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A Comparison Between Epoxidation and Degradation of Ethylene and Propylene over Silver
Guni, L el al. (Editors), New Fronrkrs in Catalysis
Proceedings of the 10th International Congrcsa on Catalysis, 19-24 July, 1992,Budapest, Hungary 8 1!393 Elsevier Science Publishers B.V. All rights rcserved
A COMPARISON BETWEEN EPOXIDATION AND DEGRADATION OF El'HYLENE AND PROPYLENE OVER SILVER C. Henrquesa, M.F. Porteld, C. Mauocchiab and E. GuglielminoniC aGrupo de Estudos de Catalise Heterogenea, Centro de Processes Quimicos (INIC), Dep. Enga Quimica, Instituto Superior Tecnico, Av. Rovisco Pais, 1096 Lisboa Codex; Portugal bDipartimentodi Chimica Industriale ed Ingegneria Chimica Giulio Natta, Politecnico di Milano, P.za L da Vinci 32, 20133 Milano, Italy CDipartimentodi Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universita di Torino, via P. Guria 7, 10125 Torino, Italy
Abstract Oxidation of ethylene and propylene by dioxygen has been studied over two different silver catalysts, wich exhibited markedly different selectivities for propylene epoxidation. A kinetic study shows that orders in olefin and oxygen do not change with the catalyst, for ethylene reactions, but they clearly change with the catalyst for propylene degradation. TPD of oxygen evidence that the amounts of oxygen desorbed from the two catalysts, although of the same types, are very different and infrared study shows that C02 formation proceeds via acetate and formate intermediates, with both olefins. 1. INTRODUCTION
Silver catalysts have very significant industrial application for the epoxidation of ethylene by dioxygen. However, epoxidation of propylene over the same catalysts, leads to very low selectivities. Experimental evidence indicates that epoxidation mechanims for ethylene and higher olefins should be similar [l-21. To shed light on such matter, a comparative study of two different unsupported silver catalysts with markedly differenr propylene epoxidation selectivities, was carried out with ethylene and propylene.
2. EXPERIMENTAL Two unsupported silver catalysts were prepared: catalyst A, by precipitation from a silver nitrate/ammonium hydroxyde aqueous solution, with formaldehyde 131 and caralysr B by gas phase reduction of Ag2O by a 10%olefin in nitrogen flow at 408K. TPD of oxygen was carried out in a GC-MS system. FTlR experiments were undertaken in a Pyrex IR cell and performed with a supported silver catalyst (18%wt. silver on y-alumina). This step proved to be necessary to obtain acceptable oprical rransparency ; a blank test with y-alumina was carried out.
1996
3.RESULTS AND DISCUSSION The reaction products observed in the catalytic tests were only ethylene oxide (EO), propylene oxide (PO), COP and H20. With both catalysts and olefins, the rates of C02 and epoxides pass through a maximum with increasing olefin pressure, at constant oxygen pressure; the maximum is observed for 2.5-3% of feed composition, for both oleflns. On the other hand, catalysts activity always increases with oxygen pressure. Tables 1 and 2 shows the apparent orders for the reactions with ethylene and propylene. The olefin orders change to negative, in the higher pressure range, evidencing a competition between reactants on silver surface. Table 1 Apparent reaction orders for the oxidation of ethylene Reaction QtalYwl C2H4 02 0.50 r(CO2) 0.21 *-0.27 0.33 r(E0) 0.48 * -0.1 5
CatalvrtR
C2H4 0.21 0 - 0 . 2 7 0.48 m -0.22
Table 2 Apparent reaction orders for the oxidation of propylene Reactlon CWYUA Catalvrta C3"6 02 C3H6 r(CO2) 0.21 --0.19 0.50 0.50 0 -0.13 rPO) 0.36 0 -0.1 7 0.44 0.36 m -0.1 5
02 0.50 0.33
02 1.1 0.44
Table 1 shows that the reaction orders are similar for ethylene with both catalysts. As shown in Table 2, C02 formation, from propylene, is the reaction that exhibits the major differences, with respect to the orders in 0 2 and olefin, with both catalysts. Conversely, for PO formation, the orders are similar for both catalysts. For ethylene, the global activity (measured as olefin reaction rate) of catalyst A is about 2-3 times higher than that of catalyst B. For propylene, it is considerably higher, depending markdly on the oxygen partial pressure (from about 25 times at low pressures to less than 10 times for the higher pressures). The acriviry for degradation , for catalyst A is quire similar for ethylene and propylene and for epoxidation is about 30 times higher for ethylene. For catalyst B the degradation activity is about 1 0 times higher with ethylene and, concerning epoxidation, about 30 to 75 times higher depending on oxygen partial pressure. With catalyst A, at 473K the mean values for PO selectivity, are 1-1.5%, and 6-15% with catalyst B. At the same temperature , the EO selectivity is 1030% with catalyst A and 20.50% with catalyst B. With this catalyst, the EO and PO selectivities decrease, when 02 pressure increases, towards the lower levels exhibited by caralysr A. The increase of EO and PO selectivities, observed with catalyst B, is due to a decrease of COP formation fare, as epoxidation fares are similar for both ceraiysrs and with borh olefins.
