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.