1. Introduction

Volatile organic compounds (VOCs) are produced in industrial manufacturing (e.g., inks, paints, films, and cosmetics) and have a high vapor pressure at room temperature and low boiling point [1-4]. Common VOCs include ethylene glycol, formaldehyde, methylene chloride, toluene, xylene, benzene, and ethyl acetate (EA). Some VOCs have harmful effects on human health and the environment [1,2]. Regenerative thermal oxidizer (RTO) technology has been utilized as a representative process for removing VOCs [5,6]. This process oxidizes and decomposes VOCs at 800 °C or higher, thus providing high removal efficiency of VOCs (> 98%). However, the high temperature of RTO requires high energy consumption, resulting in high processing costs. As an alternative technology, a catalytic thermal oxidizer (CTO) process has also been suggested for removing VOCs [7,8]. High-performance catalysts enable oxidizing VOCs at a relatively low temperature of 300 –450 °C with a high removal efficiency of > 95%.

Numerous catalytic materials have been exfoliated to remove VOCs in the CTO process, including noble metal (e.g., Pt, Pd, Ru, and Au) and transition metal (e.g., Cu, Mn, Co, Ti, Ni, V, and Fe) oxides [9,10]. Noble metals are expensive and hazardous; in contrast, transition metal oxides are relatively low-cost, environmentally friendly, and highly active for the combustion of VOCs[1-3]. Among the transition metal oxides, Cu-Mn bimetallic catalysts exhibit high catalytic performance with a removal efficiency of > 95% at a low operating temperature of 250 °C[4]. Numerous Cu²⁺ and Mn⁴⁺ cations play a critical role in the catalyzed oxidation of VOCs[11]. The composition ratio of Cu to Mn leads to different phases, morphologies, and structures, influencing the catalytic performance[12]. Therefore, an optimized synthesis method is required to control the formation of the Cu-Mn catalyst (CMC).

In this study, we prepared a CMC using a simple and facile co-precipitation method. During the synthesis of the heterogeneous catalyst, the Mn: Cu weight ratio of 4:1 was maintained by controlling the concentration of the Cu precursors (2.5~80 mM). CMC with 5 mM Cu concentration exhibited a large surface area of 230.8 m²/g with a mesopore structure. The resultant CMC was used as a heterogeneous catalyst for the selective oxidation of EA molecules to acetic acid and ethane. The CMC exhibited an excellent conversion efficiency of 100% after 1 h at 220 °C.

2. Experimental

2.1. Chemicals

The chemicals required for the study, Mn(CH₃COO)₂⋅H₂O, KMnO₄, and cupric Cu(NO₃)₂⋅H₂O were purchased from Sigma- Aldrich (USA). Acetone and ethanol were procured from Junsei Chemical Co. Ltd. (Japan). All the chemical reagents were used without further purification.

2.2. Synthesis of CMC

CMC was synthesized by co-precipitation method[Figure 1(a)] using Mn(CH₃COO)₂⋅H₂O, KMnO₄, and Cu(NO₃)₂⋅H₂O in an Mn: Cu weight ratio of 4:1. Mn(CH₃COO)₂⋅H₂O and KMnO₄ were mixed in deionized water, and Cu(NO₃)₂⋅H₂O was added to the resultant mixture. Following the co-precipitation process, the products were washed several times with acetone, ethanol, and deionized water. CMC powder samples were obtained after aging at 80 °C for 12 h. CMC synthesis was systematically investigated by controlling the concentration of the copper precursor (2.5~80 mM, fixed at a weight ratio of Mn:Cu = 4:1), reaction time (1~30 min), and reaction temperature (25~80 °C). Note that previous reports described the effects of composition ratio of Mn and Cu on the catalytic performance[13]. Also, a commercially available hopcalite Mn/Cu catalyst includes 4:1 composition ratio of Mn and Cu.

2.3. Material characterization

Scanning electron microscopy (SEM) images were obtained using a field-emission scanning electron microscope (JSM-6701F, JEOL, Ltd.). Optical microscopy images were obtained using an optical microscope (BX53MTRF-S; OLYMPUS). N2 adsorption/desorption was determined by Brunauer–Emmett–Teller (BET) measurements using an ASAP‒ 2010 surface area analyzer. X-ray diffraction (XRD) data were obtained using a Rigaku D/MAX‒2500 (40 kW) diffractometer with a θ/ θ goniometer equipped with a Cu Kα radiation generator.

2.4. Catalyst characterization

The catalytic activity of CMC for the catalytic decomposition of EA was investigated using a gas-phase fixed-bed reactor in the temperature range of 150~250 °C[Figure 1(b)]. The flow rates of each gas path were calculated based on the saturated vapor pressure of the EA and were controlled by mass flow meters. The CMC samples were tested in parallel, using the same reactant feed stream. The concentration of EA in the air was 1000 × 10^‒6 and the gas hourly space velocity was 12500 h^‒1. The conversion efficiency of EA was analyzed using a gas chromatograph (Tremetrics 9001) equipped with an HP-5 column and FID detector. The conversion efficiency of EA was calculated as follows:

where C_in and C_out represent the inlet and outlet concentrations of EA, respectively.

