Friday, February 14, 2020

Developing Advanced Methods for the Synthesis of Zeolite From Coal Fly Ash


Zeolite is aluminosilicates, micro porous crystalline, with TO4 tetrahedral (T = Al, Si) and oxygen atoms are linked to each other with the corners. Zeolite has been unique ion properties as an adsorption and molecular sieving, dehydration and rehydration, biological activity, catalytic, and cation exchange and many more because of it has a wide application. The numbers of scientists have been developing various methods for the synthesis of zeolite. There are many ways for the synthesis of zeolite, which mainly included treatment of natural zeolite, pertaining minerals of natural clay, fusion with alkali, zeolite synthesis by hydrothermal conventional, and microwave assisted zeolite synthesis. The review’s objectives are depending on recent trends of zeolite synthesis from fly ash.


Zeolite are crystalline aluminosilicates consist of a tetrahedral framework of [SiO4]4- and [AlO4]5-, linked each other with the corners by sharing oxygen atoms. Composition of zeolite:

M= monovalent cation, D= divalent cations, m= number of water molecules per unit cell.
x= numbers of cations (Monovalent), y= number of cations. (Divalent).

Zeolite term was firstly disclosed in 1765 by Swedish scientist ‘A. F. Cronsted’. The further studies of zeolite were started in from early 1950 onwards. In1959, at Union Carbide ‘Milton et al’ founded low silica zeolite-A and X. In 1964, ‘Brecks’ reported the synthesis of Intermediate silica zeolite-Y. In the 1960s and 1970s, Zeolite ZSM-5 with a high silica ratio was reported by ‘Mobil Research and Development Laboratories’. There were almost 180 types of zeolite was known in zeolite history. Now a day’s include synthesized zeolite was ZSM-5, zeolite -X, zeolite -A, zeolite -Y, zeolite -L. In 1995, there was first zeolite synthesized from fly ash. Fly ash is generated by coal combustion in thermal power station as a waste or by-product. In India there are more than 90 million tones of fly ash produced. The one of approach of utilization of fly ash is that in to zeolite synthesis, because fly ash having tendency to convert in  to zeolite. Zeolite has wide applications as an adsorption, catalytic activity, ion exchange (waste water treatment), air purification and many more. For the using of fly ash there are many type of zeolite like zeolite-X, zeolite-A, zeolite-P, Na-P1, F-linde was synthesized by various methods, which having industrial applications.


The synthetic zeolite was most preferable in commercial grade rather than natural zeolite because of it has uniform particle size and a huge variety of chemical properties. The researchers, in the field of zeolite have a one most common problem is that, the cost of raw material (silica source) and its availability from which they want to synthesized zeolite [1].

Coal fly ash is an industrial by product/waste which generated from the coal-combustion having an application as a raw material for the synthesis of zeolite, in glass and ceramics manufly ashcturing, in mesopourous material formation and many more [2]. Zeolite produced from coal fly ash is a very interesting product, with having environmental related applications. Zeolite could be synthesized from the fly ash, economically and the process was rapid [3]. The numbers of scientists have been developing advanced methods for the synthesis of zeolite from fly ash. Which mainly including:

  1. Hydrothermal synthesis.
  2. Fusion method.
  3. Membrane extraction.
  4. Microwave irradiation process.
All these above synthesis have affected by the various fly ash factors like, nature of reactants, the composition of Si/Al ratio, pH, temperature, time of reaction and other fly ash factors [1, 4].

