Design and synthesis of nano-catalysts for catalytic transformation of biomass derivatives

The energy crisis is a major challenge for the 21st century due to fossil fuel depletion. Biomass, the waste material from plants or animals containing renewable and abundant carbon resources, is considered as an ideal alternative for fossil fuels. However, the biomass utilization is limited by the lack of cost-effective technology for biomass valorization. Therefore, the objectives of this dissertation are to rationally design novel nanocatalysts to selectively convert biomass-derived carbohydrates into value-added chemicals and fuels and to understand the catalyst structure-property relationship and the underlying reaction mechanisms.

5-hydroxymethylfurfural (HMF) derived from biomass is considered as a renewable chemical platform to produce liquid fuels and fine chemicals, which could be produced from glucose, the main unit of cellulose, via Lewis and Brønsted acid sites (LAS and BAS). In Chapter 3, the stable solid acid catalysts with Lewis-Brønsted acidity were designed for efficient catalytic conversion of glucose to HMF. A series of metal (Fe3+, Cu2+, Zn2+ and Sn4+) exchanged Keggin-structure phosphotungstic acids (MPW), were confined in mesopores of siliceous MCM-41 for efficient glucose conversion. The selectivity to HMF decreases with the reducing Lewis acid strength of metal ions in the order: Sn4+> Fe3+> Zn2+> Cu2+. The LAS dominated Sn(Ⅳ)PW/MCM provides twice higher HMF selectivity than that obtained with HPW/MCM containing mainly BAS. Increasing the density and strength of LAS on Sn(Ⅳ)PW/MCM catalyst by elevating pretreatment temperature promote the glucose conversion and HMF selectivity.

Moreover, the confinement effect of mesoporous MCM-41 affords high resistance of leaching and dissolving for Sn(Ⅳ)PW in polar dimethyl sulfoxide (DMSO) solvent, therefore resulting in an outstanding stability of Sn(Ⅳ)PW/MCM catalyst.  

The production of another kind of important platform chemicals in biomass utilization, α-hydroxycarboxylic acids or their derivatives, from α-keto aldehydes conversion using solid acids was studied and addressed in Chapter 4.The ordered mesoporous zirconium oxophosphate (ZrPO) catalysts with tunable acidity were prepared using one-pot evaporation-induced self-assembly (EISA) method and their catalytic performance in α-hydroxycarboxylic acid production was evaluated using phenylglyoxal (PG) conversion to ethyl mandelate (EM) as a model reaction. The roles of LAS and BAS of ZrPO were investigated by kinetic studies of PG conversion combined with temperature-programmed desorption of ammonia (NH3-TPD) and solid-state nuclear magnetic resonance (NMR) characterizations. It is found that the ratio of LAS to strong BAS on the ZrPO plays a dominant role in this reaction. ZrPO-0.75-500, with a LAS/strong BAS ratio of 2.1, is found to be the best catalyst. The reaction pathways, i.e., direct isomerization of PG to EM by LAS via the formation of an intermediate  hemiacetal by strong BAS, are therefore proposed. Moreover, the effect of water on catalytic activity was studied. A moderate content of water-induced either by catalyst pre-treatment at the proper temperature or deliberately dosed on the ZrPO materials achieved a maximum catalytic activity. The highest catalytic activity, i.e., 82 % of PG conversion and 92 % of EM yield, was obtained on ZrPO-0.75-500 dosed by 15 μmol water per 50 mg of the catalyst. It is concluded that ZrPO with a suitable combination of LAS and BAS is required to catalyze the conversion of α-keto aldehydes efficiently and selectively to α- hydroxycarboxylic acid derivatives. It is feasible, from a practical point of view, to tune the density of LAS and BAS on the catalysts for achieving a better catalytic performance.

For a better and cleaner environment, scientists have turned to make use of solar energy to satisfy the demands of modern society aiming at reaching the dual Holy Grail of energy: harvesting solar energy via artificial photosynthesis and achieving lowtemperature biomass conversion. The combination of biomass utilization and the green and cost-effective photocatalysis is considered as a promising approach to achieve sustainable chemical transformation processes. In Chapters 5 and 6, photocatalytic process under sunlight irradiation is applied instead of the thermal catalytic process for the oxidation of biomass-derived alcohols.  

In Chapter 5, platinum (Pt) and bismuth (Bi) bimetallic modified mesoporous titanium dioxide (TiO2) was successfully prepared via EISA synthetic method. It was used as a highly active and selective catalyst for the photooxidation of alcohols using O2 as oxidant. Remarkably, the Pt single sites and Bi promoter co-modified TiO2 catalyst exhibited a good activity on benzyl alcohol oxidation, giving a turnover frequency (TOF) of 372.2 h-1, about 1.2 and 3.9 times higher than that of Pt subnano clusters (300 h-1) and nanoparticles (95.9 h-1) stabilized catalysts. The selectivity to benzaldehyde was up to 100% for all catalysts. The superior performance can be attributed to the strong interaction between Pt single sites, promoter Bi and mesoporous TiO2, which favors the formation of a high density of positively charged Pt even at a high reduction temperature. Positively charged Pt could play an important role in promoting the charge carrier transition and separation as well as improving the absorption of visible light. The optimized catalyst can perform various light-driven alcohol oxidation leading to the selective formation of aldehyde. 

Considering the limited practical application of TiO2 due to low quantum efficiency and large bandgap, various novel visible light-responsive semiconductor photocatalysts have been developed. The naturally abundant and non-toxic bismuth-based oxide (BiOX, X=Cl, Br, I, W or Mo), has emerged as a potential visible-light-driven photocatalyst in various photocatalytic reactions due to its narrow gap (~2.8 eV) and better visible light absorption (λ > 400 nm). In Chapter 6, various BiOW composites with different morphologies and Bi/W ratios were synthesized via a facile hydrothermal precipitation process and applied in photocatalytic benzyl oxidation to produce benzaldehyde. The systematic research has explored how the morphologies of Bi2WO6 and the components of  BiOW composites affect the photocatalytic activity. The UV/Vis absorption spectra and photoelectrochemical performance showed that the bandgap of BiOW composites is mainly governed by the components of catalysts. However, the efficiency in the separation and transfer of photo-generated electron-hole pairs is dominated by the morphology of the nanostructure. As a result, the photocatalytic activities of BiOW composites are strongly dependent on their morphologies and structures. It is found that the flower-like Bi2WO6 exhibited superior performance in benzyl alcohol oxidation to benzaldehyde, achieving ca. 2.2- and 2.7-times higher activity compared to Bi2WO6 in nanosheet and nanoplate structure. In contrast, the Birich Bi2WO6/Bi3.84O6.24W0.16 composite and Bi2O3 in block-like structure were almost inactive in this reaction. Besides, the light absorption and the separation and transfer of photogenerated charge carriers could be further enhanced via loading of co-catalyst Pt nanoparticles on the surface of the flower-like Bi2WO6, thus resulting in highly efficient photocatalytic oxidation. This study provides structural optimization strategies for bismuth tungstate photocatalysts which can be applied in fine chemicals synthesis.