1400 RICHARD 11.1. ROBERTS Vol. 63 ments. As a result, the occupancy of the levels is is largely a mass effect associated with the zero essentially restricted to the zero level. Considering point energy for the motion of the adsorbed moiethat uncertainties of the order of 10% exist in this cule in the degree of freedom perpendicular to the type: of calculation, further refinement of the calcu- surface. lation does not appear justified. Comparison of the Acknowledgment.-We are indebted to Dr. W. observed heats of adsorption a t low coverage with D. Schaeffer of Godfrey L. Cabot Inc., Boston, the computed energy of the zero level shows dif- Massachusetts for graciousIy providing us with the ferences of the expected order of magnitude. sample of Graphon and the relevant X-ray data and Of greater interest, however, is the fact that the to Mr. Robert Shepard and the National Carbon computed difference in zero point energies of 60 cal. mole-' is in good agreement with the observed dif- Company, Cleveland, Ohio, for the liquid helium ference of 40 cal. mole-'. Therefore, it appears used in the ortho-para conversion of the hydrogen reasonable to conclude that the difference in the ex- and deuterium. The research work has been supperimental heats of adsorption which has been ob- ported by the Atomic Energy Commission under served between parahydrogen and orthodeuterium Contract No. AT(30-1)-824.
CATALYTIC ISOMERIZATION OF CYCLOPROPANE BY RICHARD M. ROBERTS Contributionfrom the Shell Development Co., Emeryville, Calif. Received January $6, 1959
Catalytic isomerization of cyclopropane was found t6 occur rapidly on acidic solids. At 135O and 22 sec. contact time in a flowing system, hosphotun stic acid, silicotungstic acid, silica-alumina, boria-alumina, activated bentonite, silica-zirconia-alumina, pkosphomolyfdic acid, silicomolybdic acid were active catalysts, while silica-magnesia, alumina gel, silica gel and activated carbon were inactive. On silica-zirconia-alumina the activation energy of isomerization was 19 kcal./mole. Side reactions a t high conversion on this catalyst were dimerization, disproport,ionation and hydrogen transfer. Anthracene or acridine adsorbed on silica-zirconia-alumina inhibited the isomerization, and there was evidence of preferential adsorp tion of these compounds on catalytic sites. Analysis.-The products were analyzed for cyclopropane and propylene by infrared spectrophotometry. The absorption was measured at two wave lengths, 10.35 p for propylene and 11.9 p for cyclopropane, at 150 mm. pressure in a 10 cm. cell. The presence of propane was checked if necessary at 13.3 p and 500 mm. pressure. Closures of analyses of products by this method were slightly low in some cases, probably because of side reactions (see below). Catalysts.-Each of the following solid materials wa8 heated for four hours a t 150" before being placed in the reactor. Phosphotungstic acid, H7PW~20az.6Hz0, 14-60 mesh, Silica-alumina I, 25% AlnOaj 75% SiOz, 6-14 mesh, calReaction System.-The experiments were carried out by cined 6 hours at 565O, surface area 480 m.Z/g. Silica-alumina 11, Chicago Chemical Company cracking Bowing cyclopropane at atmospheric pressure and a known rate over a fixed bed of solid catalyst. The reaction cham- catalyst. 3 X 3 mm. pellets, calcined for 3 hours at 550°, ber was a cylindrical copper tube, 1.3 cm. i.d., with a pre- surface area 456 m.Z/g., about 127, Alto3. Silicotungstic acid, H4SiW&~.7H20, 14-60 mesh. heating section, a catalyst section 6.5 t o 7.5 cm. in length, Activated bentonite, Filtrol Corporation Grade D crackand a central thermocouple well. The reaotor was maintained at constant temperature by a surrounding bath of ing catalyst. 