Role of Carbonaceous Supports and Potassium Promoter on Higher Alcohols Synthesis over Copper–Iron Catalysts

Abstract

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.

CO2‐Promoted Catalytic Process Forming Higher Alcohols with Tunable Nature at Record Productivity

“…% of the active metals and a Cu/Fe ratio of 2 were prepared by sol‐gel and dry impregnation methods, respectively, as reported earlier . A molar K/(Cu+Fe) ratio of 0.005 was applied in the former system, while it was 0.01 in the latter, as found optimal for the two systems . The compositional, porous, and structural properties of these solids after activation in diluted H 2 at 673 K and after use in HA synthesis from CO are in line with expectations and previous evidence (Table S1 and Figure S3).…”

“…Accordingly, (K‐promoted) CuFe catalysts supported on carbon nanofibers (denoted as (K)CuFe/CNF) and a K‐promoted CuFe catalyst with a silicalite carrier (coded as KCuFe/Z1000) featuring 5 wt. % of the active metals and a Cu/Fe ratio of 2 were prepared by sol‐gel and dry impregnation methods, respectively, as reported earlier . A molar K/(Cu+Fe) ratio of 0.005 was applied in the former system, while it was 0.01 in the latter, as found optimal for the two systems .…”

“…To fulfil their surging demand, the development of more sustainable synthesis routes becomes indispensable, whereby the direct conversion of syngas (CO+H 2 ), obtained from abundant or sustainable sources such as shale gas, biomass, and CO 2 , stands as an appealing option. Revitalized research on this reaction uncovered promising catalysts based on CoCu and CuFe but an industrially amenable process has yet to be demonstrated.…”

Catalytic Conversion of Carbon Oxides in Confined Spaces: Status and Prospects

“…From references, the modulation of active sites to improve selectivity for CO x conversion owing to confinement effect is generally through the adjustment of size of metallic NPs, which has been used in diverse fields like the production of higher alcohol, and higher hydrocarbon from syngas by suppressing the formation of side-products. [66][67][68][69][70] Moreover, the confinement can also promote the selectivity through stabilizing active phase like CuÀ xÀ Fe 5 C 2 phase in confined spaces. [71] Furthermore, it is generally accepted that many reactions take place at the interface.…”

“…The confinement effect of the channels in carbon nanofibers efficiently tailors the metal particles size, which suppresses the formation of C1 compounds and CO 2 , and maximizes the higher alcohols selectivity [67] CoZrP/KIT-6 CO hydrogenation to hydrocarbon…”