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Fu et al. In situ Catalytic Transformation SCHEME 3 | CO2 capture and transcarboxylation in the synthesis of carbamates and ureas. CO2 allows the release of CO2 possible, thus promoting the CO2 hydrogenation. By employing pentaethylenehexamine (PEHA) as CO2 absorbent and Ru-based complexes as catalyst, Olah and Surya Prakash group develops a process that combines CO2 capture and the following hydrogenation in an ethereal solvent for the production of MeOH (Kothandaraman et al., 2016a). CO2 from air can be captured by an aqueous solution of PEHA and up to 61% yield of MeOH can be obtained in the triglyme/H2O mixtures at 155◦C in the following hydrogenation (Scheme 7). The resulting MeOH can be easily separated by simple distillation from the reaction mixture. Later, the same group captures CO2 with aqueous amine solution and then in situ hydrogenates the resulting ammonium bicarbonate/carbonate utilizing Ru- and Fe-based pincer complexes in a biphasic solvent system (water/Me-THF) (Kothandaraman et al., 2016b). The superbases (DABCO, TMG, and DBU) shows to be efficient for both CO2 capture and hydrogenation with more than 90% yield of formate under moderate reaction conditions (50 bar H2 at 55◦C) (Scheme 7). The biphasic system features easy separation of product and catalyst and the catalyst can be reused for at least five cycles. Hydrogenation Using Heterogeneous Catalysts Besides homogeneous catalysts, the heterogeneous catalysts were also used in the hydrogenation of CO2 capture products. For example, H. Lin group applies Pd/AC catalyst to the hydrogenation of CO2 capture products originated from ammonia. In the hydrogenation step, the dependence of the activity of CO2 capture products on the solvent is observed (Su et al., 2015a,b). For example, the ammonium bicarbonate in water and ammonium carbamate in 70 wt% ethanol-water solution can offer more than 90% yield of formate under high H2 pressure (5.52 and 2.75 MPa, respectively) at 20◦C. The ammonium carbonate presents similar activity with ammonium carbamate. Identification of the species in the reactant solutions suggests the bicarbonate ion and ethyl carbonate ion, instead of the carbamate ion, are the activation forms of CO2 in the hydrogenation (Scheme 8). Coincidently, Enthaler finds that sodium bicarbonate in methanol can be hydrogenated to sodium formate catalyzed by the nickel hydride complex while CO2 cannot be hydrogenated in the identical conditions, which further confirms the activity of the captured CO2 in hydrogenation (Enthaler et al., 2015). Mertens and coworkers report the in situ hydrogenation of the captured CO2 using Cu/ZnO-Al2O3 as catalyst under retrieval of the CO2 capture reagent N,N-diethylethanolamine (DEEA) (Reller et al., 2014). In the reaction, DEEA can also function as a trapping reagent for the resulting formic acid and drives the hydrogenation forward. The authors find that the generation of the products 2-diethylaminoethylformate and methanol can be regulated by the reaction temperature (Scheme 9). The combination of CO2 capture and hydrogenation realizes the energy integration by using the reaction heat of CO2 hydrogenation in the energy demanding CO2 stripping process. In addition to the liquid absorption system, the alkali metal and alkali earth metal based solid CO2 adsorbents are also developed (Li et al., 2010, 2011; Lee et al., 2011) and applied in the CCU strategy recently. Duyar et al. design a series of novel dual function materials (DFM) consisting of the catalyst and adsorbent components to couple the endothermic CO2 desorption step with the exothermic hydrogenation of CO2 (Duyara et al., 2016). The results show that DFM with the composition of 5% Ru 10% K2 CO3 /Al2 O3 and 5% Ru Frontiers in Chemistry | www.frontiersin.org 5 July 2019 | Volume 7 | Article 525PDF Image | CO2 Capture and in situ Catalytic Transformation
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