*Correspondence to:

Supporting Information for Carbonate-promoted hydrogenation of carbon dioxide to multi-carbon carboxylates Aanindeeta Banerjee 1 and Matthew W. Kanan 1 * 1 Department of Chemistry, Stanford University, Stanford, California *Correspondence to: mkanan@stanford.edu Experimental methods Index Figure S1. Representative NMR spectra for CO 2 hydrogenation promoted by Cs 2 CO 3 Figure S2. Representative NMR spectra for CO 2 hydrogenation promoted by Cs 2 CO 3 seeded with (Cs + ) 2 oxalate Figure S3. Representative NMR spectra for CO 2 hydrogenation promoted by Cs 2 CO 3 seeded with Cs + acetate-2-13 C Figure S4. 1 H NMR spectra for thermal annealing of Rb + formate- 13 C and Rb + formate under CO 2 and H 2 O Figure S5. NMR spectra for CO 2 hydrogenation promoted by Rb 2 CO 3 seeded with (Rb + ) 2 oxalate- 13 C 2 and (Rb + ) 2 oxalate Figure S6. NMR spectra for thermal annealing of Na + acetate- 13 C 2 and Cs 2 CO 3 under CO 2 hydrogenation conditions and Na + acetate- 13 C 2 Figure S7. Representative 1 H NMR spectrum of liquid sample formed in background reaction Figure S8. Representative GC-FID chromatogram of the product gas mixture for CO 2 hydrogenation reaction promoted by Cs 2 CO 3 seeded with Cs + acetate-2-13 C Figure S9. Two-step procedure for selective synthesis of oxalate Table S1: Carbonate-promoted CO 2 hydrogenation data Table S2: Demonstration of volatility of acetic acid-2-13 C from a mixture of M + acetate-2-13 C and M 2 CO 3 under CO 2 hydrogenation conditions page S2-S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S1

Experimental Methods Materials Cesium carbonate (99.995%, trace metal basis), cesium oxalate ( 99.9%, trace metal basis), oxalic acid (98%), phosphoric acid solution (85 wt. % in water) and 2-ethybutyric acid (99%) were purchased from Sigma Aldrich; rubidium carbonate (99.8+%) was purchased from Strem Chemicals; potassium carbonate (99.8%), sodium L-(+)-tartrate dihydrate (99.7%), HPLC grade water, HPLC grade acetonitrile and potassium phosphate monobasic ( 99%) were purchased from Fisher Scientific; carbon dioxide (99.99%) and hydrogen (99.998%) was purchased from Praxair; acetic acid-d 4 (99.5%), acetic acid-2-13 C (99%), formic acid-d 2 (98%) (<5% D 2 O), formic acid- 13 C (99%) (<5% H 2 O) and oxalic acid- 13 C 2 (99%) were purchased from Cambridge Isotope Laboratories. Chemicals were used as received without further purification. Instruments and analysis Experiments under pressurized CO 2, H 2 or N 2 were performed in a stainless steel 300 ml high-temperature, high-pressure Parr reactor (model 4561-HT-FG-SS-115-VS-2000-4848). The temperatures reported in the manuscript have been recorded by a J-type thermocouple height-adjusted by Parr such that the tip lies just above the reactor floor. The gaseous products were analyzed using a gas chromatograph (SRI Instruments) equipped with packed MoleSieve 5A column and a packed HaySep D column. Argon (99.9999%) was used as carrier gas. A flame ionization detector (FID) with a methanizer was used to quantify CO, CH 4, C 2 H 4 and C 2 H 6 and a thermal conductivity detector (TCD) was used to quantify H 2. 1 H NMR and 13 C NMR spectra were recorded at 23 C on a Varian Unity Inova 600 MHz spectrometer, Varian Unity Inova 500 MHz spectrometer, Varian Direct Drive 400 MHz spectrometer, Varian Mercury 400 MHz spectrometer, or Varian Unity Inova 300 MHz spectrometer. 1 H chemical shifts (δ) are reported in parts per million (ppm) downfield from tetramethylsilane and referenced to residual protium in the NMR solvent (D 2 O, δ = 4.79). 13 C chemical shifts (δ) are reported in ppm downfield from tetramethylsilane. Quantitative 13 C NMR was performed to quantify the amount of oxalate using cesium 2-ethylbutyrate as the internal standard. Spectra were obtained using a 90-degree pulses with a 30 sec delay time, keeping the decoupler off during delay time. The oxalate concentration was determined by comparing its peak area with that of the methyl peak of the internal standard. Oxalate yields were also independently measured using an Agilent 1200 HPLC equipped with diode array detector and Agilent Eclipse XDB-C18 reverse phase column. The HPLC samples were prepared by diluting an aliquot of the D 2 O solution of crude product mixture with freshly prepared 1N H 2 SO 4. Analysis was done at 40 C using 10 % CH 3 CN and 90 % 50 mm phosphate buffer (ph 2.1) as the mobile phase at flow rate of 1 ml min 1. The peak areas were calculated using Agilent OpenLAB CDS Chemstation HPLC software. The oxalate concentration measured by HPLC was found to be consistent with the 13 C NMR analysis. Safety Statement No unexpected or unusually high safety hazards were encountered. Cs 2 CO 3 promoted CO 2 hydrogenation at 230 C An oven-dried glass liner containing Cs 2 CO 3 (3.26 g, 10.0 mmol) was sealed in the Parr reactor. The reactor was heated to 300 C under a stream of N 2 with evacuation after every 15 min. After 3 h at 300 C, the reactor was cooled to room temperature and kept under vacuum overnight. It was then filled with 8 bar CO 2 followed by 8 bar H 2, and heated to 230 C. After 4 h at 230 C, the reactor was cooled to ambient temperature. The gaseous products of the reaction were quantified by gas chromatography (GC) using the following procedure. The Parr was vented to 1.2 bar and then diluted to 10 bar with N 2. Approximately 1.5 S2

