Environmental Engineering Reference
In-Depth Information
molecules on mesoporous materials are two key parameters in determining
the density and quality of the functionalized monolayers. As can be seen from
the silane-coupling reaction schematically illustrated in Fig. 6.4, surface silanols
are essential because they are the active centers for silane condensation and
anchoring on the particle surfaces through siloxane chemical bonds. Adsorbed
surface water is necessary for the hydrolysis of APTES in toluene, which
initializes the condensation reaction process. However, the presence of excess
free water is deleterious to the formation of a clean monolayer, as APTES is
known to polymerize into white solid precipitates in the presence of water. The
precipitates can potentially block the pores and hence reduce the effective
surface area of the functionalized sorbents. For these reasons, a proper amount
of water for the hydrolysis of APTES needs to be employed to obtain a
monolayer of silanized APTES on the pore surfaces only.
Calcination at 540
o
C to remove surfactant templates and obtain mesoporous
particles (Section 6.4.4) dehydrates the mesoporous silica-coated magnetite
surface and depletes most of the silanol groups. Such a dehydrated surface
would result in a poor surface coverage of functional groups if silanized directly.
In this study, to optimize the reaction conditions for depositing alkoxysilane
monolayers on mesoporous silica-coated Fe
3
O
4
surfaces, the particles were
rehydrated carefully by steaming the samples. The amount of surface-adsorbed
water was controlled by drying. High-purity toluene was used through out the
synthesis since toluene was reported in literature to be excellent for removing
excess water and forming organic monolayers.
As shown in DRIFTS spectrum b of Fig. 6.13B, obtained with the steamed
and dried samples, the presence of a sharp H-O vibrational band at 3,750 cm
1
confirms the successful hydrolysis of siloxane bonds by steaming. DRIFTS
spectrum c of the silanized mesoporous-Fe
3
O
4
in Fig. 6.13B exhibits the char-
acteristic bands of APTES. A pair of weak broad bands at 3,400-3,250 cm
1
is
evident in the spectrum. These two bands are assigned to free amino asymmetric
and symmetric stretching vibrations. A strong band at 1,568 cm
1
is assigned to
the deformation bending vibrations of free amine groups on the surface. In
addition, two bands at 2,932 and 2,860 cm
1
assigned to asymmetric and
symmetric stretching vibrations of CH
2
in alkyl chains, along with a band at
1,483 cm
1
assigned to CH
2
bending vibrations, are evident. These spectral
features confirm the silanation of APTES on the particle surfaces. It is inter-
esting to note that the characteristic bands of Si-O-C at 1,167, 1,105, 1,083, and
959 cm
1
almost disappeared after silanation. This finding suggests that most of
the ethoxy groups in APTES were hydrolyzed. Two strong bands at 1,126 and
1,041 cm
1
(characteristic of siloxane Si-O-Si stretching vibrations) remained after
silanation, indicating that the surface binding of APTES was not by silanols but
rather by siloxane bonds. The presence of siloxane binding was further supported by
the disappearance of IR bands at 3,745 cm
1
, assigned to the stretching vibrations
of surface silanols in the spectra of silica-coated magnetic particles before calcina-
tion and steamed particles (spectrum b in Fig. 6.13B).
