Adsorption is one common and simple technique for the condensation and recovery of VOCs. Currently, activated carbons are the most useful adsorbents for VOC uptake ^[11-14], however, employing activated carbons still has practicality issues, such as flammability ^[15], difficulties in regeneration ^[16], and humidity control ^[17]. Compared with activated carbons, zeolites exhibit unique features, such as non-flammability, excellent regeneration by calcination, and controllable surface hydrophobicity hydrophilicity. Therefore, zeolites have shown to be useful alternative adsorbents for selective adsorption of VOCs ^[18].

Chintawar et al. ^[19] investigated ambient adsorption of gaseous trichloroethylene (TCE) over chromium exchanged ZSM-5 zeolites with various SiO₂/Al₂O₃ ratios under a humid air atmosphere. The study found that when increasing the SiO₂/Al₂O₃ ratio from 30 to 120 the TCE saturation capacity of the zeolite increased from 6.0 to 10.1 wt%, attributed to an increase in zeolite surface hydrophobicity. Trichloroethylene adsorption is favored over water molecules as the surface hydrophobicity increases. Bhatia et al. ^[20] studied the adsorption behaviors of butyl acetate in air over silver-loaded Y (Si/Al = 40) and ZSM-5 (Si/Al = 140) zeolites. As shown in Fig. 1, the presence of water vapor in the feed strongly suppresses butyl acetate adsorption of AgY; however, the adsorption capacity of AgZSM-5 is only marginally affected because of the higher hydrophobicity of the zeolite. Huang et al. ^[21] investigated the changes in hydrophobicity and toluene adsorption-desorption properties of a commercial ZSM-5 zeolite as a function of Si/Al ratio and reported that an increase in zeolite Si/Al ratio obviously improved the zeolite hydrophobicity, resulting in the favorable adsorption of toluene.

Fig. 1

Fig. 1. Butyl acetate uptake by zeolite AgY with Si/Al ratio of 40 and AgZSM-5 with Si/Al ratio of 140. (a) GHSV = 5000 h−1, Cvoc = 4500 ppm; (b) GHSV = 5000 h−1, Cvoc = 1000 ppm. Reproduced from Ref. [20].

To understand the adsorption of VOCs and water on zeolites, molecular simulation has been employed ^[22-24]. For example, Güvenҫ et al. ^[24] used Monte Carlo simulations to study the relationship between the Si/Al ratio of MFI-type zeolites and their adsorptive performance in the removal of hydrophilic methyl tert-butyl ether (MTBE) and hydrophobic trichloroethylene (TCE) mixed with water. Adsorption simulations on the MFI-type zeolites with three different Si/Al ratios (∞,191, and 95) were performed, showing that increasing zeolite hydrophilicity may significantly reduce MTBE removal from water; while increasing TCE concentration in the TCE-water mixtures leads to water exclusion in high silica zeolites, which may minimize the performance loss of these materials in TCE removal because of the presence of any hydrophilic defects in their structures (Fig. 2) ^[24].

Fig. 2

Fig. 2. Methyl tert-butyl ether (a) and trichloroethylene (b) together water density fields in ZSM-5/95 at maximum VOC loading. MTBE/TCE density field is displayed in white, and water density increases from blue (near-zero density) to red. Al atoms and Na+ cations are represented as pink and purple spheres, respectively. Straight and sinusoidal channels run in the vertical and horizontal directions, respectively. Reproduced from Ref. [24].

