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Oxidation of benzene to phenol by nitrous oxide over Fe-ZSM-5 zeolites
31
Applied Catalysis A: General, 82 (1992) 31-36 Elsevier Science Publishers B.V., Amsterdam
APCAT 2214
Oxidation of benzene to phenol by nitrous oxide over Fe-ZSM-5 zeolites G.I. Panov*, G.A. Sheveleva, A.S. Kharitonov, V.N. Romannikov and L.A. Vostrikova Institute of Catalysis, Novosibirsk 630090 (USSR), tel. (+ 7-383)2354442,
fax. (+ 7-363)2355756
(Received 30 July 1991, revised manuscript received 11 November 1991)
Abstract One step oxidation of benzene to phenol by nitrous oxide was studied over Fe-ZSM-5 xeolites. In addition to the case of nitrous oxide decomposition studied previously, catalytic properties of xeolitea were determined by the presence of iron. New effective catalysta were synthesized. Some of them provided 20-25% benzene conversion with 100% phenol selectivity. In addition to the chemical composition of the catalyst the nature of the oxidant (NxO, Ox) was shown to be of exceptional importance for the rate and the direction of benzene oxidation. Keywords: benzene oxidation, Fe-ZSM-5 zeolite, phenol synthesis, nitrous oxide, xeolites.
INTRODUCTION
The oxidative hydroxilation of aromatics is one of the most difficult problems in the field of organic synthesis. The simplest reaction of this type, the oxidation of benzene to phenol, is carried out by a cumene process consisting of three stages. Many attempts to accomplish a one-step direct oxidation of benzene by molecular oxygen have been unsuccessful. Interaction with oxygen is accompanied by the destruction of the aromatic ring and results in a low phenol selectivity. Much better results were obtained by Iwamoto et al. when using nitrous oxide as an oxidant [ 1,2]. Supported vanadium pentoxide proved to be the most effective catalyst: at a temperature of 550 ’ C benzene conversion was 10% at 71% phenol selectivity. In a number of publications this result has been estimated as being a fruitful base for a new one-stage phenol process [ 3,4]. However, as the selectivity was not high enough, this process was not realized in industry. The papers by Iwamoto et al. [ 1,2] stimulated new efforts to search for more effective catalytic systems. In 1988 three groups of authors [5-71 independently discovered a new type of catalysts for this reaction - H-ZSM-5 zeolites.
32
The oxidation of benzene by nitrous oxide over these catalysts proceeded at 300-400°C with a very high phenol selectivity - close to 100%. Though the maximum yield of phenol obtained in refs. 5-7 was not very high (from 8 to 16% )these results indicated that zeolites provide quite a promising foundation for developing efficient catalysts. The nature of the catalytic action of ZSM-5 zeolites is still unclear. It is rather improbable that such a fine oxidation reaction is catalyzed merely by the acid sites of an aluminosilicate lattice as is assumed in ref. 5. Study of the nitrous oxide decomposition mechanism as one of the steps in the reaction of benzene oxidation revealed the very important role of iron cations on the catalytic properties of ZSM-5. Even small iron impurities in silicate [8] or especially in an aluminosilicate matrix [9,10] cause the formation of active sites on which the decomposition is accompanied by the generation of a specific form of surface oxygen that is impossible to obtain by oxygen adsorption. This oxygen form is very reactive and has an anomalous low bond energy [ 111. At room temperature it is involved in oxygen isotopic exchange as well as in carbon monoxide and methane oxidation. One can assume that the catalytic properties of ZSM-5 zeolites in the oxidation of benzene to phenol by nitrous oxide are also due to the presence of iron which is interesting to elucidate. EXPERIMENTAL
