Ind. Eng. Chem. Res. 1987,26, 1495-1500
1495
Activity and Selectivity of Ni-Mo/HY Ultrastable Zeolites for Hydroisomerization and Hydrocracking of Alkanes? M. Isabel VBzquez and A g u s t h Escardino Departamento de Zngenieria Q u h i c a , Uniuersitat de ValPncia, 46100 Burjassot (Valencia),Spain
Avelino Corma* Instituto de Catdlisis y Petroleoquimica, CSIC, 28006 Madrid, Spain
The hydroisomerization and hydrocracking of n-heptane have been studied on a series of Ni-Mo/HY ultrastable zeolite catalysts a t 300-350 OC reaction temperature and 25 kgcm-2 of total pressure. The product distribution in the isomerized fraction and in the cracked fraction has been discussed from the point of view of a typical bifunctional mechanism. It has been found that the Ni/(Ni Mo) atomic ratio of the catalyst has a strong influence on the activity, and a maximum is found for a Ni/(Ni Mo) N 0.5. The selectivity to hydroisomerization, hydrocracking, and hydrogenolysis Mo) ratio and any of them could be maximized by an adequate also depends on the Ni/(Ni combination of the hydrogenating and the acid function.
+
+
+
Hydrocracking is one of the three basic processes in petroleum refining used to convert heavy oils into more valuable products. It does the work by converting large high boiling point molecules into lower boiling products by simultaneous hydrotreating and cracking carbon-carbn bonds. Since the process needs to carry out two functions, typical catalysts should be bifunctional and incorporate both hydrogenation and cracking components. Hydrocracking is a very flexible process which allows one to obtain a broad range of saturated products ranging from high yields of liquid petroleum gas to middle distillate with a high octane number. This flexibility is due not only to the process itself but mostly to the catalyst design (Ward, 1983). Indeed the cracking and hydrogenation capacity strongly affect the product distribution that results. Concerning the cracking function, it is well-known that cracking is an acid-catalyzed reaction; hence, it is desirable to have one acid function in which the strength and number of acid sites could be controlled. This is achieved, actually, by using some zeolites such as Y, ZSM-5, erionite, and mordenite (Bolton, 1976; Chen et al., 1977; Steijns et al., 1981; Haynes et al., 1983; Franck and Le Page, 1981; Ward, 1975; Weitkamp et al., 1983; Guisnet and Perot, 1984). Among them, Y zeolites are the most widely used, while the others are used in the cases which require reactions controlled by pore geometry. The hydrogenating function is given by either noble metals such as Pt and Pd (Gallei et al;, 1981; Riberiro et al., 1982; Weitkamp and Ernst, 1985) or combinations of non-noble metals of groups 9 and 10 (Co, Ni) and the metals of group 6 (Mo, W) (Ward, 1983; Ternan and Parson, 1979; Mooi, 1980). In the latest case the hydrogenation activity depends, besides on the particular combination of non-noble metals, on the atomic ratio of those. In the present work, we study the influence of the acid and ~~
* To whom correspondence should be addressed. t In this paper the periodic group notation is in accord with
recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12,and the p-block elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the numbering: e.g., I11 3 and 13.)
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0888-5885/87/2626-1495$01.50/0
the hydrogenation component, in a Ni-Mo/HY ultrastable zeolite, on the activity and selectivity for hydroisomerization and hydrocracking of n-heptane.
