In Situ Solid-State NMR Studies of the Catalytic Conversion of Methanol on the Molecular Sieve SAPO-34

Article abstract of DOI:10.1021/j100370a002

We have monitored the shape-selective catalytic conversion of methanol (MeOH) to low-molecular-weight olefins and aliphatics over the molecular sieve SAPO-34 using 13C and 1H magic-angle-spinning (MAS) NMR in tandem with gas chromatography.The size of eight-membered windows in the structure limits the gas-phase products to linear C1, C2, and C3 species.However, the main species present in the intracrystalline space are branched C4 and C5 saturated hydrocarbons such as isobutane and isopentane: too large to leave the intracrystalline pore system they become trapped, thus imposing an additional steric constraint on the diffusion of linear hydrocarbons.This favors the diffusion of C1 and C2 species out of the crystallites and is responsible for the apparent overall selectivity of SAPO-34 for ethylene.By contrast, intracrystalline chemistry favors the formation of propylene.Unlike on zeolite H-ZSM-5, no carbon monoxide intermediate and no aromatics are formed.We suggest modifications to the zeolite composition designed to make the zeolite more selective for C3 rather than C2 species.