In recent years, Carbon Capture and Storage (Sequestration) (CCS) has been proposed as a potential method to allow the continued use of fossil-fuelled power stations whilst preventing emissions of CO2 from reaching the atmosphere. Gas, coal (and biomass)-fired power stations can respond to changes in demand more readily than many other sources of electricity production, hence the importance of retaining them as an option in the energy mix. Here, we review the leading CO2 capture technologies, available in the short and long term, and their technological maturity, before discussing CO2 transport and storage. Current pilot plants and demonstrations are highlighted, as is the importance of optimising the CCS system as a whole. Other topics briefly discussed include the viability of both the capture of CO2 from the air and CO2 reutilisation as climate change mitigation strategies. Finally, we discuss the economic and legal aspects of CCS.

Carbon capture and storage update

The chemistry of N-heterocyclic carbenes (NHCs) has witnessed tremendous development in the past two decades: NHCs have not only become versatile ligands for transition metals, but have also emerged as powerful organic catalysts in molecular chemistry and, more recently, in metal-free polymer synthesis. To understand the success of NHCs, this review first presents the electronic properties of NHCs, their main synthetic methods, their handling, and their reactivity. Their ability to activate key functional groups (e.g. aldehydes, esters, heterocycles, silyl ketene acetals, alcohols) is then discussed in the context of molecular chemistry. Focus has been placed on the activation of substrates finding analogies with monomers (e.g. bis-aldehydes, multi-isocyanates, cyclic esters, epoxides, N-carboxyanhydrides, etc.) and/or initiators (e.g. hydroxy- or trimethylsilyl-containing reagents) employed in such “organopolymerisation” reactions utilizing NHCs. A variety of metal-free polymers, including aliphatic polyesters and polyethers, poly(α-peptoid)s, poly(meth)acrylates, polyurethanes, or polysiloxanes can be obtained in this way. The last section covers the use of NHCs as structural components of the polymer chain. Indeed, NHC-based photoinitiators, chain transfer agents or functionalizing agents, as well as bifunctional NHC monomer substrates, can also serve for metal-free polymer synthesis.

N-Heterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis

Ionic liquids have attracted a great deal of interest in recent years, illustrated by their applications in a variety of areas involved with chemistry, physics, biology, and engineering. Usually, the stabilities of ionic liquids are highlighted as one of their outstanding advantages. However, are ionic liquids really stable in all cases? This review covers the chemical stabilities of ionic liquids. It focuses on the reactivity of the most popular imidazolium ionic liquids at structural positions, including C2 position, N1 and N3 positions, and C4 and C5 positions, and decomposition on the imidazolium ring. Additionally, we discuss decomposition of quaternary ammonium and phosphonium ionic liquids and hydrolysis and nucleophilic reactions of anions of ionic liquids. The review aims to arouse caution on potential decomposition of ionic liquids and provides a guide for better utilization of ionic liquids.

Are Ionic Liquids Chemically Stable

Ionic liquids (ILs), a kind of novel green medium composed entirely of cations and anions, have attracted considerable attention due to their unique properties such as non-volatility, tunable polarity, high stability and so on. In this article, the latest progress on the absorption and subsequent conversion of CO2 by using ILs as absorbents, catalysts or promoters will be summarized. The chemical absorption performance of ILs, especially task-specific ionic liquids (TSILs) such as amino-functionalized ILs, superbase-derived protic ILs has been systematically illustrated. Although significant advances have been made, extensive energy input in the desorption process to recover absorbents would still be a crucial barrier to realizing practical carbon capture and sequestration (CCS). On the other hand, efficient applications of CO2 in the synthesis of valuable compounds such as organic carbonates, urea derivatives, oxazolidinones and formic acid can also be promoted by employing TSILs as catalysts/reaction media. We anticipate that an integration of chemical capture of CO2 with its utilization, a so-called CO2 capture and utilization (CCU) protocol would be an ideal strategy to solve the energy penalty problem in common CCS without the need for additional heat desorption. The essence of this CCU concept is to use TSILs for CO2 capture and substantial activation, which could allow catalytic transformation of CO2 to be accomplished smoothly under low pressure (ideally at 1 atm).

CO2 chemistry: task-specific ionic liquids for CO2 capture/activation and subsequent conversion

Carbon dioxide is commonly regarded as the primary greenhouse gas, but from a synthetic standpoint can be utilized as an alternative and sustainable C1 synthon in organic synthesis rather than a waste. This results in the production of organic carbonates, carboxylic acids, and derivatives. Recently, CO2 has emerged as an appealing tool for heterocycle synthesis under mild conditions without using stoichiometric amounts of organometallic reducing reagents. This Minireview summarizes recent advances on methodologies for CO2 incorporation into N-, O-, and C-nucleophiles to provide various heterocycles, including cyclic carbamates, benzoxazine-2-one, 4-hydroxyquinolin-2-one, quinazoline-2,4(1 H,3 H)-diones, benzimidazolones, α-alkylidene cyclic carbonate.

Upgrading Carbon Dioxide by Incorporation into Heterocycles

A series of 1,3-disubstituted-2-imidazolium carboxylates, an adduct of CO2 and N-heterocyclic carbenes, were synthesized and characterized using single crystal X-ray, thermogravimetric, IR, and NMR analysis. The TGA analysis of the NHC-CO2’s shows that as steric bulk on the N-substituent increases, the ability of the NHC-CO2 to decarboxylate increases. The comparison of NHC-CO2’s with and without methyls at the 4,5-position indicate that extra electron density in the imidazolium ring enhances the stability of an NHC-CO2 thereby making it less prone to decarboxylation. Single crystal X-ray analysis shows that the torsional angle of the carboxylate group and the C−CO2 bond length with respect to the imidazolium ring is dependent on the steric bulk of the N-substituent. Rotamers in the unit cell of a single crystal of ItBuPrCO2 (2f) indicate that the C−CO2 bond length increases as the N-substituents rotate toward the carboxylate moiety, which suggests that rotation of the N-substituents through the plane of ...

A systematic investigation of factors influencing the decarboxylation of imidazolium carboxylates.

Combination of 1,3-bis(2,6-diisopropylphenyl)imidazolum-2-carboxylate (IPrCO2) with the Lewis acids MBPh4, where M = Li or Na, provided two separate complexes. The crystal structures of these complexes revealed that coordination to NaBPh4 yielded a dimeric species, yet coordination of IPrCO2 with LiBPh4 yielded a monomeric species. Combination of 1,3-bis(2,4,6-trimethylphenyl)imidazolum-2-carboxylate (IMesCO2) with LiBPh4 also afforded a dimeric species that was similar in global structure to that of the IPrCO2+NaBPh4 dimer. In all three cases, the cation of the organic salt was coordinated to the oxyanion of the zwitterionic carboxylate. Thermogravimetric analysis of the crystals demonstrated that decarboxylation occurred at lower temperatures than the decarboxylation temperature of the parent NHC·CO2 (NHC = N-heterocyclic carbene). Kinetic analysis of the transcarboxylation of IPrCO2 to acetophenone with NaBPh4 to yield sodium benzoylacetate was performed. First-order dependences were observed for IPrCO...

Bret R. Van Ausdall

Papers

A systematic investigation of factors influencing the decarboxylation of imidazolium carboxylates.

Imidazolidene Carboxylate Bound MBPh4 Complexes (M = Li, Na) and Their Relevance in Transcarboxylation Reactions