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R. B. ANDERSON, K. C. STEIN,I J. J. FEENAN, and L. 1. E. HOFER
U. S, Bureau of Mines, Pittsburgh Coal Research Center, Pittsburgh, Pa.
Catalytic Oxidation of Methane Microcatalytic reactor permits rapid screening of catalysts and provides kinetic data
for the total oxidation of methane was determined as a part of a program for developing devices to monitor methane concentration in coal mine atmospheres. For this purpose a number of catalysts were tested in a microcatalytic reactor ( 4 , 6). This device facilitates screening of catalysts and also provides information on the kinetics of oxidation. Many studies ( 3 ) have been reported on the partial oxidation of hydrocarbons to valuable intermediates, but a previous investigation by the Bureau of Mines (70) seems to be the only systematic study of catalytic total oxidation of methane. Vendt and Lebedeva reported that CraQs on acid-washed pumice was an effective catalyst for the oxidation of methane and higher hydrocarbons ( 8 ) . For the oxidation of methane, Cohn and Haley ( 2 ) found the activity of catalytic components supported on alumina to decrease in the following order: rhodium, palladium, iridium, ruthenium, platinum, and silver. Nickel oxide on alumina was inactive under their testing conditions. Currently, the catalytic oxidation of methane is of interest with respect to the problem of electrode catalysis in the use of natural gas in fuel cells. T H E ACTiVITY OF CATALYSTS
Experimental
Catalysts. Thirty catalysts were tested for their effectiveness in oxidizing methane. One catalyst, silica gel. required no preparation. 'This sample was ineffective even a t the highest temperature investigated, and was not considered further. Twenty six catalysts were prepared in this laboratory, and three (8, 1 2 , and 14) were made by others. Little information is available on the manner of preparation of this latter group. Fourteen catalysts were made by impregnation of porous supports. The supports were impregnated with an aqueous solution of metal nitrates except for the following: platinum and ti1
tanium as chlorides and molybdenum as (NH&MoOp. T h e impregnated material was dried and finally heated to 250' C. in air. The supports and their surface areas were: Kaosorb activated clay, 125 square meters per gram; alumina 151, containing 6y0 SiOn, 350 square meters per gram; and alumina 622, containing 13% Sios, 35 square meters per gram. Alumina 151 was used, as obtained, as */g-inch spheres. The other supports, which were obtained as larger pellets, were crushed and sieved to 10- to 12-mesh particles before impregnation. Decomposition catalysts were prepared by evaporating a n aqueous solution of mixed metal nitrates and heating
-Editor's
the nitrates to temperatures in excess of^ 850' C. The resulting cake was crushed and screened to 10- to 12-mesh. Most of the precipitated catalysts were made by dissolving the metal nitrate in water and adding an alkali such as KzCOj or NaZC03 to precipitate the metal oxide or oxides. T h e resulting suspension was filtered and washed. T h e washed cake was first dried and then crushed and screened to IO- to 12-mesh particle size. Zinc chromite catalyst 18 was made by precipitating the chromate from Zn(NO3)z solution using (NH4)2Cr207. The precipitate was filtered, washed, and dried. After being heated in air to 400" C. in a muffle furnace, the
Note
The reader will be interested to note that similar work has been done at Mine Safety Appliances Co. by Cotabish and Flumerfelt. This work, not as well CORtrolled as that reported here and carried out more from an equipment design approach, gave comparable results. The equipment used included a tube furnace, temperature controlled from a Variac with thermocouple imbedded in the catalyst. The catalyst bed was 7 to 7.5 cc. with a cross section of 2.435 sq. cm., and air flow was 2 liters per minute measured at room temperature. Concentration was 1% methane in an air stream of 15% relative humidity at 75" F. Effluent analysis was by the method of Haldane. Engelhard Industries, Inc., has also made numerous investigations of the catalytic oxidation of the maior gaseous hydrocarbons, particularly on platinum metal catalysts. Earlier work (2) has been continued, and catalysts burning methane with efficiencies of 98% a t space velocities of 60,000 SCFH/CF are commercially available. With a specially modified catalyst, good oxidation is maintained even in the difficult case of a high steam atmosphere after several months of operation.
