New approaches to photocatalytic reaction of low concentrations of arylamines in alcohols

Article abstract of DOI:10.1007/s11164-011-0472-y

Photocatalytic interactions of a series of arylamines, for example 1,4- phenylenediamine, 1,2-phenylenediamine, 4-aminophenol, and 2-aminophenol, at low concentrations in typical primary alcohols have been investigated in the presence of anatase TiO₂ nanopowder. GC-MS analysis of the irradiated solutions showed that, except for 1,2-phenylenediamine, 10 mmol/l alcoholic solutions of the arylamines gave predominately the corresponding imines. Irradiation of 2-aminophenol under these conditions, led to oxidation and dimerization to 2-amino-3Hphenoxazin- 3-one. Springer Science+Business Media B.V. 2012.

Full text of DOI:10.1007/s11164-011-0472-y

Res Chem Intermed (2012) 38:1411–1420
DOI 10.1007/s11164-011-0472-y
New approaches to photocatalytic reaction of low
concentrations of arylamines in alcohols
•
Azarmidokht Hosseinnia Mansoor Keyanpour-Rad
Received: 5 November 2011 / Accepted: 16 December 2011 / Published online: 7 January 2012
Ó Springer Science+Business Media B.V. 2012
Abstract Photocatalytic interactions of a series of arylamines, for example 1,4-
phenylenediamine, 1,2-phenylenediamine, 4-aminophenol, and 2-aminophenol, at
low concentrations in typical primary alcohols have been investigated in the pres-
ence of anatase TiO2 nanopowder. GC–MS analysis of the irradiated solutions
showed that, except for 1,2-phenylenediamine, 10 mmol/l alcoholic solutions of the
arylamines gave predominately the corresponding imines. Irradiation of 2-amino-
phenol under these conditions, led to oxidation and dimerization to 2-amino-3H-
phenoxazin-3-one.
Keywords Photocatalytic interactions ꢀ Arylamines ꢀ 2-Amino-3H-phenoxazin-3-
one ꢀ TiO2 nanopowder ꢀ Imines
Introduction
Because of the unique properties of titanium dioxide, interest in its applications in
different fields has increased rapidly in recent years [1, 2]. Use of this oxide in
photocatalytic processes is one such application, especially when an efficient,
photo-stable, non-toxic catalyst, potentially applicable in synthesis is needed [3–6].
Among different phases of TiO2, it is predominantly the anatase phase which has
high photoactivity, particularly when it is used as a nanopowder [7–9]. By
modifying this catalyst, for example by N-doping (N–TiO2) of the anatase phase, it
becomes applicable in narrow fields of synthesis, for example chemo-selective
reduction of aromatic nitro compounds to the corresponding amines [10].
A. Hosseinnia (&) ꢀ M. Keyanpour-Rad
Materials and Energy Research Center, P.O. Box 14155-4777, Tehran, Iran
e-mail: a-hosseinnia@merc.ac.ir
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A. Hosseinnia, M. Keyanpour-Rad
Development of new photocatalytic systems based on TiO2 for different applica-
tions in photosynthesis has become one of the most attractive research areas in
photochemistry. Motsushita et al. [11] developed a photocatalytic micro-reaction
based on immobilized Pt-loaded TiO2 (TiO2/Pt) for N-alkylation of arylamines at
room temperature; the reaction enabled N-alkylation of benzylamine, aniline, and
piperidine with the alkyl groups converted to the corresponding alcohols. In another
study, Karunakaran et al. [12] reported the photo-oxidation of aniline to azobenzene
on anatase TiO2 in ethanol, by use of solar and UV (365 nm) light.
A study of the mechanism of TiO2-photocatalyzed degradation of aniline
derivatives discovered that alkaline media enhance the direct photodegradation of
aniline or N,N-dimethylaniline by UV light at k [ 290 nm, resulting in the
formation of 2-aminophenol and benzoquinone as the main photoproducts [13].
Reaction of primary amines with carbonyl compounds to give imines, also called
Schiff bases, is well known [14, 15]. We have previously proposed a new
photocatalytic method for preparation of imines from alcohols and arylamines [16].
Advances in the applications of TiO2 as an efficient photocatalyst in the synthesis of
organic compounds stimulated us to take advantage of the efficient and powerful
technique of GC–MS analysis to study the photochemistry of the interactions of
different arylamines with primary and secondary alcohols, using anatase TiO2 as
photocatalyst.
Experimental procedure
Materials and instrumentation
All chemicals were obtained from Merck Chemical Company and used without
further purification. GC–MS analysis was performed with Agilent Technologies
GC-6890 and MS-5973 N equipment; compounds were separated on a 30 m
length 9 0.25 mm inter diameter 9 0.25 lm film thickness HP-5 column.
Synthesis of the photocatalyst
Anatase TiO2 nanopowder was synthesized and dried (azeotropic distillation drying
technique) by use of the procedure reported elsewhere [17].The sizes of the TiO2
