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Oxidation of cyclohexylamine over modified alumina by molecular oxygen
Applied Catalysis A: General 367 (2009) 32–38
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Oxidation of cyclohexylamine over modified alumina by molecular oxygen Karol Rakottyay *, Alexander Kaszonyi Department of Organic Technology, Institute of Organic Chemistry, Catalysis and Petrochemistry, SUT, Radlinske´ho 9, 812 37 Bratislava, Slovakia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 December 2008 Received in revised form 8 July 2009 Accepted 10 July 2009 Available online 3 August 2009
The oxidation of cyclohexylamine over high surface alumina was studied. To characterize the prepared product, BET surface, NH3-TPD and FT-IR spectra, were used. By using different amines during hydrolysis of aluminium alcoholate one can modify the specific surface area, pore volume, pore size distribution, powder density and the acidity of the formed alumina. The changed structure of alumina significantly influences its activity and selectivity in the oxidation of cyclohexylamine. The maximum of conversion can be increased from 15% up to 35% and the selectivity to cyclohexanone oxime from 40% up to 60% by using amine during alumina preparation. Impregnation of the prepared alumina by silicotungstic heteropolyacid enhances the selectivity of oxime formation up to 70%. The main product of the reaction in the inert atmosphere is N-cyclohexylidene–cyclohexylamine at conversions of cyclohexylamine below 1%. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Cyclohexanone oxime Cyclohexylamine Oxidation Amine modified g-alumina
1. Introduction Cyclohexanone oxime is an intermediate product in the commercial production of polyamide PA6. It is converted to ecaprolactam by Beckmann rearrangement in the presence of oleum, which is obviously neutralized by ammonia. The main disadvantage of this classical manufacturing is the formation of 4.4 kg inorganic by-product per kg of polyamide [1]. Therefore, there is an interest in new processes, which would be environmentally friendlier, capable of reducing or eliminating the inorganic by-products. Cyclohexanone oxime can be prepared also by partial oxidation of cyclohexylamine or ammoxidation of cyclohexanone. Cyclohexylamine is produced from benzene via nitrobenzene and aniline, cyclohexene or cyclohexanol [1]. There are only two oxidizing agents for partial oxidation of cycloalkylamine in the literature, molecular oxygen and hydrogen peroxide, which after the reduction produce ecological by-product, H2O. Other inorganic oxidizing agents and organic hydroperoxides after they reduction produce more problematic by-products. Hydrogen peroxide or alkyl hydroperoxide were used in the liquid phase oxidation of various primary amines and cycloalkylamines in the presence of vanadium or titanium silicalite (TS-1, TS-2 or TiS), chromium silicate, tungstic acid or heteropolyacids containing tungsten. The reaction leads to the corresponding oximes and imines [2–4]. A serious problem of liquid phase oxidation is the separation of
catalyst from the reaction mixture. If alkyl hydroperoxides are used, the separation of their reduction products is also necessary. In the vapor phase cyclohexylamine can be oxidized by molecular oxygen over heterogeneous catalyst, which is automatically separated from the reaction products in the used fixed bed reactor. Silica gel, alumina or a solid catalyst comprising alumina and molybdenum, vanadium or tungsten oxide were used as heterogeneous catalyst [5–7]. Around 70% selectivity of cyclohexanone oxime formation and 20% conversion of cyclohexylamine were obtained over tungsten oxide/alumina catalysts [5,7]. The activity of alumina can be enhanced also by supporting polyoxometalates on their surface. It is believed, that the appropriate heteropolyacid can increase the concentration of oxygen and reactant on the active site [8]. An interesting feature of this reaction is that it is catalyzed also by Al2O3 or SiO2 alone. These oxides do not belong to typical redox catalysts and the presence of components with redox properties decreases their selectivity for cyclohexanone oxime formation [6]. In this report, we studied the influence of high surface alumina on the oxidation of cyclohexylamine. Catalysts were prepared from aluminiumtriisopropylate at the presence of different amines. The used amine can modify the acidity and surface structure of prepared alumina. The reaction of cyclohexylamine and alumina in the inert atmosphere was studied too. 2. Experimental 2.1. Catalysts preparation
* Corresponding author. E-mail address: [email protected] (K. Rakottyay). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.07.034
g-Al2O3 was synthesized from aluminiumtriisopropylate by hydrolysis [9]. Aluminiumtriisopropylate (Merck 99%) was
K. Rakottyay, A. Kaszonyi / Applied Catalysis A: General 367 (2009) 32–38
dissolved in the mixture of methanol (MicroChem 99%) and toluene (MicroChem 99%) in a volume ratio (VMeOH/VT = 1/13) and then deionized water (Al:H2O = 1:7) was added. For the preparation of modified alumina the appropriate amount of amine [mono-, di-, triethanolamine or cyclohexylamine (Merck 99%)] was added (according to the molar ratio Al:amine = 1:1) before the addition of deionized water. The hydrous gelatinous solid was kept at 60 8C for 24 h. The precipitated AlOOH was dried at 80 8C in vacuum drier. For comparison also commercial g-alumina was tested (Alfa Aesar, SBET = 211 m2 g 1, Vp = 0.6 cm3 g 1). Some catalysts were prepared by slurry impregnation of alumina by solution of polyoxomatelates (Merck) (NanH4 n[Si(W3O10)4]xH2O) (n = 0–4) in deionized water. The used heteropolyacid belongs to Keggin type of polyoxometalates. It was anticipated that it forms Keggin type nanoparticles on the surface of catalyst. Before use all catalysts were dried at 110 8C for 15 h and calcined at 500 8C for 1 h with a heating rate 250 8C/h. 2.2. Catalysts characterization The acidity of catalysts was determined by TPD measurements using ammonia in the temperature range 100–600 8C in nitrogen atmosphere. Measurements were carried out in a conventional flow-type apparatus. The catalysts were dried before TPD experiment in nitrogen by the same temperature program as during TPD experiment. Then the catalyst was cooled down and saturated by ammonia at 100 8C. The excess of ammonia was flushed by nitrogen (50 cm3/min N2, 12 h at 100 8C). Finally the chemisorbed ammonia was desorbed by heating in nitrogen by the rate 10 8C/min up to 600 8C (flow rate N2 50 cm3/min). The concentration of desorbed ammonia in N2 was measured by TCD. The amount of desorbed ammonia was calculated from desorption peak area and the response area of 1 cm3 ammonia injected to the stream of N2. Specific surface areas were measured by Micromeritics ASAP 2020 equipment using nitrogen as adsorptive. The pore size, based on BJH method, was calculated assuming cylindrical pore model. The samples were conditioned before measurement at p = 2 Pa, t = 350 8C for 12 h. Shimadzu IRAffinity-1 FT-IR spectrometer was used to characterize the prepared catalysts in a wavenumber range (4000– 550 cm 1). 2.3. Catalytic test The oxidation of cyclohexylamine was carried out in a fixed bed glass reactor with 2 cm inner diameter. The amount of catalyst was 2 g. The space velocity of cyclohexylamine was 0.7 h 1. The oxidation was performed by molecular oxygen (32 vol.% in the mixture with nitrogen) or in inert atmosphere (nitrogen) at atmospheric pressure and at temperatures of 180–185 8C. The flow rate of gases was 24 cm3/min. The used molar ratio of cyclohexylamine:oxygen was 14:19. The samples of the reaction mixture were withdrawn from the reactor periodically in 1 h intervals and were analyzed on HewlettPackard HP 5890 GC equipped with FID and CP-SIL 5CB capillary column (25 m 0.53 mm 1.0 mm). The used temperature program was: 60 8C (3 min), gradient 9 8C/min up to 240 8C. For quantitative analysis the method of the external standard was used. The reaction products were identified on GC/MS QP5000 (Shimadzu) with EI and capillary column (HP-1, 50 m 0.2 mm 0.33 mm), with helium as carrier gas (1 ml/min). Temperature program: from 60 8C with gradient 9 8C/min up to 240 8C.