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product was finally crushed and screened 'to 10- to 12-mesh particle size. Catalyst 17 was made by precipitating Cos04 from an aqueous solution of Co(N03)2 with KzCOa. The wet precipitate was then impregnated on H-151 alumina. T h e resulting material was heated to 250" C. in a muffle furnace. Preparation 9 was a cobalt-thoriamagnesia-kieselguhr (100 - 6 12 200) Fischer-Tropsch catalyst. Its preparation has been described previously (7). This material was used in the form of filter cake crushed and screened to 10- to 12-mesh. Surface areas were determined on the supports, as described. and on two of the catalysts: preparation 6, 14 square meters per gram; and preparation 9300 square meters per gram. Impregnated catalysts prepared in this laboratory probably had about the same surface areas as their supports. Procedure. T h e general method of testing the catalysts is patterned closely after the microcatalytic method of Kokes, Tobin, and Emmett ( 4 ) . I n this system, as modified a t this laboratory for oxidation studies (6), a continuous stream of the carrier gas (oxygen) is passed at constant rate (40 cc. per minute a t STP) through 5 cc. of catalyst contained in a quartz reactor and heated by a n electric resistance winding. From the reactor, the gases pass through a drier, a chromatographic column, and finally a thermal conductivity cell. The thermal conductivity cell and chromatographic column are maintained at 35" C. A t appropriate time intervals, a small, accurately measured volume of methane (0.66 cc. at STP) is introduced into the oxygen stream upstream from the catalyst. As the hydrocarbon-oxygen mixture passes through the catalyst, reaction takes place; this is determined by the chromatographic section of the train. Specifically, the chromatographic unit consisted of:
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Temperature, 2 O O ' C
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A j/,,-inch (outside diameter) copper tube, 4 feet long, packed with Davison high-purity iron-free silica gel, 40- to 60-mesh particle size A Gow-Mac thermal conductivity cell connected to a Brown chromatographic recorder A typical test proceeded as follows : First, the thermal conductivity recorder was adjusted to a preselected base line with pure oxygen flowing through the system. The catalyst temperature was then adjusted to the value that was to be maintained throughout the test. The methane sample was introduced, and the test proper began. The methaneoxygen mixture and the reaction products passed rather quickly through the catalyst and drier and into the chromatographic column that separated the components of the gas mixture.
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I c
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Figure 1. Chromatograms give testing d a t a for the oxidation OF methane on palladium-on-alumina catalyst 22
Additional time was requiied for the system to be flushed completely of the reaction products from the previous experiment before proceeding with the next. Usually the time interval between tests, including changing the catalyst temperature to the next test point, was about 30 minutes. The series of experiments was continued at progressively increasing temperatures (50' C. steps were the usual practice) until all the methane introduced was destroyed, or to a maximum temperature of 600" C. The final temperatures of drying some of the catalysts were lower than those employed in the oxidation studies. I n many of these tests, the sequence of increasing temperatures was followed by a sequence of decreasing temperatures. The effectiveness of the catalysts was usually essentially the same at any given temperature for both the increasing and the decreasing temperature sequences. Thus. it must be concluded either that thermal sintering of the catalyst did not occur or that it was not important to the effectiveness of the catal) st. Figure 1 represents a typical series of chromatograms showing hoiv the effectiveness of a catalyst increases with increasing temperature. At the lowest temperature (200. C.) the only peak on the graph is that for methane (a); this shows that no measurable reaction occurred. At a higher temperature (350' C.) the methane peak is much smaller, and a n appreciable COZ peak ( b ) and a very small CO peak (d) have appeared, showing that a substantial amount of oxidation has occurred. At the highest temperature (450" C.) there
