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Publicly Available Published by De Gruyter February 1, 2022

Mesoporous catalysts for catalytic oxidation of volatile organic compounds: preparations, mechanisms and applications

  • Jing Wang , Peifen Wang , Zhijun Wu , Tao Yu , Abuliti Abudula , Ming Sun , Xiaoxun Ma and Guoqing Guan ORCID logo EMAIL logo

Abstract

Volatile organic compounds (VOCs) are mainly derived from human activities, but they are harmful to the environment and our health. Catalytic oxidation is the most economical and efficient method to convert VOCs into harmless substances of water and carbon dioxide at relatively low temperatures among the existing techniques. Supporting noble metal and/or transition metal oxide catalysts on the porous materials and direct preparation of mesoporous catalysts are two efficient ways to obtain effective catalysts for the catalytic oxidation of VOCs. This review focuses on the preparation methods for noble-metal-based and transition-metal-oxide-based mesoporous catalysts, the reaction mechanisms of the catalytic oxidations of VOCs over them, the catalyst deactivation/regeneration, and the applications of such catalysts for VOCs removal. It is expected to provide guidance for the design, preparation and application of effective mesoporous catalysts with superior activity, high stability and low cost for the VOCs removal at lower temperatures.

Abbreviations

Al

aluminum

Au

gold

Ce

cerium

Co

cobalt

CTAB

hexadecyl trimethyl ammonium bromide

Cu

copper

Fe

iron

GC-MS

gas chromatography-mass spectrometry

in situ DRIFTS

in situ diffuse reflectance infrared Fourier transform spectroscopy

La

lanthanum

Mn

manganese

Mo

molybdenum

Ni

nickel

P123

poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer)

Pd

palladium

ppm

parts per million

ppmv

parts per million volume

PS-PVP-PEO

polystyrene-b-poly-2-vinylpyridine-b-ethylene oxide

Pt

platinum

PVA

polyvinyl alcohol

Ru

rubidium

SEM

scanning electron microscopy

Sr

strontium

T90%

the conversion temperature for 90% VOCs

TEM

transmission electron microscopy

Ti

titanium

V

vanadium

VOCs

volatile organic compounds

VOC-TPD

VOC-temperature programmed desorption

VOC-TPO

VOC-temperature programmed oxidation

vpm

parts per million volume concentration

Zr

zirconium

1 Introduction

Volatile organic compounds (VOCs) such as alkanes, aromatics, alkenes, halocarbons, esters, aldehydes, and ketones are a large class of easily-evaporated carbon-based organic chemicals, which are always emitted into air by a series of human activities like the extensive use of organic solutions in building materials, decorative materials, fiber materials, chemical industries, and transportation section (Drobek et al. 2015; Kamal et al. 2016; Liotta 2010). Some VOCs such as toluene and formaldehyde have carcinogenic, teratogenic, and mutagenic nature, which will not only threaten human health, but also participate in the formation of photochemical smog (Xia et al. 2018), thus, it is urgent to remove VOCs from the generation stages.

For decades, various approaches including thermal incineration, membrane separation, condensation, adsorption, absorption, and photocatalysis have been developed and applied for the VOCs removal (Li et al. 2009). However, the high cost of incineration and membrane separation, the intractable treatment of spent coolants and solvents of condensation, the solvents selection of selective adsorption for various VOCs, and the generation of secondary toxic substances with properties similar to formaldehyde during the photocatalysis of wall paints restrict their extensive applications (Bai et al. 2016; Kamal et al. 2016; Kujawa et al. 2015; Leson and Winer 1991; Shah et al. 2000). Compared with these methods, catalytic oxidation is an environmental and economically feasible way for the removal of VOCs, which can completely transform organic contaminant into harmless substances such as water (H2O) and carbon dioxide (CO2) at relatively low temperatures (<500 °C). For the VOCs catalytic oxidation, the most important issue is how to design rational catalysts with high activity and stability but low cost and environmentally friendly.

Two typical types of catalysts have been widely reported for the VOCs catalytic oxidation. One type is the supported noble metals such as platinum (Pt), gold (Au), palladium (Pd), and silver (Ag), which always show excellent performances at low temperatures. However, their high cost and limited availability restrict their large-scale applications (Masui et al. 2010). The other type is the transition-metal-oxide-based catalysts, which not only have low-cost, but also exhibit high performance and are sometimes comparable to the noble-metal-based catalysts if we can well design their structures to fully use their active sites (Zheng et al. 2021). Previous works indicated that fabrication of transition-metal-oxide-based catalysts with porous structures and special morphologies, such as nanorod, nanowire, nanotube and nanoflower-like structures, could expose more high-energy crystal faces with active sites to improve the adsorption and diffusion of the involved reactants and activate them for the further complete conversion (Pan et al. 2017).

Porous materials with high surface area and porosity are widely investigated (Pal and Bhaumik 2013). According to the definitions of International Federation of Pure and Applied Chemistry (IUPAC), the pore sizes of micropore, mesopore, and macropore are <2 nm, 2–50 nm, and >50 nm, respectively (Sing et al. 1985). For the VOCs removal catalysts, the mesopore should be more beneficial to the adsorption of VOCs molecules. Typical mesoporous materials include SBA-15, carbon nanotubes, clays, and some man-made mesoporous metal oxides such as MnO2, Co3O4, CeO2, and CeMnO x (Sanchis et al. 2018; Tang et al. 2014). As the catalysts or catalyst supports for VOCs removal, these materials should have not only large specific surface areas, tunable pore structures, and narrow pore size distributions, but also good thermal stability (Melero et al. 2002; Pal and Bhaumik 2013). Especially, large surface area could supply sufficient catalytic active sites, and thus improve the catalytic performance (Taguchi and Schüth 2005). As a result, mesoporous materials have a promising prospect in the application of catalysis. In this review, the preparation methods for the noble-metal-based and transition-metal-oxide-based mesoporous catalysts, the reaction mechanisms of the catalytic oxidations of VOCs over them, the catalyst deactivation/regeneration, and the application of such catalysts for VOCs removal will be introduced and discussed. It is expected to find more effective way to design, prepare, and apply higher-performance mesoporous catalysts for the VOCs removal at lower temperatures.

2 Preparations of mesoporous supports and catalysts

Various mesoporous materials, such as silica-based, carbon-based, phosphate-based, and transition-metal-based mesoporous materials, have been indirectly used as the catalyst supports or directly used as the catalysts for the catalytic oxidations of VOCs. The methods for the preparation of these mesoporous materials include sol–gel method, template-assisted method, microwave-assisted method and chemical etching method (Kumar et al. 2017). In the sol–gel method, three processes, i.e., sol formation, sol/gel transition and calcination, are necessary. In the template-assisted method, in order to obtain different morphologies and micropores with different sizes, various organic small molecules and triblock copolymers are always used as the templates (Niesz et al. 2005). The microwave-assisted method is a time-saving process, in which the sol–gel formation and template application could be also involved (Yao et al. 2001). The chemical etching method can be used to prepare materials with hollow-type nanostructures, in which the special porous structure or core/shell structure could be easily created and optimized by using the appropriate etchants (Zhang et al. 2012).

In general, the template-assisted method is the most widely adopted approach to fabricate the designed mesoporous materials, which can be classified into the exo-template method and end-template method when using the surfactant and porous solid as the templates respectively (Hoffmann et al. 2006). Four types of surfactants, i.e., anionic, cationic, nonionic and amphoteric surfactants, are generally used (Kim et al. 1997). The anionic surfactants, such as phosphates and carboxylates, have negatively charged polar groups, and on the contrary, the cationic ones have positively charged polar groups, which include gemini surfactants, multihead-group surfactants and so on. Nonionic surfactants stand for a group of surfactants with uncharged polar groups, which are widely used in the industrial scale due to their low price and nontoxicity. Poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer (P123) is one of the most widely used nonionic surfactants for the syntheses of silica-based mesoporous materials (El-Safty 2008; Wan et al. 2007). The amphoteric surfactants have both positive and negative charged heads, which interact with each other through neutralization to aggregate for the formation of mesoporous materials with different morphologies (e.g., spherical, rod-like micelle, or planar lamellar bi-layer structure) (Seo et al. 2004). Various amphoteric surfactants, such as cocamidopropyl betaine (CAPB) and N-dodecyl glycine, have been widely used for mesoporous material productions (Kim et al. 1997; Xiang et al. 2008).