4 X 4 mm. pellets, calcined for 8 hours a t boiling liquid. Catalyst was placed in the reactor, which 550°, surface area 240 m.2/g. Silica-zirconia-alumina, Universal Oil Products Comwas then brought to the desired temperature and purged with dry nitrogen. At the beginning of an isomerization pahy Ty e B cracking catalyst, 87.0% S O z , 9.1% Zr& experiment the gas flow was switched from nitrogen t o cyclo- 2.6% AlZ83, 3 X 3 mm. pellets, calcined fur 16 hours at 450 propane by means of a valve at the inlet of the reactor. to 50O0, surface area 409 m.2/g. Phosphomolybdic acid, state of hydration was unknown, Products were collected over successive time intervals, rneas60-150 mesh. ured volumetrically and analyzed. Silica-alumina 111 (used), American Cyanamid Company Reagents.-Cyclopropane "for anesthesia" from Ohio Chemical and Manufacturing Company was employed ; cracking catalyst, removed from a commercial fluid crackmass spectrometric analysis showed the preaence of 1% ing unit, 4 X 4 mm. pellets, calcined at 550°, surface area propane. Anthracene and acridine, Eastman Kodak Com- 75 m.2/g., about 12% A12O3. pany highest grade, were employed without further purifiBoria-alumina, 15.6% Bz03 supported on Alcoa H-40 cation. alumina gel, surface area 360 m.2/g. Silicomolybdic acid, state of hydration was unknown, 60(1) V. N. Ipatieff and tV.Huhn, Ber., 86,2014 (1903). Catalysison 150 mesh. iron also seems indicated by a report of isomerization on iron turnings Silica-magnesia, 29% MgO, 4 X 4 mm. pellets, calat 100" (V.N. Ipatieff, ibid., 86,1063 (1902)). However, the temperacined at 550", surface area 290 m.2/g. ture of this experiment is given elsewhere (V. N. Ipatieff, J . Russ. Alumina gel, Alcoa H-40 alumina gel, 6% SiOz, eurface Phys. Chem. Soo., 84, 322 (1902), and "Catalytic Reactions at High Preseures and Temperatures," The Macmillsn Co.. New York, N. Y.. area 340 m .2/g. Silica gel, a special laboratory preparation, containing p. 154) as 600°, at which temperature homogeneoue isomerization 0.001% AI, 6-14 mesh, calcined for 6 hours at 565', surwould be rapid. It therefore appears doubtful that iron is a catalyst. (2) B. Brown, Ph.D. Thesis, University of Washington, Seattle, face area 737 m."/g. Activated carbon, Union Carbide and Carbon Chemicals 1953: Dissertation Abstract@,18, 999 (1953); University Aliorofilm Corporation, Columbia activated ombon, grade 8. Public&tion No. 6404,
Catalytic isomerization of cyclopropane to propylene is known to occur on platinum,' aluminalJ and platinum supported on alumina.2 The purpose of the present article is to show that acidic solids are particularly good catalysts for this isomeiization. The reaction was found to proceed a t convenient rates in the temperature range 100 to 150" and is among the fastest hydrocarbon reactions catalyzed by acidic surfaces. Experimental Details
1401
CATALYTIC ISOMERIZATION OF CYCLOPROPANE
Sept., 1959
TABLE I CATALYTIC ISOMERIZATION OF CYCLOPROPANE AT 135O, ATMOSPHERIC PRESSURE Catalyst bulk density, doc.
Space velocity, moles cyclopropane/ I. catalyst/hr.
Phosphotungstic acid
2.21
4.6
Silicotungstic acid
2.14
4.9
Silica-alumina I
0.445
5.0
Catalyst
a
Silica-alumina, I1
0.800
5.2
Boria-alumina
0.850
5.3
Silica-zirconia-alumina
0.766
5.3
Actiwted montmorillonite
0.815
5.4
Phosphomolybdic acid
1.51
5.2
Silicomolybdic acid
1.34
5.1
Silica-aluminn, I11 (used)
0.647
5.I
Silica-magnesia
0.796
5.5
Alumina 0.701 Silica gel ... Activated carbon 0.528 By difference between input and output.
