Zirconia-based acid catalysts for biomass conversion
Zirconia-based acid catalysts have received vastly attention as both catalyst itself and catalyst support owing to its favorable physicochemical properties, and they have been widely applied in catalytic industry. Biomass conversion, such as glucose to 5-hydroxymethylfurfural (HMF) transformation, is of high value in the sustainable manufacturing of green chemicals and it is well known as a viable way to deal with the shortage of fossil fuels and control the CO2 emission by its consumption. The production of ethyl mandelate (EM) from phenylglyoxal (PG) is also attractive since it is a stereochemical mandelic derivate that has been widely applied in pharmaceutical and chemical industries. This thesis presents the studies of zirconia-based acid catalysts and their application in glucose to HMF and PG to EM conversions. First, ZrO2 with different crystalline structures and various surface catalytic properties was fabricated via tuning the thermal treatment. ZrO2 calcined at 300 °C which is in an amorphous state possesses more Lewis (LAS) and Brønsted acid sites (BAS) than ZrO2 samples calcined at other temperatures, thus presenting a better catalytic performance in glucose conversion. Second, WOx/ZrO2 with high accumulated BAS was synthesized through a single-step flame spray pyrolysis (FSP) method. FSP-made WOx/ZrO2 possesses ~80 % of BAS and a BAS to LAS ratio (B/L) of ~4, while the corresponding material prepared by conventional impregnation method exhibits ~50 % of BAS and a B/L ratio of ~1. FSP-made WOx/ZrO2 presents outstanding performance in glucose conversion expressed as its prominent HMF selectivity. Third, layered mesoporous ZrO2/SiO2 nanospheres (ZrO2/SiO2 MNS) with tunable surface acid sites are designed. Z/SZ, with ZrO2 in the outer layer to produce LAS that triggers the glucose isomerization to fructose, and ZrO2/SiO2 in the inner layer to generate BAS that promotes the fructose conversion to HMF, is considered as the ideal structure. Z/SZ can dramatically shorten the induction period within 15 min which is distinctly shorter than that of the other structured ZrO2/SiO2 MNS catalysts (more than 45 min), with over two times higher TOF. Fourth, hierarchical pore structured ZrO2/SiO2 core-shell mesoporous nanospheres (ZrO2/SiO2 CS-MNS) with different pore size distributions are designed for PG to EM conversion. Though hierarchical structures of CS-MNS-2-10Zr (larger shell than the core) and CS-MNS-3-10Zr (larger core than the shell) possess lower LAS and BAS than CS-MNS-1-10Zr (well-defined pore with uniform pore size), they can achieve a better catalytic performance in PG conversion, with over two times higher TOF. In summary, this work provides different modification methods such as tuning thermal treatment, loading metals on zirconia, developing different layer constructions and different hierarchical structures to synthesize a variety of catalytically active complex based on zirconia. It achieves many purposes including modifying the crystalline structures, adjusting the acidity features, and tuning pore size distributions in zirconia-based catalysts. As a result, it can provide different views to comprehensively understand the catalytic performance of catalysts in glucose to HMF and PG to EM conversions, offering key knowledge for enhancing the efficiency of the catalysts in biomass conversion in the future.