Application of Unconventional External-Field Treatments in Air Pollutants Removal over Zeolite-Based Adsorbents/Catalysts

Abstract

Zeolite-based materials are widely used as adsorbents and catalysts for purifying air pollutants like NO_x and VOCs due to abundant pore structure, regular pore distribution, and numerous ion exchange sites. Thermal treatment is a necessary procedure for both removing impurities in pores and promoting the metal active dispersed evenly before the zeolite-based adsorbents/catalysts were applied for purifying the NO_x/VOCs. Nevertheless, the conventional thermal field treatment (i.e., high-temperature calcination, high-temperature purging, etc.) takes large energy consumption. In contrast, unconventional external-field treatments such as non-thermal plasma and microwave show significant advantages of high efficiency, low energy consumption as well and low pollution, which were used to substitute the traditional thermal treatment in many fields. In this paper, the roles of non-thermal plasma or microwave in the adsorption/catalysis of the NO_x/VOCs are reviewed from three aspects assisting activation of materials, cooperative catalysis process, and assisting zeolites synthesis. The reasons for unconventional treatments in improving textural properties, active sites, performance, etc. of zeolite-based materials were illuminated in detail. Moreover, the influences of various parameters (i.e., power, time, temperature, etc.) on the above aspects are elaborated. It is hoped that this review could provide some advanced guidance for the researchers to develop highly efficient materials.

1. Introduction

Nitrogen oxides (NO_x) and Volatile Organic Compounds (VOCs) are mainly emitted from fuel combustion industries, traffic vehicles, and steelmaking [1]. As the main contributors to both particulate matter and ground-level ozone, the NO_x and VOCs do great harm to both the environment and the human body [2]. To eliminate these air pollutants, various technologies have been well developed [3]. Typically, the NOx could be effectively removed via adsorption and catalytic reduction (i.e., Selective Catalytic Reduction) [4], while the elimination of the VOCs was usually achieved by adsorption and catalytic oxidation [5]. Zeolites, a kind of micro-porous alumina-silicate material [6], have been widely applied for the removal of both the NO_x and VOCs as either adsorbents or catalysts [7]. The intrinsic porous structure not only provides very good places for capturing the reactant (pollutants) but also offers superior conditions for the distribution of extraneous active species (metal) to efficiently convert the pollutants to harmless substances [8]. For example, the Fe-ZSM-5 zeolite shows good selective catalytic reduction activity for the NO_x in the NH₃-SCR reaction at 300~450 °C. But its activity temperature window is relatively narrow; therefore, in recent years the Cu-SSZ-13 is generally used for the selective catalytic reduction of NO_x from vehicle emission at high temperatures of 200~500 °C [3]. To compensate for the low-temperature activity of the Cu-SSZ-13 below 200 °C, it is found that the Pd-SSZ-13 zeolite possesses the ability to store NO_x at 80~180 °C [2]. In addition, the ZSM-5-supported Co, Ni, or Pt is usually applied for the catalytic oxidation of VOCs, such as benzene and toluene to CO₂ and H₂O [4,5]. In addition, metal-supported zeolite materials are thought to be used as adsorbents/catalysts on the source of pollution; therefore, these materials should withstand extreme conditions such as high temperature and high flow concentration of reaction gas and sulfur poisoning, water poisoning, and hydrocarbon poisoning caused by reactive gas components, etc. Despite the superior properties of regular pore structure, excellent ion exchangeability, moderate acidity, and good thermal/hydrothermal stability provided by zeolites [9], some thermal treatment procedures like calcination and purging at high temperatures are also required to ensure a good combination of extraneous metal species to the zeolite supports as well as the good exposure of active sites for the zeolite-based adsorbent/catalysis materials [10]. Except for activating the materials, a heating supply is a necessary condition for the process of converting harmful pollutants to harmless substances and releasing the captured pollutants to recover the adsorption ability of materials by providing energy for catalytic reaction and desorption [11].

It should be mentioned that the temperature necessary for thermal treatment is usually very high [12]. For instance, loading the Pd onto SSZ-13 requires hydrothermal treatment at 750 °C to increase the dispersion of the Pd²⁺ for the NO capture [13,14]. After H₂ purging at 300 °C, the Pt can be dispersed in zeolite in the form of the Pt⁰ active species for the catalytic oxidation of the VOCs [15,16,17]. All these high-temperature treatment processes cause large energy consumption [18]. Moreover, high-temperature treatment may also cause the destruction of the zeolite skeleton and the migration of metal components toward the inner channel of the carriers, making the material easily deactivated as well as decreasing the accessibility of active species [19]. To solve the problems caused by conventional thermal treatment (CT for short) mentioned above, more and more research has been reported in recent years about improving the performance of materials by using some special treatment techniques [19]. Particularly, the unconventional external-field treatment measures like non-thermal plasma (NTP), microwave (MW), etc. with the characteristics of high efficiency, simplicity [20], low energy consumption, and low pollution [21], have been expected to be effective methods to promote the activation and modification of the porous zeolites [22]. Particularly, the destruction of the zeolite framework and the loss of the active components on the carrier surface could be effectively inhibited due to the relatively low processing temperature [19]. In detail, for the non-thermal plasma, lots of energetic electrons and other active substances will be generated during the discharge process, which can not only hit the outer surface of zeolites to form new pores or active groups through etching and ablation effects, probably increasing the active sites in zeolites but also can break the NO bonds to promote the reaction process [23]. The microwave irradiating throughout the zeolites could transform into thermal energy by the selective absorption of the active sites to create “localized hot spots”, during which the active centers will be strengthened; on the other hand, microwave can assist the synthesis of zeolites by promoting the formation and growth of crystal nucleus [24].

In this paper, the application status and prospect of typical unconventional external fields (NTP, MW), for removing the classic air pollutants (NO_x, VOCs) were reviewed from diverse aspects of activating the zeolite-based materials, promoting the purifying process, and facilitating zeolites synthesis. In detail, the changes in physicochemical properties and performance of zeolite-based adsorbents/catalysts caused by various external-field treatments were illustrated, hoping to provide reference and guidance for researchers in the related field.

2. Application of Non-Thermal Plasma over Zeolite-Based Materials

Plasma is an exothermic ionized gas produced by electrical discharge [25]. By injecting energy into the reaction region, electrons are released from the atoms or molecules to achieve an electrically neutral state where ions, electrons, and the original species coexist [23]. According to the energy level, temperature, and ionic density, plasma is generally classified into high-temperature plasma (for nuclear applications) and low-temperature plasma (including thermal plasma and non-thermal plasma) [26]. Particularly, the non-thermal plasma can be applied in the fields of catalyst preparation and activation, because the relatively low-temperature condition (usually tens to 200 °C) will not cause thermal damage to the material surface [23]. Additionally, since it works at atmospheric pressure, the cost of the vacuum system can be avoided [27]. Specifically, the gas temperature of thermal plasma is close to the electron temperature (about tens of electron volts), while that of non-thermal plasma can be as low as room temperature [28].

The non-thermal plasma shows great potential in decorating zeolitic structures by generating some specific properties or functions under the etch of energetic species [20]. The activation of plasma to zeolites mainly involves the following points:

(1)

During the discharge process, the molecules in the gas atmosphere will be ionized to generate many high-energy electrons, which will collide with each other to convert electric energy into internal energy and kinetic energy of particles, thus providing the activation energy of chemical reactions [29], which can process materials that cannot be activated by the conventional thermal field [18].

(2)

Numerous electrons and other active substances with high energy will attack the external surface of zeolites to form new pores through etching and ablation effects, thus increasing the porosity.

(3)

The electrons with high energy can break the chemical bonds on the zeolite surface to form new active groups and increase the active sites amount.

