1060
Znd. Eng. Chem. Res. 1 9 9 5 , 3 4 , 1060-1073
Effects of Steam and Liquid Water Treatment on the Oxidative Coupling of Methane over a Li/MgO Catalyst Saeed M. S. Al-Zahrani and Lance L. Lobban" School of Chemical Engineering and Materials Science, University of Oklahoma, 100 East Boyd, Room T-335, Norman, Oklahoma 73019
The effects of adding H2O to the gas feed on the oxidative coupling of methane over L m g O catalyst at different partial pressures of H2O and temperatures and in the presence of cofed C 0 2 have been studied. Results indicated that H2O enhanced the deactivation rate. The deactivation rate increased with increasing partial pressure of the steam in the feed as well as with increasing temperature. The deactivation rate is decreased by adding small amounts of COOto the reaction mixture. The effects of injecting different amounts of liquid water into the catalyst bed under different reaction conditions have also been investigated. Results showed that under the conditions of our experiments water significantly enhanced the activity of the catalyst. The methane conversion increased by 86%- 124% while the CZ selectivity remained relatively unchanged. The liquid water treatment significantly increased the product C2H4: C2Hs ratio. The catalyst lithium content decreased and the BET surface area increased due to the water treatment. Some of the lithium lost from the catalyst was deposited on the walls of the reactor; however, this lithium was not responsible for the enhanced activity.
Introduction The direct conversion of methane into petrochemical feedstocks andor liquid transportation fuels has been the focus of a number of active research efforts since its initial discovery almost 13 years ago. Oxidative coupling of methane to give ethane and ethylene demonstrated the possibility of this conversion. Otsuka et al. (1985) reported that rare earth metal oxides doped with sodium, potassium, and lithium exhibited high catalytic activities for this reaction. Ito et al. (1985) reported that Li/MgO catalyst exhibited high catalytic activity and selectivity to C2 products. They reported that [Li+-0-1 species were the active and selective sites for methane activation. These active sites are generated through the substitution of Mg2+cations by monovalent lithium. Iwamatsu and Aika (1987) observed that a catalyst having a lower surface area provided higher selectivity for C2 formation. Korf et al. (1990) reported that the addition of steam to the reactor feed was deleterious to the stability of the Li/MgO catalyst at T = 1073 K. They found that the lithium migrates from the catalyst, traveling in the direction of the gas stream. This caused a decrease in the catalyst activity. Kaminsky et al. (1992) also reported that the addition of steam to the feed increases the deactivation rate of Li/MgO catalyst. Matsukata et al. (1989) observed that a t T = 998 K steam accelerates the loss of the lithium from the surface of Li&O3/MgO and L i N O a g O , but enhances the activity of the catalyst without decrease in the C2 selectivity. Kimble and Kolts (1986)found that the addition of steam to the gas feed increased the C2 yield in the presence of LiNOd Mg(OH)2 at T = 973 K. Recently Chang et al. (1993) used LiNOdlUg(N03)2-6HzOa t T = 873 K and reported that the presence of steam is essential t o the formation of the coupling product at low space velocities,but their data show a very rapid deactivation of the catalyst. This variety of findings, besides the importance of studying the effect of steam (a reaction product) on the oxidative coupling of methane, prompted our effort to elucidate the role of steam in the reaction with the ultimate goal of improving the catalyst and optimizing the reactor operating conditions.
We therefore investigated the influence of adding H20 at low partial pressures to the gas feed on the oxidative methane coupling reaction over Li/MgO at different flow rates, at different temperatures, and along with C02. The effect of H2O is very significant to the stability of the catalyst. We have also investigated the effects of treating the catalyst with relatively large quantities of deionized liquid water under different reaction conditions. Single and multiple water treatments of various amounts of the catalyst with varying lithium content were investigated. Liquid water was injected under normal reactor operating conditions (i.e., T = 1023 K, CH4:Oz E 5 ) as well as in the absence of oxygen and at room temperature. We have found no studies published about the effects of rapid quenching of the catalyst by liquid water.
Experimental Section Materials. Commercial magnesium oxide (Schweizerhall, >97% purity) and lithium carbonate (Fisher, > 99.8% purity) were used as received. The procedures for preparing the Li/MgO catalyst have been described elsewhere (Tung and Lobban, 1992; Bhumkar and Lobban, 1992). Methane (99.97%), oxygen (99.99%), carbon dioxide (99.8%), and helium (99.999%) were obtained from Linde, and all gases were used without additional purification. Apparatus and Procedures. A conventional fixed bed flow-type reaction system was used. The quartz reactor consisted of a 7 mm i.d. tube tapered to 2 mm i.d. immediately below the catalyst bed to remove the reaction gases from the high temperature zone as quickly as possible in order to minimize gas phase reactions. Quartz chips and quartz wool were used as support for the catalyst bed and to reduce post-catalystbed dead volume. A thermocouple positioned against the outside wall of the reactor and a temperature controller (Omega CN1201j were used to control the reactor temperature. The actual temperature of the catalyst bed was calibrated in a separate experiment
0888-588519512634-1060$09.00/0 0 1995 American Chemical Society
Ind. Eng. Chem.Res., Vol. 34,No. 4, 1995 1061
CH4 02
c02
He He
( Feed Control Section )
( Reaction Section )
( Analysis Section )
Figure 1. Plug flow reactor experimental setup.
