- Email: [email protected]
La2O3 Catalyst Surface and its Catalytic Behavior in the Oxidative Coupling of Methane
Guczi, L. ef ul. (Editors), New Frontiers in Cufalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary (6 1993 Elsevier Science Publishers B.V. All rights reserved
THE ACTIVE OXYGEN ON THE Li/La,O, CATALYST SURFACE AND ITS CATALYTIC BEHAVIOR IN THE OXIDATIVE COUPLING OF METHANE L. Wang, J. Wang, S. Yuan and Y Wu
Changchun Institute of Applied Chemistry,Chinese Academy of Sciences,Changchun 130022, China
Abstract
by adding Li to 02species was found on the The La203, compared with the single La203. Li/Laz03 but not on the single La203. In low-temperature desorption, ethane desorbed from the Li/La203 but was not detected with the single La203. It is considered that the addition of Li gave rise to some basic sites which are favorable for the coupling reaction.
The coupling selectivity was greatly enhanced
1. INTRODUCTION Since the early work of Keller and Bhasinl’ a considerable progress has been made in the oxidative coupling of methane to ethane and ethylene. Several selective catalysts are composed of basic oxides promoted by alkali metal ions such as lithium or sodium ions. Lunsford et a12’ reported the reaction mechanism and the nature of the active site which is responsible for the activation of methane over the Li/MgO and Na/CaO catalysts, and it is suggested that centers of the type [Li’O-I or [Na’O-1 are effective for the formation of methyl radical. In the present paper, comparison between the Li/LazO3 and single La203 is made on the basis of an activity test, by the ESR study, and the low-temperature desorption, followed by discussions about the active oxygen and the activation of methane on the catalyst surface. 2 . EXPERIMENTAL
The single La203 catalyst was prepared by immersing commercial La203 powder 0 9 9 . 9 9 % ) in distilled water, and heating while stirring to dry. The Li/La203 catalyst was prepared by immei.sing La203 with a solu-
2206
I--" A
C
Fig. 1 Apparatus for quenching and ESR measurement A-ESR tube B-liquid N;! trap C-sample holder D-rotatable valve E- thermocouple F-stopping valve G - spher i ca 1 er i nd H - manometer J - water I - oxygen K - heater L- to vacuum
Fig.2 The ESR signal of 02formed on the Li/La~03 (0.6wtX) surface
ation of LiOH 1120, and heating while stirring Lo dry, then calcinatins. An apparatus shown in Fie.1 was made for the detection of Lhe active oxygen by the quenching technique and ESR measurement. In the low-temperature desorption experiments, He was used as the carrier gas, and the desorbed products were also trapped at different temperature ranges for g a s chromatography analysis. The activity test results at 780'C over the Li/La203 catalysts with different Li-contents are listed in Table 1 , where CH4:02:N2=3:1:6 (in mole) and SV=8800 h-'.
3. RESULTS A N D DISCUSSION In comparison with the single La203, all the Li-doping catalysts gave rise to the C2-selectivity and the Cz-yield. The conversion of methane decreased with the Li-content, but the Cz-selectivi ty remained c0ns ta I1 t . Nolicing the decrease of catalyst surface area due to Li-doping, we
2207
consider that the reaction mechariism is in agrecinent with tlmt methyl radicals couple in g a ~ phtlsc:”. l’hc loword surfaca a r ~ i1 imi I s Lhc contact of the radicals onto the surface so tl~atthe i‘urlher oxidation is inhibited. Table 1 Activity of the Li-Laz03 catalysts with different Li-contents Li-Content wtX
Surface Area m2/g
Conv. of CH4
X
C2-Selec.