As size exclusion is a desirable property in zeolite molecular sieves, tuning the zeolite pore size to effectively target the size of the VOCs is also an important factor for the adsorption of VOCs over zeolites. Bulky VOCs, whose size exceeds the zeolite pore dimensions hinders effective adsorption. Nicolas et al. ^[25] investigated toluene adsorption as a function of zeolite channel size and pore structure using three commercial hydrophobic zeolites: mordenite (MOR), ZSM-5, and faujasite (FAU) zeolite. They evidenced that toluene could be readily absorbed by FAU zeolite, however, decreased adsorption capacities were observed when using MOR and ZSM-5, suggesting the importance of the zeolite structures. Similar results have also been reported by Cosseron et al. ^[26]. Here they synthesized four pure silica zeolites: chabazite (CHA-structure type), SSZ-23 (STT-structure type), silicalite-1 (MFI-structure type), and beta (*BEA-structure type) for sorption of n-hexane,p-xylene, and acetone. As the pure silica CHA-type zeolite possesses smaller 8-member rings there was an obvious decrease in the adsorption capacity as a function of increasing molar volume of the target probe molecules, preventing VOC ingress to the cages. Brosillon et al. ^[27] studied co-adsorption of n-heptane and acetone over the commercial hydrophobic zeolite, HISⅣ 3000. They showed that this zeolite displayed good selectivity for acetone even though acetone is more volatile than heptane in the mixture. Corma et al. ^[28] reported interesting results that there existed two disjunctive pore structures in MCM-22, each of which result in varying VOC adsorption properties with respect to molecular sizes, such as toluene and xylene.

It is not only the wettability of the zeolite that can affect the adsorption performance, as extra-framework cations have also been reported to play a role. For example, Zhou et al. ^[29] have prepared Y zeolites with varying metal cations (Li⁺, Na⁺, K⁺, Cs⁺) to study the adsorption performance of aromatic hydrocarbons. They found that the adsorption capacity of the aromatic hydrocarbons on Y zeolites was dependent on the alkali metal ionic radius, which is attributed to the difference in cationic electronegativity.

The non-flammability properties of zeolites allow for safe regeneration when used as adsorbents. Typically, zeolites adsorbates can be easily regenerated by microwave heating in a relatively short period (rapid heating) with low consumptions of energy ^[30-32]. Kim et al. ^[33] reported a study concerning the evaluation of adsorption behaviors as a function of the physicochemical properties of zeolites and investigated the effect of zeolite pore structure on VOC desorption behaviors by microwave heating for regeneration of zeolites from polluted zeolites. They showed that HY901 zeolite had the greatest adsorption capacity because of its larger mesopore volume and microwave heating is effective for desorption of toluene and methylethylketone on the HY901 zeolite.

Remarkable progress in the adsorption of VOCs in recent years has advanced the market and contributed to the commercialization of thermal swing honeycomb rotor adsorbents. Such VOC adsorbent products are characterized by a monolith or honeycomb structure instead of by conventional particles or pellets with a rapid response to temperature swing. This technology was developed by Seibu Giken and Nichias Corporation in Japan, Munters Zeol Corporation in Sweden, and Lurgi Corporation in Germany ^[34-36]. The initial development employed activated carbon as the adsorbent; however, this was replaced soon after by high silica zeolites to avoid possible ignition during hot air thermal regeneration. A slowly rotating honeycomb adsorbent rotor is shown in Fig. 3. During the treatment process, the feed air flows through the narrow channels of the honeycomb structure and the VOC vapors are first removed from the feed air by adsorption onto the adsorbate surface. Thereafter, the adsorbate is regenerated by a hot air stream directed to the process zone. The last section of the rotor is a cooling zone, which is located between the process and regeneration zones, and serves to purge the crude product from the bulk of the fine product and to cool the rotor to maintain high adsorption capacity in the subsequent process zone. In a typical commercial application,>95% of VOCs in the feed gas are removed ^[36], which shows the significance of this technology in the removal of harmful indoor gases.

Fig. 3

Fig. 3. Integrated VOC abatement system incorporated with a thermal swing honeycomb rotor. Inlet: cross-sectional view of the honeycomb structure. Reproduced from Ref. [36].

In summary, the adsorption properties of zeolites can be tailored as a function of framework, microporous structure, Si/Al ratio, cations and the hydrophobicity/hydrophilicity nature in zeolites. The outlook of zeolites as efficient and selective adsorbents remains promising.

3 Catalytic combustion of VOCs over zeolite-supported metal catalysts

3.1 Catalytic combustion of aromatics

Aromatic VOCs, such as benzene and toluene, significantly contribute to industrial and daily life pollutants. Table 1 summarizes recent works on catalytic combustion of aromatics over zeolite-supported metal catalysts. Such catalysts are typically divided by metal type: supported noble metal catalysts and supported transition metal catalysts.

