3080
JOHNG. LARSON AND W. KEITHHALL
This indicates that the anion-cation bonding in the organic phase is of little importance in determination of the relative anion extractability. Rather, it appears that those anions which are the best proton acceptors extract most poorly because of their greater solvation
in the aqueous phase. Diamond, et a1.,18have suggested this explanation in their discussion of the selectivity of anion-exchange resin. (18) B. Chu, D. C. Whitney, and R. M. Diamond, J . ITLOTQ. Nucl. chem., 24,1405 (1962).
Studies of the Hydrogen Held by Solids. VII. The Exchange of the Hydroxyl Groups of Alumina and Silica-Alumina Catalysts with Deuterated Methane
by John G. Larson and W. Keith Hall MelZon Institute, P&8burgh, Pennsglvanda
(Received March 31, 1966)
The exchange of CD4 (CH4) with the OH (OD) groups of silica-alumina and alumina catalysts was studied and the mixing of isotopes between CH, and CD, was investigated. Over alumina, mixing took place at a readily measurable rate at room temperature with an activation energy of 5.7 kcal./mole. It was found that the reaction was catalyzed by a small number (-5 X 1012/cm.2)of active sites and involved exchange with only about 1%of the catalyst hydroxyl groups. The surface density of active sites was measured by selective poisoning with COZ, NO, HzO, C2H4, and CaHe. A primary isotope effect was found for the exchange of CHI with Dz compared with CD4 with Hz, indicating that the ratedetermining step involved the dissociation of a CH or a CD bond. The rate of the mixing reaction was limited by the slowest step, i.e., the rupture of a CD bond. Over silica-alumina, the activation energy for mixing waa 33.4 kcal./mole; the reaction did not take place at a measurable rate below about 450". As the surface hydroxyl groups are mobile at this temperature, all of the catalyst hydrogen was available for exchange. Therefore, the catalyst hydrogen content could be measured by exchange with CD4. Such results were in good agreement with those resulting from other methods. It was shown that isotopic mixing between CD4 and CH4 was a stepwise process, proceeding through exchange with catalyst hydroxyl groups, and that there was a primary isotope effect between exchange of CH4 with OD and CD4 with OH.
Introduction In 1948,Parravano, Hammel, and Taylor1 reported that CD4 and CH, could be equilibrated over a silicaThey cited the cracking pearance of CD4 as evidence that the G D bond was The Journal of Phyeieol Chsmktry
ruptured during chemisorption and suggested that this Step is a necessrtry Prerequisite to catalytic Cracking. A thorough Study W a s not made; the data were suf-
.
(1) G. Parravano, E. Hammel, and H. S Taylor, J . Am. Chem. Soc., 70,2269 (1948).
EXCHANGE OF HYDROXYL GROUPSOF ALUMINAWITH DEUTERATED METHANE
ficient to outline only the gross aspects of the kinetics. It was not possible to decide, for example, whether the equilibration of a CD4-CH4 mixture involved a bimolecular interaction or an exchange with the catalyst hydroxyl groups. While exchange between deuterated catalysts and a number of hydrocarbons has been examined, it appears that no further studies have been made with CH4. It was deemed worthwile, therefore, to extend our studies of the interactions between paraffin hydrocarbons and acidic surfaces to include this molecule. Experiments were made in which CD4 (CH4) was circulated over the catalyst; the exchange reaction with catalyst OH (OD) groups was continually monitored vicl a capillary leak into a mass spectrometer. The mixing of isotopes between CH4 and CD4was also studied.
