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Dowson and Styring CO2 Butanol and starting reagents. First, the Grignard process requires a hydrocarbon or alcohol to form the initial Grignard reagent and the product of the Grignard reaction would have to be converted from a carboxylic acid to the final alcohol product. With CDU processes in mind, the two obvious candidates for the starting hydrocarbon and alcohol shown in Figure 3 would be methane and methanol, both readily synthesized by the hydrogenation of CO2 (Figure 4) (Mikkelsen et al., 2010; Saeidi et al., 2014). The selective formation of methyl halide from the reaction of meth- ane and the elemental halogen is somewhat challenging, due to the higher reactivity of the products than starting materials, although appropriate choice of rare-earth or precious metal cata- lyst can yield the desired product (Olah et al., 1985; Podkolzin et al., 2007). By comparison, the conversion of methanol into any of the methyl halides is rather trivial using the hydrogen halide to carry out nucleophilic substitution. The former path- way remains potentially interesting as it does not require the addition of hydrogen to the process shown in Figure 3, although additional hydrogen would be needed in the CDU production of methane over methanol. With either of these compounds used as starting materials, the product of the Grignard process, after quenching, is acetic acid. This would have to then be condensed and hydrogenated to form the desired product, butanol. Condensing two equivalents of smaller molecules such as ethanol, to produce butanol has previously been reported by Guerbet chemistry (Koda et al., 2009; Dowson et al., 2013; Ho et al., 2016). However, production of butanol from acetic acid has not previously been reported. Although butanol has previously been made directly from CO2, the yield was limited (Irimescu, 2012). Acetic acid can be condensed in the Claisen self-condensation via ethyl acetate to produce the desired four-carbon chain product, as shown in Figure 5. While this reaction is readily carried out in the presence of a strong base, the final alkyl acetoacetate (ethyl acetoacetate in the case shown in Figure 5) product cannot be isolated without an acid quench, requiring a stoichiometric equivalent of base to be consumed to drive the otherwise mildly endergonic reaction. Care must also be taken that the acid quench does not hydrolyze the alkyl ester to produce the oxobutanoic acid (acetoacetic acid), as this is not stable, particularly at room temperature, decompos- ing to produce acetone and carbon dioxide (Hay and Bond, 1967). Hydrogenation of the produced acetoacetate ester to directly yield butanol had not been previously reported in the scientific literature but similar hydrogenation processes using supported Cu(0) catalysts have been reported, particularly in the reduc- tion of highly oxidized species such as low-molecular weight esters, glycerol, dimethyl maleate, and carbon dioxide (Figure 6) (van de Scheur and Staal, 1994; Brands et al., 1999; Schlander and Turek, 1999; Bienholz et al., 2011). By including all compounds generated and consumed during the overall reaction process, a basic energetic pathway of the generation of butanol from the reaction of methanol with CO2 and hydrogen, as described in the experimental section can be constructed using the available reaction enthalpy data (Figure 7). Note that this energy calculation in Figure 7 excludes the initial formation of the methanol (or methane) starting reagent, previously illustrated in Figure 4, for clarity. From the starting methanol, the reaction profile clearly demonstrates the strongly exothermic nature of the overall reac- tion pathway, but naturally does not show the energy required to regenerate the Grignard agent, which provides the majority of the driving force for the overall process, with the remaining steps driven by the by-production of water in the esterification and hydrogenation steps. Furthermore, it should be noted that FigUre 4 | Overall preparation of acetic acid via CO2 hydrogenation followed by Grignard reaction. FigUre 5 | Esterification of acetic acid and Claisen condensation of ethyl acetate. FigUre 6 | Hydrogenation of ethyl acetoacetate to produce butanol, ethanol, and minor by-products. Frontiers in Energy Research | www.frontiersin.org 7 October 2017 | Volume 5 | Article 26PDF Image | Demonstration of CO2 Conversion to Synthetic Transport Fuel
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