Gas-phase photocatalytic coprocessing of CO2 - H2O(v) to energy products promoted by the n,n-junction In2O3@g-C3N4 under VIS-light.

Carbon dioxide capture and utilization is a strategic technology for moving away from fossil-C. The conversion of CO2 into fuels demands energy and hydrogen that cannot be sourced from fossil-C. Co-processing of CO2 and water under solar irradiation will have a key role in the long-term for carbon-recycling and energy products production. This article discusses the synthesis, characterization and application of the two-phase composite photocatalyst, In2O3@g-C3N4, formed by thermal condensation of melamine in the presence of indium(III)nitrate. The composite exhibits a n,n-heterojunction between two n-type semiconductors, g-C3N4 and In2O3, leading to a more efficient charge separation. The composite has a flat band potential enabling it to effectively catalyze the reduction of CO2 in the gas phase to produce CO, CH4 and CH3OH. While the composite's overall photocatalytic efficiency is comparable to that of neat g-C3N4, its ability to promote multielectron-transfer and Proton Coupled to Electron Transfer (PCET) suggests that there is a potential for further optimization of its properties. The use of labelled 13CO2 has allowed us to clearly exclude that the reduced species are derived from the photocatalyst decomposition or the degradation of contaminants.

Improving the Enzymatic Cascade of Reactions for the Reduction of CO2 to CH3OH in Water: From Enzymes Immobilization Strategies to Cofactor Regeneration and Cofactor Suppression.

The need to decrease the concentration of CO2 in the atmosphere has led to the search for strategies to reuse such molecule as a building block for chemicals and materials or a source of carbon for fuels. The enzymatic cascade of reactions that produce the reduction of CO2 to methanol seems to be a very attractive way of reusing CO2; however, it is still far away from a potential industrial application. In this review, a summary was made of all the advances that have been made in research on such a process, particularly on two salient points: enzyme immobilization and cofactor regeneration. A brief overview of the process is initially given, with a focus on the enzymes and the cofactor, followed by a discussion of all the advances that have been made in research, on the two salient points reported above. In particular, the enzymatic regeneration of NADH is compared to the chemical, electrochemical, and photochemical conversion of NAD+ into NADH. The enzymatic regeneration, while being the most used, has several drawbacks in the cost and life of enzymes that suggest attempting alternative solutions. The reduction in the amount of NADH used (by converting CO2 electrochemically into formate) or even the substitution of NADH with less expensive mimetic molecules is discussed in the text. Such an approach is part of the attempt made to take stock of the situation and identify the points on which work still needs to be conducted to reach an exploitation level of the entire process.

Do Bio-Ethanol and Synthetic Ethanol Produced from Air-Captured CO2 Have the Same Degree of "Greenness" and Relevance to "Fossil C"?

This paper discusses the epochal change in the reputation of carbon dioxide, which is now considered as a raw material alternative to fossil C for the synthesis of chemicals, materials and fuels, as opposed to a waste material that must be confined underground. In particular, its use as renewable C is compared to biomass. In this paper, a specific point is discussed: is ethanol (or any fuel) produced via the catalytic conversion of atmospheric CO2 different from the relevant biomass-sourced product(s)? The answer to this question is very important because it ultimately determines whether or not fuels derived from atmospheric CO2 (either e-fuels or solar fuels) have the right to be subsidized in the same way that biofuels are. Conclusions are drawn demonstrating that ethanol derived from atmospheric CO2 deserves the same benefits as bio-ethanol, with the additional advantage that its synthesis can be less pollutant than its production via the fermentation of sugars. The same concept can be applied to any fuel derived from atmospheric CO2.

Sustainable Synthesis of Oxalic and Succinic Acid through Aerobic Oxidation of C6 Polyols Under Mild Conditions.

The sustainable chemical industry encompasses a shift from the use of fossil carbon to renewable carbon. The synthesis of chemicals from nonedible biomass (cellulosic or oil) represents one of the key steps for "greening" the chemical industry. In this paper, we report the aerobic oxidative cleavage of C6 polyols (5-HMF, glucose, fructose and sucrose) to oxalic acid (OA) and succinic acid (SA) in water under mild conditions using M@CNT and M@NCNT (M=Fe, V; CNT=carbon nanotubes; NCNT=N-doped CNT), which, under suitable conditions, were recoverable and reusable without any loss of efficiency. The influence of the temperature, O2 pressure (PO2 ), reaction time and stirring rate are discussed and the best reaction conditions are determined for an almost complete conversion of the starting material and a good OA yield of 48 %. SA and formic acid were the only co-products. The former could be further converted into OA by oxidation in the presence of formic acid, resulting in an overall OA yield of >62 %. This process was clean and did not produce organic waste nor gas emissions.

