Short-chain C2-C6 mixed alcohols are important fuel additives while long-chain C7-C14 alcohols are frequently used as feedstocks for plasticizers, detergents and lubricants. The large-scale production of these higher linear 1-alcohols relies on the homogeneous catalytic hydroformylation of a Cn (n≥3) 1-alkene to Cn+1 aldehydes which are then hydrogenated to the respective terminal alcohols. More recently, alternative procedures for long chain 1-alcohols have been developed, for example, anti-Markovnikov 1-alkene hydration and one-pot hydroformylation-hydrogenation using homogeneous catalysis. However, such a homogeneous catalytic process still needs 1-alkenes which are obtained from petroleum processing. Therefore, the synthesis of higher C2+1-alcohols using catalytic CO hydrogenation via the heterogeneous Fischer Tropsch process has recently become one of the most attractive routes in finding alternatives to produce straight chain hydrocarbons with terminal oxygen functionalization. The present communication provides an account of our successful attempts in tuning the selectivity to either short- or long-chain 1-alcohols.
Our approach is based on the targeted design of CoCu-based catalysts using co-precipitation of metal salts into oxalate precursors and subsequent thermal decomposition. The chain lengthening of the 1-alcohols can be tuned through changing the composition of CoCu-based catalysts by adding a third metal, such as Mn and Nb, during the precipitation. This procedure allowed us to produce ternary systems such as “CoCuMn” and “CoCuNb” catalysts, which do not contain any classical support material. 3D Atom-Probe-Tomography demonstrated single nanosized “CoCuMn” particles to have a core-shell structure, with Co forming the core, while all three metals are co-presented in a Cu-dominated outer shell. Additionally, some of the co-precipitated Mn-oxalate decomposed to MnOx which was catalytically inactive in CO hydrogenation but acted as a promoter and dispersant thus conferring the catalyst a high BET specific surface area. On the other hand, high resolution TEM of “CoCuNb” catalysts showed a bimodal nanosized particle distribution containing larger Co-Cu particles with sizes ranging from 25 to 40 nm and smaller Nb-oxides particles between 4 and 8 nm. X-ray photoelectron spectroscopy revealed NbOxto contain major amounts of Nb in the 4+ oxidation state.
With respect to the catalytic performance, we compared the activity, selectivity and chain lengthening probability (α) of “CoCuMn” with that of “CoCuNb” catalysts. Under conditions of low CO conversion (<10%), both catalyss compositions showed a higher 1-alcohols selectivity of more than 50 wt%, and the combined selectivity of 1-alcohols/1-olefins reached up to ~75 wt%. However, higher CO conversion at high temperature usually correlated with the formation of paraffins for thermodynamic reasons. In terms of the chain lengthening probability, the CoCuMn compositions showed an α-value of 0.6-0.9, so they turned out to optimize the C8-C14 slate of terminal alcohols used as feedstock for plasticizers, lubricants, or detergents. On the other hand, “CoCuNb” catalysts showed an α-value of 0.25-0.4 thus optimizing the yield of C2-C5short chain alcohols as fuel additives. Our research strategy thus proved to be viable for tuning the synthesis to either short- or long-chain 1-alcohols from CO hydrogenation [1-3].
Keywords: CO hydrogenation, higher 1-alcohols, CoCu-based catalysts, Mn, Nb, oxalate co-precipitation.
 Xiang, Y.; Chitry, V.; Liddicoat, P.; Felfer, P.; Cairney, J.; Ringer, S.; Kruse, N. J. Am. Chem. Soc. 2013, 135, 7114
 Xiang, Y.; Barbosa, R.; Kruse, N. ACS Catal. 2014, 4, 2792
 Xiang, Y.; Barbosa, R.; Li, X.; Kruse, N. ACS Catal. 2015, 5, 2929
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