CO2 Enzymatic carboxylation and reduction to methanol
The utilisation of CO2 (technological, chemical, enhanced biological) may contribute to the reduction of the emission of CO2 in terms of innovative processes that may substitute old technologies and products with a direct or indirect beneficial effect on the impact on climate change. It is the case to recall the utilisation of CO2 is not a technology that will solve the problem of CO2 accumulation in the atmosphere, but it can give a contribution to such issue through the reduction of the volume of CO2 produced.
Catalytic reduction of carbon dioxide with H2 to produce methanol is a route to produce valuable chemicals from CO2. The reaction has been shown feasible with different kinds of catalysts at high temperature (150°C-300°C) and pressure (3-14 MPa).
Alternatively, biocatalysts have been shown to be active in such reduction processes at room temperature via a cascade of reactions involving different enzymes.
In particular, in this context, I will discuss the CO2 reduction into methanol at room temperature in an aqueous environment via a three-step reaction catalyzed by enzymes under electron transfer conditions.
CO2 is first reduced to HCOOH, which is converted into formaldehyde (CH2O), and then into methanol. This research is in its infancy, but it has been demonstrated that the three enzymes can be encapsulated and used together for an easy conversion of CO2 into methanol. The limiting factor is the electron source: the NADPH+/NADP couple has been used so far. Cheap reducing agents or solar energy should be used to generate the electrons necessary for the reduction of CO2 into methanol. A couple of options will be discussed. The process above would be of great importance as the production of methanol under such conditions would represent the solution to recycling CO2 and using it as source of carbon in the energy and chemical industry.
The selective carboxylation of phenol to 4-HO-benzoic acid has been achieved using supported enzymes extracted from Thauera aromatica. The enzyme is K+ and Mn+2 dependent and is active both in water and in sc-CO2. The reaction is carried out in simple reactors based on encapsulated enzymes (low melting Agar) or using a cut-off membrane reactor.