Ind. Eng. Chem. Res. 1991, 30, 1016-1023
1016
Roots, J.; Nystrom, B.; Sundelof, L.-0.; Porsch, B. Fractional Coefficient of Macromolecules in Concentrated Solution and in the Vicinity of the Critical Solution Temperature. Diffusion and Sedimentation Studies of Polystyrene in Toluene and Trans-decalin. Polymer 1979,20, 337. Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: London, 1989. Varoqui, R.; Dejardin, P. Hydrodynamic Thickness of Adsorbed Polymers. J . Chem. Phys. 1977,66,4395-4399.
Webber, R. M.; Anderson, J. L.;Jhon, M. S. Hydrodynamic Studies of Adsorbed Diblock Copolymers in Porous Membranes. Macromolecules 1990,23, 1026-1034. Westermann-Clark, G.B.;Anderson, J. L.Experimental Verification of the Space-Charge Model for Electrokinetics in Charged Microporous Membranes. J. Electrochem. SOC.1983,130,834-847.
Receiued for reuiew July 5, 1990 Accepted January 21, 1991
Lanthanum Catalysts for CH4 Oxidative Coupling: A Comparison of the Reactivity of Phases R. Paul Taylor and Glenn L. Schrader* Department of Chemical Engineering and Ames Laboratory-USDOE, Iowa State University, Ames, Iowa 50011
The catalytic performance of A-La203, La(OH)3, II-La202C03,and La2(C0d3for the catalytic oxidative coupling of methane to C2 hydrocarbons was investigated over a range of reaction temperatures (up to 850 "C). The activity of the starting materials for methane conversion and selectivity to ethylene and ethane were in the order II-La202C03> La2(C03)3> La(OH), > A-La203. Postreaction characterization by Fourier transform infrared spectroscopy and X-ray diffraction revealed that these materials undergo significant transformations. On the basis of this information, the presence of the oxycarbonate phase appears to have a beneficial effect on catalytic performance. 1. Introduction
Currently, a global excess of natural gas exists. Methane is the major component of this resource, typically comprising over 90 mol % of the hydrocarbon fraction. When economically attractive, methane is used as a fuel; the other hydrocarbon components can be separated for use as chemical feedstocks. However, as much as two-thirds of proven natural gas reserves are located in remote areas of the world. Providing the necessary infrastructure to bring this gas to market is usually prohibitively expensive, and therefore natural gas is often flared. There is strong industrial interest in developing processes to convert methane to more easily transportable, higher value forms (Jones et al., 1987). One possible processing route is the catalytic oxidative coupling of methane to C2hydrocarbons. These products could then be either converted to gasoline (using technology related to the Mobil methanol-to-gasoline process) or employed as feedstocks (such as in the production of polyethylene). A wide variety of catalytic materials have been shown to be active for this reaction, including oxides of most of the first transition-metal series, oxides of the group IIA alkaline-earth metals, oxides of most metals of groups IIIB, IVB and VB,and oxides of almost all rare earths. Reviews by Pitchai and Klier (1986), Otsuka et al. (1986), Scurrell (1987), Burch et al. (1988), Lee and Oyama (1988), and Deboy and Hicks (1988a) have attempted to compare the relative activities and C2 selectivities of each of these catalyst systems, despite the wide variety of experimental conditions under which the catalysts have been tested. Reaction temperature, the CH4-to-0, feed ratio, and the ratio of catalyst mass to feed flow rate (W/F)have all been found to strongly influence catalytic performance (see, for example, Hinsen et al. (1984), Lin et al. (19861, Otsuka and Jinno (1986), Otsuka et al. (1986), Otsuka and Komatsu (1987a,b), Deboy and Hicks (1988a,b), and Campbell et
* To whom
correspondence should be addressed.
