Hydrotalcite-Derived Mixed Metal Oxides as Catalyst Precursors for Methanol, Ethanol and Toluene Oxidation

Abstract

One of the most serious threats linked to the development of civilization is the growing amount of human-generated pollution released into the environment. These include volatile organic compounds (VOCs), a significant group of air pollutants found in all urban and industrial regions. They are characterized by high volatility, reactivity, and toxicity, which contribute to declining health in living organisms and to the formation of tropospheric ozone and photochemical smog. In this review, the sources of selected alcohols (i.e., methanol, ethanol) and toluene as the representative VOCs are discussed. Furthermore, the oxidation of these VOCs is reviewed over an important class of catalysts, which are multicomponent hydrotalcite-derived mixed metal oxides. The discussion about structure–property activity correlations is followed by a review of the most active and selective catalyst for each model molecule. Finally, the reaction mechanisms are discussed in the view of the existing literature, followed by the outcome. This review provides useful guidance for the rational design and development of advanced metal oxide catalysts for VOCs degradation, with particular relevance to practical industrial applications.

1. Introduction

Volatile organic compounds (VOCs) are organic compounds having a vapor pressure of no less than 0.01 kPa at 293.15 K or having similar volatility under specific conditions of use. This term covers many types of chemical compounds, the most numerous of which are (i) hydrocarbons (paraffins, olefins, aromatic hydrocarbons), (ii) organic oxide compounds (alcohols, ketones, acids, esters), and (iii) halogenated organic compounds (chlorine-, bromo-, and iodine derivatives). The type and characteristics of VOCs depend on their emission source. Alcohols and aldehydes are common pollutants, and methanol and ethanol are used in gasoline and as industrial solvents. Alcohol engines emit unburned methanol and ethanol plus formaldehyde and acetaldehyde as partial oxidation products. Finally, formic acid and acetic acid are partial oxidation products of alcohols and aldehydes. Because CO is a product of the decomposition of most of these VOCs, its oxidation is frequently also studied next to model VOCs [1,2].

VOCs are recognized as major air pollutants, and many countries have already implemented legislation to limit their emissions. These regulations aim to protect both environmental quality and public health by setting emission limits and promoting cleaner technologies. Environmental regulations are imposing increasingly strict emission limits to reduce VOC emissions into the atmosphere. In the European Union, Council Directive 1999/13/EC sets binding limits on VOC emissions from industrial activities that use organic solvents and requires measures to reduce solvent use and replace high-VOC products with lower-VOC alternatives. Member states also must register and authorize installations and report on VOCs reductions [3]. The EU also adopted Directive 2004/42/EC, which restricts the VOC content in paints, varnishes, and vehicle refinishing products sold in the internal market to lower the amount of VOCs released during use [4]. In the United States, the Clean Air Act (CAA) empowers the Environmental Protection Agency (EPA) to regulate outdoor VOC emissions because they contribute to ground-level ozone (smog). Under this law, the EPA sets standards for VOC content in products like architectural coatings and other consumer items to reduce their impact on air quality [5].

VOC emissions are linked to numerous industrial processes. In addition to major emission sources like the petroleum industry, there are numerous local sources—such as painting, printing, and laundry—that release VOCs at lower concentrations.

Table 1. Sources and the main produced VOCs.

Source Type	Examples	Common VOC Compound
Anthropogenic	Industrial processes (chemical plants, petroleum refining), paints, varnishes, adhesives, cleaning products, vehicle exhaust, power plants, landfills, sewage treatment	Benzene, Toluene, Xylene, Formaldehyde, Acetone, Ethylbenzene
Biogenic (Natural)	Plants and trees (isoprene, terpenes), soil microorganisms (decomposition VOCs), wildfires, and volcanic emissions	Isoprene, Limonene, Pinene, Methane, Terpenes

gives both the source and the specific VOCs typically emitted, which is useful for research or environmental studies.

VOCs are acknowledged as significant contributors to human health and air pollution. Some of them exhibit toxic, narcotic, or carcinogenic properties.

Table 2. Common indoor VOCs and their health effects.

VOCs	Indoor Sources	Health Effects
Formaldehyde	Furniture, building materials, textiles	Eye, nose, throat irritation; allergic reactions; cancer risk
Benzene	Tobacco smoke, paints, and glues	Headaches, dizziness, blood disorders, leukemia
Toluene	Paints, adhesives, nail polish	Nausea, headaches, fatigue, and neurological effects
Xylene	Paints, varnishes, cleaning products	Eye, nose, throat irritation; dizziness; headaches
Acetone	Nail polish remover, cleaning agents	Eye and respiratory irritation, headaches, and dizziness
Terpenes	Air fresheners, scented candles	Respiratory irritation, headaches

presents how the most common indoor VOCs accumulate and affect human health. Exposure to VOCs can cause short-term effects such as eye, nose, and throat irritation, headaches, dizziness, and nausea. Long-term or chronic exposure may lead to more serious health problems, including neurological effects, respiratory issues, organ damage, and even cancer. Vulnerable populations, such as children, the elderly, and people with pre-existing conditions, are at higher risk. Managing indoor air quality and limiting VOC sources can significantly reduce these health risks [6,7]. VOCs have significant environmental impacts. They contribute to the formation of ground-level ozone (VOC(g) + NO_x(g) + O₂(g) + hν ⟶ O₃(g) + other products(g)) and photochemical smog, which can harm plants, reduce air quality, and affect ecosystems. VOCs also participate in chemical reactions in the atmosphere that produce secondary organic aerosols, contributing to air pollution and climate effects. Additionally, some VOCs persist in the environment, contaminating soil and water, which can disrupt ecosystems and wildlife. Overall, VOC emissions play a major role in air pollution and environmental degradation [6,8,9].

Therefore, reducing VOC emissions is highly desirable to protect both human health and the environment. Thus, in this review, there is a focus on various methods of VOC removal, followed by the application of transition metal-containing hydrotalcite-derived mixed metal oxides over model VOCs such as alcohols (methanol and ethanol) and toluene. This is followed by the discussion regarding reaction mechanisms and final remarks concerning the development of catalysts based on multicomponent metal oxides.

2. Removal of VOCs

Controlling VOC emissions at their sources is the most effective approach, while removing VOCs already released into the environment remains a critical and challenging task. Some VOC emissions can be reduced by eliminating or replacing the use of solvents. However, when eliminating solvents is not feasible, the only alternative is to treat the effluents before releasing them into the atmosphere. Measures to prevent and reduce VOCs emission in production processes, from an ecological efficiency perspective, include removing the VOCs from the process entirely, capturing and recycling the emitted vapors back into the process, capturing and treating the vapors, and destroying VOCs in exhaust gases (Figure 1).

Pipe-end treatment, which focuses on the removal or degradation of environmental VOCs, has led to the development and application of various technologies. These include condensation, membrane separation, adsorption, absorption, combustion, catalytic oxidation, low-temperature plasma treatment, and biological purification. The methods for reducing VOCs emission by separating them from waste gases or by destruction are the following:

1. Condensation is typically used to pre-treat VOCs from gases before final purification. Typical recovery efficiencies range from 50% to 90%, except for cryogenic cooling (which significantly exceeds 90%). The process is carried out in diaphragm condensers or by using a cold liquid spray to cool the gases [9,10,11].

2. Membrane separation can be used to separate gases and vapors, including the removal and/or recovery of VOCs. Combined compression and condensation systems, combined with membrane separation, can be used to remove many VOCs [12,13,14].

3. Absorption involves capturing VOCs with an absorbent in scrubbers. The best results are obtained when using mineral oils with low vapor pressures for absorption. This method is rarely used due to difficulties in recovering the absorbed compounds [15,16,17].

4. Adsorption is a highly effective method that utilizes various types of adsorbents: molecular sieves, zeolites, activated carbon, and organic polymers. The advantage of sorption methods is the relatively simple recovery of adsorbed compounds. The disadvantage is low selectivity. Compound removal efficiency is 50–95%. Gases must be pre-cleaned to remove aerosols, dust, and polymerizable substances, regardless of the adsorbent used [18,19,20].

5. Low-temperature plasma (LTP) treatment is an advanced technology used to remove VOCs from air streams. In this process, plasma—a partially ionized gas containing energetic electrons, ions, and radicals—is generated at near-ambient temperatures. These reactive species break down VOCs molecules into simpler, often harmless products such as carbon dioxide, water, and other small molecules [21,22].

6. Oxidation.

Thermal and catalytic combustion is not a separation method, but rather the destruction of organic compounds, which is characterized by high VOC destruction efficiency. Due to the lower temperature, it is significantly cheaper than burning VOCs in gases. The disadvantage of this method is the omission of VOC recovery and the need for additional gas purification when the burned compounds contain halogens, sulfur, nitrogen, phosphorus, or metals. Catalyst selection plays a significant role in these processes [23,24].

Biological oxidation involves the adsorption of VOCs from air, followed by the oxidation of the pollutant by heterotrophic aerobic microorganisms. The air purification process is carried out in bioscrubbers, bifilters (peat, soil), or three-phase bioreactors. Water serves as the adsorption medium, and VOC oxidation occurs in a bed inhabited by active microorganisms [25,26,27].

Thermal incineration is the most widely used method for removing VOCs from industrial air streams. Thermal incinerators operate at higher temperatures (700–1200 °C) compared to catalytic units (400–500 °C). Additionally, thermal incineration often requires diluted mental fuel to maintain a flame in dilute VOC waste streams (less than 1000 ppm). It should also be noted that these pollutants originate from diverse industrial sectors and vary widely depending on operating conditions, which differ significantly in terms of component types, flow rates, and concentrations. However, it requires very high temperatures, typically above 1000 °C, which necessitates additional fuel and the use of heat-resistant materials; it can also lead to the formation of harmful by-products such as NOx. Cost analyses indicate that thermal incineration is most suitable for processes with high flow rates or high VOC concentration. Catalytic incineration is a widely used method for pollution control.

Catalytic incineration of VOCs is preferred over thermal incineration because it achieves high VOC destruction at lower temperature, leading to lower energy use, fewer emissions, and reduced operating costs, making it more sustainable for many industrial applications. Numerous catalysts have been extensively studied for the complete oxidation of VOCs [28,29,30]; however, catalyst deactivation during VOC oxidation remains a significant challenge [31,32]. Currently, a key focus in VOC oxidation research is the development of low-cost transition metal catalysts that exhibit high activity and selectivity [1]. Research on the heterogeneous catalytic oxidation of low-concentration VOCs in air focuses on identifying active catalysts that produce environmentally benign products at lower temperatures, ideally H₂O and CO₂. Another important characteristic of these catalysts is their ability to simultaneously remove all contaminants present in the stream (e.g., [33,34]).

