2364
THE EDITOR
Hydrocarbon Adsorption Effects on the Unit Cell Constant of NaY Zeolites Publicahon costs assisted by Centre National de la Recherche Scient/f/que
Sir: The cubic unit cell constant (U.C.) of Faujasite-type zeolites mainly depends on the relative amount of Si and AI atoms present in the zeolite aluminosilicate framework.1 However, the framework slightly distorts in response to the positions of the exchangeable cations and to the nature of sorbed molecules and these short-range perturbations generally induce some change in the zeolite U.C. For instance, it was shown in a previous paper2 that the U.C. of partly dehydrated NiY zeolites is a linear function of both exchange level and nickel population of hexagonal prisms, but the X-ray investigation of a NiY zeolite containing acetylene3 proved that the relationship no longer holds, namely the U.C. reduction occuring on acetylene adsorption is not correlated to a migration of Ni2+ ions. The purpose of the present note is to determine the reasons why the zeolite lattice undergoes such a contraction on adsorption of CzH2 or other unsaturated hydrocarbons. The NaY zeolite was chosen because the effect is far more pronounced than in NiY zeolite. The NaY zeolite was a Linde SK40 sample. The preparation of the two hydrogen zeolites H29Na27Y and H54Na2Y was described previously.* Every sample was activated for 15 hr in oxygen and for 6 hr under vacuum. The heating temperature was 600" for the NaY sample and 350" for the HY samples. Then the zeolites were placed in contact for 15 hr with 100 Torr pressure of gases previously dried over activated molecular sieves. The treated powder was then transferred into capillaries for the X-ray investigation which was performed according to the procedure previously d e ~ c r i b e d . ~ Table I gives the U.C. of the samples containing various hydrocarbons. It comes out that the magnitude of the U.C. decrease is dependent on the total sodium content of the zeolite ( N a y > H29Y > H52Y = 0) and to the multiple bond character of the hydrocarbon molecule (ethyne > ethylene > ethane = 0 ) . The unsaturated hydrocarbons may interact with the zeolite either by a direct association of the molecule with the framework oxygens or through a bonding with the Na+ ions. The two eventualities have TABLE I: Cubic Unit Cell Constant of Sample
been considered by Tsitsishvili, et a1.,6 to interpret their infrared measurements on zeolites containing acetylene. A recent crystallographic study of the Na A zeolite7 has shown that the CzH2 molecules are associated with the Na+ ions while no significant interaction between the CH groups and the framework oxygens takes place. The same behavior is expected with the present samples in order to prove that the crystal structure of an NaY zeolite was determined before and after acetylene adsorption. The structures were determined from powder data (all reflections with h2 + k2 l2 5 396) according to the refinement procedure previously described.5 The final refinements give R indices of 0.07 and 0.1, respectively, for the reference sample and for the sample containing acetylene. It turns out that the cation distribution on SI, SI', and SI1 sites is not significantly changed. Moreover, no lengthening of any T-0 bond which could have indicated an interaction of a framework oxygen with an acidic acetylene proton is observed. Moreover, a bonding across the sodalite cage such as 0---HC=CH---0 is very unlikely to occur because the distances from the sodalite cage center to all the oxygen atoms of the cage wall are longer in the sample containing CzH2 whereas a shortening of some distances should have been observed. However, the decrease of the distance between the center of the 12-membered oxygens ring (supercage aperture) and the O( 1) belonging to that ring provides possible evidence for a C2Hz bridging two O(1) across the aperture. On the other hand, the Nal1')-0(3) and Na(I1)-O(2) distances are longer-and the T-O(3) and T-O(2) shorter-in the sample contacted with C2Hz than in the reference sample. These results strongly suggest that the CzHz molecules are associated with the Na+ ions of SI' and SI1 sites. This would produce a weakening of the Na(1')-0(3), and Na(I1)-O(2) bonds (which are thus elongated) and a shortening of the T-O(3) and T-O(2) bonds, The U.C. decrease occuring on C2H2 adsorption may subsequently be explained by the shortening of these bonds. The decrease of the zeolite U.C. on adsorption of other hydrocarbons probably proceeds via the same mechanism. The results given in Table I support this assumption since the hydrocarbons effects appear to be clearly related to the multiple bond character of the molecules and to the Na+ ions available as well. The unsaturated molecules are
+
Y Zeolites (in A, f0.01A)
Before gas adsorption
Acetylene
Propyne
Ethylene
1-Butene
Ethane
24.81
24.72 (24.79)a
24.72
24.76 (24 81)"
24 76 (24.79)"
24.81
24.75 24.69
24.71 (24.75)" 24.70
a The values in parentheses correspond to the
U.C.of sample outgassed at room temperature for 6 hr (after gas contact)
The Journal of Physical Chemistry, Vol. 77, No. 19. 7973
2365
Communications to the Editor loosely attached to the Na+ ions since they can be almost entirely removed by outgassing the zeolite at room temperature (the U.C. variations are nearly reversible). Therefore, the unsaturated molecules may well be associated with the Na+ ions through ions to induced-dipole forces as previously ~ u g g e s t e d .This ~ also corroborates the findings of Y a t e ~ . ~ However, -g in contrast with ethylene, the acetylene molecules cannot be entirely removed at room temperature. This again shows that the cation to electrons sorbed molecule bond is stronger when more i~ are present. This study proves that the interaction between unsaturated molecules adsorbed on Y zeolites and Na+ ions is the main cause for the observed U.C. decrease. The variation observed must be taken into consideration before applying relations such as that given in a previous paper.2 References aind Notes J. V. Smith, Advan. Chem. Ser., No. IO?, 171 (1971,). P. Gallezot and B. Imelik, J. Phys. Chem., 77, 652 (1973). P. Pichat, J. Vedrine, P. Gallezot, and B. Imelik, J. Catal., in press. P. Gallezot and B. Imelik, J. Chim. Phys., 68, 34 (1971). P. Gallezot, Y . Ben Taarit, and B. Imelik, J. Catal., 26, 295 (1972). G. V. Tsitsishvili, G. D. Bagratishvili, and N. i. Onashvili, Russ. J. Phys. Chem., 43, 524 (1969). (7) A. A. Amaro and K. Seff, J. Chem. SOC.,22,1201 (1972). (8) J. L. Carter, D. J. C. Yates, P. J. Lucchesi, J. J. Elliott, and V. Kevorkian, J. Phys. Chem., 70, 1126 (1966). (9) D. J. C. Yates, J. Phys. Chem., 70, 3693 (1966).
