Product Selectivity in Solar photocatalytic dehydrazonation of aromatic hydrazones by Tio2-based catalysts

Article abstract of DOI:10.1080/15533174.2013.768650

Product selectivity of ketone and azine in the photocatalytic dehydrazonation of aromatic hydrazones with TiO₂-based photocatalysts has been investigated using solar light. Product selectivity is dependent on the nature of the photocatalysts, substituents and the solvents. Loading of metals on TiO₂-P25 selectively enhances the formation of azine from benzophenone hydrazone. Solvents such as dichloromethane, chloroform, ethanol, and 2-propanol enhance the formation of azine under solar irradiation. Structures of products have been established by FT-IR, ₁H-NMR, ₁3C-NMR, and GC-MS spectrometry. Supplementary materials are available for this article. Go to the publisher's online edition of Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry to view the supplemental file. Copyright Taylor &Francis Group, LLC.

Full text of DOI:10.1080/15533174.2013.768650

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Product Selectivity in Solar Photocatalytic
Dehydrazonation of Aromatic Hydrazones by TiO2-
Based Catalysts
B. Krishnakumar a & M. Swaminathan a
a Photocatalysis Laboratory, Department of Chemistry , Annamalai University ,
Annamalainagar , Tamil Nadu , India
Accepted author version posted online: 14 Aug 2013.Published online: 27 Sep 2013.
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To cite this article: B. Krishnakumar & M. Swaminathan (2014) Product Selectivity in Solar Photocatalytic Dehydrazonation
of Aromatic Hydrazones by TiO2-Based Catalysts, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal
Chemistry, 44:1, 96-100, DOI: 10.1080/15533174.2013.768650
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 44:96–100, 2014
Copyright ꢀ Taylor & Francis Group, LLC
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ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2013.768650
Product Selectivity in Solar Photocatalytic Dehydrazonation
of Aromatic Hydrazones by TiO2-Based Catalysts
B. Krishnakumar and M. Swaminathan
Photocatalysis Laboratory, Department of Chemistry, Annamalai University, Annamalainagar,
Tamil Nadu, India
tion of benzophenone hydrazone was carried out using UV
light.[1] To the best of our knowledge, there is no report on the
regeneration of ketones from aromatic hydrazones with a semi-
conductor photocatalyst under solar light. Further, utilization
of solar light, a perennial source of energy makes this reac-
tion, a green chemical process. In the present work, we report
the product selectivity in the photocatalytic dehydrazonation of
substituted benzophenone hydrazones with TiO2-based photo-
catalysts using solar light.
Product selectivity of ketone and azine in the photocatalytic
dehydrazonation of aromatic hydrazones with TiO2-based photo-
catalysts has been investigated using solar light. Product selectivity
is dependent on the nature of the photocatalysts, substituents and
the solvents. Loading of metals on TiO2-P25 selectively enhances
the formation of azine from benzophenone hydrazone. Solvents
such as dichloromethane, chloroform, ethanol, and 2-propanol en-
hance the formation of azine under solar irradiation. Structures
of products have been established by FT-IR, 1H-NMR, 13C-NMR,
and GC-MS spectrometry.
Supplementary materials are available for this article. Go to the
publisher’s online edition of Synthesis and Reactivity in Inorganic,
Metal-Organic, and Nano-Metal Chemistry to view the supplemen-
tal file.
EXPERIMENTAL
Materials and Methods
Benzophenone, substituted benzophenones, and hydrazine
hydrate (Sd fine chemicals), were used as received. A gift sam-
ple of TiO2-P25 was obtained from Evonik (Germany). It is an
80:20mixture of anataseandrutile withthe particlesizeof 30 nm
and BET surface area of 50 m2g−1. Ag-TiO2-P25 catalyst was
prepared by photoreducing Ag+ ions to Ag metal on the TiO2-
P25 surface as per the procedure reported earlier.[13,14] Pt and Pd
doped TiO2-P25 were prepared by the same procedure. Silver
nitrate, hexachloroplatanic acid, and palladium chloride (ana-
lytical grade) from Merck were used as Ag, Pt, and Pd sources,
respectively. Solvents of LR grade were distilled prior to use.
