1856
Ind. Eng. Chem. Res. 1992,31, 1856-1864
Texture of Solids. J. Colloid Interface Sci. 1979,70,66. Linares-Solano, A.; Rodriguez-Reinoso,F.; Salinas-Martinez de Lecea, C.; Mahajan, 0. P.; Walker, P. L., Jr. Platinum Catalysts Supported on Graphitized Carbon Black-I, Characterization of the Platinum by Titrations and Differential Calorimetry. Carbon 1980,20, 177. Machek, V.; Ruzicka, V.; Sourkova, M.; Kunz, J.; Janacek, L. P r e p aration of PtJActivated Carbon and PtJAlumina Catalysts by Impregnation with Platinum Complexes. Collect. Czech. Chem. Commun. 1983,48,517. Mattson, J. S.; Mark, H. B., Jr. Activated Carbon: Surface Chemistry and Adsorption from Solution; Dekker: New York, 1971. Morikawa, K.;Shirasaky, J.; Okada, M. Correlation Among Methods of Preparation of Solid Catalysts, Their Structures, and Catalytic Activities. Adv. Catal. 1969,20,97.
Pradc-Burguete, C.; Linares-Solano, A.; Rudriguez-Reinoso,F.;Salinas-Martinez de Lecea,c. The Effect of Oxygen Surface Groups of the Support on Platinum Catalysts. J. Catal. 1989,115, 98. Richard, D.; Gallezot, P. Preparation of Highly Dispersed, Carbon Supported, Platinum Catalysts. In Preparation of Catalysts ZV; Delmon, B., Grange, P., Jacobs, P. A., Poncelet, G., Eds.; Academic Press: Amsterdam, 1987;pp 71-81. Rylander, P. N. Nitro Compounds. In Catalytic Hydrogenution over Platinum Metals; Academic Press: New York, 1967; Chapter 11. Rylander, P. N. Catalytic Processes in Organic Conversions. In Catalysis; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: New York, 1983;Vol. 4,Chapter 1. Received for review January 24, 1992 Accepted May 19, 1992
Diffuse Reflectance Infrared and Transient Studies of Oxidative Coupling of Methane over Li/MgO Catalyst Soujanya C. Bhumkar and Lance L. Lobban* School of Chemical Engineering and Materials Science, University of Oklahoma, Norman, Oklahoma 73019
In situ studies of surface species at steady and unsteady state were conducted using Fourier transform infrared spectrometry and transient analysis to study the oxidative coupling of methane over a Li/MgO catalyst. The diffuse reflectance Fourier transform infrared spectroscopic (DRIFTS) technique was employed to monitor the surface features of the catalyst and ita interaction with adsorbates under reaction conditions. The effect of C02on the catalyst activity and selectivity was also studied, and a model based on evidence of the heterogeneous steps and the intermediates involved in the reaction was proposed. Introduction There is much interest in the conversion of natural gas to higher value products such as ethane and ethylene (C2), and a large number of catalysts have been shown to be active for the oxidative coupling of CHI. However, in addition to the desired Cz products, the oxidative coupling process also yields undesirable products such as carbon oxides ((20,). Keller and Bhasin (1982) first proposed a mechanism for the oxidative coupling of methane over a metal oxide catalyst. Other researchers have investigated the kinetics of the reaction over various catalysts, and several mechanisms have been proposed. Lunsford and co-workers (Ito et al., 1985; Driscoll and Lunsford, 1985; Wang and Lunsford, 1986; Driscoll et al., 1985; Aika and Lunsford, 1977; Lunsford, 1984) have characterized the active centers for the formation of methyl radicals over Li/MgO catalyst. Korf et al. (1987) have studied the influence of COPon the reaction, emphasizing the effects of products on the activity and selectivity of the catalyst. The oxidative coupling of CHI over Li/MgO was studied by Tung and Lobban (1992). They used steady-state measurements to discriminate between simple reaction mechanisms. However, these mechanisms did not include the influence of products, nor was any direct information on the surface species' or intermediates' concentrations available. Many other studies have been undertaken (Amenomiya et ai., 1990; Dubois and Cameron, 1990; Garibyan and Margolis, 1990, Lee and Oyama, 1988),but, to date, there have been few direct measurements of the surface intermediates under reaction conditions. It is extremely valuable to observe surface speciea under reaction conditions to determine the changes that occur
* To whom correspondence should be addressed.
in response to changes in the operating conditions. In order to understand important catalyst characteristics and to develop a more complete reaction mechanism, therefore, in situ investigations of surface species at steady and unsteady state were carried out using Fourier transform infrared spectrometry (FTIRS). In particular, the diffuse reflectance Fourier transform infrared spectroscopic (DRIFTS)technique was employed to monitor the surface features of a Li/MgO catalyst and its interaction with adsorbates under reaction conditions. This technique has been similarly employed in other in situ studies of surface species (Ferraro and Basile, 1982; Fuller and Griffiths, 1978; Hamadeh et al., 1984; Schraml-Marth et al., 1990; Prairie et al., 1991). Transient studies were also conducted in a tubular plug flow reactor (PFR) in which differential conversion conditions were maintained. The PFR allows for a high catalyst volume to void volume ratio, which is desirable in transient and dynamic experiments. Reaction paths were distinguiehed qualitatively by the transient responses following changes in reactant partial pressure. Experimental Section FTIR experiments were carried out using a Digilab FTS-40 FTIR spectrometer equipped with a 3200 Data Station computer. Spectra were obtained at various resolutions (8, 4, and 1 cm-') and are presented in the absorbance format. The absorbance format has been shown to be more appropriate for neat samples (i.e. samplea which are not diluted in any matrix) with highly absorbing species than is the Kubelka-Munk format commonly used for diffuse reflectance measurements (Smryl et al., 1983). A Harrick Scientific evacuable diffuse reflectance accessory (DRA) with 'praying mantis" design [Model HVC-DRB] was employed as a reactor (Figure 1). K type thermocouples were used for temperature measurements, and the
0888-5885f 92f 2631-1856$Q3.QQ f 0 0 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 8,1992 1857 Dome
,
Table 11. Bulk Analysis Using Atomic Absorption Spectroscopy -. catalyst typea Li (g of Li/g of catalyst) % loss of Li fresh 0.0422 conditioned 0.0401 5 used 0.0330 22
ObservationWindow Window for IR
Catalyst Bed -Gas Inlet Port
*Heating
Element
a Key fresh, catalyst not subjected to any reaction; conditioned, catalyst subjected to oxygen treatment at 500 OC for 5 h; used, catalyst subjected to oxidative coupling reaction for 48 h.
