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catalysis --- Catalytic activity --- plant oils --- Hydrogenation. --- Hydrogenation --- Linoleic acid

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The on-purpose application of efficient catalysis is essential to obtain economically feasible and sustainable chemical processes. Zeolites are well-known catalysts in the petrochemical sector, but they also have an eminent potential for the valorization of other feedstocks such as biomass, which can be a valuable alternative for fossil carbon sources. Their conversion to chemicals, in which (part of) the original functionality is retained, is preferred. Despite the many benefits of zeolite catalysis, the microporous zeolite framework though poses a major challenge. Bulky molecules, such as typically encountered in biomass conversions, likely only reach part of the complete zeolite crystal volume due to restricted access to the small micropores or due to slow molecular transport. A solution to this problem consists in the design of hierarchical zeolites with large mesopores in or between the zeolite crystals, often resulting in an improved catalytic performance. Several synthesis methods for hierarchical zeolites have been developed, of which post-synthetic alkaline treatments of commercially available zeolites particularly have a promising potential for large-scale applications. In this respect, an important challenge though is to minimize the loss of zeolitic material obtained due to framework atom leaching. The industrially relevant USY zeolites for instance can be hierarchized by such alkaline treatments. However, their particular sensitivity to alkaline media results in difficulties to controllably form a mesoporous network without drastic amorphization, explaining the current initiatives with costly and unsustainable organic pore-directing agents. In this doctoral research, a new practical and straightforward post-synthetic alkaline treatment is developed for the design of hierarchical USY zeolites, with intrinsic potential for large-scale synthesis. More specific, a weakly alkaline NH4OH treatment is applied at mild conditions without the use of organic additives, while avoiding the need for additional ion-exchanges. The specific features of the treatment and the resulting materials have been extensively characterized. Using this NH4OH method, small mesopores (2-6 nm) are selectively formed in a controlled way, which appear to be well interconnected and largely accessible from the outside of the zeolite crystal. During the pore formation, gradual amorphization takes place, which results in varying ratios between the present micro- and mesopores. Both the treatment time and NH4OH concentration have been identified as important parameters to steer the mesopore network formation, whereby also the pore diameter can be tuned. Concurrently, no significant mass loss by leaching of framework atoms occurs, implicating an exceptionally high material yield, while the total pore volume and crystal morphology are being preserved. These findings led to the elucidation of a new pore formation mechanism through an intracrystalline transformation of the zeolite into a new phase. After extensive investigation by advanced NMR experiments, this phase was identified as a denser amorphous hydrated aluminosilicate phase. The combination of these various synthetic and material properties expressly distinguishes the densifying mild alkaline NH4OH treatment from conventional leaching methods to synthesize hierarchical USY. The catalytic potential of the novel hierarchical USY zeolites is demonstrated in two conversions of lipidic biomass to useful chemicals: the acid-catalyzed isomerization of α-pinene, and the ruthenium metal-catalyzed conjugation of safflower oil. For the α-pinene isomerization, the enhanced activity and selectivity to primary isomers after a short NH4OH treatment is ascribed to shorter microporous diffusion paths as a result of the introduced mesopores. Its associated partial amorphization hence not necessarily implicate a worse catalytic performance, as the loss of active acid sites can be overcompensated by an increased efficiency of the remaining acid sites. Besides, an enlarged indexed hierarchy factor (IHF) appears not to be a necessary prerequisite to obtain valuable hierarchical zeolites. Promisingly, a quasi-perfect reuse of the optimal shortly NH4OH treated catalyst is achieved after thermal regeneration, indicating the intrinsic stability under both the reaction and regeneration conditions. In the conjugation of more bulky triglycerides present in safflower oil, likely a combination of an enlarged amount of accessible active sites and improved mass transport in the mesopores results in the enhanced activity, stability and selectivity to primary isomers. The optimal support for highly dispersed Ru metal sites is prepared by successive alkaline NH4OH and acetate treatments, which result in a desired high mesoporosity together with a large pore diameter. Also from these results, it appears that amorphization should not be considered by definition as negative, as the optimal support is still able to highly disperse Ru metal, despite its XRD amorphous character. Eventually, this alkaline treated Ru/USY establishes the first H2-free and solventless heterogeneous conjugation process for vegetable oil, which is capable of producing exceptionally high yields of a conjugated oil.