In this work the very same samples of Fe-ZSM-5 which had been studied in nitrous oxide decomposition [8-lo] were tested in the benzene oxidation reaction. The samples had a close aluminosilicate composition (SiOJ Al,O, = 100) but a different concentration of iron which was introduced at the hydrothermal synthesis stage. Designation of samples reflects wt.-% content of iron calculated for FezO,. Thus, for example, Fe (Al)-0.08 means that this sample is a HZSM-5 zeolite of Fe-Al-Si composition containing 0.08 wt.-% FezO,. According to the data obtained by the atomic absorption method, the content of some other transition metals (Co, Ni, Mn, Cu) is not over 0.01 wt.% calculated for corresponding oxides. Experiments on benzene oxidation were carried out in a conventional flow apparatus at atmospheric pressure. The gas flow was 60 cm3/min. Benzene was introduced with a saturator to give 5 vol.-% benzene, 20 vol.-% nitrous oxide, and balance helium. Catalyst was pressed in tablets, crushed, and sieved to obtain a particles fraction of 0.5-1.0 mm in size for use in catalytic measurements. A catalyst bed of 2.0 cm3 catalyst volume was located in the reactor made of 7.0 mm I.D. quartz tube. The apparatus was equipped with an on-line chromatograph fitted with a thermal conductivity detector and a flame ionization detector. Hydrocarbon components of the reaction mixture were separated in a glass capillary column
33
with XE-60 liquid phase coating. Separation of NzO, Nz, CO and CO, was completed in a stainless steel column with porapak Q as a packing material. To obtain more accurate data for the carbon monoxide and carbon dioxide analyses these molecules were exposed to hydrogenation over a nickel catalyst and then analyzed as methane by the flame ionization detector. In some experiments reaction products were collected in a trap filled with ethanol and analyzed by liquid chromatography and chromatographymassspectrometry. RESULTS AND DISCUSSION
As in the case of the vanadia catalyst [ 121 the occurrence of benzene oxidation by nitrous oxide over Fe-ZSM-5 zeolites is accompanied by coke deposition and the deactivation of samples. Catalytic activity in the course of 3 h experiments usually decreases by 1.5-3 times, selectivity to phenol being unchanged. Coke burn out in the oxidizing atmosphere results in the recovery of catalytic activity. Table 1 presents the data on benzene conversion (X) and selectivity (S) towards partial oxidation products taken 25 min after beginning the experiments. The samples exhibit quite low activity at very small iron content (0.0040.015 wt.-% Fe,O,). Benzene conversion considerably increases over Fe (Al) 0.07 and Fe(Al)-0.08 samples and does not change much with a further increase in iron content. The samples of ZSM-5 containing 0.07-0.72 wt.-% FezO, show much more superior results than those known in literature [ 1,2,5-7,121. Even at low temperatures such as 300°C benzene conversion is about 10% and it increases remarkably with increasing temperature. High selectivity is of special significance for this reaction. Over the best catalysts phenol is practically the only product of benzene oxidation up to a conversion degree of 20-25%. At greater X values the selectivity drops due to the successive oxidation of phenol. It brings about the appearance of such oxygenation compounds as benzoquinone, catechol, hydroquinone, and dibenzofurane in partial oxidation products within a few percent of their total selectivity. As has been shown by studying the reactions of isotopic oxygen exchange and nitrous oxide decomposition, the catalytic properties of iron in ZSM-5 zeolites are drastically changed if compared to Fe203 [ 91. When introduced into the zeolite matrix the iron atoms lose their ability to activate oxygen and to catalyze isotopic exchange but gain much in their ability to activate nitrous oxide. The atomic rate of nitrous oxide decomposition over Fe-ZSM-5 increases by several orders of magnitude. The data listed in Table 2 show that these features with respect to nitrous oxide and oxygen activation impose a pronounced effect on the reaction of benzene oxidation by these molecules. Thus, in the case of nitrous oxide ben-
34 TABLE 1 Benzene oxidation by nitrous oxide over Fe-ZSM-5 zeolites Sample
Iron content (wt.-% Fe,O,)
T (“C)
X (%)
s (%)
Fe(Al)-0.004
0.004
350 375 400
2.4 7.0 12.8
100 100 100
Fe(Al)-0.007
0.007
300 350 375
0.7 3.6 9.0
