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Despite its high crystalline stability, mordenite (MOR) is relatively easily dealuminated. Calcination of NH 4 -MOR to H-MOR at 500°C results in 20% dealumination with the production of octahedral, nonstructural aluminum located within the zeolite pores. The octahedral aluminum is nonacidic and does not affect the acid strength of the remaining structural aluminum ions. Calcination at 735°C results in over 50% dealumination, creating internal, nonacidic pentacoordinate aluminum. In H-MOR (735), 60% of the acid sites are equivalent in strength to H-MOR (500); however, the remaining acid sites are reduced in strength. Pentacoordinated aluminum may interact with the portion of structural aluminum, reducing their acid strength. Dealumination by strong acid removes 75% of the structural aluminum; the aluminum is completely removed from the mordenite, leaving no nonframework species. Strong acid creates many SiOH defect sites; however, the SiOH defects did not affect the acid strength. High-temperature steam produces extensive dealumination. In the presence of steam, nonstructural aluminum migrates to the crystallite surface and restricts the pore aperture. Eventually, the pore aperture becomes sufficiently restricted that large molecules, e.g., n -hexane or pyridine, cannot diffuse to the zeolite interior. In steam-dealuminated mordenite, pentacoordinate aluminum also reduces the acid strength of 40% of the remaining acid sites. Qualitatively, the catalysts' activities for hexane cracking could be correctly ordered by either the number of strong acid sites (independent of the maximum acid strength) measured by NH 3 TPD or infrared spectroscopy. |