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is no methane peak; only combustion products appear on the chromatogram. Line c results from a slight disturbance of the flow of carrier gas during sample introduction. Methane remaining was estimated from the height of peak a. The oxides of carbon products usually accounted for the methane consumed. Results
I n the microcatalytic reactor the differential reaction rate, r, may be expressed as Qdx/dV, where Q is the feed rate of methane in cubic centimeters per second at STP, x is the fraction of methane consumed, and V is the bulk volume of catalyst. The flow of the slug of methane should be essentially that of the carrier gas, and Q may be approximated by the flow of carrier gas. O n this basis the hourly space velocity of methane in these experiments is 480. The differential reaction rate may be approximated by empirical rate equations of the type 1 = k ( l - x)i, where k is a constant. For first-order kinetics i = 1, and the integrated equation becomes : k
=
- ( Q / V ) In (1 -
x)
(1)
If the temperature dependence of the raie constant can be expressed by the Arrhenius equation, k = A exp(-E/RT), the activation energy, E, for the firstorder kinetics is given by: -E = R d l n k / d ( l / T ) 2.303 R d{loglo logio(1
=
- x)-'}/d(l/T) (2)
M E T H A N E OXIDATION which can be evaluated from the slope of plotsofloglologlo(1 x)-'against 1/T. A rate equation similar to Equation 1 was developed by Bassett and Habgood for the catalytic isomerization of cyclopropane on a chromatographic column (7). I n the present assaying program, catalysts were tested a t different temperatures, using a constant value of Q/V. If the activation energy is assumed to be constant for a given catalyst over the temperature range studied, the apparent order of the reaction may be selected as that producing the best linear plot of the Arrhenius equation. This criterion will lead to erroneous conclusions if the activation energy changes with temperature. For processes in which diffusion in catalyst pores, coupled with reaction at the surface, is rate determining, the activation energy may vary with temperature (9). For the microcatalytic data of methane oxidation, satisfactory linear Arrhenius plots were generally obtained using first order Equation 1. Zero and second order plots showed pronounced curva-
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ture a t higher values of conversion. Half order plots were frequently as nearly linear as those for first order. Typical first order Arrhenius plots, chosen to avoid overlapping lines, are shown in Figure 2. O n these plots, where data are available, points determined during increasing and decreasing temperature sequences are shown. Usually these data show no greater deviations than duplicate experiments a t the same temperature. This type of plot is sensitive to experimental uncertainties for values of conversion below 0.1 and above 0.9, as the scale is greatly expanded in these regions. Table I gives the activation energies, rate constants a t 300' and 450' C., frequency factors, and phases detected by x-ray diffraction. Rate constants are presented per unit bulk volume and per unit weight of catalyst and the frequency factor per unit bulk volume of catalyst. Methane is the most difficult hydrocarbon to oxidize. Table I1 compares the temperature required for complete oxidation of methane, 2-pentene, and
~
Table 1.
~
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benzene on four of the most active catalysts in the microcatalytic unit; 2-pentene represents an easily oxidized higher hydrocarbon and benzene one of the most difficult to oxidize.
Discussion The microcatalytic reactor has pro\ ed useful in screening catalysts and, in addition, provides information on kinetics. This reactor is particularly advantageous for highly exothermic reactions, as the heat evolved is limited by the quantity of reactant introduced. For the present system using inlets of 0.66 cc. (STP) of methane, the heat evolved is less than 7 cal., and the overall temperature increase of the catalyst bed should be less than 3' C. The present data, as well as the few data reported by Rosenbaum ( 5 ) a t 570' C., are reasonably approximated by an empirical first order rate equation with respect to methane concentration. For the more active preparations. the apparent activation energies were between 15 and 25 kcal. per mole and for
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Rate Constants and Other Pertinent Data Were Obtained for Catalysts Tested in Oxidations of Methane Aotivation Rate Constants X Density, Energy, Phases from X-Ray Grams/ Ked./ Cc./Cc.-8ec. Cc./Gram-Sec. ______.Diff Taction" Cc. Mole 3OOOC. 450OC. 300' C. 450' C.
Bulk
Catalyst NO.
Catalyst Components and Wt. % Active Metal
Log10 8 , cc./cc.Sec.