表 1 (Table 1)

Table 1 Catalytic activities of zeolite-supported metal catalysts for the combustion of aromatics. Catalyst VOCs VOCs concentration(ppm) GHSVa(h−1) T50b(℃) T98b(℃) Ref. Pt/Meso-Beta toluene 1000 60000 186 195 [37] Pt/Beta toluene 1000 60000 197 212 [37] Pt/KZSM-5-60c toluene 1000 60000 167 170 [38] Pt/KZSM-5-100c toluene 1000 60000 162 169 [38] Pt/Meso-KZSM-5 toluene 1000 60000 172 175 [39] Pt/KBeta-SDS toluene 1000 60000 142 150 [40] Pd/Beta-PSd toluene 860 30000 273 — [46] Pd/FAU toluene 1000 60000 — 200 [47] NaX toluene 800 30000 357 394 [52] aGas hourly space velocity. bT50 and T98: the temperatures at 50% and 98% conversion of VOCs. cSi/Al ratio. d PS: pure siliceous.

Table 1 Catalytic activities of zeolite-supported metal catalysts for the combustion of aromatics.

Supported noble metal catalysts have been widely applied in the catalytic combustion of aromatic VOCs, with Pt and Pd being the two commonly used metals practical applications as a result of their catalytic activity.

Chen and co-workers ^[37-41] have systematically investigated the catalytic performance of toluene combustion over zeolite-supported Pt catalysts. To improve activity, a series of Pt/ZSM-5 zeolites functionalized with different cations (H⁺, Na⁺, K⁺, Cs⁺) with varying Si/Al ratios (60,100,300,) were prepared ^[38]. As shown in Fig. 4, they observed that the activities of Pt/KZSM-5 and Pt/CsZSM-5 catalysts yielded higher conversions when compared with Pt/HZSM-5 and Pt/NaZSM-5 catalysts, which can be attributed to the difference in the cationic electronegativity. The decrease in cationic electronegativity in the catalysts is favorable for the formation of catalytically active Pt0 species. Catalytic activity as a function of Si/Al ratio in the Pt/KZSM-5 catalysts shows a Si/Al ratio of ~100 to have the highest activity, which is attributed to the hydrophobicity level and K⁺ concentration in the ZSM-5 zeolites. Generally, higher zeolite Si/Al ratios improves zeolite hydrophobicity, which is favorable for the adsorption of hydrophobic aromatics. However, further increases to the zeolite Si/Al ratio would result in a significant decrease in the K⁺-exchange ability, which in turn, remarkably reduces Pt⁰ concentration in the catalysts. Therefore, a balance between the Si/Al ratio and K⁺ concentration needs to be considered, and comprehensive studies have shown that the Si/Al ratio at ~100 with suitable hydrophobicity and K+ content exhibits the highest activity.

Fig. 4

Fig. 4. Catalytic combustion of toluene over (a) zeolite-based catalysts as a function of cation type (Pt/HZSM-5-60, Pt/NaZSM-5-60, Pt/KZSM-5-60, and Pt/CsZSM-5-60) and (b) zeolite-based catalysts with various Si/Al ratios (Pt/KZSM-5-60, Pt/KZSM-5-100, Pt/KZSM-5-300, and Pt/S-1). Operating conditions: toluene concentration 1000 ppm, SV = 60000 mL/(gh). Inset: Histogram of catalyst activity at 165 ℃. Reproduced from Ref. [38].

It is not only ZSM-5 zeolites that have been used as supports for the preparation of zeolite-supported metal catalysts. Recently, Chen et al. ^[40] prepared Pt/KBeta-SDS catalysts using a template-free and seed-directed synthesis (SDS). They showed that the Pt/KBeta-SDS catalyst was highly active, giving a T₉₈ value for toluene combustion as low as 150 ℃, which is directly attributed to high K⁺ content in the Pt/KBeta-SDS catalyst. To increase mass transfer in the catalytic combustion, mesoporous Beta zeolite was employed as the metal support ^[37]. As shown in Fig. 5, the mesoporous Beta zeolite-supported Pt catalyst (Pt/Meso-Beta) exhibits significantly improved catalytic activity (T₉₈ at 195 ℃) and longer catalyst life, in addition to a lower apparent activation energy in the catalytic combustion of toluene than the corresponding Pt/Beta catalyst prepared with conventional microporous zeolites (T₉₈ at 220 ℃). Obviously, the presence of mesoporosity in the catalysts is a desired property to enhance both the catalytic activity and lifetime.

