A method for characterizing effective pore sizes of catalysts

The temperature dependent effective catalytic pore size can be determined b y the comparison of (13)(C) MAS NMR spectra of heterogeneously catalyzed, s hape selective reactions and data collected from GC analysis of the product stream of reactions under similar reaction conditions. The concept of effe ctive minimum molecular dimensions has been set forth in a previous paper. (Webster, C. E.: Drago, R. S.; Zerner, M. C. J. Am. Chem. Sue. 1998, 120, 5 509-5516.) In this investigation, it is concluded that the effective cataly tic channel size of HZSM-5 is between 6.62 and 7.27 Angstrom at 300 degrees C, the MIN-2 dimensions of p-xylene and o-xylene, respectively. At 370 deg rees C, the effective catalytic channel size is increased to a minimum of 7 .64 Angstrom, the MIN-2 dimension of 1,2,3-trimethylbenzene. We discuss the apparent increase in the effective catalytic channel intersection size wit h increasing temperature. This novel approach presents an a priori descript ion of relative pore sizes and can be used to investigate and describe nume rous catalytic and adsorbent systems as discussed in this paper. The compar ison of observed isomer distributions of the trimethylbenzenes to their the rmodynamic equilibrium distributions provides experimental evidence for the existence of the previously reported nest effect, which describes the shap e selectivity of the exterior surface of the HZSM-5 structure.