Preparation of Mesoporous Zeolites and Their Applications in Catalytic Elimination of Atmospheric Pollutants

: With increasing environmental awareness, the issue of atmospheric pollution has gained significant attention. Specifically, three types of atmospheric pollutants, namely, nitrogen oxides, volatile organic compounds, and carbon monoxide, have become the focus of widespread concern. In addressing these pollutants, mesoporous zeolites have emerged as promising materials due to their large specific surface area, which enables effective dispersion of active sites, and their large pore volume, which facilitates efficient diffusion. This article provides a comprehensive overview of the preparation methods of mesoporous zeolites and their applications in removing nitrogen oxides, volatile organic compounds, and carbon monoxide. It also highlights the challenges and limitations faced by the application of mesoporous zeolites in pollutant removal and emphasizes their potential as efficient catalysts.


Introduction
Human activities and natural processes generate various pollutants that can be released into the atmosphere and become part of the atmospheric circulation.These pollutants are eliminated from the atmosphere through chemical reactions, biological activities, physical sedimentation, and other ways.When the rate of pollutant removal is lower than the rate of production, these substances will accumulate in the atmosphere, leading to the formation of atmospheric pollution once a certain concentration is reached [1][2][3][4].Atmospheric pollution has adverse effects on the ecological environment and poses significant risks to human health.Therefore, it is of great significance to eliminate and reduce atmospheric pollution [5,6].Currently, the main pollutants contributing to atmospheric pollution are nitrogen oxides (NO x , including NO, NO 2 , and N 2 O), volatile organic compounds (VOCs), and carbon monoxide (CO).
NO x sources can be categorized into stationary sources (such as industrial boilers and coal-fired boilers) and mobile sources (vehicle emissions, etc.).High concentrations of nitrogen oxides in the air can lead to the formation of acid rain and photochemical smog, as well as damage to the central nervous system [7][8][9].Various technologies are employed for NO x removal, including selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), non-thermal plasma (NTP), exhaust gas recirculation (EGR), and wet flue gas denitration (WFGD) [10][11][12][13].Among them, SCR is a widely used technology in denitration.Another type of pollutant is volatile organic compounds (VOCs), which include many kinds of organic atmosphere substances, and they can stay in the atmosphere for up to 60 days.VOCs originated from various exhaust gases, mainly painting, printing, fuel storage, petroleum refineries, and motor vehicles.When the concentration of VOCs in the air is high, it will cause photochemical smog and destroy the ozone layer [14,15].Several methods for removing VOCs have been developed, among which catalytic oxidation is widely used due to its certain advantages.It occurs at moderate temperatures (165-440 • C), which allow lower operating costs; in addition, gas flows with low oxygen content are required, and the formation of NO x in the combustion process is decreased [16][17][18].Currently, catalysts used for VOC oxidation are usually metal particles or metal oxides-supported catalysts [19,20].Finally, carbon monoxide (CO), which usually comes from the inadequate combustion of carbon-containing substances, is a colorless, odorless, and neurotoxic gas.The CO in the air can cause tissue hypoxia, leading to acute or chronic poisoning and even death [21].The main method for eliminating carbon monoxide is catalytic oxidation.Among all the supported metal catalysts, zeolite-supported metal catalysts display excellent thermal/hydrothermal stability, and the ultrasmall and stable metal species can be dispersed [22].In addition, the catalytic conversion of CO under mild conditions is also an interesting subject [23,24].
Zeolites possess significant potential for the treatment of atmospheric pollutants due to their remarkable adsorption capacity, high stability, and selective catalytic capability.Their three-dimensional structure is composed of interconnected tetrahedral networks of [SiO 4 ] 4− and [AlO 4 ] 5− linked by oxygen atoms [25][26][27].The pore size of zeolites is less than 2 nm [28,29].According to the definition provided by the International Union of Pure and Applied Chemistry (IUPAC), inorganic porous materials with pore sizes smaller than 2 nm are referred to as microporous materials, while those with pore sizes ranging from 2 nm to 50 nm are known as mesoporous materials, and materials with pore sizes exceeding 50 nm are categorized as macroporous materials [30,31].The scaling up of zeolite-based catalysts is challenged mainly by the high cost required for its synthesis, and there have been efforts to reduce costs for zeolite preparation and enable high-value applications, such as using bagasse fly ash [32], palm oil mill fly ash [33], coal fly ash [34], and so on [35].Despite possessing strong acidic sites, high thermal and hydrothermal stability, as well as unique "shape selective catalysis" properties, zeolites face limitations in applications within macromolecular reaction systems due to the narrow channels and high diffusion resistance [36].Introducing a mesoporous structure into zeolites is considered an effective strategy for overcoming this constraint.
Mesoporous zeolites exhibit a large specific surface area and versatile pore structures, facilitating the diffusion and transportation of substances, thus enabling their utilization in macromolecular reactions [37].They can be classified into disordered mesoporous zeolites and ordered mesoporous zeolites based on the level of mesoporous organization.Among these, ordered mesoporous zeolites such as the MCM series (Mobil Composition of Matter), SBA-n series (Santa Barbara USA), and MSU series (Michigan State University) have been extensively investigated due to their well-defined and interconnected mesopores [38,39].Currently, the preparation methods for mesoporous zeolites have reached maturity, and their applications in catalysis have been widely explored.In catalytic conversions involving three types of atmospheric pollutants, mesoporous zeolites are typically loaded with specific metal particles to enhance their catalytic performance.For example, the addition of copper ions can effectively improve the catalytic conversion ability of NO x at low temperatures.In these catalytic reactions, mesoporous zeolites enhance catalytic performance by providing a large specific surface area that promotes the dispersion of metal particles.
This article introduces the preparation and application of mesoporous zeolites.Among them, the article provides a detailed summary of the catalytic ability of mesoporous zeolites in three main types of atmospheric pollutants (NO x , VOCs, and CO).The aim of this article is to elucidate the potential of mesoporous molecular sieves in environmental protection through investigation and research and to provide a theoretical basis for such research.

