FT-IR pyridine spectroscopy has been applied to obtain the surface acid amount of various catalysts. The FT-IR pyridine spectra recorded at two adsorption temperatures of 200 °C and 350 °C are displayed in Fig. S14. As reported, the peak appearing at 1540 cm-1 can be ascribed to the Brönsted acid sites (B). The peak at about 1490 cm-1 is resulted from both Brönsted and Lewis acid sites. Two characteristic peaks at 1450 cm-1 and 1610 cm-1 can be ascribed to Lewis acid sites (L).[47] In addition, a shoulder at about 1620 cm-1 appeared in the patterns of the NiMo catalysts containing Al species can be assigned to Lewis acid sites,[48] which may be caused by exposed Al3+ ions. It should be noteworthy that the intensity of peaks belonging to B and L acid sites increase after Al and Ti modification. Therefore, it can deduce that the incorporation of Al and Ti atoms into SBA-16 material can increase the amount of B acid sites and L acid sites. The amounts of Brönsted, Lewis, total acid sites and the B/L ratios determined from Fig. S14 are concluded in Table 2. The adsorption intensity of pyridine molecules degassed at 200 °C and 350 °C on the surface of NiMo catalysts should be ascribed to the amount of total acid sites and the amounts of medium and strong acid sites, respectively.[14]From Table 2, the amounts of B acid sites, L acid sites and B+L acid sites for other catalysts increase after Al and Ti modification. The total amounts and the amounts of medium and strong acid sites follow the order of NiMo/SBA-16 < NiMo/AT-0 < NiMo/AT-2.5 < NiMo/AT-5 <NiMo/AT-7.5 <NiMo/AT-10. Therefore, incorporation of Al species into SBA-16 materials can generate more acid sites than those of Ti species. Moreover, the B/L ratio for NiMo/SBA-16 catalyst is higher than those of NiMo/Al-Ti-SBA-16 catalysts, indicating that Al and Ti modification can generate more L acid sites than B acid sites. For NiMo/SBA-16 and NiMo/AT-0 catalysts, the amount of medium and strong acid sites of B type are 0 µmol·g-1, confirming that the incorporation of Ti atoms into SBA-16 material can mainly generate L type acid sites by Ti species appending to the support through surface silanol groups.[49]
Table 3 the proportion of Mo species over different sulfide catalysts, as determined by XPS spectra.