1997 Oxygen TPD tests were carried out with both catalysts. Figure 1 shows 0 2 TPD spectra obtained withy a mass spectrometer (mass 3 2 fragments). The TPD oxygen spectra evidences that, for both catalysts, the desorption processes occurs in the same temperature range 450-773 K: t w o main desorption processes were recorded, the first with a maximum at 538K and the second at 583K, but for catalyst B the global amount of desorbed oxygen is much smaller than with catalyst A. 2.0)
a
Tomprratura (K)
Figure 1. TPD of oxygen spectra for catalyst A and catalyst B Figure 2 shows FTlR spectra obtained with oxygen/olefin mixtures over AglyA1203 at different temperatures. and exposed to 1.2 KPa of oxygen and 0.96 KPa of olefin. The spectra were obtained by subtraction of a room temperature spectrum.
A
1
2000
iaoo
I
I
1
1600
1400
1200
c rn-’
0
I
1
I
1
1800
1000
1400
1200
’
c m-’
Figure 2. FTlR spectra of Agly-A1203 catalyst: (a) after reaction with C3H6/02 mixture,heated at 373K for 3 0 min (spectrum 11, at 478K for 45 min (spectrum 2) and at 543K for 45 min (spectrum 3); (b) after reaction with C2H4/02 mixture, heated at 423K for 90 min (spectrum 1 ) and at 473K for 60 min (spectrum 2).
They show that basically three groups of IR bands are present: (1) bands at 1650 cm-l, 1435 c m - l and 1230 c m -l , assigned to carbonates andlor bicarbonates, formed by C 0 2 14-51; (2) bands at 1 5 8 0 c m - l and 1378 crn-l; (3) bands at 1570 cm-l and 1455 c m - l. These t w o last groups could be assigned, respectively, to formate and acetate structures. In fact these types of structures has been detected as surface complexes in the propylene oxidation over different oxides 151. Bands assignable to the CO at = 1700 crn-l, found by other authors and assigned to acrolein 161, were not detected. With ethylene similar spectra was obtained: (11 bands at 1646 crn-l, 1425 cm-l and 1225 crn-l assignable to carbonates and bicarbonates; (2) bands at 1594 crn-l and 1376 crn-l and (3) bands at 1579 c m - l and 1461cm-l, assignable to acetates and formates. This evidence that acetate and forrnate species are involved in C 0 2 formation 12, 7-91; no bands assignable to EO were detected. 4. CONCLUSIONS The increase of the epoxides selectivity in, with catalyst 6, is due, not to an increase of the epoxide formation rate, but to a decrease of the C 0 2 formation rates, specially with propylene 1101. This selectivity is probably related to the different amounts of available oxygen, evidenced by TPD experiment; in fact, when the amount of surface oxygen is increased (by increasing 0 2 pressure in the feed mixture), selectivity drops markedly. FTIR experiments indicate that both olefins proceeds to degradation via formate and acetate intermediates. The presented results evidence that epoxidation depends not only on the olefin but simultaneously on the olefin and catalyst.
