Hydrophobic nature of the support: A significant effect on selective oxidation of cyclohexane with pure oxygen

Article abstract of DOI:10.1524/zpch.2010.5469

The relationship between the hydrophobic characteristic of supports and the reactivity of the liquid oxidation of cyclohexane using oxygen as oxidant was investigated. Co₃O₄ supported on amorphous silica was silylated with organosilane and characterized by FT-IR, contact angle testing and the retention of cyclohexane and cyclohexanone tesing on GC. Compared with unsilylated catalyst, the conversion of cycloheaxane was greatly enhanced with the silylated catalyst. And the correlation of reactivity and hydrophobicity of was observed. by Oldenbourg Wissenschaftsverlag, Mu?nchen.

Full text of DOI:10.1524/zpch.2010.5469

Z. Phys. Chem. 224 (2010) 1363–1370 . DOI 10.1524.zpch.2010.5469
© by Oldenbourg Wissenschaftsverlag, München
Hydrophobic Nature of the Support: a
Significant Effect on Selective Oxidation of
Cyclohexane with Pure Oxygen
*
By Deqiang Wang, Yi Bo Zhang, Jing Li and Xiangguang Yang
Laboratory of Green Chemistry and Process, Changchun Institute of Applied Chemistry,
Chinese Academy of Science, 5625 Remin Street, Changchun 130022, P. R. China
(Received February 19, 2009; accepted June 23, 2009)
Cyclohexane . Oxidation . Silylation . Hydrophobicity . Retention Time
The relationship between the hydrophobic characteristic of supports and the reactivity of the
liquid oxidation of cyclohexane using oxygen as oxidant was investigated. Co3O4 supported on
amorphous silica was silylated with organosilane and characterized by FT-IR, contact angle
testing and the retention of cyclohexane and cyclohexanone tesing on GC. Compared with
unsilylated catalyst, the conversion of cycloheaxane was greatly enhanced with the silylated
catalyst. And the correlation of reactivity and hydrophobicity of was observed.
1. Introduction
Cyclohexanol and cyclohexanone(so-called KA-oil) produced by the partial oxi-
dation of cyclohexane are of the key intermediates of adipic acid and caprolactam
in the manufacture of nylon-6 and nylon-66 polymers [1]. Conventionally, using
homogenerous cobalt naphthenate as the catalyst, cyclohexane conversion is
about 3–5% with the 80% selectivity of KA-oil. The greater demand of these
oxidation products and the more serious environmental concerns attract a great
deal of researches on developing more effective and environment-friendly proc-
esses. The ideal process is the one in which heterogeneous materials and molecu-
lar oxygen were used as catalyst and oxidant in the absent of solvents or addi-
tives. Up to now, great efforts have been made to this reaction, however, it
continues to be a challenge [2].
Due to the intrinsic benefit of heterogeneous catalytic systems, much atten-
tion has been focused on both developing new recycling catalyst and heterogeniz-
*
Corresponding author. E-mail: dqwang@ciac.jl.cn
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1364
D. Wang et al.
ing the currently used homogenous ones. For example, variety of metal-contain-
ing molecular sieves were synthesized using impregnation, ion-exchange or
directly incorporating with the preparation of molecular sieves [3–16].
According to the investigation on heterogeneous catalytic reactions, the oxi-
dation proceeds over the active centers dispersed on the supports [17]. The rate
of adsorption.desorption of the reactants and products from the support strongly
influences the reactivity of reaction. As for the partial oxidation of cyclohexane,
being a nonpolar molecular, the cyclohexane is hydrophobic. Whereas the pro-
ducts, KA-oil and acids (byproducts), are polar and hydrophilic because of the
oxygen insertion in the products. In the case of using silica-base material as
supports in the catalyst, surface silanol groups of which would influence the
desorption of products and lead the lower conversion and selectivity eventually
because of the relative strong interaction of hydrogen-bond between the products
and catalyst surface.
In the present paper, the relationship between the hydrophobicity of supports
and the reactivity of oxidation of cyclohexane by modified the supports with
organosilane was investigated. It was found that an enhancement of the catalytic
performance was obtained only by the modification of the catalyst surface with-
out changing of the catalyst composition.
2. Experimental
2.1 Catalyst preparation
2.1.1 Preparation of Co3O4/SiO2(Co/Si) catalyst
The Co.Si catalyst was synthesized by incipient wetting impregnation method.
