45th International Petroleum Conference, Bratislava, June 13 - 14, 2011
SELECTIVE OXIDATION OF CYCLOHEXANONE BY MOLECULAR
OXYGEN OVER TUNGSTEN CARBIDE CATALYSTS
Martin Šustek, Dana Gašparovičová and Milan Hronec
Institute of Organic Chemistry, Catalysis and Petrochemistry, Department of Organic
Technology, Slovak University of Technology, Radlinského 9, 812 37 Bratislava, Slovak
Republic, e-mail: martin.sustek@stuba.sk
Key words: Oxidation, cyclohexanone, adipic acid, tungsten carbide catalysts
Abstract: Tungsten carbide catalysts have been tested in the liquid phase oxidation of
cyclohexanone by molecular oxygen as an alternative way of adipic acid preparation. The
experiments were carried out at 413 K and the pressure of oxygen 0.35 MPa. The results
demonstrated positive impact of Ni addition and higher temperature of carburization during
catalyst preparation on adipic acid formation.
I.
Introduction
Adipic acid, a valuable raw material used in the production of nylon-6,6, fibers,
plasticizers, and food additives is manufactured by the oxidation of cyclohexanone, obtained
mainly by the oxidation of cyclohexane [1]. The oxidation of cyclohexane is industrially
carried out in the temperature range of 423 - 453 K and pressure of 1.0 - 1.6 MPa in presence
of cobalt salts as catalyst [2]. Cyclohexanone in a mixture with cyclohexanol (KA - oil) is
oxidized with nitric acid (60 %) using homogeneous catalysts (NH4VO3, Cu(NO3)2) in the
temperature range of 323 to 353 K at atmospheric pressure [3].
The main disadvantages of the most widely used industrial method of preparation of
adipic acid include energy consumption, low conversion of cyclohexane, the use of large
quantities of nitric acid and production of relatively large amount of by-products and the
formation of NOx. The direct air-base oxidation of cyclohexane to adipic acid using
heterogeneous catalysts has attracted much attention in recent years. Adipic acid was obtained
under mild conditions (393 K and 0.91 MPa) by oxidation of cyclohexane using novel system
composed of isoamyl nitrite, Co(acac)3 and Mn(acac)3 with conversion of cyclohexane 9,7 %
and selectivity to adipic acid 35,4 % [4]. The direct oxidation of cyclohexane over
heterogeneous catalysts, CoAPO-5 molecular sieves, results in a low selectivity of adipic acid
[5]. Oxidation of KA - oil with air in acetic acid in the presence of soluble cobalt salts is an
alternative to industrial oxidation by corrosive nitric acid. Mixture of cyclohexanone and
cyclohexanol is oxidized in the temperature range from 80 to 85 ° C, and pressure 0.6 MPa.
Selectivity to adipic acid is similar to industrial process. This process has also been tested on a
commercial scale, but seems to have resulted in adipic acid of poor quality [6]. The use of
metal carbides as catalysts increased as a result of the observed similarity of their catalytic
activity with noble metals. Research in recent years showed that tungsten carbides are useful
catalysts for many reactions [7].
The aim of this work was the oxidation of cyclohexanone to adipic acid using
molecular oxygen under mild conditions without using a solvent in the presence of the
catalysts based on tungsten carbide.
II.
Experimental
Tungsten carbide catalysts were prepared by impregnation of activated carbon (Vulcan XC
72) with an aqueous solution of ammonium metatungstate (WO3 min. 85 %) and dried at 393
K. Nickel promoted tungsten carbide catalysts were prepared by adding Ni(NO3). 6 H2O to
the aqueous solution. The catalyst was then calcinated in H2 (973 K) or N2 (1273 K) flow.
Some catalysts were prepared by two - step impregnation process. Tungsten was added to
activated carbon in the first step and prepared precursor was carburized in nitrogen flow (1273
K). Nickel was added in the second step. Ni - W/C catalytic precursor was then carburized in
nitrogen or hydrogen flow. The nominal weight loading of tungsten was 30 % and Ni loading
was 2 %. The characterisation of prepared catalysts was performed by XRD and TPR
techniques. X-ray powder diffraction patterns were obtained using an apparatus with an X-ray
source CuKα; λ = 0.154 nm. The temperature programmed measurements were carried out
using a Pulse chemisorb 7000 apparatus. The experiments were performed in oxidation
apparatus with glass – lined stainless steel reactor equipped with the system for monitoring
the oxygen consumption. In experiments, 3 ml of cyclohexanone and 150 mg of catalyst were
added into the reactor. The reactor was pressurized with O2 to 0.35 MPa and heated to 413 K.
After finishing the reaction, the reactor was cooled down to room temperature and
depressurized. The reaction mixture was analysed by gas chromatography using Chrompack
CP 9001 equipped with a flame ionisation detector. Before analysing, the acids were
converted to methyl esters by treating the sample with methanol in the presence of
concentrated H2SO4. The esterification reaction was carried out at room temperature.
III.
Results and discussion
XRD analysis of prepared catalysts (Fig. 1) confirmed the presence of W2C phase in catalysts
prepared by carburization in hydrogen and nitrogen atmosphere at different temperatures.
