Kinetics of the reactions of the light alkenes over SAPO-34

Many petrochemical processes produce light olefin byproducts that can be used to produce propene. This study reports the kinetics of the reactions of ethene, propene and n-butene isomers over a SAPO-34 catalyst at 450 °C and different space velocities and discusses suitable modes of reaction to maximize propene yield. The shape selectivity of SAPO-34 prevented the formation of i-butene and high alkenes. When a single alkene (ethene, propene, or n-butene) was used as the reactant, the other alkenes were rapidly produced by dimerization and cracking such that the final compositions of the light alkenes were close to the equilibrium composition, with propene as the main species. Hydrogen transfer reactions to form an alkane from an alkene were slower than alkene inter-conversion. A kinetic model for alkene inter-conversion was deduced. Experimental and simulation results indicated that a lower partial pressure of the reactant gave increased propene yield when ethene was the feed.

A SAPO-34 molecular sieve catalyst can increase propene yield from light alkenes, including ethene, propene and linear butenes, by imposing shape selectivity on their inter-conversion. The different carbenium ion intermediates can be treated as a carbenium ion lump (Cx+) in a kinetic scheme where the light alkenes are first converted to Cx+ and then Cx+ is cracked to light alkenes, methane and C5+ in parallel reactions. Figure optionsDownload as PowerPoint slide