2234
J. N. FINCHAND ALFREDCLARK
influenced by a salt effect. Such an effect of the ionic medium amounts to about $0.6 pK unit when going from 0.5 ionic strength to zero ionic strength. Bacarella, et al.,zOreported that the shift in pK of acetic acid in 95 vol % methanol-water from its value in pure water is 3.1 pK units at zero ionic strength. On the other hand, studies carried out in waterz1 and the sol~ * ~ ~ that salt vent 40 vol % m e t h a n o l - ~ a t e r ~indicate effects usually are not greater than 0.1 pK unit when going from 0.5 to 1.0 M ionic strength. In conclusion, the present results indicate that a rather dramatic change in the composition of the solvent is required before we can expect anomalous proton
dissociations from protein amino and imidazole groups. Protein carboxyl groups, on the other hand, seem to be fairly sensitive to a medium change involving water. Acknowledgment. This work was supported by a Grant from the Swedish Medical Research Council (Project No B68-13X-2060). (20) A. L. Bacarella, E. Grunwald, H. P. Marshall, and E. L, Purlee, J . Org. Chem., 20, 747 (1955). (21) R. A. Robinson and R. H. Stokes, "Electrolyte Solutions," 2nd ed, Butterworth Scientific Publications, London, 1959. (22) R. G. Bates and D. Rosenthal, J . Phys. Chem., 67, 1088 (1963). (23) D. Rosenthal, H . B. Hetzer, and R. G. Bates, J. Amer. Chem. SOC.,86, 549 (1964).
The Effect of Water Content of Silica-Alumina Catalyst on 1-Butene Isomerization and Polymerization by J. N. Finch and Alfred Clark Phillips Petroleum Company, Bartlesuille, Oklahoma (Received October 26, 1968)
The effect of variations in water content of silica-alumina was investigated for the polymerization and isomerizations of I-butene. The water contents of the catalysts were varied by heat treatments ranging from 150 to 800'. The effect of structural changes in the catalyst was eliminated by a preliminary heat treatment at 800' prior to rehydration and subsequent activation in the selected temperature range. Studies with I-butene indicate that on all catalysts prepared here there was rapid formation of a residue (polymeric complex) which ceased after a few minutes. The results of ammonia-blocking studies imply that this complex is associated with isomerization activity. Both polymeric complex formation and isomerization showed maximum activity on catalysts activated a t 300". Catalysts activated a t lower temperatures were less active presumably because of poisoning or blocking of active sites. For catalysts activated in the 400-600' range the reactions show different linear dependencies on water content. The presence of water above a certain threshold value, 0.45%, appears necessary for the isomerization reaction. The results of the studies with ammonia suggest either that the average degree of polymerization declines from 4 to 2 on dehydration of the catalyst from 400 to 800" or that it remains constant a t about 4 by utilizing fewer active sites. The data here indicate that isomerization activity depends on both the presence of the polymeric complex and the degree of dehydration of the surface. It is concluded that the water serves as a cocatalyst with the polymeric complex t o provide the protons necessary for carbonium ion formation and subsequent reaction.
I. Introduction Numerous investigators have demonstrated that the activity of silica-alumina used in various acid-type catalytic reactions depends in part on the degree of hydration of the In particular the rate of double-bond isomerization of 1-butene has been shown to increase with increasing hydroxyl content of the catalyst.* I n contrast the results of several deuteriumtracer studies indicate that (1) less than 0.2% of the hydrogen available on the surface is directly involved in the reaction and that (2) the isomerization is associated with a strongly adsorbed phase (polymeric complex) The Journal of Physical Chemistry
present on the surface.9-12 Ammonia-blocking studies conducted in this laboratory show that polymeric complex formation and isomerization activity appear (1) R. C. Hansford, Ind. Eng. Chem., 39, 849 (1947). (2) R . C. Haldeman and P. H. Emmett, J . Amer. Chem. Soc., 78, 2917, 2922 (1956). (3) 9. G. Hindin, A. G. Oblad, and G. A. Mills, ibid., 7 7 , 535, 538 (1955). (4) M. Miseno and Y. Joneda, Bull. Chem. SOC.Jap., 40, 42 (1967). (5) V. C. F. Holm, G. C. Bailey, and A. Clark, J. Phys. Chem., 6 3 , 129 (1959). (6) K. V. Topchieva and E. V. Rosolskya, Neftekhimiya, 2 , 298 (1962).
