Corona-induced photoxidation of alcohols and hydrocarbons over TiO 2 in the absence of a UV light source - A novel and environmentally friendly method for oxidation

Article abstract of DOI:10.1039/b418812h

Corona-induced photooxidation is a novel oxidation methodology for the efficient oxidation of alcohols and hydrocarbons utilizing the advantage of both the high oxidizing power of ozone formed in the reactor as well as the photooxidation capability of the UV light generated during the corona discharge. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b418812h

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Corona-induced photoxidation of alcohols and hydrocarbons over TiO2
in the absence of a UV light source – A novel and environmentally
friendly method for oxidation{
Unnikrishnan R. Pillai{ and Endalkachew Sahle-Demessie*
Received (in Cambridge, UK) 15th December 2004, Accepted 24th February 2005
First published as an Advance Article on the web 14th March 2005
DOI: 10.1039/b418812h
used in this study consists of a cylindrical glass tube of 28 cm
length and 4 cm i.d closed at lower end and inserted inside the
outer ceramic tube described above with proper bolted sealing at
both the ends closing the corona compartment. A TiO2 coated pad
prepared by dip-coating method on a pre-shrunk silica quartz fiber
mat of dimension 7.6 6 35 cm2 is wrapped on a 26 cm long glass
tube of 2 cm i. d and inserted inside the outer glass tube.12 The
ceramic and glass tube reactors were connected in such a manner
that the ozone generated inside the ceramic corona discharger is
carried into the glass tube along with the vapors of the substrate to
be oxidized. The products come out through the vent and are
condensed in a collector that is pre-cooled using a liquid N2–
isopropanol bath (240 uC) and analyzed by GC-MS. The
temperature inside the reactor near the catalyst surface is measured
using a thermocouple placed inside the reactor just above the TiO2
pad and measured to be around 80 uC throughout the reaction. It
is found that approximately 5% ozone is generated in the corona
reactor at the maximum operating power of 254 Watts (2.1 A) and
an oxygen flow of 0.50 L min21. The amount of ozone generated
increases from approximately 4% to 5% as the oxygen flow rate
increases from 0.10 to 0.50 liter per minute.
Corona-induced photooxidation is a novel oxidation metho-
dology for the efficient oxidation of alcohols and hydrocarbons
utilizing the advantage of both the high oxidizing power of
ozone formed in the reactor as well as the photooxidation
capability of the UV light generated during the corona
discharge.
Corona is a low energy (10–20 eV) electrical discharge around a
conductor in a gas that occurs when the electric field around the
conductor exceeds the value required to ionize the gas, but is
insufficient to cause a spark. The corona discharge also produces a
low power UV light of the order of y2.0 W in comparison to the
high power UV light obtained from a UV source (1000 kW).
Corona induced non-thermal plasma can be produced by using
pulsed streamer corona or by dielectric barrier discharge.1,2 The
corona reactors have been primarily used for the purification of air
and water as well as in the exhaust gas treatment for the
decomposition of VOCs and removal of SO2 and NOx.3–8
However, this potential technology has not been explored for
other applications such as the oxidation of alcohols and
hydrocarbons, which are of immense importance industrially.9–11
This could be highly efficient and economical due to the powerful
oxidizing property of ozone combined with its low energy
utilization. The process is also environmentally friendly, as it does
not involve the use of any harmful materials or precursors. Herein
we report the application of a corona discharge reactor for the
oxidation of alcohols and hydrocarbons to useful intermediates
over a TiO2 catalyst that can act as a photocatalyst in the presence
of UV light generated during the corona discharge.
The results of corona-induced photooxidation of alcohols in the
presence of TiO2 photocatalyst are shown in Table 1. It is apparent
that the corona oxidation method is exceptionally useful in the
selective oxidation of alcohols. Both activated and non-activated
alcohols are converted to the corresponding carbonyls efficiently.
