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Oxidative coupling of methane over sodium-salt-promoted zirconia catalysts prepared by the mixed solution method
ELSEVIER
Applied Catalysis B: Environmental
7 ( 1996) 237-250
Oxidative coupling of methane over sodium-salt-promoted zirconia catalysts prepared by the mixed solution method Ki June Yoon *, Sang Won Seo Department of Chemical Engineering, Sung Kyun Kwan University. Suwon 440-746, South Korea Received 22 March 1995; revised 26 July 1995; accepted 28 July 1995
Abstract Sodium-salt-promoted zirconia catalysts prepared from mixed solutions of zirconyl chloride and sodium salts (chloride, carbonate, nitrate or sulfate) are found to exhibit effective catalytic performances in the oxidative coupling of methane. The highest C 2+ yield ( 16.6%) and ethylene-to-ethane ratio are obtained over the Na,CO,-added catalyst having an appropriate Na/Zr ratio; however, the active promoting species in this catalyst are actually identified to be Na+ and Cl _. The mixed solution method is considered to be an effective preparation method when proper precursor compounds of zirconia and promoters are chosen, giving a desirable interaction between them. The reaction and characterization results indicate that the chlorine ion plays a more prominent role than the sodium ion, and the X-ray photoelectron spectroscopy results show the presence of multiple chlorine species on the surface. It is suggested that the Cl- bound to Na+ is capable of activating both methane and ethane whereas the Cl- bound to Zr“’ IS only capable of activating ethane. The sodium ion is considered to play an important role in stabilizing the chlorine ion. Keywords: Zirconia
Chlorine
species;
Methane;
Mixed solution
method;
Oxidative
coupling;
Sodium-salt-promoted:
1. Introduction Following the pioneering work of Keller and Bhasin [ 11, the oxidative coupling of methane (OCM) has received a great deal of attention as a promising route for direct conversion of methane to C2 hydrocarbons. Most of the research on this reaction has been directed to developing suitable catalysts which give high yields of higher hydrocarbons, and numerous kinds of catalysts have been tested for their * Corresponding
author. Tel.
( + 82-33 1) 2905424, fax. ( + 82-33 I ) 2905616.
0926.3373/96/$15,00 0 1996 Elsevier Science B.V. All rights reserved SSDlO926-3373(95)00050-X
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oxidative coupling activity [ 2,3]. The oxides of transition metals are generally more effective for the nonselective oxidation to the carbon oxides than for the selective oxidation to C2 hydrocarbons. However, when promoted with alkali metal compounds, some transition metals, notably ZnO and MnO,, exhibit a significant effect in the OCM process [ 4-61. ZrO, has attracted relatively little attention and a few results available on ZrO,, obtained by using it either as a support or as a component of mixed oxides, showed low performance [ 7-101. Recently, however, it has been shown that alkali-metalcompound-promoted ZrO, catalysts exhibit noticeable C,, selectivities [ 1 l-131. The performance of these catalysts depends strongly on the nature of alkali salts (the cations and the anions) and the method of preparation [ 131. Of the cations and the anions in the promoter tested, Na + is far more effective than the other alkali metal ions and Cl- renders a higher ethylene-to-ethane ratio than the carbonate, nitrate or acetate ion. Of the methods used for the preparation of NafZrO,-Cl _ , the sol-gel method is more effective than the other preparation methods. The relatively high performance of Na+-ZrO*-Cl_ prepared by the sol-gel method may be attributed to a better incorporation of Na+Cl - into the ZrO, matrix [ 12,131. However, the effects of the nature of the alkali salts and the method of preparation are not fully understood. These earlier works have prompted us to further investigate sodium-salt-promoted ZrO,. The effects of the promoters may be different, depending on their chemical properties and thermal stabilities as well as on the method of preparation. In this study, four sodium salts (NaCl, NaNO,, Na,CO,, and Na,SO,) were used as the promoters or the promoter precursors. The catalysts were prepared by a method different from that of the earlier works in which the catalysts were usually prepared using Zr02 powder [ 12,131 or precipitated Zr( OH), [ 111 as a starting material. In the present work, a mixed aqueous solution was first prepared by adding a solution of each sodium salt to a solution of zirconyl chloride, and this was followed by drying, calcination and crushing. We call this the mixed solution method. The effects of the nature of the promoter (or the precursor) compound and the physical properties of the systems on the catalyst performance were examined. In addition, the effects of the promoter content and the reaction conditions (temperature, CH,/O, ratio, etc.) are presented. Furthermore, the roles of sodium and chlorine and the nature of active surface species are discussed.
2. Experimental 2.1. Catalyst preparation Zirconyl chloride ( ZROCL2. 8H20, Chemical Pure Grade) was purchased from Junsel Chemical. NaCl (99.5%)) NaNO, (99%)) Na2C03. 10H20 (99 + %) and Na2S04 (99%) were purchased from Shiny0 Pure Chemical. An aqueous solution
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of a sodium salt was added dropwise under stirring to an aqueous solution of zirconyl chloride having a pH of 0.7. The water was evaporated from the mixture by heating under vigorous stirring until a thick paste was formed. This was followed by drying at 393 K, calcination in air at 673 K for 2 h and at 1073 K for 6 h, and then crushing to 40-80 mesh particle sizes. The Na/Zr atomic ratio in the mixture was typically 0.6 or 1.2. No attempt was made to analyze the elemental composition of the prepared catalysts. The mixture always formed a homogeneous solution except for the Na,CO,added solution. When the solution of sodium carbonate, having a pH of 11.5, was added, the mixture stayed homogeneous until the Na/Zr ratio exceeded 1.2. Above this, precipitation started to occur increasingly with the Na/Zr ratio. Even though the precipitate was present, the same procedure of preparation as above was followed. While the solution of sodium carbonate was added evolution of CO, was observed. 2.2. Activity testing and product analysis The oxidative coupling reactions were performed at 923-l 123 K under atmospheric pressure by cofeeding the reactant gases (methane, air and nitrogen as a diluent) into a l/2-inch stainless steel tube reactor mounted vertically and heated by an electric furnace. Typically, the reaction temperature was 1023 K, PCH4 16.5 kPa, P,, 8.3 kPa (CH,/O, = 2)) the total gas flow lOOcm”/min (NTP), andcatalyst loading 2.0 g, giving rise to a space velocity 3000 cm3 g-r hh ‘. The product effluents were analyzed by two on-line TCD-equipped gas chromatographs, one fitted with a Chromosorb 102 column to analyze carbon dioxide, methane and higher hydrocarbons and the other with a Carbosieve S-II (Supelco) column to analyze oxygen, carbon monoxide, carbon dioxide and methane. 2.3. Catalyst characterization The specific surface areas of the catalysts were measured by the BET method using a Micrometrics 2000. X-ray diffraction (XRD) measurements were taken with a Rigaku Rotaflex D/Max-C diffractometer using Ni-filtered Cu Ko radiation. For the X-ray photoelectron spectroscopy (XPS) study of some samples, binding energies of photoelectron lines, Zr 3d, Na 1s, Cl 2p and 0 1s were obtained by a VG Escalab Mark II photoelectron spectrometer using Al Ka radiation with the reference of C 1s at 284.6 eV, and the surface compositions were calculated by the integration of each peak. The samples for the XPS analysis, either fresh or used, were treated at 673 K for 30 min in airjust before introduction into the spectrometer. Energy dispersive X-ray spectroscopy (EDS) analysis was also performed with a TN-5500 (Tracer Northern) spectrometer.
