Heterogeneous catalysis using low cost and waste derived reagents for sustainable chemistry applications

Abstract

Concepts related to sustainability and the circular economy are more important today than ever, with focus on reducing primary extraction of resources, geopolitics and the high energy demand for primary resource exploitation and shipping [1-4]. In this thesis, sustainability and circular economy concepts are explored through the study of the ketonic decarboxylation of fatty acids and the use and characterisation of low-cost catalysts.
In Chapter 1, a review of the emergence of the need for circular economy principles, and how this couples with energy demand and climate change is given, along with an overview of the role of biomass and catalysis within a circular economy framework. A detailed breakdown of what the circular economy is and how biomass could play a crucial role to produce bio-derived added value products in the future is shown. Moreover, the chapter goes further in exploring the different products that can be obtained from biomass, such as ketones derived from fatty acids through the ketonic decarboxylation reaction. Thus, the introduction explores in detail the different mechanisms that could potentially happen during the latter ketonic decarboxylation reaction and the catalyst used to activate such reaction. A wide variety of the catalysts mentioned in the literature and the different reaction conditions in which the catalysts related to the ketonic decarboxylation reaction were used is also covered. The final part of the review explores a particular family of catalytic materials named the layered double hydroxides (LDHs), which can potentially be developed out of waste materials to deliver the diverse range of applications the latter LDHs can be applied for.
Chapter 2 explains in detail all the analytic methods used in this thesis, from the methods employed to synthesise and characterise the catalysts, the reactants through to the methods employed to quantify the latter obtained reaction products. Among the techniques used and detailed in Chapter 2 are powder X-ray diffraction, Fourier-transform infra-red spectroscopy, thermogravimetric analysis, inductively coupled plasma optical emission spectroscopy, gas chromatography, mass-spectrometry, surface area analysis and scanning electron microscopy.
In Chapter 3, MgO was used to catalyse the ketonic decarboxylation of dodecanoic acid to form 12-tricosanone at moderate temperatures (250 °C , 280 °C and 300 °C ) with low catalyst loads of 1% (w/w), 3% (w/w) and 5% (w/w) with respect to the dodecanoic acid, with a reaction time of 1 hour under batch conditions. Three different particle sizes for the MgO were tested (50 nm, 100 nm and 44 µm) and the ketone yield was found to increase with increasing reaction temperature, reaching approximately 75% yield for all the samples tested. The temperature was found to be the main control on reaction yield, rather than surface area or particle size, which we show is related to recrystallization of the catalysts during the reaction such that all particles sizes show similar crystalline domain sizes.
Chapter 4 continues to explore the use of MgO as an accessible and highly basic catalyst to catalyse a more complex reaction mixture, the cross-ketonisation reaction of different carboxylic acids (i.e. having different carbon chain lengths, unsaturated acids, lacking α-hydrogens). This allows the study of the effect of catalyst-substrate and substrate-substrate interactions in a given solvent system. The reactions were carried out at 300 ˚C for 1 hour, using a 5% (w/w) of MgO with respect to the carboxylic acid mixtures. The carboxylic acids lacking α-hydrogens did not react to form ketones, whereas the saturated carboxylic acids favoured the formation of symmetrical and asymmetrical ketone products, in clean post-reaction mixtures. When carboxylic acids containing unsaturated C-C bonds were used, a Diels-Alder reaction seemed to be outperforming the ketonic decarboxylation reaction, favouring the formation of a variety of different products which were not fully identified.
Starting to explore the concept of circular economy, in Chapter 5, Mg/Fe layered double hydroxide (LDH) materials, were successfully synthesised via the co-precipitation method from a dissolved acid mine drainage precipitate waste mineral assemblage, known as “ochre”, which is normally put to landfill. The prepared LDH materials were tested as a heterogeneous base catalyst to promote the production of the ketone, 12-tricosanone, from lauric acid, a component of plant oils, through a ketonic decarboxylation reaction, giving a circular economy route to catalysts for sustainable ketone production. The highest yield of the ketone observed was 80%. In order to examine other sustainable uses, an energy application was also considered in Chapter 6, where a Mg-Fe layered double hydroxide material (LDH) of ratio 4 was synthesized from the acid mine drainage waste “ochre”, which otherwise would end as landfill. The prepared ochre LDH was fully characterised and compared against an LDH made of synthetic salts as an active photocatalyst for the hydrogen evolution reaction, producing up to 1272 µmol.h-1.g-1 of hydrogen.

Chapter 7 concludes ideas explored in Chapter 5 and Chapter 6, where a LDH catalyst was prepared from ochre, by preparing a material wholly out of waste. Using a waste material obtained from water treatment plants (i.e. struvite, a Mg mineral) and the Fe containing mining waste ochre, LDHs of Mg/Fe ratio 2 and ratio 4 were synthesised. Although the materials were not further tested as catalysts or adsorbents, obtaining a material made wholly of waste was successful and the latter material was fully characterised.
Finally, Chapter 8 summarises the key findings from this study. Among some of the achievements, it was concluded that the crystallite size might be playing a major role in the ketonic decarboxylation reaction of dodecanoic acid in conjunction with the temperature parameter, with the surface area and the active site availability having less impact on the product yield. Moreover, it was also observed that the presence of unsaturation within the carbon chain of carboxylic acids might influence the Diels-Alder reaction to outperform ketonic decarboxylation reactions, however, as the literature is not fully clear on that process, more experimental work was needed to give stronger conclusions. Finally, exploring the concept of circular economy yielded the production of different LDH materials, partially and wholly made of waste, producing the first ochre-struvite derived LDH material, which can potentially be used in a wide variety of applications. The partial waste LDH was successfully used for catalytic applications such as the production of ketones from carboxylic acids and the production of hydrogen through the hydrogen evolution reaction. Moreover, the future work section discusses ways in which the gaps from this work could be addressed through future work from other members of the group.