ai. (1988)). Furthermore, the lack of rigorous catalyst characterization has led to uncertainties concerning the stability of specific phases and/or the formation of new materials under catalytic reaction conditions. It appears to be generally recognized, however, that on the basis of the yield of hydrocarbons, alkaline-earth-promoted rareearth oxides are superior to unpromoted rare-earth oxides, which are, in turn, superior to alkali-metal-promoted alkaline-earth oxides, alkali-metal-promoted rare-earth oxides, and unpromoted alkaline-earth oxides, respectively. Work performed in our research group has focused on unpromoted rare-earth oxides. The phase composition of these catalysts is known to be highly dependent on the preparation method. There is also a particular sensitivity to the presence of gas-phase H20and C02, as shown in the studies conducted by Bernal and co-workers (1983a, 1985). In their research, A-La203was shown to undergo hydration and carbonation (in their terminology, "carbonation" refers simply to the uptake of COPinto the solid, by incorporation into either the bulk or the outer surface layers). Bernal concluded that under atmospheric conditions A-La203is converted to a partially carbonated hydroxide. Carbonation was found to involve formation of a hydroxycarbonate material on the outer layers of a bulk hydroxide. In evaluating these materials as catalysts for oxidative coupling it is important to consider that even if preparation and storage procedures are employed that preclude such contamination, hydration and/or carbonation can result when these materials are used as catalysts, In situ formation of phases other than the hydroxycarbonate material examined by Bernal is also possible. As discussed elsewhere (Taylor, 1989), earlier studies employing "lanthanum oxide" as a catalyst for the oxidative coupling of CH4 have not addressed the importance of catalyst preparation, storage, and rigorous characterization. Otsuka et al. (1985) and Otsuka and Nakajima (1986) used La203supplied by Asahi Chemical Co. No catalyst pretreatment was reported; for W / F = 0.002 g s/cm3, CH4/O2 = 45.5, and T = 700 "C, selectivities to C02,
0888-5885191/ 2630-1016$02.50/0 0 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 5, 1991 1017 CO, C2H4, and C2H6of 12.2, 14.8, 29.1, and 56.1%, respectively, were reported. Hutchings et al. (1989) obtained their catalyst from BDH, pretreated it for 1 h at 450 "C in flowing 02,and then performed catalytic studies at 710 "C with CH4/02= 3.0. Selectivities to C02,CO, CzH4,and C2H6were 70.3, 13.9,4.9, and 15.870, respectively. Burch et al. (1988) obtained La203from AnalaR, heated it at 750 "C in 02/N2,and then pretreated it for 1 h at 750 "C in flowing reactant gases. For W / F = 0.375 gs/cm3, CH4/O2 = 20.0, and T = 750 "C, selectivities of 41.0, 19.0, 10.3, and 29.7%, respectively, were observed. Deboy and Hicks (1988) obtained their catalyst from Union Molycorp, pretreated it for 1 h at 750 "C in flowing Ar, and then performed reactions at 750 "C with CH4/O2 = 6.0. Selectivities to the same four products were 32.8, 8.1, 25.7, and 30.99'0, respectively. Finally, Campbell et al. (1988) obtained La203catalyst from Aldrich; they heated this material for 15 h at 900 "C in vacuo, 4 h at 100 "C in H20-saturated Ar, 8 h at 25 "C in H20-saturated Ar, 2 h at 450 "C in flowing 02,and finally 15 h at 700 "C in the reactant stream. For W/F = 0.006 gs/cm3, CH4/02 = 12.3, and T = 700 "C, selectivities of 23.0,26.6,6.7, and 43.8%, respectively, were measured. Only the study by Campbell et al. (1988) used a pretreatment temperature sufficiently high to ensure the decomposition of all phases other than A-La203(Taylor, 1989). Furthermore, the pretreatment temperature used in most of the other studies was lower even than the subsequent reaction temperature. Work by Bernal and co-workers has shown that H20and COPcan be continuously evolved from rare-earth oxide materials as they are heated to the temperatures used in these studies (Alvero et al., 1983, Bernal et al., 1983b, and Carrizosa et al. 1984). Although several lanthanum phases may actually exist in the catalytically active materials examined in this previous research, no rigorous characterization of the catalysts before or after reaction has been reported. This may account for some of the obviously apparent differences in catalytic performance reported by the research groups. The goal of this work with rare-earth catalysts has been to relate catalyst-phase composition to catalytic performance in order to provide a basis for more clearly understanding methane oxidative coupling. Pure samples of A-La203,La(OH)3,II-La202C03,and La2(C03)3were prepared and characterized by BET surface area analysis, X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis, and laser Raman spectroscopy. (Results using the latter two techniques are discussed elsewhere (Taylor 1989). Catalytic performance for methane oxidative coupling was determined over a range of reaction temperatures. In order to identify the changes in catalyst compositions that occur with use at these temperatures, the catalysts were characterized at specific intermediate temperatures. 2. Experimental Methods 2.1. Catalyst Synthesis. La203 (99.99%) was obtained
from Aldrich Chemical Co. and was stored in an argonfilled drybox. A high-temperature treatment decomposed a minor amount of La(OH), present in this material. This treatment consisted of flowing Ultra-pure Carrier grade N2 (Air Products) at 100 standard cubic centimeters per minute (sccm) over the sample and raising the temperature from 25 to 900 "C over a period of 16 min. The temperature was maintained at 900 "C for 12 h. The sample was allowed to cool to room temperature and stored in the drybox. La(OH)3was prepared by exposing A-La203to water vapor, H 2 0was distilled, deionized, and boiled for 30 min
