Direct Conversion of Syngas to Higher Alcohols via Tandem Integration of Fischer-Tropsch Synthesis and Reductive Hydroformylation

Abstract

The selective conversion of syngas to higher alcohols is an attractive albeit elusive route in the quest for effective production of chemicals from alternative carbon resources. We report the tandem integration of solid cobalt Fischer-Tropsch and molecular hydroformylation catalysts in a one-pot slurry-phase process. Unprecedented selectivities (>50 wt %) to C₂₊ alcohols are achieved at CO conversion levels >70 %, alongside negligible CO₂ side-production. The efficient overall transformation is enabled by catalyst engineering, bridging gaps in operation temperature and intrinsic selectivity which have classically precluded integration of these reactions in a single conversion step. Swift capture of 1-olefin Fischer-Tropsch primary products by the molecular hydroformylation catalyst, presumably within the pores of the solid catalyst is key for high alcohol selectivity. The results underscore that controlled cooperation between solid aggregate and soluble molecular metal catalysts, which pertain to traditionally dichotomic realms of heterogeneous and homogeneous catalysis, is a promising blueprint toward selective conversion processes.

Keywords: Cascade Reactions; Higher Oxygenates; Plasticizer Alcohols; Syngas Conversion; Tandem Catalysis.

Scheme 1

Production of higher alcohols from syngas via the integration of Fischer–Tropsch synthesis (FTS) and reductive hydroformylation (RHF) in a tandem process. The active sites of the solid catalyst are responsible for chain growth (number of C _n in R). The ratio between β‐H elimination and hydrogenation as chain termination steps determines the primary hydrocarbon selectivity, i.e. olefin‐to‐paraffin ratio. The activity of the molecular catalyst in olefin reductive hydroformylation relative to secondary olefin hydrogenation pathways determines the final alcohol selectivity. For completeness, the CO insertion chain‐termination pathway on the FTS catalyst is also indicated as a source for primary, strictly linear higher alcohols.

Figure 1

Structural characterization of an olefin‐selective cobalt Fischer–Tropsch catalyst (NaPr‐CoRu/AOmM). a) SEM image for catalyst microparticles. b), c) Cross‐sectional SEM images after Focused‐Ion‐Beam (FIB) milling of the resin‐embedded catalyst showing macropore opening cross‐sections (black regions) delimited by the γ‐Al₂O₃ backbone (light gray). d) Differential (blue scatter) and cumulative (red line) Hg intrusion pore size distributions. The contribution at >10⁴ nm corresponds to voids between the catalyst microparticles. e) Histogram for the maximum transport distance through mesopore regions to the nearest boundary with the network of macropores as derived from 3D image analysis of the FIB‐SEM tomogram. The inset shows the 3D contour plot for the Euclidean distance to nearest macropore (top half) overimposed to the reconstructed tomogram (bottom half), with mesoporous Al₂O₃ domains displayed in blue. f) C_S‐HAADF‐STEM image and g)–i) EDS analysis maps recorded at the Al‐, Pr‐ and Co‐K emission lines, respectively, on ultramicrotomed catalyst cross sections. Darkest regions (lowest Z‐contrast) correspond to the resin‐filled macropore openings. j) High‐magnification C_S‐HAADF‐STEM image showing cobalt nanoparticles (brightest speckles with higher Z‐contrast) confined to the network of γ‐Al₂O₃ sheet‐like nanocrystallites. k) Cobalt particle size histogram.

Figure 2

Higher olefin‐selective, fixed‐bed Fischer–Tropsch synthesis with cobalt catalysts. a) Linearized Anderson–Schulz–Flory hydrocarbon product distributions. b) Evolution of the olefin‐to‐paraffin molar ratio with hydrocarbon chain length in the C_3–10 product range; and c) evolution of the terminal‐to‐internal molar ratio for olefin products in the C_4–10 product range for a conventional mesoporous CoRu/γ‐Al₂O₃ (CoRu/AOm) and a dually promoted hierarchically porous NaO_x, PrO _x ‐CoRu/γ‐Al₂O₃ (NaPr‐CoRu/AOmM) cobalt‐based Fischer–Tropsch catalysts. Reaction conditions: T=473 K, P=20 bar, H₂ : CO=2, CO conversion=20±3%.

Figure 3

Direct syngas conversion to higher alcohols via the tandem Fischer–Tropsch synthesis/reductive olefin hydroformylation syngas conversion process. a) Product selectivity (left y‐axis) and CO conversion (right y‐axis) for slurry‐phase tandem FTS/RHF experiments as a function of the n(Co_FTS)/n(Co_HyFo) catalyst ratios. FTS catalyst: NaPr‐CoRu/AOmM; RHF catalyst: Co₂CO₈+P(Cy)₃ (L : M=1.0 (P/Co (at/at)). The extreme cases, i.e. the FTS and RHF catalysts tested independently, are also included for reference. Reaction conditions: T=473 K, P=120 bar (initial, measured at RT), stirring rate 700 rpm, syngas feed H₂:CO=2,2‐methyl pentane as solvent. [a] C_3–10 olefin selectivity ≤0.2 C% in all cases. The test with the RHF catalyst alone led to a too low (0.2 %) CO conversion, at which the C balance closed only at 80 %. b) Representative two‐dimensional gas chromatogram for the liquid products in a slurry‐phase FTS/RHF tandem reaction test under optimized reaction conditions.

Figure 4

Time‐resolved evolution of CO conversion and alcohol product (regio)selectivity during the slurry‐phase tandem Fischer–Tropsch synthesis/reductive olefin hydroformylation syngas conversion process. FTS catalyst: NaPr‐CoRu/AOmM; RHF catalyst: Co₂CO₈+P(Cy)₃ (L : M=1.0 (P/Co)). Reaction conditions: T=473 K, P=120 bar (initial, measured at RT), stirring rate 700 rpm, syngas feed H₂:CO=2,2‐methyl pentane as solvent, catalyst ratio n(Co_FT)/n(Co_HyFo)=170/70. Dotted lines are included as guides to the eye.