, , I
4
8
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5
3
.____ Mass Flow Controllers
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-
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Figure 2. Experimental setup for FTIR experiments. Table I. Specific Surface Area4 of Catalyst and Its Components specific surface area (m2/e) SamDle Li/MgO catalyst (fresh) 0.86 Li/MgO catalyst (used) 0.67 Li2C03powder 1.46 MgO powder 39.79 52.39 MgO catalyst (20-40 mesh size) a
Determined using BET method of area analysis.
catalyst temperature was controlled using an Omega CN 300 Series temperature controller. Catalyst temperatures as high as 720 "C were achieved by appropriately modifying the sample holder. The FTIR was connected in series to a Balzers quadrupole mass spectrometer (QMS 420) for transient analysis and a Carle gas chromatograph (Series 400 AGC) for on-line analysis of product gases. The experimental setup used for FTIR experiments is shown in Figure 2. The PFR setup consisted of a 7-mm quartz tube with inner diameter tapering to 2 mm immediately downstream of the catalyst which helped eliminate the postbed gasphase reactions. The PFR was placed in a furnace, and reactant flow rates were controlled using electronic mass flow controllers. Step changes in reactant flow rates to the PFR were made using an air actuated switching valve to switch two flows. The experimental setup for PFR experiments is similar to the setup shown in Figure 2, with the modification that the FTIR is replaced by the PFR. Other gas flow and analytical systems are unchanged. The catalyst was prepared by the impregnation technique as described elsewhere (Tung and Lobban, 1992). Surface area measurements were carried out on the catalyst using the BET method with a Micromeritica Flowsorb I1 2300 surface area analyzer. The specific surface areas of MgO and Li2C03 powder and MgO particles (20-40 mesh size) were also determined and are listed in Table I. Bulk analyses of the catalyst were also carried out using a Varian atomic absorption (AA) spectrometer. AA analyses of a fresh catalyst and a catalyst subjected to 48
Table 111. Experimental catalyst catalyst particle size lithium composition of fresh catalyst density of catalyst operating temperature pressure reactants products carrier gas
Detail6 lithium carbonate-doped magnesium oxide 20-40 mesh 7 w t % (Li/Li + MgO) 1.3 g/cm3 600-800 OC 1 atm CHI, 02
CzHit COzs CO, HzO helium or argon
h of reaction at 750 "C in the PFR indicated a loss of about 22% lithium (Table 11). Experimental details and other physical characteristics of the catalyst are listed in Table 111. A catalyst of 20-40 mesh size was used in all PFR experiments, but for FTIR experiments smaller particles were used. The smaller particle size decreases the possibility of spectrum variation for the same sample and also helps to minimize or eliminate reststrahlen bands ('inverted bands") caused by specular reflection from large particle faces. DRIFTS measurements of neat samples usually do not provide as much information as measurements of a sample diluted in an infrared transparent matrix such as potassium halides (KBr, KI, etc.) because the high IR absorbance of bulk modes obscures spectral features of low concentration species. However, ion exchange would probably occur between KBr matrix ions and the lithium-containing catalyst at the relatively high reaction temperatures (600-750 "C). Therefore, experiments were conducted using neat samples of small particle size. The FTIR sample compartment was purged of water vapor and carbon dioxide using air treated by a Balston air filtration system, and thus the spectra obtained were devoid of interferences from atmospheric COz and water vapor. It is also necessary to remove the artifacts from measured spectra caused by thermal expansion of the sample holder. A strategy to overcome this problem is described by White and Nair (1990) in which the effects from thermal expansion of the sample holder are corrected by subtracting reference spectra obtained at the same temperature. Subtraction spectra were obtained by subtracting the spectrum of the catalyst before a change from the spectrum after the change was completed and steady state reached, both spectra were obtained at the same temperature. Thus in the subtraction spectrum, features below the base line represent a decrease in surface concentration, and features above the base line indicate an increase.
Results Results of FTIR Experiments. Blank experiments were carried out in the DRIFTS reactor while CHI, 02,and AI were flowed through the reaction chamber. A t temperatures below 720 OC, no conversion of CHI was observed. Above 725 "C,C02 was observed in the reactor effluent, and hence experimental studies were conducted up to 720 "C. Reactor characterization studies were also conducted in order to understand the flow behavior of the DRIFTS reactor. Changes in gas-phase concentrations
1858 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992
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Figure 3. Spectrum of fresh Li/MgO catalyst at room temperature. I.EO
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Figure 5. Subtraction spectrum of Li/MgO catalyst made by subtracting the reference spectrum (690 "C with only Ar flowing) from the spectrum under reaction conditions (T= 690 "C, Pch = 0.257 atm, and Po, = 0.038 atrn). Features below the base line represent a decrease in surface concentration, and features above the base line indicate an increase.
