585
Ind. Eng. Chem. Prod. Res. Dev. 1982,21, 585-590
in the reduction of Ti3+to Ti2+. As Table IV shows, the values of hp in the MgC12-supported catalysts are larger than the values of h in the conventional catalysts based on 6-TiC1, by one orjer of magnitude. The MgC1, support must influence the electronic structure of the active titanium-polymer chain bond by inductive effects. The electron-withdrawing capability of the MgC12 support seems to instabilize an active titanium-carbon bond, resulting in the higher value of k . An activation effect of MgC12 may be supported by t i e fact that an apparent activation energy (5.3 kcal mol-') for polymerization with the MgCl,-supported catalysts is lower than the value (12 kcal mol-') of polymerization with the 6-TiCl3/A1(C2H6), catalyst (Doi et al., 1973). Literature Cited Bovey, F. A. Acc. Chem. Res. 1968, 1 , 175. Brockmeier, N. F. I n "Polymerization Reactors and Processes"; Henderson, J. N., Ed.; ACS Symposium Series 104, Washington, DC, 1979; p 201. Caunt, A. D. Br. Polym. J. 1981, 13, 22. Dol, Y.; Okura, I.; Keii. T. Chem. Lett. 1972a, 327. Doi, Y.; Yoshimoto, Y.; Keii, T. Nippon Kagaku Kalshl 1972b, 495. Doi, Y.; Kobayashi, H.; Keii, T. Nippon Kagaku Kaishi 1973, 1089. Doi, Y.; Morinaga, A,; Keii, T. Makromol. Chem. RapM Commun. 1980, 1 , 193. Doi, Y.; Suzuki. E.; Keii, T. Makromol. Chem. RapM Commun. 1981, 2 , 293.
Doi, Y. Makromol. Chem. RapMCommun. 1982, 3 , 635. Duck, E. W.; Grant, D.; Kronfli, E. €or. Polym. J. 1979, 15, 625. Eley, D. D.; Keir, D. A.; Rudham, R. J. Chem. SOC.,Faraday Trans. 1977, 73, 1738. Fachinetti, G.; Florianl, C.; Soeckli-Evans, H. J. Chem. Soc ., Dalton Trans. 1977, 2297. Galli, P.; Luciani, L.; Cecchln, G. Angew. Makromol. Chem. 1981, 9 4 , 63. Giannini, Y. Makromol. Chem. Suppl. 1981, 5 , 216. Kanetaka, S.;Takagi, T.; Keii, T. Kogyo Kagaku Zasshi 1984, 6 7 , 1436. Keii, T.; Takai. T.; Kanetaka, S. Shokubai 1981, 3 , 210. Muiioz-Escaiona, A.; Villalba, J. Polymer 1977, 18, 179. Nagei, E. J.; Kirillov, V. A.; Ray, W. H. Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 372. Prabhu, P.; Shindler, A,; Theii, M. H.; Gilbert, R. D. J. Polym. Sci., Polym. Lett. Ed. 1980, 18, 389. Prabhu, P.; Shlndler, A.; Theil, M. H.; Gilbert, R. D. J. Polym. Sci., Polym. Chem. Ed. 1981, 19, 523. Schnecko, G.; Dost, W.; Kern, W. Makromol. Chem. 1969, 121, 159. Schnecko, H.; Jung, K. A.; Kern, W.I n "Coordination Polymerization"; Chien, J. C. W., Ed.; Academic Press: New York, 1975; p 91. Sivaram, S. Ind. Eng. Chem. Prod. Res. Dev. 1977, 16, 121. Soga, K.; Terano, M.; Ikeda, S. Polym. Bull. 1979, 1 , 849. Soga, K.; Izumi, K.; Terano, M.; Ikeda, S. Makromol. Chem. 1980, 181, 657. Soga, K.; Terano, M. Polym. Bull. 1981, 4 , 39. Suzuki, E.; Tamura, M.; Dol, Y.; Keil, T. Makromol. Chem. 1979, 180, 2235. Wlsseroth, K. Angew. Makromol. Chem. 1989, 8 , 41. Wisseroth, K. Chem. Ztg. 1977, 101, 271.
