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Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
Supported Palladium Catalysts for Methanation M. Albert Vannlce' and Robert L. Garten' Corporate Research Laboratories, Exxon Research
Engineering Company, Linden, New Jersey 07036
-
Pd/AI,03 catalysts exhibit specific activities (molecules.s-'.site-') in the methanation reaction 70-fold greater than those for unsupported Pd and 35-fold greater than Pd/SiO, catalysts. Most surprising is the finding that the specific activities of Pd/AIz03 are only a factor of 3 less than those for typical nickel methanation catalysts. A much lower activity reported previously for a Pd/AI2O3catalyst cannot be attributed to poisoning by sodium, since the behavior of a Pd/AIzO, catalyst containing 0.7 wt YO Na was virtually identical with that with no Na added. Activity maintenance studies indicate that at high conversions and temperatures of 623-673 K, these Pd/AIzO3catalysts deactivate rapidly. However, at lower conversions and 573 K Pd/AI,O, catalysts exhibit stable activity and an activity per unit weight which is comparable to nickel catalysts because Pd/AI,03 can be prepared and maintained with a higher fraction exposed than Ni. Even though the costs of Pd catalysts are significantly greater than those of Ni, their use in methanation may be justified when other attributes such as absence of bulk carbide and volatile carbonyl formation, in situ regeneration and redispersion capability, and anticipated better sulfur tolerance than nickel catalysts are considered.
Introduction Of the various options for the production of synthetic fuels from coal, gasification/methanation has received much attention in recent years in both fundamental and applied research. The development of an SNG industry would be facilitated if catalyst improvements could be made in two areas-activity maintenance and sulfur tolerance. In this paper, results on a catalyst system which may offer some improvement in both of these areas are presented. In a previous study of CO hydrogenation over group 8 metals, it was reported that the specific activity (rate per surface metal atom) of highly dispersed P d on A1203 supports was much greater than that of unsupported P d or P d supported on silica (Vannice, 1975b). In fact, the specific activities of Pd/v-Al2O3catalysts were comparable to those measured over a number of typical nickel catalysts (Vannice, 1976). This unexpected enhancement in activity suggested the possibility of developing well-dispersed Pd on alumina catalysts which may be competitive with nickel catalysts in the methanation reaction (Vannice and Garten, 1978). Because highly dispersed Pd/A1203catalysts can be routinely prepared, whereas nickel catalysts typically have low fractions exposed, activities per gram of Pd in the A1203catalysts are comparable to nickel, even at low P d concentrations. These results were especially surprising since earlier studies had consistently reported Pd to be the least active of the group 8 metals for methanation. For example, the U.S. Bureau of Mines found a Pd/A1203 catalyst to be so inactive that it was discounted as having commercial potential. We undertook, therefore, additional experiments in an effort to elucidate the enormous differences in activity between the Bureau of Mines results and our own. In addition, the high methanation activity of Pd/A1203and other attributes such as absence of bulk carbide or volatile carbonyl formation, in situ regeneration and redispersion capability, high selectivity to methane, and an anticipated better sulfur tolerance than Ni catalysts provided incentive to investigate the behavior of Pd/A1203 in relation to Ni methanation catalysts. *Department of Chemical Engineering, The Pennsylvania State University, University Park, Pa. 16802. Catalytica Associates, Inc., 3255 Scott Boulevard, Suite 7E, Santa Clara, Calif. 95051. 0019-7890/79/1218-0186$01.00/0