The most commonly used catalysts for VOC abatement are noble metals (Pt, Pd, and Rh) and transition metal oxides (Mn₂O₃, CuO, Co₂O₃, NiO, Cr₂O₃, V₂O₅) dispersed on high specific surface area supports such as (mesoporous) alumina, silica, and titania, zeolites or on ceramic or metallic monoliths [35,36,37,38,39,40,41,42,43]. The support material is a crucial factor in developing an efficient catalyst system, significantly influencing the dispersion of the active component on its surface. The catalytic activity of nanoparticles is strongly determined by their microscopic structure, the presence of co-adsorbed species, and the nature of metal–support interactions. In many instances, the distinctive reactivity of these surfaces is ascribed to the concurrent existence of multiple active sites [44,45,46,47].

Noble metal catalysts are highly active at lower temperatures and are effective for diluting VOC streams, while transition metal oxides offer thermal stability and cost advantages for industrial applications. Catalyst activity depends on the type and dispersion of the active metal, particle size, support interactions, and the presence of promoters or poisons. Many commercial catalysts use nanoparticle structures, which provide multiple active sites and enhanced reactivity. Additives such as ceria or zirconia are sometimes included to improve oxygen storage and durability. These catalysts are typically manufactured as pellets or coated monoliths to maximize surface area and facilitate efficient VOC oxidation to CO₂ and H₂O [48,49]. However, the results indicated that the activity of commercial catalysts in laboratory-scale investigations differs from the specifications provided by the catalyst supplier [50]. There are numerous commercial-based catalysts, e.g., catalysts offered by Clariant such as EnviCat^TM VOC [51], BASF—VOCat™ and Camet™ [52], etc. Furthermore, it is important to evaluate the catalytic activity not only in the oxidation of a single organic compound in an air stream but also in the oxidation of multicomponent mixtures. The effectiveness of a catalyst in practical use depends not only on its activity but also on its thermal stability and resistance to deactivation by poisons. Catalyst deactivation in the oxidation of VOCs occurs for several key reasons [31,38,53,54]:

• Poisoning: Certain compounds in the gas stream, like sulfur- or chlorine-containing species, bind strongly to the active sites of the catalyst, blocking them and reducing activity;
• Sintering/thermal degradation: High temperatures during oxidation can cause the catalyst’s metal particles or support structure to agglomerate or collapse, lowering the specific surface area and active sites;
• Carbon deposition (coking): VOCs can decompose on the catalyst surface to form carbonaceous deposits that physically block active sites;
• Structural changes: oxidation reactions can alter the chemical or crystal structure of the catalyst, reducing its effectiveness over time;
• Moisture: Some catalysts are sensitive to water vapor, which can lead to hydrolysis or leaching of active components.

Thus, even a highly active catalyst can lose efficiency quickly in VOC oxidation because the reaction environment is harsh, containing heat, reactive intermediates, and possible poisons. Clearly, catalytic oxidation of trace amounts of VOCs in air is more cost-effective when conducted at lower temperatures. Achieving this requires VOC oxidation catalysts that exhibit high activity, excellent selectivity, low activation energy, and long-term durability. Furthermore, a fundamental understanding of catalytic oxidation mechanisms is valuable for developing effective methods to use catalysts in controlling air pollution.

In the present review, these challenges are addressed over hydrotalcite-derived mixed metal oxides for alcohols (methanol and ethanol) and toluene. Methanol and ethanol are commonly released from industrial solvent processes and as unburned fuels from mobile sources. Acetaldehyde, a partial oxidation product, is frequently emitted from ethanol-fueled engines. Similarly, formic acid and acetic acid are partial oxidation products of methanol and ethanol, respectively. Toluene was chosen as a third probe molecule for catalytic oxidation because it is commonly found in industrial exhausts and has a high photochemical ozone creation potential (POCP) [55,56]. Nevertheless, hydrotalcite-like compounds were broadly investigated in the incineration of other VOCs, such as methane [57], acetone and acetaldehyde [58,59], benzene [60,61,62,63], etc.

3. Hydrotalcite-Derived Mixed Metal Oxides

Hydrotalcite-like compounds (HTs), which belong to the group of layered double hydroxides (LDHs), are made up of positively charged metal hydroxide layers that are separated by interlayer anions and water molecules. The layers are composed of metal cations present in at least two distinct oxidation states. The chemical composition of hydrotalcite-like compounds can be expressed by the general formula of [M²⁺_1−xM³⁺_x(OH)₂]^x+[Aⁿ⁻_x/_n · yH₂O]^x−, where M²⁺ and M³⁺ denote divalent and trivalent metal ions, Aⁿ⁻ represents an anion with a charge of n-, and the value of x typically ranges from about 0.25 to 0.33 [64,65]. Their structure resembles that of brucite, Mg(OH)₂, in which each Mg²⁺ ion is octahedrally coordinated by six OH⁻ ions, and the octahedra share edges to form continuous, infinite sheets. The sheets are stacked atop one another and are held together by weak interactions through hydrogen bonding. In hydrotalcite-like compounds, the isomorphous substitution of M²⁺/M³⁺ ions in the octahedral sites of the hydroxide layers generates a net positive charge, which is balanced by interlayer anions and water molecules [66,67]. Co-precipitation is the most used method for preparing hydrotalcite materials because it is simple, quick, cost-effective, and operates under mild conditions [68,69,70]. Additionally, many mixed oxide catalysts can be produced through the controlled decomposition of such compounds. The diverse chemical compositions and physico-chemical properties of synthetic LDHs enable a wide range of applications for these materials. The calcined materials exhibit a high specific surface area, strong basicity, homogenous metal dispersion in the precursor matrix (which cannot be achieved using other preparative techniques), and resistance to sintering. After calcination, the material typically restores its hydrotalcite structure through a rehydration and reconstruction process commonly referred to as the memory effect [71,72].

Hydrotalcite-like compounds, whether in their as-prepared form or after high-temperature calcination, have been utilized in a wide range of practical applications. They are frequently employed as catalysts or catalyst precursors, ion exchangers, adsorbents, polymer stabilizers, gas sensors, and more [65,73,74,75]. Moreover, there is significant interest in using calcined hydrotalcites (oxides) as catalysts for environmental applications, such as NH₃ oxidation, NH₃ decomposition, NO_x reduction with ammonia, or N₂O decomposition [76,77,78,79]. Many hydrotalcite-like compounds are employed in heterogeneous catalysis as precursors for mixed oxide catalysts, whose catalytic performance is highly influenced by the method of thermal treatment (Figure 2). Therefore, the thermal behavior of hydrotalcite-like compounds is studied in detail, as the nature of the precursors can significantly influence the final composition of more complex oxide mixtures (e.g., [80,81]). The catalytic activity of hydrotalcites primarily depends on the M²⁺/M³⁺ ratio, the types of cations in the structure, the nature of the compensating anions, and, among other factors, the activation temperature. The combined use of different metallic phases can produce a synergistic or cooperative effect, often enhancing catalytic properties. Using metallic mixtures with rare-earth promoters (also incorporated into the hydrotalcite during their reconstruction) can improve the oxygen storage capacity of the solids, thereby enhancing their oxidation activity. Various LDH-derived mixed oxides have been reported as catalysts for the complete oxidation of VOCs, commonly using alcohols (methanol, ethanol) or toluene as model compounds. Furthermore, the activity of hydrotalcite-derived mixed metal oxides is correlated with the phases involved.

3.1. Oxidation of Methanol and Ethanol

Methanol is commonly used as a representative VOC in oxidation studies due to its simple chemical structure [83,84]. CH₃OH oxidation produces a variety of products, such as formaldehyde, methyl formate and CO₂, depending on the catalyst type and operating conditions [85,86]. Ignition curves are the most used method for evaluating catalytic activity in studies of the complete oxidation of VOCs.

Table 3. Results of the alcohol oxidation over hydrotalcite-derived mixed metal oxides.

