Renewable sources for CO2 valorization through reductive processes

Two different strategies were investigated in order to address the energetic demand in CO2 reduction to useful chemicals and fuels. The first approach was based on the use of bio-glycerol, a byproduct of bio-diesel production process, as renewable H-donor for the hydrogenation of CO2. Ruthenium catalysts have been proved to have high activity in CO2 hydrogenation to formic acid, and interestingly these catalysts are in some cases the same which are used for homogenous hydrogenation of organic substrates with 2-propanol as hydrogen donor.[1] In order to use glycerol in the place of 2-propanol and to combine the processes of glycerol oxidation and CO2 reduction a Ru(II) mediated Hydrogen transfer was performed under controlled conditions. A Ru(II) precursor, i.e. RuCl2(PPh3)3 or RuCl2(COD)/PPh3 (COD = 1,5-cyclooctadiene) was reacted with aqueous glycerol in basic ambient affording a dihydrido-carbonyl Ru(II) complex, i.e. RuH2(CO)(PPh3)3. The complex was isolated and characterized by multinuclear NMR analysis. RuH2(CO)(PPh3)3 is known to catalyze the hydrogenation of CO2.[2] The product of glycerol simultaneous decarbonylation and dehydrogenation was identified by 13 C-NMR to be glycolic acid in form of potassium salt.[3] The overall process represents an interesting reaction for the conversion of two waste into added value products. The second strategy concerned the photo-electrochemical regeneration of NADH for the enzymatic CO2 to methanol through the dehydrogenase enzyme cascade: FateDH, FaldDH and ADH.[4] Although, this process occurs under very mild conditions (water, 37°C, pH=7) and with optimal yield and selectivity (close to 100%), there still is a limitation associated with the consumption of the cofactor NADH. Enzymatic, chemical and photo-chemical approaches have been attempted,[5] but over these, electrochemical regeneration is considered the most attractive solution.[6] We have employed p-type semiconductor electrodes in order to utilize solar energy for photoelectrochemical NADH regeneration. While bare semiconductors were shown to produce only enzymatically inactive dimers (NAD2), modification of the surface by electro-deposition of a thin layer of Pt or Ru metal caused the formation of 1,4-NADH as the main product. In particular red-light illuminated (>600 nm) Pt/p-GaAs showed an increased efficiency at low overpotentials (-0.75V vs Ag/AgCl) when compared to metal electrodes (> 7 fold), with no dimer detection. This study represents the first example of NADH regeneration at an illuminated semiconductor electrode. The absence of a mediator allows the direct coupling of this regeneration system with the enzymatic CO2 reduction apparatus, modeling the light and dark reactions occurring in a chloroplast.