A Family of Molecular Sieves Containing Framework-Bound Organic Structure-Directing Agents

Abstract

Zeolites and related molecular sieves, with crystallographically well-defined channels and cavities of molecular dimensions, are not only extending their commercial uses as catalysts and separation media, but also finding new applications. To further improve their functional properties for expanded use, considerable efforts have been devoted over more than a decade to the synthesis of inorganic-organic hybrid materials with organic moieties incorporated into the inorganic framework. Indeed, if successful, new organic functionalities with tunable geometrical features can be introduced as catalytic and/or adsorption sites into the void spaces of this important class of crystalline, microporous materials. The strategies developed to date, in the search for such hybrid solids, involve either the direct synthesis using organosilanes containing organic functional groups as a (partial) silica source and alkylammonium-bound organosilanes as structure-directing agents, or the post-synthetic treatment using similar organosilane species as pillaring or silylating agents. Thus, they rely on the use of organic species which already contain an atom susceptible of incorporation as a tetrahedral atom in the zeolitic framework. In this study, we have reported the discovery of framework-bound organic structure-directing agent-containing molecular sieves (FOMSs) and a plausible formation pathway for this novel class of inorganic-organic hybrid crystals. Based on the study of FOMS, in addition, the first solid-solution of a zeolite and a FOMS has been demonstrated and their crystallization mechanism have been investigated. These studies can provide invaluable opportunities not only for extending our knowledge of many important aspects of the synthesis, structures, and modification of ordered microporous materials, but also for creating new applications. In the fourth part of this thesis, we demonstrate that ECR-40A, currently regarded as a typical silicoaluminophosphate (SAPO) molecular sieve synthesized using tris(2-hydroxyethyl)methylammonium ions with three OH groups, is a new family of inorganic-organic hybrid networks in which the organic structure-directing agent (OSDA) molecules employed in their synthesis are covalently bonded to the inorganic framework. In fact, OSDAs, such as quaternary ammonium cations and amines, used in the synthesis of zeolites and related crystalline microporous oxides usually end up entrapped inside the void spaces of the crystallized inorganic host lattice. But none of them is known to form direct chemical bonds to the framework of these industrially important catalysts and adsorbents. ECR-40A crystallization begins with the formation of an Al-OSDA complex in the liquid phase in which the Al is octahedrally coordinated. This unit is incorporated in the crystallizing ECR-40A. Subsequent removal of frameworkbound OSDAs generates Al-O-Al linkages in a fully tetrahedrally coordinated framework. In the fifth part of this thesis, we show that several members of the ECR-40 family are solid solutions of a zeolite and a FOMS than pure FOMSs. we have also analyzed a series of mother liquors and solid products separated as a function of time during the crystallization of ECR-40C, which has the largest zeolite portion (~ 60%) among the members of the ECR-40 family, using various characterization methods in order to understand the crystallization mechanism of zeolite-FOMS solid solutions. The structure of the as-made, hydrated form of ECR-40C, synthesized in the presence of (2-hydroxyethyl)trimethylammonium (HTMA+) ions as an OSDA and 2 wt% (of the alumina in the synthesis mixture) of aluminosilicate zeolite UZM-22 with the MEI topology as seeds, has been determined using synchrotron powder X-ray diffraction and Rietveld analyses. Two different types of organic species were suggested to exist in ECR-40C: the encapsulated HTMA+ ions with one intramolecular C–H•••O hydrogen bond, typical of OSDA molecules in as-made UZM-22, and the framework-bound cations. A combination of elemental and thermal analyses, Na+ ion exchange, and multinuclear MAS NMR and IR spectroscopies clearly shows the coexistence of the zeolite and FOMS domains with a proportion of approximately 3:2 in ECR-40C crystals. Na TEM elemental mapping reveals that the Na+ ions exchanged with the HTMA+ ions into as-made ECR-40C are uniformly distributed throughout the ECR-40C crystals. Therefore, ECR-40C is not a pure FOMS but a solid solution of a zeolite and a FOMS (i.e., UZM-22 and ECR-40-type FOMS), which has never been recognized or addressed before. The overall characterization results of this work demonstrate that the proportion of the zeolite domain in such solid-solutions varies notably with the number of OH groups in OSDAs.