Dealumination of small- and large-port mordenites: a comparative study

Large-port (LP) and small-port (SP) mordenites with different particle size and shape have been submitted under similar conditions to known dealumination procedures in view of comparing their respective behavior, and establishing the similarities and discrepancies among the dealuminated solids. The dealuminated zeolites have been characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), acidity measurements with ammonia and X-ray photoelectron spectroscopy (XPS). The particle size of the starting mordenite markedly influenced the extraction of Al when dealumination was performed by direct acid attack or dry thermal treatment. Admission of steam during calcination noticeably improved the removal of Al from SP mordenite compared with other methods. Elimination of the extra-framework Al species produced at the calcination step required acid concentration as high as 6 M. (Hydro)thermally dealuminated LP mordenites exhibited similar surface and bulk chemical composition. Conversely, dealuminated SP mordenites showed surface enrichment in Al up to Si/Al ratios of about 40, and depletion in Al at higher ratios. These surface/bulk heterogeneous distributions were suppressed when dealumination combined preliminary acid leaching and steaming. Dealuminated small-and large-port mordenites exhibited differences in their textural characteristics, and for both, a contraction of the unit cell volume and the development of meso- and secondary microporosity. (C) 1997 Elsevier Science B.V.