Metallkatalysierte Hydrogenolyse von biogenen Butandiolen

• Metal-catalyzed hydrogenolysis of biogenic butanediols

Stomps, Andrea; Palkovits, Regina (Thesis advisor); Liauw, Marcel (Thesis advisor)

Aachen (2017)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2017

Abstract

The hydrogenolysis of butanediols was investigated and heterogeneous noble metal catalysts were synthesized for this reaction. Based on the model reaction of 2,3-butanediol to 2-butanol, the commercially available catalyst ruthenium on carbon (Ru/C) was used to determine reaction parameters that allowed a kinetic analysis of the system without the influence of mass-transfer limitations. Subsequently, a kinetic investigation of the dehydration/hydrogenation sequence leading to the desired product 2-butanol was performed. In addition to the rate constants, the reaction enthalpy was determined using the Eyring–Polanyi equation. The substrate scope was then expanded to include 1,2-, 1,3-, and 1,4-butanediol. In contrast to 2,3-butanediol, those substrates favored the C-C-bond cleavage by decarbonylation, leading to the various isomers of propanol. Additionally, in the case of 1,3-butanediol it was possible to strongly influence the selectivity of the hydrogenolysis by changing the metal component of the catalyst to nickel or copper. For 1,4-butanediol, cyclodehydration to tetrahydrofuran occurred in addition to the decarbonylation but could be suppressed by using a basic support material such as ceria. For the decarbonylation of the three additional butanediols, a kinetic analysis was also performed, as well as a reaction network analysis of all four butanediols. Deuteration experiments allowed the investigation of the reactivity of the different positions on the carbon chain. The experiments showed that the terminal positions possess a higher reactivity than the internal positions, and carbon atoms bearing a hydroxyl substituent are considerably more reactive than unsubstituted ones. The second part of this work was focused on the development of ruthenium catalysts for the hydrogenolysis, once again using 2,3-butanediol as a model substrate. Different classes of support materials were investigated. Carbon-based materials such as activated carbons, covalent triazine frameworks, and carbon nanotubes showed mostly low activities as well as low selectivities to the target product 2-butanol. Moreover, several of them suffered from leaching of the catalytically active species during the reaction. Another group of support materials that were investigated are oxides. Acidic oxides such as alumina, silica, or zirconia afforded moderate conversions but led to the formation of side products. Basic oxides such as hydrotalcites were virtually inactive and mainly promoted the retro-aldol reaction. Only the catalyst based on amphoteric ceria showed high activity as well as ca. 80% selectivity to 2-butanol, and consequently an improved performance compared to the commercially available Ru/C. Therefore, Ru/CeO2 was characterized using TPD, DRIFTS, TPR, and TEM to reach an insight into structure–activity relations. The good performance of the catalyst likely depends on two factors. One is the special interaction between support and metal species, the other a combination of basic and weakly acidic centers on the surface of the support material. For Ru/CeO2, a kinetic analysis was performed as well as recycling experiments to investigate the long-term stability of the catalytic system. In these experiments, Ru/CeO2 demonstrated good stability over several runs, which in addition to its high activity and selectivity makes it a promising catalyst for hydrogenolysis.