Progress in Adsorptive Removal of Volatile Organic Compounds by Zeolites

Volatile Organic Compounds (VOCs) are a class of pollutants that have recently received much attention in atmospheric and indoor air pollution problems. They are precursors for PM 2.5 and ozone generation. The removal of VOCs emitted from exhaust gases is an urgent problem to solve air pollution. Adsorption is one of the most promising VOC abatement technologies, with the advantages of high purification efficiency, low cost, and simple equipment. The adsorbent plays a critical role in VOCs removal efficiency. Zeolite is a rapidly developing material due to its highly ordered and flexible microporous structure, good stability, and abundant surface modification. In this paper, the influence of zeolite properties, including framework structure, pore properties, and surface cations, on VOCs adsorption performance is analyzed. The VOCs adsorption performance on different zeolite adsorbents in the presence of water vapor is compared. The influential factors on the mass transfer kinetic properties of VOCs adsorption are summarized. Finally, an overview of zeolite honeycomb adsorbent applications for industrial use is presented, including multi-tower fixed bed adsorption and zeolite rotor adsorption.


INTRODUCTION
Volatile organic compounds (VOCs) are organic compounds that are readily volatile in indoor environments (Gelles et al., 2020).VOCs are a major component of air pollution and have a boiling point between 50 and 260°C at 133.322 Pa (Li et al., 2009).VOCs sources include natural and anthropogenic sources, with natural sources referring to natural plant emissions.Anthropogenic VOCs emissions mainly include spraying, printing, electronics, furniture, rubber, chemical, and incomplete combustion of exhaust from automobiles and other vehicles.VOCs can undergo atmospheric chemical reactions under sunlight, generating ozone and organic aerosols, exacerbating regional haze, and even causing photochemical smog (Zhang et al., 2016).Many VOCs are highly toxic, carcinogenic, and hazardous to respiratory health.With accelerated industrialization and rapid global economic growth, the emission of VOCs has increased dramatically in recent years, and countries worldwide have set increasingly stringent emission standards.For example, China's 14 th Five-Year Plan and the outline of the 2035 Vision proposed to strengthen the synergistic control of multiple pollutants and regional collaborative management, and accelerate VOCs emissions comprehensive improvement, by 2025, the total amount of VOCs emissions decreased

BASIC PHYSICAL AND CHEMICAL PROPERTIES OF VOCS
VOCs can be broadly classified into alkanes, aromatic hydrocarbons, alkenes, halogenated hydrocarbons, esters, aldehydes, and ketones according to their chemical structures.Since the structure and physicochemical properties of VOCs vary, and some physicochemical properties of VOCs directly affect the adsorption properties of VOCs on the adsorbent.Usually, the pore size of zeolite molecular sieve is between 0.3 nm and 1 nm micropores, so VOCs may have the problem of not being able to enter the pores of the adsorbent and cannot be adsorbed.For example, the size of ethanol is only 0.45 nm, while the size of NMP, a VOCs produced during the production of lithium batteries, is 0.76 nm.If 5A zeolite with a pore size of 0.5 nm is used as the adsorbent, NMP cannot enter the adsorbent pore channel for adsorption.In contrast, ethanol can penetrate the pore channel of the 5A zeolite molecular sieve for adsorption due to its small molecules.The boiling point is an important parameter affecting VOCs' adsorption performance.The size and structure of VOCs molecules are different, and the boiling point of VOCs varies.Generally, the higher the boiling point, the stronger the adsorption capacity on the adsorbent.However, the higher the boiling point, the harder it is for VOCs molecules to desorb and the higher the energy consumption required in the VOCs desorption process.Temperature-programmed desorption (TPD) experimental study pointed out that the desorption peak temperature of toluene with a boiling point of 110.4°C is higher than that of MEK with a boiling point of 79.6°C (Kim et al., 2006).Also, both the dipole moment and polarization of molecules affect the adsorption capacity parameters of the adsorbent.The higher the value of the dipole moment and polarization, the stronger the adsorption force of molecules on a zeolite molecular sieve (Epiepang et al., 2016).
The basic properties of typical VOCs are given in Table 1.