using a second thermocouple positioned in the center of the catalyst bed. Methane, oxygen, carbon dioxide, and helium flows were controlled by electronic mass flow controllers (Porter Instrument Co.) which could also be monitored using the computer data acquisition system. Figure 1 is a schematic diagram of the apparatus. The total pressure in the reactor was maintained at 1 atm with a total feed gas flow of 1000 SCCM (standard cubic centimeters per minute) for all the experiments. One gram of 20-40 mesh L a g 0 catalyst was used. The catalyst was pretreated for 2 h by heating to 873 K in the presence of 0 2 (30SCCM) before starting experiments in order to exclude the effects of the surface carbonates and to activate the catalyst. Then a total flow of 1000 SCCM He was started at the desired reaction temperature until no CO2 was detected. The gaseous H2O experiments were carried out by flowing helium through a water bubbler maintained at constant temperature in a water bath. The rate of water addition was adjusted using the helium flow rate and the temperature of the bath. The tube carrying the helium-water mixture was heated to 353 K to prevent any further condensation before the reaction zone. The gases were preheated to ~700-900K in the preheating zone located upstream of the reaction zone. The water was trapped in a water condenser afier the reactor. The liquid water treatment experiments were carried out using a syringe pump to inject deionized liquid water at a rate of 25 cm3/minimmediately upstream of the catalyst bed while at reaction conditions. Different quantities of liquid water were injected. The measured temperature (outside the reactor wall) dropped to =673 K and returned to reaction temperature within 1-2 min. The temperature inside the bed dropped to ~ 3 4 8 K and also returned to reaction temperature within 1-2 min. A mixture of ice and calcium chloride was used to maintain a water condenser at 253 K to remove H20 from the reactor effluent. The entire treatment process of injection, temperature drop and recovery, and water recovery took approximately 3-5 min. The analysis of the product stream started about 8 min after the injection when the system temperatures were stabilized.
Analysis. Products were analyzed via gas chromatography (CARLE S400 AGC equipped with thermal conductivity detectors). The bulk analyses of the catalysts were carried out using a Varian atomic absorption (AA) spectrometer. The BET surface areas of the catalysts were measured using a Micromeritics Flowsorb I1 2300. Results and Discussion
I. Steam Analysis. 1.1. Effect of Variation of P H ~Experiments . with two different water flow rates were carried out at p H 2 0 = 0.003 atm and p H 2 0 = 0.030 atm, otherwise identical operating conditions (2' = 1073 K and P c =~0.0825 atm and Po2 = 0.0175 atm). In both experiments, the deactivation of the catalyst was observed. The higher the feed partial pressure of water the faster the deactivation rate. Figure 2 shows that when the partial pressure of water in the feed was 0.03 atm, the methane conversion dropped from 6.8% to 0.4% during 30 h of operation. Within this period the 0 2 conversion dropped from 15.5% to 1% while the C2 selectivity varied between 85% and 90%. Figure 3 shows that when the partial pressure of H2O in the feed was 0.003 atm, the methane conversion dropped to 1.5% in approximately the same time while the C2 selectivity remained virtually constant at a value of 83% throughout. The data show that the steam is clearly very detrimental to the stability of the catalyst. It is hypothesized that volatile LiOH was formed and evaporated from the Li/MgO catalyst. When the H20 flow was stopped at t = 30 h, the catalyst activity shows no recovery over the next 2 h even though sufficient lithium is present that, under other circumstances, the catalyst might be active, as we will discuss later. After each steam experiment, it was noticed that the reactor walls had deteriorated. Potentially this is from the volatile lithium diffusing into the quartz to form an inert lithium silicate phase. This lithium depletion also probably contributes to catalyst deactivation. 1.2. Effect of Temperature. Experiments were conducted at two different temperatures (2' = 1073 K
1062 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995
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Figure 2. Change in CH4 and 02 conversions and CZselectivity as a function of time on stream over 1 g of 7% Li/MgO catalyst a t T = 1073 K,P C H=~0.0825 atm, Po, = 0.0175 atm, and P H ~= O 0.03 atm.
T i m (hr) Figure 3. Change in CH4 and 0 2 conversions and Cz selectivity as a function of time on stream over 1 g of 7% Li/MgO catalyst at T = 1073 K, PCH, = 0.0825 atm, Po, = 0.0175 atm, and PH,O = 0.003 atm.
and T = 1023 K),keeping the other operating conditions constant ( P a 0 = 0.003, Po2 = 0.0175, P c =~ 0.0825 atm). Comparison of the data plotted in Figure 3 and
Figure 4 show that the higher the temperature, the faster the deactivation rate. During the first 30 h of reaction period the methane conversion dropped by 56%
Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 1063 6
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Tima (M Figure 4. Change in C& and 0 2 conversions and C2 selectivity as a function of time on stream over 1 g of 7% Li/MgO catalyst at T = 1023 K,PCH,= 0.0825 atm, Po, = 0.0175 atm, and PH,O= 0.003 atm.