EFFECTOF WATERCONTENT OF SILICA-ALUMINA CATALYST simultaneously.12 One possible explanation consistent with observations is that the hydroxyl groups on the surface function as a cocatalyst.la I n the present investigation we have used ammoniablocking techniques to investigate the polymeric complex formation and isomerization reactions of 1-butene over silica-alumina of varying water content. Our objective was to clarify the roles played by catalyst water content and the polymeric complex in the isomerization reaction.
11. Experimental Section The silica-alumina, designated M-46 by the Houdry Process Corp., contained 87% Si02 and 13% A1203. The water contents of the catalysts were controlled by heat treatments ranging from 150 to 800". This procedure has been used by Holm and Clark.' To minimize the possible effect of progressive structural changes that might affect catalyst behavior at various high temperatures, the catalyst was heat treated initially a t 800" for 4 hr in a stream of dry air. This material was rehydrated by soaking in distilled water for 24 hr and then dried a t 110". About 2.5 g of catalyst was activated in situ for each experiment. The activations were of 16-hr duration a t the selected temperatures up to 800" in a stream of air dried by passage through a tower filled with Drierite and Ascarite, a Dry Ice trap, and finally through a bed of anhydrous magnesium perchlorate. The water contents were determined by similar heat treatments of 1-g samples. These were quickly transferred under flowing helium to tared weighing bottles which were immediately stoppered and reweighed. Later the weight of each sample was determined after ignition at 1200" for 4 hr. Small samples were also prepared for surface area measurement a t each activation temperature. These results are combined in Table I. The catalysts were pretreated with ammonia under static conditions. After activation the catalyst was Table I: Water Content, Sample Weight, Surface Area, and Ammonia Saturation Concentration for Each Catalyst Activation Temperature Aotivation
temp, O C
Unactivated 150 200 300 400 500 600 700 800
Sample wt at aotivation temp
Wt % Hz0
Surfaoe area, m*/g
NHa satn oonon, X IOL-7 ZOOo 300° 400°
--mol/g
7.28 2.38 2.37 2.35 2.34 2.33 2.33 2.33 2.32
2.45 1.91
1.41 0.96 0.64 0.51 0.37 0.20
267 260 280 266 271 260
1.06 1.07 1.07 1.02 1.07
0.554 0.532 0.545 0.532 0.551
0.330 0.340 0.340 0.358 0.369
2235
cooled under vacuum to the desired temperature, and then a known amount of ammonia was admitted to the system. The adsorption process was allowed to continue a t the elevated temperature for 1 hr. The quantity of ammonia required to produce a residual pressure less than 50 p Hg after equilibration at a given temperature was taken as the saturation concentration. The catalyst was isolated from the vacuum system and allowed to cool to room temperature before the flow studies were commenced. The Phillips research grade 1-butene was used as received. The helium carrier gas, Matheson ultra high purity, was dried by passage through a Dry Ice trap. The continuous flow apparatus used in the investigation has been described previ0us1y.l~ It consists of a gas injection system and flow measuring devices coupled to a gas chromatograph (Perkin-Elmer 154D) in which the column normally used for analysis was replaced with a column of catalyst. The column or reactor is attached to a gas-handling system making it possible to evacuate the catalyst and treat it statically with gases at a variety of temperatures and pressures. A second chromatograph arranged to sample the effluent from the flow apparatus was used to determine the concentration of butene isomers in the product. Figure 1is a schematic drawing of a typical flowgram showing the quantities measured with the continuous flow apparatus. Argon was used to determine the "dead space" of the reactor. The following quantities are defined: A = B - C , the amount of residue (polymeric complex) remaining on the catalyst after helium stripping; B = the amount of olefin adsorbed and reacted on catalyst