Photocatalytic reaction of arylamines/alcohols on TiO2 nano-particles

Article abstract of DOI:10.1007/s11164-010-0147-0

In this article, the photocatalytic reaction of aniline and 4-amino N,N-dimethyl aniline with methanol, ethanol and isopropanol on anatase TiO ₂ nano-particles under UV (365-nm wavelength) irradiation was examined. The concentration of unreacted arylamines and products was measured by gas chromatography picks integration, and then the products were identified by mass spectroscopy analysis. By making a comparison within the rates of photocatalysis of each arylamine in different alcohols under various irradiation times, it was revealed that, in all cases, the sequence of photocatalysis rate was methanol > ethanol > isopropanol. In reactions where the concentrations of arylamine were lower than 10 mmol/l, imines were the main products and the alkylation of amines was not observed. In the higher concentration of arylamines, oxidation and dimerization was occurred. Springer Science+Business Media B.V. 2010.

Full text of DOI:10.1007/s11164-010-0147-0

Res Chem Intermed (2010) 36:373–381
DOI 10.1007/s11164-010-0147-0
Photocatalytic reaction of arylamines/alcohols on TiO2
nano-particles
Azarmidokht Hosseinnia
Ketauon Karimian
•
Mohammad Pazouki
•
Received: 23 December 2009 / Accepted: 23 April 2010 / Published online: 17 June 2010
Springer Science+Business Media B.V. 2010
ꢀ
Abstract In this article, the photocatalytic reaction of aniline and 4-amino
N,N-dimethyl aniline with methanol, ethanol and isopropanol on anatase TiO nano-
2
particles under UV (365-nm wavelength) irradiation was examined. The concen-
tration of unreacted arylamines and products was measured by gas chromatography
picks integration, and then the products were identified by mass spectroscopy
analysis. By making a comparison within the rates of photocatalysis of each
arylamine in different alcohols under various irradiation times, it was revealed that, in
all cases, the sequence of photocatalysis rate was methanol[ ethanol [ isopropanol.
In reactions where the concentrations of arylamine were lower than 10 mmol/l,
imines were the main products and the alkylation of amines was not observed. In the
higher concentration of arylamines, oxidation and dimerization was occurred.
Keywords Photocatalytic reaction Á Arylamine Á TiO nano-particle Á
2
GC-MS analysis
Introduction
Heterogeneous photocatalysis has been extensively investigated as a possible method
for pollutant removal [1–3], organic synthesis [4–6] and bactericidal activity [7].
Nano-sized titania powder is widely used a semiconductor photocatalyst, owing to its
non-toxic nature, chemical stability, availability, low cost and capability of easy
preparation in the nano-particulate form with high surface area [8].
The initial step in TiO -mediated photocatalytic degradation is proposed to involve
2
-
the generation of an (e /p ) pair, leading mainly to the formation of hydroxyl radicals
?
A. Hosseinnia (&) Á M. Pazouki Á K. Karimian
Environment and Energy Group, Materials and Energy Research Center, Meshkin-Dasht Road,
P.O. Box 31787/316, Karaj, Iran
e-mail: a-hosseinnia@merc.ac.ir
123

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A. Hosseinnia et al.
•
•-
•
( OH), as well as super oxide radical anions (O2 ) and hydroperoxy radicals ( OOH).
These radicals are oxidizing species in the photocatalytic oxidation processes [9, 10].
Among the above-mentioned radicals from which TiO are produced by photocatal-
2
ysis, hydroxyl radicals are the most powerful oxidizing species that can attack organic
contaminants present at or near the surface of TiO particles [11]. In heterogeneous
2
photocatalysis studies involved with semiconductor powders, thermodynamic and
kinetic aspects, the surface area and photocatalyst particle size are important factors
for the identification of the reaction product [12].
In recent years, photocatalysis reaction of TiO on arylamines has attracted many
2
researchers due to its wide range of application in the organic dyes industries [12–16].
The mechanism of TiO -photocatalysed degradation of anilinederivative in acidic and
2
alkaline mediums has been studied by Canle et al. [12]. They have reported that
photocatalytic degradation of arylamines in acidic medium was inhibited due to
electrostatic repulsion between the positively charged surface and protonated amines,
while in alkaline medium, the main photoproduct of degradation of aniline is
nitrobenzene. As an example, in alkaline medium, the degradation of aniline has been
investigated using an alkaline aqueous TiO suspension containing carbonate ions
2
[
adsorption sites on the TiO particle surface. The major products in this reaction were
17]. The presence of carbonate in TiO suspension increases the number of active
2
2
identified to be azobenzene, p-benzoquinone, nitrobenzene and NH . Although aniline
3
at the ppm level is mineralised by illuminated TiO , at higher concentrations, it is
2
oxidized to azobenzene [13]. N-alkylation of amines by photocatalytic reaction in a
microreaction system using Pt-free TiO and Pt-loaded TiO has been investigated
2
2
[
TiO in a conventional batch reactor. To make a long story short, there has been a lot of
18]. It was reported that the N-alkylation did not occur by the irradiation of Pt-free
2
interest in the photocatalytic reaction of arylamines on TiO , but the results reportedin
2
,
the literature are partly controversial.
In the current work, we have investigated the photocatalytic reaction of two
arylamines (aniline and 4-amino N,N-dimethyl aniline) using TiO nano-powder
2
suspension in alcohols (methanol, ethanol and isopropanol) and the products of
photoreaction were followed by GC-MS analysis.
Experimental
Synthesis of photocatalyst
Nano-powder anatase TiO samples were synthesised by the precipitation method
2
and dehydrated with the azeotropic distillation technique, as we reported in previous
work [8]. The sizes of nano-powders obtained after calcinations were 15–25 nm and
2
the surface area measured by the BET method was 75–80 m /g.
Photocatalytic activity
The arylamines (aniline and 4-amino N,N-dimethyl aniline) were dissolved
separately in methanol, ethanol and isopropanol with 1–10 mmol/L concentrations.
1
23

