Document Detail


Investigation of the physicochemical changes preceding zeolite nucleation in a sodium-rich aluminosilicate gel.
MedLine Citation:
PMID:  19572709     Owner:  NLM     Status:  MEDLINE    
Abstract/OtherAbstract:
All industrially available zeolites are obtained from hydrogel systems. Unfortunately the level of understanding of the events preceding zeolite crystallization is far from satisfactory. In this respect, revealing the nature of the processes taking place in the precursor gel is of paramount importance to understanding zeolite nucleation. The investigation of the gel structure, however, is a difficult task due to the complexity of the object in terms of both composition and topology. Therefore, a combination of hyperpolarized (HP) (129)Xe NMR-N(2) adsorption-high-resolution transmission electron microscopy-energy-dispersive spectrometry methods complemented by X-ray diffraction, infrared spectroscopy, scanning electron microscopy, and chemical analyses has been employed to study the changes in composition and structure of sodium hydroxide rich aluminosilicate gel yielding zeolite A. The role of each component in the system and the entire sequence of events during the induction, nucleation, and crystallization stages have been revealed. The high concentration of sodium hydroxide in the studied system has been found to control the size and structure of the gel particles in the beginning stage. During the initial polymerization of aluminosilicate species a significant part of the sodium hydroxide is expelled from the gel into the solution, which restricts extensive polymerization and leads to formation of small aluminosilicate particles with open pore structure. The induction period that follows is marked by incorporation of Na back in the bulk gel. The combined action of the Na ion as a structure-directing agent and the hydroxyl group as a mobilizer results in partial depolymerization of the gel and formation of voids with mesopore sizes. The nucleation maximum coincides temporally with development of pores with sizes in the range of 2-5 nm. The amorphous gel undergoes into crystalline zeolite only after these pores have disappeared and the chemistry of the gel has evolved to reach the stoichiometric zeolite composition. It was established unambiguously by high-resolution transmission electron microscopy and HP (129)Xe NMR that the nucleation of zeolite occurs in the solid part of the system and the succeeding crystallization commences only after the nuclei are released into the liquid, which is consistent with the autocatalytic mechanism. Also this investigation has demonstrated the unrivaled sensitivity of HP (129)Xe NMR that is capable of identifying presence of small amounts of crystalline zeolite material in amorphous medium with detection limit extending below 1 wt %.
Authors:
Lama Itani; Yong Liu; Weiping Zhang; Krassimir N Bozhilov; Luc Delmotte; Valentin Valtchev
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Publication Detail:
Type:  Journal Article; Research Support, Non-U.S. Gov't    
Journal Detail:
Title:  Journal of the American Chemical Society     Volume:  131     ISSN:  1520-5126     ISO Abbreviation:  J. Am. Chem. Soc.     Publication Date:  2009 Jul 
Date Detail:
Created Date:  2009-07-22     Completed Date:  2009-10-14     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  7503056     Medline TA:  J Am Chem Soc     Country:  United States    
Other Details:
Languages:  eng     Pagination:  10127-39     Citation Subset:  IM    
Affiliation:
Institut de Science des Matériaux de Mulhouse, LRC 7228 CNRS, Université de Haute Alsace, 3 rue Alfred Werner, 68093 Mulhouse, France.
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MeSH Terms
Descriptor/Qualifier:
Adsorption
Aluminum Silicates / chemistry*
Chemistry, Physical
Gels / chemistry
Magnetic Resonance Spectroscopy
Nitrogen / chemistry
Particle Size
Sodium / chemistry*
Surface Properties
Temperature
Time Factors
X-Ray Diffraction
Zeolites / chemistry*
Chemical
Reg. No./Substance:
0/Aluminum Silicates; 0/Gels; 1318-02-1/Zeolites; 1327-36-2/aluminosilicate; 7440-23-5/Sodium; 7727-37-9/Nitrogen

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