bar was vented to clear the gas lines and then the Parr was connected to the sampling loop of the GC via a mass flow controller. The gas mixture was flowed (at 5 sccm) for 10 min to equilibrate the lines before recording. A GC run was initiated every 35 min. The concentrations of the gaseous products were quantified by comparing the ratio of the GC peak areas of the sample (average of 3 runs) with that of a standard. After collecting the GC data, the reactor was vented to ambient pressure and disassembled. The crude solid products were dissolved in D 2 O, filtered through a 0.2 µm PTFE syringe filter and analyzed by 1 H NMR and 13 C NMR. The amount of formate was quantified from 1 H NMR of the D 2 O solution using sodium L- (+)-tartrate dehydrate as an internal standard. The same experiment was performed in presence of added water. A glass liner containing Cs 2 CO 3 (3.26 g, 10.0 mmol) and 1.1 ml of deionized H 2 O was sealed in the Parr reactor. The reactor was flushed with CO 2 several times before being filled with 8 bar CO 2 and 8 bar H 2. It was heated to 230 C, maintained at that temperature for a 4 h, and then cooled to ambient temperature. The gaseous products of the reaction were quantified by GC and the solid products were quantified by 1 H NMR and 13 C NMR as described above. M 2 CO 3 promoted CO 2 hydrogenation 300 C Two mmol of M 2 CO 3 (M=Cs/Rb/K) was weighed out into a glass liner for the Parr reactor, dissolved in minimum amount of deionized water, and evaporated to dryness by heating on a hot plate at 150 C for at least 2 h. The liner was then sealed inside the Parr reactor. The reactor was evacuated and backfilled with CO 2 three times. It was then filled with CO 2 followed by H 2 at ambient temperature at appropriate pressures to attain the target pressures at the reaction temperature (e.g. 16 bar CO 2 and 16 bar H 2 at ambient temperature to reach 30 bar CO 2 and 30 bar H 2 at 320 C). The reactor was heated to the desired temperature, maintained at that temperature for a given period of time, and then cooled to ambient temperature. The approximate time taken for temperature ramp from ambient temperature to 320 C is 1h 15 min, of which the ramp from 300 to 320 C is ~20 min. The gaseous products of the reaction were quantified by GC using the procedure described above. The reactor was then vented and disassembled. Because water formed by RWGS condenses in the liner, the liner contained an aqueous solution upon disassembling the reactor. This aqueous solution was evaporated to dryness to afford the crude solid products, which were dissolved in D 2 O, filtered through a 0.2 µm PTFE syringe filter and analyzed by 1 H NMR and 13 C NMR. The amount of formate, acetate, propionate and succinate formed were quantified from 1 H NMR of the D 2 O solution using sodium L-(+)-tartrate dehydrate as an internal standard. Quantitative 13 C NMR was performed to quantify the amount of oxalate using cesium 2-ethylbutyrate as the internal standard. Representative spectra and data are shown in Figure S1 and Table S1. CO 2 hydrogenation promoted by M 2 CO 3 seeded with (M + ) 2 oxalate In a Parr liner, an appropriate amount of M 2 CO 3 and oxalic acid were dissolved in minimum amount of deionized water. The solution was evaporated to dryness on a hot plate at 150 C for 2 4 h such that the final mixture consisted of 2 mmol of M 2 CO 3 and the desired amount of (M + ) 2 oxalate. The liner was sealed in the Parr reactor and the reactor was evacuated and backfilled with CO 2 three times. It was then filled with 16 bar CO 2 followed by 16 bar H 2, at ambient temperature. It was heated to 320 C, maintained at that temperature for 8 h, and then cooled to ambient temperature. The gaseous products were analyzed by GC and the solid products were analyzed by 1 H NMR and 13 C NMR as described above. Representative spectra and data are shown in Figure S2 and Table S1. CO 2 hydrogenation promoted by M 2 CO 3 seeded with M + acetate-2-13 C In a Parr liner, an appropriate amount of M 2 CO 3 and acetic acid-2-13 C were dissolved in a minimum amount of deionized water. The solution was evaporated to dryness on a hot plate at 150 C for 2 4 h such that the final mixture consisted of 2 mmol of M 2 CO 3 and the desired amount of M + acetate-2-13 C. The liner was sealed in the Parr reactor and the reactor was evacuated and backfilled with CO 2 three times. It was filled S3

