Interplay between Reaction and Phase Behaviour in Carbon Dioxide Hydrogenation to Methanol

Condensation promotes CO2 hydrogenation to CH3OH beyond equilibrium through in situ product separation. Although primordial for catalyst and reactor design, triggering conditions as well as the impact on sub-equilibrium reaction behaviour remain unclear. Herein we used an in-house designed micro-view-cell to gain chemical and physical insights into reaction and phase behaviour under high-pressure conditions over a commercial Cu/ZnO/Al2O3 catalyst. Raman microscopy and video monitoring, combined with online gas chromatography analysis, allowed the complete characterisation of the reaction bulk up to 450bar (1 bar = 0.1 MPa) and 350 degrees C.Dew points of typical effluent streams related to a parametric study suggest that the improving reaction performance and reverting selectivities observed from 230 degrees C strongly correlate with (i)a regime transition from kinetic to thermodynamic, and (ii)a phase transition from a single supercritical to a biphasic reaction mixture. Our results advance a rationale behind transitioning CH3OH selectivities for an improved understanding of CO2 hydrogenation under high pressure.