Catalytic processes with potential in lignin biorefining

The research in this thesis relates to the preparation and characterization of different families of catalysts and processes aimed at reaction of lignin model compounds via hydrogenolysis or oxidative cleavage. Two reactivities were investigated, thermal catalysis and photocatalysis. Attempts were made to correlate catalyst activities with physico-chemical properties. The concept of biorefining and a discussion regarding the need to improve sustainability in the chemical industry are presented in chapter 1. The synthesis protocols and the characterization techniques used in this work are presented in chapter 2. In chapters 3 and 4, the properties and reactivities of different sets of H-ZSM-5-based catalysts in the hydrogenolysis of benzyl phenyl ether are discussed. Unmodified zeolite catalysts with different acidities were studied first and high loading monometallic (Ni, Cu and Co) H-ZSM-5 catalysts secondly. The difference in acidity was associated with different substrate activation levels. The metal-loaded materials promoted the reaction more efficiently by promoting dissociation of H2 molecules. Ni-ZSM-5 materials with different loadings were then investigated under different reaction conditions. Higher T and/or P led to higher conversions, with no effect on reaction selectivity. Isopropanol was shown to act as a H-transfer solvent itself promoting the hydrogenolysis reaction. Higher substrate conversion was obtained over a 10 % Ni/ZSM-5 catalyst than over a 20 % Ni/ZSM-5 analogue. This was attributed to the formation of Ni nanoparticles with different morphologies and different exposed facets over both materials. In chapter 5, two sets of Ni-Cu/ZSM-5 bimetallic alloy catalysts with different Ni:Cu ratios and total nominal loadings were prepared. Different dispersions of the Ni and Cu atoms within the nanoparticles had an effect on the materials’ reactivity for the reaction with a higher dispersion of Ni atoms leading to higher conversion. The hydrogenolysis of other model compounds was studied and the presence of substituent groups in the reactants containing a β-O-4 moiety had a direct effect on conversion. Another set of materials was prepared with a fixed Ni:Cu content but prepared at different reduction temperatures. It was shown that this parameter has an effect on the materials’ reactivity for the conversion of benzyl phenyl ether with preparation conditions yielding NPs with relatively more dispersed Ni atoms being optimal. The photocatalytic conversion of 2-phenoxy-1-phenylethanol to benzaldehyde and phenyl formate under an O2 atmosphere over g-C3N4-based catalysts was investigated in chapters 6 and 7. Bulk and nanosheet materials were prepared using different carbon nitride precursors. Acetonitrile was the optimal solvent. Parallel reactions forming acetals occurred when hole scavenging solvents were used. The nanosheet materials were more reactive than their bulk counterparts due to their larger SSA and different structures. For the bulk equivalents, the generation of superoxide species was the reaction limiting step whilst for the nanosheet materials the presence of different levels of nitrogen vacancies was more important. Plasmonically active Ag/g-C3N4 materials were studied. Low substrate conversion and high selectivities for the compounds of interest were obtained. WO3/g-C3N4 direct-Z-scheme photocatalysts were highly efficient in the promotion of the reaction. The production of acetals in the presence of a hole scavenger solvent was further studied in chapter 8 over a direct-Z-scheme photocatalyst. A screening of both solvents and substrates was performed. It was shown that different acetal compounds can be prepared selectively using this method.