4679
J . Phys. Chem. 1986, 90, 4679-4683
Deuterium Solid-state NMR Study of the Dynamics of Molecules Sorbed by Zeolitest Richard R. Eckman*l and Alexander J. Vega Central Research and Development Department, Experimental Station, E. I . du Pont de Nemours and Company, Inc., Wilmington, Delaware 19898 (Received: February 24, 1986)
The dynamics of small organic molecules sorbed by zeolite molecular sieves has been investigated by using solid-state deuterium NMR. T h e systems studied include methanol, benzene, toluene, and p-xylene sorbed by zeolites ZSM-5, mordenite, Y , and erionite. T h e dynamics of uptake or filling of the zeolite void volume has been observed. Intracrystalline molecular motion has been observed and used to characterize the molecular sieve effect. For example, a preferred diffusion of p-xylene in the straight channels of ZSM-5 has been observed, which tends to support the theory of molecular traffic control. The physical state of physisorbed species is described in the light of the deuterium NMR results. The behavior of sorbed molecules is characterized a s that of surface-adsorbed species. The effects of site-selective adsorption and diffusion on the state of sorbed species is described.
I. Introduction We report here t h e investigation of the dynamics of small organic molecules sorbed by zeolite molecular sieves. While the structure of crystalline zeolites is usually well understood by employing modern methods of diffraction and crystallography, what is lacking is an understanding of the dynamics of sorbed s p i e s . This study can lead to further understanding of the many selective effects t h a t can be produced with these materials in chemical reactions, catalysis, molecular separations, and adsorption phenomena. The dynamic processes of interest include thermodynamic adsorption, reorientation or internal molecular motion, and the process of intracrystalline migration or diffusion. We have recently demonstrated t h a t solid-state deuterium NMR is a useful method for the study of t h e dynamics of molecules sorbed by zeolites.' The use of pulsed FT wideline deut e r i u m NMR is well established, and it has been used to study molecular motion in many system^.^-^ The deuterium nucleus has spin I = 1 and a nuclear electric quadrupole tensor interaction which dominates the nuclear spin Hamiltonian. Thus the NMR line shape generally results only from the effect of the quadrupole coupling. Molecular motion can change the line shape, and a model for the motion can be developed t h a t is consistent with the observed line shape. Zeolites are crystalline aluminosilicates represented by the empirical oxide formula M,,,0.Al20,.xSiO2.yH~0, where n is the cation v a l e n ~ e . ~ They consist of a three-dimensional network of AIO, and SiO, tetrahedra linked by shared oxygens and exhibit microscopic pores or channels of molecular dimension. For example, zeolite Y has a three-dimensional internal channel system having super cage^.^ The supercage is some 13 A in diameter with an opening of 7.4 A, and there are eight cages per unit cell. The pentad type synthetic zeolite ZSM-5 also has a three-dimensional channel system, b u t one t h a t does not involve supercages.6 The channel system is composed of straight channels parallel t o the b crystal axis that are intersected at right angles by zigzag channels running in the direction of t h e a axis. The straight channels are near circular with diameters of about 5.5 A, and the zigzag channels are elliptical with dimensions of 5.1 by 5.5 A. An angle of 1 1 2 O is formed at the corner in t h e zigzag channel. The mordenite used has the large port structure with a system of straight channels of 6.7-A dimension parallel to the c axiss 11. Experimental Section
The zeolites used were L i n d e erionite, ELZ-E-10; Linde mordenite, L Z - M - 8 ; Linde Y, LZ-Y82; and ZSM-5 synthesized according to ref 7. All samples were hydrogen-form zeolites prepared from the ammonium-exchanged form and calcined in a shallow-bed flowing N, system at temperatures up t o 550 O C overnight. S t a n d a r d physical characterization was made, and t h e 'Contribution No. 3863. 'Present address: Exxon Chemical Co., 5200 Bayway Drive, Baytown, TX 77520.
0022-3654/86/2090-4679$01.50/0
TABLE I: Zeolite Characterization surface. m 2 / e I
zeolite erionite' (ELZ-E-10) mordenite' (LZ-M-8) Y" (LZ-Y82) ZSM-5' Linde.
I "
crystallite SiO2/AI2O3, Hg N2, Fe3+, size SEM, pm molar intrusion BET ppm
