CATALYTIC ACTIVITYANI) SIKTERING OF PLATINUM BLACK
April, 1968
841
ture range. The calorimetric lines include AH by Humphrey, Todd, Coughlin, and King'" and entropy and enthalpy data of Kelley.8~0 For temperatures of 1700-2000°K, their data for either a or p may be represented by the equation
AFOf = ARzss - 11,850 4- 8.887' Of 1he dissociation pressure determinations, those of Davie, Anthrop, and Searcy'l and of Grieveson and Alcock12 agree well with the present results while those of Drowart, de Maria, and Inghram'3 and of Vidale'd differ by about 2.5 kcal. in opposite directions. Our results are shown as individual points and by an average line whose equation is
Sic(@; AFO
=
-27,400
4-8.887'
The limits of error are not readily assessed. The accuracy of Method I depends upon the activity coefficient of Si in Fe in which the uncertainty is about 3 ~ 0 . 8kcal. The solubilities reported here are more dependable than those of Chipman, et al.,4from which they differ by 3y0. Recent solubility data of Kirkwood and Chipman16 in Pb and of d'Entremont and Chipman' in d g are in better agreement, particularly the latter. Method I11 depends upon the heat of formation of SiOz, probably good to f0.3 kcal. a t 298' but less precise a t high temperatures, and upon interpolation to obtain T*. The latter is confirmed within 8' by the work of Kay and Taylor's and the over-all result of this method is about as good as that of Method I. The accuracy of Me1,hod I1 is inferior to either of the others in that it involves both of the subsidiary data and ysl appears in the "/Z power. In view of the agreement among independent methods, it seems unlikely that the proposed equation for AFo can be in error by as much as 1 kcal. The derived heat of formation AH298 =
-1
AFo K cal. -I
-121
A ' d/
d'Entrernont
+
Kay and Taylor SiOp+2SiC(p)=3Si(inFe)+ 2 C O
0 sio2+ 3 c = S I C @ )
-14
I
t co
I
1
J
(10) G. L. Humphrey, S. B. Todd, J. P. Coughlin, and E. G. King, U. S. Bureau of Mines, Report of Investigations 4888, 1952. ( I 1) S.G. Dayis, D. P'. Anthrop, and A . IT, Searcy, J . Ciwm. P h y s . , 34,659 (1961). (12) P. Grieveson and C. B. Alcock, "Special Ceramics," Hayw-ood & Co., Ltd., London, 1961, p. 183-208. (13) J. Drowart, G. de Mrwia, and 31. G. Inghram. J. C h e m . Phys., 89,1015 (1958).
(14) G. 12. Vidale, General Electric Co., Missile & Space Vehicle Dept., T I S Report R60 SO333 (1960). (15) D. H. Kirkwood and J. Chipman, J. P h y s . C h e m . , 65, 1082 (1961). (16) D. A. R. Kay and J. Taylor, Trans. P a ~ a d a ySoc., 66, 1372 (1960).
CATALYTIC ACTIVITY AKD SINTERISG OF PLATINUM BLACK. I. ICIXETICS OF PROPAXE CRACKISGl BY D. V. MCKEE General Electric Research Laboratory, Scheneclady, Xew York Received September 26, 1962 The activity of unsupported platinum black for the catalytic cracking of propane has been studied between 100 and 200" by a static volumetric technique. The results indicated that the specific activity of platinum (activity per unit area of catalyst surface) was considerably less than that of nickel for this reaction. However, the activity of platinum black was very dependent on the degree of sintering, a process which occurred readilv with the reduced metal a t temperatures above 100". The specific activity of the metal decreased rapidly as sintering proceeded, indicating that the sintering process involved reduction in both surface area and density of surface sites (vacancies or point defects) which were responsible for the catalytic activity. The ratio of ethane:methsne in the products of cracking of propane was generally greater for platinum than for nickel. The activation energy for the cracking reaction was found to be 24 kcal./mole but this value decreased with increasing density of active sites.
