On the role of potassium in Prussian blue analogue-derived Mn–Co catalysts applied in the CO hydrogenation to higher alcohols

Abstract

The lowering of the K content of phase-pure Mn–Co-derived Prussian blue analogue (PBA)-based catalysts applied in higher alcohol synthesis was achieved by an additional washing step and by substituting K⁺ cations by (NH₄)⁺ cations prior to synthesis. Adding the washing step to the microemulsion-assisted co-precipitation route resulted in a decrease from 3.1 wt% to 0.6 wt% K after pyrolysis, whereas substitution achieved a K content of only 0.2 wt%. During pyrolysis the washed PBA precursor decomposed in a single step resulting in a sharp mass loss at 540 °C, whereas the NH₄-based precursor gave rise to a major broad peak at 390 °C. Correspondingly, strong changes of the textural and structural properties of the resulting porous carbon matrix with embedded Co⁰ were observed. Under HAS conditions the formation of bulk Co₂C in close contact with Co_hcp occurred as shown by in-depth structural investigations of the spent catalysts. Washing resulted in an increased total oxygenate selectivity up to 35%, whereas the CO₂ selectivity of the substituted sample was decreased to 15% at a temperature of 260 °C, a pressure of 60 bar and a H₂/CO ratio of 1. Both methods led to higher CO conversion, and the lowered K content of 0.6 wt% also resulted in a significantly higher selectivity of primary and secondary alcohols of 20% and 11%, respectively, due to the presence of more acidic Co–N–C sites.