Table 1. Lithium Content and Surface Area for the Fresh and Used Catalysts in Steam Experiments" P ~ ino PCOZin feed (atm) feed (atm) Li (wt %) time (h) T (K) fresh catalystb 3.0 used catalyst 30 1073 0.03 0.0 0.54 34 1073 0.003 0.0 0.78 44 1023 0.003 0.0 1.01 7.5 1073 0.03 0.001 1.84 a
Conditions: 1 g of 7% Li/MgO, P C H=~ 0.0825atm, PO,= 0.0175atm, total flow = 1000 SCCM,and P,,,
at 1023 K and by 74% at 1073 K,while the oxygen conversion dropped by 55% a t 1023 K and by 80% at 1073 K. The C2 selectivity in both cases remained nearly constant. It is believed that at the higher temperature the lithium gradually evaporates from the surface of the catalyst. No measurable reaction was found at T = 873 K and PH~O = 0.03atm while PCHJPO~ varied between 1 and 10 with a total flow rate of 1000 SCCM. 1.3. Addition of H20 with C02. In a previous work (Al-Zahrani et al., 1994) we have shown that carbon dioxide adsorbed competitively on the active sites. The addition of carbon dioxide to the gas feed resulted in a poisoning effect of the catalyst, but it also stabilized the catalyst against deactivation. In the present experiment carbon dioxide was introduced to the system at PCO,= 0.001 atm in the feed. The other reactants were flowed a t the following partial pressures: PCH,= 0.0825,Poz = 0.0175,and P H ~=O0.03 atm. Reaction temperature was 1073 K. The methane and oxygen conversions before and after introduction of the carbon dioxide are shown in Figure 5. It is clear from this figure that the catalyst deactivation rate decreased when COa was introduced into the system. C2 selectivity after introduction of the carbon dioxide remained constant at 86%. This value is higher by 1.5%
surf. area (m2/g) 0.64 0.34 0.55 0.40 0.41
= 1 atm. After calcination.
than that before introduction of C02 to the system. These results indicate that the carbonate species were formed instead of the volatile species. The carbonate species are more stable but less catalytically active. The lithium content and the surface area of the fresh and used catalyst are listed in Table 1. Previous work (Al-Zahrani et al., 1994)indicated that the intermediate surface OH species postulated in our model (Al-Zahrani, 1994)which form when the surface oxygen species reacted with adsorbed methane, do not give rise t o the loss of lithium species from the catalyst directly. The results presented here indicate that the deactivation of the catalyst is enhanced a t higher P H ~ Pcoz and diminished a t low P H ~ ~ratio. ~ ~ c o ~ The overall results showed that the activity of the catalyst was influenced by the addition of steam to the gas feed although the Cp selectivity was relatively unchanged. Such results indicate that the total number of active sites in the catalyst was changed but the nature of the active sites was not changed appreciably. These results also support our proposed model (AIZahrani et al., 1994)which suggests the availability of a single active site for selective and nonselective methane conversion. We have observed in agreement with literature (Erekson et al., 1992;Van Kastern et al., 1989)that only
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1064 Ind. Eng. Chem. Res., Vol. 34,No. 4,1995
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very small amounts of lithium are sufficient to create an active and selective Li/MgO catalyst. The lithium amounts shown in Table 1 are sufficient for catalyst activity under other conditions, but the activity of the catalyst is nevertheless almost completely diminished. This probably indicates that the remaining lithium is in the form of the relatively inactive LizC03. Only lithium which is incorporated into the MgO structure forms the substitution type of solid solution, and correspondingly Li+O- centers. This is also in agreement with results (Krylov, 1993; Wolf, 1992) which reported that only a small part of the lithium was in the form of Li+O-, which formed during the preparation period. This explains why in this work, when HzO ffow into the system was stopped, the activity of the catalyst did not return to its initial value. 11. Liquid Water Treatment. 11.1. Activity of
the Catalyst before and after One Water Treatment. One gram of pretreated 7% L m g O catalyst was tested at 1023 K with the methane and oxygen flow rates 82.5 and 17.5 SCCM, respectively. The activity and selectivity of the catalyst before and after the injection are shown in Figure 6. The methane and oxygen conversions were nearly constant for a period of -24 h prior to the injection of HzO. Thirty milliliters of deionized liquid water a t room temperature was then injected into the reaction system immediately upstream of the reactor. Product stream analysis began 8 min after the injection and continued for 16 h. Methane and oxygen conversions in the first sample were several times higher than prior to the injection; subsequent samples showed first a rapid then a gradual drop in catalyst activity which leveled off to nearly constant values after about 15 h, at which time the catalyst activity was about double that before the injection. The CZ selectivity showed an initial drop, following the
Table 2. Catalyst Activity, Cg Selectivity, Lithium Weight Percent, and Surface Area before and after One Water Treatment
time (h) CH, conv (%) 0 2 conv (%) C2 sel (%I C2 yield (%) surf. area (m2/g) Li (wt %) HzO injected (mL)
before water treatment 24.0 3.9
9.5 79.1 3.1 0.64 3.0
after water treatment 15.0 7.3 18.2 74.2 5.4 0.98 0.44 30
injection, after which the selectivity increased to a constant value slightly less than that prior to the treatment. Table 2 shows the methane conversion, oxygen conversion, Cp selectivity, and Cp product yield before and after the water treatment. These values are the average of the last three gas chromatographic measurements (i.e., the numbers reported in Table 2 are the average of the final three nearly constant values before the injection and the average of the nearly constant final three values 15 h after the injection). The results suggest that the activity of the catalyst was significantly enhanced by direct water treatment of the catalyst bed under reaction conditions. Methane conversion was more than doubled with almost no loss in CZ selectivity. In this experiment, the enhanced catalyst activity attained a nearly constant value for a period of 16 h with no indication of further decline at the end of this period. Ethylene production is affected the most by the water treatment compared to the other products. The rate of ethylene production after the water treatment is around
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”(hr) Figure 6. C& conversion, 02 conversion, and CZselectivity versus time over 1 g of 7% Li/MgO catalyst at P C H=~ 0.0825 atm, Poz = 0.0175 atm, and T = 1023 K, with 30 mL injected a t t = 24.0 h.