with 16 bar CO 2 followed by 16 bar H 2, at ambient temperature. It was heated to 320 C, maintained at that temperature for 8 h, and then cooled to ambient temperature. The gaseous products were analyzed by GC and the solid products were analyzed by 1 H NMR and 13 C NMR as described above. Representative spectra and data are shown in Figure 3c, Figure S3 and Table S1. H/D isotope exchange between acetate-d 3 and H 2 In a 20 ml septum capped glass vial, acetic acid-d 4 (28.7 µl, 0.5 mmol) and Cs 2 CO 3 (163 mg, 0.5 mmol) were dissolved in 0.5 ml of D 2 O. The septum-capped vial was placed in an aluminum block and the D 2 O was removed in vacuo using a Schlenk line at 150 C for 2 h to form a solid that consisted of cesium acetated 3 and 0.5 equivalents of Cs 2 CO 3. The vial was placed inside the Parr reactor, which was then dried by heating at 150 C under a stream of N 2 while keeping its gas release valve open. After 12 h, the reactor was heated to 230 C under 2.5 bar of H 2 for 1 h. After cooling to room temperature, the crude product was dissolved in D 2 O, filtered through a 0.2 µm PTFE syringe filter and analyzed by NMR. >99% exchange was observed by 1 H NMR. 1 H spectra of the reactant mixture and the product mixture are shown in Figure 2b. To test if the proton source for the above experiment is from adventitious moisture, a control experiment was performed where a mixture of 0.5 mmol cesium acetate-d 3 and 0.5 equivalents of Cs 2 CO 3, prepared in the same way as described above, was heated at 230 C under 2.5 bar of N 2 for 1 h. 1 H NMR analysis of the solid products indicated 20% exchange, which indicates that exchange with adventitious moisture contributes a minor portion of the protonation of acetate-d 3. A second control experiment was performed to test whether Cs 2 CO 3 is necessary for isotopic scrambling. In a 20 ml septum capped glass vial, acetic acid-d 4 (28.7 µl, 0.5 mmol) and Cs 2 CO 3 (75 mg, 0.23 mmol) were dissolved in 0.5 ml of D 2 O and evaporated as described above to form cesium acetate-d 3. The mixture was heated in the Parr reactor under N 2 stream overnight at 150 C followed by at 230 C under 2.5 bar of H 2 for 1 h. After cooling to room temperature, the crude product was dissolved in D 2 O, filtered through a 0.2 µm PTFE syringe filter and analyzed by NMR. 80% exchange was observed, which indicates that acetate is a competent base to mediate its own H/D exchange with H 2. H/D isotope exchange between formate-d and H 2 In a 20 ml vial, formic acid-d 2 (18.9 µl, 0.5 mmol) and Cs 2 CO 3 (163, 0.5 mmol) were dissolved in minimum amount of D 2 O. The septum-capped vial was kept under vacuum for 1 h at 200 C before transferring to the Parr reactor. The reactor was sealed and maintained at 200 C under a stream of N 2 overnight to further dry the reactants and the reactor. This mixture comprising of 0.5 mmol cesium formate-d and 0.5 equiv. Cs 2 CO 3, was heated to 250 C under 2.5 bar of H 2 for 1 h. After cooling to room temperature, the crude product was dissolved in D 2 O, filtered through a 0.2 µm PTFE syringe filter and analyzed by NMR. 22 % H/D exchange was observed by 1 H NMR. (Figure 2a). To confirm that protonation of cesium formate-d observed in the above experiment arises from H 2 instead of adventitious moisture, a dry mixture of 0.5 mmol cesium formate-d and 0.5 equiv. Cs 2 CO 3 was prepared in the same way as described above. The mixture was heated first under N 2 at 200 C for 12 h and then at 250 C under H 2 for 1 h. Less than 1 % H/D exchange was observed in this case. Another control experiment was performed to test whether Cs 2 CO 3 is necessary for isotopic scrambling between formate-d and H 2. Formic acid-d 2 (18.9 µl, 0.5 mmol) and Cs 2 CO 3 (75 mg, 0.23 mmol) were dissolved in minimum amount of D 2 O and evaporated to form cesium formate-d. After drying the vial in the reactor at 200 C overnight, it was heated to 250 C under 2.5 bar of N 2 for 1 h. After cooling to room S4