Adsorbent Structure and Effect on Adsorption Capacity
Zeolites are a class of crystalline silica-aluminate adsorbents with uniform micropores, mainly composed of silicon, aluminum, oxygen, and some other metal cations, whose chemical formula can be expressed as Mx/m[(AlO2)x-(SiO2)y]-zH2O.Due to the large adsorption capacity, high mass transfer rate, good hydrophobicity, easy regeneration, good thermal stability, and hydrothermal stability of zeolite molecular sieves, it has become an ideal adsorbent for VOCs adsorption and purification (Yang et al., 2019a).
Zeolites are skeletal topologies formed by TO4 (T for Si, Al, Ti, Sn, etc.) oxygen tetrahedral units connected in a specific way (Mekki and Boukoussa, 2019).Different combinations of zeolite molecular sieve skeleton topologies can form different zeolite molecular sieves.As of 2016, the International Zeolite Association-Structure Commission (IZA-SC) has published 231 skeleton topologies of Zeolites (Ilić and Wettstein, 2017).Usually, zeolite forms 6-, 8-, 10-, 12-, and 14membered ring pore channels through the combination of oxygen tetrahedra, and the pore sizes formed are generally 0.35-0.9nm.Typical zeolites that can be used for VOCs include FAU, MFI, DDR, MOR, LTA, BEA, CHA, and many others, as shown in Fig. 1.
The pore sizes of different zeolite vary, and some VOCs molecules cannot enter the adsorbent pore channels to achieve adsorption on zeolite due to their large diameters.Table 2 gives the pore matching of several common VOCs gases adsorbed on zeolite molecular sieves.
FAU-type zeolites consist of cubic octahedra called sodalite cages or β-cages.β-cages linked together by hexagonal prisms form 13 Å diameter super cages linked by 12 oxygen atom apertures with a pore size of 7.4 Å (Brodu et al., 2015).Due to the large size of the molecular sieve pores, most common VOCs can enter the pores for adsorption.The BEA consists of two mutually perpendicular through channels, each containing a cross-section and a sinusoidal channel with dimensions of 6.6 × 6.7 Å and 5.6 × 5.6 Å, respectively, forming three-dimensional intersecting cavities with sizes between 12 and 13 Å.The MOR contains two types of channels, including the 12-membered ring channel, which has a 6.5 × 7.0 Å elliptical cross-section, interconnected in one direction by an 8-element ring channel with a diameter of 2.6 × 5.7 Å (Ektefa et al., 2022).The MFI consists The suitable molecular sieve structure and pore size play an essential role in the adsorption of VOCs molecules.Brodu et al. (2015) investigated the adsorption performance of three types of zeolites, FAU, MFI, and BEA, on toluene (5.9 Å) and found that FAU with larger pore size (7.4Å) and "supercage" structure had 2.3 times and 3.0 times the adsorption capacity of MOR and MFI, respectively.Baek et al. (2004) compared the dynamic adsorption of toluene at 1000 ppm on Y, Beta and MOR zeolites and showed that the adsorption capacity of Y > Beta > MOR for toluene and Y zeolite has a faster adsorption speed and lower desorption temperature.Cosseron et al. (2013) synthesized four pure silica molecular sieves, CHA, SSZ-23 (STT), Silicalite-1 (MFI), and Beta (BEA), and investigated their adsorption capacities for n-hexane, p-xylene, and acetone.Among them, CHA (3.8 Å) has a significantly weaker capacity than the remaining three for the adsorption of VOCs due to its small pore size, and the adsorption capacity of the