Recent Progress on the Synthesis and Applications of Zeolites from Industrial Solid Wastes

Abstract

Zeolites have been increasingly applied in various fields such as energy conversion, environmental remediation, chemical production, and so on, being used as membranes, catalysts, and supports due to their large specific areas and strong gas adsorption. And, developing low-cost strategies for the preparation of zeolites has attracted the extensive attention of researchers. Coal fly ash, waste glass, discard zeolites, and slags are typical industrial wastes and rich in Si and Al, indicating that these industrial wastes can be utilized as alternative raw materials of zeolite synthesis. Firstly, the chemical composition and properties of these industrial wastes are summarized. Then, the strategies involved in synthesizing different zeolites from various industrial wastes are discussed. In addition, the applications of these zeolites are comprehensively reviewed.

1. Introduction

A zeolite molecular sieve is a kind of crystalline aluminosilicate with a three-dimensional pore structure. Artificial zeolites were first synthesized in 1862. According to relevant data from the International Zeolite Association, in addition to the 46 natural zeolite minerals that have been discovered, more than 250 artificially synthesized zeolites have been produced. Some zeolites such as ZSM−5, beta, Y, CHA, and x have been successfully commercialized. In China, the production of zeolites reached 2.04 million tons in 2022. The zeolite structure consists of [SiO₄]⁴⁻ and [AlO₄]⁵⁻ tetrahedra linked by a single oxygen atom, resulting in the formation of various cages with different dimensions [1]. These zeolites can be identified as belonging to one of three categories according to their pore size: small pores, medium pores, and large pores [2]. Additionally, Si and Al are the primary elements in zeolites, indicating that the structure and properties of zeolites are significantly influenced by the contents of Si and Al [3]. According to the different Si/Al ratios in zeolites, the International Zeolite Association (IZA) classifies these zeolites into low-silica zeolites (Si/Al = 1–2), medium-silica zeolites (Si/Al = 3–10), and high-silica zeolites (Si/Al ≥ 10).

Generally, the traditional aluminosilicate zeolites can be described as M_x/n[(AlO₂)_x(SiO₂)_y]·mH₂O, where M represents an exchangeable charge-compensating cation and n refers to the charge of this cation. These cations can be exchanged with other cations, enabling zeolites to be applied in various catalytic industries.

The ion-exchange properties of zeolites arise from the presence of charge-compensating cations within their framework. These cations are responsible for balancing the negative charge of the framework when heteroatoms like aluminum are incorporated into the silicate structure.

Extra-framework cations (EFCs) can significantly influence the characteristics of zeolites. These cations are usually located in the pores or at the surface of the zeolites. They can affect the adsorption properties of zeolites by altering the surface charge and hydrophobicity or hydrophilicity of the pore walls. Therefore, zeolites with different chemical compositions and pore structures can be applied in different fields, such as CO₂ capture, wastewater treatment, petroleum catalytic cracking, NO_x treatment, and so on [4,5].

Due to their high purity, excellent selectivity, and high efficiency, zeolites have attracted a great deal of attention. To achieve higher adsorptive and catalytic efficiency, chemical raw materials are usually applied to prepare zeolites [6]. Generally, the preparation of zeolite requires a substantial quantity of energy and water. Additionally, this process generates a significant volume of wastewater, which contains the alkaline solution. The release of the wastewater into the environment can lead to environmental pollution. This is inconsistent with the principles of “green chemistry” that emphasize the use of harmless and low-cost raw materials and less energy consumption [7]. Therefore, the development of low-cost raw materials, optimization of the synthesis process, and reduction in costs have become key challenges that need to be addressed in the future for the development of zeolites.

In China, the enormous industrial production and consumption capacity contribute to the large production of industrial by-products, and some of them contain abundant silicon and aluminum, such as waste glass, metallurgical slag, coal fly ash, and so on. The accumulation of industrial solid wastes not only occupies the land but also poses a potential threat to the environment. Several attempts have been made to reuse these solid wastes, and various products such as road materials, building decoration ceramics, and insulation materials have been prepared from these solid wastes. However, the low added value of these products greatly reduces the enthusiasm of producers. Therefore, developing a high-value-added utilization method for industrial solid wastes has also attracted a great deal of attention.

In fact, the research on zeolites from industrial wastes has consistently garnered significant attention, resulting in numerous related articles being published. However, the research progress on these zeolites is being rapidly developed, making it necessary to provide a concise introduction and summary of the relevant research in the most recent five years. To avoid redundancy with the existing literature, this review focuses on a selection of representative industrial solid wastes. The physicochemical properties, synthesis methods, and applications of the zeolites are discussed. This review can serve as a supplementary material and provide a valuable reference for scholars.

2. Research Methodology

This review employs a systematic approach to examine the existing literature in the field of zeolites from industrial solid wastes. The research primarily involves a comprehensive search of scientific papers published within the past five years, with particular attention paid to studies exploring the synthesis of zeolites from selected industrial solid wastes.

