Photocatalytic reaction of aryl amines/alcohols on TiO2 nanoparticles

Article abstract of DOI:10.1007/s11164-010-0206-6

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 aryl amines 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 aryl amine 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 aryl amine were lower than 10 mmol/L, imines were the main products and alkylation of amines was not observed. In the higher concentration of aryl amines, oxidation and dimerization occurred.

Full text of DOI:10.1007/s11164-010-0206-6

Res Chem Intermed (2010) 36:937–945
DOI 10.1007/s11164-010-0206-6
Photocatalytic reaction of aryl amines/alcohols
on TiO2 nanoparticles
•
Azarmidokht Hosseinnia Mohammad Pazouki
Ketauon Karimian
•
Received: 24 December 2009 / Accepted: 17 September 2010 / Published online: 28 November 2010
Ó 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 TiO2 nano-
particles under UV (365 nm wavelength) irradiation was examined. The concen-
tration of unreacted aryl amines 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 aryl
amine in different alcohols under various irradiation times, it was revealed that, in
all cases, the sequence of photocatalysis rate was methanol [ ethanol [ isopropa-
nol. In reactions where the concentrations of aryl amine were lower than 10 mmol/
L, imines were the main products and alkylation of amines was not observed. In the
higher concentration of aryl amines, oxidation and dimerization occurred.
Keywords Photocatalytic reaction Á Aryl amine Á TiO2 nanoparticle Á
GC–MS analysis
Introduction
Heterogeneous photocatalysis has been extensively investigated as a possible way
for removal of pollutants [1–3], organic synthesis [4–6] and bactericidal activity [7].
Nanosized titania powder is a widely used semiconductor photocatalyst owing to its
non-toxic nature, chemical stability, availability, low cost and capability of easy
preparation in nano-particulate form with high surface area [8].
The initial step in TiO2-mediated photocatalytic degradation is proposed to
involve the generation of an (e-/p?) pair, leading mainly to the formation of
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
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A. Hosseinnia et al.
hydroxyl radicals (ÁOH) as well as super oxide radical anions (OÁ2-) and hydroperoxy
radicals (ÁOOH). These radicals are oxidizing species in the photocatalytic oxidation
processes [9, 10]. Among the above-mentioned radicals of which TiO2 are produced
by photocatalysis, hydroxyl radicals are the most powerful oxidizing species that
can attack organic contaminants present at or near the surface of TiO2 particles [11].
In heterogeneous photocatalysis studies involved with semiconductor powders,
thermodynamic and kinetic aspects, surface area and photocatalyst particle size are
important factors for the identification of the reaction product [12].
In recent years, photocatalysis reaction of TiO2 on aryl amines has attracted many
researchers due to its considerable application in organic dye industries [12–16]. The
mechanism of TiO2-photocatalyzed degradation of aniline derivatives in acidic and
alkaline mediums has been studied by Canle et al. [12]. They have reported that
photocatalytic degradation of aryl amines in acidic medium was inhibited due to
electrostatic repulsion between the positively charged surface and protonated
amines, while in an alkaline medium, the main photoproduct of degradation of
aniline is nitrobenzene. As an example in an alkaline medium, degradation of aniline
has been investigated using an alkaline aqueous TiO2 suspension containing
carbonate ions [17]. The presence of carbonate in TiO2 suspension increases the
number of active adsorption sites on TiO2 particle surfaces. The major products in
this reaction were identified to be azobenzene, p-benzoquinone, nitrobenzene, and
NH3. Although aniline at ppm level is mineralized by illuminated TiO2, in higher
concentrations it is oxidized to azobenzene [13]. N-alkylation of amines by
photocatalytic reaction in a microreaction system using Pt-free TiO2 and Pt-loaded
TiO2 has been investigated [18]. It was reported that the N-alkylation did not occur
by the irradiation of Pt-free TiO2 in conventional batch reactor. To cut a long story
short, there has been a lot of interest in the photocatalytic reaction of aryl amines on
TiO2, but the results reported in the literature are partly controversial.
In the current work, we have investigated the photocatalytic reaction of two aryl
amines (aniline and 4-amino N, N dimethyl aniline) using TiO2 nonopowder
suspension in alcohols (methanol, ethanol and isopropanol); the products of
photoreaction were followed by GC-Mass analysis.
Experimental
Synthesis of photocatalyst
Nanopowder anatase TiO2 samples were synthesized by the precipitation method
and dehydrated with the azeotropic distillation technique, as we have reported in
previous work [8]. The sizes of nanopowders obtained after calcinations were
15–25 nm and the surface area measured by BET method was 75–80 m2/g.
Photocatalytic activity
The aryl amines (aniline and 4-amino N, N dimethy aniline) were dissolved
separately in methanol, ethanol and isopropanol with 1–10 mmol/L concentrations.
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Photocatalytic reaction of aryl amines/alcohols
939
A sample of 20 mg synthesized TiO2 nanopowder was added to each Pyrex
volumetric balloon containing 10 ml of the aryl amine solution. The contents of the
balloons were sonicated in an ultrasound bath for 10 min. The aryl amines solution,