Experimental Catalysts. The silica-alumha catalyst was from a batch of Houdry M 4 6 used previously2in related studies with isobutane. It contained about 12.5% M203and was found to have a surface area of 270 mS2/g. The high punty silica gel sample had a surface area of 565 m.2/g.; spark spectral analysis indicated a total metallic impurity of less than 10 p.p.m. The alumina sample was prepared from the neutral hydrolysis of aluminum isopropoxide. Its surface area was 158 m.Z/g. and its total metallic impurity level was less than 50 p.p.m. Further information concerning its properties is detailed elsewhere.S Catalyst Pretreatment. Oxygen at 100 torr was circulated over the catalyst at 515" for 16 hr. A liquid nitrogen cooled trap in the circulating loop collected any HzO or COz formed. When a deuterated catalyst was required, the catalyst was then repeatedly exchanged with Dz gas at 515" until the catalyst hydrogen approached the deuterium content of "pure" DZ(99 atom % D or better), as indicated by the mass spectral analy~is.~The catalyst was again treated with oxygen, after the hydrogen exchange, and finally was subjected to a 16-hr. evacuation a t 515". Gases. A center cut of Phillips research grade methane was taken during vacuum transfer. Methane&, methane-&, m e t h a n d , and methane-& were from Merck Sharp and Dohme of Canada, Ltd. These were further purified by the same method. The propylene and ethylene were both Phillips research grade and were used without further purification except for "degassing" by repeated cycles of freezing, pumping, and thawing. Carbon dioxide was obtained from the Matheson Co. After degassing, mass spectral analysis showed no detectable impurities. Matheson nitric
308 1
oxide was purified by passage through a silica gel column, followed by fractional distillation. Deuterium was obtained from General Dynamics Corp. Its isotopic purity was specified to be 99.5 atom % D. Matheson hydrogen was electrolytic grade. Both hydrogen and deuterium were given a final purification by diffusion through separate heated palladium thimbles. Equipment and Procedures. The exchange experiments were conducted under conditions of a differential reactor. The gas phase was circulated in a loop over the catalyst by a small all-glass pump.6 The reaction was monitored by allowing the gas phase to leak continuously into a modified CEC 21-611 mam spectrometer. The capillary leak was constructed from a 6-in. length of copper tubing with an i.d. of 0.008 in. It was constricted at one point to give the proper flow rate. The end of the capillary was located in the center of the flow tube to ensure representative sampling. The calculated time delay through the capillary was of the order of a few seconds. The rate of circulation was rate limiting only when reaction half-times became less than a few minutes. The catalyst was thermostated to 1 0 . 5 " by an automatically controlled electric furnace above 100" and by liquid baths below this temperature. Specific quantities of gases or mixtures of gases could be transferred quantitatively from a calibrated gas buret attached to the circulating loop. In poisoning experiments, the quantity of the poison added to or removed from the catalyst was determined by measurement in the buret system, which was also used for the preparation of mixtures. Treatment of Data. The methane exchange was followed by scanning m / e peaks 14 to 20. In cases where the composition was changing rapidly, m / e values were plotted against time and smooth curves were constructed to get the relative peak heights at the same instant of time. The isotopic composition was then calculated using cracking patterns determined for each of the isotopic methanes. Relative sensitivities for the isotopic methanes were determined from binary mixtures. Corrections to the observed spectra for background were made (primarily H20 a t m / e 18 and 17). The good agreement between a statistically calculated methane equilibrium distribuand W. K. Hall, J. Am. Chem. SOC.,85, 3570 (1963). (3) W.K. Hall and F. E. Lutinaki, J. Ciztdysia, 2,518 (1963); W.K. Hall, E'. E. Lutinski, and H. R. Gerberich,ibid., 3,512 (1964). W. K. Hall, H. P. Leftin, F. J. Cheselske, and D. E. O'Reilly, rbzd., 2, 506 (1963). (5) W. K.Hall, W. E. Wallace, and F. J. Cheselske, J . Phy8. C h m . , 63, 505 (1959);65, 128 (1961). (2) J. G. Larson
(9
Volume 69,Number 9 September 1966
3082
JOHN G. LARSON AND W. KEITHHALL
tion with that determined experimentally (see Table 11)is evidence that the method was satisfactory.
Results Xilica-Alumina Catalysts. The results of an experiment in which 1.56 cc. (NTP) of methane-d4 was circulated over 4.47 g. of catalyst at 496" are presented in Figure 1. Considering that 1.5 X 1014 OH/cm.2 were initially present,* the available H amounted to about 75 cc. (NTP). During the reaction (after 140 min.), only 1.6 cc. had appeared in the gas phase so that only about 2% of the catalyst hydrogen was exchanged. The initial product was CD3H; CD2H2, CDH3, and CH4 were formed through subsequent secondary reaction. (The partial pressure of CDIH does not extrapolate quite to zero because of the initial 2% CD3Himpurity.) Figure 2 shows that the disappearance of CD, follows first-order kinetics very closely; the data can be accurately described by a first-order rate constant. Evidently, back reaction can be neglected because of the small fraction of the total hydrogen exchanged. Therefore, the first-order rate constant is identical with that relating the rate to the distance from equilibriums for this special case. The results of a related experiment, in which 1.4 cc. (NTP) of CH4was circulated over 4.07 g. of a deuterated catalyst at 490", are shown in Figure 3. The points are the experimental data; the lines were drawn on the basis of the theory described below. A comparison of the first-order rate constant for the dismin.-') to that for appearance of CH, (18.8 X the disappearance of CD, (9.05 X min.-l) indicated a primary isotope effect. When the rate of appearance of D into the gas phase was plotted for this experiment, a linear curve resulted (Figure 4). The lack of deviation from linearity indicates that a large fraction of the total deuterium in the catalyst was equally available for exchange. At the experimental temperature, hydrogen is mobile on the catalyst s~rface.~ Hence, methane may be activated at a number of sites which is small compared with the total hydrogen present, provided that the mixing of hydrogen on the surface is fast compared with the rate at which it is removed from the surface. When a 2.12-cc. (NTP) mixture of CD4 (42.5%) and CH4 (57.5%) was circulated over an undeuterated catalyst, Figure 5 resulted. These data show that there is no direct exchange between CD, and CHI molecules. The experiment of Parravano, et U Z . , ~ was similar to this, except that they only analyzed for CD4 and C D a . These data were insufficient to distinguish between possible exchange paths. The Journal of Physical Chemistry
e
0.8
0 u)
?