Received for review March 29, 1982 Accepted August 17, 1982
Ethylene Addition to Lower n-Olefins with Potassium-Graphite Catalysts John B. Wllkes Chevron Research Company, Richmond, California 94802
Potassium-graphite intercalation compounds were shown to catalyze the addition of ethylene to C, to C, n-olefins to give a preponderance of linear olefinic products. Addition of ethylene to 2-pentenes with KC,-KC,, mixtures made from pure graphite gave 57% linear 3-heptenes and 37% 3-ethyI-1-pentene compared to 19% 3-heptenes and 54 % 3-ethyl-1-pentene produced at similar conditions with catalyst prepared from potassium metal dispersion. With catalysts varying from KC8 to KC24, the rate of reaction was proportional to the amount of KC, and not to the amount of potassium. Varying the reaction temperature had only a small effect on reaction rates. Olefin isomerization was catalyzed by potasslum-graphite catalysts made from graphite containing a significant amount of ash. A new method of preparing potassium-graphite intercalation compounds by reaction of graphite with molten potassium dispersions in liquid hydrocarbon is described.
Introduction The reaction of lower olefins with alkali metal catalysts gives a variety of valuable dimers and codimers. The olefinic isomers produced are usually not those which are most stable thermodynamically (Pines and Stalick, 1977). Potassium, rubidium, and cesium are the most active catalytic materials. In most studies of these reactions, the catalyst has been formed from potassium metal introduced into the reaction mixture. Potassium hydride has occasionally been used. Supported or fiiely divided potassium metal reacts with propylene to form organopotassium compounds (Wilkes, 1967a). Organopotassium compounds, rather than potassium metal or potassium hydride, are probably the actual catalysts in most of these potassiumcatalyzed oligomerization reactions of olefins. Potassium and graphite react to form intercalation compounds which are catalytically active (Boersma, 1974). Although these intercalation compounds are substantially different in structure from supported or unsupported potassium, and
may act differently as catalysts, graphite has often been considered to act only as a support in these olefin-oligomerization reactions. Propylene dimerization at 150 "C was studied by Hambling (1969) with both graphite and potassium carbonate as supports. Sodium-graphite had considerably less selectivity than sodium on potassium carbonate (which presumably was reacted to form potassium and sodium carbonate) for dimerization of propylene to 4-methyl-l-pentene, but a catalyst of potassium on potassium carbonate gave results similar to those obtained with KCs potassium-graphite catalyst. Although the KCs catalyst was only 42% as active as potassium on K2C03, only minor differences were found in the product distributions. KC8 catalysts dimerized propylene to give 4% n-hexenes, compared to 8% n-hexenes with potassium on K,C03, and the two catalysts gave similar selectivities of 79% and 75% to 4-methyl-1-pentene. KCZ4and KC60catalysts showed considerable activity for iscmerization of the 4-methyl-1-pentene to the more
0196-4321/82/1221-0585$01.25/00 1982 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982
Table I. Properties of Graphites Used for Catalysts
graphite
_____
Dixon Microfyne (DM) Dixon 1132, Amorphous (DA) Ultracarbon UCP-2-325 (UCP)
surface area (BET), m'ig
particle
19
1-6; 25-30
27
1-2
11
1-15
diam, pm ________
ash, wt Y
- .-.
