Selective synthesis of dimethylamine over small-pore zeolites

Zeolite H-RHO is highly selective for the synthesis of dimethylamine (DMA) from methanol and ammonia. Shallow-bed dry calcination of NH4-RHO at temperatures from 400 to 700 °C results in progressive deammoniation, dehydroxylation, and dealumination and in dimethylamine (DMA) and trimethylamine (TMA) selectivities that progressively change from 50 to 67% and 25 to 5%, respectively. Concurrent changes in dealumination and internal and external acid sites suggest a process whereby DMA selectivity arises from destruction of nonselective sites on impurity phases such as pollucite and/or from hindered TMA diffusivity by either nonframework Al species (NFA) or NFA/methylamine adsorption complexes. Shallow-bed steam calcination at temperatures from 400 to 700 °C results in higher degrees of dehydroxylation and dealumination than shallow-bed dry calcination and in DMA and TMA selectivities that change from 50 to 71% and 20 to 2%, respectively. The higher DMA selectivities result from the higher degree of dealumination and/or the more effective deactivation of impurity phases. Increased activity of shallow-bed steamed H-RHO is correlated with destruction of most of the highly acidic 3610 cm−1 Brønsted sites and to their replacement with weakly acidic 3640 cm−1 OH groups. High dimethyl ether yields observed in samples calcined above 6̃50 °C are associated with an OH band at 3680 cm−1 and attributed to the condensation of nonframework Al species.