CO2 Hydrogenation to Methanol in Different Reactor Concepts Using Metal-doped Indium-based Catalysts

Abstract

CO2 hydrogenation with H2 from water electrolysis to methanol (MeOH) is a key strategy for chemical energy storage and for CO2 utilization. MeOH serves various applications and is an important feedstock for the chemical industry as well. This work investigates the application of indium-based catalysts for MeOH synthesis via the hydrogenation of CO2 in various reactor concepts focusing on the reaction conditions and catalyst properties necessary to achieve high activity and selectivity. Industrial methanol production mainly uses two-phase (gas solid) or three-phase reactors (gas solid liquid). Two-phase reactors are categorized inro adiabatic an isothermal reactors based on the heat dissipation in the catalyst bed. Three phase reactors, such as slurry and trickle bed reactors, enable more efficiency in heat removal and minimize mass transport limitations. First, the catalytic performance of In2O3 and In(OH)3 was studied in a fixed bed reactor. The impact of thermoelement positioning in the catalyst bed (top, middle or bottom position) was studied. Using catalytic data received at 200, 250 and 300 °C, along with material characterization (XRD and TGA) after the reaction, a catalytic model cycle was defined concerning the phase transition. CO2 hydrogenation produces water as a by-product, leading to the degradation of In2O3 to In(OH)3. The model postulates stable In(OH)3 at low temperatures and higher conversions. In contrast, In2O3 remains stable at higher conversions and temperatures. The separation of In(OH)3 into two identical segments, with a layer of glass wool between the segments, investigated the influence of the phase transition at 200, 250, 275 and 300 °C. Fresh gas flows through the top segment, being partially converted into MeOH, CO and water and flowing through the bottom segment. XRD and TGA analyses revealed an inversion of the trend between 250 °C and 275 °C, while the computational model predicts this occurrence at 285 °C. Lastly, the validated model was employed to predict the effects of hydrogen drop out in the feed due to fluctuating production from renewable energies on catalyst stability. In further studies within the fixed bed reactor, two different ZrO2 were used as catalyst support and impregnated with In2O3 using different synthesis methods. In detail, the combination of wetness impregnation according to Martin et al. (M) and the ZrO2 (SG) with a lager BET surface area and CO2 adsorption capacity exhibited the best catalytic performance. At 300 °C, 75 bar, CO2/H2 = 1/3 and 8600 h-1 the maximum MeOH production reached 4.25 g(MeOH)/(g(In)∙h). To increase the MeOH productivity, various metals (Cu, Ni, Mg, Ce) were investigated as promoters for the In2O3/ZrO2 (M-SG) catalyst. NiO In2O3/ZrO2 (M-SG) demonstrated an enhanced catalytic activity. Due to enhanced H2 uptake the H2 spillover effect increases, which promotes H2 dissociation and migration to the carrier surface and favors the formation of oxygen vacancies, the active centers for CO2. This was confirmed by chemisorptive analyses (H2-TPR and CO2-TPD). NiO on In2O3/ZrO2 prepared by wetness impregnation (WI) resulted in catalysts that were catalytically more active. NiO In2O3/ZrO2 (WI) with 0.76 wt.% Ni produced with 4.42 g(MeOH)/(g(In+Ni)∙h) more MeOH than pure In2O3/ZrO2 (M-SG) with 4.25 g(MeOH)/(g(In)∙h) at 300 °C, 75 bar, CO2/H2 = 1/3 and 8600 h-1. No methanation was observed. Over 100 h time on stream the NiO In2O3/ZrO2 (WI) remains stable and active. In the Compact Profile Reactor (CPR), the reaction profiles of In2O3/ZrO2-based catalysts were investigated and compared with the industrially used Cu/ZnO/Al2O3 catalyst. The reactor design enables spatially resolved analysis of the reaction profile for every species and temperature inside the catalytic bed during high-pressure MeOH synthesis by the hydrogenation of CO2. The influence of reaction conditions, including total pressure, gas hourly space velocity (GHSV), and temperature, was studied using the most active catalyst, Ni-In2O3/ZrO2. In this study, higher pressures, temperatures, and GHSVs led to increase the MeOH productivity. Shorter residence times enhance MeOH selectivity. At 50 bar, 275 °C and 63,000 h-1, Ni-In2O3/ZrO2 produced 4.90 g(MeOH)/(g(In+Ni)∙h) with 73 % selectivity of MeOH. The remaining carbon is converted into CO via the rWGS. MeOH synthesis requires a lower activation energy (49 kJ mol-1) compared to the reverse water-gas shift reaction (71 kJ mol-1). In a suspension (slurry) reactor (SR), pure In2O3 and In2O3/ZrO2 was doped with nickel using different synthesis methods and used for three-phase MeOH synthesis. Ni-In2O3/ZrO2 prepared by co-precipitation (CP) and suspended in mineral oil achieved the highest catalytic activity and selectivity for MeOH for both CO and CO2 hydrogenation. Using synthesis gas with industrial combustion (H2/CO/CO2) and a molar ratio of 70/28/2, a very high productivity of MeOH (6.84 g(MeOH)/(g(cat)∙h)) was achieved, which is significantly more efficient than the productivity of the commercial Cu/ZnO/Al2O3 catalyst (1.25 g(MeOH)/(g(Cu)∙h)) in the SR related to the quantities of active metals. Finally, the catalyst and the mineral oil remained stable in a recycling study with four replicate runs. Intensive catalyst material characterization using ICP-OES, XRD, XPS, N2-physisorption, CO2 TPD, H2-TPR and TEM or SEM-EDX mappings provide insights in the difference in catalytic activity of the catalysts in the various reactor concepts.