nanoparticles after calcination were found to be 15–25 nm and the catalyst surface
area, which was measured by the BET method, was 75–80 m2/g.
Photocatalytic experiments
The arylamines, including 1,4-phenylenediamine, 1,2-phenylenediamine, o-amino-
phenol, and p-aminophenol, were dissolved in methanol and ethanol to prepare
5 mmol/l solutions in each alcohol. TiO2 nanopowder (20 mg) was then added to
10 mL of each solution in volumetric flasks, followed by sonication of each flask for
10 min to ensure homogenous suspensions were obtained. The suspensions, with
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New approaches to photocatalytic reaction
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blank solutions (containing only arylamines in the alcohols), were irradiated inside a
UV box, equipped with an array of 5 UVA lamps (k = 360 nm, 40 W).
Analytical procedure
The arylamine suspensions after filtration, with the blanks, were then analyzed by
GC–MS. The GC conditions were: initial temperature 100 °C held for 3 min, ramp
8 °C/min, and final temperature 250 °C. The concentrations of unreacted aryl-
amines and products were obtained by integration of the chromatogram peaks.
Results and discussion
The results from photo interactions of the arylamines with the alcohols are
summarized in Tables 1 and 2.
The products obtained by photocatalytic reaction of 1,4-phenylenediamine with
methanol in the presence of TiO2, after irradiation in the UV-Box for 3 h, were
filtered and the filtrate was analyzed by GC–MS. The mass spectra obtained, shown
in Fig. 1, suggested the presence of two major products, denoted ‘‘a’’ and ‘‘b’’,
which were identified as mono and disubstituted methylene on 1,4-phenylenedi-
amine, respectively. As reaction time was increased, formation of compound ‘‘b’’
gradually predominated, increasing to more than 95% and becoming the sole
product (m/z = 132) of the reaction. Controlling photocatalytic reactions is an
important step, and hence UVA (near visible) light with very low irradiation power
(40 W) was chosen as the source of radiation for this selective reaction. The lower-
wavelength light results in the formation of phenolic compounds [13]. Reaction of
the diamine with ethanol proceeded via a more complex path and afforded a variety
of products after 20 h (Eq. 1). As discussed elsewhere [16], in photocatalytic
oxidation reactions, alcohols are converted first to aldehydes and ketones, as
intermediates species [18, 19], followed by coupling of the resulting aldehydes and
amines, with loss of a molecule of water, to form the corresponding imines.
CH3
TiO2/UV
N
N
+
+ etc.
N
+ C2H5OH
H2N
NH2
H2N
20hrs
H3C
H3C
28.5%
12%
45%
ð1Þ
Photocatalytic reaction of 1,2-phenylenediamine with methanol afforded N-
methylene 1,2-phenylenediamine after 3 h. Another major product which started
to form when the reaction time was increased was analyzed by GC–MS and shown
to be a compound having m/z = 162 (it was unknown for us).
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Table 1 Products and yields from interactions of arylamines (5 mmol/l) and alcohols under UVA light
Arylamine
Irradiation Interacting m/z
time (h)
Product
Yield
(%)
alcohol
a
10
20
20
CH3OH
105 C6H5–N=CH2
119 C6H5N=CHCH3
133 C6H5N=C(CH3)2
[95
[95
[95
C2H5OH
iC3H7OH
NH2
a
30
40
40
CH3OH
148 CH2=NC6H4N(CH3)2
[95
[95
*50
H3C
N
C2H5OH
iC3H7OH
162 CH3–CH=NC6H4N(CH3)2
176 (CH3)2C=NC6H4N(CH3)2
NH2
H3C
10
24
20
CH3OH
CH3OH
C2H5OH
120 1,4CH2=NC6H4NH2?
132 CH2=NC6H4N=CH2
132 CH2=NC6H4N=CH2
134 CH3CH=NC6H4NH2?
*31
*52
[95
*12
H2N
NH2
160 CH3CH=NC6H4N=CHCH3 *28.5
10
CH3OH
120 1,2-CH2=NC6H4NH2
162
*20
*50
NH2
?
NH2
20
20
CH3OH
121 HOC6H4N=CH2
[85
*20
C2H5OH
135 HOC6H4N=CHCH3
HO
NH2
20
20
CH3OH
121 Db
*21
*14.5
*12
*21
*6
OH
C2H5OH
119
212
135
133
212
E
Q
F
NH2
G
Q
*13
a
From our previous work [16]
b
The formulas of these compounds are given in Table 2
Figure 2 shows the mass spectra of the major products from photocatalytic
reaction of 4-aminophenol with methanol and ethanol. This reaction was found to be
selective for synthesis of 4-N-methyleneaminophenol (Eq. 2). It was found that this
product was formed in approximately 83% yield after irradiation for 20 h, and
formation of 4-(N-dimethyl)aminophenol as the side product was insignificant
(\4%).
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New approaches to photocatalytic reaction
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Table 2 Products from
Codea Name
Formula
photocatalytic reaction of
2-aminophenol with methanol
and ethanol for 20 h
D
2-(Methyleneamino)
phenol
N
CH2
OH
N
E
F
1,3-Benzoxazole
O
N
2-(Ethyleneamino)
phenol
CH3
OH
G
Q
2-Methyl-1,3-
benzoxazole
N
CH3
O
N
2-Amino-3H-
phenoxazin-3-one
NH2
O
O
a
Given in Table 1
H3C
H2C
N
TiO2 /UV
20hrs
N
OH
+
OH
+ CH3OH
OH
H2N
H3C
4%
83%
ð2Þ
As expected, reaction of 4-aminophenol with ethanol was more complex than with
