A Short Review on Synthesis, Characterization, and Applications of Zeolites

&e review emphasizes on synthesis, characterization, and application of zeolite. Zeolite is a hydrated aluminosilicate having a tetrahedral structural framework; it contains channels and cages which are occupied by exchangeable active metal ions and water molecules. Zeolite was synthesized through different synthesis methods, particularly, hydrothermal and green synthesis methods. &e review also has tried to address the structure of zeolite such as morphology, functional group, and particle size using different characterization methods as reported via different authors. &e characterization results verify that zeolite shows many unique properties such as uniform pore size, acidic properties, thermal stability, mobile extra cation, hydrophilicity, and hydrophobicity. &ese lead to a number of applications in catalysis, water purification, adsorption, and agriculture.


Introduction
Zeolites are hydrated aluminosilicate that is made from tetrahedral alumina (AlO 4 5-) and silica (SiO 4 4-) through interlinkage of oxygen atoms [1,2]. e term zeolite is derived from Greek words "Zeo" and Lithos" [3][4][5] which means boil and stone, respectively; zeolites have high ability of absorbing water and releasing it when they get heated [4]. ey have an open void with 3D crystal structure having aluminum, silicon, and oxygen coordination with active metals [5]. Zeolites are composed of central atoms (Al, Si or P), and terminal oxygen atom in tetrahedral structure to form primary building blocks (as indicated in the left side of Figure 1); they can also form secondary building blocks by bridging through oxygen-oxygen atoms to form rings, prisms, and various sizes (as it is seen at the right side in Figure 1).
Zeolites are composed of an elementary structure of an aluminosilicate framework, which comprises a tetrahedral arrangement of silicon ion (Si 4+ ) and aluminum ion (Al 3+ ) that are surrounded by four oxygen anions (O 2-) [2,4,7]. Each oxygen ion within Si-O and Al-O bonds connects with two cations and which are shared between two tetrahedron structures [6,8].
is results in the tetravalent electroneutral Si in SiO 4/2 and the trivalent negatively charged Al in AlO 4/2 [9]. e negative charge and the pores of zeolites can be occupied by group IA or IIA metal ions and water molecule as it is denoted via the general formula, Mx/n {(Al 2 O) x (SiO 2 ) y .zH 2 O} [10][11][12], where M represents an alkali or alkaline Earth cation, n represents the valence of the cation, z is the number of water molecules per unit cell, and x and y are the total numbers of tetrahedral per a unit cell.
Zeolites can be classified into two broad groups, namely, natural and synthetic zeolites [8,12,13]. Natural zeolites are mostly formed from volcanic and sedimentary rocks such as chabazite, clinoptilolite, and mordenite [13]. On the contrary, synthetic zeolites are prepared through heating of China clay, feldspar, soda ash, and other sources [13,14]. ere are also different types of synthetic zeolites such as zeolite A, zeolite X, zeolite Y, and zeolite P, which can be synthesized from different resources [15,16]. Zeolites A, zeolites X, and zeolite Y consist of a cubo-octahedral (i.e., p-cages) having SiO 4 and AlO 4 tetrahedral at each corners of the framework [14,17]. Zeolites X and Y are well known for their remarkable stability and rigid structure with large void space [6,15]. is group of zeolites plays an important role in gasoline production as catalytic cracking of gas oil [18,19]. e other familiar zeolite is Zeolite A, which is synthesized from sodium aluminosilicate usually referred as zeolite NaA or zeolite 4A, represented by the formula Na 12 (AlO 2 ) 12 (SiO 2 ) 12 ·27H 2 O [14,17]. It is the type of synthetic zeolite that is used in detergent production for treatment of hard water. It is also used to substitute sodium tripolyphosphate which is an environmentally unfriendly detergent product [15]. Furthermore, zeolite 4A has large ion exchange capacity with an average particle size ranged from 4 to 10 μm, which prevents graying and dye transfer from one cloth to another cloth during washing in laundry [18,19].
Currently, zeolites were synthesized from different natural sources, such as clay, kaolin, fly ash, coal, natural oxide, bauxite, feldspar, activated carbon, and other silica sources [19][20][21]. Zeolites, synthesized from natural sources, have highly porous cavities in their structure, high ionic exchange capacity, large specific surface area, low cost, and hydrophilic in nature [3,18,22]. For instance, the cost comparison of obtaining a silica and alumina source from a chemical feedstock and kaolin/metakaolin has been carried out, and the result shows that zeolite A synthesized from clay has a comparative cost advantage of 15% over a commercial chemical such as sodium silicate [1,3,15]. Moreover, these zeolites are cheaper and more abundant and are employed as the sole source of silica and alumina as compared to synthetic zeolites [23,24].
Zeolites, both natural and synthetic, have huge applications in industrial, agricultural, and biomedical processes [25,26]. Many researchers have turned their attention to synthesis, characterization, and surface modification of zeolites along with their applications. For instance, Olaremu, et al. [1] encapsulated the composition of zeolites using active metals mainly group alkali and alkali Earth metals to relive the variation of Al to Si ratio. Wang et al. [10] synthesized Cr-doped TiO 2 supported zeolites in order to improve their structures, pore size, channels and absorption ability during photocatalytic activities [13,27,28]. Nyankson et al. [12] also reported Zn (II) supported zeolite as the Zn-zeolite framework that generates a secondary pore; the secondary pore results larger surface area and has the ability to removal dyes, heavy metals, pesticides, and other pollutants from water [13,29]. erefore, this review explores the synthesis, characterization, and applications of zeolite [13,22,29].

