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I.K. Mbaraka et al. / Journal of Catalysis 219 (2003) 329–336
catalyzed reactions [14–16]. The direct co-condensation
synthesis technique in which the mesostructure and func-
tional group are simultaneously introduced appears to be
a desirable route for incorporating functional groups be-
cause it has been shown that it increases the concentration
of the sulfonic groups in the mesoporous silica relative to
grafting [15,17]. One approach demonstrated previously
involves one-step synthesis based on the simultaneous hy-
drolysis and condensation of tetraethoxysilane (TEOS) with
room temperature for 24 h followed by washing with EtOH
and H2O. The wet cake was acidified in 0.1 M H2SO4 for
an additional 4 h before being washed thoroughly with H2O.
The product was finally dried at 393 K.
2.2. SBA-15–SO3H
SBA-15–SO3H was prepared by dissolving 4 g of Plur-
onic (P123 or L64) in 125 g of 1.9 M HCl at room temper-
◦
ature under stirring with subsequent heating to 40 C before
(
3-mercaptopropyl)trimethoxysilane (MPTMS) in the pres-
adding TEOS. Approximately 45 min was allowed for pre-
hydrolysis of TEOS prior to addition of the MPTMS–H2O2
solution. The resulting mixture with a molar composition
of 0.0369 TEOS, 0.0041 MPTMS, and 0.0369 H2O2 was
ence of template surfactant using in situ oxidation of the thiol
groups with H2O2. Melero et al. have shown that the acid
strength of the sulfonic groups in the mesoporous materials
can be adjusted by choice of the organosulfonic precur-
sor [18]. For example, incorporation of a more electron-
withdrawing group (e.g., phenyl group) with the sulfonic
group will significantly increase the acidic strength of the
resulting mesoporous material. Due to their large pore diam-
eters, these acid-functionalized mesoporous silicas provide
improved accessibility to large reactants such as fatty acids
and their esters [16].
Herein, are described the synthesis and utilization of sil-
ica mesoporous materials modified with sulfonic groups for
the pretreatment esterification reaction of high free fatty acid
oils. The results for the catalytic performance of the meso-
porous materials are also compared to commercial acidic
catalysts.
◦
stirred for 24 h at 40 C and thereafter aged for 24 h at
◦
1
00 C under static conditions. The product was collected
and subjected to the same extraction method as previously
described.
2
.3. SBA-15–ph-SO3H
SBA-15–ph-SO3H mesoporous silica functionalized with
benzenesulfonic acid groups was synthesized by dissolv-
ing Pluronic P123 (4 g) in 125 ml of 1.9 M HCl at room
temperature while stirring. After complete dissolution, the
◦
solution was heated to 35 C. TEOS (8.76 g) was added
◦
dropwise to the solution at a constant temperature of 35 C.
After a TEOS prehydrolysis of 45 min, 2.66 mL of 2-(4-
chlorosulfonylphenyl)ethyltrimethoxysilane (CSPTMS) so-
lution in methylene chloride (50%, Gelest) was added drop-
wise (to prevent phase separation). The resulting mixture
2
. Experimental
◦
◦
was stirred for 20 h at 35 C following by aging at 95 C for
another 24 h. The molar composition of the mixture for 4 g
of copolymer was 7.4 TEOS:1 CSPTMS:48 HCl:1466 H2O.
The solid was isolated via filtration, washed extensively with
methanol, and dried in air. The surfactant template was re-
moved by suspending the solid material in ethanol and re-
fluxing for 48 h. The sulfonyl chloride groups underwent
hydrolysis in the acidic media of the reaction.
The mesoporous materials were synthesized following
the procedures of Bossaert et al. [16] and Melero et al.
18] with only slight modification. Tetraethoxysilane (98%,
[
Aldrich) was used as the silica source. The mesoporous sil-
icas were modified using (3-mercaptopropyl)trimethoxysil-
ane (85%, Fluka) without further treatment. The surfactants,
n-dodecylamine (Aldrich), Pluronic L64, and Pluronic P123
(
BASF Co., USA), were used as purchased to tailor the tex-
tural properties of the mesoporous materials. Mesoporous
silica synthesized using the amine surfactant was denoted as
HMS, while those synthesized with the tri-block copolymers
were abbreviated SBA-15 [12].
2
.4. Characterization
The textural properties of the mesoporous materials were
measured using the BET procedure. Nitrogen adsorption–
desorption isotherms were taken at 77 K using a Micromerit-
ics ASAP 2000 system. The ion capacities of the sulfonic
acid groups in the functionalized mesoporous silica were
quantified using 2 M NaCl (aq) as the ion-exchange agent.
Approximately 0.05 g of the sample was added to 15 ml of
the salt solution and allowed to equilibrate. Thereafter, it was
titrated by dropwise addition of 0.01 M NaOH (aq) [15].
2
.1. HMS–SO3H
A molar composition of 0.08 TEOS, 0.02 MPTMS,
0
.0275 n-dodecylamine, 0.89 EtOH, and 2.94 H2O was
used to synthesize HMS–SO3H. The amine was dissolved
in an alcohol–water mixture prior to addition of the TEOS–
MPTMS mixture. The mixture was aged for 24 h at room
temperature under continuous stirring. The resulting solid
product was filtered and air-dried. The template was ex-
tracted by refluxing in boiling EtOH for 24 h. The thiol
groups were oxidized with H2O2 (2.04 g/g solid) in a
methanol–water mixture. The suspension was stirred at
2.5. Catalytic tests
The reagents used for the catalytic test included palmitic
acid (PA, ꢀ 95%, Sigma), refined soybean oil (SBO, Wes-
son), and methanol (MeOH, ꢀ 99.9%, Fisher Scientific).