5.2 4.5h 5.3
Process per/od, min.
15 30 45 60 15 30 45 80 11
29 45 53 14 32 40 56 11 28 43 58 10 20 30 60 17 33 41 57 15 30 45 60 15 30 45 60 14 30 46 54 15 30 45 60 61 15 60
% Unconverted
3.3 2 6 2.3 2.4 6.3 5 1 4.0 3.4 15.9 12.6 10.7 9.9 6.5 9.2 9.7 10.1 21.3 38.0 46.0 50.4 33.0 47.5 54.8 64.0 48.0 49.5 49.4 48.8 64.6 71.5 75.4 77.9 80.8 84.7 86.5 88.0 94.8 92.0 91.9 91.8 91.1 94.1 95.1 95.5 99.8 100 77.5
Cyolopropane charged Converted to Retained by propylene catalysts
86.7 85 5 85.3 85.0 44.3 46.1 48.5 50.7 53.3 70.6 77.0 78.9 48.5 63.3 66.2 69.9 22.9 19.9 17.4 16.1 37.5 33.7 31.3 28.6 34.8 37.6 39.1 41.5 27.6 21.7 19.2 17.6 16.8 13.0 11.3 10.1 5.2 8.0 8.1 8.0 0.9 .9 .9 .9 0.0 0.0 0.0
10.0 11.9 12.4 12.6 49.4 48.8 47.5 45.9 30.8 16.8 12.3 11.2 45.0 27.5 24.1 20.0 55.8 42.1 36.6 33.5 29.5 18.8 13.9 7.4 17.2 12.!) 11.5 9.7 7.8 6.8 5.4 4.5 2.4 2.3 2.2 I .D
0.2 8.0 5.0 4.0 3.6 O.?
.. 22.5
147'.
Results and Discussion The equilibrium in the isomeriz,ztion reaction, cyclopropane(g) = propylene(g), lies far to the right. At 250, log K for the reaction is estin,ated to be +7.36.3 The equilibrium constant increases (3) The free energy of formation of cyclopropane was taken as +24.99 kcal./mole, cf. J . W.Linnett, J. Chem. P h w , 6 . 701 (1938), and J. W. Knowlton and F. D. Rossini, J. Research Natl. Bur. Standards, 48, 113 (1949). The free energy of propylene was taken from "Se-
lected Values of Properties of Hydrocarbons," Nat. Bur. Standards
with increasing temDerature. and log K is between 9 and IO in the temperat.urerange of the present experiments, 100 to 150'. Absence of catalysis on the reactor surface was shown by passing cyclopropane through the empty reactor at 147" and 70 sec. contact time; the conversion was less than 1%. The resu1ts Of isomerization experiments with a Circular C 481,U. 8. Government Printing Office, Washington, 1947, p. 384.
1402
RICHARD M. ROBERTS
number of solids are given in Table I. These experiments were all carried out at 135" and a space velocity of about 5 moles of cyclopropane per liter catalyst per hour, corresponding to a contact time of 22 seconds. The activity of the more active catalysts was not constant, and in four cases actually increased with time, despite a gradual increase in amount of material adsorbed by the catalyst. A large fraction of the cyclopropane was retained by some of the solids studied. I n the case of activated carbon, where no propylene was detected in the gas product, the material retained was probably physically adsorbed cyclopropane. I n other cases, when retention accompanied isomerization activity, the adsorbed material may have been largely chemisorbed cyclopropane or polymeric products. Alumina, silica and silica-magnesia are only weakly acidic14and had no activity for isomerization of cyclopropane. Activated carbon appeared to have no isomerization activity (see above). The remaining materials were acidic solids and all displayed more or less activity. Silica-alumina I11 (used), which contained about 12% A1203, had a low surface area as a result of use in a commercial catalytic cracking unit and was a comparatively poor catalyst for cyclopropane isomerization. The activity of the four unsupported heteropolyacids was remarkable. Phosphotungstic acid was the most active catalyst tested. The surface areas of the heteropolyacids were not measured. However, a sample of phosphotungstic acid similar to that tested was found by krypton adsorption to have a surface area of 4.5 m.2/g. The surface areas of all four heteropoly acids are probably low, as compared with the other solids studied, and they may therefore have much higher isomerization activities per unit surface area than the other catalysts. Cyclopropane isomerization on silica-zirconiaalumina was measured a t several space velocities a t each of the temperatures 100, 112, 124 and 135". An Arrhenius plot of the space velocities required for 20Y0 conversion to propylene, a t 1 hour process period, gave an activation energy of 19 kcal./mole. This