Abstract
Generally, the microporous structure of synthetic zeolite is affected by the nature of used reactants and their pretreatments, as well as by composition of the reaction mixture. Our recent investigation of hydrothermal transformation of natural Georgian clinoptilolite demonstrated possibility of preparation of zeolites with high silicon content (mordenite-like materials) without organic template, directly from aged gels having suitable chemical composition and prepared by acid treatment of raw material. The aim of present work was preparation of zeolites with high aluminium content on the same basis without organic templates. The study tested possibility of synthesis of the LTA type zeolites in following steps: treatment of the natural clinoptilolite ((Na 3.3 K 1.15 Ca 0.75 Mg 0.25 [Me] 0.55 )(A l7.0 Si 29.3 O 72 )22.5H 2 O, where Me = Cu, Zn, Ba, etc.) in HCl water solution; gel preparation by suspension in NaOH solution; its hydrothermal crystallization to the sodalite structure (SOD) with Si/Al=1, and then re-crystallization of the sodalite into the NaA zeolite (LTA). Chemical elemental analyses confirm a good accordance with the UPAC SOD and LTA chemical formulas for prepared materials having nearly monocationic composition (Na – 78 % SOD and 89 % LTA). In XRD patterns there are no additional peaks from zeolitic or other impurities, prepared materials have high crystallinity, their FTIR spectra are typical for SOD and LTA structures, developed zeolitic crystal microporous structure is confirmed by comparatively high averaged value of water adsorption capacity (0.08 cm 3 /g for SOD, and of 0.24 cm 3 /g for LTA at p/p S = 0.4). SEM images show uniform LTA micrometric crystallites (average diameter 4 μm) with fairly narrow distribution of sizes produced by lasting (> 10 h) low temperature crystallization, as well as nanoscale spherical zeolites (0.2 μm) and fibrous aggregates (0.08 μm) produced at low temperature, that can be used for preparation of composite and hierarchical structures for various catalytic and adsorptive applications.
Highlights
At the end of the 20th century, the United States Environmental Protection Agency published a technical bulletin [1], in which it made recommendations on the use of different systems to adsorb volatile organic compounds (VOC) and other pollutants from relatively dilute concentrations in air to control emissions.It was noted that carbon and polymers have a linear adsorption isotherm relative to vapor pressure, and this linearity makes either carbon or polymers the better adsorbent when the vapor pressure is higher, while zeolite has a very non-linear adsorption isotherm relative to vapor pressure for the molecules it has an affinity to, and this makes zeolite the better adsorbent when the vapor pressure is lower
The type of the zeolite is affected by the nature of reactants and their pretreatments, as well as by composition of the reaction mixture (Si/Al ratio, OH, inorganic cations), while the sizes of products and their morphology are affected by conditions of the hydrothermal process
General characteristics of target material are in strong dependence on the chemical composition of gel prepared for aging and crystallization: the SiO2/Al2O3 ratio determines the type of microporous structure to be produced, and application of sodium hydroxide gives a possibility to prepare nearly monocationic sodium forms; water content generally is rather high to ensure suitable physical properties for crystallization process, but water molecules are compulsory units to built zeolite structure and play a significant role
Summary
The microporous structure of synthetic zeolite is affected by the nature of used reactants and their pretreatments, as well as by composition of the reaction mixture. Our recent investigation of hydrothermal transformation of natural Georgian clinoptilolite demonstrated possibility of preparation of zeolites with high silicon content (mordenite-like materials) without organic template, directly from aged gels having suitable chemical composition and prepared by acid treatment of raw material. The study tested possibility of synthesis of the LTA type zeolites in following steps: treatment of the natural clinoptilolite ((Na3.3K1.15Ca0.75Mg0.25[Me]0.55)(Al7.0Si29.3O72).22.5H2O, where Me = Cu, Zn, Ba, etc.) in HCl water solution; gel preparation by suspension in NaOH solution; its hydrothermal crystallization to the sodalite structure (SOD) with Si/Al=1, and re-crystallization of the sodalite into the NaA zeolite (LTA). In XRD patterns there are no additional peaks from zeolitic or other impurities, prepared materials have high crystallinity, their FTIR spectra are typical for SOD and LTA structures, developed zeolitic crystal microporous structure is confirmed by comparatively high averaged value of water adsorption capacity (0.08 cm3/g for SOD, and of 0.24 cm3/g for LTA at p/pS = 0.4). SEM images show uniform LTA micrometric crystallites (average diameter 4 μm) with fairly narrow distribution of sizes produced by lasting (> 10 h) low temperature crystallization, as well as nanoscale spherical zeolites (0.2 μm) and fibrous aggregates (0.08 μm) produced at low temperature, that can be used for preparation of composite and hierarchical structures for various catalytic and adsorptive applications
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