V. Antochshuk et M. Jaroniec, Adsorption, thermogravimetric, and NMR studies of FSM-16 material functionalized with alkylmonochlorosilanes, J PHYS CH B, 103(30), 1999, pp. 6252-6261
Two series of alkyl-modified molecular sieves were prepared using the FSM-1
6 mesoporous silica. The attachment of alkyl groups to the surface, the str
ucture of the bonded layer, and changes in the mesoporous structure of the
FSM-16 material upon functionalization were studied by means of high-resolu
tion thermogravimetry, elemental analysis, nitrogen adsorption, and C-13 an
d Si-29 NMR. Surface coverages of attached ligands were in the range of 0.8
5-2.60 mmol/g. All modified samples showed a decrease in the pore size, whi
ch was in relation to the size of attached ligands. The uniformity of pores
upon modification proves that the mesoporous structure of FSM-16 is retain
ed after modification. The attachment of ligands rakes place in a solvent e
ither with a small amount of water or anhydrous indicating a direct reactio
n between the chlorosilane molecule and silanol groups and/or a reaction au
tocatalyzed by the small amount of water present on the FSM-16 surface. Nit
rogen adsorption isotherms and the adsorption energy distributions calculat
ed from these isotherms revealed the presence of highly ordered alkyl ligan
ds and the existence of the screening effect produced by alkyl groups as a
result of weak interactions of nitrogen probe molecules with attached long
alkyl chains. Thermal stability of modified samples greatly depends on the
conformation of attached ligands. By using appropriate solvent and/or therm
al treatment of modified samples, it was possible to achieve higher orderin
g of attached groups and consequently higher thermal stability of the resul
ting bonded phases. The studies of silicon spin-lattice relaxation times T-
1 support the formation of ligand structures on the surface and show that t
he surface accessibility greatly decreases with increasing length of attach
ed ligands and with refinement of the ordering of ligands on the surface. M
esoporous FSM-16 material appeared to be a good model for studying function
alization of the silica surface.