Precipitated and Sintered 6 18 9 25 33
Cos04 COS0 4 ZnCrOc (48 Zn, 27.5 Cr) ZnO, Cr20J' Co-Th02-Mg0-kieselguhr (24.7 Co) C 0 3 0 4 ~ CuCrOr (33.4 Cu, 17.9 Cr) A PbCrOe PbCrOa
0.80 0.310 0.213 0.541 1.745
Decomposition-All 24
9.4 co-61 Fe
19 31 20 29
50 CO-26 CU 34 CU-30 A1 30 Cu-45 Be 38 Be-39 Cd
14 8
Pd on alumina (0.5 Pd)d P t on alumina (0.5)d Cor01 on Kaosorb clay (6.5 Co) COaO4 on alumina-622 (7.2 Co) PdO on alumina-I51 (3.5 Pd) CnOa on alumina-I51 (3.1 Cr) Mm03 on alumina-I51 (13.6 Mn) CuO on alumina-I51 (7.0 Cu) Pt on alumina-151 (0.51 Pt) Cos04 K ~ C O on I alumina-151 (2.4 coy CeOz on alumina-I51 (1.2 Ce) Cos04 on alumina-151 (2.0 Co) Fez03 on alumina-151 (2.9 Fe) VZOSon alumina (8.8 V ) d NiO on alumina-151 (3.8 Ni) Ag on alumina-I51 (4.7 Ag) MOO8 on alumina-151 (3.2 Mo) Ti02 on alumina-I51 (7.9 Ti)
FezOa, FesO4; or CoFe20p COaO4, CUO CuAIz04 CuFezOa, CuO CdFe204
16.5 15.4 19.2 23.1 25.0
7.2
... ... ... ...
144 5.3 3.3 0.84 0.056
Mixed Oxides
1.30
21.1
1.41 0.712 1.30 1.073
18.4 22.9 13.0 30.7
... ...
...
... *..
0.64 0.51 0.48 0.37 0.16
9.0
... ... ... ...
180 17.1 15.5 1.55 0.032
5.12 3.35 4.29 4.87 4.26
...
0.50
4.15
0.36 0.67 0.28 0.15
3.24 4.57 1.48 6.44
...
... ... 0 .
Impregnated Catalysts
1
5 22 10
7 15 28
17 23 13 32 11 21 16 27 26
+
0.765 0.861 0.636 1.186 0.851 0.817 0.912 0.836 0.775 0.845
21.8 23.5 22.2 20.9 20.9 22.6 24.4 22.1 24.6 18.4
0.827 0.899 0.753 0.877 0.852 0.784
28.9 21.9 30.9 23.9 31.3 50.6 35.9 32.4
0.770 0.792
76 2.8 1.6 1.1 0.88 0.43 0.25
... ...
... ... ... ... ...
... ... ... I . .
3960 199 90 49 39 26 21 6.4 5.5 4.0 0.79 0.75 0.61 0.43 0.13 0.073 0.069 0.069
99.3 3.3 2.5 0.93 1.03 0.53 0.25
... ... ... ... ... ... ... ... ... ... ...
5180 235 140 41 46 32 23
8.14 7.35 6.62 5.97 5.88 6.20 6.66 5.45 6.13
7.7 7.1 4.7 0.96 0.83 0.81
0.49 0.15 0.093 0.090 0.087
4.13
6.59 4.46 7.07 4.82 6.41 12.07 7.63 6.58
a A = amorphous, y = 7-AlnOa, a = a-AlnOa. X-Ray Powder Data File 6-0532. Spinels are isomorphous, and exact coniposition cannot be distinguished by x-ray diffraction. For most supported cobalt catalysts this phase presumed to be CoAl204, as color is blue. Catalysts from commercial sources. e Cobalt basic carbonate precipitated before addition to support.
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Table II. Oxidize
Methane Is More Difficult to on Catalysts than Higher Hydrocarbons
Complete Disappearance of Parent Hydrocarbon Peak, C.