molecular sieve for VOCs molecules gradually decreases as the molecular size of VOCs increases.Calero and Gomez-Alvarez (2015) investigated the adsorption properties of MFI and FAU type zeolites for small organic molecules such as propane, butane and propylene using molecular simulation calculations and found that due to size effect, the FAU with large pore size possesses a greater adsorption capacity for these small VOCs molecules, while the channeled MFI zeolite has a faster adsorption rate due to the similarity of the molecular sieve pore size to that of small VOCs.Deng et al. (2020) found that the adsorption forces of four different zeolite for toluene were in the order of NaX > USY > Beta > Silicate-1, while for dichloromethane the adsorption forces were in the order of NaX > USY > Beta > Silicate-1 > USY by using the first nature calculation principle.Kang et al. (2018) found that FAU zeolite has strong interaction with dichloromethane molecules mainly through H atoms at the pore windows, while for high-silica MFI zeolite molecular sieve, dichloromethane molecules can only interact with the pore walls.Therefore, the adsorption force of dichloromethane with FAU zeolite is greater than that of MFI.
The silica-alumina ratio also has a significant influence on the selective adsorption performance of zeolites.In a study by Megías-Sayago et al. (2020), the adsorption of ZSM-5 on toluene gradually increased as the silica-alumina ratio decreased because the most important parameters affecting the adsorption of BTEX on ZSM-5 zeolites have been shown to be the number of available acid sites (Al content), hydrophobicity and zeolite pore size, and as the silica-alumina ratio decreased, the available acid sites increased and the adsorption of toluene on ZSM-5 increased.In contrast, the silica-alumina ratio does not change much for xylene adsorption since its zeolite adsorption mainly depends on the skeletal structure.
Conventional zeolite adsorbents have limited adsorption capacity due to their small pore capacity.Hierarchically porous zeolite was proven to be a better VOCs adsorption material.
The resultant hierarchically porous structure was beneficial for improving diffusion performance and mass transport efficiency.Yu et al. (2015aYu et al. ( , 2015b) ) use diatomite (Dt) and MFI prepared hierarchically porous composites Dt/MFI-type zeolite shows higher benzene adsorption capacity of 62.5 mg g -1 at 25°C in comparison with Dt.Yuan et al. (2016) also synthesized Dt/silicalite composite for benzene adsorption.The results showed that the composite exhibited considerably high benzene adsorption capacity (246.0 mg g -1 ) at 25°C, which was much higher than that of Dt or silicalite.Li et al. (2019) prepared ZSM-5/MCM-41 and ZSM-5/Silicalite-1 hierarchical composites by hydrothermal method, Under 50% humidity conditions, the breakthrough time of ZSM-5/MCM-41-75% and ZSM-5/Silicalite-1-75% was 1.6 times and 1.2 times longer than that of pure ZSM-5, respectively.Hu et al. (2009) synthesized Silicalite-1/SBA-15 hierarchical composites.The hierarchical composites adsorbent has an adsorption capacity of 0.794 mmol g -1 for benzene, lower than SBA-15 (0.83 mmol g -1 ) for dry gas.While the samples are under wet conditions of RH13%, the adsorption capacity drops to 0.689 mmol g -1 , higher than SBA-15 and silicalite-1.The adsorption performance of common zeolite for VOCs molecules is shown in Table S1.

