Davies, G. R., Mais, R. H . B., O'Brien, S., Owston, P . G . , Chem. Commun. 1967, 1151. Dent, W. T., Long, R., Wilkinson, A . J., J . Chem. SOC. 1964, 1585. Donati. M., Conti: F., Inorg. Nucl. Letters 2, 343 (1966). Fischer, E. O., Vogler, A., J. Organomet. Chem. 3, 161 (1965). Fritz, H. P., Chem. Ber. 94, 1217 (1961). Green, M., Haszeldine, R. S . , Lindley, J., J . Organomet. Chern. 6, 107 (1966). Green, M. L. H., Nagy, P. L. I., Aduan. Organomet. Chem. 2, 325 (1964). Guy, R. G., Shaw, B. L., Aduan. Inorg. Chem. Radiochem. 4, 411 (1962). Helden, R. van, Kohll, C. F., Medema, D., Verberg, G., Jonkhoff, T., Rec. Trau. Chim. 87, 961 (1968). Huttel, R., Christ, H., Chem. Ber. 96, 3101 (1963). Huttel, R., Dietl, H., Chem. Ber. 98, 1753 (1965). Huttel, R.: Christ, H., Herzog, K., Chem. Ber. 97, 2710 (1964a). Huttel, R., Dietl. H., Christ, H . , Chem. Ber. 97, 2037 (196413). Huttel, R., Kratzer, J., Bechter, M., Chem. Ber. 94, 766 (1961). Kasahara, A., Tanaka, K., Asamiya, K., Bull. Chem. SOC. (Japan) 40, 351 (1967). Ketley, A. D., Braatz, J., Chem. Commun. 1968, 169. Kitching, W., Rappoport, Z., Winstein, S.,Young, W. G.. J . A m . Chem. SOC.88, 2054 (1966). Lukas, J., Coren, S.,Blom, J. E., Chem. Commun. 1969, 1303.
Lupin, M. S., Powell, J., Shaw, B. L., J . Chem. Soc. 1966a (A), 1410. Lupin, M. S., Powell, J., Shaw, B. L., J . Chem. SOC. 196613 ( A ) , 1687. Medema, D., Kohll, C. F., Helden, R . van, Inorg. Chim. Acta 3, 255 (1969). Moiseev, 1. I., Vargaftik, M. N., Syrkin, J. K., Izcest. Akad. N a u k . S S S R 1964, 775. Morelli, D., Ugo, R., Conti, F., Donati, M., Chem. Commun. 1967, 801. Xicholson, J. K., Powell, J., Shaw, B. L., Chem. Commun. 1966, 174. Parshall, G. W., Wilkinson, G., Inorg. Chem. 1, 896 (1962). Robinson, S. D., Shaw, B. L., J . Chem. SOC.1963, 4806. Robinson, S. D., Shaw, B. L., J . Organomet. Chem. 3, 367 (1965). Schnabel, W., Kober, E., J . Organomet. Chem. 19, 455 (1969). Schultz, R . G., Tetrahedron 20, 2809 (1964). Stern, E. W., Proc. Chem. SOC.1963, 111. Tsuji, J., Hosaka, S.,J . A m . Chem. SOC.87, 4075 (1965). Tsuji, J., Kiji, J., Imamura, S., Morikawa, M., J . A m . Chem. SOC.86, 4350 (1964). Volger, H . C., Rec. Trau. Chim. 86, 677 (1967). Volger, H. C., Rec. Trau. Chim. 87, 501 (1968). RECEIVED for review June 25, 1969 ACCEPTED January 22, 1970 Division of Petroleum Chemistry, 157th Meeting, ACS, Minneapolis, Minn., April 1969.
Nickel-Zeolite Catalyst Inhibition of Susceptibility t o Sintering and Poisoning
J. David Lawson' and Howard F. Rase Department o f Chemical Engineering, The Uniaersity o f Texas, Austin, Tex. 78712
The metal-dispersion a n d related poison-resistance a n d size-selective characteristics of
1 weight Yo nickel-loaded, Type Y zeolite can b e destroyed during the process of reducing the nickel. Electron microscope a n d x-ray studies show t h a t the reduced nickel migrates t o form larger crystallites with increasing severity of pretreatment. A significant portion of these crystallites appears on the outside surface, destroying possible size-selective characteristics. Mild pretreatment reduces the rate o f crystal g r o w t h a n d produces a catalyst with slightly improved poison resistance. By incorporating CR~OIinto the catalyst a greatly improved poison-resistant catalyst is obtained.