____I__
2.4
0.0005
thermodynamically stable internal olefins. Hambling reported that ash content of the graphite was important in this alkene isomerization, and low ash content favored formation of 4-methyl-1-pentene. Yeo et al. (1967) claim that dimerization of propylene by KC,, where n has a value between 8 and 24, gave high yields of 4-methyl-1-pentene when the K, complex was derived from graphite containing less than 0.1% by weight of ash. With an ash content of 0.6% by weight, extensive isomerization of the double bond of the methylpentenes took place. Forni (1973) claims that the activity of potassium graphite for isomerization of these olefinic double bonds can be largely eliminated if the graphite (of an unspecified ash content) is first boiled with aqueous KOH solution and then dried before reaction with potassium metal to form KC,. Although ethylene reacts with difficulty to form oligomers in the presence of alkali metal catalysts, it codimerizes readily with propylene and other olefins which contain allylic hydrogens. Bush et al. (1965) reported the ethylation of olefins ranging from propylene to heptenes with potassium catalysts. In the present work, potassium-graphite catalysts were studied in similar ethylations under milder conditions, and the results were compared to those obtained at similar reaction conditions with catalysts formed from stabilized potassium metal dispersions. It was found that ethylation of linear olefins with potassium-graphite catalysts gives products with a much higher content of linear olefins than are obtained with dispersed potassium catalysts. A simple new method of preparing potassium-graphite catalysts was also developed in this work. Catalyst Preparation a n d Characteristics Dispersed and stabilized potassium catalysts were prepared by high-speed stirring in white mineral oil (heavy liquid petrolatum), which contained added dispersing and stabilizing agents. The flash points of white oils are typically above 190 "C. These high flash points permit safe handling of the potassium dispersions in white oil at temperatures well above the melting point of potassium metal. The viscous white oil also effectively protects these reactive catalysts from air and moisture over considerable periods of time. The techniques for preparing these catalysts have been described previously (Wilkes, 1967a,b). The potassium particles prepared in this manner are less than 10 .um in diameter. Finely divided talc or calcium carbonate was used to aid stability of the dispersions. These catalysts are referred to in this work as K-talc or K-CaC03 catalysts. Presumably, the potassium was converted to organopotassium compounds during the reactions, but this was not investigated. Two grades of commercial graphite were used for preparing the potassium-graphite catalysts. Some catalysts were also prepared from high-purity graphite powder obtained from the Ultra Carbon Corporation, Bay City, MI. Some properties of these graphites are given in Table I. The ash content of the high-purity graphite in Table I is that provided by the supplier. The graphite particle sizes
were determined by microscopic examination. The Dixon Microfyne graphite used was a mixture of graphite particles with two size distributions. Potassium-graphite intercalation compounds can be made by dry mixing potassium and graphite at high temperatures or by use of reactive solvents or catalysts (Klein et al., 1980). Dry-mixing graphite with molten potassium in a nitrogen atmosphere as described by Pines and Stalick (1977) was used in the first part of this study. It was found to be quite difficult to obtain a uniform product by stirring dry graphite with molten potassium. Furthermore, only relatively small batches of potassium-graphite could be made at one time in conventional equipment. This necessitated testing olefin codimerization with different batches of catalysts, and there was no assurance that the batches had exactly the same properties. Surprisingly, it was found that potassium-graphite intercalation compounds can readily be made by high-speed mixing of graphite with molten potassium dispersions in white oil. The preparation of the potassium dispersions requires the use of an emulsifying agent such as oleic acid, as described by Fatt and Toshima (1961). In a typical preparation, 300 g of dried graphite, 900 g of white oil, and 0.6 cmJ of oleic acid were heated to 120 OC in a 2-L flask while purging with dry nitrogen. Sixty grams of clean potassium, rinsed with hexane, was added and the mixture was stirred at high speed with a high-shear stirrer, adequate to give a fine potassium dispersion, at 120-130 OC for 20 min. The product mixture was cooled and stored for use. This procedure gave a relatively large amount of uniform, well-mixed catalyst. Potassium-graphite intercalation compounds were formed, as shown by the following observations. Microscopic examination of the product did not reveal the presence of metallic potassium. The characteristic colors of the potassium-graphite intercalation compounds appeared. This color change