TiO2 (anatase) has been prepared by the reported method.[15]
All benzophenone hydrazones were prepared according to the
literature procedure.[16]
Keywords azine, dehydrazonation, product selectivity, solar light
INTRODUCTION
Regeneration of carbonyl compounds from stable and readily
prepared oximes, hydrazones, and semicabazones have received
a considerable attention in the last two decades.[1–3] There has
been great interest in the development of mild techniques for
the conversion of oximes into their corresponding carbonyl
compounds.[4,5] A number of methods have been developed for
the regeneration of carbonyl compounds from hydrazones,[6–8]
but most of these methods suffer some disadvantages such as
large reaction time, higher reflux temperature, difficulties in
isolation of products and low yield. The classical method of
regeneration by acid hydrolysis is not suitable for acid sensitive
compounds.[9]
Apparatus
IR spectra were recorded using an Avatar-330 FT-IR
spectrophotometer 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% methyl polysiloxane), 30 m
length, 0.25 mm internal diameter, 0.25 μm 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 spectrom-
eter operating at 500 MHz for all the samples. Powder X-ray
diffraction patterns of TiO2-P25 and metal-doped catalysts
TiO2-based photocatalysts have been used for selective or-
ganic transformations.[10–12] In our earlier study, dehydrazona-
Received 15 December 2012; accepted 16 January 2013.
The authors are thankful to CSIR, New Delhi for financial support
through research grant No. 21(0799)/10/EMR-II. Dr. Krishnakumar
is thankful to CSIR, New Delhi, for the award of Senior Research
Fellowship.
Address correspondence to M. Swaminathan, Photocatalysis Labo-
ratory, Department of Chemistry, Annamalai University, Annamalaina-
gar, Tamil Nadu, 608 002, India. E-mail: chemres50@gmail.com
96

DEHYDRAZONATION OF AROMATIC HYDRAZONES
97
were obtained using X’Pert PRO diffractometer equipped with
a CuKα radiation (wavelength 1.5406 Å) at 2.2 kW Max. Peak
positions were compared with the standard files to identity the
crystalline phase. Diffuse reflectance spectra were recorded
using Shimadzu UV-2450. The intensity of solar light was
measured using LT Lutron L×10/A Digital Lux meter.
Dehydrazonation Procedure
All photocatalytic experiments were carried out under simi-
lar conditions on sunny days between 11 am and 2 pm. An open
borosilicate glass tube of 30 mL capacity, 40 cm height, and
20 mm diameter was used as the reaction vessel. The photocat-
alytic dehydrazonation was carried out by irradiating a mixture
of appropriate amount of catalyst (150 mg/25 mL), 50 mg of hy-
drazone, and 25 mL of acetonitrile (CH3CN) in a long reaction
tube with solar light in open air condition and was continuously
aerated by a pump to provide oxygen and for the complete
mixing of the solution. During the illumination time (60 min)
the reaction temperature raised from 32 to 40◦C. As the reac-
tion is photocatalytic, this temperature change will not affect the
reaction significantly. During irradiation the volatility of the sol-
vent was compensated by the addition of solvent. The progress
of the dehydrazonation was monitored using thin layer chro-
matography (TLC). After completion of reaction, the catalyst
was removed by centrifugation and filtration and the filtrate was
subjected to GC and GC-MS analysis for the determination of
the yield of the products. Structures of the products had been
confirmed by FT-IR, 1H NMR, and mass spectral data.
FIG. 1. XRD pattern of a) TiO2-P25, (b) Ag-TiO2-P25, (c) Pt-TiO2-P25,
(d) Pd-TiO2-P25 (color figure available online).
entry 1) with a small amount of azine derivative of benzophe-
none (Scheme 1, path C). In control experiments, irradiation
of hydrazone in acetonitrile without catalyst led to no reaction
product. When the reaction was carried out without irradiation
in presence of catalyst, no product was obtained even by heating
(Scheme 1, path A). Further, azine was not formed by the ad-
dition of benzophenone under the same conditions (Scheme 1,
RESULTS AND DISCUSSION
The metal-doped (Ag, Pt, and Pd) TiO2-P25 were character-
ized by FT-IR, X-ray diffraction (XRD), and diffuse reflectance
spectroscopy (DRS) techniques. The FT-IR spectra of all metal
loaded TiO2-P25 catalysts exactly match with pure TiO2-P25
(Figure S1). However, the peak at 1638 cm−1 in pure TiO2-P25
is shifted to a range of 1630 to 1633 cm−1 in all metal-loaded
catalysts.
