Eliminating carbon dioxide emissions at the source by the integration of carbon dioxide capture and utilization over noble metals in the liquid phase

Abstract

Carbon dioxide (CO2) capture and utilization (CCU) offer a response to greenhouse gas emissions whereas they are often conducted separately, resulting in a high energy demand for the CO2 separation process. The capture reagent chemically binds the CO2 molecule, activating the stubborn CO2 to a more active species. The direct conversion of the captured and activated CO2 to the value-added chemicals will simplify the CO2 utilization process, leading to a strong energy-saving effect by omitting the CO2 separation process. In this paper, the commercial and the self-synthesized noble metal catalysts were evaluated in the hydrogenation of amine captured CO2 in ethanol at 140–165 °C, which is demonstrated to produce the main product methanol in the presence of K3PO4 or using MgO as a catalyst support. The Pd-based catalyst was superior to the Rh, Pt or Ru based catalyst in the hydrogenation of amine captured CO2 towards methanol. The superior Pd/MgO catalyst was characterized by XRD, XPS, SEM, HRTEM, and DRIFTS analysis. NMR measurement and the ATR FTIR measurements were used to determine the initial intermediate to be ethyl carbonate. Mechanistic insight to the methanol formation indicates that the synergistic effect of Pd and a base cascaded the reactions: the reduction of ethyl carbonate to formic acid, the dehydration between formic acid and ethanol to form ethyl formate, and the hydrogenolysis of ethyl formate to methanol and ethanol. The ethyl formate intermediate was hydrogenated to methanol by Pd, wherein basic condition (K3PO4 or MgO) was responsible for the activation of the carbonyl group of the ethyl formate.