Selective hydrodeoxygenation of carbonyl-substituted aromatic substrates using multifunctional catalysts

Offner-Marko, Lisa; Leitner, Walter (Thesis advisor); Chaudret, Bruno (Thesis advisor)

Aachen (2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020


Efficient hydrodeoxygenation (HDO) catalysts have to feature sites for the activation of the dihydrogen molecule and the oxygenated moiety of the substrate. In the case of the presented bifunctional catalysts, these tasks are accomplished by metal nanoparticles (NPs) and acid-functionalized supported ionic liquid phases (SILPs), respectively. In previous studies, we discovered that intimate contact of the active sites within bifunctional Ru@SILP-SO3H catalysts was crucial for a high HDO activity. Interestingly, experiments discussed in this thesis revealed that the interactions resulting from the close proximity of the Ru NPs and the SILP's sulfonic acid groups within these bifunctional catalysts were not solely advantageous for their HDO activity. Testing a series of Ru@SILP-SO3H catalysts showed a clear correlation between their acid:metal ratios and their activity towards the HDO of the model substrate, benzylideneacetone. This emphasizes the importance of a well-balanced ratio of hydrogenation and dehydration sites within the bifunctional catalysts. Comparing the HDO activity of Ru@SILP-SO3H and Ru@SILP+IL-SO3H catalysts indicated that the unfavorable interactions between the acid and the metal sites were not only present within the bifunctional catalysts, but did also occur during NP synthesis. The synthesis of FeRu NPs on a SILP featuring a chemisorbed, non-functionalized IL and the post-synthesis physisorption of an acidic IL allowed to separate the stabilization effect of the SILP from its functionalization. This approach facilitated the synthesis of Fe containing NPs from the highly acid sensitive {Fe[N(Si(CH3)3)2]2}2 precursor and ensured intimate contact of the acid and metal active sites on the surface of the bifunctional catalyst. It also paves the way for the preparation of multimetallic NPs on SILP materials and the assembly of multifunctional catalytic systems with tailor-made reactivity for challenging catalytic transformations. The structural similarity of the chemisorbed and physisorbed IL molecules led to the formation of a stable Fe25Ru75@SILP+IL-SO3H catalyst, which combined high activity and selectivity for the formation of aromatic HDO products with a broad substrate scope. Furthermore, the catalyst exhibited a unique preference for the HDO of non-benzylic over benzylic ketones. In contrast to the Ru@SILP+IL-SO3H and Fe25Ru75@SILP+IL-SO3H catalysts, the post-synthesis functionalization of Fe25Ru75@SILP with IL-SO3- and Hf(OTf)4 did not result in the formation of a stable Fe25Ru75@SILP+IL-SO3-Hf(OTf)3 catalyst. Leaching of Hf(OTf)3+OH- during the HDO of 4-methylacetophenone indicated that the immobilization of the Lewis acid via an ionic bond between the hafnium cation and the sulfonate anion was unsuccessful.