1600
Wavenumbers ( c m - 1 )
Figure 4. Spectrum of Li/MgO catalyst at 690 "C, in the presence of Ar, CHI (Pch = 0.257 atrn), and O2(Poz = 0.038 atm).
were completed in 5-15 s depending on the flow rates. These changes are rapid compared to the changes observed in surface concentrations. The spectrum of the fresh Li/MgO catalyst at room temperature with argon flowing at a rate of 14.5 cm3(STP)/min is shown in Figure 3. The spectrum shows a broad band at 1335-1710 cm-' and a peak at 1084 cm-' which are the indicators of Li2C03. The band is characteristic of a symmetrical carbonate environment (antisymmetric C-0 stretch of the CO2- ion). The bands at 1801 and 2950 cm-I are the overtones and combination bands of Li2C03(Smryl et al., 1983). Comparison of this spectrum with those of pure MgO and pure Li2C03suggests that the major features associated with the catalyst are those which are present in Li2C03. It is reasonable to assume, in agreement with earlier researchers (Peng et al., 19901, that Li2C03disperses over the MgO catalyst and the catalyst surface has features essentially characteristic of Li2C03. The catalytic behavior of Li/MgO varies significantly from that of pure Li2C03,however, suggesting that MgO does play a role in the catalytic activity. The -OH bands a t 3640 and 3695 cm-' observed in the room temperature spectrum are due to adsorbed water. When the catalyst sample is heated to 690 "C in argon, the -OH bands are lost while the carbonate band and ita overtones remain. On flowing the reactant gases, viz., Ar, CHI (partial pressure (PCHI) = 0.257 atm), and O2 (Po, = 0.038 atm), new bands develop at 1304,2364, and 3014 cm-' and a broad band in the region 3253-3700 cm-' with the major peak at 3565-3586 cm-' (Figure 4). The spectra wdre obtained after steady state was achieved. The two sharp peaks in Figure 4 at 1304 and 3014 cm-' are due to gasphase methane (Szymanski,1964). Peaks due to adsorbed CHI may not be evident due to the masking by the gasphase peaks. The development of new features following the introduction of the reaction mixture is better seen in the sub-
3200
2800
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2000
Wavenumbers i c m - I )
Figure 6. High resolution (l-cm-') spectrum of the catalyst under reaction conditions, Le. T = 690 "C,CHI (PCQ= 0.257 atm), and O2 (PO,= 0.038 atm).
traction spectrum, i.e. the difference between the spectrum of the catalyst under reaction conditions (690 "C with Ar, CHI, and O2flowing) and the spectrum of the catalyst at 690 "C with only Ar flowing. This subtraction spectrum (Figure 5) reveals the following important features: (1) A peak a t 2364 cm-I which is characteristic of adsorbed C02. The adsorbed nature of C02was confirmed experimentally by conducting a "pumping experiment" in which the DRIFTS reactor was subjected to a vacuum of 0.1 Torr after the development of the peak at 2364 cm-I. After the vacuum was applied, the 2364-cm-I peak persisted while other peaks associated with gas-phase components rapidly disappeared. Moreover, this peak is a single blunt peak with a shoulder in contrast to absorption by gas-phase C02 which typically is a split band at approximately 2350 cm-' (White and Nair, 1990). Also, typically, gas-phase peaks have rotational fine features, e.g., such as can be clearly seen surrounding the CH4 peak a t 3014 cm-'. (2) A high resolution scan of the region 200-3300 cm-' (Figure 6) shows fine rotational features in the region of 2815-3226 cm-I indicating the presence of gas-phase CHI. This scan more clearly shows the features of adsorbed COP at 2364 cm-'. Because of high absorbance by gas-phase CHI, any absorbance by adsorbed CHI is masked. The pumping experiment indicated that if adsorbed CHI exists, it is not strongly adsorbed on the catalyst surface since all absorbance by CH, rapidly disappeared from the spectrum after the vacuum was applied. Figure 6 also indicates some gas-phase CO as indicated by the fine peaks in the 20502200-cm-' wavenumber region.
Ind. Eng. Chem. Res., Vol. 31, No. 8,1992 1859
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Figure 7. Subtraction spectrum indicating the effecta of an increase in O2partial pressure made by subtracting the reference spectrum (T = 690 "C, Pcb = 0.266 atm, PO,= 0.089 atm) from the spectrum obtained at higher 0,partial pressure (T= 690 "C, PcHl= 0.266 atm, PO,= 0.446 atm).
(3) The broad band in Figure 5 in the -OH region (3253-3700 cm-') with a major peak at 3565-3590 cm-' suggests the presence of hydrogen bonded hydroxyl species on the surface under reaction conditions. (4)No significant changes in carbonate features were caused by introduction of the reactant gases. Experiments were carried out by varying either the oxygen or methane partial pressure or the system temperature in order to determine the effects of these changes on the surface features described above. (1) Effect of O2 Partial Pressure. The O2 partial pressure was increased from 0.089 to 0.446 atm while flowing argon and methane, keeping methane partial pressure constant at 0.266 atm. Figure 7 shows the subtraction spectrum obtained by subtracting the spectrum of the catalyst prior to the change from the spectrum of the catalyst after the new steady state had been reached. Increasing the O2 partial pressure caused an increase in adsorbed C02 (2364cm-')as well as a decrease in the -OH (3565-3590cm-') absorbance suggesting a decrease in -OH coverage. The gas-phase absorbance of CHI (3014cm-') and the carbonate content (1335-1590cm-') also decreased. The changes were reversible, i.e., when the O2 partial pressure was returned to 0.089 atm, the C02 and -OH absorbances returned approximately to their previous values. For COz, the decrease in peak intensity took 195 s compared to the time required for peak intensity to increase which was 90 s. (2) Removal of Oxygen under Reaction Conditions T = 720 OC, P o = 0.079 atm, and PCH4 = 0.348 atm. With these conditions, when the O2 flow was shut off completely (0 cm3(STF9/min) with Ar and CHI still flowing, a gradual depletion in surface C02 coverage as well as the regeneration of the carbonate band occurred. Kimble and Kolts (1986,1987)have proposed that C02 reacts with the catalyst to form carbonate at sufficiently high temperature. The subtraction spectrum (Figure 8) comparing the surface in the presence and absence of oxygen shows the development of the carbonate band while the adsorbed COz band decreases. (3) Dependence on CHI Partial Pressure. The partial pressure of methane was increased from 0.211 to 0.282 atm while argon and oxygen were kept flowing and the oxygen partial pressure was maintained constant at 0.096a h . Beaides an increase in the gas-phase absorbance of methane, this caused an increase in surface -OH coverage and a decrease in C02 coverage. (4) Dependence on Temperature. When the reaction temperature was increased from 690 to 720 OC with argon,
Wavenumbers ( c m - I )
Figure 8. Subtraction spectrum comparing the surface of the catThe spectrum was obtained alyst in the presence and absence of 02. by subtracting the reference spectrum (2' = 720 "C, Pcb = 0.348 atm, PO,= 0.079 atm) from the spectnun at T = 720 "C, PcHl= 0.348 atm, and Po, = 0.0 atm.