is especially apparent for the bronze-colored KC8 compound. X-ray analysis of a KCN intercalation compound produced by the new method showed the same lattice spacing reported for KC24in the literature, along with evidence of some unreacted graphite. The reactivity and selectivity in the ethylation reactions of olefins were the same with potassium-graphite made by conventional dry-mixing process and by this new method. Potassium and graphite form a series of intercalation compounds; KC8, KC24,K&, KC48, and KC,,,. In this work, the use of KCs gave somewhat erratic results. This was attributed to the occasional presence of free potassium metal. In most of the work, twice as much graphite was used as that necessary to give KC,. This mixture is referred to in this work as KC16and is presumably a mixture of KCd and KC24. Experimental Methods All the ethylation runs were conducted in a 300-mL magnetically stirred autoclave. The autoclave was heated externally and the temperature was controlled automatically by passage of water through internal cooling coils. This technique permitted rapid heating to reaction temperature without exceeding the desired reaction temperature. The autoclave was charged with the catalyst slurry in white oil, extra solvent, if used, and liquid olefin feed. The autoclave was sealed and purged with nitrogen. The pentenes and hexenes used in this work were Phillips Petroleum Co. pure grade olefins. Ethylene, propylene, and butenes were Matheson CP grade. These materials were used without further purification, as previous work (Wilkes, 1967b) had shown that impurities were low enough relative to the amounts of catalyst used to have
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982 587
z
+
I 6
Figure 1. Predicted products via allylic ethylation mechanism.
a negligible effect in the batch experiments. If propylene or butene were used, they were pressured into the autoclave after purging. The autoclave was heated to reaction temperature and ethylene was added after reaction temperature was reached, in most cases. The reaction was usually monitored by the pressure drop in the reactor. Additional ethylene was added during the run if the pressure fell below the desired level. In some cases, ethylene was fed a t constant pressure from a small gas reservoir, and the rate was followed by pressure drop in the reservoir. The reaction was terminated by cooling by passing water through the cooling coils in the autoclave. After venting gaseous olefins, the catalyst was usually destroyed by pressuring water into the autoclave and stirring the autoclave contents for several minutes before opening the autoclave. The products were analyzed by gas chromatography, usually by using a 30-m surface-coated open tubular (SCOT) column coated with UCON oil LB-550X followed by a 30-m SCOT column coated with SF-96 or DC-200 liquid using l-octene as internal standard. Selected samples were hydrogenated over Pdlcarbon to give the paraffins, which were more readily separated and identified. Because of the high volatility of the product mixtures, care was needed to avoid losses during analysis. With proper handling, about 95% material balances were obtained in most cases. Effects of Potassium-Graphite on Product Distribution The principal products obtained by the ethylation of olefins with catalysts made from dispersed or supported potassium metal can be predicted from the well-known allylic ethylation mechanism. With typical catalysts, reaction of a 2-pentene (1)with ethylene gives primarily the products shown in Figure 1. The predominant reaction is abstraction of a proton from the allylic methyl group to give the allylic anion, which can be represented by resonance isomers 2a and 2b. Ethylene adds to the anion mainly at the 3 position to give 3-ethyl-l-pentene (6) as the main product. Transfer of a proton from an allylic olefin to the ethylpentenyl anion to give the product and regenerate the catalyst is a required reaction step which is not shown in Figure 1. Addition of ethylene to the 1 position of the pentenyl anion gives the linear 3-heptenes (5a,b). Abstraction of a proton by the catalyst from the 4 position of 2-pentene gives the anion (3), which adds ethylene to yield the 4-methyl-2-hexenes (4a,b). The use of l-pentene instead of 2-pentene gives similar products, as abstraction of a proton from the 3 position of l-pentene gives the same anion (2a,b) as in obtained from 2-pentene. Methylhexenes (4a,b) are also obtained from l-pentene, presumably via a prior isomerization to 2-pentenes by the
