American Journal of Chemistry and Applications
2015; 2(2): 36-43
Published online February 10, 2015 (http://www.openscienceonline.com/journal/ajca)
Mechanism and anaerobic propan-1-ol oxidation
reaction over Au/TiO2 catalysts
Abdullahi Nuhu1, Albert Carley2
1
2
Department of Pure and Applied Chemistry, Bayero University, Kano P. M. B 3011, Kano State, Nigeria
School of Chemistry, Cardiff University, Cardiff, CF10 3AT UK
Email address
[email protected] (A. Nuhu)
To cite this article
Abdullahi Nuhu, Albert Carley. Mechanism and Anaerobic Propan-1-ol Oxidation Reaction over Au/TiO2 Catalysts. American Journal of
Chemistry and Applications. Vol. 2, No. 2, 2015, pp. 36-43.
Abstract
Our recent results concerning the propan -1-ol oxidation to propanal and other side products on gold based catalyst has
motivated us to investigate and explore the anaerobic oxidation of propan -1- ol over Au/ TiO2 catalysts. In our study we
found that gold on titania based catalyst are capable of converting propan –1-ol to propanal and other side products. It is
suggested that the conversion of propan-1-ol to propanal by dehydrogenation and propene by dehydration by gold on TiO2
catalyst prepared by deposition precipitation. The adsorption of propan-1-ol over TiO2 (P25) indicated a full monolayer with
much of it in a dissociated state, forming propoxy group on the cationic site and hydroxyl group at anions. The propoxy is
relatively stable until about 250oC, at which dehydration to propene occurs by bimolecular surface reaction. As the
concentration of propoxy on the surface disappear, so mechanism reverts to a decomposition pathway, producing CO2 and
H2O. However, the presence of gold on the catalyst is marked with complete conversion of propan-1-ol at low temperature
(230oC) lower than Titania (300oC).
Keywords
Au/TiO2, TPD, XRD, DRITFS and TPFRP
1. Introduction
The adsorption and reaction of propan -1-ol over Au/TiO2
catalysts was investigated using pulse flow reactor. The
masses monitored were chosen because propan-1-ol can be
dehydrogenated to produce CH3CH2CHO in the presence of
oxygen (oxidation) or it may be decomposed to produce
CH3CH3, CO, H2 and sometime CO2 and water, similar to
ethanol. Ethanol can be oxidised and dehydrogenated to
acrolein (propenal). Similarly, propanal can also be oxidised
to produce propanoic acid and simultaneously, can further
react with propan-1-ol to produce isopropyl ester. However,
propan-1-ol can also be deoxygenated to propane or
dehydrated to propene. Several studies were conducted in the
investigation of anaerobic reaction of propan-1-ol over gold
containing catalysts.
2. Materials and Methods
2.1. Catalyst Preparation
The catalysts used in this study were prepared by incipient
wetness impregnation [1, 2] of titania (P25) treated in air at
500oC for 2h, using a suitable volume of an aqueous solution
of HAuCl4 (0.705 ml gcat-1). The sample was dried in air for
two hours and ground using a pestle and mortar. The catalyst
sample was passed in to a disc, crushed and then sieved
between 850 µm and 600 µm particle aggregate size.
2.2. Anaerobic Propan-1-ol Oxidation
The reactivity of the catalyst was tested using a pulse flow
reactor, which has been described in detail elsewhere [4].
American Journal of Chemistry and Applications 2015; 2(2): 36-43
3. Results and Discussion
catalyst (figure 1) is more of dehydration to propene
(appearance of mass 41 and 27 amu signals) and
deoxygenation to propane (appearance of mass 29, 28, 27, 44,
and 43 amu signals). When the temperature was increased
further, mass 28, 27 and 30 amu signals were obtained due to
decomposition of propanol to ethane, CO and hydrogen.
When the data in figure 1 was integrated and analysed, figure
(2) was obtained.