while being sparged with Ultra-pure Carrier grade NF N2 was then passed through the H20 (maintained at 60 "C) and flowed over the A-La203for 18 h at room temperature. The resulting white powder was removed immediately to the drybox. Preparation of II-La202C03involved exposing A-La203 to a flow of dry C02. The sample was heated according to a temperature profile consisting of a linear 5 "C/min ramp from 50 to 650 "C, a linear 2 "C/min ramp from 650 to 700 "C, and finally an isothermal heating at 700 "C for 30 min. This procedure yielded the hexagonal (11)polymorph of La202C03.Two other forms, the tetragonal (I) and monoclinic (la) structures, have also been shown to exist (Turcotte et al., 1969). Preparation of anhydrous La2(C03)3was performed by thermally decomposing lanthanum carbonate octahydrate, La2(C03)3.8H20,(99.99%, Johnson Mathey), under a C02 atmosphere (Wendlandt and George, 1961). A temperature profile was imposed that consisted of a linear 5 "C/min ramp from 50 to 300 "C, a linear 2 "C/min ramp from 300 to 350 "C, and finally an isothermal heating at 350 "C for 30 min. The latter step improved both the purity and crystallinity of the product. 2.2. Catalyst Characterization. Each catalyst used in this study was extensively characterized prior to use (Taylor, 1989). A portion of this characterization [BET surface area analysis (BET), X-ray powder diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR)] is reported here. BET surface area measurements were performed using an Accusorb 2100E instrument. Samples were evacuated overnight a t 150 "C and then exposed to the krypton adsorbate at liquid N2temperature. X-ray powder diffraction patterns were obtained on a Siemens D500 diffractometer using Cu Ka radiation. The diffractometer was interfaced to a Digital Equipment Corp. DPD 11/23 computer using Siemens Diffract V software for data manipulation. Infrared spectra were obtained by using a Nicolet Instruments 60SX spectrometer. Samples were prepared in the form of pressed wafers approximately 0.003 cm thick (2 wt % sample in KBr). All spectra involved the accumulation of 512 scans at 5-cm-l solution. As discussed previously, prior work has demonstrated that some of the lanthanum phases may be unstable in the presence of H 2 0 and C02. In order to determine the catalytic conditions under which phase transformations occur, a series of studies were conducted in which each catalyst was characterized after catalytic evaluation at specific intermediate reaction temperatures. 2.3. Catalyst Performance. The apparatus used for evaluating catalytic performance consisted of a gas-feed system, a fmed-bed reactor, and a gas chromatograph. The reacting feed stream had a total flow rate of 100.4 sccm, of which the component flows were as follows: helium (Air Products, 99.997% ), 80.1 sccm; methane (Matheson, 99.99%), 18.4 sccm; oxygen (Air Products, 99.6%)),1.9 sccm. The gas-feed system for each gas stream included a silica gel water trap, a 5-pm particulate filter, a Tylan mass flow controller (Model FC-260 or FC-280), and a check valve. The quartz reactor consisted of two 200-mm-long quartz tubes fused together. The upper tube had a 7-mm inside diameter (i.d.) and a &mm outer diameter (0.d.); the lower quartz length was a 6-mm-0.d. capillary tube with a l-mm i.d. Use of the narrow bore tube significantly reduced gas-phase reactions that can occur downstream of the catalyst bed. An l&in.-long, O.OZin.-o.d., K-type thermocouple with a O.OZin.-i.d. quartz sheath was forced through a high-temperature septum at the reactor inlet;
1018 Ind. Eng. Chem. Res., Vol. 30, No. 5, 1991
the tip was positioned at the center of the catalyst bed. The reactor was positioned vertically along the axis of a 12-in.-long,960-W (Lindberg, Model 55031-12-W/OB) tube furnace. All lines downstream of the reactor were wrapped in heating tapes and maintained at 150 "C. Gas analysis was accomplished by an Antek 300 Series gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) and temperature ramping. Two chromatographic columns (Alltech Associates, Inc.) were arranged in series. The first column was a 7-ft-long, l / 8-in.-o.d., 80/100-mesh Porapak Q column, and the second column was a 6-ft-long, l/s-in.-o.d., 60/80-mesh molecular sieve column. The quartz reactor was loaded in the drybox with 50 mg of catalyst (corresponding to W/F= 0.0299 g/cm3). The feed stream was analyzed after stabilization for 2 h at room temperature. The reaction temperature was then increased to 600 "C; the temperature was raised by 50 "C incrementa until the conversion of the oxygen feed approached 100%. At each temperature, 2 h were allowed for catalyst stabilization. Following the catalytic measurements, the reactor was cooled to room temperature, disconnected from the reactor system, and sealed. The reactor was immediately placed in the drybox for postreaction characterization by XRD and FTIR. The small amount of catalyst used precluded postreaction of the surface area. The same procedure was used to prepare samples for postreaction characterization of catalyst tested at intermediate reaction temperatures. Feed conversions for reactant i (Xi) were calculated as the ratio of the number of moles of the component reacted to the number of moles in the feed. The selectivity to product i (Si)was calculated on a per-carbon-atom basis expressed as the number of moles of carbon in a product compared to the total number of moles of carbon in all products. The yield of product i ( Yi)was calculated as the product of the methane conversion, XCH, and Si. The conversion occurring in the absence of a catalyst was determined by using an empty reactor. No combustion or coupling products were produced at temperatures up to 800 "C. At 850 "C a small amount of C2H6 was detected in the product stream, corresponding to a methane conversion of 0.55%. The oxygen conversion was approximately the same, although no oxygenated products such as CO or COPwere detected. At 900 "C the C2H6 produced corresponded to Xch = 0.95% and Xoz= 1.18%; no CO or C02 was detected. 3. Results 3.1. Catalyst BET Surface Area Determination. BET surface areas for the four starting catalysts were determined as follows: A-La203,0.9 m2/g; La(OH)3,9.0 m2/g; II-La202C03,1.1 m2/g; La2(C03)3,18.3 m2/g. 3.2. Catalytic Activity and Selectivity. 3.2.1. ALa203-BasedCatalyst. Conversions and selectivities as a function of reaction temperature for the studies with A-La203starting material are shown in Figure 1. The combined selectivity to all hydrocarbons with two or more carbon atoms, Scz+,is also shown. The highest selectivity to coupling products occurred at 750 "C for C2H6 (51.8%) and at 800 OC for C2H4 (7.4%). C3H8was detected only at 800 OC. Maximum yields of C2H6,C2H4, and C3H8occurred at 800 "C, corresponding to 0.0396, 0.0061, and 0.0013 mol of product produced/mol of CHI, respectively. The total selectivity and yield to all C2+products were observed to have maxima of 56.7% and 0.0471 mol of product/mol of CHI, respectively. At 850 "C, the yield of these products became secondary to the production of COP Dominance
C
,g 40 B u 30 20
'600 650
700
750 800
850
900
Reaction Temperature ('C)
Figure 1. Catalytic performance of the A-La203catalyst: reactant conversions and product selectivities as functions of reaction temperature.
