Role of acidity in acid-clay catalysts for the phosgene-free synthesis of methylene diphenyl dicarbamate (MDC)

Abstract

The phosgene-free synthesis of methylene diphenyl diisocyanate (MDI) from dimethyl carbonate (DMC) involves the acid-catalyzed condensation of methyl N-phenylcarbamate (MPC) with formaldehyde (HCHO) to produce methylene diphenyl carbamate (MDC). This study investigates the catalytic properties of a commercial acidic clay (HM-X), with a focus on its acidity and Brønsted/Lewis (B/L) balance. Characterization by XRD, N₂ adsorption, NH₃-TPD, and pyridine-IR shows that HM-X has a high density of medium and strong acid sites, with a notable Brønsted component. Under certain reaction conditions in DMC with co-fed water, HM-X achieved an 88.1% MDC yield at 90 °C after 6 hours. However, excessive acidity promoted side reactions, decreasing MDC selectivity. These findings highlight the importance of optimizing Brønsted and Lewis sites to balance activity and selectivity, providing insights for designing efficient, recyclable solid acids for sustainable, phosgene-free MDI production.