under steady-state conditions; C = the amount of olefin desorbed after termination of injection. All flow experiments were conducted at atmospheric pressure and 25". The helium carrier flow was approximately 50 cc/min (STP). At 35 mm partial pressure of 1-butene the space velocity was about 700 vol of feed per volume of catalyst per hour. Injection of the olefin into the carrier gas stream was usually terminated (7) V. C. F. Holm and A. Clark, Preprints, 2nd Joint Meeting of the American Institute of Chemical Engineers and Instituto De Ingenieros Quimicos De Puerto Rico, Tampa, Fla., May 19-22, 1968,paper 27e. (8) H. R. Gerberich and W. K. Hall, J . Cutul., 5, 99 (1966). (9) A. Ozaki and K. Kimura, ibid., 3, 395 (1964). (10) H. R. Gerberich, J. G. Larsen, and W. K. Hall, ibid., 4, 523 (1965). (11) J. W.Hightower and W. K. Hall, J . Amer. Chem. SOC.,89,778 (1967). (12) A. Clark, Ind. Eng. Chem., 59, 29 (1967). A. Clark and J. N. Finch, Preprints of Fourth International Congress on Catalysis, Moscow, June 1968,paper no. 75. (13) M. R. Basila, T. R. Kantner, and K. H. Rhee, J . Phys. Chem., 68, 3197 (1964). (14) A. Clark, J. N. Finch, and B. H. hshe, Proceedings 3rd International Congress on Catalysis, North-Holland Publishing Co., Amsterdam, 1965, p 1010. Volume 75,Number 7 July 1060
2236
J. N. FINCH AND ALFREDCLARK ACTIVATION TEMPERATURE,C
4.61
800 700 640500 I
','I
I
400
I
300 .v
I
200
!SO8
L
I1
7
j.2
-
w
TIME
Figure 1. Adsorption of 1-C4Hs on silica-alumina.
after 87 min. The isomer distributions used in the rate calculations were determined at the time of the termination of olefin injection. Other investigators have demonstrated that the isomerization of l-butene over silica-alumina follows first-order k i n e t i c ~ . ~ ~ Therefore, J~ the rate constants reported here were calculated from the expression Ict = 2.3 log Xe/(Xe - X) where Xe is the fraction of 2butene in an equilibrium mixture a t 25") X is the fraction converted, and t i n seconds is the time spent by the butene in the catalyst bed. The latter is based on the time required for the helium-butene mixture to flow through 2.5 g of catalyst having a void volume of 2.25 cc. These rate constants are significant for comparison purposes only.
III. Results and Discussion The reactions of l-butene were studied on silicaalumina activated in the temperature range 150-800". This corresponds to varying catalyst water content from 2.45 to 0.201 wt %. I n all cases there was rapid initial formation of a polymeric complex which terminated after a few minutes. The isomerization reaction then continued under steady-state conditions. Some decline in isomerization activity with time was noted. The results-of this study are plotted in Figure 2. Both polymeric complex formation and isomerization show maxima with water content of approximately 1.4 wt % or an activation temperature of 300". The activity of both reactions declines as the water content is increased above the optimal value presumably because of poisoning or blocking of active catalyst sites. Both polymeric complex formation and isomerization decline almost linearly but a t different rates as the water content is decreased from about 1 to 0.5%- The increase in the rate of isomerization with increasing water content supports the published work of Gerberich, et aL8 The increase in isomerization activity with increasing polymeric complex formation in the 0.5-1.0% H20 range is consistent with the previously reported deuterium tracer studies showing that the complex is involved in the isomerization rea~tion.9-l~This interrelationship is based on the following evidence. When 1butene is passed over exhaustively deuterated silicaalumina, the products are found to contain a negligible fraction of the available deterium. Obviously few catalyst deuterons are transferred directly to the isomThe Journal of Physical Chemistry
.2
4
"
3
f
2 =
2 :
-
- ff .I
.3
.5
.7
I
I
I
I
.9
t.1
4.3
t.5
I
I
t J i.9
I
I
2.i
2.3
25
HzO CONTENT, WEIGHT PERCENT
Figure 2. Polymeric complex formation and isomerization of 1-butene over silica-alumina.
