abstract
The electron density maps calculated from X-ray diffraction patterns of the mesoporous silica material
MCM-41 present averaged mesostructure images which in contrast to transmission electron microscopy
(TEM) images are exceptionally repeating and represent the whole sample. It was shown that the aver-
aged mesostructure parameters such as a unit cell parameter, a pore diameter, a wall width, a pore shape
estimated from the X-ray powder diffraction data in combination with the continuous electron density
function approach allow monitoring continuous set of the silica framework states at different stages of
the material synthesis.
Ar ti c l e h i st or y:
Recei v ed 8 F eb r u ar y 2 0 1 4
Recei v ed i n r e vi s ed f or m 2 4 Mar ch 2 0 1 4
Accep t ed 6 Ap r i l 2 0 1 4
Ava i l ab l e o n l i n e 16 A pr i l 2 0 1 4
Keywo rd s :
Mes o s tr u ctu r e
Hy d r ot h er mal s t ab i l i ty
MC M-4 1
X- ra y d i ff ra cti o n
TE M
The mentioned technique supplemented by N2-adsorption measurements and transmission electron
microscopy was applied for consideration of the MCM-41 hydrothermal stability. Attention has been given
tothe variationsofthe synthesis conditionsaffecting the hydrother mal stability, inparticular the maintain-
ing the basicity of the synthesis solution as well the substitution of the synthesis solution with water or a
salt solution at hydrothermal treatment. The averaged pore shape was observed to be changed from cylin-
drical to hexagonal-prismatic form. The observed wallthickness wasin the range from 0.75 to1.25 nm. The
competition of silica polycondensation and surface hydrolysiswas shown tobe responsible for the variety
of framework geometry and hydrothermal stability. It has been established that the pore diameter
increases generally due to the osmotic pressure of water. If the pores acquire the average prismatic hexag-
onalshape, the sample has low hydrothermalstability. Under the conditions favorable for the polyconden-
sation the pores have averaged cylindricalshape and material demonstrateshigher hydrothermal stability.
Cooperative mechanism of mesostructure destruction under hydrothermal conditions was observed using
TEM data and was discussed in connection with irregular polycondensation.
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