68
P. E. EBERLY, JR.
Vol. 65
The second model used by Barron also leads to a series expansion similar to (5) but contains fewer terms since fewer moments were evaluated. This series is y
-
ym
-0.62~’
(6)
There is a notable difference (6) and (8) in that the term linear in y is missing from equation 6. This arises because two of the moment coefficients, r(2), ymlbwere taken by Barron to be equal and this makes the term in y vanish. y is computed from u by means of the relation given below.” A comparison between (5) and (6) with the’? experimental data, is shown in Fig. 2 (curves 1 and 2, respectively). The variation in y - y m predicted by either model is smaller than that observed 0 0.2 0.4 0.6 0.8 experimentally. However, the maximum values of y-ym for the nearest neighbor model has been Y. Fig. 2.-Comparison of theoretical and experimental greatly exceeded by the experimental values. This Griineisen parameters: 1, nearest neighbor model; 2, discrepancy cannot be remedied by considering sodium chloride model. the effects of more neighbors or by adjusting the values of the parameters’s within reasonable limits. It has been pointed out recently by Marruddin, The NaCl type model seems to be the more proWeiss and Sacklo that the expansion of Cv in a mising one since the curvature of the theoretical similar series which converges for large values of u can be made by an application of Euler’s trans- curve qualitatively appears to be about correct. It would seem worthwhile to compute some of the f~rmation.’~In view of this result, Euler’s transformation has been applied to the series of equation higher moments for the NaCl structure so that a more reliable value of the high temperature limit, 4 giving the series y m , could be estimated as well as furnishing more y - y m = -~(0.177 0.107~ 0 . 0 6 9 ~ ~0 . 0 4 5 ~ ~ )(5) terms for the series.7,’8~1g*20 where y = u2/(1 u2). This new variable is a It is a pleasure to acknowledge the help and enconvenient one. It is employed in the subsequent couragement offered by E. N. Lassettre of this dediscussion of the data. partment.
+
+
+
+
(16) A. A. Marruddin, G. H. Weiss and R. Sade, Bull. Am. Phys. SOC. in Detroit, 1960.
(17) Bromwich, “Theory of Infinite Series,” The Macmillan Co., New York, N. Y.,1947,pp. 62. 2nd ed.
(18) G. Liebfried and W. Brendig, Z. Phyaik, 134, 451 (1953). (19) E. W. Kellerman, Phil. Trans., A.235,613 (1940). (20) E. W.Kellerman, Proc. Roy. SOC.(London). 1188, 17 (1941).
HIGH TEMPERATURE ADSORPTION STUDIES ON 13X MOLECULAR SIEVE AND OTHER POROUS SOLIDS BY PULSE FLOW TECHNIQUES’ BYP. E. EBERLY, JR. Esso Research Laboratories, Esso Standard, Division of Humble Oil & Refining Co., Baton Rouge, Louisiana Received June 0, 1960
The adsorptive properties of solids a t high temperatures are conveniently studied by allowing a pulse of adsorbate to be transported through a packed column of adsorbent by an inert carrier gas stream. The concentration of adsorbate in the effluent stream is continuously measured by a sensitive thermal conductivity cell. Modifications of such a technique permit the detection of an adsorption process and enable one to determine its degree of reversibility. In addition, heats of.adsorption can be determined by measuring the pulse retention times a t a series of temperatures. This flow method is particularly useful for studying adsorption at high temperature conditions where static methods cannot be used because of the long contact times involved which lead to decomposition of the adsorbates. In this manner, it has been found that materials such as 13X molecular sieve, silica gel, platinum on alumina and alumina itself adsorb significant quantities of benzene at temperatures as high as 427’. The adsorption capacity of 13X molecular sieve (0.632 mmole/g. a t 91 mm.) at these conditions.is markedly greater than those of the other adsorbents. With the exception of platinum on alumina, the adsorption is readily reversible and the heats of adsorption are typical of those ordinarily associated with a physical adsorption process. The heats of adsorption of benzene on 13X molecular sieve, silica gel and 7-aiumina were determined to be 15.5,Q.Sand 6.8 kcal./ mole, respectively, in the range of 260-454’.