Experimental Section Materials. The zeolite used in this study was a HY ultrastable zeolite (HYUS), with a unit cell dimension, a. = 24.40 A. It was prepared from a NaY Linde SK-40 (Si/Al = 2.4) zeolite by repeated ion exchange with ammonium acetate. Each exchange was followed by deep bed calcination at 550 "C for 3 h, until the final Naf content of the zeolite was lower than 2% of the initial value. The BET area of the HYUS zeolite was 450 m2.g-I, and this value was not influenced by removal of Na+. The nickel and molybdenum were incorporated in the HYUS zeolite separately by vacuum impregnation at 70 "C from an aqueous solution of nickel nitrate and ammonium heptamolybdate, respectively. After each impregnation, the sample was dried at 110 "C for 6 h and then calcined in two steps. The first, to decompose the salt, was carried out in air flow at 450 "C for 2 h and the second at 550 "C for 3 h. The final NiO and Moo3 content of the catalyst was determined by the following procedure: 0.5 g of the moisture-free sample was introduced into a platinum crucible and covered by 10 g of an equimolecular Na2C03-KzC0, mixture and 0.5 g of B03H3. This mixture was heated at 1100 "C and maintained for about half an hour until complete solution was obtained. When the melt had partially cooled, this was dissolved with HCl (l:l), and the solution was gauged and analyzed by atomic absorption spectroscopy. In order to study the influence of the Ni/(Ni + Mo) atomic ratio on the activity, various catalysts with different NiO content and a 8 wt % Moo3 content were prepared. High-purity n-heptane (>99.5%) and Hz (>99.9%) were used as reactants without further purification. Apparatus and Procedure. The experiments were carried out, at temperatures of 300,330, and 350 "C and 25 kg.cm-2 total pressure, in a continuous tubular, plug flow, stainless steel reactor. After preparation, the catalysta were pelletized, crushed, and sieved, and the particle size fraction of 0.125-0.250 mm was selected. In each experiment the weight of catalyst was varied for obtaining different conversions and the W / F oratio (weight of cat0 1987 American Chemical Society
1496 Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987
alyst/molar flow of n-heptane fed) was comprised between 150 and 6000 kg-s-kmol-l. Before the catalyst was introduced into the reactor, this was mixed with glass chips of the same particle size in order to keep the volume of the bed constant in all the experiments, and to minimize the thermal effects due to the reaction. Then, the temperature was raised to 450 "C in Hz flow and was kept at this temperature for 2 h. After this pretreatment, the temperature was set to the desired reaction temperature, the hydrogen flow was regulated to obtain a H,/n-heptane molar ratio of 5, and the hydrocarbon flow was started. The reactants were preheated and then fed to the reactor. The products of the reactor were cooled, and the liquid fraction was separated from the gas fraction and then collected and analyzed. The liquid products were analyzed by gas-liquid chromatography using a 4.3-m column with silicone gum rubber (SE-30) on Chromosorb P at a temperature program of 80-170 "C. The gas products were analyzed with a 2-m column of silica gel and Porapak Q at temperatures of 70 and 170 "C. Only the experiments with mass balances of 100% f 5% were considered. Experiments were repeated to test the experimental reproducibility. The molar yield curves for different cat/oil (weight of catalyst/total weight of n-heptane fed in an experiment) ratios were obtained from the representations, obtained experimentally, of the conversion and the molar yield of a product vs. the time on stream (T.O.S.) at different W/FO ratios, by cat W 1 1 - -oil F0 T.O.S. Mn.heptane where M is the molecular weight. Conversion was defined here as the number of n-heptane moles reacted by each n-heptane mole fed, while the yield to each product was defined as the number of moles of this product obtained by each n-heptane mole fed and selectivity was defiied as the number of moles obtained by each n-heptane mole reacted.
