LaSBA-15 for Alkylation
361
silica and so on [10]. However, HPW has the tendency to
leach from carriers into polar reaction media, which leads to
the poor catalytic reusability.
2 Experimental
2.1 Mesoporous Materials Preparation
It is desired to develop a support with controlled
strength and concentration of surface basic sites can pre-
vent HPW leaching without destroying its acidity. Grafting
method, mainly involving the pre-functionalization of
carrier with organic components and chemical linkage of
HPW, has been chosen to anchor HPW on carriers for
avoiding leaching. Based on this premise, Pizzio et al. [11]
revealed that HPW supported on amine-functionalized
silica and SiMCM-41 exhibited negligible solubility and
high activity in the synthesis of isoamylacetate. Kim et al.
[13] has also reported that H3PMo12O40 was chemically
immobilized on a kind of porous carbon by forming a
positive charge on the support via surface modification. Jin
et al. [12] anchored Keggin-structured tungstovanadoger-
manic HPA (H5GeW11VO40) on the aminesilane func-
tionalized SBA-15, through which HPA clusters were
attached firmly to the surface of SBA-15 evidenced by
spectroscopic characterizations. SBA-15, a well-ordered
hexagonal mesoporous silica with an uniform pore size up
to *30 nm, is an ideal support for acid catalyst for its
thicker pore walls and higher thermal and hydrothermal
stability [14, 15]. Very recently, our research group suc-
cessfully incorporated rare-earth element La into SBA-15
silicate framework by direct synthesis method [16]. Com-
pared with the SBA-15, Lanthanum doped SBA-15 (LaS-
BA-15) possesses some weak Lewis acid and higher
thermal stability, besides a tunable pore and large surface
area, which is ideal for the dispersion of catalytically active
entities, thus being widely used as a catalytic support [17]
or catalyst [18, 19]. However, to the best of our knowledge,
no attempt has been made to immobilize Keggin-structured
phosphotungstic acid (HPW) on the aminopropyl-functio-
nalized mesostructured LaSBA-15 materials. Furthermore,
no systematic investigation on the amount of amino-groups
used for the surface functionalization of mesoporous
materials has been conducted yet.
Lanthanum-substituted mesoporous SBA-15 materials has
been prepared by direct synthesis. 9.4 g of tetraethyl-
orthosilicate and 0.97 g of Lanthanum nitrate (Si/La = 20
molar ratio) were added to 10 mL of HCl aqueous solution
at pH 1.5. This solution was stirred for over 3 h and then
added to a second solution containing 4 g of amphiphilic
triblock copolymer poly(ethylene oxide)-poly(propylene
oxide)-poly(ethylene oxide) (EO20PO70EO20) (Pluronic
123 from Aldrich) in 120 mL of HCl aqueous solution at
pH 1.5 at 40 °C. The mixture was stirred vigorously for
24 h and then transferred into a Teflon-lined autoclave and
aged for 48 h at 120 °C. The resulting solid was filtered,
washed and dried at 120 °C for 24 h. The final mesoporous
LaSBA-15 products were obtained after calcination in air
at 540 °C for 6 h (heating rate: 2 °C min-1).
The adsorption of HPW on the surface of LaSBA-15
was carried out by the following procedure: 1 g of LaSBA-
15 was added to an aqueous solution containing HPW
(0.60 g) with vigorous stirring at 80 °C. After being stirred
for 8 h, the solid was filtered, washed with distilled water
and then it was dried overnight at 80 °C. The obtained
sample is designated as HPW/LaSBA-15 catalyst.
2.2 Surface Modification of LaSBA-15
and Immobilization of HPW
Figure 1 shows the schematic procedures for the surface
modification of LaSBA-15 materials and the subsequent
immobilization of HPW on the surface of modified LaS-
BA-15 (NH2-LaSBA-15) materials. The surface modifica-
tion of the LaSBA-15 materials was achieved by reacting
the silanol group of the LaSBA-15 materials with APTES
under a nitrogen atmosphere. A known amount of APTES
was slowly added to a dry toluene solution containing 1 g
of LaSBA-15 materials with constant stirring at room
temperature. After the solid product was filtered and dried,
it was calcined at 180 °C for 2 h to yield the NH2-LaSBA-
15 samples. A series of NH2-LaSBA-15 samples (NH2-
LaSBA-15-1.0, NH2-LaSBA-15-1.5, NH2-LaSBA-15-2.0,
NH2-LaSBA-15-2.5, and NH2-LaSBA-15-3.0) were pre-
pared by adjusting the amount of APTES added to 1 g of
NH2-LaSBA-15 materials. For example, NH2-LaSBA-15-
1.0 denotes the NH2-LaSBA-15 materials prepared by the
addition of 1.0 mmol of APTES to 1 g of NH2-LaSBA-15
materials.
In this work, mesostructured LaSBA-15 was prepared
via a surfactant templating method. The as-obtained LaS-
BA-15 silica was then modified by grafting 3-aminopro-
pyltriethoxysilane (APTES) to provide sites for the
immobilization of HPW. By taking advantage of the
overall negative charge of [PW12O40]3-, the HPW catalyst
was chemically immobilized on the surface of modified
LaSBA-15 (NH2-LaSBA-15) materials as
a charge
matching component. The characteristics of HPW catalyst
immobilized on the NH2-LaSBA-15 materials were char-
acterized in terms of various physicochemical techniques.
The catalytic properties of the catalysts were assessed in
the alkylation of o-xylene with styrene. Special attention
was paid to catalyst stability and reusability.
The immobilization of HPW on the NH2-LaSBA-15
support was carried out as follows (shown in Fig. 1). 1 g of
NH2-LaSBA-15 was added to an aqueous solution con-
taining HPW (0.60 g) with vigorous stirring at 80 °C. After
123