Catalytic (De)Hydrogenation of Heterocyclic Compounds through Porous Metal Oxides

Abstract

Heterogeneous catalysis plays a critical role in modern industries including chemicals and energy. Recently, it is also closely connected to green chemistry, which is for overcoming the global crises in energy and environments. Among many materials, metal oxides are involved in a great part of the important catalytic processes. They were applied as both catalysts and supports in heterogeneous catalysis. Metal oxides as catalysts exhibit specific surface properties, including oxygen redox behavior, acid-base sites, and defective sites. Surface properties enhance catalytic activity of metal oxides. When they are used as supports, metal oxides affect catalytic performances because of thermal/electron conductivity or/and synergistic effects from metal-support interactions. These catalytic contributions of metal oxides are derived from exposed surfaces. Metal oxides are improved by increasing the amount of exposed catalytic sites by adopting porous structures. Furthermore, defective sites of porous metal oxides enhance catalytic performances by promoting intrinsic activity of the material. In this context, porous metal oxides have been brought attention from many catalytic and other applications, so numerous synthetic methods were developed. Apart from preparing different porous structures (i.e. size and shape), many methods for adjusting surface sites through the fabrication of porous structures were suggested to enhance the intrinsic activity of metal oxides. As-synthesized porous metal oxides are widely applied to various heterogeneous catalysis. Further investigations of them are still required to expand their utilization for developing an efficient catalyst. In this dissertation, we present development of porous metal oxides as an efficient catalyst or support in heterogeneous (de)hydrogenation reactions. Conversion of heterocyclic compounds is very essential in many industries, which are pharmaceutical, chemical, or even green chemistry. This dissertation describes two different applications regard to heterocyclic compounds, biomass conversion, and hydrogen storage system. Both reactions are important to alternate petroleum-based industries and energy systems. The first application is furfural (FAL) hydrogenation to convert biomass-derived furfural into market-based ones. The second application is liquid organic hydrogen carrier (LOHC) system for massive hydrogen storage and transport in hydrogen economy. Following chapters 2 and 3 deal with the application of porous metal oxides in biomass conversion. In Chapter 2, we developed efficient catalysts for FAL hydrogenation with mesoporous cobalt oxides (m-Co3O4), and investigated major active sites among different Co phases in catalytic FAL hydrogenation. As-synthesized m-Co3O4 series were prepared by the nanocasting method with mesoporous silica, SBA-15 (p6mm) and KIT-6 (Ia3d), as templates. Each m-Co3O4 was reduced at different temperatures (350 ℃ and 500 ℃) under H2 flow. The replicas show different reduction behavior depending on reduction temperature and its space structure. After reduction treatment, the original crystal structure of Co3O4 is changed to CoO and Co, respectively. Here, we examine the effect of structure, porosity, and oxidation state of m-Co3O4 to identify active species in FAL hydrogenation. Among cobalt oxides having different crystal structures and symmetry, m-CoO(p6mm) exhibits the highest FAL hydrogenation activity. CoO phases of the Co-based catalysts induce subsequent FAL hydrogenolysis by selective production of 2-methylfuran (MF), while Co3O4 and Co phases promote preferential hydrogenation of the side chain (carbonyl group) of FAL to furfuryl alcohol. Overall, CoO sites are demonstrated as major active sites, which responsible for selective FAL conversion to MF. Developed m-CoO catalyst are suggested as efficient catalyst in FAL hydrogenation after further reduction treatment. Chapter 3 presents mesoporous copper oxides (m-CuO) as another efficient noble-metal-free catalyst for FAL hydrogenation. In this study, density functional theory (DFT) calculations were simulated to the whole mechanism of FAL hydrogenation over Cu-based catalysts. DFT results demonstrated that Cu2O(100) surfaces have higher adsorption energy of FAL (1.63 eV) and negligible H migration barrier (0.64 eV) than other surfaces, Cu(111) and CuO(100). FAL hydrogenation reactions were conducted over m-CuO series, preparing by the nanocasting method and controlled reduction conditions (150 ℃, 250 ℃, and 350 ℃). Reduced m-CuO shows a sequential phase transformation from CuO, Cu2O, and metallic Cu. Exposed surface compositions of m-CuO catalysts were adjusted with reduction treatment. Each exposed Cu fraction was identified and quantified with surface characterization analysis such as CO-DRIFTS. According to the reaction results, the overall activity of m-CuO catalysts is determined by the concentration of exposed Cu+. In this report, Cu2O phase is a major active site, promoting catalytic activity of FAL hydrogenation. Based on these results, reduced m-CuO is developed for noble-metal-free FAL hydrogenation to convert biomass into valuable chemicals. In Chapter 4, we introduce the utilization of porous metal oxides as catalytic support for LOHC system. Hierarchical titanate nanosheets (HTN) were synthesized by a self-assembly process through solvothermal synthesis. The Pd/HTN catalyst was prepared with a urea-assisted deposition-precipitation followed by NaBH4 reduction. In accordance with the characterization results, HTN has a larger amount of surface acid-base sites and oxygen vacancies than hierarchical TiO2. The surface properties of HTN facilitate hydrogen activation and hydrogen spillover. Therefore, Pd/HTN catalyst has higher catalytic performances in both hydrogenation and dehydrogenation reactions for the NMID-based LOHC system. This study shows that porous metal oxides with improved surface properties are applied as an effective catalytic support to enhance catalytic activity.