Synthesis of Mesoporous Zeolites
Since Barrer's pioneering synthesis of artificial zeolites in the 1940s, extensive research has been conducted on zeolite synthesis.From the mid-1950s to the early 1980s, zeolites experienced a pinnacle in scientific investigation, practical application, and industrial advancement.Presently, the Structure Commission of the International Zeolite Association (IZA-SC) has cataloged 255 distinct zeolite framework structures [40].In 1992, Mobil company reported the groundbreaking synthesis of mesoporous zeolites, sparking widespread interest in their design, preparation, and application.As of now, researchers have developed a variety of synthesis methods for mesoporous zeolites, which can be mainly divided into two categories: the post-synthesis method (also known as top-down) and the in situ method (also known as bottom-up) [41][42][43][44].The post-synthesis method involves the removal of silicon or aluminum atoms from the synthesized zeolite to introduce a mesoporous structure.Conversely, the in situ method necessitates the utilization of templates to generate mesopores during the zeolite synthesis process.

Post-Synthesis Method (Top-Down)
The post-synthesis method requires a chemical treatment process, which can be divided into dealumination [45] or desilication [46,47] according to different methods.During this process, a fraction of the zeolite framework may collapse to form mesopores [48,49].Typically, aluminum-rich zeolites are more conducive to dealumination, and the distribution of Si and Al atoms within the zeolite also influences the arrangement of mesopores generated through desilication or dealumination.That is to say, when Al atoms are randomly dispersed within the zeolite, the resulting mesopores tend to be disordered.Conversely, if the Al atoms are uniformly distributed, ordered mesopores are typically obtained [50][51][52].

Dealumination
Dealumination is a process used to remove aluminum from zeolite structures, resulting in the formation of mesopores and modification of the zeolite properties.Acid leaching is a commonly employed method for dealumination, and different types of acids can be used, including inorganic acids such as nitric acid and hydrochloric acid, as well as organic acids like oxalic acid and citric acid.The efficiency of dealumination can vary depending on the type of acid used and the specific zeolite involved.When weak acids with low concentrations of free H + ions are used, they have a limited effect on removing amorphous silicon and aluminum from the surface and pore walls of zeolites.As a result, the decrease in crystallinity, expansion of pore volume, and increase in specific surface area are not significant.However, as the acidity of the leaching solution increases, the dealumination effect becomes more pronounced, leading to greater pore enlargement.Babic et al. conducted in-depth research on the influence of acidity and different commercial zeolites with varying framework topology on mesoporous structure formation.They treated three types of commercial zeolites (CHA: Chabazite, the structure is made of a double 6-ring (D6R) as the basic building, and its large CHA cavity is accessed by six eightring windows measuring around 0.38 nm × 0.38 nm [53]; MFI: (Mobil Five)-type zeolite, which has two types of decametric ring pore channels: a straight pore channel with a pore size of 5.6 × 5.3 Å and a sinusoidal pore channel with a pore size of 5.5 × 5.1 Å [54]; LTL: Linde type L zeolite, composed of composite building units of D6R, CAN and LTL [55]) with chromic acid (H 2 CrO 4 ) solutions at concentrations ranging from 0.1 to 10 w/%.Among the samples, only LTL treated with 10 w/% chromic acid successfully formed mesoporous structures, while the others did not.This suggests that the formation of mesopores through dealumination is specific to certain types of zeolites [56].Another study by Wei et al. explored the effects of organic and inorganic acids on mesoporous Y zeolite synthesis.They successfully synthesized mesoporous Y zeolite by treating commercial Y zeolite with citric acid (C 6 H 8 O 7 ) and ammonium tetrafluoroborate (NH 4 BF 4 ) as agents (Figure 1).The treated Y zeolite exhibited a higher degree of interconnected mesopores compared to the commercial Y zeolite.This is attributed to the penetration and corrosion of the zeolite crystal by the complexes formed by C 6 H 8 O 7 and NH 4 BF 4 .The scale-up test also displayed a significant number of connected mesopores, indicating the suitability of this catalyst for industrial applications [57].
The treated Y zeolite exhibited a higher degree of interconnected mesopores compared to the commercial Y zeolite.This is attributed to the penetration and corrosion of the zeolite crystal by the complexes formed by C6H8O7 and NH4BF4.The scale-up test also displayed a significant number of connected mesopores, indicating the suitability of this catalyst for industrial applications [57].

Desilication
Alkaline desiliconization is the process of alkaline treatment over the synthesized zeolite within the range of 50-80 °C.High-silicon zeolites with a Si/Al ratio greater than 20 usually have better desiliconization effects [58].The alkaline treatment desiliconization process is usually accompanied by a small amount of aluminum leaching, but the aluminum content on the outer surface of zeolite usually leads to uneven silicon dissolution during alkaline treatment [59].Currently, alkaline media used in alkali treatment can be divided into strong bases (NaOH, LiOH, KOH, and CsOH) and weak bases (NaHCO3, Na2CO3, and NH3•H2O).Treating with different alkaline substances will lead to different results [60].It was reported that zeolites treated with alkaline have a lower Si/Al ratio and an increased ion-exchange capacity [61].Nguyen et al. prepared mesoporous ferrierite zeolite (FER) by using NaOH solution as a desilication agent.The results showed that mesoporous structures in the range of 5-40 nm can be formed on the surface of the FER zeolite.Alkaline desilication can effectively introduce interconnecting mesoporous pores.But it also has the disadvantages of easy destruction of zeolite skeleton and difficult adjustment of secondary porosity [62].The mesopore size obtained from the NaOH solution is relatively large, and the pore channel is severely damaged.To avoid this problem, Zhang et al. prepared mesoporous β zeolite via protective desilication using quaternary ammonium hydroxide (TAAOH) and explored the mechanism of protective desilication of TAAOH.The protection of TAA + coexists with the desilication function of OH− and the two act in different stages.As shown in Figure 2, TAA + occupies the entrances of micropores of β zeolite and displays a protection effect.Then, OH − extracts the Si atom in dominance, thereby steadily forming abundant mesopores.Meanwhile, if β zeolite is treated with NaOH first, the added TAA + cannot protect the β zeolite.Moreover, OH − concentration is also an important factor in the formation of mesopores in β zeolite.The mesoporosity of β zeolite can be tuned by adjusting the length of the TAAOH alkyl chain [63].

