Properties of supports
Small angle XRD patterns of various supports are displayed in Fig. S1. It showed that all samples present a sharp peak ranged from 0.5o to 1o, which is an indicative of (110) reflection and a 3D cubic meso-structure with Im3m space group. Meanwhile, a poor-resolved shoulder at about 1.2o should be assigned to (200) reflection.[22] It can deduce that SBA-16 silica is successfully synthesized and the structural order degree of SBA-16 material can be maintained after incorporation of Al and Ti species.
N2 adsorption–desorption isotherms of different supports are shown in Fig. S2. The textural properties of different supports, determined from N2 adsorption–desorption isotherms, are displayed in Table 1. The BET surface area and pore volume of SBA-16 pure silica are the highest of 975.7 m2·g-1 and 1.38 cm3·g-1, respectively. The pore size of SBA-16 pure silica is lower than that of Al-Ti-SBA-16 composites except AT-0 support, which may be resulted from the longer calcination time for AT-7.5, AT-10, AT-5 and AT-2.5 supports and further generation of intergranular pores. Moreover, the cubic unit cell parameters a0 of Al-Ti-SBA-16 composites modified by duplex metals are higher than those of SBA-16 pure silica and only Al or Ti modified SBA-16 materials. The mesopores void fractions εmes of Al-Ti-SBA-16 composites ranged from 0.66 to 0.69 are lower than that of SBA-16 material. The diameter of the spherical cavities Dme for AT-2.5 support presents the highest value of 3.80 nm. The wall thickness hw of Al-Ti-SBA-16 composites composed of both Al and Ti atoms are higher than other materials. Above all, the incorporation of Al and Ti species through the stepwise method can protect the textural properties and maintain the highly ordered degree of SBA-16 silica.
Table 1 Textural properties of serial supports.