NOTES Acidic Sites on Mordenite
469 1
.
An Infrared
Study of Adsorbed Pyridine by F. R. Cannings The British Petroleum Company, Limited, British Petroleum Research Centre, Sunbury-on-Thamerr,Middlesex, England (Received June 2.4, 1968)
Parry' and Basila, et a1.,2 have used pyridine to elucidate acid-site structures on the surfaces of alumina and silica-alumina. In particular, they were able to differentiate bietween adsorption centers that were either Brfinsted or Lewis acid in character. The rele1667 1429 vant information was obtained from the 1700-1440Wave number, crn-4. cm-1 range of the infrared spectrum and arises from Figure 1. Spectrum of pyridine adsorbed on mordenite after variations in the position of one of the "in-ring" vibrapyridine dosing at 25' and evacuating a t 500" (see Table I). tions of the chemisorbed pyridine, indicating the presence of either pyridinium ions or coordinately bound indicated by the relative ease of removal upon evacupyridine. ation and calcination. We have extended this work to an examination of The generation of the 1462-cm-1 band occurred with the surface of mordenite. The changes in the spectrum mordenite after evacuation above 300". I n Figure 1, of adsorbed pyridine and acid-site numbers after evacuthe separation into two definite absorption bands is ation of the sample at temperatures up to 600" are conclear, and approximately equal numbers of the two sistent with the scheme proposed by Hall and coforms of Lpy are detected (given equal extinction workers3 to account for the formation of defect sites coefficients). on decationated zeolites. An assessment of the numbers of acid sites present on Sodium mordenite was obtained from the Norton Co., the specimen after calcination up to a temperature of Worcester, Mass. It was leached with dilute sulfuric 600" was also undertaken. This was possible with a acid to give an aluminum content of 5.05 wt %, a knowledge of the relative extinction coefficients of sodium content of 0.94 wt %, and a BET surface area bands at 1546 and 1455 cm-l, typical of Brfinsted and of 385 m2/g. A 13-mm diameter disk weighing 6.5 Lewis species, respectively. A satisfactory conversion mg/cm2 was formed a t 10 tons/cm2 and was examined for calculating numbers on one arbitrary scale, factor in a Grubb-Parsons GS2 infrared spectrometer, using namely, Lpy, was obtained in a previous investigation.6 ancillary equipment and techniques already r e p ~ r t e d . ~ The use of the band at 1490 cm-', as outlined by Basila The successive treatments of the disk are described in and Kantner,e is not suitable, as only Brfinsted pyridine Table I, and absorbances, calculated from maximum a band a t this wave number. The (Bpy) produces band heights, are quoted therein. All spectra were run corresponding vibration for Lewis pyridine occurs at at 25". 1496 cm-l. (Refer to Figure 1.) Results and Discussion Table I lists the absorbances of the 1546-, 1462-, and 1455-cm-I bands, calculated on the Lpy scale, and The spectrum of pyridine retained by mordenite after these values are taken to be equal to the relative evacuation at 500" is shown in Figure 1. Bands a t number of acid sites of each type present an the disk. 1623, 1496, and 1455 cm-l are typical of pyridine coIt shows, for example, that after calcination a t 500" and ordinated to a Lewis acid site (Lpy). The additional feature to note i.n the spectrum is the peak at 1462 cm-'. The appearancs of a band at this wave number can be (1) E. P. Parry, J . Catal., 2 , 371 (1963). interpreted as arising from a second distinct type of (2) M. R. Basila, T. R. Kantner, and K. H. Rhee, J . Phys. Chem., Lewis acid site, of greater coordinating power than that 68, 3197 (1964). (3) J. B. Uytterhoeven, L. G . Christner, and W. K. Hall, ibid., 69, characterized by the band at 1455 cm-l. This con2117 (1965). forms with the regular transition in wave number value (4) F. R. Cannings, J. Phys. Chem., 7 2 , 1072 (1968). for the 19b vibration of pyridine adsorbed in a progres(5) When pyridine absorbed on a silica-alumina was contacted with sively stronger manner, commencing with physisorbed water vapor, a complete interconversion of Lewis to Br$nsted acid sites was observed. F r o m the respective absorbances of the 1466pyridine (1440 cm-l), to H-bonded pyridine (1446 and 164fhm-* bands, a conversion factor of 2.61 was observed. cm-l), to coordinated pyridine (1455 cm-l). The More details will be given in a future publication. strength of attachment of the pyridine to the surface is (6) M. R. Basila and T. R. Katner, J . Phys. Chem., 70, 1681 (1966). Volume 78, Number 13 December 1068