The data collection was mainly conducted through scientific databases such as Web of Science, ScienceDirect, and Google Scholar, using keywords such as “solid waste”, “zeolite”, “synthesis”, and “application” for the search. The collected studies were initially screened to exclude documents not directly related to the research topic. Subsequently, a detailed content analysis was conducted on the selected studies to extract key information about the synthesis methods, characteristics, applications, and environmental impacts of zeolites from solid waste.

Additionally, the research methods were evaluated for their consistency and reliability, and the findings of the study were subjected to critical analysis. By conducting an integrated analysis of the existing literature, the objective of this review is to uncover the present state and emerging patterns in solid waste zeolite research, and to offer a reference for future studies on solid waste zeolites.

3. Some Typical Industrial Solid Wastes

Although there are various types of industrial waste, only a limited number of them can be utilized for recycling in zeolites for adsorbents and catalytic applications. The primary constituents of zeolites are silicon (Si) and aluminum (Al). Therefore, industrial waste used for zeolite synthesis should possess stability and a high concentration of Si or Al. Furthermore, the production yield of the industrial waste should be sufficient to meet the future demand for zeolite products. After evaluating numerous industrial solid wastes, this review focuses on four representative industrial solid wastes that have been selected and will be discussed in the subsequent sections.

3.1. Coal Fly Ash

Coal fly ash is typically generated as a by-product during the process of coal combustion. In China, thermal power generation continues to dominate the energy structure in order to meet the growing energy demand, resulting in an increase in the emission of coal fly ash. According to the mineral resources report of the Ministry of Natural Resources, in 2022, the total primary energy production reached 4.66 billion tons of coal equivalent, representing a 9.2% increase compared to the previous year. Coal accounted for 67.4% of the energy production structure. Although the majority of these solid emissions were recycled through the synthesis of various low-value-added products such as ceramics and concrete admixture, approximately 160 million tons of coal fly ash were not properly disposed of in China [8]. Given the environmental problems and resource waste associated with landfilling coal fly ash, the efficient utilization of coal fly ash has attracted increasing attention from researchers.

Since coal fly ash is a kind of solid waste that is produced after combustion, its surface is rough and its color is closely related to the carbon content in coal. The physical properties of coal fly ash are mainly determined by the type of coal and the conditions under which it was burned. The particle size of coal fly ash ranges from 0.5 μm to 300 μm, and its density varies from 1.9~2.9 g/cm³. Crystal phase analysis has indicated that the main crystal phases present in coal fly ash are quartz, mullite, magnetite, and quicklime. The main elements of the glass phase in coal fly ash are O, Si, and Al. Coal fly ash can be classified as either F coal fly ash in which the silicon–aluminum–iron content is higher than 70% or C coal fly ash in which the silicon–aluminum–iron content is between 50% and 70%. Specifically, bituminous coal and anthracite coal contain higher amounts of SiO₂ and Al₂O₃ compared to other coal. Numerous experimental studies have been conducted on the synthesis and application of zeolites derived from coal fly ash, some of which are shown in

Table 1. The main chemical compositions in coal fly ash and the synthesized zeolites from coal fly ash.

SiO2	Al2O3	Zeolite	Application	Reference
41.24	45.05	4 A	Removal of As (V) in solution	[9]
50.77	31.70	X	Adsorbent of CO2	[10]
50.35	35.90	X	Removal of dioxins	[11]
43.34	47.60	NaP1	Adsorbent for Pb (Ⅱ)	[12]
54.40	29.20	ZSM−5	Water treatment	[13]
50.67	23.20	P	Removal of glyphosate in water solution	[14]

.

In comparison to the zeolites from commercially available chemical reagents, the zeolites from coal fly ash sometimes exhibit superior adsorption and catalytic behaviors. A comparative study on the removal of heavy metals using zeolites from different raw materials confirmed this. The research demonstrated that the zeolites from coal fly ash had a stronger affinity for Pb²⁺ but a weaker affinity for Cu²⁺ [15]. Therefore, the development of various specific zeolites from coal fly ash has been a focal point for zeilites.

3.2. Metallurgical Slag

Industrial slags are major by-products of metallurgy, mining, and combustion processes, and they are produced in large quantities. These slags contain various oxides, making them suitable for recycling as raw materials for specific products. However, different kinds of slags possess distinct chemical compositions and properties. Even within a specific type of slag, the chemical composition can be influenced by the raw materials and the production technology. Moreover, different elements may exist in both the amorphous phase and crystals. Therefore, proper extraction and homogenization processes are necessary. Blast furnace slag, steel slag, and waste coal gasification slag are common metallurgical slags that have high production capacities and are rich in Si and Al.

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Blast furnace slag

Blast furnace slag is generated during iron smelting, and its main chemical composition consists of SiO₂, CaO, Al₂O₃, and MgO. Depending on the cooling technology, these oxides may exist in either the glassy phase or crystal phase [16]. The hydrothermal method, alkali-fusion method, and crystal seed method have been proven to be effective in synthesizing zeolites from blast furnace slag [17,18,19].

