*2.2. Results of Hydrothermal Reactions with Sodium Bicarbonate as Carbon Source*

In the reaction of SB with glucose, typical products derived from the hydrothermal reduction of glucose were observed [29,33–37]. Formic acid (FA), acetic acid (AA) and lactic acid (LA) were the main compounds produced in the reactions. In minor amounts, glyceraldehyde, glycolaldehyde, formaldehyde, ethylene glycol, acetone, pyruvaldehyde, galacturonic acid and 5-HMF were obtained. The yields of the three main products of the catalyzed reactions are shown in Tables 1–3. Each experiment was repeated at least twice, the average error being around 5%.

After carrying out the reduction of CO2 captured as SB, it was found that the highest yields of FA were obtained by using C and Fe3O4 as catalysts, reaching yields of 53% and 52%, respectively (Table 1). The conditions at which the maximum values were obtained were 200 ◦C and 30 min of reaction for C and 250 ◦C and 30 min of reaction for Fe3O4.

The highest yield for AA was obtained in the sample without catalyst: 45% at 250 ◦C and 30 min. This was followed by Ni and Cu catalysts, which achieved yields of 45% and 44%, at 250 ◦C and 30 min and 250 ◦C and 120 min (Table 2).

For LA, the maximum yield was achieved with Fe3O4, 43% at 250 ◦C and 30 min of reaction (Table 3).

It is remarkable that most of the catalysts promoted similar or less yield of FA in comparison to the sample without catalyst; in fact, only C and Fe3O4 improved the yield of FA over AA and LA over the sample with no catalyst.


**Table 1.** Yields of formic acid obtained after the hydrothermal reaction of NaHCO3 with glucose in the presence of catalysts. The higher yields obtained after the hydrothermal reactions are marked with an asterisk (\*).

**Table 2.** Yields of acetic acid obtained after the hydrothermal reaction of NaHCO3 with glucose in the presence of catalysts. The higher yields obtained after the hydrothermal reactions are marked with an asterisk (\*).


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**Table 3.** Yields of lactic acid obtained after the hydrothermal reaction of NaHCO3 with glucose in the presence of catalysts. The higher yields obtained after the hydrothermal reactions are marked with an asterisk (\*).

Results of Hydrothermal Reactions with Ammonium Carbamate as Carbon Source

As in the previous case, the main products of the reaction with AC were FA, AA and LA. The yields achieved for each compound are shown in Tables 4–6. The experiments were repeated at least twice, the average error being around 3%.

**Table 4.** Yields of formic acid obtained after the hydrothermal reaction of NH4[H2NCO2] with glucose in the presence of catalysts. The higher yields obtained after the hydrothermal reactions are marked with an asterisk (\*).


After the hydrothermal reduction of AC, it was found that the highest yield of FA was 26% and was obtained by using Fe3O4 at 200 ◦C and 120 min (Table 4).

The maximum value for AA was obtained with Ni, 15% at 250 ◦C and 180 min. This was followed by that obtained with Cu, 14% at 200 ◦C and 60 min (Table 5).


**Table 5.** Yields of acetic acid obtained after the hydrothermal reaction of NH4[H2NCO2] with glucose in the presence of catalysts. The higher yields obtained after the hydrothermal reactions are marked with an asterisk (\*).

**Table 6.** Yields of lactic acid obtained after the hydrothermal reaction of NH4[H2NCO2] with glucose in the presence of catalysts. The higher yields obtained after the hydrothermal reactions are marked with an asterisk (\*).


For LA, the highest value was 16% and was obtained by using Fe3O4 at 200 ◦C and 90 min. This was followed by that obtained with C, 13% at 200 ◦C and 30 min (Table 6).

Once again, Fe3O4 promoted the maximum yields of FA over AA and LA in comparison to the rest of the catalysts. Only Fe3O4 and C improved the yield of FA compared to the sample with no catalyst.

