P. E. EBERLY, JR.
1042
High-Temperature Infrared Spectroscopy of Pyridine Adsorbed on Faujasites by P. E. Eberly, Jr. Esso Research Laboratories, Humble Oil & Refining Company, Baton Rouge Refinery, Baton Rouge, Louisiana (Receicrd April 4, 1967)
Results are presented on the infrared spectral studies of pyridine adsorbed on various ion-exchanged faujasites at 100-260". Pyridinium ions reflecting Brinsted acidity are detected by an infrared band near 1540 CITI-~. Coordinately bound pyridine has a band near 1450 cm-l which can be used as a measure of Lewis acidity. With hydrogen-faujasite, the adsorbed species consist predominantly of pyridinium ions formed by reaction with the structural OH groups having an infrared absorption frequency of 3635 cm-l. Little reaction is observed with the groups at 3540 crn-l. With the alkali metal exchanged faujasites, no pyridinium ions are observed, indicating the surface acidity to be predominantly Lewis in nature. The K form appears to be the least reactive toward pyridine. The Ca, Mg, and Cd forms after evacuation at elevated temperatures exhibit both Brgnsted and Lewis acidity. With Ca and Mg faujasites, the ability to form pyridinium ions is greatly enhanced by the addition of small amounts of water. These cations are believed to perturb neighboring hydroxyl groups enabling the hydrogen atoms to protonate adsorbed pyridine. The marked similarity between the spectrum of pyridine on the water-treated Ca form and that on hydrogen-faujasite suggests that the materials may have some of the same structural features. The formation of -Ca-OH and -OH groups upon exposure to water at 260" is indicated.
I. Introduction The relationship between catalyst acidity and activity in carbonium ion type reactions such as cracking and isomerization has been the subject of a number of investigations.' Although a wide variety of techniques to measure acidity have been reported, the recent method involving infrared studies of adsorbed pyridine has several advantages. I n view of the distinct difference between the spectrum of the protonated form of pyridine and that of coordinately bound pyridine, one can readily distinguish between the Br@nstedand Lewis types of acidity. Parry2 and, more recently, Basila, Kantner, and Rhee3 have employed this technique for characterizing the surfaces of silica, alumina, and amorphous silica-alumina cracking catalysts. The interaction between pyridine and silica is very weak, mainly involving the formation of hydrogen bonds. Alumina, on the other hand, possesses strong Lewis acidity. Silica-alumina catalysts exhibit both Br@nsted and Lewis acidity, and small amounts of water can transform the Lewis into Br@nstedsites. Liengme and Hall4 and, more recently, Hughes and White6 demonstrated that pyridine reacts with hydroxyl groups on hydrogen-faujasite to form pyridinium ions. The extent of this reaction is reported to decrease as the hydroxyl groups are removed by more severe calcination. The present investigation deals with infrared studies of the interaction of pyridine a t temperatures up to 260" with various ion-exchanged faujasites. The effect of small amounts of water on the distribution of Br@nsted and Lewis type sites is explored. The experimental techniques and equipment used for hightemperature infrared studies have been previously describedSG The Journal of Physical Chemistry
11. Experimental Section Materials. Two different samples of Na-fauj asite ( S a y ) were investigated and their compositions are given in Table I. Although the composition of NaY(11) was nearly the same as S a Y ( I ) , its infrared transmission properties were inferior. Possibly, it had a greater concentration of larger particles causing more scattering of the incident radiation. HY(1) was prepared by first exchanging the sodium with ammonium ions. Upon calcination, the NHdY liberates n", forming HY. For this purpose, 1SO g of NaY(1) was treated with a solution of 333 g of ;1;H4N03dissolved in 3 1. of water. The exchange was conducted for 2 hr at 70". After allowing it to settle and decanting the supernatant liquid, a fresh solution of ",NO8 was added and the treatment was repeated. After a total of five such treatments, the solid was filtered, thoroughly washed, and oven-dried at 150" overnight. Analysis of this sample is given in Table I. For our earlier studies on other ion-exchanged faujasites, KaY(I1) was used as the starting material. The various forms were prepared by ion exchanging with the chloride salts of the desired ion. I n general, a threefold excess of the ion was employed and the treatment was repeated three times. The degree of exchange accomplished is included in Table 11. (1) L. B. Ryland, M. W. Tamele, and J . N. Wilson, "Catalysis," 7, Reinhold Publishing Corp., New York, N . Y., 1960. (2) E. P. Parry, J. Catalysis, 2 , 371 (1963). (3) M. R . Basila, T. R. Kantner, and K . H. Rhee, J . Phys. Chem., 6 8 , 3197 (1964). (4) B. V. Liengme and W. K. Hall, T i a n s . Faraday Sac., 6 2 , 3229 (1966). ( 5 ) T. R. Hughes and H. M.White, J . Phys. Chem., 71, 2192 (1967). (6) P. E. Eberly, Jr., ibid., 71, 1717 (1967).