a
I
\ I
v
I
I
Figure 9. Subtraction spectrum indicating the effects of temperature, obtained by subtracting the reference spectrum (T= 690 "C, P c =~0.25 atm, PO,= 0.125 atm) from the spectrum at T = 720 "C, P c b = 0.25 atm, and PO,= 0.125 atm.
CH4 ( P c H=~ 0.25 atm), and O2 (PO= 0.125 atm) flowing over the catalyst, the adsorbed do2peak intensity increased, as shown by the peak at 2364 cm-' (Figure 9). The increase in temperature also caused decomposition of carbonate (1365-1665cm-') and an increase in surface -OH coverage (3565-3590 cm-l). This agrees with the observations of Lunsford and co-workers, which correlated the higher catalyst activity at higher temperatures to carbonate decomposition. The carbonate decomposed to its new level in less than 45 s after the increase in temperature. (5) Introduction of COz, CO, or CzHs. When C 0 2 (Pco, = 0.116 atm) and helium were deliberately introduced over the heated catalyst maintained at 620 "C, a strong broad band appeared at 2321-2370 cm-' due to gas-phase absorbance which masks adsorbed C02. Introduction of CO and He over the heated catalyst (2' = 620 "C) leads to the formation of adsorbed C02 (peak at 2364 cm-'). Introduction of C2H6and He over the heated catalyst leads to formation of new bands in the 2900-3150-cm-' region which are associated with asymmetric and symmetric stretching vibrations of methyl groups of gas-phase or surface C2H6. Peaks are also observed at 2364 and 21W2200 cm-' which are characteristic of adsorbed COz and either adsorbed or gas-phase CO, respectively (Figure 10). (6) Lithium Loss from the Catalyst. The subtraction spedrum (Figure 11)obtained by subtracting the spectrum of a %sedw catalyst, i.e., a catalyst that has been involved
1860 Ind. Eng. Chem. Res., Vol. 31, No. 8,1992 2
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Figure 10. Spectrum of Li/MgO catalyst in the presence of C2Hs at 620 OC.
0 4000
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Figure 11. Subtraction spectrum of a fresh and a used catalyst obtained by subtracting the spectrum of the used catalyst from the spectrum of the fresh catalyst.
in oxidative coupling reaction for 48 h in the PFR reactor, from the spectrum of 'fresh" catalyst (Le. a catalyst not subjected to any reaction) indicated a loas of lithium from the surface as shown by the peak at 1084 cm-' which indicates higher Li concentration in the fresh catalyst compared to that in the used catalyst. This subtraction spectrum also reveals a net loss of carbonate, as shown by the band at 1300-1700 cm-I which indicates higher carbonate content in the fresh catalyst compared to that in the used catalyst. However, no significant loss in catalyst activity was measured. (7) Involvement of Lattice Oxygen. In order to check the involvement of lattice oxygen in the oxidative coupling reaction, the reaction of CHI with Li/MgO in the absence of gas-phase O2was studied. O2was first passed over the catalyst for 30 min at 550 "C, and the system was then purged with Ar for an additional 30 min. CHI (PcH,= 0.091 atm) and argon were then passed over the heated catalyst (at 550 "C), and no conversion of CHI was observed at this temperature. The temperature was then increased to 630 "C resulting in formation of CO and adsorbed COP. The flow of CHI and Ar was continued, the adsorbed C02 peak gradually disappeared after a period of time (225 s), and CO decreased to an undetectable level. After all adsorbed C02 had disappeared, the CHI partial pressure was increased to 0.171 atm. No new peaks were observed. Upon a subsequent increase in O2 partial pressure, adsorbed C02coverage increased but CO did not reappear. The adsorption characteristics of O2 could not be observed due to high absorbance of carbonate bands below lo00 cm-', the wavenumber range in which adsorbed O2 features are typically observed. Also, it was not possible to observe adsorbed CHI since the gas-phase absorbances
200
.
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'
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Time (s) Figure 12. (a, top) Transient response of C2He,C02and H20 following a step increase in O2partial pressure from 0 to 8.75 X 10-9 atm at 690 OC at t = 34 8. (b, bottom) Transient response of CPHB following a step increase in CH, partial pressure from 0 to 0.035 atm at 690 OC at t = 29 a.
