1146
~ I A R V IF. X L. JOHNSON ASD JOHN S. MELIK
Vol. 65
DEALKYLATIOX OF t-BUTYLBENZENE BY CRACKING CATALYSTS BY ~ I A R V II:X . L. ,JOHSSON AND JOHN S. MELIK Sindair Research, Znc., Harvey, Illinois Received December 17. 1960
A low-conversion flow reactor system is used to measure rates of dealkylation of t-butylbenzene over a variety of catalysts, as a function of temperature and of added water vapor, in an attempt to classify catalysts according to Bronsted acid activity. With but, one exception, activation energies are the same for all catalvsts studied. Differences in activity between catalysts are believed due to differences in the number of sites capable of catalyzing this reaction. The addition of water vapor with the hydrocarbon has little effect, up to a point, upon the activity of silica-alumina, kaolinite-based catalyst, mixed base catalyst, or silica-alumina-zirconia. Water is harmful to boria-alumina or fluoride-alumina, by causing volatilization of the promoter. Water enhances the activity of silica-magnesia or montmorillonite-based catalyst.
I. Introduction The cat.alytic acidity of cracking catalysts has been the subject of much work in recent years. In attempting to measure what one terms catalyst acidity the concepts of number of acid sites and of the strengthc; of these sites must be considered. Techniques mch as the non-aqueous titration with n-butylamine' or quinoline adsorption, among others, give a measure of the number of such sites. 13enesi3has extended the butylamine t,itration t'o permit estimation of relative acid strengths, by using a series of indicators of varying pK. These techniques, while they serve a definit'e purpose in characterizing cat'alysts, suffer from the fact that t'hey may not represent the actual interactions involved in a catalytic reaction. Another aspect of the characterization of acidic properties of catalysts is t>hequestion of whether the sites are Lewis or Bronsted acids. Tamele4 has suggested that silica-alumina is a Lewis acid in the absence of water, Bronsted acid in the presence of water. On the other hand, Milliken, et u L , ~ postulate the presence only of potential Lewis acids, until the approach of a base (e.g., a hydrocarbon) causes a shift to the Lewis acid form. This picture completely eliminat'es t,he possibility of Bronsted acids. Hansford6 has given a rebuttal to this theory, pointing out that there is a considerable body of evidence that protons can and do exist on crackhg catalyst's a t elevated t'emperatures, and that it is difficult, to see how t'he polarizing ability of any but the most polar molecules (e.g., water) could exert, the postulated effect on catalyst st.ructure. iblapes and Eischens7 report'ed infrared spectral evidence for both Lewis and Bronsted acids (or hydrated Lewis acids) in silica-alumina, with the Lewis acids predominating. Haldeman and Emmett',3in a study of the mater content, of silica-alumina, found that t'here are not enough protons present to furnish one for each A + 3 , hence a t least some of the sites must be the Lewis type. Trambouze, et aZ.,9 made measurement,s of (1) 0. Johnson, J. Phys. Chem., 69, 827 (1955). (2) G. A . hlills. .E. R. Boedeker and A . G. Oblad, J . Am. Chem. Soe., 72, 1554 (1950). (3) H. A. Benesi, J . Phys. Chem., 61, 970 (1957). (4) M. W.Tarn&, Disc. Faraday SOC.,8 , 270 (1950). ( 5 ) T. H. Milliken, ibid., 8, 279 (1950). (6) R. C:. Hansford, Advances in Catalysis. Il', 1 (1952). (7) 3 . E. RIapes and R. P. Eischens, J . Phys. Chem., 68, I059 (1954). (8) R. 0 . IIaldeman and P. H . Emmett, J . 4 m . Chem. Soc., 7 8 , 2917 (195G). (9) Y. Trambou'ze. L. DeMourges and M. Perrin, J . chim. phys., 61, 723 (1954).