Synthesis Methods of Zeolites
e synthesis approach of zeolites can be divided into two main assembles on the basis of secondary porous formation, namely, bottom-up and top-down approaches [14]. e bottom-up approach includes hard templating, soft templating, and nontemplating processes. In this approach, the secondary porous will be created during templates' stage; modification is achieved under synthesis conditions (nontemplating methods). It is a direct and one-step approach that results in a sequential rotational intergrowth of zeolite frameworks [4,14]. In the "top-down" approach, the secondary porous is created at the post-synthesis stage [4,14,18].
Pramiry build unit of Zeolites e PBUs are the (SiO 4 ) 4and (AlO 4 ) 5-tetrahedral. e PBU is combined by sharing oxygen atom with adjacent tetrahedral to form the SBUs. SBUs might be single rings, double rings, polyhedra or more complex units.   [4,12]. e crystallization stage also consists of four steps: (i) condensation of polysilicate and aluminate anions, (ii) nucleation of zeolites, (iii) growth of nuclei, and (iv) crystal growth of zeolites, as shown in Figure 2 [18,19,25].
In short, the synthesis procedures are depicted in Figure 2; amorphous aluminum and silicate were transformed into a sol gel or clear solution through dissolving with water under alkaline or acidic condition (I). e sol-gel mixture was heated in an autoclave until the crystal forms preordered subunit (II). A small crystal of zeolite having long-range order was formed during the nucleation stage (III). Finally, well-grown crystalline zeolites were formed (IV).
Conventionally, the hydrothermal method involves crystallization of aluminosilicate gels (a mixture of aluminate and silica solution in the presence of alkali hydroxides and organic bases) [23,25]. Nonetheless, the synthetic type and nature of zeolite are highly affected by different factors [23] such as composition of silica and alumina, nature of precursors and their pretreatment [28], temperature [29,30], reaction time, and pH of the reaction mixture [26].
Nyankson, et al. [12] synthesized zinc-exchanged zeolite A (Zn-Zeolite A) from alumina and silica-based deposits (kaolin, bauxite, silica, and feldspar) sampled from three regions in Ghana through the hydrothermal method. e authors concluded that the hydrothermal method required high crystallization time around 7 : 00 hrs for the formation of zeolite phases [12,23,28]. However, the hydrothermal synthesis method used organic templates and high pressure. Besides, this method needs high pressure, has low efficacy, has high potential of polluting, and emits high greenhouse gases during combustion of organic templates [14,30]. It also has low concentrations of silicon and aluminum, which requires multiple recycling treatments and releases excessive residual solution into the environments [14,30,31]. e authors also did not compare other zeolite synthesis methods such as the dry gel method, the vapor-phase transformation method, or the solvent-free method.