Catalyst (Preparation Method)	Crystalline Phase (BET/m2 g−1)	Reaction Conditions	T50/°C	T90/°C	Ref.
Methanol
Cu-Mg-Al (co-precipitation, calcined at 700 °C in air)	MgO, CuAl2O4 (106)	c(CH3OH) = 0.5 vol.-%, c(O2) = 4.5 vol.-% diluted in He, mcat = 0.1 g, GHSV not provided	232	286	[87]
K/Cu-Mg-Al (co-precipitation, calcined at 800 °C in air)	MgO, CuAl2O4 (-)	c(CH3OH) = 0.5 vol.-%, c(O2) = 4.5 vol.-% diluted in He, mcat = 0.1 g, GHSV not provided	198	232	[87]
Cu(5)-Mg-Al (co-precipitation, calcined at 600 °C in air)	MgO (121)	c(CH3OH) = 4.0 vol.-%, c(O2) = 19.0 vol.-%, diluted in N2, mcat = 0.1 g, FTOT = 20 cm3 min−1 WHSV = 12 L h−1 g−1	250	280	[88]
Pd0.5/Cu(5)-Mg-Al (co-precipitation, calcined at 600 °C in air)	MgO (84)	c(CH3OH) = 4.0 vol.-%, c(O2) = 19.0 vol.-%, diluted in N2, mcat = 0.1 g, FTOT = 20 cm3 min−1 WHSV = 12 L h−1 g−1	193	225	[88]
Ethanol
CuCr (co-precipitation, calcined at 600 °C in air)	CuO, CuCr2O4 (-)	c(C2H5OH) = 2–4 g m−3, mcat = 0.7 g GHSV = 50,000 h−1	163	174	[89]
Co4Mn1.5Al0.5 (co-precipitation, calcined at 500 °C in air)	spinel-like phases (90)	c(C2H5OH) = 1 g m−3, mcat = 0.75 g FTOT = 40 cm3 min−1 WHSV = 3.25 L h−1 g−1	154	200	[90]
Co4Mn2 (co-precipitation, calcined at 500 °C in air)	spinel-like phases (44)	c(C2H5OH) = 1 g m−3, mcat = 0.75 g FTOT = 40 cm3 min−1 WHSV = 3.25 L h−1 g−1	165	210	[90]
Co4Al2 (co-precipitation, calcined at 500 °C in air)	spinel-like phases (84)	c(C2H5OH) = 1 g m−3, mcat = 0.75 g FTOT = 40 cm3 min−1 WHSV = 3.25 L h−1 g−1	221	250	[90]
CuMn0.25 (co-precipitation, calcined at 600 °C in air)	MgO, MgMnO3 (-)	c(C2H5OH) = 0.1 vol.-%, diluted in air mcat = 0.2 g FTOT = 200 cm3 min−1 WHSV = 60 L h−1 g−1	209	256	[91]
MnCu24 (co-precipitation, calcined at 500 °C in air)	CuO, Mn2O3, Cu1.5Mn1.5O4 (22)	c(C2H5OH):c(O2): c(He) = 1:20.8:78.2, mcat = 0.3 g FTOT = 100 cm3 min−1 WHSV = 20 L h−1 g−1	177	212	[92]
CoMn0.25 (co-precipitation, calcined at 600 °C in air)	MgO, MgMnO3 (-)	c(C2H5OH) = 0.1 vol.-%, diluted in air, mcat = 0.2 g FTOT = 200 cm3 min−1 WHSV = 60 L h−1 g−1	198	252	[91]
Mn5CoAl2-O (co-precipitation, calcined at 500 °C in air)	Mn2CoO4, Al2CoO4, Mn5O8, Mn3O4, MnO (171)	c(C2H5OH) = 0.1 vol.-%, diluted in air, mcat = 0.1 g FTOT = 100 cm3 min−1 WHSV = 60 L h−1 g−1	140	148	[93]
Mn5CoAl2-N2-O (co-precipitation, calcined at 500 °C in air)	Mn2CoO4, Al2CoO4, Co3O4 (189)	c(C2H5OH) = 0.1 vol.-%, diluted in air, mcat = 0.1 g FTOT = 100 cm3 min−1 WHSV = 60 L h−1 g−1	142	155	[93]
Co-Ni-Mn (co-precipitation, calcined at 500 °C in air)	spinel-like phases (54)	c(C2H5OH) = 0.075 vol.-%, diluted in air mcat = 0.1–0.4 g WHSV = 20–80 L h−1 g−1	100	154	[94]
Co-Ni-Al (co-precipitation, calcined at 500 °C in air)	spinel-like phases (131)	c(C2H5OH) = 0.075 vol.-%, diluted in air mcat = 0.1–0.4 g WHSV = 20–80 L h−1 g−1	167	204	[94]
CoCu41 (co-precipitation, calcined at 500 °C in air)	Co3O4, CuO (21)	c(C2H5OH) = 0.075 vol.-%, diluted in air, mcat = 0.2 g WHSV = 20 L h−1 g−1	91	159	[95]
Ce/CuCoMgAl (co-precipitation, reconstruction, calcined at 500 °C in air)	MgO, Co3O4 (181)	c(C2H5OH) = 0.1 vol.-%, diluted in air, mass of catalysts not provided FTOT = 100 cm3 min−1	226	261	[96]
CePr/CuCoMgAl (co-precipitation, reconstruction, calcined at 500 °C in air)	MgO, Co3O4 (201)	c(C2H5OH) = 0.1 vol.-%, diluted in air, mass of catalysts not provided FTOT = 100 cm3 min−1	216	260	[96]
1Au/CoAl HT(B) (deposition–precipitation, calcined at 250 °C in air)	spinel-like phases Co3O4, CoAl2O4, and Co2AlO4 (206)	c(C2H5OH) = 0.1 vol.-%, diluted in air, mcat = 0.1 g FTOT = 100 cm3 min−1 GHSV= 30,000 h−1	165	183	[97]
1 wt.-% K/Co-Mn-Al (co-precipitation, impregnation, calcined at 500/500 °C in air)	spinel-like phases (103)	c(C2H5OH) = 1.0 g m−3, diluted in air, mcat = 0.75 g WHSV = 10 L h−1 g−1	158	160	[98]
Co-Mn n(Co)/n(Co+Mn) = 0.2 (deposition, calcined at 500 °C in air)	CoMnO3 (12)	c(C2H5OH) = 0.077 vol.-%, diluted in air, 8 meshes WHSV = 20 L h−1 g−1	185	244	[99]
Ni-Cu-(Mn)-Al/pH 6.8 (deposition, calcined at 500 °C in air)	NiO (37)	c(C2H5OH) = 1.0 g m−3, diluted in air, mcat = 0.4 g WHSV = 20 L h−1 g−1	204	228	[100,101]
Co-Cu-(Mn)-Al/pH 6.8 (deposition, calcined at 500 °C in air)	spinel-like phases (70)	c(C2H5OH) = 1.0 g m−3, diluted in air, mcat = 0.4 g WHSV = 20 L h−1 g−1	268	307	[100,102]
Mn4Mg2Al2-O (co-precipitation, calcined at 500 °C in air)	spinel-like phases, Mn3O4, MgAl2O4, MnAl2O4, MgMn2O4 (108)	c(C2H5OH) = 0.1 vol.-%, diluted in air, mcat = 0.1 g FTOT = 100 cm3 min−1 GHSV = 30,000 h−1	152	164	[103]

gathers the results of the alcohol oxidation over hydrotalcite-derived mixed metal oxides. If not specified in the respective studies, the T₅₀ and T₉₀, which refer to the temperature reach of 50% and 90% of the total oxidation of alcohol, were estimated from the graphics. Many catalysts were provided with the scope of the studies, while only data for one or two of the most representative examples were gathered in Table 3. Mostly, the catalytic oxidation of alcohols, either individually or in mixtures, is presented for several materials, with particular attention given to the formation of by-products.

The studies of methanol oxidation over hydrotalcite-based catalysts are scarcer compared to other VOCs. For example, catalysts derived from hydrotalcite-based Cu-Mg-Al, Co-Mg-Al, and Cu-Co-Mg-Al oxide systems were tested for the total oxidation of mono-carbon VOCs, including methane, methanol, and formic acid. Both the calcination temperature (700, 800 °C) of the hydrotalcite precursors and the doping of the catalysts with a K promoter (0.09, 0.9 and 2.0 wt.-%) influenced their catalytic activity. CO₂ and H₂O were the only detected products of methane and methanol oxidation, whereas the conversion of formic acid also produced hydrogen. The catalytic activity of the samples, containing 0.9 wt.% and 2.0 wt., was nearly identical. K doping enhanced the activity of Cu-Mg-Al and Cu-Co-Mg-Al, likely by increasing the low-temperature reducibility of copper oxide but it did not affect Co-Mg-Al [87]. In the following studies, the hydrotalcite-derived (Cu, Mn)-Mg-Al mixed metal oxides were investigated for methanol incineration. In contrast, copper or manganese species supported on Al₂O₃ and MgO showed much lower activity compared to catalysts derived from hydrotalcites. The catalytic activity of hydrotalcite-derived mixed metal oxides was enhanced by depositing a small amount of Pd species (0.5 wt.%) [88].

Ethanol promotes the formation of aldehydes, which can lead to irritation, shortness of breath, mucous membrane irritation, and chest tightness [104]. C₂H₅OH is a major indoor pollutant released from various materials and often accumulates in airtight buildings [105]. Ethanol is also extensively utilized in the chemical industry and as a fuel additive, reflecting its strong demand in the global market. Therefore, developing efficient systems to remove C₂H₅OH and its derivatives from automobile exhaust, industrial emissions, and indoor environments is a crucial societal challenge. Ethanol, now widely used as a renewable biofuel for buses and cars, was selected as a model VOC for catalytic experiments. The selectivity of deep oxidation catalysts is crucial, as the reaction by-product can often be more harmful than the original VOCs. Concentrations of harmful compounds at high VOC conversion are particularly important. In the case of ethanol oxidation, such by-products can include acetaldehyde, acetic acid, ethylene, ethyl acetate, acetone, diethyl ether, and carbon monoxide. The formation of by-products depends on the properties of the catalyst and reaction conditions.

Bahranowski et al. [89] investigated the Cr-containing hydrotalcite-like materials calcined at 600 °C and found that they exhibited high catalytic activity for the complete oxidation of toluene and ethanol. Among them, Cu-Cr samples with a n(Cu):n(Cr) ratio of two, composed of copper oxide and copper chromite, reached 50% conversion of toluene and C₂H₅OH at temperatures of 45 and 15 °C, respectively, lower than the reference commercial catalysts. Catalytic investigations using a mechanical mixture of CuO and CuCr₂O₄ demonstrated that the use of a hydrotalcite-like precursor is essential for achieving optimal VOC conversion. Toluene oxidation proceeded directly to CO₂, whereas in the case of ethanol, acetaldehyde formed as a partially oxidized intermediate. The main products of C₂H₅OH oxidation over all the catalysts investigated were acetaldehyde and CO₂, while carbon monoxide was detected only in trace amounts.

Kovanda et al. [90] reported calcined Co-Mn-Al (n(Co)/n(Mn)/n(Al) molar ratios of 4/2/0, 4/1.5/0.5, 4/1/1, 4/0.5/1.5, and 4/0/2) hydrotalcite-like compounds as catalysts for C₂H₅OH incineration. Mg-containing materials, i.e., Co-Mg-Mn (2/2/2), Co-Mg-Mn-Al (2/2/1/1), and Co-Mg-Al (2/2/2), were also investigated. They reported that hydrotalcite synthesis was unsuccessful in systems containing only Co and Mn (n(Cu)/n(Mn) = 4/2), otherwise stated that a high (Co + Mn) content was essential for achieving complete C₂H₅OH conversion at lower temperatures. The high activity of the Co-containing catalysts were attributed to an optimal content of reducible components.

A Co-Mn-Mg-Al catalyst series was compared to Cu-containing samples, in which the n(Mn)/n(Co) or n(Mn)/n(Cu) ratios in the solids were varied from 0.05 to 0.5 [91]. The highest n(Mn)/n(Co) ratio shows the best catalytic activity. The observed behavior in the ethanol oxidation reaction was attributed to the formation of amorphous phases and redox cycles arising from cooperative interactions between the active species. The catalysts were evaluated for the total oxidation of three VOCs, ethanol, toluene, and butanol, with the ease of oxidation following the order: butanol < ethanol < toluene. The catalytic activity of manganese-modified mixed oxides depended on the redox properties of the Mn⁴⁺/Mn³⁺ system present on the oxide surface, which was likely influenced by the amount of manganese used in the synthesis as well as the presence of another metal capable of promoting redox cycles. Acetaldehyde was not detected in the chromatographic analysis, and the only oxidation products were CO₂ and H₂O, (allowing the results to be evaluated based on the conversion of ethanol to CO₂). The formation of by-products depends on both the chemical structure of the compound and the properties of the catalyst. The addition of small amounts of copper species can prevent manganese oxide from adopting a crystalline structure, and promote the formation of oxygen vacancies and the mixed Cu_1.5Mn_1.5O₄ phase with enhanced reducibility, thereby improving catalytic activity in C₂H₅OH combustion in the series of Mn-Cu-type oxides [92,106]. Also, the catalytic activity of MnCoAl oxides depended on both the n(Mn)/n(Co) ratio and the gas environment applied during their synthesis and heat treatment strategies [93]. The formation of Mn₂CoO₄ spinel and Mn₅O₈ layered phases enhanced the redox properties and increased the number of active sites in the MnCoAl oxides (Figure 3a). Employing an N₂ atmosphere during material preparation was an effective strategy to inhibit the formation of unwanted phases in the hydrotalcite precursors and their derived oxides. This further enhanced the complete conversion of ethanol to CO₂ over the obtained oxides. The CO₂ yield curves indicate that air-calcined binary oxides synthesized under a nitrogen atmosphere (Mn₆Al₂-N₂-O, Figure 3b) achieved complete ethanol conversion to carbon dioxide at the lowest temperature. C₂H₅OH conversion was significantly enhanced after completely replacing cobalt with manganese species. The temperature required for full C₂H₅OH conversion (T₁₀₀) decreased by 38 °C, shifting from 200 °C for Co₆Al₂-O to 162 °C for Mn₆Al₂-O. At low temperatures, acetaldehyde was the predominant product, with yields ranging from 500 to 700 ppm over Mn_xCo_(6−x)Al₂-O catalysts. At higher temperatures, CO₂ became the main product.