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3. Results and discussion 3.1. Properties of amine modified alumina The basic structure of alumina is predetermined during formation of Al–O–Al and –Al(OH)– bonds. If it is prepared from aluminium alcoholate, the basic structure of alumina is formed during hydrolysis of alcoholate and is markedly influenced by the conditions of hydrolysis. Amine can replace water or alcohol in the coordination sphere of Al and modify the structure of the formed Al-oxide. Xue et al. [9] found out that the density of surface Lewis acid sites is modified too. It should be mentioned that during activation at 500 8C and at the presence of air the used amine is totally evaporated from the structure of alumina (or oxidatively decomposed to CO2). The measured FT-IR spectra (Figs. 1 and 2) do not contain any IR band which are characteristic for used alcoholate, amine or solvents, i.e. all organic compounds were lost from the prepared alumina. According to this result, in the FTIR spectra a strong broadening band occurs at 3800–3000 cm 1 due to the hydrogen bond between various hydroxyl groups in the alumina (Fig. 1). The intensity of this broad band is increased by adsorbed amount of water and practically disappears if alumina is carefully dried. Similarly a broad band centered at 1640 cm 1 is seen in all samples corresponding to the bending mode of the adsorbed water molecule. The stronger broadening bands (Fig. 2) below 1000 cm 1 correspond to vibration in octahedric AlO6 (with
Fig. 1. FT-IR spectra of (a) commercial g-alumina (CA); (b) prepared without amine (C); (c) alumina modified by cyclohexylamine (C-CHA); (d) triethanolamine (CTEA); (e) ethanolamine (C-EA); (f) diethanolamine (C-DEA).
Fig. 2. FT-IR spectra of (a) commercial g-alumina (CA); (b) prepared without amine (C); (c) alumina modified by cyclohexylamine(C-CHA); (d) triethanolamine (CTEA); (e) ethanolamine (C-EA); (f) diethanolamine (C-DEA).
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K. Rakottyay, A. Kaszonyi / Applied Catalysis A: General 367 (2009) 32–38 Table 1 Physicochemical properties of commercial and prepared alumina; BET surface [SBET], pore volume [Vp], and acidic sites measured by NH3-TPD. Catalyst CA C C-EA C-DEA C-TEA C-CHA CA-h
SBET (m2 g 211 403 410 437 474 406 199
1
)
Vp (cm3 g 0.60 2.87 2.51 1.35 1.13 2.75 0.59
1
)
Acidic sites (mmol NH3 g
1
)
1.02 1.32 1.2 1.23 1.22 1.16 0.96
CA g-aluminaAlfa Aeser, other alumina were prepared using ethanolamine C-EA; diethanolamine C-DEA; triethanolamine C-TEA and cyclohexylamine C-CHA as modifier, or without amine, C, respectively; CA-h was impregnated by heteropolyacid (15 wt.%). Fig. 3. FT-IR spectra of (a) silicotungstic heteropoly acid; (b) g-alumina after impregnation by 15 wt.% of heteropoly acid; (c) g-alumina before impregnation.
maximum below 700 cm 1) and tetrahedric g-AlO4 (maximum between 825 cm 1 and 845 cm 1) [10]. The maximum of g-AlO4 in alumina prepared without amine and of commercial alumina is at 835 cm 1. DEA shifts this maximum to 846 cm 1, other used amines to 824–827 cm 1. Fig. 3 shows the FT-IR spectra of silicotungstic heteropolyacid (HPA), commercial g-alumina impregnated with 15 wt.% of HPA and commercial g-alumina. The tungsten oxygen bands (W–Oe–W, W–Oc–W, W5 5Ot) and Si–O band for pure heteropolyacid appear at 730, 906, 976 and 1016 cm 1, respectively, which are fingerprints of silicotungstic heteropolyacid [11] in the region between 500 cm 1 and 1500 cm 1. These bands are not seen after supporting 15 wt.% of HPA on alumina. The FT-IR spectra of alumina before and after impregnation with HPA are practically the same, however, the
intensity of alumina bands are lowered by 12%. These results are in agreement with data published by Selvakumar and Singh [11]. Table 1 and Fig. 4 show the dependency of specific surface area, pore volume size, pore size distribution and surface acidity of activated catalyst on the type of used amine. In comparison with commercial g-alumina (assigned for catalysis by Alfa Aeser) the BET surfaces of alumina prepared from aluminiumtriisopropylate in this work are more than 2 times broader and the total pore volumes are 2–4.8 times higher. By increasing molecular volume of used ethanolamine the BET surface and the acidity of prepared alumina is slightly increased, but the total pore volume size is decreased. The pore size distribution profiles of some catalysts are shown in Fig. 4. The commercial alumina has narrow pore distribution centered around 2.6 nm. For alumina prepared from aluminiumtriisopropylate without amine modifier the maximum of pore size distribution was centered around 12 nm with wider peak. During the alumina formation at the presence of diethanolamine and triethanolamine not only narrow pores, but also pores
Fig. 4. Pore size distribution from BJH method: dV/d log(D) pore volume of; CA – commercial g-alumina; C – alumina prepared without amine; C-EA – modified by ethanolamine; C-DEA – diethanolamine; C-TEA-triethanolamine; C-CHA – cyclohexylamine.
