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cm2, and Y ~ ~ ~ v / Y 0.4. L ~ According to Grosse, there are apparently two types of systems to consider. It may be that the noble metals including Cu, Fe, Co, Ni, Pb, W, T1, Al, Sn and the alkali metals obey an expression similar to eq 3, while metals such as Hg, Sb, Cd, Zn, hlg, and Bi, being of the other class, where n = 2, would obey an expression of the form
(5) =
;.*(E) VL"8
The anomalous behavior of bismuth, as noted by Grosse6 may result from its being the only hexagonal or rhombohedral metal in Table I to undergo a contraction upon melting. BELLTELEPHONE LABORATORIES, INC. MURRAY HILL,NEWJERSEY 07971
Figure 1. Nitrogen adsorption isotherms at 78°K: curve A is amorphous gel, activation 11 to 400' at 50°/hr, hold for 12 hr; B is microcrystalline a-chromia, activation I to 450°, hold for 1 hr; C is amorphous gel, autoclaved at 300" for 1 hr, activation as A; D is aerogel, activation as A. Weights after activation: A and B, 34.15 mg; C , 11.92 mg; H. SCHONHORN and D, 4.39 mg.
RECEIVED JULY3, 1967
The Texture of Chromium Oxide Catalysts Sir: Catalytic activity of chromium oxide gel for isotopic exchange between deuterium and alkanes and for hydrogenation of olefins develops after activation to about 275' and increases with increasing temperatures of activation. The activation process must generate coordinatively unsaturated chromium ions on the surface. A study of the effects of activation upon chemisorption offered the prospect of measuring the number of catalytically active sites and of learning more about the chemical nature of these sites. Using a gas chromatographic technique, Dr. J. F. Read found that development by activation of the capacity to adsorb oxygen and carbon monoxide a t room temperatures roughly paralleled that of catalytic activity. Adsorption of several other gases, including ammonia and hydrogen sulfide, did not. I n further studies of adsorption, we have used a Cahn recording vacuum microbalance working in a stream of flowing helium purified by diffusion through heated quartz. Results of a study of the physical adsorption of nitrogen at 78°K are of importance to the use and understanding of chromia as a catalyst. Heating chromium oxide gel prepared by ureal to about 400" in flowing hydrogen (activation I) leads to microcrystalline a-chromia, as judged by X-ray diffraction. If, at 300", hydrogen is replaced by helium (activation 11) or nitrogen, the material remains amorphous to 450" or higher. Perhaps hydrogen forms a few ions of Cr(I1) which labilize the structure by their coordinative lability. The Journal of Physkal Chemistry
The Langmuir-like adsorption isotherm on the amorphous gel by urea' is given in Figure 1-4. Gel prepared by addition of ammonia t o a solution of chromium nitrate behaves similarly. Almost all of the surface is associated with micropores which are completely filled below a relative pressure of 0.4. Figure 2A is the de Boer and Lippens t plot2 of these data. Here, t is the average thickness of the adsorbed nitrogen layer a t a particular relative pressure, PIPo,taken from a master equation, which is valid for a number of adsorbents of open textures at values of PIPQbetween 0.0s and 0.75. The slope of the initial section of the t plot appears to pass through the origin, and it gives a surface area of 231 m2/g. If a monolayer is taken as formed at PIP0 = 0.09,2 73% of the porosity is associated with pores which can hold only one monolayer on each face. After the micropores are full, the residual surface is very small; there are few macropores. The adsorption of propane at -78" fits the Langmuir isotherm well between P/PQof 0.03 and 0.6. The area is 250 m2/g if the molecular area of propane is 26 b2. Adsorptiondesorption equilibrium was slow with nitrogen and propane. Similar adsorption isotherms are shown by many charcoals and a few aluminas and silica^.^ Microcrystalline a-chromia exhibits a type IV isotherm with hysteresis loop, Figure 1B. The slope of the initial linear region (Figure 2B) gives a surface area (1) R. L. Burwell, Jr., A. B. Littlewood, XI. Cardew, G. Pass, and C. T. H. Stoddart, J. A m . Chem. Soe., 82, 6272 (1960). (2) J . H. de Boer, B. C. Lippens, B. G . Linsen, J . C. P. Broekhoff, A. van den Heuvel, and T. H. Osinga, J . Colloid Sei., 21, 405 (1966); references are given here to earlier work of this group. (3) K. S. W Sing, Chem. Ind. (London), 829 (1967) ; G. C. Bye and K. S. W. Sing, ibid., 1139 (1967); these papers give a particular interpretation of adsorption in micropores and its relation to the t plot.