Fig. 5

Fig. 5. Catalytic combustion of toluene over Pt/Meso-Beta and Pt/Beta showing (a) catalytic activities and (b) catalyst life. Operating conditions: toluene concentration 1000 ppm, SV = 60000 mL/(gh). The catalyst life was measured at 193 ℃. X, SCO2, and SSP correspond to toluene conversion, selectivity for CO2, and selectivity for side products, respectively. Reproduced from Ref. [37].

Pt particle size plays a key role in how efficient it performs as a catalyst and the effect of Pt particle size on zeolite supports has been widely investigated. For example, Chen et al. ^[41] have successfully synthesized 1.3-2.3 nm Pt nanoparticles supported on ZSM-5 zeolites to obtain a series of Pt-x/ZSM-5 (x relates to the Pt particle size). As shown in Fig. 6(a), the catalytic activities of the samples are strongly dependent on Pt nanoparticle size. For Pt nanoparticle sizes ranging from 1.3-1.9 nm, the catalytic activities of the samples are enhanced significantly, reaching an apex at 1.9 nm. However, further increasing the Pt particle sizes to 1.9-2.3 nm results in a remarked reduction in the sample activities. This activity ordering is obviously related to the trends of both the sample Pt⁰ proportion and the Pt dispersion (Fig. 6(b)), instead of a comprehensive and independent consideration of both factors. As a result, the Pt-1.9/ZSM-5, combining both a high degree of Pt⁰ and high levels of Pt dispersion, displays the highest activity among the all catalysts, although possessing a lower degree of Pt⁰ than that of Pt-2.3/ZSM-5 and a lower Pt dispersion than that of Pt-1.3/ZSM-5. Therefore, optimizing Pt particle sizes plays a significant role for industrial applications of Pt-based catalysts.

Fig. 6

Fig. 6. (a) T5, T50, and T98 toluene dependence in the complete oxidation of toluene and (b) dependences of Pt dispersion and Pt0 proportion on Pt particle size over the Pt-x/ZSM-5 catalysts. Reproduced from Ref. [41].

Various studies have evidenced that VOC catalytic oxidation in a mixture differs from its simple oxidation and, in general, an inhibiting effect can be observed ^[42-44]. Beauchet et al. ^[45] evaluated FAU zeolite performance in the presence or absence of Pt in the oxidation of isopropanol and o-xylene alone and mixtures thereof. They described a basic mechanism for propene formation from isopropanol over NaX (Scheme 1). Thereafter, propene can be directly oxidized or transformed into acetone and/or aldehyde before further oxidation into CO2 and H2O. While isopropyl dimethylbenzene formed from the alkylation of o-xylene by propene, resulting from the dehydration of isopropanol via an acidic mechanism (Scheme 2) over HY zeolite. Afterwards, isopropyl dimethylbenzene can be further oxidized into dimethylphenol and acetone (Scheme 3).

Scheme1

Scheme1. Basic mechanism for propene formation from isopropanol over NaX. Reproduced from Ref. [45].

Scheme2

Scheme2. Dehydration of isopropanol via an acidic mechanism. Reproduced from Ref. [45].

Scheme3

Scheme3. Isopropyl dimethylbenzene oxidized into dimethylphenol and acetone. Reproduced from Ref. [45].

In addition to Pt, Pd is an alternative noble metal that can also be supported on zeolite supports for the catalytic combustion of aromatics owing to its relatively low cost ^{[46, 47]}. Zhang et al. ^[46] reported the catalytic combustion of toluene over Pd-HBeta catalysts with various Si/Al ratios. Remarkable improvements in catalytic activity was observed as a function of increased Si/Al ratio in the Pd-HBeta catalysts. Optimum catalytic activity is found when employing a pure siliceous analogous Beta—related to the strong hydrophobicity of the pure Beta zeolite.