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3. Results 3.1. Catalytic peeormance The results of the reaction performed over the unpromoted and promoted catalysts are presented in Table 1. The unpromoted ZrOz had a high activity for methane, but almost all of the methane was nonselectively converted to carbon oxides. When ZrO, was promoted with the sodium salts noticeable increases in the C2 + selectivity were observed, but the methane conversion increased or decreased depending on the promoter. The Na,CO,- and NaNO,-added catalysts exhibited a methane conversion comparable to the unpromoted catalysts, but the NaCl-added catalyst gave a higher methane conversion while the Na,SO,-added catalyst showed a considerably lower methane conversion than the unpromoted catalyst. The C2 + selectivity obtained over the NaN03-added catalyst was considerably lower than that obtained over the other promoted catalysts. The highest Cz + selectivity (63.9%)) C2 + yield ( 16.6%) and ethylene-to-ethane ratio ( 1S2) were obtained over the Na,CO,-added catalyst catalyst. This C,, y ield is higher than that reported for the Na+-ZrO,-Clprepared by the sol-gel method [ 12,131. As Na,CO,- and NaCl-added catalysts were found to be effective in the OCM, the stability of these two catalysts with time on-stream was examined. The changes in the methane conversion and the C,, selectivity are presented in Fig. 1. For the Na,CO,-added catalyst the methane conversion and the C2 + selectivity were maintained nearly constant (ca. 26% and 60%, respectively) for 10 h. The C, selectivity remained constant at around 3%, and about two times more propene was produced than propane. For the NaCl-added catalyst, however, a slow decrease in the methane conversion and a more rapid decrease in the C,, selectivity were observed. The ethylene-to-ethane ratio decreased with time on-stream for both catalysts. Fig. 2 shows the effect of the promoter content on the OCM over the Na,CO,and NaCl-added catalysts. For the NaCl-added catalyst the CZ+ selectivity was little affected with the promoter content. For the Na,CO,-added catalyst, however, Table 1 Oxidative coupling of methane over sodium-salt-added Added salt (Na/Zr ratio)
Surface area (m*/g)
none
Na,CO,( 1.2) NaCl (1.2) NaCl (0.6) NaNO, (0.6) Na,SO,( 1.2)
1.1 4.1 3.0 1.0
CH, con”.
ZrQ prepared by the mixed solution method
Selectivity
C,,
(Yo)
(%)
24.6 26.0 33.5 30.4 24.4 5.5
C,He
C,H,
C,
Co,,,=,.21
2.1 24.0 17.2 22.3 6.8 29.2
1.0 36.4 22.4 18.2 3.1 6.6
3.5 1.2 1.6 -
96.9 36.1 59.2 57.9 90.1 64.2
Conditions: temperature = 1023 K, PcHl = 16.5 kPa, CH,/O,=2, results presented are after 70 min of reaction.
yield
(%)
C,HJC,H, ratio
0.8 16.6 13.6 12.9 2.4 2.0
0.48 1.52 1.30 0.82 0.46 0.23
space velocity=3000cm7
g-’ h-’ (NPT): the
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241
20
oi 0
’ 2
’ 4 Time on
’ 6
’ 8
10
- stream , h
Fig. 1. Changes in methane conversion and C2 + selectivity with time on-stream over Na,C03- and NaCl-added ZrO, (Na/Zr = 1.2) The conditions are as in Table 1 except the time on-stream. (0) C, + selectivity for Na,CO,ZrO,, (A) C,, selectivity for NaCl-ZrOz (The numbers in parentheses represent the ethylene-to-ethane ratios.), (0) methane conversion for NazCO,-ZrOz, ( A ) methane conversion for NaCI-ZrO,.
a volcano-type pattern was observed in the Ca+ selectivity with the promoter content, the maximum being at 1.2 of the added Na/Zr ratio. The maximum ethylene-to-ethane ratio was also obtained at the same Na/Zr ratio. Since the NazC03-added catalyst prepared with the Na/Zr ratio of 1.2 was found to be the most effective catalyst, further investigation on the effects of the reaction temperature and the CH4/02 ratio was performed on this catalyst. Fig. 3 shows the effect of the reaction temperature. With the increase in temperature the methane conversion increased and reached a plateau above 1050 K. This plateau is considered to come from the fact that the oxygen was almost completely exhausted above this temperature. However, the C, + selectivity passed through a maximum at 1023 K. This may be explained by the suggestion of Otsuka et al. [ 141 that the deep oxidation of ethane and ethylene, which are the primary products in the OCM
0'
0
I
1
I
I
I
1
2
3
4
5
Promoter Content,
Na/Zr atomic ratio
Fig. 2. Effect of promoter content on methane conversion and Cz+ selectivity for Na&O,The conditions are as in Table 1 and the symbols are as in Fig. 1.
and NaCI-added
ZrO,.
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20
900
1000
950
1050
Temperature
1100
1150
K
Fig. 3. Effect of reaction temperature on OCM over Na,CO,-added 210~ (Na/Zr= Table 1 except the reaction temperature and the symbols are as in Fig. 1.
1.2). The conditions are as in
process, becomes dominant at high temperatures. Fig. 4 shows the effect of the CH4/02 feed ratio on the methane conversion and the C,, selectivity over the Na,CO,-added catalyst at 1023 K while holding the partial pressure of oxygen at 8.3 kPa. The methane conversion tended to decrease with the CH4/OZ ratio, but the C2+ selectivity had a reverse trend: this is a commonly observed behavior in earlier works [3,13]. At the CH,/O, ratio of 7 the C,, selectivity was 69.3% and this is the highest value obtained in this study. However, the C,, yield at this CHJ O2 ratio was only 9.1%, and this is well below the maximum Cz+ yield ( 16.6%) obtained in this study at the CH,/O, ratio of 2. 3.2. XRD analysis The crystalline phases in the unpromoted and promoted catalysts are presented in Table 2. The ZrO, in all the catalyst samples was mostly the monoclinic phase
0’ 0
1
’
’
2
3 CH,/O,
’ 4
5
’ 6
A
7
8
feed ratio
Fig. 4. Effect of CH,/O, feed ratio on OCM over Na,CO,-added ZrO, (Na/Zr= Table 1 except the partial pressure of CH, and the symbols are as in Fig. I.
1.2). The conditions
are as in
K.J. Yom, S.W. Seo /Applied
Table 2 Crystalline
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phases identified by XRD
Added salt
Status
Major phase
Minor phase
none
Fresh Used Fresh Used Fresh Fresh Fresh
m-ZrO; m-ZrOz m-ZrO,, m-ZrOz, m-ZrO,, m-ZrOz m-ZrO,,
NaCl NaClb NaCl
t-z&I; t-zr0: t-ZrO, _ _
Na,SO,
t-Z& t-ZrO,
Na,CO, NaCl NaNO, Na*SO,
NaCl
a m-ZrOz = monoclinic ZrO, phase (Baddeleyite), t-ZrOz = tetragonal ZrOL phase. h The intensity was weaker than that in the fresh sample.
(Baddeleyite) . A small fraction of the Zr02 was present as the tetragonal phase, but this phase was gradually transformed under the reaction conditions to the monoclinic phase, in agreement with previous findings [ 15-l 81. In the NaCl- and Na,SO,-added samples the presence of the corresponding sodium salt was identified. In the NazC03-added sample, however, a large amount of NaCl was found to be present. This NaCl is considered to be formed by the reaction of Na,C03 with zirconyl chloride during the preparation process, and virtually no Na2C03 seems to be present in the bulk of the sample. A significant loss of the NaCl in the Na,CO,added sample (Na/Zr= 1.2) was observed after the reaction at 1023 K for 10 h, as indicated by the decrease of the peak intensity ratio of NaCl (220) /m-ZrO, ( 111) from 1.35 in the fresh sample to 0.45 in the used sample. In the NaN03added sample a small amount of NaCl existed, but no lines of other sodium compounds were observed. As NaN03 is unstable under the calcination or reaction conditions most of the NaNOx in the sample may have been decomposed to the oxide, but it could not be identified by XRD, perhaps due to being present as small crystallites or amorphous phases. 3.3. XPS analysis Binding energies of Zr 3d5,*, Na Is, Cl 2p and 0 1s levels on the surfaces of the NaCl- and Na,CO,-added catalysts are shown in Table 3. The binding energies of the Zr 3dS12 and Na 1s are in agreement with the reported values [ 13,191. The 0 1s spectra indicate that at least two kinds of oxygen species are present on the surfaces. The lower binding energy of 0 1s (529.6-529.8 eV) is ascribed to the lattice oxygen O* ~ in ZrO, and the higher value (53 1.4-532.0 eV) to the carbonatetype oxygen [ 13,20-231. A weak peak appeared at 536-537 eV: however, this peak is considered to be an Auger line of Na KL1L23 rather than to arise from another kind of oxygen species [ 19,201, and no further consideration of this peak was given. The Cl 2p spectra are shown in Fig. 5. For the NaCl-added catalyst the peak was rather broad (Fig. 5a) and hence it is considered that at least two different lines are
244
Table 3 Binding-energy
K.J. Yoon, S. W. Seo /Applied
results for NaCl- and Na,CO,-added
Catalyst (added Na/Zr ratio)
NaCl-added ZG (1.2) Na,CO,-added ZQ (1.2)
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Status
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ZrO, catalysts Binding energy (eV) Zr 3d,/,
Na 1s
Cl 2p (%)
0 1s (%)
Fresh
181.7
1071.7
198.6 (100)
Used
181.5
1071.1
198.5 (100)
FresH
181.8
1072.0
Used
181.2
1071.4
198.5 199.6 198.2 199.8
529.7 531.4 529.6 531.5 529.8 532.0 529.7 531.7
(47) (53) (59) (41)
(86) (14) (84) (16) (89) (11) (91) (9)
superimposed. The broad peak was deconvoluted by assuming that two symmetric peaks were superimposed, giving one with a binding energy of about 198.5 eV and the other with about 195.5 eV. The former is ascribed to the Cl- bound to Naf [ 19,241. Sarma and Rao [ 251 have observed a weak peak from ZrO, at about 14 eV above the Zr 3d,,, line, i.e. at around 196 eV, and suggested that this peak originates from a plasmon excitation process. Therefore, the peak at around 195.5 eV in Fig. 5 is assigned to the plasmon excitation line associated with the Zr 3d
206
204
200
196
Binding Energy
192
166
, eV
Fig. 5. Cl 2p and Zr 3d plasmon excitation lines in NaCI- and Na,CO,-added catalysts (Na/Zr ‘= 1.2). (a) fresh NaCl-added catalyst, (b) fresh NazCO,-added catalyst, (c) used Na&O,-added catalyst.