Blast furnace slag with a Ti bearing can also be recycled in zeolite synthesis. Weizao Liu compared the synthesis of FAU and MFI zeolites from Ti-bearing blast furnace slag using the alkali-fusion hydrothermal method and the seed-assisted method, respectively. In MFI zeolites, Ti can exist in the form of TiO₂, which enhances the efficiency of catalytic oxidation [20].

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Steel slag

Steel slag is another kind of metallurgical slag that is produced in large quantities. Its chemical composition and phase composition differ from those of blast furnace slag. The CaO and Fe₂O₃ contents in steel slag usually exceed 50 wt. % of the steel slag, which hinders its recycling. The Al₂O₃ content in steel slag is low, so Al sources like red mud need to be added in the synthesis of zeolite [21]. Various zeolites derived from steel slag have been successfully prepared, and they have exhibited excellent adsorption efficiency and catalytic behavior for wastewater treatment. The alkali-fusion hydrothermal method is usually used to produce zeolites from this kind of slag. Firstly, the steel slag is fused with alkali at a relatively high temperature. Then, the above mixture is modified with chemical reagents containing silicon and aluminum, such as sodium silicate and sodium aluminum, to obtain polymeric gel. Finally, the gel is hydrothermally treated to promote the crystallization of zeolite. Meanwhile, the CaO in steel slag reacts with H₂O and produces complex calcium salts that attach to some silicate ions [22]. Mihir K. Purkait prepared FAU type zeolite A from basic oxygen furnace (BOF) slag using the alkali-fusion hydrothermal method [23]. Furthermore, they applied ultrasonic energy to enhance the hydrothermal efficiency (as shown in Figure 1) [24].

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Coal gasification slag

Coal gasification slag (CGS) is generated during the process of coal gasification. According to the manufacturing technique, it can be classified as fine slag (CGFS) or coarse slag (CGCS). The coarse slag has a lower carbon content and can be recycled in road materials. However, the fine slag with higher carbon content and lower calcium content poses challenges for efficient recycling [25]. CGS contains various oxides such as SiO₂ and Al₂O₃, making it suitable for the synthesis of zeolite [26]. Recently, there has been significant interest in utilizing CGS for the preparation of zeolites [27].

Alkali fusion and calcination are effective pretreatment measures for CGS. After pretreatment, CGS can be transformed into a zeolite framework via hydrothermal synthesis. This results in a large surface area, excellent homogenization of metal ions, and a synergistic effect [28].

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Some other slags

In addition to the aforementioned slags, mineral slags such as lithium slag can also provide Si and Al for zeolites. Zeolite X was successfully prepared from lithium slag using the alkali fusion hydrothermal method, and it exhibited a comparable adsorption capacity for water vapor to commercial zeolite X [29].

Coal-fired slag is also rich in SiO₂. After the acid pretreatment, the slag can be used to synthesize 13X zeolite using the alkali-fusion hydrothermal method [30]. Aluminum saline slag and aluminum scrap can be recycled as an aluminum source for zeolites. Korili A. Gil designed a multifactor experiment on aluminum-saline-slag-derived zeolites (NaX and LTA) and determined the best synthesis conditions with low hydrothermal temperature and longer hydrothermal time [31]. In addition to inorganic materials, some organic materials can also serve as Si or Al sources. Isabel Padilla optimized the synthesis conditions when salt slag and rice husk ash were added as the Al source and Si source, respectively [32]. Some scholars attempted to decrease the synthesis temperature and avoid the alkali fusion. Caixia Liu synthesized high-silica SSZ−13 from coal gangue using a one-step hydrothermal method (as shown in Figure 2) [33].

3.3. Waste Glass

Due to their excellent optical properties, chemical stability, decorative nature, and other outstanding properties, glass products have been utilized in various industries. Meanwhile, the generation of a significant amount of glass waste remains a challenge. Some of the waste glass has been recycled in processes such as glass remelting, porous ceramic production, pavement materials, and others [34]. However, the commercial value of the products from waste glass has not been fully developed. The utilization of waste glass as a raw material for zeolites presents promising prospects.

Generally, the primary oxides in flat glass and bottle glass are identified as SiO₂, Al₂O₃, Na₂O, CaO, K₂O, and MgO. Specifically, the SiO₂ content in waste glass is much more than Al₂O₃. Therefore, an additional Al source such as aluminum scraps needs to be introduced for the synthesis of zeolites [35]. Furthermore, the structural analysis of these glass products indicates that SiO₂ acts as a network former and Al₂O₃ acts as a network former or network modifier, contributing to the chemical stability to some extent. Accordingly, the effective extraction of Si and Al is another key issue that needs to be addressed.

Under alkaline conditions, the glass network undergoes continuous destruction. Si and Al dissolve slowly and participate in the formation of crystals. Nichola J. Coleman found that, regarding the hydrothermal reaction proceeding, 63% of the glass was transformed into crystals such as hydroxysodalite and hydroxycancrinite on the tenth day [36].