Even though noble metal catalysts display high activities they are not economically viable to be widely applied ^{[48, 49]}. Therefore, the market still requires development of highly ac-tive and alternative non-noble metal catalysts. To circumvent this issue, researchers have focused on non-noble metals with empty d orbitals, such as Ni, Cu, Cr, Co, and Mn. Soylu et al. ^[50] have prepared clinoptilolite zeolite (CLT)-supported Cu, Fe, Co and Mn transition metals as catalysts for toluene combustion, showing that the zeolite-supported Mn catalyst exhibits relatively high activity, giving T₉₀ at 297 ℃. Ozcelik et al. ^[51] systematically studied the catalytic activities of toluene combustion over CLT zeolite-supported Co, Fe, and Mn catalysts. They found the Na-CLT-supported Mn catalyst (9.5MnO₂/NaCLT) to give a T₉₃ of toluene at 350 ℃. Douglas et al. ^[52] synthesized a series of zeolite-supported Cu catalysts with varying Cu loadings (1-4 wt%), and catalytic tests showed the optimum catalyst to be a zeolite X-supported Cu catalyst (Cu loading at 1 wt%) giving a T₉₈ at as low as 263 ℃. This feature is strongly related to high dispersion of Cu species in the micropores. Notably, although these supported transition metal catalysts show good catalytic activities in the combustion of aromatics, their activities are still much lower than those of supported noble metal catalysts. Therefore, supported noble metal catalysts are still dominant in industrial applications.

3.2 Catalytic combustion of methane

Methane is typical greenhouse gas, whose greenhouse effect is 23 times that of CO₂ ^{[53, 54]}. Table 2 presents the catalytic activity with respect to methane combustion over various zeolite-supported Pd catalysts. Park et al. ^[55] studied catalytic conversion of methane as a function of zeolite structure, suggesting that the intersecting 10-member and 8-member ring porous structures of zeolites have better dispersed and stable PdO nanoparticles than zeolites with uniform 10-member ring and 12-member ring porous structures. As a result, Pd/H-TUN-10 exhibits the highest combustion activity, together with excellent durability in the presence of water vapor from the catalysts studied. Later, Li et al. ^[56] reported Pd/HZSM-5 to have excellent combustion activity, giving T₅₀ for methane as low as 255 ℃. The high activity is dependent on the weak Pd-O bond in the catalyst, which has been confirmed by CO temperature profile reduction tests of the catalyst.

表 2 (Table 2)

Table 2 Catalytic activities in methane combustion over various zeolite-supported catalysts. Catalyst VOCs concentration(ppm) GHSV(h−1) T50(℃) T90(℃) Ref. 1.3Pd%/H-TNU-10 10000 171326 380 425 [55] 1.2Pd%/H-ZSM-5 10000 171326 420 485 [55] 2.5Pd%/H-Beta 10000 171326 395 450 [55] 2.5Pd%/H-USY 10000 171326 415 475 [55] 3.4% Pd/HZSM-5 10000 30000 255 — [56] 4.2% Pd/γ-Al2O3 10000 30000 325 — [56]

Table 2 Catalytic activities in methane combustion over various zeolite-supported catalysts.

3.3 Total oxidation of formaldehyde

Formaldehyde is a well-known indoor pollutant, which poses significant risk to human health ^[57-63]. Particularly, total oxidation of formaldehyde at room temperature has attracted much attention, where the major active centers are noble metals and the major supports being lattice-oxygen rich oxides, such as TiO₂ ^{[64, 65]}, Fe₂O₃ ^[66] , CeO₂ ^[67] and MnO₂ ^[68]. More recently, Park et al. ^[69] employed zeolites to support Pd as catalysts for the catalytic combustion of formaldehyde, as presented in Table 3. Interestingly, under the same conditions, the zeolite-supported Pd catalysts exhibit much higher activities in formaldehyde combustion than the oxide- supported Pd catalysts. Furthermore, it has been observed that the catalytic conversion also depends on the zeolite structure. For example, the Beta zeolite-supported Pd catalyst (0.25%Pd/Beta) has a conversion of 80% 40 ℃, while a conversion of only 30% was observed when employing a ZSM-5 zeolite-supported Pd catalyst (0.25%Pd/ZSM-5) under the same conditions. Notably, one future scientific goal, which still remains a challenge in this field, is the full conversion of formaldehyde at room temperature over the zeolite-supported metal catalysts.

