Applications of High-Resolution 13C-NMR and Magic-Angle Spinning NMR to Reactions on Zeolites and Oxides

Abstract

Conventional high resolution 13C-NMR is shown to provide quantitative information on the conformation of 1-and 2-butene molecules adsorbed on zeolites and on mixed tin-antimony oxides. The kinetics of isomerization of 1-butene into the two 2-butene isomers is readily determined and enables the proposition of a reaction mechanism involving an intermediate cyclic complex. Rate constants can be related to the acid-base properties of the mixed oxide surfaces. An in-situ characterization of reaction products is readily achieved in the conversion of methanol, ethanol and ethylene on the highly acidic and shape selective ZSM-5 zeolite. In addition, 13C isotopic labeling proves to be a powerful technique to discriminate between possible reaction pathways of ethylene. High resolution magic angle spinning 13C-NMR appears as a superior technique to investigate the nature of carbon-containing residues in which molecular motion is highly reduced. There exists a relationship between the nature of these species and the acidic and molecular shape selective properties of the zeolites which are considered (ZSM-5 and mordenite). This technique allows a distinction between hydrocarbon molecules which are trapped inside the zeolite framework as a result of pore plugging and strongly chemisorbed or surface alkoxide species. Large molecules or ions can also be trapped inside the zeolitic framework during cristallization. Tetrapropylammonium ions in ZSM-5 zeolite and tetrabutylammonium ions in ZSM-11 zeolite (intact in their respective frameworks) occupy the channel intersections and their alkyl chain extend in the linear and zig-zag channels in ZSM-5 zeolite or in the perpendicular linear channels of ZSM-11 zeolite.