Fundamental Insights and Recent Advances in Catalytic Oxidation Processes Using Ozone for the Control of Volatile Organic Compounds

doi:10.21203/rs.3.rs-4011138/v1

The development of technologies for highly efficient treatment of emissions containing low concentrations of volatile organic compounds (VOCs) remains an important challenge. Catalytic oxidation with ozone (catalytic ozonation) is useful for the oxidative decomposition of VOCs, particularly aromatic compounds, under ambient temperature conditions. Only inexpensive transition metal oxides are required as catalysts, and Mn-based catalysts are widely used for catalytic ozonation. This review describes the oxidation reaction mechanisms, reaction pathways of aromatic hydrocarbons, and dependence of the catalytic ozonation activity on the reaction conditions. The reasons why Mn oxides are effective in catalytic ozonation are also explained. The structure of the catalytic active sites and the types of supporting materials contributing to the reaction are also discussed in detail, with the aim of establishing a VOC control technology. In addition, recent progress in catalytic oxidation processes using ozone as an oxidant has been outlined, focusing on catalyst materials and reaction conditions.

Ozone

Catalytic oxidation

Complete oxidation

VOC control

Manganese

Metal oxides

Ambient air pollution from volatile organic compounds (VOCs) remains an important social problem because it causes photochemical oxidants and human health damage. Currently, adsorption and combustion methods are in practical use as VOC treatment processes in the industry. Although all these methods are effective for treating high concentrations of pollutants, they are not suitable for treating low concentrations of VOCs emitted from small-scale facilities. The adsorption method requires frequent replacement of adsorbents, such as activated carbon, and the subsequent treatment process is expensive. In the combustion method, fuel supply is essential for the treatment of low-concentration VOCs. In addition, precious metals are generally used as active ingredients when catalysts are used. Therefore, there is a strong need to develop alternative VOC treatment processes that use inexpensive catalyst materials. In addition, from the viewpoint of reducing CO₂ emissions, there is a need to improve energy efficiency (i.e., reduce the power consumption required to process a unit gas flow) compared to conventional methods.

Ozone has high oxidizing ability and can be easily produced by low-temperature plasma chemical reactions in air, in which ozone is generated by the application of high voltage and discharge at atmospheric pressure. The chemical properties of ozone have long been known, and ozone is widely used for wastewater treatment, disinfection, deodorization, and washing clothes. The reaction between ozone and organic compounds in solvents has long been used as a synthetic reaction for organic compounds (Bailey PS (1958)). Ozone is highly reactive with compounds with C = C double bonds and produces aldehydes, ketones, and carboxylic acids via ozonide (the Criegee mechanism). The reactivity of ozone with organic compounds in the gas phase has been extensively studied in atmospheric environmental chemistry (Atkinson R, Carter WPL (1984)). In the gas phase, the reaction of O₃ with alkenes proceeds rapidly, and O₃ becomes an active species. When ozone is introduced into a gas stream containing ethylene or cyclohexene, these substrates react quickly. However, depending on the structure of the substrate, aromatic hydrocarbons and alkanes are less reactive with ozone, and their rate constants in the gas phase are approximately 10^− 2 to 10^− 8 smaller than those of alkenes.

The stability and reactivity of ozone depend on the ambient conditions, and the decomposition reaction accelerates as the temperature increases; however, it is relatively stable at room temperature with a long half-life of more than a dozen hours. However, when a third body, such as a catalyst, is present, the decomposition reaction proceeds to produce atomic oxygen (O), which is more reactive than ozone. Taking advantage of this property, catalytic oxidation reactions using ozone as an oxidant (catalytic ozonation) have been investigated since the 1990s. To date, much research has been done on catalytic ozonation processes to control VOCs, and a comprehensive review has been published by 2000 (Dhandapani B, Oyama ST (1997); Oyama ST (2000)). Since ozone can be easily synthesized by atmospheric pressure discharge, it is easy to construct a compact system that can be applied to small- to medium-scale VOC sources. Furthermore, the catalytic ozonation process has the advantage that inexpensive transition metals such as Mn can function as active ingredients. This is in marked contrast to catalytic thermal oxidation, in which Pt and Pd metals are generally used as active species. Catalytic ozonation processes are promising efficient methods for treating low-concentration VOCs. Additionally, catalytic ozonation process can be combined with nonthermal plasma reactors (Einaga H, Ibusuki T, Futamura S (2001); Ogata A, Saito K, Kim H-H, et al (2010); Guo H., Liu X., Hojo H., et al (2019)).

In this review, we mainly discuss the catalytic oxidative decomposition of aromatic hydrocarbons by ozone, detailing the reaction process and mechanism, and describing the reaction conditions for effective reaction progression. Next, based on these results, effective catalytic materials are discussed: active point components for efficient reaction progression, especially transition metals, and types of support materials. We also explain why manganese oxide is an effective catalyst for ozone oxidation in terms of reaction rate and effective ozone utilization. These fundamental findings are important for the development of energy-efficient VOC oxidation and decomposition systems. Finally, examples of recent studies on the catalytic ozonation of other types of aromatic hydrocarbons such as toluene, formaldehyde, and chlorinated hydrocarbons are presented.

Catalytic ozonation has initially been vigorously examined intensively by Bulgarian researchers (Naydenov A, Mehandjiev D (1993); Naydenov A, Stoyanova R, Mehandjiev D (1995)). Compared with the conventional catalytic oxidation method, the catalytic oxidation method allows the oxidation reaction to proceed under mild conditions near room temperature, making temperature control easier. Subsequently, in the late 1990s, Oyama et al. studied in detail the mechanism of ozone oxidative decomposition on supported manganese oxide catalysts, as described later (Li W, Oyama ST (1997); Li W, Gibbs GV, Oyama ST (1998); Li W, Oyama ST (1998)). With the tightening of emission regulations from fixed sources, it has become necessary to upgrade treatment technologies for low concentrations of VOCs, especially aromatic hydrocarbons, which are important target compounds.