Temp. for
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Catalyst 6
Cos04 (un-
200
200
400
supported) C ~ Z Oon S
...
350
500
200
200
400
250
300
300
8
I4
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12
14
16
16
20
2 2
1.000 I T‘K
Figure 2. methane
Arrhenius plots show temperature dependence in catalytic oxidation of Activation energies in kilocalories per mole are given on the graph
the active impregnated catalysts between 21 and 25 kcal. per mole. Catalysts with low activity had higher activation energies. About half of the catalysts in Table I (most of those impregnated on alumina 151) were prepared for this study, and the remainder were available from other investigations or commercial sources. T h e effectiveness of impregnated catalysts of about the same chemical composition varied widely with the method of preparation and type of support. In one case, the first order rate constants differed as much as a 100-fold.
Catalysts impregnated on alumina 151 form a consistent group of preparations; however, a standard preparative procedure may not be equally effective for different active components. For this group of catalysts, frequency factors and first order rate constants a t 450’ C. were computed per gram of active metal, and the logarithm of the frequency factor is plotted as a function of activation energy in Figure 3. T h e activity of the metals or metal oxides per gram of active metal decreases in the following order: Pt, Pd. Cr. Mn, Cu, Ce, Co. Fe, Ni. .4g.
r--1 -
Meth-
Catalyst
10
10
2-Pen- Bentene Bene
No.
alumina Pi on alumina Pd on alumina
m e
Although C0304 is the most active single-component catalyst in oxidation of hydrocarbons, its activity is decreased by impregnation on alumina, possibly because of the formation of CoA1204. Precipitation of the C o ( K 0 J solution with K&Oa prior to addition to the support increased the activity fivefold. T h e supported silver (Table I) probably has a n abnormally low activity, because part of the silver reacted with residual chloride in the support to form AgCl. The present results from the microcatalytic reactor were consistent with available published data. Yant and Hawk (70) examined pure metal oxides and mixtures of oxides and found that C0304 was the most effective. For catalysts supported on alumina, the order of effectiveness in these tests was essentially the same as reported by Cohn and Haley (2). Supported C r z 0 3 was one of the more effective catalysts, as also described in a Russian study ( 8 ) . literature Cited (1) Bassett, D. W., Habgood, H. Mi., J . Phvs. Chem. 64. 769 (1960). (2) cohn, J. G. &ley, ’ A , J., Can. Patent 597,459 (May 3, 1960). (.3,) Dixon, J. K., Longfield, J. E.. in
B.,
’ /
Figure 3. Plots of the logarithm of frequency factor, A, as a function of activation energy for metal oxides and metals supported on alumina-1 51
“Catalysis,” P. .H, Emmett, ed., Vol. VII, pp. 183-281, Reinhold, New York, 1960. (4) Kokes, R . J., Tobin, H., Jr., Emmett, P. H., J . Am. Chem. Sac. 77, 5860 (1955). (5) Rosenbaum, E. J., Adams, R. W., King, H. H.? Jr., Anal. Chem. 31, 1006 (1959). (6)’ Stein, K. C., Feenan, J. J., Thompson, G. P., Shultz, J. F., Hofer, L. J. E., Anderson. R. B., IND.ENG. CHEM.5 2 , 671 (1960). (7) Storch, H. H., Anderson, R. B.. Hofer, L. J. E., Hawk, C. O., Anderson, H. C . , Golumbic, N., U. S. Bur. Mines Tech. Paper 709, p. 85,1948. (8) Vendt, V. P., Lebedeva, T. A., Zaoodskaya Lab. 24, 915 (1958). (9) Wheeler, A,, in “Catalysis,” P. H. Emmett, ed., Vol. 11, Chap. 2, Reinhold, New York, 1954. (10) Yant, W. P., Hawk, C. O., J . Am. Chern. SOC. 49, 1454 (1927).
Oblique lines represent constant values of rate constant a t 450’ C. Rate constants and frequency factors a r e expressed per gram of active metal
RECEIVED for review January 5, 1961 ACCEPTED -4pril 5, 1961
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