Green Synthesis Methods.
e hydrothermal synthesis method produces substantial quantity of wastes and generates carbon dioxide and nitrogen-containing gases [14]. erefore, an alternative synthesis approaches had been designed to overcome such challenges. us, newly developed approaches are used to increase the crystallization rate, develop continuous-flow synthesis, simplify post-treatment of a residual solution after synthesis, and enhance the production yield [14,28,30]. ese synthesis approaches are generally noted as green synthesis of zeolite [16,20,22]. e ionothermal synthesis method is characterized using ionic solvent and template to avoid high pressure, which is caused by water vapor [18,30]. In 2004, Wang et al. reported the first synthesis of aluminophosphate and silico-aluminophosphate zeolites through the ionothermal method [10].
is resulted in high efficient and high surface area zeolite. e other green synthesis method is solid-state or quasisolid-state synthesis [17,18]. Typically, quasi-solid-state synthesis method is a dry gel conversion approach that was developed by different authors such as Deng et al. [17], using a bit of solvent [17]. Xiao et al. [31] also reported the solidstate synthesis method in which the initial solid raw material is grounded and mixed without adding any solvent [31]. e microwave-assisted method is characterized by having mild reaction conditions and high crystallization rate [14,30,31]. is method has been developed to promote the nucleation rate of zeolites through increasing of the crystallization rate [31][32][33]. Omisanya et al. [2] synthesized zeolite A from Kankara kaolin under microwave conditions using adsorbent in a solar refrigerator. e continuous-flow synthesis method is characterized by a complete crystallization within seconds or minutes due to the large heat transfer coefficient that carried out in a tubular reactor [13,18]. Pan et al. [14] reported the crystallization rate of zeolite Socony-Socony Mobil-5 (ZSM-5) from amorphous aluminosilicate gel within a minute. e authors verified that this synthesis method provides great potential for facilitating the mass production of industrial zeolites [2,14,16]. e other synthesis method is the solvothermal method that involves a solvent such as polar solvents and nonpolar solvents [14,30,31]. Xiao et al. [31] synthesized faujasite zeolite from silica and pearlite. Both the silica and the pearlite were mixed in sodium hydroxide solution, and commercial aluminum-hydroxide was dissolved in sodium hydroxide to produce synthesized gel. Crystallization of the produced gel was done at a temperature of 90°C for 120 hrs; then, mixed phases of zeolite Y, zeolite P, and gmelinite were produced [2,31].
Odebunmi et al. [1] synthesized zeolite from kaolin clay in Erusu Akoko south western Nigeria. e kaolin clay was converted to metakaolin at 600°C and then leached with sulfuric acid to achieve the required silica-alumina ratio [1,14]. An alkaline fusion was carried out to transform the metakaolin into Zeolites Y, X, and P [1,10,19]. In the fusion process, kaolin is thermally activated by the addition of sodium hydroxide to form an active Al and Si species [14,19]. ese species tend to react with each other followed by reorganizing to form a ring-like structure that used as basic units for the zeolite framework. e authors deduced zeolites were successfully synthesized from local source such as clay through alkaline treatment of its metakaolin under the solvothermal process, which is important in the petroleum industry as a catalyst and wastewater treatment as an adsorbent [1,23,34]. Nevertheless, authors did not introduce secondary pores (meso-or macropores) to reduce the zeolite crystal size to the nanometer scale to give hierarchical and nanosized zeolites and enhance diffusion limit of bulky molecules into the pores and out of the pores.
Luo et al. [25] synthesized sodalite from alkali-activated coal fly ash through the hydrothermal process for the removal of lead ions and realized the effect of temperature, pH, initial concentration, and adsorption time during the adsorption process [16,25]. However, still the authors did not realize the effects of competing metal ion (more active metals than lead ion). Deng et al. [17] also synthesized zeolite Advances in Materials Science and Engineering through hydrothermal followed by fusion methods [14,16] from municipal solid waste fly ash (MSWFA) and showed that there is no significant difference on the produced zeolite as compared to the zeolite synthesized through the two methods. Nevertheless, the hydrothermal method consumed high energy and was time-consuming during zeolite synthesis from MSWFA [17,23].
Wang and Peng [35] reported Cr-supported and TiO 2doped photocatalytic natural zeolites through displacing of TiO 2 lattice with Ti 4+ sites due to the close ionic radius of Ti 4+ and Cr 3+ [10]. ey bring enhancement to the binding force of Cr/O/Ti through the increment of calcination temperature. is improves the photocatalytic activity of the synthesized zeolites [10,25]. e authors inferred that the gab energy is significantly reduced in Cr/TiO 2 -doped zeolite than the undoped one. us, doped Cr/TiO 2 zeolite exhibits higher de-colorization and photocatalyst activity than the undoped TiO 2 zeolite [10,17,31].
Ruíz-Baltazar et al. [18] synthesized zeolite from natural precursors such as clinoptilolite, mordenite, and feldspars and further encapsulated with iron to increase adsorption capacity of zeolite as iron nanoparticles [18]. e authors inferred that lower concentration of iron nanoparticles have greater interaction with the surface area of the prepared zeolite [18,20]. However, further increments of the iron concentration diminished adsorption of processes due to a saturation of iron on the surface of the zeolite. e authors verified optimum concentration of iron which has higher adsorption of pollutants; the result agrees with the Lagergren model [18,19,31].
Sudiyarmanto [36] also synthesized natural zeolite through the wet impregnation method using hydrogen chloride (HCl) solution and metal catalyst [36,37]. e authors successfully prepared Ni-based natural zeolite (Ni/ ZAB) through the wet impregnation method [13,36]. e prepared zeolite showed good hydrogenation reaction with citronellol and high adsorption activity for 3, 7-dimethyl-1octanol compound. Nevertheless, the catalytic performance of the prepared catalyst varies with temperature and pressure towards yield and selectivity of 3, 7-dimethyl-1-octanol production [36]. Moreover, authors did not explain the catalytic activities of zeolites in comparison with metalencapsulated zeolite or presence of extra pores in zeolite. Yongquan et al. [38] synthesized NaY zeolite in a rotating packed bed (RPB); authors verified zeolites NaY in RPB which shows short reaction time, small particle size, high catalytic performance, and feasible industrial application than zeolite NaY synthesized through conventional synthesis methods.
Hongwei et al. [39] synthesized nanocrystals' templatefree zeolites (zeolite Y and X) using reverse microemulsionmicrowave method in comparison with the template synthesis method. e result revealed that the template approach is normally carried out by calcination, which leads to irreversible aggregation of the nanocrystals; the use of the template in the synthesis tends to change the Si/Al ratio of the final products that could drastically affect their applications. However, template-free zeolite synthesis has the advantages of short reaction time, producing small particles with a narrow size distribution and high purity.
Sajo et al. [40] synthesized zeolitic mesoporous materials by dry gel conversion under controlled humidity; the dry gel conversion (DGC) method, sometimes called the vaporphase transport method, is typically carried out in a reactor where a porous support stands above a reservoir of water or other volatile liquids. e authors deduced that the DGC method generates less waste and requires less reactor volume and is very effective if the product is to be made into selfbonded pellets or membranes than the conventional hydrothermal method. Naser Azizi et al. [41] synthesized template-free zeolite P and its phase transformation to two types of high crystalline zeolites Y and analcime with changing crystallization temperature (80-160°C). ey also observed the effect of Si/Al ratios and crystallization times on transformed zeolites because the Si/Al ratios have a great influence in the phase transformation during template-free synthesis of zeolite P.
Ghadamnan et al. [42] compared the microporous cavities and channels of zeolites at nanoparticles' scale for application of ion exchange and catalysis through encapsulation of cations, complexes, and metals. Encapsulation of zeolite can provide a strict control of the nanoparticle size as well as a limitation of aggregation at high temperature. e authors also summarized the hierarchical structures or unusual morphologies of zeolites as well as perspectives in the design of zeolite-containing metal nanoparticles, which differ from traditional materials by the originality of the synthesis method or the morphology of the support. Yuanyuan et al. [43] presented green fabrication of hierarchical zeolites from natural minerals through a versatile mesoscale depolymerisation-reorganisation strategy. is hierarchical zeolite synthesis method offers an effective solution to overcome the drawbacks of conventional zeolites in catalytic applications zeolites [44].

