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Integrated CO2 Capture and Hydrogenation to Produce Formate in Aqueous Amine Solutions Using Pd-Based Catalyst

https://doi.org/10.3390/catal12080925

“Integrated CO2 capture and hydrogenation to produce formate offers a sustainable approach for reducing carbon dioxide emissions and producing liquid hydrogen carriers (formate) simultaneously. In the current study, three different types of aqueous amine solutions including monoethanolamine (MEA), diethanolamine (DEA) and triethanolamine (TEA) were investigated as CO2-capturing and hydrogenation agents in the presence of a Pd/NAC catalyst. The effect of amine structures on the CO2 absorption products and formate yield was investigated thoroughly. It was found that the formate product was successfully produced in the presence of all three aqueous amine solutions, with tertiary amine TEA accounting for the highest formate yield under the same CO2 loadings. This is due to the fact that primary and secondary amine moieties in MEA and DEA are responsible for the formation of CO2 adducts of carbamate and bicarbonate, whereas the tertiary amine moiety in TEA is responsible for the formation of hydrogenation-favorable bicarbonate as the solo CO2 absorption product. A high yield of formate of 82.6% was achieved when hydrogenating 3 M TEA with 0.3 mol CO2/mol amine solution in the presence of a Pd/NAC catalyst. In addition, the physio-chemical properties of the Pd/NAC catalyst analyzed using TEM, XRD and XPS characterization were applied to rationalize the superior catalytic performance of the catalyst. The reaction mechanism of integrated CO2 capture and hydrogenation to produce formate in aqueous amine solutions over Pd/NAC catalyst was proposed as well.”