K. Rakottyay, A. Kaszonyi / Applied Catalysis A: General 367 (2009) 32–38
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of aluminiumtriisopropylate reduces the intensity of the first peak, elevates the amount of adsorbed ammonia in the second zone, and has practically no influence in the last zone. An addition of triethanolamine practically has no influence in the first and second region, but moderately elevates the amount of desorbed ammonia over 500 8C. The presence of ammonia in outlet gas was confirmed during TPD by acid/base indicators up to 600 8C. The impregnation of alumina by silicotungstic acid increases mainly the first peak of NH3-TPD curves (Fig. 6). The ammonia desorbed in the second zone is overlapped by broader and higher peak of the first zone. The moderate increase of last peak is probably connected with acidic protons of silicotungstic acid. An impregnation of alumina by silicotungstic acid increases the calculated total acidity of catalyst by 10–15%, which is linear function of the area of NH3-TPD curve. Fig. 5. NH3-TPD profiles of alumina prepared (a) without any amine (C); (b) with triethanolamine(C-TEA); (c) diethanolamine (C-DEA); (d) monoethanolamine (CEA) as modifier.
with larger diameters are formed. The modification by these amines brought bimodal distribution with high population of pores having diameters centered around 2.6 and 35 nm. These pores help increase the specific surface of alumina. The alumina prepared at the presence of ethanolamine and cyclohexylamine have practically the same pore size distribution as alumina prepared without amine. In Fig. 5 it is obvious that the used amine influenced not only the total acidity of activated catalyst calculated from the chemisorbed amount of ammonia, but also the relative amount of acidic sites with different acid strength. In the first cycle of TPD-NH3 experiment alumina is activated at 600 8C during 1 h, cooled down and saturated by ammonia at 100 8C. In the second cycle ammonia is desorbed from alumina in one broad peak, which can be divided into three zones. The first contains the maximum of peak around 200 8C and belongs to the least acidic sites on the boundary of chemically and physically adsorbed ammonia. The second between 300 8C and 500 8C belongs to the sites with medium acidity and the last around 600 8C to the most acidic sites and probably to the further slow dehydroxylation of g-alumina, which was not finished in the first cycle. The addition of cyclohexylamine, mono- and diethanolamine during hydrolysis
3.2. Influence of alumina modification to the oxidation process It is important to emphasize that without catalyst the oxidation of cyclohexylamine is negligible at used reaction condition, i.e. there is no oxidation in the gas phase, neither on the reactor wall nor on the surface of glass balls used for preheating of reaction mixture. The determined total acidity of prepared catalysts was below 1.4 mmol/g. Over commercial alumina impregnated by 2 mmol/g of NaOH no cyclohexanone oxime formation was detected. Only traces of N-cyclohexylidene–cyclohexylamine were found in the reaction products. Thus the acidity of alumina is essential for the oxidation of cyclohexylamine. According to the obtained data, high basicity of the catalyst has negative impact to the oxidation process. Because the impregnation of alumina by NaOH did not eliminate the presence of eventual transition metal impurities in alumina, from the mentioned results also follows that the catalysis of cyclohexylamine oxidation to cyclohexanone oxime over alumina is not a result of a trace element content with redox properties but it is the result of catalytic activity of acidic sites of alumina. The effect of alumina modification on the conversion of cyclohexylamine and selectivity to cyclohexanone oxime is shown in Fig. 7 and Fig. 8, respectively. The main reaction products over tested catalyst are: cyclohexanone oxime, cyclohexanone and Ncyclohexylidene–cyclohexylamine, which is the Schiff base of cyclohexanone and cyclohexylamine. Traces of cyclohexanol,
Fig. 6. Comparison of NH3-TPD profiles of alumina before and after impregnation by 15 wt.% of HPA (for legend to catalysts see Fig. 4; the catalyst with HPA are marked by ‘‘h’’).
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Fig. 7. The influence of modifier amine on the conversion of cyclohexylamine over alumina (CA – commercial alumina, C – prepared without modifier, CEA – modified by monoethanolamine, CDEA – modified by diethanolamine, CTEA – modified by triethanolamine, CCHA – modified by cyclohexylamine).
Fig. 8. The influence of modifier amine on the selectivity of cyclohexanone oxime formation (for legend to catalysts see Fig. 7).
cyclohexene, nitrocyclohexane, cyclohexylaniline, dicyclohexylamine, methylidenecyclohexylamine, cyclohexylformamide and cyclohexylacetamide were detected in the reaction products, too. The conversion of cyclohexylamine and the selectivity of cyclohexanone oxime formation, calculated from the composition of reaction products increases during the first 5 h on stream. Then the conversions begin to decrease as a result of deposition of high molecular weight by-products mainly on the acidic sites of the catalyst. From the opposite direction of the change in the amount of deposits and cyclohexylamine conversion follows, that in this reaction the formed tar does not play the essential role of initiator, like in the gas phase ammoxidation of cyclohexanone over amorphous silica, where it increases the formation of cyclohexanone oxime [12]. The acidic surface of the catalyst is deactivated also by less volatile higher molecular weight amines and their condensation products (minor by-products of the reaction). Part of the deposits can be dissolved in the organic solvents. The whole tar can be oxidized to CO2 by molecular oxygen at 500 8C and regenerate the initial activity of catalysts. Fig. 9 shows the FT-IR spectra of cyclohexylamine, g-alumina before reaction, after deactivation by cyclohexylamine oxidation and after its reactivation at 500 8C by air. The FT-IR spectra of tars on the alumina contain some typical bands of cyclohexylamine mainly in the range of C–H bands. All bands of tar disappear after its total oxidation by molecular oxygen and the catalyst is reactivated. By modification of alumina with amine its catalytic activity is significantly changed (Figs. 7 and 8). The maximum of cyclohexylamine conversion rose from nearly 10% to 35%. The lowest
Fig. 9. FT-IR spectra of (a) pure cyclohexylamine; (b) g-alumina after catalytic test; (c) g-alumina after reactivation at 500 8C; (d) g-alumina before catalytic test.
conversion was observed over unmodified alumina. The commercial alumina with narrow pores has nearly the same catalytic activity despite of its significantly lower BET surface. Because the observed conversions over both catalysts were nearly the same, the BET surface is not the most important factor which determines the oxidation activity of alumina. The highest conversions were achieved over alumina modified by cyclohexylamine and ethanolamine. These two amines practically neither not change the BET surface, nor the pore volume, nor the distribution of pore sizes. Both alumina have relatively high pore volume with wide pores centered around 12–14 nm. Alumina prepared by di- and triethanolamine as modifier have lower activity, significantly lower pore volume size and bimodal pore size distribution with maxima centered around 2.6 and 35 nm. The best results obtained over alumina prepared by cyclohexylamine can indicate that during the formation of alumina specific sites were created in its structure (like key and lock) were cyclohexylamine is more easily adsorbed for activation. The aminic modifier of alumina enhances not only the conversion of cyclohexylamine but also the selectivity of cyclohexanone oxime formation. The highest selectivity above 60% was achieved by di- and triethanolamine as modifier. However, this increase is lower than increase in the conversions for catalyst prepared by cyclohexylamine and so the highest yield of cyclohexanone oxime was found over catalyst modified by cyclohexylamine and ethanolamine. In our previous work [13] the positive influence of silicotungstic acid, deposited on the surface of alumina was confirmed by improving the conversion of cyclohexylamine and selectivity of
Fig. 10. The influence of modifier amine on the conversion of cyclohexylamine over alumina impregnated with heteropolyacid (H4[Si(W3O10)4]xH2O) (for legend to catalysts see Fig. 7).