K. J. Yom. S. W. Sea /Applied
Table 4 Surface composition Catalyst
of the elements in the NaCI- and NazCOz-added
(added Na/Zr ratio)
NaCl-added
ZrOz
(1.2) NazCO1-added (1.2)
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ZrO,
Status
Fresh Used Fresh Used
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7 (I 996) 237-250
ZrOz catalyst determined
by XPS
Surface atomic ratio
Comp. (%) Zr
Cl
O/Zr
NalZr
CllZr
CI/Na
19.8 17.9 14.8 18.3
5.9 5.5 8.9 8.6
2.25 2.40 2.34 2.11
0.18 0.35 0.34 0.47
0.30 0.3 I 0.60 0.47
0.89 1.76 1.00
I.67
line. The spectrum of the used NaCl-added catalyst in the Cl 2p region was almost identical with that of the fresh catalyst. The Zr 3d plasmon excitation line was also observed in the Na,CO,-added samples. The peak intensity ratios of the plasmon excitation line to the parent Zr 3d line (Zr 3d5,2 + 3d3,2) in the four samples were fairly constant and the average ratio was estimated to be 0.09. The relative concentration of surface chlorine was corrected from the peak intensity by excluding the Zr 3d plasmon excitation line. For the Na,CO,-added samples a second chlorine species which had a binding energy of ca. 199.6 eV appeared (Fig. 5b and 5~). This chlorine species is ascribed to the Cl- bound to Zr4’, since the Cl 2p binding energy in the transition metal chloride has been found to be higher than the value for the alkali metal chloride and to become higher as the electron density of the metal cation decreases [ 26,271. The concentration of this second species in the fresh sample was slightly higher than that of the Cl- bound to Nat, but in the used sample it decreased to about 2/3 of that of the Cl- bound to Naf . The surface compositions of the elements in the NaCl- and Na,CO,-added catalysts are presented in Table 4. Both of the Na/Zr and Cl/Zr ratios in the Na,CO,added catalyst were higher than those in the NaCl-added catalyst. In both of the fresh samples the surface concentration of chlorine was about 1.7 times higher than that of sodium: after the reaction the surface concentration of sodium increased noticeably whereas that of chlorine decreased to some extent, giving Cl/Na ratios near unity. 3.4. EDS analysis As EDS has a probing depth of a few micrometers, the analysis results in Table 5 provide the composition of the elements having a value between the surface and bulk. The Na and Cl concentrations in the Na&O,-added catalyst were 4-5 times higher than those in the NaCl-added catalyst. For both of the NaCl- and Na,CO,added catalysts the Cl/Na ratios were somewhat greater than 1, indicating that most of the sodium and the chlorine exist as the NaCl salt. After the reaction the Na/Zr and Cl/Zr ratios increased but the Cl/Na ratios decreased. This indicates that the NaCl diffuses out from the bulk to the surface during the reaction and that the chlorine disappears at a faster rate than the sodium from the surface. In contrast,
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Table 5 EDS results for the sodium-salt-added Catalyst
(added Na/Zr ratio)
NaCl-added (1.2) Na,CO,-added
ZrO, ZrO,
( 1.2) NaNO,-added
ZrOZ
(1.2) Na$O,-added
ZrO,
(1.2)
Catalysis B: Environmentcrl 7 (1996) 237-250
ZrO, catalysts Status
Fresh Used Fresh Used Fresh Used Fresh Used
Atomic ratio Na/Zr
CI/Zr
S/Zr
CI/Na
0.14 0.3 1 0.73 1.53 2.52 0.76 4.38
0.19 0.34 0.88 I.65 0.97 0.23 0.89 0.15
_
1.34 1.1 I
_
1.20 I .08 0.38 0.32 0.20 0.08
1.85
_ _ _ 0.18 0.31
the Cl/Na ratios in the NaNO,- and Na,SO,-added catalysts were considerably lower than 1. The sodium concentrations in the fresh samples of these two catalysts were much higher compared to those in the other two fresh samples, but they decreased rapidly during the reaction. The chlorine concentrations in the fresh NaNO,- and Na,SO,-added catalysts were relatively high, but they decreased more rapidly than the sodium concentrations.
4. Discussion 4.1. Effect of the nature of sodium salt The most effective catalyst system in this work was the Na,CO,-added catalyst having a proper Na/Zr ratio, but the prepared sample contained a large amount of NaCl and virtually no Na,CO,. Therefore it is apparent that the best promoting species are Nat and Cl-. The NaCl-added catalyst, however, exhibited a lower catalytic performance than the Na,C03-added catalyst, even though the chemical components present in both of the prepared samples appear identical. The difference in the catalytic performance might be due to different interactions between ZrO, and NaCl which arise from different structural changes (reaction in solution, nucleation, decomposition, etc.) during the preparation process. Na2C03 is a basic salt and CO2 evolution was observed while preparing the mixed solution of Na,CO, and zirconyl chloride. Hence, after drying during the preparation process, it is certain that the composition of the NaCl-added sample is different from that of the Na,C03-added sample in which additional zirconium compounds formed by the reaction of Na,CO, with zirconyl chloride are present. We speculate that during the subsequent calcination the zirconium compounds are decomposed to Zr02, accompanied by ZrO, interacting with the NaCl but to a different extent depending on the nature of the zirconium compounds present prior to the decomposition. The interaction appears to be greater in the Na,CO,-added catalyst than in the NaCl-
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added one. The reaction, XPS and EDS results, which show a better catalytic performance, presence of a second chlorine species, and higher concentrations of sodium and chlorine for the Na,CO,-added catalyst, are consistent with this suggestion. The results in this study also suggest that the intimate interaction has an effect to enhance the stability of the chlorine. This kind of interaction may be considered to be the intimate mixing between Na+Cll and ZrO, or the incorporation of Na+Cl- into the ZrO, matrix, as suggested in other studies which have shown good and stable performances of Nat-ZrO,-Clcatalysts [ 12,131. The effectiveness of the mixed solution method appears to depend on the nature of the precursors of ZrO,. The NaCl-added catalyst prepared in this work exhibits a good catalytic performance which is comparable to the performance of the Na+ZrO,-Cll catalyst prepared by the sol-gel method in Refs. [ 121 and [ 131. A common feature in both of the preparation methods is that the precursors of ZrO, undergo drastic chemical changes (i.e. decomposition) during the preparation processes, and this might result in better incorporation of Na+Cll into the ZrO, matrix compared to the other methods such as impregnation and physical mixing in which ZrOz is used as the starting material. It has also been shown in an earlier study that for some chlorine-containing systems chemical and physical changes of the precursors of active components under calcination and reaction conditions lead to the formation of active catalytic systems for the OCM [ 281. The Na,CO,-added catalysts which were prepared with added Na/Zr ratios exceeding 1.2 exhibited relatively poor catalytic performances (Fig. 2). As it was observed that an appreciable portion of the zirconyl chloride was transformed to a precipitate [probably as Zr( OH),] in the solution, contrary to the solution prepared with the added Na/ Zr ratio of 1.2 staying homogeneous, it is considered that this might have resulted in weaker interactions and poorer promoting effects. The effectiveness of the mixed solution method seems to increase when using a basic sodium salt (e.g. Na,CO,) rather than a neutral salt. Neutral sodium salts such as NaNO, and Na,SO, are chemically and/or thermally more stable, and hence the interactions of ZrO, with these salts would occur to a lesser extent. This may have resulted in smaller amounts of chlorine left after calcination, as observed by XRD and EDS analyses. 4.2. Roles
qf sodium and chlorine
The formation of Li+O- or Na+O- sites which are responsible for methane activation has been identified in alkali-doped alkaline earth metal oxide systems [ 22,29,30] or suggested in lithium-doped titania catalysts [ 21,3 11. However, the XPS results in this study do not provide direct evidence for the existence of Na+ Osites on the surface of the sodium-containing Zr02 systems. The reaction and EDS results show that the NaN03- and Na,SO,-added catalysts exhibit poor catalytic performance even though they appear to have high surface concentrations of sodium. Thus, it is suspected that, as suggested by some investigators [ 211, the