In general, the yields of FA, AA and LA obtained by the reduction of AC are much lower (less than 25%) than those observed with SB (less than 53%). Some other works have shown that sodium bicarbonates and carbonates required high-temperature reactions to achieve higher yields of FA. SB and AC are decomposed easily into HCO3−, which is the species that is going to be reduced in the reaction. In the case of AC, not only HCO3<sup>−</sup> is formed. There is another step in which AC is also decomposed because the H<sup>+</sup> protons of the ion NH4 <sup>+</sup> are being donated to other compounds, and then the yield to FA is reduced because there is a competition between two reactions: the reduction of AC and the thermal

decomposition of AC [13,16]. It was observed that the experiments held at 200 ◦C showed higher yields of FA than the reactions at 250 ◦C. The reduction of CO2 is favored by the reaction in alkaline media; when the temperature rises, NH4 <sup>+</sup> dissociates into NH3 and H+, which are species that reduce the alkalinity and might reduce the solubility of CO2 in water [10,38].

#### *2.3. Nuclear Magnetic Resonance Spectroscopy Results*

It is known that FA can be generated from sugars at lower temperatures in basic aqueous media [32–34] and can be also obtained by the reduction of SB at temperatures higher than 300 ◦C [29]. In order to understand the reactions, it is necessary to check whether the FA is coming from SB or from glucose and if the catalysts are favoring or disfavoring one or the other reaction. To do so, experiments with an isotope of sodium bicarbonate (NaH13CO3; SB-13C) were performed with the different catalysts. 13C-NMR analyses were carried out to identify the fraction of formic acid that possesses 13C, which comes from the reduction of the carbon source, and the fraction that comes from glucose. The experiments were conducted at 250 ◦C and 2 h.

The fraction of formic acid coming from the SB-13C when using each of the catalysts is presented in Figure 2.

**Figure 2.** Fractions of formic acid coming from the reduction NaH13CO3 and from the oxidation of glucose for each of the catalysts at 250 ◦C and 2 h. Gray bars represent the total yield of formic acid of each sample (obtained by HPLC); black dots represent the fraction of formic acid coming from NaH13CO3 (obtained by 13C-NMR). The average error in the measure of the fraction of formic acid was 5%.

It was observed that although Fe3O4 is the catalyst that provides the highest yield of total FA (49%, 250 ◦C and 2h, measured by HPLC), its proportion of reduced SB-13C is lower (0.32) in comparison to the fraction of FA obtained with Pd/C 5%, Ru/C 5% and Ni (0.81, 0.76 and 0.69, respectively).

The metal supported catalysts (Pd/C 5% and Ru/C 5%) presented the highest selectivity in reducing CO2 in comparison to the performance of the activated carbon support (C), which reached a fraction of 0.34.

There were catalysts that did not improve the reduction of SB-13C; in fact, the reaction without catalyst (fraction FA-13C: 0.37) showed a slightly higher capability to reduce CO2 than Cu, Fe3O4 and Fe2O3 (0.37, 0.32 and 0.34, respectively).

The order in which catalysts were able to reduce CO2 captured as SB-13C was as follows: Pd/C 5% > Ru/C 5% > Ni > Cu > C ≈ Fe2O3 > Fe3O4.

In all the experiments, the only products that came from the direct reduction of carbon source (SB-13C) were FA-13C at δ = 163 ppm and an unidentified compound at δ = 173 ppm (this peak was absent in Fe2O3 and in the sample with no catalyst). At δ = 127 ppm, another peak was observed; according to the literature [39,40], this compound could be 13CO2 dissolved in the sample.

An AA-13C standard was injected. AA-13C standard peak was observed at δ = 184 ppm, and then the possibility that the unidentified peak at δ = 173 ppm was AA-13C was excluded.

## Possible Mechanisms of Reaction

In the NMR spectra, it was confirmed that the reduction of the carbon source led mostly to the formation of FA, while byproducts and FA were obtained from the oxidation of glucose.