100 100 100
Fe(Al)-0.015
0.015
300 350 375 400
1.4 7.2 12.5 16.5
100 100 100 98
Fe(AI)-0.07
0.07
300 350 375 400
9.0 23.8 30.0 36.0
100 99 97 93.5
Fe(Al)-0.08
0.08
300 325 350 375 400
11.5 18.0 27.0 28.0 36.5
100 100 99 97 94.5
Fe(Al)-0.29
0.29
300 325
10.0 17.0 23.0
100 98 95
Fe(Al)-0.50
0.50 325 350
9.5 17.4 28.0
100 97 85
300 350 375
11.5 20.0 26.5
98 90 85
Fe(Al)-0.72
0.72
zene oxidation over the Fe (Al) -0.08 sample proceeds at 300” C yielding phenol as the only product, while with regard to oxygen the oxidation is hardly noticeable even at 500” C yielding carbon dioxide and no trace of phenol. There is a reverse order in the activity over Fe203. The rate of benzene oxidation by oxygen is higher than that of oxidation by nitrous oxide which is consistent with the data for isotopic oxygen exchange and nitrous oxide decomposition for this catalyst [ 91. Both oxidants over Fe20, produce only products of complete oxidation. These data therefore show the crucial influence of the chemi-
35 TABLE 2 Properties of Fe (AI) -0.08 and FecOs in the catalytic activation of nitrous oxide and oxygen. Comparison with the reaction of benzene oxidation by these molecules Sample
Fe(Al)-0.08
Atomic rate” of N,O decomposition and Ox isotope exchange [9]
Benzene conversion at oxidation by NcO and O2
W NnO
wo,
T
X NzO
X02
(molecule/Fe*s)
(molecule/Fe~+)
(“C)
(%)
(%)
4.4.10-x
Inactive ( <8.10-6)
300 500
11.5 36.5 -
0.0 o..o . 0.2
300
2.5
10.0
4.2.10-’
Fe&
.400
8.0*10-4
‘For Fe (Al)-0.08 the rate value referred to the total number of iron atoms in the sample, for Fe203 - to the number of surface iron atoms. TABLE 3 Benzene oxidation by nitrous oxide at 400” C over metal doped ZShl-5 zeolites Metal
Metal content (wt.-% of metal oxide)
Iron content (wt.-% Fe,O,)
x (%)
s (%)
V Cr Mn co co Ni Ti Zn
0.035 0.018 0.07 0.05 0.12 0.06 1.45 1.3
0.010 0.004 0.015 0.009 0.004 0.007 0.15 0.50
11.6 4.5 6.5 11.7 7.5 4.3 1.2 1.5
100 100 100 100 100 100 100 109
cal nature of the oxidants and the degree of their activation on the direction and rate of benzene oxidation. It is interesting to learn whether the reaction of benzene oxidation to phenol is possible over ZSM-5 zeolite doped with metals other than iron. We synthesized and tested a number of zeolite samples containing transition metals Ti, V, Cr, Mn, Co, Ni and Zn (Table 3). As with iron, metal salts were introduced at the hydrothermal procedure stage [ 13 1. The metal content in the zeolite was determined by the atomic absorption method. Comparing the data presented in Tables 1 and 3 one can see that phenol formation over these samples can be entirely accounted for by the presence of iron. Transition metals have either no effect on catalytic activity or even a negative one which becomes quite pronounced at high concentrations of the metals. Thus in the case of titanium and zinc even at 0.15 wt.-% Fe,O, and 0.5
36
wt.-% Fe203 these samples exhibit a very low activity. Special experiments with zinc indicate that this effect does not depend on the method by which the metal is introduced. In addition to introducing zinc during the hydrothermal procedure was also used an impregnation and a cation-exchange procedure to introduce zinc into the Fe-ZSM-5 zeolite. All the methods produced samples with approximately the same level of activity, being much lower than the FeZSM-5 activity without zinc. A similar effect of transition metals was reported by Suzuki et al. [ 51. They observed no phenol formation over Co2+ or Cu2+ cation-exchanged ZSM-5 zeolite instead of H-ZSM-5 zeolite. These data suggest that the catalytic activity of iron in Fe-ZSM-5 zeolite in the benzene oxidation reaction is an exceptional rather than a common property. One possible explanation can be referred to the specific form of surface oxygen generated on Fe-ZSM-5 under nitrous oxide decomposition. However, this assumption needs to be supported by convincing experimental evidence since according to generally accepted ideas the features of this oxygen atom especially its low bond energy and high reactivity are not consistent with a process of partial oxidation [ 141.