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sodium by itself does effectively block the lattice oxygen which may lead to the nonselective oxidation. Rather, the reaction and characterization results suggest that the promoting effect of chlorine is more prominent than that of sodium. It is considered that the major role of sodium would be to assist chlorine, perhaps by increasing the stability of chlorine or enhancing the function of chlorine in some fashion [ 121, which is discussed below. The Cl 2p binding energies for the NaCl- and Na,C03-added sample indicate the presence of one kind of Cl- species in the former sample and two kinds in the latter. As the NaCl-added catalyst exhibited a good catalytic performance, it is certain that Cll bound to Naf is active for the catalytic action. From the XPS results, it may be reasonable to consider that this chlorine species is relatively stable owing to the presence of the Na cation which acts to retard the rapid loss of chlorine from the surface. Lunsford et al. [ 321 have reported that Li f-MgO-Clcatalysts which have Cl/Li ratios near unity are referred to as good and stable catalysts, and from this one may speculate that the alkali metal cations present at an appropriate level could play an important role in stabilizing the Cll ions. The Na,CO,-added catalyst exhibited a lower methane conversion, a higher C2 + selectivity and a higher ethylene-to-ethane ratio than the NaCl-added catalyst. Therefore, it is apparent that the Cl- species bound to Zr4+, which is observed only in the Na,CO,-added catalyst, also plays a role to increase the C,, selectivity or the ethylene-to-ethane ratio. This will be discussed below in more detail. The loss of chlorine from chlorine-containing catalysts under the OCM reaction conditions is a commonly observed phenomenon and it has been attributed to decreases of the activity and selectivity [ 12,28,32-351. From the XRD, EDS and XPS results, it is considered that during the reaction NaCl imbedded under the surface diffuses out, gradually decomposes and then disappears from the surface. The activity and selectivity of the NaCl-added catalyst decreased with time onstream, but the surface sodium concentration increased considerably while the surface chlorine concentration decreased slightly. The initial high activity for methane conversion may be attributed to the high Cl/Na ratio. However, this observation of the surface Cl/Na ratio in the fresh sample greater than unity suggests that a significant part of the surface chlorine may be less stable (but might be more reactive) due to deficiency of the counter cation. The relatively rapid decreases of the activity and selectivity during the early time on-stream may be explained by the rapid disappearance of this unstable Cl-. The higher Cz+ selectivity of the Na,CO,-added catalyst could also be attributed to the higher chlorine concentration. However, the activity and CZ+ selectivity over the Na,CO,-added catalyst decreased little while the ethylene-to-ethane ratio decreased significantly (Fig. 1) . Lunsford et al. [32] have observed a similar trend in the activity and selectivity catalyst and suggested that changes with time on-stream for a good Li+ -MgO-Cla type of active site which is responsible for the activation of only ethane is operative. The Cl- bound to Zr4’ has decreased to a greater extent in the used sample, and thus it is considered that the major role of this Cll species would be
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249
to dehydrogenate ethane to ethylene. It appears that the stability of the Cl- species bound to Na+ is more stable than the Cl- bound to Zr4’. Several functions of chlorine have been proposed by many studies ( [ 12,27,32,36,37], and references therein) : (i) to increase the methane conversion activity, probably by increasing the number of active sites, (ii) to enhance the C,, selectivity, perhaps by blocking the deep oxidation sites, (iii) to increase the ethylene-to-ethane ratio, etc. The reaction results in this work are in agreement with these proposed functions. However, the precise mechanistic role of chlorine has not been well established. Recently, it has been suggested that two types of active sites - one capable of activating both methane and ethane, and the other only capable of activating ethane - are operative on Li+-MgO-Cll catalysts [ 321 and oxychloride catalysts [ 381, although the exact nature of the active sites or the active chlorine species has remained a matter of speculation. The results in this study suggest that the Cl- species bound to Na + is associated with the former type of active sites, and the Cll species bound to Zr4’ with the latter. Although it cannot be ruled out that Na+ might, to some extent, contribute to the catalytic functions ratio, their such as enhancement of the CZ+ selectivity and the ethylene-to-ethane effects are considered to be minor. 5. Conclusions Sodium-salt-promoted zirconia catalysts prepared from mixed solutions of zircony1 chloride and sodium salts were found to be active and selective in the oxidative coupling of methane. The NaCl- and Na,CO,-added catalysts exhibited relatively high performances, and the most effective catalyst was the Na,CO,-added one prepared with an appropriate Na/Zr ratio, which gave a C,, selectivity of 63.9% and a C 2+ yield of 16.6%. The effective promoting species in the catalysts were actually identified to be Naf and Cl-, the latter playing a more prominent role than the former. The presence of multiple chlorine species on the surfaces was observed. It is suggested that the Cl- species bound to Na+ is capable of activating both methane and ethane whereas the Cl- species bound to Zr4’ is only capable of activating ethane. The sodium ion is considered to play an important role in stabilizing the chlorine ion. The structural changes during preparation of the Na,CO,-added catalyst may give rise to intimate interactions between zirconia and Na+Cl _ , resulting in high surface concentrations of sodium and chlorine, stabilization of the chlorine and appearance of the Cl- species bound to Zr4’, and thus yielding a high catalytic performance. Acknowledgements The authors are grateful to the Korea Science and Engineering Foundation and the Research Center for Catalytic Technology for financial support (RCCT-93608) and to Dr. Yun Soo Kim for the XPS data.
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References [ 1 ] G.E. Keller and M.M. Bhasin, .I. Catal., 73 ( 1982) 9. [2] J.S. Lee and ST. Oyama, Catal. Rev.-Sci. Eng., 30 ( 1988) 249. [3] Y. Amenomiya, V.I. Buss, M. Goledzinowski, J. Galuszka and A.R. Sanger, Catal. Rev.-Sci. Eng., 32 (1990) 163. [4] I. Matsuura, Y. Utsumi, M. Nakai and T. Doi, Chem. Lett., ( 1986) 1981. [5] C.A. Jones, J.J. Leonard and J.A. Sofranko, J. Catal., 103 (1987) 311. [6] K. Otsuka and T. Komatsu, J. Chem. Sot., Chem. Commun., (1987) 388. [7] K. Aika, T. Moriyama, N. Takasaki and E. Iwamatsu, J. Chem. Sot., Chem. Commun., (1986) 1210. [8] K. Machida and M. Enyo, J. Chem. Sot., Chem. Commun., (1987) 1639. [9] D.J. McNamara, S.J. Korf, K. Seshan, J.G. van Ommen and J.R.H. Ross, Can. J. Chem., 69 (1991) 883. [IO] K. Aika, N. Fujimoto, M. Kobayashi and E. Iwamatsu, J. Catal., 127 ( 1991) 1. [ 111 H. Matsuhashi, K. Sakurai and K. Arata, Chem. Lett., (1989) 585. [ 121 AZ. Khan and E. Ruckenstein, Appl. Catal. A, 90 (1992) 199. [ 131 A.Z. Khan and E. Ruckenstein, J. Catal., 139 ( 1993) 304. [ 141 K. Otsuka, K. Jinno and A. Morikawa, J. Catal., 100 ( 1986) 353. [ 151 R.C. Garvie, J. Phys. Chem., 69 (1965) 1238. [ 161 W. Hertl, Langmuir, 6 ( 1989) 96. [ 171 J.R. Sohn, S.G. Ryu and Y.I. Pae, Hwahak Konghak, 30 ( 1992) 586. [ 181 M. Inoue, H. Kominami and T. Inui, Appl. Catal. A, 97 (1993) L25. [ 191 C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Mullenberg (Editors), Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, Eden Prairie, Minnesota, 1979. [20] K.C.C. Kharas and J.H. Lunsford, J. Am. Chem. Sot., 111 (1989) 2336. [21] G.S.Lane,E.MiroandE.E. Wolf,J. Catal., 119 (1989) 161. [22] X.D. Peng, D.A. Richards and P.C. Stair, J. Catal., 121 (1990) 99. [ 231 W. Ding, Y. Chen and X. Fu, Appl. Catal. A, 104 ( 1993) 61. [24] W.E. Morgan, J.R. Van Wazer and W.J. Stec, J. Am. Chem. Sot., 95 ( 1973) 751. [25] D.D. Sarma and C.N.R. Rao, J. Elect. Spect. Related Phenom., 20 (1980) 25. [26] J.R. Blackbum, R. Nordberg, F. Stevie, R.G. Albridge and M.M. Jones, Inorg. Chem., 9 ( 1970) 2374. [27] W.M. Riggs, Anal. Chem., 44 (1972) 830. [28] A.Z. Khan and E. Ruckenstein, J. Catal., 138 (1992) 322. [29] T. Ito, J.-X. Wang, C.-H. Lin and J.H. Lunsford, J. Am. Chem. Sot., 107 ( 1985) 5062. [30] C.-H. Lin, J.-X. Wang and J.H. Lunsford, J. Catal., 11 I ( 1988) 302. [31] A.M. Efstathiou, D. Papageorgiou and X.E. Verykios, J. Catal., 144 (1993) 352. [32] J.H. Lunsford, P.G. Hinson, M.P. Rosynek, C. Shi, M. Xu and X. Yang, J. Catal., 147 ( 1994) 301. [33] K. Otsuka, Q. Liu, M. Hatano and A. Morikawa, Chem. Lett., ( 1986) 903. [34] R. Butch, G.D. Squire and SC. Tsang, Appl. Catal., 46 (1989) 69. [35] S.J. Conway and J.H. Lunsford, J. Catal., 131 (1991) 513. [36] R. Burch, S. Chalker and S.J. Hibble, Appl. Catal. A, 96 ( 1993) 289. [37] B.K. Warren, Catal. Today, 13 (1992) 311. [38] R. Burch, S. Chalker, P. Loader, J.M. Thomas and W. Ueda, Appl. Catal. A, 82 (1992) 77.