1701.
HIGH-TEMPERATURE
INFRARED
SPECTROSCOPY O F
PYRIDINE
Table I: Composition of Na and PI"4 Faujasites Compn as expressed by empirical formula, moles NaY(1) N&Y(II) NH4Y(I)
Na20
1.13 0 1.00 4.72
("4)@
Altos Si02
1.07 0 1.00 4.88
0.08 0.87 1.00 4.67
Table 11: hbsorbance of Pyridine Bands a t 150'" Ion-exchanged form
HY(I) CdY (11) MgY ( I1) CaY(I1) Li(I1) Na(I1) WI)
% exchange
92 73 67 75 64 100
95
e/?, A-1
... 2.1 3.1
2.0 1.7 1.1 0.75
--Absorbance/g of solid-1545 om-1 1490 om-' 1450 om-1
7.0 0.57 0.41 0.33 0 0 0
m
1.8 1.3 1.0 0.29 0.27 0
0.8 6.8 3.0 4.1 2.1 1..2 0
a Exposed to 0.1 mm of pyridine and then evacuated for 30 min.
Experimental Procedure. For infrared examination, the solids were ground with a mortar and pestle. Then they were compressed under 30,000 psi pressure into 1.25 in. diameter disks. These disks were about 8-16 mils thick and contained 13-19 mg of solid/cm2. These were placed in a sample holder and inserted into the high-temperature infrared cell. Details of the cell and associated equipment have been previously described.6 The samples were initially degassed for several hours at 427". The temperatures were then lowered to the desired value and spectra were recorded with a Cary-VVhite Model 90 infrared spectrophotometer. Generally, spectra were obtained in the region of 4000-1200 cm-l at a spectral slit width of 4 cm-1 and a scan speed of 3 cm-1 sec-l. Spectra were measured both before and after exposure to 0.1 mm of pyridine vapor. I n certain cases, the samples were pretreated with small amounts of water to determine its effect on the distribution of Breinsted and Lewis sites. To cancel out the absorption of infrared radiation by the gas phase, a dummy cell was placed in the reference beam and connected to the same vacuum and gas dosing system. 111. Results P y r i d i n e Adsorption on NaY(1) and H Y ( 1 ) . From previous studies on the spectra of pyridine complexes and pyridine adsorbed on various solids, it has been possible to distinguish between various types of adsorbed specie^.^-^ The protonated form of pyridine is best characterized by a band near 1540 cm-'. Coordinately bound pyridine formed by interaction with Lewis type sites has a band near 1450 cm-1.
