The metathesis activity and deactivation of heterogeneous metal oxide catalytic systems

Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2004.

During this investigation it was found that the 8 wt% W03/SiO2 catalyst was effective for the metathetical conversion of 1-octene or 1-heptene to longer chain internal olefins in the detergent range. Various factors were investigated and the following conclusions were drawn: (a) Characterisation of catalysts with different metal loadings showed that crystalline W03 is the predominant species On catalyst with higher loadings. The W03 is well dispersed at lower loadings and is present a surface species. The activity of the catalyst increase with loading up to 6 wt% and is independent of any further increase in metal loading. This gives an indication that crystalline material does not contribute significantly to the metathesis activity. The stability of the catalysts is greater at loadings of 8 wt% and higher which suggests that crystalline material may play a stabilising role by preventing over-reduction of the active species. (b) Alkali metal ion doping can be used to significantly reduce the formation of branched metathesis products and is thus useful for curbing skeletal isomerization activity. Excessive doping (> 0.5%) results in loss of Bronsted acidity and hence a dramatic loss of metathesis activity. (c) The 8 wt% WO3/Si02 catalyst has a long lifetime (700 h) when operating in the recycle mode using the optimised conditions used (460°C, 16 h-I and 1:5.6 feed: recycle ratio). Regeneration of the catalyst results in a longer lifetime (1200 h) suggesting a decrease in acidity or better dispersion. Coke formation seems to be the cause of deactivation. The catalyst seems to coke from inside the pores and these act as reservoirs for the deposits. (d) Carbon maps (EFTEM) of the coked catalyst showed that carbon was located around the W03 clusters and did not cover them. This explains why the catalyst is still active even after the accumulation of excessive amounts of coke (e) Coke formation is dependent on a number of factors including temperature, time online, LHSV and amount of olefin. Trace quantities of oxygenates (100 ppm) can CONCLUSIONS 117 be used as coke retarding additives. These may act by blocking acid sites that result in reactions that lead to coke formation. (f) The catalyst is sensitive to the typical oxygenates (300 ppm in the recycle mode) present in an FT-derived feed stream however the effect of these poisons is reversible upon reintroduction of a pure feed stream. The oxygenates lower the intensity of the yellow colour of the product which is believed to be caused by polyaromatics. This effect may prove to be beneficial in terms of the quality of the final product.

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