Applied Catalysis B: Environmental
7 ( 1996) 237-250
Oxidative coupling of methane over sodium-salt-promoted zirconia catalysts prepared by the mixed solution method Ki June Yoon *, Sang Won Seo Department of Chemical Engineering, Sung Kyun Kwan University. Suwon 440-746, South Korea Received 22 March 1995; revised 26 July 1995; accepted 28 July 1995
Abstract Sodium-salt-promoted zirconia catalysts prepared from mixed solutions of zirconyl chloride and sodium salts (chloride, carbonate, nitrate or sulfate) are found to exhibit effective catalytic performances in the oxidative coupling of methane. The highest C 2+ yield ( 16.6%) and ethylene-to-ethane ratio are obtained over the Na,CO,-added catalyst having an appropriate Na/Zr ratio; however, the active promoting species in this catalyst are actually identified to be Na+ and Cl _. The mixed solution method is considered to be an effective preparation method when proper precursor compounds of zirconia and promoters are chosen, giving a desirable interaction between them. The reaction and characterization results indicate that the chlorine ion plays a more prominent role than the sodium ion, and the X-ray photoelectron spectroscopy results show the presence of multiple chlorine species on the surface. It is suggested that the Cl- bound to Na+ is capable of activating both methane and ethane whereas the Cl- bound to Zr“’ IS only capable of activating ethane. The sodium ion is considered to play an important role in stabilizing the chlorine ion. Keywords: Zirconia
Chlorine
species;
Methane;
Mixed solution
method;
Oxidative
coupling;
Sodium-salt-promoted:
1. Introduction Following the pioneering work of Keller and Bhasin [ 11, the oxidative coupling of methane (OCM) has received a great deal of attention as a promising route for direct conversion of methane to C2 hydrocarbons. Most of the research on this reaction has been directed to developing suitable catalysts which give high yields of higher hydrocarbons, and numerous kinds of catalysts have been tested for their * Corresponding
author. Tel.
( + 82-33 1) 2905424, fax. ( + 82-33 I ) 2905616.
0926.3373/96/$15,00 0 1996 Elsevier Science B.V. All rights reserved SSDlO926-3373(95)00050-X
238
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oxidative coupling activity [ 2,3]. The oxides of transition metals are generally more effective for the nonselective oxidation to the carbon oxides than for the selective oxidation to C2 hydrocarbons. However, when promoted with alkali metal compounds, some transition metals, notably ZnO and MnO,, exhibit a significant effect in the OCM process [ 4-61. ZrO, has attracted relatively little attention and a few results available on ZrO,, obtained by using it either as a support or as a component of mixed oxides, showed low performance [ 7-101. Recently, however, it has been shown that alkali-metalcompound-promoted ZrO, catalysts exhibit noticeable C,, selectivities [ 1 l-131. The performance of these catalysts depends strongly on the nature of alkali salts (the cations and the anions) and the method of preparation [ 131. Of the cations and the anions in the promoter tested, Na + is far more effective than the other alkali metal ions and Cl- renders a higher ethylene-to-ethane ratio than the carbonate, nitrate or acetate ion. Of the methods used for the preparation of NafZrO,-Cl _ , the sol-gel method is more effective than the other preparation methods. The relatively high performance of Na+-ZrO*-Cl_ prepared by the sol-gel method may be attributed to a better incorporation of Na+Cl - into the ZrO, matrix [ 12,131. However, the effects of the nature of the alkali salts and the method of preparation are not fully understood. These earlier works have prompted us to further investigate sodium-salt-promoted ZrO,. The effects of the promoters may be different, depending on their chemical properties and thermal stabilities as well as on the method of preparation. In this study, four sodium salts (NaCl, NaNO,, Na,CO,, and Na,SO,) were used as the promoters or the promoter precursors. The catalysts were prepared by a method different from that of the earlier works in which the catalysts were usually prepared using Zr02 powder [ 12,131 or precipitated Zr( OH), [ 111 as a starting material. In the present work, a mixed aqueous solution was first prepared by adding a solution of each sodium salt to a solution of zirconyl chloride, and this was followed by drying, calcination and crushing. We call this the mixed solution method. The effects of the nature of the promoter (or the precursor) compound and the physical properties of the systems on the catalyst performance were examined. In addition, the effects of the promoter content and the reaction conditions (temperature, CH,/O, ratio, etc.) are presented. Furthermore, the roles of sodium and chlorine and the nature of active surface species are discussed.
2. Experimental 2.1. Catalyst preparation Zirconyl chloride ( ZROCL2. 8H20, Chemical Pure Grade) was purchased from Junsel Chemical. NaCl (99.5%)) NaNO, (99%)) Na2C03. 10H20 (99 + %) and Na2S04 (99%) were purchased from Shiny0 Pure Chemical. An aqueous solution
K.J. Yom,
S. W. See /Applied
Cafa/ysis
B: Environmmtcd
7 (1996)
237-250
239
of a sodium salt was added dropwise under stirring to an aqueous solution of zirconyl chloride having a pH of 0.7. The water was evaporated from the mixture by heating under vigorous stirring until a thick paste was formed. This was followed by drying at 393 K, calcination in air at 673 K for 2 h and at 1073 K for 6 h, and then crushing to 40-80 mesh particle sizes. The Na/Zr atomic ratio in the mixture was typically 0.6 or 1.2. No attempt was made to analyze the elemental composition of the prepared catalysts. The mixture always formed a homogeneous solution except for the Na,CO,added solution. When the solution of sodium carbonate, having a pH of 11.5, was added, the mixture stayed homogeneous until the Na/Zr ratio exceeded 1.2. Above this, precipitation started to occur increasingly with the Na/Zr ratio. Even though the precipitate was present, the same procedure of preparation as above was followed. While the solution of sodium carbonate was added evolution of CO, was observed. 2.2. Activity testing and product analysis The oxidative coupling reactions were performed at 923-l 123 K under atmospheric pressure by cofeeding the reactant gases (methane, air and nitrogen as a diluent) into a l/2-inch stainless steel tube reactor mounted vertically and heated by an electric furnace. Typically, the reaction temperature was 1023 K, PCH4 16.5 kPa, P,, 8.3 kPa (CH,/O, = 2)) the total gas flow lOOcm”/min (NTP), andcatalyst loading 2.0 g, giving rise to a space velocity 3000 cm3 g-r hh ‘. The product effluents were analyzed by two on-line TCD-equipped gas chromatographs, one fitted with a Chromosorb 102 column to analyze carbon dioxide, methane and higher hydrocarbons and the other with a Carbosieve S-II (Supelco) column to analyze oxygen, carbon monoxide, carbon dioxide and methane. 2.3. Catalyst characterization The specific surface areas of the catalysts were measured by the BET method using a Micrometrics 2000. X-ray diffraction (XRD) measurements were taken with a Rigaku Rotaflex D/Max-C diffractometer using Ni-filtered Cu Ko radiation. For the X-ray photoelectron spectroscopy (XPS) study of some samples, binding energies of photoelectron lines, Zr 3d, Na 1s, Cl 2p and 0 1s were obtained by a VG Escalab Mark II photoelectron spectrometer using Al Ka radiation with the reference of C 1s at 284.6 eV, and the surface compositions were calculated by the integration of each peak. The samples for the XPS analysis, either fresh or used, were treated at 673 K for 30 min in airjust before introduction into the spectrometer. Energy dispersive X-ray spectroscopy (EDS) analysis was also performed with a TN-5500 (Tracer Northern) spectrometer.