(4)

The high-energy particles produced by plasma discharge can promote the excitation, ionization, and interaction of gas-reactant molecules to form new chemical bonds [30].

Compared to the traditional thermal treatment method, non-thermal plasma is fast, clean, energy-saving, convenient, and relatively low-cost [18] despite improving the activity, life, and selectivity of materials [31]; therefore, it is viewed as an ideal treatment method. At present, the non-thermal plasma is generally applied for activating the adsorbents/catalysts or assisting the purification process for enhancing the removal efficiency of the VOCs or NO_x. The general non-thermal plasma technologies used for activating materials include dielectric barrier discharge plasma (DBD), glow discharge plasma (GDP), and corona discharge plasma (CDP) [32], whose experimental devices are shown in

Table 1. Schematic diagrams of non-thermal plasma experimental devices.

Number	Category	Generation Scheme	Discharge Status	Reference
1	Dielectric barrier discharge plasma (DBD)			[33]
2	Glow discharge plasma (GDP)			[34]
3	Corona discharge plasma (CDP)			[33]

, respectively. DBD is the phenomenon of electron penetration in the presence of a strong electric field in the medium, where the electrons are accelerated by the electric field and obtain enough energy. The GDP is caused by adding enough voltage to the gas to cause the collision of atoms, producing ionized electrons to form an electric current. The CDP, occurring in an uneven electric field near a tip electrode with a large radius of curvature, is caused by the ionization and excitation of the gas due to the local electric field strength exceeding the gas ionization field strength.

2.1. NTP Assisted Activation of Materials

Non-thermal plasma could effectively modify the surface properties of materials [25] due to the large number of energetic electrons and other active substances produced during the discharge process attacking the zeolite surface to form new pores and active groups. Meanwhile, chemical reactions take place between the excited species and acid-base groups on the zeolite carriers, thus changing the acidity and alkalinity of the carriers [19]. Further, the alkaline metal's active components can regulate the metal dispersion through the interaction with acid sites [35]. In addition, by modifying the textural properties of zeolites, ultimately, non-thermal plasma can assist the zeolite-based materials to improve the reactivity, selectivity, stability, and yield of the target products [36]. More significantly, compared to the traditional thermal treatment at high temperatures that easily destroy the zeolite framework, the plasma can avoid such a situation due to the low temperature, maintaining the stability of zeolites [28].

It has been proved that more active groups such as hydroxyl will be generated on the surface of the zeolites, accompanied by the growth of porosity and the corresponding increase of active sites after the electric discharge. Material activation by non-thermal plasma is mainly applied in assisting the purification of the VOCs. Liu et al. [32] found that the methane conversion rate of the Pd/HZSM-5 catalyst grew from 25% to 90% at 400 °C after being treated with non-thermal plasma, which resulted from the increase of active sites [37]. Wang et al. [38] proved that the plasma treatment enhanced the initial activity of the Pd/Al-MCM-41 zeolite catalyst in the methane combustion reaction compared to the untreated catalyst (Figure 1A), which was caused by the higher acidity of the catalyst (Figure 1B) and the better dispersion of the PdO particles active species. Moreover, the improved stability of the plasma-treated Pd-based catalyst was proven to be closely related to the stronger interaction between the palladium oxide and the zeolite support. Using the non-thermal plasma, Wang et al. [39] significantly increased the pore size from 0.62 to 0.72 nm, the specific surface area from 222.14 to 618.06 m²/g, and the pore volume from 0.19 to 0.36 cm³/g, respectively, accompanied with the greatly increasing adsorption capacity of 167% for naphthalene. It was also ascribed to those abundant active substances (i.e., energetic electrons, ions, protons, ultraviolet photons, and active radicals) produced in the discharge region bombarded the outer surface of the adsorbents to promote the formation of new pores and reconstruction of the original pores by etching effect. On the other hand, the high energy of accelerating electrons in the electric field can break chemical bonds and form new bonds, resulting in an increase in the active site amount, thus improving the activity of the zeolites.

In addition to changing the pore structure and surface properties of zeolites, another important application of non-thermal plasma is to introduce a certain amount of protonic acid sites to improve the acidity and activity of zeolites [41]. Free radicals and active ions containing carbon, oxygen, and nitrogen possessing high kinetic energy can easily react with acidic or alkaline groups in the zeolites [42], thus changing the acidic or alkaline properties of the zeolites [19]. Xia et al. [43] found that compared to the Fe-Mo/HZSM-5 zeolite obtained by the conventional treatment, the Brönsted acidity of the non-thermal plasma-treated zeolite was up to more than 265%, which significantly enhanced methane conversion from 11% to 24% [44]. For the Mo-Fe/HZSM-5 zeolite, Zhu et al. [45] also found that the Brönsted acid intensity and density of the plasma-treated catalyst were higher than that of the untreated sample, and thus the methane conversion was increased. Liu et al. [32] confirmed that the non-thermal plasma treatment significantly increased both the Brönsted and Lewis acids contents of the Pd/HZSM-5 zeolite. The concentration of both the acidic sites was 1.13 times and 1.21 times higher than that of the fresh sample, respectively. As a result, the treated zeolite showed 50% higher catalytic activity and stronger stability than the fresh agent in methane combustion [37]. In addition to the above applications for plasma-assisted methane removal, Takeuchi et al. [46] proved that for the iso-butane catalytic cracking, the plasma treatment for 5 min could enhance the activity by 10% compared to the treatment by conventional calcination at 600 °C for 2 h, which was attributed to the fact that the Lewis acid sites could be effectively formed in the HY zeolite after plasma treatment.

The non-thermal plasma treatment can also increase the dispersion and stability of the metal active components loaded on the materials, thus providing more reactive centers [19]. Liu et al. [32] also found that after treatment with the non-thermal plasma, the particle size of the PdO on the surface of the Pd/HZSM-5 catalyst decreased from 21.0 to 16.0 nm and the Pd dispersion increased by 2.5 times, effectively avoiding the aggregation of PdO particles, and blocking the pores of ZSM-5. Thereby, the catalyst showed higher catalytic activity and better stability than that without the plasma treatment in the methane combustion. It was concluded that the plasma treatment could make the PdO cluster (alkaline oxide) better attracted to the increased Brönsted and Lewis acidic sites, facilitating the PdO dispersion on the zeolite. In addition, the enhanced interaction between the PdO and acidic sites made the PdO more stable during the reaction, significantly improving the stability of the catalyst [37]. Cao et al. [40] found that compared to the calcination, the plasma treatment could promote the formation and dispersion of small-size Co₃O₄ on Co₃O₄/H-ZSM-5 (TEM shown in Figure 1C,D). Moreover, it was obvious that the content of the adsorbed oxygen species O_β for 20-P (5.2) was more than 7 times higher than that for 20-C (0.7), which indicated that plasma could improve the number of lattice oxygen of Co₃O₄ on the catalyst surface (XPS spectra shown in Figure 1E). Therefore, the catalytic activity of methane was simultaneously improved (the conversion rate of 100% at 550 °C) (Figure 1F). Wang et al. [47] reported a high performance of the plasma-enhanced Zn/MCM-41 catalyst for acetylene hydration, showed that plasma treatment could effectively enhance the interaction between the Zn active sites and MCM-41 carrier, promote the dispersion of the Zn active component, and inhibit the loss of Zn species.

There is also some research on the activation materials by non-thermal plasma to assist NO_x purification, although are not as widespread as the VOCs removal. Yoon et al. [48] found that the active sites on the NaY catalyst increased via non-thermal plasma treatment, which greatly improved the NO_x conversion rate of the catalyst from 20% to 55% at 225 °C in the lean NO_x reduction [49]. Zhang et al. [50] proved that the dispersion of the Pd active component on the surface of the Pt/NaZSM-5 catalyst increased markedly from 4.2% to 63.3% after plasma treatment. Correspondingly, the particle size decreased significantly from 27.0 to 1.8 nm, and the NO redox activity enhanced observably from almost zero to 61.3% at 400 °C.