Experimental Section Equipment. The chemisorption measurements were conducted in a glass, mercury-free adsorption system capable of achieving a dynamic vacuum of ca. 3 x lo-' torr (1torr = 0.133 kPa) by the use of an oil diffusion pump bracketed by two liquid nitrogen cold traps. A Texas Instruments precision pressure gauge was used for pressure measurements. Matheson CO (99.99% purity) was used for these adsorption runs. The kinetic data at 1 atm total pressure were gathered using a reactor system which has been described previously (Vannice, 1975a). The kinetic data'at higher pressure were obtained using a single-pass, 3/8-in. stainless steel tube reactor containing approximately 0.5 g of catalyst. A I / 16-in.thermocouple probe was positioned in the middle of the catalyst bed. The reactor and preheater section were positioned in a fluidized sandbath which provided very sensitive temperature control. The feed stream consisted of a mixture of H2 and CO (H,/CO = 3) which was prepared and analyzed by Matheson. Before entering the reactor the feed gas flowed through a 13X molecular sieve trap operated at 353 K to remove any water and decompose any carbonyl impurities. The reactor exit stream passed through two metering valves, for controlling the flow rate, to a heated gas sampling valve where periodic analyses were conducted with a Hewlett-Packard 7620 gas chromatograph and Hewlett-Packard 3380 electronic integrator. The use of Chromosorb 102 columns and subambient temperature programming produced easy separation of both organic and inorganic species. After exiting the gas sampling valve, the gases passed through a bubble meter and a wet test meter in series. This allowed both instantaneous and integrated flow rates to be determined for conversion calculations. Catalyst Preparation. The 7-A1203support used in the catalyst preparations described below was prepared by heating Davison tri-hydrate of alumina in air for 4 h at 863 K and had a surface area of 180 m2 g-l. The silica was HS-5 Cab-0-si1 with a surface area of 300 m2 g-' obtained from Cabot Corporation. The supported P d catalysts were prepared using typical incipient wetness techniques. The appropriate quantity of the metal salt, PdC12,was dissolved in concentrated HC1 and the resulting solution diluted with distilled water to a volume corresponding to the incipient wetness of the support. For A1203catalysts the incipient wetness volume 0 1979 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
187
Table I. Enhancement of Methanation Reaction over Pd/q-A1,0, Catalysts (P= 1 0 1 kPa; H,/CO = 3 ) catalyst
T,K
2% Pd/A1,0, 4.75% Pd/SiO, Pd black 4.75% Pd/SiO, Pd black 5% Ni/q-Al,O, 42% Ni/a-A1,0, 30% Ni/a-Al,O,
548 548 54gb 617 617 548 548 548
specific activity," molecule CH,/site.s
" Based on CO uptakes on used catalyst samples. for Pd black.
0.012 0.00035 0.00017b 0.0041
0.0023 0.032 0.027 0.019
rate, Mmol of CH,/s.g of metal
13 0.62 0.0041 7.3 0.055 70 14 37
co
2
formation ( % CO conv.)
CO/metal (used cat.)
0.25 0.01
0.22 0.38 0.005 0.23 0.005 0.13 0.029 0.12
_____
0.06 0.94 2.0 -_-__
_____
Calculated from rate at 617 K and E , = 105.8 kJ/mol (25.3 kcal/mol)
is typically -0.5 mL g-'. An A1203catalyst containing 0.7 w t 5% Na was prepared by sequential impregnation steps. A NaOH solution was added to the alumina by incipient wetness and after drying at 120 "C this sample was calcined in air for 4 h at 873 K. The PdClz solution was then added, and after drying at 393 K the catalyst was heated in air for 4 h a t 533 K. The P d black was that used and described by Vannice (197513). The 5% Ni/7pAlZO3catalyst was prepared in our laboratory while the two cu-A1203-supportednickel catalysts were commercial samples. All three catalysts have been described earlier (Vannice, 1976). P r o c e d u r e . For chemisorption measurements a standard pretreatment was used and consisted of heating the sample in flowing hydrogen (50 cm3 min-') to 723 K, reducing at this temperature for 1h, evacuating at 698 K for 1 h, then cooling under dynamic vacuum to 298 K. Metal surface areas were determined using CO adsorption and the dual isotherm technique described by Yates and Sinfelt (1967). A similar pretreatment was used for the kinetic studies to reduce and activate the catalyst samples. For the runs at atmosphere pressure, the treatment consisted of heating the catalyst in flowing Hz to 723 K, reducing at this temperature for 1h, then cooling under flowing Hzto the desired temperature. For the runs at 2130 kPa (294 psig), the catalyst samples were pretreated as above, then the Hz/CO feed was introduced at atmosphere pressure. The flow rate was adjusted to the desired value, then the pressure was increased to 2130 kPa while the flow rate was maintained constant by adjusting the two micrometering valves in series downstream from the reactor. Results a n d Discussion Although much research has been devoted to the study of CO hydrogenation over metal surfaces, few of these studies have involved palladium. Most likely this