K. Rakottyay, A. Kaszonyi / Applied Catalysis A: General 367 (2009) 32–38
Fig. 11. The influence of modifier amine on the selectivity of cyclohexanone oxime formation over alumina impregnated with heteropolyacid (H4[Si(W3O10)4]xH2O) (for legend to catalysts see Fig. 7).
oxime formation. The impregnation of commercial alumina by HPA (H4[Si(W3O10)4]xH2O) prominently increases its activity mainly in the first hours of time on stream without significant influence on the selectivity for cyclohexanone oxime (compare Fig. 7, Fig. 8 and Fig. 10, Fig. 11, respectively). In the case of alumina prepared from alcoholate with or without aminic modifier the situation is more complex. After impregnation with mentioned HPA the conversions of cyclohexylamine are slightly lower (with maximum around 30%) and selectivity for cyclohexanone oxime is higher (with maximum over 70%). The observed lower conversions are probably the result of decreased contact time of cyclohexylamine and reaction products with catalyst, which have significantly higher powder density. The lower powder density results for the same weight of catalyst in the higher catalyst bed. The powder density of alumina prepared by hydrolysis of aluminiumtriisopropylate and commercial alumina are 0.07 and 0.56 g cm 3, respectively. At the same flow rate of cyclohexylamine and reaction products their contact time is prolonged, which allows higher conversions. However, after impregnation all catalysts had powder density around 0.55 g cm 3 and as a result nearly the same contact time during cyclohexylamine oxidation. The used HPA adds highly acidic protons and some form of tungsten oxide to the catalysts. Over strong acidic sites cyclohexylamine and the formed oxime are strongly adsorbed. The protonized amine is stable against oxidation. As a result, the acidic protons of HPA are neutralized and an equivalent amount of amine is captured. The highly acidic protons of HPA can catalyze deamination of cyclohexylamine to cyclohexene, which reacts with cyclohexylamine to dicyclohexylamine, one of the
Fig. 12. The influence of catalyst on the conversion of cyclohexylamine in inert gas (N2) and air (O2) (C – alumina prepared without modifier, CA – commercial alumina).
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Fig. 13. The influence of catalyst on the selectivity of N-cyclohexylidene– cyclohexylamine formation in inert gas (N2) and air (O2) (C – alumina prepared without modifier, CA – commercial alumina).
by-products of the reaction. However this reaction has very low rate in comparison with the rate of amine oxidation. 3.3. Reaction of cyclohexylamine in the inert gas over modified alumina Alumina is an atypical catalyst for oxidation reactions. Chemisorption and possible activation of molecular oxygen on the surface of alumina is mentioned in the literature. By XPS spectra Al0 and Al3+ were found in alumina [14]. However, we were not able to find evidence of participation of Al from alumina in redox cycles with oxygen and organic compounds in the available literature. We checked the ability of alumina to oxidize cyclohexylamine in some experiments, where before the start of reaction the physically adsorbed oxygen was desorbed from the catalyst surface by flushing of catalyst by inert gas during several hours at the reaction conditions. After removal of oxygen the feeding of cyclohexylamine was started in nitrogen atmosphere and the samples of products collected directly below catalyst bed were analyzed. During this process like at the presence of oxygen, the catalyst prepared from aluminiumtriisopropylate showed weaker activity than the commercial one (Fig. 12). The conversions of cyclohexylamine over both catalysts were less than 1% in inert atmosphere. The main product of cyclohexylamine oxidation under these conditions was N-cyclohexylidene–cyclohexylamine (Fig. 13), which is in equilibrium with cyclohexanone and cyclohexylamine as their Schiff base. Traces of cyclohexanone oxime, cyclohexene, benzene, dicyclohexylamine, cyclohexylaniline and cyclohexanol were also observed in the samples. The selectivity of Schiff base production over commercial alumina reached 85% (Fig. 13). When the catalyst was flushed by nitrogen only several minutes before feeding of amine, significantly higher amount of cyclohexanone oxime was observed during the first 3 h of reaction. Thus the chemisorbed oxygen plays a very important role in the oxidation of cyclohexylamine to cyclohexanone oxime. The formation of cyclohexanone (N-cyclohexylidene–cyclohexylamine) lasted more than 20 h at conversions of amine around 0.7%. From this result follows, that cyclohexanone and cyclohexanone oxime are formed by different reaction mechanisms. In agreement with it the selectivity of N-cyclohexylidene–cyclohexylamine formation rises in inert atmosphere, however, at the presence of oxygen decreases by time on stream (Fig. 13). The production of other reaction by-products can be rationalized by the set of hydrogenation–dehydrogenation reactions (Scheme 1):
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K. Rakottyay, A. Kaszonyi / Applied Catalysis A: General 367 (2009) 32–38
Scheme 1. Scheme of reactions.
4. Conclusions The preparation technique of alumina plays an important role for its activity in the oxidation of cyclohexylamine. By using different amine during formation of Al–O–Al and –Al(OH)– bonds one can modify the specific surface area, pore volume, pore size distribution, powder density and the acidity of formed alumina. The changed structure of alumina significantly influences its activity and selectivity in the oxidation of cyclohexylamine. The maximum of conversion can be increased from 15% up to 35% and the selectivity to cyclohexanone oxime from 40% up to 60%. The impregnation of prepared alumina by silicotungstic heteropolyacid enhances the selectivity of oxime formation up to 70% though the conversion of cyclohexylamine besides commercial alumina is decreased. During this impregnation the powder density of alumina prepared from aluminiumtriisopropylate increases from 0.07 g cm 3 to 0.55 g cm 3, which markedly decreases the height of catalysts bed and contact time. The main product of the reaction in the inert atmosphere is N-cyclohexylidene–cyclohexylamine at conversions of cyclohexylamine below 1%. The other reaction byproducts are formed by hydrogenation–dehydrogenation and deamination processes. The catalysts can be reactivated by total oxidation of formed tars. Acknowledgement Financial support from the Slovak Grant Agency VEGA 1/0768/ 08 is gratefully acknowledged.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apcata.2009.07.034.
