1 Department of Chemical and Biochemical Engineering, Technical University of Denmark2 CHEC Research Centre, Department of Chemical and Biochemical Engineering, Technical University of Denmark
This work has been an investigation of the catalytic conversion of syngas into mixed alcohols with Mo-based catalysts. The primary focus has been on the use of alkali promoted cobalt-molybdenum sulfide as a catalyst for the alcohol synthesis. The alcohol synthesis is a possibility for the production of gasoline additives/replacements from biomass via a gasification process. It is observed that the sulfide catalyst is able to operate both with and without a sulfur source in the syngas feed, but the presence of a sulfur source like H2S can exert a significant influence on the catalytic properties. The presence of 103 ppmv or more of H2S in the syngas feed stabilizes a large fraction of higher alcohols in the product. With 57 ppmv or less of H2S in the feed the production of higher alcohols is however gradually declining, while the methanol production increases. The present investigations could suggest that these changes in the product distribution, which occur in sulfur-free or sulfur-poor syngas, are related to changes in the state of cobalt, which is added to the catalyst to promote chain-growth. The distribution of methanol and higher alcohols in the product after 25 hours on stream is largely independent of the cobalt content in the catalyst, although the fraction of higher alcohols initially benefits significantly from an increased presence of cobalt. In catalysts that have operated in sulfur free syngas, cobalt is incorporated into larger, coagulated structures, and signs of crystalline Co9S8, which is considered to be inactive, can be observed in the spent catalyst. It is hypothesized that the loss of sulfur from the catalyst in the reducing atmosphere is driving the conversion of cobalt from its active form (possibly a mixed cobalt-molybdenum sulfide) into larger, more sulfur-deficient structures and into Co9S8. It must however be added that X-ray absorption spectroscopy investigations have not provided direct evidence for a Co-Mo coordination in neither the fresh nor the spent sulfide catalysts. That the catalyst requires the presence of a sulfur source in the feed to stabilize a large fraction of higher alcohols in the product introduces a dilemma, because the presence of a sulfur sauce like H2S in the gas can lead to an undesirable incorporation of sulfur species into the alcohol product. It is observed that the sulfur content in the condensed alcohol product increases linearly with the H2S level in the syngas feed from 1250 ppmw S with 46 ppmv H2S to 1905 ppmw S with 460 ppmv H2S. Without H2S in the feed sulfur species are also incorporated into the condensed reaction product, but in this case the products sulfur content decreases over time. The primary sulfur species in the alcohol product are the thiols corresponding to the formed alcohols. With the increasingly stringent regulations for sulfur in motor fuels this incorporation of sulfur into the alcohol product is an important issue for the use of the alcohol product as a fuel additive/substitute. It has been discovered that alcohol coupling reactions, which appear to occur via aldol condensation pathways, contribute to the chain-growth over the sulfide catalyst. Such coupling reactions via aldol condensation pathways can explain the observed presence of branched alcohols (e.g. iso-butanol) in the product. This discovery arose from the observation that ethanol co-fed along with the syngas especially causes an increased production of 1-butanol. Various investigations have been carried out to clarify the effect of the feed composition on the catalytic properties of the sulfide catalyst. In a sulfur free syngas it is observed that the production of higher alcohols is optimal with an equimolar mixture of CO and H2 in the feed, while the methanol production benefits from an increasing hydrogen content in the feed. It has often been argued that the sulfides ability to operate in a sulfur containing atmosphere may enable the user to employ a less thorough and therefore less costly syngas cleaning. To evaluate, to which extent a removal of other components in the raw syngas is necessary, the influence of NH3 and H2O in the feed has also been investigated. Ammonia (741 ppmv) in the feed is observed to cause a general and largely reversible deactivation of the catalyst. Operation with elevated water levels in the syngas feed (4.7-13.4 mol%) is observed to cause a deactivation of the catalyst, and it is especially the chain-growth, which is affected. A permanent, general deactivation is observed once the water is removed from the feed – a deactivation which could be caused by an accelerated sintering in the presence of water. Since the use of the sulfide catalyst encompasses the risk of sulfur being incorporated into the alcohol product, carbide catalysts have been investigated as a possible nonsulfided alternative. Various catalysts based upon the bulk carbides Mo2C, WC and NbC have been synthesized and evaluated with respect to the catalytic behavior in highpressure CO hydrogenation. NbC is largely inactive, and K2CO3/WC produces mainly methanol and methane with a low activity, while K2CO3/Mo2C produces a mixture of methanol and higher alcohols, but also significant amounts of hydrocarbons. The role of the choice of alkali cation in Mo2C promoted by an alkali salt has also been evaluated at a promoter level of Alkali/Mo = 0.164±0.001 mol/mol. At 275 °C-300 °C the behavior of catalysts promoted by Cs2CO3 and K2CO3 is qualitatively similar, although K provides a markedly better activity (31 % at 300 °C, 100 bar, 5000 h-1) and a better selectivity at identical conditions. At 275 °C an Li(CH3COO) promoted catalyst is very active and produces only hydrocarbons. If the effect of the different alkali promoters is compared at the same general activity level, corresponding to the Li-containing catalyst being operated at a lower temperature, the Li-promoted catalyst is however only slightly inferior to the K-promoted catalyst in terms of the alcohol selectivity. Finally different multiply promoted Mo2C catalysts have been evaluated in terms of the CO hydrogenation properties. Addition of Re (1 wt%) or Mn (1 or 5 wt%) to the K2CO3/Mo2C system results in a slight reduction in the catalytic activity. The addition of Cu (0.84 wt%) was on the other hand observed to improve the activity (by 33 % at 275 °C, 100 bar, 5000 h-1) and improve the selectivity of the K2CO3/Mo2C system without altering the distribution of the alcohol product. Mo2C modified with La (5 wt%) and V (3 wt%) produces mainly hydrocarbons.
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Jensen, Peter Arendt, Jensen, Anker Degn
Technical University of Denmark, Department of chemical and Biochemical Engineering, 2011