1906
Lai Sin Yuan et al.
number of problems, such as leaching and metal poi- 2. Experimental
soning. Previous studies have proven that the catalytic
2
.1 Materials
activity of catalysts containing Ti(IV) can be greatly
enhanced for bulky and long chain organic com-
pounds. However, the leaching of Ti(IV) active sites
The chemicals used are salicylaldehyde (98%, Sigma-
Aldrich), absolute ethanol (Merck), 3-aminopropyltri-
methoxysilane (97%, Sigma-Aldrich), titanium(IV)
sulphate solution (24%, Kanto Chemical Co., Inc.), and
octadecyltrimethoxysilane (90%, Acros Organics) and
molecular sieves 3Å (25 g, Sigma-Aldrich).
10
still can occur and the efficiency of hydrogen per-
11
oxide (H O ) decreases in organic-aqueous oxida-
2
2
tion. Support materials, i.e. silica, contain high density
of silanol groups (SiOH) which is highly hydrophilic.
The water molecules produced from H O easily form
2
2
strong hydrogen bonds with Si-OH and compete with
12,13
H O .
Apart from that, water molecules are able
2
2
2
.2 Methods
to open the oxirane ring of epoxides to form diol,
which can strongly adsorb to the titanium(IV)’s active
2.2a Synthesis of silica functionalized with non-
12,13
sites, causing catalyst deactivation.
results were significantly affected and thereby, some
The catalytic
silylated Ti-salicylaldimine complex: Silica function-
alized with Ti-salicylaldimine complex was synthe-
sized as follows: first, salicylaldimine ligand, which
is generally known as Schiff base ligand, was pre-
pared under nitrogen flow. Salicylaldehyde (0.05 mol,
8
,13–18
researchers
have come out with various silylating
agents to be grafted or attached to the surface of catalyst
to induce hydrophobicity.
Previously, it was reported that the long hydropho-
bic chain of octadecyltrichlorosilane (OTS), which was
attached to fluorinated-TiO -ZrO (F-TiO -ZrO ), man-
5
.30 mL) in absolute ethanol (20.00 mL) and 3-
aminopropyltrimethoxysilane (0.05 mol, 9.30 mL) in
absolute ethanol (20.00 mL) were added drop-wised
into a three-neck round bottom flask containing molec-
ular sieves 3Å (25 g) and absolute ethanol (50.00 mL).
Salicylaldehyde 98% (0.05 mol, 5.3 mL) reacted with
2
2
2
2
aged to achieve higher TON for titanium(IV) active sites
as compared to F-TiO -ZrO in 1-octene oxidation by
2
2
8
aqueous H O . Another research showed that the short
2
2
hydrophobic chain of N-methyl-N-(trimethylsilyl)tri-
fluoroacetamide (MSTFA) has also increased the epox-
ide yields in the epoxidation of cyclohexene with
3
-aminopropyltrimethoxysilane (0.05 mol, 9.3 mL) to
form bright yellowish ligand under mild stirring at room
temperature for a few minutes. The mixture was cen-
trifuged at 4000 rpm for 5 min and then vacuumed to
remove the ethanol solvent. Dark yellowish ligand was
formed after the removal of the solvent. Titanium(IV)
sulphate solution (0.01 mol, 8.9 mL) in 10.0 mL of
absolute ethanol was added into the yellowish ligand
14
13,15–18
H O . Several researches
showed that cata-
2
2
lysts with surface modification of hydrophobic sily-
lated titanium(IV) catalysts performed remarkable
higher activity than the non-silylated counterpart of
titanium(IV) catalysts. These examples proved that
the surface properties1
3–18
of catalysts play signif-
(0.02 mol) in 40.0 mL absolute ethanol. The solution
icant roles in achieving high catalytic activity and
selectivity.
quickly turned into colloidal yellowish suspension. It
was stirred overnight at room temperature to complete
the reaction. The resulting powder was centrifuged at
Herein, this research highlights the types of silylating
agents and catalysts utilized in the oxidation of small
aromatic alkene. Most of the silylating agents used in
previous reports were of short alkylsilyl chain with
trimethylsilyl and hexamethylsilyl groups in the oxida-
tion of small alkenes by aqueous H O . To date, the
4000 rpm for 10 min and washed with ethanol, fol-
lowed by hexane, twice each. The amorphous com-
plex formed was dried at room temperature for a few
days. The weight of the silica functionalized with non-
silylated Ti-salicylaldimine complex was 0.58 g with
2
2
findings of silylation with long alkylsilyl chain are
lessj reported. Besides that, catalysts commonly used
to improve hydrophobicity are metal oxides, especially
2
4% of titanium(IV) in the catalyst.
12–14,17
silylated Ti-MCM-41 and silylated Ti-MCM-48.
2.2b Silica functionalized with silylated of Ti-
Thus, an attempt to produce a different type of cat-
alyst, i.e. silica functionalized with silylated Ti-salic-
yladimine complex is shown here. A series of varia-
salicylaldimine complex: The silylation of Ti-
salicylaldimine complex by octadecyltrimethoxysilane
(OTMS) with Ti:OTMS molar ratio of 1:4 was car-
tions in the amount of octadecylsilyl (C ) group is used ried out as follows: Ti-salicylaldimine complex (10.00
18
to silylate Ti-salicylaldimine complex for the oxida- mmol) was added with OTMS (2.50 mmol, 1.18 mL)
tion of limonene (small, aromatic alkene) by aqueous and dissolved in toluene (10.00 mL). The mixture
H O .
was then stirred overnight. To prevent hydrolysis of
2
2