Structured molecular sieves :synthesis, modification and characterization
Abstract (Summary)The work presented concerns synthesis of structured molecular sieves. The preparation procedure is based on the use of ion exchange resins as macrotemplates. Macroporous cation or anion exchange resin types were used depending on the molecular sieve type to be synthesized. Anion exchange resin beads were employed as macrotemplates in the synthesis of zeolite type molecular sieves since negatively charged precursor species are present in the initial zeolite synthesis solutions. Cation exchange resins were used for the preparation of aluminophosphate molecular sieves, where precursor species in the synthesis solutions are positively charged. The method involves a number of steps, which can be summarized as: (i) introduction of precursor species into the resin by ion exchange; (ii) secondary treatment to transform the precursor into a desired phase; and (iii) removal of the ion exchanger by calcination. The first two steps can be performed simultaneously by hydrothermally treating the resin with synthesis solution, where inorganic material is initially ion exchanged into the resin and transformed into a zeolite upon hydrothermal treatment. The steps (i) and (ii) may also be separated by firstly treating the resins with precursor solutions and then converting the exchanged species into a molecular sieve by secondary treatment with additional components and/or structure directing agents. Upon calcination the ion exchange resin and zeolite structure directing agents are removed leaving porous molecular sieve structures with a shape determined by the shape of the resin macrotemplate. The calcined macrostructures can be employed in secondary synthesis for the preparation of composite macrostructures containing two types of molecular sieves. Modified molecular sieve macrostructures were also prepared by the method. These macrostructures correspond to molecular sieve macrostructures containing various transition metal oxides. The modification is performed as a post-synthesis procedure. It is based on the fact that as synthesized resin-molecular sieve composites retain a certain degree of ion exchange capacity, which facilitates the introduction of charged metal ions. The procedure was demonstrated for both: (i) metal cations, which were exchanged into the cation exchange resin-molecular sieve composite; and (ii) negatively charged metal species, which were exchanged into anion exchange resin-molecular sieve composites. Upon calcination of a complex metal precursor-resin-molecular sieve composite modified molecular sieve macrostructures were obtained. The properties of non-modified and modified molecular sieve macrostructures were extensively characterized by various methods (X-ray diffraction, scanning electron microscopy, Raman and UV-vis DRS spectroscopy, nuclear magnetic resonance, nitrogen adsorption and chemisorption measurements) to study the crystallization mechanism, sample crystallinity and morphology, nature of the metal species, etc. The non-modified molecular sieve macrostructures are of interest for application as catalysts and catalyst supports, adsorbents and packing materials, whereas modified macrostructures are of interest mainly in the area of catalysis.
School:Luleå tekniska universitet
Source Type:Doctoral Dissertation
Date of Publication:01/01/2003