Calcination
is necessary because kaolinite resists leaching, even
under highly acidic conditions [8]. The metakaolinite was
leached in 2.5 M H2SO4 solution at 90 C for 3 h with
stirring. These leaching conditions give an optimal SBET
value of the resulting MicroPS. After leaching, the product
was washed with dilute H2SO4 (0.5 M), washed three times
with deionized water and centrifuged. The Si/Al ratio of
the resulting MicroPS was 41 and its SBET value was
370 m2/g, arising from slit-shaped micropores 0.6 nm wide
[8]. The second silica source was reagent grade TEOS
(Wako), used without pre-treatment.