Direct synthesis of mesoporous M-SBA-15 (M = Al, Fe, B, Cr) and application to 1-hexene oligomerization
M-SBA-15 materials (M = Al, B, Cr, Fe) were prepared by different direct synthesis methods, characterized and tested catalytically in the 1-hexene oligomerization reaction at 125ºC. Al-SBA-15 were synthesized within the range Si/Al=12-86 using aluminium isopropoxide as aluminium source. 27Al MAS NMR spectra point out that the three Al-SBA-15 samples contain more tetrahedral aluminium than octahedral and the ratio tetrahedral / octahedral diminished with increasing aluminium contents. B-SBA-15 was prepared with a Si/B=51-106 using two direct synthesis methods (sol-gel and hydrothermal) and three different boron sources (solid boric acid, aqueous boric acid and boron isopropoxide). The best results in terms of boron incorporation (Si/B = 51) were achieved with solid boric acid and the sol-gel method, although a lower degree of mesoscopic ordering was obtained compared to the hydrothermal procedure. 11B MAS NMR showed that trigonal boron is the principal coordination state obtained after calcination. Cr-SBA-15 was also achieved by direct synthesis method at different pH (1.5, 3, 5) but the incorporation degree was low, at best Si/Cr = 240 at pH = 5. UV-vis spectroscopy indicated that all Cr species were Cr6+ formed during the calcination step. Oligomerization of 1-hexene at 125ºC showed that the highest conversion (~ 30%) was attained over Al-SBA-15 (Si/Al=30) although Cr-SBA-15 material exhibited close conversion (21%) despite its low heteroatom content (Si/Cr = 240). Dimers were the major products over Al-SBA-15, Cr-SBA-15 and Fe-SBA-15 catalysts (selectivity ¿ 40%) while strikingly, sol gel B-SBA-15 lead mostly to heavy oligomers (> 60%), with less than 10 % of dimers.
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