Carbon Dioxide Utilisation -The Formate Route

UIDB/50006/2020 CEEC-Individual 2017 Program Contract.The relentless rise of atmospheric CO2 is causing large and unpredictable impacts on the Earth climate, due to the CO2 significant greenhouse effect, besides being responsible for the ocean acidification, with consequent huge impacts in our daily lives and in all forms of life. To stop spiral of destruction, we must actively reduce the CO2 emissions and develop new and more efficient “CO2 sinks”. We should be focused on the opportunities provided by exploiting this novel and huge carbon feedstock to produce de novo fuels and added-value compounds. The conversion of CO2 into formate offers key advantages for carbon recycling, and formate dehydrogenase (FDH) enzymes are at the centre of intense research, due to the “green” advantages the bioconversion can offer, namely substrate and product selectivity and specificity, in reactions run at ambient temperature and pressure and neutral pH. In this chapter, we describe the remarkable recent progress towards efficient and selective FDH-catalysed CO2 reduction to formate. We focus on the enzymes, discussing their structure and mechanism of action. Selected promising studies and successful proof of concepts of FDH-dependent CO2 reduction to formate and beyond are discussed, to highlight the power of FDHs and the challenges this CO2 bioconversion still faces.publishersversionpublishe

Fe(II) and Fe(III) dithiocarbamate complexes as single source precursors to nanoscale iron sulfides: A combined synthetic and in-situ XAS approach

Nanoparticulate iron sulfides have many potential applications and are also proposed to be prebiotic catalysts for the reduction of CO2 to biologically important molecules, thus the development of reliable routes to specific phases with controlled sizes and morphologies is important. Here we focus on the use of iron dithiocarbamate complexes as single source precursors (SSPs) to generate greigite and pyrrhotite nanoparticles. Since these minerals contain both iron(III) and iron(II) centres, SSPs in both oxidation stateshave been utilised. Use of Fe(CO)2(S2CNR2)2is novel and it readily loses both carbonyls in a single step providing an in situ source of the extremely air-sensitive [Fe(S2CNR2)2]. Decomposition of [Fe(S2CNR2)3] alone in oleylamine affords pyrrhotite, although by careful control of reaction conditions a window exists in which pure greigite nanoparticles can be isolated. With cis-[Fe(CO)2(S2CNR2)2] we were unable to produce pure greigite, with pyrrhotite formation dominating, a similar situation being found with mixtures of Fe(II) and Fe(III) precursors. In-situ X-ray absorption spectroscopy (XAS) studies showed that heating [Fe(S2CNiBu2)3] in oleylamine resulted in amine coordination and, at ca. 60 oC, reduction of Fe(III) to Fe(II) with (proposed) elimination of thiuram disulfide (S2CNR2)2. We thus carried out a series of decomposition studies with added thiuram disulfide (R = iBu) and found that addition of 1-2 equivalents led to the formation of pure greigite nanoparticles between 230-280 oC with low SSP concentrations. Average particle size does not vary significantly with increasing concentration, thus providing a convenient route to ca. 40 nm greigite nanoparticles. In-situ XAS studies have been carried out and allow a decomposition pathway for [Fe(S2CNiBu2)3] in oleylamine to be established; reduction of Fe(III) to Fe(II) reduction triggers substitution of the secondary amide backbone by oleylamine (RNH2) resulting in the in situ formation of a primary dithiocarbamate derivative [Fe(RNH2)2(S2CNHR)2]. This in turn extrudes RNCS to afford molecular precursors of the observed FeS nanomaterials. The precise role of thiuram disulfide in the decomposition process is unknown, but it likely plays a part in controlling the Fe(III)-Fe(II) equilibrium and may also act as a source of sulfur allowing control over the Fe:S ratio in the mineral products