Mechanistic and steric aspects of direct amination of methanol over Brønsted acidic mordenites were investigated using in situ infrared (IR) spectroscopy and kinetic studies. It was found that the rate of formation of the primary product monomethylamine is linearly correlated to the total concentration of available methyl groups from methylammonium ions rather than the acid site concentration. This indicates scavenging of methyl groups by ammonia and amines is likely to be the major pathway for amine formation. Differences in selectivity to the different amines, i.e., mono-, di- and trimethylamine, are related to differences in hindrance for the diffusion of the larger molecules. Higher concentration of alkylammonium ions in the pores and higher substitution of sorbed molecules suppress formation of trimethylamine. The selectivity to dimethyl ether seems also to be controlled by such steric factors. Lowering the effective available space for formation and transport retards dimethyl ether formation or leads to more dimethyl ether reacting to amines. A concerted bimolecular mechanism involving weakly sorbed methanol species is proposed to be the main pathway for ether formation.
- IR spectroscopy
- Acidic mordenite