2.2. NTP Cooperative Catalysis Process

In addition to the above-mentioned activation level of material modification, during the process of activating zeolite materials by non-thermal plasma, reaction gas can be accelerated to activated when injected in the discharge region, NO_x/VOCs can be cooperatively removed, and the conversion rate can be improved. The basic principle of this technology is that a high-intensity electric field provides energy for the electrons so that electrons can obtain high kinetic energy. Then the high-speed moving electrons transfer energy to molecules through collision, so that some gas molecules in the gas are excited and ionized. Through direct generation or ionization, many active particles such as ions, excited atoms, molecules, electrons, and free radicals are generated, and these active particles undergo a series of complex physicochemical reactions with the NO_x or VOCs molecules to achieve the purpose of removal [51]. The energy of the active particles in the plasma is higher than that of most chemical bonds, which indicates that these extremely high-energy active particles can completely promote the fracture of the old bonds of reactants, thus generating new chemical bonds [30].

The NTP has been widely applied for purifying the NO_x via the routes of direct decomposition [52], oxidation combined with absorption [53], adsorption combined with in-situ decomposition [4], selective catalytic reduction [54], etc., the core roles of which in above actions are mainly decomposition, oxidation, and reduction. The NO decomposition by the NTP is realized through colliding and making N-O bonds broken by the electrons or high-energy species generated from the NTP to produce N₂ and O₂ [52]. The NO oxidation by the NTP is to use the oxidizing species O, O_3, or OH generated by NTP to oxidize NO to NO₂ or nitrate [53]. NO reduction by NTP is to use the N atom generated by NPT to react with NO to produce N₂ and O₂; or introduce NH₃, HC, or C_xH_y into the reaction gas to reduce NO to N₂ and O₂ [54]. The specific reaction process can be seen in Equations (1)–(3). The possible ways of combining the discharge plasma with the catalyst are shown schematically in Figure 2A,B [55]. The single-stage method is also known as the plasma-driven catalyst (PDC) system [19]. In this method, the catalyst is placed directly in the plasma reactor. The PDC system will induce both the gas-phase chemical reaction and the catalytic reaction on the catalyst surface. On the other hand, in the two-stage method, the plasma reactor is followed by the catalyst bed. This method is referred to as plasma-enhanced selective catalyst reduction (PE-SCR) method. In this case, NO_x is oxidized by plasma and then reduced by the catalyst.

NTP decomposition of NO: 2NO + e→N₂ + O₂

(1)

NTP oxidation of NO: NO + O+M→NO₂ + M, NO + O₃→NO₂ + O₂

(2)

NTP reduction of NO: NO + N→N₂ + O

(3)

It has been demonstrated that the main pathway of plasma to deal with NOx is HC-SCR, NH₃-SCR, CH₄-SCR, etc. by producing electrons or free radicals. Coupling the NTP technology with the NH₃-SCR system by Byun et al. [59], the low-temperature reaction activity of NO_x was greatly improved under the synergistic effect, and the reaction temperature window was also widened from 300–400 °C to 100–500 °C. This tendency could be attributed to the fact that the OH and H radicals generated from the NTP will react rapidly to decompose NH₃ at low temperatures to produce NH₂ radicals, followed by selective non-catalytic reduction through the reaction of NO with NH₂ to produce N₂. The NTP in collaboration with the NH₃-SCR technology was also used by Schmidt et al. to remove NO_x from diesel exhaust [56]. It was regarded that the conversion of NO is mainly triggered by the electron impact dissociation of molecular oxygen and molecular nitrogen. By comparing the infrared spectra of the gas mixture before and after being treated with plasma, it was found that the byproducts are probably formed due to the action of the plasma. With this method, the species produced by plasma can be found since new absorption lines would occur in the spectrum as depicted in Figure 2C. It has been reported by Singh et al. [60] that the reaction system cooperated the NTP with NH₃-SCR could produce some long-lived and short-lived active substances, such as the gas-phase electronic excited states of NO_x, hydrocarbon fragments, some oxygen-containing derivatives, and vibration-excited substances. These active species, which exist in gas or surface phases, are a key factor for the significant synergistic effect of the NTP and SCR catalysts. Fan et al. [57] studied the removal of NO_x by C₂H₂-SCR on an H-MOR catalyst in the DBD plasma. The results showed that the synergistic effect of the catalyst and DBD significantly increased the low-temperature catalytic activity of NO_x from 54.0% to 91.4% (Figure 2D). A common understanding about the mechanism of C₂H₂-SCR is plenty of HCN by-products, derived from key surface intermediate species, such as -CN, -NCO, -NC_aO_bH_c, which would react with NO_x and O₂ to produce N₂. By introducing hydrocarbon, Niu et al. [58] studied the synergistic effect of the non-thermal plasma on the denitrification efficiency of the Co/ZSM-5 catalyst in HC-SCR and found that when 500 ppm NO_x was introduced, the NO_x removal rate of Co-HZSM-5 catalyst applied plasma discharge was better than Co-HZSM5 catalyst alone, which increased by 30% at most in the temperature range of 150–450 °C. The synergistic effect showed that the excess oxygen generated by NTP led to the massive conversion of NO to NO₂, which is then effectively reduced to N₂ by hydrocarbons (Figure 2E).

Similar to the NO_x removal via the NTP technique, the VOCs molecules are excited and ionized by the oxidizing substances produced by NTP (such as atomic oxygen, ozone, hydroxyl radicals, active ions, or high-energy electrons, etc.), so that they can be broken down into small molecular substances, even harmless substances such as CO₂ and H₂O. Previous studies strongly suggested that the removal of VOCs proceeds mainly on the surface of the metal-loaded zeolites. During the discharge process, dissociative chemisorption of O₂ onto metal-loaded zeolites produces reactive surface species (such as O₂⁻ and O⁻) [61], which lead to the oxidative removal of pollutants, which was shown in Equations (4) and (5). Here the subscripts (g) and (s) indicate the species in the gas phase and surface, respectively. When plasma is applied to the catalyst, reactive surface oxygen species can be also generated from the lattice oxygen [62] and direct interaction with O_3(g) [63], or O_(g) radicals as well [64], as shown in Equations (6)–(8). The NTP can also promote the desorption of reaction products, which helps keep the catalytic activity, shown as in Figure 3A [65].