Desilication
Alkaline desiliconization is the process of alkaline treatment over the synthesized zeolite within the range of 50-80 • C. High-silicon zeolites with a Si/Al ratio greater than 20 usually have better desiliconization effects [58].The alkaline treatment desiliconization process is usually accompanied by a small amount of aluminum leaching, but the aluminum content on the outer surface of zeolite usually leads to uneven silicon dissolution during alkaline treatment [59].Currently, alkaline media used in alkali treatment can be divided into strong bases (NaOH, LiOH, KOH, and CsOH) and weak bases (NaHCO 3 , Na 2 CO 3 , and NH 3 •H 2 O).Treating with different alkaline substances will lead to different results [60].It was reported that zeolites treated with alkaline have a lower Si/Al ratio and an increased ion-exchange capacity [61].Nguyen et al. prepared mesoporous ferrierite zeolite (FER) by using NaOH solution as a desilication agent.The results showed that mesoporous structures in the range of 5-40 nm can be formed on the surface of the FER zeolite.Alkaline desilication can effectively introduce interconnecting mesoporous pores.But it also has the disadvantages of easy destruction of zeolite skeleton and difficult adjustment of secondary porosity [62].The mesopore size obtained from the NaOH solution is relatively large, and the pore channel is severely damaged.To avoid this problem, Zhang et al. prepared mesoporous β zeolite via protective desilication using quaternary ammonium hydroxide (TAAOH) and explored the mechanism of protective desilication of TAAOH.The protection of TAA + coexists with the desilication function of OH− and the two act in different stages.As shown in Figure 2, TAA + occupies the entrances of micropores of β zeolite and displays a protection effect.Then, OH − extracts the Si atom in dominance, thereby steadily forming abundant mesopores.Meanwhile, if β zeolite is treated with NaOH first, the added TAA + cannot protect the β zeolite.Moreover, OH − concentration is also an important factor in the formation of mesopores in β zeolite.The mesoporosity of β zeolite can be tuned by adjusting the length of the TAAOH alkyl chain [63].
Based on the above discussion, the essence of dealumination and desilication is to sacrifice the parts of zeolite crystals for generating mesopores, which will reduce the crystallinity of zeolites and change the original Si/Al ratio and the micropore connectivity, resulting in uncontrollable secondary pores.It is also worth noting that dealumination is usually only applicable to aluminum-rich zeolites, which greatly narrows the application range of forming mesopores through dealumination [64].Based on the above discussion, the essence of dealumination and desilication is to sacrifice the parts of zeolite crystals for generating mesopores, which will reduce the crystallinity of zeolites and change the original Si/Al ratio and the micropore connectivity, resulting in uncontrollable secondary pores.It is also worth noting that dealumination is usually only applicable to aluminum-rich zeolites, which greatly narrows the application range of forming mesopores through dealumination [64].

In Situ Method (Bottom-Up)
The in situ method, also known as the bottom-up method, mainly includes the hard template method and the soft template method, which obtain materials with mesoporous structure by adding template agents in the process of zeolite synthesis [65,66].In general, the template method is one of the ideal ways to synthesize mesoporous zeolites.

Hard Template Method
The hard template method refers to the addition of a hard template during the synthesis of mesoporous zeolites.Although a variety of hard template agents have been developed, the most commonly used material is still carbon.Cho et al. successfully prepared ordered mesoporous MFI zeolites by using mesoporous carbon as a hard template.The mesopores are highly ordered and have a diameter of approximately 10 nm.Moreover, the pore size of the template and skeleton stiffness are also important factors affecting the mesopores during synthesis [67].Abdulridha et al. synthesized mesoporous Y zeolite by using multi-walled carbon nanotubes (CNTs) and nanocrystalline cellulose (NCC) as hard templates, respectively.The micropore and mesopore size distributions of Y zeolites in Figure 3a,b show that all the zeolites prepared with different templates have mesoporous structures.And the nitrogen adsorption-desorption curves also proved that the parent Y zeolite showed a type I curve, while the zeolites prepared by the hard template method showed a typical type IV curve with hysteresis loop, indicating that the mesoporous structure on the Y zeolite was successfully introduced by using the hard template method [68].In addition, White et al. prepared mesoporous ZSM-5 zeolite by using the nitrogendoped carbonaceous monolithic (NDC) as a hard template (Figure 3c).The NDC template was composed of sugar and egg protein precursors.The mesopore diameters of the prepared Meso-ZSM-5 were located at the range of 12-16 nm [69].

In Situ Method (Bottom-Up)
The in situ method, also known as the bottom-up method, mainly includes the hard template method and the soft template method, which obtain materials with mesoporous structure by adding template agents in the process of zeolite synthesis [65,66].In general, the template method is one of the ideal ways to synthesize mesoporous zeolites.

Hard Template Method
The hard template method refers to the addition of a hard template during the synthesis of mesoporous zeolites.Although a variety of hard template agents have been developed, the most commonly used material is still carbon.Cho et al. successfully prepared ordered mesoporous MFI zeolites by using mesoporous carbon as a hard template.The mesopores are highly ordered and have a diameter of approximately 10 nm.Moreover, the pore size of the template and skeleton stiffness are also important factors affecting the mesopores during synthesis [67].Abdulridha et al. synthesized mesoporous Y zeolite by using multi-walled carbon nanotubes (CNTs) and nanocrystalline cellulose (NCC) as hard templates, respectively.The micropore and mesopore size distributions of Y zeolites in Figure 3a,b show that all the zeolites prepared with different templates have mesoporous structures.And the nitrogen adsorption-desorption curves also proved that the parent Y zeolite showed a type I curve, while the zeolites prepared by the hard template method showed a typical type IV curve with hysteresis loop, indicating that the mesoporous structure on the Y zeolite was successfully introduced by using the hard template method [68].In addition, White et al. prepared mesoporous ZSM-5 zeolite by using the nitrogen-doped carbonaceous monolithic (NDC) as a hard template (Figure 3c).The NDC template was composed of sugar and egg protein precursors.The mesopore diameters of the prepared Meso-ZSM-5 were located at the range of 12-16 nm [69].2) biomass-derived NDC monolithic templates [69].Copyright 2014, ACS Publications.