x
&-Yield ?:
In the Li/La203 catalyst calcined at 350‘C, six phases were found Li~C03, L i d , and LiLaOa. Although some of them disappeared in the sample calcined at 850.C. these transit ion phases wou Id probab 1y emerge again under the reaction con -dition and play as some basic sit,es favorable for the coupling. BY using the apparatus given in F i e . 1 , the sample of Li/La203 (O.GwtX) was treated in vacuum (L) at 800% for 8 h. The surface-cleat1 -ed sample was recalcined in situ at G50’C in an atmosphere of oxygen I ( I ) , then poured into 1 iquid oxygen below for quenching at 77 K. After recxhausting, the sample was transferred into the ESR tute(A) and examined by ESR. The signal thus ob -tained is shown in Fig.2, which is the typical 02- signal with g,= F i g . 3 Desorption profile of the 2.031, gyy=2.000, g,,=1.995. The La203 (02 pretreatment temperature: same procedure was repeated using 1--400’C ; Z--GOO*C ; 3--800’C, nitrogen instead of oxygen (I), but CH4 adsorption: 1.33 kpa, R.T.) no similar signal like 02- was by
XRD, i.e. La2O3, La(011)3, La(C03)011,
I
\
2208
x
observed, indicating that the 02species came from the o x m e n adsorb -ed on the catalyst surface. On the CHd(I6X) CH, (18:) other hand, no 0 2 - species was co ( 1 5 x 1 CO ( 7 6 % ) found in the same condition in the CzHs (9x1 CzHs(6X) case of single L a d s instead of I L i /La 203. 298K The activation of methane on the surface of La203 and Li/LazOs Fig.4 Desorption profile of the catalysts was also investigated by means of a low-temperature desorp Li/La203 ( 02 pretreatment: 13.3 kpa, 500%; CHn adsorption: -tion from 77 K to room tempera1.33 kpa, R.T. 1 ture. After the pretreatment of La203 catalyst in vacuum for 8 h and the preadsorption of oxygen for 2 h and exhausting gaseous oxygen, methane (1.33 kPa) was adsorbed on the catalyst which was cooled with 1 iquid nitrogen. The low-temperature desorption started when the 1 iquid nitrogen was moved away. There are 2-4 peaks in the profile on LazOs, as shown in Fig.3, suggesting that different active sites for methane activation exist on the catalyst surface. From the analysis of products collected at different desorption tem -perature ranges, as given in Fig. 3, it was noted that methane was activated on the La203 surface and converted into complete oxidation products only even at low-temperature. In the case of Li/LazO3 (O.Gwt%) catalyst., three desorption peaks occurred in the desorption profile, as shown in Fig.$. The desorption products responding to the former two peaks contained 9% of ethane in addition t o CO and methane, and that responding to the last big peak contained ethane (6%) too. It was evident that the Li-doping in the Li/La~03catalyst did promote oxidative coupling of methane and repress the complete oxidation.
4. REFERENCES 1) G. E.Ke 1 ler , M. M. Bhas in, J. Cata 1. ,73,9 (1 982) 2 ) J-X. Vang and J.H. Lunsford, J. Phys. Chem., 90(17), 3890(1986): ibid, 90(22) ,5883(1986) 3) T.Ito, J-X.IJang, C-H.Lin, J.H.Lunsford, J. Am. Chem. SOC., 107(18) ,5062(1985)
THE ACTIVE OXYGEN ON THE Li/La,O, CATALYST SURFACE AND ITS CATALYTIC BEHAVIOR IN THE OXIDATIVE COUPLING OF METHANE L. Wang, J. Wang, S. Yuan and Y Wu
Changchun Institute of Applied Chemistry,Chinese Academy of Sciences,Changchun 130022, China
Abstract
by adding Li to 02species was found on the The La203, compared with the single La203. Li/Laz03 but not on the single La203. In low-temperature desorption, ethane desorbed from the Li/La203 but was not detected with the single La203. It is considered that the addition of Li gave rise to some basic sites which are favorable for the coupling reaction.
The coupling selectivity was greatly enhanced
1. INTRODUCTION Since the early work of Keller and Bhasinl’ a considerable progress has been made in the oxidative coupling of methane to ethane and ethylene. Several selective catalysts are composed of basic oxides promoted by alkali metal ions such as lithium or sodium ions. Lunsford et a12’ reported the reaction mechanism and the nature of the active site which is responsible for the activation of methane over the Li/MgO and Na/CaO catalysts, and it is suggested that centers of the type [Li’O-I or [Na’O-1 are effective for the formation of methyl radical. In the present paper, comparison between the Li/LazO3 and single La203 is made on the basis of an activity test, by the ESR study, and the low-temperature desorption, followed by discussions about the active oxygen and the activation of methane on the catalyst surface. 2 . EXPERIMENTAL
The single La203 catalyst was prepared by immersing commercial La203 powder 0 9 9 . 9 9 % ) in distilled water, and heating while stirring to dry. The Li/La203 catalyst was prepared by immei.sing La203 with a solu-
2206
I--" A
C
Fig. 1 Apparatus for quenching and ESR measurement A-ESR tube B-liquid N;! trap C-sample holder D-rotatable valve E- thermocouple F-stopping valve G - spher i ca 1 er i nd H - manometer J - water I - oxygen K - heater L- to vacuum
Fig.2 The ESR signal of 02formed on the Li/La~03 (0.6wtX) surface
ation of LiOH 1120, and heating while stirring Lo dry, then calcinatins. An apparatus shown in Fie.1 was made for the detection of Lhe active oxygen by the quenching technique and ESR measurement. In the low-temperature desorption experiments, He was used as the carrier gas, and the desorbed products were also trapped at different temperature ranges for g a s chromatography analysis. The activity test results at 780'C over the Li/La203 catalysts with different Li-contents are listed in Table 1 , where CH4:02:N2=3:1:6 (in mole) and SV=8800 h-'.