In catalytic ozonation over manganese oxide catalysts, benzene and toluene were oxidized even at room temperature (22°C). We conducted studies to elucidate the reaction pathway of oxidation of aromatic compounds on alumina-supported manganese oxide (MnOx/Al₂O₃) catalysts using ozone and to clarify the mechanism of catalyst deactivation (Einaga H, Futamura S (2004a)). In the benzene oxidation reaction at room temperature, the only products in the gaseous phase were CO₂ and CO (Fig. 1). However, catalyst deactivation is generally observed because of the formation of many byproducts on the catalyst surface. The byproducts on the catalyst surface were analyzed using temperature-programmed desorption/oxidation (TPD/TPO) techniques, in situ FTIR spectroscopy, and GC-MS analysis. The byproducts were oxidized to CO_x by heating the MnO_x/Al₂O₃ catalyst at 500°C. FTIR studies also revealed the presence of strongly bound surface carboxylates (Fig. 2). Thus, benzene was oxidized to form phenol, carboxylic acid, and acid anhydride. Formic acid and adsorbed formic acid species were also detected as major byproducts, suggesting that although cleavage of the aromatic ring occurred rapidly, oxidative decomposition of these species to CO₂ and CO was relatively slow. Catalyst deactivation was attributed to the poisoning of the active site by unreacted byproduct compounds that accumulated on the catalyst at room temperature. MnO_x/Al₂O₃ has also been reported to be deactivated in toluene oxidation at room temperature (Rezaei E, Soltan J (2012)). Alcohols and carboxylic acids have been identified as catalyst surface byproducts that cause catalyst deactivation.

The oxidative degradation behavior of aliphatic alkanes has been studied (Einaga H, Futamura S (2004b)). Using cyclohexane as the reaction substrate, the products were easily identified, and partial oxidation products such as alcohols, ketones, carboxylic acids, and acid anhydrides were detected on the catalyst surface (Fig. 3). These products also reduce the VOC oxidation rate but are more easily oxidized than the benzene oxidation products. Thus, the formation and decomposition behavior of partial oxidation products on the catalyst surface are important factors controlling the oxidation rate of aromatic compounds, and these products must be removed quickly to reduce the decomposition rate.

Improvement of activity by controlling reaction conditions

As described above, catalysts suffer from severe deactivation when the air pollutants contain aromatic compounds. Partial oxidation products generated by the oxidative decomposition of the substrate accumulate on the catalyst surface during the benzene oxidation reaction at room temperature. These catalysts can be quickly removed by heating the catalysts; increasing the temperature of the MnO_x/Al₂O₃ catalyst bed to 100°C suppressed the decrease in the benzene decomposition rate. In this case, a temperature of less than 100°C was sufficient for the heat treatment. When the temperature of the reaction tube is raised too high, the ozone autolysis reaction in the gas phase will be accelerated, and ozone may not be effectively used for the catalytic reaction (Einaga H, Futamura S (2006)).

The humidity of the reaction gas also affected the VOC oxidation reaction (H. Einaga H, Futamura S (2006)). During the oxidative decomposition of benzene, humidifying the reaction gas suppressed the formation of partial oxidation products and promoted decomposition, thereby suppressing the decrease in the reaction rate (Fig. 3). FTIR spectroscopic studies revealed that formic acid adsorption on MnO_x/Al₂O₃ resulted in bands attributed to the surface formate species on the catalyst surface (Fig. 4). This band did not disappear completely, even when ozone was circulated and in contact for a long time (Fig. 4(a)). However, when ozone was introduced under humid conditions, these bands disappeared completely and only those attributed to adsorbed water were observed (Fig. 4(b)). In these reactions, both the rate of benzene oxidation and the amount of CO₂ produced increase with increasing humidity. Under dry conditions, ozone completely decomposed, but adsorbed formate species remained. However, ozone decomposition coincides with the formation of CO under humid conditions. The surface formate species efficiently decomposed to CO₂ under humid conditions, even on alumina with low ozone decomposition activity. These findings reveal that humidifying the reaction gas promotes the oxidation of strongly bound surface formates on the catalyst surface, which are formed during benzene oxidation, suppressing the build-up of byproduct compounds that cause catalyst deactivation. Thus, optimizing the reaction conditions is important for controlling the VOC oxidation process.

Reactive Oxygen Species Contributing to the Ozone Oxidation Reaction

As described above, aromatic hydrocarbons and alkanes were rapidly oxidized and decomposed on the catalyst with ozone. Here, the question arises: What are the reactive oxygen species that contribute to this reaction? The oxidation of benzene over MnO_x/Al₂O₃ catalysts at different ozone concentrations showed good linearity with respect to the ozone decomposition rate (Fig. 5), indicating that benzene degradation behavior strongly depends on ozone decomposition behavior. Benzene oxidation did not occur when the catalysts did not exhibit ozone decomposition activity, indicating that ozone decomposed on the catalyst surface and the active oxygen species produced contributed to the reaction.

The decomposition behavior of ozone on the catalyst was investigated using spectroscopic methods, and CO oxidation was promoted by ozone on Ag₂O and CeO₂. The formation of O₃⁻ was observed by ESR during ozone decomposition, suggesting the formation of O⁻, a highly reactive oxygen species (Imamura S, Ikebata M, Ito T, Ogita T (1991)). Oyama et al. used in situ Raman spectroscopy to identify the oxygen species produced and kinetic analysis to clarify the ozone decomposition behavior over MnO_x/Al₂O₃ catalysts (Li W, Gibbs GV, Oyama ST (1998); Li W, Oyama ST (1998)).

Oyama et al. also carried out the oxidative decomposition of acetone and found that the rate of formation and decomposition of peroxide did not match the rate of oxidation of acetone (Reed C, Xi Y, Oyama ST (2005); Reed C, Lee YK, Ted Oyama S (2006)). They conducted an oxidative degradation reaction of acetone and concluded that the active species contributing to the oxidation reaction were atomic peroxide (O*) rather than peroxide. In the oxidation of aromatic hydrocarbons and alkanes, these active oxygen species are also considered to contribute to the reaction. Based on the reaction mechanisms, assuming that one atom of atomic oxygen is produced from one molecule of ozone to oxidize and decompose benzene, the stoichiometric equation for the decomposition of benzene is expressed in Eq. (1), and the ratio of ozone to benzene decomposition was 15.

The ratio of the amount of O₃ decomposed to that of benzene was approximately 10 for various supported manganese oxide catalysts. This value is also lower than that of 15, implying that O₂ participated in the oxidation reaction. It is plausible that the reaction proceeds via a radical reaction, and the organic radical species generated from benzene and active oxygen species (O*) are further oxidized by molecular oxygen. This indicates that both ozone and molecular oxygen (O₂) play important roles in catalytic ozonation.

