PTCs promoted biphasic oxidation of ethylbenzene catalyzed by NHPI and CoSPc

Article abstract of DOI:10.1016/j.cclet.2012.05.026

An easy workup and environmentally friendly method to convert ethylbenzene into the corresponding oxides, catalyzed by a combined catalytic system consisting of NHPI and CoSPc, was developed in a water involved liquid-liquid heterogeneous system. Experime

Full text of DOI:10.1016/j.cclet.2012.05.026

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 895–898
www.elsevier.com/locate/cclet
PTCs promoted biphasic oxidation of ethylbenzene
catalyzed by NHPI and CoSPc
*
Yu Lu Zhou , Sha Sha Lin, Dao Hong Xia, Yu Zhi Xiang
College of Science, State Key Laboratory of Heavy Oil, China University of Petroleum, Qingdao 266580, China
Received 13 March 2012
Available online 4 July 2012
Abstract
An easy workup and environmentally friendly method to convert ethylbenzene into the corresponding oxides, catalyzed by a
combined catalytic system consisting of NHPI and CoSPc, was developed in a water involved liquid–liquid heterogeneous system.
Experimental results showed that the reaction performed well facilitated by PTCs at multigrams material level, under which the
conversion rate of ethylbenzene and the selectivity of acetophenone reached 60.6% and 95.2% after 0.5 h of reaction in oxygen
atmosphere. An oxidation process was proposed at the end of this paper.
# 2012 Yu Lu Zhou. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Ethylbenzene; Aerobic oxidation; NHPI; CoSPc; PTC
Though there are many advantages of utilizing oxygen as the oxidant in organic substrates oxidation, large-scale
effective processes of transforming alkane or alkyl with molecular oxygen into the corresponding oxides have been
limited due to the inert activity of alkane or alkyl and low oxidation ability of oxygen, so the development of effective
and selective catalytic system for the oxidation of alkane or alkyl still faces a big challenge [1]. Recently, more and
more attention has been paid to N-hydroxyphthalimide (NHPI) for its catalytic ability in the oxidation of hydrocarbons
[2], and which is usually employed combined with some transition metals, such as cobalt, manganese, copper, iron,
etc. [3]. The transition metals usually presented in the forms of carboxylic salts, rarely in their more stable complex
compounds forms [4]. Herein, we chose a widely used catalyst cobalt sulfonated phthalocyanine (CoSPc) [5] as one
part of the catalytic system, and in order to facilitate the separation process of products and catalysts, water was
inducted as an environmentally friendly solvent for catalysts, which formed a liquid–liquid biphasic system with
ethylbenzene and oxidation products, phase transfer catalysts (PTCs) were employed as the crucial additives aiming to
promote the mass transfer between aqueous and organic phases. Until now, there is no report concerning the
combination of CoSPc and NHPI for the oxidation of ethylbenzene in a water involved liquid–liquid biphasic system,
and which represented a good catalytic activity and selectivity, a 60.6% of conversion rate and a 95.2% of
acetophenone selectivity were achieved after 0.5 h of reaction in oxygen atmosphere (Scheme 1). The separating
workup process for this approach was simple only by an ordinary separation funnel. Currently, the most developed
heterogeneous oxidations of hydrocarbons were concerning liquid–solid and gas–solid biphasic systems [6], they were
* Corresponding author.
E-mail address: zhouyl@upc.edu.cn (Y.L. Zhou).
1001-8417/$ – see front matter # 2012 Yu Lu Zhou. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2012.05.026

