Expanded view of zeolite structures and their variability based on layered nature of 3-D frameworks

Historically, zeolites have been formed and viewed as continuous, 3-dimensional, 4-connected frameworks. Originally these appeared not to be amenable to non-destructive structural modification post-synthesis. By contrast, the synthesis of zeolite MCM-22 followed an indirect pathway to framework formation via a layered precursor MCM-22P comprising stacked MCM-22 monolayers. Gradually other spatial arrangements of these layers have been synthesized, accidentally or by post-synthesis modification, such as: pillared, delaminated, stabilized in expanded form and disordered. MCM-22 can be formed by the conventional direct assembly in 3-D pathway as well (i.e., MCM-49). Subsequently, other frameworks have been recognized as having the layered precursor including well known classical zeolites ferrierite, sodalite and recently ZSM-5. This new approach has been suggested to extend to most, possibly all zeolites. An expanded view of zeolites structures is presented in an integrated scheme based on various materials identified with MWW structure. It is shown that X-ray powder diffraction can be used to unequivocally identify various types of structures generated by different layer arrangements. This is based on self-consistent detailed analysis of selected peaks that reflect inter-layer distances and ordering. (C) 2010 Elsevier Inc. All rights reserved.