REFERENCES
8 9
10 11 12 13 14
M. Iwamoto, K. Matsukami and S. Kagawa, Jpn. Patent, 58-146 522 (1982). M. Iwamoto, K. Matsukami and S. Kagawa, J. Phys. Chem., 87 (1983) 903. Chem. Econ. Eng. Rev., 14 (1982) 47. Process Econ. Intern., 4 (1983) 48. E. Suzuki, K. Nakashiro and Y. Ono, Chem. Lett., (1988) 953. M. Gubelmann and P. Tirel, European Patent EP, 341 165 (1988). A.S. Kharitonov, T.N. Aleksandrova, L.A. Vostrikova, K.G. Zone and G.I. Panov, USSR Authorship Certificate, 4 445,636 (1988). V.I. Sobolev, G.I. Panov, A.S. Kharitonov, V.N. Romannikov, A.M. Volodin and K.G. Ione, J. Catal., submitted for publication. G.I. Panov, V.I. Sobolev and AS. Kharitonov, J. Mol. Catal., 61 (1990) 85. G.I. Panov, V.I. Sobolev and A.S. Kharitonov, in S. Yoshida, N. Takezawa and T. Ono (Editors), Catalytic Science and Technology, Vol. 1, Kodansha, Tokyo, 1991, pp. 171-176. V.I. Sobolev, O.N. Kovalenko, A.S. Kharitonov, Yu.D. Pankrat’ev and G.I. Panov, Mendeleev Commun., 1 (1991) 29. A.S. Kharitonov, A.I. Yartsev, E.A. Paukshtis, G.S. Litvak, E.N. Yurchenko and G.I. Panov, React. Kinet. Catal. Lett., 37 (1988) 7. L.S. Tchumachenko, V.N. Romannikov and K.G. Ione, USSR Authorship Certificate, 1192 225 (1984). G.K. Boreskov, Heterogeneous Catalysis, Nauka, Moscow, 1986, p. 197.
Applied Catalysis A: General, 82 (1992) 31-36 Elsevier Science Publishers B.V., Amsterdam
APCAT 2214
Oxidation of benzene to phenol by nitrous oxide over Fe-ZSM-5 zeolites G.I. Panov*, G.A. Sheveleva, A.S. Kharitonov, V.N. Romannikov and L.A. Vostrikova Institute of Catalysis, Novosibirsk 630090 (USSR), tel. (+ 7-383)2354442,
fax. (+ 7-363)2355756
(Received 30 July 1991, revised manuscript received 11 November 1991)
Abstract One step oxidation of benzene to phenol by nitrous oxide was studied over Fe-ZSM-5 xeolites. In addition to the case of nitrous oxide decomposition studied previously, catalytic properties of xeolitea were determined by the presence of iron. New effective catalysta were synthesized. Some of them provided 20-25% benzene conversion with 100% phenol selectivity. In addition to the chemical composition of the catalyst the nature of the oxidant (NxO, Ox) was shown to be of exceptional importance for the rate and the direction of benzene oxidation. Keywords: benzene oxidation, Fe-ZSM-5 zeolite, phenol synthesis, nitrous oxide, xeolites.
INTRODUCTION
The oxidative hydroxilation of aromatics is one of the most difficult problems in the field of organic synthesis. The simplest reaction of this type, the oxidation of benzene to phenol, is carried out by a cumene process consisting of three stages. Many attempts to accomplish a one-step direct oxidation of benzene by molecular oxygen have been unsuccessful. Interaction with oxygen is accompanied by the destruction of the aromatic ring and results in a low phenol selectivity. Much better results were obtained by Iwamoto et al. when using nitrous oxide as an oxidant [ 1,2]. Supported vanadium pentoxide proved to be the most effective catalyst: at a temperature of 550 ’ C benzene conversion was 10% at 71% phenol selectivity. In a number of publications this result has been estimated as being a fruitful base for a new one-stage phenol process [ 3,4]. However, as the selectivity was not high enough, this process was not realized in industry. The papers by Iwamoto et al. [ 1,2] stimulated new efforts to search for more effective catalytic systems. In 1988 three groups of authors [5-71 independently discovered a new type of catalysts for this reaction - H-ZSM-5 zeolites.