was due to the early study of Fischer et al. (19251, which reported that P d was the least active of the group 8 metals for methanation. However, this study involved unsupported metals whose surface areas were not measured since it was conducted before the development of the BET method and the now well-known H2 or CO chemisorption techniques. The low activity of unsupported Pd was verified 40 years later by McKee, who observed that unsupported Ru, Rh, and Ir were more active for methanation than Pd (McKee, 1967). At the time the current work was initiated, the only reported study of supported P d in the methanation reaction was that of Shultz, Karn, and Anderson (Shultz et al., 1967). They found that the two catalysts with the lowest activity were Pd/Al2O3 and Os/Al,O,, and concluded that these catalysts were not worthy of consideration as commercial methanation catalysts. In both the studies by McKee'and Shultz et al., metal surface areas were not determined so that activities per gram of catalyst
were reported rather than the desired specific activities, which are necessary for proper activity comparisons. With these precedents, we were surprised to find that the specific activity of alumina-supported Pd for methanation is comparable to that for typical synthesis catalysts such as Ni, Co, and Fe (Vannice, 1975a). The specific activities for methanation ( N C H , = molecules CH,.s-'-site-') over the most active nickel catalysts were only 3 times greater than that for P d (Table I). This observation and the knowledge that supported Pd catalysts can be readily prepared with initial metal dispersions significantly greater than those for Ni catalysts suggested the possibility of developing practical P d catalysts for methanation. Such catalysts could also have additional benefits because Pd is expected to exhibit greater sulfur tolerance than Ni and does not form bulk carbides or volatile carbonyls as does Ni. As indicated earlier, alumina supports were required to obtain the high activity P d catalysts (Vannice, 1975b). Other supports such as silica produced only a small activity enhancement. This effect of support is shown in Table I. The data in Table I pertain to conversions of 5% or less where artifacts due to heat and mass transfer were avoidable. The turnover frequency of the Pd/A1203 catalyst is 70 times that of unsupported P d and 35 times that of Pd/Si02. The low specific activity of unsupported Pd is readily apparent, in agreement with expectations from the older studies (Fischer et al., 1925; McKee, 1967). Activity Maintenance of Pd Catalysts. Recognizing the enhanced specific activity of P d dispersed on acidic supports, it was important to consider next the stability and activity maintenance of such catalysts under working conditions. Clearly, the specific activity advantage of such catalysts would be lost if metal particle growth occurred under operating conditions so that the specific activity of the P d approached that of the unsupported material. It would thus be desirable to operate the Pd/A1,03 catalysts under conditions where the high dispersion and activity could be maintained. Short term activity maintenance studies combined with chemisorption measurements were therefore conducted at several temperatures. Chemisorption measurements were carried out on both fresh and used catalyst samples to allow the calculation of metal dispersions and specific activities. An adsorption stoichiometry of one adsorbed CO per surface site was assumed for relative comparisons of the catalysts. Table I1 gives the results. The CO/Pd ratio of 0.99 for a freshly prepared, reduced 0.5% Pd/A1203 sample indicates essentially complete dispersion and P d crystallites smaller than 1.5 nm (1 nm = 10 A). However, after use a t temperatures of 673, 623, and 573 K, it is apparent that a significant decrease in the fraction exposed has occurred under reaction conditions. Concurrent with such a decrease in metal surface area, one would expect the initial
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
Table 11. Chemisorption Data for Methanation Catalysts
a
reaction CO uptake, temp, " C pmol/g
catalyst
run
0.5% Pd/q-A1203 0.5% Pd/q-Al,O, 0.5% Pd/q-Al,O, 0.5% Pd/q,-Al,O, 0.5% Pd/0.7% Na/q-Al,O, 0.5% Pd/0.7% Na/q-Al,O, 5% Ni/q-A1,0, 5% Ni/q-Al,O, 42% Ni/a-Al,P, 42% Ni/a-Al,O, 30% Ni/a-Al,O, 30% Ni/a-Al,O,
fresh sample 1 2 3 fresh sample 4 fresh sample
H, uptake, pmollg
CO/Pd
H/Ni
fresh sample fresh sample
Obtained from Vannice (1976).
