Abstract:
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.