1043
Infrared spectra of pyridine adsorbed on HY(1) at 260" are given in Figure 1. HY(1) was prepared from the ammonium form by evacuation at 427" and has three characteristic OH bands at 3740, 3635, and 3540 cm-l. Because of the nature of these bands, the uncertainty in frequency assignment is estimated to be f5 cm-'. Other investigator^^-^ have reported their occurrence at 3750-3744, 3677-3640, and 35703540 cm-l, respectively. Upon exposure to 0.1 mm of pyridine at 260°, large changes in the spectrum can be observed. Pyridine reacts quite selectively with the OH groups responsible for the 3635-cm-I band. Very little change occurs in the intensity of the two remaining OH bands. This interaction results in the formation of pyridinium ions almost exclusively as evidenced by a strong band at 1540 cm-l (not shown) and the near absence of a band at 1450 cm-'. This is consistent with previous spectral studies at lower t e m p e r a t ~ r e . ~I n~ ~contrast to the broad ill-defined bands generally seen on amorphous silica-alumina catalysts12J three distinct XH stretch vibrations are observed at 3230, 3160, and 3130 cm-l. These probably reFult from varying degrees of hydrogen bonding and indicate some degree of order in the interaction of the adsorbed species with oxygen ions in the lattice. These are nearly 30 cm-' lower than those previously listed for the N-H stretch for chemisorbed protonated pyridine.2-4 In addition, a new band is observed near 2160 cm-1 (not shown) which is in the region characteristic of amine salts.lOJ1 Pyridine hydrochloride, for example, has a pronounced band a t 2100 cm-I. In the CH stretch region, the band at 3080 cm-l is characteristic of such vibrations in the pyridine molecule. It is surprising, however, to observe bands at 2970 and 2880 cm-' showing the formation of saturated CH linkages. In fact, the positions of the bands appear to be more nearly characteristic of CH, rather than CH2 groupings. This indicates that HY(1) at 260" possesses some hydrogenation-dehydrogenation activity. The adsorbed material is rather tenaciously held on the surface and is not extensively removed even after a 16-hr evacuation a t 260". Some desorption occurs, however, as indicated by a general lowering in intensity of the infrared bands and also by the partial reappearance of the OH band at 3635 cm-l. A similar set of experiments was performed with (7) C. L. Angel1 and P. C. Schaffer, J. Phys. Chem., 69,3463 (1965). J. B. Uytterhoeven, L. G. Christner, and W. K. Hall, ibid., 69, 2117 (1965). (9) J. L. White, A . N. Jelli, J. M. And& and J. J. Fripiat, Trans. Faraday Soc., 63,461 (1967). (10) R. N. Jones and C. Sandorfy, "Chemical Applications of Spectroscopy," W. R. West, Ed., Interscience Publishers Inc., New York, N. Y.,1956,Chapter V. (11) C. N. R. Rao, "Chemical Applications of Infrared Spectroscopy," Academic Press, Inc., New York, N . Y.,1963, Chapter IV. (8)
Volume 78, Number 8 March 1908
P. E. EBERLY, JR.
1044 I
I
I
1
I
I
I
I
I
-
3800
3600
3400
3200
3000
2800
FREQUENCY I N CM.-'
Figure 1. Infrared spectra of pyridine adsorbed on HY(1). Spectra were recorded a t 260' with a disk originally containing 19 mg/cm2. The lines have the following significance: , original spectrum under vacuum; , spectrum under 0.1 mm of pyridine; - - - -, spectrum after overnight evacuation.
-.-.-.