3.4. Waste Catalyst

Nowadays, catalysts are widely utilized in various fields such as chemical engineering, wastewater treatment, VOCs adsorption, and CO₂ capture. Every year, a significant number of discarded catalysts and carriers are produced worldwide, causing an increasing focus on their disposal. In order to maximize the adsorption and catalytic effects, catalysts are designed and prepared with precise chemical compositions and specific crystal structures. Some of these solids can serve as ideal sources of Si and Al and can be recycled in the synthesis of zeolites.

Among the various waste catalysts, Fluid Catalytic Cracking (FCC) catalysts have been proven to be suitable for zeolite recycling. FCC catalysts are commonly used in the petroleum industry and consist of zeolite, a kaolin matrix, and a binder of silica and alumina [37]. The zeolites in FCC catalysts serve as the active component and typically comprise 15–50% of the catalyst. Various methods such as hydrothermal synthesis, the alkali-fusion-assisted method, and microwave-assisted hydrothermal synthesis are commonly employed. Wenfu Yan proposed a conversion strategy for converting SFCC to zeolite X at a temperature of 200 °C (as shown in Figure 3) [38]. This approach avoided acid treatment and gradually decreased the synthesis temperature compared to previous studies [39,40]. Except for zeolite NaX, the successful syntheses of zeolite NaY and zeolite NaA from FCC catalysts have also been achieved, demonstrating excellent properties [40,41,42,43].

4. Synthesis of Zeolites from Industrial Wastes

4.1. Synthesis Methology

Since the discovery of zeolite, its synthetic methods have been continuously improved, among which the hydrothermal method is widely used as a classic method. With the development of synthesis technology, increasing auxiliary techniques are being applied to improve the efficiency of zeolite synthesis. Some common synthesis strategies are introduced as follows.

In recent years, zeolite with a hierarchical pore structure has attracted a great deal of attention. Hierarchical pore molecular sieves are based on nanomolecular sieves and possess a multi-level pore system that spans multiple scales. Typically, larger mesopores or macropores are introduced in addition to the zeolite molecular sieve micropores, and these different levels of pores are directly interconnected. Hierarchical pore molecular sieves can significantly enhance the diffusion and mass transfer performance, as well as mitigate the catalyst carbon deposition deactivation in catalytic reactions [44,45].

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Hydrothermal method

This classical method for synthesizing zeolites was first proposed by Holler and Wirsching [46]. They synthesized zeolite materials from coal fly ash under alkaline (NaOH or KOH) hydrothermal conditions. Subsequent research revealed that the process consisted of three stages, including the dissolution of Si and Al in coal fly ash, the formation of aluminosilicate gel, and the formation of zeolite crystals [47]. A variety of zeolites, like NaA, Na-P1, and GIS, can be obtained with this method [48,49,50].

Generally, this method involves a relatively low synthesis temperature and allows for the easy adjustment of the chemical composition, such as Si/Al, through the addition of other chemical reagents. However, the crystallization and Si/Al are strongly influenced by the synthesis temperature, which should be precisely controlled. Furthermore, hydrothermal synthesis is both time-consuming and energy-consuming, resulting in the emission of greenhouse gases and the discharge of alkaline wastewater.

The industrial solid wastes discussed in this review have stable chemical properties. An acid solution and alkali solution are always used to destroy the structure and improve the dissolution of Si and Al. In an acid solution (such as HCl, H₂SO₄, or HNO₃), oxides may react with H⁺, as shown in the following Equation (1) (where M represents the metal elements in the solid wastes). Similarly, in an alkali melt solution, oxides like SiO₂ and Al₂O₃ will react with OH^- to form aluminosilicates, as shown in Equations (2) and (3).

$${\mathrm{M}}_{\mathrm{a}}{\mathrm{O}}_{\mathrm{b}}+{2\mathrm{b}\mathrm{H}}^{+}\to \mathrm{a}{\mathrm{M}}^{{\displaystyle \frac{2\mathrm{b}}{\mathrm{a}}}}+\mathrm{b}{\mathrm{H}}_2\mathrm{O}$$

(1)

$$\mathrm{S}\mathrm{i}{\mathrm{O}}_2+2{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{S}\mathrm{i}\mathrm{O}}_4^{2-}+{\mathrm{H}}_2\mathrm{O}$$

(2)

$${\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3+2{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_4^{2-}+{\mathrm{H}}_2\mathrm{O}$$

(3)

In fact, the chemical composition and crystal structure of the industrial solid wastes are relatively complex, which means the reactions in acid or alkali conditions are more intricate than the aforementioned three equations.

To enhance the crystalline fraction, Hollman first proposed the two-step hydrothermal method. In this strategy, the chemical composition of the solution obtained from the one-step hydrothermal treatment was first adjusted by chemical reagents, and then the formed silicon aluminum gel continued to react under hydrothermal conditions [48]. Compared with the one-step hydrothermal method, zeolites prepared from this method usually have high purity. However, there is still significant consumption of energy and water, and the raw materials are not fully utilized.

