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Heterogeneous catalytic ammoximation of cyclohexanone with ammonia and molecular oxygen
T. Inui et al. (Editors),N e w Aspects of Spillover Effect in Catalysis 0 1993 Elsevier Science Publishers B.V. All lights reserved.
241
Heterogeneous catalytic ammoximationof cyclohexanonewith ammonia and molecular oxygen G. Buscaa, E. GiamelldD. D. PinelliCand F. Trifidc
aIstituto Chimico, Facolta di Ingegneria, Fiera del Mare, Pad.D, &nova (Italy) bDepartment of Inorganic Chemistry, Physical Chemistry and Chemistry of Materials, V. Pietro Giuria 9, 10125 Torino (Italy) ‘Department of Industrial Chemistry and Materials, V.le Risorgimento 4, 40136 Bologna (Italy)
1. INTRODUCTION Cyclohexanone oxime is the reactant for the production of caprolactam, the monomer for nylon 6. Cyclohexanone oxime is produced in a multi-step process involving a liquid phase reaction of cyclohexanone with hydroxylamine catalyzed by dilute sulfuric acid [l]. Two alternative ways of production of oxime have been proposed: i) ammoximation of cyclohexanone with ammonia in the presence of silica as catalyst [2-51 and ii) ammoximation with ammonia and hydrogen peroxide with titanium silicalite as catalyst [6,7]. The present paper is a report of a pan of our research on the ammoximation with molecular oxygen following the patent and papers of Armor et al. [2-51. They found that, using a high-surface-area commercial amorphous silica (Porasil A) as the catalyst, the cyclohexanone converts to the corresponding oxime at 2W C in the presence of 02 as oxidant and N H 3 as the source of nitrogen instead of hydroxylamine. The low yields obtained and the formation of heavy products which can not desorb and remain on the catalyst surface as tars, causing fast deactivation, prevented this process to be developed. Much work [8-15] has been done more recently by our research group to study the ammoximation of cyclohexanone to the corresponding oxime in the gas-phase with 0 2 over a commercial amorphous silica (AKZO F- 7). In particular, in a fist phase of the research, several efforts have been made in order to investigate the reaction network on the basis of flow-reactor experiments [8,9].
2. STUDIES ON THE REACTION NETWORK Amorphous AKZO F-7 Sio2 was used as catalyst in the catalytic tests. This catalyst was chosen because of its similarity to the amorphous silica Porasil A, the best catalyst tested by Armor [3,4]. The main characteristics given by the producers are the following: surface area 472 m2/g, pore volume 2.0 cm3/g. apparent density 0.246 g/ml, A1203 0.07%,Na2O 0.01%.
248 g tars4 5102
l 80
-
0
:
l
4
conversion t tars content
Y leld CHO -1
10
0
20
40
30
time-on-stream (h)
Figure 1 Time evolution of the catalytic behaviour of Akzo F-7.
The catalyst was charged as powder (0.075-0.300 mm). The catalytic tests were carried out in a tubular glass fixed-bed plug-flow micro-reactor (maximum capacity 4.0 ml. 1.Og of catalyst) which has been described in detail elsewhere [9]. It was designed as to reduce as much as possible the incidence of homogeneous non-catalytic reactions. The typical reaction conditions were: NH3=34% mol, @=lo% mol, cyclohexanone = 2.8% mol, and the remainder nitrogen, T=170-250'C, catalyst weight W 4 . 5 g loaded as powder (0.125-0.150mm), contact time=3.Os (GHSV=1200 h-', W/F=175 g Wmol). The catalytic tests were carried out following the catalytic performance with the time-on-stream. During the entire life of the catalyst, four main types of products are found [9]. Together with the oxime, the cyclohexanone imine, heavy products which remain irreversibly adsorbed on the catalyst as tars and other condensation products (dimers and aimers of the ketone or the imine) are produced. Typical evolutions of the conversion and the yields of the products in the standard conditions with the time-on-stream are reported in Figure 1; a complex evolution with time-on-stream was found in standard conditions. The oxime yield increases with time in the fiist 10 h and then decreases to zero more rapidly than the rate of formation of tars. On the other hand, the yield of the other products, consisting mainly in condensation products (aldol condensation