Lewis and Bronsted acid concentrations by room temperature titrations; they found an increase in the former upon heating to successively higher temperatures, the sum of the two types remaining const'ant. It appears most likely that in act'uality silicaalumina cont'ains, a t least potentially, both Lewis and Bronsted acid sites, the actual amounts of the two types depending upon the degree of hydration. That there is an effect of water on the activity of silica-alumina is well-known. The possibility exists that a similar effect may be observed with other types of acidic oxide catalysts. The work reported here is an attempt to classify catalysts according to Bronsted acid activity as defined by the rate of a catalytic reaction which is believed to be catalyzable only by Bronsted acids, e.g., the dealkylat,ion of aromatics. A classification of Lewis acid activity by a catalytic reaction would be more difficult; a paraffin, for example, can be converted to a carbonium ion by hydride abstraction by a Lewis acid, or by proton donation from a Bronsted acid to a small amount of olefin produced by thermal cracking.
11. The Aromatic Dealkylation Reaction Not only is the catalytic dealkylat.ion of cumene a very rapid reaction, but it is highly selective; furthermore, the rate does not decrease with time due t'o coke f o r m a t i ~ n . ' ~ . It ' ~ is therefore well suited for a test reaction. One of the drawbacks to the use of cumene is, however, the fact t>hathydroperoxides can form fairly easily, and serve as inhibitor^.'^ For this reason, and because its rate of reaction is much higher (ea. 10 X), we have chosen to use t-but~ylbenzene(TBB). It is generally believed, although without direct evidence, that this reaction is cat>alyzed by a Bronsted acid, so that, t'he rate of reaction can serve as a measure of Bronsted acid activity. A mechanism not involving proton transfer does not appear reasonable in view of t'he effect of hydrocarbon substituent on dealkylation ratel6 and in view (10) R. G. Haldeman and P. H. Emmett, J . Am. Chem. Soc., 78, 2922 (1956). (11) R. C. Hansford, Ind. Eng. Chem.. 39, 849 (1947). (12) R. C. Hansford, P. G. Waldo, L. C. Drake and R. E. Honig. ibid., 44, 1108 (1952). ( 1 3 ) S.C . Hindin. G. .I. Mills and A . G . Oblad, J . A m . C'hem. SOC. 73, 278 (1951). (14) R. W. Maatman, R. M. Lago and C. D. Prater, Advances in Catalysis, I X , 531 (1957). (15) C. J. Plank and D. M.Naoe, I n d . Eng. Chem., 47, 2374 (19.55). (le) B. S. Greensfelder, H. H. Voge and G. K. Good. ibid.. 37, 1168 (1945).
DEALKPL.ITION OF ~-BUTYLBENZENE BY CRACKIXG CATALYSTS
July, 1961
of the effect of aromatic substitutions on the cracking of unsymmetrical diarylethanes." I n addition, our own observai ions of the effect of added water in certain cases support the viewpoint of proton transfer. The kinetics of cumene dealkylation over silicaalumina catalyst have been discussed by Prater and Lago.18 The following scheme is believed to represent the steps involved in dealkylation ; this essentially follows Prater and Lago, except that the first step of adsorption of reactant is included here TBB(gas) +TBB(adsorbed) (1) TBB (adsorbed) H +(catalyst) --+ protonated TBB
+
(z1
The protonated TBB ultimately decomposes, perhaps by way of R and c complexes; regardless of the details, the over-all reaction in this step will be CH3
I
IC
+ benzene (3) + The catalyst acid is regenerated by step (4) Protonated TBB +CHa-C-CH3
CH3
CH3
I
I
CH3-C-CH3
CH3-C=CH2
+ H+(catalyst)
(4) + Step (a),that of proton transfer to the hydrocarbon, is the important one as far as the catalyst acid is concerned if we can assume equilibrium in the adsorption steps. The equilibrium constant of this step, K , is a measure of the strength of the acid, when working with a given base (hydrocarbon), K will have a temperature-dependency of the form K = Fe-H/RT (5) where F is a constant containing the entropy of interaction, H iis the heat of reaction. Since entropies of interaction between a given base and a series of acids tend to be constant, the heat of interaction is a measure of acid strength. In carbonium ion chemistry the formation of the catalyst complex has never been found to be the rate-determining step. The rate-determining reaction will then be steps (3), with the rate given by +e
Rat e = k(protonated TBB)
(6)
Since K =
(Protonated TBB)
BO
(7)
Then Rate = kBoK
(8)
where Bo = nuimber of sites. This is essentially the equation proposed by Prater and Lago except that the interaction-equilibrium constant, K , has replaced their adsorption-equilibrium expression. Since k =
De-E/RT
(9)
where D contain? the entropy of activation and E is the energy of ,activation (17) D. R . May, X . W Saunders, E. L. Kropa and J. K. Dixon, Dzsc. Faraday Soc., 8 , 290 (1950). (18) C. D. Prater and R. RI. Lago, Advances zn Catalyst?, V I I I , 293 (1956).