masked any adsorbed CH4. However, as previously noted, any adsorbed CHI must be only weakly adsorbed. Transient experiments using the PFR were employed to gain additional information on the reaction mechanism. Results from Transient Plug Flow Reactor Experiments. (1) Step Changes in O2and CHI Partial Pressure. Step change experiments were carried out in the PFR loaded with 0.51 g of catalyst. CHI (Pch = 0.035 atm) and helium (carrier gas) were flowed over the catalyst at 690 "C. After steady state had been attained, O2partial pressure was stepped up from 0 to 8.75 X atm. As shown in Figure 12a, the CzH6 signal increases initially in almost step fashion and then increases more slowly, requiring about 18 s to achieve steady state. C02and H20 increase only slowly following the increaee in 02,requiring about 140 s to approach their new steady-state valuea. The C& response following a step increase in CHI feed is shown in Figure 12b. In this experiment, the methane partial pressure was stepped up from 0 to 0.035 atm with O2and He flowing at a steady rate (PO?= 8.75 X atm) at 690 "C. Compared to the response in Figure 12a, C& reaches its new steady-state level much faster but overshoots and slowly relaxes back. C02 and H20 responses (not shown for the CHI step experiment) are similar to those shown in Figure 12a. Additional experiments were conducted making a step decrease in either the oxygen or methane feed rates. A significant difference was noted in the effluent C02signal depending on which reactant was decreased. In Figure 13a is shown the C02 signal following a step decrease in CHI partial pressure from 0.035 to 0 atm at t = 95 s with O2 (8.75 X atm) and helium constant, while Figure 13b shows the COPsignal following a step decrease in O2partial
Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 1861
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Figure. 14. Transient responses of COz and C2HBin the effluent following addition of C02 (Pco, 6.16 X atm) to feed (Pch = 0.046 atm, Po,= 9.33 X atm) at t = 20 8.
Time (s)
-
Time (s) Figure 13. (a, top) Transient response of C02 following a step decrease in CHI partial pressure from 0.035 to 0 atm at 690 OC at t = 95 8. (b, bottom) Transient response of COP following a step to 0 atm at 690 OC decrease in O2 partial pressure from 8.75 X at t = 91 a.
pressure from 8.75 X W3to 0 atm at t = 91 s with methane (0.035atm) and helium constant. (2) Effect of C02 and C2H6. COBwas suddenly introduced into the PFR feed stream using the air actuated switching valve. The system was initially at steady state with CH4 (PpH, = 0.046 atm), O2 (Po, = 9.33 X atm), and He f l o m g over the catalyst at 760 "C. C02was then introduced in three separate experiments (COz partial 3.42 X and 6.15 X 10" atm). pressures: 1.45 X In each experiment the C2H6partial pressure in the effluent immediately decreased, although increased C02did not immediately appear in the product. Figure 14 shows the C02 and C2H6 responses in the effluent following addition of C02 to the feed at t = 20 s. While the C2H6 response began to decrease almost immediately, the C 0 2 signal did not increase for 240 s. The highest partial pressure of C02 was sufficient to completely poison all coupling activity, but in all cases the effect was reversible. With the system at the same initial condition as above atm, balance He), (i.e. Pc& = 0.046 atm, Po, = 9.33 X C2H6 was suddenly introduced to the feed stream (PcSHd = 9.27 X lo4, 2.7 X lV3,and 0.0108a b ) in three separate experimenta. The lowest partial pressure of CzH6 caused no apparent effect on the reaction. At higher partial pressures, however, the increase in CzH6 partial pressure caused an increase in C02 and H 2 0 production. (3) Involvement of Lattice Oxygen. Several experiatm) menta were carried out in which O2 (P = 8.67X was passed over the heated catalyst = 675 "C) for 2 h and then shut off. In approximately 15 s the effluent O2 partial pressure dropped to 0 atm. CHI was then introduced over the catalyst. No measurable C2H6 was produced.
8
Discussion Previous investigators have concluded that an oxygen species at the surface abstracts a hydrogen from methane to form the active methyl radical and that the catalyst active sites can be converted to a less active carbonate by reaction with COP Our resulta provide evidence that some of the Li2C03is transformed to LiOH (strongly bonded -OHgroups seen on the surface) and LizOor Li+O- (the active component for methane activation) and that a complicated relationship exists between these and gasphase 02,C02, and HzO. This relationship plays an important role in determining the activity and selectivity of the catalyst. C02 in the gas phase causes a decrease in the activity of the catalyst (Figure 14) due to both C02 adsorption (Figure 4 and Korf et al. (1987,1989)) and subsequent formation of less active carbonate. The initial rapid decrease in coupling activity (Figure 14) is caused by reversible adsorption of COPon the active sites, probably 0centers (Korf et al., 1989). Adsorbed C02can subsequently react associatively with 0-to form COB-(Figure 9;Warman, 1967;Che and Tench, 1983). The COS- formation is reversible but is reversed more slowly than simple desorption because COS- reacts further to form Li2C03. Figure 8 shows disappearance of adsorbed C 0 2 as carbonate coverage increases due to reaction between adsorbed C02 and either Li20 or Li+O-. Only the reaction with Li+O- is expected to significantly affect catalytic activity. Decreasing the C02 partial pressure allows desorption of C02 and decomposition of the carbonate (Peng et al., 1990). In addition, carbonate decomposition to Li20 and C02 is favored by increasing temperature. C02 will be present on the surface either due to gasphase oxidation of CH3' radicals to C02 which subsequently adsorbs on the catalyst or due to surface oxidation of CH; radicals by surface oxygen species. An additional source of COPis ethane oxidation. C2H6 oxidizes on the catalyst surface to form CO and COP(Figure lo),and hence a high partial pressure of can significantly increase the surface C02. Our observations that CO is rapidly oxidized to C02even in the absence of gas-phase oxygen suggest that CO adsorbs and reacts with lattice oxygen to form chemisorbed COP This was also suggested by Yatea and Zlotin (1988). The ability of the carbonate catalyst to rapidly oxidize CO to C02 may explain why we do not observe CO in steady-state PFR experiments conducted under differential conversion conditions. Changes in O2 partial pressure affect catalyst activity and selectivity. Increasing the O2partial pressure increases the rate of Carbonate decomposition at a fied temperature, decreasing the carbonate content on the surface (Figure