(1) (2) (3) (4) (5) (6)
lnstitut de Recherches sur la Catalyse, C. N. R.S. 69100 Villeurbanne, France
P. Gallezot B. Imelik*
Received June 29, 1973
Selective Hydrogen Atom Abstraction by Hydrogen Atoms in Neopentane-Alkane Mixtures at 77 K Publication costs assisted by Faculty of Engineering, Nagoya University
Sir: Recently quite interesting phenomena have been reported on the hydrogen atom abstraction reaction by radicals in the sollid phase at 77 K.Is2 Here we report that H atoms produced by the photolysis of hydrogen iodide in neopentane containing a small amount of alkane react selectively with the solute alkane at 77 K. Experimental procedures were identical with those described in the previous ~ t u d i e s . When ~ , ~ the photolysis of hydrogen iodide (0.05 mol %) is performed in neopentane with 2537-A radiation at 77 K, an esr spectrum of the neopentyl radical is obtained. H atoms produced by the photolysis of hydrogen iodide abstract hydrogen atoms from neopentane to form the neopentyl radical. When the photolysis of hydrogen iodide is performsd in neopentane containing a small amount of an alkane such as ethane, propane, or isobutane, a quite different esr spectrum of the solute radical, such as ethyl, propyl, or the t-butyl radical, is obtained. The H atoms produced by the photolysis react selectively with the solute alkane to form the solute radical, even if the solute concentration is very low. The yields of solute radicals in the photolysis of neo-CsHlz-iC4Hlo(l%)-HI(O.O5%) are 35 times as high as those in the
photolysis of neo-CeHlz -HI(0.05%). Therefore, most of the H atoms produced by the photolysis cannot react with neopentane in the pure neopentane matrix, while they can react with the solute alkane in the neopentane-alkane mixture. One possible explanation for the selective formation of solute radicals is that hydrogen iodide and the alkane form a complex and dissolve in juxtaposition in the neopentane matrix. Though the possibility cannot be neglected at present, it may be small for the following reasons. First, there is no evidence to support the idea that the hydrogen iodide forms a complex with the alkane except neopentane. Since the yields of solute radicals in the photolysis of neo-C5H12-i-C4Hlo(l%)-HI(O.l%) are about 6 times as high as those in the photolysis of neo-C~Hl2C3H,(l%)-HI(O.l%), it is expected from the complex hypothesis that isobutane is more favorable for the formation of the complex than propane. Propyl radical, however, is selectively formed in the photolysis of i-CIHIOC3Hs(l%)-HI(O.l%) at 77 K. Therefore, the results cannot be explained by the hypothesis of the complex formation. Secondly, it was found that solute radicals were selectively formed in the radiolysis of neopentane containing a small amount of alkanes.3 The results show that the selective C-H bond scission does occur in the absence of hydrogen iodide and that it is a phenomenon characteristic of the neopentane-alkane mixtures at 77 K. Thirdly, it has been observed that a large fraction of 3-methylpentyl radicals produced by the photolysis of HI in 3-methylpentane glass at 77 K decays within a few minutes, apparently by combination with iodine atoms formed near the same locations in the matrix when the HI is dissociated.* The ~ - C ~ H radicals Q produced by the photolysis of HI in a neopentane-isobutane(l%) mixture, however, do not decay at all at 77 K, even if the sample is stored for 5 hr after the photolysis. Therefore, it seems that the H atoms produced by the photolysis of hydrogen iodide migrate through the neopentane matrix and react selectively with the solute alkane. The yield of t-C4H9 radicals in the photolysis of ~ ~ O - C ~ H ~ ~ - ~ - C ~ H ~ Oat-77 H IK(was O . studO~%) ied as a function of concentration of isobutane. Since the yields become a plateau over 0.2 mol % of isobutane, most of the H atoms react selectively with isobutane at 0.2%. The selective hydrogen atom abstraction by H atoms was also observed in the isobutane-propane mixture. The relative probabilities for hydrogen atom abstraction by H atoms in a variety of alkane mixtures at 77 K are summarized in Table I. It is surprising that the relative probabilities for hydrogen atom abstraction from the solute alkanes are much larger than that from neopentane. The relative probabilities for deuterium atom abstraction in the solid phase are shown in the third column of Table I.5 The relative rate constants of hydrogen atom abstraction by hot D atoms in the gas phase are also given in the fourth column of Table 1.6 The reported relative probabilities for hydrogen atom abstraction from alkanes are of the same order of magnitude as that from neopentane even in the solid phase. The extremely large values for the alkanes in the present work may be caused by the specific physical property of the neopentane-alkane mixture in the solid phase. Classical activation energies of hydrogen atom abstraction by thermal H atoms are shown in the last column in Table I. It should be noted that the activation energy for neopentane is smaller than that for ethane. The present results obtained for the solid alkane mixture are quite mysterious for us. It is conceivable, howThe Journalof Physical Chemistry, Vol. ??, No. 19, 1973
