STERLING E. VOLTZAND SOLW. WELLER
574
Appendix Integrated Forms for Equation 5.-A. metric condition, a = b mknt = 4(a ~
1
- x)
[ (m - 1) + "2(a -1 - x)
Vol. 62
StoichioThe equation is valid for b > a; when a > b the equation is valid for Ib - a1 < K/16. When Ib - a1 > K/16, d r becomes imaginary and in place of the second term is substituted
-
The various symbols in these equations are m = ks/2kd = 4.2 t
~
B. Non-stoichiometric condition, a # b B 4
mkd = - In
B
4
a(b - x) b(a - x) ~
+
fi
=
residence time in reaction space
=
m
+
b-a
a
B = - A b-a
+ dF
u
+
=*E + Zi(b - 2) 16
uo =
r
i-g + Zib
K2
=E+
K(b - a )
EFFECTS OF AMMONIA ON THE ELECTRICAL RESISTIVITY OF SILICA-ALUMINA CATALYSTS BY STERLING E. VOLTZAND SOLW. WELLER
4
Contribution from the Houdry Process Corp., Marcus Hook, Pa. Received November BO, 1967
Ammonia and amines markedly increase the conductivity of silica-alumina catalysts below 200". The increased conductivity is closely associated with large amounts of physical adsorption, which probably increases the mobilities of the current carriers. The irreversible adsorption of ammonia below 200' is independent of temperature and is a measure of the acid sites on the catalyst. The reversible adsorption increases with decreasing temperatures. The increase in conductivity depends on the properties of both the adsorbate and the catalyst.
Introduction The work reported in this paper is concerned with the effects of ammonia and amines on the electrical resistivity of silica-alumina catalysts. Cook and co-workers' have reported that quinoline increases the conductivity much more than water. They attributed this effect to the influence of quinoline on the shift in coordination of surface aluminum ions from the 6- to the 4-coordinated state. Schmidt2 also has reported that water vapor increases the conductivity of both catalyst dust and cement dust. Recently Weisz and co-workers3 found that sodium and potassium increase the conductivity of silica-alumina catalysts a t about 400" and higher. They think the increased conductivity results from ionic conduction of sodium and potassium ions. These results are similar to those found with glasses. Numerous attempts have been made to measure (1) M. A. Cook, R. 0. Daniels, Jr., and J. H. Hamilton, THIS JOURNAL, 68, 358 (1954). (2) W. A. Schmidt, Ind. Eng. Chem., 41, 2428 (1949). (3) P. B. Weiss, C. D. Prater and K. D. Rittenhouse, J . Chem. Phys., 21, 2236 (1953); 23, 1965 (1955).
the acidic properties of silica-alumina catalysts.4-8 These studies have included aqueous titrations, non-aqueous titrations and gas adsorptions. I n a great deal of this work, correlations between measurements of acidity and catalytic activities (for acid catalyzed 'reactions) were observed. The additions of basic solutions to silica-alumina decrease both the acidity of the catalyst and the activities for various acid catalyzed reactions. Direct correlations between acidity and activity have been observed in many instances. 4 , 9 , lo The mechanism of the chemisorption of compounds such as ammonia on a silica-alumina catalyst is not yet completely clear. A recent approach to this problem that shows promise is the (4) A. G. Oblad, T. H. Milliken, Jr., and G. A. Mills, "Advances in Catalysis," Academic Press, Inc., New York, N. Y . , Vol. 111, p. 199, 1951. This reference cites most of the previoua literature. 67, (5) C. J. Plank, Anal. Chem., Z4, 1304 (1952); THISJOURNAL, 284 (1953). (6) 0. Johnson, ibid., 69, 827 (1955). (7) H. A. Benesi, J . A m . Cham. Soc., 78, 5490 (1950). 61, 405 (8) R. L. Richardson and 6 . W. Benaon, THISJOURNAL, (1957). (9) P. Stright and J. D. Danforth, ibid.. 67, 448 (1953). (10) J. D. Danforth, ibid., 68, 1030 (1954).
