Abstract
This PhD research serves to implement both known and novel catalytic systems for the purpose of cyclic carbonate synthesis from biomass-derived substrates. Such products have been earmarked as potential monomers for non-isocyanate polyurethanes (NIPUs), amongst other uses. Particular attention has been placed on operating under the guidance of the 12
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principles of Green Chemistry as posited by Anastas et al. Following an in depth literature discussion of catalytic cyclic carbonate synthesis in Chapter 1, the use of Mg-Al hydrotalcites as heterogeneous base catalysts for the synthesis of diglycerol dicarbonate from diglycerol and dimethyl carbonate is described in Chapter 2. Both self-synthesised and commercial material of varying Mg-Al ratios are characterised and tested, with a number of them selected to showcase the reusability of such a system. Chapters 3 and 4 concentrate on using a CO2-masked N-heterocyclic carbene (NHC) system based on substituted imidazole for the synthesis cyclic carbonates from a number of different biomass-derived vicinal diol substrates, including crude glycerol. Chapter 3 demonstrates the homogeneous version of this system, exploring the effect of ring substitution on the catalyst’s activity. The use of 13C-labelled dimethyl carbonate to identify the key catalytic species also led to a proposed mechanism being postulated. Chapter 4 extends the work on this system by heterogenising the NHC on siliceous mesostructured cellular foam. A different protecting group/mask, (H)HCO3, compared to the homogeneous system is required to allow for facile handling and subsequent reuse of the catalyst. If properly treated after reaction by a simple ion exchange procedure, , the original activity can be maintained. Chapter 5 investigates the one-pot synthesis of cyclic carbonates from fatty acid methyl esters (FAME), namely methyl oleate. This process involves two sequential reactions, firstly an epoxidation step, followed by a subsequent carboxylation to yield the desired cyclic carbonate. For each step, known catalysts were applied; alumina was utilised as the epoxidation catalyst and tetraalkyl ammonium halides as the carboxylation catalyst. Self-synthesised alumina was optimised for methyl oleate epoxidation, having previously been shown to be an efficient system for cis-cyclooctene conversion, by varying the water-to-aluminium ratio during the synthesis procedure. The presence of the carboxylation catalyst was revealed to have a detrimental effect on the first step, inhibiting the epoxidation catalyst, depending on the counter ion selected. Consequently, a one-pot two-step protocol was developed in order to yield carbonated methyl oleate, although this has yet to be fully optimised. Chapter 6 summarises the main findings described in this PhD thesis. In addition, perspectives for the future of this field of chemistry are given, as well as some concluding remarks regarding the strides made towards enhancing the green credentials of the routes for the production of cyclic carbonates. The findings of this thesis are put in the wider context of the literature, by making a comparison of some key criteria.
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