The ternary mixed oxides containing Mn species were more active than the binary Cu-Mn, Co-Mn, and Ni-Mn systems, as well as the ternary catalysts containing Al, among mixed metal oxides of n(M²⁺)/n(M³⁺) with a molar ratio of two (M²⁺ = Cu, Co, Ni, Cu-Ni, Cu-Co, and Co-Ni; M³⁺ = Mn or Al) [94]. Acetaldehyde was the main by-product of ethanol oxidation. Catalysts containing Al species produced lower amounts of acetaldehyde compared to those containing Mn species. The Cu-Ni-Mn sample exhibited the highest activity among the investigated mixed oxides, achieving nearly complete conversion (90% at 154 °C) to CO₂ and H₂O. In comparison, the same level of conversion was reached at 173 °C and 181 °C for the Cu-Co-Mn and Co-Ni-Mn catalysts, respectively.

The Co-Cu oxide catalyst with a n(Co)/n(Cu) molar ratio of 4/1 demonstrated the highest activity in gas-phase C₂H₅OH oxidation, achieving the lowest T₅₀ (91 °C) and T₉₀ (159 °C), corresponding to conversion to CO₂, respectively. This enhanced activity and selectivity were attributed to the presence of finely dispersed CuO particles on the surface of Co₃O₄. Additionally, a strong correlation was previously observed between the position of the first reduction peak in H₂-TPR measurements and the T₅₀ value during C₂H₅OH oxidation (Figure 3c,d) [95]. Pérez et al. [96] combined the co-precipitation with reconstruction in the presence of chelates [Ce-EDTA] and/or [Pr-EDTA]. They linked the highest catalytic activity to increased oxygen storage capacity and enhanced reducibility in solids containing rare-earth elements (Ce-Pr), as well as to improvements in specific surface area. Moreover, the high selectivity toward CO₂—the main product of total oxidation—indicated that solids modified with Cu and/or Co species and promoted with rare-earth elements were highly effective catalysts for C₂H₅OH oxidation, highlighting a cooperative interaction among the metals. The presence of Ce species, either alone or in combination with Pr, on the catalyst surface significantly promoted complete conversion in all cases below 270 °C. The concentrations of combustion by-products (such as ethanol, acetaldehyde, and acetic acid, among others) did not exceed 2%.

Studies related to the modified hydrotalcite-derived mixed metal oxides, either with noble metals or alkali metals, are scarce for the oxidation of alcohols (e.g., [97,98]). For example, the catalytic activity of the Co-Mn-Al mixed oxide catalyst (n(Co)/n(Mn)/n(Al) molar ratio of 4/1/1) modified with different potassium loadings (0–3 wt.-%) was investigated for the total oxidation of toluene and ethanol [98]. The highest conversion in C₇H₈ oxidation was achieved with the catalyst containing approximately 1 wt.-% of K species, with no detectable reaction by-products other than CO₂ and H₂O. The highest activity in toluene oxidation was obtained with the catalyst containing approximately 1 wt.-% K, whereas in C₂H₅OH oxidation, the activity gradually increased with increasing potassium content up to 3 wt.-%. Potassium species in the Co₄MnAl mixed oxides influenced the formation of acetaldehyde during ethanol oxidation, with its concentration increasing as the potassium content increased. No reaction by-products were detected during C₇H₈ oxidation.

Structured cobalt oxide catalysts supported on stainless steel meshes were readily prepared via hydrothermal synthesis under mild conditions [99,107], while the mixed oxides supported on stainless steel meshes were prepared by calcining carbonate precursors that crystallized on the supports during hydrothermal treatment. The supports were immersed in aqueous solutions of Co and Mn nitrates in the presence of hydrolyzing urea. The exceptional activity of the catalyst with a n(Co)/n(Mn) ratio of 0.5 was ascribed to a synergistic effect arising from a high concentration of oxygen vacancies and enhanced redox properties. Due to the formation of a thin film of the active phase and, consequently, the minimal influence of internal diffusion limitations, this catalyst exhibited higher activity (48 to 114 times higher catalytic activity) in C₂H₅OH oxidation than pelletized commercial Co₃O₄ (despite containing 50 times less cobalt oxide). Acetaldehyde was identified as the primary by-product of C₂H₅OH oxidation across all supported catalysts. The highest concentrations of acetaldehyde were observed between 217 and 243 °C, ranging from 485 to 619 ppm [99].

Relatively broad studies concern structured hydrotalcite-derived mixed metal oxides [100,101,102], prepared by calcining the precursor deposited on Al₂O₃/Al supports (anodized aluminum foil). The precursors were deposited onto the supports under hydrothermal conditions at 140 °C using aqueous solutions of Ni, Co, Cu, and Mn nitrates. M²⁺-(Mn)-Al (M²⁺ = Ni, Co, Ni-Co, Ni-Cu, and Co-Cu) with only small amounts of Mn species were obtained. Increasing the pH of the deposition solution promoted the formation of hydrotalcite-like phases. After heating at 500 °C, spinel-type and/or NiO-type oxide phases were identified in the supported mixed oxides. Among the supported catalysts, the Ni-Cu-(Mn)-Al mixed oxide showed the highest activity for the total oxidation of C₂H₅OH. Increasing the pH of the solutions used during hydrothermal deposition of the hydrotaclite-like precursors enhanced both the catalytic activity and selectivity of the supported mixed oxides. However, controlling the preparation conditions (i.e., pH) led to higher loads of active components and improved catalytic activity. The mixed oxides deposited on Al₂O₃/Al supports at pH 8.5 showed a slight shift in the reduction maxima toward lower temperatures, likely due to the higher Mn species content in these samples. Acetaldehyde was the main reaction by-product identified in the reaction mixture, while acetic acid, ethyl acetate, and ethylene were either not detected or were present only in negligible amounts (ethylene). Increasing the solution pH during hydrothermal deposition of the precursors improved both the catalytic activity and selectivity of the supported mixed oxides.

The combination of three transition metal cations (Cu, Co, or Ni) with Mn can increase the concentration of lattice defects and modify oxygen vacancies in mixed oxides derived from co-precipitated precursors, likely due to differences in ionic radii. The activity of the investigated mixed oxides showed a linear dependence on the amount of easily reducible species (reduced between 25 and 500 °C) as well as on the quantity of oxygen adsorbed on the catalyst surface. Mn-based hydrotalcite-like compounds with a n(Mn)/n(Mg)/n(Al) ratio of 1/5/2 were synthesized and applied as catalysts for the simultaneous oxidation of C₂H₅OH, CH₃CHO, and C₄H₁₀S [108]. The removal efficiencies for the three gaseous pollutants reached 100% for durations of 420, 450, and 360 min, respectively. Meanwhile, the breakthrough capacity for VOCs was 426 mg g⁻¹. Mn³⁺/Mn²⁺ redox pairs and oxygen vacancies served as active sites for C₂H₅OH/CH₃CHO and C₄H₁₀S, respectively. The oxidation of C₄H₁₀S produced CH₃CHO and elemental sulfur. The elemental sulfur was further oxidized to sulfuric acid at elevated temperatures, which led to the consumption of active Mn³⁺/Mn²⁺ sites and accelerated catalyst deactivation. The experimental findings have been consistent with theoretical results, providing a mechanistic basis for the simultaneous catalytic removal of sulfur-containing VOCs.

Summarizing, C₂H₅OH oxidation as a model volatile organic compound over hydrotalcite-derived mixed metal oxides is an important catalytic process for air pollution abatement and for understanding oxidation mechanisms on redox-active surfaces. Hydrotalcite-like compounds are commonly used as precursors to prepare calcined highly dispersed mixed metal oxides. Upon thermal decomposition, hydrotalcite-like compounds form porous, high specific surface area oxides with well-mixed cations (e.g., Mg-Al, Co-Al, Cu-Mn, Cu-Ce systems). This intimate mixing led to strong metal–metal interactions, abundant oxygen vacancies, and enhanced redox properties, all of which are beneficial for VOC oxidation. Key advantages of these catalysts include the high dispersion of active sites, tunable acidity/basicity, improved thermal stability, and resistance to deactivation. Activity is typically evaluated in terms of light-off temperature, conversion efficiency, and selectivity toward complete oxidation to CO₂ and H₂O. These materials are therefore promising for the low-temperature catalytic combustion of C₂H₅OH and other oxygenated VOCs in environmental applications. They are frequently reported to show better activity and repeatability of catalytic cycles, demonstrating their efficiency compared to commercially available catalysts [103].

3.2. Oxidation of Toluene

Studies on BTEX (benzene, toluene, ethylbenzene, and xylenes) oxidation, particularly toluene, have been widely reported in the literature [109,110,111]. C₇H₈ was chosen as a probe molecule for VOC oxidation because it is commonly found in industrial exhaust and has a POCP of 63.7 [56]. Toluene is widely used as a solvent in chemical and processing industries, and controlling its emissions into the atmosphere is becoming increasingly important [112,113]. C₇H₈, a typical VOC pollutant, has been linked to various human health issues due to its neurotoxic and carcinogenic effects [114]. Therefore, developing effective strategies to remove toluene is essential. Mainly Mn-containing hydrotacite-derived mixed metal oxides are investigated for the oxidation of C₇H₈ (e.g., [115,116]). T₅₀ and T₉₀ values, corresponding to the temperatures required to achieve 50% and 90% conversion to CO₂, respectively, are used to compare the catalytic activity of the oxides (

Table 4. Results of the toluene oxidation over hydrotalcite-derived mixed metal oxides.

