CHANGE OF PORE-OPENING STRUCTURE OF MORDENITE UPON DEALUMINATION BY HYDROCHLORIC-ACID

The structure of dealuminated mordenites prepared at 340-358 K of hydrochloric acid was studied . Al-27 MAS n.m.r. and XPS were measured to determine the species and depth profile of alumina, and these were corroborated with adsorption experiments using adsorbates of different sizes. Adsorption capacity of nitrogen was unaltered so that the structure of mordenite was retained, while that of 2,2,4-trimethylpentane with a larger kinetic diameter increased below 358 K of dealumination temperature, but then decreased above that. The H-mordenite (HM, Norton 100-H) contained a large amount of octahedral aluminum species (ca. 28%), and the nonframework aluminum was removed preferentially by the dealumination. Aluminum depth profile showed the homogeneous distribution on the inherent HM, but the enrichment on the samples prepared above 350 K. Based upon these findings, the following conclusions were drawn: (1) On the inherent HM, a small portion of nonframework aluminum near the pore opening disturbs the adsorption of large molecules: (2) The pore-opening size of the sample dealuminated at 358 K was large enough to adsorb 2,2,4-trimethylpentane: (3) However, upon the dealumination above 358 K, the pore opening was partly destroyed or blocked by nonframework alumina again to inhibit the adsorption of large molecules. Most of the aluminum impurity species seemed to be occluded as blocks or clusters in the mordenite. These variations of structure of the pore opening can be well related to those of the catalyst activity.