포스페이트계 분자체의 합성, 특성 분석 및 구조 분석에 관한 연구

Abstract

Zeolites and related microporous molecular sieves still continue to be of particular importance in industry for their potential applications in detergents, catalysis, separations and ion-exchange. Since many important properties of zeolitic materials are primarily determined by their unique framework structures, therefore, over the past several decades there has been a considerable drive to search for novel zeolite structures not only to improve existing technologies but also to develop new processes. The discovery of aluminophosphate (AlPO4) molecular sieves by scientists at Union Carbide in the early 1980s was one of the considerable achievements in this field, which brought about great progress in both structural and compositional realms of phosphate-based crystalline microporous materials. In this thesis we present synthetic methods for a number of phosphate-based molecular sieves with novel topologies and/or compositions. We have investigated the structure-directing ability of diethylamine (DEA) in the AlPO4 system to discover novel materials and described a series of chemically feasible hypothetical structures based on their structures. We have established a charge density mismatch (CDM) approach in the charged AlPO4 based system to utilize combined alkali-organoammonium structure-direction of the intermediate and/or high charge density heteroatom-containing AlPO4 molecular sieves. 1. We report the synthesis of four novel AlPO4 molecular sieves by using DEA as an organic structure-directing agent (OSDA). Their structures were initially solved and refined using rotation electron diffraction method, and further confirmed and refined by Rietveld analysis of synchrotron powder X-ray diffraction data. The structures of PST-5 and PST-13 contain penta-coordinated framework Al atoms bridged by hydroxyl groups and thus edge-sharing 3- and 5-rings. Upon calcination, PST-5 and PST-13 undergo a transformation to PST-6 and PST-14 with loss of bridging hydroxyls and occluded organic species, respectively. The pore systems of medium-pore AlPO4 materials PST-5 and PST-6 with 36 crystallographically distinct tetrahedral sites can be transformed from a two-dimensional to one-dimensional structure through the removal of hydroxyl groups. Inspired by the PST-5 to PST-6 structural change, we propose a new synthesis approach based on a unique 3D-3D topotactic transformation. By adopting this approach several hypothetical systems have been also proposed, and the structural models of the resulting new zeolites have been shown to be experimentally feasible. The structures of both PST-13 and PST-14 consist nonjointly of pairs of previously undiscovered 1,5- and 1,6-open double 4-rings (d4rs) which are mirror images of each other. We also present a series of novel chemically feasible hypothetical structures built from these 1-open d4r (sti) or 1,3-open d4r (nsc) units exclusively, as well as from these two enantiomeric structural building units. 2. The charge density mismatch concept was applied to the synthesis of high charge density silicoaluminophosphate SAPO-69 (OFF) and SAPO-79 (ERI), as well as zincoaluminophosphate PST-16 (CGS), PST-17 (BPH), PST-19 (SBS), and ZnAPO-88 (MER) molecular sieves, by using a combined alkali-organoammonium structure direction in these systems. Structure direction is treated from the perspective of stabilizing an ionic framework, the relationships between reaction charge density (OH-/H3PO4), alkali and organoammonium content, and ionicity of tetrahedral framework atoms in structure direction are presented.