Results and Discussion Reaction Products and Initial Selectivities. The reaction products obtained during hydroisomerization and hydrocracking on noble metals (Pt, Pd)/Y zeolite bifunctional catalyts have been largely covered in the literature (Steijns et al., 1981; Weitkamp et al., 1983; Gallei et al., 1981;Ribeiro et al., 1982; Weitkamp and Ernst, 1985; Giannetto et al., 1985; Weitkamp, 1982; Weitkamp et al., 1984; Jacobs et al., 1980). However, with non-noble metals (Ni, Mo), detailed work is scarce (Franck and Le Page, 1981; Jothimurugesan and Bhatia, 1984; Choudhary and Saraf, 1978). In this work, the molar yields to the different reaction products at different levels of conversion have been obtained, with a 4 wt % Ni0-8 w t % Mo03/HYUS catalyst, by changing the amount of catalyst, and the results, at 350 "C, are given in Figure 1. In these curves the slope of the tangent, when conversion goes to zero, represents the initial selectivities to the different reaction products (Table I). There we see that the selectivity to branched heptanes is 78.5%, the rest of the products being hydrocarbons with a number of carbon atoms lower than seven and which are formed by cracking on acid sites and hydrogenolysis on the NiO and MOO,, as will be discussed later. Product Distribution in the C, Isomerized Fraction. In Table I it can be seen that 2- and 3-methylhexane account for -91% of the branching isomers of n-heptane, while the 2/3-methylhexane ratio (at 350 "C is 0.76. From
Table I. Initial Selectivities to the Different Reaction Products in the Hydrocracking of II -Heptane with a 4 wt '70 Ni0-8 wt % M o 0 3 / H W S Catalyst initial selectivity at 350 "C 300 "C methane 0.018 0.001 ethane 0.003 0.001 propane 0.111 0.026 isobutane 0.125 0.027 n-butane 0.021 0.002 butenes 0 0 C4 fraction 0.146 0.029 isopentane 0.021 0.002 n-pentane 0.018 0.003 pentenes 0 0 C5 fraction 0.039 0.005 2-methylpentane 0.015 0.002 3-methylpentane 0.008 0.001 hexane 0.038 0.016 C6 fraction 0.061 0.019 2-methylhexane 0.309 0.396 3-methylhexane 0.405 0.487 other i-C7 0.071 0.053 i-C7 fraction 0.785 0.936 Table 11. Thermodynamic Equilibrium Distribution of C, Isomers at 350 "C equilibrium distribution, mol % 2-methylhexane 17.23 3-methylhexane 21.27 2,2-dimethylpentane 21.19 2,3-dimethylpentane 26.71 2,4-dimethylpentane 5.86 3,3-dimethylpentane 7.75
Figure 2 one sees that if the formation of 2- and 3methylhexane would only take place by protonated cyclopropanes (PCP) (Weitkamp and Jacobs, 1981),a 2/3methylhexane ratio of 0.5 should be expected. The ratio observed experimentally, and which is very close to the thermodynamic equilibrium (see Table 11), indicates that the following 1,2-methylshift (Fajula, 1985)
should also take place and is very fast on this catalyst. Furthermore, in Table I it is also shown that the amount of the other branched isomers of n-heptane is quite low, and it is far away from the equilibrium composition (Table 11). This result can be a consequence of a lower rate of formation of dibranched prqducts, a faster cracking of the dibranched isomers of n-heptane, or both effects. From the point of view of a PCP mechanism, the isomerization of monobranched to dibranched heptanes goes through secondary and tertiary carbocations (Figure 3) and therefore it should be thermodynamically favorable. However, it has been found (Baltanas et al., 1983) that the rate constant for the formation of multibranched (MTB) isomers of n-octane from monobranched (MB) is 1.5 lower than the rate of formation of monobranched isomers from n-octane. Taking into account this fact and the influence of the hydrocarbon chain length on the relative rate of formation of mono- and multibranched isomers, we would expect that, in the case of n-heptane, k,TB/kMB 1/2. This factor plus the fact that the isomerization follows one scheme of the type, n-heptane MB MTB, and that we are considering initial selectivities would explain the low values of MTB found at low levels of conversion.
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Ind. Eng. Chem. Res., Vol. 26,No. 8,1987 1497
"'i 02
O1
ETHANE
k IO
20
30
L
LO
m %3
i-10
A
w ~
20
30
2MHexane
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3 M Hexane
01
1
10
20
30
IO
LO
I SOPE N TANE 10
IO
20
M
LO
I 2MHeane
10
0
20 5
30
LO E
TOTAL 03
t
1
I
OI
->
I
20
33
2.3DM Pentane
2MPENTANE
+-
2 and 3Elhylpenlane
ZZDMRnlane
40
0.5 l
3MPENTANE
5
E
IO 20 30
1
+
(%I
HEXANE
0
0
CONVERSION
PENTENES
10
Figure 2. Formation of 2-methylhexane and 3-methylhexane from C7 carbocations through PCP intermediates.
o
IO
do
p
20 30
2,PDMPcntane
'
/)&'
-
LO
2MHEXANE
2Mrthylhcxane
2and9Elhylpcntane
2,ZDMPentan
A
D i branched
3Methylhexane
-
DNbranched
Figure 3. Formation of dibranched isomers from 2-methylhexane and 3-methylhexane through PCP intermediates.