value is to be compared with 65 kcal./mole for the homogeneous uncatalyzed reaction in the temperature range 440 to 520°.6 Data obtained a t 135" over a range of space velocities were tested for kinetics in which the isomerization rate was proportional to one of the following quantities: (1) Pcyclopropane, (2) P'cyclopropane, (3) Pcyclopropaiielpropylene. The data certainly did not conform to (l), and did not agree fully with either (2) or (3), although one would expect propylene to inhibit the reaction. The material retained by the catalysts was prob(4) H. A. Benesi, J . A m . Chem. Boc., 7 8 , 5490 (1954). The alumina
If the silica were present as "copolymer," the catalytic activity might be expected to be higher than t h a t of pure alumina. However, activity is known to increase much less rapidly on addition of silica to alumina tnan oies versa; see V. C. F. Holm and R. W. Blue, Ind. Eng. Chem., 48, 496 (1951); V. C. F. Holm, G. C. Bailsy and A. Clark, Am. Chem. Soc., Div. of Petroleum Chsm., Preprints of Genaral Papers, Vol. 2, No. 1, Marcn, 1957, page gel tested contained 6% SiO,.
334. (5) E. 6.Corner and R. N. Pease, J . A m . Chem. Soc., 67, 2071 (1945); cf. also R. H. Lindquist and G. K. Rollefaon, J . Chem. Phys., 84, 729 (1956).
Pentenes n-Pentane Hexenes Hexanes Benzene Heptenes Heptanes
Vol. 63
10 2 Remainder 3 to 10 0.02 1 to 2 0.5 to 5
'
,
Sept., 1959
CALCULATED BONDCHARACTERS IN OXAMIDE AND OTHERAMIDES
1403
cene or acridine the formation of propylene from cyclopropane was reduced to about the same extent, to 22 and 18%) respectively, of the value found with fresh catalyst, Retention of material on the catalyst was slightly increased by the presence of anthracene and slightly decreased by acridine, a$ the end of 30 minutes. Assuming an area of 70 A.2 for the anthracene or acridine molecule, the quantities of these substances adsorbed amounted to 30 and 38%, respectively, of the B.E.T. nitrogen surface area. Actually, the entire nitrogen surface area is probably not accessible to these large molecules, resulting in a non-uniform distribution on the catalyst surface. At all events, the isomerization experiments show that the activity is reduced to a greater extent than would be expected from random coverage of the surface, pointing to specificity of poisoning by anthracene and acridine. Because of the basic character of these molecules, adsorption may have occurred preferentially on the acidic regions of the catalyst surface. It is not surprising that solid acids catalyze the isomerization of cyclopropane. Cyclopropane is absorbed with cleavage by sulfuric or perchloric acids at room temperature to form propyl esters.6 Further, it has long been known that ring cleavage of alkyl cyclopropanes by halogen acids to form alkyl halides obeys the Markovnikov Rule, in that the ring is broken between the most and least substituted carbon atoms.’ Terpenoids containing the cyclopropane ring, such as carane, are also attacked by acids, with opening of the cyclopropane ring.8 A molecular orbital treatment of the cyclopropane molecule shows that the orbitals forming the carbon-carbon bonds in the cyclopropane ring may be regarded as hybrids making an angle of 22’ with the line joining adjacent carbon atoms.g The greater p character of these orbitals as compared
with sp3 hybrids gives rise in substituted cyclopropanes to conjugative effects with an adjacent carbon-carbon double bond or benzene ring.’O It therefore seems reasonable that the greater p character would also facilitate proton attack at a ring carbon atom. Cyclopropane isomerization in the present case may be imagined to occur by way of a carbonium ion intermediate
(6) C. D. Lawrence and C. F. H. Tipper, J . Chsm. SOC.,713 (1955). ( 7 ) R. A. Raphael, in “The Chemistry of Carbon Compounds,” E. H. Robb, Editor, Elsevier Publishing Co., Amsterdam and N. Y.. 1953,Vol. IIA, p. 26. (8) P.de Mayo, Perfumery and Essential Oil Rcc., 49, 238 (1958). (9) C.A. Coulson and W. E. Moffitt, Phil. Mag., 40, 1 (1949).
