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J. W. WARDAND H. W. HABGOOD
fessor Harrison Shull for stimulating discussions and for his kind support. He also wishes to thank Dr.
Darrell D. Ebbing and Dr. Norman T. Huff for their very useful comments regarding this work.
The Infrared Spectra of Carbon Dioxide Adsorbed on Zeolite X
by J. W. Ward' and H. W. Habgood Contribution No. 323from the Research C o u d l of Alberta, Edmonton,Alberta, Canada (Received October 21, 1966)
Spectra of carbon dioxide adsorbed on the Ca, Sr, and Ba forms of zeolite X showed no evidence of bent, or carbonate-like, structures such as were previously found for carbon dioxide adsorbed on LiX, NaX, and KX. This difference is believed to be due to the absence, in zeolites containing divalent cations, of any cations in the highly exposed typeI11 sites. Carbon dioxide on MgX did show some carbonate-like bands in the 17501300-cm-l region but there was evidence that only partial replacement of monovalent cations had been achieved. The spectra of carbon dioxide on both group I-A and group II-A zeolites showed a strong band between 2375 and 2350 em-', the frequency being higher the stronger the electrical field of the cation, which is believed to be due to a linear species held by an ion-dipole interaction. This band was accompanied (in most cases) by two pairs of weaker side bands symmetrically spaced about the central band.
A previous communication from this laboratory2 described the infrared spectra of carbon dioxide adsorbed at low coverages on the lithium, sodium, and potassium forms of zeolite X. These spectra each included a band near 2350 cm-l attributed to carbon dioxide adsorbed in a linear or near-linear configuration onto the exchangeable cations and at least two pairs of bands in the region 1750-1250 cm-l ascribed to carbon dioxide chemisorbed in a bent configuration onto a surface oxygen to give a carbonate-like structure. The bands near 2350 cm-l showed a slight dependence of frequency upon the nature of the cation while those ascribed to the bent forms varied widely from one cation to another. These observations have now been extended to the alkaline earth ion-exchanged forms of the zeolite and also to higher carbon dioxide coverages, and a number of unexpected results have been found. The Journal of Physical Chemistry
Experimental Section The procedures were similar to those reported previously. The various zeolites were prepared by ion exchange of the sodium form (Linde Lot No. 13916) using 5% solutions of the appropriate chlorides. LiX, KX, CaX, SrX, and BaX were essentially 100% exchanged while in the magnesium zeolite only 70% of the sodium was exchanged and the amount of magnesium taken up corresponded to 35% of the magnesium being added as MgOH+ rather than as Mg2+. A sample of RbX was also prepared but because of the small amount of material available, the degree of exchange was not determined. The pellets were outgassed at 450-500", and doses of carbon dioxide were added at room temperature up (1) Research Council of Alberta Postdoctoral Fellow, 1962-1963. (2) L. Bertsch and H. W. Habgood, J. Phys. C h m . , 67, 1621 (1963).
INFRARED SPECTRA OF COz ADSORBED ON ZEOLITEX
Table I: Observed Frequencies in Vicinity of
Zeolite
First band (strongest sites)
LiX NaX KX RbX MgX CaX SrX BaX
2365 2351 2349 2349 2374 2367 2355 2350
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Band of Carbon Dioxide Adsorbed on Various Cationic Forms of Zeolite X Additional bands in order of appearance (with displacement from initial band)
2425 (+74), 2425 (+76), 2415 (+66), 2403 (+29), 2433 (+66), 2422 (+67), 2415 (+65),
to an equilibrium pressure of a few millimeters. Spectra were obtained after each dose and occasionally several spectra were taken at various intervals up to 1 or 2 days to detect any slow processes. As in the previous work, a Perkin-Elmer 221G spectrometer was used.