2.1 Silica-based mesoporous catalyst supports

Silica-based mesoporous materials such as SBA series, MCM series, PDU series, and MSU series (Li et al. 2013; Zhu et al. 2009) always have highly porous and amorphous inorganic framework structures, tubular channels with well-defined mesopores and large surface areas. The typical one is SBA-15, whose preparation method is shown in Figure 1, in which P123 is always used as the structure directing agent, and after hydrolyzing, condensing, and removing the surfactant directing agent at the elevated temperature, SBA-15 can be obtained (Zhao et al. 1998). It always possesses large surface area with two-dimensional mesoporous structure, well-defined cylindrical channels, and good hydrothermal stability (Singh et al. 2018; Ungureanu et al. 2013), which can serve as a good support for catalysts. It was reported that metal nanoparticle catalysts could be encapsulated in the porous inorganic shells of SBA-15, which could protect metal nanoparticles from growing larger, and thus promote the catalytic performance of the involved catalysts (Parlett et al. 2017). The properties of SBA-15 can be further improved by the modifications either during the direct synthesis (Piumetti et al. 2010) or by post-treatment (Orlov and Klinowski 2009). Therefore, the modification provides a new perspective on the application of silica-based mesoporous materials.

Figure 1: 
Illustration for the formation of mesoporous SBA-15 support. Reprinted with permission from Singh et al., copyright 2018, Elsevier B.V.
Figure 1:

Illustration for the formation of mesoporous SBA-15 support. Reprinted with permission from Singh et al., copyright 2018, Elsevier B.V.

2.2 Carbon-based mesoporous materials

Carbon-based mesoporous materials always have excellent physical and chemical properties and are widely used as either catalyst supports or catalysts in many important reactions. In general, micropores are also contained in the carbon-based mesoporous materials such as activated carbon and carbon black, but they may not be favorable for the mass transfer of the reactant VOCs molecules during the catalytic oxidation. Thus, carbon-based materials with more mesopores have been developed for the preparation of heterogeneous catalysts (Liu et al. 2012).

Mesoporous carbon materials with the designed structure can be prepared by using either soft-template (e.g., amphiphilic molecules and block copolymers) or hard-template (e.g., mesoporous silica) synthesis method (Coughlin et al. 1969). The main synthesis process includes silica gel prepared with well controlled pore structures, formation of monomer or polymer precursors under the action of the silica template, carbonization of the carbon precursors and removal of the silica template (Liang et al. 2008). For example, Liu et al. (2012) prepared monolithic mesoporous carbons by assembling block copolymers with resorcinol-formaldehyde, which exhibited the hexagonal structures with uniform pore size distribution. Meanwhile, Qiu et al. (2019) fabricated a novel N-doped carbon nanospheres with mesoporous structures (NMCS) by a soft-template-assisted method, as depicted in Figure 2, in which hexadecyl trimethyl ammonium bromide (CTAB) was used as the soft template. The obtained material had a large specific surface area of 1093 m2 g−1 with a mesopore size of around 4 nm. Additionally, Bedia et al. obtained mesoporous carbon materials by chemical activation of kraft lignin using H3PO4, and found that Pd nanoparticles could be well dispersed and supported on it, resulting in a high catalytic performance (Bedia et al. 2010). Thus, in situ or post treatment of mesoporous carbon materials should be an effective way to improve its performance as the catalyst support.

Figure 2: 
Synthesis process of the N-doped mesoporous carbon nanospheres (NMCSs). Reprinted with permission from Qiu et al., copyright 2019, John Wiley & Sons, Inc.
Figure 2:

Synthesis process of the N-doped mesoporous carbon nanospheres (NMCSs). Reprinted with permission from Qiu et al., copyright 2019, John Wiley & Sons, Inc.

2.3 Phosphate-based mesoporous catalysts

Phosphate-based materials are a potentially important family of catalysts for the VOCs removal since they always have a neutral framework with large surface area (Bhaumik and Inagaki 2001). Transition metal (e.g., V, Ti, Zr, Al, and Co) phosphate can be prepared by mixing metal species containing compounds with the phosphate anions to form metal phosphate precipitates (Bhaumik 2002; Moffat 1978). Besides using as the VOCs oxidation catalysts, these materials can be also applied as effective catalysts for a variety of other reactions such as dehydration, hydrolysis, photocatalysis, and coupling reactions (Bhaumik and Inagaki 2001; Joshi et al. 2005; Moffat 1978).

Various methods for the preparation of mesoporous metal-containing phosphates have been widely reported. For examples, Bhaumik et al. (2002) synthesized mesoporous titanium phosphate molecular sieves by using cationic surfactants of octadecyltrimethyl ammonium halides as the structure directing agents and titanium isopropoxide as the Ti source. The obtained materials had high surface areas as well as high cation ion exchange capacities, and could be used as ion exchangers and catalysts. Sinhamahapatra et al. (2011) prepared a mesoporous zirconium phosphate by a microwave-assisted method, and found that the obtained material had excellent catalytic activity due to the acidic property with a large amount of Brönsted acid sites. Meanwhile, Bastakoti et al. (2016) prepared the mesoporous material of iron phosphate (FePO4) by using the soft-template method on the basis of the polymeric micelles’ assembly. As displayed in Figure 3, the mesoporous FePO4 was achieved by the effective interaction between the negatively charged PO43− ions derived from inorganic precursors and the positively charged PVP+ units derived from polymeric micelles, then the evaporation of organic solvent induced assembly of composite micelles, and finally the template was removed by calcination. Herein, the exisited PO43− ions provided a bridge between the micelle surface and the Fe3+ ions.

Figure 3: 
Illustration of mesoporous iron phosphate (FePO4) synthesis process. Reprinted with permission from Bastakoti et al., copyright 2016, John Wiley & Sons, Inc.
Figure 3:

Illustration of mesoporous iron phosphate (FePO4) synthesis process. Reprinted with permission from Bastakoti et al., copyright 2016, John Wiley & Sons, Inc.

2.4 Transition-metal-based mesoporous catalysts

There is growing interest in the synthesis and application of transition-metal-based mesoporous materials as catalysts, catalyst supports, adsorbents and sensors owing to their confining d-electrons, electrical and optical properties with the ordered pore structures as well as crystalline walls (Jiao et al. 2006; Jiao et al. 2008). The synthesis of such materials is similar to that of other mesoporous materials, which also involves the utilization of soft or hard template for the forming of mesoporous structure and subsequently the removing of the template. As displayed in Figure 4, mesoporous Ni–Mn mixed oxides were prepared by a nano-casting method templated with SBA-15, which possessed a large surface area of around 603.2 m2 g−1 and a small crystal size with a large pore volume of 0.89 cm3 g−1 (Tang et al. 2015). Aluminum-based porous materials with well-defined mesopores are easily achieved by the processes such as high-temperature dehydration, ordered mesoporous carbon as the template, and porous structure directing by self-assembly with block copolymers (Fang et al. 2004; Kuemmel et al. 2005; Liu et al. 2006). For example, Morris et al. (2008) prepared ordered mesoporous alumina materials by using the soft template of triblock copolymer as the structure directing agent, accompanied by self-assembly between the metal precursor and aluminum isopropoxide. The obtained materials exhibited well-developed mesoporosity, large surface areas, and crystalline pore walls. In addition, doping with the secondary metal could enhance the thermal stability.

Figure 4: 
Preparation of Ni–Mn mixed oxide templated by SBA-15. Reprinted with permission from Tang et al., copyright 2015, Elsevier B.V.
Figure 4:

Preparation of Ni–Mn mixed oxide templated by SBA-15. Reprinted with permission from Tang et al., copyright 2015, Elsevier B.V.

Other transition metals (e.g., Fe, Co, Ce, Mn, and Mo) can also be synthesized as the mesoporous oxide materials. For instance, ordered mesoporous Fe3O4 and Co3O4 have been synthesized by using the hard template of KIT-6, and both possessed the mesoporous structures with crystalline walls (Jiao et al. 2006; Sa et al. 2013). Meanwhile, mixed-metals-based mesoporous materials (e.g., Cu–Ce, Cu–Mn, Co–Mn, and Cu–Mn–Ce) have also been widely investigated and reported in the literature due to their superior catalytic performances. Herein, the synergistic effect of the mixed metal elements can improve the properties of the materials when they are used as catalysts or electrodes (Ma et al. 2012). In terms of VOCs removal, the mesoporous mixed metal oxides always show superior catalytic activity as the noble-metal-based catalysts due to the improved properties, such as larger surface areas, higher resistance ability to coke formation, higher oxygen storage capacity as well as redox cycling properties (Delimaris and Ioannides 2009; Hosseini et al. 2014). Meanwhile, the porous TiO2, clays, diatomite, perovskite, and many other transition-metal-based mesoporous materials are also widely used in the catalytic oxidations of VOCs.

