Shape-selective pathways in methanol conversion over zeolite catalysts

Abstract

Mechanistic studies of the catalytic conversion of methanol over HZSM-5 have so far concentrated mostly on the intriguing problem of the first C-C bond formation and less attention has been paid to final mixtures. However, in this multistep reaction, precyclization pathways are clearly not intracrystalline diffusion controlled. Thus, olefin formation, growth, and cracking, unlike formation and distribution of the largest attainable molecules-naphthenics and aromatics- cannot reflect the special shape-selective properties of HZSM-5. The upper cutoff in product size over HZSM-5 presents a problem: HZSM-5 catalysts effective in the production of gasoline from methanol are non-para-selective providing, e.g., in the case of xylene, the thermodynamic equilibrium mixtures of positional isomers whereas under comparable conditions a highly para-selective HZSM-5 converts methanol mainly to small olefins. In this note the authors report on final product mixtures obtained in comparative batch experiments of methanol conversion over HZSM-5, HY, and HM and in comparative continuous-flow experiments using HZSM-5 samples of different properties. The results are believed to be in accord with the idea of shape-selective external activity nest effect in HZSM-5.