Table 111. Kinetic Data for CO Conversion Using 0.5% Pd/A1,0, (P = 2130 kPa; H 2 / C 0= 3) run 1
2 3 4 (with Na) Bureau of Mines
T,K
GHSV,a h-'
681 67 9 625 624 574 571 577 67 5 672 772 57 3
4500 5300 6800 6900 3000 4500 4600 3200 1200 318 4600
time on stream, h I/2
3'/2 '/4
5'/2 ' I 2
1 6'12 1 3'/2
_-__-
_____
CO conv. to CH,, %
cat. activity, pmol of CH,/min.g of cat.
71 18 17 5.0 25 4.3 3.4 43 30 44 (35.5)b
1740 258 866 259 363 142 117 950 262 50 (est.) 0.044 (calcd)c
_____
(H, t CO) conversion t o all hydrocarbon products. Based o n gas volume a t STP. (19.7 kcal/mol) obtained from Vannice (1975a).
catalytic activity to decline, and this behavior was observed, as shown in Figure 1, for runs conducted at 2130 kPa total pressure. During the run at 673 K, the high initial activity drops precipitously with time on stream. The decline in activity parallels the decrease in fraction exposed as both decrease by an order of magnitude. It should be noted, however, that the reaction conditions were severe, because initial CO conversions to CHI were over 70%. Considering the exothermicity of this reaction, the local temperatures generated in the catalyst bed may have been extraordinarily high, thereby greatly facilitating the loss of P d surface area. Lowering the reactor temperature to 623 K reduced the initial activity, as expected, but also reduced the rate of deactivation and the loss of metal surface area. After 5.5 h on stream, the catalyst in run 2 had a CO/M ratio of 0.16 compared to 0.08 for the catalyst in run 1after 3.5 h on stream. Because of this, the activity per gram of catalyst was actually higher at 623 K than at 673 K after 3 h on stream. However, even at 623 K stable activity was not obtained after 6 h on stream. Run 3 at 573 K showed an activity decline only during the first hour with stable activity for the remainder of the run. The decrease in fraction exposed was smaller for run 3 in that a final CO/M ratio of 0.25 was obtained after 7 h on stream. In all these runs, no attempts were made to avoid the high initial conversions which were obtained (Table 111). The heat generation at the high initial reaction rates would certainly be expected to facilitate sintering of the highly dispersed Pd initially present, and such an approach would not be used commercially. To reduce sintering, large temperature excursions are avoided when today's state of the art nickel catalysts are used, and conversions per pass
Based on E, = 82.4 kJ/mol
8 4W.C
RUN
4CQ'CRUN (with 07%
0
350'CRUN
A3WCRUN
a 0
A
A
A 2
A
A
4 TIME C N - S T R E A M l h r l
A
A
A 6
Figure 1. Methanation activity of 0.5% Pd/q-Al,O, as a function of time on stream. Reaction conditions: P = 2130 kPa; H*/CO = 3.