The selectivity to the corresponding carbonyl product is almost
complete for secondary and cyclic alcohols. Primary alcohols, on
the other hand, are converted mainly to their formic esters. The
initial high reaction rate declines after a pass time of 2 h and then
more or less stabilizes. One interesting aspect of TiO2 containing
corona reactor is in the oxidation of primary alcohols, which form
the corresponding formate ester as the main product unlike the
acid product that is usually observed in the conventional
photooxidation or oxidation using other traditional methods.
The selectivity of primary alcohol to its corresponding aldehyde
may be improved by varying the corona power, amount of ozone
generated (O2 flow through the reactor) or the flow rate (space
velocity) of the alcohol in the reactor (Fig. 1). Primary aldehyde
selectivity increases with decrease in corona power (Fig. 1a) and
ozone amount generated (Fig. 1b) or by increasing the feed flow
rate (Fig. 1c).
The corona discharger system includes a power supply with a
variable voltage controller, a high-voltage transformer, and the
treater station through which the material to be reacted passes (see
ESI{). The treater station consists of a cylindrical ceramic tube of
30 cm length and 6 cm i.d containing a series of electrodes on its
inside wall with a 0.5–1.0 cm gap on either side. The electrodes
emit high-voltage corona discharge into the gap, ionizing the gas
(oxygen and the feed vapor) inside. The ceramic tube containing
the electrodes is placed in a plastic jacket, which is cooled by
flowing water during its operation. The corona reactor assembly
{ Electronic supplementary information (ESI) available: experimental
details and pictures of the corona reactor assembly, power supply and
inside view of the corona generator. See http://www.rsc.org/suppdata/cc/
b4/b418812h/
{ Present address: New technology Research, Philip Morris USA,
Richmond, VA 23234.
*Sahle-Demessie.Endalkachew@epa.gov
The relevance of this oxidation technology is further verified by
extending the protocol to the oxidation of hydrocarbons especially
the cycloalkanes such as cyclohexane, cycloheptane and cyclooctane
(Table 2). Alkane oxidation is usually very difficult to achieve
2256 | Chem. Commun., 2005, 2256–2258
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a
Table 1 Corona-induced photooxidation of alcohols in the presence of TiO2
Selectivity (%)
Entry No.
1
Substrate
Products
Time(h)
Conversion (%)
A
B
C
D
1-Pentanol
Pentanal- A
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
52
42
37
38
72
60
46
45
82
71
55
55
62
54
52
52
88
72
60
60
98
89
79
75
100
90
88
85
94
80
80
78
91
89
88
88
36
27
20
19
95
95
95
95
96
95
96
95
—
—
—
81
19
16
15
16
—
—
—
—
—
—
—
—
2
47
44
45
—
—
—
—
8
6
10
7
—
—
—
—
—
5
5
6
—
—
6
6
25
5
—
—
11
13
15
16
—
10
8
—
30
51
49
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
46
52
51
—
—
—
—
Pentyl formate-B
Pentyl acetate-C
Pentanoic acid-D
2-Pentanone-A
54
34
35
—
—
—
—
—
—
—
—
86
45
49
48
—
—
9
11
14
11
15
13
—
—
—
—
100
39
34
33
—
7
—
2
3
2-Pentanol
100
100
100
100
100
100
100
100
12
Cyclopentanol
1-Hexanol
Cyclopentanone-A
4
Hexanal-A
Hexyl formate-B
Hexanoic acid-C
8
7
7
5
2-Hexanol
2-Hexanone-A
Formic acid sec-butyl ester-B
100
100
91
89
78
83
75
80
100
100
100
100
—
10
8
10
100
93
87
88
75
95
100
100
78
81
79
81
—
—
—
—
6
Cyclohexanol
Cycloheptanol
1-Octanol
Cyclohexanone-A
Cyclohexyl formate-B
Cyclohexyl acetate-C
7
Cycloheptanone-A
8
Octanal-A
Octyl formate-B
Octyl acetate-C