To remove impurities and facilitate the hydrothermal reaction, the solid wastes always need to be washed and sieved. Generally, the properties and structure of zeolites are influenced by factors such as the alkali solution, pH of the solution, hydrothermal time, and temperature. It has also been reported that the liquid/solid and the stirring style (such as magnetic stirring or impingement stream reactor) affect the adsorptive and catalytic efficiency [10,51].

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Alkali fusion–hydrothermal synthesis [52]

Due to the presence of insoluble crystals such as mullite and quartz in certain raw materials, the extraction of Si and Al using one-step or two-step hydrothermal methods is extremely challenging. Furthermore, Si and Al are found in the glass phase of certain raw materials like coal fly ash. The alkaline sintering hydrothermal method destroys the structure of the glass phase and crystals, which thereby accelerates the dissolution of Si and Al and reduces the synthesis time [53]. Especially for some slags, geopolymers, which are precursors, are easily formed and contribute to the nucleation and crystallization of zeolites. It should be noted that the raw materials require calcination pretreatment, and the melting temperature is generally above 400 °C.

There is also a glassy phase in some industrial solid wastes. Due to the difficulty in the homogenization of the glassy phase in an alkali solution, alkali fusion is typically employed prior to hydrothermal treatment [54,55]. Si and Al can be effectively extracted from liquid crystal display waste glass, solar panel waste glass, sandblasting waste, aluminum scraps, alum sludge, and other wastes [56,57]. The synthesis process can be simply classified as the dissolution of aluminosilicate, initial recrystallization, and aggregation and growth of zeolite crystals [58].

To mitigate the generation of wastewater during zeolite synthesis, it has been proposed to mix salts (such as NaCl and Na₂CO₃) and alkali with the raw materials and then melt them [59]. Consequently, Si and Al are extracted from the raw materials and zeolite is formed under hydrothermal conditions. Although this approach reduces the waste liquid, the removal of residual salt requires a great deal of water, and numerous impurities are generated.

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Assisted hydrothermal synthesis

In certain conditions, the efficiency of zeolite synthesis is unsatisfactory. Various assisted technologies such as microwave and ultrasound have been adopted in the pretreatment process.

Microwave-assisted hydrothermal synthesis combines microwave treatment and hydrothermal synthesis. The use of ultra-high-frequency microwave radiation instead of direct heating reduces the amount of water needed for washing the products. Meanwhile, the microwave radiation promotes the formation of silica alumina gel and increases the nucleation rate of zeolite, leading to high crystallization in zeolite [60]. This method simplifies the operation process and significantly reduces the reaction time.

However, the crystal growth is not accelerated throughout the entire process. It has been found that only in the early stage is the formation of zeolite promoted, whereas, in the intermediate and later stages, the microwave radiation actually hinders the crystal growth [61]. Additionally, other disadvantages of this method include the challenge in controlling the temperature and the limited yield of high-quality crystals.

In addition to microwave-assisted synthesis, ultrasound is another assisted technology that is utilized in zeolite synthesis. It is similar to the microwave-assisted hydrothermal method. The raw materials, after treatment, are mixed with an alkali solution and then subjected to a hydrothermal reaction. Ultrasound accelerates the dissolution of Si and Al in the raw materials, contributing to the formation of a silica alumina gel due to the cavitation effect and perturbation effect [62]. Meanwhile, zeolites treated with ultrasound exhibit a more complete framework and better adsorption performance.

Different assisted technologies can also be applied simultaneously. Ziwei Li and Sibudjing Kawi utilized microwave- and ultrasonic-irradiation-assisted technologies to treat coal fly ash. The silica gel and alumina formed the initial solution for the growth of Y-type (FAU) zeolites. The combination of microwave and in situ ultrasound contributed to the homogenization of silica gel and the extraction of Si and Al in coal fly ash. The surface area reached 916.1 m²/g, and the adsorption of CO₂ at 0 °C reached 107.14 cm³/g [63]. Compared with the traditional hydrothermal method, the surface area was increased and the synthesis time was reduced.

Microwave-assisted technology has been proven to be effective in promoting the nucleation and crystal rate of ZSM−5 zeolite from CGCS [64]. The crystal surface was much smoother and the grain homogeneity was strongly improved [65].

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Synthesis with crystal seeds

In this strategy, crystal seeds for specific zeolites need to be prepared first and then added into the materials [66]. The addition of crystal seeds provides crystal nuclei for the growth of zeolites, enabling the rearrangement of Si and Al in the same structure as the crystal seeds. This approach reduces the synthesis time and the presence of impure crystals. Additionally, the Si/Al can be adjusted by the crystal seeds.

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Other new methods

In addition to the aforementioned methods, scholars are still developing new strategies to continuously improve the synthetic efficiency of zeolites. It has been reported that in situ inter-zeolite transformation (IZT) can also be used in the preparation of zeolites from coal fly ash. Lina Han found that coal fly ash was first converted into ANA or FAU zeolites and then continuously transformed into CHA zeolites in a hydrothermal alkaline solution. They applied this method in the synthesis of Cu-SSZ−13 for NH₃-SCR of NO, using TMAda-OH and Cu-TEPA as dual templates (as shown in Figure 4) [67]. Furthermore, when in situ technology was introduced, Cu-SSZ−13 from coal fly ash was synthesized in one step. The obtained zeolites exhibited more excellent NO conversion, N₂ selectivity, and enhanced hydrothermal stability [68].

