Homogeneous and heterogeneous catalysis for biomass upgrading to platform chemicals and end products
Date
2020
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Publisher
University of Delaware
Abstract
Today, transportation fuels and lubricants, plastics, fabrics, and many other products are derived from petroleum, a nonrenewable feedstock and contributing to greenhouse gas emissions. Bioderived, renewable feedstocks can mitigate the environmental impact of manufacturing commodity products, while providing comparable or even superior properties to their petroleum-derived counterparts. Among the renewable carbon sources, nonedible lignocellulosic biomass is one of the most promising feedstock alternatives; however, its high oxygen content remains the most significant barrier for its conversion into low-oxygen containing fuels and chemicals traditionally derived from petroleum. As a result, multiple processes have been studied for oxygen removal, such as the preparation of oxygenated furans from biomass-derived sugars. From there, oxygenated furans can undergo additional transformations to form high-value products, such as lubricants. All of these transformations require a suitable homogeneous or heterogeneous catalyst to enable highly selective and energy efficient processes. To this end, the objective of this thesis is to investigate selected catalytic systems and processes to obtain valuable products from biomass. ☐ Chapter 3 of this thesis begins by identifying and investigating the molecular species derived from the homogeneous metal salt, AlCl3, that enable the conversion of sugar (glucose, C-6) in biomass to 5-hydroxymethylfurfural (HMF). While homogeneous metal salts have been shown to catalyze sugar chemistries, direct experimental evidence in support of a specific catalytic species remains elusive. Here, direct speciation measurements are coupled with kinetics to provide convincing evidence that [Al(H2O)4(OH)2]1+ is the active species for glucose conversion in water. A speciation model is used to predict aluminum species as a function of composition, while simultaneously an experimental protocol is used to quantify the various aluminum species. Linear scaling between the glucose conversion rate and the speciation measurements at sufficiently high temperatures indicates that the [Al(H2O)4(OH)2]1+ complex is the active species in glucose conversion. Knowledge of the active species can help improve future catalyst development for this and other reactions. ☐ In Chapter 4, biomass-derived platform chemicals are used to produce bio-lubricant base oils. Our strategy involves coupling 12-tricosanone, obtained from bioderived fatty acids, with furfural, a C-5 species, obtained from hemicellulose, to form a highly branched bio-lubricant base oil. I show that the viscous properties of the final product are comparable to commercial petroleum-derived Group III and Group IV base oils. ☐ Another major challenge in biomass conversion is catalyst deactivation. Despite its importance, in situ characterization of catalytic materials is often difficult. In Chapter 6 of this thesis, the effects of mild hydrothermal treatment on siliceous nanomaterials are monitored in situ by infrared reflection absorption spectroscopy (IRRAS). Well-ordered, siliceous materials called zeosils are employed as supports for metal catalysts in biomass upgrading, but require frequent regeneration to remove carbonaceous by-products from their pores. Monitoring the effects of mild hydrothermal conditions, in situ, on siliceous materials has been challenging to observe by IRRAS, which requires flat surfaces and benefits from electrically conductive substrates. To address this challenge, I use 2-D siliceous nanomaterials deposited on metal single crystals. In Chapter 6, it is shown that elevated temperatures and water pressures increase the formation of silanol (SiOH) groups in the MFI nanosheets, but do not change the polymorphous bilayer silicate. The effects are fully reversible in the MFI nanosheets. The implications shown here may provide insights into the effects of mild hydrothermal treatment applied to siliceous 3-D materials, which are challenging to study using surface science.
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Keywords
Bio-lubricants, Heterogeneous catalysis, Homogeneous catalysis, Hydrothermal treatment, Lewis/Bronsted acids, Surface science