Green Chemistry COMMUNICATION Cite this: DOI: 10.1039/c5gc00397k Received 19th February 2015, Accepted 19th March 2015 DOI: 10.1039/c5gc00397k www.rsc.org/greenchem High yield production of formate by hydrogenating CO 2 derived ammonium carbamate/carbonate at room temperature† Ji Su, Mi Lu and Hongfei Lin* A sustainable economy is needed to recycle carbon rather than to irreversibly use stored carbon in fossil fuels. Instead of releasing CO 2 into the atmosphere, its value can be recognized as a base C1 material for chemicals. In this study, a new CO 2 utilization strat- egy is developed via the hydrogenation of CO 2 derived ammonium carbamates/carbonates, which are the intermediates in industrial urea production or carbon capture processes, to produce value- added formate chemicals. A high yield of formate, ∼92%, was achieved after hydrogenating ammonium carbamates in the ethanol–water solution at room temperature with the carbon sup- ported palladium nano-catalyst. The ethyl carbonate ions in the ammonium carbamate/carbonate solutions were speciated by 13 C NMR and were attributed to the superior hydrogenation efficiency. Ammonium ions promote the formation of ethyl carbonate ions in the presence of ethanol. The solvent affects the distribution of the reactive intermediates. The use of carbon dioxide (CO 2 ) as a chemical feedstock reduces the amount of oil or gas required to make the product. 1 Carbon recycling, instead of carbon sequestration, lays out the basis of a fully sustainable carbon economy. If the energy inputs into the process are renewable, a sustainable carbon recycling can be realized. In this regard, a variety of chemicals or fuels, including formic acid, methanol, methane, etc., can be produced by hydrogenation of CO 2 with hydrogen from the electrolysis of water using renewable electricity from solar or wind energy. 2 Among the CO 2 hydrogenation products, formic acid is an important commodity chemical used for silage preservation, animal feed additives, and leather tanning. 3 Moreover, formic acid and its formate salts are highly attractive as the carbon-neutral H 2 storage materials. 4 A plethora of studies were focused on the hydrogenation of CO 2 in aqueous or organic solvents to produce formic acid or formate salts with homogeneous catalysts. 2i,4b,5 Recently immobilized homogeneous catalysts 6 were found to be easier to handle than the homogeneous ones for the hydrogenation of bicarbonate salts. However, the TOFs of these pseudo- heterogeneous catalysts decreased to one or two orders of magnitude compared to the homogeneous counterparts. 7 In addition, there are concerns about the stability of such immobilized catalysts that could not withstand industrial applications. 6a,c Supported palladium heterogeneous catalysts have been proved to be stable for the reduction of sodium bicarbonate to sodium formate in water. 2a,8 However, the development of heterogeneous catalysts for CO 2 hydrogenation in liquid solutions has lagged in the last decade mainly because the activity of heterogeneous catalysts was rather low. Moreover, the reduction of the CO 2 derived intermediates over heterogeneous catalysts in liquid solvents is still unclear. It is well accepted that a bicarbonate ion (HCO 3 - ) was the common intermediate during the hydrogenation of CO 2 in aqueous alkaline solutions. 5b,d However, hydrogenation of other CO 2 derived compounds, such as carbamic acid, carba- mate, and alkyl carbonate formed by the contact of CO 2 with an inorganic base or an amine in water or organic solvents, lacks systematic studies. The controversial results are not uncommonly displayed in the literature. For example, Jessop’s group found that the hydrogenation of methyl carbonate was not facile, 7a while another group proved that methyl carbonate was rather active towards hydrogenation to produce formate. 9 To date, there have been only a few industrial processes, such as the production of urea, salicylic acid, and polycarbo- nates, which employ CO 2 as the C1 chemical building block. 10 The major obstacle is the high cost of conversion since CO 2 has the low energy state and thus is very stable. In general, significant cost reduction can be achieved by leveraging the existing industrial infrastructure. We thus are motivated to examine the utilization of CO 2 derivatives in commercial pro- cesses, aiming to graft the new carbon recycling strategy onto the existing industrial processes. In our previous study, we discovered that an ammonium cation (NH 4 + ) promoted the hydrogenation of aqueous † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c5gc00397k Department of Chemical and Material Engineering, University of Nevada, Reno, 1664 N. Virginia St. M/S388, Reno, NV 89557, USA. E-mail: HongfeiL@unr.edu; Fax: +1 7753275059 This journal is © The Royal Society of Chemistry 2015 Green Chem. Published on 19 March 2015. Downloaded by Lawrence Berkeley National Laboratory on 17/04/2015 18:57:02. View Article Online View Journal