Effect of Cations on Adsorption Performance of Adsorbents
Zeolite often require the addition of cations outside the skeleton to balance the negative charge in the skeleton during the synthesis process to maintain the electrical neutrality of the silica-alumina molecular sieve skeleton and also to change the active cationic center of the zeolite by ion exchange (Antúnez-García et al., 2023).The addition of cations can cause a change in the electrostatic adsorption energy between the zeolites and the VOCs molecules.Oliveira et al. (2009) modified NaY zeolite using Zn, Ni, and Ag ions and determined the single component apparent isotherms of toluene on NaY, NiY, ZnY, and AgY zeolite.The experimental results showed that the adsorption capacity for toluene following NaY < ZnY < NiY < AgY.The performance of the modified Y-type zeolites for toluene adsorption were all better than that of the original zeolites, indicating that the cationic modification has obvious significance for improving the performance of Y-type Zeolite for VOCs adsorption.
At the same time, the cations in the zeolite may also have a steric hindrance effect on the adsorption of VOCs, blocking the pores of the molecular sieve.Beerdsen et al. (2003) studied the adsorption characteristics of alkanes on MFI zeolites containing cations such as Li + , Na + , K + , Cs + , Ca 2+ , and Ba 2+ , respectively.The results showed that the adsorption capacity of alkanes decreased with the increase of cations introduced, while for the same zeolite, the adsorption capacity of alkanes increased with the decrease of the cation size.However, in a study by Nigar et al. (2015), the introduction of the same cation (Na + ) on Y zeolite facilitated the adsorption of n-hexane by zeolite.Zeng and Ju (2009) used GCMC (Grand canonical ensemble Monte Carlo) to simulate the adsorption  of benzene on MFI zeolite.The results showed that Na + would block the intersection of the straight channels and Z-shaped channels of MFI zeolite and reduce the adsorption capacity of benzene.The results were consistent with the research of Beerdsen et al. (2003).The above studies suggest that cations in zeolites with larger pores can act as additional adsorption sites.Conversely, cations in zeolites with smaller pores may block the pores and reduce the porosity to the detriment of adsorption.Benchaabane et al. (2022) prepared LiNa-, CaNa-, and MgNa-LTA by ion exchange and tested the adsorption performance of propane and propylene on three zeolites at 303 K and 5 bar.The results show that the adsorption performance of these three zeolites was improved compared with pure silica zeolite (Si-LTA), and CaNa-LTA was the most effective for propylene/propane CaNa-LTA had the best adsorption effect on propylene/propane.Furthermore, it is shown that the adsorption performance of zeolites can be adjusted very precisely by the cation exchange process.
Zeolites for VOCs adsorption should be modified with suitable cations.In the study by Tidahy et al. (2007), using Cs + exchange with Na + in FAU and BEA zeolites resulted in a distortion of the zeolite skeleton with excessive Cs + , thus leading to a decrease in both surface area and pore volume of the zeolite, leading to a reduction in adsorption performance.García-Pérez et al. (2010) showed a significant increase in the adsorption capacity of Na + -exchanged zeolites for 2,2-dimethylbutane with increasing amounts of Na + , while the adsorption capacity of Ca 2+ -exchanged zeolites remained almost unchanged.This study showed that Na + favored the adsorption of 3-methylpentane, while Ca 2+ favored the adsorption of hexane.
In summary, metal cations affect the adsorption performance of molecular sieves by changing the electric field, the adsorption capacity of VOCs may increase with the decrease of the cation size and increase of the charge of the cation on zeolite.And the metal cations will also lead to a decrease in both surface area and pore volume of the zeolite, leading to a reduction in adsorption performance.