catalyst. The allylic tertiary hydrogen of 3-ethyl-l-pentene (6) is unreactive, and this product is quite stable under the reaction conditions. The 4-methyl-2-hexenes (4a,b) contain allylic methyl groups so these compounds react readily with ethylene to give nonenes. The 3-heptenes are less reactive but can undergo ethylation to nonenes. The products of ethylation of other simple linear olefins can be similarly predicted by the allylic ethylation mechanism. Ethylation of propylene gives only n-pentenes, along with the products of pentene ethylation. Ethylation of n-butenes gives 3-methyl-l-pentene and n-hexenes as the initial products. Some dimerization of ethylene to n-butenes occurs at reaction conditions, and these butenes are ethylated to give traces of hexenes and octenes, which are found in the products of ethylation of propylene or n-pentenes. This is especially noticeable a t high pressures of ethylene. These trace products have not been investigated in this study. The present study shows that use of potassium-graphite catalysts in place of simple potassium catalysts has a marked effect on the types of products formed in the ethylation of C3 to C6 linear olefins. The potassium-graphite catalysts give products having a much higher content of linear olefin isomers than is found in products made with dispersed potassium catalysts. There is also a tendency for the olefinic products from potassium-graphite catalysts to contain larger amounts of products resulting from the addition of two or three ethylene molecules to the C3 to C6 olefin feeds than are found in the products made with dispersed potassium catalysts. The distribution of products obtained by ethylating n-pentenes or propylene with potassium-graphite or dispersed potassium catalysts are shown in Table 11. Propylene, l-pentene, and 2-pentene are expected to give the same type of products upon ethylation, if the same catalysts are used, and experimental results bear out this expectation. The dispersed potassium catalysts (runs A21, A13, and WHL8) all gave heptene fractions containing 13-19% normal heptenes and 54-69% of 3-ethyl-l-pentene. With potassium-graphite catalyst made from highpurity graphite (run B41), the heptene fraction contained 57% of linear 3-heptenes. The content of both 3-ethyll-pentene and of the 4-methyl-2-hexenes in the heptene fraction was considerably lower with this potassium-graphite catalyst than with the dispersed potassium catalysts. Very little isomerization of the product olefins was observed when using potassium-graphite catalyst made from graphite with very low ash content. Substantial isomerization of the products occurred when using potassiumgraphite catalysts made from commercial Dixon Microfyne graphite, which contained 2.4% ash. It appears that potassium-graphite catalysts are poor isomerization catalysts unless inorganic impurities are present. The isomerization may be due to a potassium-clay material or to some specific interaction of the inorganic compounds with the potassium-graphite catalysts. Ethylation of n-pentenes with dispersed potassium catalysts gives high yields of 3-ethyl-l-pentene, which is resistant to further ethylation. Consequently, these catalysts produce only small amounts of nonenes from npentenes. The potassium-graphite catalysts produce relatively large amounts of heptene isomers containing primary or secondary allylic hydrogens. These compounds are more reactive to further ethylation. Consequently, larger amounts of nonenes are obtained with potassiumgraphite catalysts than with the catalysts made from potassium dispersions. The potassium-graphite catalysts
588
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982
U U
.r
u
R
x
U
cl U
made from commercial grade graphite can also isomerize olefins such as 3-ethyl-1-pentene to more reactive structures, such as the 3-ethyl-2-pentenes, and these can then ethylate to nonenes under the reaction conditions. Decreasing the reaction temperature from 150 "C to 100 "C (run B1) had only a modest effect on the product distribution. A so-called amorphous graphite (run B21) was tested because of the low cost of this type of graphite. Isomerization was more extensive than with the catalyst made from other graphites, but the catalyst made from amorphous graphite still showed improved selectivity for production of n-heptenes compared to the use of the catalysts made from potassium dispersions. A limited investigation showed that the high linearity of ethylated products with potassium-graphite catalysts also is found when ethylating 2-butenes or linear 2-hexenes, as shown in Table 111. The product structures in these cases were determined by hydrogenation, and olefinic isomers were not identified. However, the ethylation mechanism predicts that the methylpentene produced from 2-butenes with the simple potassium-talc catalysts should be 3-methyl-1-pentene. This is unreactive to further ethylation. Consequently, only small amounts of octenes are formed with this catalyst. Relatively large amounts of octenes are formed upon ethylating 2-butenes with potassium-graphite catalysts. These probably are formed from the linear hexenes and from the 3-methyl2-pentenes formed by the isomerization of 3-methyl-1pentene by the potassium-graphite catalyst. The ethylation of mixed 2-hexene isomers with potassium-graphite catalyst gave octenes containing 72 % linear compounds. The lower linearity of octenes produced by diethylation of 2-butenes indicates that much of the octene made by ethylation of 2-butenes is formed by ethylation of the methylpentenes produced from the butene ethylation.