O_2(g) + Catalyst*→O_2(s)⁻ + Catalyst

(4)

O_2(s)⁻ +e→O_(s)⁻ + O⁻_(s)

(5)

Lattice-O_(s) + plasma→O_(s)⁻ + Catalyst

(6)

O_3(g) + Catalyst*→O_(s)⁻ + O_2(s) + Catalyst

(7)

O_(g) + Catalyst*→O_(s)⁻ + Catalyst

(8)

The following examples all show that the plasma discharge will produce active substances consisting of reactive short-lived intermediates and long-lived radicals that participate in induced VOCs decomposition reactions. Wallis et al. [66] studied the non-thermal plasma-assisted catalysis for the destruction of dichloro-methane (CH₂Cl₂), demonstrating that the combination of the plasma and HZSM-5 catalyst improved dichloro-methane conversion (36%) compared to plasma processing alone (27%), which is attributed to the synergistic action of zeolite surface with oxygen active species dissociated in the plasma. The schematic diagram of the reaction mechanism for CH₂Cl₂ decomposition in gaseous air has been given in Figure 3B. In a combined zeolite plasma reactor, Oh et al. [67] proved that toluene was decomposed not only by direct plasma decomposition in the plasma region but also by oxidation with O₃ on the zeolite outside the plasma region. With increasing reactor temperature, toluene decomposition increased, and the formation of CO was promoted owing to the enhanced decomposition of HCOOH. The reaction mechanism of the zeolite-plasma compound system for toluene decomposition was also studied by Teramoto et al. [68]. It was found that O₃ directly decomposed toluene adsorbed in the inner of the zeolite micropores because it had a long lifetime, while the contributions of short-lived radicals and fast electrons to the decomposition of adsorbed toluene were low. Trinh et al. [69] proposed and described the main reaction pathways responsible for the formation of gaseous byproducts in Figure 3C. The gaseous and adsorbed acetone molecules are first dissociated into CH₃, CH₃CO, and atomic hydrogen (H) radicals by plasma-induced energetic species. Subsequently, the recombination of these fragments in the gas phase leads to the formation of CH₃CHO, HCHO, and CH₄. Meanwhile, further oxidations of the intermediates by the oxidizing agents such as O atomic, OH radicals, and O₂ finally lead to the formation of CO_x through reactions. About other VOCs, Francke et al. [70] described the synergistic application of plasma treatment and NH₄-mordinite zeolite for the catalytic removal of butyl acetate and dichloroethene as typical VOCs oxidized to CO₂ at 100 °C. This synergy was partly due to the catalytic oxidation with ozone produced in the discharge. In addition, a plausible mechanism for the plasma-driven catalysis of the VOCs was summarized by Kim et al. [71] in Figure 3D. The models explaining the catalytic reactions are the Langmuir–Hinshelwood (L–H) model and the Eley–Rideal (E–R) model. In the L–H model, both of the reactants need to be absorbed on the surface and followed by migration to the active site. In the E–R model, only one reactant is adsorbed on the surface and the other exists in the gas phase. The adsorbed surface species can also migrate from one site to the other and eventually disappear via recombination and chemical reactions.

Figure 3. (A) The working principle of the cyclic adsorption−plasma catalytic process for VOC removal [65]. (B) A schematic diagram of the reaction pathways for plasma destruction of CH₂Cl₂ in air [66]. (C) Reaction pathways for the oxidation of acetone on the Ag surface [69]. (D) A plausible mechanism for the plasma-driven catalysis of VOCs [71].

Figure 3. (A) The working principle of the cyclic adsorption−plasma catalytic process for VOC removal [65]. (B) A schematic diagram of the reaction pathways for plasma destruction of CH₂Cl₂ in air [66]. (C) Reaction pathways for the oxidation of acetone on the Ag surface [69]. (D) A plausible mechanism for the plasma-driven catalysis of VOCs [71].

2.3. Influence Parameters of NTP

Whether in the modification process of material structure or the reaction process of pollutant gas, to maximize the performance and simultaneously reduce the energy consumption of non-thermal plasma, it is essential to determine the appropriate parameters such as discharge power, voltage, and time. Above all, the input power is one of the most important parameters affecting the adsorption/catalysis capacity of the plasma-modified materials. With the growth of power, the reactivity of the zeolites will climb up and then decline. Li et al. [72] found that with the enhancement of the plasma power, the NO_x conversion of NaY zeolite first increased and then decreased, reaching the maximum at 7.6 W owing to the maximum number of high-energy particles produced. However, when the power was too strong, the side reaction of the NO_x synthesis from substantial N₂ and O₂ might occur, which would reduce the NO_x conversion rate. Xiang et al. [73] investigated the change of the hexanal removal rate of MnO_x/SBA-15 with the discharge voltage under the coordination of the NTP and proved that the higher the discharge voltage, the higher the energy density, and the higher the concentration of active particles and O₃ produced by the plasma field, which could promote the complete oxidation of hexanal. However, when the discharge voltage was greater than 8 kv, the hexanal removal rate of the MnO_x/SBA-15 catalyst no longer changed significantly (Figure 4A).

Voltage is an influential factor that affects the goal of a plasma catalytic system to achieve an acceptable removal efficiency with low energy cost. Zhu et al. [74] found that synergy between the NTP and catalyst enhanced the conversion efficiency by 18–40%, especially at a discharge voltage of 5 kV. At low applied voltage, the numbers of energetic electrons and reactive species are relatively low. In addition, the concentration of oxygen resulting from O₃ decomposition was low, and UV radiation was too weak to effectively activate the catalyst in the reactor. Consequently, the concentration of O atoms on the catalyst surface was low. When the discharge voltage increased, the gas discharge became more intense, and catalytic activity increased, resulting in higher removal efficiency (Figure 4B).

The discharge time of non-thermal plasma also exhibits an important effect on the reaction, and specifically, the optimal discharge time can not only improve the reactivity of zeolites but also can reduce energy consumption. The catalytic performance of the Zn-based catalysts with different oxygen plasma treatment times for acetylene hydration was studied by Wang [47]. The activity and selectivity of the catalysts were shown in Figure 4C,D. When t = 30 min, it showed that the highest conversion of acetylene was over 95% and selectivity to acetaldehyde was about 80% after 12 h of reaction. The conversion and selectivity of the catalyst were reduced after 12 h of reaction when t = 60, 90 120 min compared to t = 30 min. It may be explained that the long period of the plasma treatment may lead to the agglomeration of the active components or the destruction of the catalyst structure, thus obstructing the better catalytic performance of the catalyst to be achieved. Wang et al. [39] verified that with the increase of plasma discharge time, the naphthalene adsorption capacity of the NaY zeolites increased rapidly, achieving the maximum at 25 min, which was improved by 56% compared to the unmodified NaY. It could be attributed to the generation of plentiful acid sites and surface chemical functional groups, which can facilitate the adsorption of naphthalene to the external surface of zeolite. However, when the discharge time was up to more than 25 min, the adsorption capacities of the NaY did not change significantly, which was mainly due to the number of active sites reaching saturation and no longer increasing. Here, the changes in the activity of zeolite adsorbents/catalysts for purifying the NO_x/VOCs under non-thermal plasma treatment are summarized in

Table 2. Changes in adsorption/conversion rates of the zeolites before and after the low-thermal plasma treatment.

Reactants	Adsorbents/Catalysts	NTP Condition		Removal Efficiency with/without NTP (%)		Ref.
		Power (W)	Time (min)	Without NTP	NTP
NOx	NaA	-	40	20.0	55.0	[48]
	Pt/NaZSM-5	-	30	0.0	61.3	[50]
	Cu-ZSM-5	80	-	80.0	99.0	[75]
	Co-ZSM-5	-	90	15.0	75.0	[55]
	BaY	-	-	55.0	74.0	[76]
	NH4-MOR	-	-	50.0	85.0	[70]
Acetone	Ag/β-zeolite	28	15	66.8	97.0	[69]
Benzene	FER	-	25–40	20.0	33.0	[71]
	13X	-	25–40	40.0	43.0	[71]
	HY	-	25–40	75.0	84.0	[71]
	Ag/HY	-	25–40	80.0	89.0	[71]
	TiO2-MnO2/zeolite	10.33	10	52.7	81.0	[77]
	FER	9.72	-	-	58.0	[78]
	Ag/MOR	9.72	-	58.0	83.6	[78]
	Ag/HY	9.72	-	61.6	96.0	[78]
	Ag/13X	9.72	-	71.1	88.0	[78]
	FER	-	60–90	-	78.0	[79]
	Ag/HY	-	60–90	68.0	75.0	[79]
	Ag/HZSM-5	4.7	13	87.0	100.0	[80]
	AgMn/HZSM-5	6	24	78.0	96.0	[81]
Chloro-benzene	CeO2/HZSM-5	63		72.6	96.0	[82]
	CeO2/HZSM-5	-	-	32.3	72.6	[74]
Dichloro-methane	HZSM-5	0.9	20	27.0	36.0	[66]
	NaZSM-5	0.9	20	27.0	32.0	[66]
Ethylene	Ag-Fe/13X	19	30	90.0	100.0	[83]
	Ag/13X	-	20	42.0	63.0	[84]
Formaldehyde	AgCu/HZSM-5	2.3	40	72.0	100.0	[85]
Methane	Pd/HZSM-5	-	30–60	25.0	90.0	[37]
Toluene	NaY	5	40	15.0	52.0	[68]
	NaY	5	10	50.0	60.0	[67]
	Co/13X	20	60	90.0	93.9	[86]
	5A	20	60	90.0	95.0	[86]
	HZSM-5	2.4	480	70.0	100.0	[87]
	Ag-Mn/HZSM-5	2.4	480	93.5	100.0	[87]
	Co/13X	20	-	81.6	92.7	[88]
	Hβ	3.3–3.7	42	69.0	98.4	[89]
	CuCeZr/ZSM-5	43		35.4	78.8	[90]