-
20 F
\
'600 650
700 750 800 850
900
Reaction Temperature ('C)
Figure 2. Catalytic performance of the La(OH)3 catalyst: reactant conversions and product selectivities as functions of reaction temperature.
by the combustion reactions above 800 "C was also revealed by the high conversion of oxygen. 3.2.2. La(OH),-Based Catalysts. For the studies using La(OH)3as the starting material, maximum selectivities to C2H6, C2H4,and C3H8occurred at 750,800, and 700 "C with values of 45.9, 12.6, and 2.0%, respectively (Figure 2). The corresponding product yields showed a similar trend with maxima of 0.0640, 0,0190, and 0.0030 mol of product/mol of CH4occurring at the same temperatures. The total selectivity and yield to C2+products had maximum values at 59.6% and 0.0865 mol of product/mol of CH4, respectively. In these experiments, the yields of products from the coupling reactions did not diminish as rapidly at the higher temperatures, compared to results for the A-La203starting materials: even at 850 "C, Sc,+ exceeded SCO,.Furthermore, C3H8was produced over a much wider temperature range (700-800 "C). 3.2.3. II-La202C03-BasedCatalysts. As shown in Figure 3, the maximum selectivities to C2H6,C2H4,and C3H8using 11-La 0 COBas the starting material occurred at 740,800, and 7kOZoCwith values of 49.5,14.5, and 3.3%, respectively. The corresponding maximum yields occurred at 800,825, and 750 OC with values of 0.0761,0.0240, and 0.0045 mol of product/mol of CH,, respectively. The maximum selectivity to C,+ products was 64.0% for 760
Ind. Eng. Chem. Res., Vol. 30, No. 5, 1991 1019
2
/
701
-
1
900 Reaction Temperature ("C)
Figure 3. Catalytic performance of the II-LazOzC03 catalyst: reactant conversions and product selectivities as functions of reaction temperature.
-
'600 650 700 750 800 650 900 Reaction Temperature C '(
1
Figure 4. Catalytic performance of the Laz(C03)3catalyst: reactant conversions and product selectivities as functions of reaction temperature.
"C. The corresponding maximum yield occurred a t 800 "C with a notably high value of 0.1030 mol of product/mol of CHI. Observed in these experiments were trends similar to those in the previous studies. C3H8was produced more selectively and over a wider temperature range. As for the La(OH),-based catalysts, combustion failed to dominate the coupling reaction at high reaction temperatures. The higher methane conversions also resulted in significantly higher product yields over the entire temperature range. 3.2.4. La2(C03)3-BasedCatalysts. For the studies using La2(C03)3as the starting catalyst, the maximum selectivities to C2Hs, C2Hr,and C3H8(shown in Figure 4) occurred at 750,850,and 700 "C with values of 50.3,14.3, and 2.0%, respectively. The total selectivity to C2+ products had a maximum of 61.5% at 780 OC. The corresponding maximum yields to coupling produds occurred at 820, 850, and 700 "C with values of 0.0620, 0.0229, and 0.0018 mol of product/mol of C H I , respectively. The total yield was highest at 850 "C with a value of 0.0827 mol of product/mol of CH,. An apparent "discontinuity" in the product selectivities can be observed in Figure 4 between 700 and 750 "C. 3.3. Catalyst Characterization as a Function of Reaction Temperatures. On the basis of characterization of the catalysts by XRD, FTIR, and other techniques
(Taylor, 1989),all four starting catalysts were unchanged after exposure to reactant gases at 200 "C. The XRD patterns and the FTIR spectra after this treatment are included in the figures that follow. Characterization performed after reaction at 850 "C indicated that the hydroxide, oxycarbonate, and carbonate materials were converted to A-La203. 