Why is Mn effective for catalytic ozonation?

In catalytic ozonation processes, transition metal oxides are used as the active species, and ozone can be decomposed on their surfaces even at room temperature, yielding active oxygen species that contribute to the oxidation of organic compounds. We prepared SiO₂-supported catalysts on which inexpensive metal oxides (MnOx, FeOx, CoOx, NiOx, and CuOx) were dispersed by an impregnation method using metal nitrate precursors and investigated the catalytic activities for benzene oxidation, as shown in Table 1 (Einaga H, Maeda N, Nagai Y (2015)). All the supported metal oxide catalysts efficiently decomposed ozone, and no decrease in activity was observed at 70 C. In contrast, the benzene oxidation activity was completely different depending on the metal oxide used. MnOx/SiO₂, which exhibited the lowest ozone decomposition activity, exhibited the highest ozone benzene oxidation activity. CoOx/SiO₂ showed low activity but steady-state activity, whereas the other metal oxides gradually deactivated. It is worth noting that the MnO_x/SiO₂ and CoO_x/SiO₂ catalysts consumed ozone efficiently during benzene oxidation, whereas autolysis of ozone to O₂ proceeded on NiOx, CuOx, and FeOx.

In situ FTIR and TPO studies clarified that the aromatic rings were promptly cleaved on MnO_x/SiO₂ during the catalytic ozonation of benzene, producing oxygen-containing byproduct compounds covering the catalytically active sites. However, these byproducts were more rapidly oxidized with ozone to form CO and CO on MnO_x/SiO₂ than on the other supported oxides (Fig. 7).

For CoOx/SiO₂, the initial rate of aromatic ring cleavage was higher than for Mn/SiO₂. However, the oxidation rate of the byproducts formed on the catalyst surface was much lower than that of Mn/SiO₂. The initial aromatic ring cleavage rates in NiO_x/SiO₂ and CuO_x/SiO₂ were similar to the initial aromatic ring cleavage rate in MnOx/SiO₂. However, both catalysts were rapidly and significantly deactivated when a small amount of byproduct compounds was formed on the catalyst surface. Aromatic ring cleavage on FeO_x/SiO₂ was slower than that on MnO_x/SiO₂. Furthermore, the catalytic oxidation process is significantly retarded by the build-up of byproducts. Thus, the activity of SiO₂-supported metal oxides is greatly affected by the build-up of oxygen-containing byproducts during the catalytic ozonation process.

Table 1

Activities for benzene oxidation with ozone over SiO2-supported metal oxide catalysts
Catalyst	Benzene conv. / %	Product selectivity / %			O₃/C₆H₆ ratio
Catalyst	Benzene conv. / %	CO₂	CO	HCOOH	O₃/C₆H₆ ratio
MnO_x/SiO₂	68	66	32	2	10
CoO_x/SiO₂	25	57	29	14	10
CuO_x/SiO₂	9	48	31	21	23
NiO_x/SiO₂	9	59	28	13	35
FeO_x/SiO₂	4	36	34	30	44
SiO₂	2	―	―	―	―
^a[C₆H₆] = 150 ppm, [O₃] = 2250 ppm, [O₂] = 10%, catalyst weight 0.10 g, gas flow rate 500 mL/min (WHSV = 30 L/g h). ^bRatio was determined from the amount of ozone consumed divided by that of benzene consumed.

Catalyst design for catalytic ozonation

Effect of manganese oxide structure

Supported Mn oxides are active in the catalytic ozonation of various hydrocarbons at relatively low temperatures, particularly at low concentrations. The structure of the manganese oxide on the support can be tuned by changing the type of manganese precursor and the loading method (Radhakrishnan R, Oyama ST, Chen JG, Asakura K (2001); Xi Y, Reed C, Lee YK, Oyama ST (2005); Reed C, Lee YK, Ted Oyama S (2006)). For instance, when manganese nitrate is used to obtain γ-Al₂O₃-supported catalysts, aggregated oxides are formed. When manganese acetate was used, isolated oxides were dispersed at low loadings, whereas agglomerated species were formed at high loadings (Einaga H, Futamura S (2004)). The catalytic properties of the prepared MnOx/γ-Al₂O₃ were investigated, and both the isolated and agglomerated oxides were found to be active sites for benzene oxidation by ozone. Subsequent studies focused on the effect of Mn loading on the catalytic properties of benzene oxidation to determine the factors controlling the catalytic activity for ozone oxidation. The rate of benzene oxidation was not significantly dependent on the manganese oxide loading (Fig. 8), but the isolated manganese oxides produced under low loading conditions were the most effective active sites for the reaction. The benzene oxidation reaction was followed by temperature-programmed oxidation and the profiles were compared. It was found that benzene oxidation behavior was not significantly affected by Mn loading.

Razaei and Soltan performed catalytic ozonation of toluene in the temperature range of 22–100°C and found a direct relationship between Mn loading and the average oxidation state of Mn; catalysts with Mn loadings below 10% were mostly Mn₂O₃, whereas those with Mn loadings above 10% contained a mixture of MnO₂ and Mn₂O₃ (Rezaei E, Soltan J (2012)). The lower the Mn loading, the higher the toluene oxidation activity. It was suggested that the lower oxidation state of manganese favored the decomposition of ozone, resulting in a faster oxidation rate of toluene. It was also suggested that the activity order of the catalyst is related to the oxidation state of Mn on the catalyst: catalysts with lower Mn loading have a lower oxidation state and are therefore more active in transferring electrons to ozone to initiate the ozone decomposition reaction. Consequently, the decomposition rate of ozone to atomic oxygen increased, and the oxidation rate of toluene improved.

We performed X-ray absorption fine structure (XAFS) spectroscopic studies on the supported Mn oxide catalysts. EXAFS and XANES spectra were obtained to investigate the structure of Mn sites and their changes during ozone decomposition (Einaga H, Harada M, Futamura S (2005)). For the MnOx/g-Al₂O₃ catalyst in which isolated Mn oxide was dispersed, only the band of M-O was observed at R = 1.5Å in the Mn K-edge EXAFS spectra. In ozone decomposition under dry conditions, the band structure was unchanged by the reaction with ozone. On the other hand, under humid conditions, the intensity of the Mn-O band increased, and the absorption edge shifted to a higher energy (Fig. 9). These findings indicate that the oxidation state of Mn and the coordination number of Mn-O bonds increased (Fig. 9). In separate experiments, we found that the Mn species on the used catalysts were easily removed by washing with water, indicating that the Mn–O–Al bonds were cleaved by the chemical adsorption of water to Mn sites. We also found that the MnOx/g-Al₂O₃ catalyst could be regenerated by heat treatment in an O₂ stream at 450°C: XANES and EXAFS spectra of the regenerated catalysts and fresh catalysts were the same.