Rate =
BaDFe-(cJrH)/RT
1147 (10)
The observed activation energy, Ea, will then be E+H. E, is obtained from the slopes of log rate us. 1/T plots, log A from the intercepts, where A is the pre-exponential factor, BoDF. If it can be assumed that the entropy factors are constant, then variations in 14are due only to Bo,the number of acid sites. Since E will probably be the same for all catalysts, differences in Ea will represent differences in H , or acid strength of the surface sites. 111. Experimental and Data Analysis A. Equipment and Procedure.-The measurements are obtained in a differential reactor, that is, a t low conversions and high space velocities. The advantage of a differential reactor system is that kinetic data can be obtained with a single run, since conversions are so low that composition of the vapor is relatively unchanged; in an integral reactor, composition changes through the bed, so that runs at several space velocities must be made to obtain kinetic data. In a differential reactor system one measures the rate of formation of product, not the degree of conversion. The observed rates equal the rate constants of the reaction. The present equipment is suitable for a reaction which produces a gaseous product from a liquid; the rate of gas production is measured. Whereas it was desired to hold conversions at the 1% level, in actuality some catalysts (e.g., silica-alumina) were so active that even with 50 mg. of catalyst and TBB flow rates of 5 ml./min. conversions approaching 10% were obtained. However, this level does not markedly affect the results. The a p p a r a t u is depicted in Fig. 1. 50-500 mg. of catalyst, ground to pass 250 mesh, are placed in the reactor a t the tip of the thermowell, supported by a small quantity of glass wool. Tabular alumina is placed above the catalyst, to serve as preheat; no reaction is observed over tabular alumina in the absence of catalyst, below 500". The reactor is surrounded by a fluidized sand-bath, to minimiz: temperature gradients, the whole being placed in a 2 Hoslrins furnace, controlled by a Gardsman temperature controller. Catalyst samples are prepared for testing by drying the ground and sieved catalyst in flowing dry air, 3 hours a t 565'; they are then stored in a desiccator until ready for use. After charging the system with catalyst, the latter is dried in sztu with flowing dry Nz, either 1 hour a t 482' or 12 hours at 565"; no differences in activity resulted from the two types of drying, as the data plots demonstrate. The nitrogen also serves the purpose of flushing air from the syetem before starting hydrocarbon flow. The catalyst is then cooled to the appropriate temperature, K2 flow stopped, and TBB flow started, normally a t 5.0 ml. liquid/min.; some runs a t 4.0 ml./min. were made. .4 Corson-Cerveny bellows pump is used to feed the TBB. A 5-10' drop in temperature will be observed, due to vaporization, before the temperature is lined out. The reactor effluent passes through a helical cooler held a t 190200" by means of an external winding, thence to the Utube used for the liquid-gas separation. The latter is held a t , or close to, the boiling point of TBB ( l i g ' ) , to minimize solution of isobutylene in the liquid; control of this temperature is very critical to precision of the data, since too low a temperature will permit isobutylene to remain in the liquid, while a t too high a temperature the constant-level liquid seal will not be maintained. Liquid overflowing the U-tube drains off in one direction, to be repurified later, Phile the gas passes in the other direction through a condenser to remove benzene, thence to the buret where it is collected over water. Rate measurements are made using a stopwatch to time the evolution of a measured volume of gas. Gas volumes are corrected to standard conditions, and the rates expressed as cc.(STP)/g./min. I t is assumed that the gas is 100% isobutylene. Periodic mass spectrographic analyses show its concentration alwaos to be greater than 96-97qG, a difference less than other eyperimental errors. As shown in the drawing, provision is made for the addition of water a t very low flow rates, by means of an Aminco Compensator. This consists of a motor-driven ground-glass
MARVINF. L. JOHNSON AND JOHN S. MELIK
1148
MP SANO
BATH -
1-
I
WATEL INJECTOR
DRAIN
Fig. 1.--Differential
TIC
I "DL
'
n
EFFLUENT