Results The frequencies of the observed absorption bands of carbon dioxide are listed in Tables I and 11. Table I summarizes the observations between 2500 and 2200 cm-l for both the group I-A and group 11-A cationic forms of zeolite X, and Table I1 gives the bands observed below 2000 cm-' for the group 11-A forms. The bands between 1800 and 1300 cm-l for carbon dioxide on LiX, NaX, and KX were described previously. Of the group 11-A zeolites, MgX behaves differently with carbon dioxide than do the others. Initial adsorption of a dose of 200 F of carbon dioxide produced bands at 2374, 1734, and 1382 cm-'. On standing up to 96 hr, the 2374-cm-l band decreased in intensity, whereas the band at 1734 cm-l increased in a manner analogous to that reported for group I-A zeolites. The band a t 1382 cm-' became more intense and split into a doublet, whereas after 23 hr a band at 1625 cm-1 was also observed. The adsorption of a further 400 p of gas followed the same pattern. On increasing the pressure irhially to 10 mm, a band comparable in intensity to that at 2374 cm-' appeared at 2350-2345 cm-l, togethe? with weaker bands at 2403, 2305, and 2290 cm-l. At this stage the presence of a small amount of COz in the gas phase was detected. Expansion of the cell contents several times into a bulb of equal volume caused a decrease in the intensity of the 2350-2345-cm-1 band and a smaller decrease in the 1380-1360-cm-l doublet; similar but more marked intensity decreases were observed on evacuation for 1 min to 150-p pressure. On evacuation for 10 min to mm, intensity changes were negligible except for the 2374-cm-l band which decreased in absorbance from 0.74 after 1 min to 0.30 after 10 min. All bands
2287 ( -64); 2280 ( - 6 9 ) ; 2280 ( -69); 2305 (-69); 2300 ( - 6 7 ) ; 2288 (-67); 2285 (-65);
2370 (+21), 2330 (-19) 2370 (+21), 2330 (-19) 2340 2290 ( -84); 2350 2350 2370 (+15), 2335 (-20); 2375 (+as), 2320 ( - 3 0 )
2350
Table 11: Observed Frequencies below 2000 Cm-1 of Carbon Dioxide Adsorbed on Various Cationic Forms of Zeolite X Zeolite
MgX CaX SrX BaX RbX
1734
1700
1625
1645
1382 1382 1380 1378 1382
1363 1270 1265 1265 1333
were removed by evacuation overnight at room temperature. The adsorption of carbon dioxide on Ca, Sr, and BaX followed a different form in that no discrete absorption bands in the 2000-1400-~m-~region were observed. Occasionally, broad bands develaped on adsorption of carbon dioxide but their appearance was not consistent, they appeared to have little effect on the rest of the spectrum, and they were easily removed by desorp tion. These bands are thought to be connected with possible changes in the solid. On the addition of 200 p of carbon dioxide, a band appeared near 23502370 cm-1, the frequency depending on the cation. As more carbon dioxide was added, the intensity of this band increased and new bands appeared equally spaced, 66 cm-1, on either side of the main band. At the same time, two weak bands also appeared near 1380 and 1265 cm-l. On further additions of carbon dioxide, other pairs,of bands more or less equally spaced on either side of the v3 band also appeared. The development of the band system around 2350 cm-' for BaX is shown in Figure 1. The principal side band on the low-frequency side is much sharper than that on the high-frequency side. The growth of the low-frequency side band at 2285 cm-l as a function of amount of carbon dioxide adsorbed is shown in Figure 2. The total absorbance at the peak is plotted and the values for high coverage will be influenced by broadening of Volume 70, Number $,
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J. W. WARDAND H. W. HABGOOD
1180
0.06
0.8
-
1.0 1.5 I -a2500 2400
I
I
2300 2200 FREQUENCY, Cm"
Figure 1. Spectra of carbon dioxide adsorbed on BaX (pellet density 19 mg/cm2; standard slit program 927 on PE 221 spectrometer) redrawn from observed spectra to correct for pellet background.
T
1.0
us Q)
lu lu
0
I 2 3 4 5 MOLECULES PER CAVITY
Figure 2. Maximum absorbance a t 2285 cm-l, sharp lower side band, against amount of carbon dioxide adsorbed by BaX.