The structures of catalytically active sites have also been investigated with SiO₂-supported Mn oxides (MnO_x/SiO₂) by XRD and XAFS studies (Einaga H, Yamamoto S, Maeda N, Teraoka Y (2015f)). The catalytic ozonation of benzene was performed using MnO_x/SiO₂ catalysts prepared by the impregnation method with Mn nitrate and Mn acetate as Mn precursors. The structure was not significantly influenced by the Mn loading. For instance, Mn₂O₃ is formed when Mn nitrate is used. The XANES studies clarified that the average oxidation state was almost the same for both catalysts with different Mn loadings. The activity of the catalyst prepared from the Mn acetate precursor was much higher than that of the catalyst prepared from the Mn nitrate precursor at all Mn loadings, indicating that the reaction rate on the isolated Mn oxide was higher than that on the aggregated Mn oxide on SiO₂ (Fig. 10). Conversely, the selectivity of CO₂ production and efficiency of ozone utilization were not influenced by the Mn oxide structures. In contrast, CO₂ selectivity and ozone utilization efficiency were unaffected by the Mn oxide structure.

Reaction conditions: catalyst weight 0.10 g, benzene 150 ppm; ozone, 2250 ppm, O₂ 10%. Temperature 343 K.

Effect of support materials

It is important to develop catalyst materials with high activity and durability for the oxidation of aromatic compounds under ambient conditions. The catalytic oxidation of gaseous benzene with ozone was carried out over Mn oxides supported on various types of supporting materials to investigate the factors governing catalytic activities (Einaga H, Ogata A (2009)). A strong relationship between the benzene oxidation rate and catalyst surface area was observed for the supported Mn oxide catalysts, independent of the type of catalyst support (Fig. 12). We also investigated whether ozone was effectively utilized for the oxidation of benzene over these supported oxide catalysts. On the other hand, the ratio of benzene oxidation to ozone decomposition was used as an indicator of effective ozone utilization, and this value was highest for the SiO₂-supported catalyst (Fig. 12). These findings indicate that Mn oxides that are highly dispersed on SiO₂-based supports with high surface areas are expected to be particularly effective catalytic materials. Furthermore, we reported methods to improve the activity of supported Mn oxide catalysts for benzene oxidation with ozone, including increasing the catalyst bed temperature and humidifying the reaction gas. Benzene conversion was drastically increased by increasing the catalyst temperature from room temperature to 70–100°C. However, further increasing the reaction temperature accelerated ozone autolysis in the gas phase and reduced the oxidation-promoting effect of ozone.

Based on the findings described above, the target materials were SiO₂-base zeolites with ordered pore structures for supported Mn oxide catalysts because these supporting materials have high surface areas and adsorption capacities for VOCs. Using various techniques, we designed and obtained high-performance catalytic materials comprising Mn oxide and zeolite supports for VOC oxidation (Einaga H, Futamura S (2007); Einaga H, Teraoka Y, Ogata A (2011); Einaga H, Teraoka Y, Ogata A (2013)). Table 2 summarizes the activities of Mn-exchanged zeolites for benzene oxidation with ozone at 70°C. The activities were measured at steady state and under conditions where the W/F ratio was proportional to benzene conversion. The activities of the Mn/SiO₂ catalysts are listed for comparison. The Mn-loaded Y zeolite (SiO₂/Al₂O₃ ratio of 5.1) prepared by the ion exchange method (Mn-Y) showed the highest activity among the catalysts tested in this study, with CO₂ as the main product. In contrast, when Mn was introduced into ZSM-5 and MOR using the ion exchange method, zeolites with small pore sizes showed little activity. Thus, zeolite structures with appropriate pore structures are essential for the formation of active sites with stable activity during catalytic ozonation of benzene.

Although Mn-Y efficiently catalyzed benzene oxidation, the catalyst was completely deactivated under humid conditions, in marked contrast to amorphous SiO₂-supported Mn oxides, owing to the highly hydrophilic properties of Y zeolite, in which strongly adsorbed water inhibited the adsorption of benzene in micropores. Therefore, ultrastable Y (USY) zeolites with a higher Si/Al ratio (SiO₂/Al₂O₃ = 180) were used instead of MnY, because USY zeolites are hydrophobic and have a lower adsorption capacity for water vapor and a high adsorption capacity for hydrocarbons. Mn/USY exhibited a lower activity than Mn-Y under dry conditions. In contrast, humidifying the reaction gas improved the benzene oxidation rate of Mn/USY (Fig. 13).

Table 2

Catalytic activities for benzene oxidation with ozone over Mn-loaded catalystsa
Catalyst	Rate^b	CO₂^c / %	Mn/Al ratio	SiO₂/Al₂O₃ ratio
NaY	0.1	―	―	5.1
USY	0.06	―	―	180
Mn-Y	19.1	82	0.336	5.1
MnO_x/USY	13.7	67	(5 wt%)	180
Mn-b	18.1	72	0.356	18
Mn-MOR	1.1	―	0.276	20
Mn-ZSM-5	0.6	―	0.328	23
MnO_x/SiO₂	4.8	62	(5 wt%)	―
MnO_x/SiO₂	3.3	60	(15 wt%)	―
MnO_x/SiO₂-Al₂O₃	1.3	63	―	(6.5%Al)
^a[C₆H₆] = 300 ppm, [O₃] = 1500 ppm, [O₂] = 10%. ^bRate / 10^− 5 mol g^− 1 min^− 1. ^cCO₂ mole fraction/%= [CO₂]/([CO₂]+[CO]).

The XRD and EXAFS studies indicated that highly dispersed Mn oxides were supported on the Mn/USY catalyst. It should be noted that zeolite itself has also been reported to exhibit ozone catalytic oxidation activity, and ozone oxidation of benzene is also decomposed on Y-type zeolite, but complete oxidation to CO₂ is difficult, and formic acid and CO are easily formed. On the other hand, the oxidation rate of benzene on the Mn/USY catalyst was much higher than that on the USY catalyst, indicating that Mn oxide serves as the active site.

