Metal phosphates and metal-oxides based heterogeneous catalysts for fuels and chemicals production from biomass and CO2

Abstract

The global demands for commodity chemicals, medicines, and energy are fulfilled by the raw materials obtained from fossil fuels. However, the stock of fossil fuels on the earth is limited and depletes faster. The consumption of fossil fuels has a negative environmental impact, as it produces greenhouse CO2 and other harmful gases. The CO2 concentration is continuously increasing, and it is expected to become double by the end of this century. The toxic effluent gases are deteriorating the health of living creatures. Moreover, as the population and industrial growth increase, global energy consumption is continuously increasing. Therefore, for the survival of future generations on the earth the environmental and industrial sustainability is necessary. For ecological sustainability, fossil fuels must be replaced with renewable energy sources such as wind, solar, hydropower, geothermal, and biomass. Among the various renewable energy sources, biomass is considered the most reliable energy alternative to fossil fuels. Further, biomass can also fulfill the other requirements; for example, a wide range of value-added chemicals can also be derived from biomass that has applications as green solvent, fuel and fuel precursor, polymer precursor, fuel additive, pharmaceuticals, etc. The photocatalytic pathway employing solar energy is another way for energy production. However, if both biomass and solar energy are collectively utilized, it will be the energy-saving and economical pathway for efficient biomass and solar energy utilization. After considering the above aspects, the first objective was to develop multifunctional catalytic materials based on metal phosphates and metal oxides for biomass transformations. The aim of choosing these materials was their availability, stability under the employed reaction conditions, and easy tuning of their active sites by changing their synthesis strategy, metal ratio (especially in metal phosphates). Metal phosphates and metal oxides with different chemical and physical properties were synthesized by employing different synthesis strategies. The synthesized metal phosphates have different Lewis ratios to Brönsted acid sites and different amounts of acidity and basicity. The acidity ratio was crucial for higher and selective biomass conversion through dehydration, isomerization, transfer hydrogenation, oxidation, and condensation pathways. Simple and mixed metal oxides were also synthesized for the higher and selective transformation of biomass through isomerization and condensation pathways. Furthermore, the applications of metal phosphates and metal oxides as support were explored for noble and non-noble metals decoration was also studied in biomass transformations, especially in oxidation and hydrogenation reactions. However, for these applications, attempts have been made to minimize the noble metal content and to achieve similar activity using nonprecious metals. The second objective was to develop a photocatalytic pathway for efficient conversion of biomass and CO2, using metal oxides or their nanocomposite materials. The photocatalytic activity of metal oxides and g-C3N4 was investigated in biomass oxidation and photocatalytic production of cyclic carbonates from CO2 and epoxides. The metal phosphate and metal oxide-based catalysts were synthesized for biomass transformations via isomerization, condensation, transfer hydrogenation, and oxidation reactions. In order to improve their catalytic activity, especially in hydrogenation reactions, different noble and non-noble metal nanoparticles were decorated over the external surface of metal phosphates and metal oxides. The catalytic activity of supported metal phosphates and metal oxides were evaluated in hydrogenation and oxidation of biomass-derived carbonyls and alcohols, respectively, under mild reaction conditions. Moreover, hydrogenated products were prepared by changing the support, metal nanoparticles, solvent, or temperature of the reaction. Further, the photocatalytic activity of metal oxides and graphitic carbon nitride was also evaluated in photocatalytic CO2 insertion into epoxide and oxidation of biomass-derived carbonyls in presence of artificial and natural solar light. The structure-activity relationship was established, and reaction mechanisms were proposed based on the catalytic data, control experiments, and other physicochemical characterizations. Overall, this thesis reports the synthesis of a wide range of multi-functional catalytic materials based on metal phosphates and metal oxides. Acidic and basic multi-functional metal phosphates and metal oxides reported in this thesis were used as catalysts in the synthesis of fuel additives, precursors, and value-added chemicals. The work presented in this thesis contributes to the area of green and sustainable chemistry.