Zwitterionic alcoholic solutions for integrated CO2 capture and hydrogenation

Abstract

An integrated CO₂ capture and conversion (ICCU) process using zwitterionic bases (ZBs) in alcohol solvents was developed. Inner salts composed of a 1,3-dimethylimidazolium cation covalently attached to a phenolate moiety react with CO₂ to form alkyl carbonates (AKCs) in alcohols. The ZBs yield up to 70% AKCs and retain chemisorbed CO₂, whereas tertiary amines with similar conjugate acid pK_a values capture only small amounts of CO₂ and quickly release it in the alcohols tested. The AKCs were hydrogenated to HCOO⁻ or methanol, depending on the reaction temperature and the Ru catalyst selected. Using isopropanol as the solvent, the ICCU process combining 1,3-dimethyl-2-(4-oxyphenyl)imidazolium (ZB-p) and cis-[Ru(dppm)₂Cl₂] quantitatively converted the respective AKC to formate at 50 °C and 10 bar H₂ after 35 minutes of reaction. This represents one of the most active ICCU systems reported to date, with a turnover frequency of 660 h⁻¹ under these mild conditions. Methanol was obtained with a 66% yield after 20 hours by hydrogenation of the AKC of ZB-p in ethylene glycol using the catalyst Ru-MACHO-BH at 140 °C and 70 bar H₂.