2013 AIChE Annual Meeting
(603e) Selective Synthesis of C2+ Terminal Alcohols By CO Hydrogenation Over Cocu-Based Catalysts
While short-chain C2-C6 mixed alcohols play an increasingly important role as fuel additives long-chain C8-C14 1-alcohols form the feedstock for plasticizers, detergents and lubricants. Generally, 1-alcohols can be obtained from natural feedstocks such as fatty acid esters or synthezied from petrochemical sources through hydroformylation and subsequent hydrogenation of aldehydes. More recently, alternative procedures for long chain 1-alcohols production have been developed as, for example, anti-Markovnikov 1-olefin hydration and one-pot hydroformylation-hydrogenation using homogeneous catalysis. On the other hand, given the continuing problems to comply with the Markovnikov rule and the noble metal recovery the question may be asked if a heterogeneous process based on the Fischer Tropsch technology can be developed to produce strictly 1-alcohols. We delt with this question in the recent past and provide here an account of our successful attempts to boost the selectivity of short-chain and long-chain 1-alcohols synthesis.
Our approach is based on tuning the composition of CoCu-based catalysts. In a first step, our efforts concentrated on increasing the mixibility of both metals (only a maximum of 9% Cu can be dissolved in Co metal) by adding a third metal. Moreover, we chose to co-precipitate all metals as oxalates and to decompose these precursors by hydrogen-assisted TPDec (temperature programmed decomposition). This procedure allowed us to produce ternary systems such as “CoCuMn”. It should be noted that these systems do not contain a classical support material. 3D Atom-Probe-Tomography demonstrated single nanosized “CoCuMn” particles to have a core-shell structure, with Co forming the core. Most interestingly, atom mapping using the same technique revealed the thin outer shell to contain all metals but to be otherwise dominated by Cu atoms. It should also be noted that some of the co-precipitated Mn-oxalate decomposed to Mn-oxides (mainly Mn5O8) which was catalytically inactive in CO hydrogenation but acted as a dispersant and thus conferred the catalyst a high BET specific surface area.
In this paper, we compare the catalytic performance of “CoCuMn” with that of “CoCuMo” and “CoCuNb”. Under conditions of low CO conversion (<10%), catalyst compositions of Co1Cu1Mn1, Co1Cu1Mo1 and Co1Cu1Nb0.4 showed 1-alcohols selectivities of more than 50 wt% and sometimes reach more than 90%. The α-ASF chain lengthening probability for alcohols over these catalysts was quite different, however. While a unique α value was obtained for “CoCuMn” (an optimum C8-C14 1-alcohol slate with α≈0.65 and a CO conversion close to 20% was achieved at T=240 °C and a total pressure of 60 bar (H2/CO=2)) both Co1Cu1Mo1 and Co1Cu1Nb0.4 ASF plots showed two chain lengthening probabilities depending on the chain length. For Co1Cu1Mo1 we found a behavior very close to that of pure CoCu (α≈0.28) in the range of C1 to C7. For C7+, however, the α-values usually exceeded those of Co1Cu1Mn1. Just the opposite ASF chain lengthening behavior was observed for Co1Cu1Nb0.4. In that case, the α-values started high up to C4 and then remained close to those of pure CoCu.
Our research strategy thus proved to be viable for tuning the synthesis to long-chain 1-alcohols for plasticizers, surfactants or detergents production, or to short-chain C1-C5 alcohols with a possible use as fuel additives.
Keywords: CO hydrogenation, long- and short-chain 1-alcohols, CoCu-based catalysts, Mn, Nb and Mo promotion.