Catalysis of Organic..

Mantilla, Tzompantzi, Torres and Gómez 63With respect to deactivation, sulfated TiO 2 catalyst showed a high stability,mostly at GHSV of 8h -1 , where no deactivation occurs after 6 hours on stream(Figure 1). On the other hand, at high GHSV 64 h -1 the sulfated TiO 2 deactivation infunction of time and the conversion goes down from 42 to 23%. Stable activity wasobserved for GHSV of 16 y 32 h -1 . In the NiY catalyst, the lost of activity is notablypronounced for GHSV values of 32 and 64 h -1 , and after 7 h on stream the catalystwas totally deactivated (Fig. 2). The isobutane was fed to the reactor as a diluent andit did not show conversion in any experiment. The selectivity to triisobutylene washigher for the “in situ” sulfated titania than for the NiY catalyst in all the cases (Fig.3).Figure 3 Selectivity to C 12 = fraction over sulfated TiO 2 and NiY catalystsThe isobutene oligomerization is a highly exothermic reaction, carried out viathe carbenium ion mechanism, which is thermodynamically favoured at lowtemperature. The kind of products obtained as well as the conversion and stability atconstant temperature and pressure will depend on the reaction GHSV, whichdetermine the intermediate carbenio ion formed during the first steps.Another important factor in the behaviour of the catalysts is the nature and forceof their acid sites. Both Lewis and Brönsted acid sites are necessary to carry out thereaction, but an important fact is the relative abundance of them as it has beenrecently reported for sulfated TiO 2 [12].Figure 4 shows the reaction pathway for the isobutene oligomerization. After thedimer formation, the addition of another one butene molecule will depend mainly ofBrönsted acidity to stabilize the formation of the carbocation. The oligomerization toheavier olefins will be favored on catalyst showing a low L/B acids sites ratio [12].