/ 6- 0 \ 6' Tars
activ. 0
0
NH
+NH3
activ. 0
2
_.____)
-H2O
Condensation
Figure 2 Simplified reaction network
Producto
249
and other kind of condensations), changes only slightly with time-on-stream up to 40 h when they are the only products of the reaction. Several catalytic tests were carried out in different conditions and the information collected on the mechanism of selective reaction and on the influence of the reaction parameters in the reactions allowed to simplify the complex reaction network and to draw the scheme reported in Figure 2 which describes satisfactorily the reaction in the conditions chosen to obtain maximum yield in oxime. The cyclohexanone imine is produced by interaction of the cyclohexanone and ammonia over the catalytic surface. The intermediate imine is in equilibrium with the ketone and is transformed by three parallel pathways. It is either oxidized to oxime or transformed into tars by a reaction in which some activated oxygen species are involved. The third parallel parasitic pathway transforms the imine intc condensation products, mainly aldol condensation products. 3. ROLE OF BRONSTED ACIDITY OF SILICA
A complete investigation [ 101 was carried out by adsorption of base with different basicity and FT-IR measurements to study the surface acidity. The IR studies showed that, as usual for silicas, free silanol groups are present on the surface of the Akzo F-7 silica. The IR spectra evidenced other absorptions in the region between 4000 and 3000 cm-l in which the OH stretchings are present. These bands are normally present in silica samples and are assigned in the literature to H-bonded and/or "internal" hydroxy groups [16,17]. However, the surface concentration of free silanols and above all of the H-bonded silanols of the silica Akzo F-7 seems to be unusual. In order to characterize the strength of the surface acidic sites of the catalyst, IR spectroscopy was used to study the adsorption of probe molecules of different strengths (butyl-amine, ammonia, pyridine, deuterated acetonimle). The results indicated an Bronsted acidity on the catalyst, unusual on pure silica surfaces such as Cabosil. This extra-acidity was atmbuted to the presence of small amounts of A13+ in the sample. These data were in strength and the concentration of the Bronsted acidic sites on the catalyst had an important role in determining the catalytic behaviour. Therefore, some catalytic tests were carried out with other amorphous silica samples with different acidities in order to verify the importance of Bronsted acidity on the catalytic performance. The catalytic behaviour of AKZO F-7 silica has been compared with those of other two commercial silica samples: Cabosil silica and silica GRACE Nr.2. The strengths and surface concentrations of free and H-bonded silanols were measured by comparison of the absorptions in the region corresponding to the OH stretchings and by adsorption of a weak base as deuterated Table I - Catalytic performance and Bronsted acidity of the tested catalysts. Catalyst
T (.c)
(g)
&OF-7
220
0.5
2.8
175
Grace Nr.2
220
0.5
2.8
Cabosil
220
0.5
2.8
W
ketone WIF Conv. Y-oxime Y-tars Bronsted (kmol) (g/h.mol) (%) (%mol) (%mol) acidity 72.1
32.2
18.9
strong
175
55.4
13.2
25.9
medium
175
42.0
1.3
13.6
weak
Notes: W=catalyst weight, F=molar flow rate, Ketone=ketone concentration in the test.