Soft Template Method
The soft template method can be traced back to 1992, when Mobil's Kresge reported that the M41S series mesoporous materials were synthesized using surfactants as templates [70,71].At present, the commonly used soft templates include silyliertes polymer [72], hydrophilic cationic polymer [73], cationic organosilane [74], and surfactant [75].The synthesis method involves the interaction between structure-directing agents and templates within the synthetic gel, leading to the self-assembly and formation of mesopores.Hence, the selection of appropriate structure-directing agents is crucial for the successful synthesis of mesoporous zeolite.Miyake et al. prepared a uniform mesoporous MFI zeolite using poly vinyl alcohol with surface activity (PVA) as a secondary template agent and explored the optimum PVA contents in the starting mixture.MFI zeolite crystals with uniform mesopores (15 nm) can be prepared in the medium PVA content while low PVA content (1.5 wt%) or high PVA content (4.5 wt%) are not.Meanwhile, the diameter of mesopores is approximately consistent with the end-to-end distance of PVA molecules (10.8-14.4nm), indicating that one PVA molecule is incorporated in one mesopore [76].Zhao et al. prepared hierarchical ZSM-5 zeolite by using organic functionalized nano silica and organic silane mesoporous structure directing agents as raw materials through hydrothermal synthesis.The results showed that the crystallinity of MFI zeolite decreases with the increase of silicone content.The nanocrystalline size and surface topography of ZSM-5 are related to organic functionalized silica and organic additives, and the mesoporous volume and pore size of the catalyst are related to the degree of silanization of silica [41].Guo et al. successfully prepared mesoporous silicalite-1 with the mesoporous size range of 5-20 nm using polyacrylic acid (PAA) as a template.The preparation process of mesoporous silicalite-1 is shown in Figure 4a; the positively charged alkylammonium moiety has a strong interaction with the negatively charged PAA, whereas the reaction between the hydrolysable methoxysilyl moiety and growing crystal domains is strong through covalent bonds with silica sources.Therefore, PAA chains introduced into zeolite can form  2) biomass-derived NDC monolithic templates [69].Copyright 2014, ACS Publications.

Soft Template Method
The soft template method can be traced back to 1992, when Mobil's Kresge reported that the M41S series mesoporous materials were synthesized using surfactants as templates [70,71].At present, the commonly used soft templates include silyliertes polymer [72], hydrophilic cationic polymer [73], cationic organosilane [74], and surfactant [75].The synthesis method involves the interaction between structure-directing agents and templates within the synthetic gel, leading to the self-assembly and formation of mesopores.Hence, the selection of appropriate structure-directing agents is crucial for the successful synthesis of mesoporous zeolite.Miyake et al. prepared a uniform mesoporous MFI zeolite using poly vinyl alcohol with surface activity (PVA) as a secondary template agent and explored the optimum PVA contents in the starting mixture.MFI zeolite crystals with uniform mesopores (15 nm) can be prepared in the medium PVA content while low PVA content (1.5 wt%) or high PVA content (4.5 wt%) are not.Meanwhile, the diameter of mesopores is approximately consistent with the end-to-end distance of PVA molecules (10.8-14.4nm), indicating that one PVA molecule is incorporated in one mesopore [76].Zhao et al. prepared hierarchical ZSM-5 zeolite by using organic functionalized nano silica and organic silane mesoporous structure directing agents as raw materials through hydrothermal synthesis.The results showed that the crystallinity of MFI zeolite decreases with the increase of silicone content.The nanocrystalline size and surface topography of ZSM-5 are related to organic functionalized silica and organic additives, and the mesoporous volume and pore size of the catalyst are related to the degree of silanization of silica [41].Guo et al. successfully prepared mesoporous silicalite-1 with the mesoporous size range of 5-20 nm using polyacrylic acid (PAA) as a template.The preparation process of mesoporous silicalite-1 is shown in Figure 4a; the positively charged alkylammonium moiety has a strong interaction with the negatively charged PAA, whereas the reaction between the hydrolysable methoxysilyl moiety and growing crystal domains is strong through covalent bonds with silica sources.Therefore, PAA chains introduced into zeolite can form mesopores.As shown in Figure 4b-e, the mesopores in the zeolite crystal are abundant and uniformly distributed in the range of 5-20 nm [77].In summary, the hard template method can strictly control the size and morphology of nanomaterials without considering the interaction between template agents and inorganic species nor controlling the hydrolysis and condensation of inorganic silicate species.However, the rigid framework exhibits a relatively simplistic structure [78,79].The soft template method can flexibly adjust the pore size of mesopores, but compared to the hard template, the stability of the soft template structure is weaker.

Removing NOx
Nitrogen oxides (NOx) mainly contain nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O).Among them, the content of NO accounts for over 90% of NOx emissions, and NO2 accounts for 0.5-10% [80].The nitrogen oxides in the atmosphere pose significant hazards to the environment and human health.They can form a haze, acid rain, and photochemical smog and cause bronchitis, skin aging, and lung cancer.The main sources of NOx vary in different countries.For example, mobile sources account for 57.5% of NOx emissions in the US, while stationary sources account for 71% of NOx emissions in China [81].To solve NOx emissions, many regulations and policies have been implemented by governments worldwide.Mainly, three ways can be used in the removal of NOx: source control, process control, and end-of-pipe control.Among them, selective In summary, the hard template method can strictly control the size and morphology of nanomaterials without considering the interaction between template agents and inorganic species nor controlling the hydrolysis and condensation of inorganic silicate species.However, the rigid framework exhibits a relatively simplistic structure [78,79].The soft template method can flexibly adjust the pore size of mesopores, but compared to the hard template, the stability of the soft template structure is weaker.

