Catalytic behavior of sulfated TiO₂ in light olefins oligomerization

Isobutene oligomerization was carried out at room temperature over sulfated-titania sol-gel catalysts prepared by three methods; (i) "in situ" sulfation using sulfuric acid, (ii) gelling with nitric acid and sulfation (a), and (iii) gelling with ammonium hydroxide and sulfation (b). XRD spectra identified anatase as the crystalline phase in all the sulfated samples. The specific surface areas were 126, 115 and 62 m²/g, and FTIR-pyridine total acidity was 280, 105 and 160 mmol/g, showing L/B ratios of 3.65, 1.82 and 1.25, for the "in situ", sulfated (a) and sulfated (b) samples, respectively. For the isobutene oligomerization carried out in a continuous flow reactor, the "in situ" sample showed 100% of conversion even after 90 h in stream, with selectivity to C₈-C ₁₂ olefins of 76%. In TiO₂ sulfated (a) the initial conversion was 100%, and it was maintaining during 40 h with selectivity to C₈-C₁₂ olefins of 88%. On the other hand, TiO₂ sulfated (b) sample showed a continuous and pronounced lost in activity. In this case the selectivity to C₈ olefins, it evolves with time and reaches values of 70%. It has been proposed that the catalytic behavior of the sulfated TiO₂ catalysts strongly depends on the relative Lewis/Brönsted sites abundance. At higher L/B ratio corresponds higher catalyst stability.