250
acetonitrile (CD3CN) as described in the experimental section of reference 12. The adsorption of d3- acetonitrile on the samples results in the interaction of proton- donor centers (free silanols Bronsted sites) with the d3- acetonitrile, which in turns influences the CN bond with the appearance of a new band whose frequency is shifted as compared to that of free CD3CN proportionally to the strength of the adsorption site. When comparing the intensities of the corresponding bands between the three silica samples (Akzo F-7, Grace Nr.2 and Cabosil), it was easily evidenced that the concentration of proton-donor centers is much higher in the case of the AKZO sample than in the case of Cabosil, especially for the component due to the perturbed and/or hydrogen bonded hydroxy groups, while the Grace Nr.2 exhibits intermediate Bronsted acidity. On the other hand, the strengths were found to be comparable [121. Catalytic tests were carried out in the standard conditions in order to compare the catalytic behaviours of the three samples. The results of the tests are summarized in Table I. The same phenomenology found in the case of Akzo F-7 was also found in the cases of the other commercial silica samples. The conversion data confiied the relationship between activity and Bronsted acidity. Moreover, the data of the yields in oxime and tars suggested that no relation exists between Bronsted acidity and the oxidant capabilities (both with regards to the selective and the parasitic pathway). Bronsted acidity plays an important role in the first ster of the reaction, the production of the imine, but it is not important in the following determinant step consisting in the imine oxidation to the oxime. 4. THE ROLE OF RADICAL SPECIES: E.P.R. DATA The experiments described above clarified the mechanism of formation of the cyclohexanone imine. However, poor information was available on the following oxidation to the corresponding oxime, in particular with regards to the nature of the oxidant species involved in the reaction. In order to improve the knowledge in this field, some E.P.R. experiments were carried out trying to evidence possible oxidant species present or formed on the fresh and the aged catalyst [ 111. A first experiment was carried out in order to verify the possibility that oxygen radical species could be generated by the silica surface during the catalytic test. A sample of AKZO silica was activated by annealing overnight at 200°C under dynamic vacuum. Then, molecular oxygen (200 torr) was added at room temperature, kept in contact with the solid for one hour and, finally, pumped off. After this treatment, an E.P.R. spectrum was recorded in vacuum, which was the typical anisotropic spectrum of the superoxide radical anion (@.-) adsorbed on a surface positive ion [18]. The signal exhibited the following principal values of the g tensor: gzz=2.0262, gy~=2.0096.gxx=2.0046. The values of the tensor found are, indeed, very near to those reported in the literature for superoxide radicals on Ti4' ions [ 19.201. A second E.P.R. experiment was carried out to investigate the presence of organic radical species on the catalyst surface dirtied by the tars, and the changes occurring at the silica surface due to the tar deposition. For this purpose, six samples of discharged catalysts, containing different amounts of tars deposited on the catalyst surfaces, were prepared by running the reaction in the standard conditions and then stopping the reaction at different time-on-stream values (0.5,2.0,4.0,8.0, 16.0,32.0 h). All samples gave rise to the same EPR spectrum consisting of a single broad line, with a peak to peak width of 0.56 mT,
25 I
characterized by a g value of 2.0035,whose intensity increased with time-on-stream with the same trend as the yield of oxime during the activation process. The EPR evidence presented showed that the amorphous silica sample used as a catalyst for the gas-phase ammoximation of the cyclohexanone to the oxime with molecular oxygen can be a real oxidation catalyst since it activates 02 in the form of a superoxide radical ion (02-.) which is known as a very reactive electrophilic species. It is remarkable that this activation is achieved in experimental conditions very similar, indeed, to the real reaction conditions. On the other hand, the principal values of the g tensor seem to indicate a possible role of Ti incorporated into the lattice or segregated on the silica surface. The EPR results also show that the silica surface covered by tars is rich in organic radicals. The signal intensity can not simply be correlated to the tar content (g tars/g Si02) but seems to be more easily related to the activation process of the catalyst which takes place during the fist 10-15 h of reaction, suggesting that these radicals may play a role in the catalytic process. Therefore, in the samples investigated, two types of active species are present at the surface of the catalyst and involved in the oxidation process: i) a m-.species formed on silica and trapped on tetrahedral cations, probably Ti4', and ii) organic radicals contained in large amounts in the catalyst tars whose possible role in H- abstraction from the imine or in 0-insertion can not be ignored.
5. SPILLOVER OF OXYGEN SPECIES OR OF IMINE ? All the data collected on the three main class of products oxime, tars and other volatile compounds allowed to formulate a fi st tentative superficial model which can account for the catalytic behaviour observed. In the standard conditions, the rate limiting step is assumed to be the oxidation of the adsorbed imine, in equilibrium on $he silica surface with the ketone, to the adsorbed oxime by some activated oxygen species 0 ,probably superoxide radical ions. These species are assumed to be generated by active sites whose concentration increases with the total tar content and to migrate and react with the adsorbed imine. Alternatively, it can be supposed that the oxidant species formed on the tars react with the free imine present in the gas phase or in the adsorbed phase. In any case, the following scheme may be suggested:
In spite of the great effort spent to understand the mechanism of the reactions involved in the ammoximation of cyclohexanone with molecular oxygen, there is still a great lack of knowledge on the reaction, in particular, with regards to the mechanism of the imine oxidation to oxime. The future works will try to determine the real nature of the oxidative species present on the tars and involved in the mechanism of formation of the oxime, to discriminate between the two possible mechanisms proposed, and to clarify if the superoxide radicals may indeed have a role in the catalytic behaviour. We will try to gain the required information investigating the oxidation power of the fresh and the aged catalysts in some
252 model reactions of oxidation such as: methanol oxidation, cyclohexylamine and cyclohexane oxidative dehydrogenation and cyclohenanol oxidation. The data which will be collected will be used to complete the surface model and to draw new kinetic equations to be verified in a fitting of the experimental data and to design new more selective catalysts. 6. ACKNOWLEDGMENTS. THE FINANCIAL SUPPORT FROM C.N.R. - "PROGETTO FINALIZZATO - CHIMICA FINE 2"(ROME) IS GRATEFULLY ACKNOWLEDGED.