I. Introduction I n the petroleum and allied industries, many chemical reactions and separation processes involving hydrocarbons frequently are conducted in the (1) Presented a t the Southwest ACS Regional Meeting, Capitol Home, Baton Rouge, Louisiana, December 3,4 and 6. 1959.
presence of solid materials a t elevated temperatures. Adsorption measurements under such conditions become of interest in elucidating the kinetics and mechanism of the reaction. Also, they are important in determining the various processes which must be taken into account in describing the mass
Jan., 1961
ADSORPTIVE PROPERTIES OF SOLIDS AT HIGHTEMPERATURES
transfer kinetics in gas-solid systems such as b e d bed or fluidized-solids processes. Up to the present time, however, very little information has been published on high temperature adsorption of hydrocarbons. Most of the previous experiments were conducted in static systems which limited the study to those lower temperature regions where decompclsition did not occur. Hara and Ikebe,2 however, were successful in studying the adsorption of propane on a silica-alumina catalyst up to 400'. In a series of papers, Emmett and his co-worker~~-~ reported additional studies on the adsorption of hydrocarbons on the silica-alumina catalyst. To minimize decomposition in their high temperature work, they used a flow system in which the hydrocarbon vapor was transported through a packed column of catalyst. After a steady-state had been reached, they disconnected the column and measured its weight from which they were able t o calculate the amount adsorbed. The present report describes a gas-solid chromatographic technique for studying high temperature adsorption. In such a flow system, the contact time of the hydrocarbon vapor with the solid surface can be made quite short, thereby minimizing or even eliminating decomposition reactions. Using a pulse injection technique, an adsorption process can be detected easily. Also, data on the heat of adsorption as well as the adsorption equilibrium constant can be obtained readily. Results are presented on the study of benzene adsorption on 13X molecular sieve, silica gel, alumina and platinum on alumina reforming catalyst in the temperature range of 260454'.
II. Experimental Apparatus.-The a paratus used for the pulse injection experiments was sim!!ar in basic design to that of a conventional gas chromatograph. Helium, the carrier gas, was allowed to flow in senes through a &port Perkin-Elmer injection valve, the packed column, and a Perkin-Elmer thermal conductivity cell. The helium flow was accurate1 meaaured by observing the movement of a soap film througg a graduated buret. The packed columns were constructed of standard 1/4t' aluminum tubing (i.d. = 0.18 in.). The admrbenta were pellethed and ground to a 250 to 300 p particle size to avoid excessive pressure drops. The packed columna were held at the desired temperature by means of a fluidized sand-bath, which was similar to that described by Adams, Gernand and Kimberlina and consisted of a bed of finely divided SiOrA1*Oacatalyet (40 to 150 p ) fluidized by a stream of hot air. The bath was heated electrically and controlled to a temperature variation of no more than f2' at the 427' level. The conductivity cell was obtained from the Perkin-Elmer Corporation and had a fast time constant of 0.5 second. It was housed in an oil-bath at an arbitrarily selected temperature of 55 f 0.05'. A Minneapolis-Honeywell recorder having a pen speed of one second for full-scale deflwtion wm connected to the output of the conductivity cell by a conventional bridge circuit. Pulaes, having a strictly reproducible volume, were injected into the helium stream by means of the &port, Perkin-Elmer injection valve. This valve is the type used
69
