Green and Sustainable Chemistry, 2011, 1, 26-30
doi:10.4236/gsc.2011.12005 Published Online May 2011 (http://www.SciRP.org/journal/gsc)
Selective and Clean Cyclohexene-Promoted
Oxidation and Photooxidation by Air
Grigoriy Sereda, Vikul Rajpara
Department of C hemi st ry , University of South Dakota, Vermillion, USA
Received March 17, 2011; revised April 17, 2011; accepted April 24, 2011
Abstract
A simple and environmentally friendly selective procedure for cyclohexene-promoted photooxidation of
p-xylene, ethylbenzene, and cumene by air in the presence of a pristine or oxidized carbonaceous material is
reported. Depending on the catalyst and conditions, the reaction yields either of the following industrially
important products: 4-methylbenzyl hydroperoxide, 4-methylbenzoic acid, 1-phenylethyl hydroperoxide,
2-phenyl-2-propanol, acetophenone with high selectivity and practical extent of conversion. Exposure the
reaction mixture to ambient light further increased the yields. Improved performance of oxidized graphite
has demonstrated the potential of surface modification for the design of novel carbonaceous catalysts.
Keywords: Oxidation, Synthesis, Catalysis, Hydrocarbons
1. Introduction
Alkylbenzenes industrially produced by the petroleum
processing, are known as key commodities for the or-
ganic synthesis. Products of their benzylic oxidation
have countless practical applications. To name just a few,
4-methylbenzyl hydroperoxide 1 was found to act as
oxygen donor in cytochrome c [1] and inhibit alcohol
dihydrogenase [2]. 4-Methylbenzoic acid 2 is a common
plasticizer [3], steel anti-corrosion agent [4], and a sig-
nificant component of writing ink [5]. 1-Phenylethyl
hydroperoxide 3 is a good oxygen donor for alkene ep-
oxidation [6]. 2-Phenyl-2-propanol 4 is a component of
optical lenses [7] and a catalyst for styrene polymeriza-
tion [8]. Acetophenone 5 is commonly used as a fra-
grance [9] and anticorrosion agent [10]. Synthesis of
these compounds usually involves toxic tungsten and
palladium-based catalysts [11,12]. Here we report a series
of simple, environmentally friendly, and selective proce-
dures for the synthesis of compounds 1-5 by oxidation of
alkylbenzenes with air in the presence of a carbonaceous
catalyst, ambient light, and the cyclohexene promoter.
Recently we reported [13] that oxidation of p-xylene 6
by air on carbonaceous materials can be significantly
affected by ambient light that may lead to either activa-
tion or passivation of the catalyst along with significant
shifts in the product composition. Due to the adsorption
of the key intermediate 1 (Figure 1), carbonaceous ma-
terials with the elevated defects-to-basal plane ratio and
highly sorptive surface (carbon black (CB) and oxidized
graphite (OG)), provided higher yields of the hydroper-
oxide 1 comparing with graphite [13]. However, notice-
able loss of their catalytic activity upon exposure to light
significantly curtailed application of these materials for
organic synthesis.
The herein reported procedures have taken advantage
of the free-radical promoting activity of cyclohexene that
we have shown before on the example of the graphite
catalyst [14]. Graphite-catalyzed oxidation of p-xylene 6
led to hydroperoxide 1 as the major product along with
the acid 2, alcohol 7, aldehyde 8, and ester 9 (Figure 1).
However, the relatively low yield and selectivity led us
to the exploration of alternative carbonaceous catalysts.
2. Results and Discussion
We found that in the presence of cyclohexene, carbon
Figure 1. Oxidation and photooxidation of p-xylene.

G. SEREDA ET AL.
27
black has demonstrated excellent selectivity toward hy-
droperoxide 1 (Table 1, Entry 2), however, significant
photopassivation of the catalyst made the extent of con-
version unpractical. In contrast, catalytic performance of
oxidized graphite struck the best balance between the
extent of conversion and selectivity despite noticeable
photopassivation (Table 1, Entries 3, 4). Adsorption of
the intermediate hydroperoxide 1 by the polar surface of
oxidized graphite was apparently counteracted by the
ability of cyclohexene to generate free radicals due to its
allylic oxidation. Ultrapure graphite comparable with
oxidized graphite by the cost, presents an alternative ef-
ficient catalyst for the synthesis of hydroperoxide 1 (Ta-
ble 1, Entries 5, 6), which has a potential for scaling the
process up to the industrial level.
As we reported earlier [13], graphite nanofibers (GNF)
belong to the group of carbonaceous materials activated
by ambient light. In the presence of cyclohexene, GNF
kept their photocatalitic properties toward oxidation of
p-xylene (Table 1, Entries 7, 8) and produced acid 2
with high conversion and selectivity similar to that we
observed in the absence of cyclohexene [13].