Rates of Ethylation of 2-Pentenes Ottmers and Rase (1966) determined the rate constants for the isomerization of 1-pentene to cis and trans-2pentenes over potassium-graphite catalysts and found that the rates, expressed relative to the amounts of potassium present, were 3.4 to 4.0 times higher for KC, than for KCB. The rates, based upon potassium present, were only slightly higher for KCC0than for KC&. The rates of isomerization, based upon KC,, were quite similar for KC8, KC24, KC36,and KC4@Similarly, Hambling (1969) found that the dimerization of propylene with KC%,with the rate expressed as (g of hexene)/(mol K), h, was approximately twice as fast as with KCs. In this work, it was similarly found that the rate of ethylation of 2-pentene, based on the amount of KC, used, is essentially the same for KC8 and KCZ4,as shown in Table IV. Based upon the POtassium used, the rate of ethylation ;> 2.5 times faster with KC24 than with KC8. Ottmers and Rase postulated that the potassium in the potassium-rich compounds was less effective because of steric hindrance by the adsorbed olefins. This would appear to be an acceptable explanation for these observed effects of potassium to graphite ratios on rates, but it does not account for the low energies of activation found for these isomerizations. The activation energies for isomerization of 1-pentene, as determined by Ottmers and Rase, were 2.3 to 3.1 kcal/g-mol for isomerization to cis or trans-2-pentenes for all of the potassium-graphite compounds. In the present work, the rate of ethylation of 2-pentenes between 100 and 135 "C, expressed as amount of product per gram of catalyst per hour, increased by only 70%, as shown in runs B1 and A47 (Table IV). If these rates are treated as rate constants, this rate increase corresponds to an activation
Ind. Eng. Chem. Prod.
Res. Dev., Vol. 21, No. 4, 1982
589
Table 111. Ethylation of 2-Butenes and 2-Hexenes 2-butenes
feed
catalyst " temperature, "C Dressure. atm product distributions, wt %
K-talc
KC 16( )"' 150
150
2-hexenes
KC,, (DM)
150
42-18
58
54 35 8
88
67
octenes
7 5
28 5
75 22 3
3 96
27 71
34 62
11
55 45 B4 3
43 57 A4 1
methylpentenes octene types, % linear
branched run number
135
47-24
hexenes
decenes hexene types, % linear
KC,,(DM
1
72
28 B26
" Letters after catalyst composition show type of graphite (Table I). Table IV. Rates of Ethylation of 2-Pentenes with Potassium-Graphite Catalysts variable tested K/graphite ratio temperature catalyst composition", b KC, KC,, KC% KC,, KC,, KC,, weight, g 16 15 14 15.5 15.5 15.5 preparation mode in oil in oil in oil in oil in oil in oil
pressure
KC,, 22 dry
KC,, 22 dry
feeds, g
2-pentenes white oil temperature, "C pressure, atm reaction time, h recovered olefins, g c pentenes hep tenes nonenes
ethylation rates K, h g of (C, + C,)/g of KC,, h run number g of (C, iCJg of
70 40 150
98-60
70 38
70 37
150 77-54
150
2.4
3.0
33 44 11
25 52 15
5.1 1.4
8.8 1.5
A39 " Dixon Microfyne graphite used for all catalysts. Products normalized to 100%recovery. A30
82-48 3.0
70
35 100 48-43 5.3
70
70
70
35
35
135 73-47
150 77-48 3.0
30 150
3.0
24
24
25
48
49
49
13
17
14
13.0
4.8 0.80
8.1 1.4 A47
1.5 B9
B1
25 52 15
8.8 1.5 A39
46-38 4.0
49
23 8 2.2
0.35 A34
70 30 150
99-73 1.7 25 51 16 10.9 1.8 A36
Composition shown K/graphite ratio, not compounds present.
