Abstract

A heterogeneous catalyst, rhodium hydrotalcite (Rh-HT), was synthesized, characterized, and explored for kinetic studies for the hydrogenation of CO₂ to formic acid using molecular hydrogen in an autoclave. The catalyst was efficient for the selective formation of formic acid at a moderate temperature with up to 5 catalytic cycles without any significant loss in catalytic activity. The detailed kinetic investigations were performed by determining the rate of formic acid formation as a function of time, catalyst amount, total pressure, partial pressure of CO₂, partial pressure of H₂, reaction volume, v/v ratio of the mixed solvent of methanol and water, agitation speed, and temperature in a wide range of variation. The rates were found to be dependent on all these parameters. The formic acid formation rate followed the first-order kinetics with respect to the partial pressures of CO₂ and H₂, and catalyst amount. The best reaction conditions obtained from the kinetic parametric optimisation were, 50 bar total pressure (1/1 p/p, CO₂ and H₂); 60°C temperature; a mixture of methanol:water solvent (5/1 v/v, 60 mL); 24 h time; and 500 rpm agitation speed to get a TON of 15840 for formic acid with no additional base. The thermodynamic performance of the heterogeneous catalyst Rh-HT was appreciably associated with highly negative entropy. The performance of the catalyst was effectively enhanced by the mixture of water and methanol as a solvent. The mechanistic routes for CO₂ hydrogenation to formic acid are proposed and discussed based on the kinetic and experimental observations involving the role of the molecular effect of water used in the solvent.

中文翻译：

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铑水滑石 (Rh-HT) 催化剂选择性 CO2 加氢制甲酸的动力学研究

摘要

合成、表征了一种非均相催化剂铑水滑石(Rh-HT)，并探索了在高压釜中使用分子氢将CO _{2加氢为甲酸的动力学研究。}该催化剂可在中等温度下有效选择性地形成甲酸，最多可进行 5 次催化循环，而催化活性没有任何显着损失。通过确定作为时间、催化剂量、总压、CO 2 分压、_H 2 分压的函数_{的甲酸形成速率来进行详细的动力学研究。}、反应体积、甲醇和水混合溶剂的v/v比、搅拌速度和温度在宽范围内变化。发现速率取决于所有这些参数。_{甲酸形成速率遵循关于CO 2}和H ₂分压以及催化剂量的一级动力学。动力学参数优化得到的最佳反应条件为，50 bar总压（1/1 p/p，CO ₂和H ₂）；60℃温度；甲醇：水溶剂的混合物（5/1 v/v，60 mL）；24小时时间；和 500 rpm 搅拌速度，在没有额外碱的情况下得到 15840 的甲酸 TON。多相催化剂 Rh-HT 的热力学性能与高负熵显着相关。水和甲醇的混合物作为溶剂有效提高了催化剂的性能。基于涉及溶剂中水的分子效应的动力学和实验观察，提出并讨论了CO ₂加氢生成甲酸的机理路线。