Advances in Materials Science and Engineering
Nowadays, a tremendous progress has been made in zeolite synthesis for sustaining the synthesis factors such as sources of silica and alumina, mobile ions of alkali/alkaline Earth metals, mineralizing agents of hydroxide/fluoride, concentration of gel, and conditions of synthesis processing.
is leads to the development of variety of zeolite-templating approaches and the discovery of a novel synthesizing methods of zeolite such as the dry gel method, vapor-phase transformation method, solvent-free method, and templatefree method. Besides, researchers find a cheaper zeolite source [15] such as clay minerals, natural zeolites, coal ashes, municipal solid waste, incineration ashes, and industrial slags as a starting material for synthesizing of zeolite using those novel synthesis methods [31].
In perspective, the hydrothermal method causes greenhouse gas emissions' problem and high energy consumption during calcination, recycling reagents in the residual solution and results in pollution. On the contrary, the synthetic route via solid-state or quasi-solid-state synthesis (i.e., both template-free and solvent-free routes) appears to be a sustainable choice. However, the mass production of zeolites followed by solvent-free (solid-state synthesis) (quasi-solid-state synthesis) routes is still challenging because of the difficulty of the heat and mass transfer involving solid-state reactions. Continuous-flow synthesis and microwave-assisted synthesis methods indeed promote production efficiency by enhancing heat transfer, and their application in solid-state or quasi-solid-state synthesis is worthwhile. Microwave synthesis methods are now widely used in many zeolite syntheses due to their fast and energy efficient techniques to avoid competitive reactions in many known processes. Sol-gel synthesis process also advanced the method for zeolite synthesis due to easy encapsulation of elements (metals or metal oxides) into zeolite cavities. However, this may lead to formation of aggregation and diffusion limitations; thus, using the hierarchical approach is too important to enhance the diffusion rate and catalytic properties of zeolite.

Powder X-Ray Diffraction (XRD)
. X-ray diffraction (XRD) is an efficient technique used to identify crystallographic structure of soiled states [10,13,31]. XRD is used to study the crystal structure of zeolite, crystal size of zeolites, approximate extent of heteroatom substitution, and presence of defect in zeolites [2,4]. Melaningtyas et al. [16] synthesized NaY zeolite from Bayat natural zeolite and showed the effect of pH on NaY zeolite synthesis process and characterized; XRD patterns of synthesized zeolite (NaY) at pH 11-13 revealed the formation of NaY zeolite [16]. e result was confirmed through comparison of the XRD pattern of synthesized NaY with NaP zeolite obtained from the International Zeolite Association (IZA) database (Figure 3). As the graph revealed, NaY zeolite was not completely formed at pH 11 and 12, but shows some similarities to the standard NaY zeolite at 2θ values of 6.73°, 10.91°, 23.23°, and 27.47° [16,28].
is is probably due to the presence of cations other than Na, such as K + , Mg 2+ , Ca 2+ , and Fe 2+ , which were not removed during pretreatment processes [16,45]. Zeolite NaY prepared at pH 11 and 12 showed uneven baseline pattern; this indicates the presence of mixed amorphous fraction in zeolite. Nevertheless, zeolite NaY, prepared at pH 13, showed high crystalline structure similar to that of NaP zeolite, as shown in Figure 3. Hence, the authors concluded that the XRD patterns revealed that the Bayat natural zeolite structure was successfully destroyed and converted into a different zeolite structure mainly zeolite NaY. is confirms the crystalline structure and the pore uniformity, and the pore size of zeolite NaY is affected by pH values [16,25,45].