Introduction Platinum catalysts for use in gas phase hydrocarbon reactions are generally employed in the supported state, the essentially inert support providing mechanical (1) This work was made possible by the S u p p O t t of the Advanced Research Projects Agency (Order Number 247-61) through the United States Army Engineer Research and Development Laboratories, Ft. Belvoir, Virginia, under Contract Kumber DA-44-009-EKG-4853.
strength and inhibiting the sintering of the finely divided metal. For certain purposes, however, it is necessary to use the metal in the unsupported powdered form. For example, in the preparation of electrodes for fuel Cells, the nletal particles must be in good eleCtrical contact and possess a high surface area; hence platinum black is widely used as an electrode material2
D. W. MCKEE
832 20
' 16
I
I
"
I
I
A
I
I 150-C I
-
UNREDUCED ME$
i\
Y
22DT
-
4t
0
I
I
1
2
I 3
I
I
I
I
4
5
6
7
I 6
TIME ( HOURS ).
Fig. 1.-Sintering
of platinum black: surface area (m.S/g., us. time (hr.).
in these devices. For this reason and because of the lack of available information on the change of catalytic activity of platinum black as a function of sintering, it was decided to study this effect for the cracking of propane at elevated temperatures. I t was hoped to compare the results of this work with those of previous investigations of the kinetics of propane cracking on unsupported nickel3*and on evaporated rhodium films prepared under ultra-high v a c u ~ r n . ~ b Although it has been known for many years4 that hydrocarbons crack on metals a t elevated teniperatures, there have been few detailed studies of these reactions. Nost of the literature on catalytic cracking has been devoted to the behavior of acidic materials such as aluminosilicates, which are commercially more important than metals for this purpose. However, the cracking of hydrocarbons on metallic catalysts is important in fuel cell technology as methane is inert electrochemically except a t high temperatures and extensive cracking of hydrocarbon fuels on the electrodes could limit the performance of hydrocarbon fuel cells above 100'. Experimental Apparatus.-The static volumetric technique used in this m*ork has been described previously." Samples of platinum black were sealed into the apparatus and protected from mercury vapor by means of gold foil traps. Small plugs of glass wool prevented he platinum black blowing of the catalyst during evacuation. 'I was found to be initially covered with a layer of chemisorbed oxygen and this was removed by reduction with hydrogen, obtained by diffusion through a palladium thimble. Considerable difficulty was experienced, however, in maintaining the high surface area of the catalyst during the reduction, as the heat liberated during the removal of the chemisorbed oxygen was often sufficient to cause appreciable sintering and consequent loss in area of the metal, as is discussed below. The most favorable procedure was found to consist in initially cooling the metal ( 2 ) E. J. Cairns, D. L. Donglas, and L. W.Niedraoh, B.1.Ch.E. J . , 7, 551 (1961). (3) (a) D. W. McKee, J . Am. Chem. Soc., 84, 4427 (1962); (h) R. IT. Roberts, J . Phgs. Chem., 66, 1742 (1962). (4) E.g., F. E. Fres and D. F. Smith, Ind. Eng. Chem.. 20, 948 (1928).
Vol. 67
t o 0' in a bath of melting ice and carrying out the reduction slowly with about 10% hydrogen in a stream of nitrogen at 15-20 cm. pressure. After reduction a t 0" for 30 min., the metal vias then allowed to warm up to room temperature and the reduction continued for a further 30 min. Finally the reduced metal was evacuated a t 100" to a residual pressure less than 1 X 10-6 mm. before the catalytic measurements were carried out. Even with this technique it was found that a t least 10% of the initial surface area w m lost during the reduction. The adsorption and craching of propane was studied by allowing a mettsured quantity of propane to stand over the metal for varying periods of time, changes in gas pressure being followed by means of a wide bore mercury manometer t o m-itliin 1 0 . 0 1 mm. The temperature of the catalyst was kept constant during runs by means of a Honeywell controller and furnace to within f0.5". Analysis of the products of reaction were carried out on condensed portions of the gas phase by means of an F & M temperature programmed gas chromatograph, using an 8 ft. silica gel column and helium as carrier gas. I n most of the cracking experiments, the initial pressure of the propane in the storage part of the apparatus was adjusted to about li .5 mm. After expansion into the catalyst bulb, the initial pressure over the metal was about 14.6 mm. in a total gas volume of 117.7 ml. These values, however, depended somewhat on conditions. Although liberation of methane by catalytic cracking generally resulted in a rise in gas pressure over the metal, an initial rapid chemisorption of propane occurred in every case, so that the pressure fell to a minimum value during the first fev minutes of contact. After generally one hour contact the final gas pressure was read, a known fraction of the gas phase condensed into sample bulbs cooled in liquid nitrogen, and the composition of the gas phase was calculated from the gas chromatographic analysis. !Ihe residual non-condensable pressure after cooling in the traps was found to be due only to methane, no hydrogen being detected in any of the measurements. The change in surface area of the platinum black was followed by nitrogen adsorption at -195" and application of the B.E.T. method, the generally accepted value of 16.2 A.2 being used for the cross-sectional area of the nitrogen molecule in the surface area calculations. Surface area determinations and cracking activity measurements were often carried out alternately, but oR-ing to the fact that the cracking reaction generally resulted in the formation of a non-volatile carbonaceous deposit on the metal surface, which was only removed by prolonged reduction at elevated temperatures, fresh samples were generally used for each catalytic run. n'itrogen and physically adsorbed propane could, however, be removed reversibly by evacuation at room temperature and hence the sintering process could be followed by alternate heating periods and nitrogen adsorption measurements. The carbonaceous residue mentioned above could be removed by reduction for 2 hours at temperatures above 150", but this treatment generally resulted in a large decrease in surface area of the black. Materials.