methanol, and the aldehydes and ketones formed as intermediates led to a variety of
products.
Photocatalytic reaction of 2-aminophenol with methanol and ethanol was more
interesting. GC–MS analysis of the products from the photocatalytic reaction with
ethanol after different irradiation times are shown in Fig. 3.
The mass spectra obtained from the products of this reaction (Fig. 4) revealed
that photo-oxidation of the intermediate formed in the course of reaction caused
cyclization and formation of 2-methyl-1,3-benzoxazole. At the same time, coupling
oxidation of 2-aminophenol resulted in the formation of product (Q), as shown in
this figure. GC separation of this mixture afforded 20% of the corresponding imine
(retention time = 6.95 min), 7% 2-methyl-1,3-benzoxazole (retention time =
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A. Hosseinnia, M. Keyanpour-Rad
Fig. 1 Mass spectra of the products from photocatalytic reaction of 1,4-phenylenediamine and methanol:
a monosubstituted (MW 120) and b disubstituted (MW 132)
4.77 min), and 13% 2-amino-3 phenoxazone (retention time = 21.67 min) after
irradiation for 20 h.
2-Amino-3-phenoxazin-3-one (Q) is formed by oxidative coupling of 2-amino-
phenol followed by loss of 3 molecules of hydrogen; it cannot be formed without the
presence of an oxidant species [20–22]. Therefore, formation of the imine
intermediate (G) seems to be a crucial step in formation of compound Q. This
process (Eq. 3) was confirmed by increasing the reaction time beyond 20 h. It was
found that increasing the reaction time caused a gradual decrease in the
concentrations of compounds (G) and (H) to the point they could not be detected
by GC, while the concentration of compound Q reached its maximum value. It was
interesting to discover that increasing the concentration of the starting material,
2-aminophenol, did not result in increased yield of compound (Q). Although
increasing the amount of TiO2 up to 80 mg/10 cc resulted in increased yield of Q
(Fig. 3), the yield of the compound started to decrease when more than 80 mg/
10 mL TiO2 was added, because of a increase of opacity of the solution and hence
hindrance of the passage of radiation throughout the solution [15].
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New approaches to photocatalytic reaction
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Fig. 2 Mass spectra of products of photoreaction of 4-aminophenol with methanol (MW 121) (a) and
ethanol (MW 135) (b) after irradiation for 20 h
O
N
O
N
NH2
OH
N
O
TiO2/UV
20 hrs
+
C2H5OH
+
+
NH2
OH
D
Q
G
ð3Þ
Maximum yield of compound Q under these conditions was 40% after irradiation
for 24 h. A report on preparation of compound Q has claimed the best yield of this
compound, prepared by a similar reaction, using solar irradiation as the source of
light, was only 12% after irradiation for several days [20]. Figure 5 illustrates how
the amount of the catalyst affects the yield of compound Q. The figure shows that
although increasing the amount of photocatalyst has the direct effect of increasing
the formation of compound Q, when the amount of photocatalyst is increased
beyond approximately 80 mg/10 mL formation of this compound starts to decrease
because of increased solution opacity.
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A. Hosseinnia, M. Keyanpour-Rad
Fig. 3 Chromatogram obtained from photoreaction of 2-aminophenol and ethanol in the presence of
TiO2: a before irradiation, b after irradiation for 20 h with 20 mg TiO2, and c after irradiation for 40 h
with 80 mg TiO2
Conclusions
Typical arylamines were reacted with primary alcohols in the presence of anatase
TiO2 nanoparticles under UVA irradiation. The concentrations of the arylamine and
alcohol had an important effect on the products formed, especially the amount of the
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New approaches to photocatalytic reaction
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Fig. 4 Mass spectra of the products obtained from photocatalytic reaction of 2-aminophenol and ethanol:
(retention time in chromatogram 4.77 min) 2-methyl-1,3-benzoxazole, (retention time in
chromatogram 21.96 min) 2-amino-3H-phenoxazin-3-one
G
Q
Fig. 5 Effect of amount of photocatalyst on the yield of 2-amino-3H-phenoxazin-3-one (Q)
123

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A. Hosseinnia, M. Keyanpour-Rad
corresponding imine (\10 mmol/l). When 2-aminophenol was the starting amine,
cyclization of the imine occurred, and with increasing irradiation time, 2-amino-3H-
phenoxazin-3-one was formed. Increasing the concentrations of arylamines in such
reactions enhances the formation of dimers, azo compounds, etc. The progress of
the reactions was monitored by GC analysis and each component was analyzed by
mass spectroscopy.
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