THE
unique qualities of metal-loaded zeolite catalysts (size or shape selectivity, and atomic dispersion of the metal) can be destroyed by reduction or activation. To activate an ion-exchanged loaded metal-zeolite catalyst such as nickel-zeolite, the nickel cations must be reduced to nickel metal. This can be done by heating in hydrogen, a process which ruptures the bonds between the metal
' Present address, Pan American Petroleum Corp. Research Center, Tulsa. Okla. 74102
and the zeolite framework that were originally responsible for creating the high degree of metal dispersion. Free metal thus produced is subject t o diffusion and the natural tendency to agglomerate into crystallites. If diffusion is rapid enough, large amounts may migrate to the exterior surface of the zeolite crystal, destroying much of the size or shape selectivity involving catalysis by the metal. Since high metal dispersion within the zeolite matrix favors not only high activity and possible selectivity hut also poison resistance, there is an incentive for preserving Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 3, 1970 3 17
the integrity of this dispersion, particularly for nickelzeolites. Conventional nickel catalysts are most economical and widely used industrially, hut are highly sensitive to poisoning by sulfur compounds. This study, therefore, monitered the growth of nickel crystallites on nickel-zeolit e catalyst and observed the effects on catalytic activity of a variety of reaction and pretreatment procedures. Ethane hydrogenolysis, the conversion of ethane to methane and hydrogen, was selected as the test reaction and thiophene as a typical poison. Nickel crystallites were observed to form on the surface of the zeolite particles when subjected to reduction procedures adequate for activating the catalyst, and in this form the nickel was not poison-resistant. A modified chromia-promoted nickelzeolite was developed, however. which had a greatly enhanced life in poisoned feeds. This discovery suggests a feasible means for preserving the unusual and desired characteristics of nickel-zeolite catalysts, not found in nickel on conventional supports. Previous Investigations on Metal Agglomeration in Zeolites
Although the cation exchange properties of zeolites have been studied (Barrer, 1959; Barrer and Bratt, 1959; Barrer and Meier, 1958), less is known about the stability of the cat,ions in the zeolite structure. Early patents (Breck. 1964; Breck and Milton. 1958a,b,c) tell of zeolites ion-exchanged with Ni'., Ag' , Cd' , and other metal cations. The Cd zeolite lost some of the Cd when heated. An Ag zeolite produced 100AAg crystallites when reduced in hydrogen. An 8.6 weight c i Ni Type X zeolite was heated in hydrogen for 3 hours a t 662" F, then for 3 more hours at 932" F. X-ray diffraction showed elementary nickel crystals of unreported size in an intact zeolite lattice (Breck and Milton, 1958a,b,c). Yates (1965, 1967) found that easily reduced metal cations such as Cd' , Hg'., and ZnL-, which have been ion-exchanged into a Type X zeolite, can be removed by heating in hydrogen. Ion-exchanged Ag and Ni Type X zeolites formed Ag and Ni crystallites, respectively. upon heating in hydrogen. The Ni and Ag were judged to have too low a vapor pressure, however, t o leave the zeolite. The Ni Type X zeolite was reduced a t 7520F. The average nickel crystal size was 240 A. I n each case, hydrogen was necessary to form metal crystallites; no free metal was formed when the samples were heated in a vacudm. Rabo et al. (1965) make brief reference to reduction of an ion-exchanged 0.5 weight C;. Pt Type Y zeolite. A hydrogen atmosphere a t 572" F apparently reduced the Pt' to Pto but did not agglomerate the Pt atoms. Reduction a t or above 932°F caused P t crystallites to form. Lewis (1968) studied Pt crystallite formation in a Pt Type Y zeolite equivalent to that of Rabo et al. (1965), using reduction conditions believed to be identical to those used by Rabo. I n contrast to Rabo's reports, Lewis concluded that the resultant zeolite had 405- of its Pt present as 60 A crystallites. The balance of the Pt was described as Pt crystallites of 10 A size or less, small enough to allow them to be inside the pores of the zeolite. Rabo et a/.(1966) noted that the reaction between hydrogen and Ni Type Y became appreciable a t 482°F. hut gave no other details. Kabo et al. (1966, 1967) have also studied a Na Type Y zeolite exchanged with different amounts of Ni' with 318