Photocatalytic reaction of arylamines/alcohols
375
A sample of 20 mg synthesised TiO2 nano-powder was added to each Pyrex
volumetric balloon containing 10 mL of the arylamine solution. The contents of
balloons were sonicated in an ultrasound bath for 10 min. The arylamines solutions,
in the presence and in the absence of TiO , were irradiated for 1–40 h by five UV
2
lamps (k = 360 nm) with 40 W power, in an irradiation box.
Analysis
The reaction products were analysed by gas chromatography–mass selective
detector (GC-MS) from Agilent Technologies, GC-6890N and MS-5973N model,
with the column HP-5, length 30 m, I.D. 0.25 mm and film thickness 0.25 lm. The
employed GC condition was: initial temperature 100 ꢁC, hold for 3 min with the
ramp rate 8 ꢁC/min and the final temperature was 250 ꢁC. The concentration of
unreacted arylamine and products was obtained from the integration of chromato-
gram picks.
Results and discussions
Arylamines are widely used as raw materials in organic dye industries and are
known to be highly toxic chemicals. The two arylamines which were chosen for this
experiment are aniline (An) and 4-amino N,N-dimethyl aniline (ADA).
An amount of 10 mL of ADA solutions in alcohol (methanol, ethanol,
isopropanol) with 5 mmol/L concentration and 20 mg TiO2 nano-powder was
sonicated for 10 min, and then irradiated under 40 W UVA. The rate of decrease of
the ADA concentration is shown in Fig. 1. As shown in this figure, the rate of
photocatalytic degradation of ADA in methanol is higher than ethanol and
isopropanol, respectively. After 40 h irradiation in the presence of TiO nano-
2
powder, the ADA had completely disappeared in methanol and ethanol. However,
the concentration of ADA in isopropanol reduced slowly and after 24 h, only 20%
of the initial concentration was decomposed. To compare the results, a blank
experiment was carried out on ADA/ethanol solution in the absence of TiO2
particles. ADA did not effectively degrade under similar conditions (see Fig. 1).
The chromatogram of photocatalytic reaction products of ADA and methanol is
displayed in Fig. 2. The pick at the 8.96 min retention time (RT) corresponds to ADA
and the pick at RT = 9.83 min corresponds to the major product. The mass spectra of
major product are shown in Fig. 3. All of the picks in the mass spectra are matched
with 4(methylene amino) phenyl dimethyl amine (1). The photocatalytic reaction
products of ADA/ethanol solution after 30 h irradiation are shown in Fig. 4. This
chromatogram shows two major products at RT = 11.01 and RT = 11.68. The mass
spectrum of the first product is shown in Fig. 5. This spectrum confirms the formation
of 4(ethylene amino) phenyl dimethyl amine (2) and dialkenylation may have
occurred in ethanol (RT = 11.68). So, from the obtained results, we can propose the
reaction process as shown in Scheme 1 for imines formation.
However, the rate of reaction in isopropanol was lower than the other two
alcohols. The chromatogram of this reaction product after 24 h irradiation is shown
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A. Hosseinnia et al.
Fig. 1 Variations of ADA concentrations (%) in alcohol as a function of the irradiation time. The
samples were 10 mL ADA in alcohol with 5 mmol/L concentration and 20 mg TiO
2
Fig. 2 Chromatogram of
photocatalytic reaction product
of ADA and methanol after 20 h
irradiation. The pick at
RT = 8.96 min corresponds to
ADA and RT = 9.83 is the main
product
2
Fig. 3 Mass spectra of the main product of ADA and methanol with TiO (from GC-MS spectroscopy,
RT = 9.83). The mass spectra completely matched with 4(methylene amino) phenyl dimethyl amine
in Fig. 6 and the mass spectrum of the major product (RT = 11.71 min) is shown in
Fig. 7. The addition of isopropenyl on amine is confirmed by characteristic picks in
m/e 176, 161, 145, 120, 105 and 77.
1
23