.

3. Application of Microwave over Zeolite-Based Materials

A microwave, an electromagnetic wave between radio waves and infrared rays, possesses a wavelength of 0.1 mm–1 m as well as a frequency of 300 MHz to 300 GHz [24]. In addition to the application for communication, the microwave has been gradually used to heat and activate substances, especially for those with low thermal conductivity [24]. During the traditional heat conduction, thermal energy is transferred from the outside to the inside of materials, which will inevitably cause the internal temperature lower than the outside of materials, thus producing the thermal gradient, resulting in the uneven heating temperature [91]. Different from the traditional heating method, the microwave radiating onto the material surface can directly penetrate the materials and be selectively absorbed by the active sites, which will be converted into thermal energy, heating the materials rapidly and evenly at a relatively low temperature, thus reducing energy loss [92]. The selective heating for metal active sites has been confirmed by the metal aggregation and the coke species distribution observed on the spent materials in the microwave reactor [93]. Microwave heating has the advantages of environmental protection and simple operation [94], which is considered to be a relatively desirable method to promote the activation and modification of porous skeleton materials such as zeolites [22]. The schematic diagram of the microwave device is shown in Figure 5A [95].

The microwave has been applied in assisting the synthesis of the zeolite materials for the catalysis of NO_x and VOCs [96], whose main role is to shorten the induction period, increase the rate of nucleation and nuclear growth, and then shorten the reaction cycle, thus reducing energy consumption. At the same time, microwaves can also reduce the formation of impurity crystals, thus improving the structure and properties of the zeolite [95]. The schematic diagram of the synthesis of the zeolites by microwave is shown in Figure 5B. In addition, the frequent collision between plenty of moving neighboring ions during microwave irradiation will improve the porosity, thereby promoting the activity of zeolites [98]. At the same time, the selective radiation of microwave to metal catalytic sites as “localized hot spots”, can enhance the interaction between the target reactants and active sites, thus improving the catalytic activity and selectivity of metal/zeolite catalysts [99]. It has been proved that microwave energy can selectively activate the NO molecules and directly decompose NO into the N₂ and O₂ at active sites. The schematic diagram of the microwave catalytic decomposition system is shown in Figure 5C.

3.1. MW Assisted Synthesis of Zeolites

At present, the zeolite synthesis method includes traditional hydrothermal synthesis, sol-gel, precipitation, and the newly developed trans-crystallization, seed-assisted synthesis as well as microwave-assisted synthesis [100]. The hydrothermal synthesis has the disadvantage of generating alkali-containing effluent and toxic gases [7]. Trans-crystallization has the disadvantage of complex preparation for the precursor zeolite and the high cost [95]. Moreover, the yields of the seed-assisted synthesis are relatively low. The defect of the sol-gel method is that the process steps are complicated, and the preparation cycle is long [101]. The possible problem of the precipitation method is that the pore structure of the prepared zeolites is not regular enough and the crystal size is not uniform enough [102]. It is meaningful to develop microwave synthesis to improve synthetic efficiency and reduce costs [96]. The characteristics of fast and precise heating make microwave successfully applied in the synthesis process of the various zeolites. Different from the traditional method, the polar molecules in the microwave synthesis system rotate rapidly under the induction of a high-speed microwave field, which greatly speeds up the ion diffusion and transport speed between the reactant interfaces, thus promoting the formation of crystal nucleus and the growth of grains, so the crystallization time can be significantly shortened. In addition to shortening crystallization time, the obtained zeolites by microwave have higher purity, more regular morphology, more uniform particle dispersion, larger specific surface area, and stronger acidity, thus possessing higher adsorption/catalytic performance compared to the hydrothermal synthesis [103]. It should be pointed out that the structure properties of zeolites (such as pore size, specific surface area, porosity, acidity, particle size, crystal defects, etc.) strongly influence the dispersion of the metal species and surface groups, which finally greatly affect the activity of zeolites. For example, large pores are more conducive to metal species diffusion in the zeolites than small ones. Large specific surface areas can help zeolites to improve the dispersion of active cations and surface groups than small ones [95]. Higher porosity is conducive to the improvement of specific surface area and the diffusion of metal active sites on the zeolites. More acid sites of zeolites could also do a favor in providing more reactive sites [103]. Moreover, small particle sizes can increase the acid sites and thus improve the dispersion of active species [104]. The presence of crystal defects will also have a significant impact on the properties of zeolite crystals, such as the possible generation of new active sites.

At present, ZSM-5 [104], SAPO-34 [105], SSZ-13 [92], MCM-41 [106], MCM-22 [107], SBA-15 [73], NaY [108], and NaX [109] zeolites, etc. have been successfully synthesized by the microwave-assisted method [110]. At present, the ZSM-5, SAPO-34, and SSZ-13 zeolites loaded with Cu have been used in ammonia selective catalytic reduction of NO_x [111]. There are also several zeolites synthesized by microwave methods for the adsorption or catalytic purification of VOCs [112], such as MCM-41, MCM-22, NaX, and NaY [113]. For instance, MCM-41 zeolite after loading Pd can remove VOCs by quickly adsorbing acetaldehyde [114]. MCM-22 zeolite after loading Pt has been studied extensively as the catalyst for N-hexane isomerization [115]. NaY zeolite can adsorb toluene, acetone, and other VOCs [116]. NaX zeolite can be used for catalytic oxidative degradation of VOCs such as benzene [117].

