Catalytic reduction of CO2 into fuels and fine chemicals

Abstract

With the progressive increase in atmospheric CO₂ over the years and owing to its potential environmental threat, researchers have focused their attention on the fruitful utilization of CO₂ into value-added chemicals and feedstocks. Although CO₂ conversion reactions are intensively studied through several electrochemical, photochemical and photo-electrochemical pathways, chemical reduction of CO₂ is more challenging to achieve due to the involvement of breaking of high energy CO bonds without any applied potential together with the accomplishment of green chemistry perspectives. Considerable progress in the chemical reduction of CO₂ in the presence of reducing agents over homogeneous and heterogeneous catalysts has been achieved over the years. However, this technology has several pitfalls to overcome before it can be utilized in large scale industrial processes. We show here the recent progress in CO₂ reduction to essential fuels [CO, CH₃OH, CH₃CH₂OH, HCO₂H, CH₄, dimethylether (DME), dimethylcarbonate (DMC) and lower hydrocarbons] as well as valuable chemicals via nucleophilic addition reactions. We also emphasize the direct conversion of CO₂ from ultra-diluted sources like ambient air as a possible roadmap to solve carbon emission problems from the real world. The entire discussion is divided into two parts where in the first part we summarize several homogeneous catalytic processes involving the nucleophilic addition of CO₂, resulting in C–C and C–H bond formation leading to the synthesis of 2-oxazolidinones, aminals, terminal carboxylated products and indolelactone derivatives that are potentially sound for the pharmaceutical industry. Other reduction products, such as methane, methanol, and methoxides, are also listed using Frustrated Lewis Pairs (FLP) as catalysts. The second part extensively highlights heterogeneous catalysts to reduce CO₂ with H₂. However, significant efforts are still needed to develop active, selective and stable catalysts on a pilot plant scale by judicial consideration of the thermodynamics and kinetics of the reactions. CO₂ reduction can proceed over a range of immobilized metallic nanoparticles on inorganic supports (CeO₂, Al₂O₃, TiO₂etc.) and nanostructured porous frameworks (zeolites, porous polymers, mesoporous silica). Thus, thorough investigation on the reaction mechanism of the overall process involving different active sites is necessary. Primarily, this review brings together the major advancements made in the CO₂ reduction processes together with a focus on the utility and challenges in achieving the activation of the CO₂ molecule.