Notably, the Mn/USY catalyst efficiently oxidized and decomposed benzene in a steady state at room temperature under humidified conditions. Under humid conditions, steady-state activities were obtained with a 5wt%-Mn/USY catalyst at 27 C, and catalytic deactivation did not occur after prolonged reaction times (Fig. 14). The reaction proceeded catalytically without deactivation even after a long benzene oxidation reaction. Moreover, Mn/USY was effective in terms of the catalytic activity and CO₂ selectivity. Mn/USY exhibited high CO₂ selectivity and organic byproducts did not evolve, whereas MnOx/SiO₂ afforded formic acid in the gas phase. It has also been confirmed that toluene, another aromatic compound and a major pollutant from various sources, is efficiently oxidized to CO₂ by the Mn/USY catalyst.

Mixed metal oxides

As described in the preceding section, Mn-based catalysts effectively catalyzed low concentrations of hydrocarbons. One method to improve the activity of Mn oxides is to combine them with other metal oxides. Mixed oxides containing Mn have been reported to have a higher catalytic oxidation capacity than Mn oxides because Mn oxidation state is easily changed (Mehandjiev D, Naydenov A, Ivanov G (2001)) and catalytic surface area can be increased (Einaga H, Maeda N, Teraoka Y (2013)). The catalytic activities of the mixed oxides of these transition metals were also compared. The NiMnO₃ ilmenite was more active for benzene oxidation. Mixed oxides containing Mn were also prepared and the effects of preparation method and chemical composition of catalysts and on their activities were investigated (Einaga H, Maeda N, Teraoka Y (2013)). The catalysts were prepared from metal nitrate precursors using evaporation-drying and co-precipitation techniques and heated at 400–900°C. The most important catalytic properties are catalytic activity, CO₂ selectivity, and ozone utilization efficiency. Various catalysts were prepared and their properties were compared. For Mn monoxide, the benzene oxidation rate, CO₂ selectivity, and decomposition ratio of ozone to benzene were largely independent of preparation method and calcination temperature. For Fe-Mn, Co-Mn, Ni-Mn and Cu-Mn mixed oxides, these properties depended on the preparation conditions and chemical composition of the catalysts. Among the binary oxides prepared by the evaporation-drying method, the Co-Mn mixed oxides showed high CO₂ selectivity and ozone utilization efficiency. In the case of SiO₂-supported catalysts, Mn oxides combined with transition metals are effective for benzene oxidation with ozone (Einaga H, Maeda N, Yamamoto S, Teraoka Y (2015)). Moreover, when Cu, Ni, and Fe were added to Mn oxides, the steady-state activity at 70 C improved, whereas the Co-Mn oxide sites were ineffective for benzene oxidation. The most active catalyst was Cu-Mn/SiO₂, with a Cu-Mn molar ratio of 1:1 (Fig. 15). The catalytic activity of the Cu-Mn mixed oxides increased with increasing spinel-type Cu-Mn mixed oxide phase as the calcination temperature increased.

Silver nanoparticles on alumina supports were also found to show high activity and stability for benzene oxidation at 40 C under humid conditions (Einaga H, Ogata A (2010)). High-performance catalysts for ozonation can be obtained by optimizing the active metal and support materials.

Recent progress in catalyst design

Examples from recent studies on catalytic ozonation are listed in Table 3. Toluene- and Cl-containing hydrocarbons were the main VOCs considered in this study. In catalytic ozonation, toluene is slightly less reactive than benzene, which differs from heat-driven catalytic oxidation. During the catalytic oxidation process, toluene was oxidized at lower temperatures than benzene because of the presence of methyl groups. However, in the catalytic ozonation process, the reactivity of toluene is similar to that of benzene, and the ozone oxidation reaction over supported Mn oxide catalysts cleaves the aromatic rings to form carboxylic acids and carboxylates. These are quickly oxidized and decomposed to obtain steady-state activity. Generally, Mn oxides with high-surface-area supports (mesoporous silica, zeolite, SiO₂-based, and carbon-based materials) are used as catalysts, and the reaction temperature is set to 50°C or higher, or water vapor coexists in the reaction gas. As mentioned earlier, these promote the rapid oxidative decomposition of the above byproduct compounds and contribute to maintaining a steady-state reaction. Catalytic oxidation is also promoted by the addition of precious metals (Pt and Ru) and CeO₂, which have high oxidation potentials.

A recent notable example is that surface hydroxylation of MnO_x/Al₂O₃ catalysts promotes the oxidative degradation of toluene (refs). Specifically, using the in-situ AlOOH reconstitution method, a novel hydroxyl-mediated MnO_x/Al₂O₃ catalyst was developed and tested for toluene oxidation with ozone. The activity of the hydroxylated MnO_x/Al₂O₃ catalyst for toluene oxidation was better than that of most modern catalysts. Additionally, the catalyst exhibited a high CO₂ formation rate during catalytic ozonation and stability with 100% toluene removal. in situ DRIFT and ESR studies revealed that surface hydroxyl groups efficiently generated active oxygen species and significantly enhanced aromatic ring cleavage and CO₂ formation. Furthermore, the hydroxyl groups served as anchor sites for the homogeneous dispersion of MnO_x and significantly enhanced toluene adsorption and ozone activation.

Mn oxide-based catalysts are also effective for ozone oxidative decomposition reactions of containing oxygen-containing hydrocarbons such as alcohols, aldehydes, and ketones (Table 4). In addition, these compounds are more reactive than aromatic hydrocarbons, indicating that the reaction proceeds quickly at temperatures as low as room temperature.

