Shape selective methylamines synthesis: Reaction and diffusion in a CMs-SiO 2-Al 2O 3 composite catalyst

Abstract

Product shape selective effects, to date observed only with zeolites and a few other crystalline molecular sieves, should be generalizable and thus useful in the design of other novel catalyst materials. To be a viable approach in general for a given reaction, the only requirement for product shape selectivity is that the molecules involved be sufficiently different in size that they transport through micropore structures at different rates. Here, it is reported for the first time that non-crystalline composite catalysts, consisting of silica-alumina and amorphous carbogenic molecular sieves, are able to produce mono- and dimethylamines from methanol and ammonia with higher selectivities than the silica-alumina catalyst, currently used in the commercial practice. With the standard catalyst at ∼ 85% methanol conversion a ratio of (MMA + DMA)/TMA = 1 results, while the new catalyst, designated CMS-SiO 2-Al 2O 3, at the same conditions and methanol conversion, produces ratios of at least (MMA + DMA)/TMA = 2.7. Although the rate of conversion per gram on the new catalyst is only one-third that of the standard catalyst, this is more than adequate activity for a commercial fixed-bed process. A model that accounts for reaction on silica-alumina and diffusion in the molecular sieve carbon is described. By adjusting the diffusivities of the three amines good agreement between prediction and experiment is obtained.