SYNTHESIS OF LTA TYPE ZEOLITES FROM GEORGIAN CLINOPTILOLITE

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 ((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 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/g for SOD, and of 0.24 cm/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.


INTRODUCTION
At the end of the 20 th 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 (from tens to thousands of parts per million by volume) 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 (or concentration) 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 (or concentration) is lower. According to the authors of the report, this property allows carbon or polymer adsorbent in a "sacrifial" (or first) bed followed in the air flow by a suitable zeolite in a "polishing" bed to produce the lowest vapor pressure of VOC in the outflow.
The synthetic zeolites are used commercially more often than natural zeolites due to the purity of crystalline products and the uniformity of particle sizes. The sources for early synthesized zeolites were standard chemical reagents, but today attention is paid to synthesis from low cost materials, such as clay minerals, natural zeolites, coal and other incineration ashes, etc. 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 (temperature, reaction time, pH of mixture). Modern trends in the synthesis of "bulk" and "hollow" zeolites have been considered in review [2].
The aim of present work was to demonstrate possibilities to carry out synthesis of zeolitic materials with necessary system of pores and channels, as well as with suitable morphology of crystallites reproducing mesoporous structure of natural zeolites, having a number of useful properties, including possibility to bound macromolecules and even microorganisms.

EXPERIMENTAL
Preparation of synthetic zeolite material was carried out using Georgian natural clinoptiloliteheulandite-containing rock from the Rkoni plot of Tedzami deposit [3]  Processing of raw in target material includes following steps: Treatment of raw material & preparation of suspension. Natural zeolite powder was treated at room temperature by 20 % HCl aqueous solution under stirring, washed by water before the complete disappearance of Clions, and dried in oven at 100-105 °C; water suspension of treated material was prepared with the solid to liquid ratio of 1:3.
Gel formation. Prepared suspension was treated at room temperature by 10 % NaOH aqueous solution, solid to liquid ratio of 1:6, gel homogenization takes 30 min, details are given in [4]. General characteristics of target material are in strong dependence on the chemical composition (kNa 2 O:mSiO 2 :Al 2 O 3 :nH 2 O) of gel prepared for aging and crystallization: the SiO 2 /Al 2 O 3 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 (viscosity, etc.) for crystallization process, but water molecules are compulsory units to built zeolite structure and play a significant role.
Gel aging. Generally the process of gel aging at room temperature and without application of seed crystals takes several days, details are described in [5].
Crystallization. Crystallization of aged gel was carried out in the Teflon flasks at different temperatures up to 110 °C, duration of the process -up to 90 hours; both temperature and duration affect to crystalline sizes -low temperature prolonged synthesis gives high quality large crystals, high temperature fast crystallization is a way to produce "nanozeolites" and their fibrous aggregates.
Separation and cleaning. Separation of produced crystalline material was carried out by filtration of mother solution, solid material was cleaned by water until pH 8.0-8.5, and dried at 90-100 °C.
Recently [6] synthesis of mordenite-like materials from gels aged during one week and having chemical composition of SiO 2 /Al 2 O 3 = 9.8, Na 2 O/SiO 2 = 0.08, H 2 O/Na 2 O = 250 has been described, but obtaining of materials with high aluminum content is impossible in such single step.
However, preparation of synthetic zeolitic material of the type A (LTA structure) was carried out by two-stage re-crystallization of the same HEU type natural zeolite firstly to the sodalite (SOD) structure, and then in the target structure: HEU → SOD → LTA, in following steps: preparation and acid treatment of raw material; gel formation and its aging; hydrothermal crystallization; separation of intermediate SOD product, new gel formation without any acid or basic treatment, but including gel aging; crystallization and separation of target product, its washing and drying.
Chemical composition of prepared samples (Table 1) was determined by elemental analyses carried out using a Spectromom 381L plasma spectrometer and a Perkin-Elmer 300 atomic absorption spectrometer, as well as by energy dispersive X-ray (EDS) analysis. X-ray powder diffraction patterns were obtained from a DRON-4 diffractometer, employing the CuK α line and scanning at 1° per minute, FTIR spectra in the wavenumber range 4000-400 cm -1 were recorded on a Perkin-Elmer FTIR spectrometer (version 10.4.2) using the KBr pellet technique for sample preparation, SEM images were obtained by using a Jeol JSM6510LV scanning electron microscope (parameters are given on figures) equipped with an Oxford Instruments X-Max 20 analyzer for EDS. Water adsorption capacity was measured under static conditions (p/p S = 0.40, 20 °C).

