Ultrastable and highly acidic, zeolite-coated mesoporous aluminosilicates.

The acidity and hydrothermal stability of mesoporous aluminosilicates (MesoAS) are relatively low compared to those of zeolites, which limits their potential applications as catalysts in petroleum refining and fine chemicals synthesis.[1±7] Much effort has therefore been undertaken to synthesize a new type of materials, which combines the advantages of both zeolites and mesoporous molecular sieves. One might expect to improve both the stability and acidity of these materials if zeolite-like order could be introduced into the mesopore walls. Recent advances showed that amorphous walls of the mesostructure could indeed be converted to a partially zeolitic product.[8±13] For example, MCM-41/MFI composites containing interconnected mesopores and micropores were prepared by using dual templates and exhibited enhanced acidity and steam stability compared to the corresponding MCM-41.[8, 9] Furthermore, the use of zeolite seeds as precursors for the assembly of mesoporous aluminosilicates with high hydrothermal stability and acidity was reported by Pinnavaia et al.[10] A similar zeolite beta type of materials was also prepared in the presence of co-templates of tetraethylammonium hydroxide and cetyltrimethylammonium bromide.[11] Recent results from our group showed the preparation of a new type of materials with semicrystalline zeolitic mesopore walls based on a templated solid-state secondary crystallization of zeolites starting from the amorphous walls of SBA-15. Brightand dark-field transmission electron microscope (TEM) images recorded on the same area of the sample indicated that nanocrystals were embedded in the continuous amorphous inorganic matrix to form semicrystalline wall structures while preserving the mesoporous structure. The resulting mesoporous materials had much stronger acidity than those of the corresponding amorphous alumosilicates and a much improved hydrothermal stability.[12] Herein, we describe a new approach for the production of unusual zeolite-coatedmesoporous aluminosilicates (ZCMesoAS) using diluted clear solutions containing primary zeolite units. Hydrothermally ultrastable and highly acidic ZCMesoAS result owing to the nanocrystalline zeolitic nature of their pore wall surface. These features open new possibilities for using this type of materials as acid catalysts. It is of special concern that due to the size of primary ZSM5 units templated by tetrapropylammonium ions (2.8 nm in diameter),[13±14] the pore diameter of the mesoporous precursor molecular sieves should be higher than 30 ä. The methods in references [2] and [3] are therefore useful in this context for the preparation of mesoporous precursors, such as SBA-15 and siliceous mesostructured cellular foams (MCFs). The N2 adsorption/desorption isotherms obtained from the calcined sample before and after coating are shown in Figure 1A. The parent and ZSM-5-coated samples display the typical behavior of a mesoporous molecular sieve with a mesopore volume saturation capacity of about 1.56 and 0.78 cm3g 1, respectively (Table 1). The P/P0 position of the inflection point is related to a diameter in the mesopore range, and the sharpness of these steps indicates the uniformity of the pore size. A narrower distribution of the pore diameter of the coated sample compared to that of the parent sample was observed (Figure 1B). Furthermore, a significant decrease in pore diameter (from 70 to 54 ä) and in surface area (from 800 to 465 m2g 1) could conceivably be ascribed to the ZSM-5 nanocrystals coated inside the mesopore channels of the host. Figure 2 shows a TEM image of the ZSM-5-coated MesoAS. A uniform pore size with a highly ordered structure is still observed for this sample after the coating procedure. The ZSM-5-coated MesoAS sample shows a FTIR absorption band at 550 cm 1, which is not present in the parent MesoAS sample (Figure 3). The band around 550 cm 1 has been assigned to the asymmetric stretching mode in double[10] Representative recent calculations on Cope rearrangements: a) D. J. Tantillo, R. Hoffmann, J. Org. Chem. 2002, in press; b) O. Wiest, K. A. Black, K. N. Houk, J. Am. Chem. Soc. 1994, 116, 10336 ± 10337; c) H. Jiao, P. von R. Schleyer, Angew. Chem. 1995, 107, 329; Angew. Chem. Int. Ed. Engl. 1995, 34, 334 ± 337. [11] A. P. Scott, L. Radom, J. Phys. Chem. 1996, 100, 16502 ± 16513. [12] Only TMchairlike∫ (as opposed to TMtwist∫) isomers[13] of 1,5-cyclooctadiene substructures were considered due to the geometric constraints of the polymers. [13] Experiments and force-field calculations on 1,5-cyclooctadiene: a) G. M. Whitesides, G. L. Goe, A. C. Cope, J. Am. Chem. Soc. 1967, 89, 7136 ± 7137; b) D. Boeckh, R. Huisgen, H. Noeth, J. Am. Chem. Soc. 1987, 109, 1248 ± 1249; c) N. L. Allinger, J. T. Sprague, J. Am. Chem. Soc. 1972, 94, 5734 ± 5747. [14] Barriers for Cope rearrangement of species with terminal acyclic hexadiene units were found to converge rapidly upon cyclobutane fusion to approximately 30 kcalmol 1 (from 42 kcalmol 1 in the parent boatlike cyclohexadiene,[10] Scheme 2). Also, [n]-ladderane oligomers were generally found to be 4 kcalmol 1 more stable than the broken [n]-ladderane isomers with central cyclooctadiene substructures derived from them (i.e. those species whose relative energies are set to 0.0 in Scheme 2). [15] We have not yet investigated substituent effects on this rearrangement; appropriately designed, they may lower the barrier even further. [16] a) R. Pettit, J. S. McKennis, L. Brener, J. S. Ward, J. Am. Chem. Soc. 1971, 93, 4857 ± 4958; b) previous calculations predict similar rearrangement barriers: B. Das, K. L. Sebastian, Chem. Phys. Lett. 2000, 330, 433 ± 439, and references therein. [17] a) N. C. C. Yang, B. J. Hrnjez, M. G. Horner, J. Am. Chem. Soc. 1987, 109, 3158 ± 3159; b) N. C. C. Yang, M. G. Horner, Tetrahedron Lett. 1986, 27, 543 ± 546; c) HF/6-31G* calculations on the structure and energy of 7 relative to other C12H12 isomers have also been reported: G. W. Schriver, D. J. Gerson, J. Am. Chem. Soc. 1990, 112, 4723 ± 4728. [18] Such systems include, for example, the asteranes[1a]–isomers of large prismanes with combinations of cis and trans fused cyclobutanes. [19] The size of these molecules has, so far, precluded computations on their putative fluxionality.