32
The oxidation of benzene by nitrous oxide over these catalysts proceeded at 300-400°C with a very high phenol selectivity - close to 100%. Though the maximum yield of phenol obtained in refs. 5-7 was not very high (from 8 to 16% )these results indicated that zeolites provide quite a promising foundation for developing efficient catalysts. The nature of the catalytic action of ZSM-5 zeolites is still unclear. It is rather improbable that such a fine oxidation reaction is catalyzed merely by the acid sites of an aluminosilicate lattice as is assumed in ref. 5. Study of the nitrous oxide decomposition mechanism as one of the steps in the reaction of benzene oxidation revealed the very important role of iron cations on the catalytic properties of ZSM-5. Even small iron impurities in silicate [8] or especially in an aluminosilicate matrix [9,10] cause the formation of active sites on which the decomposition is accompanied by the generation of a specific form of surface oxygen that is impossible to obtain by oxygen adsorption. This oxygen form is very reactive and has an anomalous low bond energy [ 111. At room temperature it is involved in oxygen isotopic exchange as well as in carbon monoxide and methane oxidation. One can assume that the catalytic properties of ZSM-5 zeolites in the oxidation of benzene to phenol by nitrous oxide are also due to the presence of iron which is interesting to elucidate. EXPERIMENTAL
In this work the very same samples of Fe-ZSM-5 which had been studied in nitrous oxide decomposition [8-lo] were tested in the benzene oxidation reaction. The samples had a close aluminosilicate composition (SiOJ Al,O, = 100) but a different concentration of iron which was introduced at the hydrothermal synthesis stage. Designation of samples reflects wt.-% content of iron calculated for FezO,. Thus, for example, Fe (Al)-0.08 means that this sample is a HZSM-5 zeolite of Fe-Al-Si composition containing 0.08 wt.-% FezO,. According to the data obtained by the atomic absorption method, the content of some other transition metals (Co, Ni, Mn, Cu) is not over 0.01 wt.% calculated for corresponding oxides. Experiments on benzene oxidation were carried out in a conventional flow apparatus at atmospheric pressure. The gas flow was 60 cm3/min. Benzene was introduced with a saturator to give 5 vol.-% benzene, 20 vol.-% nitrous oxide, and balance helium. Catalyst was pressed in tablets, crushed, and sieved to obtain a particles fraction of 0.5-1.0 mm in size for use in catalytic measurements. A catalyst bed of 2.0 cm3 catalyst volume was located in the reactor made of 7.0 mm I.D. quartz tube. The apparatus was equipped with an on-line chromatograph fitted with a thermal conductivity detector and a flame ionization detector. Hydrocarbon components of the reaction mixture were separated in a glass capillary column
33
with XE-60 liquid phase coating. Separation of NzO, Nz, CO and CO, was completed in a stainless steel column with porapak Q as a packing material. To obtain more accurate data for the carbon monoxide and carbon dioxide analyses these molecules were exposed to hydrogenation over a nickel catalyst and then analyzed as methane by the flame ionization detector. In some experiments reaction products were collected in a trap filled with ethanol and analyzed by liquid chromatography and chromatographymassspectrometry. RESULTS AND DISCUSSION