5 times that before the treatment, while all other production rates are approximately doubled. II.2. T w o Water Treatments in Series. To further illustrate water’s effect, two other experiments were carried out under conditions similar to the above experiments. In the first experiment, the catalyst bed was treated twice with 30 mL of water each time. In the second experiment, the bed was treated twice with 15 mL of liquid water. In both cases, the methane and oxygen conversions were increased sharply immediately after each treatment in a fashion qualitatively similar to that already described. An interesting result is that carbon monoxide appeared in the effluent gas stream only after the second water treatment. Following the second treatment the effluent CO concentration gradually increased to a constant value. In the first experiment (two treatments with 30 mL), the methane conversion increased following the first treatment but was unaffected by the second treatment. The oxygen conversion after the second water treatment was slightly higher than that after the first water treatment. This behavior can be explained by the production of carbon monoxide as will be discussed later. The Cp selectivity decreased slightly following the first water treatment and decreased from 75.4% to 64.0% following the second treatment as illustrated in Figure 7 and Table 3. The Cp yield was 2.64% before the water treatments and increased to 4.5% after the second treatment. The cause of the Cp selectivity drop following the second water treatment is believed to be further oxidation of Cp to produce carbon monoxide which was observed after the second water treatment. Similar results of the second experiment (treatment two times with 15 mL of HpO) are shown in Figure 8
Table 3. Catalyst Activity, CZSelectivity, Lithium Weight Percent, and Surface Area before and after Two Water Treatments of 30 mL before water treatment time (h) CHI conv (5%) 02 conv (%)
CZsei(%) CZyield (%) surf. area (mz/g) Li (wt %)
4.0 3.5 11.5 75.4 2.6 0.64 3.0
HzO injected (mL)
after first water treatment 10.0 7.2 19.5 75.6 5.4 30
after second water treatment 12.0 7.0 20.4 64.0 4.5 1.02 0.30 30
Table 4. Catalyst Activity, Cz Selectivity, Lithium Weight Percent, and Surface Area before and after T w o Water Treatments of 16 mL before water treatment time (h) CHI conv (%) 02 conv (%) CZsel (%I CZyield (%) surf. area (mz/g) Li wt (%) HzO injected (mL)
4.0 3.6 7.8 77.9 2.8 0.64 3.0
after first water treatment 4.0 4.9 12.7 79.0 3.9 15
after second water treatment 10.5 7.0 20.7 74.8 5.2 0.97 0.49 15
and Table 4. The final methane conversion after the second treatment was 95.8% higher than the initial conversion (prior to any water treatment). The final Cp selectivity for the second experiment is greater than the final Cp selectivity in the first experiment probably due t o less Cp oxidation to CO. Lithium loss and decrease in CZ selectivity were observed in both of these experiments as shown in Table 3 and Table 4. Interestingly, the lithium content of
1066 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 20
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n m (hr) Figure 7. CHI conversion, 02 conversion, and CZ selectivity versus time over 1 g of 7% L m g O catalyst at PCH,= 0.0825 atm, Poz = 0.0175 atm, and T = 1023 K, with 30 mL injected at t = 4.0 and t = 14.0 h.
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Figure 8. CH4 conversion, 02 conversion, and C2 selectivity versus time over 1 g of 7% L m g O catalyst at P C H=~0.0825 atm, Po, = 0.0175 atm, and T = 1023 K, with 15 mL injected at t = 4.0 and t = 8.0 h.
these catalysts is less than that of the steam-treated catalysts (Table 11,yet the activity of these catalysts is quite high. Further experiments were carried out in
which small amounts of liquid water were injected to the catalyst bed. Also, the lithium content of the fresh catalyst was increased. These experiments were at-
Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 1067 20
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Time (hr) Figure 9. CH4 conversion, 02 conversion, and C Z selectivity versus time over 1 g of 7% W g O catalyst at Pc, = 0.0825 atm, PO, = 0.0175 atm, and T = 1023 K, with 5 mL injected a t t = 3.5, t = 8.0, and t = 12.5 h. Table 5. Catalyst Activity, CZSelectivity, Lithium Weight Percent, and Surface Area before and after Three Water Treatments
time (h) CH4 conv (%) 02 conv (%) CZsei(%) CZyield (%) surf. area (m*/g) Li (wt %) HzO injected (mL)
after before after after water first water second water third water treatment treatment treatment treatment 3.5 4.5 4.5 3.0 3.9 6.6 6.5 8.6 9.5 14.2 9.5 20.3 78.6 79.5 78.7 73.5 3.0 5.2 5.1 6.3 0.64 1.10 3.0 0.51 5.0
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tempts to achieve and maintain higher Cp selectivity. The results will be given in the following sections. Under the same operating conditions, the catalyst was subjected to three consecutive treatments of 5 mL each. The methane conversion after the third water treatment was increased by 123.3%above the conversion prior to any water treatment. The C p selectivity decreased from 79% to 74%. The Cp yield was 3.03%before the water treatment and increased to 6.34%after the third water treatment. Experimental results are shown in Figure 9 and Table 5. From this series of experiments, we conclude that injection of multiple small amounts of liquid water gives better results for methane conversion and Cp selectivity than injection of a single large amount. Carbon monoxide appeared after the second water treatment in all cases but in smaller amounts in the case of 5 mL treatment than the other treatments. The water treatments of 30 and 15 mL yielded larger transient peaks of methane and oxygen conversion than did the 5 mL treatment. Following the peaks, the decline in methane and oxygen conversions to the new steady state took a longer time following the 30 and 15 mL treatments than following the 5 mL treatment.