in their Model 154-C vapor fractometer. The pulse volume was approximately 0.5 cc. In our experiments, the injected pulses consisted of a mixture of about 8 mole % of the hydrocarbon in a non-adsorbable gas such as argon or helium. Detection of Irreversible Adsorption.-The pulse technique can be used to detect the occurrence of irreversible adsorption provided the process occurs at such a rate .that a measurable number of molecules are adsorbed within the contact time allotted. For these experiments, a pulse consisting of a mixture of helium and the hydrocarbon. is injected into the helium stream. Thus, the conductinty cell will respond only to the presence of the hydrocarbon. The area of the effluentpulse is directly proportional to the amount of hydrocarbon issuing from the column. If the pulse area obtained when flowin through the packed bed is less than that with a completefy empty column, irreversible adsorption has occurred, With our particular system, the disappearance of 0,001 mmole of adsorbate from the helium stream could be detected easily. Using 20 g. of solid, this allowed the measurement of 0.00005 mmole of irreversible adsorption per gram of solid. Those adsorptions, however, which occur at a Slow rate would have been beyond the limits of detection of our apparatus. The residence time in a 25 cc. packed bed was estimated to be on the order of 10 seconds at a flow rate of 50 CC./@. In this report, the only system that exhibited any irreversible adsorption w a that of benzene on platinum-alumina reforming catalyst. For the purposes of this investigation, the amount of irreversible adsorption was arbitrarily deb e d as the material which could not be eluted from the column within an hour's time using a flow rate of 50 e$./&. Detection of Reversible Adsorption.-A slightly mfferent technique is used to detect a reversible adsorption process. In this case, a pulse consisting of a mixture of argon and the hydrocarbon is injected into the helium stream. The conductivity cell will now respond to either argon (a nonadsorbable gas) or the hydrocarbon. If only one &uent pulse is recorded, i t is evident that most of the hydrocarbon molecules traveled through the column at the same rate as argon and, hence, no adsorption occurred. If, on the other hand, the hydrocarbon molecules are retarded in their passage by adsorption on the surface, two peak6 will result the first being that of argon and the second that of the h drocarbon. In this manner, reversible adsorption can-be otserved for systems exhibiting only a very small adsorption capacity. It has been found's8 that the movement of'the maximum of a pulse through a packed column obeys the relation
L 1 t,U. = F where
L = length of packed column in cm. t, e
= retention time of pulse maximum in seconds - nu erficial linem gas velocity in cm./sec., &.e., veloc-
g y that would result if column were completely empty 5' = adsomtion eauilibrium constant J no. of molecules ads./cm. o =. a t equilibrium no. of molecules in gas/cm.' of gas
E uation 1 is important because it. relatea the chromatogra&c data to the adsorption equilibrium constant 8'. Ths constant is directly proportional to the slope of the adsorption isotherm and is a true constant only for those systems having linear adsorption isotherms. In the case of Langmuir isotherms, i t progressively decreases in value up to a monolayer surface coveraqe. Since the partial pressures normally involved in gas-sohd chromatography are quite small, the value of 5' aa calculated from equation 1 should be expected to correspond to the limiting Slope of the isotherm in the low pressure region. Eberly and Spenceld have shown that this is essentially the case. As previously reported,o.10 the constant 8' can be treated
(2, N. Hara and M. Ikebe, J . Chem. Sac. Japan, Ind. Chem. Sect., 66, 120 (1853). (3) R. C. Zabor and P. H. Emmett, J . Am. Cham. SOC.,13, 5639 (1961). (4) D.8. MacIver, P. H. Emmett and H. S. Frank, THISJOURNAL, (7) A. J. P. Ma& and R. L. M. Synge, Biochem. J . (London), 36, 1368 (1941). 68, 936 (1958). (8) P. E. Eberly. Jr., and E. H. Spencer, in procesa of publication. (5) D. 9. MacIver, R. C. Zabor and P. H. Emmett, ibid., 63, 484 (9) A. J. M. I(edem8na. "Gam Chromatography." Reinhold Publ. (1969). Corn., New York. N. Y.,1957. (6)C. E. Adam% M. 0. Carnand and C. N. Eimberlin, Jr.. 2nd. &no. (10) 8. A. QCWM d E.PUB&T m JOVBXAL, (Lp, 66 (lQWsl. C b r . 46, 2458 (lQ54).