1862 Ind. Eng. Chem. Res., Vol. 31, No. 8,1992
7) and increasing catalytic activity. The mechanism by which oxygen aids carbonate decomposition probably involves increased H 2 0 partial pressure due to faster gasphase oxidation reactions. The higher H20partial pressure causes transformation of carbonate to LiOH (Kimble and Kolts, 1987) which is more easily converted to the active Li+O- than is Li2C03. Additional work is currently underway in our laboratory to clarify this point. Increase in 0,partial pressure also decreases the -OH coverage (Figure 7) by faster regeneration of "H occupied" active sites such as LiOH. The higher catalytic activity caused by increasing 0,partial pressure is moderated by the higher CO, partial pressure due to gas-phase reactions leading to higher CO, surface coverage (Figure 7) with the net effect that increases in 0,partial pressure typically decrease the C2 selectivity. The presence of f d y bound -OH bands on the surface under reaction conditions (Figure 5) and the increase in surface -OH coverage with an increase in CHI partial pressure support the claim that methane activation occurs via H abstraction from CH,, by an active oxygen on the forming surface -OH and a methyl radical surface (0-), (CH,') (Ito et al., 1985). The -OH features are due to hydroxyl formed and are not those associated with water, as indicated by the absence of the H-0-H bending band which typically shows up at 1590 cm-l when H20is present (Smryl et al., 1983). In the absence of gas-phase oxygen, methane reacts with lattice oxygen to form CO, as demonstrated by FTIR experiments in which CHI was passed over the catalyst at high temperature. We observed no C2H6in the absence of gaseous OF Moreover, in the PFR experiments, the C2H,response very closely resembled the O2step behavior. When 0,partial pressure was stepped up suddenly, C2H6 partial pressure rapidly increased as well (Figure 12a); when O2partial pressure was stepped down to zero, C2H6 production rapidly disappeared. These experiments indicate that O2adsorption is a necessary precursor step for formation of a pool of active sites. Methane adsorption is less important in the reaction sequence. Following the removal of methane from the feed, methane oxidation products (CO,) were observed for only a very short time (Figure 13a), indicating there is little or no significant pool of reactive CHI on the catalyst surface. Following removal of O2from the feed, however, C02was observed for a much longer time (Figure 13b). Since slow CO, desorption would be expected to occur in either case, the long tail of C02 following the removal of feed O2 is probably caused by continued oxidation of CHI by a pool of oxygen species active for CO, formation but not for methane coupling. Our observations confirm that, at high temperatures (5" > 700 "C), a complicated equilibrium exists between the composition of the gas phase and the composition of the catalyst surface. O2adsorbs on the catalyst forming active sites such as Li+O-. These 0-species can abstract H from CHI to form CH3' radicals and surface -OH species, probably via an Eley-Rideal type of mechanism. CH3' radicals couple in the gas phase to form C2H, but can also react with surface 0-to form CO and C02. Gas-phase CO, can adsorb on 0-decreasing the catalyst activity. Adsorbed CO is rapidly oxidized either by 0-or by the lattice oxygen. Adsorbed CO, reacts with 0-to form the carbonate which is inactive for hydrogen abstraction. H 2 0 is formed by oxidation of CH3' radicals and by regeneration of Li+OH-. H20 reacts with Li2C03to form LiOH. In the presence of 02,LiOH (i.e., -OH species) reacts to form Li'O-, the desired active site. CzH6is also oxidized by either lattice oxygen or 0-to form COPand H20.
On the basis of our results, we have postulated a reaction mechanism which describes the steps taking place in the reaction system. Our direct observations using in situ FTIR analysis document the heterogeneous steps and the intermediates involved in the reaction. Other steps in the mechanism are based on the significant pointers obtained from transient experiment results and from the literature. Mechanism. (1)active site generation
kl
02(g) + 2e-
20-
kl
(2) methane activation (rate-limiting step)
0-+ CH4(g)
k3
CH,'(g)
+ OH-(a)
(3) carbon dioxide adsorption
0-+ COp(g)
k4
O-.C02(a)
k6
(4) carbonate formation
O-C02(a) + e-
k8
C03,-(a)
k7
(5) radical oxidation
2CH3'
+ 3.502 -% 2Coz(g) + 3H@(g)
(6) active site regeneration
20H-(a) + 0.502(g)
220- + H 2 0
(7) coupling reaction 2CHsYg)
C2&(g)
(8) carbonate transformation
cos2- + H 2 0
5 20H- + COP
Since it has been observed that ethylene production occurs at the expense of C2H6(Ito et al., 1985), ethylene formation is not presented in this mechanism. Development of Rate Equation. In developing the rate expression, it was assumed that under differential conversion conditions, the H 2 0 partial pressure is insignificant and hence step 8 in the mechanism can be neglected. Since C2H4 and CO formation were not observed under differential conditions of our experiments, they are not considered in the mechanism. Steps 1 and 3 are assumed to be fast and reversible. The irreversible steps 5 and 6 are assumed to follow first-order dependence on reactants' partial pressures. It has been verified by other researchers that step 2 is the rate-determining step (Cant et al., 1988) and the intrinsic rate for step 7 follows a second-order dependence on the partial preasure of methyl radicals. Further, it is also assumed that step 4 is reversible and is slow compared to step 3. The intermediates, viz., 0-, OH-, C032-,and O-CO, are assumed to be at quasisteady state, i.e. -d9o= - - dBoH-
dgCo32- --d80-.co2 ---0 dt dt dt dt where B is the fractional surface coverage. Using the expressions for generation and consumption of each of these intermediates in the above equation and solving give
Ind. Eng. Chem. Res., Vol. 31, No. 8,1992 1863 gas-phase oxygen is involved in regeneration of active sites, while H20 aids in carbonate decomposition. However, gas-phase oxygen also leads to COz formation. Our results point out the need to quantify these steps in the mechanism. Work is underway in our laboratory for this purpose. A model including the effects of C02 and H 2 0 was proposed to predict the rates of formation of the desired and undesired products. The model will then be used to examine reactor configurations, operating conditions, and catalyst modifications to improve C2 yield.