Scanning Electron Microscopy (SEM).
Scanning electron microscope (SEM) is used to study the surface of solids and give information about their morphology and topological via producing various signals [12,13,27]. Chunfeng et al. [13] studied the morphology of zeolites A and zeolite X using SEM and compared one another. ey observed that zeolite A was formed in chamfered-edged cubes' structure while zeolite X exhibits octahedral structure ( Figure 4) [10,13].
Magdalena [45] characterized the growth rate of zeolite by comparing the one-step synthesis method with the multistep synthesis method using SEM [10,45]. e authors concluded that the films of synthesized zeolites through a multistep procedure show more clear morphology than the corresponding films synthesized through the one-step method [45]. Furthermore, they realized that the films synthesized by multistep are more qualified than films synthesized through one-step ( Figure 5). Nevertheless, the authors did not notice the diffusion limitation and aggregation of particles during characterization.
Omisanya et al. [2] examined the morphology of prepared zeolite A from metakaolin of the kaolin sample in comparison with commercial zeolite 4A. e authors noticed the presence of some cubic crystal structure in prepared zeolite A along with the gel, and a plate-like structure for the kaolinite clay indicates the silica and alumina are sliding over one another [2,16,45]. e SEM images revealed a uniform particle size with a regular shape found in the prepared zeolite A sample. e commercial and synthesized zeolites' SEM images give a narrow distribution of particle size with an average crystal size of <20 µm [3,12,22]. Nevertheless, the authors did not use the computational method to clarify structure of zeolite due to sliding of silica and alumina over one another (aggregation of particles).
Nyankson et al. [12] characterized the morphology and the textural properties of zeolites A, magnetite nanocomposite, and zeolite/nanocomposite (zeolite A/Fe 3 O 4 ) using SEM analysis ( Figure 6). e authors concluded that the SEN analysis confirms the presence of cubic crystals' structure of zeolite, as shown in Figure 6(a); spherical morphology of the Fe 3 O 4 nanoparticles endorse the formation of the magnetite, as shown in Figure 6(b). Distortion of the original cubical occurred due to the incorporation of the magnetite nanoparticles into the zeolite framework, as shown in Figure 6 Figure 3: Comparison of NaY zeolite XRD pattern at (a) pH 11, (b) pH 12, (c) pH 13, and (d) pH 13 with that of standard NaP zeolite adopted from [16].
(a) (b) Figure 4: SEM images of synthetic (a) zeolite A and (b) zeolite X adopted from [13]. 6 Advances in Materials Science and Engineering morphological structural changes [12,16]. However, the authors did not explain the causes of distortion such as elongation during zeolite/Fe 3 O 4 nanocomposite formation.

Fourier Transform Infrared (FTIR) Spectroscopy.
Fourier transform infrared spectroscopy (FT-IR) is used to confirm the fundamental vibrations of silica-alumina in a zeolite structure [35,46]. FTIR spectroscopy is used to specify the functional units of zeolites and to predict the reactions mechanism in the zeolite framework. e FTIR spectrum is also used to indicate the secondary building units which were found in the zeolite structure [5,12]. Wang et al. [10] characterized and compared TiO 2 and 10% Cr/TiO 2 composite zeolites using FT-IR, as shown in Figure 6. e broad peak around 3437 cm −1 and 1637 cm −1 indicates the stretching and bending of O-H bond, respectively. e peak at 1049 cm −1 indicates the Si-O-Si stretching and the weak peaks between 800 and 400 cm −1 correspond to Si-O-Si and Ti-O bending vibration. us, the FT-IR peaks confirm the elemental interrelationships of the zeolite, TiO 2 /zeolite, and 10%Cr/TiO 2 /zeolite (Figure 7).
Melaningtyas et al. [16] synthesized NaY zeolite at pH values of 11, 12, and 13 with comparison of commercial NaY zeolite using FT-IR (Figure 8). e prepared NaY zeolite at pH 13 exhibited the highest H-OH intensity at 1600 cm −1 .
is result was agreed with the energy-dispersive X-ray spectroscopy (EDX) data. e band formed at 3460 cm −1 on the zeolite product attests to the attachment of hydroxyl groups to the zeolitic framework [4,10,31]. e peak at 1500 cm −1 indicates the formation of aluminum oxide compound; this result indicates the aluminasilicate framework collapses and affording smaller fragments [16]. e 1636 cm −1 band indicates the presence of adsorbed water on zeolite, typically deformation band. e peak about 1000 cm −1 is probably due to internal tetrahedral zeolite structure vibration of asymmetrical stretch, whereas the peaks at the range of 720-650 cm −1 show the symmetrical stretch vibrations of the internal tetrahedral structure of zeolite [16,19,35].
Gaidoumi et al. [3] characterized natural clay and modified clays; the FTIR spectrum indicates the presence of the pyrophyllite phase. e absorption peaks at 3674, 854, and 835 cm −1 are assigned to the OH stretching and vibration of Al-OH [3,16]. e peak at 3446 cm −1 may be attributed to O-H stretching surface water molecule and the peak at 1639 cm −1 may be due to O-H bending of the adsorbed water [16]. e band at 1428 cm −1 indicates the impurity of carbonate, whilst the peak at 980 cm −1 corresponds to the intense nonbridging Si-OH stretching vibration [16]. e bands at 797, 779, and 693 cm −1 indicate the presence of quartz [16,25]. e band at 518 cm −1 is ascribed to Si-O-Si bending vibration, while the bending of Si-O groups appears at 440 cm −1 [13,27]. e spectrum of PZ indicates the appearance of characteristic vibrational bands of the hydroxyl sodalite phase [16]. e peaks between 3000 and 4000 cm −1 are attributed to OH related to structural water molecules in the case of synthetic HS [1,16]. is was confirmed by the peak at 3626 cm −1 by the XRD study which indicates the presence of the hydroxy sodalite (HS) phase with four water molecules. e bands less than In [18], FT-IR spectra of the natural zeolite showed the band associated with symmetric and asymmetric stretching vibrations of the hydroxyl functional group at 3614 cm −1 and at 1645 cm −1 the vibration of the bond Si-O, respectively ( Figure 10). e band at 1087 cm −1 confirms the vibrations of Al-O bonds, [18,22]. e authors concluded that the presence of allotropic phase of SiO 2 at 797 cm −1 and the Si-O and O-Al bonds are tectosilicates and is very strong, which suggests that the zeolite has a large surface area [18].

