Dynamic equilibria in supported ionic liquid phase (SILP) catalysis: in situ IR spectroscopy identifies [Ru(CO)xCly]n species in water gas shift catalysis

Abstract

Ru-based supported ionic liquid phase (SILP) systems efficiently catalyze the low-temperature water-gas shift reaction (WGSR). While previous studies suggest that Ru-carbonyl species play an important role in the mechanism, detailed knowledge on the catalytically active species is still missing. To identify these carbonyl complexes, we apply in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in combination with density functional theory (DFT). Investigations of an as-prepared [Ru(CO)₃Cl₂]₂/[C₄C₁C₁Im]Cl/Al₂O₃ catalyst indicate splitting of the dimer induced by Cl⁻. Subsequently, an equilibrium between several [Ru(CO)_xCl_y]_n species is established, in which the IL serves as an effectively infinite Cl⁻ reservoir. We find that the major species in the system freshly-prepared from [Ru(CO)₃Cl₂]₂ is indeed [Ru(CO)₃Cl₃]⁻. A smaller amount of [Ru(CO)₂Cl₃]⁻ and chloride-rich species [Ru(CO)₂Cl₄]²⁻ or [RuCOCl₄]²⁻ are also found in the SILP. Similar Ru-carbonyl species are observed during carbonylation of RuCl₃/[C₄C₁C₁Im]Cl/Al₂O₃, another potential WGSR catalyst. The response of [Ru(CO)₃Cl₂]₂/[C₄C₁C₁Im]Cl/Al₂O₃ to heating and/or CO dosing was probed in situ and the results confirm the presence of the equilibrium proposed above.