a
.-c0 2
IL.
-al
0.6
0.4
0
I 0.2
7
p 2 H2
0
20
40
60
80
100
1%
Time (Minutes)
Figure 1. Distribution of products during exchange of CDd with hydroxyl groups of silica-alumina.
I.o
Icm H a
0.4
0
0.2520 cm Hg 4 0
60 .
80
100
I20
1405
Time (Minutes)
Figure 2. Firsborder dependence of exchange rate.
A model was derived to describe our results. Assuming that only one hydrogen (deuterium) atom is exchanged per interaction with the catalyst, and that the available OD is large compared with methane, the following reactions may be written
+ -OD -% CHID + -OH CH3D + -OD -%- CHzD2 + -OH CHzDz + -OD "a, CHD3 + -OH CHD3 + -OD -% CD4 + -OH CH,
These reactions lead to the rate equations
(1) (2)
(3)
(4)
EXCH~LNGE OF HYDROXYL GROUPS OF ALUMINA WITH DEUTERATED METHANE
d(CHr)/dt = -kl(CH4)
(5)
- k2(CHsD) d(CDzHz)/dt = kz(CH3D) - ks(CDzH2) d(CHDJ/dt = ka(CDzH2) - k4(CD3H)
(6)
d(CD4)ldt = k4(CD3H)
(9)
d(CHsD)/dt = kl(CH4)
P
0000
&a
(7)
h LII
h
n
CH4
7000
(8)
Equation 6 predicts a maximum at d(CH8D)/dt = 0 when (CH4)/(CHD) = k2/kl. While the preceding equations may be solved exactly without further as-
3083
6000 0) 0
n u)
-8e
5000
I c
-
P" 4000
16000 I
c
0
14000
P B
-
-E
r
3000
2000
CHD3
X
12000
6
'5
Theory CH4 CH,D CH,D2 Theory corrected for loss to Mass Spectrometer
I000
z 10
8000
a
20
30 4.0 Time (Minutes)
50
60
r
Figure 5. Exchange of CE-CD, mixture over silica-alumina.
.5 6000 W
r 4000
2000 IO00
0
20
40
60
120
100
80
140
Time (Minutes)
Figure 3. Distribution of products during exchange of CH, with deuterated silica-alumina.
r
%!
a
20
I
60
l
l
l
f
l
l
100 140 180 Time ( M i n u t e s )
Figure 4. Integrated rate of appearance of deuterium in methanes.