NaY (I). No structural hydroxyl groups, however, could be observed by infrared techniques. The interaction of pyridine at 260" was considerably weaker. Much less material was adsorbed and nearly all of this existed in the form of Lewis-bonded rather than protonated species. The adsorbed material could be completely removed by only a 15-min evacuation at 260". Pyridine Adsorption o n Other Ion-Exchanged Faujasites. Changing the nature of the cation in the faujasite cages is known to affect markedly the material's catalytic properties.12J3 Consequently, it becomes of interest to investigate the effect of ion exchange on the surface acidity as determined by pyridine adsorption. A summary of intensities of key infrared bands recorded at 150" is given in Table I1 for a series of ionexchanged faujasites. The 1540-cm-l band reflecting the amount of Brqinsted acidity is most pronounced on HY(1). With the divalent faujasites, its intensity is reduced almost 10-20-fold. The existence of this small amount of Brqinsted acidity indicated that some hydroxyl groups still exist on the surface and are capable of protonating the adsorbed pyridine. Several investigations have demonstrated the great difficulty in completely removing the hydroxyl groups by extensive evacuation a t elevated temperatures.*J4 The most pronounced band on the divalent faujasites is that at 1450 cm-' characteristic of coordinately bound pyridine. Its intensity is much larger than that on HY (I). With the monovalent faujasites, no Brqinsted acidity is observed even though trace amounts of hydroxyl The Journal of Physical Chemistry
-
groups would still be expected to be present.* Lewis acidity, however, is still present in LiY(I1) and NaY(11). The potassium form is the least acidic and exhibits no pyridine absorption bands at these conditions. Effects of H20 o n Pyridine Adsorption. With amorphous silica-alumina catalysts, it has been shown that Lewis sites can be partially transformed to Brqinsted sites by exposure to small amounts of water.3 This effect is not observed with S a y . However, with CaY and AlgY, trace amounts of water have a very pronounced effect on the relative distribution of the two types of acidity. Spectra illustrating this for CaY(I1) at 260" are shown in Figure 2 for the 38002800-cm-' region and in Figure 3 for the 1500-1200cm-' region. From the initial spectrum (A) under vacuum, little evidcnce is seen for hydroxyl groups in the OH stretch region of 3800-3500 cm-'. Upon exposure to 0.1 mm of pyridine (spectrum B), the adsorbed species consist primarily of coordinately bound pyridine as evidenced by a strong band near 1440 cm-'. Some pyridinium ions, however, are present having the characteristic 1540-cm-l absorption band. The concentration is not sufficient to produce observable vibrations in the NH stretch region. Spectrum C was (12) V. J. Frilette, P. B. Weisz, and R. L. Golden, J. Catalysis, 1, 301 (1962). (13) P. E. Pickert, J. A. Rabo, E. Dempsey, and V. Schomaker, Proc. Third Intern. Congr. Catalysis, 714 (1964). (14) J. L.Carter, P. J. Lucchesi, and D. J. C. Yates, J. Phys. Chem., 68, 1385 (1964).
1045
HIGH-TEMPERATURE INFRARED SPECTROSCOPY OF PYRIDINE
10
8
6
4
2
4 GY
ul
p
a
I
I
I
I
I
1
I
I
I
E bp
8
6
3800
3600
3400
3200
FREQUENCY IN CY.
-'
3000
2800
Figure 2. Effect of HzO on pyridine adsorption on CaY(I1) a t 260'. Spectra were recorded on a disk containing 15 mg/cm2. The various spectra, were obtained in series a t the following conditions: A, initially under vacuum; B, exposed to 0.1 mm of pyridine; C, reevacuated for 16 hr; D, exposed to 1.5 mm of HzO; E, reevacuated briefly and exposed to 0.1 mm of pyridine; F, evacuated 1 hr.
obtained after evacuation for 16 hr and is nearly identical with the original spectrum (A). When the solid is exposed to a pressure of 1.5 mm of water, OH groups are formed and also some bulk water is adsorbed. The spectrum taken under these conditions (D) shows OH groups near 3550 and 3650 cm-' and also a water bend vibration near 1630 cm-l, The faujasite was then briefly evacuated (10 min) a t 260" and the material was reexposed to 0.1 mm of pyridine. The resulting spectrum (E) is totally different from that originally observed before water addition. The final spectrum (F) was taken after an evacuation of 1 hr. The creation of Brprnsted acidity by water addition is clearly seen by the pronounced increase in intensity
of the 1540-cm-l band. Additional evidence is provided by the appearance of three NH stretch vibrations a t 3230, 3160, and 3130 cm-l. This type of acidity apparently results from water reacting with the sites formerly responsible for Lewis acidity since the 1440cm-l band is much less pronounced after the water addition. Similar phenomena are also observed with the magnesium form of faujasite. Initially, the material reacts with pyridine to produce largely coordinately bound species. However, water transforms the Lewis to Brprnsted sites resulting in a much increased intensity of the 1540-cm-' band. Volume 78,Number 9 March 1068
P. E. EBERLY, JR.
1046
30
ao
10
3! B
4
o
h
be
30
20
10
0 1800
1600
1400
FREQUENCY IN CM.-l
Figure 3. Effect of H20 on pyridine adsorption on CaY(I1). Spectra were recorded under the same conditions as listed in Figure 2.