NOTES
4692 Table I : Absorbances of Pyridine Retained by Mordenite after a 1-Hr Evacuation a t the Quoted Temperatureazb
._--_.---"Temp of evacuation,
-1546
cm-l--
1462 om -1
(BPY)
OC
-1455
Total absorbance omitting the 1462-om-' band
(LPY)
0.67 0.53
sh
0.13
0.28
N O . 15
0.01
0.48 0.58 0.66 0.62
0.48 0.58 0.66 0.62
0.09
0.63
0.23
0.22 0.03 0.04 0.03 0.05
Nil
0.78
0.70 0.83
0.70 0.83
0.20
0.30
0.12
0.12
0.12 0.30
0.03 0.04
0.09 0.26
0.84 0.48
0.20 0.03 0.03
0.45 0.54 0.63 0.57
0.66
0.66 0.84
0.02
0.67 0.80
0.84
0.84
0.04
0.82
0.78
0.78
0.11
0.80
Total absorbance including twice the absorbance of the 1462-om-' band
0.81
0.81
0.05
0.76
29 200 29 300 25d 400 25d 25" 500 25d 25' 25e 29 600 25d 25"
om-1-
0.12 O\.30
Br9nsted pyridine at 1546 cm-1 and Lewis pyridine a t 1455 a The values were obtained after successive treatments of the disk. cm-l are calculated on the same scale ' The sample after pyridine and water vapor dosing (each a t m. 3 torr) for 72 hr and then The sample after pyridine and water vapor dosing (each at ca. 3 torr) for 5 min and then evacuation. E The sample evacuation. after pyridine and water vapor dosing (each a t ca. 3 torr) for 16 hr and then evacuation. The sample after pyridine and water vapor dosing (each at ca. 3 torr) for 1 hr and then evacuation.
'
Structure I11 represents a strong Lewis acid and one site only is produced from two of intermediate strength. The apparent loss of acid sites upon calcination could be accounted for in this way. The close agreement between calculated acid site population after high-temperature evacuation, when this total includes twice the absorbance of the 1462cm-' band (final column of Table I) and the number detected on subsequent equilibration with pyridine and water vapor at 25" supports this mechanism. The reason for the large decrease in acidity upon calcination at 600" is not known, but it may indicate major structural changes. The mechanism of the reverse reaction for eq 2 is not fully understood. From our work, it appears likely that the change is slow and that the initial product of combination of structure I11 with excess water is a single Brgnsted site. The increase of total acidity of mordenite from 0.48 unit after 5 min to 0.62 unit after
subsequent equilibration with pyridine and water vapor at 25", mordenite has lost some 25% of the total number of acid sites present at lower temperatures. So a system exists where the total number of intermediate strength Lewis acid plus Brpinsted acid sites decreases while some very strong Lewis sites have been formed during the preceding calcination. The number of the latter produced appears less than the quantity of the former lost. The chemical reaction proposed by Hall and coworkersa to explain their observations of changes in calcined X and Y zeolites can also account for the transformations we have noted. The assumption must be made that some sites having structure I1 (eq 1) behave as intermediate strength Lewis acids and that they retain their identity upon adsorbing pyridine, producing a band at 1455 cm-'. Reduction in over-all acid site numbers occurs on disproportionation along the path outlined by eq 2.
0 '
'"'
H
(Bdneted acid)
+
HzO
+
O\
o/
O\
AI
/O Si (intermediate strength Lewis acid)
o/ \o
0 '
I1
I
H 2
O\
AI
O\
o/ \o o/
/O
Si
-+
H,O
(strong Lewis acid)
4-
'0 111
The Journal of Physical Chemistry
(1)
NOTES
72 hr of conta,ct with pyridine and water vapor following evacuation at 500” may be a function of this slow site rearrangement. Total acidity alters little after 16 hr of dosing, and the value obtained after this time is regarded as a correct final measure. Acknowledgment. Permission to publish this paper has been given by The British Petroleum Co., Ltd.
A n Apparatus for Degassing Liquids by Vacuum Sublimation
by T. N. Bell, E. L. Cussler, K. R. Harris, C. N. Pepela, and Peter J. Dunlop’
4693
heptane by keeping the distilling liquid at about 0”, but in addition it was found necessary to degas the mixtures two or three times to obtain reproducible results. The hydrocarbons were always degassed in the presence of sodium wire. The results of static measurements of the vapor pressures of water, benzene, n-hexane, n-heptane, n-octane, and cyclohexane samples degassed in this manner are given in Table I, together with literature values. Table I : Vapor Pressures a t 25’ of Several Hydrocarbons and Water Vapor pressure, torr This work Lit.