3.3.1. Characterization of A-La203Materials. The XRD data for the A-La203starting material at intermediate reaction temperatures revealed that A-La203 is present throughout the entire range of reaction temperatures. This is confirmed by FTIR spectroscopy: no bands associated with the hydroxide, oxycarbonate, carbonate, or hydroxycarbonate phase were observed. Since no changes in either the XRD or FTIR data were observed with increasing reaction temperature and since the characterization of A-La203is well-known, the data collected in this study is not presented here. 3.3.2. Characterizationof La(OH)3Materials. The XRD data for the La(OH)3 starting materials at intermediate reaction temperatures are presented in Figure 5. At 200 "C, La(OH)3 is present; this is confirmed by the corresponding FTIR spectrum (Figure 6), which has strong bands a t 644 and 3610 cm-I identified with the hydroxyl groups. The FTIR data also indicated the presence of a small amount of carbonate species (broad bands near 1090 and 1450 cm-'). However, by 400 "C these bands have largely disappeared. Infrared bands for I-La202C03(Turcotte et al., 1969) were observed. Analysis by XRD indicated a poorly crystalline sample, the best potential match being LaOOH (six peaks are common to LaOOH while other peaks could not be assigned). It is possible that I-La202C03and LaOOH exist simultaneously at 400 "C. For example, La(OH)3is known to decompose in vacuum to A-La203via the intermediate LaOOH (Rosynek and Magnuson, 1977); the transformation to LaOOH is complete by 350 "C,and A-La203is formed at 450 "C. However, if C02is present (asin these catalytic studies), formation of an oxycarbonate material is possible. After reaction at 600 "C, FTIR analysis revealed the presence of Ia-La202C03(Turcotte et al., 1969). The corresponding XRD data indicated that poorly crystalline A-La203was present (relatively low signal-to-noise ratio). As the reaction temperature was increased from 650 to 700 "C and finally to 800 "C, the XRD data indicated formation of A-La203. FTIR analysis at 650 "C revealed the presence of a small amount of carbonate species (broad bands centered at 1386 and 1479 cm-'). However, at 700 "C these bands are almost indistinguishable from the background noise; at 800 "C, the bands are not observable. 3.3.3. Characterization of II-La202C03Materials. The XRD data (Figure 7) indicated that II-La202C03was present up to 600 "C. A-La203was formed at 700 "C. The corresponding FTIR spectra, presented in Figure 8, confirmed the presence of II-La202C03from 200 to 700 "C. In addition, between 200 and 600 "C,a shoulder was observed at around 1270 cm-', which corresponds to the most intense band of Ia-La202C03as a stable phase. At 750 "C XRD indicated only A-La203was present, but weak infrared bands corresponding to II-La202C03(856, 1087, 1464, and 1499 cm-l) remain observable. 3.3.4. Characterization of La2(C03),Materials. The XRD data, presented in Figure 9, indicated a loss in crystallinity of the sample up to 400 "C. The corresponding FTIR spectra (Figure 10) revealed the presence of Ia-La202C03up to 400 "C. At 600 "C both XRD and FTIR spectroscopy confirmed the presence of Ia-La202C03
1020 Ind. Eng. Chem. Res., Vol. 30, No. 5, 1991 Table I. Catalyst Compositions at Intermediate Reaction Temperatures" major (minor) phases identified in catalyst at intermediate reaction temperatures starting material 200 OC 400 'C 600 'C 650 "C 700 "C 750 OC 800 OC A-Laz03 A-La203 A-La203 A-La203 na na na A-La203 A-La2O3 A-La203 na A-LazO, A-La2O3 La(OH), LaCOH La(OH), (CO32-) (I-La20,CO3) (Ia-La202C03) (Cot-) (COBz-) (A-LazOd II-La2O2CO3 II-Laz0zC03 II-La202C03 II-LaZO2CO, na II-La2O2CO3 A-La20, na (A-La2O3) (II-La202C03) LaZ(CO3), La2(C03), Laz(C03), Ia-LazO2CO3 na A-La203 A-LaZO3 A-LaZO3 (II-La202C03) (II-La2O2CO3) (II-Laz02C03) (II-Laz0zC03)
(cot-)
850 OC A-La203 A-Laz03
A-Laz03
A -L a 0
(cot-)
na = characterization not performed for these temperatures.
8.838
4.438
2.870
TWO
-
THETA
2.m
--
d
SPACING
1.-
1.541
;OO
1.343
Figure 5. Composition of the La(OH)3starting catalyst at intermediate reaction temperatures (XRDpatterns): (a) 200, (b) 400, (c) 600, (d) 650, (e) 700, (f) 800 'C.
A-La203,based on the FTIR spectra, with a possible trace contamination by a carbonate species (weak bands near 1394 and 1487 cm-'). These bands disappeared at 800 OC; the FTIR spectrum at this temperature was indicative of pure A-La203. The XRD data for 700, 750, and 800 "C demonstrated that the A-La203 present became more crystalline. However, a t all three temperatures, the presence of II-La202C03continued to be detected by XRD. 4. Discussion of Results
I
I
A
I
I
I
,
,
I
4000 3600 3200 2800 2400 2000 1600 1200 800 400 Wavenumbers
Figure 6. Composition of the La(OH), starting catalyst at intermediate reaction temperatures (FTIRspectra): (a) 200, (b) 400, (c) 600, (d) 650, (e) 700, (f) 800, (9) 850 "C.