temperature, the crude product was dissolved in D 2 O, filtered through a 0.2 µm PTFE syringe filter and analyzed by NMR. 5.8 % H/D exchange was observed by 1 H NMR. H/D isotope exchange between acetate-d 3 and propionate In a 40 ml septum capped glass vial, acetic acid-d 4 (28.7 µl, 0.5 mmol), propionic acid (37.4 µl, 0.5 mmol) and Cs 2 CO 3 (325.8 mg, 1 mmol) were dissolved in 0.5 ml of D 2 O. The septum-capped vial was placed in an aluminum block and the D 2 O was removed in vacuo using a Schlenk line at 150 C for 1 h to form a solid that consists of cesium acetate-d 3, cesium propionate and 0.5 equivalents of Cs 2 CO 3. Next, the vial was heated to 320 C for 0.5 h under N 2 atmosphere. After cooling to room temperature, the crude product was dissolved in D 2 O, filtered through a 0.2 µm PTFE syringe filter and analyzed by 1 H NMR. Complete H/D exchange was observed between acetate-d 3 and the 2 position of propionate, but no deuteration was observed in the 3 position of propionate. 1 H spectra of the reactant mixture and the product mixture are shown in Figure 2c. Annealing Rb + formate- 13 C under CO 2 and H 2 O vapor A 20 ml vial was charged with Rb 2 CO 3 (289 mg, 1.25 mmol) and formic acid- 13 C (18.9 µl, 0.5 mmol). The solid was dissolved in minimum amount of deionized water and evaporated to dryness by heating on a hot plate at 150 C for at least 2 h. The vial was then sealed inside the Parr reactor with 0.8 ml water pipetted on the base of the reactor. The Parr was evacuated and backfilled with CO 2 three times and then filled to 16 bar. It was then heated to 295 C for 1 h, then cooled to ambient temperature, vented and disassembled. The crude reaction mixture, in the vial, was evaporated to dryness, dissolved in D 2 O, filtered through a 0.2 µm PTFE syringe filter and analyzed by 1 H NMR and 13 C NMR. The spectra are shown in Figure 3a and S4a. The amounts of 13 C at the 1 and 2 positions of acetate were quantified by integration of the 13 C satellite peaks in the 1 H NMR spectrum. To confirm that the 13 C-enrichment in acetate originated from formate- 13 C, a control experiment was performed with Rb 2 CO 3 (289 mg, 1.25 mmol) and unlabeled formic acid (18.9 µl, 0.5 mmol) under the same conditions. In this case, 105 µmol formate and 3 µmol acetate were observed by 1 H NMR with no isotopic enrichment (Figure S4b). CO 2 hydrogenation promoted by Rb 2 CO 3 seeded with Rb + oxalate- 13 C 2. Rb 2 CO 3 (578 mg, 1.25 mmol) and oxalic acid- 13 C 2 (46 mg, 0.5 mmol) were weighed out into a glass liner for the Parr reactor, dissolved in minimum amount of deionized water, and evaporated to dryness by heating on a hot plate at 150 C for at least 2 h. The liner was then sealed inside the Parr reactor. The reactor was evacuated and backfilled with CO 2 three times, then filled with 16 bar CO 2 followed by 16 bar H 2 at ambient temperature. The reactor was heated to 320 C, maintained at this temperature for 15 min, then cooled to ambient temperature, vented and disassembled. The crude reaction mixture in the liner was evaporated to dryness, dissolved in D 2 O, filtered through a 0.2 µm PTFE syringe filter and analyzed by 1 H NMR and 13 C NMR (Figure 3b and S5). Annealing Na + acetate- 13 C 2 under CO 2 hydrogenation conditions. Cs 2 CO 3 (326 mg, 1.0 mmol) and sodium acetate- 13 C 2 (19.3 mg, 0.23 mmol) were weighed out into a 20 ml glass vial, dissolved in minimum amount of deionized water and evaporated to dryness by heating on a hot plate at 150 C for at least 2 h. The vial was then sealed inside the Parr reactor. The reactor was evacuated and backfilled with CO 2 three times. It was filled with 16 bar CO 2, followed by 16 bar H 2 at ambient temperature and heated to 320 C. Right when the temperature reached 320 C, the reactor was cooled to ambient temperature, vented and disassembled. The crude reaction mixture in the liner was evaporated to dryness, dissolved in D 2 O, filtered through a 0.2 µm PTFE syringe filter and analyzed by 1 H NMR (Figure S6). 166 µmol of acetate-2-13 C and 59 µmol of malonate-2-13 C was observed along with trace amount of S5