Application of Zeolites
e intrinsic properties of zeolites have led vast applications of zeolites such as absorbability, water purification, coagulation activity, membrane separation, and antimicrobial activities; these are associated with their porosity and structural diversity [18,22], their uniform pore size and shape, their mobility of cations, and the hydrophilic and hydrophobic nature of the absorbents and absorbates [12,46]. Nowadays, zeolites continue to find various applications in solving environmental, scientifical, and industrial problems [47][48][49].

Catalytic Properties of Zeolites.
e catalytic activities of zeolites are due to the presence of their active sites (known as bronsted acid) on OH bridging framework between silicon and aluminum channel [50,51]. Substitution of Si atoms in the zeolite framework by trivalent elements creates a negative charge on the lattice that needs to be balanced by a positively charged counterion [43]. When the charge balancing is provided by a proton, a Brønsted acid site is formed. e sites are referred to as bridging hydroxyl which is formed at each oxygen bridge site near the clustering of the Si-O-Al where the cation that neutralizes the negative charge is represented by protons (Scheme 1). e Si-OH-Al sites/ hydroxyl groups are a proton donor [52,53].
Ethyl acetate was synthesized from acetic acid and ethanol through esterification process using zeolite (zeolite A) as the heterogeneous catalyst [49][50][51]. e zeolite served as a surfactant and is used to shift the equilibrium reaction towards the product by removal of unwanted molecule during the reaction (Scheme 2). e adsorbed acetic acid on the zeolite catalyst forms a chemical linkage with the ethanol, followed by intramolecular dehydration to generate water and ethyl acetate [49,50,52] (Scheme 2). However, the authors did not mention the cause of poor catalyst utilization and decreased catalytic rate such as small pore sizes and long Wavenumber (cm -1 ) Transmittance (%) Figure 9: FTIR spectra of samples: natural clay (PB) and pyrophyllite (PZ) adopted from [3]. Transmittance (%) Figure 10: FT-IR spectra of the natural zeolite clinoptilolite type adopted from [18]. diffusion path lengths which reduce transport efficiency of catalytic process, enhancing the accessibility of bulky molecules to the active sites located in the zeolite channels and reducing the impact of diffusional limitations while maintaining high selectivity and catalytic activity too important.
Fattahi [54] reviewed the catalysts' role of zeolites in the production of biodiesel through transesterification and esterification reactions, as shown Scheme 3.
Li et al. [49] reviewed the catalytic activity of zeolites on the conversion of microbial produced lactic acid into biodegradable and renewable poly-lactic acid which is lactide (Scheme 4) [51]. e authors concluded that the traditional lactide synthesis method from lactic acid is time and energy consuming, involving condensation and transesterification process at high temperature which leads side products [24,51]. Besides, the authors did not consider the effects of aggregation and sinter of impurity into zeolite active site; this causes loss of catalytic performance in practical reaction coordination.
Corma et al. [55] reviewed that the catalyst activity as activated and deactivated during the catalytic process is strongly dependent on the reaction conditions, and numerous factors include the temperature, reactants, pressure, support, metal loading, and particle size. e authors summarized that metal atom may migrate from one porous zeolite site to another across the channel, pore, or cavity in the porous carrier material, as shown in Figure 11. (A) e atomicity of the metal species may change from isolated metal atoms to metal clusters with a few atoms to nanoparticles with tens or hundreds of atoms during reaction conditions and the physicochemical properties of the catalyst. (B) Potential structural transformation of supported single-atom alloy nanoparticles under reaction conditions or change of the atmosphere; segregation may occur with single-atom alloy nanoparticles. (C) By contrast, bimetallic nanoparticles with chemical segregation may be transformed into single-atom alloy nanoparticles under the proper reaction conditions.
In current situation, encapsulation of transition metal and their complexes into zeolites is a promising process to enhance the catalytic activity in different reaction routes. Pan et al. [14] reviewed on microporous and hierarchical zeolites to improve the catalytic activity, especially to enhance catalyst lifetime for potential industrial applications. Risheng et al. [56] reviewed the microporous and mesoporous zeolites in hierarchical zeolites that are favorable to overcome the mass-transfer issues associated with microporous zeolites. us, the diffusion and steric limitations of bulky molecules and the pore-blocking effect of deposited coke species can be reduced, affording high utilization efficiency, prolonged catalytic lifetime, and improved catalytic performance of zeolite catalysts. ese synthetic strategies afford hierarchical zeolite characteristics (e.g., distribution, dimensionality, connectivity, and even ordering of secondary pores) to be tuned to enhance the diffusivity of bulky molecules and the accessibility of the substrates to the active sites of the catalysts. Besides, the authors highlighted that hierarchical zeolites offer the potential to reduce steric limitations for converting bulky molecules, increase the rate of intracrystalline diffusion, inhibit deactivation due to coking, maximize catalyst utilization, and modulate [54].

