A PROBE OF BRONSTED SITE ACIDITY IN ZEOLITES - C-13 CHEMICAL-SHIFT OFACETONE

C-13 NMR spectroscopy has been used to characterize the adsorption of acetone (C-2 C-13-labeled 2-propanone), at coverages of less than one molecule per Bronsted-acid site, in the molecular sieves H-ZSM-5, H-mo rdenite, H-ZSM-12, H-ZSM-22, chemically dealuminated H-Y, SAPO-5, MgAP O-5, and BeAPO-5. It is shown that the trace of the chemical shift ten sor for the carbonyl carbon at 125 K measures the ability of the site to form a hydrogen bond with the carbonyl oxygen and, therefore, can b e used as a measure of the intrinsic acidity of the site. The sites in all of the materials investigated were nearly homogeneous, with the i sotropic shifts for most of the molecular sieves falling in the range observed for solutions of acetone in trifluoroacetic acid. In addition , the lineshape and linewidths of the C-13 NMR powder patterns at room temperature provide a measure of the molecular motion of adsorbed ace tone and are strongly affected by the size of the framework channels. (C) 1994 Academic Press, Inc.