Green synthesize of fly ash-based zeolite X: a potential microwave absorbent

Zeolite X is a potential electromagnetic wave (EMW) absorption material owning to its advantages of good dielectric properties, light weight and large specific surface area. Herein, high-purity zeolite X was prepared from fly ash using a green synthesis approach, in which only trace water was used and achieved zero discharge of waste water. Physiochemical properties of fabricated zeolite X were comprehensively evaluated through XRD, PSD, SEM, TG-DSC and BET test. XRD pattern shows the successful preparation of zeolite X with good crystallinity and high purity, and the average particle size is ~ 2.45 μm, which conforms to Gaussian distributions. The fabricated zeolite X exhibits typical octahedral structure and good thermal stability. It is noteworthy that the specific surface area is up to 473.56 m2/g, representing porous structure, which is beneficial to EMW attenuation. What’s more, the abundant crystal water and adsorbed water existing in the sample is conductive to dipole polarization, which further consume EMW energy. The effective absorption bandwidth (EAB) of the synthesized zeolite X is 2.08 GHz (13.36–15.44 GHz) at the thickness of 2.5 mm. This study provides an eco-friendly approach to change waste into valuables, and moreover the synthesized zeolite X is a potential EMW absorbing material.


Introduction
With the wide use of electronic devices, electromagnetic wave (EMW) pollution is becoming more and more serious, and hence EMW absorbing materials have attracted wide attention [1,2]. So far, various materials have already been prepared for EMW adsorption [3][4][5][6][7][8][9][10]. As a kind of low-cost material, natural and synthetic zeolites have great application potential in EMW adsorption field owning to the advantages of unique dielectric properties, low density, high specific surface, and strong mechanical and chemical stability. Recently, Shang et al. reported a sepiolite-based EMW adsorption composites with superior EMW absorption performance, in which the minimum reflection loss of -50.23 dB and effective absorption bandwidth of 5.01 GHz [11]. Using lowcost light-weight and natural sepiolite as starting material, the synthesis procedure is facile, economic and commercially practical. What's more, the large surface area of sepiolite is convenient for multiple reflections and scattering, which can increase the propagation path of incident EMW and promote EMW dissipation. As another kind of porous mineral material, the research of zeolite/zeolite-based materials as islanding or regional dielectric loss absorbers is of great significance in the research field of new EMW absorbing materials, while the relevant reports are still rare.
Zeolite X is one of the most widely used zeolites due to its large specific surface area and high adsorption capacity [12][13][14]. As a kind of porous inorganic material, zeolite has the characteristics of crystal and open framework structure with many pores in it. These pores are connected with each other through channels, and the pore size distribution is highly uniform. The weak interaction between porous AlO 4 /SiO 4 structure and non-framework cations endows zeolite with unique dielectric properties. The characteristics above are beneficial to EMW loss. Fly ash, a solid waste with huge discharge, has been widely used for the synthesis of zeolite X owing to its high silicon and aluminum content [15][16][17]. Numerous studies have been established to synthesize zeolite X, such as one-step method [18,19], two-step method [20,21], microwave-assisted two-step method [22,23] and ultrasonic-assisted method [24]. Normally, a large amount of solvent involves water or organic solution is required in these processes, which inevitably results in wastewater pollution [25]. In light of this, a pioneering process of solvent-free (trace water system) approach is proposed to solve this problem [26]. As a green approach, solvent-free method possesses significant advantages as high yield, sufficient utilization of autoclaves, and reduction of pollutants and so on [27].
In this study, low-cost zeolite X was synthesized from fly ash using a solvent-free method. The prepared zeolite X possesses outstanding physical and chemical properties, and notably, the specific surface area is up to 473.56 m 2 /g, which represents porous structure and is beneficial to EMW attenuation. The work supplied a green and low-cost method to prepare zeolite X, which was expected to be an EMW absorbing material.

Materials
In this work, fly ash was obtained from Inner Mongolia, China. Analytical purity sodium aluminate, sodium hydroxide and sodium silicate were purchased from Aladdin reagent Co., Ltd.