Adsorption Properties of Zeolites.
e adhering mechanism of zeolite is affected via adsorption equilibrium, rate of adsorption selectivity, adsorbate shape, ion exchanging processes (adsorption), and reactivity of the adsorbate and adsorbents [57]. Nyankson et al. [12] studied the feasibility of synthesized Zn exchangeable zeolite A to adsorb methylene blue (MB) from solution through adsorption kinetics [44,57,58]. e authors concluded that the adsorption of MB on to Zn-exchanged zeolite A is affected by contact time; the adsorption equilibrium reached within 180 min (Figure 12). e synthesized Zn-exchanged zeolite shows strong  adsorption for methylene blue dye at optimum contact time, as shown in [57,59]. However, the authors did not recognize the effects of competing agents during adsorption of MB.
Chojnacki et al. [22] studied the sorption mechanism (adsorption and ion exchange) of natural zeolite (Clinoptilolite) for removal of mercury from industrial effluent [22]. e authors' inferred zeolite capable for removal of mercury ion from water by trapping the cation with negative end of sorbent surface (zeolite) [49,51]. Nevertheless, the ion exchange and adsorption process of mercury ion via zeolite is highly pH dependent [58,59]. Besides, the authors did not mention the presence of the hindering agent (more active metal ions) during the sorption process. Ghadamnan et al. [46] studied the effect of competitive ions adsorption in the nano-LTA zeolite using Ca 2+ and Mg 2+ ions in the binary system [46]. As can be seen in Figure 13, the adsorption capacity of ions increased with initial concentration of Ca 2+ and Mg 2+ ions as well as adsorption time [46,57]. e authors concluded the adsorption rates of Ca 2+ ion are faster than Mg 2+ ions to exchange at 15 min and rapidly reached the equilibrium point. Furthermore, the exchangeability of Ca 2+ is higher than Mg 2+ on nano-LTA zeolite; this is because Mg 2+ exists in the form of hydrate in the solution, which makes it hard to exchange with Na + in zeolite [28,58]. According to the competitive exchange data, nano-LTA zeolite possesses a large amount of Ca 2+ ions that competes with Mg 2+ in the adsorption process [29,60].
Many researchers, such asTran et al. [57], modified zeolite adsorption using dual-electronic adsorbent for removal of cationic, oxyanionic, and organic compounds. Na-H-zeolite (composite zeolite) can be regarded an amphiphilic and dual-electronic adsorbent, which is used to remove toxic pollutants from wastewater [46,57]. Zeolites Absorbance (au) Figure 12: Absorbance intensity curve at various time intervals adopted from the work of Nyankson et al. [12].
with high ion exchange capability have high level of selective during adsorption. is was achieved using high silica zeolite absorber which is specific for hydrophilic and hydrophobic adsorbates [12,22,57].

Ion Exchange Properties of Zeolites.
e primary building units (PBU) of zeolites are the SiO 4 and AlO 4 tetrahedra structure [2]. ey connect via oxygen ions into secondary building units (SBU), which are then linked into a 3D crystalline structure of zeolite [2,18]. Substitution of Si by Al results in the negative charge of the zeolite framework, which is compensated by alkaline and Earth alkaline metal cations [4,5]. e created net negative charge which exists on zeolite ends is balanced by active metal ions such as Na + , K + , Ca 2+ , and Mg 2+ . ese positive ions are rather loosely held and can be readily exchangeable between zeolite (Z) in a contact solution, as shown in the reaction below [61]. e ion exchange properties of zeolites are widely used in detergents and treatment of waste and hard water [49,50]. e replacement of insoluble Cr and As ions by active metals is used for removal of Cr (VI) and As (V) from solution [57].

Water Purification.
Zeolites are used in purification of wastewater from various sources including natural, industrial, agricultural, and municipal waste that contain metal ions Sb, Cr, Cu, Pb, Zn, Co, and Ni, together with waste liquids which are generated by metal finishing [58]. e dissolved cations are removed from water by being exchanged with the cations on a zeolite's extra-framework exchange active sites [29,58]. Tsitsishvili et al. [37] and Wang and Pen [35] carried out a study on many natural zeolites (clinoptilotite, mordenite, philtipsite, and chabasite) for removing toxic metals from industrial waters (Cu 2+, Ag + , Zn 2+ , Cd 2+ , Hg 2+ , Pb 2+ , Cr 3+ , Mo 2+ , Mn 2+ , Co 2+ , and Ni 2+ ); these zeolites are highly selective for NH 4+ ion even in the presence of competing cations [29,54]. e authors revealed that toxic metal ions are removed from water and replaced with biologically acceptable cations such as Na + , K + , Mg 2+ , Ca 2+ , or H + from the zeolite exchange sites [60].
Pitcher et al. [52] studied the removal of heavy metals from motorway storm water using synthetic zeolites [50,52]. Zeolite is effective in removing heavy metals, but is affected by pH of solution [58]. At very low pH values, alumino groups in zeolite are protonated, whereas at high pH, they are deprotonated by hydroxyl ions (Scheme 5). e heavy metal ions react with protonated or neutral alumino groups, as shown in Scheme 5.
Hui et al. [50] showed comparative study on zeolites that is prepared from coal fly and commercial zeolite 4A for removal of heavy metals from aqueous solution [50]. ey concluded zeolites that were prepared from coal fly are used as an alternative adsorbent of heavy metal ion from wastewater; the sorption rate and process of metal ions are high at pH value 4 [3,50]. Figueiredo and Quintela [61] studied on the adsorption of heavy metal ions and oxyanions via metal modified zeolite (MMZ). is adsorption process is either a simple ion exchange or through formation of metal oxide on zeolites [54]. Counter ions mainly groups IA and IIA metals' presence in zeolites which are easily soluble and replace via insoluble Cr (VI), As (V), Ag (I), Al (III), Ba (II), Fe (III), Pb (II), and other less soluble metal ions [46,54,60]. e authors concluded MMZ used for removal of As (V), Ag (I), Al (III), Ba (II), Fe (III), and Pb (II) by trapping on the internal and external sites of the modified zeolites ( Figure 14).
Jiahui et al. [62] modified zeolite in terms of physical modification, chemical modification, and composite modification to increase the adsorption potential of zeolite for various pollutants from wastewater. e authors concluded physical modification was only related to ammonium removal whilst chemical modification and composite modification were widely for eliminating varied kinds of pollutants. Modified zeolite as a substrate can greatly improve the removal rate of pollutants and alleviate the clogging conditions of pollutants. However, the authors did not support the physical and chemical modification through adsorption models.
Nowadays, zeolites have been playing a great role in wastewater treatment, in industrial production, in hard water treatment, and in catalytic process due to their internal and external porous [46]. Zeolites have a potential for adsorption of impurities, removal of metal ions, and removal of pollutants from wastewater mainly ammonium, anions, phosphorus, and heavy metals, as shown in Figure 15.  Figure 13: e Ca 2+ and Mg 2+ competitive exchange data with different initial concentration adopted from the work of Ghadamnan et al. [46]. microbial nitrification; mixing zeolite into soil reduced the rate of nitrification likely because of NH 4 + adsorption in the zeolite mineral lattice. us, mixing zeolite into soil may reduce the leaching of inorganic N and improve the soil water status, although initial leaching of zeolite-borne Na may be necessary before growing crops. e authors compared the growth of corn using urea fertilizer and zeolite mixed urea fertilizer; results showed that zeolite mixed with urea fertilizer increases corn weight and seed yield than unmixed urea fertilizer. is is probably due to high selectivity of zeolites for NH 4 + and K + .