formate and acetate. No 13 C enrichment was noticed at the 1 position of acetate or malonate, indicating its rapid exchange with unlabelled CO 2. Selective formation of oxalate A 20 ml vial charged with Cs 2 CO 3 (326 mg, 1.0 mmol) was sealed inside the Parr reactor. The reactor was evacuated and backfilled with CO 2 three times, then filled with 16 bar CO 2 followed by 16 bar H 2 at ambient temperature. The reactor was heated to 320 C and maintained at that temperature for 2 h. The reactor was then cooled to 200 C and the gas mixture was vented while starting to flow in N 2. The reactor was kept at 200 C for 1.5 h under N 2 stream to remove water and regenerate CO 3 2 from HCO 3 formed in the first step. The reactor was then cooled to room temperature, filled with 30 bar CO 2, and reheated to 320 C. After 2 h, it was then cooled down, vented, and disassembled. The crude product was analyzed by HPLC. (Figure S9) Annealing Cs 2 CO 3 with CO and CO 2 An oven dried 20 ml vial charged with dry Cs 2 CO 3 (325.8 mg, 1.00 mmol) was sealed in the dry Parr reactor. The reactor was evacuated and backfilled with CO three times. It was then filled with 2.1 bar CO followed by 16.3 bar CO 2 at ambient temperature. The reactor was heated to 340 C, maintained at that temperature for 2 h, then cooled to ambient temperature, vented and disassembled. The crude product was dissolved in D 2 O, filtered through a 0.2 µm PTFE syringe filter and analyzed by 1 H NMR and HPLC. No formate or oxalate was observed. Background CO 2 hydrogenation An oven-dried glass liner was sealed in the Parr reactor. The reactor was evacuated and backfilled with CO 2 three times, then filled with 16 bar CO 2 followed by 16 bar H 2 at ambient temperature. The reactor was heated to 320 C, maintained at that temperature for 8 h, and then cooled to ambient temperature. GC analysis of the headspace indicated the formation of 52 mmol CO, 8 mmol methane, 20 µmol ethylene and 350 µmol ethane. The reactor was then vented and disassembled. The liquid condensed on the cooling loop was washed down in the liner with 0.5 ml D 2 O. Sodium L-(+)-tartrate dehydrate was added as the internal standard and the solution was analyzed by 1 H NMR after filtering through a 0.2 µm PTFE syringe filter. The spectrum was collected by suppressing the proton peak of water, which is the main product (from RWGS), using a presaturation method. 1 µmol formic acid, 4 µmol acetic acid, 12 µmol methanol and 5 µmol ethanol was observed. A representative spectrum is shown in Figure S7. Demonstration of volatility of acetic acid-2-13 C from a mixture of M + acetate-2-13 C and M 2 CO 3, under CO 2 hydrogenation reaction condition In a 20 ml vial, 1.25 mmol of M 2 CO 3 (Cs + or Rb + ) and 0.5 mmol acetic acid-2-13 C were dissolved in a minimum amount of deionized water. The solution was evaporated to dryness on a hot plate at 150 C for at least 2 h such that the final mixture consisted of 1 mmol of M 2 CO 3 and 0.5 mmol of M + acetate-2-13 C. Separately, 2 mmol of M 2 CO 3, meant to serve as the trap for volatile acetic acid-2-13 C, was deposited on a Parr liner by evaporating an aqueous solution to dryness. Next, the upright vial was placed inside the liner and it was sealed in the Parr reactor. The reactor was evacuated and backfilled with CO 2 three times, then filled with 16 bar CO 2 followed by 16 bar H 2 at ambient temperature. The reactor was heated to 320 C, maintained at that temperature for 1-2 h, then cooled to ambient temperature, vented and disassembled. The crude products in the vial and the liner were seperately dried, dissolved in D 2 O, filtered through a 0.2 µm PTFE syringe filter and analyzed by 1 H NMR to determine the final distribution of M + acetate-2-13 C. Representative data are shown in Table S2. S6

