Silication of γ-alumina catalyst during the dehydration of linear primary alcohols

The dehydration of alcohols to alkenes over heterogeneous catalysts such as γ-alumina occurs at temperatures of around 300 °C, and it could be accompanied by undesirable reactions such as double-bond shift and, in severe cases, skeletal isomerization of the alkene products. In the dehydration of 1-hexanol and of 1-octanol (separately) over γ-alumina, we found that the selectivity to 1-alkenes decreased substantially with time-on-stream (TOS), while the alcohol conversion remained virtually constant. The loss in 1-alkene selectivity is a result of an increase in the extent of double-bond isomerization with increasing TOS. Upon analysis of the catalysts (fresh and used) and of the alcohol feeds (all by ICP spectroscopy), it was found that the feeds contained on average 1 ppm silicon and that the silicon content in the used catalysts was as high as 15 times that in the fresh one (3000 ppm as compared to 200 ppm, in one of our examples). The results show, therefore, that there is accumulation of silicon on the catalyst during the reaction and that this leads to the formation of additional and stronger acid sites, such as those found in silicated aluminas, which account for the increasing isomerization activity of the γ-alumina catalyst.