Boosting dimethylamine formation selectivity in a membrane reactor by in situ water removal

Mono-, di-, and trimethylamine are the products of the successive methylation of ammonia. Using narrow-pore acidic catalysts of the CHA family like H-SAPO-34 or H-SSZ-13, the formation of the thermodynamically but bulky trimethylamine can be suppressed due to steric effects; thus, methylation is stopped at dimethylamine. In this work, the continuous in situ removal of the byproduct water through the 4 Å wide pores in an LINDE Type A (LTA) (grown on an a-Al2O3 support) membrane reactor further increased the selectivity toward the economically desired product dimethylamine by 50%. This experimental finding can be explained by the release of adsorbed water blocking the catalytic site. Water removal through the hydrophilic LTA zeolite membrane allows methanol to adsorb at the acidic catalyst sites, which in turn accelerates the methylation rate of monomethylamine to the desired product dimethylamine. Further methylation to trimethylamine as the thermodynamically most favored product is not possible in narrow-pore catalysts because of space restrictions, but it takes place in the 12-membered-ring H-Mordenite (H-MOR) catalyst.