In literature were found some of the possible mechanisms of reaction of glucose at high water temperatures, subcritical and supercritical water [41–44]. Glucose can be transformed in two different ways: by following a retro-aldol condensation reaction to produce glycolaldehyde or through the isomerization of the glucose into fructose (favored by basic media) which can be dehydrated to form 5-HMF (favored by the acid media) or can produce glyceraldehyde by means of another retro-aldol condensation reaction. Finally, the glyceraldehyde can be isomerized into pyruvaldehyde, which could be a precursor of lactic acid.

Besides the retro-aldol reactions that can lead to the production of lactic acid and glycolaldehyde, 5-HMF can be transformed into formaldehyde and furfural in acid media [43], but in our case, reactions were performed in basic media, so this step may or may not be occurring.

Some other works [42] described that glucose can also dehydrate to form 1,6-anhydroglucose. This molecule can be a precursor of acids or can be transformed into D-fructose and follow a reverse aldol condensation reaction to form erythrose and glycolaldehyde that can produce acids as well. In Figure 3, the main mechanisms of oxidation of glucose are represented. According to Kabyemela et al. [42], some of the products derived from the oxidation of glucose that can be identified according to these mechanisms are fructose, erythrose, glyceraldehyde, glycolaldehyde, pyruvaldehyde, dihydroxyacetone, 1,6-anhydroglucose, 5-HMF, acetic acid and formic acid. Kabyemela et al. [42] also have identified some products of the decomposition of fructose such as pyruvaldehyde, erythrose, glyceraldehyde, dihydroxyacetone, acetic acid and formic acid. Erythrose and 1,6-anhydroglucose can also be the precursors of acetic and formic acids.

Glucose has five -OH (hydroxyl) groups. In a previous work, it has been proposed that alcohol groups act as reducing agents for CO2 [29]. According to Shen et al., the reduction of the carbon source is mainly due to the alcohol moiety [8,45]. According to other studies, compounds with primary alcohol groups presented slightly higher yields in comparison with compounds with secondary alcohol groups. Because of the steric effects, the position of the hydroxyl group in the compound could be of importance in the reduction of the carbon source [29]. Shen et al. proposed a mechanism of reduction of CO2 through alcohol molecules in which, through a cyclic transition state, a H<sup>−</sup> from the α-carbon of −OH moiety is transferred to the ion bicarbonate and the resulting species dehydrate quickly into formate [8,29,45]. Most of the products of glucose decomposition contain alcohol groups (fructose, glyceraldehyde, glycolaldehyde, lactic acid), and Andérez et al. [29] proved that formic acid is rendered in appreciable yields.

Regarding catalysts, it can be found in the literature that in reactions that use metals as reductants, HCO3 - is adsorbed on a Pd/C surface, promoting the formation of C1 intermediates species to produce FA and traces of CH4 and improving the generation of C-C bonds to form C2 compounds [46]. Cu and Ni have showed also good performance in reducing HCO3 - into C1 compounds when using metals such as Fe as reducing agents [47,48]. In other studies, experiments with a mix of Fe and Fe3O4 were performed to reduce CO2. In these works, Fe is reduced into Fe3O4, generating hydrogen. Fe3O4 is transformed into Fe3O4-x, and then hydrogen and C=O of HCO3 − are adsorbed in the surface of the metal oxide and react to produce formic acid [49].

As seen before, catalysts can influence the performance of the hydrothermal reactions for CO2 reduction and glucose oxidation.

#### **3. Materials and Methods**

#### *3.1. Chemicals*

Ammonium carbamate (AC) (99%), sodium bicarbonate (SB) (100%), sodium bicarbonate 13C (SB-13C) (100%) and acetic acid 13C (AA-13C) were used as sources of captured CO2. D-(+)-Glucose (100%) was used as reducing agent. Fine powder of commercial Cu, Ni, Pd/C (5 wt% of metal loading), activated carbon, Ru/C (5 wt% of metal loading, 50% water wet paste), Fe2O3 and Fe3O4 were used as catalysts. Sodium bicarbonate was purchased from COFARCAS (Spain), Ru/c 5% was provided by Strem Chemicals and the rest of the chemicals were acquired from Sigma-Aldrich. Deionized water was used to prepare the dilutions.