Complete oxidation of formaldehyde (HCHO) is essential for controlling low-concentration indoor air pollution. Recent studies on formaldehyde oxidation by ozone (O₃/HCHO = 3–4) at low temperatures have suggested that Mn oxides can catalyze this reaction at ambient temperatures. Zhang et al. (2019) investigated the use of one-dimensional rod-like MnO₂ catalysts prepared via a hydrothermal method, and found that α-MnO₂ displayed better catalytic performance than β- and γ-MnO₂, achieving 90% HCHO conversion, 100% CO₂ yield, and 100% O₃ decomposition. The superior catalytic performance of α-MnO₂ was attributed to its suitable tunnel structure for HCHO adsorption, leading to increased mobility of adsorbed oxygen species due to the high content of Mn³⁺. Wang et al. (2018) addressed the challenge of a core-shell C@MnO catalyst for the simultaneous oxidation of low-concentration of HCHO by ozone. The catalyst possesses a unique hierarchical core–shell structure, in which porous unsaturated MnO is evenly immobilized on the carbon core surface. The formation of an intimate interface between MnO and carbon protected the carbon sphere against O₃ etching, enabling simultaneous HCHO and ozone removal with 100% efficiency and high CO₂ selectivity. Zhang et al. (2019) synthesized a MnCeO_X solid solution using the Pechini method and investigated its effectiveness in formaldehyde and ozone decomposition in humid environments. The MnCeO_X catalyst exhibited higher performance for HCHO oxidation and ozone decomposition, attributed to the formation of a solid solution of Mn and cerium, which resulted in abundant oxygen vacancies and surface lattice oxygen species. MnCeO_X achieved ∼100% HCHO conversion to CO₂ even under high humidity conditions, attributed to continuously replenished surface hydroxyl groups resulting from the interaction of water vapor and ozone on the catalyst surface, making it suitable for indoor air purification.

Methyl ethyl ketone (MEK) is a widely used VOC in industry owing to its solubility and cost-effectiveness. However, atmospheric emissions are hazardous to both human health and the environment. Currently, most techniques for facilitating MEK removal through ozonation (O₃/MEK = 10) at room temperature involve the use of Mn-oxide catalysts. Kim et al. synthesized mesoporous MnO_x/γ-Al₂O₃ catalysts using Mn acetate and nitrate precursors by solvent-deficient method for MEK ozonation (Kim et al. 2021). The catalyst prepared from the acetate precursor demonstrated 99.4% MEK removal efficiency at 30 min, which decreased slightly after 120 min. The higher catalytic activity of the acetate precursor is attributed to its larger surface area, pore volume, and improved dispersion of MnO_x particles. MnO_x/γ-Al₂O₃ is an effective catalyst for aromatic ozonation, proving its effectiveness in the ozonation of oxygen-containing hydrocarbons. Hwang et al. focused on MEK removal by ozone using recycled porous spent fluid catalytic cracking (SFCC) supporting Mn-Cu oxides catalysts (Hwang et al. 2023). Mn–Cu/SFCC and Mn/SFCC exhibited approximately 60% MEK decomposition activity at room temperature. SFCC has a structure similar to that of zeolite Y, and these catalysts can reuse the waste from petroleum refineries. Ha et al. synthesized Mn and Cu oxide-loaded zeolite catalysts (Mn-Cu/ZSM-5) that exhibited 90% MEK conversion and 80% O₃ conversion, which was attributed to the abundant production of Mn³⁺ species and the availability of adsorbed oxygen sites (Ha et al. 2023). Additionally, the Mn-Cu/ZSM-5 catalyst demonstrated durability of 420 min, confirming its prolonged effectiveness.

Ethyl acetate is a common VOC emitted from both industry and laboratories, but there are still few studies on the use of Mn oxides for its removal. Xu et al. investigated the catalytic ozonation of ethyl acetate using mesoporous Mn oxide (MnO_x) catalysts synthesized through the sonochemical method (Xu et al. 2021). Different MnO_x crystal phases were obtained by calcining samples at various temperatures. Among these catalysts, β-MnO₂ exhibited 90% removal activity for ethyl acetate and 100% O₃ conversion at room temperature. The results indicated that β-MnO₂ maintained long-term stability for ethyl acetate catalytic ozonation owing to the dynamic equilibrium between the accumulation and decomposition of intermediate products.

Catalytic ozonation with Mn oxide-based catalysts has also been applied for the decomposition and removal of chlorinated hydrocarbons. The C-Cl bond energy is higher than the bond energy of aromatic hydrocarbons; therefore, the reaction temperature is 120°C, which is higher than that of the oxidative decomposition of aromatic hydrocarbons. The reaction temperature was 120°C, which was higher than that required for the oxidative decomposition of aromatic hydrocarbons. The hydrogen chloride and Cl₂ produced during the decomposition of chlorine-containing hydrocarbons, chlorinated Mn oxide, and the formation of Mn chloride on the catalyst surface are problematic. The surface deposition of chlorine species tends to occupy the active sites on the catalyst, leading to the formation of polychlorinated byproducts through the nucleophilic substitution of chlorine. MnO_X-based catalysts are widely used in the catalytic ozonation of Cl-containing VOC because of their excellent O₃ decomposition ability and resistance to Cl poisoning. The susceptibility of Cl-VOCs to degradation is mainly affected by the surface acidity, redox capacity, and oxygen species of the catalyst (Lin et al. 2021)(Lin, Xiang, et al. 2022). In particular, surface acidity is strongly related to catalytic performance. Lewis acidity and Brønsted acidity play different roles. For example, Lewis acidity promotes the breaking of C-C and C-Cl bonds, whereas the adsorption of Cl-VOC molecules, oxidation of intermediates, and desorption of Cl species are accelerated by Brønsted acid sites.

Zhuang et al. (2022) and Zhang et al. (2023) focused on the catalytic ozonation of 1,2-dichloroethane (DCE) at room temperature. (Zhang et al. (2023) utilized Ce_xMn_1−xO₂ supported on tubular titanium nanotubes (TNTs). Ce doping of MnO₂/TNTs increased the density of oxygen vacancies, but decreased the number of strongly acidic sites. (Zhuang et al. 2022) investigated Mn_xCe_1−xO₂ prepared using SBA-15 as a template, which resulted in a solid solution with an increasing number of surface oxygen vacancies. Both the optimized catalysts demonstrated high catalytic performance and long-term stability, achieving high DCE conversion (96.7% and 96%, respectively) and CO₂ selectivity (86% and 82%, respectively). These studies highlighted the synergistic effect between surface acidity and oxygen vacancies in the catalytic ozonation of DCE. Chlorine species from the decomposition of DCE easily bind to Mn oxides, forming MnO_xCly and leading to deactivation.