表 3 (Table 3)

Table 3 Catalytic activities in formaldehyde combustion over zeolite-supported Pd catalysts [64]. Catalyst VOCs concentration(ppm) GHSV(h−1) T50(℃) T100(℃) 0.25%Pd/Beta 40 50000 <40 100 0.25%Pd/Zeo-13X 40 50000 <40 100 0.25%Pd/USY 40 50000 <40 100 0.25%Pd/ZSM5 40 50000 >60 100 0.25%Pd/HM 40 50000 >140 —

Table 3 Catalytic activities in formaldehyde combustion over zeolite-supported Pd catalysts [64].

3.4 Catalytic combustion of halogenated VOCs

The highly poisonous halogenated VOCs—usually designated as CVOCs—have higher catalytic combustion temperatures than those of conventional VOCs. In the reported CVOC catalysts, zeolite-supported metal catalysts are particularly studied. Table 4 presents catalytic activities for the catalytic combustion of CVOCs over various zeolite-supported metal catalysts.

表 4 (Table 4)

Table 4 Catalytic activities for the combustion of CVOCs over zeolite-supported metal catalysts. Catalyst VOCs VOC concentration (ppm) GHSV (h−1) T50 (℃) T98 (℃) Ref. Pt/HFAU Chlorobenzene 667 18000 350 — [71] Co/ZSM5-107 Chloromethane 1500 15000 <375 450 [74] Ce/ZSM5-107 Chloromethane 1500 15000 <400 450 [74] Mn/ZSM5-107 Chloromethane 1500 15000 400 500 [74] Fe/ZSM5-107 Chloromethane 1500 15000 >400 550 [74] Ce/ZSM5-30 Chloromethane 1500 15000 325 450 [74] CeO2/USY 1,2-Dichloroethane 1000 10000 215 270 [75] CeO2/HY 1,2-Dichloroethane 1000 10000 280 360 [75] CeO2/SSY 1,2-Dichloroethane 1000 10000 210 290 [75] HZSM-5 1,2-Dichloroethane 1000 15000 — 500 [76] 9%Cr/HZSM-5 1,2-Dichloroethane 1000 15000 — 325 [76] 12%CeO2/HZSM-5 1,2-Dichloroethane 1000 15000 — 350 [76] 9%Cr-12%CeO2/HZSM-5 1,2-Dichloroethane 1000 15000 — 269 [76]

Table 4 Catalytic activities for the combustion of CVOCs over zeolite-supported metal catalysts.

It has been reported that acidic zeolites (H-form) are catalytically active to convert CVOCs. For instance, Fonseca et al. ^[70] reported the catalytic combustion of dichloromethane over HY, HZSM-5, and HMOR, giving the temperature for full conversion of the dichloromethane at ~400 ℃. The high temperatures required means that the processes are energy intensive, therefore zeolite-supported metal catalysts that reduce the temperature for equivalent conversion of CVOCs, and thereby reducing energy consumption, is of interest. Early research focused on the preparation of zeolite-supported noble metal catalysts. Taralunga et al. ^[71] reported a HFAU zeolite-supported Pt catalyst, which showed much higher activity in the combustion of chlorobenzene than those of SiO2 and Al2O3-supported Pt catalysts.

Pinard et al. ^[72] investigated the influence of the acid-base properties of the zeolite in the catalytic oxidation of dichloromethane (DCM) over PtNaX and PtNaY catalysts. They proposed a general reaction mechanism involving mainly the transformation of DCM species adsorbed on acidic (H⁺, alcaline cation) and basic (zeolite, framework oxygens) sites into chloromethoxy then hydroxymethoxy species, as shown in Scheme 4. Chloromethoxy species resulted from the elimination of MCl (or HCl) from DCM adsorbed species undergoing a SN₂ substitution leading to HCl and to hydroxymethoxy species in the presence of water. Thereafter, the hydroxymethoxy species, corresponding to formaldehyde molecules adsorbed on protonic sites of zeolites, decompose into formaldehyde and bridging hydroxyl groups.

