3.1. Textural Analysis
The textural characteristics of the as-prepared samples were studied by BET surface analysis, and their constituent morphologies were analyzed by HR-TEM. Figure 1 shows N2 adsorption-desorption isotherms and pore size distribution (PSD) over the pristine SBA-15 as a reference material, SBA-15 immobilized ionic liquids IL1/SBA-15 and IL2/SBA-15. No marked difference can be noticed in the isotherms for pristine silica SBA-15, IL1/SBA-15 and IL2/SBA-15. All isotherms are predominately of type IV, exhibiting IUPAC type H1 hysteresis loop which is a characteristic of porous materials consisting of agglomerates or particles of uniform and regularly packed materials (Gu , 2007). The results of TEM images (Fig.2) and PSD curves support these findings. The immobilization of ionic liquids onto SBA-15 surface seemed to change the adsorption characteristics of IL1/SBA-15 and IL2/SBA-15 through the remarkable modification in the pore structure. This may be due to the existing organic layer screening Si-O-Si bonds and thus impeding the adsorption process. Such findings can be demonstrated in terms of the pore dimension and surface area values (Table 1), where the specific surface area decrease from about 668m2/g for SBA-15 to about 484 m2/g and 502 m2/g for IL1/SBA-15 and IL2/SBA-15, respectively. The pore volume VP, also decrease in the same manner. In opposite to this result, the average pore diameter (Dp) increased with immobilization of the ionic liquids due to the decrease in population of pore fractions centered at 3.6 nm which increase the average pore diameter, see PSD curves.
Figure 2 shows the constituent morphologies of the presented structures in this article. From the image, the pure SBA-15 possesses a regular pore system with a two-dimensional, long-range hexagonal, ordered open-frame structure that confirms to the results of BET surface analysis. The pore lengths seem to be nearly straight and can be estimated by matching with the image scale (~150 nm). The straight path length of the pores of the support surface, promotes the catalytic activity by diminishing the diffusional limitation. It is difficult to observe the pore dimensions after the loading of ionic liquids due to the coverage of support surface with the functionalizing sheet.