Zirconia-based Catalysts for Biomass Conversion

The detrimental impact of carbon dioxide emissions from fossil fuels on global warming, combined with the increasing global demand for energy due to population growth, has intensified the search for renewable, low-carbon energy alternatives. Biorefineries, which aims to convert biomass to energy and other valuable by-product, are a sustainable means of chemical manufacturing in which multiple bioenergy products (e.g. fuels, food, specialty chemicals) are generated from naturally occurring biomass feedstocks such as forestry and agricultural waste through the integration of clean processes. In this regard, the conversion of biomass-derived C6 sugars into 5-(hydroxymethyl)furfural (HMF)—a pivotal platform chemical used to produce fuel precursors, additives, and value-added chemicals—has garnered significant attention. However, the high production cost of HMF continues to impede its commercial viability. Therefore, the development of a heterogeneously catalysed process to efficiently convert glucose to HMF in aqueous media remains highly desirable. HMF is synthesised by the cascade conversion of glucose over Lewis and Brønsted acid sites to respectively initiate isomerisation of glucose and subsequent dehydration of reactively formed fructose to HMF. Zirconia is a promising catalyst for such reactions; however, the impact of acid properties of different zirconia phases in these transformations is poorly understood. In this thesis, we unravel the role of the zirconia crystalline phase in glucose isomerisation to fructose and subsequent dehydration to HMF. The Lewis acidic monoclinic phase of zirconia is revealed to preferentially facilitate glucose isomerisation, while the nanoparticulate tetragonal phase possesses Brønsted acid sites which favour fructose dehydration. The synergy of both zirconia phases facilitates HMF production in a cascade process, with both catalysts investigated as physical mixtures in batch and flow reactor configurations. Using a physical mixture of only 15 wt% m-ZrO2 with 85 wt% t-ZrO2 in either batch or packed bed reactor configuration is sufficient to reach equilibrium conversion of glucose for subsequent dehydration by the t-ZrO2 component. Under continuous flow, a six-fold increase in HMF production was obtained when operating with a physical mixture of m- and t-ZrO2 compared to that from a single bed of t-ZrO2. Further improvement in catalytic performance has been achieved using bifunctional catalysts through the synthesis of surface sulphation on calcined zirconium hydroxide, monoclinic and tetragonal zirconia. Sulfated zirconia (SZ) is produced via surface sulfation on calcined zirconium hydroxide, where monolayers of sulfate groups are formed from sulfuric acid. Analysis indicates that sub-monolayer SO4 coverages provide an optimal balance of Lewis and Brønsted acid sites, which are essential for the two-step cascade conversion of glucose to HMF. However, the influence of the zirconia crystalline phase on surface sulphation remains poorly understood. We then demonstrate a systematic control on Lewis-Brønsted acid properties of sulfated monoclinic (SZ(m)) and tetragonal zirconia (SZ(t)). Results show that surface sulfation enhances both Lewis and Brønsted acidity in SZ(m), leading to constant catalytic activity for glucose isomerisation to fructose—the rate-determining step in the cascade glucose conversion to HMF, even at high concentration sulfate coverage. While non-acidic sulfate species were formed in SZ(t) when surface S content exceeds 2.0 wt%. Additionally, SZ(m) catalysts demonstrate remarkable stability under continuous flow conditions at 150 °C for 6 h, with optimal activity observed in samples treated with 0.01 M H2SO4. The valorisation of biomass-derived citral is an important application in sustainable chemical manufacturing. Citral can undergo catalytic transfer hydrogenation to produce geraniol and nerol, both of which are valuable compounds used predominantly in the fragrance industry. Zirconia, particularly when calcined from Zr(OH)4 at 400 °C, has demonstrated high activity in the selective hydrogenation of the C=O bond in citral, achieving nearly 100% selectivity toward the desired products in both batch and flow conditions. Weak Lewis acid sites on the zirconia surface have been identified as the active sites responsible for this reaction. Additionally, citral can be converted to p-cymene through a cyclodehydration pathway using SZ catalysts. This cascade reaction requires a specific surface sulfur content, with 1.6 wt% sulfur necessary to initiate the process. Complete conversion of citral occurs when the surface sulfur content exceeds 2.9 wt%.<p></p>