energy of roughly 4.5 kcai. An insignificant additional increase in rate was found upon increasing the temperature from 135 to 150 "C (run A39, Table IV). In these tests of the effects of temperature on the rate, the pressure was increased with temperature in order to maintain a nearly constant concentration of ethylene in the liquid phase. At constant temperature, the reaction rate increases rapidly with pressure, as shown in Table IV. The low apparent activation energies found for these reactions are typical of the apparent activation energies of diffusion-controlled reactions. All diffusion-controlled reactions appear to be first order, regardless of their intrinsic kinetics, siqce mass transfer is a first-order process (Satterfield, 1970). The rates of ethylation of pentene-2 (Table IV) were determined by following the consumption of ethylene, and the reactions were found to be of apparent first order in pentene-2. No decrease of reaction rate was observed during any of the tests, but an occasional slight acceleration of the rate was found in some cases. This increase in reaction rate may be due to conversion of some of the potassium to catalytically more active species, such as has been observed in the dimerization of propylene to 4-methyl-1-pentene (Wilkes, 1967a,b). Although no conclusions as to the intrinsic kinetics of the ethylation reaction can be drawn from the apparent first-order kinetics which were observed, the observed kinetics demonstrate
that catalyst activity was constant during the runs, as constant catalyst activity is required to obtain the apparent first-order kinetics characteristic of diffusion-controlled reactions. The relatively high surface areas relative to particle size of the graphites used in the present work indicate that small pores may be present in the graphites. Diffusional limitations in such pores could explain the low energies of activations found with the potassium-graphite catalysts. If olefin isomerization is principally caused by the inorganic (ash) components in potassium-graphite, as seems to be the case, then the low activation energies found by Ottmers and Rase for isomerization of pentenes could be caused by diffusional limitations through pores on the access of the organic reactants to the inorganic ash components in the graphite particles. An alternative explanation of both the selectivity of potassium-graphite catalysts for linear products in the ethylation reactions and of the low activation energies observed in both the liquid-phase ethylations and in the gas-phase pen-tene isomerizations is that one or more of the olefins must insert between the graphite layers before or during the reaction. Ternary lamellar compounds containing graphite, an alkali metal, and a polar organic molecule such as tetrahydrofuran (THF) or 1,2-dimethoxyethane have been reported. Beguin et al. (1979) have
Ind. Eng. Chem. Prod. Res. Dev. 1982, 2 1 , 590-591
590
reported that graphite-alkali metal compounds react rapidly with liquid T H F at room temperature to give compounds such as KC24(THF)3.Much of the THF can be removed upon heating to 100 "C. In view of this reversible insertion of organic compounds into potassiumgraphite compounds, it seems possible that ethylene and/or the C3to C6 olefins, such as used in this work, could reversibly insert into the potassium-graphite layers before reacting. Such interlamellar insertions of olefinic molecules should be diffusion limited and would probably have preferred orientations, which would result in product selectivity such as observed in this study. Considerable additional work on the reaction of potassium-graphite compounds with olefins would be required to elucidate the nature of the organic compounds formed, if any, and to determine the causes of the high selectivity for linear products in the ethylation reactions and of the low energy of activations of ethylation and isomerization reactions
