Mordenite aluminum-deficient

Combination of thermal and chemical dealumination. This is a two-step method which was applied in the preparation of aluminum-deficient mordenite (4,5) and Y zeolites (28,29). In some instances the two-step treatment was repeated on the same material, in order to obtain a higher degree of dealumination (5,28). 

The spectrum of the aluminum-deficient mordenite shows no signal for Si(2 Al) and a considerable decrease in intensity of the Si(l Al) signal, due to framework dealumination. 

Sorption. The sorption properties of aluminum-deficient mordenite are strongly affected by the dealumination procedure used and by the degree of dealumination. Materials prepared by procedures that do not involve high temperature treatments show a relatively high sorption capacity for water (15,70), due to the presence of silanol groups, which are hydrophilic centers. However, aluminum-deficient mordenite zeolites prepared by methods requiring heat treatment show a lower sorption capacity for water due to fewer silanol groups. This was shown by Chen (71), who studied the sorption properties of aluminum-deficient mordenite prepared by the two-step method. 

The formation of such bonds during the heat treatment of dealuminated mordenite has also been suggested by Rubinshtein et al. (72-74), in some instances without the intermediate formation of SiOH groups. The hydrophobic nature of the zeolite also increases with progressive dealumination. Chen (71) has shown that aluminum-deficient mordenite zeolites with SiO /Al O ratios over 80 absorb little or no water at low pressure. These highly silicious zeolites are truly hydrophobic and in this respect are similar to highly silicious zeolites prepared by direct synthesis (e.g. ZSM-5) (75). 

Acid properties. It was reported that the acidity of aluminum-deficient mordenite, measured by ammonia chemisorption, decreases linearly with aluminum content (78,82,83). 

Using calorimetric and n-butytlamine titrations, Ghosh and Curthoys (105) have reported that aluminum-deficient mordenite exhibits strong acid sites. However, the location of these sites is uncertain. 

They found that aluminum-deficient mordenite (SiO /Al O ratio of 18), pre-calcined in a flow of dry air, is twice as active for toluene disproportionation and coke formation as compared to regular mordenite. The aluminum-deficient form also has a higher rate of deactivation. It is assumed that the formation of new, strong acid sites during the process of moderate dealumination is responsible for the observed change in activity. 

Published data by different authors suggest that the catalytic activity of aluminum-deficient mordenite zeolites depends primarily upon the following factors ... 

These are some of the more important factors that should be considered in order to explain the catalytic activity of aluminum-deficient mordenite zeolites. In general, the same factors will affect the catalytic properties of aluminum-deficient Y zeolites, although fewer data are available with regard to the catalytic properties of these materials. 

Kincal, S., and Culfaz, A., Sorption of sulfur dioxide on hydrogen-form and aluminum-deficient mordenites, Sep. Sci. Technol., 16(3), 229-236 (1981). 

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