B. Krishnakumar, M. Swaminathan / Journal of Molecular Catalysis A: Chemical 334 (2011) 98–102
99
Scheme 1. Protonation of TiO2 by aqueous H2SO4.
Scheme 2. Semiconductor catalyzed N-formylation of aniline with formic acid.
the phase. In general the surface area of the catalysts is the most
important factor influencing the catalytic activity. The surface area
of TiO2-SO4 was determined using the nitrogen gas adsorption
the starting material. 12.5 mL of tetraisopropyl ortho-titanate was
dissolved in 100 mL of 2-propanol and to this solution 3.2 mL of 1 M
H2SO4 was added drop wise under vigorous stirring. The resulting
colloidal suspension was stirred for 4 h. The gel obtained was fil-
tered, washed and dried in an air oven at 100 ◦C for 12 h. Addition
of BaCl2 to filtrate gave no precipitate indicating that all the sulfate
ions were completely loaded on the gel. The sample was calcinated
at 400 ◦C in a muffle furnace for 1 h. This catalyst contained 5 wt% of
SO42−. Similarly catalysts with 3 and 7 wt% of SO42− were prepared
with the same procedure. The bare TiO2 was prepared with water
instead of H2SO4.
2−
method. The BET surface, pore volume and median pore width of
2−
2−
are given in Table 1. BET surface area of TiO2-SO4
TiO2-SO4
(142 m2 g−1) is higher than the Degussa TiO2-P25 (50 m2 g−1). The
crystallite sizes of both prepared TiO2 and TiO2-SO42− are 34.7 and
11.6 nm, respectively. The size of TiO2-P25 is 30 nm [4]. Among
2−
the three catalysts TiO2-SO4
is having the minimum size. Both
size reduction and retardation of aggregation result in increase in
its surface area. This increases the photocatalytic activity of the
catalyst in this reaction. Herein, we report the green synthesis of
N-formylation of amines at room temperature using nano-TiO2-
2−
SO4
.
2.4. General procedure for N-formylation of an amine
2. Experimental
To a mixture of amine (1 mmol) and formic acid (3 mmol) in ace-
tonitrile (8–10 mL), 0.1 g of TiO2-SO42− or TiO2-P25 was added and
the mixture was stirred at room temperature. The progress of the
reaction was monitored by TLC. After completion of the reaction,
ethylacetate was added to the solidified mixture and the insolu-
ble catalyst was separated by filtration. The filtrate was dried over
anhydrous Na2SO4. The solvent was evaporated and the product,
obtained, was subjected to GC and GC–MS analysis for the deter-
mination of the yield of the products. The structure of products
obtained had been confirmed by FT-IR, 1H NMR, 13C NMR and
GC–MS analysis.
2.1. Materials and methods
Aniline, 4-methylaniline, 4-methoxyaniline, 4-chloroaniline,
4-fluoroaniline,4-carboxylaniline, 4-nitroaniline, piperazine, N-
methyl aniline, cyclohexyl amine, n-butyl amine, monoethanol
amine, 4-methylpiperazine, 1,2-phenylenediamine and formic acid
(85%) (Aldrich chemicals) were used as received. A gift sample of
TiO2-P25 was obtained from Degussa (Germany). It is a 80:20 mix-
ture of anatase and rutile. It has a particle size of 30 nm and BET
specific area 50 m2 g−1. Solvents of LR grade were distilled prior
to use. AnalaR grade titanium isopropoxide (Himedia 98.0%), 2-
propanol (Spectrochem 99.5%) and H2SO4 (Fischer 98%) were used
as such.
3. Results and discussion
and formic acid (3 mmol) was stirred for 48 h in acetonitrile at
room temperature. However, addition of a catalytic amount of TiO2-
P25/TiO2-SO42− to this mixture has rapidly induced N-formylation
was carried out in the presence of TiO2-P25, prepared TiO2 (with-
45 min) (Table 2; entries 4 and 5). It was surprising to find that the
quantitative yield of 99.2 (Table 2; entry 7). Structure of this product
has been confirmed by spectral and GC–MS data [26]. The percent-
age yields of the product with 3 and 5 wt% of sulfate concentrations
are 98.2 and 99.2, respectively (Table 2; entries 6 and 7). 5 wt% of
sulfated titania was found to be the most efficient. When the sulfate
2.2. Apparatus
IR spectra were recorded using Avatar-330 FT-IR spectropho-
tometer using KBr pellets. For GC analysis, Perkin-Elmer GC-9000
with a capillary column of DB-5 and flame ionization detector
was used. GC/MS analysis was carried out using GC model: Varian
GC–MS-Saturn 2200 Thermo, capillary column VF5MS (5% phenyl-
95% methylpolysiloxane), 30 m length, 0.25 mm internal diameter,
0.25 m film thickness, temperature of column range from 50 to
280 ◦C (10 ◦C min−1), and injector temperature 250 ◦C. Proton and
carbon NMR spectra were recorded on a BRUKER AVIII FT-NMR
spectrometer operating at 500 MHz for all the samples. The specific
surface areas of the samples were determined through nitrogen
adsorption at 77 K on the basis of BET equation using a micrometrics
ASAP 2020 V3.00 H.
Table 2
2.3. Preparation of sulfate loaded TiO2 photocatalysts
Effect of different catalysts (0.1 g) and solvent on N-formylation of amine (aniline)
(1 mmol) at room temperature.
2−
The catalyst with 5 wt% of SO4
was prepared by sol–gel
Entry
Catalyst
Solvent
Yielda (%)
method, taking tetraisopropyl ortho-titanate (Himedia 98.0%) as
1
2
3
4
5
6
7
8
TiO2-P25
TiO2-P25
TiO2-P25
TiO2-P25
Ethanol (45)
CHCl3 (45)
DCM (45)
CH3CN (45)
CH3CN (45)
CH3CN (30)
CH3CN (30)
CH3CN (30)
98.4
97.6
98.8
99.2
98.0
98.2
99.2
98.3
Table 1
2−
Surface properties of TiO2-SO4
.
Prepared TiO2
Properties
Values
2−
2−
2−
3%-TiO2-SO4
5%-TiO2-SO4
7%-TiO2-SO4
BET surface area
Maximum pore volume
Median pore width
142 (m2 g−1
)
0.162300 cm3/g
˚
35.725 A
a
Yields with respect to amine. Values within parentheses are indicating reaction
time in minutes.
Molecular cross-sectional area
0.1620 nm2