Liquid phase catalytic ketonization of fatty acids with titanium dioxide

Promovendus/a: Bert Boekaerts

Promotor(en): Prof. dr. ir. Bert Sels

In our joint pursuit of facing current global warming and climate change challenges, development of sustainable chemistry and engineering using renewable energy and carbon sources is key. Besides utilization of plastic waste and CO2, chemical conversion of renewable biomass sources to provide chemicals and materials that can substitute, and ideally even outperform, our current fossil-based portfolio is one of the most important cornerstones of this societal transition. In this context, oleochemical biomass consisting of vegetable and algae oils and animal fats, either in purest form or as waste, is one of the most promising alternative (carbon) resources. These triglycerides and long-chain carboxylic acids are available on a global scale, including waste streams, and their specific chemical structure with a high C to O ratio is appropriate to serve as substrates for the synthesis of renewable hydrocarbons, more particularly here of interest being waxes and their derivatives.
For this purpose, the combination of fatty acid substrates (C12-C18) and the catalytic ketonization reaction seems an ideal fit, as this unique C-C coupling reaction converts two carboxylic acids into a long internal ketone (C23-C35), with carbon dioxide and water as by-products. Without the necessity of additives, solvents and the creation of harmful/toxic products, this valorisation conversion step has potential to be sustainable, creating bio-based long tail ketones with untapped application potential. It is therefore interesting and surprising to learn through the critical review on catalytic ketonization presented in this dissertation that the fatty acid ketonization research field, though already mentioned in the 19th century, is still small-scaled and not researched enough. A revival of this chemistry in recent years is noted due to its ability to upgrade the new coming bio-oils, which contain large quantities of short carboxylic acids, to longer carbon molecules.
To address the need for sustainable bio-based products in the current fossil wax market, a solvent-less liquid phase ketonization study was conducted in this work, using commercially available anatase TiO2 catalysts and industrial fatty acid feedstock. Favourable reaction conditions were found, achieving full substrate conversion of palm fatty acid distillate (PFAD) substrate, and others, to ketone bio-waxes on laboratory scale. Extensive product characterization showed the impact of the presence of unsaturation in the feedstock on the ketone selectivity, which can be tuned by a prior hydrogenation step. These bio-waxes were benchmarked against current paraffin standards in hydrophobization tests, revealing their competitive performance.
For the first time, these lab scale results were translated into an industrial scale design (5 kiloton) for on-purpose large scale fatty acid ketonization, including full integration of material inputs, mass and energy balances and description of required process technologies such as continuous by-product removal and catalyst separation. Green Chemistry analysis of the catalytic results revealed excellent use of the substrate carbon, reaching up to 95% carbon efficiency and 89% atom economy values. The broader environmental impact of the ketone bio-waxes was determined by a cradle-to-grave life cycle assessment (LCA), which is based on the technical analysis of the designed standalone process. Regarding the carbon footprint of PFAD based waxes, an 87% reduction can be achieved compared to paraffin wax in the best case scenario, whereas an increase of up to 16% is possible in the case of uncareful feedstock selection. More generally, the LCA impacts of the bio-waxes are governed by three hotspots, viz. biomass cultivation, catalyst fabrication and the product end-of-life faith, and not the gate-to-gate ketonization process.
To achieve a better understanding of the underlying catalysis governing the fatty acid ketonization in liquid phase when employing TiO2 catalysts, a kinetic study using anatase and rutile polymorphs was conducted. Although the intrinsic activity of the rutile phase was almost 6 times higher compared to that of anatase, it also experienced the largest negative impact of product inhibition by all three ketonization products, viz. ketone, water and carbon dioxide, due to competitive adsorption on the catalytically active surface sites. These are Ti-O Lewis acid-base pairs, more specifically coordinatively unsaturated species on the active surface, of which the rutile catalyst had the highest surface density. For both TiO2 polymorphs, the ketonization rate expression could be explained by a Langmuir Hinshelwood model, with C-C coupling of two adsorbed fatty acids at the TiO2 surface as rate determining step for this liquid phase ketonization reaction. The conversion rate is inversely related to the fatty acid chain length, showcasing the presence of steric effects and the likely importance of the Ti-to-Ti site distances at the surface of the catalyst.
Our LCA study demonstrated the significant burden of the ketonization catalyst. More active catalysts thus cannot only improve the economics, but also the environmental sustainability of the ketone bio-waxes. To this end, three post-synthetic modifications were investigated to boost the fatty acid catalytic ketonization activity of anatase TiO2. Unlike the findings of reports studying the ketonization of short-chain carboxylic acids in light of bio-oil upgrading, the addition of a metal capable of allowing hydrogen spillover effects (Ru, Pt, Ni) is not the preferred strategy for the liquid phase reaction of fatty acids. Despite the higher activity of the obtained catalysts, the ketone selectivity is reduced significantly when employing these types of materials due to unwanted decarboxylation side reaction, catalysed by the additional metal. Conversely, we observed full ketone selectivity for anatase catalysts that underwent thermal treatment in either nitrogen or hydrogen gas. For the latter, a near seven-fold increase in ketonization rate (per available specific surface area) was achieved, owing to the formation of additional Lewis acid surface sites. By varying the thermal treatment conditions, modified catalyst with different surface acid densities were obtained, revealing a quadratic relationship with the observed ketonization rate. This correlation further corroborates the hypothesis that the bimolecular C-C coupling of two adsorbed fatty acids is the rate determining step in the liquid phase ketonization mechanism.
This work thus shows new insights in the ketonization chemistry at the surface of TiO2 catalysts, that has led to better performing systems in the context of sustainable chemistry.