= drag coefficient ratio, dimensionless = emissivity, dimensionless = fraction of flame energy released by radiation,
dimensionless bending angle of flame from the vertical, angle of tilt, dimensionless; also, time variable in momentum balance, sec. = kinematic viscosity, sq. ft., sec. = flame roughness factor, dimensionless = density, lb. mass, cu. ft. = Stefan-Boltzmann constant, B.t.u., hr.-sq. ft.-" R4. = wind stress, Ib. force,'sq. ft. = angle between surface normal and radius vector. dimensionless =
SUBSCRIPTS 1, 2
=
surface 1, 2
a
= surrounding air
b
= =
f
=
4
=
I
= = = =
0 0
LC
body field type force or signifies flame when used with drag coefficient and density gas ith contribution in out property a t source or surroundings wood element
Literature Cited
(3) Blinov. V. I.: Khudiakov, G. N.. .4cad. Nauk L7SSR 113, 1094-8 (1957). (4) Burgess. D. S., et al., Fire Research Abstr. Rev. 3, 177-92 (1961). (5) Burgess, D. S.. et al., Natl. .\cad. Sci. -Natl. Res. Council, NAS-NRC Pub. 786 68-75 (1961). (6) Burgess, D. S..Zabetakis, M. G., U. S. Bur. Mines, Rept. Invest. 6099 (1962). (7) Churchill. S. LV.. Sliepcevich, C. M.. private communication. Aug. 3, 1961. (8) Cmmons, H. LV., Natl. Acad. Sci.-Natl. Res. Council, NASN R C Pub. 786, 50 67 (1961). (9) Fons. I V . L., et al.. Conrbust. Flame 5 , 283-7 (1961). (10) Hirst. K., Sutton, D., I h d . 5 319-30 (1961). (11) Priestley. C. €1. B., Quart. J . Roy. Meteorol. SOC.82, 1 6 5 7 6 (1956). (12) Priestley, C. H. B., Ball, F. K., Z62d., 81, 144-57 (1955). (13) Putnam. A. A , . private communication, Aug. 14, 1963. (14) Putnam. A. A , Speich. C . F., NBS Contract CST-717, Battelle Memorial Institute, Summary Rept. 2, 3-2'7 (1963). (15) Rankine, 4 . 0.. "F.I.D.0. Investipations," Petroleum Lt'arfare Department, Great Britain, 1945. (16) Rouse. H.. Yih. C:. S.. Humphreys. H. LV.. 7ellus 4, 201 -10 (1952). (17) Simms, U. L.. Combust. Flame 4, 293-300 (1960). (18) Streeter, V. I,.. "Handbook of Fluid Dynamics," McCkawHill, New York, 1961. (19) 'Iaplor, G. I., "Fire Under the Influence of Natural Convection." Natl. Acad. Sci.- Natl. Res. Council. NAS-NKC Pub. 786, 10--31 (1961). (20) 'I'homas. P. H.. Insf. Fire Eng. Quart. 21, 197-219 (1961). (21) Zabetakis, M. G.. Burgess, I). S.. U.S. Bur. Mines, Rept. Invest. 5707 (1961).
(1) Ball. F. K . , Quart. J . Roy. Meteoroi. SOC.84, 61-5 (1958). (2) Berl. \V. G.. ed.. "Use of Models in Fire Research," Natl. Acad Sci. Natl. Res. Council. NAS-NRC Pub. 786 (1961).
RECEIVEDfor review July 8. 1963 ACCEPr m February 4, 1964
MAGNETIC STUDY OF COBALT MOLYBDENUM OXIDE CATALYSTS J A M E S 1. R I C H A R D S O N FlumblP Oil €3 Rejiiirring Co , Baytown. T e x a s
Magnetic susceptibility measurements were used to determine the nature and composition of active and inactive components in cobalt molybdate-alumina desulfurization catalysts containing 1 0% Moo3, with initial Co:Mo ratios from 0.1 to 1.0, and heat treated in the range of 538" to 816" C. Below an initial C o : M o ratio of 0.3, the fresh catalysts contain no CoA1204 and less than 10% of the cobalt exists as COO,the remainder forming an active complex with molybdena. Above an initial ratio of 0.3,the final composition depends on both heat treatment and cobalt content. In the desulfurization of West Texas gas oil, an intrinsic rate constant correlates with the active Co:Mo ratios, passing through a maximum at an active C o : M o ratio of 0.1 8. Several commercial catalysts also fit this correlation.