3 Mechanisms of catalytic oxidation of VOCs

Simply, the complete oxidation of VOCs can be expressed by the following equation:

(1) C x H Y + 4 x + y 4 O 2 x C O 2 + y 2 H 2 O

When this reaction occurs over the catalyst, three mechanisms have been proposed (as listed in Figure 5): (1) Eley–Rideal (E–R) mechanism, which illustrates that the catalytic reaction is controlled by the absorbed molecules and those molecules in the gas phase; (2) Langmuir–Hinshelwood (L–H) mechanism, which explains that the reaction is promoted by the absorbed VOCs and the absorbed oxygen; and (3) Mars–van Krevelen (MVK) mechanism, which indicates that the reaction takes place between the absorbed VOCs molecules and the lattice oxygen in the catalyst (Balasubramanian and Viswanath 1975; Golodet͡s 1983).

Figure 5: 
Schematic illustration of the mechanisms for VOCs catalytic oxidation (replotted based on Balasubramanian and Viswanath 1975; Golodet͡s 1983).
Figure 5:

Schematic illustration of the mechanisms for VOCs catalytic oxidation (replotted based on Balasubramanian and Viswanath 1975; Golodet͡s 1983).

In general, the oxidations of VOCs over different catalysts should follow different mechanisms (Spivey 1987). For examples, Minicò et al. (2000) performed the catalytic oxidations of different VOCs over the supported catalysts of Au/Fe2O3, and found that the high catalytic activity was related to the lattice oxygen, thus, they considered that the reaction followed the MVK mechanism. However, Burgos et al. (2002) confirmed that the complete oxidation of 2-propanol over Pt/Al2O3/Al monolith followed the E–R mechanism since 2-propanol was chemisorbed on the Al2O3 and directly oxidized by the chemisorbed oxygen atom from the gas phase. Moreover, Chen et al. (2018a) prepared mesoporous Cr2O3-supported platinum (Pt@M-Cr2O3) catalysts for toluene catalytic oxidation, which followed the L–H mechanism, as displayed in Figure 6. As seen in the figure, O2 molecules adsorbed and dissociated on 0.82Pt@M-Cr2O3 catalyst, and afterwards, the chemisorbed toluene molecules immediately reacted with the formed active oxygen atom and transformed into benzylic and aldehydic species, then into benzoates species; while the benzoates species were gradually converted into maleic anhydrides and CO species, which adsorbed on Pt nanoparticles and finally oxidized to CO2 by O2.

Figure 6: 
Possible reaction mechanism of toluene catalytic oxidation over 0.82Pt@M-Cr2O3 catalyst. Reprinted with permission from Chen et al., copyright 2018a, Elsevier B.V.
Figure 6:

Possible reaction mechanism of toluene catalytic oxidation over 0.82Pt@M-Cr2O3 catalyst. Reprinted with permission from Chen et al., copyright 2018a, Elsevier B.V.

Additionally, the reaction mechanism for the oxidation of VOCs can also be affected by the types of VOCs because of their different nucleophilic characteristics. Especially, the unsaturated hydrocarbon can be strongly adsorbed on the active sites of Pt or Pd surface through their π-bonds (Yao 1984). As such, the most favored reaction mechanism over noble-metal-based catalysts should be the L–H mechanism, and the controlling step is the adsorptions of the VOCs and oxygen molecules on the active sites. Moreover, the L–H mechanism can also be classified into the single-site L–H model (adsorptions occur on the similar active sites) and the dual-site L–H model (adsorptions occur on the two different kinds of active sites) (Liotta 2010).

Meanwhile, the reaction mechanism over the transition-metal-oxide-based catalysts can also be possibly explained by the MVK mechanism. In this case, the absorbed VOC molecules will interact with the oxygen species from the involved catalysts, resulting in the metal oxide to be reduced. Then, the reduced metal oxide will be oxidized by the oxygen from the gas phase again. As such, the MVK mechanism over the transition-metal-oxide-based catalysts is also known as the redox mechanism (Song et al. 2001). As shown in Figure 7, the reaction process over Co/Sr-CeO2 catalyst can occur through the following steps: firstly, active surface oxygen species were generated from the gaseous oxygen through oxygen vacancies and metal ions of Co2+. Then, the big π bonds derived from toluene made phenyl species adsorbed on cobalt ions, leading to the methyl and phenyl groups reacted with the adsorbed active oxygen species, which was activated by the abstracted H-atoms and water formed. Subsequently, nucleophilic O2− inserted oxygen into toluene, and nondestructive by-products were generated in this selective oxidation process; meanwhile, transient surface oxygen species (e.g., O2− and O) reacted with carbon atoms which derived from the weakened C–C bonds, generating intermediate product, such as benzene. Finally, active oxygen species of O and ads-O2− oxidized the carbon atoms and promoted the destructive products selectively convert into CO2 and H2O (Feng et al. 2018). Other reaction mechanisms should also exist for the oxidation of VOCs over the transition-metal-oxide-based catalysts. It is still necessary to obtain a clear understanding on the mechanism so that the novel effective catalysts can be better designed.

Figure 7: 
Proposed reaction mechanism for the toluene catalytic oxidation over Co/Sr–CeO2 catalyst. Reprinted with permission from Feng et al., copyright 2018, Elsevier B.V.
Figure 7:

Proposed reaction mechanism for the toluene catalytic oxidation over Co/Sr–CeO2 catalyst. Reprinted with permission from Feng et al., copyright 2018, Elsevier B.V.

It is worth noting that VOCs derived from the real industrial emission always contain impurities such as H2O, CO2, and SO2, which may also have an effect on the real catalytic reaction mechanism. Wang et al. (2020g) conducted systematic work and proved that water vapor cannot change the reaction pathway during the catalytic oxidation of toluene, and the decreased catalytic activity could be fully recovered after stopping H2O introduction. While, for the impurities of CO2 and SO2, the experimental results showed that both of them also did not affect the reaction pathway of the toluene oxidation, however, they poisoned the catalysts, leading to the unrecovering of catalytic activity. Many researchers also investigated the reaction mechanism in the oxidation of the mixed VOCs by using various characterization methods such as GC-MS, VOC-TPD, VOC-TPO, and in situ DRIFTS analyses. The results indicated that the reaction mechanism also followed the catalytic oxidation pathway of single VOC oxidation (Wang et al. 2020f). Therefore, the main factors affecting the reaction mechanism of catalytic oxidation of VOCs may be the types of VOCs and catalysts since they can determine the reaction path.

4 Progress in applications of mesoporous catalysts for the oxidation of VOCs

As the most effective and economical way for the removal of VOCs, the catalytic oxidations over various mesoporous catalysts have been widely investigated. Herein, the recent progress in the catalytic oxidation of VOCs over the main mesoporous catalysts will be critically reviewed.

