Selective Methylation of Phenol with CO2/H2 to o -Cresol and 2,6-Xylenol

Abstract

Employing CO2/H2 as a sustainable methylating agent offers a transformative strategy for advancing conventional methylation processes, yet achieving controllable C–C coupling with high efficiency remains a formidable challenge. Here, we report a novel tandem process that integrates CO2 hydrogenation with phenol methylation over a Zr-doped In2O3 solid-solution catalyst. The optimized In7Zr3Ox catalyst exhibits >95% selectivity toward ortho-methylation with tunable product distribution, ranging from 92% o-cresol at 16% conversion to 86% 2,6-xylenol at 97% conversion, while maintaining stable operation for 140 h. Mechanistic studies identify surface formaldehyde species (CH2O*) generated from CO2 hydrogenation as the active methylating intermediates that kinetically couple CO2 reduction to phenol methylation. This pathway exhibits a markedly lower apparent activation energy of 58.8 kJ mol–1 than the conventional methanol-mediated route of 116.3 kJ mol–1. Continuous consumption of CH2O* also shifts the CO2 hydrogenation equilibrium, imparting a thermodynamic driving effect absent in CO2 hydrogenation alone. These findings demonstrate the potential of CO2-based methylation for selective C–C bond formation and expand opportunities for CO2 valorization.