are controlled to prevent bed temperatures from exceeding 400-450 "C. This is usually accomplished using recycle reactors (Mills and Steffgen, 1973), but the use of reactors in series with no recycle has been proposed (White et al., 1974). The reactor conditions in this study, therefore, were probably much more severe than those normally encountered in commercial reactors. Comparison to Previous Studies of Pd Catalysts. Despite the decline in activity (Figure l ) , the Pd/A1203 catalysts used in this study were significantly more active
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
than the catalyst investigated by the Bureau of Mines workers, This is shown in Table 111. At 772 K and a GHSV of 318 h-', they measured an overall feed stream conversion of 35.5%. Presuming this represents the conversion of H2 + CO in the feed to hydrocarbon products, a catalyst activity of 50 mol of CH4-min-'.g of cat.-' can be estimated if a void fraction of about 0.5 is assumed for the catalyst bed. The catalyst studied at the Bureau of Mines was therefore much less active ut 773 K than our catalysts were after 6-7 h on stream at ternperatures of 573 t o 673 K. Their catalyst is calculated to be more than three orders of magnitude less active using an activation energy of 82.4 kJ/mol(l9.7 kcal/mol) for the methanation reaction (Vannice, 1975a) to adjust the rate to 573 K. Two possible explanations for the low activity of the Bureau of Mines catalyst are sintering and poisoning by an impurity such as sodium which is found in many aluminas. To test the latter possibility, sodium was purposely added to our 7-Alp03support. After reduction, CO adsorption on the fresh 0.5% Pd/0.7% Na/7-A1203was one-third that of the fresh, Na-free Pd catalyst. The initial activity of the catalyst with 0.7 wt % Na was also lower; however, for times longer than 2 h on stream, the activities of both catalysts were identical. The metal surface area measurement was consistent with the activity behavior since CO adsorption also was equal on both used samples, as shown in Table 11. The presence of alkali metal in the catalyst does not appear to inhibit the activity of Pd, at least after several' hours on stream, and cannot satisfactorily explain the low activity of the Bureau of Mines catalyst. In view of the evidence from the present study for sintering of Pd/A1203 catalysts during the methanation reaction, it appears that the low activity of the Bureau of Mines catalyst was due to metal particle growth which decreased both surface area and specific activity. As discussed in a previous publication (Vannice, 1975b), a metal-support interaction apparently occurs between Pd crystallites less than -12 nm in size and 7-A1203to significantly increase the specific activity of the Pd. The specific activity enhancement was rationalized on the basis of infrared studies (Van Hardeveld and Hartog, 1972) which showed that for Pd/A1203the more weakly bound linear form of adsorbed CO predominated over the strongly bound bridged form for particle sizes less than 19 nm. The linear form of adsorbed CO seems to correlate with higher specific activity in methanation (Vannice, 1975b). The important point here is that the growth of Pd crystallites significantly larger than 12 nm would be expected to decrease or even eliminate the cooperative metalsupport effect thereby altering the specific activity to that measured on large, unsupported Pd crystallites. Because the activity per gram of the Pd/A1203 catalysts is determined by the number of surface Pd atoms multiplied by their specific activity, P d agglomeration to produce 50-100-nm particles could account for a 1000-fold decrease in the activity per gram of catalyst-one order of magnitude decrease in dispersion and two orders of magnitude decrease in turnover frequency due to elimination of the favorable metal-support interaction. A 1000-fold decrease in Pd dispersion would not be required to account for the very low activity of the catalyst studied by Shultz et al. A recent paper by Poutsma et al. on CO-Hp reactions over P d catalysts provides an interesting contrast to this study (Poutsma et al., 1978). They reported that Pd supported on silica or alumina produces predominantly methanol, with only small amounts of methane and other
-
-
189
organic compounds formed, a t temperatures between 548 and 623 K and pressures between 1015 kPa (150 psig) and 108 mPa (16000 psig). We observed only the formation of methane with nothing but trace quantities of other products-results consistent with those reported by the Bureau of Mines workers (Shultz et al., 1967). At our conditions of 573 K, 2130 kPa pressure, and a H2/CO feed ratio of 3, the equilibrium CO conversion to CH30H is -5% (Thomas and Portalski, 1958). Our chromatograph columns had been calibrated for methanol, dimethyl ether, and other hydrocarbon products. Any of these products would have been detected if formed in significant amounts. In fact, dimethyl ether has been observed as a product of CO/H2 reactions over Pt/A1203 catalysts (Vannice and Garten, 1976). It is important to note that the palladium catalysts studied by Poutsma et al. were more than an order of magnitude less active than the 0.5% Pd/A1203 catalyst of the present study. The lowest activity observed at 623 K for our catalyst was 259 pmol of CH4*min-'.g of cat.