콜린을 사용한 제올라이트의 합성 및 특성 분석에 관한 연구

Abstract

Zeolites and related crystalline materials have a wide variety of commercial applications in catalysis, ion exchange and gas adsorption and separation. Their performance is inherently associated with the shape, size, accessibility and/or dimensionality of the crystallographically well-defined cavities and channels. Over the past several decades, therefore, extensive efforts have been devoted to the synthesis of zeolites with novel framework structures and/or compositions using numerous different organic amines and alkylammonium caions as organic structure-directing agents (OSDAs). In this thesis we present the synthesis of previously undiscovered aluminosilicate zeolites using choline ions (Ch+, (2-hydroxyethyl)trimethylammonium) as an OSDA by promoting a synergistic cooperation between inorganic structure-directing agents (ISDAs) and/or OSDAs. We have determined the structure of our novel materials, and described a possible phase transformation pathway of two-dimensional (2D) zeolite to 3D zeolite in the crystallization medium. We have also investigated the host-guest interactions occurring within a series of as-made zeolites with different framework topologies synthesized using Ch+ as an OSDA in order to check whether the conformation of OSDAs is closely related to the structure type of the crystallized zeolite host. 1. Zeolite EU-12, whose framework structure has remained unsolved during the past 30 years, is synthesized at a specific SiO2/Al2O3 ratio using Ch+ as an OSDA, with both Na+ and Rb+ ions present. Synchrotron powder X-ray diffraction and Rietveld analyses reveal that the EU-12 structure has a 2-dimensional 8-ring channel system. Among the two distinct 8-ring (4.6 × 2.8 and 5.0 × 2.7 Å) channels along c-axis, the smaller one interconnects with the sinusoidal 8-ring (4.8 × 3.3 Å) channel along a-axis. The other large one is simply linked up with the sinusoidal channel by sharing 8-rings (4.8 × 2.6 Å) in the ac-plane. The proton form of EU-12 was found to show a considerably higher ethene selectivity in the low-temperature dehydration of ethanol than H-mordenite, the best catalyst for this reaction. 2. Understanding the molecular-level mechanisms of phase transformation in solids is of fundamental interest for functional materials such as zeolites. 2D zeolites, when used as shape-selective catalysts, can offer improved access to the catalytically active sites and a shortened diffusion length in comparison with their 3D analogues. However, few materials are known to maintain both their intralayer microporosity and structure during calcination for OSDA removal. Herein we report that PST-9, a new 2D zeolite which has been synthesized via the multiple inorganic cation approach and fulfills the requirements for true layered zeolites, can be transformed into the small-pore zeolite EU-12 under its crystallization conditions through the single-layer folding process, but not through the traditional dissolution/recrystallization route. We also show that zeolite crystal growth pathway can differ according to the type of OSDAs employed. 3. A high-silica (Si/Al = 14) version of zeolite ERS-7 has been synthesized using Ch+ as an OSDA via an excess fluoride approach. We also report the physicochemical properities of this cage-based small-pore zeolite and its catalytic and adsoprtion properties. 4. The host-guest interactions in as-made zeolites Y, UZM-4, UZM-22, offretite, ferrierite, phillipsite, EU-12 and levyne, all of which were synthesized using Ch+ as an OSDA, have been investigated by a combination of different experimental techniques, including Raman, 1H-13C CP MAS NMR and variable-temperature IR spectroscopies, together with theoretical calculations. The conformation of this asymmetric quaternary ammonium cation was shown to differ significantly according to the pore topology of the zeolite host and the intrazeolitic location of the organic guest molecule. Theoretical calculations using the pure-silica zeolite model reveal that among its three representative conformers (i.e., gauche, trans and trans´ forms), the conformer, which was experimentally found to dominantly or exclusively exist in zeolite structures studied, always has a lower interaction energy with the surrounding zeolite framework. Our work provides the first example in which the conformation of OSDAs plays an important kinetic role in governing the phase selectivity during zeolite nucleation.