Selective Oxidation of 5-(Hydroxymethyl)furfural to DFF Using Water as Solvent and Oxygen as Oxidant with Earth-Crust-Abundant Mixed Oxides.

5-Hydroxymethylfurfural (5-HMF) can be considered a prominent building block: because of the presence of the alcohol and aldehyde moieties, it can be used to generate useful molecules as chemicals of industrial interest with high added value, monomers for polymers, and even fuels. This article shows how building up mixed oxides of different complexities and properties may drive the selectivity toward one of the possible products generated from 5-HMF. In particular, mixed oxides based on cerium and other metals abundant on the earth-crust perform the selective oxidation of 5-HMF to 2,5-diformylfuran (94%), using oxygen as oxidant and water as solvent. The roles of the reaction conditions (temperature, reaction time, oxygen pressure, concentration of the substrate), the chemical composition, the acidic/basic properties, and redox properties of the catalysts are discussed.

Biocatalytic and Bioelectrocatalytic Approaches for the Reduction of Carbon Dioxide using Enzymes.

In the recent decade, CO2 has increasingly been regarded not only as a greenhouse gas but even more as a chemical feedstock for carbon-based materials. Different strategies have evolved to realize CO2 utilization and conversion into fuels and chemicals. In particular, biological approaches have drawn attention, as natural CO2 conversion serves as a model for many processes. Microorganisms and enzymes have been studied extensively for redox reactions involving CO2. In this review, we focus on monitoring nonliving biocatalyzed reactions for the reduction of CO2 by using enzymes. We depict the opportunities but also challenges associated with utilizing such biocatalysts. Besides the application of enzymes with co-factors, resembling natural processes, and co-factor recovery, we also discuss implementation into photochemical and electrochemical techniques.

Synthesis and Characterization of Fe0 (2,2'-bipyridine) (2-aminoethyl-pyridine) and its Reaction with Dihydrogen.

Fe0 (bpy)(pyea) (2; bpy=2,2'-bipyridine, pyea=2-aminoethyl-pyridine), a 16-electron species, was synthesized by reduction of FeCl2 (bpy)(pyea) (1) using Na-strips. It is a diamagnetic low-melting solid (m.p. 295 K) stable under N2 and easily decomposed by radiations even at low temperature. It was fully characterized by elemental analyses and multinuclear NMR. Complex 2 acts as an active hydrogenation catalyst, but has a very short lifetime. In fact, it reacts with H2 (0.1-1 MPa) at room temperature in toluene and affords in a few minutes a new Fe0 complex characterized as Fe0 (bpy)(η6 -picoline) (3), inactive to hydrogenation. Picoline is derived from the sp3 -sp3 C-C bond cleavage of the aminoethyl arm of the pyea ligand. The rapid evolution of the putative intermediate FeH2 (bpy)(pyea) (4) has not allowed the isolation such Fe-hydrido species. The interaction of H2 with 2 has been studied by DFT, which has allowed to demonstrate that 3 is lower in energy than 2+H2 , justifying the fact that the intermediate dihydride was not isolated. Interestingly, 3 was also obtained by reaction of 1 with NaBH4 or with glycerol-KOH. Complex 2 is one of the rare examples of Fe0 complex stabilized by a set of only N-donor atoms. The reaction with glycerol confirms the potential role of Fe in catalytic hydrogenation reactions using bio-glycerol as a H-source.

Photocatalytic Carbon Dioxide Reduction at p-Type Copper(I) Iodide.

A p-type semiconductor, CuI, has been synthesized, characterized, and tested as a photocatalyst for CO2 reduction under UV/Vis irradiation in presence of isopropanol as a hole scavenger. Formation of CO, CH4 , and/or HCOOH was observed. The photocatalytic activity of CuI was attributed to the very low potential of the conduction band edge (i.e., -2.28 V vs. NHE). Photocurrents generated by the studied material confirm a high efficiency of the photoinduced interfacial electrontransfer processes. Our studies show that p-type semiconductors may be effective photocatalysts for CO2 reduction, even better than extensively studied n-type titanium dioxide, owing to the low potential of the conduction band edge.

Selective Aerobic Oxidation of 5-(Hydroxymethyl)furfural to 5-Formyl-2-furancarboxylic Acid in Water.

A simple, cheap, and selective catalyst based on copper/cerium oxides is described for the oxidation of 5-(hydroxymethyl)furfural (5-HMF) in water. An almost quantitative conversion (99 %) with excellent (90 %) selectivity towards the formation of 5-formyl-2-furancarboxylic acid, a platform molecule for other high value chemicals, is observed. The catalyst does not require any pretreatment or additives, such as bases, to obtain high yield and selectivity in water as solvent and using oxygen as oxidant. When a physical mixture of the oxides is used, low conversion and selectivity are observed. Air can be used instead of oxygen, but a lower conversion rate is observed if the same overall pressure is used, and the selectivity remains high. The catalyst can be recovered almost quantitatively and reused. Deactivation of the catalyst, observed in repeated runs, is due to the deposition of humins on its surface. Upon calcination the catalyst almost completely recovers its activity and selectivity, proving that the catalyst is robust.