X-ray diffractograms of TiO2-P25, Ag-TiO2-P25, Pt-TiO2-
P25, and Pd-TiO2-P25 are given in Figure 1. Since TiO2–P25 is a
mixture of 80% anatase and 20% rutile, XRD pattern shows both
anatase and rutile lines. Metal modification does not change the
phase. Crystallite size of catalyst was determined using Scher-
rer equation[17] for each peak and the average was taken. The
average crystallite sizes of Ag-TiO2-P25, Pt-TiO2-P25, and Pd-
TiO2-P25 were found to be 16.0, 12.9, and 17.3 nm, respectively.
Metal modification reduces the size of pure TiO2-P25 (30 nm).
The diffuse reflectance spectra (Figure 2) reveal the increased
absorbance from 400 nm to entire visible region by metal load-
ing on TiO2-P25.[18] Absorbance in the visible region is higher
for Ag-TiO2-P25 (curve b) and Pt-TiO2-P25 (curve c) than for
Pd-TiO2-P25 (curve d).
Irradiation of a suspension of TiO2-P25 in acetonitrile con-
taining benzophenone hydrazone with solar light for one hour
gave corresponding benzophenone in 84.7% of yield (Table 1,
FIG. 2. DRS spectra of (a) TiO2-P25, (b) Ag-TiO2-P25, (c) Pt-TiO2-P25,
(d) Pd-TiO2-P25 (color figure available online).

98
B. KRISHNAKUMAR AND M. SWAMINATHAN
SCH. 1. Dehydrazonation of benzophenone hydrazone (color figure available online).
path B). This shows that both light and catalyst are essential dichloromethane, 2-propanol, ethanol, and chloroform (entries
for the formation of ketone and azine. The products, ketone and 3–6) favor the formation of azine. Conversion efficiency and
azine, formed on solar irradiation with TiO2-P25 have been con- product selectivity are lowest in hexane. The product selectivity
firmed by FT-IR, 1H NMR, and 13C NMR and GC-MS spectral may be affected by the photostability of the solvents. It was al-
data. The products, ketone and azine, formed on solar irradiation ready reported that the formation of azine required the slow con-
1
with TiO2-P25 have been confirmed by FT-IR, H NMR, and version of benzophenone hydrazone to benzophenone.[1] If the
13C-NMR and GC-MS spectral data. The spectral data for the dehydrazonation is fast initially, the formation of azine will be
products formed from benzophenone hydrazone is given below. less due to low concentration of unconverted hydrazone. Com-
Benzophenone. m.p. = 47–49◦C; FT-IR (KBr) (cm–1) = paring the product selectivity of acetonitrile with other solvents,
3054, 3002 (aromatic C H str.), 1652 (C O str.), 1445 formation of benzophenone is high in acetonitrile, whereas azine
(C C str.), 1317, 1282, 1152, 1073 (other str. frequen- formation is high in other solvents. This indicates that when ace-
cies) (Fig S2); GC-MS (m/z) = 181.9 (M+) (Figure S3). tonitrile is used as a solvent, the dehydrazonation is fast, leading
Bis(diphenylmethylidene)hydrazine. m.p. = 163–165◦C; FT-IR to selective formation of ketone. Acetonitrile is highly photo-
(KBr) (cm−1) = 3079, 3054, 1638, 1564, 1487, 1442, 1318, stable and inert to light irradiation. CHCl3 is reported to undergo
767, 659 (Figure S4); 1H NMR (CDCl3, 500 MHz) δ = aromatic decomposition with TiO2 forming CCl3..[19] As the photons are
protons 7.28–7.53 (m, 20H) (Figure S5); 13C NMR (CDCl3, used to decompose chloroform, the initial conversation rate of
125 MHz) δ = aromatic carbons 127.88, 128.04, 128.67, 128.69, benzophenone hydrazone is reduced, resulting in the formation
129.34, 129.61, 135.57, 138.20 and C N 158.97 (Figure S6); of azine as a major product. The same effect is operative with
GC-MS (m/z) = 361.2 (M+1) (Figure S7).