with potassium-graphite compounds. Literature Cited Beguin. F.; Setton, R.; Hamwi, A.; Touzain. P. Mater. Sci. f n g . 1979, 4 0 , 167. Boersma, M. A. M. Catal. Rev. Sci. Eng. 1974, IO, 243. Bush, W. V.; Holzman, G.; Shaw, A. W. J . Org. Chem. 1965, 3 0 , 3290. Fatt, I.; Tashima, M. "Alkali Metal Dispersions"; D. Van Nostrand Co.: Princeton, NJ, 1961; pp 38-47. Fornl, L. U.S. Patent 3758416, 1973. Hambling, J. K. Chem. Br. 1969, 5 , 354. Klein, H. F.: Gross, J.; Besenhard, J. P. Angew. Chem., Int. Ed. fngl. 1960, 19,491. Ottmers, D. M.; Rase, H. F. Ind. Eng. Chem. Fundam. 1966, 5 , 302. Pines, H; Stalich, W. M. "Base-Catalyzed Reactions of Hydrocarbons and Related Compounds"; Academic Press: New York, 1977; p 19. Satterfield, C. N. "Mass Transfer in Heterogeneous Catalysis"; M.I.T. Press: Cambridge, MA, 1970; pp 5, 18. Wilkes, J. B. J . Org. Chem. 1967a, 32, 3231. Wilkes, J. B. R o c . 7th World Pet. Congr. 1967b, 5 , 299. Yeo, A. A.; Hambling, J. K.; Alderson, G. W. U S . Patent 3325559, 1967.
Received for reuiew April 21, 1982 Accepted August 17, 1982
Contact Angle of Mercury against Catalyst Materials for Use in Intrusion Porosimetry Aifons Balker' and Alwln Relthaar Swiss Federal Institute of Technology (ETH), Department of Industrial and Engineering Chemistry, 8092 Zurich, Switzerland
The contact angle of mercury against 20 different catalyst materials was determined. An average contact angle of 129 f 6" has been found and this angle is suggested to be used for pore size distribution determination on catalyst materials by mercury intrusion.
Introduction Mercury intrusion porosimetry (Van Brake1 et al., 1981) is one of the most popular methods of determining pore size distributions in porous catalysts. The method consists essentially of measuring simultaneously the capillary pressure and the volume of mercury that has penetrated the porous sample up to that pressure. The operating equation (1)was first derived by Washburn (1921) and is a special case of the well-known Young and Laplace equation. 2 ilp = -2 cos 0
The contact angles were determined employing the method first described by Kossen and Heertjes (1965) and later modified by Schubert (1968). It consists of measuring the maximum height a drop of mercury can attain on a horizontal surface of the solid sample. The contact angle is calculated from the measured maximum height h of the drop and the porosity t of the solid material by employing eq 2
AP is the excess pressure which is required to force a liquid with the surface tension Y into cylindrical capillaries of radius r and 0 is the contact angle of wetting. As indicated by eq 1, the contact angle plays an important role for determining the pore size distribution of porous solids by mercury porosimetry. In practice a contact angle of 140" is frequently used. For some materials this contact angle may be appropriate, but for others this value may lead to erroneous results in the pore size distribution. Thus, the aim of the present investigation was to determine the contact angle which is most appropriate for use in pore size distribution measurements on catalyst materials.
with B = pg/2u, where g is the acceleration due to gravity, is the density, and Y is the surface tension of the liquid a t the temperature of measurements. For the measurements round slabs of 5 cm diameter and 0.5 cm height were compressed from powders obtained by grinding the commercial catalysts. The powders consisted of particles of less than 100 pm size, and a pressure of lo4 kPa was applied for compression of the slabs. Before measurement the slabs were outgassed at 150 "C for 6 h a t 10 Pa. The porosities of the samples were calculated from the apparent densities pa and the solid densities pa using the relation t = 1 - paf pa. The densities were de-
Experimental Section
cos 8 = -1
r
0196-4321/82/1221-0590$01.25/0
+
[
p
0 1982 American
Chemical Society
]
4 - 2h2B ' I 2 3(1 -
c)
(2)