10
5
IO
20
30
10
40
20
30
40
OTHERS i-C7
TOTAL
CONVERSION
(*/e)
Figure 1. Selectivity plots for products obtained during hydrocracking of n-heptane on a 4 wt % Ni0-8 wt % MoOa/HYUS catalyst, at 350 OC. Cat/oil ratios (weight of catalyst/total weight of n-heptane fed in an experiment): (0)1.2 X (0) 2X (A) 3 x 10-3.
On the other hand, it is expected that MTB isomers crack faster than the MB ones (Baltanas et al., 1983; Steijns and Froment, 1981). Indeed, from Figure 4, where the different cracking possibilities involving only secondary and tertiary carbonium ions are presented, it can be seen
that the MB isomers always crack through a C-type mechanism (secondary-econdary carbocation), while the MTB can also crack through the more probable B1 and B2 mechanism (secondary-tertiary and tertiary-secondary carbocations, respectively; see Figure 4) (Weitkamp et al., 1983). All this is reflected in the cracking products obtained (Table I), which show that the C4 fraction is mainly formed by isobutane, which can only be formed by cracking of 2,2'- and 2,4-dimethylpentane through a B1 and B2 mechanism, respectively. Product Distribution in the Cracked Fraction. The formation of methane as a primary product is difficult to explain by an acid-catalyzed mechanism if we take into account that the production of methane involves primary carbonium ions and, moreover, no methane is observed as a primary product when the reaction is carried out on the purely acid H W S zeolite (Corma et al., 1984). Therefore, we believe that, in this system, methane could be formed by hydrogenolysis on the metal part of the catalyst, while when catalysts containing noble metals are used, methane is not observed (Weitkamp et al., 1983; Weitkamp and Ernst, 1985; Jacobs et al., 1980; Martens et al., 1985).
1498 Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987
2MHexane
I
3MHexane
A
05t
N
O3
Oil
A
t
01
A * A + AI
L
2,2 DMPentane
A
CARBON NUMBER OF CRACKED PRODUCTS
Figure 5. Selectivity of the different cracking fractions at 350 "C 0, (0) 0.248, (v) for catalysts with Ni/(Ni + Mo) atomic ratio: (0) 0.441,( 0 )0.50,(W) 0.539, (A)0.569, (A)1.0. HYDROGEWLYSIS
2,3 DMPentane
N
2 -
2J&DMPentane
i
A Figure 4. Cracking possibilities of the monobranched and dibranched C, isomers, involving only secondary and tertiary carbonium ions.
The isobutaneln-butane ratio can be taken as an indication of the relative rate of cracking by B (Bl,B2) and C mechanisms (see Figure 4). If this is so, then by comparing the isobutaneln-butane ratio observed at 300 and 350 "C reaction temperatures (Table I), it is obvious that the cracking through mechanism type C (formation of n-butane) has a higher apparent activation energy than cracking through mechanisms of type B (B1 and B2, responsible for the formation of isobutane). This conclusion would agree with the structures involved in those mechanisms and also with previously reported results (Corma et al., 1985; Weitkamp et al., 1983). Another interesting feature which can be observed in the results of Table I is that the c6 + C5 + C4/C1+ C2 + C3 ratio is higher than 1. This result indicates that the C6, C5, and C4fractions are not only produced by cracking or hydrogenolysis of C7 but also by other reactions involving consecutive alkylation-cracking processes, as has been observed during cracking of alkanes on Y zeolites (Bolton and Bujalski, 1971; Weisz, 1970; L6pez Agudo et al., 1981). On the other hand, when hydrocracking is carried out on Pt or Pd/HYUS zeolite catalysts, the product distribution is perfectly symmetrical with respect to the number of carbons in the products (Weitkamp et al., 1983). From this point of view, the Ni-Mo/HYUS catalysts present an intermediate behavior between that of a monofunctional cracking HYUS zeolite catalyst and an ideal bifunctional Pt/HYUS catalyst. This can be due to the lower hydrogenating activity of the Ni-Mo with respect to the noble metals. The consequence of this lower hydrogenation capacity is the presence of small amounts of olefins in the products, never detected on P t / H W S catalyst, which, on the other hand, are the origin of the consecutive alkylation-cracking reactions observed in the Ni-Mo/HYUS catalyst.
