DOI: 10.1002/chem.201404568

Communication

& Zeolites

Ordered Mesoporous ZSM-5 Employing an Imidazolium-Based Ionic Liquid Alexander Sachse,* Caroline Wuttke, Elzio Lissner, and Michle Oberson de Souza[a] however, showed no characteristic diffraction peaks in the wide-angle range, thus mesoporous materials with high thermal stability but without zeolitic structure were formed. The use of ammonium salts was investigated in combination with SZM-5 based seeds, though yielding materials without ordered arrangement of mesopores within the zeolitic network.[15, 16] Ionic liquids have lately attracted a great deal of attention throughout an important range of disciplines in chemistry[17] and in recent years for the synthesis of inorganic materials.[18–21] Yet, ionic liquids have so far never been disclosed for the attainment of ordered mesoporous zeolites. Unordered mesoporous zeolites employing surfactants based on ionic liquids with bulky substituents were recently obtained by Srivastava et al.[22, 23] It was reported that the imidazolium base headgroup shows an improved templating behavior in hydrothermal syntheses, compared to commonly used cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC) possessing identical alkyl chains.[24] Herein we describe for the first time the achievement of ordered hexagonal arranged mesopores within a ZSM-5 framework by the combination of zeolite seeds and imidazoliumbased ionic liquid. The synthetic strategy that we have followed for the elaboration of ordered mesoporous zeolites comprises a two-step synthesis. Firstly, zeolite seeds were obtained by hydrothermal treatment at 100 8C for 16 h of a mixture with the molar composition: 1 SiO2/17 H2O/0.065 Na2O/ 0.022 TPA2O. The seeds were thereafter combined with the ionic liquid and with an aluminum source in a molar ratio: and 1 SiO2/0.037 Al2O3/0.037 Na2O/125.8 H2O/0.22 [C16Mim]Cl subjected to hydrothermal treatment in a Teflon-lined stainless steel autoclave under autogenous pressure for 24 h at 190 8C. Solids were filtered and washed with deionized water. Final products were dried overnight at 80 8C and calcined at 550 8C for 8 h to remove the organic part from the materials. The materials synthesized in the presence or absence of [C16Mim]Cl are hereafter referred as meso-ZSM-5 and ZSM-5, respectively. For catalytic applications materials were proton exchanged by using an aqueous solution of NH4NO3 followed by calcination 550 8C for 8 h. Obtained samples are named H + -ZSM-5 and H + -meso-ZSM-5. As can be evidenced from the nitrogen adsorption and desorption isotherms at 77 K (Figure 1) the employment of ionic liquids during the crystallization process has an important impact on the textural properties of the final material. The material synthesized in the absence of ionic liquid (ZSM-5) shows a typical type I isotherm as expected for an entirely microporous solid. Differently, the meso-ZSM-5 reveals a type IV iso-

Abstract: Hierarchically porous ZSM-5 was achieved by using a simple bottom-up strategy combining zeolite seeds with imidazolium-based ionic liquids. The bimodal ZSM-5 with hexagonal arranged mesopores (3 nm) shows important activity in the acid catalysis of bulky compounds relative to conventional ZSM-5.

The quest for the achievement of hierarchically porous organized zeolites is a strongly covered issue within materials science, as outlined in many recent review articles.[1–3] The necessity for disclosing synthetic pathways for the incorporation of a secondary, mesoporous system within the microporous zeolitic network is given by the fact that mass transfer as well as accessibility to active sites for bulky compounds can importantly be enhanced.[4, 5] Theoretical considerations indicate that hierarchical porous networks appear to be optimal entities for zeolite catalysis.[6, 7] Zeolites, figuring a secondary porous network can be achieved by either top-down or bottom-up strategies. The former relies on destructive methods, such as dealumination and/or desilication, which implies the treatment of preformed zeolites under harsh conditions.[8, 9] An important drawback of the obtained materials by such destructive techniques represents the difficult control of the introduced porosity as a majority of the mesopores form as internal cavities and will thus not enhance diffusivity in the material when used as heterogeneous catalysts. On the other hand, constructive techniques refer generally to strategies employing hard or soft templates.[1–3] By removal of the template the porosity forms as imprints of the latter. The obtained materials present in many cases a narrow size distribution of mesopores and in some cases ordered arranged mesoporosity. Mesoporous ZSM-5 by soft templating techniques has so far been obtained by the employment of large copolymers (polystyrene-co-4-polyvinylpyridine),[10] by the use of ammonium functionalized silanes as [(CH3O)3SiC3H6N(CH3)2C16H33]Cl,[11] or by biotemplates as modified chitosane.[12] An alternative approach is the combination of zeolite seeds with surfactant molecules as firstly described by Liu et al. to achieve mesoporous aluminosilicates.[13, 14] These materials,

[a] Dr. A. Sachse, C. Wuttke, Dr. E. Lissner, Dr. M. Oberson de Souza Institute of Chemistry, UFRGS, Av. Bento GonÅalves 9500 Porto Alegre, 91501-907, Box 15003 (Brazil) E-mail: [email protected] Chem. Eur. J. 2014, 20, 1 – 5

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Communication

Figure 3. TEM images of thin slices of meso-ZSM-5.