IV. Discussion The hydroxyl group structure of HY has been the subject of a number of i n v e s t i g a t i ~ n s . ~ - ~The J ~ lowintensity band near 3740 cm-‘ is almost universally observed on silica-containing materials and hence it is not necessarily characteristic of HY. Since zeolites are rarely completely free of amorphous material, it could easily result from an OH group on the impurity. Different interpretations have been applied to the remaining two intense OH frequencies near 3635 and 3540 cm-l. Angel1 and Schaffer7 have attributed the lower frequency band to interaction between two adjacent hydroxyl groups. Liengme and Hall,4 however, suggested that the two bands may result from OH groups in different crystallographic positions. Our present results on the preferential interaction of pyridine with the OH groups at 3635 cm-l are similar to those of the latter authors. Consequently, the data favor the explanation that the OH groups a t 3635 cm-l are inside the adsorption cavities in a position capable of direct interaction with pyridine. Similar groups at 3540 cm-1 could be in inaccessible bridge positions located between sodalite units. However, in a recent investigation by Hughes,6 the lower frequency groups were found to interact with piperidine and, conseThe Journal of Physical Chemistry
quently, must be close to the large adsorption chambers. The variation in the type and amount of acidity with the nature of the cation is particularly interesting. The Brpinsted acidity, for example, appears to increase with the polarizing power of the cation as defined by the ratio of ionic charge to radius ( e / r ) . This latter quantity can only be considered as an estimate since the ionic radius has been shown to depend on the nature of the cry~ta1.l~Since no information is currently available on the size of these ions in zeolites, Pauling’s crystal radii were used.16 These results suggest that cations of high polarizing power are able to perturb nearby hydroxyl groups causing the hydrogens to become acidic enough to protonate pyridine. Richards o d 7 has suggested this type of reaction and his esr results on the effect of polarizing power of the cation on the ionization of aromatics are in line with our acidity measurements. With the monovalent ions, no Br@nsted acidity is observed. Basila and K a n t n e P reported on the acidity of amorphous silica-alumina materials and for a K-poisoned sample containing only Lewis acidity, the absorption coefficient for the 1490cm-l band was only one-fourth of that for the 1450cm-’ band. This is the same ratio that we observed on sodium faujasite. However, with LiY(I1) which also shows only Lewis acidity, the intensity of the 1490-cm-‘ band is only about one-eighth of the 1450cm-’ band. Apparently, the ratio of these bands cannot be considered to be independent of the nature of the cation and complicates the estimation of the relative amounts of Lewis and Brgnsted acidity. The effect of trace amounts of water on the distribution of the two types of acidity for the Ca and Mg faujasites is especially significant. Since these ions are thought to perturb OH groups resulting in their protonation of adsorbed pyridine, the amount of Brpinsted acidity is limited by the number of OH groups. When these are increased by addition of water, the pyridine bound to Lewis sites becomes protonated and the intensity of the infrared band at 1540 cm-’ increases. The striking similarity between the spectrum of pyridine on water-treated CaY (spectrum F, Figure 2) and pyridine on HY (dashed line, Figure 1) suggests that the materials may have some of the same structural features. For example, water could be reacting with the calcium ions to form -Ca-OH and -OH (15) K. B. Harvey and G. Porter, “Introduction to Physical Inorganic Chemistry,” Addison & Wesley, Reading, Mass., 1965, Chapter 11, p 24. (16) L. Pauling, “The Nature of the Chemical Bond,” 3rd ed, Cornel1 University Press, Ithaca, N. Y . , 1960, p 514. (17) J. T. Richardson, paper presented a t Symposium on Mechanisms of Heterogeneous Catalysis, 153rd National Meeting of the American Chemical Society, New York, N. Y . , Sept 1966, Preprint Vol. 11, NO. 4, A-123 (1966). (18) M. R. Basila and T. R. Kantner, J. Phys. Chem., 7 0 , 1681 (1966).