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 3, 1970
respect to reduction of Ni' to N i - by sodium vapor. Electron spin resonance spectra showed that N i - and Ni' were formed a t 1085°F. Excessive heating favored the formation of Nio. Conventional nickel catalysts, such as nickel on silica, exhibit crystallite growth upon high temperature treatment. A 42 weight 5 Ni-on-silica gel catalyst had an average nickel crystallite size determined by x-ray diffraction of 38 A after sintering for 1 hour a t 842°F (Selwood et al., 1955). After sintering for I hour a t 1022"F, the average size had increased to 70 A. Another 10 weight C; Ni-on-silica-alumina catalyst had an average nickel crystallite size of approximately 30 A when sintered at 698O F (Carter et al., 1966). Again, x-ray diffraction was used. Heating to 1076'F increased the average size to 71 A. Apparently there are no published, systematic studies of the effects of reduction and siibsequent agglomeration of nickel on the catalytic properties of a nickel-zeolite catalyst . Experimental Plan
The precise goals of this investigation were:
T o define the nature and extent of agglomeration of nickel in nickel-loaded zeolite under various conditions of pretreatment and reaction. T o establish procedures for minimizing agglomeration of nickel to preserve the unique selectivity and poison resistance inherent with highly dispersed metal located within the caged structure of a zeolite. (Feed containing a given amount of poison will deactivate a catalyst with large crystallites of active metal much more rapidly than one with the same amount of highly dispersed metal. A single poison molecule adsorbed on a large crystallite of metal can by steric hinderance prevent efficient use of a large fraction of the metal contained in that aggregate. If the aggregate is smaller, less active metal is removed from use. Thus, with highly dispersed metal catalysts the steady-state quantity of adsorbed poison for a feed with modestly low poison content may not cause an appreciable decline in activity.) T o accomplish these goals, samples of a sodium Type Y zeolite ion exchanged with nickel to 1.0 weight % Ni were pretreated and reduced a t varying temperatures. Each sample was tested for ethane hydrogenolysis activity. Type Y zeolite consists of SiOl and A104 tetrahedra arranged in the shape of a truncated octahedron. The octahedra are in turn arranged tetrahedrally t o produce elliptical voids of 13 A entered through apertures. This aperture is large enough to accept a variety of organic molecules and was selected to avoid diffusion limitations in the ethane reaction. Observations with this type of zeolite should be applicable t o Type A zeolites with smaller apertures and thus greater capacity for size selectivity. Size selectivity has been reported for Ni Type Y zeolite, however, in selective hydrogenating of soybean oil, where large and complex fat molecules are involved (Riesz and Weber, 1964). Detailed electron microscope and x-ray diffraction studies of the Ni Type Y zeolite samples were used t o define the extent and possihle location of agglomerated nickel. Poisoning tests were also conduct,ed to determine the extent of poison resistance, since this characteristic is not only an indication of metal dispersion but also an important potent,ial attribute of metal-loaded sieves.
Experimental Equipment and Experimental Procedures
Catalyst Samples. The zeolite catalysts were donated by the Linde Division, Union Carbide Corp. The 1.0 weight % Ni-on-sodium Type Y was identified as Linde sample 12967-43A. The sodium Type Y was from Linde lot 3606-220. The catalyst samples were received as dehydrated powders of 0.1- to 3-micron particle size. The commercial catalyst was prereduced Harshaw NI-0104P containing 58 weight Yo nickel on kieselguhr. Portions of zeolite catalyst were reduced in flowing hydrogen in the microreactor according to the different heating schedules to obtain reduced samples for analysis by electron microscopy and x-ray diffraction. Heating was started a t 15P F with a heating rate of 150" F per hour. Maximum temperatures were held for up to 20 hours and varied from 58P to 100P F. Reaction Rate Measurements. An automatic flow microreactor was used to measure reaction rates. The original