Synthesis of zeolite X
As we can see from XRF results (Table S1), Si/Al in fly ash is 1.70 which exhibits ideal raw materials for the synthesis of zeolite X, and no additional Si or Al source was necessary. The synthesis process was conducted in the following steps: Firstly, fly ash was homogeneously mixed with sodium hydroxide and the mixture was calcined in muffle furnace at 750°C for 2 h. After cooling, trace amounts of water and prepared seeds (5% wt) were added into the fused mixture to make a smooth paste. The final molar ratio of Na:Al:Si:H 2 O is 1:1:1.6:3.5 (in accordance with typical zeolite X (JCPDS card No. 12-0228): Na 2 Al 2-Si 3.3 O 10.6 Á7H 2 O). Then transferred the paste into autoclave, and heating in an oven at 120°C for 12-72 h. After that, filtered and washed with distilled water for three times. Finally, the solid was dried at 105°C for 24 h, and Zeolite X powder was obtained. The scheme of the synthesis process was shown in

Characterization
Elements of raw ore, synthesized zeolite X and residue were tested by X-ray fluorescence spectrometer (XRF, AXIOS Max , PANalytical). The phase structure of the samples were analyzed by X-ray Diffractionmeter (XRD, D8-FOCUS, Bruker). Morphological characteristics were performed by a Scanning Electron Microscope (SEM, SU8010, HITACHI). The particle size distribution (PSD) of samples was determined using laser particle size analyzer instrument (Mastersizer 2000, MALVERN). The Brunauer-Emmett-Teller (BET) surface area, pore volume and pore size distribution were performed by N 2 adsorption at 77 K with an ASAP2020 (TSI) Automatic Volumetric Sorption Analyzer. Thermogravimetry/Differential Scanning Calorimetry (TG-DSC) analyses were characterized by Netzsch STA 409 thermal analysis system. The EMW absorption properties of NiCo 2 O 4 samples were investigated by the Anritsu MS46322B vector network analyzer (Japan) at 2-18 GHz.

XRD patterns and XRF analysis of synthesized zeolites
To rapidly obtain pure zeolite X, synthesis time was controlled between 12 and 72 h. When the reaction time is 12 h, the product is completely amorphous. Then raising the reaction time to 24 h, zeolite X (JCPDS card No. 12-0228 ) with low crystallinity and zeolite P (JCPDS card No. 39-0219) are obtained, and the content of zeolite X is higher than zeolite P. Thus, continuing to increase the reaction time to 48 h, zeolite X with high crystallinity is prepared. Although there is still tiny amount of zeolite P, the purity and crystallinity are excellent enough. Further improving the reaction time to 72 h, intensity of zeolite X weakens remarkably, however, intensity of zeolite P strengthens by contrast. In addition, peaks of sodalite (JCPDS card No. 73-1733) appear as well.
It is obvious that high temperature is unfavorable to the crystallization of zeolite X. Therefore, zeolite X synthesized at 48 h is taken for the further study, which is named SZX. X-ray fluorescence spectrometer (XRF) was used to conduct the chemical compositions of SZX. As is shown, apart from the structural components of zeolite X (Si, Al and Na), small amount of Ti, Fe, Mg, Ca, K are also concluded. The Si/Al ratio of SZX is 1.28, presents a typical zeolite X. (Table 1).

Morphology analysis of synthesized zeolites
The SEM images and EDS result are shown in Fig. 2. When the reaction time is 12 h (Fig. 2a), the obtained sample particle is irregular. When the reaction time reaches to 24 h (Fig. 2b), it can be seen that the particle size of zeolite X is about 2.5 lm, and the morphology is irregular. Thus, continuing to increase the reaction time to 48 h (Fig. 2c), the morphology of zeolite X is of regular octahedron and uniform particle size (about 2.5 lm). Zeolite X synthesized at 72 h presents regular octahedron as well (Fig. 2d), but the particle size (about 2.6 lm) is a little larger than that of 48 h. The EDS result of SZX is exhibited in Fig. 3e, and it shows that the Si/Al ratio is about 1.28, which fits the XRF result perfectly.

Particle size distribution of synthesized zeolites
According to the data of PSD (Fig. 4), the particle size distribution of SZX follows gaussian distributions, and the average particle size of SZX is about 2.45 lm, which was in good agreement with the SEM results.
The results above indicate that SZX was smaller and more uniform compared to that obtained by hydrothermal method [28,29].

Thermal behavior analysis
TG-DSC curves of SZX are shown in Fig. 5. Before 100°C, there is a weak endothermic peek, which shows adsorbing steam of air on SZX [30]. TG curve Fig. 1 The scheme of synthesize zeolite X of the sample shows smooth mass loss and was devoid of any distinct kinks. And a significant endothermal valley appears at 136°C. Meanwhile, there is a dramatic weight loss on the TG curve.
When it reaches to 246°C, the free water is removed completely. Obviously, the content of free water is high, which will produce dipole polarization and relaxation under the action of electromagnetic field so as to consume EMW energy [31,32]. The strong exothermal peak at 246°C may be caused by the combustion of organic impurities, which is another source of dipole polarization. After 600°C, mass loss is stable at about 20.61%. Although the TG-DSC analysis was conducted under 800°C, however, the tendency of DSC curve hints that there exists at least an exothermal peak after 800°C which is caused by the phase transformation and the decomposition of zeolite structure [33]. Hence the heat treatment temperature of SZX should be controlled below 600°C.