CRACKING
ISOMERIZATION
p,< 1 /\, w , ,
az
,
,
,
02
04
a6
4
OL
06 i o
02 NiINt.Mo
ah
n6
Iafomic ratio1
Figure 6. Initial selectivity for hydrogenolysis, cracking, and isomerization reactions as a function of the Ni/(Ni + Mo) atomic ratio of the catalyst.
Influence of t h e Ni/(Ni + Mo) Atomic Ratio on Selectivity. In Figure 5 the selectivity to the different cracking products is presented for Ni-Mo/HYUS catalyst with different Ni/(Ni + Mo) atomic ratios. There can be seen that on all the Ni-Mo/HYUS catalysts a maximum in selectivity is obtained for the C4 fraction. Moreover, a nonsymmetric distribution is found in any of the NiMo/HYUS catalysts, being, in all cases, the ratio C4 + C5 + c6/c3 + Cz+ C1 of higher than one. In Figure 6 the initial selectivities for hydrogenolysis, cracking, and isomerization, as a function of the Ni/(Ni + Mo) atomic ratio, are given. The initial selectivity to hydrogenolysis seems to increase, in the limits of the experimental error, when the amount of nickel increases, being maximum for the catalyst containing only nickel, in agreement with the well-known ability of nickel as a hydrogenolysis catalyst. The cracking and isomerization initial selectivities go through a maximum for catalysts with a Ni/(Ni + Mo) atomic ratio in the 0.4-0.5 range, for which the acidity is also maximum, as it should be expected if one takes into accoynt that in these reactions the controlling step is that involving the acid-catalyzed carbonium ion reaction. A change in the total conversion is also observed when the Ni/(Ni + Mo) ratio was modified. Indeed, in Figure 7 it can be seen that a maximum in the activity (referred to the conversion of n-heptane) is obtained with NiMo/HWS catalysb with a Ni/(Ni + Mo) ratio of 0.44-0.5. It has been established (Coonradt and Garwood, 1964; Jacobs et al., 1980) that the hydroisomerization and hydrocracking of alkanes go through a series of consecutive steps involving, first, a dehydrogenation of the paraffin to give one olefin which isomerizes and cracks, with the olefinic products formed being hydrogenated in the last step. If the hydrogenating function is active enough, then the reaction is controlled by the acid function and the reaction rate should be related with the concentration of Bronsted acid sites. In this way, the amount of Bronsted
Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 1499 150 lLOr
I
3p 1
A
100
I_j[ 100
Loy&e&
I
I
I
'
2
1550cm-1
3
au
Figure 9. Relation between the initial n-heptane reaction rate (activity) and the Bronsted band intensity for catalysts with a Ni/ (Ni + Mo) atomic ratio 2 0.5.
2ot
&---
01
02
03
04
05
06
IO
Ni/Ni+Mo (atomic ratio/
Figure 7. Variation of the initial n-heptane reaction rate vs. the 300 OC, (0)330 OC, Ni/(Ni + Mo) atomic ratio of the catalyst: (0) (A)350 O C reaction temperatures.
lyst. The inital selectivities for hydrogenolysis, cracking, and isomerization vary as a function of the Ni/(Ni Mo) ratio of the catalyst. Finally, a direct relation is found between activity and acidity for catalysts with Ni/(Ni Mo) 2 0.5.