4.1 Single-noble-metal-based catalysts

Single-noble-metal (e.g., Pd, Au, Pt, and Ru)-based catalysts have been widely confirmed to have superior activity for the complete conversion of VOCs to harmless substances of CO2 and H2O at low temperatures (Avgouropoulos et al. 2006). It was found that the immobilizing and/or well dispersing of noble metal nanoparticles on the mesoporous support with a high surface area could improve the catalytic performance greatly (Centi 2001). To date, many researches have already been done on the investigation of the performance of mesoporous materials supported single-noble-metal-based catalysts for the VOCs catalytic oxidation. Table 1 summarizes some typical results reported in recent years, with the corresponding surface and pore structure information listed in Table S1. For example, Bendahou et al. (2008) supported noble metals of Pd and Pt on the mesoporous lanthanum (La)-doped SBA-15, both of which completely and selectively converted toluene to H2O and CO2 at a low temperature of around 200 °C. Herein, the La doping effectively modified the electronic density of the supported metals and increased the Pd (or Pt)–O bonding strength, which was conducive to the improvement of the catalytic activity of these catalysts. Abdelouahab-Reddam et al. (2015) supported Pt and CeO2 on the mesoporous carbon with a large surface area, and found that both Pt and Ce were highly dispersed on the carbon, and the obtained catalysts of Pt/Ce-C resulted in the complete conversions of ethanol and toluene to harmless substance of CO2 at 160 and 180 °C, respectively. Herein, the excellent catalytic performance can be assigned to the effective interaction between the well-dispersed Pt and Ce species. However, as displayed in Figure 8, when Pt was supported on bulk CeO2 (Pt/CeO2) with a low specific surface area and very few mesopores, much lower catalytic activity was exhibited when compared with that of the optimum sample Pt/10Ce-C, indicating that the surface area of the support and the well dispersion of Pt active species should play an important role in the catalytic oxidation of VOCs. Wang et al. (2018) supported ruthenium (Ru) oxide on a mesoporous material of cobalt phosphate-SiO2 cellular foams (CoPO-MCF) for the catalytic oxidation of vinyl chloride. Since the CoPO-MCF support possessed an ordered mesoporous structure with a large surface area, which made a great contribution on the well-dispersed active species of Ru, the prepared Ru/CoPO-MCF catalyst exhibited excellent catalytic performance in the oxidation reactions, namely, a low conversion temperature for T90% (the conversion temperature for 90% of vinyl chloride) at around 313 °C was achieved with good stability for a long-term test. This could contribute to the enhanced reducible and acidic properties of CoPO-MCF support and the strong interaction of ruthenium active species and the support.

Table 1:

Typical single-noble-metal-based catalysts reported in the literature.

Catalyst VOCs VOCs concentration WHSV Temperature (°C) Conversion (%) References
Pt/γ-Al2O3 N-hexane 250 ppm 108,000 mL g−1 h−1 ∼320 100 Ihm et al. (2004)
Pd/TiO2 Toluene, chlorobenzene 1000 ppm, 1000 ppm 20,000 mL g−1 h−1 164, ∼363 50 Tidahy et al. (2006)
Pt/TiO2 Formaldehyde 100–400 mg/m3 288,000 mL g−1 h−1 120 100 Peng and Wang, (2007)
Pd/SBA-15 Toluene 1000 ppm 6000 mL g−1 h−1 167 50 Bendahou et al. (2008)
Au/Ce–Ti–O Propene, toluene 6000 ppm, 1000 ppm 6000 mL g−1 h−1 ∼230, ∼330 100 Gennequin et al. (2009)
Pd/Co3AlO Toluene 800 ppm 30,000 mL g−1 h−1 190 90 Li et al. (2011)
Au/Ce oxides Ethyl acetate, ethanol, toluene ∼466 ppm, ∼930 ppm, ∼266 ppm 53,050 mL g−1 h−1 230, 250, 300 100 Bastos et al. (2012)
Au/Co3O4 Benzene, toluene, O-xylene 1000 ppm 20,000 mL g−1 h−1 189, 138, 162 90 Liu et al. (2014)
Pt–CeO2/activated carbon Ethanol, toluene 1000 ppm 40,000 mL g−1 h−1 160, 180 100 Abdelouahab-Reddam et al. (2015)
Pd/Al-doped TiO2 Ethanol 0.5 vol% 20,000 mL g−1 h−1 175 50 Zhu et al. (2017)
Pd/CoO O-xylene 1000 ppm 40,000 mL g−1 h−1 173 90 Xie et al. (2018)
Au/Fe2O3 Propene 8000 ppm GHSV = 100,000 h−1 370 90 García et al. (2019)
Pd/CeO2 Benzene 1000 ppm 20,000 mL g−1 h−1 187 90 Guo et al., (2019)
PdCu/ZSM-5 zeolite Toluene 50 ppm 36,000 mL g−1 h−1 152 90 He et al. (2020)
Pt–MnOx/CeO2 Toluene 1000 ppm 40,000 mL g−1 h−1 171 90 Fu et al. (2020)
Ag–Co3O4@carbon monoliths Formaldehyde 100 ppm GHSV = 30,0001 h−1 80 90 Wang et al. (2020e)
IrFe/meso-CeO2 Acetylene 1000 ppm 20,000 mL g−1 h−1 165 90 Li et al. (2021)
Pt–Co/HZSM-5 Dichloromethane 600 ppm 15,000 mL g−1 h−1 249 90 Su et al. (2021)
  1. WHSV, weight hourly space velocity; GHSV, gas hourly space velocity.

Figure 8: 
Catalytic results of Pt supported catalysts for total oxidation of ethanol. Reprinted with permission from Abdelouahab-Reddam et al., copyright 2015, Elsevier B.V.
Figure 8:

Catalytic results of Pt supported catalysts for total oxidation of ethanol. Reprinted with permission from Abdelouahab-Reddam et al., copyright 2015, Elsevier B.V.

Meanwhile, the preparation method could also play an important role in the tuning of the physiochemical properties of catalysts, thereby influencing their catalytic performances. For example, Yang et al. (2019) supported single-atom Pt on mesoporous material of Fe2O3 (meso-Fe2O3) by a PVA (polyvinyl alcohol)-protected reduction method for benzene combustion. The obtained catalyst of 0.25Pt/meso-Fe2O3 (with a 0.25 wt% of Pt loading amount) displayed excellent catalytic activity with a low conversion temperature of T90% = 198 °C in the benzene catalytic oxidation, which should be contributed to the well dispersion and good utilization of Pt atoms (Figure 9A). Additionally, the favorable interaction between Pt single-atom and the efficient support of meso-Fe2O3 also made huge contribution to the superior catalytic stability. For comparison, as Pt nanoparticles rather than Pt atoms were loaded on the meso-Fe2O3 (PtNP/meso-Fe2O3) support by using the general method, the obtained catalysts showed much higher conversion temperature of T90% = 250 °C (Figure 9B). Therefore, the performance of catalysts could be enhanced by the improved preparation method.

Figure 9: 
Diagram of single-atom Pt on 3D ordered mesoporous Fe2O3 (Pt/meso-Fe2O3) (A), catalytic result of meso-Fe2O3 supported Pt catalysts for benzene combustion (B). Reprinted with permission from Yang et al., copyright 2019b, Elsevier B.V.
Figure 9:

Diagram of single-atom Pt on 3D ordered mesoporous Fe2O3 (Pt/meso-Fe2O3) (A), catalytic result of meso-Fe2O3 supported Pt catalysts for benzene combustion (B). Reprinted with permission from Yang et al., copyright 2019b, Elsevier B.V.

4.2 Mixed-noble-metals-based catalysts

In order to improve the catalytic performance of the noble-metal-based catalysts for the catalytic oxidation of VOCs, mixed-noble-metals-based catalysts were also designed and applied. The synergistic effect between the different noble metals was found to be favorable for the catalytic activity (Barakat et al. 2011; Hosseini et al. 2009). Tables 2 and S2 summarize the reported typical mixed-noble-metals-based catalysts. For examples, Hosseini et al. (2007) supported Au and Pd together on the mesoporous support of TiO2, which resulted in lower toluene conversion temperatures (50% conversion, T50% for propene = 208 °C, and T50% for toluene = 219 °C) than those of single metal-based ones of Pd/TiO2 (T50% for propene = 250 °C, and T50% for toluene = 230 °C) and Au/TiO2 (T50% for propene = 332 °C, and T50% for toluene = 367 °C), respectively. Matějová et al. (2012) and da Silva et al. (2015) also proved the similar results. Chen et al. (2019) supported Pd–Pt mixed noble metals on the mesoporous material of Ce modified γ-Al2O3 for the catalytic oxidation of benzene, and also found that the obtained catalysts exhibited better catalytic activity than the corresponding single-noble-metal-based catalysts. When compared with the non-mesoporous catalyst of Pt–Pd/CeZr/cordierite honeycomb ceramics ((T95% = 205 °C) (Jiang et al. 2013), it also exhibited lower conversion temperature (T100% = 200 °C) in the catalytic oxidation of benzene, indicating that the mesoporous structure should be more favorable to the catalytic oxidation of VOCs. Barakat et al. (2012) also conducted the similar work, in which they doped Nb and V for the improvement of the catalytic activity. Consequently, the noble-metal-based catalysts exhibited higher catalytic activity by the doping of the second metal. To date, large amounts of efforts have already been made on the noble metal supported catalysts for the oxidation of VOCs, especially on the investigation of catalyst support effect, which is one of the pivotal factors affecting the supported active species.