dichloromethane. Similarly, the alcoholic solvents undergo ox-
The dehydrazonation was carried out in various solvents and idation leading to the formation of azine as a major product.
the results are shown in Table 1. The conversion efficiencies We used acetonitrile for dehydrazonation process because of its
are not much different (89.4–97%), but their product selectiv- photostability and less volatility.
ity differ significantly. Among these solvents acetonitrile and
The dehydrazonation of benzophenone hydrazone with dif-
n-hexane selectively (entries 1 and 2) formed ketone whereas ferent TiO2-based photocatalysts using solar light was carried
TABLE 1
Percentages of products and their selectivities in various solvents in solar light
% of
benzophenone
% of benzophenone
azine
% of
% of product
% of product
Entry
Solvent
conversion selectivity of azine selectivity of ketone
1
2
3
4
5
6
Acetonitrile
n-Hexane
Dichloromethane
2-Propanol
Ethanol
84.7
63.1
10.5
16.0
13.8
7.5
5.5
26.3
86.5
80.0
80.5
84.9
90.2
89.4
97.0
96.0
94.3
92.4
6.1
29.4
89.2
83.3
85.4
91.9
93.9
70.6
10.8
16.7
14.6
8.1
Chloroform
[Benzophenone hydrazone] = 50 mg/25 mL, TiO2-P25 = 150 mg/25 mL, irradiation time = 60 min, airflow rate = 8.1 mL s−1
.

DEHYDRAZONATION OF AROMATIC HYDRAZONES
99
TABLE 2
Effect of various photocatalysts on dehydrazonation of benzophenone hydrazone with solar light in acetonitrile
% of
benzophenone
% of benzophenone
azine
% of
conversion
% of product
selectivity of azine
% of product
selectivity of ketone
S. No.
Catalyst
1.
2.
3.
4.
5.
Ag-TiO2-P25
TiO2-P25
Pt-TiO2-P25
Pd-TiO2-P25
TiO2(anatase)
10.2
84.7
7.5
11.1
33.2
80.1
5.5
75.3
60.1
13.3
90.3
90.2
82.8
71.2
46.5
88.6
6.1
90.9
84.4
28.6
11.4
93.9
9.1
15.6
71.4
[Benzophenone hydrazone] = 50 mg/25 mL; catalyst suspended = 150 mg/25 mL; irradiation time = 60 min; airflow rate = 8.1 mL s−1
.
out in acetonitrile under identical conditions and the results are Hence 150 mg/25 mL is the optimum for ketone formation
given in Table 2. These photocatalysts play an important role in and 100 mg/25 mL is the optimum for azine formation. Higher
the relative product distribution of ketone and azine. The order catalyst concentration favors the formation of ketone while
of activities for overall conversion of benzophenone hydrazone lower catalyst concentration favors azine formation.
with photocatalysts is Ag-TiO2-P25 ≈ TiO2-P25 > Pt-TiO2-
In order to show the generality and scope of this new protocol,
P25 > Pd-TiO2-P25 > Prepared TiO2 (anatase). As the anatase we had carried out the dehydrazonation of various substituted
form of prepared TiO2 was least efficient, anatase TiO2 and its benzophenone hydrazones both in solar and UV light and the
modified forms were not used for this reaction. Azine formation results obtained are summarized in Table 3. In case of ben-
was high with doped catalysts, where as TiO2-P25 favored the zophenone hydrazone, ketone formation occurred selectively
formation of ketone. The trend observed in products percentage with TiO2-P25, whereas doped catalysts favored the formation
and selectivity is same as in UV light irradiated dehydrazonation of azine in solar and UV light. But there is no difference in
by the doped catalysts.[1]
product selectivity between TiO2-P25 and doped catalysts in
We investigated the influence of the amount of TiO2-P25 the dehydrazonation of substituted benzophenone hydrazones
on the dehydrazonation under solar light (Figure 3). The but conversion efficiency is different. Conversion efficiency is
formation of ketone increases from 100 to 150 mg, whereas almost same with 4-methyl and 4-chlorobenzophenone hydra-
azine formation increases slightly from 50 to 100 mg/25 mL. zones, but it is decreased with 4-nitrobenzophenone hydrazone.