l
l
220
l
l
sumptions, four rate constants are involved and only two of these (kl and h)are experimentally determined. Therefore, the model was further restricted by the assumption that the rate constant for exchange per hydrogen atom is k1/4 regardless of the molecule reacting; ie., that kt = ( 3 / & ~ , k3 = (I/z)kl, and k4 = ('/4)kl. This is equivalent to assuming that secondary isotope effects are negligible, Le., that a hydrogen atom of CD2Hz has the same probability of reacting as a hydrogen atom of CH4. The integrated forms of eq. 5-9 now reduce to two parameters: kl and the initial pressure of CHI. The lines drawn in Figure 3 were calculated using this theory, taking kl from the fist-order plot for the disappearance of CH4. The approximation to the experimental data was remarkably good, but could be improved further by taking into account the decrease in total gas pressure due to the leak into the mass spectrometer. This is indicated by the dotted line for CHD. An activation energy associated with the firstorder rate constant for the disappearance of CD4 was determined to be 33.4 kcal./mole over the range 460 to 540". The hydrogen held by silica gel did not exchange with methane below 600". When the data for silica-alumina were extrapolated to this temperature, it was determined that the exchange process over silica, gel is only l / ~ o o o as fast as that over silica-alumina. Volume 60,Number 0 September 1066
3084
JOHNG. LARSON AND W. KEITHHALL
Alumina Catalysts. The hydroxyl groups of alumina rapidly equilibrated with D2 at 5oOo4; the catalyst hydrogen content could be measured readily from the isotope dilution which occurred. When CD4 was circulated over the catalyst at 525") it equilibrated with the OH groups of the catalyst as rapidly as it flowed over the catalyst. This behavior was in marked contrast to that with silica-alumina under the same conditions where the exchange proceeded at a modest but readily measurable rate. The catalyst hydrogen content could be measured from theanalysis of thedeuterated methanes. A comparison of the results from the two gases is made in Table I. Table I1 demonstrates that the equilibrium distribution of methane was obtained. Table I: Measurement of Hydroxyl Content of the Alumina Catalyst OH/om.z X
Method
Dt exchange (before CDr exchange) CDI exchange Da exchange (after CDI exchange)
2.5 2.6 2.9
* Values of 2.5 and 3.1 x 1014/cm.2 were found for the same catalyst using the DHA method; see ref. 3. -~ ~
Table 11: Methane Equilibration over Alumina Mole % Compound
Measd.
caiod.5
CD4 CDaH CDJL CDHi CH4
6.33 25.45 37.5 24.45 6.31
6.39 25.2 37.6 24.8 6.13
* Equilibrium calculation for D/H
=
1.011.
When CD4 was circulated over the same catalyst at 135")there was an initial rapid exchange followed by a slower process (Figure 6). The extent of exchange is indicated by the fraction of molecules which have picked up one hydrogen atom (for this extent of exchange, essentially no CDZHZwas present). These resulb demonstrated the heterogeneous character of the catalyst hydrogen; the fraction of the hydrogen which exchanged rapidly at this temperature mounted to only a few per cent of the total. Nevertheless, as the extent of exchange increased, it became evident that the hydrogen removed from the catalyst was equilibrated among the methanes. Hence, the equilibration reaction is fast compared with the surface reaction The J O U Tof~Physical Chemistry
2o
P
P I
0
l
l 20
t
l 40
t
l
l
l
60
EO Time (Minutes)
l
r 100
l
r 120
l
l 140
Figure 6. Rate of exchange of alumina hydroxyl groups with CD4 at 135".
(exchange of the hydroxyl groups). The surface density of hydroxyl groups corresponding to the initial rapid process (zero time intercept of Figure 6) amounted to about 7 X 1OI2 OH/cm.2. CH, and CD4 equilibrated rapidly at room temperature. This was quite interesting because carbonhydrogen bonds must be broken in order for this to occur and methane is not easily dissociated. The results presented in Figure 7 indicate that isotopic mixing occurred through the exchange of only one hydrogen (deuterium) atom per interaction with the surface. Methane-& and methane-& were formed at approximately the same rate; methane-& was formed as secondary product. It is of particular interest that CH4 and CDI disappeared at the same rate (Figure 8) whereas there was a strong isotope effect when they reacted separately. When a mixture of CD4 and Hz (61.5% CD,) was circulated over the catalyst, isotopic mixing occurred at about the m e rate found for the CD4-CH4 equilibration. From this and the fact that the HzDz equilibration was much faster (virtually instantaneous at -78")) it was inferred that the rate-controlling step was probably the rupture of a C-D bond in the CD4. Since the mixing of CHI with Dz proceeds about 1.8 times faster than the reaction of CD, with Hz (Table 111))it follows that the rate of mixing of CH, with CD4 is limited by the rate of breaking of the C-D bond, v&., the slowest step. In another experiment, CD, was exchanged with the catalyst hydrogen at 138" and the number of D atoms transferred to the catalyst was calculated from the analysis of the gas phase at the end of the experiment; this amount (3 X 1012/cm.2)was only about 1% of the total catalyst hydrogen. When the catalyst was
EXCHANGE OF HYDROXYL GROUPSOF ALUMINA WITH DEUTERATED METHANE
Time (Minutes]
Figure 7. Distribution of methanes during exchange of equimolar CD4 and CH4 over alumina at 26".
