TRANSFORMATION OF NATURAL ANALCIME AND PHILLIPSITE DURING THEIR HYDROTHERMAL RECRYSTALLIZATION INTO ZEOLITES A AND X.

The objective of the present work was to study transformation of Georgian natural zeolites, analcime and phillipsite, during their recrystallization in the aim to obtain zeolites A and X, widely used for adsorption, separation, ion exchange and catalysis. It is found that phase-pure zeolite can be prepared in the form of cubic/rhombus crystallites with uniform micrometric (3-5 μm) dimensions by hydrothermal crystallization (95 o C) of aged (72 hr) at room temperature gel (4.5Na 2 O: 0.45Al 2 O 3 : 1SiO 2 : 178H 2 O) obtained from natural analcime, treated with hydrochloric acid before suspending in water and mixing with sodium hydroxide. Phase-pure zeolite (Al Si 114(4) O 384 )) with specific surface area of 589 m 2 /g and total pore volume of 0.578 cm 3 /g can be prepared in the form of octahedral crystallites with uniform micrometric (2-7 μm) dimensions by hydrothermal crystallization (95 o C) of aged (96 hr) at room temperature gel (2.9Na 2 O: 0.26Al 2 O 3 : 1SiO 2 : 150H 2 O) obtained from water suspension of natural phillipsite, treated with hydrochloric acid and mixed with sodium hydroxide. The resulting zeolites in their characteristics are competitive with commercially available materials.

Zeolite X (Si/Al<3) and zeolite Y (Si/Al>3) are analogues of the rare natural zeolite faujasite (the crystal chemical formula |(Ca,Mg,Na 2 ) 29 (H 2 O) 240 | [Al 58 Si 134 O 384 ]-FAU) having the largest unit cell (cubic, a=24.74Å) containing 192 T-atoms. Channels in the FAU crystal structure are running perpendicular to each other in the x, y, and z planes similar to LTA (Fig 1), and are made of 4-and 6-member ring SBUs, CBUs are d6r (double 6-membered ring containing 12 T atoms) and sod (Baerlocher et al., 2007). The channel diameter is large at 7.4Å since the aperture is defined by a 12-member ring, and leads into a larger cavity of diameter 12Å. The cavity is surrounded by ten sodalite cages connected on their hexagonal faces. There have been many studies on synthesizing zeolites from natural minerals, such as smectite (Abdmeziem and Siffert, 1994) Natural zeolites have a fairly constant composition and controlled impurities, so that they can be used to produce synthetic zeolites. Recently it was shown (Tsitsishvili et al., 2016a), that zeolites with high silicon content and low ion exchange capacity, such as mordenite (|Na 8  The reason for such behavior may be a comparatively high Si/Al ratio (>4.0) in raw material, so to produce the A and X zeolites, it is better to use common zeolites with a comparatively low Si/Al ratio, such as chabazite (Si/Al=3.0), analcime and laumontite (Si/Al=2.0), or phillipsite (Si/Al= 5 / 3 ). Another reason for the impossibility of recrystallization of clinoptilolite in zeolite A may be the fact that the HEU structure has only 4-member ring SBU of 4-4=1 type containing 9 T-atoms, and CBU of bre type containing 10 T-atoms.
The aim of our work was to study the recrystallization of the Georgian natural zeolites analcime and phillipsite to obtain the A and X zeolite structures in one step without application of crystallization seeds and organic templates. Analcime structure contains 4-and 6-member ring SBUs, has no CBUs and may be suitable for the preparation of zeolite A. Phillipsite has 4-and 8-member ring SBUs, double crankshaft chain as CBU and may be applied to obtain zeolite X. Both zeolites are widespread in Georgia, but have no practical application.

Materials and Methods:-Materials:-
Preparation of synthetic zeolite materials was carried out using following Georgian natural zeolites described and characterized (Tsitsishvili et al., 1998)  Sodium hydroxide and the other chemicals used in the experiments were purchased from Merck KGaA (Darmstadt, Germany). All chemicals were of analytical reagent grade and used without any further purification. Deionized water was used throughout this study.

