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Oxidative coupling of methane over oxide catalysts with layered structure
CatalysisToday,(1992)599-602 EleevierSciencePubIiehersB.V.,Am8terdam
599
OXIDATIVE COUPLING OF METHANE OVER OXIDE CATALYSTS WITH LAYERED STRUCTURE E.N.Voskresenskaya, L.I.Kurteeva, V.P.Zhereb and A.G.Anshits Institute of Chemistry of Natural Organic Materials, K.Marx str. 42, 660049 Krasnoyarsk, USSR
Abstract The catalytic properties of complex oxides with a layered structure BizGeOs, BizSiOs, Bi~Ti8012, BizCaSrCusOa+x and YBazCu306+8 in oxidative coupling of methane (OCM) have been studied. BizEOs metastable compounds have been found to possess the high activity and C&selectivity 53-70%. It has been assumed that the intergrowth boundaries for the BizEOs decomposition products, on which active catalytic sites may be located, play a specific role on the catalytic performance of the oxides. Bi6Ti3018 and BizCaSrCusOs+x have close catalytic activity values but BizCaSrCusOa+x as well as YBazCu806+6 are not selective in OCM reaction. INTRODUCTION Previously 111 layered Bi-containing oxyhalides have been suggested as promising catalysts for OCM. An application of oxyhalides impedes the process because very reactive hydrohalides appear as by-products. BizGeOs and BizSiOs compounds which belong to the Auriwillius phase family are similar in structure to the bismuth oxyhalides. The common structural peculiarity for these compounds is [BizOzl layers separated from each other by LEO41 tetrahedrons. Bi4Ti3012 as well as high-temperature superconductors (HTSC) BizCaSrCusOs+x and YBazCu306+6 also have a layered structure closed to the Auriwillius phase. Previously the catalytic behaviour of YBazCu806+6 has been studied in the process of partial methane oxidation [21 and tolu-' ene ammonolysis 131. The catalyst selectivity has been shown to depend on the d value (oxygen defectness degree). At 6 - 0 this catalyst is active and selective to form benzonitrile, at 6- >1 it is active in deep oxidation 131. The present work is devoted to the study of catalytic properties for complex oxides with a layered structure and to elucidate the effect of their structure peculiarities on catalytic properties in OCM reaction. EXPERIMENTAL For catalysts preparation the substances of high purity (99.99%) were used. Samples of BizEOs (E-Ge,Si) metastable compounds were prepared by spontaneous crystallization of the overcool melt having the Biz03.EO2 composition in platinum crucibles 141. Bi4Ti3012 , BiaCaSrCu308+x and YBazCu306+8 were synthesized using a conventional ceramic technology. The composition and structure of catalysts prepared were analysed by XRD, DTA, X-ray microanalysis and scanning electron microscopy (SEM) before the OCM reaction and after it. The catalytic experiment was carried out in a flow microcatalytic set-up with a quartz reactor.
m20-5861/92/$05.00 0 1992-ElsevierSciencePublishersB.V.AIlrightareserved.
600 RESULTS AND DISCUSSION metastable compounds in an Catalysts presenting BizGeOs and BizSiOs similar catalytic properties initial state have been shown to possess (Table 1). Their activity exceeds more than in an order of magnitude of that of bismuth oxide.According to XRD data (Fig.11 and DTA data catalysts are BizEOs thermodynamically metastable compounds with a layered structure in an initial state and after the OCM reaction these catalysts are the mixture of stable compounds with cubic structure namely BiizEO20 sillenite and Bi~E2012 eulitine. The same behaviour is characterized for BizEOs decomposition during heating in air [41. Relationships between the OCMproduct formation selectivity and methane conversion for BizGeOs and Bi4Ge2012 are presented in Figure 2. Schemes of the methane conversion over these catalysts are analogous (C2H6 and CO2 are primary products, C2H4 and CO are second products) as well as for the other Bi-containing catalysts. Some differences are ovserved only for product fothe catalytic activities of Bi4Ge2012, However, rmation selectivity. BiizGeOzo single crystals and mixture of these compounds are essentially’ lower than ones obtaned on the products of metastable phase decomposition (Table 11. By the methods of SEM (Fig.31 and X-ray microanalysis it has been shown that the Bi2GeOs sample after heating has converted into twophase system formed by alternating phases inriched and depleted by Bi and Ge. Phase boundaries are well developed (Fig.3b), and sillenite phase fills These results allow to suppose that acspace between eulitine particles. tive zones of the catalysts obtained by decomposition of metastable phases which are likely to be zones of non-coherent are phase boundaries parameters intergtowth, where a continuous joining of phase with different of defect ordering of lattice cells occurs. Probably, structural changes
Figure
1. X-ray
of Bi2EOs: b)Bi2Ge0s b
tial ter.