+
+
Registry No. Ni, 7440-02-0; Mo, 7439-98-7; H3C(CH2),CH,, 142-82-5; CHI, 74-82-8; HSCCH, 74-84-0; H&CH2CH3,74-98-6; (H,C)2CHCH,, 75-28-5; H,C(CH2)2CH,, 106-97-8; (H&)&HCH2CH3, 78-78-4; H&(CH2)3CH3,109-66-0; (H3C)*CH(CH,)&H,, 107-83-5; H&CH&H(CH,)CH,CH,, 96-14-0; H&(CH2),CH3, 110-54-3; butene, 25167-67-3;pentene, 25377-72-4; 2-methylhexane, 591-76-4; 3-methylhexane, 589-34-4; 2,2-dimethylpentane, 590-35-2; 2,3-dimethylpentane, 565-59-3; 2,4-dimethylpentane, 108-08-7; 3,3-dimethylpentane, 562-49-2.
Literature Cited
Ni/Ni+Mo (atomlc r a t l o )
Figure 8. Intensity of the Bronsted band for catalysts with different Ni/(Ni + Mo) atomic ratio. Temperature desorption of pyridine: (0) 150 " c , (0) 350 " c , (A)450 "c.
acidity for the different catalysts, measured by pyridine adsorption-desorption (1550-cm-l band frequency),is given in Figure 8. Comparing both figures it can be concluded that there is not a direct relation between the activity and the acidity of the Ni-Mo/HYUS catalysts studied. A direct relation is found however when considering only the samples with a Ni/(Ni + Mo) 2 0.5 (Figure 9). These results would be in agreement with a previous work (Vbquez et al., 1986), which presented, on the basis of kinetic data, that for Ni/(Ni + Mo) < 0.44-0.5, the controlling step of the hydrocracking is the dehydrogenation of n-heptane on the metal, while for samples with Ni/(Ni Mo) 2 0.5 the reaction of carbonium ion on the acid centers is the controlling step. In conclusion it can be said that on a 4 wt % Ni0-8 wt % Mo03/HYUS catalysts, the 1,2-methyl shift reaction is very fast and the 2/3-methylhexane ratio observed experimentally is very close to the thermodynamic equilibrium. On the other hand, according to the cracked fraction distribution, the dibranched isomers crack preferably to the monobranched ones. The formation of methane as a primary product could be explained by hydrogenolysis in the metal part of the catalyst. This fact and the nonsymmetric distribution found in the cracked products indicate that the Ni-Mo/HYUS catalyst presents an intermediate behavior between that of a monofunctional cracking H W S zeolite catalyst and an ideal bifunctional Pt/HYUS cata-
+