equipment (Harrison et al., 1965) was modified by the addition of two mass flowmeters and metering valves to measure and control feed rates of unpoisoned ethane and poisoned ethane. The small concentrations of poison in the feed ethane were obtained by adding a known amount of thiophene to a quart cylinder of liquid ethane. Unpoisoned and poisoned ethane could then be fed simultaneously in any desired proportion. Equal moles of hydrogen and ethane were fed, and space velocities were approximately 1200 grams of feed per hour per gram of nickel. Reaction rates with poisoned feed were obtained after first establishing the reaction rate a t a selected temperature using unpoisoned feed. Poisoned ethane was then blended into the feed with a like reduction in unpoisoned ethane to keep the same total feed rate. The conversion of ethane to methane was calculated from chromatogram peak areas generated from samples of reactor effluent. The average reaction rate was calculated from a material balance. Feed conversion in the reactor was kept below 5%, so that the calculated average reaction rate more closely approximated the actual reaction rates. Electron Microscope Examinations. Electron microscope specimens of reduced zeolite catalyst were prepared using two techniques: ultramicrotome sectioning of catalyst particles embedded in an epoxy plastic, and surface replication with vacuum-evaporated platinum and carbon films. An RCA EMU-3G electron microscope was used. Ultramicrotome sections were made to reduce the thickness of catalyst specimens to about 0.1 to 0.3 micron. In this way contrast could be obtained between the zeolite and the more dense nickel crystallites. Under ideal conditions, nickel crystallites as small as 25 A could be seen. A very intense electron beam had the ability to reduce the nickel cations and cause nickel crystallite formation in previously untreated zeolite catalyst samples. Therefore, to avoid alteration of the specimen by the electron beam, a lower beam intensity was used, which caused no observable nickel crystallation. The surface replicas consist of a thin sheet of carbon the shape Of the and platinum grains which away surface after the has in hvdrofluoric acid. The redicas then reuresent the catalyst particle surface in as fine detail as the grain
size of the platinum coating permits. They are unaltered by the electron beam. Surface nickel crystallites as small as 100 A could be detected. X-Ray Diffraction. A Siemens Crystalloflex IV was used with a capper tube, horizontal goniometer, motor-driven step-scanner, and an automatic digital printer to collect powder x-ray diffraction data. Powder patterns for the reduced catalyst samples were superimposed upon the pattern of untreated zeolite t,o determine the contribution of nickel crystallites near the (111) nickel reflection a t 2fl = 44.6". Because the nickel reflection was Xveak and partially obscured by overlapping zeolite crystal reflections, the counting time needed to determine the patterns accurately was several hours. The average nickel crystallite sizes were calculated from the (111)nickel peak width, as descrihed by Klug and Alexander (1954). Results
Time and Temperature Dependence of Nickel Agglomeration. Electron micrographs and x-ray powder diffraction patterns of samples pretreated in hydrogen under various time-temperature schedules definitely show growth in nickel crystallite size with severity of treatment. Most importantly, electron micrographs of surface replicas show that nickel crystals grow, a t least in part, on the surface of zeolite crystals (Figure 1). This apparent migration and subsequent growth of nickel crystallites on the surface can be expected to have a deleterious effect on size seleetivity of the nickel-zeolite catalyst. Further, any poison resistance attributable to a high degree of nickel dispersion is also in jeopardy.
Figure 1. Surface replica of Ni-Y particles heated in Hz for 2.5 hours at 1000" F 6oo.p. and ~maller nickel crystallites seen 01 humps on zeolite surface. plotinurn spray, blocked, leaves light are0 on "blind" ride of nickel crystal1 1 / 1 1 .
Magnlficotion 49.000 X
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 3, 1970 319
.
-
Figure 2. Surface replica of Ni-Y particles heated in H? for 1 hour a t 785" F Smoller nickel crystallites, approximately 200 A, f o r m e d m zeolite surface. Magnificotion, 49,000 x
.