Effect of Water Vapor on the Adsorption Performance of Zeolite Adsorbents
A major advantage of zeolite over activated carbon is hydrophobicity.That activated carbon can absorb large amounts of water in high-humidity environments, thus limiting the effectiveness of VOCs' absorption.Hydrophobic zeolites have a higher affinity for VOCs adsorption both at high temperatures and humid conditions (Bhatia et al., 2009).In high-humidity environments, water molecules will present competitive adsorption to VOCs.Water vapor preferentially occupies the active sites of zeolites, thereby reducing the number of active adsorption sites available for VOCs molecules and decreasing the adsorption capacity.Kraus et al. (2018) studied the adsorption of 22 zeolites under different humidity conditions for VOCs in the presence of water vapor.The results showed that a small amount of water vapor in the feed gas caused a significant decrease in the adsorption capacity of zeolites for toluene.Fig. 2 shows the change in toluene adsorption capacity due to the addition of water vapor (Kraus et al., 2018).13X and NaY zeolites decreased the adsorption capacity for toluene by more than 95%, while the highly silica superstable USY showed relatively less decay in the adsorption capacity for toluene.
Usually, zeolites with more structural defects (e.g., nano-sized Zeolites and multistage pore molecular sieves) exhibit hydrophilic properties.This is because the bridge-connected Si-OH-Al structure in zeolite can form strong hydrogen bonds with water molecules, resulting in a significant decrease in the adsorption performance of zeolite on VOCs.Bal'zhinimaev et al. (2019) showed that the silanol group of FAU-type zeolite with a low Si/Al ratio will form hydrogen bonds with water molecules and further form a large number of water clusters to hinder the mass transfer of toluene to FAU-type zeolite.
The increase in the silica-alumina ratio helps to improve the adsorption performance of zeolite for VOCs in the presence of water, according to Table S1.Bhatia et al. (2009) investigated the adsorption performance of AgY (Si/Al = 40) and AgZSM-5 (Si/Al = 140) for different concentrations of butyl acetate under dry and humid conditions at 28°C, respectively.The results showed that the adsorption capacity of AgY to butyl acetate was strongly inhibited with the increase of humidity, and the adsorption amount decreased by 42%.However, the uptake capacity of AgZSM-5 for Fig. 2. Variation of toluene and water adsorption capacities for various zeolites due to competing adsorption of the second component (at 22°C, pH2O = 1.66 kPa, ptoluene = 0.02 kPa) (Kraus et al., 2018).
butyl acetate was only slightly affected, with only a 7% decrease in the adsorption capacity.This is due to the low hydrophobicity of the AgY surface, where the adsorption sites act as nucleation sites for further water adsorption, leading to competitive adsorption with molecules of VOCs, which reduces the adsorption performance of VOCs.Yazaydin and Thompson (2006) and Güvenç and Ahunbay (2012) also confirmed through molecular simulation studies that an increase in the silica-alumina ratio would contribute to the adsorption of VOCs under wet air conditions.Yin et al. (2020) increased the Si/Al ratio of NaY using high-temperature water vapor dealumination, and the toluene adsorption capacity of NaY (SiO2/Al2O3 = 13.76)compared with untreated NaY (SiO2/Al2O3 = 5.48) at a relative humidity of 50% was increased from 11.26 mg g -1 to 128.85 mg g -1 , and the adsorption capacity of water vapor decreased from 237.24 mg g -1 to 12.81 mg g -1 .
There are also structural coating methods and surface modification methods to improve the hydrophobicity of zeolites.Hu et al. (2009) obtained superhydrophobic adsorbent MSs (silicalite-1-coated SBA-15 particles) by coating SBA-15 with Silicalite-1 zeolites (pure silicon MFI), and the MSs increased the adsorption capacity of toluene by 22% at a relative humidity of 13%.Wang et al. (2019) structurally coated silicalite-1 as a hydrophobic shell on silica-alumina ZSM-5 molecular sieve by 3D printing, and the resulting shell-core structured molecular sieve adsorbent exhibited a saturation capacity of toluene adsorption at 50% relative humidity.The saturation adsorption capacity of toluene was increased by 38% compared with that of the parent ZSM-5.The grafting of silane groups on the surface of the molecular sieve can also effectively improve the hydrophobicity of the molecular sieve.Han et al. (2011) used various long-chain alkyl trichlorosilane such as octyltrichlorosilane (OTS), decyltrichlorosilane (DTS), dodecyltrichlorosilane (DDTS) and cetyltrichlorosilane (HDTS) to modify the surface of ZSM-5 molecular sieve, and the hydrophobicity of zeolite was enhanced with the increase of alkyl trichlorosilane chain length.

