbonding of interlayer silanols. This makes the layers shifted or twisted to result in a MCM-56-like structure with lateral disorder. The MCM-56 analogues possess more external silanols and a larger external surface than conventional 3D MWW zeolites. This enables MCM-56 to catalyze reactions involving bulky molecules. The aluminosilicate MCM-56 analogue, maintaining the basic units of MWW zeolite and containing the tetrahedral Al species in the framework position, shows a higher catalytic activity in the cracking of TIPB than 3D MWW. Thus, the aluminosilicate MCM-56 analogues postsynthesized by the present simple method are expected to be potential solid acid catalysts for processing the large molecules. Acknowledgment. We gratefully acknowledge NSFC of China (20673038, 20873043, 20925310), Science and Technol-

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