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Oxidation of cyclohexylamine over modified alumina by molecular oxygen Karol Rakottyay *, Alexander Kaszonyi Department of Organic Technology, Institute of Organic Chemistry, Catalysis and Petrochemistry, SUT, Radlinske´ho 9, 812 37 Bratislava, Slovakia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 December 2008 Received in revised form 8 July 2009 Accepted 10 July 2009 Available online 3 August 2009
The oxidation of cyclohexylamine over high surface alumina was studied. To characterize the prepared product, BET surface, NH3-TPD and FT-IR spectra, were used. By using different amines during hydrolysis of aluminium alcoholate one can modify the specific surface area, pore volume, pore size distribution, powder density and the acidity of the formed alumina. The changed structure of alumina significantly influences its activity and selectivity in the oxidation of cyclohexylamine. The maximum of conversion can be increased from 15% up to 35% and the selectivity to cyclohexanone oxime from 40% up to 60% by using amine during alumina preparation. Impregnation of the prepared alumina by silicotungstic heteropolyacid enhances the selectivity of oxime formation up to 70%. The main product of the reaction in the inert atmosphere is N-cyclohexylidene–cyclohexylamine at conversions of cyclohexylamine below 1%. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Cyclohexanone oxime Cyclohexylamine Oxidation Amine modified g-alumina
1. Introduction Cyclohexanone oxime is an intermediate product in the commercial production of polyamide PA6. It is converted to ecaprolactam by Beckmann rearrangement in the presence of oleum, which is obviously neutralized by ammonia. The main disadvantage of this classical manufacturing is the formation of 4.4 kg inorganic by-product per kg of polyamide [1]. Therefore, there is an interest in new processes, which would be environmentally friendlier, capable of reducing or eliminating the inorganic by-products. Cyclohexanone oxime can be prepared also by partial oxidation of cyclohexylamine or ammoxidation of cyclohexanone. Cyclohexylamine is produced from benzene via nitrobenzene and aniline, cyclohexene or cyclohexanol [1]. There are only two oxidizing agents for partial oxidation of cycloalkylamine in the literature, molecular oxygen and hydrogen peroxide, which after the reduction produce ecological by-product, H2O. Other inorganic oxidizing agents and organic hydroperoxides after they reduction produce more problematic by-products. Hydrogen peroxide or alkyl hydroperoxide were used in the liquid phase oxidation of various primary amines and cycloalkylamines in the presence of vanadium or titanium silicalite (TS-1, TS-2 or TiS), chromium silicate, tungstic acid or heteropolyacids containing tungsten. The reaction leads to the corresponding oximes and imines [2–4]. A serious problem of liquid phase oxidation is the separation of
catalyst from the reaction mixture. If alkyl hydroperoxides are used, the separation of their reduction products is also necessary. In the vapor phase cyclohexylamine can be oxidized by molecular oxygen over heterogeneous catalyst, which is automatically separated from the reaction products in the used fixed bed reactor. Silica gel, alumina or a solid catalyst comprising alumina and molybdenum, vanadium or tungsten oxide were used as heterogeneous catalyst [5–7]. Around 70% selectivity of cyclohexanone oxime formation and 20% conversion of cyclohexylamine were obtained over tungsten oxide/alumina catalysts [5,7]. The activity of alumina can be enhanced also by supporting polyoxometalates on their surface. It is believed, that the appropriate heteropolyacid can increase the concentration of oxygen and reactant on the active site [8]. An interesting feature of this reaction is that it is catalyzed also by Al2O3 or SiO2 alone. These oxides do not belong to typical redox catalysts and the presence of components with redox properties decreases their selectivity for cyclohexanone oxime formation [6]. In this report, we studied the influence of high surface alumina on the oxidation of cyclohexylamine. Catalysts were prepared from aluminiumtriisopropylate at the presence of different amines. The used amine can modify the acidity and surface structure of prepared alumina. The reaction of cyclohexylamine and alumina in the inert atmosphere was studied too. 2. Experimental 2.1. Catalysts preparation
* Corresponding author. E-mail address: [email protected] (K. Rakottyay). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.07.034
g-Al2O3 was synthesized from aluminiumtriisopropylate by hydrolysis [9]. Aluminiumtriisopropylate (Merck 99%) was
K. Rakottyay, A. Kaszonyi / Applied Catalysis A: General 367 (2009) 32–38
dissolved in the mixture of methanol (MicroChem 99%) and toluene (MicroChem 99%) in a volume ratio (VMeOH/VT = 1/13) and then deionized water (Al:H2O = 1:7) was added. For the preparation of modified alumina the appropriate amount of amine [mono-, di-, triethanolamine or cyclohexylamine (Merck 99%)] was added (according to the molar ratio Al:amine = 1:1) before the addition of deionized water. The hydrous gelatinous solid was kept at 60 8C for 24 h. The precipitated AlOOH was dried at 80 8C in vacuum drier. For comparison also commercial g-alumina was tested (Alfa Aesar, SBET = 211 m2 g 1, Vp = 0.6 cm3 g 1). Some catalysts were prepared by slurry impregnation of alumina by solution of polyoxomatelates (Merck) (NanH4 n[Si(W3O10)4]xH2O) (n = 0–4) in deionized water. The used heteropolyacid belongs to Keggin type of polyoxometalates. It was anticipated that it forms Keggin type nanoparticles on the surface of catalyst. Before use all catalysts were dried at 110 8C for 15 h and calcined at 500 8C for 1 h with a heating rate 250 8C/h. 2.2. Catalysts characterization The acidity of catalysts was determined by TPD measurements using ammonia in the temperature range 100–600 8C in nitrogen atmosphere. Measurements were carried out in a conventional flow-type apparatus. The catalysts were dried before TPD experiment in nitrogen by the same temperature program as during TPD experiment. Then the catalyst was cooled down and saturated by ammonia at 100 8C. The excess of ammonia was flushed by nitrogen (50 cm3/min N2, 12 h at 100 8C). Finally the chemisorbed ammonia was desorbed by heating in nitrogen by the rate 10 8C/min up to 600 8C (flow rate N2 50 cm3/min). The concentration of desorbed ammonia in N2 was measured by TCD. The amount of desorbed ammonia was calculated from desorption peak area and the response area of 1 cm3 ammonia injected to the stream of N2. Specific surface areas were measured by Micromeritics ASAP 2020 equipment using nitrogen as adsorptive. The pore size, based on BJH method, was calculated assuming cylindrical pore model. The samples were conditioned before measurement at p = 2 Pa, t = 350 8C for 12 h. Shimadzu IRAffinity-1 FT-IR spectrometer was used to characterize the prepared catalysts in a wavenumber range (4000– 550 cm 1). 2.3. Catalytic test The oxidation of cyclohexylamine was carried out in a fixed bed glass reactor with 2 cm inner diameter. The amount of catalyst was 2 g. The space velocity of cyclohexylamine was 0.7 h 1. The oxidation was performed by molecular oxygen (32 vol.% in the mixture with nitrogen) or in inert atmosphere (nitrogen) at atmospheric pressure and at temperatures of 180–185 8C. The flow rate of gases was 24 cm3/min. The used molar ratio of cyclohexylamine:oxygen was 14:19. The samples of the reaction mixture were withdrawn from the reactor periodically in 1 h intervals and were analyzed on HewlettPackard HP 5890 GC equipped with FID and CP-SIL 5CB capillary column (25 m 0.53 mm 1.0 mm). The used temperature program was: 60 8C (3 min), gradient 9 8C/min up to 240 8C. For quantitative analysis the method of the external standard was used. The reaction products were identified on GC/MS QP5000 (Shimadzu) with EI and capillary column (HP-1, 50 m 0.2 mm 0.33 mm), with helium as carrier gas (1 ml/min). Temperature program: from 60 8C with gradient 9 8C/min up to 240 8C.