in the presence and in the absence of TiO2, were irradiated for 1–40 h by five UV
lamps (k = 360 nm) with 40 W power, in an irradiation box.
Analysis
The reactions products were analyzed by gas chromatograph–mass selective
detector (GC–MS) from Agilent Technologies, GC—6890 N and MS—5973 N
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 3 min
with the ramp rate 8 °C/min and final temperature 250 °C. The concentration of
unreacted aryl amine and products was obtained from integration of chromatogram
picks.
Results and discussions
Aryl amines are widely used as raw materials in organic dye industries and are
known as highly toxic chemicals. Two aryl amines which were chosen in this
experiment are aniline (An) and 4-amino N, N dimethyl aniline (ADA).
First, 10 ml of ADA solutions in alcohol (methanol, ethanol, isopropanol) with
5 mmol/L concentration and 20 mg TiO2 nanopowder were sonicated for 10 min,
and then irradiated under 40 W UVA. The rate of decreasing of 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 TiO2 nanopowder, ADA completely disappeared in
methanol and ethanol. However, the concentration of ADA in isopropanol reduced
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 TiO2
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A. Hosseinnia et al.
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
slowly and,after 24 h,only 20% of 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 in the 8.96-min retention time (RT) corresponds to
ADA and the pick in the RT = 9.83 min corresponds to the major product. The
mass spectra of major product are shown in Fig. 3. All the picks in 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 in 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),
while dialkenylation may be occurring 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 products, after 24 h irradiation, is
shown in Fig. 6, and 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.
Fig. 3 Mass spectra of the
main product of ADA and
methanol with TiO2 (from
GC–MS spectroscopy,
RT = 9.83). The mass spectra
completely matched with
4(methylene amino) phenyl
dimethyl amine
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Photocatalytic reaction of aryl amines/alcohols
941
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
RT = 11.68 are the main
products
Fig. 5 Mass spectra of the
main product of ADA and
ethanol with TiO2 (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
length of time, ADA in isopropanol was converted to various compounds. Although
in the photodegradation process, especially in higher concentration (C50 mmol),
dimerization of aryl amine with the violet color may be occurring, in the lower
concentration (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
TiO2. As shown in this figure, aniline remained unchanged in the absence of TiO2
after 5 h irradiation and, similar to the ADA, photocatalytic reaction of aniline–
methanol and aniline–ethanol is more effective than isopropanol. The chromato-
gram of 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 in the RT = 2.97 corresponds to the
aniline and the pick in the 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
additional to N-ethylene amino phenyl. In the higher concentration of more than
50 mmol/L, various products, e.g., azo benzene, azoxy benzene and 4-phenyl azo
phenol, were formed. Scheme 2 shows the reaction process of 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 TiO2 under UV (365 nm) irradiation
To investigate the reason for the lower reaction rate of aryl amine in isopropanol,
the pure isopropanol with TiO2 nanopowder was irradiated under UVA lamps. As a
result, various compounds were produced. The products of reaction were acetone,
isopropyl ortho formate 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 produced imine or enamine
compounds. The photocatalytic reaction of alcohol with longer chain 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.
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Photocatalytic reaction of aryl amines/alcohols
943
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
Fig. 7 Mass spectra of the
main product of ADA and
isopropanol with TiO2 (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 TiO2
Conclusions
Samples containing aryl amine/alcohol and TiO2 nanopowder suspensions were
irradiated with 40-W UVA lamps. The GC–Mass analysis showed the imine
formation in the concentration of lower than 10 mmol/L. The imines can be
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A. Hosseinnia et al.
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
Fig. 10 Mass spectra of the
main product of aniline and
methanol with TiO2 (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 TiO2 under UV (365 nm) irradiation
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Photocatalytic reaction of aryl amines/alcohols
945
isomerized to the enamines form. The sequence of photocatalytic reaction rate was
methanol [ ethanol [ isopropanol. Investigation into these reaction mechanisms
showed that active radicals of alcohols form in the presence of TiO2 under UVA
irradiation and that these radicals initiate the reactions. The reason for the difference
in reaction rate corresponds to the stability of formed radicals from alcohol. Also,
the variety of radicals from isoprpanol is more than two other alcohols, so the
radicals react together perfectly. In the higher concentration (more than 10 mmol),
oxidation of aryl amine 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 aryl amines is not observed in any of the reaction studies.
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