First, microwave radiation can promote nucleation and shorten the crystallization time during the zeolite synthesis process, because in addition to the thermal effect, specific microwave effects will also be produced simultaneously, which can strengthen the Brownian motion and rotational dynamics of water molecules, causing the hydrogen bond of water molecules to break [96]. Therefore, these activated water molecules with high activation energy can lead to rapid gel dissolution [95]. Jun et al. [111] found that microwaves could promote the formation of crystal nuclei during the aging process of ZSM-5. The SAPO-34 with different morphologies was synthesized by Lin et al. [105] after rapid crystallization at 180 °C for 1 h in a microwave radiation device, which greatly shortened the crystallization time from 2.5 h. Shalmani et al. [110] synthesized the SAPO-34 by combining the microwave with a traditional hydrothermal method and concluded that the microwave played a main role in promoting nucleation and shortening crystallization time from 24 to 3 h. The microwave heating radiation was also applied by Yu et al. [92] to the synthesis of SSZ-13, and the crystallization time was greatly shortened from 72 h under traditional hydrothermal crystallization to 9 h. The main reason was that microwaves could promote the rapid dissolution of the gel mother liquor, activate the water molecules, and improve the reaction rate, thus shortening the induction period and nucleation period during the process of crystallization. As for zeolites for the VOCs removal, the MCM-41 zeolites were microwave-assisted prepared by Wu et al. [112] at 150 °C by shortening the synthesis time from 5 to 7 days in a conventional autoclave to 1 h under microwave heating, which was because microwave heating could accelerate the condensation reaction of the silicate network. On this basis, Zhang et al. [106] crystallized the MCM-41 zeolite via the microwave radiation heating method for 15 min, during which the template agent could rapidly be removed without the destruction of the zeolite structure. Compared to the traditional hydrothermal preparation method, the crystallization cycle and energy loss were greatly reduced. The reactant molecules in the system were rapidly diffused and transmitted under the induction of the high-speed microwave field, thus accelerating the synthesis reaction rate. The MCM-22 zeolite was successfully synthesized by Wu et al. [115] through the microwave method for the first time. Compared to the ultrasonic-assisted, dynamic hydrothermal, and static hydrothermal methods, it was found that the microwave method had the fastest crystallization rate (Figure 6A). It is attributed that in microwave-assisted heating of synthesis gel mixture, water absorbs the microwaves and then the microwave energy transfers to the hydrogel either through dielectric heating or resonance absorption, and the activated water molecular should easily attack the Si-O and Al-O bonds to enhance the dissolution of the Si and Al ions into the solution. This results in a faster formation of abundant nuclei, which consequently leads to a much shorter nucleation period and shorter crystallization time afterward. Ling et al. [107] synthesized the MCM-22 zeolite by microwave for 29 h, which greatly reduced the crystallization time compared to the traditional hydrothermal synthesis which required 120 h, and part of the template agent was decomposed by microwave. This might be due to the fact that the hydroxyl groups of solvent water molecules were activated under the action of microwave, forming an activated “water”, which could better combine with Si-O and Al-O in the sol, so that the induction period was significantly shortened. The NaY zeolite with uniform and small size was obtained by Arafat et al. [108] under microwave radiation heating at 100–120 °C for 10 min. It was shown that the crystallization induction period was greatly shortened, thus avoiding the formation of sodiolite, hydrocalcineolite, P-type zeolite, and other impurities that often appear in the traditional hydrothermal method. NaX zeolite was obtained by Song et al. [109] after crystallization by microwave heating for about 30 min. Compared to the traditional hydrothermal method with a crystallization time of more than 17 h, the crystallization period was significantly shortened. It was suggested that the main role of the microwave in the synthesis of zeolite is to shorten the induction period, increase the rate of nucleation and nuclear growth, and then reduce energy consumption. Ansari et al. [113] successfully synthesized the pure nano-NaX zeolite by the microwave heating technique at 90 °C for 3 h. The microwave heating required a much shorter induction period of zeolite synthesis compared to the conventional heating method because it was easy to react with free silicon and aluminum species and then rapidly grow into the target crystal once the crystal nucleus of zeolite was formed in the sol.

In addition to shortening the crystallization time, the microwave can also optimize the structural morphology of the zeolites during the preparation process, so that the grains will obtain small and uniformly dispersed, maintain intact crystallinity and purity, increase the number of pores, and improve the surface area of the zeolites. Compared to the uneven temperature distribution of conventional heating, because the microwave can directly penetrate the samples, the reactants can be heated uniformly inside and outside at the same time, so the synthesized zeolites can possess a more regular structure and better thermal stability [106]. Koo et al. [104] used microwave heating to synthesize the ZSM-5 zeolite with uniform particle dispersion and regular morphology. Simultaneously, the microwave heating method generated additional secondary porosity, especially micropores and mesopores in the ZSM-5 zeolite, and promoted a narrow pore size distribution in the range of 4.08–4.69 mm compared to conventional. This caused the BET surface area to add from 328 to 398 m²/g under microwave in comparison to the hydrothermal method. The ZSM-5 zeolite prepared by Lima et al. [118] through microwave heating produced new secondary pores, and the specific surface area and the total pore volume were improved by 3.97% and 7.14%, respectively, compared to that obtained by the traditional method, thus improving the catalytic conversion of ethylene. The preparation reaction was carried out by Lin et al. [105] via microwave heating, obtaining the SAPO-34 nanoparticles with different morphologies. The crystal products were more uniform and less aggregated compared to the particles formed by conventional heating at 180 °C due to the uniformity of microwave heating. The MCM-41 zeolite with high crystallinity was prepared by Wu et al. [112] under microwave-assisted heating. The homogeneous and rather small crystal size of the product is probably the result of the fast and homogeneous condensation reactions occurring during microwave heating. The MCM-22 zeolite with high purity and uniform morphology was synthesized by Ling et al. [107] under the microwave, accompanied by the formation of many small crystal nuclei due to the uniform heating principle of the microwave. Arafat et al. [108] also obtained the NaY zeolite with uniform and small sizes by heating it under microwave radiation. The NaX zeolite was obtained by Song et al. [109] after crystallization by microwave heating. Compared to the traditional hydrothermal method, the grain size was more uniform, and the specific surface area increased to double, showing excellent catalytic performance. Because of the uniformity of microwave heating, the nucleation rate of zeolites is the same, so the grain size of microwave-synthesized zeolites is homogenous. Ansari et al. [113] successfully synthesized the pure nano-NaX zeolite using the microwave heating technique. The microwave heating produced more uniform and small zeolite nanoparticles (Figure 6B,C), which owned relatively narrower particle size distribution (95 nm) compared to the conventional heating method (112 nm) as shown in Figure 6D, and did not significantly change composition or crystallinity.

Moreover, the microwave can also strengthen the acidity and metal dispersion, thereby enhancing the catalytic performance of zeolites. Jun et al. [111] found that the concentration of the acid sites on H-ZSM-5 assisted by microwave was higher than that of conventional hydrothermally synthesized H-ZSM-5. It was proved by Yu et al. [92] that the Brönsted and Lewis acidic sites of the SSZ-13 samples prepared by microwave were higher than those by traditional methods. It might be attributed to the periodic motion of polar substances caused by the periodic change of microwave, changing the distribution of Si and Al in SSZ-13 products, thus promoting the increase of B/L acidic sites. Moreover, according to the change of the desorption amount of NH₃, it was found that the acidity of the samples prepared by MW changed little after aging, showing that the acidic sites provided by the zeolite skeleton and metal active components were less damaged, which indicated that they had strong resistance to hydrothermal aging. Additionally, the loading capacity of the active component Cu was also significantly increased from 1.68% to 0.88% compared to the hydrothermal synthesis, which might be related to the fact that the distribution of Si and Al elements in the skeleton was changed by microwave radiation so that the Cu ions exchange sites in the skeleton was increased. As a result, the catalysts prepared by microwave showed better low-temperature activity and hydrothermal stability compared to the traditional method, whose active temperature window was expanded from 150–580 °C to 200–620 °C (Figure 6E,F). Zhang et al. [106] used the microwave radiation heating method to crystallize MCM-41 zeolite, which increased the surface acid content of the zeolite. The MCM-22 zeolite with more effective protonic acid sites was synthesized by Ling et al. [107] via microwave in contrast to the hydrothermal method, which was because Al entered the zeolite skeleton more effectively and uniformly in the form of tetrad coordination. To sum up, a comparative table with the properties parameters of zeolites synthesized via microwave method and hydrothermal method (HT for short) is in

Table 3. Comparative table with the properties parameters of zeolites synthesized via MW and HT.