As in the case of the vanadia catalyst [ 121 the occurrence of benzene oxidation by nitrous oxide over Fe-ZSM-5 zeolites is accompanied by coke deposition and the deactivation of samples. Catalytic activity in the course of 3 h experiments usually decreases by 1.5-3 times, selectivity to phenol being unchanged. Coke burn out in the oxidizing atmosphere results in the recovery of catalytic activity. Table 1 presents the data on benzene conversion (X) and selectivity (S) towards partial oxidation products taken 25 min after beginning the experiments. The samples exhibit quite low activity at very small iron content (0.0040.015 wt.-% Fe,O,). Benzene conversion considerably increases over Fe (Al) 0.07 and Fe(Al)-0.08 samples and does not change much with a further increase in iron content. The samples of ZSM-5 containing 0.07-0.72 wt.-% FezO, show much more superior results than those known in literature [ 1,2,5-7,121. Even at low temperatures such as 300°C benzene conversion is about 10% and it increases remarkably with increasing temperature. High selectivity is of special significance for this reaction. Over the best catalysts phenol is practically the only product of benzene oxidation up to a conversion degree of 20-25%. At greater X values the selectivity drops due to the successive oxidation of phenol. It brings about the appearance of such oxygenation compounds as benzoquinone, catechol, hydroquinone, and dibenzofurane in partial oxidation products within a few percent of their total selectivity. As has been shown by studying the reactions of isotopic oxygen exchange and nitrous oxide decomposition, the catalytic properties of iron in ZSM-5 zeolites are drastically changed if compared to Fe203 [ 91. When introduced into the zeolite matrix the iron atoms lose their ability to activate oxygen and to catalyze isotopic exchange but gain much in their ability to activate nitrous oxide. The atomic rate of nitrous oxide decomposition over Fe-ZSM-5 increases by several orders of magnitude. The data listed in Table 2 show that these features with respect to nitrous oxide and oxygen activation impose a pronounced effect on the reaction of benzene oxidation by these molecules. Thus, in the case of nitrous oxide ben-
34 TABLE 1 Benzene oxidation by nitrous oxide over Fe-ZSM-5 zeolites Sample
Iron content (wt.-% Fe,O,)
T (“C)
X (%)
s (%)
Fe(Al)-0.004
0.004
350 375 400
2.4 7.0 12.8
100 100 100
Fe(Al)-0.007
0.007
300 350 375
0.7 3.6 9.0
100 100 100
Fe(Al)-0.015
0.015
300 350 375 400
1.4 7.2 12.5 16.5
100 100 100 98
Fe(AI)-0.07
0.07
300 350 375 400
9.0 23.8 30.0 36.0
100 99 97 93.5
Fe(Al)-0.08
0.08
300 325 350 375 400
11.5 18.0 27.0 28.0 36.5
100 100 99 97 94.5
Fe(Al)-0.29
0.29
300 325
10.0 17.0 23.0
100 98 95
Fe(Al)-0.50
0.50 325 350
9.5 17.4 28.0
100 97 85
300 350 375
11.5 20.0 26.5
98 90 85
Fe(Al)-0.72
0.72
zene oxidation over the Fe (Al) -0.08 sample proceeds at 300” C yielding phenol as the only product, while with regard to oxygen the oxidation is hardly noticeable even at 500” C yielding carbon dioxide and no trace of phenol. There is a reverse order in the activity over Fe203. The rate of benzene oxidation by oxygen is higher than that of oxidation by nitrous oxide which is consistent with the data for isotopic oxygen exchange and nitrous oxide decomposition for this catalyst [ 91. Both oxidants over Fe20, produce only products of complete oxidation. These data therefore show the crucial influence of the chemi-
35 TABLE 2 Properties of Fe (AI) -0.08 and FecOs in the catalytic activation of nitrous oxide and oxygen. Comparison with the reaction of benzene oxidation by these molecules Sample
Fe(Al)-0.08
Atomic rate” of N,O decomposition and Ox isotope exchange [9]
Benzene conversion at oxidation by NcO and O2
W NnO
wo,
T
X NzO
X02
(molecule/Fe*s)
(molecule/Fe~+)
(“C)
(%)
(%)
4.4.10-x
Inactive ( <8.10-6)
300 500
11.5 36.5 -
0.0 o..o . 0.2
300
2.5
10.0
4.2.10-’
Fe&
.400
8.0*10-4
‘For Fe (Al)-0.08 the rate value referred to the total number of iron atoms in the sample, for Fe203 - to the number of surface iron atoms. TABLE 3 Benzene oxidation by nitrous oxide at 400” C over metal doped ZShl-5 zeolites Metal