3.1. Activity Test
In order to explore the role of gold on TiO2, Similar to
oxidation reaction of propan -1-ol over Au/TiO2 catalyst, the
anaerobic reaction were carried out on TiO2 catalyst alone as
a control experiment. Details of the experiments were
discussed as follows:
The anaerobic reaction of propan- 1- ol over the TiO2
mass 27 amu
mass 2 amu
mass 28 amu
mass 43 amu
mass 30 amu
37
mass 18 amu
mass 29 amu
8mass 32 amu
mass 15 amu
mass 58 amu
Temperature 癈 (corrected)
mass 44 amu
mass 31 amu
mass 41 amu
mass 16 amu
400
-7
Mass Spec Response (Arb. Units)
1.4x10
350
-7
1.2x10
300
-7
1.0x10
250
-8
8.0x10
200
-8
6.0x10
150
-8
100
-8
50
4.0x10
2.0x10
0
0.0
0
20
40
60
80
100
Time(min)
Figure (1). Temperature Programmed Pulse Flow Reaction for anaerobic propan-1-ol reaction in He gas flow over TiO2 catalyst
Selectivity to CO
Selectivity to CO2
Selectivity to CH3CH2CH3
Selectivity to CH3CH=CH2
Selectivity to CH3CH3
Conversion (%)
100
80
80
60
60
40
40
20
20
0
Conversion (%)
Selectivity (%)
100
0
200
250
300
350
o
Temperature C
Figure (2). Selectivity and conversion with temperature for anaerobic propan-1-ol reaction in He flow over TiO2 catalyst
38
Abdullahi Nuhu and Albert Carley: Mechanism and Anaerobic Propan-1-ol Oxidation Reaction over Au/TiO2 Catalysts
Mass Spec Response (Arb. Units)
Temperature 癈 (corrected)
mass 27 amu
mass 2 amu
mass 28 amu
mass 15 amu
mass 58 amu
mass 18 amu
mass 29 amu
mass 41 amu
mass 16 amu
mass 44 amu
mass 31 amu
mass 43 amu
mass 30 amu
2.0x10
-7
1.8x10
-7
1.6x10
-7
1.4x10
-7
1.2x10
-7
1.0x10
-7
200
8.0x10
-8
150
6.0x10
-8
4.0x10
-8
2.0x10
-8
400
350
300
250
100
50
0
0
10
20
30
40
50
60
70
80
90
Time(min)
Figure (3). Temperature Programmed Pulse Flow Reaction for anaerobic propan-1-ol reaction in He gas flow over a 1wt% Au/ TiO2 catalyst
Selectivity to CO
Selectivity to CO2
Selectivity to CH3CH=CH2
Selectivity to CH3CH2CH3
Selectivity to CH3CH2CHO
Conversion (%)
100
80
80
60
60
40
40
20
20
0
Conversion (%)
Selectivity (%)
100
0
180
200
220
240
260
280
300
320
340
360
o
Temperature C
Figure (4). Selectivity and conversion with temperature for anaerobic propan-1-ol reaction in He flow over a 1%wtAu/TiO2 catalyst
Figure 2 shows that as the conversion of propanol was
60%, the catalyst was selective towards dehydration of
propanol to propene, with selectivity value being 70% and
the temperature was 200oC. However, when the temperature
was increased to 250oC, 100% conversion of propanol was
reached, with
selectivities to CO and propene being 15% and 70%
respectively and continued throughout in a steady state.
Similarly, the CO2, ethane, and propane selectivities
remained small with selectivities values being not more than
15%.
When the anaerobic reaction for propan-1ol was carried
out over the Au/TiO2 catalyst (figure 3), we observe more
decomposition of propan-1-ol to CO at about 70 minutes and
American Journal of Chemistry and Applications 2015; 2(2): 36-43
further decomposition to CO2 above 80 minutes due to
dehydrogenation. However, the reaction is accompanied by
the dehydration of propan-1-ol to propene (appearance of
mass 41, 27, and 29 amu signals) from 70 minutes and
deoxygenation to propane (appearance of mass 29,28,27, 43
and 44 amu signals) and dehydrogenation to propanal
( appearance of 29,28,27 and some little 58 amu signals )
from 80 minutes.
When the data in figure 3 were integrated and analysed,
figure 4 was obtained. Figure (4) shows propan-1-ol
conversion to be 50% as the temperature was about 190oC
and the catalyst is selective towards CO. The CO selectivity
increases until the temperature reached about 200oC in which
the selectivity changes towards dehydration to propane and
deoxygenation to propane, with selectivities value of about
<20% each respectively. However, when the temperature was
increased to 250oC, the CO selectivity was increased to 40%
and the propanal and CO2 selectivities begin to increase to
almost 20% respectively. The high selectivity to propanal and
CO2 is due to dehydrogenation and decomposition of propan1-ol. However, the CO and propene selectivities decrease
when the dehydrogenation and decomposition begin, with
values being less than 40% and 20% respectively.