Every effort was made to ensure t h a t the electron micrographs were representative of many observations over a wide area of the bulk surface. The observed humps were judged to be nickel crystallites, because. Na Type Y zeolite treated in an identical manner exhibited no humps, and Ni Type Y zeolite samples pretreated in hydrogen a t low temperatures (143" F) were also free of humps. Neither of these samples produced (111) nickel reflections with x-ray diffraction, while all samples with visible humps did. Ni Type Y zeolite samples treated a t conditions less severe, but adequate for reducing nickel, exhibited smaller humps (Figure 2). Electron micrographs with transmission viewing of ultramicrotome sections afford a chance to observe all the nickel crystallites large enough to be resolved but do not differentiate between crystallites within or on the surface. Specimens of Ni Type Y zeolite heated in hydrogen had distinct spots of high density material. A sample heated in hydrogen for 2 hours a t 9 7 P F is shown in Figure 3. The spots approximately 300 A in size are rounded or hexagonal and cubic in shape, indicative of the stable face-centered cubic structure of nickel crystals. Numerous other electron micrographs of Ni Type Y zeolite samples were examined (Tabie I). Nickel crystallites as small as 25 A were detectable. For each sample viewed, the average crystallite size or sizes and the relative amount of detectable nickel crystallites are listed from measurements on both the surface replicas and the ultramicrotome sections. These data clearly indicate a tendency for the amount of detectable nickel crystallites to increase as reduction time and temperature are increased. T o avoid misleading observations, wide areas of eacb specimen consisting of approximately 20 particles were scanned. A specific region judged to be representative of the average appearance of the specimen was then selected for photographing and further study. Although any particular photograph and count may not be precisely representative of the total sample, the observed increase in nickel crystallite size and amount of detectable nickel in proceeding from mild to severe reducing treatments is valid. These general conclusions were confirmed by x-ray diffraction studies in which (111) nickel line hreadth was used to calculate average nickel crystallite size and the area under the peak used to determine the total mass of nickel present as crystallites of 80 A or larger. The results are included in Table I and should be compared with the ultramicrotome sections. These x-ray diffraction data confirm the same general trend with increasing severity of treatment that was determined from the electron micrographs. Effect of Activation Procedures on Activity. Ni-zeolite activity was tested using ethane hydrogenolysis (C,H, + H, ZCH,). Time of reduction, temperature of reduction, and catalyst water content all affected the activity of the catalyst. EFFECTOF WATERCONTENT.The water content of the zeolite catalyst prior to reduction had a marked effect on the ultimate activity. Although the catalyst as received had been dried thoroughly and transferred to the shipping container in a dry box, small amounts of moisture could readsorb in loading the test reactor. Figure 4 illustrates the marked effect of initially adsorbed water on the final reduced catalyst. The partially hydrated sample was handled briefly in the room atmosphere while transferring from the container to reactor. Fully hydrated catalyst
-
Figure 3. Ni-Y zeolite heated 2 hours at 970" F Dirtinct high~denrityorem ore 300A nickel crystallites which have formed under the more severe reduction ireomen+. Mogniiicotion 100,000 x
320 Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 3, 1970
~~~
~~~
Table I. Summary of Electron Microscope and X-Ray Diffraction Data Showing Nickel Crystallite Growth on NiY
F.
Hour
X-ray diffraction
1000 1000
20 2.5
210 210
970 970 970
20 13.5 2
230 180 190
970 970 785
I 0 2
170 170 150
785 785 580
1 0 2
160 140
1 0
... ... ...
580 580 143 Untreated NaY 1000
13.5 2
Average Amount of Detectable Nickel Crystallites"
Average Nickel Crystallite Size(s), A
Reduction TimeTemperature Program
-
...
Ultramicrotome sections
Many 275 Some 70 Many 320 Some 70 Many 250 Some 50 Some 250 Many 50 Some 50 None None None
Surface replicas
X-ray diffraction
Ultramicrotome sections
Surface replicas
400
100 100
60
50
300
90 100 80
100
100
300 220 200
80 70 40
70
60 20 30
220 180 260
30 20 5
180 160 Kone None Kone
5 0 0
20 3
30
)
0 10 0 0
0 0 0 0
0
Arbitrary units. Catalyst with most observable nickel assigned value of 100. Amounts on micrographs proportional to mass density of nickel. X-rav measurements proportional t o area under (111)nickel reflection.
containing about 25 weight % water was prepared by treating the catalyst in situ with a stream of saturated air for 50 minutes. The helium-purged sample was a partially hydrated sample that was purged prior to reduction with helium for 2 hours a t 250" to 400°F and then for 2 additional hours a t 1000°F or a t least 12 hours a t 800°F. The initial low temperature purge is necessary to prevent damage to the faujasite crystals while in contact with appreciable partial pressures of water (Barrer and Stuart, 1959). Essentially all the water can be removed by these procedures (Turkevich, 1967; Union Carbide Corp.). The activity increases as much as 15 times when the catalysts are purged with helium prior to reduction. Even the small amount of water (1%) in the partially hydrated catalyst had a marked effect. Based on these results, a helium purge was used on all runs reported, since it assured better reproducibility. Water content prior to reduction does not affect the size of resulting nickel crystallites as detected by x-ray diffraction (see Table 11). Although effects such as competitive adsorption between water and ethane may be invoked to explain the decline in activity, the faujasite structure may have been partially destroyed. The work of Barrer and Stuart (1959) suggests that this could occur at longer times with smaller quantities of water than they employed. EFFECT OF REDUCTIONTEMPERATURE. Ni-zeolite activity increased with increasing reducing temperatures for reductions held a t the maximum temperature for 2 hours. Arrhenius-type plots of low-conversion reaction rates for catalysts reduced a t different temperatures are given in Figure 5 . Unreduced nickel-zeolite had no activity. EFFECTOF REDUCTION TIME.Ni-zeolite samples reduced for 20 hours at 970°F were inactive for ethane hydrogenolysis. Water content was not a controlling factor, for reductions with and without a prior helium purge produced inactive catalysts. The increased time for reduction a t 97O'F. however, did not cause greater crystal
Table II. Effect of Ni Type Y Prereduced Water Content on Nickel Agglomeration as Detected by X-Ray Diffraction Water Content
Fully hydrated Partially hydrated Dehydrated by He purge
Av. Nickel Crystal Size, A
of Detectable Ni
Relative Amount
230 190 210
90 80 90
U
5W:
t 16 17 18 20
001 15
I/T
x IO3
19 T, R e a c t i o n T e m p , O
K
Figure 4. Effect of water content of prereduced Ni-Y on activity Reduction schedule, 970" F, 2 hours Ind. Eng. Chem. Prod. Res. Develop., Voi.
