Double-bond isomerization of hexadecenes with solid acid catalysts

• Active and selective Nafion/Al2O3 catalysts were synthesized.
• Selectivity of Nafion/SiO2 were traced to a TPD range of a standard probe base.
• Oversulfonated polymer resin catalysts were more active.
• We identified cooperative effects from closely-spaced acid sites.
• Degree of crosslinking greatly affects turnover frequencies.

The positional isomerization of hexadecenes was investigated using the following catalysts: perfluorinated ion exchange resins (Nafion®) supported on SiO2 or Al2O3, sulfonated poly(styrene-co-divinylbenzene) resins (PS-DVBs), tungstated zirconias, and acidic zeolites. Acid sites were characterized by propan-1-amine (1-PA) temperature programmed desorption, wet titration, and decomposition of sulfonate groups. Catalysts were evaluated in a packed bed reactor at 130 °C for 3 h with flowing hexadecenes. Active Nafion/Al2O3 catalysts were synthesized by wet impregnation and, in one case, such a catalyst was selective for double-bond isomerization, although less active than silica-supported Nafion. Selected catalysts underwent lifetime studies to examine the effects of structure and acid strength on deactivation. We found that oversulfonated PS-DVBs (e.g., Amberlyst 35) were reasonably selective with long lifetimes, only gradually deactivating due to poisoning by surface oligomers. Amberlyst 70, Amberlyst XN1010, ZSM-35 and SAPO-11 were evaluated because of their lower acid site densities and (for the Amberlysts) varying degrees of crosslinking. None of these catalysts were optimal for various reasons. Beneficial cooperative effects arising from the close proximity of acid sites in certain catalysts were observed.

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