The data for each water level and each catalyst were fitted to an Arrhenius equation (log rate us. 1 / T ) by least squares. This calculation provides values for E,, log A , and for the standard deviations of E., log A and of log rate. (The standard deviation of the observed rates from the lea& squares line has no meaning, since rates in a given set vary over a 50-fold range.) The standard deviations correspond to variations in the rates which average about 35%. C. Catalysts.-A listing of the catalysts used for this work may be found in Table I, together with some of their properties. Surface areas and pore volumes were obtained from nitrogen adsorption-desorption isotherms; by application of a modified Barrett-Joyner-Halenda calculation*8 the desorption isotherms were converted to pore volume distributions, from which the positions of the peaks were taken as the most probable pore radii. One may note the fairly good agreement between this value and t h a t of the average calculated from area and pore volume.
TABLE I
reactor system for t-butylbenzene dealkylation.
syringe, with graduations, connected t o the reactor by means of hypodermic tubing which passes through a rubber stopper into the to of the reactor; the end of this tubing contacts the wall o t t h e reactor, to avoid surges due to drop formation a t the tip. Water rates from 0.8-12.1 ml./hr. are obtained, by varying the size of the syringe and the gear ratio of the motor. To avoid excess ebullition in the U-tube when using water, excess water is removed periodically through the drain stopcock. Normally, rate measurements on the same charge of catalyst, at the same temperature, are made with varying water rates; some runs were made starting dry, then going to different water rates, while for others the reverse procedure was used. It (cannot be stated for certain whether the previous water history has an effect on the rate at a given water level. In! practice, all data for a given catalyst at a given water level are grouped. The dealkylation reaction is unique among cracking reactions in that activity usually does not decline with time due t o coke; this has been discussed earlier. After an initial period of high activity the rate becomes constant for a t least an hour or so. Our practice has been t o take four to six rate measurements over a period of 30-60 min., and average the results. This serves the purpose of checking on constancy. Certain catalysts will rapidly lose activity when run in the absence of added water, as will be discussed later. As a simple means of obtaining data as a function of temperature, it was, first thought to make rate measurements a t successively higher temperatures with a single catalyst charge. Howeber, not only was the reproducibility very poor, but rates a t the higher temperatures were lower than for a series in which separate charges of catalyst were used for each temperature. All data reported here were obtained by using fresh catalyst for each run a t a sinQ temperature. Temperature was varied from 260 to 500 , with most data i n the 300-450' range. T B B is obtained as the Pure Grade from Phillips, and is further purified by refluxing with liquid sodium, and distilling a t atmosphcric pressure, discarding 570 at either end; jugt prior to use it is passed through a column of F-20 alumina. Reactor effluent, containing perhaps 5y0 benzene, is recovered for further use by distillation a t sub-atmospheric pressure in a 2" Stedman column, at 21/1 reflux ratio; this also is passed through an alumina column just prior to use. Chromatographic analyses of the purified materials indicate a purity of a t least 99.950/0. B. Treatment of Data and Error Analysis.-The errors in these measurements are such that i t is difficult to distinguish small differences between catalysts or between water vapor levels by comparing single runs, but even relatively small changes in activity due to changing water levels can be observed in .t single run. The chief source of error is probably the liquid-gas separation; 0.1 mole yoisobutylene in the effluent leads to an error of 15 to 150 cc. (STP)/ g./min., depending on catalyst weight. At one time, small differences in the state of hydration were felt to be responsible; this was ruled out by the observation that wide variations in tho severity of the i n situ pretreat seem to have no effect.