the main band. Within this limitation, however, the growth of the band is linear with concentration. I n all cases, reduction of the pressure produced a weakening of the bands in the reverse order of their appearance, and after continued evacuation a t room temperature only the initially formed 2350-2375-cm-' band was detectable; complete removal of this latter band required evacuation a t temperatures up to about 200". The Journal of Physical Chemistry
The difference in behavior between the alkaline earth group X zeolites and the previously investigated LiX, NaX, and KX zeolites prompted an examination of RbX and the former three zeolites a t higher pressure. During initial doses, RbX behaved analogously to LiX and KX. However, on further addition of COZ, a pair of bands on either side of the main band a t 2349 cm-' appeared with frequencies of 2415 and 2280 cm-l. Further addition of CO, produced a broad band at 2335-2345 cm-l. Evacuation removed the bands in the 2400-2300-cm-' region but left those between 1650 and 1300 em-' unaffected. These latter bands were removed by evacuation a t elevated temperature. Similar results were obtained for NaX and KX, but with LiX, only the one band at 2365 cm-l was observed. The development of side bands around the v 3 band thus appears to be a general phenomenon accompanying the adsorption of carbon dioxide on all group I-A and group II-A X zeolites with the exception of LiX. Chemisorption to carbonate-like species on group I-A zeolites was previously found to be accelerated by the presence of small amounts of water.2 I n a further examination of this effect, the spectrum of adsorbed DzO was observed while carbon dioxide was added to a pellet of NaX. The use of DzO instead of HzO brought the water spectrum into a more convenient region. The pellet was dehydrated overnight a t 500" under vacuum, cooled to loo", and approximately 0.5 molecule of DzO per cavity was added. A dose of carbon dioxide corresponding to approximately 0.5 molecule per cavity was introduced and the region 2800 to 2300 cm-' was scanned. Chemisorption was complete within the approximately 1 min required for the first scan and the water spectrum showed significant changes due to the presence of carbon dioxide. Figure 3 shows the spectrum before and after the carbon dioxide admission; new bands were produced a t 2682 and 2640 em-'; the sharp band a t 2728 cm-1 was unchanged, but the broad band centered around 2500 cm-1 appeared to decrease in intensity. The strong bands at 1715 and 1365 cm-l, attributed to the carbonate-like species, were similar in appearance to those observed on dry zeolite.2 In this particular experiment the amount of carbon dioxide was sufficient that a small concentration of linearly adsorbed carbon dioxide was also present as indicated by the band at 2350 cm-l.
Discussion The adsorption of carbon dioxide on alkali and alkaline earth X zeolites appears to be of three different types : physical adsorption, chemisorption to a car-
INFRARED SPECTRA OF C02 ADSORBED ON ZEOLITEX
0.5
I
2800
I
I
1
I
I
2600
2400 FREQUENCY, Cm-'
Figure 3. Spectrum of D20 adsorbed on NsX (curve 1) showing the effect of added carbon dioxide (curve 2 ) .
bonate-like species, and adsorption by an ion-dipole interaction. First, a band a t or slightly below 2350 cm-' observed a t higher pressures can be attributed to the physical adsorption of carbon dioxide on the zeolite surface. The pressure dependence, ease of removal by evacuation, and proximity to the gas-phase carbon dioxide frequency are all consistent with this assignment. Chemisorption of a bent carbon dioxide to form a carbonate-like species was previously postulated2 to account for the appearance of a series of sharp bands in the 1750-1300-~m-~region with LiX, NaX, and KX. These observations have been confirmed and extended to RbX which, with carbon dioxide, gives bands a t 1645, 1382, and 1333 cm-', somewhat less well developed than those of K X but fitting into the series from LiX. With the exception of MgX, alkaline earth zeolites do not exhibit well-defined bands for carbon dioxide in this region. For MgX, several bands were observed in the 1750-1300-cm-' region (cf. Table I) similar to those found with the alkali metal zeolites. The apparent absence of carbonate-like species for carbon dioxide absorbed on alkaline earth zeolites is believed to result from the different distribution of cations in these forms as compared with the group I-A zeolites. In terms of the published structure3 of the X zeolites and of calculations of the relative potential energies of the various sites,* it appears that sites of type 11,5 within the six-membered rings, are more stable than sites of type 111, on the cavity walls. While type I11 sites have not yet been confirmed crystallographically, they can be assigned with reasonable probability. I n the group I-A X zeolites some type I11 sites must be occupied by the cations; in the group II-A zeolites only half as many cations are present and these will probably be found only in sites of types I and 11.It was apparent from the work with LiX,
1181
