METHYLAMINE SYNTHESIS OVER SOLID ACID CATALYSTS - MICROCALORIMETRIC AND INFRARED SPECTROSCOPIC STUDIES OF ADSORBED SPECIES

Microcalorimetry was used to determine the differential enthalpy chang es of adsorption versus coverage on H-ZSM-5 and H-mordenite of reactan ts and products of methylamine synthesis. The enthalpy changes of adso rption of dimethylether, ammonia, monomethylamine, and dimethylamine o n Bronsted acid sites vary linearly from -90 to -250 kJ/mol with the g aseous proton affinities of these basic molecules. The enthalpy change s of adsorption of water and methanol vary from -60 to -90 kJ/mol. In situ infrared spectroscopy indicates that methoxyl species are present on the catalyst in flowing methanol at temperatures near 600 K; howev er, adsorbed ammonia and methylamines are associated with the Bronsted acid sites in gas mixtures of methanol and ammonia typically used for methylamine synthesis at these temperatures. These results suggest th at dimethylether formation from methanol may occur through surface met hoxyl species, while methylamine synthesis most likely involves adsorb ed ammonium cations. (C) 1994 Academic Press, Inc.