0,No. 3, 1970
321
Although the resistance t o poison that might have been hypothesized for nickel dispersed throughout the pore structure was not observed because of the marked diffusion of nickel into larger crystallites. it is apparent that this disadvantage can be partially overcome by prolonged low temperature reduction of the Si Type Y zeolite. This mild and prolonged procedure will produce a high degree of reduction without producing large crystallites. The susceptibility of Ni Type Y zeolite to poisoning is not necessarily contrary to the reported sulfur resistance of certain Pt- and P d - zeolites (Mays P t al., 1965; Rabo et al., 1961, 1965). P t is intrinsically more resistant t o poisons than Ni for hydrogenation (Maxted. 1951), and Ni and P t should not be expected to show the same degree of poison resistance when incorporated into zeolites. No comparison is available between the sulfur resistance of Ni and Pd. Rabo et a!. (1965) hypothesized, but did not confirm by infrared studies, that Pt in Pt-zeolite forms PtH, under hydrogen atmospheres and that this hydride is poison-resistant. Our infrared studies, however, of Ni Type Y zeolite reduced under a 580" F-1-hour reduction schedule and Pt Type Y zeolite reduced under conditions used by Rabo showed no evidence of hydride absorbance in the characteristic regions.
L
t
x al
4 0.I O
0 Lo
al -
0
E
m
c 0
LL
c
.-0 c
0
0
m
K
I/t
x IO3
T, R e a c t i o n T e m p , " K
Figure 5. Effect of reduction temperature on ethane hydrogenolysis reaction rates growth or increased crystallite formation in the size range observable by x-ray diffraction. Some alteration in the characteristics of the nickel crystallites must have occurred, such as coating of the surface by a silica skin as reported for standard nickel catalysts (Schuit and van Reijen. 1958). Yates (1965, 1967) observed nickel crystallites after 16-hour reduction a t 752°F of a 10% Ni Type X zeolite and an apparent loss of crystallinity of the zeolite base, but we observed no such change in the zeolite of the Ni Type Y zeolite. Perhaps the lower nickel content of the Ni Type Y zeolite permitted greater zeolite lattice stability. Long reductions (20 hours) a t low temperature (580" F ) produced a sample twice as active as that reduced for only 2 hours a t the same temperature. One can speculate that under these mild conditions not all the nickel is reduced in 2 hours and prolonged treatment produces more reduced nickel and thus higher activity. Posion Resistance of Various Samples. Poison resistance is not only important in evaluating a catalyst for commercial use but can be an indicator of the degree of dispersion of active metal. Thiophene was selected as a poison because of its thermal stability up to 1600°F (Faragher. 1929). Mildly reduced and more severelv reduced catalyst samples were tested. Amounts of thiophene equivalent to 160 ppm of sulfur or more caused immediate total loss of activity in all samples. Lower quantities of poison permitted observing rate decline with time, as shown in Figure 6 for 20 ppm of sulfur as thiophene. A well-known commercial nickel catalyst, Harshaw 585 nickel on kieselguhr. is presented for comparison. Since this catalyst was partially prereduced and was used in this state, the observed higher poison resistance might be caused by new nic,kel surface created by further reduction during use oi t h e large miount of available nickel. 322
Ind Eng Chem Prod. Res Develop., Vol 9 No 3, 1970
Effect of Nickel Crystallite Size on Catalyst Behavior
The experimental results suggest a casual connection between observable nickel crystallite size and catalyst behavior which deserves further comment. Although the analytical methods were limited to observing crystallites of 25 A and larger, it is safe to conclude that nickel remaining smaller than this must have also experienced some agglomeration. The amount of nickel in crystalline form and the average crystallite size increased over the reduction range of 580° to 1000"F and for reduction times up to 20 hours. Unreduced Ni Type Y zeolite is inactive for ethane hydrogenolysis until sufficient hydrogen has passed over the catalyst to reduce some of the chemically bonded nickel. Single nickel atoms may also be inactive. There is reason to believe that nickel in Ni Type Y zeolite which has been reduced to single nickel atoms but has not migrated from initial cation sites in the zeolite lattice would be unavailable for catalysis of reactant molecules because of its suspected shielded position. The distribution of the nickel