3. RESULTS A N D DISCUSSION In comparison with the single La203, all the Li-doping catalysts gave rise to the C2-selectivity and the Cz-yield. The conversion of methane decreased with the Li-content, but the Cz-selectivi ty remained c0ns ta I1 t . Nolicing the decrease of catalyst surface area due to Li-doping, we
2207
consider that the reaction mechariism is in agrecinent with tlmt methyl radicals couple in g a ~ phtlsc:”. l’hc loword surfaca a r ~ i1 imi I s Lhc contact of the radicals onto the surface so tl~atthe i‘urlher oxidation is inhibited. Table 1 Activity of the Li-Laz03 catalysts with different Li-contents Li-Content wtX
Surface Area m2/g
Conv. of CH4
X
C2-Selec.
x
&-Yield ?:
In the Li/La203 catalyst calcined at 350‘C, six phases were found Li~C03, L i d , and LiLaOa. Although some of them disappeared in the sample calcined at 850.C. these transit ion phases wou Id probab 1y emerge again under the reaction con -dition and play as some basic sit,es favorable for the coupling. BY using the apparatus given in F i e . 1 , the sample of Li/La203 (O.GwtX) was treated in vacuum (L) at 800% for 8 h. The surface-cleat1 -ed sample was recalcined in situ at G50’C in an atmosphere of oxygen I ( I ) , then poured into 1 iquid oxygen below for quenching at 77 K. After recxhausting, the sample was transferred into the ESR tute(A) and examined by ESR. The signal thus ob -tained is shown in Fig.2, which is the typical 02- signal with g,= F i g . 3 Desorption profile of the 2.031, gyy=2.000, g,,=1.995. The La203 (02 pretreatment temperature: same procedure was repeated using 1--400’C ; Z--GOO*C ; 3--800’C, nitrogen instead of oxygen (I), but CH4 adsorption: 1.33 kpa, R.T.) no similar signal like 02- was by
XRD, i.e. La2O3, La(011)3, La(C03)011,
I
\
2208
x
observed, indicating that the 02species came from the o x m e n adsorb -ed on the catalyst surface. On the CHd(I6X) CH, (18:) other hand, no 0 2 - species was co ( 1 5 x 1 CO ( 7 6 % ) found in the same condition in the CzHs (9x1 CzHs(6X) case of single L a d s instead of I L i /La 203. 298K The activation of methane on the surface of La203 and Li/LazOs Fig.4 Desorption profile of the catalysts was also investigated by means of a low-temperature desorp Li/La203 ( 02 pretreatment: 13.3 kpa, 500%; CHn adsorption: -tion from 77 K to room tempera1.33 kpa, R.T. 1 ture. After the pretreatment of La203 catalyst in vacuum for 8 h and the preadsorption of oxygen for 2 h and exhausting gaseous oxygen, methane (1.33 kPa) was adsorbed on the catalyst which was cooled with 1 iquid nitrogen. The low-temperature desorption started when the 1 iquid nitrogen was moved away. There are 2-4 peaks in the profile on LazOs, as shown in Fig.3, suggesting that different active sites for methane activation exist on the catalyst surface. From the analysis of products collected at different desorption tem -perature ranges, as given in Fig. 3, it was noted that methane was activated on the La203 surface and converted into complete oxidation products only even at low-temperature. In the case of Li/LazO3 (O.Gwt%) catalyst., three desorption peaks occurred in the desorption profile, as shown in Fig.$. The desorption products responding to the former two peaks contained 9% of ethane in addition t o CO and methane, and that responding to the last big peak contained ethane (6%) too. It was evident that the Li-doping in the Li/La~03catalyst did promote oxidative coupling of methane and repress the complete oxidation.
4. REFERENCES 1) G. E.Ke 1 ler , M. M. Bhas in, J. Cata 1. ,73,9 (1 982) 2 ) J-X. Vang and J.H. Lunsford, J. Phys. Chem., 90(17), 3890(1986): ibid, 90(22) ,5883(1986) 3) T.Ito, J-X.IJang, C-H.Lin, J.H.Lunsford, J. Am. Chem. SOC., 107(18) ,5062(1985)