RESULTS AND DISCUSSION
Chemical composition of prepared materials is in a good accordance with corresponding chemical formula with the exception of small "lack" of the Al atoms in the frame; prepared materials are nearly pure Na-forms (78 % SOD and 89 % LTA), traces (<1 %) of Cu and Zn have been observed for SOD, other metals are removed in full during first crystallization.
XRD. The framework type of prepared material was testified by X-ray powder diffraction patterns. No additional diffraction peak at 2Θ = 7.74° indicating the formation of the BEA type structure or at 2Θ = 15.8° indicating the formation of ANA type structure as impurities have been observed after the first crystallization. X-ray powder diffraction pattern of target product was compared ( Fig. 1) with calculated one taken from the "Database of Zeolite Structures" of the International Zeolite Association Structure Commission (http://www.iza-structure.org/). Calculated XRD pattern include a large number of low-intensity peaks that are not observed experimentally, so for comparison with recorded pattern only peaks of comparatively high intensity (over 0.09I max ) have been taken into consideration ( Table 2).  Experimental XRD pattern has the same peculiarities as was mentioned in [7]: high intensity peaks not only at 2Θ = 7° and 10°, but at 2Θ = 24, 27, 30, and 34°. With the molar ratio Si/Al nearly equal to one, kaolin is considered as an ideal raw material for preparing NaA zeolite [8], but LTA materials prepared from natural kaoline contain easily recognized in XRD patterns quartz (strong peak at 2Θ = 26.63°) and the SOD type zeolite (characteristic peaks at 2Θ = 14.14° (0.53), 24.62° (1.00), 31.96° (0.98), and 35.1° (0.78)) as impurities, not observed in XRD patterns of the samples obtained by recrystallization of SOD produced from HEU. No improvement in crystallinity like noted in [9] for high-silica zeolite A sample treated for five minutes with 12.5 % NaOH solution at room temperature is observed.
Developed zeolitic crystal microporous structure in synthesized samples has been confirmed also by comparatively high averaged value (0.08 cm 3 /g for SOD, and of 0.24 cm 3 /g for LTA) of water adsorption capacity under static conditions at the "plateau" pressure.
FTIR. The mid infra red peak pattern shows a typical vibration pattern for the sodalite framework FTIR spectra of obtained NaA zeolite shows the band at 465 cm -1 assigned according to [12] to T-O bending vibration, band at 552 cm -1 (double ring vibration), bands between 660-800 and 1000-115 cm -1 assigned to symmetric and antisymmetric T-O-T stretching vibration, broad band in the region of 3470 cm -1 due to asymmetric stretching of OH group and the band at 1648 cm -1 due to bending vibration of H-OH.  (Fig. 2-4) show crystalline morphology of obtained samples and testify the possibility of obtaining different crystallites depending on the crystallization rate and other parameters. Long (> 10 h) low temperature process results in uniform crystallites with fairly narrow distribution of sizes (Fig. 2, crystallite average diameter 4 μm), fast (< 2 h) process gives different fibrous aggregates (Fig. 3, leftcomparatively low temperature, right -high temperature).   (Fig. 4, left -crystallites with average diameter 0.2 μm produced at low temperature, right -fibers with average diameter 0.08 μm produced at comparatively high temperature) belonging to nanoscale zeolites. Obtained micrometric and narrow fibrous aggregates are a good raw material for preparation of composite and hierarchical structures [13] for various catalytic and adsorptive applications.
The proposed methods are based on the use of natural silica-alumina raw materials and inexpensive reagents (HCl, NaOH), are characterized by the relative rapidity, low energy expenditures and low Sheldon's factor E. According to the modern review [14], "template-free synthetic routes such as desilication/recrystallization will be used industrially in the future".