70
P. E. EBERLY, JR.
240 bh 1220 pi g 200 v
6 u
180
3-160 Q
8 140 4 120
5 100 0
2 3
80 60
L- 40 20 I I I 0 0.2 0.4 0.6 0.8 1.0 P/PLl. fig. 1.--Nitrogen adsor tion isotherms at 195.8'. The symbols have the folyowing significance: 0 , 13X molecular sieve; 0, silica gel; V, alumina; 0 , alumina calcined a t 980°F. for 16 hours; and A, 0.6% Pt on alumina. The dark, solid symbols represent desorption points.
-
4,000 3,000
21000
I
-
I
,
I
I
Vol. 65
This method is of great value because it represents about the on1 way in which heats of adsorption can be convenient$ measured at high temperatures. The heats of adsorption obtained in this manner are believed to be average values over the range of partial pressures involved. Since the partial pressures of hydrocarbon in the present work were on the order of 1 mm. or less, the heats of adsorption should correspond to those at very low surface coverage. Green and Pust,lO working a t lower temperatures, found good agreement between the heats of adsorption evaluated in this way and those obtained calorimetrically and isosterically. Materials.-The silica gel and 0.6% Pt on alumina catalvst were obtained from the Davison Chemical Co. The "alumina was supplied by the National Aluminate Corporation and consisted essentially of the 7-crystalline phase. A portion of this alumina was calcined a t 980" for a period of 16 hours. X-Ray diffraction patterns indicated that the calcined alumina contained approximately 20% of the a-Alz03 crystalline phase. The 13X molecular sieve was obtained from Linde Air Products Co. in powder form. The solids existing in powder form were first pelleted and then ground to obtain material in the 250-300 fi particle size range. Nitrogen adsorption isotherms for these materials a t -195.8' are shown in Fig. 1. Pore volumes were determined by measuring the amount of nitrogen adsorbed a t saturation pressure. These are listed in Table I together with the calculated B.E.T. surface arem. The benzene used in this investigation was the reagent grade available from J. T. Baker Chemical Co. The nhexane was the research grade hydrocarbon supplied by Phillips Petroleum Company. Both of these were used without any further purification. The gases were commercially available. The purities of the helium and argon were 99.99 and 99.995%, respectively. These gases were used as such and no attempt was made for further purification.
111. Results Table I1 lists the data obtained with the five different columns of adsorbent. I n each case, helium gas was flushed through the column for a period of 16 hours a t 427' before the experimental measurements were taken. The amount of material adsorbed a t the indicated partial pressures was determined by a continuous flow method reported previously. l1 No decomposition was observed in TABLE I ADSORBENT PROPERTIES Adsorbent
1.5 1.6 1.7 1.8 1.9 2.0 I/T ( " ~ - 1 ) x 103. Fig. 2.-Pulse flow retention times as a function of tempaature. All experiments were made with helium as the carrier gas floq;ing at a rate of 50 cc. (STP)/min. The adsorbents havlng particle size of 250-300 p were packed into 0.18 in. i.d. tubing. The symbols have the following significance: m, benzene on 8.7 cm. long column of 13X; 0 , n-hexane on 8.7 cm. long column of 13X; 0, benzene on 122 cm. long column of silica gel;. 0 , benzene on 152 cm. long column of the calcined alumna; A, benzene on 152 cm. long column of 7-alumina. as a thermodynamic equilibrium constant and by making the appropriate substitution, equation (2) can be obtained. 1.3
1.4
log tM(cor.) = C
AH 1 -2.303B F
(2)
where tu(cor .) represents the observed retention time corrected to 25' arid atmospheric pressure in the manner described by Green and Pust.'O The constant C is a function of the entropy of adsorption, the dimensions of the column and the carrier gas flow rate. If these factors are kept constant, then a plot of the logarithm of the corrected retention time against the reciprocal of the absolute temperature 1/T should yield a straight line the slope of which as proportional to the heat of adsorption.