-The platinum black used in this work was obtained from Fisher Co. and was of high purity (>99.8%). The unreduced metal had a B.E.T. nitrogen surface area of 19.5 m.%/g. Examination by electron microscopy revealed that the black consisted of roughly spherical particles, probably microcrystallites, of approximately 100 A. diameter. Phillips "Research Grade" 99.987, pure propane was used throughout, the gas being condensed and fractionally distilled from a trap cooled in liquid nitrogen before being admitted into the apparatus.
Results Sintering of Platinum Black.-The loss of surface area of the pIatinum black as a function of time a t different temperatures is shown in Fig. 1, the surface area being measured by nitrogen adsorption as described above. Owing to the rapid and unavoidable decrease in surface area during the reduction process, it was difficult to obtain reproducible results or to examine quantitatively the kinetics of the sintering process. However the metal lost up to 50% of its surface area during the first 30 min. of heating a t the sintering temperature, this rapid decrease being followed by a more gradual loss in area Tvhich continued for a con-
April, 1963
CATALYTIC ACTIVITY ASD SINTERING OF PLATINUM BLACK
siderable period. Ehen after sintering at 200' for over 8 hours, the metal showed a residual porosity and surface area of over 6 m.2/g. Similar characteristics are shown during the sintering of other materials, such as magnesium oxide.s Figure 1 also illustrates the effect of an adsorbeld film of oxygen on the sintering process, the unreduced metal remaining unchanged in surface area after heating for over 8 hours a t 150°, whereas the reduced metal showed a decrease of over 40% during the same period. It is probable that cheinisorbed layers of other substances, for example hydrocarbons, would. also tend to inhibit the sintering process. Measurements of sintering below 150' were difficult owing to the fact that temperatures of at least 100O were necessary to remove chemisorbed hydrogen after the reduction plrocess. For this reason maxima were sometimes found in the curves of surface area v8. time for temperatures below 100'. These results are not shown in Fig. 1. Electron micrographs were also used to follow the changes which took place during sintering. The particles in the orig;inal unsintered black were quite uniform, roug)ly spherical in shape and of approximately 100 A. in diameter. The particles readily aggregated into loose clusters in which, however, the individual particles remained discernable. After sintering a t 200' for four hours, the particles had fused together into irregular amorphous conglomerates which still retained some porosity. This coalescence of the metallic particles was attended by a contraction and increase in density of the catalyst mass. Cracking of Propane on Platinum Black.-The products of the cracking of propane on platinum black between 100 and 200" were found to be methane, small amounts of ethane and a carbonaceous residue of variable composition (CH,). which remained on the solid surface. No hydrogen, olefins or higher paraffins were detected in the gaseous products. Identical products were obtained from the cracking of propane on evaporated rhodium films a t As with nickel,sa the surface residues remaining after reaction tended to poison the metal. 'I'hese residues were not desorbed by evacuation a t the reaction temperature but reduction with hydrogen a,t, 150' and above for several hours was necessary to regenerate the activity of the metal. However, such drastic treatment usually resulted in substantial sintering of the metal and, for this reason, a fresh sample was generally used for each catalytic run. At the beginning of this investigation it became apparent that the catalytic activity of the metal depended t o a large extent on the degree of sintering which had taken place. If this involved merely the reduction in surface area without decrease in the density of catalytically active sites, then the specific activity of the metal (activity/surface area) should be a constant throughout the sintering process. Figure 2 illustrates the resuIts of a number of measurements of platinum black samples with various degrees of sintering, the BET nitrogen surface area being determined before each catalytic run. The catalytic activity of the metal was determined by measuring the amount of methane formed after one hour coiitact with propane a t the same initial pressure. As the rate of methane evolution was found to be approximately linear with respect to ( 5 ) 8. J. Gregg, (1953,
R. K. Piokrr, and X. H. Wheatley. J. Chem. SOC.,46
X
-0 -4 P
843
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6-
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e
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B
5
3-
2-
I
0
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4
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8
I 1 IO 12 SURFACE AREA (metre'lg.1,
14
16
Fig. 2.-Change of catalytic activity of Pt black during sintering: initial C& pressure = 14.6 mm.; 1 hr. contact time. 9,
I
I
I
I
I
I
TEMPERATURE 'C.