Acknowledgment
where PO,PcH,,and PCO, are the partial pressures of 02, CHI, and Cop, respectively, and X = APcH, (B
This work has been supported by the University of Oklahoma Sarkeys Energy Center and the Oklahoma Center for the Advancement of Science and Technology. We also thank Dr. Robert White, Associate Professor, Department of Chemistry and Biochemistry, for helpful discussions and recommendations.
Assuming that the partial pressure of methyl radicals is also at steady state leads to the following: [k;po, %~I$CH~BO-]~’~ - k$o, PCHr = 4k10 Then, R1 = rate of formation of COz = k$opcH,
Nomenclature atm = atmosphere (g) = gas phase (a) = adsorbed phase ki = rate constant for reaction in the proposed mechanism, e.g., k l , k2, etc. Bz = fractional surface coverage for surface species 2 , e.g.,, ,e BOH-9 et“.. Py = partial pressure of component y, e.g., PO,,PcH,,etc. Registry No. C&, 74-82-83 C2&, 1484-0;Li, 1439-93-2; MgO,
+
+
1309-48-4; COZ, 124-38-9; CHS’, 2229-01-4. \L
Literature Cited Aika, K.; Lunsford, J. H. Surface Reactions of Oxygens Ions. 1. Dehydrogenation of Alkanes by 0-on MgO. J. Phys. Chem. 1977, 81,1393.
Also:
Rz = rate of formation of C2He = klOPCHs.2
One of the limitations of this model is that gas-phase reactions,which can become important at high conversions, are not considered. Studies are currently underway to estimate the individual rate constants in the rate expressions using steady-state and transient kinetic studies and using steady-state and transient FI’IR studies. The effects of additives on surface species and on the catalyst activity and selectivity can then be studied in order to more clearly define the desirable features of a catalyst. The results from these studies will be used to compare catalysts to obtain a direction for new catalyst development.
Summary In situ FTIR studies were used to characterize the surface species during oxidative coupling of CHI over Li/Mgo and their interactions with gas-phase components. The absorbance spectra of the catalyst indicated the presence of adsorbed COP, strongly bonded -OH, and carbonate under reaction conditions. The effects of reaction conditions (gas partial pressures and temperature) on the adsorbed species were also studied. Gas-phase COz causes a decrease in catalytic activity via adsorption and reaction to produce carbonate. The results suggest that
Amenomiya,Y.; B h s , V. I.; Goledzinowski,M.; Galuszka, J.; Sanger, A. R. Conversion of Methane by Oxidative Coupling. Catal. Rev.-Sci. Eng. 1990, 32, 163. Cant, N. W.; Lukey, C. A.; Nelson, P. F.; Tyler, R. J. The Rate Controlling Step in the Oxidative Coupling of Methane over a Lithium-promoted Magnesium Oxide Catalyst. J. Chem. SOC., Chem. Commun. 1988,766. Che, M.; Tench, A. J. Characterization and Reactivity of Molecular Oxygen Species on Oxide Surfaces. Adu. Catal. 1983, 32, 1. Driscoll, D. J.; Lunsford, J. H. Gas-Phase Radical Formation during the Reactions of Methane, Ethane, Ethylene, and Propylene over Selected Oxide Catalysts. J. Phys. Chem. 1985,89,4415. 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.
Duboii, J. L.; Cameron, C. J. Common features of oxidative coupling of methane cofeed catalysts. Appl. Catal. 1990,67,49. Ferraro, J. R.; b i l e , L. J. Fourier lkansform Infrared Spectroscopy In The Study of Catalysts. In Fourier Transform Infrared Spectroscopy-Techniques Using Fourier Transform Znterferometry; Academic: San Diego, CA, 1982; Vol. 3, Chapter 1, pp 1-36.
Fuller, M. P.; Griffiths, P. R. Diffuse Reflectance Measurements by Infrared Fourier Transform Spectrometry. Anal. Chem. 1978,50, 1906.
Garibyan, T. A.; Margolis, L. Y. Heterogeneous-Homogeneous Mechanism of Catalytic Oxidation. Catal. Reu.-Sci. Eng. 1990, 31, 355.
Hamadeh, I. M.; King, D.; Griffiths, P. R. Heatable-Evacuable Cell and Optical System for Diffuse Reflectance FT-IR Spectrometry for Adsorbed Species. J. Catal. 1984, 88, 264. 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. Keller, G. E.; Bhasin, M. M. Synthesis of Ethylene via Oxidative Coupling of Methane 1. Determination of Active Catalysts. J. Catal. 1982, 73, 9. Kimble, J. B.; Kolts, J. H. Oxidative Coupling of Methane to Higher Hydrocarbons. Energy Progress. 1986,6,226.
