About this book The role of carbon dioxide in our changing climate is now hard to ignore, and many countries are making pledges to reduce or eliminate their carbon output. Chemical valorisation of carbon dioxide, as an alternative to sequestration, is likely to play an important part in reaching these targets, and as such is one of the fastest developing areas of green chemistry and chemical reaction engineering. Providing a comprehensive panorama of recent advances in the methods and technologies for chemical valorisation of carbon dioxide, this book is essential reading for anyone with an interest in sustainability and green chemistry. Both the technological improvements in traditional processes and new methods and concepts are discussed, including various (renewable) electricity-based methods, as well as novel catalytic, photocatalytic and biocatalytic approaches.

The electrocatalytic epoxidation of alkenes at heterogeneous catalysts using water as the sole oxygen source is a promising safe route toward the sustainable synthesis of epoxides, which are essential building blocks in organic chemistry. However, the physicochemical parameters governing the oxygen-atom transfer to the alkene and the impact of the electrolyte structure on the epoxidation reaction are yet to be understood. Here, we study the electrocatalytic epoxidation of cyclooctene at the surface of gold in hybrid organic/aqueous mixtures using acetonitrile (ACN) solvent. Gold was selected, as in ACN/water electrolytes gold oxide is formed by reactivity with water at potentials less anodic than the oxygen evolution reaction (OER). This unique property allows us to demonstrate that a sacrificial mechanism is responsible for cyclooctene epoxidation at metallic gold surfaces, proceeding through cyclooctene activation, while epoxidation at gold oxide shares similar reaction intermediates with the OER and proceeds via the activation of water. More importantly, we show that the hydrophilicity of the electrode/electrolyte interface can be tuned by changing the nature of the supporting salt cation, hence affecting the reaction selectivity. At low overpotential, hydrophilic interfaces formed using strong Lewis acid cations are found to favor gold passivation. Instead, hydrophobic interfaces created by the use of large organic cations favor the oxidation of cyclooctene and the formation of epoxide. Our study directly demonstrates how tuning the hydrophilicity of electrochemical interfaces can improve both the yield and selectivity of anodic reactions at the surface of heterogeneous catalysts.

A pyrazole–based ligand substituted with terpyridine groups at the 3 and 5 positions has been synthesized to form the dinuclear cobalt complex 1, that electrocatalytically reduces carbon dioxide (CO₂) to carbon monoxide (CO) in the presence of Brønsted acids in DMF. Chemical, electrochemical and UV–vis spectro–electrochemical studies under inert atmosphere indicate pairwise reduction processes of complex 1. Infrared spectro–electrochemical studies under CO₂ and CO atmosphere are consistent with a reduced CO–containing dicobalt complex which results from the electroreduction of CO₂. In the presence of trifluoroethanol (TFE), electrocatalytic studies revealed single–site mechanism with up to 94% selectivity towards CO formation when 1.47 M TFE were present, at –1.35 V vs Saturated Calomel Electrode in DMF (0.39 V overpotential). The low faradaic efficiencies obtained (<50%) are attributed to the generation of CO–containing species formed during the electrocatalytic process, which inhibit the reduction of CO₂.

Ammonia synthesis is a thermodynamically and kinetically demanding chemical process, and the senescent Haber–Bosch process has been the only way to manufacture ammonia under high pressure and temperature until recently. The electrochemical nitrogen reduction reaction (NRR) is a fungible method of ammonia production that requires strategic tailoring of a highly active, robust, and efficient electrocatalyst to overcome the challenges of high nitrogen (N₂) bond dissociation energy, high specific surface area, more catalytic adsorption sites for N₂, specificity to produce ammonia (NH₃) as a major product, competitive hydrogen evolution reaction (HER), and long-term durability. Herein, Graphene wrapped nickel phthalocyanine (NiPc) nanohybrid (NiPc–RGO) was developed as an electrocatalyst, with a high ammonia yield of 23.9 µg h^-1 mg^-1cat and a Faradaic efficiency of 18.8% at –0.3 V vs. RHE, demonstrating outstanding durability and good selectivity. Isotopic labelling studies and numerous control trials are used to establish the validity of nitrogen sources in NH3 generation. These experimental results showed that NiPc-RGO hybrids could potentially serve as efficient NRR electrocatalysts for N₂ fixation under ambient conditions.

Novel energy and atom efficiency processes will be keys to develop the sustainable chemical industry of the future. Electrification could play an important role, by allowing to fine-tune energy input and using the ideal redox agent: the electron. Here we demonstrate that a commercially available Milstein ruthenium catalyst (1) can be used to promote the electrochemical oxidation of ethanol to ethyl acetate and acetate, thus demonstrating the four electron oxidation under preparative conditions. Cyclic voltammetry and DFT-calculations are used to devise a possible catalytic cycle based on a thermal chemical step generating the key hydride intermediate. Successful electrification of Milstein-type catalysts opens a pathway to use alcohols as a renewable feedstock for the generation of esters and other key building blocks in organic chemistry, thus contributing to increase energy efficiency in organic redox chemistry.

N₂ splitting is achieved with a simple Mo complex at a carbon electrode. Controlled reduction (a 3 electron transfer process) at −1.4 V (vs. SCE) and ambient conditions (room T and atmospheric P) afforded a Mo nitride complex in 30 % yield. DFT studies support the transient formation of a Mo^I−N₂-Mo^I bridging dimer.

Electrochemical CO₂ reduction is an attractive option for storing renewable electricity and for the sustainable production of valuable chemicals and fuels. In this roadmap, we review recent progress in fundamental understanding, catalyst development, and in engineering and scale-up. We discuss the outstanding challenges towards commercialization of electrochemical CO2 reduction technology: energy efficiencies, selectivities, low current densities, and stability. We highlight the opportunities in establishing rigorous standards for benchmarking performance, advances in in operando characterization, the discovery of new materials towards high value products, the investigation of phenomena across multiple-length scales and the application of data science towards doing so. We hope that this collective perspective sparks new research activities that ultimately bring us a step closer towards establishing a low- or zero-emission carbon cycle.

A Co-quaterpyridine (Coqpy) and a Co-tris(2-pyridylmethyl)amine cobalt (CoTPA) complexes, known for their ability to reduce CO₂ to CO under visible-light irradiation in homogeneous conditions, were immobilized using the impregnation method in two different Zr-based metal-organic frameworks, UiO-67-bpydc and PCN-777, respectively. The successful synthesis of Coqpy@UiO-67-bpydc shows that linker deficiencies can allow catalytic complexes (Cat) to enter into the cavities of the Zr-based MOF while they would not in the defect-free MOF. The composites were characterized through solid state characterization techniques complemented by Density Functional Theory calculations in order to locate the catalysts into the MOF’s pores and estimate the host/guest interactions involved. The two noble-metal free Cat@MOF composites reduce CO₂ to CO under visible-light irradiation in CH₃CN/H₂O (5:1) solutions, in the presence of [Ru(phen)₃]²⁺ as solution-dissolved external photosensitizer and dimethylphenylbenzimidazoline (BIH) as electron and proton donor. CO yields, TON and selectivity reached 619 µmol g^-1 h^-1, 268 and 93% respectively for Coqpy@UiO-67-bpydc and 368 µmol g^-1 h^-1, 482 and 94% for CoTPA@PCN-777 after 24 h solar simulated illumination. These photocatalytic studies evidenced that the composite materials are efficient, selective and recyclable and that the MOF scaffold offers a favorable environment for constant CO production rate over a long time period.