Fig. 3.-Gaseous products from cracking of propane on sintered Pt black (surface area 4 m.2/g.): initial CjHs pressure = 14.6 m.: 1 hr. contact time.
time for at least one hour a t the beginning of the reaction, the specific activity was then calculated in molecules CHI set.-' cm.-2. The total amount of methane produced was always less than 30% of the propane present, so that an excess of the latter remained in contact with the metal at all times. The results of Fig. 2 indicate that, far from being constant, the specific activity decreases rapidly with loss of surface area, the rate of decrease increasing with increasing temperature. Although some additional sintering may have taken place during the catalytic run, this effect is likely to have been reduced by the presence of the film of adsorbed propane. However, if the measured nitrogen
844
D. W. MCKEE
surface area values are slightly high, the true values would tend to accentuate the effect shown in Fig. 2. Sinal1 amounts of ethane were also produced during the cracking reaction, as shown in Fig. 3, which gives the average yields of the two gaseous products as a function of temperature for sintered samples of approximate area 6 m.”g. The volume of ethane produced, although small, reached a maximum at about 170’. The yield of methane increased rapidly above 150’ and from the rate of formation of methane with temperature the apparent activation energy was calculated to be 24 f l kcal./mole and the frequency factor A (in molecules sec. cm. +) was given by log A = 20.5 i 0.3. The liberation of methane was always observed to be preceded by adsorption of propane on the metal surface and it was possible to measure the extent of this adsorption at temperatures too low for cracking to be appreciable. At 0’ a platinum black sample, having a B.E.T. nitrogen surface area of 16.7 m.2/g., required 1.47 ml. STP/g. of propane to cover the surface with a monolayer (calculated from the BET plot for the propane adsorption isotherm), whereas a sintered sample with 6.8 m.2/g. surface area adsorbed 0.6 ml. STP/g. propane in the monolayer. The ratios of nitrogen surface area to propane monolayer values are identical in the two cases, indicating that no “molecular sieve” effect was responsible for excluding propane from the pores of the sintered sample, the loss in catalytic activity being entirely due to the elimination of active sites. The adsorption of propane at 0’ was physical in nature and entirely reversible, the effective crosssectional area f! the propane molecule? in the nionocompared with 16.2 for nitrogen. layer being 41 This valuz is reasoiiable in comparison with the value of 27.4 A 2 which is frequently shown by propane physically adsorbed a t its boiling point of -42°.6 Discussion Mechanism of Sintering of Platinum Black.-The sintering of a finely divided metal results in a decrease in the surface free energy and a coilsequent loss in the area of the solid-gas interface. This process is of great importance in ceramics and pon-der metallurgy and the subject has an extensive literature.’ The material transport involved during sintering caii take place by several different mechanisms, for example, (i) evaporation and condensation as a result of differences in vapor pressures over the curved solid surfaces, (ii) viscous flow and plastic deformation at points of contact, and (iii) surface diffusion and migration of vacancies. In the present case, mechanisms (i) and (ii) seem unlikely owing to the low vapor pressure and high melting point of platinum, however the physical properties of platinum in fine dispersion may be quite different from those of the bulk metal. Plastic flow is not generally considered appreciable at least until the Taininaiiii temperature (0.5Tln,where TI,,is the irieltiiig poiiit of the solid) is reached but migration of defects can take place at much lower temperatures. Thus lattice vacancies could migrate away from the contact area between two spherical particles and be discharged a t the surface or at grain boundaries (if (6) D. W. McKee, J Phgs. C h e m , 63, 1236 (1959) 17) E.g., W. D. Kingery, “Introduction to Ceramics,” John Kiley and
