Deactivation and stability of K-CoMoSx mixed alcohol synthesis catalysts

• Desulfurization of K-CoMoS MAS catalysts results in BOTH oxidation and reduction of the catalyst surface.
• In operation, surface carbide is formed and is distinct from coke.
• Continuous sulfidation may prevent oxidation and coking but not carburization.
• Oxidation is reversible until coke is formed, resulting in permanent activity loss.
• H2S maintains an active and selective catalyst while organic sulfides may not.

Potassium-promoted cobalt molybdenum sulfide (K-CoMoSx) mixed alcohol synthesis catalysts were operated from 118 to 3969 h for the purpose of studying catalyst deactivation. Continuous and discontinuous sulfiding with H2S and methyl sulfides was considered. Fresh and discharged catalysts were analyzed via X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). When continuously sulfided, selectivity was maintained for thousands of hours. Without sulfur cofeed, catalysts exhibited a change in selectivity from alcohols to hydrocarbons over periods of hundreds of hours. Sulfur deprivation resulted in oxidization, carburization, and coking of the catalyst surface and segregation of cobalt into crystalline Co9S8. It is suggested that in the absence of a sulfiding agent, the catalyst surface becomes more acidic (oxidized) promoting dehydration of alcohols (selectivity change) and coking (blocking active sites). Reintroduction of H2S may reverse oxidation on non-coked surfaces. Proper sulfur maintenance may render catalysts operable for years without need of regeneration or replacement.

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