CO2 hydrogenation by heterogeneous catalysts

Environmental issues have pushed the necessity to reduce CO2 emissions caused by the use of fossil fuels. CO2 hydrogenation has been more intensively investigated recently, due to its multiple chemicals and fuel products which could be obtained from the reaction such as alkanes (methane, liquefied petroleum gas and gasoline), alkenes, aromatics, formic acid, dimethyl ether, methanol, and higher alcohols (Fig. 1) [1]. The hydrogenation of CO2 into CH4 is called methanation reaction. CO2 methanation is exothermic with high equilibrium conversion between 25 and 400 °C. CO2 methanation can reach 99% CH4 selectivity through use of appropriate catalysts, avoid the subsequent product separation and overcome the difficulty of dispersed product distribution. Methanation can be achieved using metallic catalysts that are either based on noble metals such as Rh and Ru or the transitional counterparts containing Ni-species as the active components. Silica, alumina, ceria, zirconia and modified mixed oxides have been used as support [2]. In other route, CO2 is hydrogenated into methanol (i.e. CO2 + 3H2 = CH3OH + H2O). Variable reaction conditions that include 200-300 °C of temperatures, high pressures and supported oxide catalysts are normally employed. Catalysts based on Fe and Cu-particles doped with different oxides (including TiO2, ZnO and ZrO2) or promoted with metals have been substantially studied as materials for the CO2 to CH3OH process. However, sintering, low CO2 conversion rates and low methanol selectivity are accounting for a shift of interest to other catalysts including carbides noble bimetallic systems. Catalyst systems based on Cu-particles are widely given preference than Fe based catalysts due to realized activity-selectivity [3].CO2 hydrogenation to C2+ hydrocarbons can be catalyzed through modified FTS route or methanol mediated route to promote hydrocarbon chain growth. For the modified FTS metal-based catalysts, appropriate active metal should be chosen, such as Fe, to get the best hydrogenation capacity. For the methanol-mediated route, bifunctional catalysts combining metal oxides and zeolites are crucial for obtaining a higher selectivity of long chain hydrocarbons. Acid sites are important for the conversion of methanol to hydrocarbons and the channel diameter can influence product selectivities due to the shape-selectivity characteristic. SAPO-34 with 8-ring pore structure is beneficial for C2-C4 formation and ZSM-5 with 10-ring pore structure will lead to C5-C11 formation [4]. For CO2 hydrogenation to olefins, avoiding the formation of side products such as CO, CH4, C2-C4 alkanes and C5+ hydrocarbons is crucial for industrial implementation. In the modified FTS route, iron catalysts are preferred (usually doped with alkali metals). γ-Al2O3 is the most commonly used support. The conversion of these catalysts can be increased by adding metals such as Cu, Co, Mn or Zn. Temperatures around 400 °C were suitable for this mechanism. Conversely, the methanol mediated route is catalyzed by group IIIA metal oxides such as In2O3 and Ga2O3. ZrO2 is added to increase the number of vacancies and the overall stability. SAPO-34 is an essential part of the bifunctional catalyst for olefin synthesis. The combination of both processes (CO2 to methanol + MTO) requires a compromise in the reaction temperature where, ideally, CO formation is avoided (≤ 300 °C) but the zeolite is active for C-C coupling (≥ 350 °C).[95] Theses reactions are carried out between 380-400 °C [5].

Keywords:

catalytic hydrogenation , catalyst , methanation , methanol , hydrocarbons , olefins