Carbon dioxide to methanol: stoichiometric catalytic hydrogenation under high pressure conditions
- Autores: Rohit Gaikwad
- Directores de la Tesis: Atsushi Urakawa (dir. tes.)
- Lectura: En la Universitat Rovira i Virgili ( España ) en 2018
- Idioma: español
- Tribunal Calificador de la Tesis: Dorota Koziej (presid.), Jonathan Albo Sánchez (secret.), Jan-Dierk Grunwaldt (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencia y Tecnología Química por la Universidad Rovira i Virgili
- Materias:
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- Resumen
CO2 utilization for the synthesis of chemicals or fuels is expected to significantly contribute to reduce anthropogenic CO2 emission and thus limit its substantial impact on global warming. Methanol, among other chemicals, is one of the most promising future chemical energy carriers as well as C1 feedstock, thus drawing global attention as the target molecule produced from CO2. High-pressure advantages under over-stoichiometric CO2:H2 ratio (1:>3) have been reported previously by drastically increasing the reaction kinetics and even reaching the thermodynamic conversion. However, the major drawback of such processes is the treatment of unreacted hydrogen. In addition, there are obvious necessities to improve the catalyst and also to understand reaction mechanisms towards rational catalyst and process development. Reflecting this background, this thesis aims to (i) critically evaluate the advantages of the high pressure approach in stoichiometric CO2:H2 (1:3) ratio by examining different reaction and process parameters, (ii) investigate the reaction mechanism characteristic to high-pressure conditions, and (iii) develop thermally stable and highly active catalysts comprising of Cu-ZnO core-shell nanomaterials.
First, a high-pressure lab scale reactor setup for the continuous catalytic hydrogenation of CO2 to methanol at pressures up to 510 bar was successfully constructed. The operation of the reactor system was controlled by software implementing safety measures, thus allowing unattended catalytic tests for a long period of time. Using this reactor system, advantages of high-pressure conditions under the stoichiometric reaction condition were evaluated in-depth using a commercial Cu/ZnO/Al2O3 catalyst. A strong interplay between kinetics and thermodynamics in the reaction performance were evidenced. At kinetically favorable high temperature (>260 °C) especially at lower GHSV, it was possible to enter the regime where thermodynamic equilibrium plays dominant roles in determining the catalytic activity. A good weight time yield (WTY) of 0.92 gMeOH gcat-1 h-1 was achieved at 442 bar with 88.5% CO2 conversion and 97.2% methanol selectivity using our standard size of catalyst particles (100-300 μm). However, above 331 bar there was a formation of dense reaction mixture due to product condensation and thus the overall reaction rate was limited by internal mass transfer. When smaller catalyst particles (10-20 μm) were used instead, the limitation could be effectively removed. Thus obtained catalytic performance fully benefited from the high-pressure advantages of high reaction rate (kinetics), high equilibrium conversion (thermodynamics) and enhanced conversion (phase separation). Under these conditions of negligible mass transfer limitations at 442 bar, a very good WTY of 2.4 gMeOH gcat-1 h-1 could be observed with 87.7% CO2 conversion and 97.6% methanol selectivity. At a very high GHSV (100000 h-1), an extraordinary WTY of 15.2 gMeOH gcat-1 h-1 could be achieved.
To gain insights into the reaction mechanisms under high pressure, a mechanistic study using the commercial Cu/ZnO/Al2O3 catalyst was performed by space-resolved sampling of the reaction mixture from three different locations along the axial direction of the catalytic reactor. The results showed that CO2 was directly converted to methanol at low temperature (180 °C) and a small amount of detected CO resulted from methanol decomposition. A contrasting mechanism was observed at 340 °C, where the endothermic reverse water-gas shift (RWGS) reaction dominated, producing CO as the major product. Importantly, this CO could then be hydrogenated to produce methanol. At 260 °C catalytic activity was high, the results showed RWGS and CO to methanol reactions are in equilibrium and resulted into high methanol concentration. These mechanistic insights were further verified by operando Raman concentration profiling using a sapphire capillary reactor and looking into the void space between the catalyst beds at 200 bar. A similar trend as the case sampled by GC was confirmed; at 180 °C preferential direct methanol formation and later methanol decomposition took place as confirmed by the change in H2/CO2 ratio. At higher temperature (260 °C) initially H2/CO2 ratio was increased due to CO formation by RWGS and later lowering ratio indicates CO transformation to methanol. A liquid product condensation was observed at this condition at the end of the reactor, which was beneficial and responsible for high catalytic activity.
Cu-based catalysts are widely known for their excellent activity in the methanol synthesis reaction; however, copper agglomeration at higher temperature can lead to catalyst deactivation. With the aim to enhance the stability of Cu-based catalysts, Cu-ZnO core-shell catalyst, where Cu2O spherical core is coated with ZnO nanoparticles, was synthesized by a newly designed protocol based on the non-aqueous sol-gel method. This morphology separates the Cu particles by thermally stable Zn component and also provides high C@ZnO interfacial area, which is considered to be an important factor for methanol synthesis.
A series of 15, 30, 50 and 70 wt% Cu-ZnO catalysts were synthesized and tested under various methanol synthesis conditions.
30 wt% Cu@ZnO showed the highest CO2 conversion (52%) and methanol selectivity (84%) at 300 °C, 360 bar. Further increase in the copper concentration reduced the catalytic activity due to agglomeration of active Cu metal. The catalysts before and after the reaction was characterized by XRD, STEM-EDX. Interestingly, 30% and 50% Cu@ZnO catalyst showed emergence of ZnCO3 phase after the reaction. The formation of ZnCO3 was found general under high-pressure conditions as verified also for the commercial Cu/ZnO/Al2O3 catalysts by high-pressure operando XRD. The ZnCO3 phase, formed likely due to the liquid phase formation which can become acidic, does not influence the catalytic activity significantly, and rather it helps to stably isolate the copper particles.