ordered one-dimensional pores from which a hexagonal arrangement is deduced. The mesopore size derived from the TEM images is comparable to those obtained from the nitrogen adsorption and desorption isotherm showing a mean correlation distance of 4.5 nm. EDX analysis gave for both materitherm with a sharp N2 uptake in the relative pressure range of als ZSM-5 and meso-ZSM-5 a Si/Al ratio of 13. 27 Al and 29Si MAS NMR were recorded for ZSM-5 and mesop/p0 = 0.3–0.5 ascribable to the presence of mesopores within the material. The BJH method applied to the N2 adsorption ZSM-5 materials (Figure 4). For both samples a single peak is obtained in the respective 27Al MAS NMR spectra centered at branch confirms this observation and reveals a narrow size disd = 55 ppm that is characteristic for tetrahedral coordinated tribution of mesopores with an average of 3 nm (Figure 1). The alumina, which is expected for a purely zeolitic material. The porous volume was calculated employing the t-plot method to 29 Si MAS NMR spectra show for both samples the presence of calculate the thickness of the adsorbed N2 layer.[25] The ZSM-5 material achieves 0.13 mL g 1 corresponding to microporous a wide peak at d = 110 ppm ascribable to the Q4 term in volume. The total volume of the meso-ZSM-5 material amounts which each Si atom is surrounded by four OSi and/or OAl. The to 0.28 mL g 1 with a mesoporous contribution of 0.18 mL g 1. NMR spectra thus indicates a similar chemical nature of both ZSM-5 and meso-ZSM-5 materials. The equivalent BET surface area was calculated to be To assess the catalytic activity of meso-ZSM-5 we preformed 490 m2 g 1. the acetalization of benzaldehyde with pentaerythritol The powder X-ray diffractograms exhibit at wide angles for (Scheme 1). This is an important test reaction as the product ZSM-5 and meso-ZSM-5 characteristic peaks of the highly crysmolecules are too bulky to diffuse within the narrow channels talline MFI phase with large crystal sizes as revealed by the of classically microporous ZSM-5. As depicted in Table 1 the sharp diffraction peaks (Figure 2B). By applying the Scherrer material synthesized in the absence of the ionic liquid (H + formula mean crystal sizes of 60 nm were deduced. The diffraction pattern for the meso-ZSM-5 at low angles presents a peak -ZSM-5) gave a very low conversion (1 %) within the first two centered at 1.98 2q and a broader peak that is centered at 3.78 hours of reactions. Whereas, the H + -meso-ZSM-5 gave high 2q, which can be ascribed to the presence of an organized conversions of up to 78 % with a selectivity of diacetal/monoamesoporous network (Figure 2A). The first diffraction peak can cetal of 25 and amounting to a productivity of 15.6 mol g 1 h 1 be attributed to the (100) plane, which indicates the presence after 1 h. Full conversion to the diacetal product is achieved of a hexagonal arrangement. within two hours of reaction. The presence of mesopores was furthermore confirmed by The catalytic performance of these materials compares well electronic microscopy. Figure 3 shows TEM images evidencing to those previously reported for the acetalization of benzaldehyde with pentaerythritol in similar conditions. A mesoporous ZSM-5 (with Si/Al = 37, SBET = 365 m2 g 1, 17 nm pore diameter, 0.02 g of catalyst) obtained through the use of large copolymers (based on polystyrene-co-4-polyvinylpyridine) shows full conversion after 4.5 h of reaction, amounting to a productivity of 12 mol g 1 h 1.[10] The group of Ryoo reported the use of a mesoporous ZSM-5 Figure 2. Small angle (A) and wide-angle (B) XRD patterns of ZSM-5 (black) and meso-ZSM-5 (gray) prepared at synthesized by the use of an 190 8C during 24 h and autogenous pressure.

Figure 1. A) N2 adsorption/desorption isotherms at 77 K of ZSM-5 (&) and meso-ZSM-5 (&). The insert shows the BJH pore-size distribution of the adsorption branch of meso-ZSM-5.

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Chem. Eur. J. 2014, 20, 1 – 5

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 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Communication Experimental Section Materials synthesis

Figure 4. 27Al MAS NMR (A) and 29Si MAS NMR (B) spectra of ZSM-5 and meso-ZSM-5 materials.

For the synthesis of meso-ZSM-5, NaAlO2 (0.068 g) was dissolved in distilled water (30 mL) and combined with [C16Mim]Cl (1 g) and 5 g of the previously synthesized gel. The solution was transferred to a Teflon-lined stainless steel autoclave and was treated at 190 8C for 24 h under autogenous pressure. Solids were filtered under vacuum and washed three times with deionized water (200 mL). Final products were dried overnight at 80 8C and calcined at 550 8C for 8 h under air to remove the organic part from the materials. Proton exchange was carried out by treating 1 g of samples with 100 mL of a 1 m aqueous solution of NH4NO3 for two 2 h at 80 8C followed by filtration, washing, and drying at 80 8C for 16 h. Exchange cycles were repeated three times prior to calcination at 550 8C for 8 h under air.

Scheme 1. Acetalization of benzaldehyde with pentaerythritol.

Table 1. Acetalization of benzaldehyde with pentaerythritol over H + -ZSM-5 and H + -meso-ZSM-5.

t [h]

H + -ZSM-5 monoacetal diacetal [%] [%]

H + -meso-ZSM-5 monoacetal diacetal [%] [%]

1 2

0 0

3 0

Ordered mesoporous ZSM-5 employing an imidazolium-based ionic liquid.

Hierarchically porous ZSM-5 was achieved by using a simple bottom-up strategy combining zeolite seeds with imidazolium-based ionic liquids. The bimoda...
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