3502
J. Phys. Chem. 1980, 84, 3562-3567
Study of Oxygen Mobility in Some Synthetlc Faujasltes by Isotopic Exchange with COP C. Gensse, T. F. Anderson,* and J. J. Friplat Centre de Recherche sur les Solldes Ci Organisation Crlstalllne Imparfaite, C.N.R.S.,45045 Orleans Cedex, France (Received: August 13, 1980)
Oxygen isotopic exchange between C 0 2 and Na-Y, Na-X, and the germanic homologue of Na-X was studied at 25-500 O C by mass-spectrometric analysis of the change in the 1sO/160ratio in the gas circulated across the zeolite. Infrared spectra of C02 adsorption and “reaction” with the X zeolites were obtained under similar experimental conditions. Isotopic exchange is measurable in all of the zeolites at room temperature. However, exchange in Na-X and NaGe-X was much more rapid under comparable conditions than exchange in Na-Y. This difference in behavior must be attributed to the effects associated with the substitution of A13+ and Ge4+ for Si4+in the tetrahedral sites of X zeolites. In particular, the infrared results demonstrate that various chemisorbed “carbonate” species can form on the X varieties. Isotopic exchange is promoted by some of these species, whereas others can evidently block active exchange sites on the zeolite surface. The absence of chemisorbed C 0 2 species on Na-Y accounts for the relatively low rate of exchange in this molecular sieve.
Introduction Zeolites are active catalysts in an extraordinary variety of hydrocarbon conversion Their catalytic activity is due in part to their high capacity for selective adsorption of reactants and cocatalysts. The activity of zeolites in acid-catalyzed reactions, and the poisoning of many zeolite catalysts by bases indicate that acid sites are i n ~ o l v e d .Both ~ Lewis and Bronsted acid sites have been shown to exist, but neither can completely explain the extraordinary activity of zeolite^.^^^ It has been proposed6p7 that zeolite catalytic activity is the result of a dual mechanism involving both Lewis and Bronsted sites. Oxygen isotope exchange reactions are potentially important ways of investigating the nature of active oxide sites on zeolites. The pioneering work of demonstrated the efficacy of isotope exchange in characterizing the reactivity of oxide surfaces. There are very few comparable studies on oxygen mobility in zeolites.lOJ1 Peril’ used oxygen isotope exchange with ClSO2to investigate the active oxide sites on zeolites and several oxide catalysts. Since the adsorption of C02 on zeolites results in many cases in the formation of very stable carbonate specie^'^^'^ it is likely that isotope exchange in C02-zeolite systems is selective for unusually reactive sites. The results suggested that the mechanism of isotopic exchange involved the formation of carbonate species on active sites. Perill used infrared spectroscopy not only to characterize the modification of the surface of the catalyst during the exchange reaction but also to evaluate the extent of isotopic exchange between the adsorbed C02and solid. The lack of precision and sensitivity in estimating changes in isotopic composition by this technique limited the determination of reliable rates of exchange. In order to further assess the potential applicability of isotope exchange in characterizing the catalytic properties of zeolites, we investigated the kinetics of exchange between COz and three synthetic faujasites by using mass-spectrometric assay of the isotopic composition of the gas. The high precision and sensitivity of this method permit us to follow the progress of the exchange reaction over a wide range of reaction time and temperature. In addition, interactions between C02 and two of the zeolite samples under the *Address correspondence to this author at the Department of Geology, University of Illinois, Urbana, IL 61801.