-' (run 2, Table 111). Using a first-order dependence on total pressure as found in previous work (Vannice, 1975a) the turnover frequency for the 0.5% Pd/A1203catalyst at the conditions used by Poutsma et al. in their run 9, Table I1 [623 K, 10.15 mPa (1500 psig)], is calculated to be 2.8 molecules of CH4.s-'.site-'. This compares to a turnover frequency of 0.15-0.21 molecules of CH,OH.s-'.site-', calculated from run 9, Table 11, in Poutsma et al. for the 4.5% Pd/SiOz a t the same conditions. These specific activities were based on a fraction exposed of 0.16 for our used catalyst and of 0.05 to 0.07 for the catalysts in Poutsma et al. since the Union Carbide workers reported a range of crystallite sizes between 15 and 20 nm for their used catalysts. In another comparison between run 3, Table I11 of this study and runs 4, 5, 7, and 8, Table V in Poutsma et al., the higher specific activity of our catalysts is again apparent. The lowest activity measured in the present study at 573 K was 117 pmol of CH,-min-'.g of cat.-' (Table 111) for a sample with a final CO/M of 0.25. Adjusting this activity to the temperature used by Poutsma et al., (563 K) with an activation energy of 82.4 kJ/mol (19.7 kcal/ mol) (Vannice, 1975a), a turnover frequency of 0.122 molecule of CH4.s-'.site-' is obtained at 2130 kPa. The average specific activity for the four runs on the 4.6% Pd/Si02 catalyst studied by Poutsma et al., adjusted to 2130 kPa total pressure assuming a first-order dependency, molecule CH30H.s-'-site-'. The fraction is 6.9 X exposed for this used catalyst was assumed to be near 0.06 since an actual value was not reported. For this comparison the 0.5% Pd/A1203 catalyst has a turnover frequency nearly 20 times that of the 4.6% Pd/Si02 catalyst which is in good agreement with the previous comparison. We do not know the reason for the differences in behavior of the Pd catalysts from this study and that of Poutsma et al. In our study, methane was essentially the only hydrocarbon formed and other products, if present, existed only in trace amounts. It is possible that the significantly lower activity is responsible for this behavior, especially if the entire deactivation process affected only the methanation reaction. It is also possible that the alumina support alters the selectivity of the palladium catalyst, compared to silica-supported Pd. The origin of the differences in catalytic behavior between our catalysts and those of Poutsma et al. (1978) will have to await clarification by further work. Comparison of Pd and Ni Catalysts. It is instructive to compare the relative merits of Pd and Ni catalysts for
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
190 100
I
I
I
I
53
-
P
E
7
10
0
0
5
0 0 0
0 I
I
0.5
1
I
5
I
10
L
50
0 0 0 0
TOTPI. PRESSURE I A l X I
Figure 2. Activity dependence on pressure for 42% Ni/a-A1203. Reaction conditions: T = 503 K; H2/C0 = 3; CO conversion 620%.
methanation, in view of the enhanced specific activity of Pd/A1203 catalysts. The dispersions and turnover frequencies for several commercial nickel catalysts have been determined and are shown in Table I. Using these values and the rate equations which were determined previously (Vannice, 1976), the more active 42% Ni/a-A1203catalyst would have a calculated activity of 0.025 mol of CHI. min-'.g of cat.-' at 2130 kPa and 623 K which is the same as that calculated for a 2% Pd/A1203catalyst, assuming a dispersion of 100%. The low dispersion of the 42% Ni/a-A1203catalyst is normal for lined-out methanation catalysts. Nickel crystallite growth typically stabilizes at 30-50-nm particles under reaction conditions, which corresponds to a dispersion range of 1-3% (Bridger and Woodward, 1974; White, 1973). In fact, if the methanation rate equation determined by Lee for a large number of commercial nickel catalysts, including this 42 % Ni/a-Alz03 sample, is used to calculate activity, a rate of 0.015 mol of CH4.min-'.g of cat.? is obtained at 2130 kPa and 623 K (Lee, 1973). This is in excellent agreement with the rate estimated from Vannice (1976) since the dispersion of nickel in the catalysts analyzed by Lee is expected to be even lower than that of the 42% Ni/a-A1203 catalyst studied by Vannice. It is interesting to note that the activity of the 2% Pd/A1,03 catalyst, on a g metal basis, is comparable to activities of commercial nickel catalysts, as shown in Table I. Both commercial nickel catalysts were studied at higher pressures and exhibited two characteristics-a continual loss of activity and little or no activity increase as the total pressure increased. In fact, a noticeable decline in activity occurred as pressures increased from 1013 kPa to 3039 kPa, as shown in Figure 2 for the 42% Ni/a-A1203catalyst. Much of this behavior can probably be attributed to a deactivation process superimposed on these pressure dependency runs, as indicated by the difference in the two rates determined at each pressure. A t each pressure the second rate measurement, 0.5 h after the initial measurement, was always lower. Sintering was certainly responsible for much of this deactivation behavior as the final CO/M ratio of this catalyst was only 0.0073 (Table 11),a loss of 70% of the initial Ni surface area. With the Ni catalysts, we could not obtain activity stabilization during 10-15-h runs. Figure 3 shows the relationship of activity vs. time on stream at 548 K for the 30% Ni/a-Alz03catalyst. A decline of over 70% occurred during the first 12 h-behavior clearly in contrast to the run at 573 K with our Pd/~pA1,0, catalyst. In addition,
Figure 3. 30% Ni/a-A1203deactivates continuously as time on stream increases. Reaction conditions: T = 548 K; P = 1010 kPa; H2/C0 = 3; CO conversion 6 6%.
