An insight of CO 2 hydrogenation to methanol synthesis: Thermodynamics, catalysts, operating parameters, and reaction mechanism

Catalytic hydrogenation of CO2 to methanol is an exciting avenue to curb the rising CO2 emissions and generate renewable energy or value-added products. Methanol synthesis via the thermal catalysis route gets increasing emphasis due to its fast kinetics and flexible combination of active components. In the last decade, many studies on CO2 hydrogenation to methanol have been reported with different kinds of catalysts that have been synthesized and characterized using state-of-the-art surface science tools and techniques. In situ analysis techniques as well as theoretical (eg, density functional theory, Monte Carlo simulations, and Micro-Kinetics modeling) studies have been performed to understand the insights of morphology changes, the interaction of active sites, and the formation of intermediate species under the reaction conditions. In the present review, the advancements on CO2 to methanol via hydrogenation route have been presented taking into consideration different perspectives spanning across thermodynamic aspects, the influence of reaction temperature, pressure, feed composition, space-velocity, and morphologically tuned novel catalyst on CO2 conversion and methanol selectivity. Among the reported catalysts, the Al2O3-supported Cu-Zn catalyst showed better performance with 25% CO2 conversion and 73% methanol selectivity at 170°C and 50 bar pressure. The CeO2-supported Pd-Zn catalyst showed 14% CO2 conversion and 97% methanol selectivity at 220°C under 20 bar pressure. Also, CeO2-nanorods supported Cu-Ni catalyst showed good performance at 260°C and 30 bars, with around 18% CO2 conversion and 73% methanol selectivity. Additionally, the mechanistic insights of the process are emphasized with necessary figures and diagrams. The rate-determining steps for each mechanism are also highlighted with chemical structures for further clarity. This review also summarizes potential catalysts and their optimum operating conditions to achieve maximum CO2 conversion and methanol selectivity. We believe, the review is unique and not found in any article which addressed the above aspects altogether in a compact form.