Preparation of zeolites:-
Processing of raw in target material included following steps: preparation of material, preparation of suspension, gel formation and aging, hydrothermal crystallization, and separation of product.
In the experiments were used zeolite-containing rocks, crushed in the planetary micro mill Pulverisette 7 premium line (Fritsch Laboratory Instruments, Idar-Oberstein, Germany) to a size less than 0.063 mm (250 BSS mesh).
Analcime powder was treated at room temperature by HCl water solution (12%) under stirring, washed by water before the complete disappearance of Clions, and dried in thermostat oven at 100-105 o C; water suspension (the 222 solid to liquid ratio of 1 : 3) of homogeneous amorphous (XRD tested) material was prepared in Teflon flack; suspension was treated at room temperature by NaOH water solution (20%) under stirring, homogenization and gel formation takes approx. 30 minutes.
Phillipsite powder was suspended in Teflon flack placed in shaking water bath OLS 26 Aqua Pro (Grant Instruments, Cambridge, UK) controlling temperature at 90-95 o C; suspension was processed with a 12% hydrochloric acid solution at the rate of 5 mL per gram of the solid raw material; activated suspension was diluted with water and treated by adding of a 25% sodium hydroxide solution, followed by the formation of a homogeneous gel for about one hour.
General characteristics of the target zeolite products are in strong dependence on the chemical composition of gel (aNa 2 O . bAl 2 O 3 . SiO 2 . cH 2 O) prepared for aging and crystallization: the Si/Al ratio determines the type of porous structure to be produced; application of sodium hydroxide provides an alkaline environment for breaking T-O bonds and gives possibility to prepare nearly pure sodium forms; high water content ensures suitable viscosity and other physical properties for crystallization process. The molar ratios SiO 2 /Al 2 O 3 , Na 2 O/Al 2 O 3 , and H 2 O/ Na 2 O, optimal for obtaining zeolite A from analcime and zeolite X from phillipsite, are given in the Table 1.
The aging of the gel in all cases was carried out at room temperature for several days; crystallization was carried out in temperature-controlled water bath; the temperature (up to 95 o C) and duration have been adjusted to prepare micrometric single crystals with diameter of 2-8 μm. The crystallization was followed by X-ray diffraction (XRD) patterns, the strongest peaks (2Θ~30° for zeolite A, according to recent results (Dolaberidze et al., 2017), and 2Θ = 6.1° for zeolite X) were observed to detect the start of zeolitization and determine the time of formation of a stable structure, shown in the Table 1.
Separation of produced crystalline material was carried out by filtration of mother solution, solid material was cleaned by distilled water until pH 8.0-8.5, and dried at 90-100 o C. Characterization:-Chemical composition of raw material and prepared samples was determined by elemental analyses carried out using a 381L plasma spectrometer (Spectromom, Hungary) and atomic absorption spectrometer (model 300, Perkin-Elmer, UK), as well as by energy dispersive X-ray (EDS) analysis. The crystalline phase was identified by powder X-ray diffraction (XRD) patterns obtained from a modernized Dron-4 X-ray diffractometer (Russia) employing the hydrothermal synthesis. Using analcime pre-activated with hydrochloric acid and forming a gel in accordance with the amounts of sodium and water given in Table 1, zeolite A can be obtained with a high degree of phase purity. This is confirmed by the powder XRD pattern (Fig 2), which corresponds to the XRD pattern of zeolite A, obtained from the rice husk ash and aluminium scrap (  The XRD pattern of phillipsite recrystallization product (Fig 3) shows not only the strongest peak at 2Θ = 6.1° (100%;111;14.28Å), but also all low intensity peaks given in the "Database of Zeolite

Optimal conditions and parameters of transformation:-
Obtaining of zeolite NaX from a gel with molar ratio SiO 2 /Al 2 O 3 = 3.8 corresponds to the results of a previous study (Zhang et al., 2013), according to which a single phase NaX zeolite was obtained from sodium silicate and sodium aluminate only with the SiO 2 /Al 2 O 3 molar ratio of 1.5-4.0. When using pure chemicals, the NaA zeolite was developed at SiO 2 /Al 2 O 3 = 1.0 in addition to the NaX zeolite, but at SiO 2 /Al 2 O 3 = 0.5 a single phase NaA zeolite was generated. However, the use of natural precursors leads to other results, the synthesis of zeolite X from coal fly ash was carried out at SiO 2 /Al 2 O 3 = 5, and zeolite A at SiO 2 /Al 2 O 3 = 1.67 (Hu et al., 2017), so that the preparation of zeolite NaA by recrystallization of analcime at SiO 2 /Al 2 O 3 = 2.2 and large molar ratio Na 2 O/Al 2 O 3 is understandable.
Generally, high alkaline concentration of the crystallization system accelerates the dissolution of silicon and aluminum components in the precursor materials (Cundy et al., 2005). The optimal conditions for the recrystallization of the analcime in zeolite A were the ratios 4.5Na 2 O/SiO 2 and ~40H 2 O/Na 2 O, while the synthesis of the same zeolite from the coal fly ash was successfully carried out at significantly lower sodium content (1.3Na 2 O/SiO 2 ) partially compensated by comparatively low dilution factor, 1.3Na 2 O/SiO 2 (Hu et al., 2017). In all likelihood, such a high alkaline concentration is needed to transform the structure of analcime, which has the highest framework density (18.5T/1000Å 3 ) among zeolites (Baerlocher et al., 2007).
The optimal conditions for the recrystallization of the phillipsite in zeolite X are 2. . Structure PHI has a rather low framework density (15.8T/1000Å 3 ), and high alkalinity is not needed for its transformation.
Aging also plays an important role in the nucleation of amorphous gel. During this stage, the aluminosilicate species included in the gel phase are transformed along with the aging conditions (Ogura et al., 2003). In the study, out of considerations of energy saving, the room temperature was chosen for aging the gel. Of course, this led to a significant increase in the duration of aging, from about six to ten hours to several days, but this saves more than 100 Joules per gram of the reaction mixture. The same energy saving considerations were taken into account when selecting the optimum crystallization temperature. In addition, it was decided to carry out recrystallization at a temperature below the boiling point of water, in this case there is no need to use an autoclave.

