Structure-activity correlation for Re- and Mo-based catalysts for the deoxydehydration of polyols

Landini, Christian; Palkovits, Regina (Thesis advisor); Liauw, Marcel (Thesis advisor)

Aachen : RWTH Aachen University (2020, 2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020

Abstract

Coal, petroleum, and natural gas are energy supplies, which are the main resources for human needs of energy, materials, and chemicals. In recent years, a significant change in the research activities occured, which brought conversion of biomass into useful liquid fuels in an attempt for total or partial substitution of dependency on fossil fuel consumption. A wide range of products can be obtained from biomass, which could be synthesized by processing the wastes of their own processes, such as bioplastics, bioenergy, and biochemicals. Shifting from fossil fuels to biomass-based fuels becomes evident that the high oxygen content in the biomass is an issue for their exploitation. Therefore, different deoxygenation strategies were developed, such as dehydration, hydrogenolysis, and DODH. The DODH is the most elegant and novel reaction since it converts vicinal diols to olefins in a single step, driven by the oxidation of a sacrificial reducing agent. The DODH of polyols is an exothermic reaction favored at lower temperatures, but a catalyst is needed due to kinetic limitations. A catalyst has to satisfy many requirements, such as cost, activity, selectivity, stability, recovery, and handling, in order to be industrial valuable. Over the last decade, DODH was intensively investigated using Re-based homogeneous catalysts, which are the conventional ones. However, the Re-based catalysts are expensive, researchers in this field focus on the development of catalysts based on non-noble metals, such as Mo and V. But, feasible solid catalysts are still missing. This work, therefore, aim to bring heterogeneous DODH catalysis in the liquid phase to a level of fundamental understanding, which is comparable to homogeneous catalysts. The research aims to unravel the nature of the catalytically active sites of Re and Mo-based supported catalysts.For the studies on ReOx catalysts, the DODH of 1,2-hexanediol (1,2-HDO) to 1-hexene (1-HE) was used as a model reaction. In order to study the effect of the supports, different inorganic materials (ZrO2-SiO2, ZrO2-WO3, Al2O3-SiO2, and TiO2-SiO2) were synthesized. The prepared materials were then characterized by Brunauer-Emmett-Teller Theory (BET), X-Ray Diffraction (XRD), pyridine-Diffuse Reflectance Infrared Fourier Transform spectroscopy (DRIFT), NH3-TPD, Scanning Trasmission Electron Microscopy (STEM)-Energy-Dispersive X-ray spectroscopy (EDX), and Raman spectroscopy. The comparative studies between all these materials revealed that the acidity of the supports plays a crucial role in the DODH of polyols. The addition of SiO2 to TiO2 improves the catalytic performance since it is possible to tailor the amount of the acid sites in the materials. Moreover, different species can be found on the Re-based catalysts, such as single sites (monooxo, dioxo, and trioxo structures) or nanoparticles (NPs). Based on the current experimental conditions investigated, only the ReOx single sites were active in DODH, and the distribution of those structures can be tailored by modifying the acidity of the support. ReOx/70TiO2 was the catalyst with the highest amount of acid sites and therefore it was able to stabilize the dioxo species, which is the most active one in the DODH of polyols. In order to study the structure-activity correlation of Mo-based catalyst, the DODH of 1,4-Anhydroerythritol (1,4-AHE) to 2,5-dihydrofuran (2,5-DHF) was used as a model reaction. Different catalysts were synthesized and characterized using techniques, such as BET, XRD, and Raman spectroscopy. Different MoOx structures can be found on Mo catalysts, like mononuclear, polynuclear, and NPs. In order to study the influence of those species in DODH, different catalysts were synthesized using different Mo precursors like MoO3, Na2MoO4, (NH4)2MoO4 (AMM), and (NH4)6Mo7O24 · 4 H2O (AHM). Among all these precursors, AMM showed the highest catalytic performance, which might be due to the dominance of the mononuclear Mo species. Furthermore, different MoOx mononuclear species can be found on the catalysts, which have from 0 to 3 bonds between the Mo center and the support. The ratio of those structures on the catalysts was tailored by modifying the impregnation pH and the calcination temperature. Among all the different structures, the delta mononuclear Mo species was found to be the most active one in DODH of polyols. AMM/100TiO2 was the catalyst with the highest amount of this structure, therefore it was the most active Mo-based catalyst.