Annealing Cs 2 CO 3 under H 2 Two mmol of Cs 2 CO 3 (651 mg, 2.0 mmol ) was weighed out into a glass liner for the Parr reactor, dissolved in minimum amount of deionized water, and evaporated to dryness by heating on a hot plate at 150 C for at least 2 h. The liner was then sealed, at room temperature, inside the Parr reactor which had been previously dried by heating at 320 C for 3 h under a stream of N 2 while keeping its gas release valve open. The reactor was evacuated and backfilled with H 2 three times. It was filled with 16 bar H 2 and heated to 320 C, maintained at that temperature for 2 h, and then cooled to ambient temperature. No gaseous products were observed on GC analysis of the reactor headspace, using the procedure described above. The reactor was then vented and disassembled. The crude solid was dissolved in D 2 O and filtered through a 0.2 µm PTFE syringe filter. No products were observed by 1 H NMR and 13 C NMR. Annealing Cs 2 CO 3 under CO 2 Two mmol of Cs 2 CO 3 (652 mg, 2.0 mmol ) was weighed out into a glass liner for the Parr reactor, dissolved in minimum amount of deionized water, and evaporated to dryness by heating on a hot plate at 150 C for at least 2 h. The liner was then sealed, at room temperature, inside the Parr reactor which had been previously dried by heating at 320 C for 3 h under a stream of N 2 while keeping its gas release valve open. The reactor was evacuated and backfilled with CO 2 three times. It was filled with 16 bar CO 2 and heated to 320 C, maintained at that temperature for 2 h, and then cooled to ambient temperature. No gaseous products were observed on GC analysis of the reactor headspace, using the procedure described above. The reactor was then vented and disassembled. The crude solid was dissolved in D 2 O and filtered through a 0.2 µm PTFE syringe filter. No products were observed by 1 H NMR and 13 C NMR. Annealing Cs 2 CO 3 in presence of CO and H 2 Two mmol of Cs 2 CO 3 (651 mg, 2.0 mmol ) was weighed out into a glass liner for the Parr reactor, dissolved in minimum amount of deionized water, and evaporated to dryness by heating on a hot plate at 150 C for at least 2 h. The liner was then sealed, at room temperature, inside the Parr reactor which had been previously dried by heating at 320 C for 3 h under a stream of N 2 while keeping its gas release valve open. The reactor was evacuated and filled with 8 bar CO and 16 bar H 2, at ambient temperature. It was heated to 320 C, maintained at that temperature for 2 h, and then cooled to ambient temperature. GC analysis of the headspace was done using the procedure described above. The gaseous products were composed of 23 mmol methane, 37 µmol ethylene, 2.5 mmol ethane and approximately 45 mmol CO 2. (CO 2 is formed from WGS as the water formed from methanation reacts with CO). The solid products, analyzed by 1 H NMR and 13 C NMR as described above, comprised of 2.6 mmol formate, 273 µmol oxalate, 9 µmol acetate, 9 µmol of propionate and 10 µmol of succinate. S7