Integrated CO2 capture and hydrogenation to produce formate process offers a sustainable approach to reduce CO2 emissions and produce liquid hydrogen carriers at the same time. When heating up the obtained ammonium formate solution, formic acid product can be generated from thermal cleavage, which is accompanied by the regeneration of aqueous amine solvents. The concept of integrated CO2 capture and hydrogenation to produce formate was firstly reported by He et al. in 2013 [9]. They reported the successful application of CO2 absorption into polyethyleneimine 600 (PEI600) sorbents in methanol and in situ hydrogenation to produce alkylammonium formate with the assistance of homogeneous catalyst RhCl3.3H2O/CyPPh2. The authors demonstrated a highest turnover number of 726, corresponding to a 55% formate yield under the reaction conditions of 40 bar H2, 60 °C and a reaction time of 16 h. Later, the integrated CO2 capture and hydrogenation process was extended to aqueous amine solutions as CO2 capture in such solvents is most widely explored and suitable for scale-up. In 2014, Hicks et al. reported the application of a polyethyleneimine-tethered iminophosphine iridium catalyst for integrated CO2 capture and hydrogenation to produce formate in triethylamine solutions, with a TON of 248 achieved under reaction conditions of 20 bar H2 and 120 °C [10]. An outstanding high TON value of 7375 was reported by Olah and Prakash et al. upon integrated CO2 capture and hydrogenation in TMG solutions with Ru-Macgo-BH as a homogeneous catalyst [11]. In the same paper, an iron-based homogeneous catalyst was also explored as a promising candidate for CO2 capture and hydrogenation in pentaethylenehexamine (PEHA) solutions with a TON of 255 obtained. Although the application of homogeneous catalysts in the CO2 capture and hydrogenation process to produce formate demonstrated superior catalytic performance with high formate yields and TON values, the common issues related to homogeneous catalysts including the recycling of catalysts and the separation of products still hindered the scale-up of such a process. On the other hand, integrated CO2 capture and hydrogenation to produce formate in aqueous amine solution using Pd-based catalysts is rarely reported in the open literature. One such example is reported by Lin’s group, who stated that they successfully produced formate with a product yield of 50.2% using piperidine as a CO2-capturing solvent and Pd/AC as a catalyst [12]. However, the effect of amine structure on the product yield was not reported in the open literature.
In the current study, three different aqueous amine solutions with different amine types including primary amine monoethanolamine (MEA), secondary amine diethanolamine (DEA) and triethanolamine (TEA) were employed as CO2-capturing and hydrogenation solvents in the presence of Pd/NAC catalysts. The CO2 absorption product distribution together with formate yield at different CO2 loadings were measured and compared between the three different aqueous amine solutions. XRD, XPS and TEM were employed to evaluate the physio-chemical properties of the Pd/NAC catalysts in order to rationalize the corresponding catalytic performance. At last, the reaction mechanism of integrated CO2 capture and hydrogenation in the presence of the Pd/NAC catalyst was proposed.
Table 1. Results of integrated CO2 capture and hydrogenation to produce formate in aqueous amine solutions of MEA, DEA and TEA in the presence of Pd/NAC catalyst.
Entry Capturing and
Hydrogenation Solvent
CO2 Loading Captured CO2 Concentration (M) Hydrogenation Results of CO2 Species
Concentration (M)
Conversion Results
HCO3 R1R2COO HCO3 R1R2COO HCOO Formate Yield (%)
1 MEA 0.15 0 0.45 0 0.31 0.14 30.8
2 MEA 0.31 0.14 0.79 0 0.55 0.38 40.8
3 MEA 0.46 0.25 1.13 0 0.63 0.75 54.1
4 MEA 0.72 1.19 0.97 0 0.89 1.27 58.8
5 DEA 0.16 0 0.48 0 0.29 0.19 35.1
6 DEA 0.31 0.17 0.76 0 0.5 0.43 50.3
7 DEA 0.48 0.45 0.98 0 0.71 0.72 46.3
8 DEA 0.78 0.70 1.64 0 1.16 1.18 60.6
9 TEA 0.15 0.45 0 0.14 0 0.31 68.5
10 TEA 0.30 0.9 0 0.16 0 0.74 82.6
11 TEA 0.46 1.38 0 0.51 0 0.86 62.5
12 TEA 0.60 1.8 0 0.84 0 0.96 53.2
Reaction conditions: 80 °C, 5.0 mL 3M amine solutions, 6 MPa H2, 100 mg catalyst and reacting time of 8 h.
To investigate the effect of amine structure on the product yield, three aqueous amine solutions including MEA, DEA and TEA were employed as capturing agents for the integrated CO2 capture and hydrogenation process. The results of integrated CO2 capture and hydrogenation in aqueous amine solutions of MEA, DEA and TEA in the presence of a Pd/NAC catalyst are shown in Table 1. In addition, we performed the integrated CO2 capture and hydrogenation process with 3M TEA solutions, 0.30 mol CO2/mol amine, with NAC as the catalyst, and the NMR results showed no characterization peak for formate was formed after the reaction.
As expected, the CO2-capturing product in MEA and DEA solutions contained both bicarbonate (HCO3) and carbamate (R1R2COO) species, as CO2 can directly react with primary/secondary amines. Meanwhile, HCO3 was the solo CO2 absorption product for TEA, since CO2 cannot react directly with tertiary amine groups. It is interesting to find that the concentrations of HCO3 species appeared to be 0 after the integrated CO2 capture and hydrogenation process in both the MEA and DEA solutions, meaning that all HCO3 species were converted into formate products. However, there were still some carbamate species left in the liquid phase after the hydrogenation process. This interesting phenomenon proves that HCO3 is a more favorable reactant for the hydrogenation process compared with the carbamate species. This agrees well with results published in the literature that show HCO3 can act directly as a reactant for the hydrogenation process; however, the direct conversion of carbamate is rather difficult due to its electron-rich nature [19]. The conversion of carbamate species can be achieved via two possible pathways including 1) the conversion of carbamate into bicarbonate then participating in the hydrogenation process, and 2) the conversion of carbamate species into gaseous CO2 under the reaction temperature of 80 °C then participating in the hydrogenation process. However, the low solubility of CO2 under reaction conditions makes the second pathway less possible. Therefore, the hydrogenation of carbamate species is assumed to be through the HCO3 pathway.

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