Cu dispersed ZrO2 catalyst mediated Kolbe- Schmitt carboxylation reaction to 4-hydroxybenzoic acid

The Kolbe-Schmitt reaction is one of the industrially promising route for the production of hydroxybenzoic acids. In the present manuscript, alkali metal hydroxide free, direct carboxylation reaction has been demonstrated over heterogeneous CuO/ZrO2 catalyst. Different basic oxide (ZrO2, TiO2 and Bi2O3) and corresponding Cu dispersed oxides (CuO/ZrO2, CuO/TiO2 and CuO/Bi2O3) were tested for direct carboxylation. The performance of catalysts were elucidated by comparing 4-HBA yield, phenol conversion, TON and TOF. Amongst the tested catalysts, ZrO2 showed highest catalytic activity with TON 0.1022 and TOF 0.0043, hence selected as an efficient catalyst support for Cu dispersion. The lowest value of E-factor (0.12) and PMI (0.39) was obtained for 5 wt% CuO/ZrO2 catalyst. A series of Cu dispersed ZrO2 catalyst were synthesized via simple co-precipitation method with variable Cu precursor loading. Of the tested catalysts 1.25 wt% CuO loaded ZrO2 catalyst showed highest yield (62.57%) and phenol conversion (64.55%) under non-stringent operating condition (60 bar, 170 degrees C). The in-depth physicochemical characteristion were also conducted by using various analytical tools which dipicted the characterstic properties of CuO/ZrO2 catalyst. The process intensification study was conducted to obtain optimum operating parameters for high yield and conversion. The catalyst recyclability performance study showed the consistency in 4-HBA yield and conversion for four consecutive recycle runs. The plausible reaction mechanism for production of 4-HBA over CuO/ZrO2 catalyst and catalytic cycle has also been proposed. Thus, the present process provides green and efficient route for alkali hydroxide free Kolbe- Schmitt reaction.

Automated summary

This manuscript demonstrates the alkali metal hydroxide free, direct carboxylation reaction over heterogeneous CuO/ZrO2 catalyst, which is a green and efficient route for the Kolbe-Schmitt reaction. The performance of the catalysts was evaluated, and in-depth physicochemical characterization and process intensification studies were conducted. The results show the potential of CuO/ZrO2 catalyst for the production of hydroxybenzoic acids.