after
Bi2Si0s;
reaction.
diffraction
alinitial
patterns
Bi2GeOs;
OCM reaction; dlBi2SiOs
after
(Ctie-irradiation,
Conditions
for
OCM Ni fil;
OCM reaction:
T=1053K, CH4:02:He = 30:15:55 C
clini-
vol.%)
601
Table 1 Catalytic properties of oxide catalysts in OCM reaction (T=1053K,mcat =O.l0.3 g, flow rate - lo-20 ml/rain,CH, conversion< 3%). CIi4:0:He = 30:15:55 vol.%
Catalysts
B.E.T. area
S
m2/g
%
CH,:os = 85:15 vol.%
w.10-l7 molec.CH4/m%
CO2 c,
CO
CO2
0.5 33 6; 0.6 BizGeOs * co.1 47 53 28.3 BizSiOs * CO.1 45 55 33.6 Bi4TisOla 0.3 20 80 1.3 BizCaSrCusOa+x 0.3 64 36 7.2 YBazCu306+6 0.1 75 25 - 206.0 BiizGeOzo(I1 CO.1 7 78 15 0.4 BirCesOia(III 0.1 2 58 40 0.1 (I+IIl** 0.1 12 68 20 0.7 Bi203
c,
S
w110-17
%
molec.CH4/m%
co2 c,2
1.3" 32.2 41.9 6.3 4.0 69.3 5.1 2.7 4.4
31 30
co2
&a
69 70
1.0 2.2 72.2 169.6
47 53 72 28
7.9 8.9 275.7 119.3
* the initial compositionis given. ** mechanical mixture of (5 BirGesOiz+ BiizGeOzoI.
80 60
60
4-o
40
20 0
2
4
6
a
IO
I=WWE__qWvERSION.% ',
0
2
4
6,
a IO
MGIHANECONvERSION.$
Figure 2. Selectivityof OCM-productformation as a function of methane conversion over a) BizGeOs ; b1 BirGesOizCT=1053K, CH4:Oa:He= 30:15:55 vol.'/.).0 C2H6; 0 C2H4; A C02; l CO. type are presented. Zones of this kind are observed at the decompositionof metastable minerals 151. It-should be noted that BizEOscompoundsdo not contain alkali metals and have no highbasicity but they are active catalysts for OCM reaction. Bi~TisOiaslightly differs from BizO3 but it shows the higher C2 selectivity (to 8016).BizCaSrCusOs+x HTSC having the same structure insignificantly differs from Bi4TisOrs(Table 1) in the catalytic activity but has lower C2 product formation selectivity.
602
Figure 3. SEM photographsof BizGeOssingle crystal: a1 initial BizGeOs at 293K; b) the same after heating (lOK/minfrom 293K to 1235K in air.)
The high activity of methane conversionis manifested by the other HTSCYBaaCu306+b.C2 product formationactivity being close to that of the BiaEOs (Table 11, however, the selectivityis 25-29% which under the reaction conditions decreases to 10% after 10 hr. This can be stipulated by the decompositionof this compound: according to the XRD data after the reaction, except for the main phase, the presence of BaCuOa and CuO to be, active in deep oxidation has been detected. The catalyst pretreatment in CH4 flow to decrease 6 value (1 hr at 1053Kl accelerates the decomposition of this compound: the CO2 formation selectivityapproaches to 99% after 5 hr of reaction. CONCLUSIONS The catalysts in a metastablestate (BizEOs,YBazCus06+6) have been found to possess a high activity. The differencesin selectivityof these catalysts may be caused by the different character of decomposition processes as well as distinctionsin the chemical composition and morphology of the decompositionproducts. It has been assumed that active catalytic sites may be located on the intergrowthboundariesfor the BizEOs decomposition products. Bi4Ti3012and BizCaSrCusOe+x have close catalytic activity values, but HTSC BiaCaSrCusOs+x as well as YBaaCusO6+6are not selective in OCM reaction. ACKNOWLEDGMENT.We thank Mr. V.F.Karginfor assistance in SEM of catalyst samples.
examination
REFERENCES W.Ueda and J.M.Thomas.Proc. 9th Intern. Congress on Catalysis. Calgary,' Canada, 1988. V.2 (19881 960. L.Lee and K.Y.NG. Catal. Lett. 2 (19891 403. S.Hadsen, J.Otamiri.J.-O.Bovinand A.Andersen.Nature. 334 (19881 143. V.P.Zhereb. Ph.D.Thesis,Instituteof General and Nonorganic Chemistry, Moscow. 1980 (in Russ.). A.Patnis, J.D.C.McConnell.Principlesof Mineral Behavior, Blackwell Scientific Publications,Oxford-London-Edinburgh-Boston-Melbourne, 1980.
