Bimetallic ruthenium- and rhodium-based nanoparticles immobilized on supported ionic liquid phases (SILPs) for selective hydrogenation and hydrogenolysis reactions

Rengshausen, Simon; Leitner, Walter (Thesis advisor); Chaudret, Bruno (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021

Abstract

This thesis deals with the development of bimetallic nanoparticles immobilized on supported ionic liquid phases (SILPs) combining noble metals (i.e. Ru and Rh) with 3d metals (i.e. Fe, Co, Ni) as catalysts for the selective hydrogenation, hydrodeoxygenation and hydrogenolysis of complex aromatic substrates using H2 as a benign reducing agent. Using an organometallic approach, bimetallic RuxM100-x@SILP (M = Co, Ni) catalysts were developed and compared to the previously reported RuxFe100-x@SILP catalytic system in order to investigate the influence of the used 3d metals on the formation of bimetallic nanoparticles on SILP as well as on their reactivity. Applying characterization techniques including TEM, SEM-EDX and STEM-HAADF-EDX allowed for a detailed analysis of the developed materials, evidencing the formation of well-dispersed bimetallic nanoparticles on the SILP support. The catalysts reactivity was investigated using the catalytic hydrogenation of benzylideneacetone as a model reaction. The Ru:M ratio was found to have a great influence on the activity and selectivity of the catalysts, especially with regards to arene and ketone hydrogenation. Interestingly, the use of 3d metals with increasing atomic number showed a consistent trend requiring decreased Ru:M ratios to enable the hydrogenation of the ketone as well as the aromatic ring. Then, the addition of a sulfonic acid-functionalized ionic liquid (IL-SO3H) to the RuxM100-x@SILP produced bifunctional bimetallic catalysts (RuxM100-x@SILP+IL-SO3H) that were applied to the selective hydrodeoxygenation of benzylic and non-benzylic ketones as well as the hydrogenolysis of diphenyl ether, with a special focus on the production of aromatic products. The investigated catalytic systems showed for optimized Ru:M ratios (e.g. Ru50Co50@SILP+IL-SO3H) a good activity and selectivity for the selective hydrodeoxygenation of the benzylic and non-benzylic ketones towards the formation of aromatic alkanes. However, a trade-off between activity and selectivity was observed in dependency of the Ru:M ratio for the hydrogenolysis of diphenyl ether, along with a rapid deactivation of the bifunctional catalysts under the considered reaction conditions. This deactivation of RuxM100-x@SILP+IL-SO3H in the hydrogenolysis of diphenyl ether was explained by the observed agglomeration of the bimetallic nanoparticles under acidic reaction conditions. Subsequently, the hydrogenolysis of substituted diaryl ethers was focused using a bifunctional monometallic Ru@SILP-SO3H catalyst to investigate the influence of different lignin-based substitution patterns on the resulting product distribution. Interestingly, asymmetric diaryl ethers possessing methoxy-functionalities were selectively cleaved adjacent to the unsubstituted ring resulting in the selective production of substituted phenols and cyclohexane. The catalytic system was also successfully used under continuous flow conditions for the hydrogenolysis of 2-methoxy-4-methylphenoxybenzene, highlighting the catalyst’s practicability and stability. Additionally, the influence of water accumulation on the catalytic properties was investigated, and the selective extraction and isolation of phenols from the reaction mixtures was demonstrated. Finally, the synthesis of bimetallic RhxCo100-x@SILP catalysts was developed finding [Rh(allyl)3] and [Co(COD)(Cyclooctadienyl)] as suitable organometallic precursors. The catalysts were used for the selective hydrogenation of benzylideneacetone and a variety of (substituted) heteroaromatics, showing a distinct switch between Rh:Co ratios of 70:30 to 75:25 to yield either partially or fully saturated products. The detailed characterization by advanced spectroscopic techniques, such as STEM-HAADF-EDX and XAS allowed for the structural elucidation of the catalysts showing homogeneously alloyed and monodisperse nanoparticles immobilized on the SILP. In summary, the controlled synthesis of bimetallic Ru- and Rh-based nanoparticles immobilized on SILP was achieved using the versatile and powerful organometallic approach. The well-defined nature of these finely tunable catalytic systems allowed for their precise tailoring to the specific task and gave detailed insights into the structure-reactivity correlations.