Zeolites	Density (g/cm3)	Crystallinity (%)		Crystallite Size (nm)		Surface Area (m2/g)		Pore Volume (cm3/g)		Ref.
		HT	MW	HT	MW	HT	MW	HT	MW
ZSM-5	1.81	-	-	4.69 × 106	4.08 × 106	328.00	398.00	0.12	0.82	[104]
ZSM-5	-	86.90	89.50	4.69 × 103	4.23 × 103	445.00	428.00	0.30	0.28	[118]
ZSM-5	-	-	-	-	4.30 × 103	360.00	333.00	0.28	0.23	[111]
SAPO-34	1.65	-	-	150.00	100.00	-	400.00	-	0.42	[105]
SAPO-34	1.72	100.00	100.00	-	400.00	-	400.00	-	0.30	[110]
MCM-41	2.20	99.00	100.00	10.00 × 103	2.00 × 103	1000.00	1090.00	0.76	0.80	[112]
MCM-41	2.50	-	-	10.00 × 103	2.00 × 103	-	1000.00	-	0.80	[106]
MCM-22	1.60	-	-	10.00 × 103	1.00 × 103	641.00	657.00	0.39	0.60	[107]
MCM-22	1.80	90.00	100.00	10.00 × 103	8.00 × 103	-	450.00	-	0.56	[115]
NaY	2.60	90.00	100.00	-	1000.00	-	295.00	-	0.60	[108]
NaY	-	-	-	150.00	280.00	690.00	716.00	-	-	[119]
NaY	-	99.00	100.00	72.60	44.40	775.00	474.00	0.34	0.35	[120]
NaX	2.10	100.00	100.00	1000.00	-	-	250.00	0.95	1.00	[109]
nano-NaX	1.90	-	-	112.00	95.00	417.00	-	-	-	[113]
SSZ-13	0.60	99.00	100.00	-	500.00	402.79	471.31	0.20	0.22	[92]
SSZ-13	-	100.00	100.00	-	-	624.00	666.00	0.23	0.24	[121]
SSZ-13	-	-	100.00	600.00	50.00	571.00	520.00	0.26		[122]

.

3.2. MW Assisted Activation of Materials

There are few studies on the effect of the microwave on the pore structure of zeolite materials, but there are still some primary conclusions from these studies. After the microwave is selectively absorbed by the active sites on the zeolites, the thermal energy produced rapidly will make the numerous moving neighboring particles frequently vibrate and collide to open the blocked pores, which can improve the porosity, enlarge the pore volume, and expand the surface area. Moreover, the acidity and the dispersion of metal species may increase simultaneously [123], correspondingly promoting the ion exchange capacity and adsorption/catalytic activity of zeolites. It was proved by Wei et al. [124] that the addition of the microwave can increase the NOx removal efficiency of FeCu-5A zeolite from 81.2% to 95.8%, which was assumed that the microwave could remove the volatile and oxidizable impurities in the pores inside the catalyst, thus making the blocked pores open. The microwave-assisted treatment by Liu et al. [123] expanded the mesopore volume of mordenite zeolite by 0.70 cm³/g due to the uniform and gentle etching process of the microwave. Moreover, the mesopores increased the amount and accessibility of acidic sites, which helped to increase the conversion rate of α-pinene in isomerization reaction from 46.1% to 94.7% compared to conventional heating treatment. Bo et al. [125] verified that microwave radiation could promote the ion exchange between Cu and Mn ions and Ca ions in the Cu-Mn-Ce/5A zeolite catalyst, thus making the Cu and Mn ions more dispersed on the surface and in the pore channels of the zeolite, which was conducive to the increase of the catalytic oxidation rate of benzene from 80% to 95% at 500 °C.

3.3. MW Assisted Catalysis Process

In addition to the above approaches to improve the catalytic activity of the zeolites through changing the textural properties of zeolites, by intensifying auto-oscillation of the molecules, thermal energy will be generated inside the zeolites, which can selectively heat the active sites to form “localized hot spots”. The NO_x or VOCs reactants can be induced to catalyze when they contact the active sites, thus the adsorption/desorption performance of the zeolite materials will be further enhanced [99]. Although less research about the microcosmic mechanism of microwave promoting direct conversion of VOCs at zeolites active sites, some studies have indicated that the NO can be motivated to an activated state (NO*) through the selection effect from microwave energy, as shown in Equation (9) [126]. NO* is originally physically adsorbed on the active sites of catalysts, and then microwave catalysis changes the adsorption form to chemisorption. Finally, the NO* is selectively activated by microwave energy and directly decomposed into the N₂ and O₂ (Equation (10)) [97]. Figure 7 shows the proposed mechanism of the adsorption-microwave catalytic decomposition method for the NO on catalysts [97].

NO→NO*

(9)

NO*→N₂ + O₂

(10)

Tang et al. [127] proved that after microwave treatment, the average NO selective reduction rate on the Co/HZSM-5 zeolite increased from 20% to 40%, and the active temperature range of NO conversion was widened by 100 °C compared to the sample after conventional thermal treatment (Figure 8A,B). It was attributed to the formation of many “local hot spots” on the zeolite surface after active sites absorbing microwave, which had high energy to promote the activation of NO_x species adsorbed on the catalyst, thus facilitating the conversion of NO_x to N₂ [128]. Additionally, there have been many investigations reported on the activation of the Fe-ZSM-5 zeolite catalyst by microwave treatment. It has been proven that the Fe active species such as the Fe-oxo with high microwave sensitivity can rapidly absorb microwave energy to form “localized hot spots” due to the ability to heat up quickly, which can enhance the interaction between the reactants and the zeolite surface, efficiently activating the catalytic performance of zeolites [129]. Ohnishi et al. [129] proved that compared to the conventional electric furnace, the NO decomposition activity over the Fe-ZSM-5 zeolite catalyst was increased from 7.5% to 11% within 670–875 K under the microwave irradiation (Figure 8C,D). In addition, microwave improved the selectivity of the product, making the proportion of N₂ in the product larger than that under conventional heating, and inhibiting the formation of the N₂O greenhouse gas, whereas the N₂O formed over the catalyst treated with the conventional heating was directly desorbed without decomposing to N₂ and O₂. Similarly, Tang et al. [126] found that compared to conventional heating, microwave heating effectively improved the NO decomposition from 35% to 70% over the Fe/NaZSM-5 catalyst, and Fe species combined with NO would form the complex [130] (Figure 8E,F). In addition, by the phenomenon that the microwave field increased the NO conversion rate from nearly 0 to 23.6% over the In-Fe₂O₃/HZSM-5 catalyst in comparison to conventional heating, Wang et al. [131] found that the active temperature window of InFe₂O₃/HZSM-5 catalyst moved significantly to a lower temperature under the action of microwave, and put forward a viewpoint that Fe₂O₃ medium could promote the microwave energy to be effectively converted into the activation energy of methane molecules, thus reducing the required temperature for NO reduction and broadening the active temperature range [132].

In addition to the above applications of the NO_x removal, for VOCs removal, Gao et al. [133] discovered that microwave heating enhanced the catalytic activity of the ZnNi/HZSM-5 zeolite for alkene molecules by more than 5% at 450 °C, which might be because the electriferous alkene molecules were easy to be adsorbed on the catalyst surface sites due to the enhancement of the dipole effect. Deng et al. [93] also verified that under microwave irradiation, the methane conversion on the H-(Fe)-ZSM-5 zeolite increased significantly from 3% to 40%, and the selectivity of the products (ethane and ethylene) was increased by 2–7 times compared to conventional heating. Meanwhile, the Fe as the methane activation center would be converted into iron oxide cluster active sites. The yield of isobutene over the ferrierite-based zeolite catalyst prepared by Zholobenko et al. [134] increased from 65% to 69% and the selectivity to isobutene increased from 19% to 22% under microwave irradiation compared to that under conventional heating. As was concluded by others, the Fe active species sensitive to the microwave could effectively absorb the irradiation to form “hot spots”, which can effectively adsorb the reactants to the catalysts, promoting the catalytic reaction balance to a more favorable direction.