[(Scheme_1)TD$FIG]
896
Y.L. Zhou et al. / Chinese Chemical Letters 23 (2012) 895–898
CH2CH3
O
OH
NHPI, CoSPc, PTCs
H2O, 0.75MPa, 110oC
O2
+
+
1
2
3
Scheme 1. Heterogeneous oxidation of ethylbenzene.
blamed for their lower catalytic activity or higher energy demand, therefore, a liquid–liquid heterogeneous system
may provide a promising way for an easy separating approach.
1. Experimental
In a general process, into a high pressure autoclave, 10 mL of ethylbenzene, 30 mL of H2O, 10 mol% of NHPI,
0.42 mol% of CoSPc and a certain amount of PTCs were added, then, air in the autoclave was exchanged three times
with O2 to sweep N2 out. Subsequently, the oxidation was conducted for 0.5 h at 110 8C under stirring and in O2
atmosphere. The organic layer was separated through a separation funnel, and the products yields were estimated
according to the peak areas based on internal standard technique by GC. The predominant product was acetophenone,
and 1-phenylethanol was scarce, they were assigned by 1H NMR and EI-MS after separation on a silica gel column [7].
The conditions of the GC (Agilent 6820 gas chromatograph with a flame ionization detector using a
30 m  0.32 mm  0.50 mm capillary column) analyses were as follows: a temperature program from 100 8C
(5 min) to 180 8C (10 8C/min), then maintaining 20 min.
2. Results and discussion
Preliminary, optimization of the reaction was performed. Results demonstrated the most optimal conditions were as
follow: V(ethylbenzene):V(H2O) = 1:3, n(NHPI):n(ethylbenzne) = 1:10, n(CoSPc):n(NHPI) = 1:24, 2.5 mol% (based
on ethylbenzene) of TBAB, temperature 110 8C, O2 pressure 0.75 MPa, stirring rate 350 r/min, reaction time 0.5 h,
under which, a moderate conversion rate 60.6% and an excellent acetophenone selectivity 95.2% were achieved, the
detailed full paper concerning optimization of the reaction will be released later.
Table 1
Effect of PTCs on ethylbenzene oxidation.
Entry
PTCs
Conversion (%)
Selectivity (%)
2
3
Total
a
1
2
–
16.4
60.6
49.7
12.6
28.7
28.7
44.6
25.4
8.7
82.8
95.2
78.4
92.2
87.6
70.9
89.6
84.8
0.7
8.8
4.5
5.6
5.6
6.0
4.2
2.9
8.4
0.1
6.2
5.1
6.5
7.3
0.1
91.6
99.7
84.0
97.8
93.6
75.1
92.5
93.2
0.8
TBABb,d
TBACb
TMABb
CTABc
CTACc
TDMBACb
SDBSc
CMC-Nad
OP-10c
3
4
5
6
7
8
9
10
11
12
13
14
24.5
18.3
24.1
10.4
74.7
75.6
82.3
83.4
90.2
74.5
81.8
87.4
89.9
97.5
74.6
PEG 6000d
Tween-60c
b-CDc
Acetic acide
a
Without present of PTC.
b
c
Conc. is 2.5 mol% of ethylbenzene.
Conc. are their CMCs: CTAB (9.12 Â 10À5 mol/L), CTAC (1.6 Â 10À2 mol/L), SDBS (1.2 Â 10À3 mol/L), OP-10 (0.4 g/L), Tween-60 (2.528 g/
L), b-CD (4 g/L).
d
Utilized at the same weight amount of TBAB 7.6%.
e
Acetic acid as solvent in absence of PTC.