ntEASUREMENT OF
ACTIVITIES IN GALLIUM-INDIUM LIQUIDALLOYS
groups on the surface. The latter would be similar to those on HY and if in cage positions could react with pyridine to form pyridinium ions. Indications of the formation of these OH groups are given in spectra D, E, and F of Figure 2. Analogous effects occur on MgY. With the Na form and presumably the Li and K forms as well, the polarizing power of the ions is not sufficient to dissociate significant amounts of water to
1047
form structural OH groups. However, Basila, Kantner, and Rhee3 do report that water on a K-poisoned amorphous silica-alumina catalyst transforms a small portion of the Lewis into Brgnsted sites. Their studies were done at 150” rather than the 260” used in the present study. This low temperature could have caused the concentration of adsorbed species to be sufficiently high to detect their existence.
Measurement of Activities in Gallium-Indium Liquid Alloys’ lby G. J. Macur, R. K. Edwards, and P. G. Wahlbeck Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois
60616
(Received September 65,1967)
Activities for both components have been determined for the Ga-In system. Activities were derived from effusion measurements utilizing the multiple Knudsen-cell effusion technique. Positive deviations from ideality occur for the activities of both components. Entropies of mixing were derived from the free-energy data of this study and calorimetric data recently reported in the literature. Integral entropies of mixing were found to be less than values for ideal solutions.
Introduction Thermodynamic information for binary liquid metal alloys is important to the advancement of theories of metals. I n the work being reported, thermodynamic activities in the Ga-In system have been determined experimentally. No direct experimental measurements have been reported in the literature. Activities were obtained from Knudsen effusion measurements leading to evaluations of partial pressures of the alloy components. The effusion measurements were performed by means of a “multiple Knudsen-cell” technique.2 Essentially, the technique consists of performing simultaneous measurements for a large number (up to 14) of alloy compositions, each of which is contained within its own effusion cell, and all cells are contained within the same isothermal zone (large molybdenum block). This technique was developed specifically to achieve for each selected temperature a high relative precision in that set of data. Thus, improved consistency should be achieved in evaluations making use of the Gibbs-Duhem integration method, since the integration appropriately is being performed for a set of data which are truly isothermal. This improvement is desirable, particularly if no direct measurement is made of the composition of the effusing vapor, but instead, compositions are obtained by the “calculation m e t h ~ d . ” ~Since both components are volatile in the Ga-In system, compo-
sitions of the gas phase must necessarily be evaluated. The calculation method was chosen because it greatly simplifies the experimentation, although it does demand data of high relative precision. Previous information bearing on the thermodynamic activities of the gallium-indium system is of an indirect nature. Svirbely and Read, utilizing information on the ternary Zn-In-Ga system, calculated activities for this binary system. Brossa and Bros, Castanet, and Laffitte5b have recently reported calorimetric measurements of enthalpies of mixing for the Ga-IA system.
Experimental Section Apparatus and Procedure. The total rates of effusion of gallium and indium from 14 alumina Knudsen cells under isothermal conditions were measured with an ap-
(1) Based on a thesis by G. J. Macur submitted to the Illinois Institute of Technology, Chicago, Ill., in partial fulfillment of the requirements for the Ph.D. degree, 1965. Presented before the Physical Chemistry Division a t the 150th National Meeting of the American Chemical Society, Atlantic City, N. J., Sept 1965. (2) G. J. Macur, R. K. Edwards, and P. G. Wahlbeck, J. Phys. Chem., 70, 2956 (1966). (3) R. K . Edwards and M. B. Brodsky, J . A m . Chem. Soc., 78, 2983 (1956). (4) W. J. Svirbely and 9. M. Read, J . Phys. Chem., 66, 658 (1962). (6) (a) J. P. Bros, Compt. Rend., 263, 977 (1966); (b) J. P. Bros, R. Castanet, and M. Laffitte, ibid., 264, 1804 (1967).
Volume 76, Number 3 March 1968