Vol. 65
CATALYST PROPERTIES Catalyst
Nalcat Si-Al:HA Nalcat Si-A1:LA Aerocat Si-A1:LA Nalcat 0783-mixed baw Clay-K Boria-alumina Silica-magnesia, XDA-30 Silica-aluininazirconia Clay-M 5 Calculated as 2 responding to peak cCf.ref 3.
n-ButylArea Pore Pore radius, A. amine titration Moat (m.z/ volume g.) (co./g.) Av.0 probableb (meqh.1
385 0.87 637 .80 676 .
45 25
266 134 394
.72 .30
54 60 45 (Broad)
..
..
.265 .16 .74
509
.535 18
22
.69
.
..
.*
52 25
..
0.45 .30 .44
442 .73 33 33 .25 330 .41 25 (Broad) ,175 X (pore volume/area) X 104. 6 Corposition in pore volume distribution.
Also shown are the values obtained by n-butylamine titration following the technique of Benesi.3 For the silicaaluminas, silica-zirconia-alumina, and boria-alumina, the titer was nearly independent of indicator pK; for the rest, the titer listed was that corresponding t o the most basic indicator, that of pK = 4.0. Most of the catalysts are commercially available, as indicated in Tables I and 11. HA refers t o high-alumina (nomiClay-K and nally 25y0), LA to low-alumina (12-13%). Clay-M are kaolinite- and montmorillonite-based catalysts, respectively, received from the Filtrol Corporation. The boria-alumina was prepared by precipitating sodium borate with aluminum sulfate, washing, drying, calcining a t 560'; it contains 10% B203. Silica-alumina-zirconia was prepared from a silica hydrogel, by adding a solution of zirconyl nitrate and aluminum chloride, then ammonium hydroxide, washing free of chloride, drying, calcining at 560". It contains 7.25% ZrOt and 3.35% A120?. Fluoridealuminas were prepared from calcined aluminas prepared by precipitation from the chloride by impregnation with ammonium fluoride solutions and subsequent calcination.
IV. Results The complete data for HA silica-alumina (Si-A1) and for silica-magnesia (Si-Mg) are shown in Figs. 2 and 3, respectively, as typical examples; similar plots were used for the other catalysts. The same results are obtained whether the in situ pretreatments were carried out a t 482' or at 565'. Table I1 summarizes the results of the least-squares analyses of all the data, including rates a t the arbitrarily chosen temperature of 455'. (19) A. Wheeler, "Catalysis," Vol. 11, ed. by P. H. Emmett, Reinhold Publ. Corp., New York, N. Y., 1955, p. 106.
July, 1961
DEALKYLATION OF ~BUTYLBENZENE BY CRACKING CATALYSTS
1149
TABLEI1 SUMMARY OF DEALKYLATION RATEDATA Catalyst
Mole % HnO
0 Nalcat Si-AI-HA 6 Nalcat Si-Al-HA 15 Nalcat Si-Al-HA 18 Nalcat Si-Al-HA 0 Nalcat Ai-&-LA 6 Nalcat Si-&-LA 15 Nalcat Si-AI-LA 0 Aerocat Si-A1-LA 0 Nalcat #0783--mixed base 6 Nalcat #0783-mixed base 15 Nalcat P0783-mixed base 0 Clay-K 6 Clay-K 15 Clay-K 0 Boria-alumina 0 Silica-magnesia 2 Silica-magnesia 6 Silica-magnesia 15 Silica-magnesia 18 Silica-magnesia 0 Silica-alumina-zirconia 6 Silica-alumina-zirconia 15 Silica-alumina-zirconia 0 Clay-M 6 Clay-hi 15 Clay-hi 26 Clay-M a-Cc. (STE’)/g./nin. Kcal./mole.