same conditions as the exchange reactions were evaluated by infrared spectroscopy. Experimental Section Materials. Na-Y zeolite (Si/A1 = 2.43) and Na-X zeolite (Si/Al = 1.23) were obtained from the Linde Division of Union Carbide Corp. The germanium homologue of the X zeolite14J5having a Ge/A1 = 1.00 was furnished by L. Lerot, Catholic University of Louvain. C02with an leO/laO ratio 2.2 times the normal abundance ratio (i.e., 0.44 atom % ls0)was prepared by equilibration at 25 “C with water having about the same oxygen isotope composition.16 Apparatus and Procedure. The isotope-exchange experiments were carried out in a Pyrex vacuum system in which the C02 was circulated continuously at a pressure of 95-120 torr over the zeolite sample maintained at a constant temperature. The reaction temperatures ranged from 25 to 500 “C and were maintained to f5 “C. The progress of the exchange reaction was monitored by the decrease in the lsO/leO ratio in small samples of the C02 taken throughout the duration of the m. Maximum times of the experiments ranged from 50 to 200 h. The zeolite powder (100 mg) was loaded in a quartz glass boat and placed in the reaction tube. The silicic zeolites were degassed at 360 “C for 12-18 h at a dynamic residual pressure of lo* torr in order to remove water and other adsorbed species.17 Because the germanic zeolite apparently undergoes structural changes at high temperat ~ r e s , it~ was ~ J degassed ~ at 300 “C and 5 X lo-* torr for the same period of time. Infrared observations by us and others indicate that this high-temperature evacuation removes essentially all adsorbed C02 and carbonate species from the silicic Na zeolites12p20but not from the germanic zeolite.15J9 The 180-enrichedC 0 2 ,which was previously purified of condensable gases, was then admitted to the system at an initial pressure of 110-120 torr, equivalent to 3.9-4.2 mmol. Immediately after introduction, an initial sample of the gas (-0.06 mmol) was taken in a gas pipet. The gas was circulated by a electromagnetically driven piston pump. Vacuum traps in the system were cooled to the temperature of dry ice-trichloroethane in order to remove any residual water vapor which may have degassed from the zeolite. At intervals throughout the exchange run, samples of COz were taken in the gas pipet, purified, and stored
QQ22-3654/8Ql2Q84-3562$Q 1 .QQlQ 0 1980 American Chemical Society
Oxygen Mobillty in Some Synthetic Faujasites
in Pyrex tubes for subsequent analysis. The total quantity of COzremoved from the system as samples was equivalent to less than 15% of the initial quantity. Before the first C02-zeolite exchange experiment and from time to time throughout the course of the experiments, the Is0-enriched C 0 2was circulated in the system while the quartz gllass reaction tube was heated to 200-500 “C. These blank iruns served to “condition” the reaction system to the isotopic composition of the C02. The COz was Circulated until its isotopic composition did not change, -2Q-30 h. After the first blank run, the maximum change in isotopic composition was negligible compared to that observed during an exchange run under the same conditions. The oxygen isotopic composition of the COzsamples was measured in a VG Micromass 602 spectrometer. This instrument collects and simukaneously compares the intensity of the maw 46 ion current and the mass 44 + 45 ion currents. The inlet system is designed to allow rapid switching between the sample of unknown isotopic composition and a startdard gas. This mode of analysis is used to determine very Eimall variations in the isotopic composition of light elemlents in geochemical research and yields a precision in the iratio of 1sO/160in the unknown to that in the standard gas of better than 50 ppm. The infrared investigation of C02-zeolite interactions was conducted with a Perkin-Elmer Model 180 infrared spectrometer. Spectra were obtained under the same conditions of C02 pressure and temperature as the isotopic exchange experiments. The zeolite was prepared as an autocoherent wafer for the transmission spectroscopy. Calculations. The fractional approach to isotopic equilibrium for the C02 at any time t is defined in eq 1,
The Journal of Physical Chemistry, Vol. 84, No. 26, 1980 3563
