A 13C NMR study of the condensation chemistry of acetone and acetaldehyde adsorbed at the Brønsted acid sites in H-ZSM-5

Abstract

Several bimolecular, acid-catalyzed condensation reactions of acetone and acetaldehyde have been examined in H-ZSM-5, along with the adsorption complexes formed by the products, using 13C NMR. For acetone, the hydrogen-bonded adsorption complex is stable at room temperature and coverages below one molecule per Brønsted acid site. Reaction to mesityl oxide occurs only at higher coverages or temperatures, which are necessary to induce site exchange. The adsorption complex exhibits reaction chemistry analogous to that observed in solution phase, forming adsorption complexes of chloroacetone upon exposure to Cl2 and of imines upon exposure to NH3 or dimethylamine. The reactions of acetaldehyde to crotonaldehyde and imines are similar, although they occur at a faster rate due to the higher mobility of this molecule. The adsorption complexes formed by acetone, acetaldehyde, and their condensation products can all be described as rigid, hydrogen-bonded complexes at low coverages. Complexes formed from imines and enamines exhibit isotropic chemical shifts nearly identical to those observed in magic acids, indicating that proton transfer is nearly complete for these molecules. The extent of proton transfer for the remaining molecules varies with the proton affinity of the molecule, ranging from close to complete proton transfer for mesityl oxide and crotonaldehyde to almost complete absence of proton transfer for the chloroacetones. The differences and similarities between these reactions in the zeolite and in solution phase are discussed, along with the implications for understanding the primary processes responsible for these reactions in zeolites. © 1995 Academic Press, Inc.