Probing the active sites of a Prussian blue analogue-derived Mn–Co catalyst in the CO hydrogenation to higher alcohols by high-pressure pulse experiments and co-feeding of ethylene

Abstract

The reaction network of higher alcohol synthesis over a pyrolised Prussian blue analogue-based catalyst was investigated by performing transient as well as steady-state kinetic experiments using a Mn-promoted Co catalyst with a molar Mn : Co ratio of 1 : 11 (Mn₁Co₁₁) at 260 °C. While the temperature variation revealed an apparent activation energy of 88 kJ mol⁻¹, the partial pressure variations of CO and H₂ resulted in reaction orders of −0.3 and +0.7 for CO and H₂, respectively. The reaction order for H₂ was similar to the value of +0.8 derived for the previously established 2CoCu catalyst (Co : Cu = 2 : 1) synthesized by co-precipitation, but the activation energy of the Mn₁Co₁₁ catalyst was lower by 52 kJ mol⁻¹ amounting to only 88 kJ mol⁻¹. While ethylene co-feeding showed that reductive hydroformylation takes place yielding 1-propanol presumably at the Co₂C/Co⁰ interface similar to the 2CoCu catalyst, high-pressure pulse experiments using methanol as probe molecule demonstrated the presence of an additional Co-based active site catalyzing the reductive carbonylation of primary alcohols over the Mn₁Co₁₁ catalyst. Correspondingly, the presence of Co–N–C sites anchored in the highly nitrogen- and oxygen-functionalized carbon matrix is assumed to result in the intertwined reaction network, comprising the carbide-based mechanism and reductive olefin hydroformylation over Co₂C/Co⁰, reductive alcohol carbonylation over the molecular sites and olefin hydration over acidic sites.