ADSORBENT KINETIC ADSORPTION PROPERTIES
The adsorption kinetics is another critical characteristic of VOCs adsorbent in terms of adsorption and desorption rates.The faster the VOCs gas adsorption speed, the higher the effective utilization rate of the adsorbent in the actual adsorption purification process.The mass transfer of VOCs in zeolite consists of three steps : firstly, the VOCs molecule is transferred from the bulk gas stream to the external surface of the adsorbent; then, the VOCs molecule diffuses from the relatively small area of the external surface into the macropores, transitional pores, and micropores within each adsorbent; at last, the VOCs molecule adsorbs to the surface in the pore.The VOCs molecule transfer in the solid phase can be considered the controlling stage of the adsorption process.The diffusional time constant (D r -2 ) or diffusion coefficient (D) is usually used to describe the speed of gas adsorption in the adsorbent.Due to the different pore structures of zeolite and the size of VOCs gases, the adsorption speed of different VOCs gases on different zeolites is also different.Zeolite has a developed microporous structure, and the microporosity and the gap between zeolite crystals influence the gas transfer rate (Chung et al., 2022).Möller et al. (2009) studied the adsorption kinetics of propane, n-butane and n-hexane on 5A and ZSM-5 using a non-isothermal model.The results show that with the increase of VOCs molecular size, the diffusional time constant of VOCs gas in zeolite adsorbent gradually decreases, and the larger the pore size of the adsorbent, the greater the diffusional time constant.For example, the diffusional time constants of n-Butane and n-Hexane on ZM-5 are 0.0337 and 0.0194 s -1 , respectively, while the diffusional time constant of n-Butane on 5A is only 0.0082 s -1 .Chung et al. (2022) analyzed the adsorption kinetics of ethylene and ethane on 13X zeolite particles using a non-isothermal adsorption model.The result shows that ethane's diffusional time constant increases with the temperature, but the diffusion time constant of ethylene is contrary.Yan et al. (2022) studied the diffusion coefficients based on molecular dynamics simulations.The result shows that methanol and benzene diffusion coefficients at 673 K in H-ZSM-5 zeolite are 2.97 × 10 −8 and 6.50 × 10 −12 m 2 s −1 , respectively.Chen et al. (2022) use molecular dynamics to study the diffusion simulation of VOCs molecules on MOR, MFI, and CON zeolites.The results show the diffusion coefficients increase with temperature.A study is performed on the adsorption and diffusion of a pure and binary mixture of methanol and ethanol through MFI membranes by GCMC and molecular dynamic simulation, respectively.Keyvanloo et al. (2022) studied the influence of temperature, acid site density (Si/Al) of zeolites on the diffusion of methanol and ethanol, pure and binary mixture (80% methanol and 20% ethanol) in silicalite-1 and HZSM-5 (Si/Al = 47 and 23) by using of the COMPASS force-field molecular dynamics method.The result show the diffusion coefficients of methanol and ethanol decreased with increasing Si/Al-ratios.This may be due to the interaction between methanol and the negative charge centers created by aluminum atoms.By increasing the aluminum atoms in the zeolite structure, the interaction between methanol (and ethanol) with the framework becomes weaker.Table 3 shows the adsorption kinetic parameters of VOCs on different zeolites.
In the process of zeolite molecular sieve adsorption of VOCs molecule, micropores are the main adsorption sites for VOCs adsorption on zeolite and dominate the adsorption of the adsorbent, but the role of macropores and mesopores cannot be ignored.Macropores contribute 5% of the adsorbent's surface area, while mesopores and micropores contribute about 95% of the surface area.Therefore, adsorbents with only micropore pores result in too long diffusion paths for VOCs molecules inside the adsorbent, with lower mass transfer coefficients (Zhang et al., 2017;Le-Minh et al., 2018).
In order to reduce the mass transfer resistance in the adsorption process of zeolite molecular sieve, decreasing their crystal-size into nanoarchitectures is an effective means to improve the mass transfer kinetic properties.Kim et al. (2021) decreased the crystal thickness of MFI zeolite to the nanometer scale, the results showed that the breakthrough time of p-xylene adsorption on MFI zeolite could be improved by more than 2.3 times by reducing the crystal thickness of zeolite to a single-unit-cell dimension (~2 nm), and the breakthrough curve become sharper.
IMIt can be considered that the shorter diffusion path and larger external specific surface area in the nano ZSM-5 zeolite molecular sieve system are conducive to the rapid adsorption of p-xylene.
Increasing the number of mesopores in microporous materials is an effective way to increase  the mass transfer rate of adsorbents.Yang et al. (2019b) pointed out that the microporous and mesoporous cross-linked pore structure is conducive to the rapid adsorption and desorption of macromolecular organic substances.Hierarchical porous materials, which have porous structures (micro-, meso-, and macro-pores) with multiple porosity levels, have been the center of interest for the last decade.Macropores promote molecular diffusion and accessibility to active sites (Li et al., 2013).Feng et al. (2020) obtained NH4HF2-etched NaY-s-x zeolites with intracrystalline mesoporosity via the surfactant-templating process in weak base NH4OH solution.It is demonstrated that the intracrystalline mesoporosity obviously improves the toluene molecular adsorption properties of zeolite samples.Xu et al. (2014) prepared the hierarchical zeolites SPP and microporous ZSM-5.The pore volume of SPP is 2.3 times that of ZSM-5.The adsorption kinetics of hierarchical porous SPP and microporous ZSM-5 to 2,2-dimethylbutane have been studied, and found that the diffusional time coefficient of hierarchical porous SPP (4.2 × 10 -3 s -1 ) was much larger than that of microporous ZSM-5 (2.6 × 10 -5 s -1 ).Kim et al. (2021) prepared CM-MFI (constricted mesopore), OM-MFI(open-mesopore) and C-MFI (commercial MFI) with similar Si/Al ratios of 22-26.OM-MFI exhibited a high diffusional time constant of 7.5 × 10 -3 s −1 for p-xylene adsorption, which is higher than that of C-MFI (5 × 10 -3 s −1 ), confirming a high diffusion resistance in CM-MFI.This shows a mass transfer enhance for the open mesopore increasing.While the constricted mesopores may hinder the diffusion, CM-MFI exhibited a diffusional time constant of 1.8 × 10 -3 s −1 , which is much lower than that of C-MFI (5 × 10 -3 s −1 ).