Octanoic acid-D
2-Octanone-A
Formic acid 1-methyl-heptyl ester-B
Heptanoic acid-C
Hexanoic acid-D
Benzaldehyde-A
Benzyl formate-B
Benzoic acid-C
9
2-Octanol
7
7
10
11
12
a
Benzyl alcohol
1-Phenyl ethanol
2-Phenyl ethanol
—
—
—
—
—
—
2
Acetophenone-A
11
6
4
—
—
—
—
—
Formic acid benzyl ester-B
Benzyl phenyl ether-C
Toluene-D
Phenyl acetaldehyde-A
Formic acid phenethyl ester-B
Acetic acid phenethyl ester-C
3
100
90
92
90
10
Reaction conditions: Substrate flow 5 9 mL h21, O2 flow 5 0.50 L min21, temperature 5 80 uC, Corona power 5 254.1 Watts
due to the very inert nature of the C–H bond.11,13,14 The
corona oxidation is also found to be highly useful for the
hydrocarbon oxidation where relatively high conversions in
the range 20–35% could be achieved (Table 2). The initial
higher reaction rate is not diminished even after 5 h of
reaction pass time, unlike in the case of alcohol oxidation
where the initial high rate declines after 2 h (Table 1).
These results reveal that the corona oxidation is an efficient
oxidation technology for the oxidation of alcohols and hydro-
carbons wherein no expensive or environmentally undesirable
materials are employed. The unreacted reactants can easily be fed
back to the reactor and recycled. A comparison of the power
utilization (Fig. 2) shows a two fold increase in the conversion and
a ten fold decrease in power consumption for the corona oxidation
as compared to that in the case of the conventional photocatalytic
oxidation using a powerful UV light (250 vs 2500 Watt per pass).
A control experiment carried out using cyclopentanol in the
absence of a photocatalyst (no TiO2) shows a much lower
conversion (50%) when compared to the reaction in the presence of
TiO2 catalyst (82%). No reaction is observed in the case of a
reaction using a corona discharger connected in series with another
glass reactor wherein the ozone generated in the former is brought
in contact with the vapors of cyclopentanol in the glass reactor
kept at the same temperature as that in the original experiment
(80 uC). These results highlight the important role of corona in
causing the oxidation reaction and also suggest that oxidation
using corona reactor is not merely due to the formation of ozone.
The presence of TiO2 improves the alcohol conversion and also
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Chem. Commun., 2005, 2256–2258 | 2257

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Fig. 2 Comparison of conversion and the power utilization for the
corona reactor and a conventional photoreactor for the oxidation of
cyclopentanol to cyclopentanone at a feed flow rate of 6.0 mL min21 and a
reaction pass time of 2 h.
Unlike most other oxidants that are stored on-site in bulk form,
ozone is produced on-site in low concentrations and immediately
consumed. Consequently, any accidental leakage can be easily
controlled, as evidenced by ozone’s long safety history in many
applications.
Fig. 1 Effect of (a) corona power (b) amount of O3 and (c) feed flow rate
on the conversion and selectivity of corona-induced photooxidation of
1-hexanol.
Table 2 Corona-induced photooxidation of hydrocarbons in the
presence of TiO2
a
In summary, a novel oxidation methodology is reported for the
efficient oxidation of alcohols and hydrocarbons using a corona
reactor. This oxidation protocol utilizes the advantage of both the
high oxidizing power of ozone formed in the reactor as well as
the photooxidation capability of the UV light generated during the
ozone formation.