a) b) Figure S1: a) 1 H NMR (500 MHz) and b) 13 C NMR (125 MHz) in D 2 O of the crude product mixture after reaction of 2 mmol of Cs 2 CO 3 under 60 bar of 1:1 CO 2 :H 2 at 320 C for 2 h. S8

a) b) Figure S2: a) 1 H NMR (600 MHz) and b) 13 C NMR (125 MHz) in D 2 O of the crude product mixture after reaction of 2 mmol of Cs 2 CO 3 seeded with 75 µmol of (Cs + ) 2 oxalate under 60 bar of 1:1 CO 2 :H 2 at 320 C for 8 h. S9

a) b) Figure S3: a) 1 H NMR (600 MHz) and b) 13 C NMR (125 MHz) in D 2 O of the crude product mixture after reaction of 2 mmol of Cs 2 CO 3 seeded with 0.5 mmol of Cs + acetate-2-13 C under 60 bar of 1:1 CO 2 :H 2 at 320 C for 8 h. S10

a) b) Figure S4: a) 1 H NMR (500 MHz) in D 2 O of the crude product mixture after reaction of 1 mmol of Rb 2 CO 3 and 0.5 mmol of Rb + formate- 13 C and b) 1H NMR (600 MHz) in D 2 O of the crude product mixture after reaction of 1 mmol of Rb 2 CO 3 and 0.5 mmol of Rb + formate under 30 bar CO 2 and 3 bar H 2 O at 295 C for 1 h. S11

a) b) Figure S5: a) 1 H NMR (500 MHz) and b) 13 C NMR (125 MHz) in D 2 O of the crude product mixture after reaction of 1.5 mmol of Rb 2 CO 3 and 0.5 mmol of (Rb + ) 2 oxalate- 13 C 2 under 60 bar of 1:1 CO 2 :H 2 at 320 C for 15 min. Integrations for overlapping peaks have been calculated by peak fitting. S12

a) b) Figure S6: a) 1 H NMR (500 MHz) in D 2 O of a mixture of 0.23 mmol of Na + acetate- 13 C 2 and 1 mmol of Cs 2 CO 3 after heating to 320 C under 60 bar of 1:1 CO 2 :H 2 and then cooling to ambient temperature. b) 1 H NMR spectrum of the Na + acetate- 13 C 2 starting material. S13

Figure S7: 1 H NMR (500 MHz) of the liquid sample collected after background reaction performed under 60 bar of 1:1 CO 2 :H 2 at 320 C for 8 h. S14

Figure S8: GC-FID chromatogram of the product gas mixture after CO 2 hydrogenation reaction promoted by 2 mmol Cs 2 CO 3 seeded with 50 µmol Cs + acetate-2-13 C under 60 bar of 1:1 CO 2 :H 2 at 320 C for 8 h. S15

Figure S9: A two-step procedure for selective formation of oxalate by carbonate-promoted CO 2 hydrogenation followed by C H carboxylation of formate. S16

Table S1: Carbonate promoted CO 2 hydrogenation data All reactions have been performed under total pressure of 60 bar. * Yields within reactor background limit. S17

Table S2: Demonstration of volatility of acetic acid-2-13 C from a mixture of M + acetate-2-13 C and M 2 CO 3 performed under 60 bar of 1:1 CO 2 :H 2 at 320 C S18