Catalyst (Preparation Method)	Crystalline Phase (BET/m2 g−1)	Reaction Conditions	T50 /°C	T90 /°C	Ref.
Toluene
CuCr (co-precipitation, calcined at 600 °C in air)	CuO, CuCr2O4 (-)	c(C7H8) = 2–4 g m−3, mcat = 0.7 g GHSV = 50,000 h−1	217	227	[89]
CuMn0.25 (co-precipitation, calcined at 600 °C in air)	MgO, MgMnO3 (-)	c(C7H8) = 0.1 vol.-%, diluted in air mcat = 0.2 g FTOT = 200 cm3 min−1 WHSV = 60 L g−1 h−1	265	360	[91]
Cu3Mn (co-precipitation, calcined at 400 °C in air)	CuO (60)	c(C7H8) = 0.1 vol.-%, diluted in air mcat = 0.2 g FTOT = 100 cm3 min−1 WHSV = 30 L g−1 h−1	210	231	[117]
Mn6Al2HT500 (co-precipitation, calcined at 500 °C in air)	spinel-like phases Co3O4, CoAl2O4 or Co2AlO4 (123)	c(C7H8) = 0.1 vol.-%, diluted in air, mcat = 0.1 g FTOT = 100 cm3 min−1 WHSV = 60 L g−1 h−1	249	250	[118]
MnMgAl (auto-combustion, calcined at 500 °C in air)	MgO, Mn3O4, Mn2MgO4, MnAl2O4, MgMn1.75Al0.25O4 (70)	c(C7H8) = 0.06 vol.-%, diluted in air, mcat = 0.2 g FTOT = 280 cm3 min−1 WHSV = 85 L g−1 h−1	235	250	[119]
CoMn0.25 (co-precipitation, calcined at 600 °C in air)	MgO, MgMnO3 (-)	c(C7H8) = 0.1 vol.-%, diluted in air, mcat = 0.2 g FTOT = 200 cm3 min−1 WHSV = 60 L g−1 h−1	271	319	[91]
Co6Al2HT500 (co-precipitation, calcined at 500 °C in air)	spinel-like phases (53)	c(C7H8) = 0.1 vol.-%, diluted in air, mcat = 0.1 g FTOT = 100 cm3 min−1 WHSV = 60 L g−1 h−1	270	295	[120]
cCoAl3–70 (co-precipitation, calcined at 550 °C in air)	spinel-like phases, Co3O4, Co2AlO4 (72)	c(C7H8) = 0.1 vol.-%, diluted in air, mcat = 0.1 g FTOT = 82 cm3 min−1 WHSV = 50 L g−1 h−1	257	275	[121]
Mn4Mg2Al2-O (co-precipitation, calcined at 500 °C in air)	spinel-like phases, Mn3O4, MgAl2O4, MnAl2O4, MgMn2O4 (108)	c(C7H8) = 0.1 vol.-%, diluted in air, mcat = 0.1 g FTOT = 100 cm3 min−1 GHSV = 30,000 h−1	250	268	[103]
Co2Mn4Al2HT(500) (co-precipitation, calcined at 500 °C in air)	spinel-like phases, Mn5O8, CoMnO3	c(C7H8) = 0.18 vol.-%, diluted in air, mcat = 0.1 g FTOT = 50 cm3 min−1 WHSV = 30 L g−1 h−1	247	250	[122]
Co1.5Mn1.5Al-500 (co-precipitation, calcined at 500 °C in air)	Co3O4 (139)	c(C7H8) = 0.1 vol.-%, diluted in air, mcat = 0.1 g FTOT = 100 cm3 min−1 WHSV = 60 L g−1 h−1	220	240	[123]
Co2.25Mn0.75O4 (hydrothermal treatment, calcined at 350 °C in air)	MnCo2O4 (41)	c(C7H8) = 0.1 vol.-%, diluted in air, mcat = 0.05 g FTOT = 33.4 cm3 min−1 WHSV = 40 L g−1 h−1	214	225	[124]
1 wt.-% K/Co-Mn-Al (co-precipitation, impregnation, calcined at 500/500 °C in air)	spinel-like phases (103)	c(C7H8) = 1.0 g m−3, diluted in air mcat = 0.75 g WHSV = 10 L g−1 h−1	145	165	[98]
Cu1.5Mn1.5Al1-LDO (co-precipitation, calcined at 500 °C in air)	Mn2O3 (71)	c(C7H8) = 0.1 vol.-%, c(O2) = 20 vol.-% mcat = 0.1 g WHSV = 60 L g−1 h−1	219	228	[125]
CPCoMn (co-precipitation, calcined at 500 °C in air)	MgO, Mn3O4, Mn2CoO4, Co2MnO4, Co3O4, CoAl2O4 (161)	c(C7H8) = 0.0250 vol.-%, c((CH3)2CHOH) = 0.0250 vol.-% diluted in air, mcat = 0.2 g FTOT = 500 cm3 min−1 WHSV = 150 L g−1 h−1	280	320	[126]
Co/Mg-Al(700)500 (co-precipitation, impregnation, calcined at 700 and 500 °C in air)	MgO (190)	F(C7H8) = 0.09 mL min−1 mcat = 0.1 g FTOT = 33.4 cm3 min−1 WHSV = 20 L g−1 h−1	320	335	[127]
CoAlCeO (co-precipitation, calcined at 500 °C in air)	Co3O4, CoAl2O4, Co2AlO4 (108)	c(C7H8) = 0.1 vol.-%, diluted in air, mcat = 0.1 g FTOT = 100 cm3 min−1 WHSV = 60 L g−1 h−1	249	250	[128]
Ce/CoMgAl (co-precipitation, reconstruction, calcined at 500, 500 °C in air)	MgO, Co3O4 (153)	c(C7H8) = 0.12 vol.-%, diluted in air, mcat = 0.2 g FTOT = 200 cm3 min−1 WHSV = 60 L g−1 h−1	262	286	[129]
MnCuAl450 (co-precipitation, calcined at 450 °C in air)	spinel-like phases, CuMn2O4 (108)	c(C7H8) = 0.08 vol.-%, diluted in air, mcat = 0.2 g FTOT = 250 cm3 min−1 GHSV = 84.75 h−1	258	275	[116]
Cu2Co2MnAl (co-precipitation, calcined at 500 °C in air)	spinel-like phases, (-)	c(C7H8) = 5.0 g m−3, diluted in air, mcat = 0.5 g FTOT = 200 cm3 min−1 WHSV = 20 L g−1 h−1	227	235	[130]
Cu4Al (co-precipitation, calcined at 450 °C in air)	CuO (37)	c(C7H8) = 1.0 g m−3, mcat = 0.75 g FTOT = 125 cm3 min−1 WHSV = 10 L g−1 h−1	273	325	[131]
Noble metal-based catalysts
Mg3AlPd (co-precipitation, calcined at 290 °C in air)	PdO (-)	c(C7H8) = 0.1 vol.-%, diluted in air, mcat = 0.2 g FTOT = 33 cm3 min−1 WHSV = 10 L g−1 h−1	214	230	[132]
Pd/Co3AlO (co-precipitation, calcined at 500 °C in air)	spinel-like phases (93)	c(C7H8) = 0.08 vol.-%, diluted in air, mcat = 0.6 g FTOT = 300 cm3 min−1 GHSV = 30,000 h−1	220	230	[133]
Pd-CoAl-O-Al (hierarchical core–shell microspheres, calcined at 600 °C in air)	Co3O4, PdO (38)	c(C7H8) = 0.2 vol.-%, diluted in air, mcat = 0.1 g FTOT = 100 cm3 min−1 WHSV = 60 L g−1 h−1	192	207	[134]
Modern synthesis methods
CuAlMW500 (microwave-assisted synthesis, calcined at 600 °C in air)	CuO (36)	c(C7H8) = 0.1 vol.-%, c(O2) = 20 vol.-%, diluted in helium, mcat = 0.1 g FTOT = 100 cm3 min−1 WHSV = 60 L g−1 h−1	260	280	[135]
Mn0.4Zr0.6 (microemuslion, calcined at 450 °C in air)	amorphous phase (250)	c(C7H8) = 0.35 vol.-%, c(O2) = 9 vol.-%, diluted in argon, mcat = 0.1 g FTOT = 60 cm3 min−1 WHSV = 36 L g−1 h−1	250	250	[136]
MnAl(im)Na-L(II)-450 (inverse microemulsion, calcined at 450 °C in air)	Mn3O4 (245)	c(C7H8) = 0.05 vol.-%, mcat = 0.5 g GHSV = 10,000 h−1	241	270	[137]
MnAl(im)CTA-L(I)-600 (inverse microemulsion, calcined at 600 °C in air)	Mn3O4 (286)	c(C7H8) = 0.05 vol.-%, mcat = 0.5 g GHSV = 10,000 h−1	260	270	[137]
CoNi-6h-350 (self-templating synthesis, calcined at 350 °C in air)	NiO, NiCo2O4 (142)	c(C7H8) = 0.02 vol.-%, diluted in air, mcat = 0.05 g FTOT = 50 cm3 min−1 WHSV = 60 L g−1 h−1	220	229	[61]
CoCoO (topochemical transformation route, calcined at 400 °C in air)	Co3O4 (56)	c(C7H8) = 0.0191 vol.-%, diluted in air, mcat = 0.2 g FTOT = 100 cm3 min−1 WHSV = 60 L g−1 h−1	185	200	[138]

).

Mixed metal oxides derived from X₆Al₂ hydrotalcites, calcined at 500 °C (where X = Fe, Cu, Zn, Ni, Co, Mn, or Mg), were investigated for the total oxidation of C₇H₈ and CO [118]. Among the materials studied, the Mn₆Al₂-HT catalyst exhibited the highest activity for C₇H₈ oxidation, achieving 50% conversion (T₅₀) at 249 °C. Upon complete conversion, only CO₂ and H₂O were detected as final products (with trace amounts (a few ppm) of benzene for all samples during the initial stages of toluene oxidation). The catalytic activity followed a decreasing order: Mn₆Al₂-HT500 > Co₆Al₂-HT500 = Cu₆Al₂-HT500 > Fe₆Al₂-HT500 > Ni₆Al₂-HT500 >> Zn₆Al₂-HT500 > Mg₆Al₂-HT500. This trend was attributed primarily to differences in reducibility and the presence of active metallic phases within the catalysts.

A series of Mg-Mn-Al ternary hydrotalcite-like compounds, with a n(Mg+Mn)/n(Al) atomic ratio of approximately three and n(Mg)/n(Mn) atomic ratios ranging from 3/0 to 0/3, were synthesized via a co-precipitation method [115]. The as-synthesized samples can be used as catalysts for the liquid-phase oxidation of C₇H₈, selectively producing benzaldehyde and benzoic acid, with small amounts of benzyl alcohol detected. The experiments over as-synthesized materials revealed that MgAl-LDHs, in the absence of Mn, showed no activity for C₇H₈ oxidation. Both the toluene conversion rate and the selectivity toward benzoic acid increased with increasing Mn content in the samples. Mn species present in the hydrotalcite-like phase exhibited higher catalytic activity for C₇H₈ oxidation, rather than samples containing crystalline phases of Mn(OH)₂ and MnCO₃. In another approach, Co-(Mg)-Al hydrotalcite-type materials (n(M²⁺)/n(M³⁺) molar ratio of 3.0, n(Co²⁺)/n(Mg²⁺) ratios of 0.5 or 2.0) were synthesized for the complete oxidation of C₇H₈ [120]. The catalytic activity of these materials depended on Co species loading. The most efficient catalyst (n(Co)/n(Al) = 3/1, Co₆Al₂HT(500)) achieved the complete conversion of toluene to CO₂ and H₂O at around 300 °C. The activity of the cobalt-containing catalysts was related to crystallite size and the relative amounts of cobalt spinel and cobalt aluminate phases. Smaller crystallites with a higher proportion of Co₃O₄ showed higher activity due to improved reducibility. A small amount of carbonaceous deposits formed during the catalytic experiment, mainly including styrene, benzaldehyde and acetophenone [120]. Further studies confirmed significantly higher activity than the same catalyst, which was not synthesized via a hydrotalcite-like phase [139]. Also, Co-Al hydrotalcite precursors were synthesized via constant-pH co-precipitation by varying the precipitation temperature (30–70 °C) and the n(Co)/n(Al) molar ratio (2–5) [121]. The sample synthesized at 70 °C with a n(Co)/n(Al) ratio of three was the most efficient catalyst for C₇H₈ combustion (50% conversion at 257 °C). The only products were CO₂ and H₂O, except for the CoAl₂-70 sample, for which trace amounts of benzene were observed (1–2 ppm at 400–450 °C). The ratios of Co³⁺/Co²⁺ and lattice oxygen to adsorbed (electrophilic) oxygen in the resulting mixed oxides increased with the temperature used during precursor co-precipitation.