Table 2:

Typical mixed-noble-metals-based catalysts reported in the literature.

Catalyst VOCs VOCs concentration WHSV Temperature (°C) Conversion (%) References
Pd–Au/TiO2 Toluene, propene 1000 ppm, 1000 ppm 60,000 mL g−1 h−1 219, 208 50 Hosseini et al. (2007)
Pd–Au/TiO2–ZrO2 Toluene 1000 ppm 60,000 mL g−1 h−1 202 50 Hosseini et al. (2009)
Pd–Au/TiO2–ZrO2 Toluene, propene 1000 ppm, 3000 ppm 60,000 mL g−1 h−1 202, 193 50 Barakat et al. (2011)
Pd–Au/TiO2 Toluene 1000 ppm 60,000 mL g−1 h−1 ∼218 100 Hosseini et al. (2012)
Pt–Pd/Al2O3–Ce Ethanol, toluene 1000 vpm, 1000 vpm 71 m3 kg−1 h−1 212, 285 90 Matějová et al. (2012)
Pd–Pt(6:1)/Ce/solid kaolin/NaY composite Benzene 1000 ppm GHSV = 20,000 h−1 205 90 Zuo et al. (2014)
Au–Pd/Co3O4 Toluene 1000 ppm 40,000 mL g−1 h−1 168 90 Xie et al. (2015)
Au–Pd/MCM-41 Benzene, toluene, O-xylene 1.2 g m−3, 0.7 g m−3, 0.5 gm−3 90,000 mL g−1 h−1 ∼213, 300, ∼485 50
Pt–Pd/MCM-41 Toluene 500 ppm GHSV = 10,000 h−1 175 90 Fu et al. (2016)
Au–Pd/Cr2O3 Toluene 1000 ppm 20,000 mL g−1 h−1 165 90 Wu et al. (2016)
Pd–Pt/SiO2 Toluene 1000 ppm 60,000 mL g−1 h−1 160 98 Wang et al. (2017)
Pt–Pd/Al2O3 Toluene 950 ppm GHSV = 19,500 h−1 160 90 Liu et al. (2018)
Pd–Pt/Ce/γ-Al2O3 Benzene 1000 ppm 20,000 mL g−1 h−1 200 100 Chen et al. (2019b)
Ag@Pd/MnO2 Toluene 1.0 g m−3 15,000 mL g−1 h−1 185 99.3 Li et al. (2019b)
AgAuPd/Co3O4 Methanol 0.1 vol% 80,000 mL g−1 h−1 112 90 Yang et al. (2019a)
Pd–Pt/CeO2-γ-Al2O3 Toluene 1000 mg m−3 GHSV = 5000 h−1 228 90 Yang et al. (2020)
Pt–Pd/MnO x Toluene 50 ppm 36,000 mL g−1 h−1 175 100 He et al. (2021)
Pd–Pt/MgO/γ-Al2O3 Toluene 243 ppm GHSV = 5000 h−1 235 90 Yang et al. (2021)
  1. WHSV, weight hourly space velocity; GHSV, gas hourly space velocity.

4.3 Supported transition-metal-oxide-based catalysts

Mesoporous material supported transition-metal-based catalysts have already been widely investigated and applied in the catalytic oxidation of VOCs, and some of the typical researches on the mesoporous material supported single-transition-metal-based catalysts and the mesoporous material supported mixed-transition-metals-based catalysts are summarized in Tables 3, S3, 4 and S4, respectively. For examples, Gaur et al. (2005) impregnated single transition metals (i.e., Co, Cr, Cu, and Ni) on the active carbon fiber (ACF) and applied them to the catalytic oxidations of toluene and m-xylene, and found that the Ni-oxide-based catalysts with a proper loading amount exhibited the best catalytic performance among all the prepared single-transition-metal-based catalysts, however, a higher loading amount would block the micro and mesopores of ACF support, thereby hindering the mass transfer during the reaction process. Chuang et al. (2010) demonstrated that the single-transition-metal-based catalyst of Cu/SBA-15 was the most active one for the catalytic oxidation of toluene. Moreover, as they doped the second metal of Mn together with Cu on SBA-5, they found that the catalytic activity was further improved significantly. Recently, more researches have been focused on the mixed-transition-metals-based catalysts in the field of VOCs removal (as those examples shown in Table 4). For instances, Yu et al. (2010) supported MnO x –CeO2 mixed oxide on mesoporous TiO2, which converted 89.4% of toluene into carbon dioxide at a temperature as low as 180 °C, and the complete toluene conversion temperature was 40 °C lower than those over the related Ce-free catalysts, suggesting that Ce addition can significantly improve the catalyst performance. Yang et al. (2013) supported Cr–Ce mixed oxide on the modified HZSM-5 zeolite for the oxidation of chlorinated VOCs, and found that the CeO2 mixed with Cr2O3 promoted the formation of Cr6+ active species, which in turn enhanced the mobility of the active oxygen species and improved the catalytic activity of the catalysts.

Table 3:

Mesoporous materials supported single-transition-metal catalysts reported in the literature.

Catalyst VOCs VOCs concentration WHSV Temperature (°C) Conversion (%) References
Cr/MCM-48 Trichloroethylene 34,000–60,000 ppm 4500 mL g−1 h−1 350 100 Kawi and Te (1998)
WOx/activated carbon Toluene 500 ppm 14,000 mL g−1 h−1 350 87 Alvarez-Merino et al. (2004)
Ni/activated carbon fiber Toluene 2000 ppm 300 ∼90 Gaur et al. (2005)
Co/porous carbons Toluene 150 ppm 30,000 mL g−1 h−1 250 100 Lu et al. (2009)
Cu/SBA-15 Toluene 200 ppm 30,000 mL g−1 h−1 250 ∼70 Chuang et al. (2010)
Ce–Al/silica spherical particles Acetone 1000 ppmv GHSV = 15,000 h−1 150 ∼80 Wang and Bai (2011)
KMn/SBA-15 Toluene 1600 ppm 265 50 Qu et al. (2012)
Co3O4/carbon nanotubes Toluene 850 ppm 18,000 mL g−1 h−1 257 100 Jiang and Song (2013)
CuO/Ce x Zr1−xO2 Toluene 4400 ppm 33,000 mL g−1 h−1 275 100 Deng et al. (2014)
NiO/SiO2 Toluene 80 ppm 1000 mL g−1 h−1 <200 100 Jeong et al. (2014)
VO x /CeO2 Chlorobenzene 1000 ppm 30,000 mL g−1 h−1 307 90 Huang et al. (2015)
Mn/KIT-6 Chlorobenzene 5000 ppm GHSV = 20,000 h−1 210.7 90 He et al. (2016)
Cu/CeO2 Ethyl acetate 466.7 ppm 600,000 mL g−1 h−1 275 100 Konsolakis et al. (2017)
Fe2O3/Al2O3 Toluene 160 ppm 120 mL g−1 h−1 350 100 Kim et al. (2017)
MnOx/SBA-15 Toluene 500 ppm 15,000 mL g−1 h−1 230 >90 Qin et al. (2019)
MnO x /Cr2O3 Toluene 1000 ppm 20,000 mL g−1 h−1 268 90 Chen et al. (2019a)
MnO x /Co3O4 nanowire Acetone 900 ppm 49,200 mL g−1 h−1 227 90 Zhao et al. (2020)
CeO2/Co3O4 Acetone 600 ppm 18, 600 mL g−1 h−1 180 90 Zheng et al. (2020)
CuO/Co3O4 Toluene 1000 ppm 20,000 mL g−1 h−1 229 90 Xu et al. (2021)
MnO x @Carbon matrix Acetone 750 ppm 56,000 mL g−1 h−1 167 90 Zheng et al. (2021)
  1. WHSV, weight hourly space velocity; GHSV, gas hourly space velocity.

Table 4:

Mesoporous materials supported mixed-transition-metal catalysts reported in the literature.