To decrease the water consumption, Zhenping Qu proposed a solvent-free synthesis strategy using coal fly ash as the source of Si and Al. Coal fly ash and sodium carbonate were treated at 700–900 °C for 2 h, and then hydrochloric acid (2–5 mol/L) was added and heated at 80 °C for 2 h. After filtration, washing, and drying, the precursor gel was mixed with sodium carbonate and tetrapropyl ammonium bromide. The mixture was heated at 180 °C for 3–48 h and then calcined at 550 °C. The obtained ZSM−5 from coal fly ash had a hierarchical structure, with a surface area of 183.1 m²/g and an adsorption capacity for toluene of 75.1 mg/g in dry stream and 63.7 mg/g in wet stream, respectively [59].

To decrease the time consumption and increase the conversion rate in hydrothermal synthesis, Yaqi Peng proposed a one-step efficient synthesis using the mechanochemical activation method. The coal fly ash, stainless steel balls, and NaOH solution (1–4 mol/L) were mixed in a mill jar with a rotation speed ranging from 10 to 30 Hz. The mixture was reacted at 100–130 °C for 1–4 h. The evaluation indicated that the maximum adsorption for Cd²⁺ reached 130.68 mg/g, which was much more effective than that of zeolites from traditional hydrothermal synthesis in the control experiment. The prominent advantages of this strategy are the high temperature and high pressure, which are generated by the mechanochemical reaction in the dissolution (as shown in Figure 5) [69].

To enhance the specific chemical composition in zeolite and improve its properties, plasma technology has also been utilized. It has been reported that the melt, obtained after alkali fusion at 800 °C for 60 min, was mixed with distilled water and homogenized using ultrasonic technology. The resulting hydrogel was then subjected to hydrothermal treatment at 90 °C for 2 h. The Fe-CFAZ crystals obtained after modification using radiofrequency CHF3 plasma technology exhibited a higher reduction (81%) compared to the initial crystals. Furthermore, the conversion of total toluene to CO₂ at 500 °C reached 98% [70].

Although various methods have been applied in zeolite preparation, each method has its advantages and disadvantages. The one-step hydrothermal synthesis is very easy to carry out, and the parameters can be simply controlled. Meanwhile, the extraction efficiency is relatively low, and some impurities are not sufficiently removed, leading to defects and low crystallinity for zeolites. Accordingly, alkali fusion, mechanical vibration, microwave, and other technologies are applied to the pretreatment of these solid wastes [71]. The major impurities can be removed, and the generation of gel is improved. Especially, alkali-fusion-assisted hydrothermal synthesis seems to be the preference in zeolite synthesis when raw materials are difficult to be dissolved. However, the application of these assisted technologies indeed increases the cost and complexity of the synthesis process. Furthermore, additional acid–base waste liquid may be generated, and more water may be consumed. To compete with commercial zeolites from chemical reagents, it is critical to balance the synthesis efficiency and the cost [72].

4.2. Some Factors in Synthesis

In different synthesis strategies, several factors influence the formation of zeolites, thereby affecting their efficiency in adsorption and catalysis. The control of synthetic parameters plays a crucial role in determining the microstructure of zeolites when using different raw materials. Generally, the integrity of crystals is strongly affected by the synthesis conditions, such as temperature, time, pressure, and so on. After Si and Al are extracted from these solid wastes and gel is formed, hydrothermal temperature, hydrothermal time, and pH are recognized as three main factors. It is found that the effects of these three factors on the crystallinity are not rigidly linear. Principally, higher temperature accelerates the growth of crystals and contributes to high crystallinity. A longer synthesis time also benefits the growth of crystals. Therefore, the impacts of temperature, acid solution, alkali solution, and Si/Al ratio are discussed as follows.

4.2.1. Temperature

In the hydrothermal process, temperature has been proven to be more significant than the molarity of the alkaline solution and hydrothermal time [17]. The polymerization state of aluminosilicates and the formation of zeolite grains are strongly influenced by temperature. Generally, lower temperatures hinder the formation and growth of zeolites. There is a clear positive correlation between hydrothermal temperature and crystallization [32]. However, excessively high temperatures contribute to the formation of sodalite, which subsequently reduces zeolite production and leads to agglomeration. Additionally, the types of zeolites vary with different temperatures. Researchers have found that the crystallization of A-type zeolite increases when the temperature rises from 80 °C to 100 °C but decreases when the temperature continues to increase to 120 °C [56].

In the alkali-fusion or salt-fusion strategy, the activation of ions in the melt is influenced by temperature. The temperature of the melt is gradually higher than that in hydrothermal synthesis, leading to the accumulation of structural damage in solid wastes. For example, when coal fly ash is pretreated in an alkali melt, the inactive alpha quartz and mullite react with alkali reagents at high temperatures. As a result, Si and Al in coal fly ash are extracted and subsequently participate in the formation of zeolite under hydrothermal conditions.