erized products. I n contrast, when l-butene is passed over catalyst having an adsorbed phase of perdeuteriobutene on its surface, the products, but not the reactants, are extensively deuterated. I n our studies slugs of l-butene, 3.9 cc, in helium flowing at 50.0 cc/min were passed at atmospheric pressure over 0.5 g catalyst pretreated with 2.7 cc of perdeuteriobutene, all gas volumes expressed at NTP. At 25", the number of deuterium atoms transferred per molecule of butene during conversion of 37% of the l-butene to 2-butene was ten times greater for the products than for the starting material. This appears t o indicate deuterium transfer at the moment of isomerization in agreement with accepted mechanisms.'O Although these studies do not show conclusively that the complex provides the actual sites for isomerization, they do demonstrate that it is involved in the reaction. These results are augmented by our ammonia-blocking studies which show that isomerization does not commence until the complex is present and the more of it that is present, the more isomerization takes place.12 However, the mere presence of the polymeric complex does not appear sufficient for catalytic activity. The present study shows that there can be considerable adsorbed polymeric complex on the surface, as much as 3.0 X mol of butene/g, and yet isomerization does not commence until the water content exceeds 0.45% or 0.56 X 1014 molecules/cm2. Holm and Clark found that propylene polymerization becomes responsive to changes in water content above 0.5 X l O I 4 molecules H 2 0 / ~ m 2 . 7These investigators also studied ammonia adsorption on silica-alumina as a function of water content and found that extrapolations of the curves for adsorption at various temperatures and pressures converged at a water content corresponding to 0.5 X l O l 4 (16) J. W. Hightower and W. K. Hall, J . Phys. Chem., 71, 1014,
(1967). (16) J. W.Hightower, H. R. Gerberich, and W. K. Hall, J . Catal., 7,57 (1967).
2237
EFFECT OF WATERCONTENT OF SILICA-ALUMINA CATALYST
.e
-
.5 -
400 A 800 0 800
o 700
PI
u s .3 d -2 -
-
A
c
I
.i
-
0200
300
400 100
TEMPERATURE, NH3 SATURATION
Figure 3. Isomerization and polymeric complex formation over ",-saturated silica-alumina.
molecules/cm2. They concluded that in each case the presence of water above this critical level was necessary for adsorption and catalysis. This appears to be true for the butene isomerization under the conditions employed here, namely 25" and a butene partial pressure of about 35 mm. The threshold water content probably depends on both the reaction and the conditions employed. For example, Gerberich and Halls found appreciable isomerization of 1-butene at our threshold water content when the reaction was carried out at 112" and at a butene partial pressure of 340 mm. Our data showing the direct dependence of isomerization on water content in the 0.45-1.4% range indicate that water serves as a cocatalyst as previously sugg e ~ t e d . ~ ,Hence, '~ we conclude that the isomerization reaction involves both the polymeric complex and water content. The polymeric complex and isomerization reactions of 1-butene were investigated as a function of both ammonia saturation temperature and catalyst water content. Catalysts activated at 400, 500, 600, 700, and 800" were saturated a t 200, 300, and 400" with the quantities of ammonia given in Table I. As would be expected, the ammonia saturation concentrations varied little with catalyst water content.' Ammonia saturation at temperatures less than 200" virtually destroyed all catalytic activity. I n Figure 3 both polymeric complex, A , and the isomerization rate constant, k, are plotted against the temperature of ammonia saturation. The curves show that for all water contents both polymeric complex formation and isomerization reactions commence on catalysts ammoniasaturated at 200" or above. The increase in isomerization activity with increasing polymeric complex formation is in accord with the deuterium tracer studies showing that the complex is involved in the isomerization process. The results of the ammonia-blocking study can be rationalized by assuming a continuous distribution of site energies on the surface in which there is a sharp
boundary with respect to kT between sites capable and incapable of holding molecules. The higher the preadsorption temperature the higher the lower boundary of the energy required for fixed adsorption. Thus ammonia adsorption at 200" marks the point at which ammonia just covers all the sites involved in polymeric complex formation and accompanying isomerization. Ammonia saturation at 300" leaves the sites capable of holding ammonia a t 200-300" available for subsequent reaction with butene. For catalysts saturated at 400", the butene reaction occurs on sites capable of holding ammonia in the 200-400" range. It follows that over an untreated catalyst the reaction takes place on sites capable of holding ammonia a t 200" and above. The curves in Figure 3 show the increase in polymeric complex formation and accompanying isomerization as more reaction sites become available for reaction.
