Higher alcohols are important compounds with widespread applications in the chemical, pharmaceutical and energy sectors. Currently, they are mainly produced by sugar fermentation (ethanol and isobutanol) or hydration of petroleum-derived alkenes (heavier alcohols), but their direct synthesis from syngas (CO + H) would comprise a more environmentally-friendly, versatile and economical alternative. Research efforts in this reaction, initiated in the 1930s, have fluctuated along with the oil price and have considerably increased in the last decade due to the interest to exploit shale gas and renewable resources to obtain the gaseous feedstock. Nevertheless, no catalytic system reported to date has performed sufficiently well to justify an industrial implementation. Since the design of an efficient catalyst would strongly benefit from the establishment of synthesis-structure-function relationships and a deeper understanding of the reaction mechanism, this review comprehensively overviews syngas-based higher alcohols synthesis in three main sections, highlighting the advances recently made and the challenges that remain open and stimulate upcoming research activities. The first part critically summarises the formulations and methods applied in the preparation of the four main classes of materials, i.e., Rh-based, Mo-based, modified Fischer-Tropsch and modified methanol synthesis catalysts. The second overviews the molecular-level insights derived from microkinetic and theoretical studies, drawing links to the mechanisms of Fischer-Tropsch and methanol syntheses. Finally, concepts proposed to improve the efficiency of reactors and separation units as well as to utilise CO and recycle side-products in the process are described in the third section.

The identification of an effective copper–iron catalyst for the direct conversion of synthesis gas into higher alcohols is hindered by the low solubility limit of Cu in Fe and the limited understanding of structural and electronic descriptors in such multicomponent systems. Here, commercial carbonaceous carriers are shown to produce an efficient material only if they enable control of the size and location of metal species through confinement in adequately sized channels, with conical carbon nanofibers being more adequate than carbon nanotubes. Application of a sol–gel route was preferred to other deposition methods to avoid excessive Cu aggregation, associated with enhanced CO2 formation. A bulk Cu/Fe ratio of 2 permitted one to balance the different tendencies of Cu and Fe toward agglomeration, i.e., to form numerous Cu particles of moderate size and hinder the dispersion of the Fe phase and in turn the Fischer–Tropsch activity. Promotion by tiny amounts of potassium was instrumental to further increase the size of the Fe particles and enhance their proximity to Cu. These structural features were associated with a more facile Cu-mediated reduction of the Fe phase and a more pronounced hydrogen activation ability, based on thermal characterization under reaction conditions, and maximized the higher alcohols selectivity (47%) and the olefins fraction among hydrocarbons (50%). An in-depth kinetic analysis over the top performer provided guidelines to optimize temperature, pressure, H2/CO ratio, and residence time, leading to a space time yield of 0.53 gHA gcat –1 h–1. This value is almost twice as high as that of the state-of-the-art bimodal silica-supported CuFe system and could be maintained for 100 h on stream.