FT-IR characterization:-
The mid infra red peak patterns in FTIR spectra (Fig 4)  Sorption properties:-Developed zeolite crystal microporous structure in synthesized samples has been confirmed also by their sorption properties. The N 2 adsorption-desorption plot at 77 K for the prepared zeolite NaX is presented in Fig 5 and corresponds to typical Langmuir isotherm with the presence of steep nitrogen uptake at very low relative pressures (p/p o~0 .05), which is attributed to the filling of micropores.
The calculated specific surface area, 589 m 2 /g, is comparable to 573 m 2 /g for zeolite X, obtained from coal fly ash (Hu et al., 2017), and is greater than the specific surface area of 453 m 2 /g reported for NaX obtained from diatomite (Yao et al., 2018).
The total pore volume of prepared NaX is 0.578 cm 3 /g, the volume of micropores with a diameter of less than 8 Å is 0.301 cm 3 /g, which is slightly higher than the volume of micropores in zeolites X obtained from coal fly ash (0.281 cm 3 /g) and from diatomite (0.284 cm 3 /g). It is noted (Chen et al., 2016) that such a volume of micropores is much higher than that of NaX zeolites synthesized with structure-directing reagents which block some of the channels. The maximum adsorption capacity of synthesized NaX measured for water vapor is up to 0.394 cm 3 /g, which is more than indicated for a commercial sample (0.3303 cm 3 /g), but water adsorption capacity of micropores (measured under static conditions at the "plateau" pressure p/p 0 =0.40) is only 0.2052 cm 3 /g, up to 48% of water molecules are adsorbed in mesopores.
In structure LTA, the pore size (0.41 nm) is much smaller than that of the FAU structure and is similar to the kinetic diameter of N 2 (0.364 nm), so the BET surface area of the synthesized NaA sample was not measured; water adsorption capacity of micropores (p/p 0 =0.40) is up to 0.24 cm 3 /g and is consistent with most of the reports on phase-pure zeolite NaA.

SEM images:-
The SEM images of NaA and NaX are shown in Fig 6. In general, more than 92% of NaA crystallites have uniform size of 3 -5 μm and cubic or rhombus morphology, as well as more than 95% of NaX crystallites have octahedral habit and uniform size of 2 -7 μm.
In the process of synthesizing zeolite NaA, a small amount (<3wt.%) of spherical or ellipsoidal nanoscale (average diameter 0.25 μm) crystallites is also formed, while long crystallization of NaX results in micrometric crystals combined into honeycomb-like structure through nanocrystal bridges. However, obtaining of "hierarchical" zeolites is the task of our subsequent research.

Conclusion:-
It is of great significance to develop cheap, energy-saving and eco-friendly routines that can synthesize zeolites A and X from low-cost raw materials.
In this study, zeolite NaA with chemical composition Na 11.25 (25)  The structure, as well as high phase purity and crystallinity of both samples is confirmed by their X-ray diffraction patterns and FT-IR spectra. Zeolite NaX is characterized by high specific surface area (589 m 2 /g) and pore volume (0.578 cm 3 /g) including micropores of LTA structure (52%) and cylindrical channels with diameter up to 67 nm (48%). SEM observation revealed that most of the NaA and NaX crystallites have uniform micrometric size.