(653a) Investigation of the Formaldehyde-Isobutene Prins Condensation over MFI Zeolites

Branched five-carbon unsaturated alcohols are attractive synthetic targets because they are versatile precursors to fine chemicals, as well as the initial point for the production of commodity chemicals such as dienes. Isoprene, in particular, is formed by the dehydration of 3-methyl-3-buten-1-ol, and is important in the preparation of synthetic rubbers (polyisoprene, butyl rubber, styrene or acrylonitrile co-polymers). Currently, isoprene is in the main obtained as a by-product from the five-carbon fraction of naphtha steam cracking; nevertheless, the yields are usually low [1]. Prins condensation of formaldehyde with isobutene has been used as an on-purpose method to isoprene, however the more common two-step process requires a liquid-phase catalysis with aqueous sulfuric acid or phosphoric acid and the one-step process suffers from catalyst deactivation due to high reaction temperatures [2-4].

We have investigated the condensation of formaldehyde with butenes using zeolite catalysts (H-ZSM-5 (MFI) and others). The thermal and Lewis acid catalyzed Prins reaction is well understood and suggested to follow a concerted mechanism through a single transition state complex with a six-membered ring geometry [5,6]. In contrast, the mechanism over Brønsted acids remains unclear. We investigated the Prins reaction of three isomers of butene (isobutene, 1-butene and cis-2-butene) with formaldehyde over H-ZSM5 zeolite using quantum chemical calculations. We were unable to find a single-transition-state mechanism for the Brønsted-catalyzed reaction, but identified a two-step process that formally entails protonation of the formyl group, electrophilic attack on the alkenyl group and deprotonation of the resulting carbocation. Isobutene is the most reactive of the three isomers and was the focus of our experimental efforts. The catalyst reactivity was measured by autoclaving formaldehyde and isobutene with anhydrous 1,4-dioxane as solvent at a temperature range of 323-453K and C₄H₈/CH₂O molar ratio=3.15. We found that H-ZSM-5 forms 3-methyl-3-buten-1-ol at significant rates and high selectivity (up to 93%) within a low-temperature range (323-348K). The Si/Al ratio had a significant impact on product selectivity; at the optimal Si/Al ratio (Si/Al=40), 3-methyl-3-buten-1-ol dehydration rate is high enough to form isoprene in a single step at 423K. Lower Si/Al ratios promote undesired side reactions leading to catalyst deactivation, while the acid site density at higher Si/Al ratios is not sufficient to promote the sequential dehydration step of the unsaturated alcohol to isoprene. Mechanistic studies led to understanding the reaction network. Side and sequential reactions of the desired compounds to form heterocycles proceed only at slow rates under all the conditions investigated.

References

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