5. REFERENCES C. Mukoid, S. Hawker, J.P.S. Badyal, R.M. Lambert, Catal. Letters, 4 (1990) 5 7 R.A. van Santen and H.P.C.E. Kuipers, Adv. Catal., 3 5 (1987) 265 K.P. de Jong and J.W. Geus, Appl. Catalysis, 4 (1982) 41. N.D. Parkyns, J. Chem. SOC. (A), (1969) 410. A.A.Davidov, V.G.Mikhaltchenko, V.D.Sokolovskii and G.K.Boreskov, J.Catalysis, 55 (1978) 299. 6 I.L.C. Freriks, Robert Bowman and Peter V. Geenen, J. Catalysis, 65 (1980) 31 1. 7 A.G. Sault and R.J. Madix, J. Catal., 67 (19811 118 8 T. Kanno and M. Kobayashi, Proc. Int. Catal. Conf., 8th, Berlin 111 (1984) 277. 9 E.L. Force and A.T. Bell, J. Catal., 3 8 (1975) 440 E.L. Force and A.T. Bell, J. Catal., 40 (1975) 356 E.L. Force and A.T. Bell, J. Catal., 44 (1976) 175 1 0 M.F. Portela, C. Henriques, M.J. Pires, L. Ferreira and M. Baerns, Catalysis Today, 1 (1987) 101
1 2 3 4 5
Proceedings of the 10th International Congrcsa on Catalysis, 19-24 July, 1992,Budapest, Hungary 8 1!393 Elsevier Science Publishers B.V. All rights rcserved
A COMPARISON BETWEEN EPOXIDATION AND DEGRADATION OF El'HYLENE AND PROPYLENE OVER SILVER C. Henrquesa, M.F. Porteld, C. Mauocchiab and E. GuglielminoniC aGrupo de Estudos de Catalise Heterogenea, Centro de Processes Quimicos (INIC), Dep. Enga Quimica, Instituto Superior Tecnico, Av. Rovisco Pais, 1096 Lisboa Codex; Portugal bDipartimentodi Chimica Industriale ed Ingegneria Chimica Giulio Natta, Politecnico di Milano, P.za L da Vinci 32, 20133 Milano, Italy CDipartimentodi Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universita di Torino, via P. Guria 7, 10125 Torino, Italy
Abstract Oxidation of ethylene and propylene by dioxygen has been studied over two different silver catalysts, wich exhibited markedly different selectivities for propylene epoxidation. A kinetic study shows that orders in olefin and oxygen do not change with the catalyst, for ethylene reactions, but they clearly change with the catalyst for propylene degradation. TPD of oxygen evidence that the amounts of oxygen desorbed from the two catalysts, although of the same types, are very different and infrared study shows that C02 formation proceeds via acetate and formate intermediates, with both olefins. 1. INTRODUCTION
Silver catalysts have very significant industrial application for the epoxidation of ethylene by dioxygen. However, epoxidation of propylene over the same catalysts, leads to very low selectivities. Experimental evidence indicates that epoxidation mechanims for ethylene and higher olefins should be similar [l-21. To shed light on such matter, a comparative study of two different unsupported silver catalysts with markedly differenr propylene epoxidation selectivities, was carried out with ethylene and propylene.
2. EXPERIMENTAL Two unsupported silver catalysts were prepared: catalyst A, by precipitation from a silver nitrate/ammonium hydroxyde aqueous solution, with formaldehyde 131 and caralysr B by gas phase reduction of Ag2O by a 10%olefin in nitrogen flow at 408K. TPD of oxygen was carried out in a GC-MS system. FTlR experiments were undertaken in a Pyrex IR cell and performed with a supported silver catalyst (18%wt. silver on y-alumina). This step proved to be necessary to obtain acceptable oprical rransparency ; a blank test with y-alumina was carried out.
1996
3.RESULTS AND DISCUSSION The reaction products observed in the catalytic tests were only ethylene oxide (EO), propylene oxide (PO), COP and H20. With both catalysts and olefins, the rates of C02 and epoxides pass through a maximum with increasing olefin pressure, at constant oxygen pressure; the maximum is observed for 2.5-3% of feed composition, for both oleflns. On the other hand, catalysts activity always increases with oxygen pressure. Tables 1 and 2 shows the apparent orders for the reactions with ethylene and propylene. The olefin orders change to negative, in the higher pressure range, evidencing a competition between reactants on silver surface. Table 1 Apparent reaction orders for the oxidation of ethylene Reaction QtalYwl C2H4 02 0.50 r(CO2) 0.21 *-0.27 0.33 r(E0) 0.48 * -0.1 5
CatalvrtR
C2H4 0.21 0 - 0 . 2 7 0.48 m -0.22
Table 2 Apparent reaction orders for the oxidation of propylene Reactlon CWYUA Catalvrta C3"6 02 C3H6 r(CO2) 0.21 --0.19 0.50 0.50 0 -0.13 rPO) 0.36 0 -0.1 7 0.44 0.36 m -0.1 5
02 0.50 0.33
02 1.1 0.44
Table 1 shows that the reaction orders are similar for ethylene with both catalysts. As shown in Table 2, C02 formation, from propylene, is the reaction that exhibits the major differences, with respect to the orders in 0 2 and olefin, with both catalysts. Conversely, for PO formation, the orders are similar for both catalysts. For ethylene, the global activity (measured as olefin reaction rate) of catalyst A is about 2-3 times higher than that of catalyst B. For propylene, it is considerably higher, depending markdly on the oxygen partial pressure (from about 25 times at low pressures to less than 10 times for the higher pressures). The acriviry for degradation , for catalyst A is quire similar for ethylene and propylene and for epoxidation is about 30 times higher for ethylene. For catalyst B the degradation activity is about 1 0 times higher with ethylene and, concerning epoxidation, about 30 to 75 times higher depending on oxygen partial pressure. With catalyst A, at 473K the mean values for PO selectivity, are 1-1.5%, and 6-15% with catalyst B. At the same temperature , the EO selectivity is 1030% with catalyst A and 20.50% with catalyst B. With this catalyst, the EO and PO selectivities decrease, when 02 pressure increases, towards the lower levels exhibited by caralysr A. The increase of EO and PO selectivities, observed with catalyst B, is due to a decrease of COP formation fare, as epoxidation fares are similar for both ceraiysrs and with borh olefins.