Dichloromethane (DCM), 1,2-dichloroethane (DCE), chlorobenzene (CB), and trichloroethylene (TCE) are widely used Cl-VOCs that are emitted from various industries and waste treatment plants. Cl substitution in Cl-VOCs contributes to a complex transformation process, leading to the formation of highly toxic by-products such as polychlorinated compounds and dioxins. In practical processes, multiple Cl-VOCs always coexist and possess different physicochemical properties, which may cause significant differences in catalytic performance and interactions. Consequently, based on catalyst research on the decomposition of a single component, simultaneous research on various Cl-VOCs has gradually gained popularity in recent years.

Zhang et al. reported that MnO₂ with an α-crystalline phase and higher Mn⁴⁺ content was the optimal candidate for the ozonation of chlorobenzene (CB) and DCE at low temperatures (30–120°C) (Zhang et al. 2021). Its abundant oxygen vacancies, strong acid sites, and high oxygen mobility behavior at low temperatures mutually contributed to its excellent ability to oxidize CB and DCE molecules. Chen et al. (2020) studied MnO_x-based catalysts with different supports for catalytic ozonation of CB at 120°C (Chen et al. 2020). The results indicate that MnO_x/Al₂O₃ exhibited the highest CB conversion efficiency (82.9%). Subsequently, catalytic ozonation of a mixture of different types of VOCs (CB, DC, DCM, and benzene) was performed. It was concluded that Cl-containing aromatics degraded more easily than Cl-containing alkanes. An increase in the number of carbon atoms decreases the difficulty of decomposition: CB∼benzene > DCE∼DCM and DCE > DCM; however, chlorinated substitution increases the difficulty, as benzene > CB. Co-ozonation improved ozone utilization efficiency while maintaining the original properties of the catalyst.

Lin, Zhang, et al. (2022) continuously investigated the catalytic ozonation of different Cl-VOCs using various supported MnO_x catalysts. Cl-VOCs exhibited significant differences in catalytic performance and byproduct formation owing to their varying molecular structures. The Mn/HZSM-5 catalyst showed superior activity and CO₂ selectivity for various Cl-VOCs, which was attributed to its numerous surface acid sites and pore structure. The catalytic ozonation of mixed Cl-VOCs showed both inhibitory and facilitatory effects, with CB conversion being preferred over alkanes. Competitive adsorption and degradation difficulties have been identified as the key factors in catalytic ozonation.

Table 3

Recent studies on catalytic ozonation of aromatic compounds
Organic Pollutants	Catalysts	Reaction Parameters	Reaction Temperature	Highest Conversion	CO₂ selectivity	Ref.
Toluene	Ag/CeO₂-in situ reduction	O₃: 300 ppm, Toluene: 30 ppm, Catalyst: 0.05 g, Flow rate: 100 mL/min	70°C	99%	93%	Shi X et al. (2022)
	Pt/MnO_X	O₃: 300 ppm, Toluene: 30 ppm, Catalyst: 0.1 g, Flow rate: 100 mL/min	70°C	100%	90%	Xu Z et al. (2022)
	MgOHF film/Ni mesh	O₃: 1000 ppm, Toluene: 100 ppm, Catalyst: 0.55 g, Flow rate: 450 mL/min, RH: 50%	105°C	100%	85%	Zhu J et al. (2020)
	3D-NiO_1−δ/Ni foam	O₃: 350 ppm, Toluene: 100 ppm, Catalyst: 0.55 g, Flow rate: 1200 mL/min, RH: 50%	25°C	100%	70.6% of CO_X	Tian S et al. (2021)
	0.07 wt%Pt-CeO₂/BEA	O₃: 350 ppm, Toluene: 35 ppm, Catalyst: 0.05 g, Flow rate: 100 mL/min	70°C	100%	80%	Wang J et al. (2021)
	5 wt%Mn/AC- fuming nitric acid	O₃: 2100 ppm, Toluene: 100 ppm, Catalyst: 0.2 g	30°C	84–87%	36%	Xu P. L. et al. (2020)
	MnCe/Al₂O₃	O₃: 300 ppm, Toluene: 30 ppm, Catalyst: 0.1 g, Flow rate: 100 mL/min	40°C	100%	~ 95%	Gan Q et al. (2022)
	Pt/FeO_X	O₃: 1019 ppm, Toluene: 72 ppm, Catalyst: 1 g, Flow rate: 150 mL/min, RH: 50%	50°C	100%	72.53%	Liu R et al. (2021)
	Ru-Mn/HZSM-5	O₃: 1000 ppm, Toluene: 100 ppm, Catalyst: 1.5 g, Flow rate: 2000 mL/min	ambient temperature	36%	40% of CO_X	Kim J et al. (2020)
	Ru-Mn/HY	O₃: 1000 ppm, Toluene: 100 ppm, Catalyst: 1.5 g, Flow rate: 2000 mL/min	ambient temperature	95%	-	Kim J et al. (2021)
	MnO_X/sawdust char	O₃: 900 ppm, Toluene: 50 ppm, Catalyst: 0.3 g, Flow rate: 2000 mL/min	ambient temperature	100% at 80 min	-	Cha JS et al. (2022)
	5 wt%MnO_X/acid treated red mud	O₃: 1000 ppm, Toluene: 50 ppm, Catalyst: 0.5 g	room temperature	100%	-	Ryu S et al. (2020)
	5 wt%MnO_X/MCM-41	O₃: 1000 ppm, Toluene: 100 ppm, Catalyst: 0.5 g, Flow rate: 1000 mL/min	20°C	100%	73%	Dong Y et al. (2022)
	hydroxyl-mediated MnO_X/Al₂O₃	O₃: 180 ppm, Toluene: 20 ppm, Catalyst: 0.1 g, Flow rate: 100 mL/min, RH: 50%	25°C	100%	-	Zhang B et al. (2022)
	amorphous Cu_0.2MnO_X	O₃: 2100 ppm, Toluene: 160 ppm, Catalyst: 0.2 g, Flow rate: 700 mL/min	100°C	91.2%	MAR 67.6%	Ye Z et al. (2022)
	5 wt%MnO_X/meso γ-Al₂O₃ MnCl₂ precursor)	O₃: 1000 ppm, Toluene: 100 ppm, Catalyst: 0.5 g, Flow rate: 700 mL/min	20°C	95%	-	Reddy KHP et al. (2021)
	3 wt%δ-MnO₂/USY	O₃: 60 ppm, Toluene: 20 ppm, Catalyst: 0.15 g, Flow rate: 250 mL/min, RH: 70%	25°C	55%	MAR 8.5%	Yang R et al. (2020)
	Cu-Mn/dealuminated zeolite Y	O₃: 3000 ppm, Toluene: 200 ppm, Catalyst: 0.1 g	30°C	95%	~ 68%	Shao Q et al. (2022)
	0.1 K-modified MnO_X/Al₂O₃	O₃: 300 ppm, Toluene/DCM: 500 ppm, Catalyst: 0.2 g, Flow rate: 1000 mL/min	80°C	Toluene: 45%, DCM: 48%	-	Wang X et al. (2023)
Binary mixture of toluene and acetone	10 wt% MnO_X/γ-Al₂O₃	O₃: 1200 ppm, total VOCs: 130 ppm, Catalyst: 0.06 g, Flow rate: 350 mL/min	90°C	Toluene 90%, acetone 95%	95%	Aghbolaghy M et al. (2018)
Benzene	MnO_X/SiO₂@AC	O₃: 300 ppm, Benzene: 30 ppm, Catalyst: 1.5 g, Flow rate: 1000 mL/min, RH: 50%	ambient temperature	100%	MAR 76.4%	Fang R et al. (2019)