B.E.T. area, m.l/g.
Pore volume, cc./g.
13X molecular sieve" 760 0.32 Silica gel 649 .38 Alumina 192 .39 Calcined aluminab 69 .37 139 .42 0.6% Pt on alumina 0 Data were calculated from the isotherm by use of the B.E.T. equation for narrow capillaries (S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 60, 309 (1938)). Heated to 980" for 16 hours.
any of these runs. This was indicated by a lack of irregularity in the shape of the effluent peak. A41so, in the independent measurements in which a continuous flow of hydrocarbon was passed through the column, no decomposition products were observed in a sample of the effluent vapor collected in a liquid nitrogen trap. With all the solids except the 0.6% Pt on alumina, two distinct and separate peaks were observed. The first corresponded to that of the unrtdsorbed argon and the second to that of the hydrocarbon. The time difference between the two peaks is listed (11) P. E.
Eberly, Jr.,
and C. N. Kimberlin, Jrr. Trans. Faraday SOC.
Jan., 1961
ADSORPTIVE PROPERTIES OF SOLIDSAT HIGHTEMPERATURES
71
TABLE I1 CHROMATOGRAPHIC DATAWITH VARIOUSADSORBENT COLUMNS AT 427"" Adsorbent b
13X molecular sieve 13X molecular sieve Silica gel Alumina Calcined aluminae 0.6% Pt on alumina
Column length, c cm.
8.7 8.7 122.0 152.0 152.0 152.0
Difference in retention time of argon and hydrocarbon peak, tm in sec.
Adsorbate
Benzene n-Hexane Benzene Benzene Benzene Benzene
Amount adsorbed, mmoles/g.d
186 25 39 29.4
0.632 at 91 mm.
17
0.007 a t 62 mm.
..........
0.053a t 62 mm.
...........
No hydrocarbon peak obsd.
Rev. 0.0421 at 92 mm. Irr. 0.02 J b The particle size of the adsorbent was between 250 0 The carrier gas was helium flowing a t a rate of 50 cc. (STP)/min. The amount adsorbed was deterto 300 p. e Columns were constructed of aluminum tubing having an i.d. = 0.18 in. mined from independent experiments using continuous flow methods aB described by Eberly and Kimberlin.11 Heated to 980' for 16 hours.
in the fourth column. The hydrocarbon retention clearly illustrates the existence of a reversible type of adsorption at this comparatively high temperature. The 13X inolecular sieve exhibits an extraordinarily large adsorptive capacity. A 186 second time difference was observed with a column only 8.7 cm. in length. With both silica gel and the aluminas, much longer columns had to be constructed in order to get a detectable time difference between the argon and hydrocarbon pulses. The effect of calcination on the alumina is illustrated by its extremely short retention time and its low capacity for benzene. The platinum--alumina reforming catalyst was the only material that showed a highly irreversible adsorption of benzene. I n fact, no effluent pulse was detected even up to one hour's time with a helium flow rate of 50 cc. (STP)/min. Since the amount of this irreversible adsorption was too large to measure conveniently by the pulse flow technique, independent experiments were made using the continuous flow method reported previously. l1 With a stream containing 91 mm. partial pressure of benzene, the total amount of material adsorbed was found to be 0.062 mmole/g. Approximately two-thirds of this amount was adsorbed reversibly and was eluted easily from the column. The remaining 0.02 mmole/g. was held very tightly to the catalyst surface and represented the extent of irreversible adsorption. This accounts for the absence of an effluent peak in the pulse experiments. When the temperature of the columns was reduced to successively lower values, it was observed that the retention times of the hydrocarbon pulse increased progres,sively whereas those of the argon pulse remained essentially constant. The retention times corrected in the previously described manner were found to obey the predicted relationship and increased logarithmically with the reciprocal of the absolute tempera1 ure. The data are shown in Fig. 2 and the cdculated heats of adsorption are given in Table 111. The calculated heats are representative of those normally associated with a physical adsorption process. Benzene on silica gel gave a value of 9.8 kcal./mole which compares favorably with the 8.5-12.0 kcal. measured previously a t much lower temperatures. The 7-alumina gave a value which was about 3 kcal /mole lower than that obtained with silica gel. I t is also of interest t o note the