4.2.2. Acid or Alkali Solution

Through acid leaching or alkali fusion, Si and Al can be extracted from blast furnace slag. Research on the leaching mechanism research has revealed that this process typically follows the unreacted core shrinking model and is identified as a non-catalytic and non-heterogeneous solid–liquid reaction [73]. In the pretreatment process, the leaching temperature and leaching time both influence the efficiency of extraction. Experimental research indicates that higher temperature and longer time are beneficial for the destruction of the slag structure. However, the excessive increases in temperature and time increase the energy consumption [74]. The obtained gel after leaching finally forms zeolites under optimal conditions (as shown in Figure 6).

The alkali solution also has an influence on the crystallization of glassy materials. The pH of the solution is always determined by NaOH. For different kinds of zeolites, the optimal pH varies, which should be precisely controlled. The best conditions are usually determined through a multi-factor experiment. It has been observed that the crystallization rate in a KOH solution is much slower than that in a NaOH solution in the early stage. However, in the later stage, the crystallization rate increases more noticeably in the KOH solution [36]. The concentration of the alkali solution also plays a role in the formation of zeolite crystal. According to Ostwald’s rule of successive transformation, the super-saturation can be increased by increasing the concentration of NaOH, which may result in the transformation of zeolites into another highly stable zeolite [75,76]. In other words, the increased NaOH concentration is beneficial for the change in zeolite type.

The NaOH solution is usually used in hydrothermal synthesis, and its molar concentration varies depending on the structure of the zeolites. To effectively adsorb sulfates in solution, Lukhanyo Mekuto prepared zeolites from F type coal fly ash at a neutral pH. The raw materials were mixed with a 2 mol/L NaOH solution in an autoclave reactor at 95 °C for 48 h. The surface area increased from 1.059 m²/g in coal fly ash to 85.53 m²/g in modified zeolites, and the maximum adsorption capacity reached 42.25 mg/g [77]. In Brahim Achiou’s research, Na-P1 zeolite for Cr (Ⅵ) adsorption was prepared at 140 °C for 50 h with a NaOH solution of 1 mol/L. The obtained zeolite had a crystallinity of 96.87% and a large surface area of 87.20 m²/g and exhibited an excellent adsorption capacity of 13.5 mg/g [49].

4.2.3. Si/Al

It is confirmed that higher Si/Al leads to higher crystallinity. However, Kae-Long Lin compared different parameters and found that a too-high Si/Al resulted in the decomposition of zeolite and decreased the crystallinity [56]. Similar research revealed that increasing Si/Al might also decrease the crystallinity. In Jinchuan Gu’s research, with increasing Si/Al, the crystallinity of zeolite first increased and then decreased when Si/Al exceeded 4 [78]. When waste glass and sludge were applied to prepare LTA zeolite, a SiO₂/Al₂O₃ higher than 1.5 also led to the decreasing crystallinity. The continuous increase in SiO₂/Al₂O₃ contributed to the formation of faujasite zeolite rather than the target zeolite [79]. To modify the Si/Al, coal fly ash can be added as raw materials with steel slag. The acquired zeolite exhibited an adsorption capacity of 292.8 mg/g and an adsorption efficiency of 97.6% under experimental conditions [80]. Additionally, it was reported that the increasing Al contributed to the transformation of the glass phase to LTA zeolite, and therefore promoted the crystallinity [81].

It was also found that Na/Al affected the synthesis mechanism and the pore radius in zeolites [82]. Yuxiang Li found that the introduction of NaNO₃ participated in the formation of chabazite, and the coupling effect between NaNO₃ and NaOH influenced the pore structure and adsorption efficiency. With increasing Na/Al, the adsorption of zeolites for Sr²⁺ initially increased and then decreased. The adsorption mechanism indicated that the adsorption followed the Freundlich isotherm models rather than the Langmuir isotherm models [83].

Apart from Si and Al, some other elements in blast furnace slag also influence the synthesis. Elements such as Mg, Fe, and K with low content have a weak impact on the synthesis. However, it was found that Ca and P influenced the crystallinity of specific zeolites [84]. Additionally, Mn and Ce could be co-doped with these zeolites to improve catalytic performance [85].

Due to the fact that the Si/Al in solid waste is inappropriate, some other materials need to be added [86]. These modification materials are usually chemical reagents, natural minerals, and solid wastes. Although chemical reagents are simpler and more efficient, they do increase the cost, which is not in line with the concept of “Green Synthesis”. Therefore, there have been a great deal of research attempts to synthesize zeolites from the mixture of different solid wastes. Undoubtedly, the precise control of the crystal structure and properties becomes more difficult, which consequently influences the catalytic efficiency and adsorption of zeolites.

