9045
J . Phys. Chem. 1993,97, 9045-9047
Effects of Temperature on 129Xe NMR Spectroscopy of Xenon in Zeolite rho Matthew L. Smith,t David R. Corbin,# and Cecil Dybowski'9t Department of Chemistry and Biochemistry and Center for Catalytic Science and Technology, University of Delaware, Newark, Delaware I971 6-2522, and Central Research and Development, Du Pont Company,$ Experimental Station, Wilmington, Delaware 19880-0262 Received: May 17, 1993'
Zeolite rho has an unusually flexible framework, which may be an important factor in its catalytic selectivity. We report studies of the NMR spectroscopy of xenon sorbed in this zeolite structure which are sensitive to temperature and loading. The results indicate that the structure is sensitive to both xenon loading and temperature.
Introduction Zeolites have been known to exhibit small structural distortions,' induced by the adsorption of small molecules2 or by temperature.3 Since many models for catalytic selectivity of zeolites rely on steric effects on diffusion, changesin the structure may play a significant role in determiningthe catalytic properties. The framework of zeolite rho provides a unique structure whose flexibility is substantially greater than that of many other zeolties.4 This flexibility of the rho structure has been implicated in the selectivecatalytic production of monomethyl- and dimethylamines over trimethylamine from methanol and ammonia.5 The details of this catalytic process are still a matter of investigation.6 Initial variable-temperature 129XeNMR measurements have shown that xenon is in rapid exchangebetween two sites in zeolite rho, with the exchangerates and residence times of xenon strongly temperature de~endent.~ We report more extensive 129XeNMR data on the gas adsorbed in zeolite rho, from which we derive thermodynamic data on the relative stability of the sites. The data are interpreted in terms of changes in the zeolite structure with both loading and temperature.
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Figure 1. 129XeNMR chemical shifts of adsorbed xenon as a function 300 of uptake for various temperatures: X 195 K; 245 K 273 K K, + 320 K.
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Results and Discussion
Figure 1 shows the 129XeNMR chemicalshift for the adsorbed xenon in H,Cs-rho as a function of gas uptake and temperature. Na,Cs zeolite rho was synthesized using a modification of the At temperatures below 320 K, the plots of xenon chemical shift method described by Robson.* Na,Cs zeolite rho was ionas a function of uptake display two regions, with the chemical exchanged with ammonium nitrate and subsequentlycalcined at shift being approximately a linear function of uptake in each 673 K to give H,Cs zeolite rho. X-ray diffraction analysis of the region. The transition from one behavior to another occurs at material show that it is highly crystalline. Chemical analysis a specific uptake at each temperature. At 195 K there is a jump showed it to have a unit cell formula of Hlo.~7Nao.4~Cso.7~All~-discontinuity in the chemical shift at the transition uptake, but Si36096. at other temperatures the chemical shift is continuous at the The sample was pretreated for NMR spectroscopy by the transition. following procedure: About 0.3g of the zeolite was loaded into We fit each region at each temperature to the truncated form a resealable NMR cell9 attached to a glass, grease-freemanifold, of Fraissard's equation: with which it was outgassed to 2 X l W Torr at 294 (f2) K. s'(n,T) = 6 , , ' ( ~ ) + 6,'(T)n (1) Subsequently,the sample temperature was raised to 673 K over 6 h with the sample under dynamic vacuum. It was maintained where60'is theextrapolatedshift at lowuptake,61'is thecoefficient at these conditions for 24 hand then cooled to 294K. The sample of shift due to xenon-xenon collisions, and the superscript i was loaded with a known amount of xenon (Air Products and indicates either the chemical shift below (Y") or above ("11") the Chemicals,99.99%), equilibrated for approximately 15 min, and transition. sealed. With a known total amount of xenon gas in the NMR Figure 2 shows how the uptake at the transition depends on sample tube, the amount of xenon adsorbed at different temtemperature. At 320 K, no transition is observed, indicated on peratures could be determinedusing published isotherms? NMR the figure by a point at zero xenon uptake for this temperature. spectroscopy was performed with a Bruker W M 2 5 0 spectrometer In Figure 3 are shown the variations of 60' and 60" with operating at 69.19 MHz for the 129Xe resonance. All chemical temperature. For region I, 60' changes by 110 ppm over the shifts are reported relative to the extrapolated chemical shift of range from 195-320 K. For region 11,the change over this same bulk xenon gas at P = 0. Downfield shifts are considered positive. temperature range is only about 60 ppm. 60' and Son converge to a common value in the region around 320 K (Figure 3). At f University of Delaware. 160 K the NMR spectrum shows xenon is not in rapid exchange Du Pont Company. between the two sites and two resonances are observed above 5 1 Contribution no. 6490. Xe per unit cell (Figure 4). *Abstract published in Aduance ACS Absrracts, August 15, 1993.
Experimental Section
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0022-365419312097-9045%04.00/0 0 1993 American Chemical Society
9046
The Journal of Physical Chemistry, Vol. 97, No. 35, 1993
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Smith et al.
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F i i 2 . Xenonuptakeat the transition point as a function of temperature.
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Figure 4. Dependence of xenon chemical shift on uptake at 160 K.
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