Loading...
Heterogeneous catalytic materials for hydrothermal waste upgrading and remediation
Cronmiller, Lauren Estelle
Cronmiller, Lauren Estelle
Citations
Altmetric:
Advisor
Editor
Date
Date Issued
2024
Date Submitted
Collections
Research Projects
Organizational Units
Journal Issue
Embargo Expires
2026-04-04
Abstract
Purification of waste streams to reuse standards is a critical challenge in achieving economic circularity. To increase the feasibility of waste stream upgrading and remediation, the advantages of both homogenous and heterogeneous catalysts should be utilized. Further, the functionalities of solid heterogeneous catalysts in relevant reactions should be understood and taken as a foundation for optimization. This thesis applies heterogeneous catalysis under subcritical hydrothermal conditions to two areas relevant for sustainable waste stream management: (1) the upgrading of waste cooking oils to fuel grade hydrocarbons and (2) the destruction of per- and polyfluoroalkyl substances (PFAS) in water.
Awareness of the consequences and limits of fossil fuel usage has driven demand of alternative energy and fuel feedstocks. Waste cooking oils are high in fatty acid content, which through deoxygenation can be converted to long-chain hydrocarbons that can be used in drop-in fuel replacements for both diesel and sustainable aviation fuel. We review the functionality of nickel based non-noble metal catalysts supported on ZrO2 for the hydrothermal hydrogenation and decarboxylation of oleic acid, a fatty acid common in many waste cooking oils. Synthesis techniques and bimetal additives were compared and a monometal Ni/ZrO2 catalyst was found to be the most effective for hydrothermal deoxygenation of oleic acid. Kinetics studies (e.g., time, temperature, catalyst concentration, etc.) were completed to optimize reaction conditions. A variety of liquid hydrogen donor sources were screened (e.g., methanol, formic acid, glycerol) and methanol was found to yield optimal selectivity for heptadecane formation via catalytic decarboxylation. Initial fatty acid feedstock was also varied to assess the influence of molecular complexity on catalyst activity, and the degree of unsaturation was found to promote undesired side reactions and decrease selectivity for hydrocarbon production. Catalyst deactivation experiments identified sintering of the active Ni phase as a primary mode of deactivation.
A similar catalyst study was also applied for the remediation of PFAS-contaminated water. In 2024, a stringent federal drinking water standard limiting a variety of PFAS was established. Still, options for full destruction of PFAS remain limited, and understanding of how heterogeneous catalysts can facilitate their degradation remains relatively unexplored. Thus, noble metal and metal oxide catalysts were initially screened to identify catalysts for the degradation of perfluoroalkylsulfonic acids (PFSAs) under hydrothermal conditions. Noble metal catalysts (supported ruthenium and palladium) were identified to have the greatest potential for destruction of perfluorobutane sulfonate (PFBS). Further kinetics studies with ruthenium-on-carbon (Ru/C) (e.g., time, temperature, catalyst concentration, etc.) were then conducted to optimize catalyst performance. Considering the relative infancy of many PFAS detection methods, the analytical approach reviewed in this chapter also provides a useful didactic framework for identifying potential transformation byproducts. Through said intermediate identification, a novel degradation mechanism was proposed wherein reaction is initiated by cleavage of perfluoroalkyl C-C bonds. Although Ru/C mediated complete degradation of the parent PFSAs, only partial defluorination is observed and the remaining fluorine remains chemisorbed to the solid Ru/C following reaction, which is attributed to coupling of C-centered radicals (resulting from catalytic homolytic C-C bond cleavage) with the carbon support material. Application of hydrothermal alkaline treatment conditions liberated the chemisorbed F as fluoride ions.
Subsequently, prompted by activity differences seen in previous catalytic studies, we compared commercially purchased monoclinic ZrO2 against in house synthesized ZrO2 dominant in the tetragonal phase. Whereas monoclinic ZrO2 showed little reactivity with PFSAs, reaction with the tetragonal form of ZrO2 (t-ZrO2) was able to achieve >85% defluorination of PFSAs of varying chain lengths (e.g., TFMS, PFBS, PFOS). Degradation of perfluoroalkyl carboxylates was found to be slower than that of sulfonates, which was attributed to the formation of more nonpolar intermediate 1H-perfluoroalkanes by thermal decarboxylation at lower temperatures (e.g., 200 ï‚°C), prior to the reactor reaching temperatures where the catalyst was active for PFAS destruction (e.g., 350 ï‚°C). Catalyst activity was attributed to its density and strength distribution of basic sites, promoting hydrolytic PFAS degradation similar to the mechanism proposed for homogeneous hydrothermal alkaline treatment (HALT) of PFAS.
Associated Publications
Rights
Copyright of the original work is retained by the author.