11.3. Water Treatment at Low 0 2 Partial Pressure and in the Absence of 0 2 . To help elucidate the mechanism of the catalyst activity enhancement, we decided to inject the water under varying reaction conditions. In the first of these experiments, the oxygen partial pressure was held very low. This situation is also of interest since the maximum Ca selectivity can be reached when the partial pressure of oxygen is very low. Other reaction conditions are similar t o previous experiments. After the catalyst pretreatment, the experiment was carried out by flowing methane and oxygen at 82.5 and 5 SCCM, respectively. The helium was flowed to keep a total flow a t 1000 SCCM. The reaction was continued for 4 h and then 30 mL of deionized liquid water was injected into the bed. Similar results for methane and oxygen conversions were observed as in the previous experiments. The Cp selectivity remained relatively constant a t an average value of 87% which is higher than that of previous experiments due to high methane to oxygen ratio in this experiment. These results are illustrated in Figure 10 and Table 6. The catalyst activity is significantly enhanced even in the presence of very low partial pressure of oxygen. An experiment was also conducted to determine the effects of the water treatment of the catalyst in the absence of oxygen under the same operating conditions as the above experiments. The aim of this experiment is to determine the effects of the liquid water on the catalyst activity in the absence of gas phase oxygen. We suspected that gaseous oxygen was necessary for generation of more active sites as will be discussed later. After the catalyst pretreatment, the experiment was carried out by flowing methane a t 82.5 SCCM with helium as balance gas to keep a total flow a t 1000 SCCM. No oxygen was flowing at this time. Thirty
1068 Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 10.0
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Figure 10. CH4 conversion, 0 2 conversion, and CZselectivity versus time over 1 g of 7% Li/MgO catalyst at P C H=~ 0.0825 atm, Po2 = 0.005 atm, and 2’ = 1023 K with 30 mL injected at t = 4 h. Table 6. Catalyst Activity, Cz Selectivity, Lithium Weight Percent, and Surface Area before and after One Water Treatment at Low Oxygen Partial Pressure
~~~
time (h) CHI conv (%I 0 2 conv (%) CZsei(%) C2 yield (8) surf. area (m2/g) Li (wt %) HzO injected (mL)
before water treatment 4.0 2.4 16.9 88.1 2.1 0.64 3.0
after water treatment 8.0 4.7 39.2 87.3 4.1 0.94 0.53 30.0
milliliters of deionized liquid water was then injected into the bed. After 5 min, the oxygen was then flowed into the system at a rate of 17.5 SCCM and the total gas flow was maintained at 1000 SCCM by adjusting the helium flow. Methane conversion, oxygen conversion, and Cp selectivity showed trends similar to those observed following treatment in the presence of oxygen as illustrated in Figure 11. The methane conversion started at a high value and then gradually decreased until it reached a constant value. Table 7 compares the effects of water treatment in the presence and in the absence of oxygen. Table 7 shows that the water treatment of the catalyst in the presence of oxygen gives a higher methane conversion and CZ yield than the treatment in the absence of oxygen. This suggests that gas phase oxygen might be important for the generation of the active sites. However, gas phase oxygen is not absolutely necessary, probably because of the availability of adsorbed and lattice oxygen. 11.4. Quenched Catalyst. In order to investigate whether the activity enhancement by water treatment
Table 7. Comparison between the Iqjection in the Presence and Absence of 0 2 after first after first water treatment in the presence of 0 14.0 time (h) CH4 conv (%) 7.3 18.2 0 2 conv (5%) Cz sel (%) 74.2 5.4 CZyield (%I surf. area mVg 0.98 0.44 Li w t (9) 30.0 HzO injected (mL)
2
water treatment in the absence of 0 14.0 5.6 20.3 73.7 4.1 0.93 0.50 30.0
2
requires reaction conditions or can be achieved solely through modifications of the catalyst preparation procedure, a “quenched catalyst’’was tested. The catalyst was 7% Li/MgO, but after the calcination of the catalyst sample at 1073 K for 6 h, the sample was quenched using deionized room temperature liquid water. The quenched sample was then dried in a vacuum oven at 413 K overnight. Afterward, the sample was prepared in the usual fashion (Tung and Lobban, 1992;Bhumkar and Lobban, 1992). The experiments were conducted under the same reaction conditions as described in the first experiment. The results obtained from this catalyst are quite similar t o the normal (no water treatment) catalyst, which indicates that quenching the catalyst during catalyst preparation did not enhance the catalyst. However, when the modified-preparation catalyst was treated with water under reaction conditions, enhanced activity was achieved. Figure 12 and Table 8 illustrate the results from this experiment compared to the normal catalyst. 11.5. 3w0Li/MgO. After several water treatments, the 7% LiiMgO catalyst suffered lithium loss and nearly complete loss of activity. We speculated whether an
Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 1069
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Tim (hr) Figure 11. CH4 conversion, 0 2 conversion, and C2 selectivity versus time over 1 g of 7% L m g O catalyst at PCH,= 0.0825 atm, Poz = 0.0175 atm, and T = 1023 K,with 30 mL injected initially in the absence of oxygen.
