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ISSN : 1225-0112(Print)
ISSN : 2288-4505(Online)

Applied Chemistry for Engineering Vol.33 No.3 pp.328-334
DOI : https://doi.org/10.14478/ace.2022.1021

Comparison of Removal Efficiency of Mn-loaded Natural Zeolites and Red Mud for the Catalytic Ozonation of 2-Butanone

Youna Park, Jung Eun Lee^*, Young-Kwon Park†

School of Environmental Engineering, University of Seoul, Seoul 02504, Republic of Korea
^*Department of Environmental Engineering, Kwangwoon University, Seoul 01897, Republic of Korea

1 Co-first authors

^† Corresponding Author: University of Seoul School of Environmental Engineering, Seoul 02504, Republic of Korea Tel: +82-2-6490-2870 e-mail: catalica@uos.ac.kr

Received April 15, 2022 ; Review May 30, 2022 ; Accepted June 2, 2022

Abstract

For the study of environmental application of natural zeolites (NZ) and red mud (RM), which are discharged from various industrial fields, the catalytic ozonation of 2-butaone (methyl ethyl ketone, MEK) was performed using the Mn-loaded NZ prepared according to the Mn content of 1, 3, 5, 7, and 10 wt%. By the addition of Mn to NZ, the BET (Brunaure-Emmett-Teller) specific surface area of Mn/NZ catalysts decreased while the ratio of Mn³⁺/[Mn³⁺+Mn⁴⁺] intensively increased. Besides, the addition of Mn component to NZ increased the ratio of adsorbed oxygen (O_adsorbed) toward lattice oxygen (O_lattice), O_adsorbed/O_lattice from 0.076 of NZ to 0.465 of 10 wt% Mn/NZ according to the amount of Mn. It is known that the proportion of two species, Mn³⁺ and O_adsorbed, would greatly affect the catalytic activity. However, the balancing between the paired species, Mn³⁺ vs. Mn⁴⁺ and O_adsorbed vs. O_lattice might be more essential for the catalytic ozonation of MEK at room temperature. Among the Mn-loaded NZ catalysts, the 3 wt% Mn/NZ showed the best activity for the removal of MEK and ozone. The 5, 7, and 10 wt% Mn/NZ catalysts are slightly inferior to the 3 wt% Mn/NZ. Compared to the pristine NZ, the Mn/NZ catalysts showed better activity for the catalytic ozonation of MEK.. In addition, the 3 wt% Mn/NZ was confirmed to have the most available acid sites among them by the analysis of NH₃-TPD (temperature programmed desorption). This might be the major reason for the best catalytic activity of 3 wt% Mn/NZ together with the adjusted distribution ratios of Mn³⁺/Mn⁴⁺ and O_adsorbed/O_lattice. Considering the result of 3 wt% Mn/NZ, the 3 wt% Mn/RM was prepared to perform the catalytic activity for the removal of MEK and ozone, but the efficiency of 3 wt% Mn/RM was significantly lower than that of the 3 wt% Mn/NZ.

