Metal-uronide interactions and their relevance to the thermolysis of kelp

Abstract

This thesis describes investigations into the coordination- and thermo-chemistry of metal-uronide complexes (namely alginates). The work probes the importance of such metal-saccharide interactions in the thermolysis of kelp, with a view to deriving valuable fuels and chemicals from this aquatic bioresource. In this regard, Chapter 1 justifies the model-compound approach adopted in this thesis, and outlines the wider context in which the work is set.

Chapter 2 describes the isolation of the composite monosaccharides of alginate (D-mannuronate and L-guluronate) and their characterisation by NMR spectroscopy. A combination of 1- and 2-D 1H and 13C experiments were utilised to provide the most comprehensive assignments of algal mono-uronates to-date. Subsequently, the well-defined mono-uronate spectra were used to probe the conditions that favour hydrothermal uronolactone formation.

Chapter 3 probed the response of the algal mono-saccharides (prepared and characterised in Chapter 2) to a range of metal ions (Na+, K+, Mg2+, Ca2+, Sr2+, Ba2+, Zn2+, and Cu2+) to explore the validity of the well-known “Egg-box model” of metal ion/alginate coordination. By observing changes to anomeric equilibria, α-L-gulopyranuronate was found to coordinate (via an axial-equatorial-axial arrangement of hydroxyl groups) to large, divalent cations, in a manner consistent with “Egg-box binding”. In contrast, in the presence of Na+, Mg2+, Zn2+, and, Cu2+ α-L-gulopyranuronate interacted via its carboxylate moiety (and possibly ring oxygen), demonstrating the unsuitability of these ions for Egg-box binding.

Chapter 4 describes the impact of the metal ions discussed in Chapter 3 on the subsequent pyrolysis behaviour of alginates (and related mono- and poly-uronides). Thermogravimetric analysis (TGA), pyrolysis-gas chromatography mass spectrometry (Py-GCMS), and solid-state studies of the post-thermolysis chars were all conducted. The uronides were generally found to demonstrate unfavourable thermal behaviour (high yields of char, CO2, and H2O, and low yields of condensable hydrocarbons). Complexation of Cu2+, however, had a beneficial impact on subsequent thermolysis, by increasing the low-temperature, selective formation of 2-furfural. The presence of Cu0 in the alginate char is indicative of Cu(II)-mediated alginate decomposition occurring via a Hofer-Moest type decarboxylation.

Chapter 5 tested the validity of the model compound approach by enriching samples of kelp with either Cu2+ or Ca2+ ions and studying the thermal degradation of the resulting materials by TGA and Py-GCMS. The thermochemical outcomes for the whole biomass mirrored those found for the model compounds studied in Chapter 4.

Finally, in Chapter 6, the results of Chapter 2 – 5 are analysed synoptically, and the success of the model compound approach is appraised. Ultimately, it is concluded that the ability of a metal ion to inhibit or promote thermal decarboxylation of a uronide (in isolation or within kelp) is more important in dictating pyrolysis behaviour than any differences in coordination to various hydroxyl groups around the saccharide ring. The results could find application in the development of a phytoremediative kelp-based thermal biorefinery.