terminology a "cobalt molybdate" catalyst is a molybdenum oxides dispersed on some support. such as alumina, and heat treated a t a n elevated temperature. Usually the exact stoichiometry and structure of the active catalytic species are unknown. especially under reaction conditions. A considerable amount of experimentation has been aimed a t rstablishing the most desirable cobalt and molybdenum concentrations. Commercial preparations cover a wide range, with 5 to 13% Mo, 1 to 6% Co, and Co : Mo ratios from 0.2 to 1.0. Nahin and Huffman (7) advocate a C o : M o ratio of 1 .O for cobalt molybdate desulfurization catalysts. whereas Engel and Hoog (4).Sulimov et 01. (72). and Porter ( 8 ) find optimum results with 0.2. Beuther. Flinn, and McKinley. N CATAI YST
I mixture of cobalt and
154
l&EC FUNDAMENTALS
investigating the hydrodesulfurization activity of promoted molybdenum oxide -alumina catalysts, find maximum activity of 0.35 ( 3 ) . T h e formation of cobalt-molybdenum and cobalt-aluminum compounds plays a n important role in the establishment of catalytic properties (5). However, the components of' these catalysts have not been identified, nor the reason for an optimum Co: Mo ratio established. 'This paper presents the results of a magnetic study of various laboratory and cornmercial cobalt molybdate catalysts. 'I he magnetic measurements are used to identify and isolate the active and inactive components of the catalysts. l ' h e Co: Mo ratios in the active component are then correlated with activities for a typical process-type application.
Experimental
A sample of pure CohloOI was prepared by a coprecipitation method. A cobalt acetate solution was mixed with a n appropriate amount of ammonium paramolybdate solution containing excess ammonia. T h e light purple-pink precipitate was washed, filtered. air-dried a t 100' C.. and heated in air a t 650' C. A series of cobalt oxide-molybdena-on-alumina catalysts was prepared by impregnating iron-free y-AlsOa with solutions of cobalt acetate .and ammonium paramolybdate. T h e concentrations of the mixtures were adjusted to give samples containing approximately 107, M o o s on .A1203 and with C o :Mo ratios in the range 0.1 to 1 .O. .4fter thorough milling. the impregnated samples were air-dried and heat treated for 24 hours in air to give Series 1-6 catalysts \vith a C o : Mo ratio of 1.0, heat treated in i.he range 538' to 816' C . ; Series II6 catalysts with Co: Mo ratios from approximately 0.1 to 0.7. heat treated a t 538@C. ; and Series 111-6 catalysts \vith Co: hlo ratios from approximately 0.1 to 0.7, heat treated a t 650@C. Magnetic susceptibility measurements Jvere made b>- the Faraday method. using an apparatus similar to one previously described ( 6 ) . Measurements kvere made a t five magnetic field values u p to 5000 oersteds to check for any field dependence. l ' h e temperature range was varied from - 196' to 700" C . by means of suitable De\var flasks and furnaces. A quartz spring with a sensitivity of about 0.5 mg. per m m . was used in the force balance. giving a measured susceptibility accuracy of 1k0.02 X 1 0 emu per gram. Approximately .TO mg. of sample \vere loaded into a small spherical silica container and suspended from the spring balance. At the same time a n aliquot portion was tveighed for subsequent chemical analysis. T h e sample in the magnetic apparatus was heated in a stream of dry air a t 450@C . until it reached a constant we.!ght. This procedure measures Ivater coiitent of the sample. Magnetic susceptibility was measured in the temperature range from -196@ to 450@ C . T h e cobalt and molybdenum in the aliquot samples ivere determined by both colorimetric a n d electrodeposition methods. T h e composition of the dry sample \vas calculated from these d a t a . Appropriate correctilms for the diamagnetism of the container, alumina (measured to be -0.42 X 10-6 emu per g r a m ) , cobalt, mol>-bdi:num. and oxygen ions were made in calculating the susceptibility per gram of cobalt, xc
T h e fraction of cobalt present as COO is simply r , as determined from the reduction experiment. This follows, since neither Co.41?04 nor the active cobalt reduces to the metal. T h e fraction of cobalt existing as "active cobalt" is then
(Co: Mo)init = 1 .O
I&EC FUNDAMENTALS
04
05
06
07
MO) INITIAL
Figure 3. Magnetic moment, fraction of cobalt reduced and (Co: Mo),,, Series Ii.
H e a t treatment 5 3 8 " C.