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3. Results and discussion 3.1. Properties of amine modified alumina The basic structure of alumina is predetermined during formation of Al–O–Al and –Al(OH)– bonds. If it is prepared from aluminium alcoholate, the basic structure of alumina is formed during hydrolysis of alcoholate and is markedly influenced by the conditions of hydrolysis. Amine can replace water or alcohol in the coordination sphere of Al and modify the structure of the formed Al-oxide. Xue et al. [9] found out that the density of surface Lewis acid sites is modified too. It should be mentioned that during activation at 500 8C and at the presence of air the used amine is totally evaporated from the structure of alumina (or oxidatively decomposed to CO2). The measured FT-IR spectra (Figs. 1 and 2) do not contain any IR band which are characteristic for used alcoholate, amine or solvents, i.e. all organic compounds were lost from the prepared alumina. According to this result, in the FTIR spectra a strong broadening band occurs at 3800–3000 cm 1 due to the hydrogen bond between various hydroxyl groups in the alumina (Fig. 1). The intensity of this broad band is increased by adsorbed amount of water and practically disappears if alumina is carefully dried. Similarly a broad band centered at 1640 cm 1 is seen in all samples corresponding to the bending mode of the adsorbed water molecule. The stronger broadening bands (Fig. 2) below 1000 cm 1 correspond to vibration in octahedric AlO6 (with
Fig. 1. FT-IR spectra of (a) commercial g-alumina (CA); (b) prepared without amine (C); (c) alumina modified by cyclohexylamine (C-CHA); (d) triethanolamine (CTEA); (e) ethanolamine (C-EA); (f) diethanolamine (C-DEA).
Fig. 2. FT-IR spectra of (a) commercial g-alumina (CA); (b) prepared without amine (C); (c) alumina modified by cyclohexylamine(C-CHA); (d) triethanolamine (CTEA); (e) ethanolamine (C-EA); (f) diethanolamine (C-DEA).
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K. Rakottyay, A. Kaszonyi / Applied Catalysis A: General 367 (2009) 32–38 Table 1 Physicochemical properties of commercial and prepared alumina; BET surface [SBET], pore volume [Vp], and acidic sites measured by NH3-TPD. Catalyst CA C C-EA C-DEA C-TEA C-CHA CA-h
SBET (m2 g 211 403 410 437 474 406 199
1
)
Vp (cm3 g 0.60 2.87 2.51 1.35 1.13 2.75 0.59
1
)
Acidic sites (mmol NH3 g
1
)
1.02 1.32 1.2 1.23 1.22 1.16 0.96
CA g-aluminaAlfa Aeser, other alumina were prepared using ethanolamine C-EA; diethanolamine C-DEA; triethanolamine C-TEA and cyclohexylamine C-CHA as modifier, or without amine, C, respectively; CA-h was impregnated by heteropolyacid (15 wt.%). Fig. 3. FT-IR spectra of (a) silicotungstic heteropoly acid; (b) g-alumina after impregnation by 15 wt.% of heteropoly acid; (c) g-alumina before impregnation.
maximum below 700 cm 1) and tetrahedric g-AlO4 (maximum between 825 cm 1 and 845 cm 1) [10]. The maximum of g-AlO4 in alumina prepared without amine and of commercial alumina is at 835 cm 1. DEA shifts this maximum to 846 cm 1, other used amines to 824–827 cm 1. Fig. 3 shows the FT-IR spectra of silicotungstic heteropolyacid (HPA), commercial g-alumina impregnated with 15 wt.% of HPA and commercial g-alumina. The tungsten oxygen bands (W–Oe–W, W–Oc–W, W5 5Ot) and Si–O band for pure heteropolyacid appear at 730, 906, 976 and 1016 cm 1, respectively, which are fingerprints of silicotungstic heteropolyacid [11] in the region between 500 cm 1 and 1500 cm 1. These bands are not seen after supporting 15 wt.% of HPA on alumina. The FT-IR spectra of alumina before and after impregnation with HPA are practically the same, however, the
intensity of alumina bands are lowered by 12%. These results are in agreement with data published by Selvakumar and Singh [11]. Table 1 and Fig. 4 show the dependency of specific surface area, pore volume size, pore size distribution and surface acidity of activated catalyst on the type of used amine. In comparison with commercial g-alumina (assigned for catalysis by Alfa Aeser) the BET surfaces of alumina prepared from aluminiumtriisopropylate in this work are more than 2 times broader and the total pore volumes are 2–4.8 times higher. By increasing molecular volume of used ethanolamine the BET surface and the acidity of prepared alumina is slightly increased, but the total pore volume size is decreased. The pore size distribution profiles of some catalysts are shown in Fig. 4. The commercial alumina has narrow pore distribution centered around 2.6 nm. For alumina prepared from aluminiumtriisopropylate without amine modifier the maximum of pore size distribution was centered around 12 nm with wider peak. During the alumina formation at the presence of diethanolamine and triethanolamine not only narrow pores, but also pores
Fig. 4. Pore size distribution from BJH method: dV/d log(D) pore volume of; CA – commercial g-alumina; C – alumina prepared without amine; C-EA – modified by ethanolamine; C-DEA – diethanolamine; C-TEA-triethanolamine; C-CHA – cyclohexylamine.