A. Hydrothermal Synthesis

‘Reos et al’ have reported an efficient synthesis of zeolite at laboratory scale by using fly ash with NaOH & KOH, however the best results were obtained by using NaOH instead of KOH. This zeolite has unique ion-exchange properties for effluent treatment [5]. The synthesis of zeolite from the class-F fly ash using a hydrothermal process has been 4g/dm3 of solid/liquid (S/L) ratio for the first 4 hour reaction. During this period 80 wt % like crystalline zeolite material was produced. NaP1 which was single phase zeolite was produced using 2M NaOH solution at 104°C temperature [6]. ‘Che pa and co- workers have synthesized Boggsite zeolite from fly ash by using the hydrothermal method by utilizing 2M NaOH for 6-9 h. Here, fly ash/NaOH ratio was 0.3:1. XRD (X-ray diffraction technique) was used for characterization of Boggsite zeolite. This zeolite was used as molecular sieve in the adsorption process [7]. ‘Shoumkovas et al.’ have disclosed zeolite from fly ash by using the combustion of lignite in the presence of alkali NaOH & KOH under the various heating temperature. The characterization of product was studied by XRD, SEM (scanning electron microscopy), and EDX(Energy-dispersive X-ray spectroscopy). In addition, Zeolite F & hydroxyl Sodalite were produced at 373-393 K temperature by using NaOH (>3m) and KOH (>6m),  while at 353K temperature using 3.1 M NaOH, the zeolite P & A was obtained with smaller amount of other impurities like Fe, Mg & S etc. [8]. ‘Mainganyes et al.’ have successfully synthesized highly crystalline zeolite Na-P1 from coal fly ash by using the hydrothermal method for 48 h at 140°C in 4-flat blade impeller at agitation for 200 rpm. [9]. ‘Jha et al.’ have developed an alkali interaction method in which fly ash was converted to salt. After 12 h treatment, there was a strong interaction between 0.5N NaOH & coal fly ash were observed. Due to this process the structure of fly ash was changed significantly [10]. ‘Wang &  co-workers have synthesized zeolite A & X from fly ash by using hydrothermal treatment using alumino silicate gel at 100°C. This selection has economic grade properties like thermal stability, high cation exchange capacity, etc., and having a tendency to remove methelene blue (cationic dye) from the solution by adsorption [11]. Huis et al. have developed a commercial detergent grade zeolite 4A uses a hydrothermal treatment. Here, the ratio of S/L was 1:10. This zeolite 4A has been efficiency to remove calcium ions during the washing process. [12]. ‘Somersets et al.’ have developed zeolite from fly ash by using an alkaline hydrothermal process. The reaction was carried out with 1:1.2 NaOH ratios, for 1-2 h at 600 °C. For the characterization of product XRD, XRF(X-ray Fluorescence spectrometry) and BET(Brunauer–Emmett– Teller theory) adsorption process [13].‘Vucinics et al.’ have discovered Na-P1 zeolite from CFLY ASH by hydrothermal treatment  with Na2O/SiO2  mole ratio at  90°C for 5 h. The characterization of the product they used XRD & IR techniques [14].

B. Fusion Followed by a Hydrothermal Method

‘Keka oza & co-workers’ have synthesized X-type zeolite from the coal fly ash by using alkali fusion followed by hydrothermal process. In this method, at 823K temperature the crystalline synthetic zeolite  was produced. The synthesized zeolite was characterized by different methods such as Fourier transform infrared spectroscopy (FT-IR), BET adsorption method, X-ray diffraction (XRD), scanning electron microscopy (SEM) etc. [15]. ‘Kazemains et al.’ have reported two stage conversion process for the synthesis of zeolite which having a high ion exchange capacity from high silica fly ash by using fusion followed by hydrothermal process. Na-P1 zeolite was produced at 120 °C for 4 h with molar ratio of 2.2 SiO2:Al2O3, 5.28 Na2O, 106 H2O under hydrothermal process. For the characterization of zeolite used SEM, XRD, thermal analysis, infrared spectroscopy (IR) and for determining ion exchange capacity of zeolite using an ammonium acetate method [16]. ‘Klamrassameel’s and co-workers’ have discovered zeolite with the alkali fusion method at 450°C from coal fly ash. The reaction carried out with 2.25 molar ratio of NaOH & treated with different temperature. The characterization component using XRD, XRF, BET adsorption techniques. Here, this synthesized zeolite was used as a water absorbent in ethanol-water mixture [17]. ‘Amalandhas et al.’ have discovered X-type zeolite from the fly ash and metal complex of ascorbic acid by using of the alkaline fusion method. The complex of ascorbic acid was encapsulated in the cages of zeolite. The characterization of this complex-zeolite was carried out using XRD, FT-IR, UV spectroscopy, thermo gravimetric analysis (TGA) and atomic absorption spectrometry (AAS). This complex was used as a heterogeneous catalyst in the phenol hydroxylation [18]. ‘Kondrus et al.’ have synthesized NaY zeolite from fly ash by using alkali fusion followed by hydrothermal processes at 550 °C fusion temperature with maximum 160 m2/g surfly ashce area of 10 h aging and the crystal of salt was produced at 100°C for 17h. NaP zeolite was produced by using hydrochloric acid instead of alkali at 750 ºC fusion temperature with 1 and 3º C/min heating rates. Fe complex of NaY zeolite was used for degradation of Congo red dye from the solution [19]. ‘Zangs & co- workers’ have developed zeolite from fly ash with alkaline fusion followed by a hydrothermal method.  It was analyzed by XRD and SEM. During the synthesis process cation exchange capacity was increased from 0.03 to 2.79 meq/g. This synthesized zeolite can be used as a removal of ammonia from waste water and also used as  an ion-exchange [20]. ‘Bhandari et al.’ have developed zeolite A & X by using coal fly ash with alkali fusion followed by hydrothermal processes using a ratio of fly ash/NaOH (1:1.2) at 550 °C fusion temperature, followed by 110 °C aging temperature for 10-12 h crystallization time. For the characterization of this produced zeolite XRD, FT-IR, BET adsorption and laser particle analyzer technique (LPAT) were used. When NaX was ion exchanged by potassium ions, it was formed KX. The application of KX was used as a catalyst for formation of biodiesel from sunflower oil [21]. ‘Changs & co-workers’ have synthesized zeolite by using alkali fusion from fly ash at 550°C.This fused solution has a high quantity of Al species produced a high quantity of fly ashujasite. The XRD results show conformation of ion-exchange capacity of fly ashujasite [22].‘Ryus et al.’ have reported the synthesis of zeolite Na-P1 from coal fly ash by using fusion followed by hydrothermal treatment at a 100°C temperature with 2M of NaOH solution. The synthesized zeolite has very higher efficiency of cation exchange [23]. ‘Yaos et al.’ have discovered Li-ABW zeolite from fly ash by using fusion followed by hydrothermal treatment with LiOH.H2O solution. The condition of higher zeolite crystallization was depended on the aging time, temperature, and concentration of LiOH.2H2O. The SEM image shows that crystal of Li-ABW was a rode shape crystal [24].