Table 8. Comparison between the Quenched Catalyst and Normal Catalyst before and after One Water Treatment
Table 9. 30% Li/MgO Catalyst Activity, Ca Selectivity, Lithium Weight Percent, and Surface Area before and after Three Water Treatments
quenched catalyst normal catalyst before after before aRer water water water water treatment treatment treatment treatment time (h) 9.0 3.0 23.0 15.0 7.3 C& conv (%) 3.6 8.6 3.9 18.2 0 2 conv (%) 8.5 23. 9.5 74.2 C2 sel(%) 78.2 74. 79.1 5.4 C2 yield (%) 2.8 6.3 3.1 0.98 surf.area (m2/g) 0.64 1.1 0.64 0.44 Li (wt %) 3.0 0.33 3.0 30.0 H2O injected (mL) 30.0
after after after before water first water second water third water treatment treatment treatment treatment time (h) 1.0 5.0 10.0 19.0 CHI conv (%) 2.1 4.0 9.0 4.5 02 conv (%) 6.9 10.2 25.0 14.2 C2 sel (%) 80.60 74.1 69.2 53.5 1.71 3.0 6.2 2.4 C2 yield (%) surf area (mz/g) 0.22 1.9 Li (wt 5%) 11.7 1.72 HzO injected (mL) 30.0 30.0 30.0
initial higher lithium content might permit even greater activity enhancement. A new catalyst was prepared using exactly the same procedures and the same conditions as those used to make the 7% Li/MgO, except this time 30% lithium was used in the catalyst. The porcelain calcining dishes used to prepare the 30% Li/ MgO catalyst showed a brown crust formation fixedly adhered to the crucible wall after the calcination period. The experiment was conducted under conditions similar to those used for the previous experiments. The results are illustrated in Figure 13 and Table 9. Deionized liquid water in the same quantity each time (30 mL) was injected. The observed methane and oxygen conversions and the selectivity are similar to that observed using the 7% Li/MgO catalyst, but there are two major differences. First, after the second water treatment, both methane and oxygen conversion increased sharply to a high level and stayed a t almost the same level for more than 10 h before the third water injection. In previous experiments immediate drop of the conversions would follow
the sharp peak. The second difference is that the CZ selectivity is less than that achieved with 7% Li/MgO. This might be due t o strongly bonded carbonate, which rendered the catalyst less active. Table 9 shows that after the second water treatment CZ yield increased from 1.71% to 6.23%, which is a greater increase than was observed using the 7% Li/MgO catalyst under the same conditions. These results suggest that the lithium content in the catalyst plays an important role in the enhancement of the catalyst activity by water treatment. It has been mentioned earlier that the rate of ethylene production after the water treatment is around 5 times the rate before treatment. Two different experiments were carried out to determine whether the water treatment enhanced the catalyst's activity for oxidative dehydrogenation of ethane. In both experiments no methane was flowing. In the first experiment, the ethane flow rate was varied between 2 and 20 SCCM, while 0 2 flow rate was maintained at 10 SCCM with helium as a balance gas in the presence and in the absence of the catalyst at T = 1023 K. No reaction was detected in the absence of
1070 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995
.
20
6
.'n '. '8,
&
15
. J
,
i 0
f
,..':
10
o...*...*...*
5
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.* 4
60
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00..
*=-*...e.............o...o
.
. nm (hr)
Figure 12. CH4 conversion, 02 conversion, and CZselectivity versus time over 1 g of 7% Li/Mg€l catalyst a t P C H=~ 0.0825 atm, PO,= 0.0175 atm, and T = 1023 K,with 30 mL injected a t t = 9 h on the quenched catalyst.
10
20
30
100 I
1
40
1
10
20
30
1
40
Tim (hr)
Figure 13. CH4 conversion, 0 2 conversion, and CZselectivity versus time over 1 g of 30% Li/MgO catalyst at PCH,= 0.0825 atm, Po2 = 0.0175 atm, and T = 1023 K,with 30 mL injected at t = 2.0,t = 7.0,and t = 16.0 h.
the catalyst. Figure 14 shows that in the presence of the catalyst the increase of C P His ~ almost linear with the increase of CzHs concentration over the whole range.
The catalyst's methane coupling activity is enhanced by the water treatment, but if its ethane dehydrogenation activity were unaltered, one would expect the ethane:
Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 1071 .12
1
I
I
1 .013
I
J
re*-..... ........ /** .p
.09 -
#*V
-0
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-011
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C,H, in the feed (mmoldmin.) Figure 14. Formation rates of CzH4 and CO, versus CzH6 inlet flow rate over 1 g of 7% L m g O catalyst a t PO,= 0.01 atm and T = 1023 K.
e
d
d 2
d
0
0"
5
I
15
10
20
z
3
4
0
Tlme (hr)
Figure 15. CzHs conversion, 0 2 conversion, and CzH4 selectivity versus time over 1 g of 7% LilMgO catalyst at Pc,% = 0.02 atm, PO, = 0.01 atm, and T = 1023 K,with 30 mL injected at t = 4.0 h.
ethylene ratio to remain unchanged by the water treatment. That the catalyst's ethane dehydrogenation activity was enhanced was confirmed by a second experiment. In the second experiment, C2H6 and 02 were flowed a t a rate of 20 and 10 SCCM, respectively. After the reaction continued for 4 h, 30 mL of liquid water was then injected into the reaction system. The ethane and oxygen conversions jumped initially to high values and then dropped gradually and leveled off at constant values. Methane appeared after the water treatment in trace quantities. The results illustrated in Figure 15 and Table 10 show enhancement of ethane oxidative dehydrogenation by the water treatment. Table 10 shows that the catalyst favored the oxidative dehydrogenation of ethane after the water treatment.