Metal content (wt.-% of metal oxide)
Iron content (wt.-% Fe,O,)
x (%)
s (%)
V Cr Mn co co Ni Ti Zn
0.035 0.018 0.07 0.05 0.12 0.06 1.45 1.3
0.010 0.004 0.015 0.009 0.004 0.007 0.15 0.50
11.6 4.5 6.5 11.7 7.5 4.3 1.2 1.5
100 100 100 100 100 100 100 109
cal nature of the oxidants and the degree of their activation on the direction and rate of benzene oxidation. It is interesting to learn whether the reaction of benzene oxidation to phenol is possible over ZSM-5 zeolite doped with metals other than iron. We synthesized and tested a number of zeolite samples containing transition metals Ti, V, Cr, Mn, Co, Ni and Zn (Table 3). As with iron, metal salts were introduced at the hydrothermal procedure stage [ 13 1. The metal content in the zeolite was determined by the atomic absorption method. Comparing the data presented in Tables 1 and 3 one can see that phenol formation over these samples can be entirely accounted for by the presence of iron. Transition metals have either no effect on catalytic activity or even a negative one which becomes quite pronounced at high concentrations of the metals. Thus in the case of titanium and zinc even at 0.15 wt.-% Fe,O, and 0.5
36
wt.-% Fe203 these samples exhibit a very low activity. Special experiments with zinc indicate that this effect does not depend on the method by which the metal is introduced. In addition to introducing zinc during the hydrothermal procedure was also used an impregnation and a cation-exchange procedure to introduce zinc into the Fe-ZSM-5 zeolite. All the methods produced samples with approximately the same level of activity, being much lower than the FeZSM-5 activity without zinc. A similar effect of transition metals was reported by Suzuki et al. [ 51. They observed no phenol formation over Co2+ or Cu2+ cation-exchanged ZSM-5 zeolite instead of H-ZSM-5 zeolite. These data suggest that the catalytic activity of iron in Fe-ZSM-5 zeolite in the benzene oxidation reaction is an exceptional rather than a common property. One possible explanation can be referred to the specific form of surface oxygen generated on Fe-ZSM-5 under nitrous oxide decomposition. However, this assumption needs to be supported by convincing experimental evidence since according to generally accepted ideas the features of this oxygen atom especially its low bond energy and high reactivity are not consistent with a process of partial oxidation [ 141.
REFERENCES
8 9
10 11 12 13 14
M. Iwamoto, K. Matsukami and S. Kagawa, Jpn. Patent, 58-146 522 (1982). M. Iwamoto, K. Matsukami and S. Kagawa, J. Phys. Chem., 87 (1983) 903. Chem. Econ. Eng. Rev., 14 (1982) 47. Process Econ. Intern., 4 (1983) 48. E. Suzuki, K. Nakashiro and Y. Ono, Chem. Lett., (1988) 953. M. Gubelmann and P. Tirel, European Patent EP, 341 165 (1988). A.S. Kharitonov, T.N. Aleksandrova, L.A. Vostrikova, K.G. Zone and G.I. Panov, USSR Authorship Certificate, 4 445,636 (1988). V.I. Sobolev, G.I. Panov, A.S. Kharitonov, V.N. Romannikov, A.M. Volodin and K.G. Ione, J. Catal., submitted for publication. G.I. Panov, V.I. Sobolev and AS. Kharitonov, J. Mol. Catal., 61 (1990) 85. G.I. Panov, V.I. Sobolev and A.S. Kharitonov, in S. Yoshida, N. Takezawa and T. Ono (Editors), Catalytic Science and Technology, Vol. 1, Kodansha, Tokyo, 1991, pp. 171-176. V.I. Sobolev, O.N. Kovalenko, A.S. Kharitonov, Yu.D. Pankrat’ev and G.I. Panov, Mendeleev Commun., 1 (1991) 29. A.S. Kharitonov, A.I. Yartsev, E.A. Paukshtis, G.S. Litvak, E.N. Yurchenko and G.I. Panov, React. Kinet. Catal. Lett., 37 (1988) 7. L.S. Tchumachenko, V.N. Romannikov and K.G. Ione, USSR Authorship Certificate, 1192 225 (1984). G.K. Boreskov, Heterogeneous Catalysis, Nauka, Moscow, 1986, p. 197.