(583d) Ultra-Low Temperature Water-Gas-Shift Reaction with Homogeneous Supported Ionic Liquid Phase (SILP) Catalysts

Supported Ionic Liquid Phase (SILP) catalysts are new materials consisting of an ionic liquid, a metal catalyst and a porous support. The catalyst is dissolved in the ionic liquid which itself is dispersed as a thin film on the inorganic support. This application combines both the advantages of homogeneous and heterogeneous catalysis and thus bridges the gap between traditional homogeneous and heterogeneous catalysis. Especially continuous, gas-phase reactions are highly suited for this novel and innovative technology. An industrially important example is the water gas shift (WGS) reaction, by which hydrogen can be generated from carbon monoxide and water. Homogeneous WGS catalysts operate at milder temperatures than commercial heterogeneous systems. This can open new fields of applications such as distributed hydrogen production or fuel reforming. Since hydrogen production via WGS is an exothermic reaction, lower temperatures result in higher equilibrium conversions. In this contribution we present investigations of homogeneous metal complexes active in the water gas shift reaction which have been immobilized by the SILP technique. Because of that an optimization strategy for this multi-parameter catalysts system was developed as depicted in the figure focusing on improving stability, activity, robustness and removal of the induction period. We tested more than 50 catalyst precursors were tested as SILP WGS catalysts. These precursors include literature-known systems and newly developed catalysts. This detailed study showed, that catalysts bearing a ruthenium center exhibit the highest level of activity. So we selected RuCl3 as simple precursor for the further optimization steps. The ionic liquid severely influences the activity and stability of the SILP WGS catalyst by its changing physic-chemical properties. Especially the different solubilities of the substrates, the different coordination strength towards transition metal complexes and the basicity are important. In a set of experiments we tested more than 20 ionic liquids on their potential in the WGS. Indeed the final level of activity and the tendency of stripping out volatile ruthenium components can be tuned. In our experiments the ionic liquids [BMMIM]Cl and [BMMIM][OTf] turned out to be the most promising candidates for the optimized system. It is known in literature, that the activity of homogeneous water-gas shift catalysts is influenced by the basiscity of the reaction environment. In case of a SILP WGS catalysts basicity can be introduced via addition of an organic base or the support basicity. Our results also indicate a promising effect on activity by increased basicity. We found that either the support material or the addition of a base leads to this effect, which also can be combined. The highly porous support material based on Alumina turned out to be a significantly better choice than the Silica which was previously used, so it was selected for the optimized system. In order to remove the induction period, we carried out infrared spectroscopy measurements on the RuCl3 based SILP WGS catalysts. For this study, we washed out the catalyst-IL phase from the support material by ethanol and produced a sample for IR by removal of the solvent. In comparison of samples of catalysts that were just prepared and ones that were already prune to the reactive atmosphere against reference spectra of various ruthenium-chloro-carbonyl complexes. It turned out, that the induction period is possibly related to the formation of [Ru (CO)3 Cl2]2 from the RuCl3. So the dimeric ruthenium species was immobilized on Alumina and indeed no induction period was observed, so that the [Ru (CO)3 Cl2]2 was selected for the optimized system. Further experiments focused on the optimization of catalyst loading, ionic liquid loading and the addition of cocatalyst. With this building-block like approach the activity was increased by more than 2 orders of magnitude. The investigated SILP systems exhibit activities and stabilities exceeding those of homogenous systems reported in literature. The newly developed catalysts showed excellent restart behavior and long-term stability for more than 750h time-on-stream. Cross-tests in typical synthesis gas environments were also carried out at the Süd-Chemie AG labs successfully. The results indicate that SILP derived WGS catalysts may become a promising alternative to conventional heterogeneous systems, especially in distributed fuel cell setups or decentralized biogas applications.