C. Membrane Extraction

‘Morenos et al.’ have synthesized zeolite A, X and NaP1 from both coal gasification & pulverized fly ash by using an alkaline fusion, direct conversion and extraction by SiO2.The high yield of zeolite was obtained by SiO2 extraction and less of direct conversion. With the alkaline fusion high purity zeolite-X with law leachable metals obtained by coal gasification fly ash [25].

D. Microwave Irradiation Technique For Utilization Of Fly Ash

For the utilization of fly ash new method has been developed for the synthesis of crystalline zeolite from fly ash by using microwave irradiation technique. The advantages of this method are time saving, very cheap, rapid heating, given a very high purity and yield etc. The wavelength of the microwave is 1mm to 1m and its frequencies are 300 GHz and 300 MHz. The synthesis of mesopourous materials like zeolite by using microwave irradiation process was very fly ash faster & economical as compared to the traditional hydrothermal method. It’s given a quick result in nucleation of homogeneous and takes a less time of crystallization of mesopourous zeolite [26].

‘Querol et al.’ have successfully discovered zeolite from fly ash by using microwave assisted hydrothermal treatment in the presence of alkali NaOH & KOH. The conventional & microwave assisted methods produced similar type of zeolite, but in microwave assisted method the activation time was less (24-48h to 30min.) rather than conventional method. The synthesized most popular type of zeolite by microwave synthesis was KM phillipsite, Na-P1 and F linde, which were used in industrial application [27].Inadaas et al.’ have synthesized zeolite by using microwave techniques in the presence of alkali but zeolitic product was not obtained. The microwave heating followed by hydrothermal heating must required to produce crystalline form of Na-P1 zeolite. The control of power of microwave from high to low was more suitable for production of zeolite from fly ash [28].‘Tankas & co- workers’ have developed single-phase Na-A zeolite from amorphous aluminosilicate of coal fly ash by using a microwave assisted two step syntheses. In a first step coal fly ash was pre-treated with NaOH solution for 1h under the microwave irradiation. Material of coal fly ash was dissolved in first step, then filters off remaining fly ash. For the second step residual solution of first step treated with NaOH-NaAlO2 at 358K for 24h. The produced single-phase Na-A zeolite ratio of SiO2/Al2O3 = 0.8 was 508 cmol/kg, which was nearer to commercial Na-A zeolite [29].


Zeolite from fly ash was a one of the alternatives of environmental protection because of its unique properties and characterizations. For the synthesis of zeolite with microwave irradiation technique is very fly ash faster & economical as compared to traditional (hydrothermal) method, it takes a less time and gave a high purity and yield of crystalline zeolite. Zeolite obtains from the fly ash has been at various kinds of environmental related properties, one of the cation exchange properties so; it was helpful to remove heavy metals and hazardous material in the waste water treatment.

Source: Khushbu G. PatelV.K. Srivastava - Pandit Deendayal Petroleum University


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