3.4. Influence Parameters of MW

Generally, the microwave treatment parameters that influence the activation effect of zeolites include power, time, temperature, etc. As the microwave power rises within an appropriate range, the activity of zeolites will be enhanced, owing to the acceleration of heating rate, facilitated the formation of crystal nucleus, the opening of blocked pores, as well as the addition of newly formed active sites. However, excessive power will not only cause massive energy consumption but also restrain the activity of active sites and destroy the functional groups on the zeolite surface due to the high penetrability of intensive the microwave.

First, microwave power has a great effect on the activity or crystallization rate of the zeolite. Wei et al. [135] investigated the influence of microwave power on denitrification using Fe/Ca-5A zeolite as the catalyst, and found the conversion of NO_x removal efficiency increased from 83.6% with 164 W to 94.8% with 280 W, decreased to 86.7% with 331 W. Because in the process of microwave-induced NO_x catalytic reduction when exceeding the optimal power 280 W, the FeCu/zeolite would absorb microwave excessively and be destroyed (Figure 9A). It has been proved by Zhang et al. [106] that in the process of synthesizing MCM-41 zeolite assisted by microwave heating when the power was 160 W, the product with high crystallinity could be obtained. At the appropriate power, the microwave could directly penetrate the sample and heat it uniformly inside and outside simultaneously, so the zeolite structure was more regular and had better thermal stability. If the microwave power was too low (≤90 W), the crystallinity became worse; if the power was too high (>350 W), the heating temperature was too high, which would result in the damage of zeolite skeleton structure and the pore collapse (Figure 9B).

The appropriate microwave treatment time can accelerate the formation of crystal nuclei [136]. However, the long-term accumulation of microwave radiation may make the internal temperature of zeolites too high, resulting in partial damage to the zeolite surface and the collapse of the pore structure inside the zeolites [137]. Shalmani et al. [110] adjusted the microwave irradiation time from 10 to 50 min during SAPO-34 preparation and concluded that the morphologies and sizes of the SAPO-34 crystals were strongly influenced by the microwave irradiation time. Not enough time might imply the aggregated particles and the less uniform nucleation. Prolonging the microwave irradiation time led to the production of numerous and homogeneous nuclei. However, further prolongation led to particle growth after nucleation (Figure 9C).

The treatment temperature is considered another important parameter of the microwave field. Wu et al. [115] investigate the influence of three different aging processes, enhancement of microwave aging temperature from 60 °C to 100 °C could increase the rate of nucleation and shorten the crystallization time from 6.5 days to 4 days and meanwhile decrease the produced the MCM-22 particle size (Figure 9D–F). This can be explained by the accelerated formation of germ nuclei due to the increase in the dissolution rate of amorphous silica in the synthesis gel with increasing the microwave aging temperature. Here, the changes in the activity of the zeolite adsorbents/catalysts for purifying NO_x/VOCs under microwave treatment are summarized in

Table 4. Changes in adsorption/conversion rates of the zeolites before and after microwave radiation.

Reactants	Adsorbents/Catalysts	MW Condition			Removal Efficiency with/without MW (%)		Ref.
		Temperature (°C)	Power (W)	Time (min)	Without MW	MW
NOx	Cu-ZSM-5	300	100	-	31.0	36.0	[129]
NOx	Co/HZSM-5	-	-	-	20.0	40.0	[127]
NOx	Fe-ZSM-5	400	100	60	7.5	11.0	[129]
NOx	Fe/NaZSM-5	300	200	-	35.0	70.0	[126]
NOx	InFe2O3/HZSM-5	-	-	-	0.0	23.6	[131]
NOx	Fe/Ca-5A	-	280	35	79.3	97.3	[135]
NOx	Fe/Ca-5A	-	259	35	85.6	98.2	[138]
NOx	Fe/Cu-5A	-	280	35	81.2	89.3	[124]
NOx	FeCoCu-5A	-	280	35	87.5	92.3	[139]
Aldehyde	Y	31	425	15	86.0	92.0	[140]
Benzene	Cu-Mn-Ce/5A	477	117.3	2	80.0	90.0	[125]
Methane	H-(Fe)-ZSM-5	550	180	-	3.0	40.0	[93]
1-butene	Ferrierite	300	300	120	30.0	33.0	[134]
α-pinene	Mordenite	65	600	120	46.1	94.7	[123]

.

4. Conclusions and Prospect

Due to the defects like large energy consumption as well as damage to the zeolitic framework caused by traditional thermal treatment, this paper systematically reviewed the application of unconventional external-field (non-thermal plasma and microwave) treatment methods on zeolite-based adsorbents/catalysts for purifying the NO_x and VOCs. The roles of the non-thermal plasma or microwave in the adsorption/catalysis of the NOx/VOCs include assisting in materials activation, reaction process, and zeolites synthesis. The effects of non-thermal plasma and microwave treatments while assisting in the activation of materials are consistent, which are all through improving the specific surface area, pore volume, pore size, porosity, and acidity of the zeolites or reducing activation energy to optimize the reactivity, selectivity, stability, etc. However, the essential mechanisms of auxiliary material activation via NTP and MW are different. The non-thermal plasma discharge can produce a large number of high-energy electrons impacting the outer surface of zeolites to increase new pores and active groups, while microwave selectively irradiating the active sites is converted into thermal energy, which can enlarge the pores of the zeolites and enhance the cation exchange capacity, thus improving the activity of zeolites. During the process of the NTP and MW-assisted catalytic reaction, the oxidizing substances produced by non-thermal plasma (such as active ions or high-energy electrons, etc.) can also directly excite and ionize the NO_x and VOCs, break the chemical bonds between gas molecules, and thus decompose them into small harmless molecules. Contrastively, the microwave can directly induce and convert the NO_x/VOCs reactants to harmless gas over the active sites which are selectively irradiated as “localized hot spots”. Moreover, in the case of significantly shortening the crystallization time, the microwave can also optimize the reaction activity by accelerating the growth rate of the crystal nucleus and forming highly purified, uniform, and small-sized grains during the preparation of zeolites. Additionally, the influence of different treatment parameters of various external fields on assisting the zeolites was elaborated in this paper, to provide some references for the activation treatment of zeolite-based materials. In general, with the increase in power, time, voltage, etc., the activity of zeolites will increase, while decreasing or remaining unchanged after exceeding the appropriate range. So, it is of great significance to find suitable parameters to maximize the performance of zeolites while reducing the loss of energy and fuel.

Although unconventional external fields offer great opportunities for zeolite-based materials to purify some types of air pollutants, there are still some challenges that need to be considered in the future. (1) Deepening the research of microscopic action mechanisms is necessary for guiding the design of highly efficient materials. (2) In addition to the effects on material pretreatment, assisted catalysis, and auxiliary zeolite synthesis described above in this review, it is worthy of studying whether unconventional external fields can replace other thermal procedures, such as calcination and drying. (3) There is a lack of systematic comparison of non-thermal plasma and microwave efficiencies for different types of zeolites. (4) The multi-fields collaborative treatment of the assisted activation materials is also worth noting. (5) We should strive to expand the laboratory research scale on the external-field treatments to the industrial application. However, at present, the main problem of scaling field treatments to industry is limited activation timeliness. Doping metals or auxiliaries could be considered to prolong the activation effect. In addition, it can also achieve batch processing and industrial scale-up by designing more efficient treatment installations, simplifying the technical process, and reducing the operating temperature as much as possible.