this deactivation occurs even though total CO conversion was well below 10% at all times during the run. The high specific activity and greater dispersion of the 2% Pd/A1203 catalyst relative to the 42% Ni/a-A1203 leads to comparable methanation activities for the two materials when compared on a per gram catalyst basis. At equal catalyst densities, of course, the volume activities, which are significant for reactor sizing in commercial application, would also be comparable. Treating Pd/A120, catalysts with the same care accorded commercial nickel catalysts should minimize deactivation due to sintering allowing the maintenance of high catalyst activity. Unfortunately, however, the cost of Pd catalysts would be significantly greater than for Ni catalysts. Although the 2% Pd/Al2O3catalyst contains 20 times less metal than the 42% Ni catalyst, the higher initial cost of P d is not offset because Pd is approximately 300 times more expensive than nickel. Additional advantages of a P d catalyst would have to be considered to justify its use. These include (1)lower C 0 2 make, (2) superior activity maintenance, (3) in situ regeneration capability, (4) absence of carbonyl and carbide formation, and (5) improved sulfur tolerance. The first advantage has been demonstrated in this study as shown in Table I, and evidence to support the second consideration is provided by Figures 1 and 3. Techniques already exist to regenerate noble metal catalysts used in reforming and hydrocracking reactions whereas nickel catalysts are not regenerated in situ. The fourth advantage is supported by thermodynamic considerations in addition to numerous physical studies which have failed to find evidence for the formation of these compounds. Finally, enhanced resistance toward sulfur poisoning is an aspect of considerable importance because nickel catalysts are easily and irreversibly poisoned by exceedingly small concentrations of sulfur. Because of this sulfur sensitivity, syngas feed streams must contain less than 0.1 ppm of H2Sto achieve satisfactory Ni catalyst lifetimes of 1-2 years. In contrast, noble metal reforming and hydrocracking catalysts routinely operate at much higher sulfur concentrations, and satisfactory lifetimes are achieved by P d hydrocracking catalysts in sulfur con-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979 191
centrations as high as 0.01-1% (Thomas, 1970). Therefore, it is anticipated that the Pd/A1203catalysts would be more sulfur-tolerant than nickel catalysts, thereby reducing the stringent purification procedures required for the latter catalysts. In addition, we would expect the P d catalysts to be less susceptible to permanent sulfur poisoning should a system upset occur causing an H2Sbreakthrough. These potential benefits could offset to some extent the higher cost of the P d catalyst. Summary and Conclusions Well-dispersed palladium on 17-A1203has a specific activity for CO conversion nearly 70-fold greater than large, unsupported Pd crystallites. This enhancement in activity is great enough to increase the turnover frequency for methane formation on Pd/A1203catalysts to a value within a factor of 3 of that on typical nickel methanation catalysts. If run under reactor conditions to minimize sintering, the Pd/A1203catalysts exhibit stable activity and activity per unit weight which is comparable to nickel catalysts because P d can be prepared and maintained with higher fraction exposed than Ni. This in itself does not balance out the higher cost of P d metal compared to Ni metal, but additional benefits may be gained from the use of Pd/Al2O3 catalysts including reduced C 0 2 formation, longer catalyst lifetimes, in situ regeneration, absence of carbonyl and carbide formation, and higher sulfur tolerance. The last benefit is inferred from the literature, but no direct experimental confirmation has yet been obtained. Even in