The need to implement an efficient biomass fractionation and full utilization based on the concept of "biorefinery" for a viable economic utilization of microalgae.

In the present work, microalgae strains, such as Scenedesmus obliquus and Phaeodactylum tricornutum grown in indoor/outdoor photobioreactors (PBRs) and in open ponds (this is the first study on such strains cultivated in the local Southern Italy climatic conditions), were fully analyzed for their protein content, carbohydrates, lipids, and fatty acid profile in order to assess their potential use for the production of biofuels, chemicals, and omega-3, and as animal feed and human food. They are compared with Nannochloropsis sp. (commercial sample) which was fully analyzed in our laboratory and Chlorella (literature data). An economic evaluation was carried out, demonstrating that the cultivation of microalgae for the production of only biofuels will not match the economic standards. Conversely, if chemicals are also produced applying the biorefinery concept and using wastewater as a source of nutrients, it will be possible to have a good positive return from microalgae.

Organic Carbonates: Efficient Extraction Solvents for the Synthesis of HMF in Aqueous Media with Cerium Phosphates as Catalysts.

We describe a process for the selective conversion of C6 -polyols into 5-hydroxymethylfurfural (5-HMF) in biphasic systems of organic carbonate/water (OC/W), with cerium(IV) phosphates as catalysts. Different reaction parameters such as the OC/W ratio, catalyst loading, reaction time, and temperature, were investigated for the dehydration of fructose. Under the best reaction conditions, a yield of 67.7 % with a selectivity of 93.2 % was achieved at 423 K after 6 h of reaction using [(Ce(PO4)1.5 (H2 O)(H3 O)0.5 (H2 O)0.5)] as the catalyst. A maximum yield of 70 % with the same selectivity was achieved after 12 h. At the end of the reaction, the catalyst was removed by centrifugation, the organic phase was separated from water and evaporated in vacuo (with solvent recovery), and solid 5-HMF was isolated (purity >99 %). The recovery and reuse of the catalyst and the relationship between the structure of the OC and the efficiency of the extraction are discussed. The OC/W system influences the lifetime of the catalysts positively compared to only water.

Solar energy utilization in the direct photocarboxylation of 2,3-dihydrofuran using CO2.

The conversion of CO2 into high energy products (fuels) and the direct carboxylation of C-H bonds require a high energy input. Energy cannot be derived from fossil carbon, in this case. Solar energy can be used instead, with a low environmental impact and good profit. We have studied the use of white light or solar energy in the photoreduction of CO2 and in photocarboxylation reactions, using different semiconductors modified at their surface. Two examples of reduction of CO2 to methanol and CO will be shortly discussed, and two cases of carboxylation of organic substrates. The case of carboxylation of 2,3-dihydrofuran will be discussed in detail.

An integrated photocatalytic/enzymatic system for the reduction of CO2 to methanol in bioglycerol-water.

A hybrid enzymatic/photocatalytic approach for the conversion of CO2 into methanol is described. For the approach discussed here, the production of one mol of CH3OH from CO2 requires three enzymes and the consumption of three mol of NADH. Regeneration of the cofactor NADH from NAD(+) was achieved by using visible-light-active, heterogeneous, TiO2-based photocatalysts. The efficiency of the regeneration process is enhanced by using a Rh(III)-complex for facilitating the electron and hydride transfer from the H-donor (water or a water-glycerol solution) to NAD(+). This resulted in the production of 100 to 1000 mol of CH3OH from one mol of NADH, providing the possibility for practical application.

Cerium-based binary and ternary oxides in the transesterification of dimethylcarbonate with phenol.

Diphenyl carbonate (DPC) plays a key role in phosgene-free carbonylation processes. It can be produced by transesterification of dimethyl carbonate (DMC) with phenol in the presence of catalysts. Methyl phenyl carbonate (MPC) is first produced that is then converted into DPC by either disproportionation or further transesterification with phenol. Cerium-based bimetallic oxides (with the heterometal being niobium, iron, palladium, or aluminum) are used as catalysts in the transesterification of DMC to synthesize MPC. The catalytic activity is affected by the type and concentration of the heterometal. XPS, IR and elementary analyses are employed to characterize the new catalysts. Differently from pure oxides, the mixed oxides produce a significant increase of the conversion and selectivity towards MPC.