A C-13 NMR-STUDY OF THE CONDENSATION CHEMISTRY OF ACETONE AND ACETALDEHYDE ADSORBED AT THE BRONSTED ACID SITES IN H-ZSM-5

Several bimolecular, acid-catalyzed condensation reactions of acetone and acetaldehyde have been examined in H-ZSM-5, along with the adsorpt ion complexes formed by the products, using C-13 NMR. For acetone, the hydrogen-bonded adsorption complex is stable at room temperature and coverages below one molecule per Bronsted acid site. Reaction to mesit yl oxide occurs only at higher coverages or temperatures, which are ne cessary to induce site exchange. The adsorption complex exhibits react ion chemistry analogous to that observed in solution phase, forming ad sorption complexes of chloroacetone upon exposure to Cl-2 and of imine s upon exposure to NH3 or dimethylamine. The reactions of acetaldehyde to crotonaldehyde and imines are similar, although they occur at a fa ster rate due to the higher mobility of this molecule. The adsorption complexes formed by acetone, acetaldehyde, and their condensation prod ucts can all be described as rigid, hydrogen-bonded complexes at low c overages. Complexes formed from imines and enamines exhibit isotropic chemical shifts nearly identical to those observed in magic acids, ind icating that proton transfer is nearly complete for these molecules. T he extent of proton transfer for the remaining molecules varies with t he proton affinity of the molecule, ranging from close to complete pro ton transfer for mesityl oxide and crotonaldehyde to almost complete a bsence of proton transfer for the chloroacetones. The differences and similarities between these reactions in the zeolite and in solution ph ase are discussed, along with the implications for understanding the p rimary processes responsible for these reactions in zeolites. (C) 1995 Academic Press, Inc.