Table 10. Effect of Water Treatment on the Oxidative DehydrogenationReaction
time (h) CzH6 conv (%) 0 2 conv (%) CzH4 sei(%) C2H4 yield (%) surf. area (m2/g) Li (wt %) HzO injected (mL)
before water treatment 4.0 12.3 13.5 92.0 11.3 0.64 3.0
affer water treatment 12.0 17.5 25.3 88.9 15.3 0.91 0.42 30.0
11.6. Wall Effect. Additional experiments were carried out to ensure that lithium deposited on the walls of the reactor was not responsible for the increased
1072 Ind. Eng. Chem. Res., Vol. 34,No. 4,1995
reaction rate. In the first experiment the catalyst was removed after one water treatment of 30 mL, while in the second experiment the catalyst was removed after two water treatments of 30 mL. The reactants were then fed to the empty reactor a t 1023 K. No reaction products were detected. In the third experiment, the catalyst was removed after one liquid water treatment of 30 mL and replaced by an equal amount of fresh catalyst to see if there are wall effects in the presence of the methyl radicals. The results of this experiment also indicate that the wall has no effect on the reaction. After this experiment we concluded that the enhancement in the catalyst activity was not related to the quartz wall or the deposited lithium. 11.7. Causes for the Catalyst Activity Enhancement. We also observed an increase in catalyst specific surface area due to the liquid water treatment. However, the increase in surface area alone does not explain our results. In a separate experiment, 1g of the catalyst was treated with 30 mL of liquid water four times. Following the fourth injection the reaction rate dropped to zero (i.e., almost no activity) and no lithium was found in the remaining catalyst. The final surface area was 1.5 m21g. In another experiment, when 2 g of the catalyst was used a t 873 K (a relatively low temperature), no detectable reaction was observed. However, following a single treatment of 1 g of catalyst, a measurable amount of reaction was observed when the operating temperature was 873 K. These experiments indicate that the enhanced catalytic activity is not due solely to increase in surface area. The experimental results have generally two features in common. The first is an immediate short-lived peak in methane conversion following the injection of water. The maximum in methane conversion corresponds to a minimum in CZ selectivity. The second feature is the long-term increase in methane conversion at high selectivity. We speculate that the first feature is caused by the following mechanism: 2Li'O-
2Li'OH-
=*
-
+20H Li+02- + L i b + H,O
+ 2H,O
2Li'OH-
(1) (2)
The overall stoichiometry of these steps is the production of hydroxyl radicals, Le.,
H,O
+ (1/2)0, - 2 0 H
(4)
The reaction of the hydroxyl radicals with methane accounts for the maximum in conversion and minimum in selectivity since the reaction of hydroxyl radicals with methane is expected t o lead to carbon oxides. We speculate that the water also reacts with the relatively inactive lithium carbonate as follows: Li,+CO,2Li'OH-
+ H,O
-
-
2Li'OH-
Li+02-
+ Li'O
+ CO, + H,O
(5)
(6)
The net of this mechanism, which requires the participation of oxygen, is the conversion of inactive
lithium carbonate to active Li+OLi2+C03-
-
+ (1/2)02
2Li'O-
+ CO,
(8)
LiOH decomposes at 723 K (Kaminsky et al., 1992). Under normal reaction conditions for the Li/MgO catalyst (2' > 923 K), Li+OH- produced during the reaction (e.g., by steam treatment) decomposes so that reactions 2 and 3 (6 and 7) do not occur. The injection of room temperature liquid water, on the other hand, provides a low temperature environment which makes the produced LiOH stable so that more active sites Li+O- can be produced by steps 2 and 3 or 6 and 7. Several liquid water treatments in series eventually decrease the catalyst activity to a low level, since with each water treatment some LiOH is volatilized before conversion to Li+O-. When steam is introduced into the system under reaction conditions, the same activity enhancement is not observed. The steam accelerates the deactivation rate and does not provide a low temperature environment which makes the intermediate LiOH stable. Recently Kaminsky et al. (1992) observed the presence of alkyllithium (CH3Li) on the surface of the catalyst. They reported that CH3Li would volatilize and migrate some distance before dissociating homolytically t o CH3 and Li radicals. During this time and in the presence of Hz0, CH3Li might react with H20 according to the following reaction: 2CH3Li
+ H20
-
2CH,
+ Li,O
(9)
The Liz0 is likely to react further with the 0 2 since it is less stable than LiOH. The net reaction to create more active sites is 2Li,O
+ 0,
-
4(Li+O-)
(10)
This is another possible source of increasing the number of active sites. It should be noted that interactions between liquid water and the solid at high temperature are probably considerably more complex. Additional insight into the nature and distribution of surface species using surface analysis techniques will be of great importance to provide more information on the effects of liquid HzO on the catalyst activity. Further investigations along these lines are currently under way in our laboratory.