(IO) L. L. Innraham, Wterio Effeats in Organic Chemistry,” edited by M. S. Newman, Jonn Wiley and Sons, New York, N. Y., 1956,p. 518. (11) 0.Johnson. T H IJOURNAL, ~ 69,827 (1955). (12) H. H. Voge, G. M. Good and B. 9. Greenafelder, Ind. Eng. Chem., 88, 1034 (19461; H.N. Dunning, ibid.. 46, 551 (1953).
H2
/C\ HZC-CH1
[
/’: ]
H&+
+H+A-+
A-
+H + A-
+ CHpCH-CHS
CHs
where H+A- repreAents the solid acid catalyst. It is worthy of remark that cyclopropane isomerization is among the fastest hydrocarbon reactions catalyzed by acidic solids. It proceeds at a rate comparable to those of several reactions of olefins. Polymerization of propylene has been shown to occur at easily measurable rates on silicaalumina catalyst in the temperature range 150 to 200°.11 cis-trans isomerization and double bond shift isomerization are particularly fast reactions of olefins.12 For example, isomerization of 1-butene to 2-butenes was found to proceed at 110”on the same silica-zirconia-alumina catalyst employed in the cyclopropane isomerization experiments described above; at a space velocity of 16 moles per liter-hr. the conversion to 2-butene was 59% of the equilibrium value during the first half-hour of the experiment. The author is indebted to 0. Johnson and L. B. Ryland for providing several catalysts, to J. R. Douglass, L. W. Smith and the late S. E. Reaume for experimental assistance, and to B. S. Greensfelder and H. H. Voge for many helpful discussions.
CALCULATED BOND CHARACTERS I N OXAMIDE AND OTHER AMIDES BY E. L. WAGNER Contribution from the Department of Chemistry, State College of Washington, Pullman, Washington Received January 87. 1969
The bond characters, bond lengths and stretching force constants for the bonds in the amide grou a of oxamide and several other amides have been calculated by the usual simple L.C.A.O. molecular orbital treatment. &e results obtained are in fair agreement with the experimental values. Significant improvement of the agreement could not be obtained by using different sets of values for the Coulomb integrals than those normally employed. Molecular diagrams showing the bond orders, electron distributions and free valences of formamide, oxamide, carbamic acid, acrylamide, urea, oxamic acid, fumaric acid diamide, meso-oxalic acid diamide and beneamide are presented which are in qualitative agreement with the known and predicted properties and chemical behaviors.
Infrared spectra’ and X-ray crystallographic data2 on solid oxamide, (CONK&, indicate that, as in other amides, the CO and CN bonds possess hybrid characters not readily describable as ordi-
nary double or single bonds, respectively. Also, it is now known that amides are not satisfactorily described in terms of ionic and covalent resonance forms since the dipole moments* and the para-
(1) T. A. Scott and E. L. Wagner, J . Chem. Phye., SO, 465 (1959). (2) E. M.Ayerst and J . R. C. Duke, Acta Cryat., 7 , 588 (1954).
(3) R. G. Bates and M. E. Hobbs, J . A m . Chem. Soc., 7 8 , 2151 (1951).
