On the elementary steps of acid zeolite catalyzed amination of light alcohols

Abstract

Potential elementary reaction steps in solid acid catalyzed amination of light alcohols are critically compared using hydrogen mordenite as (model) catalyst and the open mechanistic questions have been addressed. Transient kinetic experiments combined with in situ infrared spectroscopy and isotopic labeling techniques were the main experimental means to provide answers to these questions. While we have shown before that the ammonium ions rapidly react with methanol to produce methylammonium ions, these results show that also methoxy groups formed from methanol at strong Brønsted acid sites under reaction conditions are very low, it is concluded that the contribution of the methoxy group amination to the amine formation is very small. The rate determining step for the overall reaction is the ammonia mediated removal of the alkylammonium ions from the zeolite. It is shown that two interconnected reaction steps govern this, i.e., alkyl group scavenging by ammonia/amines and ammonia/amine adsorption assisted desorption of alkylammonium ions. The combination of both the steps determines the observed rates and selectivities to the different amines. The formation of alkenes in the synthesis of larger alkylamines over strongly acidic catalysts is shown to largely result from Hofmann elimination (decomposition of ethylamines) and not from elimination reactions of the alcohol reactant.