Catalyst VOCs VOCs concentration WHSV Temperature (°C) Conversion (%) References
Cr-Cu/H-ZSM-5 Chlorinated VOCs 2500 ppm GHSV = 32,000 h−1 400 100 Abdullah et al. (2006)
Cu-Mn/MCM-41 Toluene 3500 ppm 36,000 mL g−1 h−1 320 95.2 Li et al. (2008)
LaCoO3/SBA-15 Toluene, ethyl acetate 1000 ppm, 1000 ppm 20,000 mL g−1 h−1 284, 290 100 Deng et al. (2009)
Mn–Ce/Al-pillared clays Benzene 130–160 ppm 25,000 mL g−1 h−1 310 100 Zuo et al. (2009)
MnO x –CeO2/TiO2 Toluene 1000 ppm GHSV = 15,000 h−1 240 100 Yu et al. (2010)
Cr–Ce/Ti-pillared interlayered clays Nitrogen-containing VOCs 1000 ppm 60,000 mL g−1 h−1 180 50 Huang et al. (2010)
Cu–Mn/Al2O3 Ethanol, toluene 1000 ppmv, 1000 ppmv 71,000 mL g−1 h−1 184, 301 90 Matějová et al. (2012)
Cu–Mn/TiO2 Benzene, acetaldehyde 900 ppm, 500 ppm GHSV = 30,000 h−1 350, 200 100 Doggali et al. (2012)
Co–Ce/SBA-16 Benzene 1000 ppm GHSV = 20,000 h−1 265 100 Zuo et al. (2013)
Co–Ce/USY zeolite Benzene 1000 ppm 21,400 mL g−1 h−1 ∼250 100 Li et al. (2017)
Mn–Ce/Silica Benzene 100 ppm 12,000 mL g−1 h−1 216 90 Li et al. (2018)
Co–Ce/Silica-pillared clay Toluene 1000 ppm 20,000 h−1 200 98 Cheng et al. (2018)
CoMn/SBA-15 Propane, n-hexane 2000 ppm, 1000 ppm GHSV = 6000 h−1 ∼318, ∼314 98, 100 Todorova et al. (2019)
FeO x –CeO x /SBA-15 Formaldehyde 9.8 μg L−1 300,000 mL g−1 h−1 94.9 60 Fan et al. (2020)
Co3O4@MnOx/Nickel foam Acetone 580 ppm 13,000 mL g−1 h−1 177 90 Zhao et al. (2020b).
Co–Mn/Silica N-hexane 1000 ppm GHSV = 60,000 h−1 290 ∼100 Todorova et al. (2021)
  1. WHSV, Weight hourly space velocity; GHSV, Gas hourly space velocity.

Transition metal oxides can be not only supported on other mesoporous materials as the catalysts, but also applied as the catalyst supports. For instance, Kustov et al. (2011) doped perovskite nanoparticles of LaCoO x in the matrix of mesoporous ZrO2 and used them for the catalytic oxidation of methanol. It was found that 80% of methanol was converted at a temperature as low as 150 °C. Herein, the mesoporous structure of the ZrO2 support provided the possibility to form nano-sized particles in it and therefore improved the catalytic performance. Doggali et al. (2012) supported Cu–Mn mixed oxide catalysts on different mesoporous supports of Al2O3, TiO2, and ZrO2 for the catalytic oxidation of benzene and acetaldehyde, and found that Cu–Mn/TiO2 and Cu–Mn/ZrO2 catalysts had higher catalytic performance than Cu–Mn/Al2O3 due to the improved mass transfer and better redox properties of these two catalysts. Hence, it is important to consider the synergitic effects between different trasition metal oxides as well as the interaction between the active species and the mesoporous support so as to obtain the optimum mixed transition-metal-oxide-based catalysts for the oxidation of VOCs.

4.4 Unsupported mesoporous transition-metal-oxide-based catalysts

Unsupported mesoporous transition-metal-oxide-based catalysts can be synthesized directly so that they are not necessary to support on mesoporous materials. As summarized in Tables 5 and S5, various mesoporous transition-metal-oxide-based catalysts with special morphologies have been reported for the catalytic oxidation of VOCs. For instance, He et al. (2013) prepared catalysts of mesoporous structured CuO x –CeO2 with ordered structure via the hard-template method, and found that the most active catalysts converted epichlorohydrin into CO2 with a superior selectivity (>99%) and stability for long-term evaluation of 50 h, which was mainly assigned to the well dispersion of active species within Cu x Ce1−xO2−δ, high amount of surface-active oxygen and good reducibility at low temperatures. Deng et al. (2018) prepared MnO2@NiO composite with a core@shell structure as shown in Figure 10A–D. The experimental results confirmed that the hetero-interface existed between MnO2 and NiO could effectively enhance the reducibility at low temperature and increase the surface oxygen active species of the catalyst with special structure, and as a result, the catalytic activity and stability of core@shell structured MnO2@NiO catalyst for benzene oxidation were better than those of single MnO2 (Figure 10E and F). Li et al. (2019) prepared CoMnO x catalyst with a mesoporous coral-like morphology for the catalytic oxidation of benzene, which exhibited better catalytic activity than that of single-transition-metal-based catalysts of Mn3O4 and Co3O4. It should be attributed to its larger specific surface area, better reducibility at low temperatures and larger amount of surface-active oxygen species. Meanwhile, CoMnO x catalysts with a flower-like morphology were also synthesized (Figure 11), which were extremely active to the toluene catalytic oxidation due to the interaction between Mn and Co accompanied by the large number of active species of surface oxygen and oxygen vacancies (Wang et al. 2018). In our previous works, a series of catalysts of Co-Cu/NF (NF represents nickel foam) (Wang et al. 2020b), Co–Ce/NF (Wang et al. 2020d), Ag–CeO2@CNWs/CF (CF represents Cu foam) (Wang et al. 2020c), and Mn–Co/CF (Wang et al. 2020a), were also prepared, respectively, which possessed special morphologies of flower-like, nanosheet, nanowire and nanoparticles, and showed good catalytic activity, stability and water resistance in the catalytic oxidation of toluene. Therefore, it is a promising way to directly design and synthesize mesoporous transition-metal-based catalysts with special morphologies for the catalytic oxidation of VOCs.

Table 5:

Unsupported mesoporous transition-metal-based catalysts reported in the literature.

Catalyst VOCs VOCs concentration WHSV Temperature (°C) Conversion (%) References
3D chromium oxide Toluene 100 ppm 30,000 h−1 280–300  100 Sinha and Suzuki (2005)
Mn–Zr mixed oxides Dichloroethane, trichloroethylene 1000 ppm, 1000 ppm 35,200 mL g−1 h−1 305, 315 50 Gutiérrez-Ortiz et al.2007
Mn pillared TiP Diethyl ether 1200–1500 ppm 3600 mL h−1 290 100 Das and Parida (2007)
CuO–CeO2 mixed oxides Benzene 1000 ppm 96,000 mL g−1 h−1 230 100 Hu et al. (2008)
Mesoporous chromia Toluene, acetate 1000 ppm, 1000 ppm 20,000 mL g−1 h−1 290, 250 100 Xia et al. (2009)
Cerium oxides Naphthalene 450 ppm GHSV = 75,000 h−1 275 90 Puertolas et al. (2010)
MnO x Formaldehyde 500 ppm 160 100 Torres et al. (2011)
Iron oxide Acetone, methanol 1000 ppm, 1000 ppm 20,000 mL g−1 h−1 186, 189 90 Xia et al. (2011)
CuO x –CeO2 Heteroatom-containing VOCs 40,000 mL g−1 h−1 174 90 He et al. (2013)
Mn–Co mixed oxide Ethyl acetate, n-hexane 1000 ppm 120,000 mL g−1 h−1 220, 230 100 Tang et al. (2014)
CeO2–CuO2 Chlorinated VOCs ∼1000 ppm 9000 mL g−1 h−1 224 90 Yang et al. (2015b)
Mn3O4-based monolith Ethylene, propylene, toluene 1000 ppm 30,000 mL g−1 h−1 262 90 Piumetti et al. (2015)
CoMnAl oxides Benzene 100 ppm 30,000 mL g−1 h−1 208 90 Mo et al. (2016)
Co3O4 O-xylene 1000 ppm 40,000 mL g−1 h−1 240 83 Xie et al. (2017)
MnO2-nanowire@NiO-nanosheet Benzene 1000 ppm 120,000 mL g−1 h−1 320 100 Deng et al. (2018)
CoMnO x Benzene 1000 ppm 20,000 mL g−1 h−1 210 100 Li et al. (2019a)
Co3O4 2-propanol 6.4% in air 12,000 mL g−1 h- 157 90 Dissanayake et al. (2020)
CeMnO x Toluene 500 ppm GHSV = 60,000 h−1 246 90 Zhang et al. (2020)
Cu–Mn oxide Benzene 1000 ppm 21,740 mL g−1 h−1 219 90 Lee et al. (2020)
CoMn oxides Toluene 1000 ppm 60,000 mL g−1 h−1 247 90 Liu et al. (2021)
  1. WHSV, weight hourly space velocity; GHSV, gas hourly space velocity.