(major XRD peaks at d spacings of 3.90, 3.15, 2.91, and 2.04; major FTIR bands at 1363,1457,1507, and 845 cm-l). The XRD data also revealed the major peaks of II-La202C03at d spacings of 3.52, 3.02, 2.85, and 2.06. Between 700 and 750 "C, the catalyst appeared to be primarily
Table I summarizes the lanthanum phases detected in the catalysts as a function of temperature. Since the determination of catalytic performance for each starting material began at 600 "C, it is clear from this information that only the A-La203and II-La202C03materials were retained under reaction conditions. The La(OH), starting material was converted primarily to A-La20swith a minor amount of Ia-La202C03present at lower temperatures. The La2(C03)3starting materials at 600 "C contained IaLa202C03as the major component with II-La2O2CO3 present as a minor phase. It is also clear from these studies that regardless of the initial composition of these catalysts, A-La203was produced at a reaction temperature of 850 "C. Considering A-La203for comparison purposes, the following observations can be made about the performance
Ind. Eng. Chem. Res., Vol. 30, No. 5, 1991 1021 8
DIFFRAC V
i
11-LA202C03
I
I
I
I
1
8 II-La202C03
1 I
? P I", u
4.498
0.m
2.878
--2.252
-
II"
1
e
e
i.6a
I I
.w
i.w
TWO THETA d SPACING Figure 7. Composition of the II-La202C03starting catalyst at intermediate reaction temperatures (XRDpatterns): (a) 200, (b) 400, (c) 600, (d) 700, (e) 750 'C.
1
I
I
I
I
I
I
1
1
I
4000 3600 3200 2000 2400 2030 1600 I203 800 400 Wovenumbers
Figure 8. Composition of the II-La202C03starting catalyst at intermediate reaction temperature (FTIR spectra): (a) 200, (b) 400, (c) 600, (d) 700, (e) 750, (0 850 "C.
of the other phases. First, the hydroxide, carbonate, and oxycarbonate starting materials were about 5 0 4 5 % more active toward the conversion of CHI than A-La203 decreased in the order II-La202C03> La2(C03)3> La(0H)3> A-La203). Second, was observed at 750 "C for all materials, and although the same relative order was observed as for the values for differed by only 10%. Of course, it follows from these facts that decreased in the same order: the relative ratios (compared to A-La203) for each material were 2.19, 1.76, and 1.76. The greater activity and selectivity for C2+hydrocarbon products achievable with the II-La202C03starting catalyst resulted in an almost 120% higher yield. It is interesting to note that p;,also varied in the same manner as did SEE for the different catalysts, with the hydroxide, carbonate, and oxycarbonate starting materials being 70,93, and 96% more selective for C2H4 production, respectively, than A-La203. p$,varied in the same manner as and SEg , with the relative ratios for each starting catalyst being 3.93, 3.75, and 3.11 (Le., the greater activity and
(=e
=e,
Qr
selectivity to C2H4 of the II-La202C03starting material resulted in an almost 300% higher yield of C2H4 compared to A-La203). In these studies, the increase in activity and selectivity for one of the highly desired products, C2H4, appeared to correspond to the extent to which the oxycarbonate phases of lanthanum could be detected in the active catalyst. For example, no oxycarbonate phases could be detected for the catalyst with the poorest catalytic performance, A-La203 (which was also the most stable material). For La(OHI3 materials, oxycarbonates were identified as being minor phases at 400 and 600 "C. The La2(CO3I3starting catalyst contained Ia-La202C03as the major constituent at 600 "C and had II-La202C03as a minor phase at 600, 700, 750, and 800 "C. Finally, the most active and selective starting catalyst, 11-La202C03, apparently was relatively stable from room temperature to 700 "C; this composition was still detectable at 750 "C. Because of previous lack of extensive catalyst characterization and rigorous preparation techniques, comparison of these results with those of prior researchers is difficult. However, the performance of some of these materials appears to be superior in some respects. For example, the only previous study with higher values for S,,, is that by Otsuka et al. (1985) (85.2% versus 63.8% in this work). However, their reaction conditions differed greatly from those used in our research ( W / F = 0.002 gs/cm3 and CH4/02= 45.5, contrasted to our values of 0.030 and 9.7, respectively). A study that did use a similar CH4-to-02 ratio was conducted by Lin et al. (1986) (W/F = 2.381 g.s/cm3 and CH4/02= 8.8). At a reaction temperature of 725 "C, CHI and O2 conversions of 9.4 and 91 %, respectively, were observed in this study, along with selectivities to C02, C2H4, and C2H6of 53.4, 23.4, and 23.2% respectively. At 725 "C, the II-La202C03starting materials used in this study produced CHI and O2conversions of 11 and 5570, respectively; selectivities to C02,CO, C2H4, C&, and C3H, were 21,17,8,50,and 4%. The lower value of W / F employed in this study likely accounts for some of this difference, but catalyst composition is also probably an important factor.
1022 Ind. Eng. Chem. Res., Vol. 30, No. 5 , 1991
x
0
DIFFRAC V
LA2 (CO3)3
m 0
0
? Ln N
-0 0
*? 0
z
c n Z 3 0
U
0 h Ln
4 , A36
6.636
2.252
2.876
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-
THETA
--
d SPACING
i .E23
1.541
1.343
Figure 9. Composition of the La2(C03)$starting catalyst at intermediate reaction temperatures (XRDpatterns): (a) 200, (b) 400, (c) 600, (d) 700, (e) 750, (0 800, (g) 850 OC.
