Carbon nanofibres-supported KCoMo catalysts for syngas conversion into higher alcohols†
The development of industrially-viable heterogeneous catalysts for higher alcohol (HA) synthesis via direct syngas conversion is hindered by the limited understanding of structural and electronic descriptors of their performance. Here, K-promoted CoMo(5 wt%)-based catalysts were investigated to shed light onto property–function relations. Evaluation of impregnated oxides and C-based materials identified a higher HA selectivity for the catalyst supported on carbon nanofibres obtained by the simultaneous addition of all metals. Using this carrier, additional solids were synthesised via ball milling as well as a sol–gel method with citric acid, which enhanced the CO conversion. After confirming the positive influence of K and a unitary Co/Mo ratio, the impact of temperature (573–773 K) and pressure (0.1–5 MPa) upon activation through reduction by hydrogen was investigated. A combination of two steps, one at 723 K and ambient pressure, fostering the HA selectivity (22%), and a second, more extended, at 573 K and 5 MPa, boosting the CO conversion (14%), maximised the space time yield of HA, which matches that of the best comparable CoMo system reported in the literature (ca. 0.12 gHA gcat−1 h−1) in spite of the 13-times lower metal loading. Characterisation by X-ray diffraction, temperature-programmed reduction by H2, X-ray photoelectron spectroscopy, operando infrared spectroscopy and electron microscopy uncovered that this originated from the atomic intermixing of Co and Mo in binary oxide phases and their high dispersion, enabling proximity and an effective reduction to low oxidation states of the metal sites (0 and +2 for Mo and 0 for Co) upon activation and reaction.