Zeolitic catalysts, partially crystalline

P-12 - Partially crystalline zeolitic material as novel solid acid catalysts... 

The crystalline structure of modified zeolites determine a number of properties which are specific and favorable for catalytic reactions. The complete or partial loss of crystalline structure during catalytic reactions or regeneration is in most cases accompanied by decreased catalytic activity. Thermal stability or structural stability characteristics are therefore suitable for evaluating such catalysts or supported catalysts. 

T,he original report by Barrer and Makki (1) that aluminum in a high A silica zeolite, clinoptilolite, could be extracted with mineral acid to give a silica pseudomorph, has given rise to considerable research on acid-extracted mordenite (2-6). Hydrogen mordenite is useful as an adsorbent and a catalyst, and its properties for some purposes are improved by partial extraction of the aluminum. Further, the ability to vary aluminum content while maintaining crystallinity offers the opportunity to leam more about the nature of the active sites in mordenite. 

Zeolites are crystalline aluminosilicates that have exhibited catalytic activities ranging from one to four orders of magnitude greater than amorphous aluminosilicates for reactions involving carbonium ion mechanisms such as catalytic cracking (144). As a result extensive efforts have been undertaken to understand the nature of the catalytic sites that are responsible for the observed high activity. The crystalline nature of zeolites permits more definite characterization of the catalyst than is possible for amorphous acidic supports such as alumina and silica-alumina. Spectral techniques, in conjunction with structural information derived from X-ray diffraction studies, have led to at least a partial understanding of the nature of the acidic sites in the zeolite framework. 

The estimates in Table IX represent the minimum effect of the loss of micropore volume. Pore-mouth constrictions which lead to partial or complete micropore plugging would serve to increase the contribution of loss of micropore volume to density change. No such pore plugging is expected to occur in the (mesoporous) catalyst matrix or in the mesoporous material that is generated during crystalline zeolite destruction. 

Instead of introducing a degree of mesoporosity into a microporous catalyst, the problem can also be approached from the opposite direction. Kloetstra et al. reported the introduction of crystalline microporous domains inside mesoporous MCM-41 by the partial recrystalhzation of the pore walls.[81] The mesoporous host can be regarded as the aluminium and silicon source for the zeolite crystallization. 

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