(111g) Computational Insights into the Direct Acylation of 2-Methylfuran with Acetic Acid over Phosphotungstic Acid and H-BEA Zeolite

It has recently been demonstrated that the Friedel-Crafts acylation of furanic compounds is a low-carbon-footprint synthesis of renewable oleo-furan sulfonate (OFS) surfactants;¹ a class of surfactants with enhanced properties.¹ Current industrial acylation processes that use carboxylic chloride or anhydrides as acylating agents have several drawbacks: formation of corrosive side products and significant amounts of carbon waste requiring additional separation/recycling processes. In contrast, carboxylic acid acylating agents only generate water as a by-product and are 100% carbon atom efficient. Thereby, executing the direct acylation with carboxylic acids is of significant interest for developing renewable surfactants and green processes in general.²

A major challenge to realizing this in practice is accelerating the acyl formation, which entails dehydration of an acid. In this paper, we perform electronic structure calculations and microkinetically analyze an extensive reaction network to gain insights into the Brønsted acid catalyzed acylation of 2-methylfuran with acetic acid over phosphotungstic acid (HPW), a superacid, and compare with the catalytic activity of H-BEA zeolite. The reaction entails two steps: formation of the acyl group, either as acylium ion or stabilized acyloxy surface species; and formation of the Wheland intermediate by electrophilic aromatic addition (EAS).

Among the major predictions from our models are: (a) In HPW, the dominant pathway is through the acylium intermediate, which is very unstable and extremely reactive and as result the EAS requires no activation. (b) In H-BEA, the preferred pathway is via the stable acyloxy intermediate and the rate-limiting step is the EAS. (c) We predict higher turnover frequencies in HPW than in H-BEA even though HPW under low-coverage conditions. We compare with experimental kinetics.

References:

(1) Park, D. S.; Joseph, K. E.; Koehle, M.; et al. ACS Cent. Sci. 2016, 2 (11), 820–824.

(2) Gumidyala, A.; Wang, B.; Crossley, S. Science Advances 2016, 2 (9), e1601072.