Daniel KREMITZL1, Peter SCHULZ1, Peter WASSERSCHEID1
1Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany
Functionalized ionic liquids (ILs) have hardly been used as modifiers in catalytic applications so far, one possible reason being the increased viscosity of functionalized ILs. Classical IL/organic biphasic catalysis would suffer from severe mass transport problems in case the chemical reaction is much faster than the mass transfer involving a viscous functionalized IL. This restriction, however, is overcome in IL thin film catalysis where diffusion pathways and thus the characteristic time of diffusion for the reactants are much shorter. In Solid Catalyst with Ionic Liquid Layer (SCILL) systems, a heterogeneous catalyst is modified with ILs to achieve higher selectivities compared to its untreated counterpart, where the variety of different surface sites leads to little control over undesired side and consecutive reactions. So far, most SCILL research has focused on the use of non-functionalized ILs, however.
This project deals with selective modification of the reactive interface of supported Pd nanoparticles with ILs that carry one or several functionalities. In specific, the project targets a broad range of functionalized imidazolium-based ILs carrying polar and coordinating functionalities (e.g. nitrile, esters, thiols). These functionalized ILs are synthesized and used as reaction media creating an optimized reaction environment by assembling at the active site. A combination of effects strongly altering activity and selectivity of the catalyst will be expected: i) differential solubility of the IL film and the related diffusion processes change the effective concentration of reactants, intermediates and products at the catalyst interface; ii) the IL selectively interacts with the catalytic sites at the liquid/solid interface, thereby modifying their adsorption and reaction properties; iii) under the operating conditions the IL coating reacts with the support, altering the activity of the overall catalytic system.
The so-modified advanced SCILL catalysts will be applied in a competitive and selective gas phase hydrogenation reaction of large technical interest, i.e. of acetylene in excess ethane. The reaction will be carried out as continuous gas phase process in a fully automatized testing rig in order to provide detailed kinetic data on activity and selectivity induced by the IL functionalities.
For such systematic investigations the awareness of the active species present in the supported liquid phase is crucial. It is expected that the ILs transform via several possible reaction pathways that take place in the bulk of the IL film, at the Pd/IL interface and at the support/IL interface. These transformations can range from carbene formation[1] or C2-alkylation of the imidazolium backbone[2] over cleavage,[3] hydrogenation, or hydrolysis of the functional groups to the undesired elimination of side chains of the IL[1]. The preliminary study presented herein shall suggest potential IL candidates for further application in the subsequent kinetic studies.
References
[1] Chancelier et al. French-Ukrainian Journal of Chemistry 2016, 4 (1), 51−64
[2] Bauer et al. ChemCatChem 2017, 9, 109−113
[3] Xu et al. Angew. Chem. Int. Ed. 2017, 56, 9072−9076