Specific surface area analysis
The pore structure of SZX was evaluated by nitrogen adsorption measurements, and the results are shown in Fig. 6. According to the IUPAC classification, the isotherm presents classical type IV with H3 hysteresis loop [34,35]. The pore size distribution for SZX was estimated by the BJH method as shown in the insert Fig. 6, which shows a pore radius below 3 nm. And the broad pore size distribution might be caused by the agglomeration of zeolite particles. The textural parameters of SZX are shown in Table 2. BET surface area is measured to be 473.56 m 2 /g, and the prepared zeolite is primarily microporous with average pore size of 1.914 nm. Large numbers of micropores and channels allows EMW transmit into the material easily, and extends the propagation channel of EMW. Thus, EMW adsorption is increased. According to the literatures [36,37], large specific surface area of porous material represents good EMW absorbing performance.

Electromagnetic wave absorption properties
The electromagnetic wave absorption performance of as-obtained zeolite X was evaluated by calculating the reflection loss (RL) according to the transmission line theory [38,39] RL ¼ 20log where Z in is the input impedance of the absorber, Z 0 is the intrinsic impedance of free space, l r and e r are relative complex permeability and permittivity, respectively, f is the frequency of EMW, d is the thickness of the absorber, and c is the velocity of light in vacuum. When the RL value is below -10 dB, over 90 % of the incident EMW can be attenuated, and the corresponding absorption bandwidth is called effective absorption bandwidth (EAB). Figure 7 shows the RL curves and corresponding 3D As expected, the fly ash-based zeolite X reveals certain dissipation capability for EMW, in which the EAB is 2.08 GHz (13.36 to15.44 GHz) at a matching thickness of 2.5 mm (Fig. 7a) and the minimum reflection loss (RL min ) is -16.95 dB (Fig. 7b). The electromagnetic parameters were investigated to explore the main source of EMW absorption. As shown in Fig. S4, the real part of complex permittivity (e 0 ) decreases from 5.67 to 4.31 gradually with the increasing frequency, displaying a typical dielectric behavior. On contrary, the values of imaginary part of complex permittivity (e 00 ) first increase slowly from 1.00 to 1.38 (11.92 GHz), and then decrease to 1.00 finally. Two obvious resonance peaks at 11.92 GHz and 15.44 GHz prove the existence of Debye relaxation processes, which may originate from abundant crystal water and adsorbed water and is conductive to EMW loss. Nevertheless, the corresponding values of l 0 and l 00 fluctuate within a narrow range at 1.0 and 0 respectively (Fig. S4 c and  d), indicating the negligible magnetic loss. Hence, dielectric loss together with suitable impedance matching of porous zeolite X contributes to EMW loss. For all that, the narrow EAB and small RL min are far from the requirements of practical application. In the follow-up study, the EMW absorption performance can be improved by doping magnetic components to enhance magnetic loss.

Prospect as an EMW absorbent
The zeolite X prepared in this work is of good prospect to be EMW absorbing materials. On one hand, the special porous structure and frame structure enable zeolite X good impedance matching, which make the incident EMW enter the material as much as possible; on the other hand, in addition to dipole polarization loss caused by adsorbed water and Fig. 6 Nitrogen adsorption-desorption isotherms of SZX; Insert is BJH pore size distribution curve  7 Reflection loss of fly ash-based zeolite X impurities, the vesicular cavity and the structure between vesicular in materials will also reflect and scatter EMWs, which dissipate EMW energy greatly. What's more, Zeolite materials have the characteristics of light weight and thermal stability, which provides favorable conditions for it to become a new type of microwave absorbing material. Briefly, the zeolite X prepared in this work is of good prospect to be a EMW absorbing material. So zeolite and zeolitebased materials will be further studied in our following work.

Conclusions
High purity zeolite X was synthesized by a green method from fly ash in 48 h. For SZX, the Si/Al ratio is 1.28; the average particle size is about 2.45 lm; and the specific surface area is 473.56 m 2 /g. Besides, the SZX shows good phase, morphology and thermal stability as well. Besides, the EAB of the obtained zeolite X is 2.08 GHz at matching thickness of 2.5 mm, which is mainly caused by weak dielectric loss. And the EMW adsorption capacity of zeolite X will be further studied in following research.