In addition to the above factors, there are other factors affecting the formation of zeolites, such as particle size, solid–liquid ratio, and aluminum concentration. Smaller and narrower particle size distribution promoted the formation of zeolites according to the study in [35]. Dongwan Kim used a high-energy ball mill to decrease the particle size of windshield glass to 520 nm. After acid treatment and hydrothermal synthesis, a zeolite with an average particle size of 1.4 μm was obtained (as shown in Figure 7) [87]. It was hypothesized that decreasing particle size contributed to higher crystallization due to the enhanced effect of impurity removal.

5. Application

5.1. Degradation of Volatile Organic Compounds (VOCs)

VOCs are classified as types of common air pollutants that have high volatility and low water solubility. Examples of VOCs include xylene, benzene, toluene, and acetaldehyde. These polar or phenyl molecules can be adsorbed by the electrostatic forces within the pores of a molecular sieve. Zeolites are often regarded as effective adsorbents for VOCs due to their excellent adsorption capacity and thermal stability.

Zeolites derived from coal fly ash through alkali fusion have demonstrated similarly excellent adsorption capabilities compared to those derived from chemical materials. Zhenping Qu successfully prepared zeolites from fly ash for the removal of toluene, and the obtained zeolite exhibited excellent adsorption in both the dry and wet streams (as shown in Figure 8a) [59]. Xingxing Cheng prepared coal-fly-ash-based zeolite for the adsorption of o-xylene. The coal fly ash and NaOH were mixed in a mass ratio of 1:1.2, and the mixture was hydrothermally treated at 85 °C for several hours. The adsorbents of FA-ZTC based on the obtained FAZ zeolite were further prepared after carbonization. The obtained FA−ZTC exhibited micropores ranging from 4.95 nm to 6.74 nm and excellent adsorption of 338.6 mg/g for o-xylene [88].

Feiqiang Guo prepared zeolite-based catalysts from coal gasification fine slag (CGFS) for the steam reforming of toluene, achieving a conversion rate of 73% [89]. When Ni was doped into the zeolite catalysts, the transformation of toluene reached 81% [90]. Qikun Zhang prepared ZSM−5 zeolite from coal gasification coarse slag (CGCS) using micro-assisted synthesis, and the static adsorption capacities of the obtained zeolite (as shown in Figure 8c,d for p-xylene and butyl) were characterized as 81.44 mg/g and 106.84 mg/g, respectively [64]. In Yue Xiao’s research, red mud and steel slag were used to synthesize the Y-type zeolite, which finally exhibited a specific surface area of 685 m²/g (as shown in Figure 8b). This zeolite could be added to asphalt, reducing the asphalt VOCs by more than 45% [21]

5.2. Wastewater Treatment

The rapid development of industry has resulted in a significant increase in water consumption, leading to the generation of large amounts of wastewater containing heavy metals, ammonia nitrogen, and dye. Zeolites derived from industrial solid wastes are being considered as a promising solution to this problem due to their unique pore structure and low cost.

Lukhanyo Mekuto successfully prepared a zeolite capable of adsorbing sulfate ions in wastewater at room temperature, with a maximum adsorption capacity of 42 mg/g [77]. Scholars have also attempted to prepare composite zeolite from CGFS using an acid–alkali leaching method, and the zeolites exhibit excellent adsorption properties for dye in wastewater [91]. To improve the adsorption efficiency, ZSM−5 zeolite with an ultra-low SiO₂/Al₂O₃ and hierarchical pore structure was successfully prepared, and it exhibited excellent adsorption for methylene blue, malachite green, Rhodamine B, Congo red, and methyl orange in wastewater (as shown in Figure 9) [92].

5.3. Denitrification

If the zeolites are synthesized for catalytic reactions rather than adsorption, it is necessary to introduce certain metal elements through doping. Edwin Geo Varuvel successfully prepared mono-metallic and bi-metallic zeolites doped with Cu, Ce, and Fe; the results indicated that the doped zeolites showed excellent NOx conversions [93]. Lina Han synthesized Cu-loaded SSZ−13 zeolite, and the NO conversion rate reached more than 90% in the temperature range of 200–700 °C [68]. Mouna Sayehi prepared zeolite crystals from glass waste and aluminum waste using alkali melting and hydrothermal synthesis. When copper was loaded on this zeolite, the catalytic results indicated that the conversion rate of NO reached 98% in the temperature range of 180–550 °C [57].

6. Conclusions

Zeolites from industrial solid wastes or their by-products have been proven to be promising materials and have attracted a great deal of attention. However, there are still some challenges in the synthesis of these zeolites, including the synthesis method, the optimization of the preparation condition, and the modification of the chemical composition.

It has been widely agreed that industrial solid wastes are valuable resources that have been misplaced. The synthesis and application of zeolites from these solid wastes not only contribute to energy conservation and emission reduction but also stimulate enthusiasm for the disposition of these solids. Many kinds of industrial solid wastes have been successfully converted into zeolites. To improve the synthesis efficiency, various assisted technologies like microwave and ultrasonic have been applied. In summary, the synthesis of zeolites from industrial solid wastes is being rapidly developed.