Structural and acidity characterization of mesoporous molecular sieves (MCM41S) and amorphous silica-alumina.

摘要

The mesoporous molecular sieves recently discovered by Mobil are potentially interesting catalysts and catalyst supports (1). These materials have uniform parallel channels in the range of 15-100 A. A series of these materials with five Si/Al ratios (0.7, 1.1, 3.0, 5.0, and 16) with six pore sizes between 15-40 A were prepared. We have characterized these materials by {dollar}sp{lcub}27{rcub}{dollar}Al and {dollar}sp{lcub}29{rcub}{dollar}Si MAS NMR, TEM, low angle XRD, and FTIR. The crystallinity, porosity and acidity are functions of the source of the pseudobohemite used in the preparation and of the pore size which was varied according to the alkyl chain length of the alkyltrimethyl ammonium template used (C{dollar}sb6{dollar}, C{dollar}sb8{dollar}, C{dollar}sb{lcub}10{rcub}{dollar}, C{dollar}sb{lcub}12{rcub}{dollar}, C{dollar}sb{lcub}14{rcub}{dollar}, C{dollar}sb{lcub}16{rcub}{dollar}). It is possible to prepare precursors with only tetrahedral Al up to 20 wt. % alumina for all the pore sizes. However, after calcination, the small pore size sieves retain more alumina in a tetrahedral environment than large pore size sieves. The 2-methyl-2-pentene isomerization was performed for chemical characterization. The acidity, measured by the ratio of 3-methyl-2-pentene over 4-methyl-2-pentene, correlates with pore diameter and alumina content in a complex way.; Mid-infrared spectroscopy, {dollar}sp{lcub}29{rcub}{dollar}Si MAS NMR, XRD, and TEM have been employed to study the framework structure of mesoporous molecular sieves (MCM41S). Some MCM41S materials showed unique spectral features in IR and {dollar}sp{lcub}29{rcub}{dollar}Si NMR distinct from crystalline zeolites and amorphous aluminosilicates. These features appear to be related to the regularity of MCM41S framework in local structure and bonding of TO{dollar}sb4{dollar} (T = Si or Al) tetrahedral. The best structural regularity was achieved at intermediate pore sizes (2-3 nm) for a fixed Si/Al ratio, and the regularity was reduced progressively as Si/Al ratio decreased for a fixed pore size. Structural features varied systematically from the relatively ordered structure to amorphous ones as the pore size or Si/Al ratio was varied. ftn(1) C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359, 710 (1992).