3. Results and discussion

Figure 1(a) illustrates the preparation method for CMC by the co-precipitation of Mn(CH₃COO)₂⋅H₂O, KMnO₄, and Cu(NO₃)₂⋅ H₂O in an Mn: Cu weight ratio of 4:1. The synthesis method involved a two-step process for the efficient synthesis of CMC. Mn(CH₃COO)₂ ⋅H₂O and KMnO₄ were mixed in the first step, and Cu(NO₃)₂⋅H₂O was added in the second step. CMC was obtained after filtering and aging at 80 °C for 12 h. The co-precipitation procedure was based on the redox process and electron transfer between Cu⁺, Mn²⁺, and Mn⁷⁺ according to the following reaction: Cu⁺ + Mn²⁺ + Mn⁷⁺ → Cu²⁺ +2 Mn⁴⁺[14]. To investigate the yield of CMC, the precursor concentration, reaction time, and reaction temperature were manipulated during the co-precipitation process. The concentration of the copper precursor varied in the range of 0.0025~0.08 M with a fixed Mn: Cu weight ratio of 4:1. The synthesis was also performed at different times (1~30 min) and temperatures (25~80 °C).

Figure 2(a) shows the yield of CMC obtained from different concentrations of the Cu precursor at a reaction temperature of 25 °C and a reaction time of 30 min. A low concentration of 2.5 mM resulted in a low yield of 21.5%. When the concentration was increased to 5 mM, the yield performance increased significantly to 38%. Further, increasing the concentration from 0.005 M to 8 mM exhibited a similar yield of 41%. A concentration of 5 mM for the Cu precursor was optimized for a high yield of CMC. Therefore, the synthesis of CMC with a 5 mM Cu precursor was further investigated at different times[Figure 2(b)] and temperatures[Figure 2(c)]. Although reaction times of 1 and 3 min showed higher yields compared to other reaction times, the as-obtained CMC in a short reaction time had a poor crystalline structure and low specific surface area (< 30 m²/g) of copper and manganese oxide, which is not suitable for application in catalytic thermal oxidation reactions[15,16]. Higher reaction times of 10 and 30 min resulted in similar yields for the synthesis of CMCs. Increasing the reaction temperature from 25 to 80 °C increased the yield performance from 38% to 41.5%. The experimental parameters of time and temperature in this study exhibited a subtle change in yield performance. The crystalline structure and textural properties of catalysts can be influenced by precursor concentrations in synthetic conditions. Based on these results, we selected two CMC samples obtained using different Cu precursor concentrations of 5 mM and 80 mM at 25 °C for 30 min. The CMC samples were denoted as 5-CMC and 8-CMC for Cu precursor concentrations of 5 mM and 80 mM, respectively.

Photographs and optical images of 5-CMC showed a dark brown fine powder sample[Figures 3 (a), and (b)]. Examination of the SEM images revealed that the 5-CMC and 8-CMC samples had aggregated nanoparticles[Figures 3 (c), and (d)]. The randomly aggregated CMC nanoparticles resulted in numerous empty voids and pore structures, which is favorable for enhancing catalytic performance. The crystalline structure of CMC was investigated using XRD measurements. Figure 4(a) shows the XRD patterns of the as-synthesized CMC samples. The diffraction peaks at 2θ values of 38.4°, 48.7°, and 67.7°, which were ascribed to the (111), (202), and (113) planes, respectively, were indexed to CuO (JCPDS 01-089-2529)[3,4]. The other main diffraction peak at 18.7° corresponds to the (111) plane of manganese oxide (MnO2, JCPDS 01-083-6090)[3,4]. These diffraction peaks indicated the coexistence of copper and manganese oxides in the 5-CMC and 8-CMC samples. Nitrogen adsorption and desorption measurements were performed to investigate the BET surface areas of the CMC samples. Figure 4(b) shows the hysteresis loop of the type IV isotherm, indicating the mesoporous structure of CMC. The mesopores of CMC are mainly attributed to the aggregated voids of the self-as- sembled nanoparticles. A large surface area of 230.8 m²/g for 5-CMC was obtained using the BET equation. This surface area is two-fold higher than 103.3 m²/g of 8-CMC.

The catalytic performance of the CMC catalysts was demonstrated by the oxidation of EA in a fixed-bed reactor at atmospheric pressure. Figure 1(b) shows a schematic of the catalyst test system of a gas-phase fixed-bed reactor connected to a gas flowmeter, temperature controller, and gas chromatograph. The use of the CMC catalyst facilitated the oxidation of EA to acetate and ethane. Figure 5 shows the conversion efficiency of 5-CMC and 8-CMC measured in the temperature range of 150~250 °C. As the reaction temperature was increased, the conversion efficiency increased. In particular, the 5-CMC and 8-CMC catalysts exhibit high conversion efficiency values above 200 °C. Although 5-CMC and 8-CMC exhibited similar trends in conversion efficiency based on the reaction temperature, 5-CMC showed superior catalytic performance for the oxidation of EA compared to 8-CMC. The 5-CMC sample exhibited a high conversion efficiency of 100% during oxidation of EA at 250 °C. The superior catalytic performance of 5-CMC compared to that of 8-CMC is attributed to the higher specific surface area and pore structure of 5-CMC.

4. Conclusion

A CMC using a simple and efficient co-precipitation method was developed. A high yield and good textural properties of 5-CMC were obtained by controlling the Cu concentration to 0.005 M, reaction time to 30 min, and reaction temperature to 25 °C. The as-obtained 5-CMC comprised randomly aggregated nanoparticles, resulting in a large surface area of 230.8 m²/g with a mesoporous structure. The catalytic performance of 5-CMC was tested in the oxidation reaction of EA, and the results were compared with those of 8-CMC. Further, 5-CMC exhibited ameliorated catalytic performance than that of 8-CMC in the reaction temperature range of 150~250 °C. In particular, 5-CMC had a high conversion efficiency value of 100% for oxidation reaction of EA at a low reaction temperature of 250 °C. The synthesis method and CMC developed in this study can be used in catalytic thermal oxidizer processes for the oxidation and decomposition of many VOCs at low temperatures.