the gas and th.e solid, respectively, and y is the fraction of oxygen siteer which are exchangeable under the conditions of the exchange experiment. The initial isotopic composition o:f the zeolites, R,(O), was not measured directly. We assume that R,(O) is the average natural abundance ratio. We also assume that isotopic fractionation between C02and the zeolites is negligible, i.e., R,(eq) = R (eq). Substituting this into eq 2 and rearranging terms yielas eq 3. Elubstituting eq 3 into eq 1 and letting r =
ng/n,,we have eq 4. R,(t) (4)
In order to callculate F(t) when exchange and equilibrium occur in a stepwise fashion (model 2), we arbitrarily divide the totalnumber of exchangeable oxygen atoms in the solid into j separate volumes. Exchange proceeds by stepwise isotopic equilibration between these j volumes and the gas. Thus, at the it,h equilibration step (i.e., i = 1, 2, ...,j ) a material balance consideration yields 7% ngRg(i- 1) + j R , ( O ) = n,R,(i)
7%
+jRs(i)
(5)
Assuming that R,(i) = R,(i) it can be shown that
for j 1 10. Su’bstituting eq 6 into eq 1 yields
-_
1 R,(O) where R,(O), R,(t).,and R,(eq) are the 1sO/160ratios initially, at time t, arid at equilibrium (t = a),respectively. R,(t)/R,(O) is measured directly in the mass spectrometer. In order to evaluate F(t), it is therefore necessary to determine R,(eq)/R,.(O), a value which is constant for any given C02-zeolite exchange experiment and depends upon the relative quantities and initial isotopic compositions of the C02 and zeolite and the manner in which equilibrium is approached. We consider two models for the achievement of equilibrium: (1)Portions of the solid which have previously exchanged with the gas are maintained at equilibrium with the continuously changing isotopic composition of the gas. (2) Solid-gas exchange occurs in a stepwise manner such that after a portion of the solid has equilibrated with the gas, its isotopic composition remains constant. In model 1 (continuous equilibration) isotopic equilibrium requires that all portions of the solid are in equilibrium with the final gas. In model 2 (stepwise equilibration) only the last-reacting portion of the solid is in equilibrium with the final gas; the earlier-reaction portions will have higher l80/ l60values, reflecting the composition of the C 0 2 with which they exchanged. Therefore, R,(eq) in model 2 is less than R,(eq) in model 1. For continuous equilibrium between the gas and the solid (model l),the nsotopic ratio of the gas at equilibrium can be calculated firom a simple material balance consideration (eq 2), where ngand n, are the moles of oxygen in ngRg(0)+ rn,R,(O) = n,R,(eq) + yn,R,(eq) (2)
R&)
--I
For the experimental values of r, R,(t)/R,(O), and R,(O)/R (O), Fz(t)= 0.90 - 0.99Fl(t). The relative uncertainty in F(t1 values far either model in a single experimental run is less than dzO.CK)2. The absolute uncertainty in any single value of F(t) is -&4%. The principal sources of uncertainty are the initial isotopic composition of the solid and the weight losci of the solid during the initial vacuum dehydration.
Results The isotope exchange results are shown as plots of F(t) vs. t in Figures 1-3. Values of F ( t ) were calculated for both the continuous (1)and stepwise (2) models, assuming that all lattice oxygens were available for isotope exchange (i.e., y = 1). An estimate of the rate of isotopic exchange for each experiment is given in Table I as the time required to reach 50% exchange in the continuous exchange model. The infrared spectra of COz adsorption on Na-X and NaGe-X are shown in Figures 4 and 5. Nu-Y. The rate of isotope exchange in Na-Y increases systematically with temperature at 1200 “C (Figure 1). The extent of exchange in Na-Y was considerably less at all comparable temperatures relative to the X zeolites, in agreement with Perill There is no indication from the data that some fraction of the oxygen is unaccessible to isotope exchange (ie., y C 1).
3564
The Journal of Physical Chemistry, Vol. 84, No. 26, 1980
Gensse et al.
TABLE I: Rate of Isotopic Exchange Expressed as the Time Required t o Reach F, = 0.5 sieve Na-Y Na-X NaGe-X
temp, " C
time, h
300
>300
400
200
500 200 3 00 100
69
aa
1ga 6 1
200 300