Addition of cyclohexene significantly increases selec-
tivity of Single Wall Carbon Nanotubes (SWCNT) to-
wards the acid 2 that surpass that of GNF. However, due
to the low cost, graphite nanofibers seem to be the most
practical photocatalyst for the preparation of 4-methyl-
benzoic acid 2. Addition of cyclohexene did not change
the unique ability of Multi Wall Carbon Nanotubes
(MWCNT) to catalyze formation of “coupled” products:
di(4-methylbenzyl) ether 10 and 1,4-dimethyl-2-
(4-methylbenzyl)benzene 11. However, both the in-
creased conversion and complete suppression of uniden-
tified phenolic products [13] make MWCNT a promising
material for the design of new carbonaceous catalysts for
organic synthesis.
In order to demonstrate reproducibility of the experi-
ments performed under the ambient light, we repeated
several experiments (Table 1, Entries 8, 13, 14) two
more times and arrived to very similar yield and product
distribution.
It is also worth mentioning that without a carbona-
ceous catalyst, cyclohexene itself did not catalyze oxida-
tion of p-xylene [14].
Interestingly, the presence of cyclohexene does not
affect the relationship between photosensitivity of car-
bonaceous materials toward oxidation of p-xylene, and
their structure, discussed in detail earlier [13]. Employ-
ing cyclohexene as an activator led us to the most prac-
tical procedures for preparation of hydroperoxide 1 par-
tially due to the complete suppression of formation of
unidentified phenolic products.
Fortunately, deactivation of carbon black by light is
very sensitive to the substrate of oxidation and does not
hold for the practically important reaction of oxidation of
ethylbenzene 12 (Figure 2).
In the absence of cyclohexene, photopassivation of
carbon black reverses (Table 2, Entries 1, 2), which makes
it a practical catalyst for the synthesis of 1-phenylethyl
Figure 2. Oxidation and photooxidation of ethylbenzene.
Table 1. Oxidation of p-xylene in the presence of cyclohexene.
Molar ratio
Entry Catalyst Surface area, m2/g (average
pore size, nm)Light 1 7 8 2 9 10 11
Total yield
(g)
1 No 0.8 1.5 1 2 0.25 0 0 0.46
2 CB 80
(289) Yes 33 2 1 0.3 0 0 0 0.05
3 No 2.1 1.65 1 1 0 0 0 0.42
4 OG 13
Yes 8 1.1 1 0.8 0 0 0 0.21
5 No 2.2 1.8 1 0.72 0 0 0 0.23
6 UPG 30
Yes 12.5 1.5 1 1 0 0 0
0.23
7 No 0 0.6 1 1.1 0.1 0 0 0.21
8 GNF 19
(1.4) Yes 0 1 1 2.7 0.31 0 0 0.57
9 No 0 0.85 1 1.25 0.09 0 0 0.17
10 SWCNT 281 Yes 0 0.32 1 3 0 0 0 0.54
11 No 0 0.7 1 0.7 0.7 0.14 0.34 0.11
12 MWCNT 99
(261) Yes 0.3 0.2 1 0.5 0.7 0.14 0.5 0.13
1314 No 0 1 1 0.7 0.7 0 0 0.23
1414 Graphite 5.9
(19.6) Yes 4.5 1.5 1 1.5 0 0 0 0.23

28 G. SEREDA ET AL.
Table 2. Oxidation of ethylbenzene.
Entry Catalyst Light Cyclohexene
present 1-Phenylethyl hydroperoxide 3 1-Phenylethanol 13 Acetophenone 5 Total yield (g)
1 No No 2.9 2.2 1 0.38
2 CB Yes No 4.5 0 1 0.72
3 No Yes 1 1 1 0.51
4 CB Yes Yes 10 0.3 1 0.42
5 No Yes 2.1 1.65 1 0.51
6 OG Yes Yes 8.0 1.1 1 0.60
Table 3. Oxidation of cumene.
Entry Catalyst Light 2-Phenyl-2-propyl hydroperoxide 152-Phenyl-2-propanol 4Acetophenone 5 Total yield (g)
1 No 0 2.9 1 0.30
2 OG Yes 0 2.2 1 2.0
3 No 0 0.55 1 1.25
4 CB Yes 0 0.32 1 1.24
514 No 0.3 1.8 1 0.97
614 None Yes 0 0.7 1 1.83*
*0.1 equiv. of unidentified phenolic products are formed.
hydroperoxide 3 (Table 1, Entry 2). In the presence of
cyclohexene, both carbon and oxidized graphite provided
practical conversion and selectivity towards 3 (Table 2,
Entries 4, 6). Contrary to carbon black, graphite [14]
completely suppress oxidation of ethylbenzene 12, which
produces hydroperoxide 3 with low selectivity along
with alcohol 13 and acetophenone 5 (Table 2).