Removing NO x
Nitrogen oxides (NO x ) mainly contain nitric oxide (NO), nitrogen dioxide (NO 2 ), and nitrous oxide (N 2 O).Among them, the content of NO accounts for over 90% of NO x emissions, and NO 2 accounts for 0.5-10% [80].The nitrogen oxides in the atmosphere pose significant hazards to the environment and human health.They can form a haze, acid rain, and photochemical smog and cause bronchitis, skin aging, and lung cancer.The main sources of NO x vary in different countries.For example, mobile sources account for 57.5% of NO x emissions in the US, while stationary sources account for 71% of NO x emissions in China [81].To solve NO x emissions, many regulations and policies have been implemented by governments worldwide.Mainly, three ways can be used in the removal of NO x : source control, process control, and end-of-pipe control.Among them, selective catalytic reduction (SCR), which is end-of-pipe control, was widely used in the industry for removing NO x due to its high denitration efficiency, safe operation, and no secondary pollution [82][83][84][85].One important application in industry is the marine SCR system for ocean-going vessels.For this system, its operating costs are mainly affected by operating time, NO x inlet concentration, and emission limits [86].The preparation and application of catalysts are crucial for SCR.The catalysts used in SCR usually require high activity and selectivity toward N 2 and high stability in the presence of SO 2 and H 2 O.The traditional metal catalysts used in NH 3 -SCR (such as V 2 O 5 , manganese, and cerium) show some shortcomings, such as ammonia slip and unstable SO 2 poisoning in low temperature.Meanwhile, the zeolite catalysts containing Cu can be used in low-temperature window operation without ammonia slip and are more stable in the presence of SO 2 .Further, zeolite catalysts containing Fe are more suitable for high-temperature window operation [87][88][89].
At present, Cu-SSZ-13 zeolite catalysts with small pore chabazite (CHA) structures have been widely studied for NH 3 -SCR reaction due to their ability to resist hydrocarbon poisoning and hydrothermal deactivation [90][91][92].However, the small pore structure can restrict the transport of reactants to the active site, resulting in a decrease in reaction efficiency.Currently, one of the effective solutions for resolving diffusion limitations is to introduce mesopores into the zeolite framework [93,94].Oord et al. introduced mesopores with a diameter of 2-10 nm into the zeolite using alkaline treatment (NaOH).The results showed that the catalytic activity of Cu-based zeolite catalyst with mesoporous structures was higher than that without mesoporous structures.Specifically, the conversion of NO over catalysts with mesoporous structures is 100% in the temperature range of 250-450 • C, and the presence of mesopores improves its performance at low temperatures (i.e., at 150 • C and 250 • C) by two times.It is worth noting that the formation of a mesopore is accompanied by a loss in the microporous surface area.Therefore, when the concentration of alkalinity is high, the crystallinity of the zeolite will be decreased.In addition, Oord et al. also found that mesopores are mainly formed outside the SSZ-13 zeolite and gradually corrode towards the interior of the SSZ-13 zeolite when increasing the amount of NaOH [95].To deeply enhance activity, hydrothermal stability and propene poisoning resistance of zeolite should also be studied.Zhang et al. prepared mesoporous SSZ-13 and core-shell meso-SSZ-13@MAS (MAS: mesoporous aluminosilicate materials) via alkaline desilication and self-assembled methods (Figure 5a).The introduction of a mesoporous structure makes it easier to obtain more active Cu sites and effectively reduces the pore diffusion restriction.Meanwhile, the mesoporous zeolite catalyst shows higher SCR activity than that of traditional Cu-SSZ-13 over the whole temperature range (Figure 5b), which can be explained as follows: Cu ion exchange only occurs in the outer parts of traditional Cu-SSZ-13, and a part of the Cu ion could migrate and aggregate into CuO species on the external surface of SSZ-13.After the introduction of mesoporous structures, more ion-exchange sites in the internal parts of the SSZ-13 crystal can be exposed, and more active Cu sites can be obtained.The diffusion path length inside the SSZ-13 zeolite becomes shorter, and the diffusion resistance can be reduced remarkably [96].Liang et al. prepared Cu-SSZ-13-Meso by using carbon black as a hard template (Figure 5c).In the NH 3 -SCR reaction, Cu-SSZ-13-Meso shows excellent low-temperature activity, and the existence of mesopores can effectively solve blockage of the micropores due to NH 4 NO 3 or (NH 4 ) 2 SO 4 [97].
In addition, ZSM-5 is also one of the early catalysts used for denitration research.Vennestrøm et al. synthesized mesoporous H-ZSM-5 by using sodium hydroxide, guanidinium hydroxide, and guanidinium carbonate, finding that all base-treated samples lead to longer catalyst lifetimes in the methanol-to-gasoline reaction.Compared to conventional zeolites, mesoporous zeolites are able to accommodate a higher amount of dispersed Fe species, which act as active species.The prepared sample (FeZ(0.3)NaOH(1.0))possesses the highest degree of mesoporosity and shows the highest catalytic activity [98].Similarly, Rutkowska et al. investigated the influence of mesopores on the catalytic performance of ZSM-5 commercial zeolite.The ZSM-5 zeolite catalyst with mesoporous structure was obtained by adjusting different ratios of NaOH and TPAOH, as well as different operating times.It was found that the enhancement of catalytic performance is closely related to the generation of mesopores [99].Peng et al. prepared a Cu-ZSM-5 catalyst with a micromesoporous structure using a one-pot hydrothermal synthesis method and applied it to NH 3 -SCR.Compared with traditional Cu-ZSM-5 catalysts, the mesoporous Cu-ZSM-5 catalysts showed better catalytic performance, higher anti-SO 2 toxicity activity, and higher hydrothermal stability in the range of 150-350 • C, which were attributed to the fact that a mesoporous structure can provide a larger specific surface area, as well as improve the mass transfer of reactants and NO-adsorption capacity [100].In addition, ZSM-5 is also one of the early catalysts used for denitration research.Vennestrøm et al. synthesized mesoporous H-ZSM-5 by using sodium hydroxide, guanidinium hydroxide, and guanidinium carbonate, finding that all base-treated samples lead to longer catalyst lifetimes in the methanol-to-gasoline reaction.Compared to conventional zeolites, mesoporous zeolites are able to accommodate a higher amount of dispersed Fe species, which act as active species.The prepared sample (FeZ(0.3)NaOH(1.0))possesses the highest degree of mesoporosity and shows the highest catalytic activity [98].Similarly, Rutkowska et al. investigated the influence of mesopores on the catalytic performance of ZSM-5 commercial zeolite.The ZSM-5 zeolite catalyst with mesoporous structure was obtained by adjusting different ratios of NaOH and TPAOH, as well as different operating times.It was found that the enhancement of catalytic performance is closely related to the generation of mesopores [99].Peng et al. prepared a Cu-ZSM-5 catalyst with a micro-mesoporous structure using a one-pot hydrothermal synthesis method and applied it to NH3-SCR.Compared with traditional Cu-ZSM-5 catalysts, the mesoporous Cu-ZSM-5 catalysts showed better catalytic performance, higher anti-SO2 toxicity activity, and higher hydrothermal stability in the range of 150-350 °C, which were attributed to the fact that a mesoporous structure can provide a larger specific surface area, as well as improve the mass transfer of reactants and NO-adsorption capacity [100].