Agricultural
Reháková et al. [64] carried out the study on natural zeolite of the clinoptilolite in agrochemicals' process such as fertilizers; natural zeolites are effective in improving soil properties and treating contaminated soil. e authors concluded that feed zeolites in soil improve the efficiency of nutrient use by increasing the availability of P from phosphate rock, the utilization of N-NH 4 + and N-NO 3 -, and reduced losses by leaching of exchangeable cations, especially K. Besides, zeolites enhance the seedling growth and crop products. However, the authors did not mention the effects of zeolite on soil acidity. [65] studied the natural zeolite (clinoptilolite) in soil amendments for sandy, clay-poor soils. Clinoptilolite has highly selectivity for NH 4 + and K + and is slow-release chemical fertilizer. e author deduced clinoptilolite-rich tuff as a soil conditioner which provides significant increment in the yields of wheat (13-15%), eggplant (19-55%), apples (13-38%), and carrots (63%). e author concluded that nutrient-treated zeoponic substrate used for growing crops and the rooting of cuttings in greenhouses produced greater root systems and larger yields of strawberries, tomatoes, and peppers, without further fertilization.

Mumpton and Frederick
Currently, zeolites are used in the agricultural sector to ensure food security owing to their enrichment with N, P, and K and adequate slow release of those nutrients to plants. Zeolite is used as soil conditioner or amendment by improving soil nutrient retention, water infiltration, reduced water run-off and reduced surface water run-off, and nutrient leaching. Benefits from zeolite application could translate to increased yield, more efficient use of nutrient inputs, and reduction in environmental pollution of nitrogen (N) and phosphorus (P) in water bodies, as well as reduction in greenhouse gas emissions. However, the use of zeolites in crop production increases soil pH to undesirable levels.

Conclusion and Perspective
In conclusion, the synthesis methods of zeolites depend on many factors: composition of precursors, pH of the reaction, temperature, pretreatment of precursors, seeding time,  reaction time, and templates used. Microwave synthesis methods are now widely used in many zeolite syntheses due to their fast and energy efficient techniques to avoid competitive reactions in many known processes. Sol-gel synthesis process also advanced the method for zeolite synthesis due to easily encapsulation of elements (metals or metal oxides) into cavities. Nevertheless, employing a two-step crystallization process in hierarchical porous zeolite synthesis is promising; combining hydrothermal crystallization with microwave heating technique is one more synthesis method giving smaller and more uniform particles in a shorter time. Combinations of two or more synthesis methods are required such as solid-state or quasi-solid-state synthesis with the aid of a microwave, and it can be considered because of the high crystallization rate and high product yield. However, this may lead to formation of aggregation and diffusion limitations; thus, using hierarchical approach is too important to enhance the diffusion and the catalytic properties of zeolite materials. Beyond this, using computational approach such as density functional theory (DFT), Gaussian, and AOMix have significant contributions to elucidate the molecular nature of active sites in zeolite catalysts. e review also highlighted different characterization methods by organizing related literatures that are carried out in different chronological time sequences. On the basis of characterization results, zeolites show inherent properties such as uniform pore size/shape, acidic properties, thermal stability, mobile cation, and surface properties such as hydrophobicity and hydrophobicity. ese lead a number of applications of zeolites such as catalysis, ion exchanger, water purification, adsorption, and agricultural applications [66].

Conflicts of Interest
e authors declare that they have no conflicts of interest.