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3. Results 3.1. Catalytic peeormance The results of the reaction performed over the unpromoted and promoted catalysts are presented in Table 1. The unpromoted ZrOz had a high activity for methane, but almost all of the methane was nonselectively converted to carbon oxides. When ZrO, was promoted with the sodium salts noticeable increases in the C2 + selectivity were observed, but the methane conversion increased or decreased depending on the promoter. The Na,CO,- and NaNO,-added catalysts exhibited a methane conversion comparable to the unpromoted catalysts, but the NaCl-added catalyst gave a higher methane conversion while the Na,SO,-added catalyst showed a considerably lower methane conversion than the unpromoted catalyst. The C2 + selectivity obtained over the NaN03-added catalyst was considerably lower than that obtained over the other promoted catalysts. The highest Cz + selectivity (63.9%)) C2 + yield ( 16.6%) and ethylene-to-ethane ratio ( 1S2) were obtained over the Na,CO,-added catalyst catalyst. This C,, y ield is higher than that reported for the Na+-ZrO,-Clprepared by the sol-gel method [ 12,131. As Na,CO,- and NaCl-added catalysts were found to be effective in the OCM, the stability of these two catalysts with time on-stream was examined. The changes in the methane conversion and the C,, selectivity are presented in Fig. 1. For the Na,CO,-added catalyst the methane conversion and the C2 + selectivity were maintained nearly constant (ca. 26% and 60%, respectively) for 10 h. The C, selectivity remained constant at around 3%, and about two times more propene was produced than propane. For the NaCl-added catalyst, however, a slow decrease in the methane conversion and a more rapid decrease in the C,, selectivity were observed. The ethylene-to-ethane ratio decreased with time on-stream for both catalysts. Fig. 2 shows the effect of the promoter content on the OCM over the Na,CO,and NaCl-added catalysts. For the NaCl-added catalyst the CZ+ selectivity was little affected with the promoter content. For the Na,CO,-added catalyst, however, Table 1 Oxidative coupling of methane over sodium-salt-added Added salt (Na/Zr ratio)
Surface area (m*/g)
none
Na,CO,( 1.2) NaCl (1.2) NaCl (0.6) NaNO, (0.6) Na,SO,( 1.2)
1.1 4.1 3.0 1.0
CH, con”.
ZrQ prepared by the mixed solution method
Selectivity
C,,
(Yo)
(%)
24.6 26.0 33.5 30.4 24.4 5.5
C,He
C,H,
C,
Co,,,=,.21
2.1 24.0 17.2 22.3 6.8 29.2
1.0 36.4 22.4 18.2 3.1 6.6
3.5 1.2 1.6 -
96.9 36.1 59.2 57.9 90.1 64.2
Conditions: temperature = 1023 K, PcHl = 16.5 kPa, CH,/O,=2, results presented are after 70 min of reaction.
yield
(%)
C,HJC,H, ratio
0.8 16.6 13.6 12.9 2.4 2.0
0.48 1.52 1.30 0.82 0.46 0.23
space velocity=3000cm7
g-’ h-’ (NPT): the
K.J. Yoon, S.W. Seo /Applied
Caialysis B: Environmental
7 (1996) 237-250
241
20
oi 0
’ 2
’ 4 Time on
’ 6
’ 8
10
- stream , h
Fig. 1. Changes in methane conversion and C2 + selectivity with time on-stream over Na,C03- and NaCl-added ZrO, (Na/Zr = 1.2) The conditions are as in Table 1 except the time on-stream. (0) C, + selectivity for Na,CO,ZrO,, (A) C,, selectivity for NaCl-ZrOz (The numbers in parentheses represent the ethylene-to-ethane ratios.), (0) methane conversion for NazCO,-ZrOz, ( A ) methane conversion for NaCI-ZrO,.
a volcano-type pattern was observed in the Ca+ selectivity with the promoter content, the maximum being at 1.2 of the added Na/Zr ratio. The maximum ethylene-to-ethane ratio was also obtained at the same Na/Zr ratio. Since the NazC03-added catalyst prepared with the Na/Zr ratio of 1.2 was found to be the most effective catalyst, further investigation on the effects of the reaction temperature and the CH4/02 ratio was performed on this catalyst. Fig. 3 shows the effect of the reaction temperature. With the increase in temperature the methane conversion increased and reached a plateau above 1050 K. This plateau is considered to come from the fact that the oxygen was almost completely exhausted above this temperature. However, the C, + selectivity passed through a maximum at 1023 K. This may be explained by the suggestion of Otsuka et al. [ 141 that the deep oxidation of ethane and ethylene, which are the primary products in the OCM
0'
0
I
1
I
I
I
1
2
3
4
5
Promoter Content,
Na/Zr atomic ratio
Fig. 2. Effect of promoter content on methane conversion and Cz+ selectivity for Na&O,The conditions are as in Table 1 and the symbols are as in Fig. 1.
and NaCI-added
ZrO,.
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Catalysis B: Environmental
7 (1996) 237-250
20
900
1000
950
1050
Temperature
1100
1150
K
Fig. 3. Effect of reaction temperature on OCM over Na,CO,-added 210~ (Na/Zr= Table 1 except the reaction temperature and the symbols are as in Fig. 1.
1.2). The conditions are as in
process, becomes dominant at high temperatures. Fig. 4 shows the effect of the CH4/02 feed ratio on the methane conversion and the C,, selectivity over the Na,CO,-added catalyst at 1023 K while holding the partial pressure of oxygen at 8.3 kPa. The methane conversion tended to decrease with the CH4/OZ ratio, but the C2+ selectivity had a reverse trend: this is a commonly observed behavior in earlier works [3,13]. At the CH,/O, ratio of 7 the C,, selectivity was 69.3% and this is the highest value obtained in this study. However, the C,, yield at this CHJ O2 ratio was only 9.1%, and this is well below the maximum Cz+ yield ( 16.6%) obtained in this study at the CH,/O, ratio of 2. 3.2. XRD analysis The crystalline phases in the unpromoted and promoted catalysts are presented in Table 2. The ZrO, in all the catalyst samples was mostly the monoclinic phase
0’ 0
1
’
’
2
3 CH,/O,
’ 4
5
’ 6
A
7
8
feed ratio
Fig. 4. Effect of CH,/O, feed ratio on OCM over Na,CO,-added ZrO, (Na/Zr= Table 1 except the partial pressure of CH, and the symbols are as in Fig. I.
1.2). The conditions
are as in
K.J. Yom, S.W. Seo /Applied
Table 2 Crystalline
Catalysis B: Environmental
243
7 (1996) 237-250
phases identified by XRD
Added salt
Status
Major phase
Minor phase
none
Fresh Used Fresh Used Fresh Fresh Fresh
m-ZrO; m-ZrOz m-ZrO,, m-ZrOz, m-ZrO,, m-ZrOz m-ZrO,,
NaCl NaClb NaCl
t-z&I; t-zr0: t-ZrO, _ _
Na,SO,
t-Z& t-ZrO,
Na,CO, NaCl NaNO, Na*SO,
NaCl
a m-ZrOz = monoclinic ZrO, phase (Baddeleyite), t-ZrOz = tetragonal ZrOL phase. h The intensity was weaker than that in the fresh sample.