Removing of VOCs
VOCs are recognized as a type of harmful substance released into the air.The World Health Organization (WHO) has a clear definition of VOCs, which refer to various organic compounds with boiling points between 50 • C and 260 • C, mainly including aliphatic hydrocarbons, halogenated hydrocarbons, aromatic hydrocarbons, alcohols, aldehydes, esters, ethers, ketones, carboxylic acids, amines, and sulfur-containing organic compounds [101].Some VOCs can pass through the troposphere to the stratosphere through photolysis, physical adsorption, and other processes and react with atmospheric ozone, resulting in the destruction of the ozone layer.Under high temperatures or ultraviolet irradiation, the active free radicals from VOC cracking can participate in a series of chemical reactions and trigger a chain reaction, leading to the occurrence of ozone, photochemical smog, the greenhouse effect, and other phenomena.Among them, benzene can have a photochemical reaction with NO x under the action of light.When benzene coexists with NO x and SOx, the dry fallout becomes photochemical smoke, and the wet fallout turns into acid rain, causing great damage to the environment [102][103][104].Moreover, VOCs can also harm human health: they can damage the eyeball, nasal cavity, and throat; cause memory and visual impairment; and even pose a risk of cancer.Meanwhile, VOCs can indirectly harm the human body as a viral carrier, which promotes the formation of secondary organic aerosol (SOA) and provides a carrier for bacteria and viruses [105][106][107][108].At present, the removal of VOCs can be divided into two methods: the physical method and the chemical method.The common physical methods are absorption, condensation, and membrane separation.Physical methods, which usually enrich and separate VOCs by changing the physical conditions (such as pressure and temperature), are suitable for high concentrations (>5000 mg/m 3 ) of VOCs.Chemical methods, mainly including the incineration method, biological method, and catalytic oxidation method, can convert VOCs into CO 2 , non-toxic or less-toxic inorganic substances, which are suitable for the treatment of medium-and low-concentration (<1000 mg/m 3 ) VOCs.
Toluene is a type of VOCs.The treatment of toluene has attracted extensive attention because of its toxicity and damage to the environment.Kim et al. found that the adsorption properties of zeolite for toluene and methylethylketone (MEK) were closely related to the pore structure.The researchers removed toluene and MEK via the adsorption of mordenite and X-or Y-type FAU zeolite at 25 • C and investigated the relationship between the adsorption and desorption behavior of selected volatile organic compounds and the physicochemical properties of zeolite (acidity, Si/Al ratio, crystal structure, pore structure, specific surface area, and pore volume).The results showed that the pore volume has an important effect on the reaction.Among the selected zeolites, the commercial Y-zeolite shows the largest adsorption capacity due to its large mesoporous volume [109].Lv et al. synthesized a Pd/HMAS-1 catalyst (HMAS: hollow mesoporous aluminosilicate sphere) by loading Pd onto a hollow mesoporous silica-aluminum sphere for toluene combustion.The result indicated that small-sized Pd nanoparticles existed in the ordered mesoporous pores of HMAS.The presence of mesopores provided a large specific surface area and improved the mass transfer efficiency, which made the catalyst exhibit excellent toluene oxidation performance at 150 • C.Moreover, Pd/HMAS-1 has excellent stability and can maintain 100% conversion efficiency at 200 • C for 20 h [110].Li et al. investigated the adsorption and desorption properties of different industrial adsorbents (A zeolites, ZSM-5 zeolites, Y zeolites, X zeolites, and MCM-41 mesoporous silica) on VOCs before and after alkali modification.The p-xylene adsorption capacity of mesoporous USY zeolite after alkaline modification is improved due to the increase in mass transfer.These zeolites can also be applied to other volatile organic compounds, including benzene, chlorobenzene, and butyl acetate [111].In addition, Khawaja et al. synthesized mesoporous NaY zeolitesupported palladium (Pd) or platinum (Pt) via three steps of dealuminization, desilication, and alkaline treatment.The prepared catalysts were applied in the oxidation of ethanol and toluene.According to the characterization and test results, the introduction of mesopores enabled the partial presence of catalytically active Pt 0 species, and the temperature for oxidation of 90% toluene was 130 • C over meso-Pt/NaY, which is lower than that (151 • C) of Pt/NaY [112].Meanwhile, Zhao et al. synthesized ZSM-5 with controllable intercrystalline and intracrystalline mesopores using the hydrothermal method and studied the catalytic performance of catalysts in toluene oxidation.Mesoporous ZSM-5 materials have excellent cycling properties due to their low spatial confinement and high diffusion rate for organic molecules, especially macromolecules [41].In addition, Beta zeolites can also be used in the catalytic oxidation of toluene; for example, Chen et al. prepared mesoporous Beta zeolite (Beta-H) for the catalytic combustion of toluene.It was found that the Pt particle sizes are smaller and the Pt 0 /Pt 2+ ratio is higher on mesoporous Pt-R/Beta-H than those on the traditional zeolite, which means that the mesoporous structures are favorable for the catalytically active Pt 0 species loaded.As shown in Figure 6a,b, the catalytic activity and selectivity for toluene combustion of Beta-H zeolite was higher than that of Beta zeolite, which was related to the higher amount of Pt 0 [113].
olites can also be used in the catalytic oxidation of toluene; for example, Chen et al. prepared mesoporous Beta zeolite (Beta-H) for the catalytic combustion of toluene.It was found that the Pt particle sizes are smaller and the Pt 0 /Pt 2+ ratio is higher on mesoporous Pt-R/Beta-H than those on the traditional zeolite, which means that the mesoporous structures are favorable for the catalytically active Pt 0 species loaded.As shown in Figure 6a,b, the catalytic activity and selectivity for toluene combustion of Beta-H zeolite was higher than that of Beta zeolite, which was related to the higher amount of Pt 0 [113].In addition, researchers have also carried out research on other types of pollutants in VOCs, such as light alkane, which is a typical volatile organic compound with strong carbon-hydrogen bonds and is not easily degraded into CO2 and H2O.At present, the most effective method for the removal of light alkanes is deep oxidation, but the small pore size of traditional zeolites limits the diffusion of molecules and the accessibility of metal active sites.Therefore, zeolite application in the oxidation of light alkanes is limited.In order to improve the mass transfer efficiency and the accessibility of the active site, the introduction of a mesopore is important.Peng et al. prepared Pd@IM-S-1 material with abundant mesopores.Compared with the commonly supported catalyst (Pd/S-1), the mesoporous Pd@IM-S-1 material showed excellent deep oxidation activity of methane and propane [114].Similarly, Park et al. synthesized a mesoporous MFI zeolite-supported Pt3Y catalyst by using diammonium-type bromide surfactant [C18H37-N(CH3)2-C6H12-N(CH3)2-C4H9Br2]) and sodium-free silica source.The results indicated that more active sites existed in mesopores, which contributed to high catalytic durability, high propane conversion, and high propylene selectivity [115].