Co- and Mn-rich mixed oxides (calcined at 500 °C) were investigated while maintaining a constant Al species content to optimize their activity in C₇H₈ oxidation [122]. When toluene conversion was complete, only CO₂ and H₂O were detected as products. However, under incomplete conversion conditions, benzene was also formed, ca., 50 ppm during the catalytic experiments. The highest benzene formation occurs at 240–350 °C, depending on the catalyst used and the extent of C₇H₈ conversion. The catalytic activity follows the decreasing order: Co₂Mn₄Al₂HT(500) = Mn₆Al₂HT(500) > Co₄Mn₂Al₂HT(500) > Co₆Al₂HT(500) = Mn₆Al₂OH(500) >> Co₆Al₂OH (500). Again a direct relationship was observed between the reducibility of the mixed oxide species and their reactivity in C₇H₈ oxidation. Mn₆Al₂HT(500) and Co₂Mn₄Al₂HT(500) consumed the same amount of hydrogen and exhibited identical catalytic activity. Higher catalytic activity in C₇H₈ oxidation was observed for Mn-rich samples, particularly for Co₂Mn₄Al₂HT(500) due to its superior reducibility, as the material consisted of a mixture of nanocrystallites of Co-spinel, Mn₅O₈, and CoMnO₃ phases.

The calcination temperature has a significant impact on the catalytic activity of hydrotalcite-derived Co_1.5Mn_1.5Al mixed metal oxides. As the calcination temperature increased, catalytic activity decreased. This is evidenced by the rise in T₉₀ values from 240 °C for Co_1.5Mn_1.5Al-500 to 250 °C for Co_1.5Mn_1.5Al-600 and 270 °C for Co_1.5Mn_1.5Al-700. The Co_1.5Mn_1.5Al-500 catalyst demonstrated excellent stability under dry conditions, maintaining C₇H₈ conversion above 90% at 240 °C over a period of 100 h. However, the presence of water vapor adversely affected its activity. When 10 vol.-% of H₂O was introduced into the system, the C₇H₈ conversion decreased noticeably, dropping from 92% to 83% [123]. Similar results were reported over Cu_1.5Mn_1.5Al₁ (calcined at 500 °C) in the presence of 10 vol.-% H₂O in the feed containing both toluene and ethyl acetate [125]. Conversion of both reactants decreased significantly after 10 h of passing H₂O. Catalyst deactivation is a major challenge in catalytic oxidation processes, as catalysts can readily lose activity due to the adsorption of H₂O and intermediate species.

Otherwise, CuMn (n(Cu)/n(Mn) = 3/1) demonstrated excellent stability and durability, maintaining its performance even in the presence of 20 vol.-% H₂O [117]. The synergistic interaction between copper and manganese species (Mn³⁺ + Cu²⁺ ⟶ Mn⁴⁺ + Cu⁺) enhanced the mobility of lattice oxygen, thereby promoting catalytic activation. Catalytic oxidation likely proceeded via the following pathway: toluene ⟶ benzyl alcohol ⟶ benzaldehyde ⟶ benzoate ⟶ maleic anhydride ⟶ acetate. Among these steps, the transformation of surface benzoate was identified as the rate-determining step in C₇H₈ oxidation. A similar conclusion was reported by other researchers (e.g., [140]).

Co-Mn-Mg-Al oxides were synthesized using auto-combustion and co-precipitation methods. The composition was controlled with fixed molar ratios of n(Co + Mn + Mg)/n(Al) = 3.0, n(Co + Mn)/n(Mg) = 1.0, and n(Co)/n(Mn) = 0.5. The oxides prepared by the co-precipitation method exhibited the highest activity for the oxidation of the VOC mixture, attributed to enhanced oxygen mobility and an improved ability to promote redox processes within the material structure. The catalytic activity of the oxides was evaluated in the total oxidation of a mixture containing 250 ppm toluene and 250 ppm 2-propanol. The oxidation of the toluene and 2-propanol mixture was not fully selective toward CO₂, as acetone and propene were formed as reaction intermediates. Again, the catalytic properties of the materials were directly dependent on their redox behavior and oxygen mobility [126].

Furthermore, Co species supported on hydrotalcite-derived mixed metal oxides obtained by using the memory effect of MgAl were investigated in oxidation of C₇H₈ [127]. The Mg-Al sample was calcined at temperatures ranging from 500 to 900 °C and subsequently immersed in an aqueous cobalt nitrate solution to regenerate a surface hydrotalcite-like phase. The highest activity in oxidation of C₇H₈ was achieved with cobalt species supported on hydrotalcites calcined at 700 °C, while the activity was summarized according to the decreasing activity order: Co/Mg-Al(700)500 > Co/Mg-Al(800)500 = Co/Mg-Al(900)500 > Co/Mg-Al(600)500 > Co/Mg-Al(500)500 > Mg-Al(700) [127]. They attributed the higher activity of the second group of catalysts to a better dispersion of cobalt species. CO₂ and H₂O were the only products detected at full conversion.

The most active catalyst (full C₇H₈ conversion into CO₂ at about 300 °C) was obtained from MnCuAl hydrotalcites (n(Cu)/n(Mn)/n(Al) = 3.0/3.0/1.0) after calcination at 450 °C. Compared to other hydrotalcite-derived materials, i.e., MnZnAl (calcined at 450 and 600 °C), and MnCuAl calcined at 600 °C, it exhibited a light-off temperature below 260 °C. All catalysts were selective toward only CO₂. The catalytic experiments were conducted at a higher space velocity than in other studies, while still achieving one of the lowest reported light-off temperatures [116]. Conversions of C₇H₈ to CO₂ were reported to vary over materials prepared with varied the influence of the synthesis pH, as follows: CuMn(pH = 6) < CuMn(7) < CuMn(8) < CuMn(9) [141].

Cu_xCo_4-xMnAl mixed metal oxides with varying n(Cu)/n(Co) molar ratios were synthesized via thermal decomposition of LDH precursors [130]. The synergistic interactions among copper, cobalt, and manganese species in the Cu-Co-Mn-Al system promoted electron transfer, which enhanced both the reducibility and oxygen mobility of the catalysts. A higher concentration of Mn³⁺ species facilitated the formation of reactive lattice oxygen, which played a key role in the catalytic oxidation of C₇H₈. Among the studied materials, the Cu₂Co₂MnAl mixed oxide exhibited the highest catalytic activity across the entire temperature range, achieving complete C₇H₈ conversion at 250 °C and a T₉₀ value of 235 °C. This superior activity was attributed to catalyst’s more complex phase composition, which indicated stronger synergistic interactions between Cu, Co, and Mn species. Furthermore, Cu₂Co₂MnAl possessed the highest content of Mn³⁺ and lattice oxygen among the catalysts examined, both of which contributed significantly to its enhanced catalytic efficiency.

Kovanda et al. [131] reported high catalytic activity of hydrotalcite-derived Cu-Mg-Al (n(Cu)/n(Mg)/n(Al) = 0/4/2, 1/3/2, 2/2/2, 3/1/2, and 4/0/2) oxide systems calcined at 450 °C. The activity of the studied catalysts increased with increasing copper species loading and, as suggested by the authors, this behavior was related to the presence of easily reducible copper species. In addition, hydrotalcite-like materials containing copper, along with other transition metals, have also been reported as effective precursors of active catalysts for VOC combustion (e.g., [59,142,143,144]).

3.2.1. Noble Metal-Based Catalysts

Pd-containing catalysts are highly effective for VOC oxidation due to their strong ability to activate oxygen and operate at relatively low temperatures. The active phase is typically PdO, which participates in redox cycles between Pd⁰ and PdO during the reaction (e.g., [145,146,147]). Pd-containing hydrotalcite-derived mixed metal oxides were the most widely investigated for the oxidation of C₇H₈ (e.g., [148]). The hydrotalcite-like sample hydroxide [Mg_0.75Al_0.25(OH)₂]^0.25+[PdCl₂(OH)₂]²⁻_0.125−x x (CO₃)_x²⁻·nH₂O was synthesized via the co-precipitation of the corresponding chlorides (MgCl₂, AlCl₃, and H₂PdCl₄) using freshly boiled water under a nitrogen atmosphere [132]. Palladium species were introduced as the [PdCl₂(OH)₂]²⁻ complex, formed through partial substitution of Cl⁻ by OH⁻. This modified hydrotalcites were less stable (destruction at about 350 °C) than the conventional Mg-Al hydrotalcites without Pd species (ca. 410 °C), as their decomposition occurred at a lower temperature. Otherwise, the Pd-containing hydrotalcite calcined at 290 °C showed the highest C₇H₈ conversion when compared to a conventional palladium-containing catalyst, i.e., Pd/Al₂O₃. Furthermore, a series of Pd/Co₃AlO catalysts derived from hydrotalcite-like compounds was prepared and investigated. The Co-Al hydrotalcite precursor phases were prepared by the co-precipitation method, and Pd active species were introduced using different approaches, namely impregnation (IMP), thermal combustion method (TCB), wet ion exchange (WIE), or direct incorporation during the co-precipitation stage (COP) [133]. The activities of all synthesized catalysts followed the decreasing order: Pd/Co₃AlO (COP) > Pd/Co₃AlO (WIE) ≥ Pd/Co₃AlO (IMP) > Pd/Co₃AlO (TCB). For Pd/Co₃AlO (COP) catalyst, the C₇H₈ ignition temperature and complete conversion temperature were 190 °C and 230 °C, respectively. Enhanced catalytic activity of the hydrotalcite-derived Pd/Co₃AlO catalysts was attributed to their high specific surface area, small mean crystallite size of the support, and highly dispersed PdO particles. Also in this case, the activity was positively correlated with catalyst reducibility and the concentration of oxygen vacancies. Stability tests indicated that all Pd/Co₃AlO catalysts show no significant deactivation, and some even exhibit improved activity during the reaction due to the formation of metallic Pd. Benzene was the only organic by-product observed for all catalysts, below 400 °C. The Pd/Co₃AlO catalysts investigated in this study exhibited stable activity for C₇H₈ oxidation at 240 °C over more than 80 h, except during the initial few hours of operation.