7.REFERENCES 1 2 3 4
5 6 7 8 9 10 11 12 13 14 15
16 17 18 19 20
K. Weissermel and H.J. Ape. Industrial Organic Chemistry, Springer Verlag, 1978,pp. 222-231. J.N. Armor,U.S.Patent4,163.756(1979). J. N. Armor, J. Catal.. 70,72(1981). J. N. Armor, E. J. Carlson, S. Soled, W. D. Conner, A. Laverick, B. De Rites and W. Gates, J.Catal., 70,84(1981). J. N.Armor, P. M. Zambri and R. Leming, J.Catal., 72.66 (1982). P. Roffiaet al., US Patent 4,745,221(1988). P. Roffia, G. Leofanti, A. Cesana, M. Mantegazza, M. Pado van, G. Petrini, S. Tonti, P. Gervasutti, in G. Centi and F. Trifid (eds.) New Developments in Selective Oxidation, Rimini, 1989. D. P. DreoN, D. Pinelli, F. Trifid, in Proceedings, "12Simposio Ibero American0 de Catdisc", Rio de Janeiro 1990,Inst. Brasileiro de Petroleo, (1990).V01.2,pp.305-312. D. P. Dreoni, D. Pinelli, F. Trifid, J. Mol. Catal., 69,171 (1992). D. P. Dreoni, D. Pinelli, F. Trifirb, Busca, Lorenzelli, J. Mol. Catal., 71,11 1 (1992). D. Pinelli. F. Trifid, A. Vaccari, E.Giamello. G. F. Pedulli, Catal. Letters. 13.21 (1992). D.P. Dreoni, D. Pinelli, F. Trifid, H. Habersberger, Z. Tvaruskova, P. Jim, Catal. Letters, 11,2X5 (1992). D.P. Dreoni, D. Pinelli, F. Trifid, in New Development in Selective Oxidation by Heterogeneous Catalysis 111, book series Studies in Surface Science and Catalysis, vo1.72, P.Ruiz and B. Delnion Eds., Elsevier Science pub., Amsterdam 1992,pp, 109-116. D.P. DreoN, D. Pinelli, F. Trifid, H. Habersberger, Z. Tvaruskova, P. Jim, in Proceedings ofthe "loth International Congress on Catalysis", Budapest 1992.in pub. E. Pieri, D. Pinelli,F.Trifid, Chem. Eng. Science,47,2641(1992). G. Ghiotti, E. Garrone, C. Mo~terraand F. Boccuzzi, J. Phys. Chem., 83,2863(1979). S.Kondo. H.Yamagouchi, Y.Kajiyama, T. Ishikawa, J. Chem. SOC.Faraday Trans. 1,80,2033 (1984). M. Che, A. J.Tench, Advan. Catal.,32,l(1983). M. Shiotani, G. Mom. J. H. Freed, J. Chem. Phys., 74,2616(1981). M. Anpo, N. Aikawa, Y. Kubokawa, M. Che, C. Louis, E. Giamello, J. Phys. Chem., 89,5689(19x5).