I
I
I
1
1
1
I
I
1
I
4000 3600 3200 2800 2400 2000 1600 1200 800 400
Wovenumbers
Figure 10. Composition of the La,(C03)3 starting catalyst at intermediate reaction temperatures (FTIRspectra): (a) 200, (b) 400, (c) 600, (d) 700, (e) 750, (D800, (g) 850 'C.
The catalyst used in the study by Otsuka et al. (1985) was La203supplied by Asahi Chemical Co.; no catalyst pretreatment or characterization was reported. The catalyst employed in the study by Lin et al. (1986) was probably primarily A-La203with some carbonate incorporation. The catalyst preparation route involved hydrating the starting material (A-La203,Aldrich) in deionized water followed by drying. The resulting material was identified as La(OH)3with carbonate impurities. However, in converting the La(OH)3 to A-La203 via two heat treatments (the first at 450 "C in flowing O2and the second at 725 OC in the reactant gas mixture), it is likely that not all of the carbonate species were decomposed, since a higher temperature would be necessary. Bernal and coworkers have shown that the carbonate content of rareearth oxides can only be removed by a heat treatment at a temperature not lower than 900 "C (Alvero et al., 1983; Bernal et al., 1983b; Carrizosa et al., 1984). Heating at a temperature lower than 900 OC and maintaining that temperature until the evolution of HzO and COz ceases probably do ensure that no further HzO nor CO, will be
evolved up to that temperature during catalytic reaction. However, the catalyst will likely not be a pure rare-earth oxide; rather a material will be produced that is only less extensively hydrated or carbonated than the starting compound. Whether some of the more active phases identified in our research are present as surface layers or bulk compounds is presently being investigated. The infrared technique may be more useful than XRD in identifying surface interactions and compound formation since the latter technique focuses on the bulk crystalline structure. On this basis, it appears that the CO$-, I-La202C03,and Ia-La2OZCO3 secondary phases identified between 400 and 700 "C for the La(OH)3 starting catalyst are probably restricted to the catalyst surface. Carbonation most probably results from the C02 present in the reactor. On the other hand, the II-La202C03secondary phase detected between 600 and &IO "C for the La2(C03)3starting catalyst is believed to be present in the catalyst bulk (observed by both XRD and FTIR spectroscopy). 5. Conclusions
When the four pure lanthanum compounds studied in this work were employed as starting materials for the oxidative coupling of CHI to C2hydrocarbons, it is found that the general catalytic performance follows the following trend: II-La2O2CO3> La2(C03)3LLa(OH), > A-La203 and hence [in terms of decreasing XE& S&+, and p',": and @%,I. However, on the basis of catalyst characterization by XRD and FTIR, these starting materials undergo important changes during reaction. In summarizing the observed trends, it would appear that the extent to which an oxycarbonate material is present has an important, beneficial effect on catalytic performance. I t is interesting to observe that although the catalyst compositions of all starting materials appear to be the same after reaction at 850 OC (A-La203),the catalytic performance remained quite different. Clearly, catalytic performance can depend not only on the catalyst composition but also on its life history. It should also be pointed out that this study has not attempted to determine an optimal per-
vH,
Ind. Eng. Chem. Res. 1991,30, 1023-1032
formance for each catalyst starting material; the optimal conversion, yield, and selectively clearly would likely correspond to different reaction conditions. However, such a determination would be useful for the practical selection of a catalyst. Acknowledgment This work was conducted through the Ames Laboratory, which is operated for the U S . Department of Energy by Iowa State University under Contract W-7405-Eng-82. This work was supported by the Office of Basic Energy Sciences, Chemical Sciences Division. Literature Cited Alvero, R.; Carrizosa, J. A,; Odriozola, J. A,; Trillo, J. M. Activation of Rare Earth Oxide Catalysts. J. Less-Common Met. 1983,94, 139-144. Bernal, S.; Botana, F. J.; Rodriguez-Izquierdo, J. M. Thermal Evolution of a Sample of La203 Exposed to the Atmosphere. Thermochim. Acta 1983a,66, 139-145. Bernal, S.;Garcia, R.; Lopez, J. M.; Rodriguez-Izquierdo, J. M. TPD-MS Study of Carbonation and Hydration of Yb203(C). Collect. Czech. Chem. Commun. 1983b,48,2205-2212. Bernal, S.;Diaz, J. A.; Garcia, R.; Rodriguez-Izquierdo, J. M. Study of Some Aspects of the Reactivity of La203with COz and HzO. J. Mater. Sci. 1985,20,537-541. Burch, R.; Squire, G. D.; Tsang, S. C. Comparative Study of Catalysts for the Oxidative Coupling of Methane. Appl. Catal. 1988, 43,105-116. Campbell, K. D.; Zhang, H.; Lunsford, J. H. Methane Activation by the Lanthanide Oxides. J. Phys. Chem. 1988,92,750-753. Carrizosa, I.; Odriozola, J. A.; Trillo, J. M. Lanthanide Oxides: Yb203 Hydration. Znorg. Chim. Acta 1984,94,114-116. Deboy, J. M.; Hicks, R. F. The Oxidative Coupling of Methane over Alkali, Alkaline Earth, and Rare Earth Oxides. Znd. Eng. Chem. Res. 1988a,27, 1577-1582. Deboy, J. M.; Hicks, R. F. Kinetics of the Oxidative Coupling of Methane over 1 wt% Sr/La203. J. Catal. 1988b,113,517-524. Hinsen, W.; Bytyn, W.; Baerns, M. Oxidative Dehydrogenation and Coupling of Methane. h o c . 8th Znt. Congr. Catal. 1984,581-592.