Removal of CO
Carbon monoxide (CO) is a common toxic gas that poses a threat to the environment and human health.The natural concentration of CO in air is approximately 0.2 parts per million (ppm), which is harmless to humans.But when the concentration of carbon monoxide is higher, it combines with hemoglobin present in blood cells and converts carboxyhemoglobin (CoHb), then reduces the oxygen-carrying capacity of the human body [116].Carbon monoxide poisoning can cause headaches, fatigue, seizures, coma, nausea, vomiting, and even death [21].Carbon monoxide is also an atmospheric pollutant, which In addition, researchers have also carried out research on other types of pollutants in VOCs, such as light alkane, which is a typical volatile organic compound with strong carbon-hydrogen bonds and is not easily degraded into CO 2 and H 2 O.At present, the most effective method for the removal of light alkanes is deep oxidation, but the small pore size of traditional zeolites limits the diffusion of molecules and the accessibility of metal active sites.Therefore, zeolite application in the oxidation of light alkanes is limited.In order to improve the mass transfer efficiency and the accessibility of the active site, the introduction of a mesopore is important.Peng et al. prepared Pd@IM-S-1 material with abundant mesopores.Compared with the commonly supported catalyst (Pd/S-1), the mesoporous Pd@IM-S-1 material showed excellent deep oxidation activity of methane and propane [114]

Removal of CO
Carbon monoxide (CO) is a common toxic gas that poses a threat to the environment and human health.The natural concentration of CO in air is approximately 0.2 parts per million (ppm), which is harmless to humans.But when the concentration of carbon monoxide is higher, it combines with hemoglobin present in blood cells and converts carboxyhemoglobin (CoHb), then reduces the oxygen-carrying capacity of the human body [116].Carbon monoxide poisoning can cause headaches, fatigue, seizures, coma, nausea, vomiting, and even death [21].Carbon monoxide is also an atmospheric pollutant, which is one of the direct factors contributing to global warming.Meanwhile, it can react with hydroxyl (OH) radicals present in the environment, thereby reducing their abundance [117,118].In addition, the emissions of carbon monoxide are very extensive, mainly divided into natural sources and human sources.Human sources, including residential, industrial, and transportation, are the main factors causing air pollution [119,120].Therefore, effective treatment and conversion of carbon monoxide is crucial [121].Catalytic oxidation is considered to be the best solution to remove CO; many types of catalysts for CO oxidation have been studied, such as catalysts loaded with noble metal particles and non-noble metal particles [122].The complete conversion temperature of catalysts loaded with non-noble metal particles for CO conversion is usually higher than 150 • C, and their catalytic activity significantly decreases under harsh conditions, which limits their application.Compared to the catalysts loaded with non-noble metal particles, the catalysts loaded with noble metal particles exhibit relatively high catalytic activity and stability for CO oxidation, but the price is relatively high for industrial applications.Among them, catalysts loaded with palladium are commonly studied, but they still face the challenge of water resistance and sulfur resistance [123].
To improve the water resistance of catalysts containing Pt, Chen's group successfully prepared mesoporous zeolite (Pt/Fe-mBeta) catalysts with different iron contents using a one-pot hydrothermal method and ion-exchange method, active species Pt and Fe were highly dispersed on mesoporous zeolites.Among all the samples, the optimized sample Pt/Fe(3)-mBeta exhibits the best CO oxidation catalytic activity, and the complete oxidation temperature can be reduced to 90 • C, which is due to the introduction of mesopores increasing the specific surface area of the catalyst and providing abundant active sites, and the larger mesoporous pore volume is conducive to adsorption and diffusion of reaction gas.In addition, the possible catalytic reaction mechanisms under different conditions with/without the presence of H 2 O for CO oxidation on mesoporous zeolite (Pt/Fe-mBeta) have also been proposed.In the presence of CO and O 2 , Fe 3+ species can obtain electrons from Pt 0 ; then, CO and O 2 participate in the reaction and finally produce CO 2 .Another reaction route happened when Pt 2+ species obtained electrons from Fe 2+ ; after a similar activation process, CO 2 can be produced.When H 2 O is introduced, it can be activated into O - and/or OH groups and then participate in the reaction (Figure 7a) [123].Moreover, in order to improve sulfur resistance, Chen's group also designed and synthesized mesoporous zeolites Pt/m-Beta@TiO 2 -x with a core-shell structure via a facile photochemical reduction route.The obtained Pt/m-Beta@TiO 2 catalysts possess rich oxygen vacancy defects, which are important in catalysis due to their unique properties and remarkable redox performance.Meanwhile, mesoporous zeolites provide a large specific surface area and abundant active sites, which provide a site for the high dispersion of Pt and show excellent catalytic activity in CO oxidation.Among them, the best sample, Pt/m-Beta@TiO 2 -3, showed higher CO catalytic oxidation activity and sulfur resistance.The complete conversion of CO of Pt/m-Beta@TiO 2 -3 can be reduced to 90 • C (Figure 7b), and the temperature for CO complete conversion is as low as 105 • C in the presence of both H 2 O and SO 2 (Figure 7c).Furthermore, after three cycles, Pt/m-Beta@TiO 2 -3 still exhibits high CO catalytic activity, which demonstrates its high stability [124].In addition, carbon monoxide can also be used to produce various organic derivatives through methanation for the synthesis of various organic products.Triwahyono's group has conducted research on the use of mesoporous ZSM-5 zeolites for carbon monoxide methanation.They prepared mesoporous ZSM-5 (mZSM5) via a dual-templating method, and the prepared catalyst possesses both acidic and basic sites, which are beneficial for improving catalytic ability (Figure 8a).The catalytic performance of the CO methanation reaction is greatly influenced by both porosity and basicity.Among all the samples, the In addition, carbon monoxide can also be used to produce various organic derivatives through methanation for the synthesis of various organic products.Triwahyono's group has conducted research on the use of mesoporous ZSM-5 zeolites for carbon monoxide methanation.They prepared mesoporous ZSM-5 (mZSM5) via a dual-templating method, and the prepared catalyst possesses both acidic and basic sites, which are beneficial for improving catalytic ability (Figure 8a).The catalytic performance of the CO methanation reaction is greatly influenced by both porosity and basicity.Among all the samples, the optimal mZSM5-0.5Dexhibited the highest CO methanation rate (0.0226 µmol/m 2 s) at 723 K [125].Furthermore, the core-shell structure mesoporous ZSM-5 (FmZSM-5) was prepared to enhance the CO conversion rate (0.0708 µmol-CO/m 2 s) and the CH 4 formation rate (0.0488 µmol-CH 4 /m 2 s) at 723 K (Figure 8b).Similarly, Teh et al. also illustrated that the presence of mesopores provides a large specific surface area and pore volume, as well as sufficient base active sites [126].In addition, mesoporous ZSM-5 containing metal particles was also explored.The excellent catalytic performance of nickel-promoted mesoporous ZSM5 (Ni/mZSM5) is attributed to the presence of mesopores and basicity, which lead to the synergistic effect of Ni metal active sites and mZSM5 carriers.Two possible CO methanation mechanisms are proposed.The first mechanism is the main pathway, in which adrobed CO could react with H 2 to form CH 4 and H 2 O (Figure 8c) [127].

