Condensed-phase low temperature heterogeneous hydrogenation of CO2 to methanol

Abstract

A low-temperature CH₃OH synthesis was achieved at 120–170 °C using tertiary amine and alcohol in the presence of a Cu/ZnO/Al₂O₃ catalyst by CO₂ hydrogenation. A series of 1°, 2° and 3° amines and alcohols were screened to study their influence on the formation of CH₃OH. Particularly, 3° amines such as NEt₃ in combination with EtOH formed CH₃OH with 100% yield with respect to the amine. Unlike the traditional gas-phase heterogeneous metal catalyzed CO₂-to-CH₃OH reactions, no CO is used in the feed gas mixture in this method. In addition, the hydrogenation gives good selectivity (>95%) to CH₃OH and only trace amounts of CO and CH₄ are formed. The presence of CO in the gas mixture was attributed to the decomposition of the CH₃OH product, which was confirmed by high-temperature and high-pressure MAS NMR. The reaction was performed in the condensed phase at relatively lower temperatures, thus the RWGS reaction, which typically operates at >250 °C, was significantly reduced at these temperatures (120–170 °C). The first in situ spectroscopic evidence for the condensed phase hydrogenation of alkylcarbonate to CH₃OH via ammonium formate and alkylformate intermediates was also presented under the experimental conditions.