Design and development of metal oxide-based nanocomposites for renewable synthesis of chemicals and fuels

Abstract

The population and industrial demands rise, and global energy consumption intensifies the environmental strain. Finite resources of fossil fuels predominantly meet the world's need for commodity chemicals, medications, and energy. Unfortunately, these resources are diminishing rapidly and cause environmental degradation. The combustion of fossil fuels releases harmful gases like CO2, contributing to the greenhouse effect and increasing global temperature. To ensure the well-being of future generations, it's imperative to pursue both environmental and industrial sustainability. Transitioning from fossil fuels towards renewable energy sources such as wind, solar, hydropower, geothermal, and biomass is crucial for ecological preservation. Among these alternatives, biomass is a promising substitute for fossil fuels due to its versatility. Beyond energy production, biomass offers a plethora of value-added chemicals with applications ranging from green solvents to pharmaceuticals. Considering the aforementioned considerations, the primary goal was to design multifunctional catalytic materials, specifically metal oxides like inverse spinels, Nb-based materials, and Ce-based materials for biomass conversions. These metal oxides were selected for their accessibility, stability in varied reaction conditions, and flexibility in adjusting their active sites through synthesis methodology. Various synthesis approaches were employed to generate metal oxides with diverse chemical and physical properties. Moreover, the potential of employing metal oxides as supports for Pd and non-noble (Ni) metal catalysts in biomass conversions, particularly in hydrogenation reactions. These efforts were directed toward minimizing the activation energy and maintaining comparable activity levels for the hydrogenation of biomass model compounds. In the preliminary phase, inverse spinels, including CuFe2O4, NiFe2O4, and Fe3O4, were utilized for catalyzing the transfer hydrogenation. Subsequently, the deposition of Pd and Ni onto these spinels was conducted, followed by an investigation of their efficacy in the hydrogenation of lignin model compounds. Likewise, adjusting the composition of Nb yielded diverse, active sites and adjustable physicochemical properties, enhancing its effectiveness in catalyzing the hydrogenation of lignin model compounds and lignin bio-oil into valuable fuel range chemicals. Simultaneously, utilizing CO2 for energy generation and the production of valuable chemicals presents a viable strategy for reducing CO2 emissions. In line with this, my secondary objective entails the tailored design of metal oxides possessing adjustable physicochemical properties for CO2 conversion. Through the modification of CeO2, I achieved a remarkable breakthrough in the selective conversion of CO2 into cyclic urea and cyclic urethanes, as well as in the C-C coupling reactions. These compounds hold immense promise, particularly as agrochemicals, advancing both environmental and industrial innovation. Enhancing sustainability in biomass conversion, I started on a transformative journey through a photocatalytic pathway, which is a greener approach. Harnessing light within the visible spectrum and solar energy for selective catalysis in biomass transformation is a promising alternative route in our investigation. To minimize energy consumption and make sustainable strategies for converting biomass model compounds into high-value chemicals, a key focus lies in utilizing CeO2 and its nanocomposites within photocatalytic processes. By adjusting bandgap parameters and fine-tuning properties, I effectively activated CeO2 to absorb visible light, enhancing the biomass model compound's conversion efficiency. Throughout the thesis, metal oxide and its nanocomposites were synthesized for biomass transformations, CO2 conversion, and C-C coupling, enhancing catalytic activity by decorating noble and non-noble metal nanoparticles on their surfaces and evaluated in the hydrogenation of biomass-derived compounds, with variations in different reaction parameters. Additionally, metal oxides were synthesized to utilize CO2 and C-C coupling reactions, particularly photocatalytic Sonogashira coupling. The thesis establishes structure-activity relationships and proposes reaction mechanisms contributing to green and sustainable chemistry by synthesizing active metal oxides for fuel additives, precursors, and value-added chemicals.