the absence of H2S tolerance data, P d catalysts appear worthy of consideration for use in methanation, although the possibility of methanol formation at high pressures must now be considered. Acknowledgment We thank Donna Piano and Larissa Turaew for their expert help in conducting the experimental work. Literature Cited Bridwr, G. W., Woodward, C., Am. Chem. SOC.Div. FuelChem. Prepr., 19, 105 (1974). Fischer, F., Tropsch, H., Diithey, P., Brennst. Chem., 8, 265 (1925). Lee, A. L., "Clean Fuels from Coal" Symposium, p 341, 1973. McKee, D. W., J. Catal., 8, 240 (1967). Mills, G. A., Steffgen, F. W., Catal. Rev., 8, 159 (1973). Poutsma, M. L., Elek, L. F., Ibarbia, P. A,, Risch, A. P., Rabo, J. A,, J. Catal., 52, 157 (1978). Schukz, J. F., Karn, F. S., Anderson, R. B., U.S. Bur. Mines Rep. No. 6974 (1967). Thomas, C. L., "Catalytic Processes and Proven Catalysts", Academic Press, Chapter 17, New York, N.Y., 1970. Thomas, W. J., Portalski, S., Ind. Eng. Chem., 50, 967 (1958). Van Harteveld, R., Hartog, F., Adv. Catal., 22, 75 (1972). Vannice, M. A,, J. Catal., 37, 449 (1975a). Vannice, M. A,, J. Catal., 40, 129 (1975b). Vannice, M. A,, J. Catal., 44, 152 (1976). Vannice, M. A., Garten, R. L., US. Patent 3941 819 (1976). Vannice, M. A., Garten, R. L., US. Patent 4093643 (1978). White, G. A., private communication, Ralph M. Parsons Co., 1974. Whne, G. A., Roszkowski, T. R., Stanbridge, D. W., Am. Chem. SOC.Div. Fuel Chem. Prepr., 19, 57 (1974). Yates, D. J. C., Sinfek, J. H., J. Catal., 8, 348 (1967).
Received for review October 20, 1978 Accepted April 2, 1979
Catalytic Conversion of Alcohols. 11 Influence of Preparation and Pretreatment on the Selectivity of Zirconia Burtron H. Davis" Potomac State College of West Virginia University, Keyser, West Virginia 26726
Pasupathy Ganesan Department of Metallurgical d Materials Engineering, University
of Kentucky, Lexington, Kentucky 40506
Zirconia has been found to resemble thoria for both dehydration and 1-alkene selectivities in 2-01 conversions. Its catalytic properties differ greatly from two other members of the group 4B family, titania and hafnia. Pretreatment and preparation play a role in determining the selectivity for dehydration and 1-alkene formation. Some zirconia samples may convert 2-01s to greater than 95% of the 1-alkene isomer. Isomerization of the pure cis-2 or trans-2-methylcyclohexanol occurred and each isomer produced the same, nonequilibrium amount of 3methylcyclohexene.
Introduction While zirconia has been studied as a dehydration catalyst, the number of reports concerning this oxide are not nearly as numerous as those for oxides such as alumina or thoria. The preparation and pretreatment of thorium oxide have a strong influence on the catalytic character
* Address correspondence to this author at the Institute for Mining & Minerals Research, University of Kentucky, P.O. Box 13015, Lexington, Ky. 40583. 0019-7890/79/1218-0191$01,00/0
of the resulting solid (Davis, 1972; Davis and Brey, 1972). A survey of the formation and properties of zirconia has recently been published (Rijnten, 1970); however, this survey did not define the character of the calcined materials nor detail the catalytic properties. It has been reported that when 2-pentanol is passed over a zirconia catalyst at 300 "C the resulting water/hydrogen ratio is 5 and the l-pentene:2-pentene ratio is 0.4 (Freidlin et al., 1971; Sharf et al., 1972). Thus, the catalyst is selective for dehydration but not for 1-alkene formation from this 2-01. On the other hand, ZrOz at 200 "C has been 0 1979 American Chemical Society