241
Heterogeneous catalytic ammoximationof cyclohexanonewith ammonia and molecular oxygen G. Buscaa, E. GiamelldD. D. PinelliCand F. Trifidc
aIstituto Chimico, Facolta di Ingegneria, Fiera del Mare, Pad.D, &nova (Italy) bDepartment of Inorganic Chemistry, Physical Chemistry and Chemistry of Materials, V. Pietro Giuria 9, 10125 Torino (Italy) ‘Department of Industrial Chemistry and Materials, V.le Risorgimento 4, 40136 Bologna (Italy)
1. INTRODUCTION Cyclohexanone oxime is the reactant for the production of caprolactam, the monomer for nylon 6. Cyclohexanone oxime is produced in a multi-step process involving a liquid phase reaction of cyclohexanone with hydroxylamine catalyzed by dilute sulfuric acid [l]. Two alternative ways of production of oxime have been proposed: i) ammoximation of cyclohexanone with ammonia in the presence of silica as catalyst [2-51 and ii) ammoximation with ammonia and hydrogen peroxide with titanium silicalite as catalyst [6,7]. The present paper is a report of a pan of our research on the ammoximation with molecular oxygen following the patent and papers of Armor et al. [2-51. They found that, using a high-surface-area commercial amorphous silica (Porasil A) as the catalyst, the cyclohexanone converts to the corresponding oxime at 2W C in the presence of 02 as oxidant and N H 3 as the source of nitrogen instead of hydroxylamine. The low yields obtained and the formation of heavy products which can not desorb and remain on the catalyst surface as tars, causing fast deactivation, prevented this process to be developed. Much work [8-15] has been done more recently by our research group to study the ammoximation of cyclohexanone to the corresponding oxime in the gas-phase with 0 2 over a commercial amorphous silica (AKZO F- 7). In particular, in a fist phase of the research, several efforts have been made in order to investigate the reaction network on the basis of flow-reactor experiments [8,9].
2. STUDIES ON THE REACTION NETWORK Amorphous AKZO F-7 Sio2 was used as catalyst in the catalytic tests. This catalyst was chosen because of its similarity to the amorphous silica Porasil A, the best catalyst tested by Armor [3,4]. The main characteristics given by the producers are the following: surface area 472 m2/g, pore volume 2.0 cm3/g. apparent density 0.246 g/ml, A1203 0.07%,Na2O 0.01%.
248 g tars4 5102
l 80
-
0
:
l
4
conversion t tars content
Y leld CHO -1
10
0
20
40
30
time-on-stream (h)
Figure 1 Time evolution of the catalytic behaviour of Akzo F-7.
The catalyst was charged as powder (0.075-0.300 mm). The catalytic tests were carried out in a tubular glass fixed-bed plug-flow micro-reactor (maximum capacity 4.0 ml. 1.Og of catalyst) which has been described in detail elsewhere [9]. It was designed as to reduce as much as possible the incidence of homogeneous non-catalytic reactions. The typical reaction conditions were: NH3=34% mol, @=lo% mol, cyclohexanone = 2.8% mol, and the remainder nitrogen, T=170-250'C, catalyst weight W 4 . 5 g loaded as powder (0.125-0.150mm), contact time=3.Os (GHSV=1200 h-', W/F=175 g Wmol). The catalytic tests were carried out following the catalytic performance with the time-on-stream. During the entire life of the catalyst, four main types of products are found [9]. Together with the oxime, the cyclohexanone imine, heavy products which remain irreversibly adsorbed on the catalyst as tars and other condensation products (dimers and aimers of the ketone or the imine) are produced. Typical evolutions of the conversion and the yields of the products in the standard conditions with the time-on-stream are reported in Figure 1; a complex evolution with time-on-stream was found in standard conditions. The oxime yield increases with time in the fiist 10 h and then decreases to zero more rapidly than the rate of formation of tars. On the other hand, the yield of the other products, consisting mainly in condensation products (aldol condensation
/ 6- 0 \ 6' Tars
activ. 0
0
NH
+NH3
activ. 0
2
_.____)
-H2O
Condensation
Figure 2 Simplified reaction network
Producto
249
and other kind of condensations), changes only slightly with time-on-stream up to 40 h when they are the only products of the reaction. Several catalytic tests were carried out in different conditions and the information collected on the mechanism of selective reaction and on the influence of the reaction parameters in the reactions allowed to simplify the complex reaction network and to draw the scheme reported in Figure 2 which describes satisfactorily the reaction in the conditions chosen to obtain maximum yield in oxime. The cyclohexanone imine is produced by interaction of the cyclohexanone and ammonia over the catalytic surface. The intermediate imine is in equilibrium with the ketone and is transformed by three parallel pathways. It is either oxidized to oxime or transformed into tars by a reaction in which some activated oxygen species are involved. The third parallel parasitic pathway transforms the imine intc condensation products, mainly aldol condensation products. 3. ROLE OF BRONSTED ACIDITY OF SILICA
