Formaldehyde–isobutene Prins condensation over MFI-type zeolites

Abstract

The liquid phase Prins condensation of formaldehyde with butenes on H-ZSM-5 (MFI) zeolite catalysts was investigated showing that reaction rates follow the order isobutene > 1-butene > cis-2-butene. The catalytic rates for medium-pore zeolite H-ZSM-5 were not only a larger than for H-beta, but also selective for 3-methyl-3-buten-1-ol during formaldehyde condensation with isobutene. Isoprene forms, under the optimal reaction conditions, either in a second dehydration step of the unsaturated alcohol, or in a single step over H-ZSM-5 with optimal Si/Al ratio (Si/Al = 40). Mechanistic studies revealed that sequential Prins cyclization and hetero-Diels–Alder reactions of the desired products, forming six carbon species, occur only at slow rates over H-ZSM-5. Using DFT methods it was determined that the reaction follows a two-step mechanism: protonation of formaldehyde and electrophilic attack to the alkene, and deprotonation of the resulting intermediate carbocation. For isobutene and 1-butene, the electrophilic addition is coordinated with the protonation of the formyl group, but for cis-2-butene, it is coordinated with the proton back-donation step. The rate-limiting step switches from electrophilic addition for isobutene to proton elimination for the other two C4 isomers.