May, 1958
EFFECTS O F AMMONIA ON ELECTRICAL RESIST~VITY O F SILICA-ALUMINA CATALYSTS
11.0 application of infrared spectroscopy. 11,12 The work reported here demonstrates that ammonia and amines decrease the resistivity of a sil10.0 ica-alumina catalyst a t temperatures below 200". During the adsorptions of these gases on silicaalumina the resistivities go through maxima a t 9.0 about 100". The current carriers appear to result from the chemisorption of ammonia (or amine) d on the silica-alumina. The increased conductivity 8 8.0 at lower temperatures requires the presence of 4 ,large amounts of physically adsorbed ammonia (or amine) ; the latter probably increases the mobility 7.0 of the current carriers. Experimental 6.0
I
575
1 60
The adsorption studies were carried out in a high vacuum system. I n most experiments the silica-alumina catalysts were evacuated overnight a t 500" prior to adsorption meas5.0 lo urements. All the adsorption studies were carried out a t 1.0 2.0 3.0 400 mm. The adsorption data are given in cc. (S.T.P.)/g. catalyst. 1031~. The resistances were determined with a Wheatstone Fig. 1.-Resistivity-adsorption isobar of ammonia on bridge. The powdered catalysts were placed in a vertical silica-alumina (Houdry Type S-46). Pyrex tube between platinum electrodes. A metal weight was placed on the upper electrode. The resistivities 11.0 1 60 (volume) were calculated from the resistances and the dimensions of the powdered catalyst bed. These values are dependent on factors such as packing and particle sizeI3; 50 care was taken to minimize these effects. In this study, the most important result is the variation of the resistivity 2u with temperature and adsorption in each experiment. In each single experiment the above factors are constant. The catalysts used were Houdry synthetic silica-alumina catalysts of various surface areas and cracking activities. The cracking activities were determined by the "CAT A" test which measures the activity of the catalyst to crack a gas oil to gas01ine.l~ A catalyst designated 5-46 produces 46 wt. % gasoline under the conditions of the test. The experimental conditions used in each experiment are given in the next section along with the results. The total adsorption was that measured after a high temperature evacuation. Reversible adsorption is defined as that part of the total adsorption that can be removed by evacuation a t the temperature of the adsorption. The reversible adsorption was determined by measuring the adsorption following a previous adsorption and evacuation 0 (all a t the same temperature). Irreversible adsorption is 1.0 2.0 3.0 defined as that part of the total adsorption that is not re103/~. moved by evacuation at the temperature of the adsorption isobar of methylamine on (irreversible adsorption = total adsorption - reversible Fig. 2.-Resistivity-adsorption silica-alumina (Houdry Type 5-46). adsorption). All evacuations were carried out to pressures of 1 X mm. or less. The reproducibilities of both the adsorption and resistivity 10.0 measurements were reasonably good. In most instances, duplicate experiments were carried out.
1
Results The effect of ammonia on the resistivity of silicaalumina is shown in Fig. 1. This shows a plot of log p ( p = resistivity in ohm-cm.) versus l / T ( T = absolute temperature) for a silica-alumina catalyst (Houdry Type 5-46) cooled in ammonia (400 mm.) from 500" to room temperature. A maximum is observed a t about 100". The ammonia adsorption is also shown in Fig. 1. The adsorption is only a few cc./g. a t 500" but increases markedly with decreasing temperature. The relation of the adsorption to the increased conductivity will be discussed later. I n contrast to the results in ammonia, when the
9.0
d. 8.0 ha
s
7.0 6.0
5.0
2.0 3.0 1031~. I. B. Cutler, Fig. 3.-Resistivity-adsorption isobar of dimethylamine on silica-alumina (Houdry Type 5-48).
(11) R. 0. French, M, E. Wadsworth, M. A. Cook ibid., 68, 805 (1954). (12) J. E. Mapes and R. P. Eischens, ibid., 68, 1059 (1954). (13) T. J. Gray, "Chemistry of the Solid State," Butterworths Scientific Publications, London, Chap. 5, 1955. (14) J. Alexander, Proc. Am. Pet. Inst., 27, 111, 5 1 (1947).