The temperature programmed desorption of propan-1-ol
over TiO2 catalyst is shown in figure 5. Propan-1-ol was
being desorbed at lower temperature, but as the temperature
was increased to 250oC, the dehydrogenation occurs and
indicates the presence of propanal and the appearance of
1.60E-009
Mass Spect. Response (arbit. units)
mass 41, and 27 and 18 amu signals confirmed the
dehydration of the adsorbed propanol to propene. Similarly,
propane was also desorbed (the appearance of mass 29, 28,
27, 44, 43 and 15 amu signals).
However, the emergence and coincidence of CO2 and H2 in
the propanol TPD on TiO2 confirmed the presence of formate
as adsorbed species present as in the case of Au/TiO2 catalyst.
The TPD also confirmed that the reaction involved for
propanol on TiO2 is mainly dehydration and deoxygenation
as evident in figure 5 and is also due to high surface coverage
of 2-propoxy species adsorbed on the catalyst.
Temperature programmed desorption was also conducted
of propan -1 ol over on the Au/TiO2 catalyst. The data are as
shown in figure 6. The sample was dosed with pulses of
propan-1-ol and the uptake corresponded to about one
monolayer of propan-1-ol at saturation.
As seen in figure 6 some water and propan-1-ol desorbed
at lower temperature ~90oC and as the temperature was
increased to 180oC, hydrogen, CO, propanal, propene and
propane was evolved due to dehydrogenation of propan-1-ol,
dehydration to propene and deoxygenation to propane.
However, as the temperature further increased above 350oC,
hydrogen, water and CO2 was evolved due to decomposition
of propan-1-ol to CO2 and water. The coincident appearance
of CO2 and hydrogen confirmed the presence of formate as
adsorbed species on the Au/TiO2 catalyst. The results are
consistent with that reported by Diaz et al. (4, 5)
mass 18 amu
mass 18 amu
mass 31 amu
mass 29 amu
mass 43 amu
mass 16amu
1.80E-009
39
mass 44 amu
mass 2 amu
mass 27 amu
mass 41 amu
mass 15 amu
1.40E-009
1.20E-009
1.00E-009
8.00E-010
6.00E-010
4.00E-010
2.00E-010
0.00E+000
-2.00E-010
50
100
150
200
250
300
350
o
Temperature C
Figure (5). Temperature Programmed Desorption of TiO2 catalyst saturated with propan-1-ol at room temperature
40
Abdullahi Nuhu and Albert Carley: Mechanism and Anaerobic Propan-1-ol Oxidation Reaction over Au/TiO2 Catalysts
mass 28 amu
mass 2 amu
mass 31 amu
mass 41 amu
mass 58 amu
1.80E-009
mass 44 amu
mass 18 amu
mass 27 amu
mass 43 amu
mass 15 amu
mass 32 amu
mass 30 amu
mass 29 amu
mass 74 amu
mass 16 amu
Mass Spec. Respon. (arbit. unit)
1.60E-009
1.40E-009
1.20E-009
1.00E-009
8.00E-010
6.00E-010
4.00E-010
2.00E-010
0.00E+000
50
100
150
200
250
300
350
o
Temperature C
Figure (6). Temperature Programmed Desorption of a 1wt% Au/ TiO2 catalyst saturated with propan-1-ol at room temperature
1723
3690
2937
2964
Infrared absorption spectra were taken for propan-1-ol
over an Au/TiO2 catalyst after the catalyst was heated in
oxygen for 1 hour and allowed to cool to room temperature.
Propan-1-ol was introduced at room temperature and allowed
to stabilize in pressure then nitrogen was subsequently
introduced in order to purge the gas phase species. Figure 518 shows the spectrum taken for propan-1-ol over the 20wt%
Au/TiO2 catalyst.
1453
1426
1335
3.2. Infra Red Spectrometry
40
Kubleka munk units
30
o
400 C
20
o
300 C
10
o
200 C
o
100 C
0
2000
-1
Wave number (cm )
1500
1559
2500
1000
RT
500
1340
1375
3000
2883
2870
3500
3550
4000
Figure (7). DRIFTS spectra from the adsorbed propan-1-ol over a 20%wt Au/TiO2 catalyst at different temperatures.