Theses

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Vegetable oils are used for various applications, both in food and industrial products. Although different vegetable oils with various chemical compositions are available in nature, catalytical modification is necessary to enlarge their usage possibilities and to anticipate on the fluctuating oil production numbers and oil prices. With the use of a selective catalyst and the appropriate process conditions, the chemical and physical properties of vegetable oils can be tuned to ensure a constant supply of a large range of different oil and fat products for various uses. In this work, two important catalytic processes are studied, viz. the catalytic hydrogenation and conjugation of fatty acid methyl esters and vegetable oils, using Pt loaded ZSM-5 and Ru-USY zeolite catalysts, respectively. The design of the most appropriate catalyst was done combining catalytic measurements with a series of suitable physico-chemical characterization techniques. The selective hydrogenation of vegetable oils is used in industry to stabilize the oil against autoxidation and/or to obtain fat products with a certain melting profile. However, during this hydrogenation process also isomerisation occurs at the catalyst surface, leading to the formation of trans isomers. Although these trans fatty acids are associated with positive melting properties, the cis/trans isomerisation should be limited as some of these trans isomers, especially elaïdate (C18:1 trans-9), are considered a risk factor in coronary heart diseases. Unfortunately, literature learns that low cis/trans isomerisation is almost always associated with a lower hydrogenation selectivity.In this thesis it is proven that by using a well-designed catalyst, it is possible to hydrogenate vegetable oils with high selectivity and low cis/trans isomerisation, resulting in low-trans, stable, and plastic hydrogenated fat products. It is shown that a shape-selective Pt/ZSM-5 catalyst, which contains small nano-sized Pt-clusters mainly located in the pores of the zeolite crystal, can preferably hydrogenate trans fatty acids in a mixture containing both cis and trans fatty acids. This preference for the hydrogenation of trans over cis fatty acid chains is also seen in the hydrogenation of model triacylglycerols. Moreover, in the hydrogenation of triacylglycerols the fatty acid chain on the central position of the glycerol backbone is preferably hydrogenated, which points to a specific adsorption mode of the triacylglycerol molecules on the catalyst surface, viz. in a tuning fork configuration. Finally, the Pt/ZSM-5 catalyst shows also a more preferred stepwise reduction of polyunsaturated fatty acids to the mono-unsaturated level with reduced formation of saturated fatty acids. These selectivity concepts are also observed in the direct hydrogenation of soybean oil. It is shown that a single hydrogenation process of soybean oil at low temperature and high hydrogen pressure, using a shape selective Pt/ZSM-5 catalyst, results in thermally stable, highly nutritive, low trans, plastic fat products, suitable for shortening applications. This process meets perfectly the requirements that the food industry demands today and hence may be suitable to replace the currently used alternative processes, viz. fractionation and interesterification. The conjugation of linoleic acids and vegetable oils are important for both industrial and food applications. Industrial application requires high CLA productivity, while the CLA isomer distribution is less important. In contrast, for food applications a high selectivity for beneficial CLA isomers, especially c9,t11 and t10,c12 and from recent findings also the t9,t11 CLA, is crucial. Nowadays, CLAs are mainly produced by conjugation of oils high in linoleic acid, viz. soybean and safflower oil, using homogeneous bases or alternatively by dehydration of castor oil. However, the use of homogeneous alkali bases, solvents and neutralizing acids are disadvantageous from an ecological and economic point of view. Heterogeneous catalysts are more attractive as they can be easily recovered by filtration. However, literature learns that it is not easy to design a catalyst with high CLA selectivity as the competing hydrogenation reaction limits the CLA yield. Moreover, obtained productivities are much lower compared to the homogeneous processes.In this thesis, it is shown that high productivities of CLA from methyl linoleate can be obtained via heterogeneous catalysis, using highly dispersed Ru/USY catalysts with a high Si/Al ratio. Additionally, almost no hydrogenated products are formed as the reactions are performed in inert atmosphere. By using Cs+ as the charge compensating cation, the biologically most active CLA isomers were the main products. Because of the very high productivities obtained with the Ru/Cs-USY catalyst, this process can be a major breakthrough in the production of bio-based drying oils used in paints and varnishes and in the production of bio-plastics. Besides, as the beneficial CLA isomers are the main products, this process may be used for the production of neutraceuticals.