Vola 67
these exist in the microcrystallites of the Pt black). As a result a net transfer of material to the contact zone would occur and growth of necks betmeen particles take place. Calculations based on this models yield the relation
x r
-
-
hr-”/” t V 6
where x is the neck radius, and r is the radius of the spherical particle. N o exact formulation of this process is possible as the relation between the increase in neck radius and decrease in surface area is not known and, in any case, the variation of sintering rate with particle size and temperature for real systems of irregular particles would be too complex to be represented by this equation. However, the data of Fig. 1 were found to give straight lines of constant slope when log s was plotted against log t, for times greater than 15 min. This result suggests that the sintering process follows the relation, s = Kt-O~ll,where K is a function of teinperature. The initial rapid decrease in surface area can be attributed to the large amount of heat liberated during the reduction process. The heat evolved during the reaction PtO
+ H&)
-+
Pt
+ H20(ads.)
has recently been determined to be 42 kcal./mole of H2.9 This heat, unless dissipated rapidly, would be capable of producing high local temperatures at asperities and points of contact of the metal particles, even when the catalyst mass is cooled to temperatures as low as 0’. It may be possible t o reduce this initial sintering further by using lower concentrations of hydrogen in the carrier gas stream. Sintering and Catalytic Activity.-The marked dependence of catalytic activity on extent of sintering, shown in Fig. 2, is consistent with the idea that the sintering process involves the migration and elimination of surface defects which may be responsible for the activity of the metal in the cracking reaction. Such an effect has rarely been reported before. In a recent authoritative work on catalysis,’O the author states, “No successful attempt has been made to demonstrate an inherent change in catalytic activity per atom or per unit weight with particle size, as might be expected if very small particles had properties significantly different from those of the massive metal. Such effects as have been observed are adequately interpreted in terms of the varying surface area.” Previous failure to observe this effect may have been due to the fact that catalysts are usually chosen which have been annealed to a stable state before use. However, it has been found possible to induce catalytic “superactivity” in copper and nickel wires by flashing a t high temperature.ll It is suggested that the enhanced activity is due to the production of a high concentratioii of vacancies by the flashing procedure. Assuming that a similar process operates in the case of platinum black, ( 8 ) R L.Coble, J A m Ceram. Soc , 41, 55 (1958) (9) H Chon, R. A. Fisher, E. Momezsko, and J. G. Aston, “Proe. 2nd. International Congress on Catalys~s,”Paris 19fi0, paper 3 G C Bond, “Catalybis by hletals,” 4cadeniic I’loss, NOMY o l k , N Y , 1962, p. 31. (11) M. J Duel1 and A. J B Robeltson, T i a n s . Fnrndnq SOL.,67, 1410 (1961).