Baltanas, M. A,; Vansina, H.; Froment, G. F. Ind. Eng. Chem. Prod. Res. Deu. 1983, 22, 531. Bolton, A. P. Zeolites Chemistry and Catalysis; Rabo, J., Ed., ACS Monograph Series 171; American Chemical Society: Washington, DC. 1976; p 714. Bolton, A. P.; Bujalski, R. L. J . Catal. 1971, 23, 331. Coonradt, H. L.; Garwood, W. E. Ind. Eng. Chem. Process Des. Deu. 1964, 3(1), 38. Corma, A.; M o n t h , J. B.; Orchill&, A. V. Ind. Eng. Chem. Prod. Res. Deu. 1984,23(3), 404. Corma, A.; Planelles, J. H.; Tomls, F. J . Catal. 1985, 97, 445. Chen, N. Y.; Gorring, R. L; Ireland, H. R.; Stein, T. R. Oil Gas J. 1977, 75, 165. Choudhary, N.; Saraf, D. N. Ind. Eng. Chem. Prod. Res. Deu. 1978, 17(3), 196. Fajula, F. S t u d . Surf. Sci. Catal. 1985, 20, 361. Franck, J. P.; Le Page, J. F. Proc. Int. Congr. Catal., 7th 1981,792. Gallei, E.; Marosi, L.; Schwarzmann, M.; Lorenz, E. US. Patent 4 252 688, 1981. Giannetto, G.; Perot, G.; Guisnet, M. S t u d . Surf. Sci. Catal. 1985, 20, 265. Guisnet, M.; Perot, G. NATO ASI Ser., Ser. E 1984,80, 397. Haynes, H. W., Jr.; Parcher, J. F.; Helmer, N. E. I n d . Eng. Chem. Process Des. Deu. 1983, 22, 401. Jacobs, P. A.; Uytterhoeven, J. B.; Steyns, M.; Froment, G.; Weitkamp, J. Proc. I n t . Conf. Zeolites, 5 t h 1980, 607. Jothimurugesan, K.; Bhatia, S. Can. J . Chem. Eng. 1984, 62, 390. Ldpez Agudo, A.; Asensio, A.; Corma, A. J . Catal. 1981, 69, 274. Martens, J.; Weitkamp, J.; Jacobs, P. A. Stud. Surf. Sci. Catal. 1985, 20, 427. Mooi, J. U S . Patent 4238316, 1980. Ribeiro, F.; Marcilly, C.; Guisnet, M. J. Catal. 1982, 78, 267. Steijns, M.; Froment, G. F. Ind. Eng. Chem. Prod. Res. Deu. 1981, 20, 660. Steijns, M.; Froment, G. F.; Jacobs, P.; Uytterhoeven, J.; Weitkamp, J. Ind. Eng. Chem. Prod. Res. Deu. 1981, 20, 654. Ternan, M.; Parson, B. I. U.S. Patent 4 176 051, 1979. Vtizquez, M. I.; Escardino, A.; Aucejo, A.; Corma, A. Can. J. Chem. Eng. 1986, 64, 272. Ward, J. W. US. Patent 3926780, 1975. Ward, J. W. Preparation of Catalyst Ill; Poncelet, G., Grange, P., Jacobs, P., Eds.; Wiley: New York, 1983; p 587. Weitkamp, J. Ind. Eng. Chem. Prod. Res. Deu. 1982, 21, 550. Weitkamp, J.; Ernst, S. Stud. Surf. Sci. Catal. 1985, 20, 419.
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I n d . Eng. Chem. Res. 1987, 26, 1500-1505
WeitkamD. J.: Jacobs. A. PreDr.-Am. Chem. SOC.Diu. Pet. Chem. 1981, i6,9.’ Weitkamp, J.; Jacobs, P. A,; Ernst, S. Stud. Surf. Sci. Catal. 1984, 28, 279. Weitkamp, J.; Jacobs, P. A.; Martens, J. A. Appl. Catal. 1983,8, 123.
Weisz. P. B. Annu. Rev. Phvs. Chem. 1970,21, 175.