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Hutchings, G. J.; Scurrell, M. S.; Woodhouse, R. J. Partial Oxidation of Methane over Samarium and Lanthanum Oxides: A Study of the Reaction Mechanism. Catal. Today 1989,4,371-381. Jones, C. A.; Leonard, J. J.; Sofranko, J. A. Fuels for the Future: Remote Gas Conversion. Energy Fuels 1987,1, 12-16. Lee, J. S.; Oyama, S. T. Oxidative Coupling of Methane to Higher Hydrocarbons. Catal. Rev.-Sci. Eng. 1988,30, 249-280. Lin, H. C.; Campbell, K. D.; Wang, J. X.; Lunsford, J. H. Oxidative Dimerization of Methane over Lanthanum Oxide. J.Phys. Chem. 1986,90,534-537. Otsuka, K.; Jinno, K. Kinetic Studies on Partial Oxidation of Methane over Samarium Oxides. Znorg. Chim. Acta 1986,121, 237-241. Otsuka, K.; Nakajima, T. Partial Oxidation of Methane over Rare Earth Metal Oxides using NzO and O2 as Oxidants. Znorg. Chim. Acta 1986,120,L27-L28. Otsuka, K.; Komatsu, T. Active Catalysts in Oxidative Coupling of Methane. J. Chem. SOC., Chem. Commun. 1987a,388-389. Otsuka, K.; Komatsu, T. High Catalytic Activity of Sm203for Oxidative Coupling of Methane into Ethane and Ethylene. Chem. Lett. 1987b,483-484. Otsuka, K.; Jinno, K.; Morikawa, A. The Catalysts Active and Selective in Oxidative Coupling of Methane. Chem. Lett. 1985, 499-500. Otsuka, K.; Jinno, K.; Morikawa, A. Active and Selective Catalysts for the Synthesis of CzH6 and C2H, via Oxidative Coupling of Methane. J. Catal. 1986,100,353-359. Pitchai, R.; Klier, K. Partial Oxidation of Methane. Catal. Rev.SC~ Eng. . 1986,28, 13-88. Rosynek, M. P.; Magnuson, D. T. Preparation and Characterization of Catalytic Lanthanum Oxide. J. Catal. 1977,46,402-413. Scurrell, M. S. Prospects for the Direct Conversion of Light Alkanes to Petrochemical Feedstocks and Liquid Fuels-A Review. Appl. Catal. 1987,32,1-22. Taylor, R. P. M.S. Thesis, Iowa State University, 1989. Turcotte, R. P.; Sawyer, J. D.; Eyring, L. On the Rare Earth Dioxymonocarbonates and Their Decomposition. Znorg. Chem. 1969,8,238-246. Wendlandt, W. W.; George, T. D. The Thermal Decomposition of Inorganic Compounds IV. Rare Earth Carbonates. Tex. J. Sci. 1961,13,316-323. Received for review July 23, 1990 Revised manuscript received December 26, 1990 Accepted January 8,1991
Pressure Swing Adsorption: Experimental and Theoretical Study on Air Purification and Vapor Recovery James A. Rittert and Ralph T. Yang* Department of Chemical Engineering, S t a t e University of New York a t Buffalo, Buffalo, New York 14260
Pressure swing adsorption (PSA) air purification/vapor recovery was studied by experiments and model simulations using dimethyl methylphosphonate (DMMP) vapor and activated carbon. The most significant result was that complete cleanup of the product effluent resulted when starting from a saturated bed even for the very strongly adsorbed vapor, DMMP. Furthermore, at the same time, a concentrated DMMP vapor was produced at the exhaust effluent. Therefore, PSA cannot only be used to purify air, it can also concentrate the vapor for more efficient recovery or abatement. Also, two cyclic steady states were demonstrated both experimentally and theoretically. When starting from a clean bed, the concentration wave penetrated the bed very slowly while a cyclic steady state was being approached and much of the bed remained unused acting as a guard against product effluent contamination. However, when starting from a saturated bed, a different cyclic steady state was approached where a "heel" existed in the bed a t the product effluent end. Introduction Although fixed bed adsorption processes have existed for a very long time (see Mantell (1951)),cyclic adsorption 'Present address: Westinghouse Savannah River Company, Savannah River Laboratory, Aiken, S C 29802.
0888-5885/91/2630-lO23$02.50/0
processes are relatively new. It was not until 1959 that the first truly cyclic adsorption process was invented by Skarstrom (1959) for air drying, referred to as heatless adsorption or pressure swing adsorption (PSA). This paper is concerned with a PSA process similar to that invented by Skarstrom (1959); however, in this study PSA is used for air purification instead of dehydration. The significant 0 1991 American Chemical Society
