Supramolecular origins of product selectivity for methanol-to-olefin catalysis on HSAPO-34

Ethylene selectivity in methanol-to-olefin (MTO) catalysis is related to th e number of methyl groups on benzene rings trapped in the nanocages of the preferred catalyst HSAPO-34. By correlating the time evolutions of the cata lysts' C-13 NMR spectra and the volatile product distribution following abr upt cessation of methanol flow, we discovered that (in the absence of other adsorbates) propene is favored by methylbenzenes with four to six methyl g roups but ethylene is predominant from those with two or three methyl group s. We substantially increased ethylene selectivity by operating at lower me thanol partial pressures or higher temperatures, either of which reduces th e steady-state average methyl substitution. As a step toward a kinetic anal ysis of the MTO reaction on HSAPO-34, we treated each nanocage with a methy lbenzene molecule as a supramolecule capable of unimolecular dissociation i nto ethylene or propene and a less highly substituted methylbenzene. Additi on of a water molecule to a nanocage containing a methylbenzene produces a distinct supramolecule with unique properties. Indeed, co-feeding water wit h methanol significantly increased the average number of methyl groups per ring at steady state relative to identical conditions without additional wa ter, and also increased ethylene selectivity, apparently through transition state shape selectivity.