1.0
same silica-alumina catalyst is cooled in vucuo, oxygen or hydrogen, the plot of log p V S . 1/T is straight over the entire temperature range, with an activa-
576
STERLING E. VOLTZAND SOLW. WELLER
Vol. 62
tion energy for conduction of 17 kcal./mole. The resistivities at 500" in vacuo, oxygen and hydrogen are slightly higher than those in ammonia or amines. 10.0 The adsorptions of oxygen and hydrogen under these conditions (400 mm.) are quite small. I n both instances only about 0.1 cc./g. is adsorbed 9.0 40 a t 500". The adsorption increases as the temperature is lowered, but even at the lower temperature r; 30 the total adsorption is only about 1 cc./g. y 8.0 I4 3 Other nitrogen compounds such as methylamine 2 and dimethylamine also increase the conductivity 7.0 20-g of silica-alumina (Houdry Type S-46) a t low temperatures. Data for these compounds are given f! in Figs. 2 and 3. In both cases, maxima are ob6.0 1OFJ. served in the electrical resistivities a t about 100". The adsorptions are comparable in magnitude with 5.0 ! ammonia adsorption. 1.0 2.0 3.0 In the case of ethylamine, no maximum in resistivity was observed as the temperature was lowered 10S/T. Fig. 4.-Resistivity-adsorption isobar of ethylamine on t o 25" (Fig. 4). It is probably significant in this connection that the magnitude of the adsorption silica-alumina (Houdry Type S-46). is only about one-third of that for ammonia adsorp11.0 __ 60 tion. These results suggest the interdependence of the conductivity (at lower temperatures) and adsorption. 10.0 50 A Variations in adsorptions might be expected be@ tween these four nitrogen compounds (ammonia d and amines). The physical adsorptions would be 9.0 40 expected to increase with increasing relative pres.g 0 sure (p/pO); thus, less physical adsorption should r; Q occur a t a given temperature and pressure with 9 8.0 ammonia than with the amines. The basicity c;) 6 constants of ammonia and these amines are about the same; the chemisorptions on a strong acid 7.0 catalyst such as silica-alumina should, therefore, be E similar. The adsorptions in decreasing order are 1 0 4 ammonia > methylamine > dimethylamine > eth6.0 ylamine. This order is the reverse of that predicted from considerations of the relative boiling 5.0 0 points and basicity constants. The reason for the 1.0 2.0 3.0 observed order is not known. Other effects such as 10a/x. steric factors probably play important roles. Fig. 5.-Resistivity-adsorption isobar of ammonia on The energies of activation for conduction besilica-alumina (Houdry Type S-16). tween 300-500" in ammonia, methylamine and dimethylamine are about 24 kcal./mole. These 60 values are higher than those in vacuo, oxygen or hydrogen. The resistivity-adsorption isobar of ammonia on 5o . a silica-alumina catalyst is dependent on the cata$ lyst. Data for two less active catalysts are given 4 0 2 in Figs. 5 (Type 5-16) and 6 (Type 5-32). The 8 maxima in the resistivity curves for the 5-16 and .+ S-32 catalysts are not as sharp as with the Type S30 46 catalyst (compare with Fig. 1). The ammonia adsorption at lower temperatures decreases with ~a decreasing cracking activity. 20.g Attempts to relate ammonia adsorptions and E electrical resistivities of silica-alumina catalysts a t 500' to other properties were not too successful. lo The differences in adsorptions and resistivities a t this temperature for catalysts of different catalytic activities were negligible. However, ammonia ad1.o 2.0 3.0 sorptions on silica-alumina catalysts a t lower temperatures are larger and can be related to other lOa/T. Fig. 6.-Resistivity-adsorption isobar of ammonia on properties. Cracking activities, surface areas, quinoline chemisorption numbers and ammonia silica-alumina (Houdry Type S-32). 11.0
60
502
>
g f
304 20.i
-
$
4
May, 1958
EFFECTS OF AMMONIA ON ELECTRICAL RESISTIVITY OF SIL~CA-ALUMINA CATALYSTS 577
adsorptions for a series of silica-alumina catalysts are summarized in Table I. The ammonia adsorptions (total, reversible and irreversible) increase with increasing cracking activities, surface areas and quinoline chemisorption numbers.
TABLE 111 EFFECTS OF AMMONIAADSORPTION ON RESISTIVITY OF SILICA-ALUMINA (HOUDRY TYPES-46) AT 30"
TABLE I
Evac., 500" 3 . 5 x 107 >101' Evac., 30" NHs (400 mm.), 30" 3.9 x 108 43.P >10" Evac., 30" NHa (400 mm.), 30" 5 . 0 X IO8 32. 7b a Treatments were carried out in this order on a single Total ammonia adsorption occurring catalyst sample. after previous treatments.
