Low temperature hydrogenation of carbon dioxide into formaldehyde in liquid media

There is a growing demand for formaldehyde (HCHO) in various industrial applications, however, current industrial processes cannot be considered as green. There is an urgent need to develop an environmentally friendly and efficient method to produce this chemical. Our previous work introduced a novel HCHO production method via catalytic hydrogenation of CO in liquid media. This work demonstrates for the first time that HCHO can be synthesized via catalytic hydrogenation of CO2 in liquid media. Unlike CO conversion however, CO2 conversion is not believed to be a single step conversion. The conversion may proceed via two possible pathways: Route ACO(2) hydrogenation into formic acid (HCOOH) followed by dehydration-hydrogenation into HCHO; and Route B -CO2 conversion into CO via reverse water gas shift reaction, followed by direct hydrogenation of CO into HCHO similar to our previous report. To study feasibility of Route A, HCOOH conversion into HCHO was tested in methanol solvent and Pt-Cu/alumina showed the best conversion and highest yield. This partly confirmed our hypothesis that dehydration-hydrogenation of HCOOH into HCHO is feasible and hence HCOOH may act as an intermediate. However, the rate of HCHO production was substantially lower than the rate of HCOOH consumption, which suggests that there may be other competing reactions, such as decomposition of HCOOH. No intermediates could be detected in one-pot conversion of CO2 into HCHO in our investigation, which may be due to their low concentration and/or rapid consumption to form the products. Therefore, this study concludes that both the routes are likely for the conversion of CO2 into HCHO in a liquid phase reaction.