ions among the different types of cation sites is not known for 1 weight % nickel Type Y zeolite. However, single crystal x-ray diffraction studies of a similar 8 weight 5 nickel Type Y zeolite reported by Olsen (1968) show that Xi' prefers locating in the Type I site during the ion exchange for zeolites of low nickel content. A nickel atom at a Type I site would be completely shielded from reactant molecules by the surrounding six oxygen atoms of the hexagonal prism. I t has been suggested that Type I sites are so unavailable to substrate molecules that they do not seem to be directly active as catalyst sites (Turkevich, 1967). Even if the single nickel atoms had open access to reactant molecules, the proposed reaction mechanism for ethane hydrogenolysis lends no support to their possessing ethane hydrogenolysis activity. Proposed mechanisms for ethane hydrogenolysis (Cimino et al.. 1954; Kemball and Tavlor, 1948) and ethylene hydrogenation (Bond, 1962) involve two-point adsorption of the ethane and ethylene on adjacent nickel atoms in a nickel crystal. In addition.
-
..ct';*
x W
0
Reoction Temp - 622°F Reduction - 9 7 0 F - 2 h r
c
E
$
c _
Figure 6. Poisoning of Ni-Y a n d Harshaw nickel by 20pprri f e e d sulfur as thiophene
I O
.c '\ U 0)
c. 0
0
a, L
.I
(D
I
N
010 3
W -
0
E
$ W
c
0
n
;001
e 0 0) 0
n 002
S V = Spoce Velocity g m feed / h r / g m nickel R = Reduction Schedule
Figure 7. Poisoning o f p r o moted Ni-Y by 20-ppm feed sulfur as thiophene
0 10 20 Time f r o m Poison Addition, hr.
0 200 400 600 Elopsed Time f r o m Poison Addition, rnin
the unique crystal lat>tice spacings of nickel and other transition metals are characteristic of these excellent hydrogenation catalysts (Twigg and Rideal, 1940). The activity and poisoning tests indicate that although some agglonierating of nickel seems essential, an optimum nickel crystallite size and thus nickel distrihution exist. This is particularly apparent in the poisoning experiments. A prolonged reduction at 580" F produced a more poisonresistaiit catalyst than the short high temperature treatment. Clearly, this difference must have been due t,o differences in dispersion, for the sample reduced a t high temperature had a much greater initial activity. indicating more available nickel. The sample reduced for only 2 hours at low temperature also had high dispersion of' reduced nickel. but the amount was so small that it deactivated abruptly at the poison concentrations employed. Improved Catalyst
If an optimum dispersion of nickel exists, it might be more nearly approached by incorporating an irreducible oxide such as chromia on the surface of the catalysts, which would tend to inhibit agglomerating of nickel into large crystallites. This hypothesis was tested by vaporizing Cr(CO)Gin helium through the catalyst bed. The Cr(COI6 absorbed on the catlayst and was changed to chromia by heating the catalyst bed in flowing oxygen. This procedure avoided ion exchange phenomena that would occur with aqueous deposit ion. l ' w o samples of Ni Type Y zeolite having different amounts of chlomia were tested for poison resistance using 20-ppm feed sulfur as thiophene. The promoted catalysts had lower iiiitial activity but greatly enhanced activity maintenance i n poisoned feed. Figure 7 compares the poisoning of unpromoted and promoted samples. Each sample was reduced with a 970' E'-2-hour reduction schedule. The initial activity of the unpromoted catalyst is greater than either promoted catalyst, but after a short exposure t o
poisoned feed, the promoted catalysts have higher activity. Neither Cr201or the zeolite support in its sodium form exhibited activity for ethane hydrogenolysis. These experiments indicate a promising path for development of a greatly improved Ni Type Y zeolite catalyst. It was not possible to predict the amount of chromia that would be deposited in a given time by the method used, hut this ability could be developed by further study and improvement in the deposition procedures. Large amounts of chromia may block portions of the surface and macropores in the catalyst particles. An optimum chromia content probably exists which will produce an even more active and poison-resistant catalyst. Mechanism of Nickel Transport in NI Type Y Zeolite
