Impact of carrier acidity on the conversion of syngas to higher alcohols over zeolite-supported copper-iron catalysts

Abstract

The development of catalysts showing satisfactory performance to realize a large-scale synthesis of higher alcohols (HA) via CO hydrogenation, which promises higher sustainability compared to current industrial routes, is hindered by limited fundamental understanding. Here, the impact of acidity and porosity of MFI zeolite carriers on the properties of CuFe catalysts was investigated. Significant changes in activity and product distribution were observed at higher concentrations of Brønsted-acid sites over materials featuring similarly sized and distributed metal particles, whereas a hierarchical carrier structure did not produce relevant alterations. Individually feeding methanol, primary and secondary HA, and hydrocarbons over a zeolite with a Si/Al bulk ratio of 40 uncovered that acidity greatly fosters the formation of secondary HA via hydration of the coproduced olefins. Acidic supports are attractive if such products are desired, while are suboptimal to boost HA yields. Indeed, the selectivity to primary HA is concomitantly hindered, since they are partially dehydrated to olefins, which can rehydrate to secondary HA, but mostly undergo coupling reactions followed by hydrogenation and cracking to paraffins, and the CO conversion is lowered. Ion exchange, deposition, and their combination were explored as strategies to add K as a promoter, whereby the second was superior and increased the HA selectivity over the low-acidic CuFe/silicalite from 33 to 43%, while outstandingly minimizing the CO2 selectivity (2%). Finally, it was demonstrated that positive acidity effects can be isolated if a zeolite is used in series to a catalyst producing a high fraction of olefins, since hydration of the latter to secondary HA becomes dominant over other parallel reactions, overall boosting the total HA productivity.