A complete investigation [ 101 was carried out by adsorption of base with different basicity and FT-IR measurements to study the surface acidity. The IR studies showed that, as usual for silicas, free silanol groups are present on the surface of the Akzo F-7 silica. The IR spectra evidenced other absorptions in the region between 4000 and 3000 cm-l in which the OH stretchings are present. These bands are normally present in silica samples and are assigned in the literature to H-bonded and/or "internal" hydroxy groups [16,17]. However, the surface concentration of free silanols and above all of the H-bonded silanols of the silica Akzo F-7 seems to be unusual. In order to characterize the strength of the surface acidic sites of the catalyst, IR spectroscopy was used to study the adsorption of probe molecules of different strengths (butyl-amine, ammonia, pyridine, deuterated acetonimle). The results indicated an Bronsted acidity on the catalyst, unusual on pure silica surfaces such as Cabosil. This extra-acidity was atmbuted to the presence of small amounts of A13+ in the sample. These data were in strength and the concentration of the Bronsted acidic sites on the catalyst had an important role in determining the catalytic behaviour. Therefore, some catalytic tests were carried out with other amorphous silica samples with different acidities in order to verify the importance of Bronsted acidity on the catalytic performance. The catalytic behaviour of AKZO F-7 silica has been compared with those of other two commercial silica samples: Cabosil silica and silica GRACE Nr.2. The strengths and surface concentrations of free and H-bonded silanols were measured by comparison of the absorptions in the region corresponding to the OH stretchings and by adsorption of a weak base as deuterated Table I - Catalytic performance and Bronsted acidity of the tested catalysts. Catalyst
T (.c)
(g)
&OF-7
220
0.5
2.8
175
Grace Nr.2
220
0.5
2.8
Cabosil
220
0.5
2.8
W
ketone WIF Conv. Y-oxime Y-tars Bronsted (kmol) (g/h.mol) (%) (%mol) (%mol) acidity 72.1
32.2
18.9
strong
175
55.4
13.2
25.9
medium
175
42.0
1.3
13.6
weak
Notes: W=catalyst weight, F=molar flow rate, Ketone=ketone concentration in the test.
250
acetonitrile (CD3CN) as described in the experimental section of reference 12. The adsorption of d3- acetonitrile on the samples results in the interaction of proton- donor centers (free silanols Bronsted sites) with the d3- acetonitrile, which in turns influences the CN bond with the appearance of a new band whose frequency is shifted as compared to that of free CD3CN proportionally to the strength of the adsorption site. When comparing the intensities of the corresponding bands between the three silica samples (Akzo F-7, Grace Nr.2 and Cabosil), it was easily evidenced that the concentration of proton-donor centers is much higher in the case of the AKZO sample than in the case of Cabosil, especially for the component due to the perturbed and/or hydrogen bonded hydroxy groups, while the Grace Nr.2 exhibits intermediate Bronsted acidity. On the other hand, the strengths were found to be comparable [121. Catalytic tests were carried out in the standard conditions in order to compare the catalytic behaviours of the three samples. The results of the tests are summarized in Table I. The same phenomenology found in the case of Akzo F-7 was also found in the cases of the other commercial silica samples. The conversion data confiied the relationship between activity and Bronsted acidity. Moreover, the data of the yields in oxime and tars suggested that no relation exists between Bronsted acidity and the oxidant capabilities (both with regards to the selective and the parasitic pathway). Bronsted acidity plays an important role in the first ster of the reaction, the production of the imine, but it is not important in the following determinant step consisting in the imine oxidation to the oxime. 4. THE ROLE OF RADICAL SPECIES: E.P.R. DATA The experiments described above clarified the mechanism of formation of the cyclohexanone imine. However, poor information was available on the following oxidation to the corresponding oxime, in particular with regards to the nature of the oxidant species involved in the reaction. In order to improve the knowledge in this field, some E.P.R. experiments were carried out trying to evidence possible oxidant species present or formed on the fresh and the aged catalyst [ 111. A first experiment was carried out in order to verify the possibility that oxygen radical species could be generated by the silica surface during the catalytic test. A sample of AKZO silica was activated by annealing overnight at 200°C under dynamic vacuum. Then, molecular oxygen (200 torr) was added at room temperature, kept in contact with the solid for one hour and, finally, pumped off. After this treatment, an E.P.R. spectrum was recorded in vacuum, which was the typical anisotropic spectrum of the superoxide radical anion (@.