American Journal of Chemistry and Applications 2015; 2(2): 36-43
Figure (7) shows spectra in the range 1000cm-1 to 1750cmand in the range 25000cm-1 to 4000cm-1. The bands at 2964,
2937 2883 and 2870 cm-1 are in the CH-stretching region and
are assigned to vas.(CH3), vas(CH2) vs(CH3) and vs(CH2) of the
adsorbed propan-1-ol (C3H7OHad) respectively. In the CH
bending region, the bands δas(CH3)/δ(CH2) at 1460cm-1 and
δsCH3) at 1375cm-1 and δ(CH) at 1335cm-1 can be assigned1
(7-19). However, according Weckhuysen et al. (6), the bands
at 1090 and 1140cm-1 can be assigned as C-O-Ti stretching
vibrations for propan-1-ol. The intensities of the bands are
higher when compared with TiO2 alone, which is likely due
to the formation of formate on the gold component of the
catalyst.
Similarly, the presence of the peaks at 1723cm-1 was
assigned by Chuang et al.( 19, 20), as the v(C=O) band due
to adsorbed propanal (C2H5CHOad). The bands observed at
1559, 1453 1426 and 1340cm-1 will also be assigned to
vas(COO), δas(CH3), δ(CH2) and δs(CH3) for adsorbed
propanoate (CH3CH2COO-ad) respectively (19). The presence
of the bands at 2850 and 1340cm-1 was assigned to the v(CH2)
and vs(COO) bands for the adsorbed formate (HCOO-ad)(2123). The data obtained in figure 5-6 shows that the band at
3690, 3550 and 3634cm-1 are likely due to v(OH) of isolated
OH. (21-23 ). The spectrum is similar to TiO2 alone but with
some small differences. Higher spectrum intensities are
observed in the Au/TiO2 catalyst than when TiO2 was used
alone. The high intensities may be due to more formate and
propanoate formation on the gold and reduced coverage of
propoxy species on the surface of the catalyst (21). The result
is similar to ethanol reported earlier (21-23). The main
species present on TiO2 and Au/TiO2 catalysts are formate,
propanal and propanoate, and they appeared to have thermal
stability.
The disappearance of the pair of bands at 2971 and
2901cm-1 upon heating may be correlated with the CO, CO2,
CH3CH=CH2, CH3CH2CH3 and hydrogen seen in the
temperature programmed desorption (figure 6). The
remaining bands are probably propoxy-derived species and
are related to titania, which are responsible for the
dehydrogenation and dehydration.
1
3.3. The Mechanism of Propan-1-ol Oxidation
on the Au/TiO2 Catalyst
From the data gathered so far, the following mechanism
may be proposed for propan-1-ol oxidation on Au/TiO2,
where g and a refer to gas phase and adsorbed species
respectively:
(a) C3H7OH (g) → C3H7OH (a)
From the temperature programmed desorption (figure 6),
propan-1-ol is seen to be desorbed at low temperature. The
propan-1-ol uptake in figure (6) was measured to be about
half monolayer from the pulses taken up by the catalyst. As
in the case of ethanol, the low desorption of propan-1-ol at
low temperature is associated with a weakly bound form of
molecular ethanol on the surface, and the higher temperature
desorption states are those involved with catalytic processes
41
and involved the reaction of propan-1-ol with the surface as
follows:
(b) C3H7OH (a) + O2-(a) → C3H75O-(a) + OH-(a)
(c) C3H7OH (a) + O H-(a) → C3H7O-(a) + H2O (g)
The steps (b) and (c) are the most likely ways for the
propoxy formation, however, with (b) likely to be dominant
at ambient temperature for titania. In this case, the anion
vacancies are designated as Vo2-, although the electrons are
likely to be associated with cation sites, as Ti3+, than the
vacancy itself.
The presence of the bands at 2964, 2937 2883 and 2870
cm-1 indicate that propoxy species are formed according steps
(b) and (c) above. The adsorbed propoxy formed by the
reaction in (b) and (c) are likely the main intermediate for the
formation of gas phase products such as:
(d) C3H7O-(a) + O 2-(a) → C2H6 (g) + CO (g) + OH-(a) + Vo2(e) The adsorbed propoxy may also react with adsorbed
oxygen on the surface and give hydrogen as seen as a product
at high temperature from titania and even in the presence of
gold. The appearance of the peaks at 1723cm-1 indicated the
presence of adsorbed propanal species, while the peaks at
2850 and 1340cm-1 also indicated the presence of adsorbed
propanoate species respectively. The adsorbed propanoate
and propanal species may be formed by the following
reaction.