While cumene 14 undergoes oxidation (Figure 3) with
and without the presence of graphite [14], its cyclohex-
ene-promoted oxidation catalyzed by carbon black pro-
vides by far best selectivity toward alcohol 4 (Table 3,
Entry 2). Oxidized graphite has shown excellent photo-
catalytic performance for the preparation of another
practically important product - acetophenone 5 (Table 3,
Entry 4). Low cost of carbon black and oxidized graphite
presents a potential for their industrial application.
Presence of ambient light is not necessary for the
reactions to proceed, however it improved selectivity and
usually overall yields. For carbon black (Table 1 , Entries
1, 2) the observed photopassivation rendered the reaction
unpractical. However, oxidized graphite was less prone
to photopassivation, which was conteracted by the sig-
nificantly improved selectivity (Table 1, Entries 3, 4).
3. Experimental Part
Carbon Black (CB), Graphite Nanofibers (GNF), Ul-
trapure Graphite (UPG), Multi Wall Carbon
Figure 3. Oxidation and photooxidation of cumene.
Nanotubes (MWCNT), and Single Wall Carbon Nano-
tubes (SWCNT) were purchased from Aldrich. Oxidized
graphite (OG) was prepared by oxidation for 10 h with
aqueous nitric and sulfuric acids [15]. The reactions of
benzylic oxidation were performed by passing air at the
rate of 1 mL/min through 37 mg of a catalyst, suspended
in 5 mL of p-xylene, and 0.3 mL of cyclohexene under
reflux for 24 h. After addition of 10 ml of hexane, the
catalyst was removed by filtration. The filtrate was con-
centrated in vacuum and the residue was analyzed by
non-overlapping 1H NMR signals, characteristic for the
hydroperoxide 1 [16] (singlet at 4.95 ppm), alcohol 7 [17]
(singlet at 4.60 ppm), aldehyde 8 [18] (singlet at 9.95
ppm), acid 2 [19] (doublet at 8.0 ppm), ester 9 [20]
(singlet at 5.30 ppm), ether 10 [21] (singlet at 4.50 ppm),
and hydrocarbon 11 [22] (singlet at 3.85 ppm).
Oxidation of ethylbenzene 12 and cumene 14 was
performed by the same procedure. The reaction product
was analyzed by non-overlapping 1H NMR signals,
characteristic for the hydroperoxide 3 [23] (quadruplet at
5.05 ppm), alcohol 13 [24] (quadruplet at 4.90 ppm), and
acetophenone 5 [25] (singlet at 2.60 ppm). The methyl
groups of the hydroperoxide 15 [26] and alcohol 4 [27]
show up in the 1H NMR as singlets at 1.55 - 1.60 ppm.
We determined the molar ratio of 15 and 4 by integration
of the inverse-gated broad band decoupled 13C NMR
spectrum as we described earlier [14]. The molar ratios
of products are presented in Tables 2 and 3. Conversion
of the oxidation was characterized by the mass of the
product mixture, given in Tables 2 and 3 (Total Yield).
Although the synthetic outcome of the reported reactions
was characterized by NMR, we previously demonstrated
[14] that all components of the reaction mixtures can be
efficiently separated on a silica gel chromatography
column eluted by hexane-ethyl acetate.

G. SEREDA ET AL.
29
Surface analysis of the catalysts was performed by the
nitrogen adsorption/desorption measurements. The sam-
ples were outgassed for 1 h at 100˚C and analyzed at 77 K
using a Quantachrome Nova 2200e gas adsorption ana-
lyzer.
4. Conclusions
In conclusion, we introduced carbon black, ultrapure
graphite, and graphite nanofibers as selective, ambient
light-activated catalysts for selective oxidation of
p-xylene, ethylbenzene, and cumene by air. While the
presence of ambient light is not necessary, it significantly
improves the practical outcome in terms of selectivity
and usually overall yield. Utilization of cyclohexene as a
promoter has allowed us to circumvent previously re-
ported photopassivation of carbon black and oxidized
graphite. Depending on the catalyst and conditions, this
environmentally friendly reaction yields either of the
following industrially important products: 4-methlbenzyl
hydroperoxide 1, 4-methylbenzoic acid 2, 1-phenylethyl
hydroperoxide 3, 2-phenyl-2-propanol 4, acetophenone 5
with high selectivity and practical extents of conversion
without any use of toxic metal co-catalysts. Improved
performance of oxidized graphite has demonstrated the
potential of surface modification for the design of novel
carbonaceous catalysts. Furthermore, addition of cyclo-
hexene significantly improves the unique catalytic per-
formance of carbon nanotubes towards oxidation and
oxidative coupling that makes them perspective materials
for additional research.
5. Acknowledgements
This work has been supported by the Director, Office of
Science, Office of Biological & Environmental Research,
Biological Systems Science Division, of the U.S. De-
partment of Energy under Contract No. DE-FG02-
08ER64624, and NSF (EPSCoR Grants No. 0554609,
0903804). We also thank the group of Prof. Ranjit
Koodali (USD) for assistance in performing surface
analyses.
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