X =Mole Froction
N
-
1.0
12 18 Time (Minutes)
24
27
Figure 8. Proof that CD4 and CHI exchange at the same rate. ~~
Table III: Evidence that Rupture of the C-D Bond is the Rate-Determining Step of the Methane Equilibration Reaction Mixture used
CJ% CHd
+ D2 + CD4
Vol. reacted,
CC.
8.4 8.8 8.9 8.9 5.gb
(NTP)
l ~m,h -~1
x
10g
6.01 3.38 3.02 5.65 3.46
= k = 1 / t log [(I - zsp)/(5 - zecss)lwhere = mole fractionof CH, (CD4). 'Equivalent to 8.9 cc. (NTP)of CD4 2H2.
+
then exchanged with CH4,virtually all of the D atoms, which had been transferred the in the step, now reappeared in the gas phase. This eXPeriment proved conclusively that the exchanging hydrogen is located on specific sites; it does not mix with the
3085
bulk of the catalyst hydrogen. It may be supposed therefore, that the diffusion of hydrogen across the surface of the catalyst is slow at this temperature. The reacting hydrogen may be held as OH, adjacent to the catalytically active sites. A series of experiments was conducted to determine the activation energy associated with the equilibration of CD, and CH4 over alumina. A good Arrhenius plot, the slope of which corresponded to 5.7 kcal./ mole, was formed from rate constants determined in the range 0 to 62". This was very much lower than the value for silica-alumina (33.4 kcal./mole). The exchange between CD4 and CH4 could be completely poisoned by the adsorption of a small amount of C02. Peri6 found that COZwas adsorbed on sites essential for the isomerization and polymerization of butene-1. These processes were not poisoned by C02, however, because it was displaced by the olefin. This does not happen with methane below 300". A series of experiments was made to determine the number of sites required for the methane equilibration reaction. The catalyst was exposed to excess COz (so that some remained in the gas phase) at room temperature. After 5 min. of contact, the gas phase was condensed into a liquid nitrogen trap from which it was regenerated and measured. By difference, it was found that 2.81 cc. (NTP) of C02 had been retained by 1.61 g. of alumina. This corresponded to a coverage of 2.7 X 10la COz/cm.2. No observable exchange occurred when a mixture of CDd and CH, was circulated over the catalyst for 34 min. Moreover, no COz appeared in the gas phase (less than l X 1OI2 C02/cm.2 would have been detected). The mixture was removed and the catalyst was raised in temperature to effect desorption of a portion of the chemisorbed COz. The COZremoved was measured and the catalyst was cooled to room temperature for another equilibration reaction. These alternate processes were repeated using increasing desorption temperatures until the COP material balance was obtained (0.03 cc. (NTP) of C02 not recovered at 500"). Figure 9 is a plot of the amount of C02 left on the catalyst as a function of the temperature at which it was removed. The catalyst fist became active for the equilibration reaction after evacuation to about 3 0 ° , where the slope changes abruptly. In agreement with Peri,E this finding may be taken as evidence that there are several different kinds of chemisorbed COZ present on (6) J. B. Peri, Abstracts of Papers, Division of Colloid and Surface Chemistry, 145th National Meeting of the American Chemical society, New York, N. y., Sept. 1963; Proceedings of the 3rd International Congress on Catalysis, Amsterdam, July 1964, Preprint N ~ I,.p. 72.
Volume 69, Number 9 September 1966
3086
JOHN G. LARSON AND W. KEITHHALL
co2 MOI /Cm2 Figure 9. Retention of COSby alumina; vacuum isobar.
alumina; the portion poisoning the equilibration reaction was the more strongly adsorbed form. The firsborder rate constant for the equilibration reaction is plotted as a function of the C02 remaining on the catalyst in Figure 10. The approximately linear relationship between the rate constant and the number of sites covered by strongly chemisorbed C02 suggests that these sites are homogeneous. Little reliable information is available on this subject although one frequently reads reports referring to the distribution of acid sites (weak, medium, and strong) on these materials. Work on the isobutane exchange system2 indicated that about 80% of the isobutane was adsorbed on silica-alumina catalysts in a homogeneous manner. The curvature at the slower rates (Figure IO) was probably caused by the removal of two types of CO2 in this region, only one of which was acting as a poison for the reaction. Correcting for this, the number of sites active for methane equilibration amounted to 3-4 X 1012/cm.2. Several simple compounds were found which adsorbed selectively on the sit& required for methane equilibration. Small measured increments of the poison were circulated over the catalyst with a diluent gas (methane). The disappearance of the poison from the gas phase was followed via the leak to the mass spectrometer. The results obtained with nitric oxide are shown in Figure 11; they demonstrate the linear relationship between the rate constant for the methane equilibration reaction and the number of sites covered by poison. Similar poisoning plots were made for other gam, vk., C2H4, C a s , and HzO (H2 02); the values of the number of active sites, as obtained
+
The J O U Tof~Physical Chemistry
x
Figure 10. Poisoning plot for Cog.