3.2.2. Modern Synthesis Methods

Rationally designed hydrotalcite-like compounds with 3D hierarchical hollow structures have attracted considerable interest in catalytic oxidation, owing to the complex architecture and composition of their shells. Co-precipitation is the most widely used and reliable method for synthesizing hydrotalcites applied for VOC oxidation. However, it was shown frequently that other more sophisticated preparation precursor procedures led to more active catalysts for oxidation of C₇H₈, e.g., reconstruction method [129], auto-combustion [119], microwave-assisted synthesis, and ultrasound-assisted synthesis [135,149], etc.

For instance, Cu₆Al₂ mixed metal oxides were prepared using three different synthesis routes: conventional co-precipitation (CP), microwave-assisted synthesis (MW), and ultrasound-assisted synthesis (US). The solid obtained through MW showed the highest catalytic activity among the three samples in the reactions studied, surpassing those prepared by co-precipitation and ultrasound-assisted methods [135]. The catalytic activity followed the decreasing order, CuAlMW500 > CuAlUS500 > CuAlCT500, indicating that the microwave-treated sample exhibits the highest activity, followed by the ultrasound-treated sample, while the co-precipitated material showed the lowest activity. This material proved strong potential for industrial applications, particularly due to its reduced synthesis time and improved catalytic activity in the presence of VOCs and CO in gas mixtures. Upon complete conversion, CO₂ and H₂O were the only products detected. However, at the initial stage of toluene conversion for all samples, trace amounts (a few ppm) of benzene were also observed. Based on the T₅₀ values for C₇H₈, the catalytic activity follows the order: CuAlMW500 (mixture) > CuAlUS500 (mixture) > CuAlMW500 (alone) > CuAlUS500 (alone) > CuAlCT500 (mixture) = CuAlCT500 (alone); mixture of toluene on carbon monoxide.

Compared to conventional co-precipitation, the catalytic conversion of toluene over Mn-Zr mixed oxides prepared via the reverse microemulsion method was significantly higher, even when the catalysts have identical chemical compositions for C₇H₈ oxidation [136]. This concerns Mn_0.4Zr_0.6 synthesized by the microemulsion method and calcined at 450 °C. Specifically, Mn_0.4Zr_0.6 achieved 99% toluene conversion at 270 °C, whereas Mn_0.4Zr_0.6-CP reached only 95% conversion at 290 °C. The enhanced dispersion of the active phase achieved through the microemulsion preparation method played a key role in significantly higher catalytic activity over the resulting mixed oxides. The catalytic activity of the Mn_0.5Zr_0.5 catalyst was evaluated as a function of time on stream. C₇H₈ conversion remained stable at approximately 95% over a 26 h. This result indicated that the Mn_0.5Zr_0.5 catalyst prepared via the microemulsion method exhibited relevant catalytic stability under the reaction conditions. In another series of catalysts, Fe-Mn, Co-Mn, and Cu-Mn mixed oxides were also synthesized via the reverse microemulsion method. Among them, the Mn_0.67Cu_0.33 catalyst (33 mol.-% Cu) achieved complete C₇H₈ oxidation at 220 °C, a temperature comparable to that of widely investigated Pd-based catalysts for this reaction. When the Mn content was increased to 50 mol.-%, the temperature required for total conversion rised to 260 °C.

Napruszewska et al. [137] proposed a strategy for designing combustion catalysts in which catalytically active oxide nanoparticles, prepared via the inverse microemulsion method, were confined within randomly oriented clay layers. Mn-Al (n(Mn)/n(Al) = 3/1) hydrotalcite-like compounds were employed as precursors of the active phase, while synthetic smectite Laponite RD was used as the clay component in either its sodium form (Na-L) or as an organoclay modified through exchange with cetyltrimethylammonium (CTA) cations (CTA-L). The extent of clay exfoliation was significantly greater in materials derived from CTA-L compared to those prepared using Na-L, indicating that organoclay modification enhanced the dispersion and delamination of the clay structure in the resulting catalysts. In MnAl(im)/CTA-L samples, the MnO_x phase evolved into well-ordered, slightly oxygen-rich Mn₃O₄ nanocrystals. In contrast, MnAl(im)/Na-L materials formed less structurally ordered MnO_x particles with a broader size distribution and an average manganese oxidation state that was significantly higher than that of Mn₃O₄. The relatively less abundant but more easily reducible Mn⁴⁺ species present in organoclay-based composites were mainly associated with oxygen-rich surfaces of Mn₃O₄ crystallites. In contrast, the less reducible Mn species observed in Na-L-derived catalysts were likely located within an amorphous phase with MnO₂-like stoichiometry. CO₂ and H₂O were identified as the sole reaction products. The results confirmed the advantage of the proposed catalyst design strategy, which involved trapping MnAl(im) hydrotalcite nanoparticles between clay layers. Composites based on Na-L were found to be more susceptible to thermal degradation, showing a loss of activity when the calcination temperature increased from 450 °C to 600 °C.

Wang et al. [61] reported a simple two-step method for constructing a 3D hierarchical NiCo₂O₄/NiO nanocage. This synthetic strategy involved a partial in situ transformation of ZIF-67 (zeolitic imidazolate framework-67) into Co-Ni LDH yolk–shell structures via ethanol etching, followed by a structure-preserving transformation from Co-Ni LDH@ZIF-67 into a biphasic nanocage through calcination (Figure 4a). The reaction time and calcination temperature significantly influenced the textural and catalytic properties. Thus, CoNi-yh-T with varied reaction time and calcination temperature were evaluated. The CoNi-6h-350 sample exhibited significantly higher activity, achieving 90% C₇H₈ conversion (T₉₀) at 229 °C under a high space velocity (WHSV = 60 L h⁻¹ g⁻¹), compared to other catalysts (T₉₀ > 240 °C, Figure 4b). The enhanced catalytic activity was attributed to abundant surface high-valence Co species generated by the unique hierarchical nanostructure, along with adsorbed oxygen species and a high density of medium-strength surface acid sites. As the reaction proceeds, the CO₂ output increased and the carbon balance reached 97%, with no by-products detected except for a minimal amount of CO. The excellent activity of the CoNi-6h-350 catalyst was attributed to its retained hollow structure, high specific surface area, abundant adsorbed oxygen species, numerous medium-strength acid sites, and improved low-temperature reducibility resulting from a strong synergistic effect. The CoNi-6h-350 sample was repeatedly used three times under identical experimental conditions to evaluate the reproducibility of the catalyst in the continuous catalytic oxidation of C₇H₈, Figure 4c). The toluene conversion in the third cycle changes slightly, confirming that the catalyst exhibits excellent reproducibility.

A hierarchical Co²⁺_2.8Co³⁺₁-layered double hydroxide nanostructure was constructed via a facile topochemical transformation route under a dynamic oxygen atmosphere. A possible formation mechanism for the assembly of primary LDH nanosheets into three-dimensional self-assembled nanostructures was proposed based on interactions between organic molecules and LDH crystals [138]. Among them, the Co-CoO nanocatalyst exhibited the highest abundance of surface-adsorbed oxygen species, superior low-temperature reducibility, and the highest activity of surface Lewis acid sites. This was attributed to the high concentration of surface Co³⁺ species, resulting in the highest catalytic activity (T₉₉(benzene) = 210 °C and T₉₉(toluene) = 220 °C at WHSV = 60 L h⁻¹ g⁻¹).

Hierarchical core–shell Al₂O₃@Pd-CoAlO (Pd-CoAlO-Al) microspheres have been successfully prepared and applied for C₇H₈ combustion [134]. The core–shell AlOOH@CoAl-LDHs (CoAl-LDHs/Al) microsphere precursor was successfully prepared by first synthesizing AlOOH microspheres, followed by the in situ growth of 2D CoAl-LDH nanosheets on their surface. Furthermore, Pd nanoparticles (NPs) were immobilized on CoAl LDH precursors (CoAl-LDHs-X, X = S and Al) via an in situ redox method to obtain Pd-CoAl-LDHs-X. These precursors were then calcined at 600 °C in air to yield the corresponding spinel oxide microspheres, namely CoAlO, Al₂O₃@CoAlO, Pd-CoAlO, and Al₂O₃@Pd-CoAlO (CoAlO-S, CoAlO-Al, Pd-CoAlO-S, and Pd-CoAlO-Al). Benzoate species were the main intermediate formed during the reaction. Lifetime experiments were conducted on Pd-CoAlO/Al catalysts over a 48 h on-stream reaction at 190 and 220 °C to evaluate catalyst stability, also in the presence of 5.5 vol.-% water vapor. This result suggested that water vapor slightly reduced catalytic efficiency at 220 °C due to competitive adsorption between C₇H₈ and H₂O on the active sites.

Flower-like Co_3−xMn_xO₄ (x = 0.75, 1.0, and 1.5) mixed metal oxides were successfully synthesized using a controlled, template-free hydrothermal method [124]. The Co-rich sample exhibited significantly enhanced activity, with the 3D dandelion-like Co_2.25Mn_0.75O₄ catalyst showing the highest C₇H₈ oxidation rate and achieving 100% conversion at 239 °C. C₇H₈ was sequentially oxidized into benzyl radicals, benzaldehyde, benzene, oxalic acid, and ultimately CO₂ and H₂O. The strong interaction between Co and Mn species, together with a high concentration of surface oxygen species and abundant oxygen vacancies, effectively accounts for the enhanced catalytic activity and thermal stability of the 3D flower-like Co_3−xMn_xO₄ spinel catalysts.

3.2.3. VOC Mixtures and Non-Thermal Plasma Conditions

C₇H₈ serves as an ideal model VOCs for studying oxidation processes due to its representative structure and well-understood chemistry. Catalytic oxidation offers an efficient and environmentally friendly method for VOC removal, with ongoing research focused on developing more active, stable, and cost-effective catalysts. However, industrial applications required investigation under mixture compositions. Such studies are scarcely investigated over hydrotalcite-derived mixed metal oxides. CoAlCe mixed oxides were investigated and compared to selected palladium-based catalysts reported in the literature (namely Pd/α-Al₂O₃, Pd/HY, Pd/CeO₂, and Pd/γ-Al₂O₃) for the total oxidation of toluene, butanone, and mixtures of VOCs, including MEK/toluene and industrial VOC blends [128]. The CoAlCe (n(Co)/n(Al)/n(Ce) = 6/1.2/0.8) mixed oxide demonstrated catalytic activity comparable to that of Pd/CeO₂ and Pd/γ-Al₂O₃ systems, highlighting its potential as a cost-effective alternative to palladium-based catalysts for VOC abatement. Although the CoAlCe catalyst shows slightly lower activity when evaluated solely based on T₅₀ and T₁₀₀ values, it produced significantly lower benzene emissions. Taking this important factor into account, CoAlCe can be considered one of the most promising catalysts among those studied (Figure 5a). To simulate industrial VOC emissions, a representative liquid mixture was prepared consisting of six aromatic compounds (toluene, o-xylene, ethylbenzene, propylbenzene, 4-ethylbenzene, and 1,4-diethylbenzene), three oxygenated compounds (butanone, butanol, and butoxyethanol), and one paraffin (decane). Each component was present at a concentration of 10 vol.-% in the liquid phase. The liquid feed was introduced via a saturator, which temperature was adjusted to achieve a concentration of 7000 ppm carbon equivalent (ppm eq. C) in an air flow of 100 cm³ min⁻¹. Figure 5b presents the resulting gas-phase composition, determined by micro gas chromatography.

