Alkene Metathe­sis for Propy­lene Pro­duc­tion over W-based Cat­a­lysts: Insights from Multi-Functional Cat­a­lysts and Met­al­la­cy­clobu­tanes

2014 Spring Symposium

 
Christopher P. Nicholas

 
Abstract – Tungsten oxide supported on silica is an efficient catalyst for olefin metathesis used in industrial processes since the 1960s. Several elements point to isolated metal centers as the active sites, and from the Chauvin mechanism, it is reasonable to expect that carbene species are involved, possibly bearing an oxide ligand in the metal’s ligand sphere. Owing to the strategic importance of olefins as building blocks for the world chemical industry, development of efficient processes is of utmost relevance. More specifically, tailored heterogeneous catalysts with known structure–activity relationships may improve lifetimes and have higher numbers of active sites.

With our collaborators, we have been studying tungsten hydride supported on alumina prepared by the surface organometallic chemistry method as an active precursor for metathesis processes at low temperature and pressure. Taoufik, et.al. showed that ethylene can be converted to propylene at very high selectivities of 99% via a tri-functional mechanism involving dimerization, isomerization and cross-metathesis of ethylene and the produced 2-butene. Recently, via a contact time study we revealed that the dimerization of ethylene to 1-butene is the primary and also the rate limiting step in this reaction and results in deactivation of the catalyst due to a side reaction like olefin polymerization producing carbonaceous deposits on the catalyst.

With that knowledge, we have also investigated performance of the catalyst in the presence of ethylene and butenes. At low temperature (120 °C) in the cross-metathesis of ethylene and 2-butene, the catalyst deactivates notably with time on stream. However, at 150 °C, the catalyst was stable with time and thereby gave a high productivity in propylene. The ratio of ethylene to trans-2-butene was also studied, and the W-H/Al2O3 catalyst is stable and highly selective to propylene even at sub-stoichiometric ethylene ratios.

Surprisingly, we have also been able to obtain propylene in high yields from butene only feeds. 1-butene and 2-butene are both able to be converted into propylene at higher selectivity than expected due to isomerization and metathesis occurring simultaneously. Then, by studying isobutene / 2-butene cross-metathesis, we observed that the catalytic cycle involving the less sterically hindered tungstacyclobutane intermediate governs the conversion rate of the cross-metathesis reaction for propylene production from butenes and/or ethylene.
 

References

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Christopher_P_NicholasBiography – Chris joined UOP in 2006 after earning a Ph.D. from Northwestern University and working in the Hard Materials Center of Excellence at Sigma-Aldrich. He has worked in the Catalysis and Exploratory Research departments and is currently focused on New Materials Research. Chris is an inventor or co-inventor on 30+ US and foreign patents and coauthor of 13 peer reviewed journal articles and a book chapter. He has been involved with the Chicago Catalysis Club since graduate student days and is currently serving as the Program Chair for the Chicago Catalysis Club. Chris’ research interests encompass the gamut of inorganic and catalytic technologies ranging from materials synthesis to characterization to catalyst and process development. He has particularly enjoyed understanding the relationship between homogeneous and heterogeneous catalysts.