Figure 11. Poisoning plot for NO.
by linear extrapolation to the adsorption axis, are summarized in Table IV. All of these data lead to the conclusion that there are between 2 and 4 X 10l2 sites/cm.2. The result for H20 requires special comment. Water is a nonselective poison and, if added directly, much larger amounts would be required to poison the reaction. The sites can be selectively poisoned, however, by forming the H20 directly on the active sites by the reaction of hydrogen with oxygen. This also serves to identify the sites active for methane equilibration with those required for water formation.
EXCHANGE OF HYDROXYL GROUPS OF ALUMINA WITH DEUTERATED METHANE
I n a similar vein, C02 was found to poison the equilibration of C2D4 with c2H4 and of H2 with DS. Since linear poisoning plots (similar to Figures 10 and 11) were obtained in all cases, it was concluded that the sites are all energetically equivalent. Table IV: Number of Active Sites on Alumina as Determined by Poisoning the CDa-CH4 Equilibration Reaction Poison
con H2 and O2 Ethylene Propylene
NO
Method used
Selective desorption Reaction on site Selective desorption Selective desorption Selective desorption
Sites/om.z X 10-12
2-4 2 or 4 2 4 4
Discussion The isotopic mixing of CDh and CHI over alumina involved exchange with several per cent of the catalyst hydrogen. Moreover, this hydrogen resided at very definite sites on the alumina surface. This was shown by the fact that the deuterons introduced by exchange with CD4 could be quantitatively recovered by a subsequent exchange with CHI; had the few OD groups introduced mixed with the much larger amount of OH, this would not have been possible under the conditions of the experiment. The small amount of hydrogen which exchanged between room temperature and 135" is believed to be located at sites where methane is dissociatively adsorbed. It is instructive to consider a recent model of the alumina surface. Per? pictured the creation of active sites by the process of dehydroxylation; as water is removed, aluminum ions are exposed and adjacent surface 0 2 - ions are formed. In this way, dual acid-base sites are created which tend to rehydrate readily. The driving force for this is so high that analogous processes occur with other molecules, e.g., with ammonia which is cleaved to form adjacent NH2 and OH groups. Peri classified the residual hydrogen into five categories, depending upon whether a residual hydroxyl group had 0, 1, 2, 3, or 4 immediately adjacent 02- ions; he found infrared bands in the OH stretching region corresponding to each. The C site, which has the lowest frequency (3700 cm.-I), corresponds to isolated OH groups whose nearest neighbor sites are all vacant, i.e., sites where aluminum ions are exposed. Such OH groups have no immediately adjacent 02ions, but nevertheless exchanged with D2 most readily.s Since the nearest neighbors are not 02-,the next nearest neighbors generally are. This is exactly the situation required to afford a simple interpretation of our results.
3087
Since methane is readily deuterated over alumina, it may be supposed that the molecule is cleaved as it is chemisorbed. If it reacted with the dual acid-base surface sites in a manner analogous to NH3 and H20, then Al-CH3 and adjacent OH groups would be formed. In some cases, the resulting aluminum alkyl would be simultaneously adjacent to an OH and an OD group so that on desorption it could equally well combine with either. Presumably, this could occur with any of the types of residual OH (OD) groups considered by Peri except one, viz., the A site where the hydroxyl group is surrounded by four 02-ions so that there are no adjacent vacant sites, but the situation at the C type would be most favorable. The exchange process can be expressed very simply as CD4
+0 '\\
OH /'
Ai
DO CH3 OH '\\
I /'
(10)
-A1
The experimental data indicate that there are 3 X 1012/cm.2 active sites. A completely hydroxylated alumina surface would contain (Peri's model) 1.3 X 1015/cm.2of OH. When 90% dehydroxylated, 5 X 1014/cm.2of aluminum ions would be exposed and an equal number of 02-ions formed; the residual hydroxyl content would be 1.3 X 1014/cm.2. It follows, therefore, that a large fraction of the residual OH groups would have adjacent exposed aluminum ions with an 02-nearest neighbor. The surface density of such configurations is, however, at least an order of magnitude higher than the density of active sites derived from the poisoning experiments. Hence, it must be concluded that all such configurations are not catalytically active. It is probable that only the high energy sites, which are created at the domain bound a r i e ~ ,are ~ strong enough to dissociate methane. Nevertheless, for each active site (3 X 1012/cm.2) there will be one readily exchangeable OH group (3 to 4 X 1012/cm.2found). It seems pertinent that the strong sites which dissociate CH4 are the ones which also strongly chemisorb C02,NO, etc. Figure 6 shows that a slower exchange process occurred in addition to the rapid exchange identified with hydroxyl groups adjacent to active sites. It seems reasonable to ascribe this slower process to the diffusion of hydrogen from distant locations, to positions adjacent to the active sites. This allows one to reconcile the marked differences in behavior of silicaalumina and alumina. With the former, there was no evidence of heterogeneity of catalyst hydrogen; the (7) J. B. Peri, J . Phys. Chem., 69, 211, 220, 231 (1965). (8) J. B. Peri and R. B. Hannan, ibid., 64, 1526 (1960).