MONOLITHIC ZEOLITE ADSORBENT AND ITS APPLICATION IN VOCS PURIFICATION PROCESS
Zeolites are usually synthesized as powders with particle sizes ranging from tens of nanometers to tens of microns.However, powdered zeolites are usually not used directly in the industry.Usually, the powder must be made into granular or monolithic zeolites by a specific molding process.Among them, monolithic zeolite is used in large quantities in VOCs removal due to its low flow resistance, removal efficiency, and service life (Valencia et al., 2014).The commonly used methods for preparing honeycomb zeolite are extrusion molding and coating.

Extrusion Molding Zeolites and Applications
Extrusion is a process in which molecular sieves, binders, and specific solvents are mixed in a certain ratio and then placed in a die and extruded under physical pressure (Akhtar et al., 2014).The common zeolite honeycomb size for VOCs removal is a 100 × 100 × 100 (mm) square, as in Fig. 3. Extrusion is a simple and inexpensive process, but the binder added during the preparation process may cover the surface of the molecular sieve, thus blocking the molecular sieve pore channels and reducing the specific surface area and pore capacity of the molecular sieve.Shams and Mirmohammadi (2007) investigated the effect of binder on the adsorption performance of 5A zeolite during extrusion.They showed that the adsorption capacity of Zeolites formed with 1% binder decreased compared with those formed without binder.The adsorption capacity of saturated alkanes decreased by about 14% compared with that of zeolite formed without adding a binder.However, it is difficult to mold without a binder, and the obtained honeycomb adsorbent has low mechanical strength and is easily pulverized.Hence, extrusion molding requires reasonable control of the amount of binder added.Well-established extrusion molding processes are commercially available for producing industrial adsorbents and catalysts with high mechanical strength and wear-resistant granular and honeycomb structures, such as LTA, X-type, Y-type, ZSM-5 and BEA (Akhtar et al., 2014).
Honeycomb adsorbents prepared by extrusion have been widely used for the adsorption and capture of industrial emission VOCs, and pressure/temperature swing adsorption.Compared with the traditional particle-packed fixed bed adsorption process, the honeycomb adsorbent is less susceptible to wear and tear and has less piezoresistance.However, laminar flow through the honeycomb channel affects the mass transfer and can somewhat reduce the adsorption performance (Gadkaree, 1998).Fig. 4 gives a typical schematic flow diagram of the VOCs removal process using fixed bed zeolite adsorption in a printing plant in Foshan, China.The filtered exhaust gases are alternately adsorbed in four adsorption beds filled with zeolite honeycombs during the adsorption process.After saturation, the VOCs gas adsorbed on the zeolite honeycomb is desorbed by heating and regeneration and sent to the catalytic oxidation furnace for oxidation and destruction.Finally, the heated and regenerated honeycomb zeolite adsorption bed is cooled and adsorbed by raw material gas, or air can be used for cooling.Fixed bed honeycomb adsorbent in the process of VOCs removal is suitable for intermittent work of emission sources, not too large air volume, containing high boiling point VOCs, etc.

