The effect of organic modifiers on Ag/Al2O3 catalysts for the sequential hydrogenation of 1-octyne vs 1-octene

Abstract

The selective hydrogenation of 1-octyne (in 1-octene) to 1-octene was carried out over a 5Ag/Al2O3 catalyst coated at one monolayer (ML) coverage with 20 wt% 1‑butyl‑3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][NTf2]) as the first model system, followed by coating with 15 wt% 1‑butyl‑3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), 15 wt% 1‑butyl‑3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), 15 wt% tetrabutylammonium nitrate ([N4444][NO3]) and the equivalent organic modifier, 1-ethylimidazole [EIM] (10 wt%). The initial uncoated catalyst gave high octyne conversions (99%) with a selectivity to 1-octene at 30% initially, reaching 70% within 24 h on-stream. In an attempt to improve the selectivity and catalyst stability with time, modifying the catalyst with [EIM] gave 1-octene selectivities of 50% over 36 h time-on-stream with an average 1-octene gain of 10 wt%. The conversion of 1-octyne decreased significantly from 99% over the uncoated catalyst to 30% over the modified version as a result of diffusion limitations. However, coating with different ionic liquids ([BMIM][NTf2], [BMIM][PF6], [BMIM][BF4] and [N4444][NO3]) gave increased 1-octene selectivities reaching up to 82% over the 5Ag-15%[N4444][NO3] catalyst. The stability of most SCILLs was maintained for a period exceeding 10 h after which deactivation of the active catalytic sites by either poisoning or metal sintering led to a decline in activity.