Due to the global shortage of propylene, the chemical industry is turning to metathesis of ethylene and 2-butene to produce on-purpose propylene. Heterogeneous supported ReOx/Al2O3, MoOx/Al2O3 and WOx/SiO2 catalysts have been found to be most efficient for metathesis of olefins. Although heterogeneous supported MoOx/Al2O3 catalysts have been used in the Shell Higher Olefin Process (SHOP) to produce higher olefins since 1968, the effect of the oxide support on olefin metathesis by supported MoOx catalysts is still not clear. Mixed oxide supports (Al2O3-SiO2, Al2O3-CoO, Al2O3-Bo2O3) are reported to be highly effective supports in comparison to the one component Al2O3 support for promoting surface MoOx sites for olefin metathesis. The improved catalytic performance of the mixed oxide supports has been mostly attributed to the modified surface Lewis and Brønsted acidity, but there is no supporting evidence for these conclusions and the nature of the surface MoOx sites are still unknown. To understand the effect of mixed oxide supports, a novel TaOx promoter was used to surface modify the Al2O3 support and further improve the catalytic activity of the surface MoOx sites for propylene self-metathesis to ethylene and 2-butene. The objective of this study is to determine the origin of the enhanced performance of the surface MoOx sites on the TaOx surface modified Al2O3 support. Modern in situ molecular spectroscopy (UV-Vis, Raman, IR and XAS) and chemical probe studies (basic chemical probe molecules, C2=/C4=-titration, C3H6-TPSR and steady-state propylene self-metathesis) of the surface MoOx sites (3%) on well-defined supported TaOx (5%-15%)/Al2O3 were investigated to determine the origin of the promotional effect by the surface TaOx sites. The enhanced catalytic activity was found to be related to the greater number of activated surface MoOx sites on the TaOx/Al2O3 support resulting from modification of the anchoring surface hydroxyls and formation of Al-(OH)+-Ta Brønsted acid sites. This study demonstrates that the olefin metathesis activity by supported MoOx catalysts can be tuned by modifying the nature of the anchoring surface hydroxyls on the Al2O3 support.

Molecular design of supported MoOx/Al2O3 catalysts with TaOx promoters for olefin metathesis

ACS Spring 2022

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