TABLE 3
Product selectivity of various hydrazones with different forms of TiO2 using solar and UV light in acetonitrile
% of product
% of product
Entry
Catalyst
% of ketone
% of azine
% of conversion
selectivity of azine
selectivity of ketone
4-Methylbenzophenone hydrazone
1
2
3
4
TiO2-P25
89.5 (71.3)
98.0 (62.9)
93.5 (54.8)
82.1 (20.4)
8.4 (26.8)
1.0 (35.0)
4.1 (43.2)
14.2 (71.5)
97.9 (98.1)
99.0 (98.9)
97.6 (98.0)
96.3 (91.9)
8.6 (21.4)
1.1 (35.4)
4.2 (44.1)
14.8 (77.8)
91.4 (72.6)
98.9 (64.6)
95.8 (55.9)
85.2 (22.2)
Ag-TiO2-P25
Pt-TiO2-P25
Pd-TiO2-P25
4-Chlorobenzophenone hydrazone
5
6
7
8
TiO2-P25
21.5 (35.9)
6.4 (3.3)
27.0 (11.1)
16.5 (25.6)
76.0 (62.9)
92.1 (95.6)
70.4 (85.8)
80.4 (70.6)
97.5 (98.8)
98.5 (98.9)
97.4 (96.9)
96.9 (96.2)
77.9 (63.7)
93.5 (96.6)
72.3 (88.5)
82.9 (73.4)
22.1 (36.3)
6.5 (3.4)
27.7 (11.5)
17.1 (26.6)
Ag-TiO2-P25
Pt-TiO2-P25
Pd-TiO2-P25
4-Nitrobenzophenone hydrazone
9
10
11
12
TiO2-P25
90.0 (99.0)
65.5 (44.7)
24.0 (66.2)
2.0 (12.5)
0 (0)
0 (0)
0 (0)
0 (0)
90.0 (99.0)
65.5 (44.7)
24.0 (66.2)
2.0 (12.5)
0 (0)
0 (0)
0 (0)
0 (0)
100 (100)
100 (100)
100 (100)
100 (100)
Ag-TiO2-P25
Pt-TiO2-P25
Pd-TiO2-P25
[Hydrazones] = 50 mg/25 mL, catalyst suspended = 150 mg/25 mL, irradiation time = 60 min, airflow rate = 8.1 mL s−1. Values in
parentheses indicate the percentage in UV process. IUV = 1.381 × 10−6 einstein L−1 s−1
.

100
B. KRISHNAKUMAR AND M. SWAMINATHAN
zone. But with substituted hydrazones the product selectivity
is decided by the reactivity of the substrate. The GC-MS spec-
tra of products of 4-methyl, 4-chloro and 4-nitro benzophenone
hydrazones are given in Figures S8–S12.
CONCLUSIONS
Acetonitrile selectively formed ketone as a major product in
TiO2-P25 whereas dichloromethane, chloroform and alcoholic
solvents (ethanol, 2-propanol) enhanced the formation of azine.
Among the semiconductors tested for the dehydrazonation pro-
cesses, TiO2-P25 is found to be more efficient for formation of
ketone, whereas metal-loaded semiconductors selectively form
azine as a major product in benzophenone hydrazone. 4-methyl
benzophenone hydrazone and 4-nitrobenzophenone hydrazone
selectively formed ketone, whereas 4-chlorobenzophenone hy-
drazone selectively formed azine with all catalysts in solar and
UV light. As the substituted benzophenone hydrazones are less
reactive than benzophenone hydrazone, the product selectivity
is decided by their reactivity.
FIG. 3. Percentage of benzophenone and benzophenone azine formed with
different amounts of TiO2-P25 under solar light: [Benzophenone hydrazone]
= 50 mg/25 mL in acetonitrile, irradiation time = 60 min, airflow rate =
8.1 mL s−1
.
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