(i) C3H7O-(a) + O2-(a) → C2H5COH (a) + OH-(a) + Vo2(ii) C3H7O-(a) + O2-(a) → C3H5COO-(a) + H2 (g) + Vo2The bands observed at 2964, 2937 and 2883cm-1are
associated with the presence of adsorbed formate species
which are likely be formed either through adsorbed propanal
or adsorbed propanoate species according the following
reaction.
(f) (i) C2H5COH(a) + OH-(a) → HCOO-(a) + C2H6 (g)
(ii) C3H5COO-(a) + OH – (a) → HCOO-(a) + C2H5O-(a)
(g) The formates adsorbed on gold may subsequently
decomposed (i) or react with the adsorbed surface hydroxide
(ii) according to following reaction.
(i) 2HCOO-(a) → 2CO2 (g) + H2 (g) + 2e(ii) HCOO-(a) + OH-(a) → CO2 (g) + H2O + 2e(i) The propene, which is a dehydration product mainly
on TiO2, will be formed according to the following
reaction:
(i) C3H7O-(a) → C2H5CH2 (g) + OH-(a)
4. Conclusions
The result of propan-1-ol oxidation over an Au/TiO2
catalyst is a partial oxidation. The propan-1-ol could also be
adsorbed on TiO2 and Au/TiO2 catalyst as shown in figure 8
and 9 respectively. The adsorption of propan-1-ol over the
TiO2 catalyst surface is similar to the methanol and ethanol.
As with methoxy and ethoxy species, the propoxy may be
bonded to TiO2 and Au/TiO2 surfaces via TiO2-O-C bond.
The adsorption of propan-1-ol may occur in two ways (figure
8 and 9)
42
Abdullahi Nuhu and Albert Carley: Mechanism and Anaerobic Propan-1-ol Oxidation Reaction over Au/TiO2 Catalysts
CH3
CH3
CH2
C--------- O----H
Ti
=
=
H
H
CH2
H
O----H
C+
H
. .
. .
O
(a)
TiO2
CH3
TiO2
CH3
H
H
CH2
H
CH2
H
C
C
:O :
+
H
H
(b)
TiO2
O-
TiO2
Figure (8). Propan-1-ol adsorption over a TiO2 catalyst surface
When propanol is adsorbed on the surface of the TiO2
catalyst, it can either be adsorbed leading to the formation of
propyl group as shown in figure (8). This produces a CH3
CH2CH2+ ion adsorbed at the basic site (O) and the OH- ion,
which is basic and can react on acidic site Ti. For the other
possibility shown (Ethanol oxidation), propanol adsorption
leads to a surface propoxide which was identified by infra
spectrometry. In this case, the basic site (O), extracts the
hydroxyl hydrogen producing a propoxylate ion, which is a
strong base and can neutralize the acid centre Ti, so as mutual
neutralization attained and the adsorbed species will be
electrically neutral. However, the most probable adsorption
step of propanol was in figure (8) as shown by the infrared
spectra. However, even if gold was present, propanol
adsorption was found to be with propoxy being bonded with
acidic site (Ti) and H+ on the basic site (O) of the surface of
Au/TiO2 catalyst as shown as in figure (9). The presence of
propoxy species being adsorbed on TiO2 and Au/TiO2
catalysts is responsible for the surface reaction observed in
aerobic and anaerobic reaction of propan-1-ol.
CH3
H
CH2
H
yields products due to dehydrogenation to propanal,
deoxygenation to propane and dehydration to propene.
However, the aerobic propan-1-ol oxidation products over
TiO2 catalyst are propanal and propene (product of
dehydrogenation and dehydration) while the anaerobic
reaction involved dehydration to propene, deoxygenation to
propane and decomposition to ethane, CO and hydrogen. As
seen from the temperature programmed pulse flow reactions,
the presence of gold enhances the conversion of propan-1-ol
at low temperature; it also enhances the reactivity of the
catalyst towards dehydrogenation to propanal in the presence
of oxygen and deoxygenation to propane in the anaerobic
reaction of propan-1-ol.