TABLE I11 HEATSOF ADSORPTION DETERMINED FROM PULSE RETENTION TIMES Adsorbent
Adsorbate
AH of adsorption, kcal./mole This work Other work
Benzene 9.8 8.5-1Zb Silica gel Benzene 6.8 . ..... Alumina Benzene 3.4 ..... Calcined alumina" Benzene 15.5 16.8c 13X molecular sieve n-Hexane 10.8 . . .. . . 13X molecular sieve a Calcined at 980" for 16 hours. b A. A. Isirikyan and Akad. Sci. U.S.S.R., Sect. Phys. Chem., A. V. Kiselev, PTOC. 115, 473 (1957). " R . M. Berger, F. W. Bultitude and J. W. Sutherland, Trans. Faraday Soc., 53, 1111 (1957).
decrease in energy of the alumina surface produced by the calcination process. The highest value of the heat of adsorption of benzene was obtained with the 13X molecular sieve. This is in good agreement with the 16.8 kcal./mole obtained by Barrer and his co-workers. For comparative purposes, the heat of adsorption of n-hexane also was determined and found to be about 5 kcal./mole less than that for benzene. Due to the occurrence of irreversible adsorption, no attempt was made to conduct similar experiments on the platinum-alumina reforming catalyst'.
IV. Discussion The techniques described in this paper are particularly valuable for the detection of adsorption processes a t high temperatures. The low contact' times afforded by the flow method permit the study of adsorption without the complicating effects of decomposition frequently encountered in static systems. The method, however, can only detect those processes which occur a t a sufficiently rapid rate so that a measurable number of molecules are adsorbed within the allotted contact time. The dependence of the pulse retention time upon temperature permits the evaluation of the heat of adsorption. This quantity would be extremely difficult to measure a t these conditions by calorimetric or isosteric methods. The information available from the pulse flow experiment complements that from the previously reported continuous flow method so that a rather complete picture of high temperature adsorption can be obtained. The existence of a reversible adsorption of this order of magnitude a t high temperatures has not been greatly appreciated by previous investigators
72
JAMES A. HAPPE
in the field. This is due probably to the lack of adequate experimental facilities for measuring adsorption under these conditions. The heats of adsorption determined in the present report are similar to those normally associated with a physical adsorption process. It then becomes interesting to observe that this adsorption occurs at temperatures well in excess of the critical temperatures of benzene and n-hexane which are 288.5 and 234.8’, respectively. Similar phenomena, however, have been reported with other systems although not to the degree illustrated in the present paper. For instance, Morris and Maass12 and Edwards and Maassla found that propylene and dimethyl ether are appreciably adsorbed on aluminaat temperatures 10 to 15’ in excess of the critical temperature. In fact, even multimolecular adsorption was observed. The unusual nature of the 13X molecular sieve is illustrated by the fact that this solid adsorbs roughly six times the amount of benzene than does the more conventional adsorbent, silica gel. It also exhibited the highest heat of adsorption. Similar effects were recognized previously in moderate temperature regions, and it is interesting to observe their existence even at high temperatures. The platinum-alumina catalyst was the only material examined which exhibited an irreversible adsorption of benzene. This effect must be closely associated with its catalytic properties.
