Evaluation of surface enthalpy of porous aluminosilicates of the MCM-41 type using immersional calorimetry: Effect of the pore size and framework Si : Al ratio
Mj. Meziani et al., Evaluation of surface enthalpy of porous aluminosilicates of the MCM-41 type using immersional calorimetry: Effect of the pore size and framework Si : Al ratio, J COLL I SC, 233(2), 2001, pp. 219-226
Surface properties of porous aluminosilicates of the MCM-41 type have been
tested by immersional calorimetry. Two series of materials, referred to as
SiAL chi Cn, where chi is the Si:Al mole ratio and n the chain length of th
e surfactant template, having (1)chi = 32 and n = 8, 12, 14, 16, 18 and (2)
n = 14 and chi = 8, 32, oo, were used. The results of thermogravimetric ana
lysis on these samples served to evaluate the surface density of hydroxyl g
roups. This parameter is rather sensitive to the pore size than to the alum
inium content in the solid matrix. Based on the experimentally measured ent
halpies of immersion in n-heptane, water, and formamide per unit BET specif
ic surface area, estimates could be made of the apolar, Lewis acid, and Lew
is base contributions to the total surface enthalpy of MCR-41 materials. Th
e samples studied have a predominant surface acidic character, which is mar
kedly enhanced by incorporating aluminium into the silica matrix. Surface a
cidity is also modified by changes in the porous structure, although the tr
ends are less noticeable here. Nevertheless, the total surface enthalpy of
MCM-41 aluminosilicates appears to be small in comparison with typical inor
ganic oxides, such as silica or alumina. (C) 2001 Academic Press.