Coating Zeolites and their Application
The preparation of zeolite honeycomb by coating method refers to the use of profiles such as glass fiber, ceramic fiber, porous glass, ceramics, and metal oxides as substrates, which are infiltrated in zeolite suspensions.The zeolite powder is uniformly fixed on the surface of the substrate profile through the process of solvent evaporation and high-temperature calcination.In the zeolite coating process, inorganic colloids such as sodium silicate and silica sol are usually used as dispersants and binders.However, zeolite with a large particle size (about 2 mm) tends to precipitate during the coating process of this method, resulting in uneven surface distribution.In addition, inorganic gums can also lead to a decrease in the adsorption capacity of zeolite honeycomb bodies.Compared with the extrusion method, the pore channels in the zeolite are less likely to be blocked, and the adsorbent utilization rate is higher.Still, the disadvantage is that the zeolite coated on the surface of the substrate is easy to fall off.
In order to improve the effect of zeolite coating, some researchers used methods such as seed crystal growth method and in-situ growth method to coat and grew a thin layer of molecular sieve on the surface of the substrate to improve the stability and uniformity of the molecular sieve on the surface of the monolithic adsorbent (Meille, 2006).Yasumori et al. (2015) used a hydrothermal method to grow and coat porous glass as a substrate on the surface of the monolithic adsorbent was prepared by using a hydrothermal method with a porous glass substrate and coated with X-type zeolite.Fang et al. (2018) proposed a new technique for the in situ microwave hydrothermal synthesis of NaA zeolite monolithic adsorbent on a honeycomb ceramic substrate.The result shows NaA zeolite with cubic form and particle size of about 0.9 μm could be quickly in situ synthesized on both the surface and the void of honeycomb ceramic matrix within 35-55 min by microwave irradiation.The present coated zeolite monolithic zeolite can be applied to fixed-bed adsorption and to zeolite rotor adsorption (Lin and Chang, 2009).Rotating bed adsorbents include rotating cylinder adsorbers and rotating drum adsorbers.
The rotors in commercial applications for VOCs adsorption are zeolite rotors.The zeolite rotor can work continuously.Compared with the working mode of the fixed bed, it can handle VOCs gas with a larger air volume and lower energy consumption.The zeolite rotor operates continuously.Compared with the fixed bed, it can handle VOCs gas with a larger air volume, occupies a smaller area, and lowers the energy consumption for operation.But the zeolite rotor is not suitable for discontinuous work situations.The honeycomb bodies are usually inorganic fiber paper rolled into a honeycomb substrate profile.Next, hydrophobic zeolites are coated on the surface of the channels of the porous honeycomb structure and finally sintered to make honeycomb zeolite (Yuan et al., 2015).A photograph of a zeolite honeycomb rotor is shown in Fig. 5.The size of a zeolite rotor for VOCs purification is usually very large, with some diameters reaching 4 m or more.In this case, the rotor-type adsorber generally consists of a block of spliced fan-shaped honeycomb bodies.
The basic working principle of the rotor adsorber is shown in Fig. 6.The rotor adsorber is divided into adsorption, desorption, and cooling zones.During operation, the zeolite rotor rotates to the adsorption zone and adsorbs a low concentration of VOCs gas until it reaches the saturation state of the rotor.Then it turns to the desorption zone for high-temperature desorption and VOCs concentration.The concentrated VOCs gas is usually degraded to water and carbon dioxide by applying catalytic combustion technology or incineration to achieve environmentally friendly emissions.Finally, the rotor rotates to the cooling zone to cool down and complete a cycle.The rotor drum adsorber has the same workflow as the rotor zeolite, and the gas flows in the radial direction during operation.As the rotor drum adsorber is better sealed than the cylinder rotor adsorber, it can have a higher concentration rate than the cylinder rotor adsorber during adsorption    2012) designed a zeolite rotor for Shanghai SMIC with a VOCs gas removal efficiency of 96.6% at a concentration of 120.3 mg kg -1 .Cho et al. (2019) prepared a zeolite runner with ZSM-5 coating to remove toluene with an efficiency of 94.7%.Yamauchi et al. (2007) prepared a monolithic molecular sieve adsorbent coated with high silica molecular sieve and applied it to a honeycomb runner adsorption system with a removal efficiency of more than 95% for VOCs.The zeolite rotor is widely used in VOCs removal.For example, Nichias, Japan, has delivered more than 2000 VOCs rotors, and the main application areas are shown in Fig. 7.

CONCLUSION
(1) Zeolite molecular sieve is a microporous structure of silica-aluminate crystals, with highly ordered and flexible microporous structure, good stability, and accessible surface modification that have become the most effective adsorbent for VOCs gas adsorption and purification.The zeolite molecular sieve has been prepared as extruded honeycomb adsorbent and fiber-coated honeycomb adsorbent for VOCs gas purification in fixed bed and rotary adsorbers.
(2) The pore structure and size of the zeolite molecular sieve greatly influence the adsorption performance of VOCs gas.The selection of pore channels with a suitable skeleton structure is significant for matching the size of the VOCs molecular.The adsorbent materials with microporous and mesoporous cross-linked structures can play both the large pore volume and the adsorption characteristics of microporous structure for low partial pressure, which can significantly promote the adsorption of VOCs gas by the adsorbent.
(3) The zeolite molecular sieve can enhance the charge density on the surface of zeolite by cation exchange, thus enhancing the adsorption capacity for VOCs.However, the possible blockage effect of cations on zeolite pore channels leads to the reduction of VOCs adsorption capacity.
(4) Water vapor in the gas stream will seriously weaken the zeolite's VOCs adsorption capacity.The hydrophobic performance of the zeolite can be enhanced by increasing the silica-alumina ratio of the adsorbent or by modifying the surface silylation of the adsorbent.
(5) The adsorption kinetics is an essential parameter of zeolite molecular sieve for VOCs adsorption.Two methods can be used to improve the adsorption kinetic properties of VOCs adsorption on zeolite, one is to reduce the crystal size of the zeolite, and the other is to construct zeolite with microporous, mesoporous cross-linked multistage pore structure.

Fig. 4 .
Fig. 4. Flow chart of the principle of VOCs removal by adsorption on fixed bed honeycomb zeolites.

Table 1 .
Basic physical and chemical properties of common VOCs.

Table 2 .
Feasibility of adsorption of common VOCs by different adsorbents.

Table 3 .
Diffusion kinetic parameters of VOCs adsorption.