Volume 69, N u m b e r 9 September 1966
3088
JOHNG. LARSON AND W. KEITHHALL
activation energy for exchange was much higher and reaction did not occur at measurable rates below 400'. Evidently, on silica-alumina, the sites are not the same as on alumina; they appear to be weaker. Consequently, much higher temperatures are required to activate the methane molecule. At these temperatures (above 400'), the surface migration of hydrogen is rapid; hence, all of the catalyst hydrogen appears to be equivalent. In the case of CD4 exchange, the D left at the catalytic site by the adsorption of CD, is rapidly equilibrated with the total catalyst hydrogen and is not available for subsequent exchange with a CHI molecule (Figure 5 ) . This may be contrasted with the result from alumina where the exchanged hydrogen maintains its identity at the active site. While the exchange of methane over silica-alumina catalysts possibly does not follow the same mechanism as with alumina, the above arguments show that there is no inconsistency in a uniform picture for both. The manner in which hydrogen is activated by alumina and silica-alumina surfaces is not a t all understood. By analogy with the present methane results, hydrogen may be cleaved on the alumina surface to form an A1-H and an adjacent OH. Infrared data support the view that the analogous reaction occurs on the surface of Zn0.9 In both cases, the driving force seems to be the screening of (restoration of coordination around) the exposed cation. With silica-alumina, the situation is even less clear and anything that is written must necessarily be classified as speculation. Studies of the exchange between the hydrogen held by isobutane and deuterated cracking catalysts have been interpreted2 in terms of abstraction of the tertiary hydride ion by the catalyst to form the isobutyl carbonium ion; the nine hydrogens remaining on the carbonium ion then exchange freely with available catalyst hydroxyl groups. The identity of the tertiary ion is conserved either by hydride transfer from incoming molecules or by recombination with the carbonium ion on desorption. A model for the active sites on silicaalumina catalysts may be drawn from the developing understanding of the acidity of the X- and Y-type eeolites,1° with which they may be presumed to be related. When a zeolite is decationated, sites which function as Brplnsted acids first form. These may be described by H+ 0
0
0
\-/-\ / A1
/\o
0
,Si 0
\
HO
0
2 0
Ths Journal of PhymM Chemi&y
0
\
\/
,AI\
o,Si\
0
0
(11)
0
On degassing above 500', these sites are dehydroxylated and in the process pairs of defect sites are created. One of the pairs contains an anion vacancy and the other a cation vacancy. This pair may function to form carbonium ions from paraf%ns by hydride ion abstraction, i.e. 0
(CH3)aCH
+
0
\ I Al $3 / \ o \ 0
0
0
0
0
\/ + \-/ AI ,Si / \ o \ 0
0 0
----t
0
+ cation vacancy
anion vacancy
(CHa)aC 0 +
H
0
\ \/" O w \ / Al /si + AI ,Si / \ o \ / \ o \
0
0
0
0
0
0 (12)
0
It seems doubtful whether these ideas should be extended to methane, as the existence of a methyl carbonium ion seems very unlikely. It is, of course, possible that the over-all result is given by 0
0 CH4
+
i \ o \
0
0
/Si
0
0
0
0
0
\/
$3 /" +
\-/O\ AI
/ \ o \
0
H
\ 0
+
)SiI
\Al 0
0
0
0 CHaO
\
+
Al
0
\/" /si
0
(13)
0
These equations indicate ways in which hydrocarbons may dissociate and adsorb, but they offer no insight into the mechanism of hydrogen exchange with catalyst deuteroxyl groups. They do, however, emphasize the important point that the ideas which have been developed concerning the intermediates involved in exchange procesaes over silica-alumina do not fit well with the simple type of intermediate suggested by our data for alumina. In fact, with &lumina, the methyl group is thought to be held by the alumhum ion with concomitant formation of a hydroxyl group, whereas extension of current thoughts concerning silicaR.P. Eischens, W. A. Pliskin, and M. J. D. Low,J . Catalysis, 1, 180 (1962). (10) J. B. Uytterhoeven, L. G. Christner, rand W. K. Hall, J. Phve. C h m . , 69, 2117 (1965). (9)
3089
PHENOL INTERACTIONS WITH SUBSTITUTED PYRIDINES
alumina would suggest that the methyl group be attached to oxygen and hydrogen to the cation. While this chemistry is plausible, it is highly speculative and should be so considered.
