Conversion of CO2 captured by ammonia into value-added chemicals

Abstract

In chapter 1 the analysis of indicators of the state of progress and relevance of CO2 utilization technologies revealed that the sector of fuels and chemical products (formic acid and hydrogen among them), has the greatest relevance compared to other non-traditional emerging technologies (mineral carbonation, chemical and material processing, power sector and others). In chapter 2, the batch study of the catalytic reduction process of CO2 in hydrothermal media showed that, as starting material, ammonium carbamate had the best yield towards formic acid at a mild reaction temperature of 120 ºC, while sodium bicarbonate presented the highest yield, but at a higher temperature of 250 ºC. The formic acid yield, from ammonium carbamate, is limited by the thermal decomposition of the reducible species HCO3- and of the formate ion, for which the yield started to decrease at temperatures higher than 120 ºC. The formic acid yield was directly proportional to the catalyst (palladium over activated carbon) concentration in the reaction solution, obtaining a yield of almost 40% for the highest concentration of 60%, suggesting that the process is highly sensitive to the nature and quantity of the catalyst. Through the characterization of the spent solid after successive reuse reactions, it was identified that the aluminum is not completely consumed, even up to the fifth reuse. In addition, it was possible to elucidate the evolution of the catalyst in the different reuse cycles, with the H2-TPR analysis. This allowed to identify the formation of palladium hydride species, which can play a reductive role of the captured CO2 into formic acid. In chapter 3, the batch study of the hydrothermal reduction of captured CO2 in ammonia was extended to the evaluation of different reducing metals and catalysts, as well as their incidence in the simultaneous production of green hydrogen. Among a pool of selected catalysts, differentiated by the type of support and active metal, palladium 5% wt over activated carbon was the only one that showed activity towards the production of formic acid, for all the reducing metals used. Among these metals, zinc showed the highest autogenous pressure, corresponding to a higher yield of green hydrogen up to 26%. This chapter also analyzed the effect of the particle size of the reducing metal and its purity, using 500 μm aluminum and aluminum residue from the company BEFESA, in the form of spall (or chips). Compared to the powder size (<5 μm), the 500 μm size, both for pure aluminum and for the residue, decreased the formic yield, but this allowed demonstrating that the residue shows activity towards the in situ generation of green hydrogen. In this chapter, for the study of green hydrogen generation, two media/streams were considered, one rich in captured CO2 (CO2RS), and another free of CO2 or fresh solvent (CO2LS), in experiments with no catalyst and for 10 hours. In general, zinc presented faster H2 generation rates under both types of streams at all times, with a yield of up to 63.7% with aqueous carbamate. Aluminum activation was generally slower, it only started to be significant at 6 h with SB, for an H2 yield of 11%. The XRD analysis of the solids showed, in addition to the expected oxides of the metals used, the appearance of metal carbonates, suggesting that part of the carbamate conversion is due to this product. In chapter 4, the synthesis of metal-crosslinked aerogels (MCAs) and metal-decorated carbogels (MDCs) was performed within the framework of the doctoral stay. In the first synthesis step, hydrogel preparation, the concentration of the metal precursor solution in the gelation bath was varied, but the final metal content stabilized at a concentration of 170 mmol/L. Pd+2 showed the highest incorporation in aerogel, with a final metal content of up to 13%, which is higher than those found in the reviewed literature, and after pyrolysis the Pd-carbogel was notably the one that lost less mass, with only 58%, up to 600 ºC. At different pyrolysis temperatures (150 – 600 ºC) there was a decreasing trend in textural features, with a large change after approximately 285ºC, associated with an expected shrinkage. SEM-BSE of carbogels showed an evolution of nanoparticle clusters through the different pyrolysis temperatures, developing well-defined forms for Ni- and Cu-carbogels at 600 ºC, with average size of 14 nm +/- 7 nm and 85 nm +/- 29nm, respectively. XRD and TPR results of Ni-, Cu-, Pt- and Pd-carbogels indicated that the particles are composed of elemental metals and metal oxides in varying proportions, while Pd-aerogel was the only aerogel to show the exclusive presence of the zero-valence state by XPS and TPO analyses. When Pd-aerogel was tested as a catalyst in the hydrothermal reduction reaction of captured CO2, the yield of formic acid was 34.3%, in contrast to the blank reaction (without catalyst) that did not show H2 consumption and/or formic acid signal in the HPLC chromatogram. On the other hand, the reaction with Pd powder as catalyst (control reaction) showed a negligible formic acid yield of 2.97%. For the first time, chapter 5 proposes a novel facility operated in semi-continuous, of self-construction, for the use of the basic solvent streams that mediate in typical CO2 capture plants (CO2RS and CO2LS streams) for the generation of green hydrogen, using metals. One of the novelties is that the process is conducted with superheated water (also called subcritical water), which is defined as liquid water pressurized at temperatures ranging from the boiling point (100 ºC) and the supercritical temperature (374 ºC). The experimental study showed that the H2 production increased in the order Mn-Al-Fe-Zn, using aqueous ammonium carbamate as CO2RS stream. The yield was proportional to temperature and concentration, and indirectly to particle size. Once again, the aluminum residue in the form of spall (or chips) from the company BEFESA showed a high performance, reaching a yield of 12% for the size of 250 μm, using aqueous ammonium carbamate as CO2RS. The yield of H2 was proportional to the variables temperature and carbamate concentration, while the variable stream flow did not show a significant effect. It is noteworthy that aqueous sodium bicarbonate (a CO2RS stream) showed a higher yield compared to carbamate, reaching a relative yield of 71% of H2, using Al, in only 0.5 h of steady state operation time, at 200 ºC and a concentration of 1.0 mol/L. Likewise, 0.5 mol/L aqueous NaOH (CO2LS) showed the highest relative yield of H2 (up to 85.5%), using Al, in only 50 min. Likewise, as in chapter 3, the interaction of CO2RS with metals was detected by the formation of metal carbonates, and by ATR-FTIR the formation of aluminum hydrides was detected. The results of this chapter allowed concluding that aqueous sodium bicarbonate and sodium are strong bases that should be better used in activating less active metals like Al and Mn. For its part, highly active metals like Zn and Fe should be better used with less basic aqueous streams like ammonium carbamate, ammonia and MEA.