Conclusions
The ecological environment is closely related to human life.However, with the progress of human production and technological civilization, the ecological environment has been destroyed, and both human life and health have been greatly threatened.How to solve air pollution effectively is an important subject.Mesoporous zeolite catalysts have attracted much attention because of their advantages, such as large specific surface area, easy adjustment of pore size, and strong acidic site.At present, the preparation methods of mesoporous zeolites have been widely studied.The preparation methods of mesoporous zeolites, including the post-treatment method (dealumination and desilication) and in situ method (the hard template method and soft template method), are described in detail in this paper.Moreover, the applications of mesoporous zeolite in nitrogen oxides, VOCs, and CO are also illustrated.The presence of mesopores in zeolites provides a large specific surface area and pore volume, making them better for dispersing active species.The presence of mesopores also facilitates the diffusion of reactants and increases adsorption capacity.Thus, the mesopores in zeolites significantly improved the catalytic and adsorption ability when compared with traditional zeolites.
Although mesoporous zeolites have been widely studied in environmental purification, some problems still limit the industrial application.With the development of science and technology, research methods and technical means are constantly updated, and the

Conclusions
The ecological environment is closely related to human life.However, with the progress of human production and technological civilization, the ecological environment has been destroyed, and both human life and health have been greatly threatened.How to solve air pollution effectively is an important subject.Mesoporous zeolite catalysts have attracted much attention because of their advantages, such as large specific surface area, easy adjustment of pore size, and strong acidic site.At present, the preparation methods of mesoporous zeolites have been widely studied.The preparation methods of mesoporous zeolites, including the post-treatment method (dealumination and desilication) and in situ method (the hard template method and soft template method), are described in detail in this paper.Moreover, the applications of mesoporous zeolite in nitrogen oxides, VOCs, and CO are also illustrated.The presence of mesopores in zeolites provides a large specific surface area and pore volume, making them better for dispersing active species.The presence of mesopores also facilitates the diffusion of reactants and increases adsorption capacity.Thus, the mesopores in zeolites significantly improved the catalytic and adsorption ability when compared with traditional zeolites.
Although mesoporous zeolites have been widely studied in environmental purification, some problems still limit the industrial application.With the development of science and technology, research methods and technical means are constantly updated, and the development of more low-cost, high-stability, and high-activity catalysts will contribute to shortening the distance between basic research and practical application.By changing the raw materials used for synthesizing zeolites, the cost of synthesizing zeolites can be effectively reduced.In short, mesoporous zeolites with excellent performance can be used in the treatment of air pollutants, which is of great significance for the governance of the ecological environment and the protection of human health.

Catalysts 2024 ,
14, x FOR PEER REVIEW 7 of 19mesopores.As shown in Figure4b-e, the mesopores in the zeolite crystal are abundant and uniformly distributed in the range of 5-20 nm[77].

Figure 6 .
Figure 6.(a) Catalytic combustion of toluene over Pt-R/Beta-H, Pt-O/Beta-H, Pt-R/Beta, and Pt-O/Beta catalysts; (b) Dependences of activities on reaction time in the catalytic combustion of toluene over Pt-R/Beta-H and Pt-R/Beta catalysts [113].Copyright 2013, Elsevier B.V.

Figure 6 .
Figure 6.(a) Catalytic combustion of toluene over Pt-R/Beta-H, Pt-O/Beta-H, Pt-R/Beta, and Pt-O/Beta catalysts; (b) Dependences of activities on reaction time in the catalytic combustion of toluene over Pt-R/Beta-H and Pt-R/Beta catalysts [113].Copyright 2013, Elsevier B.V.
. Similarly, Park et al. synthesized a mesoporous MFI zeolite-supported Pt3Y catalyst by using diammonium-type bromide surfactant [C 18 H 37 -N(CH 3 ) 2 -C 6 H 12 -N(CH 3 ) 2 -C 4 H 9 Br 2 ] and sodium-free silica source.The results indicated that more active sites existed in mesopores, which contributed to high catalytic durability, high propane conversion, and high propylene selectivity [115].

Figure 7 .
Figure 7. (a) The possible catalytic pathways of CO oxidation over the catalyst Pt/Fe(3)-mBeta under different conditions [123].Copyright 2019, pubs.rsc.org;(b) CO conversion vs. T profile of different catalyst samples in dry reaction atmosphere; (c) The effect of H 2 O and SO 2 on the catalytic activity (CO conversion vs. T profile) of the Pt/m-Beta@TiO 2 -3 and Pt/m-Beta samples [124].Copyright 2013, Elsevier B.V.