Development of innovative mesostructured catalysts for CO2 conversion to dimethyl ether

The present thesis is focused on the development of mesostructured catalysts for the one-pot conversion of CO2 to dimethyl ether (DME). With the aim of reducing the emissions of CO2, several measures are being adopted; one of them is the Carbon Capture and Utilization (CCU), consisting in the capture of CO2 in order to use it to transform it into chemicals and fuels. Among fuels, DME represents a good choice since it is a non-toxic and non-carcinogenic gas that can be used as an additive or replacement for diesel fuel, resulting in lower emissions of SOx and NOx; furthermore, DME can be stored and transported using the same technologies used today for Liquified Petroleum Gas (LPG). CO2 is converted into DME through two subsequent reactions: the first one is the reduction of CO2 to methanol with hydrogen and the second one is the dehydration of methanol to DME. These two reactions require different catalysts, with Cu-based redox catalysts being used for the first reaction, and solid acidic catalysts for the second. When the two reactions are carried out simultaneously into the same reactor (one-pot or one-step process) a combination of the two catalysts is required. The catalysts can be combined in form of either physical mixtures or by obtaining composite catalysts (chemical mixtures) featuring an intimate contact between the redox and the acidic phase. This thesis work first consisted in the development of mesostructured acidic dehydration catalysts to be used in form of physical mixtures with a Cu-based redox catalyst (CZA). Particularly, mesostructured TiO2, Zr-TiO2, γ-Al2O3 and several mesostructured aluminosilicates (Al-MCM-41, Al-SBA-16 and Al-SBA-15) were synthesized using different approaches like classical sol-gel, solvothermal, and Evaporation-Induced Self-Assembly (EISA) routes. The obtained materials were then characterized with XRD, N2-physisorption, and TEM to gather information on their structural, textural, and morphological properties; particular attention was given to the characterization of the acid sites with NH3-microclorimetry and FTIR-monitored pyridine adsorption, that allowed to assess the typology, amount, and strength of acid sites. The materials were then tested as methanol dehydration catalysts for the CO2-to-DME one-pot process. The aluminosilicate catalysts were found to be the most effective, due to the presence of Brønsted acid sites. Additionally, the study revealed that the surface density of acid sites is a crucial factor in achieving high activity for methanol dehydration. Specifically, Al-SBA-16, which has the highest surface density of acid sites, demonstrated superior performance in terms of selectivity to DME. Aluminosilicates and γ-Al2O3 were then used as supports to obtain bifunctional composite catalysts by dispersing a Cu/ZnO/ZrO2 redox phase within the mesopores. Two different impregnation routes were tested, i.e. a two-solvent and a self-combustion approach. The impregnation route based on a self-combustion reaction proved to be an effective method to obtain a homogeneous distribution of the redox phase inside the pores in form of small nanoparticles. The composite catalysts were then tested for the one-pot CO2-to-DME reaction. To perform a direct comparison between the composite catalysts and the physical mixtures, a Cu/ZnO/ZrO2 redox catalyst (CZZ) with the same composition selected for the redox phase impregnated into the supports, was synthesized and used. The comparison between composite catalysts and physical mixtures pointed out how the higher dispersion of the redox phase in composite catalysts gives rise to higher values of CO2 conversion, associated to lower values of DME selectivity, presumably due to the coverage of acid sites with the redox phase. Future studies will focus on the optimization of the loading of redox phase in composite catalysts and of the impregnation approach, as well as on the synthesis of mesostructures using industrial waste as precursors.

This thesis is focused on the development of novel mesostructured catalysts for the conversion of CO2 to dimethyl ether (DME). The first part consists in the design of mesostructured acidic dehydration catalysts based on mesostructured TiO2, Zr-TiO2, γ-Al2O3 and several aluminosilicates (Al-MCM-41, Al-SBA-16 and AlSBA-15). To obtain mesostructures with various pore arrangements, pore sizes, and textural features, the materials are synthesized using different approaches like sol-gel, solvothermal, and EISA routes. The samples are then characterized with XRD, N2-physisorption, TEM, and STEM-EDX to gather information on their structural, textural, and morphological properties; particular attention is given to the characterization of the acid sites with NH3-microclorimetry and FTIR-monitored pyridine adsorption, that allows to assess the typology, amount, and strength of acid sites. The materials are then tested as methanol dehydration catalysts with a commercial redox Cu-based catalyst for the one-pot CO2-to-DME process into a bench-scale plant with a fixed bed reactor. The correlation of the catalytic performances with the acidic features shows that, among the proposed acidic catalysts, aluminosilicates present the best dehydration activity, due to the presence of Brønsted sites, and that the surface density of acid sites has a crucial role. The most promising materials are then used as supports to obtain bifunctional composite catalysts by dispersing a Cu/ZnO/ZrO2 redox phase inside the mesochannels. An impregnation route based on a self-combustion reaction allows to homogeneously disperse the redox phase inside the mesopores in form of small nanoparticles. The incorporation of the redox phase causes a partial coverage of the acid sites; however, this detrimental effect is compensated by higher values of CO2 conversion for the composites compared to the corresponding physical mixtures, due to the high dispersion of the redox phase inside the mesostructured matrix.