Table 4 Recent studies on catalytic ozonation of oxygen-containing hydrocarbons

Organic Pollutants	Catalysts	Reaction Parameters	Reaction Temperature	Highest Conversion	CO₂ selectivity	Ref.
ethyl acetate	β-MnO₂	O₃: 500 ppm, ethyl acetate: 17 ppm, Catalyst: 2.46 g	room temperature	90%	71.5%	Xu Z et al. (2021)
	1 wt%Pd/Active carbon fiber	O₃: 1000 mg/m³, ethyl acetate: 350 mg/m³, Catalyst: 0.2 g	30°C	60%	-	Cui J et al. (2021)
methyl ethyl ketone	MnO_X/γ-Al₂O₃	O₃: 1000 ppm, MEK: 100 ppm, Catalyst: 1.5 g, Flow rate: 2000 mL/min	room temperature	99.4% at 20min		Kim B S et al. (2021)
	5 wt%Mn-1 wt%Cu/spent fuild catalytic cracking	O₃: 1000 ppm, MEK: 100 ppm, Catalyst: 0.5 g	20-23°C	~60%	-	Hwang Y et al. (2023)
	5 wt%Mn–1 wt%Cu/HZSM-5	O₃: 1000 ppm, MEK: 100 ppm, Catalyst: 0.6 g, Flow rate: 1000 mL/min	25°C	90.2%		Ha M-J et al. (2023)
methanol	0.2 wt%Pt/FeO_X	O₃: 200 ppm, methanol: 380 ppm, RH: 30%	30°C	100%	-	Tian M et al. (2020)
acetone	2.5 wt%CoO_X/γ-Al₂O₃	O₃: 1200 ppm, acetone: 150 ppm, Catalyst: 0.065 g, Flow rate: 250 mL/min	25°C	85%	~90% of CO_X	Aghbolaghy M et al. (2019)
formaldehyde	1D rod α-MnO₂	O₃: 240 ppm, HCHO: 60 ppm, Catalyst: 0.05 g, Flow rate: 100 mL/min	ambient temperature	90%	100%	Zhang Y et al. (2019)
	MnCeO_X	O₃: 165 ppm, HCHO: 55 ppm, Catalyst: 0.05 g, Flow rate: 100 mL/min, RH: 60%	20°C	~100%	96%	Zhang Y et al. (2019)
	C@MnO	O₃: 180 ppm, HCHO: 60 ppm, Catalyst: 0.1 g, Flow rate: 100 mL/min	30°C	100%	100%	Wang H et al. (2018)
	mesoporous NiO	O₃: 180 ppm, HCHO: 60 ppm, Catalyst: 0.1 g, Flow rate: 100 mL/min, RH: 50%	90°C	100%	100%	Wang H et al. (2018)

In this review paper, the catalytic oxidation process using ozone as an oxidant is described in detail, focusing on the oxidation of benzene, reaction mechanism, reaction process, catalyst design, and reaction conditions to increase the reaction efficiency. One of the most important findings is that high-performance catalysts with high durability can be obtained for benzene oxidation using inexpensive Mn and zeolite Y supports. However, the reaction mechanism remains unclear as to why the addition of water vapor increases the rate of benzene decomposition. However, much remains to be investigated regarding the reaction mechanism. First, the addition of water vapor accelerates the oxidative decomposition of byproduct compounds, such as surface formate species derived from benzene. Why is Mn oxide effective as a metal oxide for efficient oxidative decomposition of such byproduct compounds? These questions have yet to be answered, and it is essential to conduct further research using computational science.

Recent advances in the catalytic ozonation process have also been reported. Aromatic hydrocarbons and hydrocarbons, such as alkanes, alcohols, and oxygenated hydrocarbons, can be oxidized and decomposed relatively easily using Mn-based supported catalysts. Recently, research on the decomposition reactions of less reactive Cl-containing organic compounds such as dichloromethane has been intensively pursued.

A VOC abatement system can be constructed using a combination of a silent discharge reactor (ozone generator) and Mn-based catalysts. However, the environmental standards for air pollution will continue to become stricter. To efficiently treat VOCs emitted from various sources, the development of systems with lower initial and operational costs is a challenge. Therefore, the development of a technology to process VOCs with lower power consumption is challenging. It is necessary to develop an ozonizer with a high ozone generation efficiency and a catalyst that can efficiently decompose VOCs with a small amount of ozone.

Acknowledgments

This work was financially supported by JST A-STEP program (JPMJTR20TE), JST SICORP program (JPMJSC18H3), and JSPS KAKENHI Grant Number 21H03635.

Ethical Approval

Not applicable.

Consent to Participate

Not applicable.

Consent to Publish

All authors declare their consent to publication in Environmental Science and Pollution Research.

Authors Contributions

Hisahiro Einaga and Xuerui Zheng contributed to manuscript preparation. Xuerui Zheng investigated recent advance in catalytic ozonation processes. All authors read and approved the final manuscript.

Funding

This work was financially supported by JST A-STEP program (JPMJTR20TE) and JSPS KAKENHI Grant Number 21H03635.

Competing Interests

The authors declare no competing interests.

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Fundamental Insights and Recent Advances in Catalytic Oxidation Processes Using Ozone for the Control of Volatile Organic Compounds

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Abstract

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Introduction

Catalytic oxidation processes using ozone as an oxidant