Acknowledgment. This work was sponsored by the Gulf Research and Development Company as part of the research program of the Multiple Fellowship on Petroleum.
Hydrogen Bonding. 11. Phenol Interactions with Substituted Pyridines'"
by Jerome Rubinlb and Gilbert S. Panson Chemistry Department, Rutgers, The State University, Newark, New Jersey
(Received April 6,1966)
The hydrogen-bonding equilibrium constants of some substituted pyridines to phenol in carbon tetrachloride at 20 and 40" are reported. The constants are correlated with Hammett's and Tafts' substituent constants, the equation of the line having been calculated by the method of least squares. Thermodynamic data for the reaction are given. A steric effect caused by di-o-tbutyl groups is demonstrated, as well as a solvent effect which shows that the equilibrium constant value is dependent upon the solvent.
The hydrogen-bond formation of phenol to proton acceptors is well known and has been studied extensively.2 Investigations have been made from merely to such the molecular association of phenol electron-pair donors as carbonyl^,^ ethers,6 amides,6 and alkyl halides.' However, as indicated by the authorsJ2 comprehensive and complete studies are seriously lacking. The work of Gramstad is a noteworthy exception as can be seen by his contributions in this particular area. He has investigated the interaction of phenol and/or pentachlorophenol with organophosphorus c o m p o ~ n d s , amides,lO ~~~ nitrogen compounds,11carbonyls and ethers,12and sulfoxides and nitroso compounds.l 3 However, a complete and thorough study of the hydrogen bonding between phenols and pyridinw had not been done. It was with this intention that the project was undertaken. The previous communication had reported the equilibrium constants of some substituted phenols with pyridine in carbon tetrachloride." It is shown that a linear correlation exists for the logarithm of the association values vs. Hammett's substituent constants. Continuing this investigation, the equilibrium constants of phenol with substituted pyridines are now
reported. A steric effect is studied as well as the effect on the value of the equilibrium constant by changing the solvent.
Experimental The technique used for evaluating the equilibrium constants is the same as that reported previously.14 (1) (a) Parts I and I1 are excerpts of the thesis submitted to the Graduate School of Rutgers, The State University, by J. Rubin in partial fulfillment of the requirements for the Degree of Doctor of Philosophy. (b) E. I. du Pont de Nemours and Co., Richmond, Va. (2) G. C . Pimentel and A. L. McClellan, "The Hydrogen Bond," W. A. Freeman and Go., San Francisco, Calif.,1980. (3) (a) M.Iro, J . Mol. Spectry., 4, 125 (1960); (b) M. M. Maguire and R. West, SpSpedrochim. Acta, 17,369 (1961). (4) G. Akanes, Acta Chem. Smnd., 14, 1475 (1960). (5) R. West, etal., J . Am. C h . SOC.,86, 3227 (1964). (6) N.D. Joesten and R. S. Drago, ibid., 84,2696 (1962). (7) R. West, et al., ibid., 84,3221 (1962). (8) G. Aksnes and T. Gramstad, A d a C h a . Scund., 14, 1485 (1960). (9) T.Grsmstad, ibid., IS, 1337 (1961). (10) T. Gramstad and W. J. Fregledt, ibid., 16, 1369 (1962). (11) T. Gramstad, Spectrochim. A d a , 16,807 (1962). (12) T. Gramstad, ibid., 19, 497 (1963). (13) T. Gramstad, ibid., 19,829 (1963). (14) J. Rubin, et al., J . Phys. Chem., 68, 1601 (1964).
Volume 69,Number 9 Septembst 1066