No. of
-Reaction
exptl.
L.S. Value
18 12 4 3 6 6 6 9 4 6 6 12 10 11 5 20 5 10 5 4 6 6 6 18 17 15 5
1550 2060 1980 990 1600 1700 1800 1750 740 760 760 440 410 490 216 67 90 127 175 87 171 148 172 74 96 107 100
points
at 4 5 5 O
With the exception of that for Clay-AI, the activation energies are approximately the same for all catalysts ; this is particularly apparent when comparisons are macle a constant water level, such as 15y0. There appears to be a tendency for the activation energy to increase slightly with increasing moisture; ai, the same time, log A increases. For any given catalyst, a plot of E, us. log A is approximately linear, a n example of the so-called “compensation effect. ”2o Reaction rates themselves provide a better means of rating catalysts in this case than considerations of the kinetic factors, since small changes in E or log A may markedly affect the rates. On this basis the order of decreasing activity is: silicaalumina, mixed-base, Clay-K, boria-alumina, silicaaluminum-zirconia, silica-magnesia, Clay-&I. This agrees with the ratings observed by cumene dealkylation for some of these by Swegler, et nL21 Water has little effect on silica-alumina activity until the 15% level is reached, beyond which further water lowers activity. No effect is apparent for Clay-I(, mixed base, or silica-alumina-zirconia, :ilthough experiments beyond 15% water were not carried out. There is apparently a dcfinite although small effect of water on the activity of Clay-RiI which increases with increasing water to a constant level. The effect is much more pronoiinced vith silica-magnesia for which a tm-ofold increase in ( 2 0 ) E. Cremer. Advances zn Catdyazs, 1/11,75 (195;). (21) F. W. 8wegler. R. L. Golden, R. M. Lago, C. D. Prater a n d P. B. Welar, A.C.S. Meeting, Petroleum Divimon, April. 195G.
rateaStd. dev. log rate
0.099 .079 ( .09) ( .I71
.20 .17 .ll
.17 ( .052)
.13 .098 .17 .17 .18 .093 .16 .072 .ll
,067 ( .OS)
.071 ,048 .060 .13 .15 .15 .os0
Pre-exp term, log A Value Std. dev.
7.40 7.87 8.2 8.7 8.72 8.49 8.26 7.61 G.6 7.98 8.14 7.17 6.77 7 .53 7.18 5.84 5.62 6.62 6.98 7.2 6.54 6.59 7.15 5.02 5.15 5.60 4.9G
0.24 0.27 (0.5) (1.4) 0.99 .82 .54 .G6 ( .48) .65 .49 .40 G4 .53 .55 .33 .59 .31 * 31 ( .4) .34 .23 .29 .27 .3G .40 .39
.
Activation energy6 Value Std. dev.
14.0 15.2 16.2 19.0 18.4 17.5 16.7 14.6 12.3 17.0 17.4 15.1 13.8 16.1 16.1 13.4 12.2 15.0 15.8 17.5 14.4 14.7 16.4 10.5 10.5 11.9 9.8
0.7 0.8 (1.5) (4.0) 2.9 2.4 1.6 1.9 (1.5) 2.0 1.5 1.4 1.9 1.5 1.7 1.0 1.9 0.9 0.9 (1.3) 1.0 0.7 0.9 0.8 1.1
1.2 1.2
Fig. 2.-&Butylbenzene dealkylation NALCAT HA, SiAl, 25% AlzOa.
activity was observed on going from 0 to 15% water; again, a t the 18% level there was a definite decrease. For both of these catalysts the increase in activity on going from no water to a given water level was more npparcnt during a given run than is the compariwn of the correlated data from a number of runs. This is because in any given run the variations in rates during the run were less than the variations between runs. Kot shon-n is the effect of water on boria-alumina, since a marked decrease in activity, due to boria removal, was observed. Another effect of water should be mentioned which is not apparent from the data as presented. That is, for the two catalysts for which water is
1150
Fig. 3.--t-Butylbenzene
MARVIN F. L. JOHNSON AND JOHN S. MELIK
dialkylatiori XDA-30 Si-Mg.
