Heterogeneous catalysts for the production of green glyceryl valerate esters

Licentiatavhandling, 2025

In response to pressing global climate change issues, the chemical sector is striving to reduce its CO₂ emissions and mitigate the environmental impact of its production processes. By combining biomass-derived feedstock with the development of more sustainable catalytic processes, significant reductions in process-related CO₂ emissions can be achieved. In this context, this thesis explores the production of biomass-derived glyceryl valerate esters via heterogeneous catalysts: supported heteropolyacids (HPAs) in Manuscript I and zeolites in Manuscript II.

In Manuscript I, tungsten and molybdenum-based HPAs at 5, 10, 15, and 20 wt% were impregnated on metal oxide supports (γ-Al₂O₃, CeO₂, ZrO₂, and SBA-15) and systematically screened for the esterification of valeric acid (VA) and glycerol under reflux condition. The results showed that the phosphorus variants with tungsten (HPW) and molybdenum (HPMo) showed similarity in catalytic activity at nominal loadings of 5-10 wt%. While the difference in P and Si central heteroatoms in the HPAs Keggin structure for the Mo series also performed similarly, HSiW performed worse when compared to HPMo and HSiW at the same nominal 15 wt% loading and 300 °C calcination temperature. The lower calcination temperature at 300 °C is preferred over 500 °C due to higher esterification activity and better maintenance of acidity, surface area, pore volume, and structural integrity. The highest combined divalerin and trivalerin yield was 51.6% for 20 wt% HPMo/γ-Al₂O₃ calcined at 300 °C under the reaction conditions: 130 °C, 1 wt% catalyst loading, a 6 h reaction time, 400 rpm stirring, 5:1 acid-to-glycerol feed mole ratio. Although higher loadings of HPMo (15 and 20 wt%) led to an enhanced glycerol conversion rate but also accelerated the leaching of Mo species. The investigation of leached Mo species revealed CeO₂ maintains HPAs on its structure better than γ-Al₂O₃, while SBA-15 and ZrO₂ suffered from severe leaching.

In Manuscript II, commercial protonated zeolites were systematically screened and tested under various reaction conditions, including temperature, reactant mole ratio, catalyst loading, reflux and continuous water removal. Zeolite catalysts demonstrated better catalytic efficiency and stability compared to HPAs. Three types of zeolites with varying framework structures-ZSM-5, Y, and BEA-were evaluated in their protonated forms (denoted as H-zeolites) at varying SiO₂/Al₂O₃ ratios. The optimal SiO₂/Al₂O₃ ratio was found to be in the range of 80-88, with HY-80 (SiO₂/Al₂O₃ =88.6) being the best performing catalyst. It achieved a 73.2% trivalerin yield under the best conditions: 140 °C, 4 wt% catalyst loading, a 6 h reaction time, 400 rpm stirring, 5:1 acid-to-glycerol feed mole ratio, and continuous water removal. The mesoporosity of zeolites and hydrophobicity associated with high SiO₂/Al₂O₃ ratios contributed to high di- and triester yields. The balance between adequate active acid sites, active site accessibility, and surface hydrophobicity rendered HY-80 highly reactive. HY-80 also exhibited only a minor loss in esterification reactivity after four consecutive esterification-cycles, with full reactivity restored upon calcination in air. The regenerated catalyst maintained structural integrity without framework destruction. This work demonstrates that protonated commercial zeolites are promising candidates for the esterification process in industrial contexts due to their superior catalytic performance, long-term stability, and easy regeneration by calcination.