Synthesis, Characterization, and Structure Determination of CDO Zeolite Precursors and Their Transformation Into 3D Zeolites

Abstract

Zeolites are a class of crystalline aluminosilicates that are finding a wide variety of commercial applications in catalysis, ion exchange, gas adsorption, and separations, which is a consequence of their well-defined cavities and channels of molecular dimensions. Therefore, the synthesis of zeolites with novel pore topologies is of major importance from a practical, as well as from an academic viewpoint. Generally, zeolites are synthesized hydrothermally in their 3D forms, which exhibit little structure discrepancies between the as-synthesized and calcined samples. Therefore, these zeolites almost have an invariable structure and hard to structural modification. Unlike traditional 3D zeolites, one of the most attractive features of two-dimentional (2D) layered zeolite precursor is modified into other types of 3D zeolites by many kinds of transformation methods. In this thesis we have synthesized a number of 2D layered zeolite precursors with novel composition and their transformation 3D zeolite by simple interlayer contraction. We have investigation the structure-directing ability of simply alkylammonium ions as an organic structure-directing agents (SDAs) in the aluminosilicate system. We have also investigated the topotactic transformation of obtained layered material into 3D zeolites with new direct interlayer contraction by ion-exchange. 1. A new CDO zeolite precursor (PreCDO) with a Si/Al ratio of 8.7, the lowest among the layered precursors of this channel-based small-pore zeolite reported so far, has been synthesized using tetraethylammonium (TEA+) and tetramethylammonium (TMA+) ions in fluoride media. However, both the elemental analysis and 13C MAS NMR results reveal the presence of TEA+ only in PreCDO, although this layered phase did not crystallize without TMA+ or F- present. The structure of PreCDO solved by a combination of synchrotron X-ray diffraction Rietveld analysis and computational modeling shows that the ferrierite (fer) layers of PreCDO are stacked with an interlayer distance of 2.7 Å. Its calcination led to a condensation along the a-axis, yielding a highly crystalline CDO zeolite. 2. A CDO zeolite precursor with Si/Al = 6.5, denoted PST-23, using 4-(dimethylamino)-1-methylpyridinium (DMAMP+) ions as an organic SDAs in fluoride media. Unlike as-made PST-23 is collapsed after calcination, PST-23 undergoes a transformation to FER and CDO-type zeolites due to direct contraction of the layers by the simple ion-exchange of inorganic and organic cations, respectively, upon calcination. Inspired by the PST-23 to FER and CDO zeolites change, we propose a new topotactic transformation approach based on simple interlayer contraction. We also describe four hypothetical zeolite structures formed of each four types of building layers (i.e., fer, pcr, heu, and cas layer), with 8- and/or 10-ring pores, which are chemically feasible and will thus be helpful in designing the synthesis of novel zeolites.