Future work should focus on the development of efficient catalytic systems capable of oxidizing a wide range of VOCs at significantly lower temperatures. Such studies should examine how different VOCs interact during oxidation, since mixtures can compete for active sites or influence reaction pathways. A key challenge is maintaining high efficiency when treating complex, real-world VOC mixtures rather than single compounds. Overall, the goal is to design robust catalysts that can handle diverse emissions in industrial and environmental applications.

Non-thermal plasma (NTP) is an effective technology for VOC removal that operates at near-ambient temperatures using energetic electrons to generate reactive species. These species, such as radicals (•OH, •O) and ozone, initiate the oxidation and decomposition of VOC molecules into simpler compounds. Unlike conventional thermal methods, NTP does not require high temperatures, making it energy-efficient for dilute VOC stream. However, incomplete oxidation can lead to by-products like CO or partially oxidized organics. To improve performance, NTP is often combined with catalysts (plasma–catalysis systems), enhancing mineralization to CO₂ and H₂O. Overall, NTP is a promising method for air purification, though challenges remain in energy efficiency and by-product control [150,151,152]. In recent years, NTP has attracted considerable attention as a promising technology for the degradation of VOCs under ambient temperature and pressure conditions. This can be attributed to its high degradation efficiency, rapid reaction rate, and enhanced activity. In addition, manganese-based hydrotalcite-like compounds with compositions of MMn-LDHs (M = Ni, Co, Fe) have been employed to investigate their strong synergistic interaction with plasma technology for the degradation of VOCs [153]. Non-thermal plasma was generated using a dielectric barrier discharge (DBD) reactor. The experimental results demonstrated that the post-non-thermal plasma (post-NTP) catalytic system significantly enhanced C₇H₈ degradation efficiency and CO₂ balance, while effectively suppressing the formation of by-products (e.g., NO_x, carbonaceous species, and other oxygenated intermediates) compared to the NTP-alone system. NiMn-LDHs exhibited the highest activity in the post-NTP–catalytic system, achieving a C₇H₈ degradation rate of 96%, a CO₂ selectivity of 57%, a carbon balance of 80%, and an energy efficiency of 3.19 g kWh⁻¹ at a specific energy density (SED) of 930.45 J L⁻¹. The findings indicated that plasma discharge generated a substantial amount of free radicals and reactive species, thereby enhancing the decomposition of C₇H₈. Subsequently, the remaining toluene and intermediate products were adsorbed onto the catalyst surface, where they further reacted with abundant hydroxyl radicals and activated metal species, ultimately decomposing into CO₂ and H₂O. Additionally, the interaction of ozone with hydrotalcite-like compounds promoted the generation of reactive oxygen species, thereby accelerating the decomposition of organic intermediates, enhancing CO₂ formation, and improving C₇H₈ mineralization. The findings of this work provided valuable theoretical support for the plasma–catalytic degradation of VOCs. Moreover, by-products such as NO_x and O₃ were analyzed and compared between the NTP-alone and plasma–catalytic systems.

3.3. Reaction Mechanisms

As mentioned above, various VOCs are involved and have a direct impact on atmospheric chemistry. Although the catalytic oxidation of VOCs has been widely studied, establishing a universal correlation and a single reaction mechanism remains challenging due to variations in pollutant properties and reaction conditions. According to Palazzolo and Tichenor [154], an increase in molecular weight corresponds to greater difficulty in oxidizing VOCs. This trend is consistent with the order proposed for oxidation ease: alcohols < aldehydes < aromatics < ketones < acetates < alkanes. Depending on the partial reaction order, several mechanisms have been proposed for VOC oxidation. These include the Langmuir–Hinshelwood (L-H) mechanism, where the rate-determining step involves a surface reaction between two adsorbed species on similar active sites; the Eley–Rideal (E-R) mechanism, in which the key step is the reaction between an adsorbed species and a molecule in the gas phase; and the Mars-van Krevelen (MvK) mechanism, a redox pathway in which VOC molecules react with lattice oxygen at redox-active sites. The latter mechanism has been widely applied to describe the oxidation of organic compounds (e.g., [155,156]). According to the Mars-van Krevelen mechanism, the oxidation rate of VOCs can be expressed by as follows [157]:

$${-r}_{VOC}={\displaystyle \frac{k_{O_2}{k}_{VOC}{P}_{VOC}{P}_{O_2}}{\gamma k_{VOC}{P}_{VOC}+k_{O_2}{P}_{O_2}}}$$

where $-r_{VOC}$—reaction rate, mol m⁻³ s⁻¹; $P_{VOC}$—partial pressure of the VOCs; $P_{O_2}$—partial pressure of oxygen; $k_{VOC}$—rate constant of VOCs oxidation; $k_{O_2}$—rate constant of catalysts re-oxidation; γ—the stoichiometry coefficient of O₂ in the oxidation.

Similarly, the kinetics are given for Langmuir–Hinshelwood (L-H) and Eley–Ridel mechanisms and can be found in the literature (e.g., [158,159]).

The Mars-van Krevelen (MvK) mechanism is a common pathway used to explain how solid catalysts, including hydrotalcite-derived materials, oxidize VOCs. First, in the catalytic mechanism for VOC oxidation over Cu_xCo_4-xMnAl-MMO, a greater number of active sites and reactive species were produced due to the synergistic redox interactions among Co³⁺/Co²⁺, Mn⁴⁺/Mn³⁺, and Cu²⁺/Cu⁺ [130]. Second, strong interactions among Mn, Co, and Cu species are crucial for maintaining the redox cycles (Co³⁺ + Mn³⁺ ⇌ Co²⁺ + Mn⁴⁺; Cu²⁺ + Mn³⁺ ⇌ Cu⁺ + Mn⁴⁺), thereby promoting electron transfer and enhancing catalytic oxidation activity. Third, lattice oxygen plays a vital role in the VOC oxidation process: surface reactive oxygen species are consumed during the oxidation of organics and are subsequently replenished by lattice oxygen. Additionally, gas-phase oxygen (O₂) can be adsorbed and activated at oxygen vacancies, forming reactive oxygen species that assisted in restoring depleted lattice oxygen and sustaining catalytic activity (Figure 6).

Similarly, strong interactions promote the redox cycle Mn³⁺ + Cu²⁺ ⇌ Mn⁴⁺ + Cu⁺ for CuMn composite oxide catalysts derived from CuMn-LDH precursor for C₇H₈ oxidation [117]. For the CMO-400 catalyst (CuMn-LDH calcined at 400 °C), both gaseous oxygen and lattice oxygen from the surface participated in the reaction. The active oxygen species are identified as free oxygen (O), which can directly take part in the oxidation of C₇H₈. Afterward, the depleted free oxygen is primarily replenished by adsorbed oxygen, which facilitates the catalyst’s subsequent re-oxidation cycle. Notably, a higher amount of adsorbed oxygen (O_ads) is expected to enhance the utilization efficiency of oxygen species. Herein, the surface reaction pathway for C₇H₈ oxidation over the CMO-400 catalyst proceeded through consecutive steps. Toluene interacts with gas-phase oxygen, followed by sequential transformation along the pathway: toluene ⟶ benzyl alcohol ⟶ benzaldehyde ⟶ benzoate/benzoic acid ⟶ maleic anhydride ⟶ acetate (Figure 7). Ultimately, these toxic VOCs are fully oxidized into H₂O and CO₂. Considering the relatively slow oxidation rate of benzoic acid, it can be inferred that the cleavage of the aromatic ring is the rate-determining step. In another studies, Hou et al. [125] upon introduction of O₂, toluene and surface intermediates undergo ring-opening reactions followed by deep mineralization. Toluene oxidation proceeds sequentially through benzyl alcohol, benzaldehyde, benzoic acid, and maleic anhydride, which are ultimately fully converted into CO₂ and H₂O. To verify the promoting effect of ethyl acetate, it was introduced into the reaction system while the C₇H₈ stream remained on, enabling the coexistence of gaseous ethyl acetate and toluene-derived intermediates on the catalyst surface. Upon introduction of ethyl acetate, the bands associated with maleic anhydride, a key intermediate in C₇H₈ oxidation, rapidly diminished. This observation is consistent with the experimental results obtained when both VOCs were introduced simultaneously, providing direct and convincing evidence of the catalytic promotion of C₇H₈ oxidation by ethyl acetate.

4. Conclusions

Metal oxide catalysts offer several advantages, making them attractive alternatives to expensive noble metal catalysts. Despite the above advances in the catalytic oxidation mechanisms of VOCs, there remain many essential challenges in understanding their reaction mechanisms, particularly for complex VOCs. In this review, multicomponent hydrotalcite-derived mixed metal oxides were evaluated for the oxidation of alcohols (methanol, ethanol) and toluene. The available literature on VOC oxidation has systematically investigated model exhaust streams in which VOC concentration and flow rate are carefully controlled; however, only a limited number of studies have focused on VOC mixture. To better approximate industrial conditions, it is necessary to investigate more complex mixtures (i.e., 10–50 compounds). However, due to the complexity of the processes occurring during the oxidation of mixtures, predicting catalyst behavior is difficult, and experimental validation is required. Catalysts can also be easily deactivated by the adsorption of water vapor and other intermediate species. Therefore, the development of highly resistant catalysts is essential to reduce operating costs and improve VOC removal efficiency. Plasma-based VOC abatement is an effective room-temperature oxidation technology that utilizes highly reactive species generated in the discharge to convert VOCs into intermediate oxygenated compounds and, ultimately, CO₂ and H₂O. While high conversion can be achieved, plasma alone may suffer from incomplete mineralization and by-product formation. The introduction of catalysts significantly enhances performance by improving selectivity, promoting full oxidation, and reducing undesirable intermediates, leading to more efficient and stable VOCs removal.

In the future, laboratory experiments should be supported by machine learning models that can be applied to analyze large experimental datasets to predict LDH-derived catalysts for VOC removal, without extensive trial-and-error experiments. In process optimization, it is necessary to develop model relationships between operating parameters (temperature, gas composition, humidity, plasma power, etc.) and VOC conversion efficiency. This enables the rapid optimization of catalytic and plasma`catalytic systems, improving degradation efficiency and energy utilization.