Scheme4

Scheme4. Mechanism of DCM hydrolysis species. Reproduced from Ref. [72].

However, the supported Pt catalysts suffer from deactivation as a result of the Pt species easily reacting with Cl in the organic compounds ^[73]. Therefore, supported noble metal catalysts are rarely used for the catalytic combustion of CVOCs. Researchers have since developed a series of supported transition metal catalysts for the combustion of CVOCs, which show good chloride-resistant properties ^[48]. Recently, zeolite- supported transition metal catalysts have received much attention because of their specific adsorptive properties.

Zhang et al. ^[74] have synthesized a series of ZSM-5 zeolite- supported metal catalysts (Co, Fe, Ce, Mn) following an impregnation method for the combustion of CH₃Cl and observed that the Co- and Ce-based catalysts possess higher activity than the corresponding Fe- and Mn-based catalysts. Later, Zhou et al. ^[75] showed that the supported transition metal oxide catalysts had higher activities than the supported transition metal catalysts, which suggests the importance of metal oxide lattice-oxygen species for the combustion of CVOCs. Recently, Yang et al. ^[76] prepared HZSM-5 zeolite-supported CeO₂ and Cr₂O₃ catalysts (9%Cr-12%CeO₂/HZSM-5), which exhibited outstanding activity in the combustion of 1,2-dichloroethane, giving a T₉₈ at 269 ℃. The high activity is explained by the stronger oxidation ability from the interaction between CeO₂ and Cr₂O₃. More importantly, this catalyst also exhibits excellent tolerance to coking and chloride poisoning. Such catalysts provide promising routes for the catalytic combustion of CVOCs.

4 Plasma assisted catalytic combustion of VOCs

Typical VOC gas concentrations are very low (ppm levels), and therefore there direct combustion is expensive and ineffective. To circumvent this problem, it is suggested to combine the processes of both adsorption and combustion, where the VOCs can be concentrated in the adsorbents, followed by the combustion of the VOCs. This combined process is thought to reduce energy costs. For example, Ogata et al. ^[77] showed that zeolite adsorbents in the presence of a plasma-assisted reactor for the combustion of VOCs should be recyclable and energy-saving process. Later, Trinh et al. ^[78] also reported the abatement of ethylene using a combination of non-thermal plasma and Ag/zeolite adsorbents. Notably, dilute ethylene was first adsorbed onto the catalyst over a period of 25 min before being plasma-catalytically oxidized. After adsorption, the plasma-oxidation of the adsorbed VOCs in the zeolite catalyst was catalytically transformed in a very short time. Since the plasma is only required during the catalytic conversion rather than adsorption process, the energy efficiency is relatively high. The outlook of VOC removal by combustion should look to combine various techniques to design more efficient and low-cost processes.

5 Summary and outlook

Recently, zeolite-based materials have been developed that exhibit many advantages for the removal of VOCs at low temperatures, which include energy-saving and environmentally friendly processes. A wealth of scientific research has been devoted to design highly efficient zeolite-based materials with excellent absorptive and catalytic properties for the removal of VOCs. Such properties are strongly dependent on the framework structure and pore size, Si/Al ratio, cation types, adsorption selectivity in zeolites, and the heterogeneity of metal distribution on the zeolite supports as a function of their size.

Notably, the use of zeolites in the total oxidation of VOCs still faces challenges to improve deactivation and the economic viability of the zeolite catalysts. Catalyst deactivation is mainly attributed to coking of the zeolite surface during the catalytic process. There has been many studies to develop tailored hierarchical zeolites ^{[79, 80]}, which have proven to significantly increase the catalysts’ tolerance to coking ^[37]. Additionally, the relatively high cost of zeolite production is in the use of relatively expensive organic templates in the synthesis and the relatively low efficiency of hydrothermal routes. Sustainable routes (e.g. organotemplate-free and solvent-free) would remarkably reduce the cost of zeolites ^{[81, 82]}, which will widen their application. The outlook for VOC removal looks promising and developing zeolite catalysts with unique microporous or hierarchical structures at a lower cost would offer good opportunities to further design and prepare a series of highly efficient adsorbents and catalysts for the removal of VOCs.

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