Mineral Mediated Catalysis of Fatty Acids

Abstract

In order to reduce reliance on fossil oil, and it’s associated problems, there is a need to develop new platform chemicals, fuels and products, in a sustainable way, from biomass. In this thesis the catalytic upgrading of fatty acids, derived from the lipid fraction of biomass, through deoxygenation reactions is studied.

Chapter 1 reviews the general area of catalytic upgrading of biomass into biofuels and bioproducts. The history and motivation for alternative sources of fuels and materials are introduced, followed by a summary of the reaction processes currently utilised for biofuels and bioproducts production. It is shown that the demand for alternative fuel sources initially led to the mass commercial production of ethanol, via fermentation of sugars, and biodiesel, through trans-esterification of lipids present in vegetable and algal oils. A selection of the catalysts and mechanisms for trans-esterification reactions is reviewed and, following an outline of the fuel properties and processes, an evaluation of “green diesel” production is given, whereby fatty acids are converted directly into hydrocarbons through decarboxylation reactions. This review culminates in an analysis of alternative conversion of fatty acids into long chain ketone bioproducts, namely ketonic decarboxylation, which details the catalysts and processes involved to date.

Chapter 2 describes the analytical methods utilised in this thesis to investigate heterogeneous catalysis of biomass conversion, along with the relevant background theory and describes the type of data that can be obtained using the techniques. The techniques include powder X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, inductively coupled plasma optical emission spectroscopy, surface area analysis, Hammett basicity, elemental analysis, Fourier-transform infra-red spectroscopy and gas chromatography.

Chapter 3 introduces a class of materials known as layered double hydroxides (LDHs) of general formula Mz+1–xM3+x (OH)2]q+(Xn–)q/n•yH2O. The ease with which these mixed oxide materials may be prepared offers significant scope for the variation of the metal cations; the M2+:M3+ ratio (denoted R-value); the counter anion(s); and crystal morphology. These LDHs consist of positively charged layers, with negatively charged counter-anions and water residing in the interlayer. A review of the commonly used synthesis methods for LDHs is given, along with the advantages and disadvantages associated with each method. Following this, the synthesis of the Mg-Al LDHs and their calcined counterparts, mixed metal oxides (MMOs) (for R-values 1-6) via a readily scalable co-precipitation (CoP) and a more environmentally-friendly co-hydration (CoH) route is described. A range of techniques, outlined in chapter 2, are utilised to study the LDH and MMO crystal and chemical structures, surface topography, surface area, pore volume and relative basicities. The crystal structures of two of the CoP-LDHs were refined to the 3R-polytype using DICVOL, however the other LDHs were not significantly ordered and could not be refined. Upon calcination from LDHs to MMOs, the interlayer counter-anions and water are lost, along with the layered structure, leading to a commensurate increase in surface area and pore volume.

In chapter 4, investigations undertaken to deoxygenate stearic acid, a free fatty acid model biomass compound, are described. As a catalyst, 5 % Pd/C was used, adapting a method found in the literature. These reactions were undertaken at 230 °C, with decarboxylation of stearic acid producing straight-chain n-heptadecane at up to 58 % conversion by gas chromatography analysis. However, in this study, issues arose due to catalyst instability and an ensuing loss of catalyst activity was observed.

In chapters 5 and 6, to increase catalyst stability and recyclability, while also reducing costs relative to the Pd/C catalyst used in chapter 4, (due to the use of precious metals), LDHs and their calcined derivatives, MMOs, were utilised for deoxygenation of the model stearic acid biomass. Thermal reactions of stearic acid controls, without catalyst, were not observed to occur, however, stearic acid conversions between 83-97 % at 250 °C were observed to occur with both LDH and MMO catalysts. However, unlike the Pd/C reaction, no decarboxylated product was evidenced and, instead, a waxy solid formed, which was subsequently analysed and found to be the ketonic decarboxylation product, stearone. A protocol was developed to separate the stearone from the catalyst. Gas chromatography analysis showed the LDH and MMO materials catalysed the conversion of stearic acid to a similar degree, allowing for the error within the extraction and analysis processes employed. The reasons for similarity in reactivity are discussed, and it is suggested that an intermediate state of catalyst is present in the reactor.
Comparing synthesis methods, the CoH materials were as effective as their CoP counterparts, despite the presence of Mg(OH)2 secondary-phases. Within the LDH phases, an indication of catalytic dependence on pore size was also recorded, with the smaller pore-sized materials leading to lower conversions of stearic acid, resulting from the bulky size and required head alignment of the long-chain fatty acid molecules. In terms of control reactions, calcined MgO led to 90 % conversion of stearic acid to stearone, however very little reaction occurred with uncalcined MgO (0.5 %) and zero reaction with the acidic Al2O3 (both uncalcined and calcined). Hence activated MgO is also an effective catalyst for ketonic decarboxylation.
Chapter 7 summarises the results and discussion given within this thesis, highlighting the milestones achieved such as the first ketonic decarboxylation reactions involving MMO catalysts and concluding that LDHs and MMOs catalyse the conversion of stearic acid via ketonic decarboxylation of free fatty acids to high value ketones, to a similar degree, within the associated errors. In addition the LDH synthesis method employed does not play a significant role in the degree of catalysis, resulting in the recommendation that the more environmentally-friendly co-hydration synthesis method should be employed for the catalyst involved in the conversion of stearic acid derived from biomass. The MMO catalysts were found to behave akin to the calcined MgO material during the ketonic decarboxylation of stearic acid, while the catalytic reactions involving LDH catalysts were potentially involving the interaction of their interlayer anions and cations. Finally, a summary of additional work for further developing this process is discussed.