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of aluminiumtriisopropylate reduces the intensity of the first peak, elevates the amount of adsorbed ammonia in the second zone, and has practically no influence in the last zone. An addition of triethanolamine practically has no influence in the first and second region, but moderately elevates the amount of desorbed ammonia over 500 8C. The presence of ammonia in outlet gas was confirmed during TPD by acid/base indicators up to 600 8C. The impregnation of alumina by silicotungstic acid increases mainly the first peak of NH3-TPD curves (Fig. 6). The ammonia desorbed in the second zone is overlapped by broader and higher peak of the first zone. The moderate increase of last peak is probably connected with acidic protons of silicotungstic acid. An impregnation of alumina by silicotungstic acid increases the calculated total acidity of catalyst by 10–15%, which is linear function of the area of NH3-TPD curve. Fig. 5. NH3-TPD profiles of alumina prepared (a) without any amine (C); (b) with triethanolamine(C-TEA); (c) diethanolamine (C-DEA); (d) monoethanolamine (CEA) as modifier.
with larger diameters are formed. The modification by these amines brought bimodal distribution with high population of pores having diameters centered around 2.6 and 35 nm. These pores help increase the specific surface of alumina. The alumina prepared at the presence of ethanolamine and cyclohexylamine have practically the same pore size distribution as alumina prepared without amine. In Fig. 5 it is obvious that the used amine influenced not only the total acidity of activated catalyst calculated from the chemisorbed amount of ammonia, but also the relative amount of acidic sites with different acid strength. In the first cycle of TPD-NH3 experiment alumina is activated at 600 8C during 1 h, cooled down and saturated by ammonia at 100 8C. In the second cycle ammonia is desorbed from alumina in one broad peak, which can be divided into three zones. The first contains the maximum of peak around 200 8C and belongs to the least acidic sites on the boundary of chemically and physically adsorbed ammonia. The second between 300 8C and 500 8C belongs to the sites with medium acidity and the last around 600 8C to the most acidic sites and probably to the further slow dehydroxylation of g-alumina, which was not finished in the first cycle. The addition of cyclohexylamine, mono- and diethanolamine during hydrolysis
3.2. Influence of alumina modification to the oxidation process It is important to emphasize that without catalyst the oxidation of cyclohexylamine is negligible at used reaction condition, i.e. there is no oxidation in the gas phase, neither on the reactor wall nor on the surface of glass balls used for preheating of reaction mixture. The determined total acidity of prepared catalysts was below 1.4 mmol/g. Over commercial alumina impregnated by 2 mmol/g of NaOH no cyclohexanone oxime formation was detected. Only traces of N-cyclohexylidene–cyclohexylamine were found in the reaction products. Thus the acidity of alumina is essential for the oxidation of cyclohexylamine. According to the obtained data, high basicity of the catalyst has negative impact to the oxidation process. Because the impregnation of alumina by NaOH did not eliminate the presence of eventual transition metal impurities in alumina, from the mentioned results also follows that the catalysis of cyclohexylamine oxidation to cyclohexanone oxime over alumina is not a result of a trace element content with redox properties but it is the result of catalytic activity of acidic sites of alumina. The effect of alumina modification on the conversion of cyclohexylamine and selectivity to cyclohexanone oxime is shown in Fig. 7 and Fig. 8, respectively. The main reaction products over tested catalyst are: cyclohexanone oxime, cyclohexanone and Ncyclohexylidene–cyclohexylamine, which is the Schiff base of cyclohexanone and cyclohexylamine. Traces of cyclohexanol,
Fig. 6. Comparison of NH3-TPD profiles of alumina before and after impregnation by 15 wt.% of HPA (for legend to catalysts see Fig. 4; the catalyst with HPA are marked by ‘‘h’’).
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Fig. 7. The influence of modifier amine on the conversion of cyclohexylamine over alumina (CA – commercial alumina, C – prepared without modifier, CEA – modified by monoethanolamine, CDEA – modified by diethanolamine, CTEA – modified by triethanolamine, CCHA – modified by cyclohexylamine).
Fig. 8. The influence of modifier amine on the selectivity of cyclohexanone oxime formation (for legend to catalysts see Fig. 7).
cyclohexene, nitrocyclohexane, cyclohexylaniline, dicyclohexylamine, methylidenecyclohexylamine, cyclohexylformamide and cyclohexylacetamide were detected in the reaction products, too. The conversion of cyclohexylamine and the selectivity of cyclohexanone oxime formation, calculated from the composition of reaction products increases during the first 5 h on stream. Then the conversions begin to decrease as a result of deposition of high molecular weight by-products mainly on the acidic sites of the catalyst. From the opposite direction of the change in the amount of deposits and cyclohexylamine conversion follows, that in this reaction the formed tar does not play the essential role of initiator, like in the gas phase ammoxidation of cyclohexanone over amorphous silica, where it increases the formation of cyclohexanone oxime [12]. The acidic surface of the catalyst is deactivated also by less volatile higher molecular weight amines and their condensation products (minor by-products of the reaction). Part of the deposits can be dissolved in the organic solvents. The whole tar can be oxidized to CO2 by molecular oxygen at 500 8C and regenerate the initial activity of catalysts. Fig. 9 shows the FT-IR spectra of cyclohexylamine, g-alumina before reaction, after deactivation by cyclohexylamine oxidation and after its reactivation at 500 8C by air. The FT-IR spectra of tars on the alumina contain some typical bands of cyclohexylamine mainly in the range of C–H bands. All bands of tar disappear after its total oxidation by molecular oxygen and the catalyst is reactivated. By modification of alumina with amine its catalytic activity is significantly changed (Figs. 7 and 8). The maximum of cyclohexylamine conversion rose from nearly 10% to 35%. The lowest
Fig. 9. FT-IR spectra of (a) pure cyclohexylamine; (b) g-alumina after catalytic test; (c) g-alumina after reactivation at 500 8C; (d) g-alumina before catalytic test.
conversion was observed over unmodified alumina. The commercial alumina with narrow pores has nearly the same catalytic activity despite of its significantly lower BET surface. Because the observed conversions over both catalysts were nearly the same, the BET surface is not the most important factor which determines the oxidation activity of alumina. The highest conversions were achieved over alumina modified by cyclohexylamine and ethanolamine. These two amines practically neither not change the BET surface, nor the pore volume, nor the distribution of pore sizes. Both alumina have relatively high pore volume with wide pores centered around 12–14 nm. Alumina prepared by di- and triethanolamine as modifier have lower activity, significantly lower pore volume size and bimodal pore size distribution with maxima centered around 2.6 and 35 nm. The best results obtained over alumina prepared by cyclohexylamine can indicate that during the formation of alumina specific sites were created in its structure (like key and lock) were cyclohexylamine is more easily adsorbed for activation. The aminic modifier of alumina enhances not only the conversion of cyclohexylamine but also the selectivity of cyclohexanone oxime formation. The highest selectivity above 60% was achieved by di- and triethanolamine as modifier. However, this increase is lower than increase in the conversions for catalyst prepared by cyclohexylamine and so the highest yield of cyclohexanone oxime was found over catalyst modified by cyclohexylamine and ethanolamine. In our previous work [13] the positive influence of silicotungstic acid, deposited on the surface of alumina was confirmed by improving the conversion of cyclohexylamine and selectivity of
Fig. 10. The influence of modifier amine on the conversion of cyclohexylamine over alumina impregnated with heteropolyacid (H4[Si(W3O10)4]xH2O) (for legend to catalysts see Fig. 7).