1997 Oxygen TPD tests were carried out with both catalysts. Figure 1 shows 0 2 TPD spectra obtained withy a mass spectrometer (mass 3 2 fragments). The TPD oxygen spectra evidences that, for both catalysts, the desorption processes occurs in the same temperature range 450-773 K: t w o main desorption processes were recorded, the first with a maximum at 538K and the second at 583K, but for catalyst B the global amount of desorbed oxygen is much smaller than with catalyst A. 2.0)
a
Tomprratura (K)
Figure 1. TPD of oxygen spectra for catalyst A and catalyst B Figure 2 shows FTlR spectra obtained with oxygen/olefin mixtures over AglyA1203 at different temperatures. and exposed to 1.2 KPa of oxygen and 0.96 KPa of olefin. The spectra were obtained by subtraction of a room temperature spectrum.
A
1
2000
iaoo
I
I
1
1600
1400
1200
c rn-’
0
I
1
I
1
1800
1000
1400
1200
’
c m-’
Figure 2. FTlR spectra of Agly-A1203 catalyst: (a) after reaction with C3H6/02 mixture,heated at 373K for 3 0 min (spectrum 11, at 478K for 45 min (spectrum 2) and at 543K for 45 min (spectrum 3); (b) after reaction with C2H4/02 mixture, heated at 423K for 90 min (spectrum 1 ) and at 473K for 60 min (spectrum 2).
They show that basically three groups of IR bands are present: (1) bands at 1650 cm-l, 1435 c m - l and 1230 c m -l , assigned to carbonates andlor bicarbonates, formed by C 0 2 14-51; (2) bands at 1 5 8 0 c m - l and 1378 crn-l; (3) bands at 1570 cm-l and 1455 c m - l. These t w o last groups could be assigned, respectively, to formate and acetate structures. In fact these types of structures has been detected as surface complexes in the propylene oxidation over different oxides 151. Bands assignable to the CO at = 1700 crn-l, found by other authors and assigned to acrolein 161, were not detected. With ethylene similar spectra was obtained: (11 bands at 1646 crn-l, 1425 cm-l and 1225 crn-l assignable to carbonates and bicarbonates; (2) bands at 1594 crn-l and 1376 crn-l and (3) bands at 1579 c m - l and 1461cm-l, assignable to acetates and formates. This evidence that acetate and forrnate species are involved in C 0 2 formation 12, 7-91; no bands assignable to EO were detected. 4. CONCLUSIONS The increase of the epoxides selectivity in, with catalyst 6, is due, not to an increase of the epoxide formation rate, but to a decrease of the C 0 2 formation rates, specially with propylene 1101. This selectivity is probably related to the different amounts of available oxygen, evidenced by TPD experiment; in fact, when the amount of surface oxygen is increased (by increasing 0 2 pressure in the feed mixture), selectivity drops markedly. FTIR experiments indicate that both olefins proceeds to degradation via formate and acetate intermediates. The presented results evidence that epoxidation depends not only on the olefin but simultaneously on the olefin and catalyst.
5. REFERENCES C. Mukoid, S. Hawker, J.P.S. Badyal, R.M. Lambert, Catal. Letters, 4 (1990) 5 7 R.A. van Santen and H.P.C.E. Kuipers, Adv. Catal., 3 5 (1987) 265 K.P. de Jong and J.W. Geus, Appl. Catalysis, 4 (1982) 41. N.D. Parkyns, J. Chem. SOC. (A), (1969) 410. A.A.Davidov, V.G.Mikhaltchenko, V.D.Sokolovskii and G.K.Boreskov, J.Catalysis, 55 (1978) 299. 6 I.L.C. Freriks, Robert Bowman and Peter V. Geenen, J. Catalysis, 65 (1980) 31 1. 7 A.G. Sault and R.J. Madix, J. Catal., 67 (19811 118 8 T. Kanno and M. Kobayashi, Proc. Int. Catal. Conf., 8th, Berlin 111 (1984) 277. 9 E.L. Force and A.T. Bell, J. Catal., 3 8 (1975) 440 E.L. Force and A.T. Bell, J. Catal., 40 (1975) 356 E.L. Force and A.T. Bell, J. Catal., 44 (1976) 175 1 0 M.F. Portela, C. Henriques, M.J. Pires, L. Ferreira and M. Baerns, Catalysis Today, 1 (1987) 101
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