Y.L. Zhou et al. / Chinese Chemical Letters 23 (2012) 895–898
897
Based on the examination of TBAB, a variety of other PTCs were investigated, results are summarized in Table 1,
PTCs are divided into three classes, one class are quaternary ammonium cationic PTCs (entry 2–7), one class are
anionic PTCs (entry 8 and 9), and the other one are nonionic PTCs (entry 10–13). The dosages of PTCs were outlined
under the table. TBAC, TMAB and TDMBAC were utilized at the same molar amount of TBAB, and CTAB, CTAC,
SDBS, OP-10, Tween-60 and b-CD were used at their critical micelle concentrations (CMC), in the cases of CMC-Na
and PEG 6000, which do not bear CMC, utilized at the same weight amount of TBAB.
Table 1 indicated that the oxidation performed unsatisfactorily without presence of PTC, only 16.4% of conversion
rate was gotten (entry 1), so PTC is a crucial additive, and among the three kinds of PTCs, cationic PTCs significantly
favored the biphasic oxidation (entry 2, 3, 5–7), except TMAB (entry 4), which bearing the shortest aliphatic chains in
its molecular, therefore, the mass transfer initiated by TMAB was not distinct. Comparing with that of in aqueous
phase, the solubility of CoSPc in ethylbenzene is limited, therefore, the mass transfer of CoSPc from aqueous phase to
organic one predominated the oxidation process, cationic PTCs might enhance the mass transfer process owing to the
sulfonic substituents on the phthalocyanine ring, which could facilitate the formation of ionic pair [8] with cationic
PTCs. Anionic and nonionic PTCs had an inconspicuous effect on the oxidation (entry 8–13) due to their helplessness
in the formation of ionic pair with CoSPc, in the cases of CMC-Na and b-CD, were even inferior to that of without
PTC. Counterion bromide in PTC (entry 2) did favored the oxidation compared to their counterpart which combined
with chloride anion (entry 3), due to the better surface activity for bromide as a counterion of quaternary ammonium
than chloride did [9], CTAC performed similarly to the result of CTAB, but with a much higher concentration than that
of CTAB.
A comparative reaction was conducted, employing acetic acid as the solvent in absence of PTC and without other
conditions changed. Empirical results indicated homogeneous system exhibited a much higher oxidation activity, but
with a much lower selectivity of acetophenone, the generation of benzoic acid via over oxidation of acetophenone was
detected by TLC, this phenomenon attributed to the better dissolubilities of O2, NHPI and CoSPc in acetic acid, at the
same time, it was also blamed for its higher catalytic activity, which resulted in the lower product selectivity. The
inducting of water involved liquid-liquid heterogeneous system separated the substrate and catalysts, and restrained
the over oxidation of acetophenone, so the products selectivities were higher then that of in acetic acid.
Ultimately, the possible process of the oxidation was proposed according to the empirical results, it is well known,
b-CD bear a lipophilic inner cavity and a hydrophilic outside surface, the cavity diameter of b-CD is between 6.0 and
˚
6.5 A [10], which favored the ethylbenzene mass transfer from organic to aqueous phase [11], nevertheless, when b-
CD was utilized as PTC, the ethylbenzene conversion rate was lower (entry 13) even than that of without present of
PTC (entry 1). It indicated that the oxidation did not mainly take place in aqueous phase. In the cases of quaternary
ammonium PTCs, they promoted the mass transfer of CoSPc from aqueous phase to organic one, which fulfilled the
oxidation. This also supported the organic phase oxidation process. NHPI can be slightly dissolved both in organic and
aqueous phases, so the influence initiated by PTCs on NHPI was not significant. In summary, a conclusion can be
drawn that PTCs promoted oxidation of ethylbenzene was mainly affected by the mass transfer of CoSPc, which is an
aqueous dissolved molecular. A proposed procedure is outlined in Scheme 2. Preliminary, CoSPc yielded hyperoxide
[(Scheme_2)TD$FIG]
Q+X-
O2
Co2+PcSO3H
+
HX
-
OOCo3+PcSO3H
OOCo3+PcSO3 Q+
Aqueous phase
Organic phase
-
OOCo3+PcSO3 Q+
Co2+PcSO3 Q+
-
2
+
3
NHPI
Ph
HOOCo3+PcSO3 Q+
-
CHOOH
Ph
PINO
NHPI
CHOO
Q+=Quaternary ammonium
CH
O2
Ph
Ph
Scheme 2. Proposed reaction procedure.

898
Y.L. Zhou et al. / Chinese Chemical Letters 23 (2012) 895–898
radical complex with dissolved oxygen in aqueous phase, then generated ionic pair with cationic PTCs, after that, it
was transferred into the organic phase, therein, PINO was generated via hydrogen abstraction by hyperoxide radical
complex, and then, PINO was involved in the oxidation circulation, similar to that of homogeneous oxidation process
[2].
3. Conclusions
In summary, an easy workup and environmentally friendly method to convert ethylbenzene into the corresponding
oxides, promoted by PTCs, was developed in a water involved liquid–liquid biphasic system, among which, TBAB
was the most favorable PTC, due to its appropriate aliphatic chain length and capability of forming ionic pair with
CoSPc, contrary to that, anionic and nonionic PTCs exhibited less helpfulness to the oxidation.
Acknowledgments
We thank the Fundamental Research Funds for the Central Universities (No. 10CX04024A) and the financial
support from Natural Science Foundation of Shandong Province (No. 2009ZRA05107).
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