Conclusions The addition of HzO to the gas feed is very detrimental to the stability of the catalyst. The higher the partial pressure of the steam, the faster the deactivation rate. Also, the higher the temperature, the faster the deactivation rate. In the presence of steam, the deactivation of the catalyst can be retarded if a low concentration of C02 is added to the reaction mixture. The steam results also show that the lithium is present at least in two forms, i.e., Li+O- and inactive LizC03. The activity of the catalyst can be enhanced significantly by liquid water treatment of the catalyst at reaction conditions. Cz selectivity remains relatively unchanged. The liquid water lowers the catalyst temperature sufficiently that the LiOH formed reacts further to create active sites rather than rapidly decomposing or being lost to the gas phase. When steam is added, LiOH is formed but at the high temperature the LiOH decomposes too rapidly for active sites to form.
Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 1073 Multiple injections of small amounts of liquid water at low oxygen partial pressure show significant improvement in both methane conversion and CZselectivity. The catalyst activity for ethane oxidative dehydrogenation is enhanced by the water treatment. The lithium content after the water treatment is remarkably low compared to that of the fresh catalyst, and it approaches zero if too many injections are carried out. The quartz reactor walls have no measurable effect on the reaction.
Acknowledgment This work has been supported by the US.Department of Energy.
Literature Cited Al-Zahrani, S. M. Oxidative Coupling of Methane over Li/MgO Catalyst : The Effects of Gas Composition and Process Conditions., Ph.D. Dissertation, University of Oklahoma, Norman, 1994. Al-Zahrani, S.; Song, Q.; Lobban, L. L. Effects of COz during Oxidative Coupling of Methane over Li/MgO: Mechanisms and Models. Ind. Eng. Chem. Res. 1994,33 (2),251. Bhumkar, S. C.; Lobban, L. L. Diffuse Reflectance Infrared and Transient Studies of Oxidative Coupling of Methane over Li/ MgO Catalyst. Ind. Eng. Chem. Res. 1992,31, 1856. Chang, Y. F.; Somorjai, G. A.; Heinemann, H. Oxidative Coupling of Methane over Mg-Li Catalysts at Relatively Low Temperature -The Effect of Steam. J . Catal. 1993,141,713. Driscoll, D. J.; Martir, W.; Wang, J.; Lunsford, J. H. Formation of Gas-Phase Methyl Radicals over MgO. J . Am. Chem. SOC.1985, 107,58. Erekson, E. J.; Lee, A. L. Conversion of Methane to Ethylene. Symposium on Natural Gas Upgrading II; American Chemical Society: Washington, DC, 1992;Vol. 37,No. 1, p 98. Ito, T.; Wang, J.; Lin, C.; Lunsford, J. H. Oxidative Dimerization of Methane over a Lithium-Promoted Magnesium Oxide Catalyst. J. Am. Chem. Soc. 1985,107,5062.
Iwamatsu, E.; Moriyama, N.; Takasaki, N.; Aka, J. Importance of the Specific Surface Area of the Catalyst in Oxidative Dimerization of Methane over Promoted Magnesium Oxide. J . Chem. SOC.,Chem. Commun. 1987,19. Kaminsky, M. P.; Huff, G. A.; Spangler, M. J.; Kobylinski, T. P. Deactivation of Li-Based Catalysts for Methane Oxidative Coupling. Symposium on Natural Gas Upgrading II; American Chemical Society: Washington, DC, 1992;Vol. 37,No. 1, p 89. Kimble, J. B.; Kolts, J. H. Oxidative Coupling of Methane to Higher Hydrocarbons. Energy Prog. 1986,6 (4),226. Korf, S.J.; Roos, J. A De Bruijn, J. G.; Van Ommen, J. G. Lithium Chemistry of Lithium doped Magnesium Oxide Catalysts Used in the Oxidative Coupling of Methane. Appl. Catal. 1990,131. Krylov, 0.V. Catalysts for Methane Oxidative Coupling and its Reaction Mechanism. Kinet. Catal. 1993,34 (11,18. Matsukata, M.; Okanari, E.; Komori, K.; Matsuda, E.; Kikuchi, E.; Morita, Y. Oxidative Coupling of Methane Over Li-Doped MgO: Enhancement of Catalytic Activity by Steam. Symposium on Methane Activation, Conversion, and Utilization, International Chemical Congress of Pacific Basin Societies; 1989;p 58. Otsuka, K.; Jinno, K Morikawa, A. The Catalysts Active and Selective in Oxidative Coupling of Methane. Chem. Lett. 1985, 499. Tung, W.; Lobban, L. L. Oxidative Coupling of Methane over Li/ MgO: Kinetics and Mechanism. Ind. Eng. Chem. Res. 1992,31, 1621. Van Kasteren, J . M.; Geerts, J. W.; Van der Wiele, K. Working Principle of Li Doped MgO for the Oxidative Coupling of Methane. Preprints First World Congress: “New Developments in Selective Oxidation”, Rimini, Italy, Sept 18-22,1989;Elsevier: New York.
Received for review September 19, 1994 Accepted January 5,1995 * I39405495
* Abstract published in Advance ACS Abstracts, March 1, 1995.