(Baddeleyite) . A small fraction of the Zr02 was present as the tetragonal phase, but this phase was gradually transformed under the reaction conditions to the monoclinic phase, in agreement with previous findings [ 15-l 81. In the NaCl- and Na,SO,-added samples the presence of the corresponding sodium salt was identified. In the NazC03-added sample, however, a large amount of NaCl was found to be present. This NaCl is considered to be formed by the reaction of Na,C03 with zirconyl chloride during the preparation process, and virtually no Na2C03 seems to be present in the bulk of the sample. A significant loss of the NaCl in the Na,CO,added sample (Na/Zr= 1.2) was observed after the reaction at 1023 K for 10 h, as indicated by the decrease of the peak intensity ratio of NaCl (220) /m-ZrO, ( 111) from 1.35 in the fresh sample to 0.45 in the used sample. In the NaN03added sample a small amount of NaCl existed, but no lines of other sodium compounds were observed. As NaN03 is unstable under the calcination or reaction conditions most of the NaNOx in the sample may have been decomposed to the oxide, but it could not be identified by XRD, perhaps due to being present as small crystallites or amorphous phases. 3.3. XPS analysis Binding energies of Zr 3d5,*, Na Is, Cl 2p and 0 1s levels on the surfaces of the NaCl- and Na,CO,-added catalysts are shown in Table 3. The binding energies of the Zr 3dS12 and Na 1s are in agreement with the reported values [ 13,191. The 0 1s spectra indicate that at least two kinds of oxygen species are present on the surfaces. The lower binding energy of 0 1s (529.6-529.8 eV) is ascribed to the lattice oxygen O* ~ in ZrO, and the higher value (53 1.4-532.0 eV) to the carbonatetype oxygen [ 13,20-231. A weak peak appeared at 536-537 eV: however, this peak is considered to be an Auger line of Na KL1L23 rather than to arise from another kind of oxygen species [ 19,201, and no further consideration of this peak was given. The Cl 2p spectra are shown in Fig. 5. For the NaCl-added catalyst the peak was rather broad (Fig. 5a) and hence it is considered that at least two different lines are
244
Table 3 Binding-energy
K.J. Yoon, S. W. Seo /Applied
results for NaCl- and Na,CO,-added
Catalyst (added Na/Zr ratio)
NaCl-added ZG (1.2) Na,CO,-added ZQ (1.2)
Catalysis B: Environmental
Status
7 (1996) 237-250
ZrO, catalysts Binding energy (eV) Zr 3d,/,
Na 1s
Cl 2p (%)
0 1s (%)
Fresh
181.7
1071.7
198.6 (100)
Used
181.5
1071.1
198.5 (100)
FresH
181.8
1072.0
Used
181.2
1071.4
198.5 199.6 198.2 199.8
529.7 531.4 529.6 531.5 529.8 532.0 529.7 531.7
(47) (53) (59) (41)
(86) (14) (84) (16) (89) (11) (91) (9)
superimposed. The broad peak was deconvoluted by assuming that two symmetric peaks were superimposed, giving one with a binding energy of about 198.5 eV and the other with about 195.5 eV. The former is ascribed to the Cl- bound to Naf [ 19,241. Sarma and Rao [ 251 have observed a weak peak from ZrO, at about 14 eV above the Zr 3d,,, line, i.e. at around 196 eV, and suggested that this peak originates from a plasmon excitation process. Therefore, the peak at around 195.5 eV in Fig. 5 is assigned to the plasmon excitation line associated with the Zr 3d
206
204
200
196
Binding Energy
192
166
, eV
Fig. 5. Cl 2p and Zr 3d plasmon excitation lines in NaCI- and Na,CO,-added catalysts (Na/Zr ‘= 1.2). (a) fresh NaCl-added catalyst, (b) fresh NazCO,-added catalyst, (c) used Na&O,-added catalyst.
K. J. Yom. S. W. Sea /Applied
Table 4 Surface composition Catalyst
of the elements in the NaCI- and NazCOz-added
(added Na/Zr ratio)
NaCl-added
ZrOz
(1.2) NazCO1-added (1.2)
Catalysis B: Environmental
ZrO,
Status
Fresh Used Fresh Used
245
7 (I 996) 237-250
ZrOz catalyst determined
by XPS
Surface atomic ratio
Comp. (%) Zr
Cl
O/Zr
NalZr
CllZr
CI/Na
19.8 17.9 14.8 18.3
5.9 5.5 8.9 8.6
2.25 2.40 2.34 2.11
0.18 0.35 0.34 0.47
0.30 0.3 I 0.60 0.47
0.89 1.76 1.00
I.67
line. The spectrum of the used NaCl-added catalyst in the Cl 2p region was almost identical with that of the fresh catalyst. The Zr 3d plasmon excitation line was also observed in the Na,CO,-added samples. The peak intensity ratios of the plasmon excitation line to the parent Zr 3d line (Zr 3d5,2 + 3d3,2) in the four samples were fairly constant and the average ratio was estimated to be 0.09. The relative concentration of surface chlorine was corrected from the peak intensity by excluding the Zr 3d plasmon excitation line. For the Na,CO,-added samples a second chlorine species which had a binding energy of ca. 199.6 eV appeared (Fig. 5b and 5~). This chlorine species is ascribed to the Cl- bound to Zr4’, since the Cl 2p binding energy in the transition metal chloride has been found to be higher than the value for the alkali metal chloride and to become higher as the electron density of the metal cation decreases [ 26,271. The concentration of this second species in the fresh sample was slightly higher than that of the Cl- bound to Nat, but in the used sample it decreased to about 2/3 of that of the Cl- bound to Naf . The surface compositions of the elements in the NaCl- and Na,CO,-added catalysts are presented in Table 4. Both of the Na/Zr and Cl/Zr ratios in the Na,CO,added catalyst were higher than those in the NaCl-added catalyst. In both of the fresh samples the surface concentration of chlorine was about 1.7 times higher than that of sodium: after the reaction the surface concentration of sodium increased noticeably whereas that of chlorine decreased to some extent, giving Cl/Na ratios near unity. 3.4. EDS analysis As EDS has a probing depth of a few micrometers, the analysis results in Table 5 provide the composition of the elements having a value between the surface and bulk. The Na and Cl concentrations in the Na&O,-added catalyst were 4-5 times higher than those in the NaCl-added catalyst. For both of the NaCl- and Na,CO,added catalysts the Cl/Na ratios were somewhat greater than 1, indicating that most of the sodium and the chlorine exist as the NaCl salt. After the reaction the Na/Zr and Cl/Zr ratios increased but the Cl/Na ratios decreased. This indicates that the NaCl diffuses out from the bulk to the surface during the reaction and that the chlorine disappears at a faster rate than the sodium from the surface. In contrast,
246
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Table 5 EDS results for the sodium-salt-added Catalyst
(added Na/Zr ratio)
NaCl-added (1.2) Na,CO,-added
ZrO, ZrO,
( 1.2) NaNO,-added
ZrOZ
(1.2) Na$O,-added
ZrO,
(1.2)
Catalysis B: Environmentcrl 7 (1996) 237-250
ZrO, catalysts Status
Fresh Used Fresh Used Fresh Used Fresh Used
Atomic ratio Na/Zr
CI/Zr
S/Zr
CI/Na
0.14 0.3 1 0.73 1.53 2.52 0.76 4.38
0.19 0.34 0.88 I.65 0.97 0.23 0.89 0.15
_
1.34 1.1 I
_
1.20 I .08 0.38 0.32 0.20 0.08
1.85
_ _ _ 0.18 0.31
the Cl/Na ratios in the NaNO,- and Na,SO,-added catalysts were considerably lower than 1. The sodium concentrations in the fresh samples of these two catalysts were much higher compared to those in the other two fresh samples, but they decreased rapidly during the reaction. The chlorine concentrations in the fresh NaNO,- and Na,SO,-added catalysts were relatively high, but they decreased more rapidly than the sodium concentrations.
