Publisher: Cardiff University

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Publication date(s): 2017 (online)

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Volume: Issue:

Journal: Doctoral Thesis

Link: http://orca.cf.ac.uk/107295/

URL: http://orca.cf.ac.uk/107295/

Porous Lewis acids are rapidly emerging heterogeneous catalysts, particularly for the upgrading of biorenewable feedstocks, due to their ability to coordinate lone-electron pairs from oxygen atom, hence inducing molecular rearrangements and cleavage. As such, this study tackles one of the most important challenges in liquid phase catalytic chemistry, namely the design of novel Lewis acidic zeolites to act as heterogeneous catalysts for liquid phase applications. Lewis acidic (Sn-BEA) zeolites are typically synthesised by highly complicated hydrothermal synthesis procedures, which have significant downsides preventing industrial application. In addition to technical difficulties, some drawbacks minimize the effectiveness of Sn-BEA in industrial interest. Amongst these limitations are 1) long crystallization time (40 days), 2) the large crystallite sizes obtained via typical hydrothermal synthesis, resulting in mass-transfer issues, and 3) the low Sn amount (typically < 2 wt.%), resulting in low space-time yield. Furthermore, a relatively amount of undesirable HF is required to induce crystallisation. Therefore, much academic and industrial research is currently devoted to the development of new methodologies for preparing Lewis-acid zeolite catalysts with higher or similar activity. The broad context at this doctoral dissertation is to investigate the potential of acid leaching (i.e., demetallation) of commercial (Al-containing) BEA zeolite as a simple, versatile, and scalable method to introduce different amounts of active centers (Sn) in zeolites, using solid-state incorporation (SSI). To evaluate the activity of the synthesized catalysts, the study focused firstly on the Meerwein-Ponndorf-Verley (MPV) transfer hydrogenation of carbonyl compounds and isomerization of glucose. Owing to the low activity of Sn-BEA micropores catalyst for the activation of bulky molecules, mesopores are subsequently created via top-down alkaline treatment to synthesis hierarchical Lewis-acid porous zeolite (Sn-h*-BEA), by employing the post-synthetic demetallation route. This catalyst, was evaluated in reactions that involve bulky molecules, such as cyclooctanone (C8) and cyclododecanone (C12). Sn-h*-BEA was found to be active, selective and more stable for continuous operation than its purely microporous analogue. Subsequent work focused on the catalytic valorisation of bio-renewable feedstock, which often relies upon multi-stage processing of highly-functionalised substrates, resulting in selectivity and process engineering challengers. Later parts of this thesis therefore report the synthesis of a novel acid-base bifunctional catalyst [Sn-Al] BEA which contains Sn-related Lewis acid sites and Al-related BrØnsted acid sites. This bifunctional catalyst has been tested, as a catalyst for the cascade catalytic transfer hydrogenation and etherification of furfural, under batch as well as continuous flow reaction condition. With this catalyst, furfural was first converted by Lewis acid Sn(Ⅳ) framework sites to form furfural alcohol via transfer hydrogenation from the solvent. Subsequently, furfuryl alcohol etherification with the solvent is catalysed by the BrØnsted acid (Al) framework to produce corresponding alkyl furfuryl ether. Such ethers are highly desirable as bio renewable fuel additives.

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