Hydrogenation of furanic compounds over heterogeneous catalysts
Aachen (2017) [Dissertation / PhD Thesis]
Page(s): 1 Online-Ressource (xvi, 116 Seiten) : Illustrationen, Diagramme
Substituted furans are valuable platform chemicals that can be derived from biomass feedstock. Thanks to the rich functionality, they can be chemically transformed into a variety of useful products. Currently, hydrogenation of furanic compounds calls a lot of attention of researchers as the furans can be upgraded by lowering the number of oxygen atoms into the molecule. In spite of a number of publications in this area, the challenges of designing of active, selective, cheap and environmentally friendly catalysts for furan hydrogenation still exist. Moreover, understanding the mechanism and kinetics of these reactions on the catalyst surface is still missing, mostly because of the rich chemistry of substituted furans and the great number of the species involved. The present dissertation reports a study on alkaline earth metal oxide catalysts for furfural hydrogenation by means of hydrogen transfer using methanol as a hydrogen donor. Interestingly, low surface area CaO and SrO pretreated at the optimal conditions show remarkable activity in the transformation of furfural to furfuryl alcohol, with nearly 100% selectivity. The careful analyses of the catalyst samples indicate that there is no correlation between the number and strength of base sites and their catalytic activity. Whereas DRIFT measurements confirm that the interaction of methanol with the catalyst surface and its subsequent activation play an important role in promoting furfural hydrogenation.Mechanistic and kinetic studies of the hydrogenation of furfural and furfuryl alcohol over conventional Ru/C using molecular hydrogen show that at lower temperature the hydrogenation of the furan ring is favored, whereas at higher temperatures the hydrogenation of functional groups goes first with the subsequent hydrogenation of the furan ring. The selectivity of reactions is also influenced by the specificity of the functional group attached to the furan ring. Applying the detailed kinetic modeling to furfural and furfuryl alcohol hydrogenation, we confirmed that the interaction of the substrate with the catalyst surface differs depending on the exact functional group attached to the furan ring that also determines the main reaction route.The study was extended to include 5-hydroxymethylfurfural as a substrate in hydrogenation over Ru/C as well as Ni/C. For both catalysts the importance of the first step, hydrogenation of the aldehyde group, is confirmed by providing mechanistic and kinetic studies. Here we also see the different reaction selectivities depending on the substrate’s functional groups, either both alcohol and aldehyde groups are attached to the furan ring or twice an alcohol group. In case of Ni/C used as a catalyst the main product of 5-hydroxymethylfurfural hydrogenation is 2,5-dimethylfuran, whereas in case of Ru/C 2,5-dimethylfuran acts as an intermediate and the kinetically favored product is 2,5-dimethyltetrahydrofuran. Therefore, understanding the nature of substrate – catalyst interaction and the kinetics of the process, it becomes possible to selectively upgrade the substituted furans to valuable products.