NaX, and KX zeolites that the carbonate-like structures were formed in intimate association with the cations. Cations in type I11 sites are less shielded than those in type I1 sites and have stronger electrical fields in their vicinity. It is reasonable to postulate that the carbonate-like species is associated with a cation in a type I11 site and that they are absent from CaX, SrX, and BaX because no type I11 sites are occupied. The apparent presence of a carbonate-like species with carbon dioxide on MgX may be due to a partial occupancy of type 111 sites resulting from the presence of a larger number of ions, either unexchanged Na+ or MgOH+ as indicated by the analysis. The MgX preparation was unusual also in that weak absorption bands were also observed in the OH stretching region (3740, 3690, and 3630 cm-l) after outgassing at 480", whereas under similar conditions no bands were found with the other zeolites.6 These observations give some support to the presence of MgOH+ but the situation is considered to be still uncertain. The experiments involving DzO suggest that the carbonate-like species may be stabilized by hydrogen bonding with adsorbed water. From the weakening of the broad 2500-cm-l band and the production of new bands at 2682 and 2640 cm-I it would seem that hydrogen bonding between water and carbon dioxide replaced hydrogen bonding of adsorbed water to surface oxygen.2 The third type of adsorption is that which gives rise to the strong band a t low pressures between 2374 and 2349 cm-I. The frequency of this band tends to be higher than that observed for the vs vibration of the free molecule and the shift to higher frequencies is greatest for the more strongly polarizing cations, e.g., Li+, Ca2+, and Mg2+. Peril7 in the adsorption of carbon dioxide on silica-alumina aerogels, found a similar shift in frequency to 2375 cm-'. With CaX, SrX, and BaX, the band intensity at a given pressure or coverage is time independent, whereas with 1lgX and group I-A X the intensity decreases with time paralleling the growth of the bands in the 1750-1300cm-l region. (3) L. Broussard and D. P. Shoemaker, J. Am. Chem. SOC.,82, 1041 (1960). (4) P. E. Pickert, J. A. Rabo, E. Dempsey, and V. Schomaker, Actes Cowr. Intern. Catalyse, Se, Amsterdam, 1964, 714 (1965). ( 5 ) D. W. Breck, J. Chem. Educ.,41, 678 (1964). (6) See J. L. Carter, P. J. Lucchesi, and D. J. C. Yates, J . Phys. Chem., 68, 1385 (1964); H. W. Habgood, ibid., 69, 1764 (1965); and J. B. Uytterhoeven, L. G. Christner, and W. K. Hall, ibid., 69, 2117 (1965),for a discussion of the observation of discrete OH bands in the spectra of dehydrated zeolite X. (7) J. B. Peri, Actes Congr. Intern. Catalyse, 9 , Amsterdam, 1964, 1110 (1965).
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The displacement of a molecular vibration toward higher frequency is seldom observed. Interactions resulting from either solution or adsorption usually lead to a decrease in frequency. Little and Ambergs observed an increase in the vibrational frequency of adsorbed carbon monoxide; they discussed some of the factors which might lead to the increase and suggested a highly polarized structure for the adsorbed molecule. Considering the proximity of the observed band to the 29 vibration of carbon dioxide and also the sensitivity of the frequency of the band to the nature of the cation and the intense electric field'gradients in the vicinity of the cation, analogy with the conclusions of Little and Amberg suggests linear adsorption by an ion-dipole interaction
Such a polarized structure with an increased electron density in the CO bonds should have an increased bond strength giving rise to an increase in frequency. The asymmetry produced in the molecule by the mode of absorption suggested in the preceding paragraph would be expected to result in the v2 vibration becoming infrared active. For CaX, SrX, and BaX, the weak bands at 1380 and 1270 cm-l vary in intensity in a manner roughly parallel to that of the band near 2350 em-]. It is therefore suggested that these bands are the v1 and 2 v z vibrations of carbon dioxide that are normally Raman active and observed in Fermi resonance a t 1388 and 1285 cm-l. I n this respect also MgX behaves differently from the other alkaline earth zeolites. The intensity of the doublet observed at 1380 and 1362 cm-1 does not parallel the intensity
The Journal of Physical Chemistry
J. W. WARDAND H. W. HABGOOD
of the 2374-cm-I band and the assignment to a carbonate-type species seems more probable. No band is observed near 1270 cm-l. The nature of the linear species of carbon dioxide adsorbed on CaX, SrX, and BaX thus appears to be somewhat different from that on MgX and the alkali metal X zeolites. The pairs of bands appearing equally displaced on either side of the main v3 band are difficult to explain. The most pronounced pair and first to appear are spaced roughly 70 cm-I from the main band and consist of a sharp low-frequency component and a much broader and weaker high-frequency componeiit (see Table I and Figure 1). Three possible explanations may be suggested. One is that these bands represent adsorption on cations in different environments but symmetrical displacements would then hardly be expected. A second possibility is that the bands are the result of mechanical resonance between two carbon dioxide molecules adsorbed on the same cation or closely neighboring cations, but this is incompatible with the observed linear increase in band intensity with coverage (Figure 2). The third possibility is that the side bands represent combination frequencies of the v3 vibration of carbon dioxide and of the vibration of the whole molecule against the surface. Such combination frequencies are, indeed, to be expected but the displacement from the central band would also be expected to vary with different cations as does the frequency of the central band, because of the different strengths of binding to the surface. Thus, none of these possible explanations for the side bands is really satisfactory and further study is indicated. (8) L. H. Little and C. H. Amberg, Can. J. Chem., 40,1997 (1962).
