Theoretical Study of 4-(Hydroxymethyl)benzoic Acid Synthesis from Ethylene and 5-(Hydroxymethyl)furoic Acid Catalyzed by Sn-BEA

A sustainable route has been reported for the production of terephthalic acid (PTA) from 5-(hydroxymethyl)furoic acid (HMFA) and ethylene, both of which can be derived from biomass. This process starts with the production of 4-(hydroxymethyl)benzoic acid (HMBA) from HMFA and ethylene catalyzed by Sn-BEA. The subsequent oxidation of HMBA leads to PTA. The present study reports the results of a detailed computational investigation of the mechanism of HMBA synthesis from ethylene and HMFA mediated by Sn-BEA. Density functional theory calculations show that the formation of HMBA proceeds via Diels–Alder cycloaddition of HMFA and ethylene, which is rate-limiting, followed by Lewis acid-catalyzed dehydration. The solution-phase reaction and six different pathways in Sn-BEA, including one pathway on the Si site and five different pathways on the Sn site, are investigated for the Diels–Alder cycloaddition of HMFA and ethylene. Energy decomposition analysis (EDA) shows that the Sn site stabilizes the transition state of the Diels–Alder reaction electrostatically instead of facilitating charge transfer between HMFA and ethylene. Therefore, the preferred pathway for the Diels–Alder reaction starts with binding HMFA to the Sn site by the carbonyl oxygen, which is the configuration that maximizes electrostatic interactions between substrates and the catalyst in the transition state. The effect of substituting Sn in the active site by Zr and Ti was examined and the highest reaction barriers were for the Ti sites. Using EDA, we found that though the barriers of the Sn and Zr site are comparable, the individual contributing effects are different: lower energy penalty associated with distortion of the geometry of the Zr site overcomes less favorable electrostatic and charge transfer effects compared to the Sn site.