Insights into the Deactivation Mechanism of Heterogeneous Mo/Hβ-Al2O3 Catalysts for Olefin Metathesis

On-stream deactivation during metathesis reaction of ethene and butene-2 to propene over Mo/H beta-Al2O3 catalysts was examined. TG, TPO, solid-state NMR, UV-vis, EPR, and XPS techniques were used to characterize the nature and location of carbonaceous deposits as well as the oxidation state of Mo species on the fresh, used, and regenerated catalysts. It is found that side reaction of olefin oligomerization takes place simultaneously with the metathesis reaction even at low temperature. Most of the carbonaceous species are formed at the beginning of the reaction, and long-chain alkanes are the predominant coke species. There are three types of carbonaceous deposits located at different positions within the Mo/H beta-Al2O3 catalysts. Most of the aliphatic hydrocarbons formed with time on stream are associated with molybdenum species of the catalyst. Active sites poisoned by coke and deep reduction of Mo species may be responsible for catalyst deactivation. The recovery of the activity of regenerated catalyst further proves our proposed deactivation mechanism.