Chabazite thermal stability

The key structural feature of the molecular sieves is the narrow, uniform, continuous channel system that becomes available after the zeolitic water has been driven off by heating and evacuation. Great thermal stability after dehydration has been observed in the rigid lattices of X- and Y-type faujasites, zeolite A, mordenite, and chabazite. The geometry of the internal channel and cavity system is characteristic of the individual zeolite. Entrance to the intracrystalline volume is through orifices (ranging from 3 to 9 A in the various zeolites) located periodically throughout the structure. It is thus apparent that access to the intrazeolitic environment is limited to molecules whose dimensions are less than a certain critical size. 

Beyer et al. [528] studied by IR spectroscopy the thermal stability of NH4-chabazite. They observed two OH bands at 3630 and 3540 cm, the behavior of which was related to that of the decreasing NH deformation bands at 1420 and 1460 cm. The OH groups indicated by the 3540 cm -band were thermally less stable than the other hydroxy groups. 

An important group of catalysts for NOx reduction through anunonia SCR is represented by copper exchanged zeolites [1-9]. Many different zeolites have been investigated, for example, Cu-ZSM-5 [10-13], Cu-faujasite [14], Cu-Beta [15,16], and Cu-Y [17]. Recently, copper zeolites with chabazite (CHA) structure have received great attention due to their high thermal stability and hydro carbon resistance [18]. Both Cu-SAPO-34 [19] and Cu-SSZ-13 [18] have a CHA structure... 

BEA, or Y), however, display a beginning of deactivation for hydrothermal treatments carried out at 773 K [63]. For processing temperatures above 973 K, more than 90% of the acidity is destroyed due to the dealumination as well as the collapse of the structure [64]. One mean of improving the stability and efficiency of these zeolitic systems is to change the zeolite structure, and research efforts have therefore focused on chabazite structure [65-67]. Recently, some papers relate the use of perovskite catalyst for SCR-NH3 due to their remarkable hydro-thermal stability [68-72]. 

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