1864
Ind. Eng. Chern. Res. 1992,31, 1864-1867
Kimble, J. B.; Kolts, J. H. Playing matchmaker with methane. CHEMTECH 1987,501. Korf, S . J.; Roos, J. A.; de Bruijn, N. A.; van Ommen, J. G.; Ross, J. R. H. Muence of COz on the Oxidative Coupling of Methane over Lithium-Promoted Magnesium Oxide Catalyst. J. Chem. SOC., Chem. Commun. 1987,1433. Korf, S . L.; R m ,J. A.; Veltman, L. J.; van Ommen, J. G.; Ross, J. R. H. Effect of Additives on Lithium Doped Magnesium Oxide Catalysts Used in the Oxidative Coupling of Methane. Appl. Catal. 1989,56,119. Lee, J. S.; Oyama, S. T. Oxidative coupling of Methane to Higher Hydrocarbons. Catal. Rev.-Sci. Eng. 1988,30, 249. Lunsford, J. H. The Role of Oxygen Ions in the Partial Oxidation of Hydrocarbons. Catalytic Materials: Relationship between Structure and Reactivity; ACS Symposium Series No. 248 American Chemical Society: Washington, DC, 1984. Peng, X. D.; Richards, D. A.; Stair,P. C. Surface Composition and Reactivity of Lithium-Doped Magnesium Oxide Catalysts for Oxidative Coupling of Methane. J. Catal. 1990, 121, 99. Prairie, M. R.; Renken, A; Highfield, J. G.; Thampi, K. R.; Gratzel, M. A Fourier Transform Infrared Spectroscopic Study of C02 Methanation on Supported Ruthenium. J . Catal. 1991,129,130. Schraml-Marth, M.; Wokaun, A.; Baiker, A. Grafting of V206Monolayers onto TiOz from Alkoxide Precursors: A Diffuse Reflec-
tance FTIR Study. J. Catal. 1990,124,86. Smryl, N. R.; Fuller, E. L., Jr.; Powell, G. L. Monitoring the Heterogeneous Reaction of LiH and LiOH with H 2 0 and COz by Diffuse Reflectance Infrared Fourier Transform Spectroscopy. Appl. Spectrosc. 1983,37, 38. Szymanski, H. A. Alkanes. Interpreted Infrared Spectra; Plenum: New York, 1964; pp 1-15. Tung, W.; Lobban, L. L. Oxidative Coupling of Methane over Li/ MgO: Kinetics and Mechanisms. I d . Eng. Chem. Res. 1992, in press. Wang, J.; Lunsford, J. H. Characterization of [Li+O-] Centers in Lithium-Doped MgO Catalysts. J. Phys. Chem. 1986,90,5883. Warman, J. M. Electron Capture by Nitrous Oxide in Irradiated Alkane and Alkene Gases. Subsequent Reactions of the 0-Ion. J. Phys. Chem. 1967, 71, 4066. White, R. L.; Nair, A. Diffuse Reflectance Infrared Spectroscopic Characterization of Silica Dehydroxylation. Appl. Spectrosc. 1990,44,69.
Yam, D. J. C.; Wotin, N. E. Blank Reactor Corrections in Studies of the Oxidative Dehydrogenation of Methane. J. Catal. 1988, 111, 317.
Received for review January 13, 1992 Accepted May 26, 1992
Gas-Liquid Mass-Transfer Coefficients in a Slurry Batch Reactor Equipped with a Self-Gas-InducingAgitator H. Hichri, A. Accary, J. P. Puaux, and J. Andrieu* Laboratoire d'Automatique et de G h i e des ProcBdBs, UniversitB Claude Bernard-Lyon I , URA-CNRS D 1328, LAGEP Bet 305-69622, Villeurbanne CBdex, France
Gas-liquid mass-transfer coefficients (IzLa)and solubilities of hydrogen in a batch stirred autoclave reactor have been measured by a method based on the gas solute physical absorption kinetics; organic systems, namely, 2-propanol/Hz, o-cresol/Hz, and the mixture (1/3 o-cresol + 2 / 3 2-propanol)/H2 between 30 and 120 "C were investigated. The influence of the main operating parameters was studied agitation speed, temperature, total pressure, gas to liquid volume ratio, concentration, and diameter of solid particles. Experimental values were correlated by an empirical relationship of the type Sh = f(Re, Sc, We, (Vg/VJ).
I. Introduction Stirred gas-liquid or gas-liquid-solid dispersions are frequently used in many industrial processes like catalytic hydrogenations, oxidations, etc., or in laboratory experimenta to develop or to set up new catalysts for heterogeneous catalytic reactions. Furthermore, and especially with very active catalysts, these systems show gas-liquid mass-transfer limitations which influence both the reaction yield and ita selectivity. So, in order to simulate or to model these systems, it is important to have precise and rapid methods for measuring "in situ" the volumetric mass-transfer coefficient kLa in the true reaction conditions. Although the literature data are large for aqueous systems (Joshi et al., 1982;Albal et al., 1983;Ledakowicz et al., 1985; Teramoto et al., 1974; Midoux and Charpentier, 1979,Mehta and Sharma, 1970,there are only a few data concerning organic systems with or without solid particles (Chaudhariet aL, 1987;Ledakowicz et al., 1984,Barbs and Satterfield, 1986;Joosten et al., 1977,Oguz and Brehm, 1988). The purpose of this paper is to report and correlate some new experimental data obtained with organic solvents in dead-end stirred autoclave reactor equipped with a selfgas-inducing agitator. This dynamic method previously used by Albal et al. (1983),Chaudhari et al. (1987),and
Table I. Reactor Geometrical Characteristics total volume V, = 0.15 X m3 D R = 5.0 X m tank diameter agitator diameter D = 2.9 X m ho = 1.4 X m turbine height from bottom 0.07 X < VI < 0.10 X liquid volume 1.1 < Vg/Vl < 1.7 gas to liquid volume ratio baffle width 1 = 1.0 x m ~~~~
ms
Ledakowicz et al. (1984)is based on the dynamics of the solute gas physical absorption and also has the great advantage of rapidly giving solubility data for the system.
11. Materials and Methods 11.1. Apparatus. It is mainly composed by a laboratory stainless steel autoclave designed to study polyphasic catalytic reactions under pressure and was described in more detail elsewhere (Hichri et al., 1991,Hichri, 1991). Gas-liquid dispersion is produced by surface aeration with a self-gas-inducing agitator mounted on a hollow shaft in order to improve the bubble recirculation between the gas space and the liquid phase. The main geometrical characteristics of the apparatus are indicated in Table I. Four baffles were placed at 90 OC to prevent vortex formation. The method principle is based on the measurement of the kinetics of the solute gas isochore absorption. The
0888-5885/92/2631-1864$03.00/00 1992 American Chemical Society
