A Nice Chemical Engineering Paper on the Hydrogenation of Carbon Dioxide to Methanol.

[NNadir]

5. From a purely mechanistic standpoint the third equation should be specified.

I don't have the reference handy, but could get it. I think it referred to MeOH, although it could have been DME. Olah showed that CO2 needed to be present at least in minute amounts, for catalysts to activate.

The direct use of CO2 of course results in the formation of more water which introduces a separation penalty, but happily, the MeOH/water system is zeotropic, unlike the azeotropic EtOH/water system.

The ease of separation from water and the vastly reduced toxicity of DME - dimethyl ether - for which MeOH is a precursor, makes DME superior, as does the easy transformation back and forth from the gas and liquid phases.

There are catalysts for the direct preparation of DME from syn gas, but heat removal has been a challenge. One can drive a turbine with this reduction, a sort of "anti-combustion."