Metal-Organic Frameworks (MOFs) Immobilised Organocatalysis for Process Intensification: Catalyst Development and Evaluation

Abstract

Abstract

Demand for mono-enantiomerically pure chiral compounds has increased in the last several decades. Asymmetric catalysis is the method for preparing those compounds, which usually use homogeneous or organometallic catalysis. However, those catalysts generate more chemicals, waste, low atom economy, difficult separation, and less energy saving, creating a new challenge. On the other hand, using heterogeneous catalysts is one of the synthetic process technologies that, in many ways, reduce production costs and help ecological protection. To allow the heterogeneous catalyst use on the industrial scale, it is necessary to be observed its role in the intensified reactor, such as in the Spinning Mesh Disc Reactor (SMDR).

Therefore, this research aims to develop, characterise and evaluate a heterogeneous catalyst that meets the above criteria by synthesising heterogeneous catalysts derived from homogeneous catalysts through organocatalysis immobilisation onto MOF and wool fabric. Furthermore, the new heterogeneous catalyst was characterised analytically to certify its reliability as a catalyst material. Finally, the catalyst’s performance must be evaluated in an intensified reactor, SMDR, which allows the immobilised catalyst to be used to produce chiral C-C compounds through an aldol addition reaction. Three ongoing projects have been completed. Immobilisation of L-proline onto MOFs, woollen fabric and the combination of the two support materials, immobilisation on wool and MOFs during the growth of MOFs on the wool surface. Heterogeneous catalysts that have been developed include Zr-MOFs-S30Pr, Wool-Pr and Wool-Zr-MOF-S30Pr.

Strategies to immobilise the L-proline within the metal-organic frameworks and woollen fabric have been investigated. In the functionalisation of L-proline in MOF (Chapter-4), the immobilisation method was developed using a solvothermal scheme modulated by acetic acid and peptide compound (L-proline). Mixed-linkers of sulfonated-bdc and bdc ligands combine intermolecular Zr nodes to construct the MOFS framework. The solvothermal method is carried out by heating a mixture of DMF (solvent) with an appropriate amount of L-proline, a mix of s-bdc and bdc (3:7) and acetic acid at 120 C for 40 hours. The crystalline powder was characterised using 1H NMR, TGA and leaching test, PXRD, N2 physisorption, SEM/EDX, XPS and FTIR. Good Zr-MOF-S30Pr chemical and thermal stability were obtained with a high BET surface area (975 m2/g). The evaluation of the catalytic parameters showed that the catalyst’s performance was good, with 4NBA conversion of up to 83%, a yield of 81% and ee up to 86% in the spinning disc reactor (SDR) and moderate in the conventional reactor (73%, 69% and 80% for conversion, yield and ee respectively). Therefore, the Zr-MOF-S30Pr catalyst can be recycled three times in the conventional and spinning disc reactor. However, unexpected results (in SDR) were exhibited after three running courses; no transformation occurred due to the catalyst agglomeration along the peristaltic tube and encrusted on the disc surface. Therefore, it is necessary to find another support material compatible with the spinning disc glass topology. Therefore wool is the next choice for L-proline functionalisation.

Furthermore, immobilisation of L-proline was carried out onto woollen fabric by reduction and amination method using a reducing and coupling agent to link the active centre of L-proline with wool surface’s reactive groups (Chapter-5). The material characterisation showed that L-proline was well integrated onto wool, resulting in excellent stability. Furthermore, FTIR and XPS spectroscopy presented covalent hydrogen bonds which connected L-proline and wool. After being applied to conventional and spinning disc reactors, it was demonstrated that conversion, yield and selectivity in SMDR were higher than in conventional reactors. Furthermore, the reaction time reaching equilibrium for complete transformation is shorter in the spinning mesh disc reactor than in the traditional reactor. The conversion of 4NBA and the ee of the Wool-Pr catalyst reached 84% and 89%, with a yield of 80% in SMDR. Meanwhile, 64% of conversion and 67% of ee, with a yield of 53%, are from the conventional reactor. The higher catalyst activity in the SMDR compared to traditional reactors and shorter time for complete transformation indicates that process intensification occurred in the SMDR. This kinetic improvement in SMDR is due to rotation and mechanical assistance adjusting the spinning disc speed and feed flow rate. Controlling the spinning disc speed and feed flow rate to be optimum generates centrifugal forces to create sheared liquid film over the disc. Shear stress between liquid film and the catalyst disc increases micro mixing, enhancing mass and heat transfer. The superiority of the spinning mesh disc in intensifying the aldol reaction to form C-C bond chiral compounds also positively impacts the novel catalyst recycling ability. As a result, the Wool-Pr heterogeneous reactivity decreased after five runs in a conventional reactor, while in SMDR, it remained stable.

Organocatalyst functionalisation was also developed by growing MOF during organocatalyst immobilisation onto woollen fabrics and modulating MOF growth with 4-hydroxyproline at room temperature (Chapter-6). The new Wool-Zr-MOF-S30Pr catalyst has excellent chemical and thermal stability with more active sites. The leaching test and TGA results show no loose catalyst component and are stable up to 500 C. The catalyst performance evaluation carried out in the SMDR and conventional reactors showed the Wool-Zr-MOF-S30Pr catalyst performances in SMDR were higher than in the traditional reactor. Conversion in SMDR reaches 87% and in a conventional reactor up to 68%, with 83% yield in SMDR and 59% in a conventional reactor. The selectivity in SMDR is also superior to that in traditional reactors, namely 89% and 75%, respectively. The outstanding performance of the Wool-Zr-MOF-S30Pr catalyst in SMDR than in batch reactor is due to the control of the spinning mesh disc speed and feed flow rate, which led to the mass and heat transfer enhancement as a primary objective of Process Intensification. Due to the occurrence of PI, the efficiency of aldol reaction time is significantly affected. The time required by the catalyst for complete conversion is only 60 min in SMDR. It takes 1400 min to reach stoichiometric equilibrium in conventional reactors until a complete transformation is achieved. Another impact of the aldol reaction PI in the SMDR is the different catalyst recyclability between the SMDR and the conventional rector. Up to seven runs, the catalyst is still stable in the SMDR; on the contrary, the activity and selectivity gradually decrease towards the seventh recycling stage in the batch reactor. The catalyst’s ability to be separated quickly, simply and reused leads to savings in chemicals and solvents. In addition, it reduces chemical waste that may be harmful to the environment. It means decreasing production costs (economic aspect) and improving environmental safety (green technology). Therefore, the goal of the process intensification is achieved, thus enabling it to be applied on an industrial scale to meet the production requirements of intermediate organic compounds, especially monostereoisomeric chiral compounds, that are extensively essential in the pharmaceutical and fine chemical industry.

Date of Award	12 Dec 2022
Original language	English
Awarding Institution	University of Bath
Supervisor	Emma Emanuelsson Patterson (Supervisor) & Andrew Burrows (Supervisor)