The textural properties of the mesoporous SBA-15 (SBA15)
adsorbent before and after its functionalization with amino groups
were studied by N2 adsorption–desorption isotherms at 77 K. As
example, the N2 adsorption–desorption isotherms of pure SBA15
and SBA15–0.4NH2 SBA15 having maximum concentration of
APTES are shown in Fig. 3. All samples exhibited irreversible type
IV adsorption–desorption isotherms with a H1-type hysteresis
loop in the partial pressure range from 0.55 to 0.80, which is characteristic
of materials with 6–8 nm pore diameter [43,30,44]. A
well-defined step occurs approximately at P/P0 0.6, which is
associated with the filling of the mesopores due to capillary condensation. After modification of SBA-15 with amino groups,
the amount of adsorbed nitrogen decreases and the inflection point
of the step shifts from 0.6 to 0.55 value of relative pressure. The reduced
value of adsorbed nitrogen suggests the pores plugging after
the grafting process, while the shift of the step to a lower value of
relative pressure is indicative of smaller pore sizes. Indeed, the isotherm
of pure SBA15 sorbent shows one step desorption branch
indicating the existence of an open pore structure whereas the
SBA15–0.4NH2 adsorbent show two steps desorption branch suggesting
the pores plugging by ligand. The pore size distributions
of pure SBA15 and SBA15–0.4NH2 sorbents, as calculated from
the adsorption branch of N2 isotherm by using the Barrett–Joyner–
Halenda (BJH) model [45], are shown in both inlets of Fig. 3.
It should be noted that both SBA15 and SBA15–0.4NH2 sorbents
show a uniform, narrow pore size distributions centered at about
7.2 and 5.8 nm, respectively, confirming again that the modification
of the SBA-15 with maximum concentration of APTES does
not damage the hexagonal structure of this material.
Table 1 lists some textural properties of SBA-15 before and after
grafting, which were calculated from nitrogen sorption studies by
applying the BET equation for specific surface area [46] and the
BJH formula for pore size distribution [45]. For the functionalized
SBA-15 samples the BET surface and volume were standardized versus pure silica weights. As seen in Table 1, the BET surface area,
the mesoporous volume and average pore diameter did not suffer
significant changes after SBA-15 grafting with –NH2 groups. This
fact suggests that no changes occur in pore characteristics in spite
of the increase of amine loading in the SBA-15.