Characteristic peak positions for W2C phase are 2θ = 34.7 °, 38.2 °, 39.8 °, 52.5 °, 62.0 °,
75.2°. It was also confirmed the presence of metallic tungsten, for which the characteristic
peak position of 2θ = 40 °. No crystalline WC phase was identified by XRD analysis.
Fig. 1 X-ray diffraction pattern of the Ni W/C (CHR, 700) and Ni W/C (N2 , 1000) catalysts
TPR profiles (Fig. 2) of 2 % Ni - 30 % W2C/C CHR 700 catalyst precursor showed the
formation of two peaks. Compared to the TPR profile of commercial nickel catalyst
containing 66% of Ni, a smaller peak at around 450 ° C can be attributed to the reduction of
nickel.
Fig. 2 TPR profiles of the 2 % Ni - 30 % W/C CHR 700 and 0 % Ni - 30 % W/C CHR 700
precursors
Table 1 presents the effect of preparation of tungsten carbide catalyst in H2 and N2
atmosphere and the doping with Ni on the conversion of cyclohexanone and the yields of
adipic, succinic and oxalic acids. The highest conversion of cyclohexanone was obtained in
the presence of Ni - W/C catalyst prepared in N2 flow. Higher yields of adipic acid produce
catalysts carburized in N2 flow than catalysts carburized in H2 flow. Positive impact of Ni
addition on adipic acid yield was observed in catalysts carburized in hydrogen flow. The
formation of oxalic acid was observed in the presence of Ni - promoted catalysts.
Table 1 Oxidation of cyclohexanone over tungsten carbide catalysts
Catalyst
Cyclohexanone
conversion (%)
Yield (%)
Adipic acid
Succinic acid
W/C H2 (973 K)
24.8
1.0
5.4
Ni-W/C H2 (973 K)
28.9
2.8
1.1
W/C N2 (1273 K)
28.5
3.4
4.1
Ni-W/C N2 (1273 K)
45.3
3.1
1.1
Reaction conditions: 413 K, 0.35 MPa, O2, 150 mg of catalyst, 160 min
Oxalic acid
2.9
4.8
Table 2 presents the influence of carburization temperature on the activity of catalysts and the
yields of dicarboxylic acids. The used Ni - promoted catalysts were prepared by two - step
impregnation process. Higher temperature of carburization and the additon of Ni to catalysts
activated in hydrogen flow had positive effect on adipic acid yield. In the presence of Ni promoted catalysts the formation of oxalic acid was also observed.
Table 2 Oxidation of cyclohexanone over tungsten carbide catalysts
Catalyst
Cyclohexanone
conversion (%)
Yield (%)
Adipic acid
Succinic acid
W/C H2 (873 K)
29.9
1.2
1.7
Ni-W/C H2 (873 K)
28.9
2.8
1.1
W/C H2 (1123 K)
27.2
1.9
2.6
Ni-W/C H2 (1123 K)
24.0
3.5
1.5
W/C N2 (1273 K)
28.5
3.4
4.1
Ni-W/C N2 (1273 K)
28.2
3.3
2.2
Reaction conditions: 413 K, 0.35 MPa, O2, 150 mg of catalyst, 160 min
Oxalic acid
2.9
7.8
1.2
Table 3 shows the influence of carburization temperature on the conversion of cyclohexanone
and yields of adipic, succinic and oxalic acids. The results indicates that at the same
conversion of cyclohexanone, higher carburization temperature of Ni - W / C catalysts has a
positive effect on the production of adipic acid.
Table 3 Oxidation of cyclohexanone over tungsten carbide catalysts
Catalyst
Cyclohexanone
conversion (%)
Yield (%)
Adipic acid
Succinic acid
Ni-W/C H2 (923 K)
37.0
2.4
1.4
Ni-W/C H2 (973 K)
28.9
2.8
1.1
Ni-W/C H2 (1023 K)
40.5
2.8
1.7
Ni-W/C H2 (1073 K)
39.0
4.2
1.9
Reaction conditions: 413 K, 0.35 MPa, O2, 150 mg of catalyst, 160 min
Oxalic acid
2.0
2.9
2.5
2.7
The influence of Ni concentration in tungsten carbide catalyst on the catalytic activity and
yields of dicarboxylic acids is presented in Table 4. Presented experiments indicate that
higher amount of Ni added to the carbide catalyst has negative effect on the yield of adipic
acid. However, this assumption does not apply in the case of 8% Ni-W/C N2 (823 K) catalyst.
Table 4 Oxidation of cyclohexanone over tungsten carbide catalysts
Catalyst
Cyclohexanone
conversion (%)
Yield (%)
Adipic acid
Succinic acid
0% Ni-W/C N2 (823 K)
28.5
3.4
4.1
2% Ni-W/C N2 (823 K)
28.2
3.3
2.2
4% Ni-W/C N2 (823 K)
26.7
2.3
5.0
8% Ni-W/C N2 (823 K)
39.6
4.5
4.4
Reaction conditions: 413 K, 0.35 MPa, O2, 150 mg of catalyst, 160 min
IV.
Oxalic acid
1.2
2.0
2.1
Conclusion
Presented results demonstrate positive effect of doping of tungsten carbide catalyst with Ni
and higher temperature of carburization during the catalyst preparation on adipic acid
formation. This was observed when the catalysts were carburized in hydrogen flow. The
formation of oxalic acid in the presence of Ni - promoted tungsten carbide catalysts was also
observed.
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