-) adsorbed on a surface positive ion [18]. The signal exhibited the following principal values of the g tensor: gzz=2.0262, gy~=2.0096.gxx=2.0046. The values of the tensor found are, indeed, very near to those reported in the literature for superoxide radicals on Ti4' ions [ 19.201. A second E.P.R. experiment was carried out to investigate the presence of organic radical species on the catalyst surface dirtied by the tars, and the changes occurring at the silica surface due to the tar deposition. For this purpose, six samples of discharged catalysts, containing different amounts of tars deposited on the catalyst surfaces, were prepared by running the reaction in the standard conditions and then stopping the reaction at different time-on-stream values (0.5,2.0,4.0,8.0, 16.0,32.0 h). All samples gave rise to the same EPR spectrum consisting of a single broad line, with a peak to peak width of 0.56 mT,
25 I
characterized by a g value of 2.0035,whose intensity increased with time-on-stream with the same trend as the yield of oxime during the activation process. The EPR evidence presented showed that the amorphous silica sample used as a catalyst for the gas-phase ammoximation of the cyclohexanone to the oxime with molecular oxygen can be a real oxidation catalyst since it activates 02 in the form of a superoxide radical ion (02-.) which is known as a very reactive electrophilic species. It is remarkable that this activation is achieved in experimental conditions very similar, indeed, to the real reaction conditions. On the other hand, the principal values of the g tensor seem to indicate a possible role of Ti incorporated into the lattice or segregated on the silica surface. The EPR results also show that the silica surface covered by tars is rich in organic radicals. The signal intensity can not simply be correlated to the tar content (g tars/g Si02) but seems to be more easily related to the activation process of the catalyst which takes place during the fist 10-15 h of reaction, suggesting that these radicals may play a role in the catalytic process. Therefore, in the samples investigated, two types of active species are present at the surface of the catalyst and involved in the oxidation process: i) a m-.species formed on silica and trapped on tetrahedral cations, probably Ti4', and ii) organic radicals contained in large amounts in the catalyst tars whose possible role in H- abstraction from the imine or in 0-insertion can not be ignored.
5. SPILLOVER OF OXYGEN SPECIES OR OF IMINE ? All the data collected on the three main class of products oxime, tars and other volatile compounds allowed to formulate a fi st tentative superficial model which can account for the catalytic behaviour observed. In the standard conditions, the rate limiting step is assumed to be the oxidation of the adsorbed imine, in equilibrium on $he silica surface with the ketone, to the adsorbed oxime by some activated oxygen species 0 ,probably superoxide radical ions. These species are assumed to be generated by active sites whose concentration increases with the total tar content and to migrate and react with the adsorbed imine. Alternatively, it can be supposed that the oxidant species formed on the tars react with the free imine present in the gas phase or in the adsorbed phase. In any case, the following scheme may be suggested:
In spite of the great effort spent to understand the mechanism of the reactions involved in the ammoximation of cyclohexanone with molecular oxygen, there is still a great lack of knowledge on the reaction, in particular, with regards to the mechanism of the imine oxidation to oxime. The future works will try to determine the real nature of the oxidative species present on the tars and involved in the mechanism of formation of the oxime, to discriminate between the two possible mechanisms proposed, and to clarify if the superoxide radicals may indeed have a role in the catalytic behaviour. We will try to gain the required information investigating the oxidation power of the fresh and the aged catalysts in some
252 model reactions of oxidation such as: methanol oxidation, cyclohexylamine and cyclohexane oxidative dehydrogenation and cyclohenanol oxidation. The data which will be collected will be used to complete the surface model and to draw new kinetic equations to be verified in a fitting of the experimental data and to design new more selective catalysts. 6. ACKNOWLEDGMENTS. THE FINANCIAL SUPPORT FROM C.N.R. - "PROGETTO FINALIZZATO - CHIMICA FINE 2"(ROME) IS GRATEFULLY ACKNOWLEDGED.
7.REFERENCES 1 2 3 4
5 6 7 8 9 10 11 12 13 14 15
16 17 18 19 20
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