Selectivity
(%)
Entry
No. Substrate
Time Conversion
(h) (%)
Products
A B
C
1
2
3
a
Cyclohexane Cyclohexanol-A
Cyclohexanone-B
Cyclohexyl
1
2
3
4
5
18
20
31
38
38
5
11
17
18
19
7
— 100 —
10 90 —
12 83
24 71
24 71
5
5
5
Unnikrishnan R. Pillai{ and Endalkachew Sahle-Demessie*
National Risk Management Research Laboratory, Sustainable
Technology Division, MS 443, United States Environmental Protection
Agency, Cincinnati, Ohio-, 45268, USA.
formate-C
Cycloheptane Cycloheptanone-B 1
— 100 —
— 100 —
— 100 —
— 100 —
— 100 —
— 100 —
— 100 —
— 100 —
— 100 —
— 100 —
2
3
4
5
1
2
3
4
5
E-mail: Sahle-Demessie.Endalkachew@epa.gov; Fax: 011-513-569-7677
Notes and references
Cyclooctane Cyclooctanone-B
1 K. Yan, H. Hui, M. Cui, J. Miao, X. Wu, C. Bao and R. Li,
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2 K. Yan, E. J. M. van Heesch, A. J. M. Pemen and P. A. H.
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3 D. R. Grymonpre, W. C. Finney and B. R. Locke, Chem. Eng. Sci.,
1999, 54, 3095; D. R. Grymonpre, A. K. Sharma, W. C. Finney and
B. R. Locke, Chem. Eng. J., 2001, 82, 189.
11
22
22
22
Reaction conditions: Substrate flow 5 9 mL h21, O2 flow 5
0.50 L min21, temperature 5 80 uC, Corona power 5 254.1 Watts
4 B. Srinivasan, S. Palanki, D. R. Grymonpre and B. R. Locke, Chem.
Eng. Sci., 2001, 56, 1035.
5 E. M. van Veldhuizen, W. R. Rutgers and V. A. Bityurin, Plasma
Chem. Plasma Processing, 1996, 16, 227.
6 T. Fujii and M. Rea, Vacuum, 2000, 59, 228.
7 B. S. Rajanikanth and S. Rout, Fuel Process. Technol., 2001, 74, 177.
8 X. Hu, J. Nicholas, J.-J. Zhang, T. M. Linjewile, P. de Filippis and
P. K. Agarwal, Fuel, 2002, 81, 1259.
9 R. A. Sheldon and J. K. Kochi, Metal-Catalyzed Oxidation of Organic
Compounds, Academic Press, New York, 1981.
10 M. Hudlicky, Oxidations in Organic Chemistry, ACS, Washington DC,
1990.
11 D. H. R. Barton, M. J. Gastinger and W. B. Motherwell, J. Chem. Soc.,
Chem. Commun., 1983, 731; J. M. Thomas, Nature, 1985, 314, 669; T. Ito
and J. H. Lunsford, Nature, 1985, 314, 721.
12 U. R. Pillai and E. Sahle-Demessie, J. Catal., 2002, 211, 434.
13 U. Schuchardt, W. A. Carvalho and E. V. Spinace´, Synlett, 1993, 10,
713.
14 K. U. Ingold, Aldrichim. Acta, 1989, 22, 69.
15 H. Gerisher and F. Willig, Top. Curr. Chem., 1976, 61, 50.
affects the product selectivity, especially in the case of primary
alcohols. Generally, corona treating is a very effective way to
increase the surface tension of virtually any material. For example,
the result after corona treatment on a surface is a surface that is
unchanged to the naked eye, but is much more receptive to inks,
coatings, and adhesives. In a similar fashion, the substrate material
may also be getting activated upon exposure to corona. Corona
discharge process also produces chemical free radicals and ions.2,3
In addition, the generation of UV light could produce electron-
hole pairs15 that is believed to have an oxidation potential of ca.
3.0 V, and has, therefore, considerable oxidizing capability.
Therefore, the combined effects of ozone and UV light generation
result in the oxidative chemical transformation. Although ozone in
the gaseous form is both toxic and corrosive, it presents no safety
or handling problems in properly designed operating systems.
2258 | Chem. Commun., 2005, 2256–2258
This journal is ß The Royal Society of Chemistry 2005