PROPERTIES OF SILICA-ALUMINA CATALYSTS Relative cracking activity
Surface area, m.z/g.
Quinoline chemisorption no., meq./g.
Ammonia adsorption,n cc./g. ReIrreTotal versible versible
0.7 4.7 4.0 0.013 42 6.5 0.9 .015 7.4 69 8.9 1.6 .024 10.5 * 28 104 20.1 14.2 5.9 .O5Ob 139 32 18.7 6.3 .041 25.0 212 40 24.7 10.4 .070 35.1 46 305 "50", 400 mm.; pretreatment = evacuation, 500", overnight. * This result appears high; however, a duplicate determination gave even a slightly higher value. 16
24
The electrical resistivities of these catalysts were also determined. Even under conditions where ammonia adsorption changed the electrical resistivity by a factor of 103, the resistivities did not reliably distinguish between catalysts of different activities, surface areas and acidities. The effects of the reversible and irreversible ammonia adsorptions on the electrical resistivity of Type S-46 silica-alumina catalyst a t 200 and 30' are illustrated in Tables I1 and 111. In each case, the series of treatments was carried out on a single sample of catalyst. At 200" the difference in the electrical resistivities in ammonia and vacuum (prior to exposure to ammonia) is at least a factor of lo3. At 200" the total and reversible ammonia adsorptions for this catalyst are 20.8 and 10.2 cc./ g., respectively. The irreversible adsorption is the difference or 10.6 cc./g. The reversible ammonia adsorption is about the same as the irreversible absorption. It is noteworthy that the reversible ammonia adsorption changes the resistivity by a factor of 100. After the last evacuation at 200" the catalyst in Table I1 was cooled to 30"; the resistivity at 150" and lower were too high to measure. TABLE I1 EFFECTS OF AMMONIA ADSORPTION O N RESISTIVITY OF SILICA-ALUMINA (HOUDRY TYPES-46) Treatment"
Resistivity, ohm-cm.
AT
200"
Ammonia adsorption, cc./g.
Evac., 500" 6.6 X lo' Evac., 200" >10" 2.2 x 108 20 :Sb NHa (400 mm.), 200' Evac., 200" 3 . 3 x 10'0 NHI (400 mm.), 200" 1 . 9 x 108 10.2b Evac., 200" 5.0 X 10'" a Treatments were carried out in this order on a single catalyst sample. Total ammonia adsorption occurring after previous treatments.
At 30" the difference between the resistivities of silica-alumina in ammonia and vacuum is also a t least a factor of lo3 (Table 111). At this temperature the total, reversible and irreversible ammonia adsorptions are 43.7, 32.7 and 11.0 cc./g., respectively. The reversible adsorption is about three
adsorption, Ammonia
Treatments
Resistivity, ohm-cm.
GO./&
times the irreversible adsorption. It is noteworthy that the irreversible adsorption a t 30" is about the same as that a t 200".
Discussion The catalytic activity of silica-alumina for acidcatalyzed reactions is thought to result from structural effects in the silica-alumina Silicaalumina exhibits acid properties even at room temperatures. Cook and co-workers' have postulated that the increased conductivity of silicaalumina in the presence of quinoline is due to mobile aluminum ions in the surface. They proposed that quinoline converts the surface aluminum ions largely from the 6-coordinated state to the 4-coordinated state and that the aluminum ions in the 4-coordinated state are extremely mobile leading to a high conductivity. The effects of ammonia and amines on the conductivity of silica-alumina catalysts reported in this paper are very similar to those reported by Cook and co-workers' for quinoline. The mechanisms of conduction in both cases are also probably quite similar. It seems more reasonable that the current carriers are ammonium ions (or equivalent) and not aluminum ions released from the silicaalumina lattice. The question of whether the acid properties of silica-alumina are due to Lewis or Bronsted type acids is still actively being investigated. Evidence suggests that both types of acids are probably present on the surf&ce. The chemisorption of ammonia on silica-alumina results in the formation of ammonium ions from the reaction of ammonia with protons on the catalyst surface. This is responsible for the lower resistivity in ammonia than in vucuo, over the temperature range 300-500". The exact mechanism of the increased surface conduction resulting from ammonia adsorption is difficult to deduce from the results of this study. The current carriers on the surface are probably ammonium ions which can migrate on the surface. The mobilities of chemisorbed species are increased by the presence of a physically adsorbed layer. This has been demonstrated in the case of hydrogen sorption on tungsten.16 Large amounts of physically adsorbed ammonia are required for the increased conductivity observed in the adsorption of ammonia on silica-alumina catalysts. The mobility of ammonium ions on the surface is probably increased (15) R. Gomer and R. Wortman, J. Chem. Phus., 23, 1741 (1955).