’10)
CATALYTIC ACTIVITY ASD SISTERIR'G OF PLATINUM BLACK
April, 1963
the rate of the cracking reaction will be related to tlie acth-ation energy for the disappearance of active sites and to the true activation energy for the cracking reaction. Hence, for a first-order reaction, the rate of methane formation in molecules cm.-2 sec.-l is given by
where c, is the density of active sites a t T'K., €0 is the activation energy of the cracking reaction a t the absolute zero, and the other terms have their usual sigiiificance.12 If the nuniber of active sites per cnx2 decreases with increasing temperature as a result of sintering according to the relation
where E D is the activakion energy for the elimination of active sites, than t h e apparent rate of the cracking reaction is
Although the rate lam for the disappearance of active sites is not known the over-simplified equation above suggests that the removal of active sites should give rise to an increase in the apparent activation energy for the cracking reaction. This effect was in fact observed, as shown in Fig. 4, the value of E varying by a factor of almost two between the fresh and completely sintered catalyst. Comparison of Propane Cracking on Nickel and Platinum Catalysts.--A comparison can be made between the rates of propane cracking on platinum black and on nickel previously reported.3" Although the measurements with nickel were carried out with a lower initial pressure of propane (5.6 mm., compared with 14.6 mm. in the present work), the rate of cracking on nickel was considerably greater than that 011 platinum, even for unsintered samples. Thus the rate constant for methane formation on Xi a t 156' was 1.27 X 10I1 niolecules se(:.-l cm.-2, compared with 2.5 X 10'0 molecules sec.-I for the maximum rate obtainable on Pt black. This generalization may not, however, apply over different temperature ranges. Comparing the nickel results with those of sintered Pt black in Fig. 3, it is apparent that cracking of propane became appreciable at lower temperatures on Si than on Pt, methane being present in about 2% concentration in the gas phase at 60' v i t h S i , but only a t above 100' with Pt. The kinetic parameters for the two metals are compared in Table I. TABLE J 2 1 td ~
N1
Pt
L'LIIII)
inngL "c'
130-1 h3 150-200
1; ( k ~ a /li n u I ~ )
log A (molecules scc -1 cm.-2)
1 5 32 1 24 f I
205&0S
18YztO3
The amount of ethane produced during cracking is also different with the two metals, the yield being somewhat greater for platinum than for nickel. Tlic (12) S Glasstone K. Laidler, and H. Eynng, "The Theory of R a t e Processes " MoGra~-H111Book Co., Nen Tork, N P , 1941, Chap VI1
r
i
3
I 4
I
545 I
I
I
I 9
I IO
II
1
16 4
I 5
I 6
I
I
7
8
I
SURFACE AREA (metre2/g ) ,
Fig. 4.--Change
of activation energy for propane cracking during sintering.
maximum concentration of ethane in the gas phase occurred with Ni a t about 130°, the rate of forniation of this product then being 0.37 X 1O1O molecules sec.-l cm.-2. By comparison, the maximum yield of ethane occurred with Pt at 170°, the rate then being 0.66 X 1O1O molecules sec.-l cm.-2. Little information can be obtained concerning the detailed steps involved in the breakup of the propane molecule, although it is probable that the reaction proceeds in a similar way for both nickel and platinum. It seems plausible that the hydrocarbon is initially chemisorbed on the metal surface, almost a monolayer being formed a t the lower temperatures before cracking becomes appreciable. Dissociation of the adsorbed hydrocarbon gives rise to mono- and di-carbon fragments and chemisorbed hydrogen atoms which migrate along the surface to hydrogenate the surface radicals to methane and ethane which are desorbed. The coiicentration of ethane in the gas phase may be limited by cracking a t elevated temperatures. According to the early work of Frey and the cracking of propane on platinum gives a higher proportion of hydrogen in the products above 400' than does nickel. Although no hydrogen was observed in the gaseous products in the present work, the amount of chemisorbed hydrogen on the metal surface may be greater for Pt than for Ni, which might tend to increase the ethane concentration in the former case. The activation energies shown in Table I above may be compared with the value of 34 kcal./mole found for the hydrocracking of propane on a supported nickel catalyst by Morikawa, Trenner, and Taylor.13 It is possible that the effect of increasing coiicentrations of hydrogen in the adsorbed layer is t o increase the actiration energy for tlie dissociation of propane. Howemr, in view of tlie different conditions under which these values were obtained it would be uiiwise to attach too much significance to this result. It has been suggested in this work that the activity of platinum black is influenced by the concentration of defects or active sites on the metal surface. However, (13) K. Rloiikana, N. R Tienner, and H. 6. Taylor, .7. B m Chem Soc 59, 1103 (1937).
,
S46
PETER J. BERKELEY, JR.,AND MELVINTV. HANXA
electronic considerations must also be responsible to some extent for the observed differences between nickel and platinum for this reaction. Both of these metals belong to group VI113 and therefore possess partially filled d-bands, but it is not possible a t this stage to
Vol. 67
assess the relative importance of morphological and electronic factors in determining the catalytic acti7-ity. Acknowledgments.-The author wishes to thank Dr. E. J. Cairns and W. T. Grubb for many helpful discussions during the course of this investigation.