599
OXIDATIVE COUPLING OF METHANE OVER OXIDE CATALYSTS WITH LAYERED STRUCTURE E.N.Voskresenskaya, L.I.Kurteeva, V.P.Zhereb and A.G.Anshits Institute of Chemistry of Natural Organic Materials, K.Marx str. 42, 660049 Krasnoyarsk, USSR
Abstract The catalytic properties of complex oxides with a layered structure BizGeOs, BizSiOs, Bi~Ti8012, BizCaSrCusOa+x and YBazCu306+8 in oxidative coupling of methane (OCM) have been studied. BizEOs metastable compounds have been found to possess the high activity and C&selectivity 53-70%. It has been assumed that the intergrowth boundaries for the BizEOs decomposition products, on which active catalytic sites may be located, play a specific role on the catalytic performance of the oxides. Bi6Ti3018 and BizCaSrCusOs+x have close catalytic activity values but BizCaSrCusOa+x as well as YBazCu806+6 are not selective in OCM reaction. INTRODUCTION Previously 111 layered Bi-containing oxyhalides have been suggested as promising catalysts for OCM. An application of oxyhalides impedes the process because very reactive hydrohalides appear as by-products. BizGeOs and BizSiOs compounds which belong to the Auriwillius phase family are similar in structure to the bismuth oxyhalides. The common structural peculiarity for these compounds is [BizOzl layers separated from each other by LEO41 tetrahedrons. Bi4Ti3012 as well as high-temperature superconductors (HTSC) BizCaSrCusOs+x and YBazCu306+6 also have a layered structure closed to the Auriwillius phase. Previously the catalytic behaviour of YBazCu806+6 has been studied in the process of partial methane oxidation [21 and tolu-' ene ammonolysis 131. The catalyst selectivity has been shown to depend on the d value (oxygen defectness degree). At 6 - 0 this catalyst is active and selective to form benzonitrile, at 6- >1 it is active in deep oxidation 131. The present work is devoted to the study of catalytic properties for complex oxides with a layered structure and to elucidate the effect of their structure peculiarities on catalytic properties in OCM reaction. EXPERIMENTAL For catalysts preparation the substances of high purity (99.99%) were used. Samples of BizEOs (E-Ge,Si) metastable compounds were prepared by spontaneous crystallization of the overcool melt having the Biz03.EO2 composition in platinum crucibles 141. Bi4Ti3012 , BiaCaSrCu308+x and YBazCu306+8 were synthesized using a conventional ceramic technology. The composition and structure of catalysts prepared were analysed by XRD, DTA, X-ray microanalysis and scanning electron microscopy (SEM) before the OCM reaction and after it. The catalytic experiment was carried out in a flow microcatalytic set-up with a quartz reactor.
m20-5861/92/$05.00 0 1992-ElsevierSciencePublishersB.V.AIlrightareserved.
600 RESULTS AND DISCUSSION metastable compounds in an Catalysts presenting BizGeOs and BizSiOs similar catalytic properties initial state have been shown to possess (Table 1). Their activity exceeds more than in an order of magnitude of that of bismuth oxide.According to XRD data (Fig.11 and DTA data catalysts are BizEOs thermodynamically metastable compounds with a layered structure in an initial state and after the OCM reaction these catalysts are the mixture of stable compounds with cubic structure namely BiizEO20 sillenite and Bi~E2012 eulitine. The same behaviour is characterized for BizEOs decomposition during heating in air [41. Relationships between the OCMproduct formation selectivity and methane conversion for BizGeOs and Bi4Ge2012 are presented in Figure 2. Schemes of the methane conversion over these catalysts are analogous (C2H6 and CO2 are primary products, C2H4 and CO are second products) as well as for the other Bi-containing catalysts. Some differences are ovserved only for product fothe catalytic activities of Bi4Ge2012, However, rmation selectivity. BiizGeOzo single crystals and mixture of these compounds are essentially’ lower than ones obtaned on the products of metastable phase decomposition (Table 11. By the methods of SEM (Fig.31 and X-ray microanalysis it has been shown that the Bi2GeOs sample after heating has converted into twophase system formed by alternating phases inriched and depleted by Bi and Ge. Phase boundaries are well developed (Fig.3b), and sillenite phase fills These results allow to suppose that acspace between eulitine particles. tive zones of the catalysts obtained by decomposition of metastable phases which are likely to be zones of non-coherent are phase boundaries parameters intergtowth, where a continuous joining of phase with different of defect ordering of lattice cells occurs. Probably, structural changes
Figure
1. X-ray
of Bi2EOs: b)Bi2Ge0s b
tial ter.