Photocatalytic reaction of arylamines/alcohols
377
Fig. 4 Chromatogram of
photocatalytic reaction product
of ADA and ethanol after 30 h
irradiation. The pick at
RT = 8.96 min corresponds to
ADA and RT = 11.01 and
11.68 are the main products
Fig. 5 Mass spectra of the main product of ADA and ethanol with TiO
2
(from GC-MS spectroscopy,
RT = 11.01). The mass spectra completely matched with 4(ethylene amino) phenyl dimethyl amine
After a long period of UVA irradiation (about 70 h) in the presence of TiO2,
ADA was completely decomposed in methanol and ethanol. But, during the same
time, ADA in isopropanol was converted to various compounds. Although in the
photodegradation process, especially at higher concentrations (C50 mmol), the
dimerisation of arylamine with the violet colour may have occurred, at the lower
concentrations (B5 mmol/L), ADA did not dimerize.
The rate of photocatalytic reaction of aniline in alcohol was higher than ADA.
Figure 8 shows the decreasing rate of aniline in the 10 mL aniline/alcohol solutions
with 1 mmol/L concentration of aniline in the presence and absence of TiO . As
2
shown in this figure, aniline remained unchanged in the absence of TiO after 5 h
2
irradiation and, similar to the ADA, photocatalytic reaction of aniline–methanol and
aniline–ethanol is more effective than isopropanol. The chromatogram of the
photocatalytic reaction of aniline in methanol after 3 h is shown in Fig. 9. During
the 3-h reaction time, more than 82.5% of aniline was converted to other
compounds. In this chromatogram, the pick at RT = 2.97 corresponds to the aniline
and the pick at RT = 2.68 corresponds to the product. The mass spectra of this
product match completely with N-methylene amino phenyl (the mass spectra are
displayed in Fig. 10). During this time, the number of products in ethanol is greater
and a small amount of 2-methyl quinoline is considered to be additional to
N-ethylene amino phenyl. At the higher concentrations (more than 50 mmol/L),
many various products, e.g. azo benzene, azoxy benzene and 4-phenyl azo phenol,
were formed. Scheme 2 shows the reaction process of the photocatalytic reaction of
aniline in alcohols.
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A. Hosseinnia et al.
Scheme 1 Alkenylation of ADA in alcohol in the presence of TiO
2
under UV (365 nm) irradiation
Fig. 6 Chromatogram of
photocatalytical reaction product
of ADA and isopropanol after
24 h irradiation. The pick at
RT = 8.96 min corresponds to
ADA and RT = 11.71 is the
main product
To investigate the reason for the lower reaction rate of arylamine in isopropanol,
the pure isopropanol with TiO nano-powder was irradiated under UVA lamps. As a
2
result, various compounds were produced. The products of reaction were acetone,
isopropyl orthoformate and acetaldehyde diisopropyl acetate, as identified by
GC-MS analysis. A condensation reaction between amines and ketones or aldehydes
(
from photocatalytic reaction of alcohols) may have afforded imine or enamine
1
23

Photocatalytic reaction of arylamines/alcohols
379
Fig. 7 Mass spectra of the main product of ADA and isopropanol with TiO
2
(from GC-MS spectroscopy,
RT = 11.71). The mass spectra completely matched with 4(isopropylene amino) phenyl dimethyl amine
Fig. 8 Variations of aniline concentrations (%) in alcohol as a function of the irradiation time. The
samples were 10 mL aniline in alcohol with 1 mmol/L concentration and 20 mg TiO
2
Fig. 9 Chromatogram of
photocatalytic reaction product
of aniline and methanol after 3 h
irradiation. The pick at
RT = 2.97 min corresponds to
aniline and RT = 2.68 is the
main product
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A. Hosseinnia et al.
Fig. 10 Mass spectra of the main product of aniline and methanol with TiO
2
(from GC-MS
spectroscopy, RT = 2.68). The mass spectra completely matched with N-methylene aniline
Scheme 2 Alkenylation of
aniline in alcohol in the presence
of TiO under UV (365 nm)
2
irradiation
compounds. The photocatalytic reaction of alcohol with longer chains is more
complex and leads to the formation of various products. This can be attributed to the
formation of numerous radical species which finally react together perfectly.
Conclusions
Samples containing arylamine/alcohol and TiO nano-powder suspensions were
2
irradiated with 40 W UVA lamps. The GC-MS analysis showed the imine formation
at the concentrations lower than 10 mmol/L. The imines can be isomerised to
enamines form. The sequence of photocatalytic reaction rate was methanol [
ethanol [ isopropanol. Investigation of these reaction mechanisms showed that
active radicals of alcohol forms in the presence of TiO under UVA irradiation and
2
these radicals initiate the reactions. The reason for the difference in the reaction
1
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Photocatalytic reaction of arylamines/alcohols
381
rates correspond to the stability of formed radicals from alcohol. Also, the variety of
radicals from isopropanol is more than two other alcohols, so the radicals react
together perfectly. At the higher concentrations (more than 10 mmol), oxidation of
arylamine occurs and some products, e.g. azobenzene, azoxy benzene, 4-phenyl azo
phenol and some of their derivatives, are formed. In the results of these reactions,
the alkylation of arylamines is not observed in any of the reaction studies.
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