The temperature programmed desorption data and infrared
spectra show that the main adsorbed species responsible for
the complete oxidation of propanol to CO2 and water was
formate similar to methanol and ethanol. The spectra
observed for propan-1-ol over TiO2 and Au/TiO2 are similar,
with a slight lower intensity being observed in the case of
TiO2. The high intensity observed in the Au/TiO2 catalyst is
due to the presence of gold in the catalyst. The uptake of
propan-1-ol is found to be 10l TiO2 and Au/TiO2 catalyst
before saturation.
References
[1]
M. Haruta, Catal. Today 36 (1997) 153
[2]
M. Bowker, A. Nuhu, and J. Soares, Catalysis Today, 122 245247 (2007)
[3]
A. Nuhu Bayero Journal of Physical and Applied Sciences
(BAJOPAS) 23 134-139 (2009)
[4]
J. Arana. J. M. Dona- Rodrigues, J. A. Herrera Melian, E.
Tello Rendon, O. Gonzalez Diaz, J. of PhotoChem. And
Photobio. A: Chemistry 174 7-14(2005)
[5]
J. E.Baile, C. H. Rochester and G. Hutchings, J. of Chem. Soc.,
Faraday Trans., 93 4389-4394 (1997)
[6]
T. A. Nijhuis, T. Visser and B. M.. Weckhysen, Angew. Chem.
Int. Ed. 44 1115-1118 (2005)
[7]
T. Kecskes, J.Rasko and J. Kiss, Appl. Catal. A: Gen. 273 5562 (2004).
[8]
A. V. Vorontsov and V. P. Dubovitskaya, Journal of Catalysis
221102-109 (2004).
[9]
F. G. Montanez, Z. Yu, S. S.Chuang and Chen Yang,
C
=O
H
O
=Ti
= Au
Au/TiO2
Figure (9). Propan-1-ol adsorption over Au/ TiO2 catalyst surface
The main products of propan-1-ol oxidation (aerobic)
reaction over the Au/TiO2 catalyst are propanal, propene and
propane, which are products of dehydrogenation, dehydration
and deoxygenation of propanol. The reaction is selective and
although is a complete oxidation to water and CO2. The
anaerobic propan-1-ol reaction over the Au/TiO2 catalyst
[10] Gamal. A. M. Hussein and N. Sheppard. M.I. Zaki and R. B.
Fahim, J. Chem. Soc. Faraday Trans., 87(16), 2661-2668
(1991)
[11] H. Idriss, Platinum Metal Rev, 48(3), 105-115 (2004)
[12] J. Rasko, M. Domok, K. Baan and A. Erdohelyi, App. Cat. A:
Gen. 299 202-211 (2006)
[13] A. Erdohelyi, J. Rasko, T. Kecskes, M. Toth, Marta Domok,
and K. Baan, Cat. Today 116 367-376 (2006)
[14] J.M.Guil, N. Homs, J. Llorca, and P. Ramirez de la Piscina, J.
Phys. Chem. B. 109 10813-10819 (2005)
American Journal of Chemistry and Applications 2015; 2(2): 36-43
43
[15] R. Hayashi, M. Onishi, M. Sugiyama, S. Koda, and Y. Oshima,
J. of supercritical Fliuds 40 74-83 (2007)
[19] J. E.Baile, C. H. Rochester and G. Hutchings, J. of Chem. Soc.,
Faraday Trans., 93 4389-4394. (1997)
[16] J. Arana. J. M. Dona- Rodrigues, J. A. Herrera Melian, E.
Tello Rendon, O. Gonzalez Diaz, J. of PhotoChem. And
Photobio. A: Chemistry 174 7-14 (2005)
[20] Z. Yu, S.C. S. Chuang, Journal of Catalysis 246 118-126
(2007).
[21] L.Parti and F. Porta, Appl. Catal. A: Gen. 291 199 (2005).
[17] T. A. Egerton, and J. A. Mattinson, J. J. of PhotoChem. And
Photobio. A: Chemistry 186 115-120 (2007)
[22] S. Biella and M. Rossi, Chem, Commun. 378 (2003).
[18] A. E. Mucients, F. J. Poblete, M. A. Rodriguez and F. Santiago,
J. of Phys. Org. Chem, 10 662-668 (1997)
[23] M.A. Centeno, M. Paulis, M. Montes and J. A. Odriozola,
Appl. Catal. A: Gen. 234 65 (2002).