ACID SITES AND HYDRATION BEHAVIOR OF DEALUMINATED ZEOLITE HZSM-5 - A HIGH-RESOLUTION SOLID-STATE NMR-STUDY

The acid sites and the hydration behavior of four dealuminated HZSM-5 zeolites (calcined at 550, 600, 650, and 700 degrees C) were characterized by high-resolution H-1 magic angle spinning (MAS),H-1{Al-27} spin-echo double resonance, as well as Al-27 MAS NMR. Besides the usually observed peaks for dealuminated zeolite HZSM-5, a narrow and a broad peak appear simultaneously at 6.9 ppm in the H-1 MAS spectra, and they exhibit different dipolar dephasing behaviors in the (HC)-H-1{Al-27} double resonance experiments. The existence of the former signal indicates that Lewis acid sites may be formed in the zeolites. By means of the H-1{Al-27} double resonance technique, we observed for the first time a previously unexpected narrow signal at 5.2 ppm, which resonates on the downfield side of the Bronsted acid signal (at 4.3 ppm) and cannot be clearly resolved in the H-1 MAS spectrum. This new peak is assigned to another kind of Bronsted acid site, probably those bridging OH groups pointing into the small cavities bounded by five-membered rings. Rehydration of the dried HZSM-5 first causes the adsorption of water molecules on Lewis acid sites (6.9 ppm) with loading up to 0.1-0.3 H2O per unit cell (uc), followed by the appearance of aluminum hexaaquo complexes (9.2 ppm) and then the formation of ''free'' hydroxonium ions (8.0 ppm). The fast proton exchange between water molecules, the bridging OH groups, and the hydroxonium ions does not obviously occur under weak hydration conditions, i.e., water loading less than 9.4 H2O/uc for the sample calcined at 650 degrees C (for the samples calcined at other temperatures, this critical loading is slightly different). Three narrow peaks at 50, 30, and 0 ppm are separately superimposed on a very broad signal in the Al-27 MAS NMR spectra of the dried zeolites. Al-27 MAS spectra of the successively rehydrated samples suggest that most parts of the four-coordinated framework aluminum atoms are transformed into an intermediate species between four- and three-coordination upon dehydration, and this transformation is reversible upon dehydration/rehydration. Calcination of the zeolite may probably turn a small amount of four-coordinated Al atoms into three-coordination, which act as the Lewis acid sites. For the signal at 30 ppm, the hydration leads to a dispersion in its chemical shift or quadrupole interaction which broadens its linewidth in hydrated samples.