A typical procedure was as follows: 0.5g cobaltous nitrate(CoNO36H2O,AR) was
dissolved with a certain quantity of deionized water, followed by adding 10g
amorphous silica gel to the solution. After impregnation with stirring 24h, the
solution was filtrated, washed and dried at 373K in vacuum drying oven. Finally,
the sample was calcined at 573k for 3h.
2.1.2 Silylation of Co.Si
Thus obtained precursor was treated with trimethylchlorosilane (TMCS) for liq-
uid-phase silytation. 1.0g Co.Si was suspended into a round bottom flask con-
taining 20g hexane, 1.0g TMCS was added into the flask, the solution was stirred
under 313K for 2h, then heated Co.Si was filtrated and washed with alcohol
and water sequentially, followed by drying at 373K for 10h. Two other catalysts
were also treated with the above mentioned method only stirring 5h and 9h
instead, all the three catalysts were labeled as Co.Si(2h), Co.Si(5h),Co.Si(9h)
respectively.
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Hydrophobic Nature of the Support
1365
Scheme 1. silylation reaction of surface silanols with TMCS.
2.2 Physic-chemical characterization of catalyst
IR spectra were recorded with Fouriertransform infrared(FT-IR) spectrometer
(Bruker Vertex 70).
The water contact angles of the samples were measured with a Drop Shape
Analyzer (DSA10, KRUSS and Hamburg 2001) at room temperature.
In order to understand the adsorption.desorption properly,retentions
time(RT) of cyclohexane and cyclohexanone on the catalyst were performed on
!
a GC equipped with a stainless steel packed column(15cm 2.0mm) in which
0.2g samples were packed [18]. The chromatographic condition was as follows:
column temperature at 423K, carrier gas (helium) flow rate at 15 mL.min, in-
jector temperature at 523K and detector temperature at 453K.
2.1 Catalytic performance tests
The catalytic oxidation of cyclohexane was carried out in a 100ml autoclave
with PTFE liner and equipped with a magnetic stirrer. In a typical experiment,
20g of substrate and 0.5g of catalyst were used. After the reaction for 5h at 308K
under 1.0MPa pure oxygen, 5ml ethyl alcohol was added into the reactor to
dissolve the products, the acid (mainly adipic acid) was determined by chemical
titration, KA-oil was analyzed by gas chromatography with a capillary column
!
!
(DM-5, 530um 1um 30m) and a flame ionized detector (FID), using chlo-
robenzene as an internal standard.
3. Result and discussion
3.1 Physic-chemical property of the supports
Fig. 1 showed the IR spectra of samples, 1099cmK1 and 809cmK1was caused
by Si-O-Si stretching vibration, and Si-O-Si bending vibration was at 470cmK1
,
the Si-OH symmetric stretching vibration at 971cmK1 was obviously reduced in
Fig. 1b, which implied the Si-OH was replaced by Si-O-Si(CH3) as shown in
Scheme 1.
The further confirmation of the formation of Si-O-Si(CH3) was Si-CH3 strect-
ching vibration at 2971cmK1in Fig. 1b [19].
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1366
D. Wang et al.
Fig. 1. FT-IR spectra of samples (a) unsilylated sample (b) silylated sample.
The surface hydrophilicity of the samples with different silylation time was
determined by water contact angles measurement, the result was shown in Fig. 2,
the unsilylated sample are completely wetting and showed a zero water contact
angle (Fig. 2a). With the increasing of silylation time, the surface of the samples
became hydrophobic and gave an increasing contact angles from 00 to 117.530.
The result showed that the surface of catalyst changed from completely hydro-
philic into a certain degree of hydrophobicity, which is implied some amount of
silanol on the surface of the SiO2 were replaced by organic silane. It is speculated
that decrease of the amount of surface silanol would be helpful for the oxygen-
ated product in the desorption from the support as they formed.