578
STERLINGE. VOLTZAND SOLW. WELLER
by the presence of a physically adsorbed layer of ammonia. The increased conduct,ivity could also be accounted for by proton transfer on the surface. It is conceivable that a proton (of an ammonium ion) could be transferred to an adjacent adsorbed ammonia molecule. Alternately, the presence of the physically adsorbed ammonia layer may change the energetics of the surface so as to increase the mobility of the surface protons. Increased mobility of the protons would lead to increased conductivity. The major defect in this latter proposal is that very few protons would be expected to exist as such in the presence of large amounts of adsorbed ammonia. The conductivity of a silica-alumina catalyst when ammonia or an amine is adsorbed depends on the factors: 1, the amounts of adsorption; 2, the numbers of ions formed on the surface; 3, the charges of the ions formed on the surface; 4, the mobilities of the ions (current carriers) on the surface; 5 , the molecular sizes of the adsorbates. Some of these factors are interdependent t o some extent. The amounts of adsorption can be easily determined experimentally and good estimates can be made for the molecular sizes of the adsorbates. The ions have unit positive changes. The numbers and the mobilities of the current carriers on the surface are difficult to estimate or determine experimentally. I n semiconductors the conduction occurs predominantly by movement of electrons or positive holes and it is relatively easy to determine both the number and mobilities of current carriers from the conductivity and Hall constant. It is also possible to distinguish between surface and bulk conductivities in many of these cases. The molecular sizes of the adsorbates can be calculated on the assumption that the adsorbed molecules have the same packing on the surface as the molecules h'ave in the solid or liquid states.16 The densities of ammonia and the amines used in this study are about 0.7 g./cc. The effective Folecular areas are about 10 A.z for ammonia, 15 A.2 for methylamine and 19 A.2 for dimethylamine and ethylamine. (16) S. Brunauer, "The Adsorption of Gases and Vapors," Vol. I, Princeton University Press, Princeton, N. J., p. 287, 1943.
Vol. 62
The total adsorptions of ammonia, methylamine and dimethylamine on silica-alumina (Houdry Type 5-46) at 30" are between 40 to 50 cc./g. catalyst. These adsorptions correspond t o surface coverages of 50% and higher. The amounts of ammonia irreversibly adsorbed a t 30 and 200" on silica-alumina (Houdry Type S46) are the same (11 cc. NHa/g. cat.). This is equivalent to 0.49 milliequivalent ammonia per gram catalyst. This figure should be a reasonable estimate of the maximum number of acid sites 011 the surface. This value is seven times larger than the quinoline chemisorption number for this catalyst. This result is not surprising since quinolineis a much weaker base than ammonia and is a much larger molecule. The acidity of a similar catalyst determined by aqueous acid-base titrations is about 0.34 milliequivalent per gram which is remarkable agreement with the irreversible ammonia adsorption. The similar resistivities and activation energies in vacuo, oxygen and hydrogen suggest that the number and kind of current carriers are the same in all three cases. The acidic properties of the catalyst would not be expected to be altered very much by the adsorption of small amounts of oxygen or hydrogen. The conductivity of silica-alumina catalysts under these three conditions may be largely due t o the migration of protons on the surface and possibly in the bulk. The presence of traces of impurities such as alkali metals may contribute appreciably t o the conductivity of silica-alumina in vacuo,oxygen or hydrogen. Weisz and co-workers3 suggested this possibility in their work. The difference in activation energies (vacuum, oxygen or hydrogen us. ammonia or amines) is probably due to the fact that the principal currentcarrying species is different in the two cases. In ammonia it seems reasonable to conclude that most of the conductivity occurs on the surface. The physically adsorbed ammonia offers an excellent medium for the passage of ions. I n addition, the relative size of the ammonium ion, the internal structure of silica-alumina, and the temperature regions studied are not favorable for ionic condition (by ammonium ions) in the bulk.