Vol. 65
In a companion report,” reversible adsorption of hydrocarbons has been shown to occur on a silicaalumina cracking catalyst and molybdena-alumina reforming catalyst at temperatures as high as 427’. This type of adsorption undoubtedly plays a very important part in mass transfer kinetics of fluidizedsolids reactors such as those used in catalytic cracking of pet,roleum fractions. In order to determine the rate constants of adsorption, Eberly and Spencer* solved the mathematics of the pulse process. From an analysis of the shape of the effluent pulse, the magnitude of these rate constants can be determined. Solid adsorbents should also be useful for analytical purposes. Recently, much work has been reported on the use of gas-liquid partition chromatography for the analysis of high boiling mixtures. The main difficulty encountered in operating columns at high temperature has been either the gradual volatilization and/or decomposition of the substrates. It appears that this difficulty could be overcome by the use of solid adsorbents as packing. These are very stable and probably could be used for indefinite periods of time. Previously, the main objection to the use of solids has been that they give in general asymmetric peaks. The peaks that we have observed are not sufficiently severely asymmetric to exclude solid adsorbents as suitable column packing. Perhaps, more attention should be given to the use of gas-solid chromatography, particularly for the high temperature analyses.
(12) H. E. Morris and 0. Maass, Can. J . Res., 9, 240 (1933). (13) J . Ednards and 0. Maass, ibid., 13B, 133 (1935).
DOUBLE RESONANCE STUDY OF PYRROLE AND OF THE PYRROLEPYRIDINE INTERACTION BYJAMES A. HAPPE’ Chemistry Division, Research Department, U.S. Naval Ordnance Test Station, China Lake, California Received June 9. 1960
The self-association of pyrrole has been studied by nuclear magnetic resonance using the double resonance method to overcome interference from the nitrogen quadrupole. The association between pyridine and pyrrole has been studied by the same technique. Tbe pyrrole-pyrrole interaction appears to be of the “r-donor” type while pyrrole and pyridine exhibit an “n-donor” interaction. For the dimerization of pyrrole the equilibrium constant K was 4.3, while for the pyrrole-pyridine association reaction measurements of K a t four temperatures gave AH0 = -4.3 kcal./mole and AS0 = -8.0 cal./mole. Chemical shifts were obtained for the associated protons in both hydrogen bonded complexes.
Introduction
1
Intermolecular association in pyrrole has been demonstrated by the infrared studies of Fuson, et a1.,2who pointed out the presence of an “associated band” a t 2.92 p . An estimate of the over-all extent of the association also was reported as a function of pyrrole concentration in CCla. Calorimetric studies by Vinogradov and LinnelP supported the self-association of pyrrole. In neither case was the nature of the association clear, however, nor could the results be interpreted in terms of 8 simple monomer-dimer equilibrium. B The latter authors included a quantitative study of the (1) Chemistry Division, Lawrence Radiation Laboratory, University of California, Livermore, California. (2) (a) N. Fuson and M. L. Joden, J . Chem. Phya., 8 0 , 1043 (1952); (b) N. Fuson, M. L. Josien, R. L. Powell and E. Utterback, ibid., 80, 145 (1952). (3) 9. N. Vinogradov and
R. H. Linnell, W., 98, 98
(1066).
.
association reaction between pyridine and pyrrole. Some uncertainty existed, however, regarding the compatibility of their calorimetric and infrared results. Nuclear magnetic resonance studies on the association of pyrrole with various donor solvents have been reported previo~sly.~JThese reports gave the effect of association on the chemical shift for pyrrole a and O-ring protons. Also the n.m.r. signals for these protons were reported to separate by about 0.2 p.p.m. on dilution with an inert solvent. The NH proton resonance signal is broadened beyond detection by quadrupole interaction with the nitrogen nucleus. In view of the above it seemed desirable to carry (4) L. W. Reevw. Can. J. Chem.. 81, 1351 (1957); M. Freymann and R. Freymann, Compt. rend., 248,677 (1959). ( 6 ) T. Sohader and W. Q. Sahneider, J . Clum. PA#.., 8% 1224 (1960).