the amount of both CoMoO, a n d active cobalt increases as the concentration of Co.412c): decreases, as evidenced by the increase in both the magnetic moment and the fraction of cobalt reduced a n d the change of color from blue to purple. T h e appearance of a positive Weiss constant is also a n indication of the formation of large CohfoO, particles on the decreasing surface. T h e results for Series I [ a n d 111 are given in Figures 3 a n d 4 . I t is obvious from the behavior of the magnetic moment and fraction of cobalt reduced that the concentration of blue C0.41.0~ increases a n d the amount of COO decreases with increasing cobalt content. This effect is more pronounced as the heat treatment increases. T h e variation of the ((20 : Mo),,~ratio calculated from these data is most informative. A s cobalt is added to the system, no C0.4120, is formed until (CO:MO),,~, exceeds 0.3. Below this level most of the cobalt enters into the production of the active cobalt molybdenum complex with the remainder present as COO. A4bove a (Co:Mo),,i, ratio of 0.3. the competing formalion as C;oA412O4maintains the (Co:Mo),,, level a t 0.3 for the 538' C. treatment a n d induces it to decrease with greater thermal activation. Below the (Co: Mo),,;~ratio of 0.3. the composition of the catalyst is essentially the same for the two heat treatments. Only for ratios above 0.3 does the thermal activation change the composition of the catalyst. T o utilize the desulfurization d a t a for the evaluation of the c a t a l y t s . it has been assumed that the reaction is first order in sulfur compounds :
and
where [SI is the concentration of sulfur compound. k o h s d is the effective or observed rate constant. L' is the charge rate (v. v.;I h r , ) ) and D is the desulfurization fraction. I n spite of the fact that the feed contains different sulfur types a n d may be 307, liquid phase a t reaction temperatures, experiments performed with varying flow rates confirm that, within the range of the experiments. Equation 8 is a good assumption a n d may be used as a sound basis for comparing the performance of these catalysts. To correct for such factors as varying molybdena contents: surface area. a n d bulk density, a n intrinsic rate constant, kin,,. has been defined as
where d is the bulk density, A is the surface area in square centimeters per gram. and p is the weight fraction of molybdena. This definition is based on the assumption that diffusion does not play a significant part in the kinetics. If diffusion is limiting, then kint, will vary as the second power of kuhsd (73), so that, with a little modification. the comparisons made below are still true. Figure 5 shows the variation of hint, a t 400' C. tvith the magnetically measured (CO:MO),,~. These data are for all the samples in Series I1 a n d 111, together ivith the commercial cobalt molybdate catalysts treated in the same way. T h e curves for lower temperatures are similar in form, but occur a t lower values. T h e curve drawn through the data points demonstrates the relationship betiveen intrinsic rate constants a n d (Co : Mo),,~ ratios. This representation corrects for the effect of molybdena content a n d heat treatment. which alters only the surface area a n d composition. There is a maximum in the intrinsic activity a t a (CO:MO),,~ ratio of 0.18. This implies, from Figures 3 and 4, that the optimum intrinsic activity would be attained by a catalyst with a (Co:Mo),,,, ratio of 0.20. This catalyst would contain no CoAluO, and would therefore not be blue. For 137, MoOa content. the cobalt composition would
I
0 6gaC 0 53°C
A 0
0
l
N A L C O 4 7 1 1/16" HARSHAW 0301 1/8" HARSHAW 0 601 118"
CCI
1
1/16'
Lo
1
00
X
I
400
01
02
03
04
05
06
07
(CO Mo) INITIAL Figure 4. Magnetic moment, fraction of cobalt reduced and (Co: Mo),,, Series Ill. Heat treatment 650' C.
L
0
02
01
03
Mo)ACTIVE
Figure 5.
K,,,
for West Texas gas oil desulfurization at
400' C. VOL. 3
NO. 2
MAY
1964
157
be 1.O6y0total, with 0.0707, as COO and the remainder as the cobalt-promoted MooBcomplex. With a surface area of 275 sq. meters per gram and a bulk packing density of 0.67. such a catalyst would be expected to give 947, desulfurization a t 400’ C. Although a catalyst with these properties was not tested in this program, the existence of this optimum composition has been demonstrated (4.9, 72).
T h e correlation between intrinsic activity and active Co : Mo ratio is independent of method of preparation. This universal relationship is a valuable aid in understanding the relationship between catalytic activity and physical properties. Extension of these techniques to catalysts under various treatments such as sulfiding. regeneration. etc., should add to our knowledge of desulfurization processes.
Conclusions
literature Cited
This investigation develops three main conclusions regarding desulfurization cobalt-molybdena-alumina catalysts. First, the catalyst may- be divided into inactive components CoA1204, COO,and CoMoO,, sulfiding under reaction conditions to give CoA41204.Cog&. and Moss. T h e active component consists of some MOOBcomplex promoted with nonreducible cobalt. T h e exact‘nature of‘this complex is as yet unknown. but presumably it sulfides to a cobalt-molybdenum sulfide. \vhich is the true catalyst for the reaction. Second, catalysts with initial Co:Mo ratios less than 0.3 yield the same composition catalysts when heat treated in the range 538’ to 650’ C. No cobalt aluminate is present in these preparations. However: above initial Co: Mo values of 0.3. the final composition is dependent on both heat treatment and cobalt content. This accounts for many of the differences’ found in catalysts with the same initial conditions and for optimum values varying from 0.2 to 1.O. Finally. a maximum in activity occurs a t a n initial C o : M o ratio of 0.20 for all heat treatments and 0.54 for a 650’ C. heat treatment. both preparations resulting in an active C o : M o ratio of 0.18.
(1) Abeledo, C. R., Selwood, P. li.Bull. . Am. Phys. Soc. 6 , 353 (1961). (2) Badger, E. H.M.. Griffith. R. H., Kewling, h i . B. S.. Proc. Roy. SOC.,4197, 184 (1949). (3) Beuther, H.. Flinn, R . A . . McKinley, J. B.. Ind. Eng. Chem. 51. 1349 (1959). (4) kngel.