found to promote thc reaction, namely, silicamagnesia and Clay-M, it was frequently observed when running in the absence of water that the activity was falling, to a much greater extent than with the other catalysts. This effect was not observed in the presence of water. It is possible, therefore, that gradual dehydration of the catalysts during processing is responsible for activity loss. Alumina i: at best only slightly active for dealkylation. In the presence of water, transient rates as high as 10 cc.(STP)ig. 'min. at 455' were obtained using a pure alumina prepared by precipitation from aluminum chloride. Another alumina, prepared by aluminum isopropoxide hydrolysis, produced similar results. Just as addition of silica or boria to alumina produces catalytic acidity, so does the addition of fluoride ion, although for different reasons. Dealkylation activity increases approximately in proportion to the amount of fluoride added. However, this activity declines quite rapidly, in contrast to the constancy observed for the other catalysts, hence the data are not shown. The addition of water accelerates the decline in activity, by removal of fluoride; a similar situation n-ab observed in the boria-alumina system. V. Discussion It seems perfectly clear from the fact that these catalysts possess dealkylation activity that Bronsted acids of some sort must exist on the surfaces of each. If the severity of drying prior to each run were to be increased markedly to drive off firmly bound water no doubt the activity would decrease. The fact that in most cases the prior treatment with reasonably dry nitrogen failed to reduce activity indicates that these sites are quite stable. If one accepts the Tamele view of silica-alumina that Bronsted acids are hydrated Lewis acids, then the
Vol. 65
water of hydration is very firmly bound; this is not too surprising. Additional evidence for this strong binding may perhaps be the observation that high partial pressures of water failed to increase dealkylation activity of silica-alumina or Clay-K ; otherwise less strongly bound hydrates would have to be postulated. The differences in activity among the various catalysts may be ascribed to differences in the preexponential factor and not to the activation energy, except for Clay-M. Hence, it is most probable that Bo, the number of sites per gram, is the factor responsible for the different activities. Prater and Lagols have as a matter of fact calculated a value of Bofrom cumene dealkylation data, using absolute reaction rate theory, which is in agreement with values obtained by quinoline adsorption. Clay-1\I is known to require mater during catalytic cracking for best performance. Silica-magnesia is known to be a material which is readily dehydrated and hydrated. Hence it is not too unexpected that one should observe an effect of moisture on the Bronsted acid activity of these catalysts. It is not necessary to postulate, however, that Lewis acids are formed upon dehydration unless demonstrated by actual experiment, such as has been done for silica-aluminaQ; rather, one can only say at this point that Bronsted acid sites are lost upoii dehydration. The water of hydration in silica-magnesia is less firmly bound than in silicaalumina. An alternative way of expressing the effect of water is to consider that the rates observed are the results of both the log A and the E, terms. Since both of these tend to increase somewhat with increasing water concentrations, whether or not a change in reaction rate occurs depends upon how well these opposing effects balance. Thus, they balance very well in the case of silica-alumina until an excess degree of hydration is reached, but not as well with silica-magnesia for which an increased activity is observed, again until overhydration occurs. According to this picture, the presence of water vapor will cause the creation of more sites upon adsorption, but these sites will be weaker than the majority of sites already present. It should be noted that there is no correlation among the various types of catalyst between dealkylation activity and n-butylamine titer. Hence these techniques measure different aspects of catalyst surfaces. Acknowledgments.-The authors are grateful to Dr. h l . L. Bender, Dr. 0. H. Thomas and Dr. S. H. Bauer for helpful discussions, to Mr. R . D. Duncan for preparation of the figures, and to Sinclair Research, Iiic., for peimissjon to publish this paper.