K. Rakottyay, A. Kaszonyi / Applied Catalysis A: General 367 (2009) 32–38
Fig. 11. The influence of modifier amine on the selectivity of cyclohexanone oxime formation over alumina impregnated with heteropolyacid (H4[Si(W3O10)4]xH2O) (for legend to catalysts see Fig. 7).
oxime formation. The impregnation of commercial alumina by HPA (H4[Si(W3O10)4]xH2O) prominently increases its activity mainly in the first hours of time on stream without significant influence on the selectivity for cyclohexanone oxime (compare Fig. 7, Fig. 8 and Fig. 10, Fig. 11, respectively). In the case of alumina prepared from alcoholate with or without aminic modifier the situation is more complex. After impregnation with mentioned HPA the conversions of cyclohexylamine are slightly lower (with maximum around 30%) and selectivity for cyclohexanone oxime is higher (with maximum over 70%). The observed lower conversions are probably the result of decreased contact time of cyclohexylamine and reaction products with catalyst, which have significantly higher powder density. The lower powder density results for the same weight of catalyst in the higher catalyst bed. The powder density of alumina prepared by hydrolysis of aluminiumtriisopropylate and commercial alumina are 0.07 and 0.56 g cm 3, respectively. At the same flow rate of cyclohexylamine and reaction products their contact time is prolonged, which allows higher conversions. However, after impregnation all catalysts had powder density around 0.55 g cm 3 and as a result nearly the same contact time during cyclohexylamine oxidation. The used HPA adds highly acidic protons and some form of tungsten oxide to the catalysts. Over strong acidic sites cyclohexylamine and the formed oxime are strongly adsorbed. The protonized amine is stable against oxidation. As a result, the acidic protons of HPA are neutralized and an equivalent amount of amine is captured. The highly acidic protons of HPA can catalyze deamination of cyclohexylamine to cyclohexene, which reacts with cyclohexylamine to dicyclohexylamine, one of the
Fig. 12. The influence of catalyst on the conversion of cyclohexylamine in inert gas (N2) and air (O2) (C – alumina prepared without modifier, CA – commercial alumina).
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Fig. 13. The influence of catalyst on the selectivity of N-cyclohexylidene– cyclohexylamine formation in inert gas (N2) and air (O2) (C – alumina prepared without modifier, CA – commercial alumina).
by-products of the reaction. However this reaction has very low rate in comparison with the rate of amine oxidation. 3.3. Reaction of cyclohexylamine in the inert gas over modified alumina Alumina is an atypical catalyst for oxidation reactions. Chemisorption and possible activation of molecular oxygen on the surface of alumina is mentioned in the literature. By XPS spectra Al0 and Al3+ were found in alumina [14]. However, we were not able to find evidence of participation of Al from alumina in redox cycles with oxygen and organic compounds in the available literature. We checked the ability of alumina to oxidize cyclohexylamine in some experiments, where before the start of reaction the physically adsorbed oxygen was desorbed from the catalyst surface by flushing of catalyst by inert gas during several hours at the reaction conditions. After removal of oxygen the feeding of cyclohexylamine was started in nitrogen atmosphere and the samples of products collected directly below catalyst bed were analyzed. During this process like at the presence of oxygen, the catalyst prepared from aluminiumtriisopropylate showed weaker activity than the commercial one (Fig. 12). The conversions of cyclohexylamine over both catalysts were less than 1% in inert atmosphere. The main product of cyclohexylamine oxidation under these conditions was N-cyclohexylidene–cyclohexylamine (Fig. 13), which is in equilibrium with cyclohexanone and cyclohexylamine as their Schiff base. Traces of cyclohexanone oxime, cyclohexene, benzene, dicyclohexylamine, cyclohexylaniline and cyclohexanol were also observed in the samples. The selectivity of Schiff base production over commercial alumina reached 85% (Fig. 13). When the catalyst was flushed by nitrogen only several minutes before feeding of amine, significantly higher amount of cyclohexanone oxime was observed during the first 3 h of reaction. Thus the chemisorbed oxygen plays a very important role in the oxidation of cyclohexylamine to cyclohexanone oxime. The formation of cyclohexanone (N-cyclohexylidene–cyclohexylamine) lasted more than 20 h at conversions of amine around 0.7%. From this result follows, that cyclohexanone and cyclohexanone oxime are formed by different reaction mechanisms. In agreement with it the selectivity of N-cyclohexylidene–cyclohexylamine formation rises in inert atmosphere, however, at the presence of oxygen decreases by time on stream (Fig. 13). The production of other reaction by-products can be rationalized by the set of hydrogenation–dehydrogenation reactions (Scheme 1):
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Scheme 1. Scheme of reactions.
4. Conclusions The preparation technique of alumina plays an important role for its activity in the oxidation of cyclohexylamine. By using different amine during formation of Al–O–Al and –Al(OH)– bonds one can modify the specific surface area, pore volume, pore size distribution, powder density and the acidity of formed alumina. The changed structure of alumina significantly influences its activity and selectivity in the oxidation of cyclohexylamine. The maximum of conversion can be increased from 15% up to 35% and the selectivity to cyclohexanone oxime from 40% up to 60%. The impregnation of prepared alumina by silicotungstic heteropolyacid enhances the selectivity of oxime formation up to 70% though the conversion of cyclohexylamine besides commercial alumina is decreased. During this impregnation the powder density of alumina prepared from aluminiumtriisopropylate increases from 0.07 g cm 3 to 0.55 g cm 3, which markedly decreases the height of catalysts bed and contact time. The main product of the reaction in the inert atmosphere is N-cyclohexylidene–cyclohexylamine at conversions of cyclohexylamine below 1%. The other reaction byproducts are formed by hydrogenation–dehydrogenation and deamination processes. The catalysts can be reactivated by total oxidation of formed tars. Acknowledgement Financial support from the Slovak Grant Agency VEGA 1/0768/ 08 is gratefully acknowledged.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apcata.2009.07.034.
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