Figure 10: 
SEM and corresponding TEM images of MnO2 nanowire (A) and (C); MnO2-NW@NiO composite (B) and (D); catalytic performance over MnO2-NW, NiO, and MnO2-NW@NiO for the catalytic oxidation of benzene (E); stability test at 320 °C of MnO2-NW and MnO2-NW@NiO (F). Reprinted with permission from Deng et al., copyright 2018, Elsevier B.V.
Figure 10:

SEM and corresponding TEM images of MnO2 nanowire (A) and (C); MnO2-NW@NiO composite (B) and (D); catalytic performance over MnO2-NW, NiO, and MnO2-NW@NiO for the catalytic oxidation of benzene (E); stability test at 320 °C of MnO2-NW and MnO2-NW@NiO (F). Reprinted with permission from Deng et al., copyright 2018, Elsevier B.V.

Figure 11: 
TEM (A), (B) and FE-HRTEM (C) images of the CoMnO
x
 sample, and EDS elemental maps for the CoMnO
x
 sample (D)–(H). Reprinted with permission from Wang et al., copyright 2018b, John Wiley & Sons, Inc.
Figure 11:

TEM (A), (B) and FE-HRTEM (C) images of the CoMnO x sample, and EDS elemental maps for the CoMnO x sample (D)–(H). Reprinted with permission from Wang et al., copyright 2018b, John Wiley & Sons, Inc.

5 Deactivation/regeneration of catalysts

5.1 Deactivation of catalysts

In a practical process for the catalytic oxidation of VOCs, the stability and reusability of the catalysts are more important than the activity. Thus, catalyst deactivation always receives extensive attention when considering an industrial catalyst. Generally, fouling, poisoning, thermal degradation, vapor–solid and/or solid–solid reactions, vapor compound formation accompanied by transport, and attrition/crushing are the six main reasons for the catalyst deactivation (Bartholomew 2001). Herein, some examples will be given to discuss the deactivation of noble-metal-based catalysts and transition-metal-oxide-based catalysts in the catalytic oxidation of VOCs and the strategies to solve the catalyst deactivation as well as to regenerate the spent catalysts.

One of the common reasons for the deactivation of noble-metal-based catalysts is sintering, which is an irreversible process. As reported by Cui et al. (2019), the catalytic activities of the aged Pd/Al2O3 catalyst and aged Pd/H-Al2O3 catalyst were significantly decreased in the catalytic oxidation of methane (CH4) as a result of the agglomeration of Pd nanoparticles (as it displayed in Figure 12A). Herein, the high temperature aging led to the particle size increasing from 7 to 13 nm. In order to solve this problem, a protecting layer of Al2O3 was deposited on Pd/Al2O3, which effectively resisted the sintering even at a high temperature of 700 °C (Figure 12B). Thus, how to avoid the sintering during the reactions and/or the catalyst preparation stage at a high temperature should be considered for the VOCs removal catalysts. Meanwhile, poisoning is another main reason for the catalyst deactivation. During the catalytic oxidation of VOCs, especially for those containing halogen, sulfur, nitrogen elements and/or some other impurities, some inactive species may be strongly chemisorbed on the active sites of the catalyst so that the catalysts are poisoned to lose the activity (Bartholomew 2001). In a practical process, the VOCs emissions from industries always contain various airborne pollutants such as sulfur, chlorine, or nitrogen, which will easily lead to the catalyst deactivation. For instance, Gélin et al. (2003) conduted the complete oxidation of methane in the presence of H2S, and found that the presence of H2S resulted in the irreversible deactivation of Pd/Al2O3 catalysts due to the poisoning of the PdO active species by those sulphate species. The similar poisoning could be happened in many practical processes.

Figure 12: 
Catalytic performance of Pd/Al2O3, Pd/H-Al2O3 and 45Al/Pd/Al2O3 catalysts with 0.5 wt% nominal Pd loading before and after aging at 700 °C for CH4 catalytic oxidation (A), and TEM images of 45Al/Pd/Al2O3 catalyst aged at high temperature of 700 °C (B). Reprinted with permission from Cui et al., copyright 2019, Elsevier B.V.
Figure 12:

Catalytic performance of Pd/Al2O3, Pd/H-Al2O3 and 45Al/Pd/Al2O3 catalysts with 0.5 wt% nominal Pd loading before and after aging at 700 °C for CH4 catalytic oxidation (A), and TEM images of 45Al/Pd/Al2O3 catalyst aged at high temperature of 700 °C (B). Reprinted with permission from Cui et al., copyright 2019, Elsevier B.V.

In order to avoid the catalyst deactivation, Ma et al. (2018) supported Pd on mesoporous SBA-15 for the catalytic oxidation of n-butylamine, and found that Pd nanoparticles could be well confined in the mesoporous channels of SBA-15 with good thermal stability, which led to the low-temperature oxidation of n-butylamine. Additionally, the high porosity and the micropore of the catalyst made a large contribution to the low production of NOx. The similar work was also conducted by Darif et al. (2016), who found that the acid properties of the catalyst support remarkably enhanced the resistance of catalysts for sulfur poisoning, and thus enhanced their activity in the catalytic oxidation of dimethyl disulfide. Moreover, Cao et al. (2018) supported Pt, Pd, and Ru on the TiO2 support for the catalytic oxidation of dichloromethane, and found that the obtained Pt/TiO2 and Pd/TiO2 catalysts suffered serious deactivation problems during the reaction process, which was assigned to the chlorine poisoning and carbon species deposition. In contrast, the Ru/TiO2 catalyst displayed much better catalytic performance and higher stability in a long-term test than the other two catalysts because the harmful species of chlorine and carbon could be effectively moved away by RuO2. They also pointed out that the suitable pore-size distribution of the catalyst should be beneficial to the by-product’s desorption during the reaction process. Consequently, the catalyst deactivation by poisoning could be avoided by using suitable active species and catalyst supports.

The deactivation also always occurs for the transition-metal-oxide-based catalysts. Huang et al. (2010) used Cr–Ce/silica-pillared clay catalysts for different kinds of nitrogen-containing VOCs oxidation, and found that the –NH2 group of VOCs was easily and strongly absorbed on the acid sites of catalysts, resulting in a sudden decrease in the catalytic activity. Water vapor in the VOCs flow also has a great effect on the VOCs removal efficiency due to the competitive adsorption with VOCs molecules on the active sites (Gelin et al. 2003; Chen et al. 2019). For example, CeO2 catalysts were prepared by Chen et al. (2018b) for the complete toluene oxidation, in which the mesoporous structure was derived from metal-organic frameworks of Ce (named as Ce-MOF) (Figure 13). It displayed excellent catalytic performance with 100% conversion at a temperature as low as 260 °C and superior stability in the presence and absence of water vapor. However, the toluene conversion decreased lower than 100% when water vapor was introduced, which was assigned to the competition adsorption between toluene molecules and water on the active sites of the catalyst. Consequently, the humid environment had a negative effect on the catalytic oxidation of VOCs. Fortunately, it is noted that the catalytic activity of the catalyst would be recovered after stopping the water vapor introduction. Coke formation is also one of the main reasons for catalyst deactivation because VOCs are carbon-based chemicals. Yang et al. (2015a) synthesized mesoporous CeO2–CrO x catalysts for the catalytic oxidation of chlorinated VOCs, in which the partial Cr element was inserted into the CeO2 lattice and formed mixed oxide of Ce–Cr–O, and the favorable interaction between cerium oxide and chromium oxide enhanced the catalytic activity. However, obvious deactivation occurred during the durability test due to the coke formation and the adsorption of Cl species on the catalyst surface, but this situation could be improved by increasing the reaction temperature.

