Ketonisation of carboxylic acids over Zn-Cr oxide in the gas phase

• Zn-Cr oxide is an efficient catalyst for gas-phase ketonisation of carboxylic acids.
• No catalyst deactivation for at least 24 h on stream.
• Carboxylic acid adsorbs on Zn-Cr oxide to form a bridging metal carboxylate.

Bulk Zn(II)-Cr(III) mixed oxides with a Zn/Cr atomic ratio of 1:1–20:1 were found to be active catalysts for the gas-phase ketonisation of carboxylic acids (acetic and propionic) to form acetone and 3-pentanone, respectively, at 300–400 °C and ambient pressure. Zn-Cr (10:1) oxide showed the best performance, significantly exceeding that of the parent oxides ZnO and Cr2O3. The catalytic activity was further enhanced by supporting Zn-Cr (10:1) oxide on TiO2 and γ-Al2O3. With 20%Zn-Cr/Al2O3, ketonisation of propionic acid occurred with 97% selectivity to 3-pentanone at 99% conversion at 380 °C. Zn-Cr oxides were characterised by BET, XRD, DRIFTS of pyridine and acetic acid adsorption and microcalorimetry of ammonia adsorption. From DRIFTS, carboxylic acid adsorbed dissociatively on Zn-Cr oxide to form a surface metal carboxylate in bidentate bridging bonding mode. A mechanism for ketonisation of carboxylic acids via β-ketoacid intermediate route has been proposed.

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