4. Discussion 4.1. Effect of the nature of sodium salt The most effective catalyst system in this work was the Na,CO,-added catalyst having a proper Na/Zr ratio, but the prepared sample contained a large amount of NaCl and virtually no Na,CO,. Therefore it is apparent that the best promoting species are Nat and Cl-. The NaCl-added catalyst, however, exhibited a lower catalytic performance than the Na,C03-added catalyst, even though the chemical components present in both of the prepared samples appear identical. The difference in the catalytic performance might be due to different interactions between ZrO, and NaCl which arise from different structural changes (reaction in solution, nucleation, decomposition, etc.) during the preparation process. Na2C03 is a basic salt and CO2 evolution was observed while preparing the mixed solution of Na,CO, and zirconyl chloride. Hence, after drying during the preparation process, it is certain that the composition of the NaCl-added sample is different from that of the Na,C03-added sample in which additional zirconium compounds formed by the reaction of Na,CO, with zirconyl chloride are present. We speculate that during the subsequent calcination the zirconium compounds are decomposed to Zr02, accompanied by ZrO, interacting with the NaCl but to a different extent depending on the nature of the zirconium compounds present prior to the decomposition. The interaction appears to be greater in the Na,CO,-added catalyst than in the NaCl-
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added one. The reaction, XPS and EDS results, which show a better catalytic performance, presence of a second chlorine species, and higher concentrations of sodium and chlorine for the Na,CO,-added catalyst, are consistent with this suggestion. The results in this study also suggest that the intimate interaction has an effect to enhance the stability of the chlorine. This kind of interaction may be considered to be the intimate mixing between Na+Cll and ZrO, or the incorporation of Na+Cl- into the ZrO, matrix, as suggested in other studies which have shown good and stable performances of Nat-ZrO,-Clcatalysts [ 12,131. The effectiveness of the mixed solution method appears to depend on the nature of the precursors of ZrO,. The NaCl-added catalyst prepared in this work exhibits a good catalytic performance which is comparable to the performance of the Na+ZrO,-Cll catalyst prepared by the sol-gel method in Refs. [ 121 and [ 131. A common feature in both of the preparation methods is that the precursors of ZrO, undergo drastic chemical changes (i.e. decomposition) during the preparation processes, and this might result in better incorporation of Na+Cll into the ZrO, matrix compared to the other methods such as impregnation and physical mixing in which ZrOz is used as the starting material. It has also been shown in an earlier study that for some chlorine-containing systems chemical and physical changes of the precursors of active components under calcination and reaction conditions lead to the formation of active catalytic systems for the OCM [ 281. The Na,CO,-added catalysts which were prepared with added Na/Zr ratios exceeding 1.2 exhibited relatively poor catalytic performances (Fig. 2). As it was observed that an appreciable portion of the zirconyl chloride was transformed to a precipitate [probably as Zr( OH),] in the solution, contrary to the solution prepared with the added Na/ Zr ratio of 1.2 staying homogeneous, it is considered that this might have resulted in weaker interactions and poorer promoting effects. The effectiveness of the mixed solution method seems to increase when using a basic sodium salt (e.g. Na,CO,) rather than a neutral salt. Neutral sodium salts such as NaNO, and Na,SO, are chemically and/or thermally more stable, and hence the interactions of ZrO, with these salts would occur to a lesser extent. This may have resulted in smaller amounts of chlorine left after calcination, as observed by XRD and EDS analyses. 4.2. Roles
qf sodium and chlorine
The formation of Li+O- or Na+O- sites which are responsible for methane activation has been identified in alkali-doped alkaline earth metal oxide systems [ 22,29,30] or suggested in lithium-doped titania catalysts [ 21,3 11. However, the XPS results in this study do not provide direct evidence for the existence of Na+ Osites on the surface of the sodium-containing Zr02 systems. The reaction and EDS results show that the NaN03- and Na,SO,-added catalysts exhibit poor catalytic performance even though they appear to have high surface concentrations of sodium. Thus, it is suspected that, as suggested by some investigators [ 211, the
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sodium by itself does effectively block the lattice oxygen which may lead to the nonselective oxidation. Rather, the reaction and characterization results suggest that the promoting effect of chlorine is more prominent than that of sodium. It is considered that the major role of sodium would be to assist chlorine, perhaps by increasing the stability of chlorine or enhancing the function of chlorine in some fashion [ 121, which is discussed below. The Cl 2p binding energies for the NaCl- and Na,C03-added sample indicate the presence of one kind of Cl- species in the former sample and two kinds in the latter. As the NaCl-added catalyst exhibited a good catalytic performance, it is certain that Cll bound to Naf is active for the catalytic action. From the XPS results, it may be reasonable to consider that this chlorine species is relatively stable owing to the presence of the Na cation which acts to retard the rapid loss of chlorine from the surface. Lunsford et al. [ 321 have reported that Li f-MgO-Clcatalysts which have Cl/Li ratios near unity are referred to as good and stable catalysts, and from this one may speculate that the alkali metal cations present at an appropriate level could play an important role in stabilizing the Cll ions. The Na,CO,-added catalyst exhibited a lower methane conversion, a higher C2 + selectivity and a higher ethylene-to-ethane ratio than the NaCl-added catalyst. Therefore, it is apparent that the Cl- species bound to Zr4+, which is observed only in the Na,CO,-added catalyst, also plays a role to increase the C,, selectivity or the ethylene-to-ethane ratio. This will be discussed below in more detail. The loss of chlorine from chlorine-containing catalysts under the OCM reaction conditions is a commonly observed phenomenon and it has been attributed to decreases of the activity and selectivity [ 12,28,32-351. From the XRD, EDS and XPS results, it is considered that during the reaction NaCl imbedded under the surface diffuses out, gradually decomposes and then disappears from the surface. The activity and selectivity of the NaCl-added catalyst decreased with time onstream, but the surface sodium concentration increased considerably while the surface chlorine concentration decreased slightly. The initial high activity for methane conversion may be attributed to the high Cl/Na ratio. However, this observation of the surface Cl/Na ratio in the fresh sample greater than unity suggests that a significant part of the surface chlorine may be less stable (but might be more reactive) due to deficiency of the counter cation. The relatively rapid decreases of the activity and selectivity during the early time on-stream may be explained by the rapid disappearance of this unstable Cl-. The higher Cz+ selectivity of the Na,CO,-added catalyst could also be attributed to the higher chlorine concentration. However, the activity and CZ+ selectivity over the Na,CO,-added catalyst decreased little while the ethylene-to-ethane ratio decreased significantly (Fig. 1) . Lunsford et al. [32] have observed a similar trend in the activity and selectivity catalyst and suggested that changes with time on-stream for a good Li+ -MgO-Cla type of active site which is responsible for the activation of only ethane is operative. The Cl- bound to Zr4’ has decreased to a greater extent in the used sample, and thus it is considered that the major role of this Cll species would be
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to dehydrogenate ethane to ethylene. It appears that the stability of the Cl- species bound to Na+ is more stable than the Cl- bound to Zr4’. Several functions of chlorine have been proposed by many studies ( [ 12,27,32,36,37], and references therein) : (i) to increase the methane conversion activity, probably by increasing the number of active sites, (ii) to enhance the C,, selectivity, perhaps by blocking the deep oxidation sites, (iii) to increase the ethylene-to-ethane ratio, etc. The reaction results in this work are in agreement with these proposed functions. However, the precise mechanistic role of chlorine has not been well established. Recently, it has been suggested that two types of active sites - one capable of activating both methane and ethane, and the other only capable of activating ethane - are operative on Li+-MgO-Cll catalysts [ 321 and oxychloride catalysts [ 381, although the exact nature of the active sites or the active chlorine species has remained a matter of speculation. The results in this study suggest that the Cl- species bound to Na + is associated with the former type of active sites, and the Cll species bound to Zr4’ with the latter. Although it cannot be ruled out that Na+ might, to some extent, contribute to the catalytic functions ratio, their such as enhancement of the CZ+ selectivity and the ethylene-to-ethane effects are considered to be minor. 5. Conclusions Sodium-salt-promoted zirconia catalysts prepared from mixed solutions of zircony1 chloride and sodium salts were found to be active and selective in the oxidative coupling of methane. The NaCl- and Na,CO,-added catalysts exhibited relatively high performances, and the most effective catalyst was the Na,CO,-added one prepared with an appropriate Na/Zr ratio, which gave a C,, selectivity of 63.9% and a C 2+ yield of 16.6%. The effective promoting species in the catalysts were actually identified to be Naf and Cl-, the latter playing a more prominent role than the former. The presence of multiple chlorine species on the surfaces was observed. It is suggested that the Cl- species bound to Na+ is capable of activating both methane and ethane whereas the Cl- species bound to Zr4’ is only capable of activating ethane. The sodium ion is considered to play an important role in stabilizing the chlorine ion. The structural changes during preparation of the Na,CO,-added catalyst may give rise to intimate interactions between zirconia and Na+Cl _ , resulting in high surface concentrations of sodium and chlorine, stabilization of the chlorine and appearance of the Cl- species bound to Zr4’, and thus yielding a high catalytic performance. Acknowledgements The authors are grateful to the Korea Science and Engineering Foundation and the Research Center for Catalytic Technology for financial support (RCCT-93608) and to Dr. Yun Soo Kim for the XPS data.
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