Benzene
95.15 I t 0 . 0 3
Department of Physical and Inorganic Chemistry, University of Adelaide, Adelaide, South Australia 6001 (Received June 18,1068)
Cyclohexane
The precision of vapor pressure and virial coefficient measurements depends in part upon the efficiency with which the liquid systems are degassed. Most methods described in the literature are repetitive and consequently often tedious and time consuming. A common procedure2 is that of freezing the liquid, pumping away the noncondensable gases, melting, and refreezing, the process being continued until some experimental parameter is constant. Experience in this laboratory has shown that up to 20 such cycles are necessary with aqueous system^.^ Hermsen and Prausnitz4 have described a method whereby their materials were “refluxed for several days. . . at low pressure,” and Cruickshank and Cutler6 described another involving “several vacuum redistillations.” The method reported in this paper is similar to the technique of “freeze drying,”6 and usually one cycle is sufficient to completely degas aqueous systems and pure hydrocarbons, The apparatus (see Figure 1) consists of a Pyrex glass dewar, the vacuum chamber being the vapor pressure cell. The material to be degassed is frozen, either in the cell itself or in the detachable sampling bulb. (This is also used to sample mixtures for analysis at the end of a run without breaking the vacuum in the system.) The cell is then evacuated and a suitable freezing mixture is placed in the central compartment of the dewar. The frozen liquid slowly sublimes onto the cold glass surface, while noncondensable gases are continuously pumped away (less than 5% of the sample is lost in this process). These gases do not seem to redissolve unless the sublimate thaws. This was found to occur with n-heptane and its mixtures with benzene when the deposit was thick; in these cases anomalous vapor pressure measurements were noted with only one degassing (up to 0.2 torr too high). The effect was prevented in the case of m
97.59 f O . 0 3
n-Hexane
151.16 f 0 . 0 2
n-Heptane
45.79 I t o . 0 2
n-Octane Water
13.97 f 0 . 0 2 23.755k0.003
95.18k0.04 95.15fO.05 95.17 95.04k0.08 95.03f0.04 95.14 i 0 . 0 3 97.583=0.04 97.81i0.04 97.41=!=0.03 151.26f0.04 151.05“ 45.72 k 0.04 45. 64n 13.98” 23.753
Ref
6 C
d e
f g
b h
f b
i 6 i b j
C. B. Willinga Extrapolated using the Antoine equation. ham, W. J. Taylor, J. N. Pignocco, and F. D. Rossini, J . Res. Nat. Bur. Stand., 35, 219 (1945); F. D. Rossini, K. S. Pitser, R. L. Arnett, and G. C. Pimental, “Selected Values of Physical and Thermodynamic Properties of Hydrocarbons,” Carnegie Press, Pittsburgh, Pa., 1953. Reference 4. R. H. Stokes, Department of Physical and Inorganic Chemistry, University of New England, Armidale, New South Wales, private communication. e Reference 2b. D. H. Everett and F. L. Swinton, Trans. Faraday Soc., 59,2476 (1963). W. J. Gaw and F. L. Swinton, Reference 5. * I. Brown, Aust. J . Sci. ibid., 64, 637 (1968). Res., A5, 530 (1952). F. G. Keyes, J . Chem. Phys., 15, 602 (1947).
’
Samples which were degassed more than the minimum number of times stated above gave identical results within the experimental error. A sample of 40 ml of liquid takes from 1 to 2 hr to be degassed. The alkanes were allowed to melt in the sampling cell during the process, but, as mentioned above, it was only (1) To whom all correspondence should be addressed. (2) E.g., (a) J. B. Gilmour, J. 0. Zwieker, J. Katz, and R. L. Scott, J. Phys. Chem., 71, 3259 (1967); (b) G. Korttim and W. Vogel, 2. Elektrochem., 62, 40 (1958). (3) H. D. Ellerton, Ph.D. Thesis, University of Adelaide, 1966. (4) R. W. Hermsen and J. M. Prausnitz, Chem. Eng. Sci., 18, 485 (1963). ( 5 ) A. J. B. Cruikshank and A. J. B. Cutler, J. Chem. Eng. Data, 12,
326 (1967). (6) M. Dixon and E. C. Webb, “Enzymes,” Longmans, Green and Co., Ltd., London, 1958, p 13.
Volume 72,Number 13 December 1068
