A new assignment of the signals in the 23Na DOR N M R spectra to the different sodium sites in dehydrated NaY zeolite is presented. 23Na DOR and MAS N M R measurements of dehydrated N a y , Ca2+-exchanged NaY samples, and NaY samples in which M o ( C O )~ is adsorbed are used for this purpose. Four distinct signals in the 23Na DOR N M R spectra of dehydrated NaY are found: the first signal with an isotropic chemical shift of -6 ppm and a quadrupolar coupling constant of 0.4 M H z is assigned to the Na+ cations at site I. The second signal, with an isotropic chemical shift of -12 ppm and a quadrupolar coupling constant of 2.3 MHz, is assigned to the Na+ cations at sites I'/II'. The third and the fourth signals, with isotropic chemical shifts of -4 and 5 ppm and quadrupolar coupling constants of 4.2 and 4.7 MHz, respectively, are assigned to the Na+ cations at sites I1 and 111.
IntroductionZeolites and related molecular sieves are becoming more and more important for a great variety of applications, such as catalysts, adsorbents, and ion exchangers. Their well-defined pore system and high specific area, in combination with their ability to act as a solid acid, make them very useful for these applications. In this respect, zeolite Y is one of the more frequently used zeolites, especially as a catalyst.' In the as-synthesized form, sodium cations are present to compensate for the negative charge of the zeolite framework (Nay). A thorough knowledge of the locations of the Na+ cations and the interactions between these cations and the zeolite lattice and/or the adsorbed reactants is a prerequisite for understanding the solid-state and catalytic properties of zeolites. This knowledge can also be very useful for the study of the incorporation of metal, metal oxide, or metal sulfide particles in the zeolite channels, for instance for the development of hydrocracking catalysts. 24 Several techniques have been used to study the locations and the interactions of the Na+ cations in the Y zeolite, such as XRD,S ND,6 IR,798 and 23Na NMR measurements.9-12 However, NMR spectra of quadrupolar nuclei, like 23Na, are limited in resolution, because of the second-order quadrupolar line broadening of the readily observablecentral+1/2 *-1/2 transition in these spectra. The double-rotation NMR (DOR) technique" is able to reduce this line broadening and thus gives a much better spectral resolution. The observed center of gravity of the MAS powder pattern, or the peak position in the DOR spectrum (6,k), is the sum of two isotropic shifts (in ppm):where the first term is the isotropic chemical shift and the second term is the isotropic second-order quadrupolar shift. In contrast with the first term, the second term is field dependent and is given by eq 2:I ( I + 1)-3/4 c,2 defined as the product of the electrical field gradient (eq) and the nuclear quadrupole moment (eQ) divided by h, and VL is the Larmor frequency of the nucleus at the magnetic field.In dehydrated NaY five different cation sites are, in principle, available for occupation...