Since nickel transport seems t o play such an important role in setting ultimate catalyst characteristics, insights into the mechanism of this transport could be useful. Y o theories of the mechanism of the observed movement of the nickel in nickel-zeolites have been advanced by previous investigators. However, predictive calculations may indicate whether nickel moves through the zeolite pores as single nickel atoms or as small nickel crystallites of 8-A size or less. Ai1 analogy to the kinetic theory of gases can be used to derive an expression for the mean free path (mfp) of nickel atoms before experiencing nickel atom-nickel atom contact on a surface. The two-dimensional mean free path formula is
mfp = [ 2 ( N () d )1
l,
where iV is the number of nickel atoms per unit of surface area and d is the diameter of a nickel atom. The original surface concentration of nickel for fully reduced 1 weight 4 Type Y zeolite is 1.5 x 10" nickel atoms per 5y. cm of zeolite surface, using 700 m' per gram as the zeolite surface area. The molecular diameter of nickel is 1.2 x 10 ' cm. 'Then mfp = 0.03 micron. Thus, for zeolite particles on the order of O.1-micron diameter, there seems Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 3, 1970
323
to be an excellent chance that a nickel atom could diffuse through the pores and onto the outer surface before coming into contact with another nickel atom. Accurate use of the mean free path expression is limited by the inability to describe reduction of nickel cations and activated surface diffusion of nickel atoms over zeolite surfaces quantitatively. However, the analysis indicates that for the simplified case of free surface diffusion, the nickel atoms have a good probability of diffusing along the surfaces of the zeolite pores and onto the outer surfaces of the zeolite crystal as single nickel atoms. Of course. some of the nickel might form small crystallites inside the pores, which then could move to the. outer surface of the zeolite crystal or simply stay in place inside the pores. The possibility of unhindered movement of small nickel crystallites is much lower because of hindrance by the zeolite pores and the attractive forces of the pore walls. Further. the diffusion coefficient for nickel crystallite migration inside the zeolite can be estimated using Carslaw’s (1947) solution to the unsteady-state diffusion equation for a one-dimensional semi-infinite medium. The value of 10 ’‘ cm’ per second obtained for a 5 x 10 cm crystallite is many orders of magnitude lower than that which would be predicted assuming free movement of crystallites. Conclusions
Initially, the nickel is dispersed throughout the zeolite lattice as nickel ions. But under reducing conditions the ions begin t o convert to atoms, which move about slowly through the zeolite pores by surface diffusion. A large fraction of the nickel atoms migrate to the outer zeolite surface without forming crystallites with other migrating nickel atoms. Some nickel atoms meet and form small crystallites within the pores, but may also eventually migrate by surface diffusion t o the outer zeolite surface. When the nickel atoms reach the outer surface, they come across and follow zeolite lattice defects until they eventually concentrate into nickel crystallites. Once the nickel crystallites are formed, the nickel atoms do not evaporate and move further. although the crystzllite itself may move. The end result is a zeolite catalyst particle with much of its nickel concentrated on its outer surface. This sequence of events can be altered by low temperature reduction of long periods which effectively reduces a major portion of the nickel but slows the surface diffusion and allows more of the crystallites to remain smaller and more widely dispersed. By adding chromia prior to reduction even more dramatic diffusion-inhibiting effects apparently result. Chromia. if successfully dispersed throughout the pore structure, could even inhibit substantial movement of small crystallites to the outside surface and favor thereby a high degree of dispersion. The resultant stabilized dispersion would not only assure a measure of poison resistance but also would. in the Type A form, preserve the selectivity between straight-chain and branched organics by maintaining the nickel within the cage structure. Much additional work is required to establish the nature of inhibition by chromia and discover the optimum means for its dispersal. Acknowledgment
We appreciate the help of Glen Williams in preparing the electron microscope specimens and the valued suggestions of G. W. Watt, Hugo Steinfink, and W. F. Bradley. 324
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RECEIVED for review January 12, 1970 ACCEPTED May 15, 1970 Fellowships for J. D. Lawson were provided through the graduate school of the University of Texas.