S.N.R. STUDIES OF HYDROGEN BOXDING. I. BISARY MIXTURES OF CHLOROFORRI ASD SITROGEN BASES BY PETER J. BERKELEY, JR.,ATD RIELVIN W. HANNA Department of Chemistry, University of Colorado, Boulder, Colorado Received October 6 , 1968
A discussion is given of the theory of the dilution-shift technique for examining 1: 1 hydrogen-bonded complexes by n.m.r. It is shown that a t infinite dilution of the proton donor in an acceptor solvent, the limiting slope and the intercept (the limiting shift) of a plot of donor concentration vs. shift of the donated proton is sufficient to determine uniquely the equilibrium constant for hydrogen-bond formation and the shift of the pure hydrogenbonded species. A method for taking solvent effects into account is included. Data for chloroform plus pyridine, ethylideneisopropylimine, acetonitrile, and E-methylpyrrolidine are reported. These data are analyzed by the above theory and the importance of solvent effects is evaluated. A qualitative discussion is given of the calculated equilibrium constants and shifts upon complex formation.
Introduction
It is now well known that the position of a hydrogenbonded proton in an n.m.r. spectrum is shifted downfield with respect to the position of a non-liydrogenbonded proton.2 This shift has become known as the hydrogen-bond shift. In recent years an abundance of experimental information has appeared on many different systems in an attempt to correlate the hydrogenbond shift with hydrogen-bond strength, infrared stretching frequencies, and other physical properties of the hydrogen-bonds3 There has been considerable variation in the means of interpreting this data, however, with the result that it is difficult to get a clear picture of the relation between n.m.r. hydrogen-bond shifts and properties of the hydrogen-bond itself. This recent growth of the literature has also introduced certain nomenclature ambiguities. The situation is aggravated by the fact that tm-o distinct types of dilution-shift experiments may be carried out. In the case of hydroxylic materials, one compound can act as both proton donor and acceptor, and here it is usual to observe the shift of the hydroxyl proton as such a compound is diluted with inert solvent. This paper concerns itself, however, with the situation where a proton donor is diluted with acceptor and the shift of the donated proton observed. It is common to utilize, in both cases, the shift at infinite dilution of the acid-base in the first case, or of the acid in the second case. These two kinds of infinite dilution shifts, however, have quite a different significance. Unless otherwise stated, all that follows will be concerned with the second case. A further nomenclature difficulty involves the meaning of the phrase “hydrogen-bond shift.” I n both kinds of experiment mentioned above, a distinction should be made between the observed dilution shift and a (1) Supported in part by Grant GM 091&7-01S1 from the National Institutes of Health, Public Health Service. (2) See for instance: J. A. Pople, W. G. Schneider, and H. J. Bernsteina “High Resolution Nuclear Magnetic Resonance,” Chapter 15, hIcGraw Hill Book Co., Ken York. N. Y., 1959. (3) G . C. Pimentel and A. L. McClellan, “The Hydrogen Bond,” W.H. Freeman and Co., San Francisco, Calif., 1960. Chapter 4.
-
quantity which can be calculated from this observed shift, the hydrogen-bond shift of pure dimer. I n this paper the former quantity will be distinguished from the latter by being called the observed H-bond shift. It is the purpose of this paper to present a simplified approach that is applicable to the study of 1: 1 hydrogen-bonded complexes by n.m.r. and to point out the significance of ‘[observedH-bond shifts” as a criterion of hydrogen-bond strength. Specifically, it will be shown that for the case of a 1: 1 complex the data relating the chemical shift of the donated proton to apparent concentration of this proton are sufficient to determine uniquely the association constant of the complex without resorting to iterative procedures, to curve fifitting, or to information from other sources (such as infrared). This theory is then applied to data for the association of chloroform with pyridine, acetonitrile, et hylideneisopropylimine, and N-methylpyrroljdine. Theory Consider the equilibrium
A
+D
AD
where A is the proton acceptor and D, the donor. If the usual4 assumption is made that the system is an ideal mixture of species (after the above equilibrium has been taken into account), concentrations can be substituted for activities and the equilibrium expression takes the form
(1)
where the X’s are the mole fractions, the No’s are the initial number of moles, and X is the moles of complex a t equilibrium. Applying equation 1 to cases where only an average spectral line is observed for the hydrogeii(4) J. E. Hildehrand and R. L. Scott, “The Solubility of Non-electrolytes,” 3rd Edition, Chapter XI, Reinhold Publ. Corp., New York, N. P.,
1950.
