**1. Introduction**

Global warming is still one of the main worldwide concerns in the present time [1,2]. Increasing the efficiency of processes, implementing renewable sources of energy and fuel switching are some of the alternatives for the reduction of greenhouse emissions [3]. However, in the transition to a decarbonated economy, oil and gas still play a relevant role in energy generation for electricity and transportation; in this context, technological initiatives as carbon capture and utilization become of high interest to mitigate these emissions by using CO2 to synthesize high value-added products [4].

CO2 is attractive as a raw material for industry because it is cheap, has very low toxicity, is available in great quantity [5] and can be used as feedstock for different processes. Physical methods are widely used to revalorize CO2 as refrigerant, solvent, dry ice, etc. Chemical methods are also used to convert it into valuable compounds such as urea and DME [6,7] One of the main difficulties faced in the chemical conversion of CO2 is the high thermodynamic stability of the compound [8]. Electrochemical and photochemical reduction for CO2 hydrogenation have shown favorable results in overcoming this issue [9]. However, the high costs and low yields of these techniques [10] have led to the study of other alternatives such as the hydrothermal treatment in which CO2 reduction takes places in water media at high pressures and temperatures [11–13]. In this process, water acts as hydrogen donor instead of H2, which is flammable and complex to store [14].

In these processes, CO2 is captured in the aqueous media in basic conditions. In most works, it is captured as NaHCO3, but it can be also in the shape of carbamates that are formed when CO2 is captured by ammonia or amines. This process opens the possibility

**Citation:** Chinchilla, M.I.; Mato, F.A.; Martín, Á.; Bermejo, M.D. Hydrothermal CO2 Reduction by Glucose as Reducing Agent and Metals and Metal Oxides as Catalysts. *Molecules* **2022**, *27*, 1652. https://doi.org/10.3390/ molecules27051652

Academic Editors: Reza Haghbakhsh, Sona Raeissi and Rita Craveiro

Received: 31 January 2022 Accepted: 1 March 2022 Published: 2 March 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

to connect carbon capture in basic solutions directly with the CO2 conversion process, avoiding costly intermediate separation steps [12,15].

In the hydrothermal reduction of CO2, the most frequently obtained product is formic acid. This is a compound of great interest for the energy sector because it is an alternative source of hydrogen and can be used directly as an energy-dense carrier for fuel cells [16]; besides, it is biodegradable and less flammable than other fuels at room temperature [17].

In order to reduce CO2, several metals have been suggested as CO2 reductants: Zn [18], Fe [19], Mn [20], Mg [20] and Al [20] (efficiency: Al > Zn > Mn > Fe) [10,21] are mentioned as favorable for the process. Metallic catalysts metals such as Ni-ferrite [22], Ni nanoparticles [23], Ni [24], Raney Ni [25], Cu [20,21], Fe2O3 [26], Ru/C [27] and Pd/C [12,27] can be used as-is or coupled with "zero-valent" metals to improve the reaction; however, the reduction of the metal after the process in order to recycle the material should be considered.

Organic compounds containing alcohol groups, such as isopropanol [8], glycerol [28], glucose, C2 and C3 alcohols, saccharides and lignin derivatives [29], are often used as reductants as well. It is known that many of these molecules can be obtained from the hydrolysis of lignocellulosic biomass in hydrothermal media.

Subcritical water can act as a basic or acidic catalyst; it has a higher ion product and lower dielectric constant than room-temperature water. In these conditions, the cellulose, hemicellulose and lignin from biomass can be isolated and depolymerized into monomeric units (mainly sugars or phenols). Alongside the decarbonization approach, the usage of biomass is of great interest mainly because of the valorization of lignocellulosic residues that can be converted into several intermediate products such as lactic acid, acetic acid and vanillin [30,31]. Carrying out the hydrolysis of biomass simultaneously with the reduction of CO2 captured as a basic solution (i.e., bicarbonate, amine carbamates of ammonia) could be interesting because many of the hydrolysis products of biomass contain alcohol groups that can act as reductants of CO2 in hydrothermal media.

So far, there has been a number of studies in which the use of catalysts (Cu, Ni, Pd/C) in hydrothermal CO2 reduction was carried out using metals as reductants (Zn, Mg, Al, etc.) [12,18,20–22,25]. This work studies, for the first time, the influence of different catalysts in the hydrothermal reduction of CO2 by using an organic (glucose) as a reducing agent. As CO2 source, sodium bicarbonate (NaHCO3) and ammonium carbamate (NH4[H2NCO2]) were used. There are literature studies stating that ammonium carbamate is reduced by using metals or hydrogen [12], but this is the first time that the reduction is performed using organics containing an alcohol group. The main objective of the present work is to develop batch screening reactions to find the best catalyst that can improve the formic acid production, as well as lowering the temperature for the reduction reactions normally fixed at 300 ◦C in other works [29]. In addition, as formic acid can be also derived by sugar hydrolysis [32–35], experiments to study the origin of the carbon forming formic acid by using NaH13CO3 as carbon source were performed.