As shown in Fig. 3, the peak of nonpolar cyclohexane is symmetric and the
absolute RT of cyclohexane is rather short (i.e. about 0.24min for all the four
typical samples), which means the adsorption of Cyclohexane on the samples is
very weak. These results confirmed that the cyclohexane is easily desorption
from the samples and silylation has little influence on the adsorption-desorption
behavior of cyclohexane. On the contrary, the cyclohexanone had a tailing peak
and long absolute RT varied from 1.734 to 28.67min, the adjusted RT of cyclo-
hexanone to cyclohexane was 1.513, 1.789, 10.552, 28.436min on Co.Si(0) to
Co.Si(9) four samples respectively. The different adsorption.desorption prop-
erty implies the longer the adjusted RT, the stronger the adsorption capacity. At
the same time, tailing of the peak was remarkably improved which can be seen
as a more homogenous surface is appeared besides the variation of adsorption.
desorption capability. Furthermore, as shown in Fig. 4b, the contact angle test
is in consistent with RT measurement on the characterization of the surface
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Hydrophobic Nature of the Support
1367
Fig. 2. Contact angles mearsurement of samples (a) Co.Si(0h),(b) Co.Si(2h),(c) Co.Si(5h),(d)
Co.Si(9h).
hydrophobicity. Both of the two improvements indicated that the desorption of
cyclohexanone became easily after silylation.
3.2 Catalytic activity of silyated catalyst for the oxidtion of cyclohexane
The oxidation of cyclohexane is known as a free radical chain mechanism [20–
22] in the reaction, cyclohexyl hrdroperoxide (CyOOH) is produced in the fast
propagation reactions (Eqs. (1) and (2)), the formation of both cyclohexnol
(CyOH) and cyclohexanone (Cy = O) are attributed to the mutual termination of
two peroyl radicals (Eq. (3)), adipic acid as a mainly byproduct, originates form
the overoxidation of CyOH and Cy = O (Eq. (4)).
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1368
D. Wang et al.
Fig. 3. Retention time of samples (a) Co.Si(0h), (b) Co.Si(2h), (c) Co.Si(5h),(d) Co.Si(9h).
Fig. 4. Relationship of contact angle vs. conversion and adjusted retention time (a) contact
angle.conversion , (b) adjusted RT.conversion.
(1)
(2)
(3)
(4)
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Hydrophobic Nature of the Support
1369
Table 1. Activities and selectivities in oxidations over various catalystsa.
Catalyst
Conversion
(mol%)
Yield (mol%)
Selectivity of
KA-oil(%)
KA-oil
Acid
1.0
Co.Si(0h)
Co.Si(2h)
Co.Si(5h)
Co.Si(9h)
3.8
6.1
8.9
9.1
2.8
4.5
6.9
7.4
73.7
73.8
77.5
81.3
1.6
2.0
1.7
aReaction condition: cyclohexane = 20.0g, catalyst = 0.5g, T = 308K, P = 1.0MPa, t = 5h
The oxidation of cyclohexane was carried out at 408K for 5h without any
initiator and solvent under the pressure of 1.0MPa oxygen, the reaction result
were listed in Table 1, for the unsilylated catalyst Co.Si(0h), the conversion was
lower to only 3.8%. It is believed that the formed oxygenated products, KA-oil,
cannot removed rapidly from the catalyst surface duo to its adsorptive property.
This can be unambiguously confirmed from the Fig. 3a, in which the long RT
indicated that cyclohexanone was very affinitive. The affinity is mostly caused
by the hydrogen bonding between cyclohexanone.cyclohexanol and silanol on
the catalyst surface. By silylation, the silanol was replaced by organosilane,
which reduced the amount of surface silanol, and was beneficial to the desorption
of the products by weakening the hydrogen bonding. Fig. 4a showed that the
conversion of cyclohexane increased with the increase of contact angle, connect-
ing with the degree of silylation. From the conversion curve, it is also concluded
that the conversion cannot be enhanced unlimited only by increasing the hydro-
phobicity of the supports. In the light of the composition of catalyst and method
for the modification of hydrophobicity, it is believed that the property of support
in the catalyst plays significant role in the reaction even though it is not decisive.
4. Conclusions
In the catalytic oxidation of cyclohexane, for a heterogeneous catalyst, the hydro-
phobic property of the support significantly influence the reaction activity and
selectivity, As a typical method for constructing hydrophobic surface, silylation
of silica containing catalyst greatly enhanced the conversion of cyclohexane from
3.8% to 9.1% by improving adsorption.desorption ability of the products. And
it suggested that for other similar reaction, such as the direct hydroxylation of
benzene, hydrophobicity will also play a role in improving the reaction activity.
Acknowledgement
This work is supported by the National Natural Science Foundation (29973041)
and the 973 Programme of the Ministry of Science and Technology
(2003CB615800) of China.
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1370
D. Wang et al.
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