after
Bi2Si0s;
reaction.
diffraction
alinitial
patterns
Bi2GeOs;
OCM reaction; dlBi2SiOs
after
(Ctie-irradiation,
Conditions
for
OCM Ni fil;
OCM reaction:
T=1053K, CH4:02:He = 30:15:55 C
clini-
vol.%)
601
Table 1 Catalytic properties of oxide catalysts in OCM reaction (T=1053K,mcat =O.l0.3 g, flow rate - lo-20 ml/rain,CH, conversion< 3%). CIi4:0:He = 30:15:55 vol.%
Catalysts
B.E.T. area
S
m2/g
%
CH,:os = 85:15 vol.%
w.10-l7 molec.CH4/m%
CO2 c,
CO
CO2
0.5 33 6; 0.6 BizGeOs * co.1 47 53 28.3 BizSiOs * CO.1 45 55 33.6 Bi4TisOla 0.3 20 80 1.3 BizCaSrCusOa+x 0.3 64 36 7.2 YBazCu306+6 0.1 75 25 - 206.0 BiizGeOzo(I1 CO.1 7 78 15 0.4 BirCesOia(III 0.1 2 58 40 0.1 (I+IIl** 0.1 12 68 20 0.7 Bi203
c,
S
w110-17
%
molec.CH4/m%
co2 c,2
1.3" 32.2 41.9 6.3 4.0 69.3 5.1 2.7 4.4
31 30
co2
&a
69 70
1.0 2.2 72.2 169.6
47 53 72 28
7.9 8.9 275.7 119.3
* the initial compositionis given. ** mechanical mixture of (5 BirGesOiz+ BiizGeOzoI.
80 60
60
4-o
40
20 0
2
4
6
a
IO
I=WWE__qWvERSION.% ',
0
2
4
6,
a IO
MGIHANECONvERSION.$
Figure 2. Selectivityof OCM-productformation as a function of methane conversion over a) BizGeOs ; b1 BirGesOizCT=1053K, CH4:Oa:He= 30:15:55 vol.'/.).0 C2H6; 0 C2H4; A C02; l CO. type are presented. Zones of this kind are observed at the decompositionof metastable minerals 151. It-should be noted that BizEOscompoundsdo not contain alkali metals and have no highbasicity but they are active catalysts for OCM reaction. Bi~TisOiaslightly differs from BizO3 but it shows the higher C2 selectivity (to 8016).BizCaSrCusOs+x HTSC having the same structure insignificantly differs from Bi4TisOrs(Table 1) in the catalytic activity but has lower C2 product formation selectivity.
602
Figure 3. SEM photographsof BizGeOssingle crystal: a1 initial BizGeOs at 293K; b) the same after heating (lOK/minfrom 293K to 1235K in air.)
The high activity of methane conversionis manifested by the other HTSCYBaaCu306+b.C2 product formationactivity being close to that of the BiaEOs (Table 11, however, the selectivityis 25-29% which under the reaction conditions decreases to 10% after 10 hr. This can be stipulated by the decompositionof this compound: according to the XRD data after the reaction, except for the main phase, the presence of BaCuOa and CuO to be, active in deep oxidation has been detected. The catalyst pretreatment in CH4 flow to decrease 6 value (1 hr at 1053Kl accelerates the decomposition of this compound: the CO2 formation selectivityapproaches to 99% after 5 hr of reaction. CONCLUSIONS The catalysts in a metastablestate (BizEOs,YBazCus06+6) have been found to possess a high activity. The differencesin selectivityof these catalysts may be caused by the different character of decomposition processes as well as distinctionsin the chemical composition and morphology of the decompositionproducts. It has been assumed that active catalytic sites may be located on the intergrowthboundariesfor the BizEOs decomposition products. Bi4Ti3012and BizCaSrCusOe+x have close catalytic activity values, but HTSC BiaCaSrCusOs+x as well as YBaaCusO6+6are not selective in OCM reaction. ACKNOWLEDGMENT.We thank Mr. V.F.Karginfor assistance in SEM of catalyst samples.
examination
REFERENCES W.Ueda and J.M.Thomas.Proc. 9th Intern. Congress on Catalysis. Calgary,' Canada, 1988. V.2 (19881 960. L.Lee and K.Y.NG. Catal. Lett. 2 (19891 403. S.Hadsen, J.Otamiri.J.-O.Bovinand A.Andersen.Nature. 334 (19881 143. V.P.Zhereb. Ph.D.Thesis,Instituteof General and Nonorganic Chemistry, Moscow. 1980 (in Russ.). A.Patnis, J.D.C.McConnell.Principlesof Mineral Behavior, Blackwell Scientific Publications,Oxford-London-Edinburgh-Boston-Melbourne, 1980.