The proposed bifunctional catalysts for efficient one-pot transformation of biomass in a aqueous solution to ethanol would contain two types of catalytic sites: Lewis acid sites and metallic sites. Tungstic acid is selected to be as the Lewis acid site and Nickel or cobalt is initially selected to be the metal sites to catalyze the hydrolysis and hydrogenation reactions, respectively, and zeolite imidazole framework (ZIF) is the selected MOF to initially based on. The development of such bifunctional catalysts was carried out experimentally in laboratories following the flowing sequence: 1) synthesis and characterisation of ZIF containing Zn, Ni, or Co, 2) synthesis and characterisation of tungstic acid hybrid ZIF (HZIF), 3) test of the catalysts synthesized, 4) model the catalysis using density function theory (Secondment at Leibniz-Institut für Katalyse e.V. (LIKAT Rostock), Germany) and 4) explore the potential to use spouted reactors for such reactions (Secondment at the Country University of Basque, Spain).
Following the proposed research, the first phase of the research was focused on the synthesis of the ZIF and HZIF interested. The salts of Zn, Co, Ni have been systematically applied in the synthesis of ZIFs. The synthesis of Zn and Co-containing ZIF was very successful and beyond of state-of-the-art methods were developed and optimised by using water as solvent at ambient conditions rather than using organic solvents at elevated temperatures and pressures for possible commercial green production. The structure-catalysis/adsorption relationship of the ZIF samples have been examined.
However, Ni-containing ZIF is hard to synthesise due to its electron orbital favour the octahedron structure formation rather than tetrahedron one, which is a new finding to the best of our knowledge. To encourage the incorporation of Ni in the framework, doping methods have been explored into Zn and Co-ZIFs.
Tungstic acid was then incorporated in the synthesis of Zn-ZIF, Co-ZIF, Ni/Zn-ZIF, Ni/Co-ZIF for WO4-HZIFs. The tungstic acid incorporation was carried out at varied composition, imidazole, solvent and reaction temperature and time. The x-ray diffraction and crystal structure analysis show that WO4-Zn-HZIF framework successfully formed. This sample was tested for its catalysis in the hydrolysis by using glucose as the model compound. The test in the aqueous solution showed the formation of water insoluble humins, which is expected as the hydrogenation catalysis is not present. However, the framework of WO4-Zn-HZIF collapsed totally. Repeated tests showed the same results. This finding shows that the WO4-Zn-HZIF framework is not so stable as Zn-ZIF in the hydrothermal conditions, not suitable to be used as catalysts for the hydrothermal reactions.
To move the research forward, we introduced Co-MOF-74 to test the hydrogenation catalysis activity of cobalt nanoparticles from MOFs in the biomass to ethanol conversion. As the hydrogenation of ethylene glycol to ethanol is the rate-limiting step for transforming cellulose to ethanol by catalysis of metal-acid bifunctional catalyst, the cobalt nanoparticle prepared from Co-MOF-74 was tested by this reaction. The MOF-derived cobalt nanoparticles by pyrolysis showed impressive catalysis activity in the hydrogenation reaction of ethylene glycol. The reaction kinetic study at different reaction temperatures gives an apparent activation energy of around 80 kJ/mol via Arrhenius equation. Both the cobalt catalysis and prepared from MOF are new in the research area covered by this project.
The Co-MOF-74 derived metallic Co catalyst may be close packed in face-centred cubic (fcc) structure and/or hexagonal close packing (hcp) structure. It is unclear which Co structure is more active for the hydrogenation of ethylene glycol. To answer this question, the Fellow did density function theory modelling during her secondment at LIKAT Rostock, Germany. The modelling results disclosed that the activity difference of the hcp and fcc Co structure in the hydrogenation reaction of ethylene glycol, as well as the catalytic reaction mechanism: the hcp Co have a much higher hydrogenation activity than the fcc Co. This is a new finding for the catalysis of ethylene glycol hydrogenation.
Towards the end of the project, the Fellow visited the Country University of Basque, Spain, and examined the feasibility of spouted reactor for the one pot conversion of biomass to ethanol.
The Fellow has drafted two papers based on the new findings for publication: 1) Control of the crystallinity and defects in ZIF-8 by ammonia-mediated aqueous synthesis for efficient catalysis applications as exemplified by the Knoevenagel condensation reaction; 2) The hydrogenation catalytic activity of MOF-derived cobalt nanoparticles in the conversion of ethylene glycol to ethanol: experimental study and DFT modelling.