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row micropores may contribute to such differences in catalytic behavior as the ratio of isomerization to hydrogenation of 1-hexene at 68" being very small with gel A (activation 11) but 5 with gel B, microcrystalline a-chromia (activation I).
Acknowledgment. This work was supported by the Air Force Office of Scientific Research and the Petroleum Research Fund of the American Chemical Society. DEPARTMENT OF CHEMISTRY NORTHWESTERN UNIVERSITY ILLINOIS 60201 EVANSTON,
'
t4inF
8
10
12
I4
Figure 2. t plots of nitrogen adsorption data: A and B are from Figure 1; curve E is for a gel in which water was replaced by pentane before drying. The data on the Y axis have been multiplied by l / g for curve D and by for curve E.
ROBERTL. BURWELL, JR. KATHLEEN C. TAYLOR GARYL. HALLER
RECEIVED AUGUST7, 1967
Secondary Valence Force Catalysis. V. Salt Effects on Certain Detergent-Catalyzed Organic Reactions
of 82 m2/g. The average pore diameter is about twice the value of t, beyond which the slope of the t plot becomes small, i.e., about 23 8. Propane isotherms at -78" are of similar shape and they lead to the same porosity, 0.11-0.12 cc/g. Amorphous gels with a more open texture were made in several ways. Autoclaving the gel in liquid water a t 300" before drying gave isotherm C in Figure 1. A small hysteresis loop is not shown. Since the t plot is linear to about 8 A, with a very small slope beyond that, the pore diameter is 16 A. The pore water in a freshly precipitated gel can be replaced by pentane via washing with ethanol or tetrahydrofuran and then with pentane. Removal of the pentane in vacuo at 25" gave isotherm E, which resembled C, but had a surface area of 300 vs. 486 m2/g. The lower surface tension of pentane us. water leads to less collapse of the initial gel structure during solvent removal. If the pentane is flashed off after heating the gel to above the critical temperature of pentane, an aerogel results. Its isotherm, D, was type I1 and, like A, exhibited no hysteresis loop on desorption. Upon pelleting, crushing, and sieving the aerogel powder to 60-80 mesh (the size used in the other samples), an isotherm like A resulted. The surface area fell from 447 to 350 m2/g. The aerogel contains both micropores and macropores. The latter are largely eliminated during peIleting. I n addition to their usual limitation of mass transport, the narrow micropores of the amorphous gel A may lead to special catalytic behavior and, possibly, to reactions involving opposite walls of a pore. The nar-
Sir: A number of investigations of the influence of micelle-forming detergents on the rates of organic reactions have established that such rates are frequently sensitive functions of the nature and concentration of the micelle-forming species.' Qualitatively, most results can be rationalized on the basis of hydrophobic and electrostatic interactions between detergents and substrates serving to alter the concentrations of reactants in the micellar phase relative to those in the bulk phase. Thus, for example, the sodium lauryl sulfate catalysis for hydrolysis of methyl o-benzoate can be accounted for by assuming that the negative charges on the micellar surface tend to concentrate the hydrated proton in the vicinity of the substrate which, in turn, is incorporated into the micellar phase as a consequence of hydrophobic interactions. Id Studies, thus far, have concentrated on establishing the existence and scope for detergent catalysis of organic reactions, but have dealt only superficially with questions regarding parameters which affect these reactions and with the details of the reaction processes. We now wish to report certain striking salt effects on the tetradecyl(1) Leading references include: (a) E. F. J. Duynstee and E. Grunwald, J. Am. Chem. Soc., 81, 4540, 4542 (1959); (b) R. L. Reeves and L. K. J. Tong, ibid., 84,2050 (1962); ( c ) M.T . A. Behme and E. H. Cordes, ibid., 87,260(1965); (d) M.T. A. Behme, J. G. Fullington, R. Noel, and E. H. Cordes, ibid., 87, 266 (1965); (e) J. L.Kurs, J. Phys. Chem., 66, 2239 (1962); (f) D.G.Herries, W. Bishop, and F. M. Richards, %bid.,68, 1842 (1964); ( 9 ) L. K. J. Tong, R. L. Reeves, and R. W. Andrus, ibid., 69, 2357 (1965); (h) L. J. Winters and E. Grunwald. J . Am. Chem. SOC.,87, 4608 (1965); (i) A. SolanoOchoa, G. Romero, and G. Gitler, Science, 156, 1243 (1967); (j) T . C. Bruice, J. Katshendler, and L. R. Fedor, J . Phya. Chem., 71, 1961 (1967); (k) M.B. Lowe and J. N. Phillips, ivature, 190, 262 (1961).
Volume 71. Number 1.9 December 1967