Figure 13: 
Long-term stability test at different reaction temperatures (A) and influence of water vapor (10, 20 vol%) on the catalytic activity of CeO2-MOF/350 catalyst (B). Reprinted with permission from Chen et al., copyright 2018a, Elsevier B.V.
Figure 13:

Long-term stability test at different reaction temperatures (A) and influence of water vapor (10, 20 vol%) on the catalytic activity of CeO2-MOF/350 catalyst (B). Reprinted with permission from Chen et al., copyright 2018a, Elsevier B.V.

5.2 Regeneration of catalysts

Deactivation is generally an unavoidable phenomenon in an industrial process. How to regenerate the deactivated catalyst is also an important issue during the catalyst development. For example, it was found that the H2S-poisoned Pd/Al2O3 catalyst could only be partly regenerated under the O2/He atmosphere at a temperature range of 673–873 K (Gelin et al. 2003). Herein, the sulphur species were redistributed on the surface of the catalyst so that the active species of PdO were partly regenerated after the regeneration. Moreover, the poisoned catalyst could still not be regenerated completely even the temperature was increased to as high as 923 K. It was found that H2S could result in the irreversible deactivation of Pd/Al2O3 because of the formation of the inactive species of PdSO4 during the reaction process. Thus, even some noble-metal-based catalysts could not be regenerated after deactivated. Currently, strategies for the regeneration of the catalysts include heat treatment, oxygen plasma treatment, chemical swing treatment, ozone injection treatment, radio frequency (RF) plasma treatment, and pin-to-plate dielectric barrier discharge treatment (HafezKhiabani et al. 2015). For an industrial process, the regeneration process should be simple and cost efficient. As it stated above, catalyst deactivation can be avoided or the poisoned catalyst could be regenerated after reaction, however, most importantly, the best approach is to prevent catalyst deactivation before the reaction occurs, rather than the regeneration of catalysts after the reaction.

6 Conclusions and outlook

In summary, mesoporous materials-based catalysts have shown good performance in the catalytic oxidation of VOCs. In this review, we focus on the preparations, reaction mechanisms and applications of the mesoporous catalysts in the field of VOCs removal, in which four typical mesoporous materials related catalysts as well as their synthesized methods are introduced and discussed. In addition, three kinds of reaction mechanisms, i.e., E–R mechanism, L–H mechanism and MVK mechanism, are introduced and discussed, which are dependent on the applied catalysts and the types of the involved VOCs in the reactions. Especially, the progress in the developments of noble-metal- and transition-metal-oxide-based catalysts with mesoporous structures are summarized. It can be concluded that the catalytic performance of mixed-noble-metal-based catalysts is better than that of single-noble-metal-based ones because of the synergistic effect between the mixed noble metals. Meanwhile, their catalytic performances are tremendously affected by the properties of the catalyst support. For mesoporous transition metal oxides, they can be not only supported on other mesoporous materials as active species, but also used as catalyst supports and catalysts directly. In particular, transition metal oxides can be synthesized with special morphologies such as core–shell, coral, and wormhole-like structures, which can make contributions to the catalytic activity of the catalysts. Moreover, catalyst deactivation is a serious problem especially during a long-term VOCs removal process, and thus, the ways to avoid the deactivation and to regenerate the deactivated catalysts are also introduced and discussed. Importantly, preventing catalyst deactivation is more important than regenerating catalysts.

It is worth noting that there is still great challenge for the development of novel catalysts with high catalytic activity, good stability, strong poisoning resistance, and pretty cost for the catalytic oxidation of VOCs. In terms of supported noble-metal-based catalysts, their catalytic performance is not only related to the nature of the noble metal, but also closely associated with the physiochemical properties of the support. As stated above, the materials with porous structures and large specific surface areas can act as catalyst supports, which are favorable for the dispersion and stabilization of the supported noble metal, and finally promote their physiochemical properties in the catalytic reaction. Many researches are concentrated on the noble-metal-based catalysts due to their high activity for the oxidation of VOCs. However, some such catalysts also have their poor resistance to poisoning and sintering, and the high costs limit their wide applications. Especially, in a practical industrial process, long-term stability is as important as high activity. Consequently, current works pay more attention to the long-term stability of the catalyst and have already made some achievements. Catalysts such as Pt–CeO2/activated carbon (Abdelouahab-Reddam et al. 2015), Pd/Co3AlO (Li et al. 2011), and Au/Co3O4 (Liu et al. 2014) have been confirmed to have excellent stability for VOCs oxidation during the long-term tests. Herein, mesoporous supports not only improve the adsorption and diffusion of the reactant molecules, but also prevent the aggregation of the nanoparticles by confining them within their mesoporous structures. Nevertheless, VOCs derived from industrial processes always contain other toxic substances such as sulfur or halogen, which can cause irreversible deactivation of the catalyst. Many efforts have been devoted to solve such issues, and the developed mesoporous catalysts of Pt/TiO2–La2O3 (Yang et al. 2017), Au/CeO2–Al2O3 (Nevanperä et al. 2016), Pd/HAP (Boukha et al. 2018), and Au–Pd/CeO2 (Zhang et al. 2019) demonstrated excellent poisoning resistance, especially for the catalytic oxidation of sulfur or chlorine containing VOCs. Thus, it can be concluded that those noble-metal-based catalysts with good resistance to poisoning and sintering can be well designed by using appropriate catalyst supports and preparation methods.

Moreover, it is always considered that the transition-metal-oxides-based catalysts are more superior than the noble-metal-based ones because of their low-cost, strong poisoning resistance, and high thermal stability for the heteroatom-containing VOCs oxidation. One of the advantages of the transition metal oxides is that their morphologies can be easily controlled by applying different kinds of preparation methods. Meanwhile, the mixed-metal-oxides-based catalysts have been proved to have high capacity for the elimination of VOCs efficiently at relatively low temperatures as a result of their improved physiochemical properties such as the low temperature reducibility and active oxygen species (e.g., surface oxygen, lattice oxygen, oxygen vacancies) of the catalyst. Catalysts of 3D mesoporous structured CuO x –CeO2 (He et al. 2013), nanocube-shaped Co3O4 (González-Prior et al. 2016), porous CeO2–TiO2 mixed oxides (Shi et al. 2016), sandwich-shaped CeO2@ZSM-5 composites (Dai et al. 2017), and La modified HZSM-5 zeolite (Liu et al. 2017) were proven to have high activity and stability for the chlorine and sulfur containing VOCs oxidation. Thus, transition metal oxides are the alternative and desirable catalysts with low-cost for various VOCs oxidation when compared with the noble-metal-based ones. However, poisoning substances and coke formation still have some adverse effects on the catalyst. Especially, the adsorption of chlorine and sulfur species could result in the loss of active sites (Yang et al. 2015a) whereas the coke formation could block the pores and active sites of the catalysts (Ji et al. 2018). In order to solve chlorine or sulfur poisoning, increasing reaction temperature, and/or ozone treatment are considered as the effective ways. Moreover, the formed coke can be easily removed by calcination in oxygen or air. Consequently, the transition-metal-oxides-based catalysts can be successfully prepared with high poisoning resistance at relatively higher reaction temperatures and good thermal resistance capability.

As mentioned above, both noble-metal-based catalysts and transition-metal-oxides-based catalysts can be well designed and prepared according to the needs of the reaction. Especially, the combination of the noble metal and/or transition metal oxide with special morphologies and porous structures can provide a possibility to apply noble-metal-based catalysts in a large scale with superior activity, high stability, and low cost. All in all, this review summarizes and provides promising ways to the synthesis and application of mesoporous materials in the field of the catalytic oxidation of VOCs. At present, the investigation and application of mesoporous materials have already given a new prospect for the use of these catalysts, and the superior nature of transition metals can make them be widely used in practical processes.


Corresponding author: Guoqing Guan, Energy Conversion Engineering Laboratory, Institute of Regional Innovation (IRI), Hirosaki University, 2-1-3 Matsubara, Aomori 030-0813, Japan, and Graduate School of Science and Technology, Hirosaki University, 1-Bunkyocho, Hirosaki 036-8560, Aomori, Japan, E-mail:

  1. Author contribution: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.

  2. Research funding: This work is supported by ZiQoo Chemical Co. Ltd., Japan.

  3. Conflict of interest statement: The authors declare no conflicts of interest regarding this article.

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Supplementary Material

The online version of this article offers supplementary material (https://doi.org/10.1515/revce-2021-0029).


Received: 2021-04-28
Revised: 2021-10-15
Accepted: 2021-11-19
Published Online: 2022-02-01
Published in Print: 2023-05-25

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