Using carbon dioxide (CO2) as a feedstock for commodity synthesis is an attractive means of reducing greenhouse gas emissions and a possible stepping-stone towards renewable synthetic fuels1,2. A major impediment to synthesizing compounds from CO2 is the difficulty of forming carbon–carbon (C–C) bonds efficiently: although CO2 reacts readily with carbon-centred nucleophiles, generating these intermediates requires high-energy reagents (such as highly reducing metals or strong organic bases), carbon–heteroatom bonds or relatively acidic carbon–hydrogen (C–H) bonds3,4,5. These requirements negate the environmental benefit of using CO2 as a substrate and limit the chemistry to low-volume targets. Here we show that intermediate-temperature (200 to 350 degrees Celsius) molten salts containing caesium or potassium cations enable carbonate ions (CO32–) to deprotonate very weakly acidic C–H bonds (pKa > 40), generating carbon-centred nucleophiles that react with CO2 to form carboxylates. To illustrate a potential application, we use C–H carboxylation followed by protonation to convert 2-furoic acid into furan-2,5-dicarboxylic acid (FDCA)—a highly desirable bio-based feedstock6 with numerous applications, including the synthesis of polyethylene furandicarboxylate (PEF), which is a potential large-scale substitute for petroleum-derived polyethylene terephthalate (PET)7,8. Since 2-furoic acid can readily be made from lignocellulose9, CO32–-promoted C–H carboxylation thus reveals a way to transform inedible biomass and CO2 into a valuable feedstock chemical. Our results provide a new strategy for using CO2 in the synthesis of multi-carbon compounds.
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We thank Stanford University and the Henry and Camille Dreyfus Foundation for support of this work through a Teacher-Scholar Award to M.W.K. G.R.D. gratefully acknowledges a fellowship through the Stanford Center for Molecular Analysis and Design, and T.Y. acknowledges a Postdoctoral Fellowship for Research Abroad through the Japan Society for the Promotion of Science. We thank T. Veltman for installation of the Parr reactor, S. Lynch for assistance with 2H NMR, and J. Du Bois for discussions. High-resolution mass spectrometry was performed at the Vincent Coates Foundation Mass Spectrometry Laboratory, Stanford University Mass Spectrometry.
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 1 NMR spectra for the carboxylation of caesium furan-2-carboxylate under flowing CO2.
a, 1H NMR (300 MHz) and b, 13C NMR (100 MHz) in D2O of the crude product mixture after the reaction of 1 mmol caesium furan-2-carboxylate and 0.55 mmol Cs2CO3 under CO2 flowing at 40 ml min–1 at 260 °C for 12 h. f1 indicates the chemical shift, δ.
a, 1H NMR (300 MHz) and b, 13C NMR (100 MHz) in D2O of the crude product mixture after the reaction of 1 mmol caesium thiophene-2-carboxylate and 0.55 mmol Cs2CO3 under CO2 flowing at 40 ml min–1 at 325 °C for 12 h.
Extended Data Figure 3 NMR spectra for the carboxylation of caesium furan-2-carboxylate in the Parr reactor.
a, 1H NMR (300 MHz) in D2O of the crude product mixture after the reaction of 1 mmol caesium furan-2-carboxylate and 0.55 mmol Cs2CO3 under 8 bar CO2 at 200 °C for 5 h. b, 1H NMR (300 MHz) in D2O of the crude product mixture after the reaction of 10 mmol caesium furan-2-carboxylate and 5.5 mmol Cs2CO3 under 8 bar CO2 at 200 °C for 10 h.
a, 1H NMR (300 MHz) and b, 13C NMR (100 MHz) in D2O of the crude product mixture after the reaction of 1 mmol caesium benzoate and 0.55 mmol Cs2CO3 under 8 bar CO2 at 320 °C for 5 h.
Extended Data Figure 5 NMR spectra for the carboxylation of potassium furan-2-carboxylate and benzene.
a, 1H NMR (600 MHz) in D2O of the crude product mixture after the reaction of 0.5 mmol potassium furan-2-carboxylate, 0.5 mmol potassium isobutyrate and 0.28 mmol K2CO3 under CO2 flowing at 40 ml min–1 at 320 °C for 8 h. b, 1H NMR (600 MHz) of the crude product mixture after the reaction in D2O of a 1.5 mmol of caesium carbonate and 1 mmol caesium isobutyrate under 42 bar benzene and 31 bar CO2 at 350 °C for 8 h.
Extended Data Figure 6 1H NMR spectra for H/D exchange between furan-2-carboxylate and deuterated acetate in the presence of Cs2CO3.
a, 1H NMR (400 MHz) in D2O of a 1:1 mixture of caesium furan-2-carboxylate and CD3CO2Cs. b, 1H NMR (400 MHz) in D2O of the crude product mixture after the reaction of a 1:1 mixture of caesium furan-2-carboxylate and CD3CO2Cs with 0.55 equivalents Cs2CO3 at 200 °C under 2 bar N2 for 1 h.
Extended Data Figure 7 Additional NMR spectra for H/D exchange between furan-2-carboxylate and deuterated acetate in the presence of Cs2CO3.
a, 13C NMR (75 MHz) and b, 2H NMR (92 MHz) in D2O of the crude product mixture after the reaction of a 1:1 mixture of caesium furan-2-carboxylate and CD3CO2Cs with 0.55 equivalents Cs2CO3 at 200 °C under 2 bar N2 for 1 h.
Extended Data Figure 8 NMR spectra for H/D exchange between furan-2-carboxylate and deuterated acetate in the absence of Cs2CO3.
a, 1H NMR (400 MHz) and b, 2H NMR (92 MHz) in D2O of the crude product mixture after the reaction of a 1:1 mixture of caesium furan-2-carboxylate and CD3CO2Cs at 200 °C under 2 bar N2 for 1 h.
Extended Data Figure 9 No H/D exchange is observed between differentially labelled caesium benzoates when heated to 320 °C in the absence of Cs2CO3.
a, 1H NMR (400 MHz) in CDCl3 of the DMFD isolated after the second carboxylation/esterification sequence. b, 1H NMR (400 MHz) in D2O of the material recovered from the aqueous phase after the second carboxylation/esterification sequence.
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Banerjee, A., Dick, G., Yoshino, T. et al. Carbon dioxide utilization via carbonate-promoted C–H carboxylation. Nature 531, 215–219 (2016). https://doi.org/10.1038/nature17185
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