A NH3 SAT'O 2OOC 0
NHJ SAT'O 300
c
z
c
2
a
3.0
-
2.0
-
v)
'b: . (
t
'I0
A
0
:4
1
I
1
. 1
1
I
I
A NH3 SAT'O 200 C
30
-
0
NH3 SAT'O 300
- ,8
0 NH3 SAT'D 400
-5
e
.20
-
.IO
-
-1
- .3
I4
&
2
- .2 - .r
3.!
3
--b
0WT. %. H20
Figure 5. Isomerization of I-butene over "8-saturated silica-alumina: variation with water content. 'Volume 73, Number 7
July 1869
2238 The polymeric complex and isomerization data shown in Figure 3 are replotted as a function of catalyst water content in Figures 4 and 5, respectively. For comparison similar data obtained in the absence of ammonia are included. These curves show that in the presence of equal quantities of ammonia preadsorbed at the same temperatures both reactions decline with catalyst dehydration. Both polymeric complex formation and isomerization over unpoisoned catalyst and catalysts ammonia-saturated at 300 and 400" show fair linearity with water content ranging from 0.5 to 1%. Only when the catalysts are dehydrated below 0.45% do the reactions become insensitive to water content. Isomerization shows greater dependence on water content than does polymeric complex formation. The role of water in the mechanism of the two reactions may be entirely different. This could arise because of the sequential nature of the reactions : polymeric complex formation terminates shortly after contact with olefin and isomerization continues under steady-state conditions. The polymeric complex formation may have altered drastically the spectrum of site energies on the surface of the catalyst and thereby modified the role of water in the isomerization reaction. The ammonia-blocking data do not give an unequivocal answer concerning the number of active sites and the average degree of butene polymerization on the surface. For example, dividing the number of butene molecules present as polymeric complex at each water concentration by the number of ammonia molecules adsorbed at 200", the amount required to suppress all activity, gives the number of butene molecules per site. Based on the average of the experimental points, these values for water concentrations of 0.969, 0.639, 0.511, 0.374, and 0.201% are 4.0, 3.6, 3.2, 2.8, and 2.4, respectively. This suggests that the average degree of polymerization
The Journal of Physical Chemistry
J. N. FINCH AND ALFREDCLARK on the surface varies from approximately dimer to tetramer over the range of water contents studied. These calculations assume one ammonia molecule occupies one site. These values may be lower bounds because the ammonia may reside on some sites that are not active. The facts that the amount of ammonia required to saturate a catalyst at a given temperature is nearly independent of water content and that both complex formation and isomerization depend on water content indicate that this may be the case. A possible explanation is that the number of sites present, protonic and aprotic, on catalysts activated between 400 and 700" may be nearly constant. Ammonia adsorption would occur on both kinds of sites and be nearly independent of water content. The dependence of complex formation and isomerization on water content would argue that these reactions are associated with the protonic sites which decline in number as the catalyst is dehydrated. Hence, the smaller quantities of polymeric complex produced as the catalyst is dehydrated may be present as tetramer adsorbed on fewer sites. I n summary, our data show that the formation of the adsorbed polymeric complex depends in part on the water content of the catalyst. The data also indicate that isomerization is dependent on both the presence of the absorbed polymeric complex and the water content of the catalyst. We believe that water serves as a cocatalyst with the polymeric complex to provide the protons to form the carbonium ions necessary for reaction.
Acknowledgment. The authors wish to express their appreciation to Phillips Petroleum Company for permission to publish this work and to Mr. R. L. Brown, who carried out most of the experimental measurements.
