A New Approach to the Reduction of Carbon Dioxide: CO2 Reduction to Formate by Transfer Hydrogenation in iPrOH

Recent advances in carbon dioxide capture, fixation, and activation by using N-heterocyclic carbenes

In the last two decades, CO2 emission has caused a lot of environmental problems. To mitigate the concentration of CO2 in the atmosphere, various strategies have been implemented, one of which is the use of N-heterocyclic carbenes (NHCs) and related complexes to accomplish the capture, fixation, and activation of CO2 effectively. In this review, we summarize CO2 capture, fixation, and activation by utilizing NHCs and related complexes; homogeneous reactions and their reaction mechanisms are discussed. Free NHCs and NHC salts can capture CO2 in both direct and indirect ways to form imidazolium carboxylates, and they can also catalyze the reaction of aromatic aldehydes with CO2 to form carboxylic acids and derivatives. Moreover, associated with transition metals (TMs), NHCs can form NHC-TM complexes to transform CO2 into industrial acid or esters. Non-metal-NHC complexes can also catalyze the reactions of silicon and boron complexes with CO2 . In addition, catalytic cycloaddition of epoxides with CO2 is another effective function of NHC complexes, and NHC ionic liquids perform excellently in this aspect.

Conversion of Carbon Dioxide into Several Potential Chemical Commodities Following Different Pathways - A Review

This article reviews the literature related to the direct uses of CO2and its conversion into various value added chemicals including high energy density liquid fuels such as methanol. The increase in the direct uses of CO2and its conversion into potential chemical commodities is very important as it directly contributes to the mitigation of CO2related global warming problem. The method being followed at present in several countries to reduce the CO2associated global warming is capturing of CO2at its major outlets using monoethanolamine based solution absorption technique followed by storing it in safe places such as, oceans, depleted coal seams, etc., (i.e., carbon dioxide capturing and storing in safe places, CCS process). This is called as CO2sequestration. Although, the CCS process is the most understood and immediate option to mitigate the global warming problem, it is considerably expensive and has become a burden for those countries, which are practicing this process. The other alternative and most beneficial way of mitigating this global warming problem is to convert the captured CO2into certain value added bulk chemicals instead of disposing it. Conversion of CO2into methanol has been identified as one of such cost effective ways of mitigating global warming problem. Further, if H2is produced from exclusively water using only solar energy instead of any fossil fuel based energy, and is used to convert CO2into methanol there are three major benefits: i) it contributes greatly to the global warming mitigation problem, ii) it greatly saves fossil fuels as methanol production from CO2could be an excellent sustainable and renewable energy resource, and iii) as on today, there is no better process than this to store energy in a more convenient and highly usable form of high energy density liquid fuel. Not only methanol, several other potential chemicals and value added chemical intermediates can be produced from CO2. In this article, i) synthesis of several commodity chemicals including poly and cyclic-carbonates, sodium carbonate and dimethyl carbonate, carbamates, urea, vicinal diamines, 2-arylsuccinic acids, dimethyl ether, methanol, various hydrocarbons, acetic acid, formaldehyde, formic acid, lower alkanes, etc., from CO2, ii) the several direct uses of CO2, and iii) the importance of producing methanol from CO2using exclusively solar energy are presented, discussed and summarized by citing all the relevant and important references.

The catalytic role of N-heterocyclic carbene in a metal-free conversion of carbon dioxide into methanol: a computational mechanism study

A density functional theory study at the M05-2X(IEFPCM, THF)/6-311+G**//M05-2X/6-31G* level has been conducted to gain insight into the catalytic mechanism of the first metal-free N-heterocyclic carbene (NHC)-catalyzed conversion of carbon dioxide into methanol. Among the various examined reaction pathways, we found that the most favorable leads to the experimentally detected intermediates, including formoxysilane (FOS), bis(silyl)acetal (BSA), silylmethoxide (SMO), and disiloxane (DSO). However, our study also revealed that formaldehyde (CH(2)O), generated from the dissociation of BSA into DSO and CH(2)O via a mechanism somewhat similar to the Brook rearrangement, should be an inevitable intermediate, although it was not reported by the experimentalists. When NHC catalyzes the reactions of CO(2)/FOS/CH(2)O with silane, there are two activation modes. It was found that NHC prefers to activate Si-H bonds of silane and push electron density to the H atoms of the Si-H bonds in favor of transferring a hydridic atom of silane to the electrophilic C center of CO(2)/FOS/CH(2)O. This holds true in particular for the NHC-catalyzed reactions of silane with FOS/CH(2)O to produce BSA/SMO. The preferred activation mode can operate by first passing an energetically unfavorable NHC-silane local minimum via pi-pi interactions or by directly crossing a transition state involving three components simultaneously. The activation mode involving initial coordination of NHC with the electrophilic C atom of CO(2)/FOS/CH(2)O is less favorable or inoperable. The predicted catalytic mechanism provides a successful interpretation of the experimental observation that phenylsilane is more efficient than diphenylsilane in performing the conversion.