Received for review June 16, 1986 Revised manuscript received March 31, 1987 Accepted April 30, 1987
Cobalt Mixed Spinels as Catalysts for the Synthesis of Hydrocarbons? Giuseppe Fornasari, Stefan0 Gusi, Ferruccio Trifirb,* and Angelo Vaccari Dipartimento di Chimica Zndustriale e dei Materiali, Viale Risorgimento 4, 40136 Bologna, Italy
The nature and the catalytic activity in the Fischer-Tropsch synthesis of cobalt, copper, zinc, and chromium mixed oxides have been investigated for a wide range of compositions. Most of the precursors showed a hydrotalcite-like structure and formed by calcination essentially a spinel-type phase (notwithstanding the high values of the M(II)/M(III) ratio (M = metal)). For all catalysts the hydrocarbons were the main products and presented typical Schulz-Flory distributions. While the Co/Cr catalysts showed very low activity, a maximum was obtained for catalysts containing comparable amounts of copper and cobalt. In all samples a spinel-type phase was present after reaction, while the formation of metallic cobalt and/or cobalt oxides was not observed. On the other hand, metallic copper after both reduction and reaction was detected by N 2 0 titrations. The catalytic activity was attributed to a synergetic effect between copper and cobalt, correlable to the presence of a nonstoichiometric spinel-type phase or to an interaction between this phase and the well-dispersed metallic copper formed in reducing conditions. The production of chemical feedstocks or fuels from syngas became an attractive alternative to petroleum supplies. The synthesis of hydrocarbons, with the exception of methane, is commonly referred to as the FischerTropsch reaction and is now utilized on a large scale in the South African Sasol plants. Many excellent books and reviews have been published, also recently, which give detailed information about the different aspects of this reaction (Storch et al., 1951; Pichler, 1952; Anderson, 1956; Vannice, 1976; Biloen and Sachtler, 1980; Dry, 1981; Rofer-De Poorter, 1981; Henrici-0liv8 and Oliv8, 1984). Groups 8-10 transition metals or their alloys, supported or unsupported, have always been employed as catalysts, and different preparation techniques have been used (Frohning, 1977; Dry, 1981); however, to achieve a highly dispersed metal surface, a reduction pretreatment must be performed, whatever the preparation method (Storch et al., 1951; Pichler, 1952; Anderson, 1956; Clarke et al., 1962, Dry, 1981). Historically, the first metals to be used were iron and cobalt, alone or alloyed together (Sabatier and Senderens, 1902; BASF, 1913; Fischer and Tropsch, 1923; Pichler, 1952), since nickel showed activity toward methanation only (Sabatier and Senderens, 1902),rhodium sharply shifted the selectivity toward oxygenated compounds (Dry, 1981), and Pd, Os, Ir, and Pt were found to have low activities (Pichler, 1952; Vannice, 1976, 1977). Ruthenium has the higher overall activity, giving rise to the formation of waxy fractions, but the methanation re-
* To whom correspondence should be addressed. In this paper the periodic group notation is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the p-block elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the numbering: e.g., I11 3 and 13.)
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action is the most favored (Pichler, 1952; Vannice, 1977). Many efforts have been successfully made to improve Ru-based systems (Vannice, 1975; Dalla Betta and Shelef, 1977; King, 1978 Everson et al., 1978; Gonzales and Miura, 1982). Among the carriers, the most commonly used are SO2, Kieselguhr, and A1203;however, some authors have developed zeolite-supported systems (Jacobs, 1980; Nijs et al., 1980; Naccache and Ben Taarit, 1980; Peuckert and Linden, 1984; Leith, 1983). While the supports are to be considered physical promoters because of their high surface area, chemical promoters such as Ni, Zr, Mn, and Cu (Dry, 1981; van Dijk and van der Baan, 1982; Bruce et al., 1983) or alkaline oxides, which partially neutralize the acidity of the supports (Fischer and Tropsch, 1930; Dry, 1981), have been widely employed. Recently, cobalt- and nickel-containing catalysts have been proposed as active also in the higher alcohols synthesis (Courty et al., 1982, 1983; Fujimoto and Oba, 1985; Uchiyama et al., 1985). In particular, the catalysts developed at the Institute FranGais du Petrole (IFP) have compositions corresponding to that of alkalized conventional copper-based methanol synthesis catalysts modified by the addition of cobalt. In this research the catalytic activity of Co, Cu, Zn, and Cr mixed oxides was investigated for a wide range of compositions,with the aim to study the catalytic behavior of the nonmetallic cobalt and the role of the other elements added. In previous works, it was shown that hydrotalcite-like phases [HY, having general formula M1I6MI1I2(OH)&03.4H20] may be useful precursors of catalysts for the synthesis of methanol at low temperature (Gherardi et al., 1983; Gusi et al., 1985). In these precursors all cations are randomly distributed in positively charged brucite-like layers alternated with negatively charged (C03.4H20)2-interlayers (Busetto et al., 1984). Therefore, we tried to prepare catalysts with different relative ratios of Co, Cu, Zn, and Cr starting from homogeneous pre-
0888-5885/87/2626-1500$01.50/0 0 1987 American Chemical Society