Key Words : Red mud , Natural zeolite , Mn-loaded zeolite , Butanone , Ozone

초록

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1. Introduction

As the environmental pollutions[1-3] caused by increasing the accumulations of various waste materials[4-6] become serious, the studies on the recycle[7,8] and the reuse of discharged waste[9-11] have been more focused[12]. Among them, the exploring the recycle or the upcycle of red mud discharged from aluminum production[13] is one of the most emerging research fields. Because the global consumption of Al increases, the discharged red mud produced from the Al related industries consistently increases every year. Since red mud contains different kinds of metals, the recovery processes of rare metals or noble metals from discharged red mud are also studied actively. The reuse or the recycling of red mud can result in saving resources and energy[ 14]. Among recycling of red mud, red mud can be used as building bricks or subgrade materials[15] because of strong solidity and durability[ 16]. However, the portion to be recycled or recycled in compared to the amount of discharged red mud is very low. Recently, it has been attempted to explore the reuse of red mud as a catalyst[13] for removal of harmful pollutants[17]. Red mud has a similar frame structure as natural zeolites. Natural zeolites have several advantages [18] such as large surface area, high pore volume, inner channels, and strong durability to be used as adsorbent[19], support, and catalyst[20] for removal of toxic materials[21], heavy metals[22] and organic pollutants[ 23]. Among various pollutants, volatile organic compounds (VOCs) [24] affect not only indoor but also outdoor air deterioration[25]. 2-butanone (methyl ethyl ketone, MEK) was selected as a representative molecule of VOCs to delve into the catalytic activity of red mud and natural zeolite. Because MEK is not only widely used in fabric industries and petroleum refineries, but also has high potential to produce more harmful intermediates by partial oxidations, it would be essential to streamline the catalytic ozonation of MEK at the room temperatures. In this work, with reference to zeolites characteristics and Mn loading effect based on our previous works natural zeolite and red mud are used as a catalyst for the removal of MEK and ozone[26] at the room temperatures. Also, the Mn-loaded[27] natural zeolite and red mud catalysts were prepared and the catalytic activities of them were compared for the catalytic ozonation of MEK.

2. Experimental setup

2.1. Materials

The red mud (RM) and the natural zeolites (NZ) performed in this work were supplied from a local company in South Korea (the KC alumina production plant). For the synthesis of Mn loaded catalysts, manganese acetate, (CH₃COO)₂Mn⋅4H₂O (Sigma-Aldrich) were purchased and used as a precursor of Mn to prepare the Mn-loaded catalysts. Except the pristine catalysts, natural zeolite (NZ) and red mud (RM), the Mn-loaded catalysts were prepared by the impregnation method according to the content of Mn, namely, 1, 3, 5, 7, 10 wt% Mn/NZ catalysts and 3 wt% Mn/RM for the removal of MEK and ozone.

2.2. Experiment for the removal of MEK and ozone

The concentration of methyl ethyl ketone (MEK) was controlled at 100 ppm with 1 L/min flow rate by Mass Flow Controller (K.M.B. Tech Co., South Korea) in the N₂ atmosphere. An ozone generator (Ozonetech, South Korea) was used for supplying 1000 ppm ozone with the flow rate of 1 L/min. After going through mixing chamber of ½ inch SUS tube, the flow rate of the gases was adjusted 2 L/min to supply for the reaction with the catalyst which was placed in catalyst bed. All experiments of removal of MEK and ozone were performed with 0.6 g catalyst at the room temperatures around 25 °C.

2.3. Characteristics analysis

The analysis of FT-IR (Fourier transform infrared spectroscopy, Nicolet iS10, Thermo Fisher Scientific, USA) including a gas cell (Nicolet 10M Gas cell, Thermo Fisher Scientific) was performed and the data obtained from an ozone analyzer (Ozonetech, South Korea) were used for understanding the catalytic activity for the simultaneous removal of MEK and ozone at the room temperatures. In addition, the adsorbed materials on a catalyst were analyzed using GC (7280A, Agilent, USA) - MSD (5977E, Agilent, USA) with the 3 mg catalyst after the reaction with each catalyst.

A Belsorp mini II (BEL JAPAN INC., Japan) was operated to estimate the value of BET (Brunaure-Emmett-Teller) specific surface area and pore volume of catalysts. A BELCAT-B analyzer was used to obtain the NH₃-TPD (temperature programmed desorption) diagram for the performed catalysts. For the investigation of x-ray diffraction (XRD) patterns, an Ultima III (Rigaku, Japan) with the Cu-K_α radiation was employed to delve into the phase differences according to the content of Mn. The XPS (X-ray photoelectron spectroscopy) data of Mn 2p and O 1s of the prepared catalysts were obtained by an ESCALab250 (Thermo Scientific).

3. Results and discussion

3.1. Analysis of characteristics of catalyst

In Table 1, the estimated values of the BET specific surface area and pore volume of the catalysts were listed. It is obvious that Mn-loaded catalysts of NZ and RM tend to show decreased values of specific surface area while the pore volumes of them might not be greatly affected by the Mn loading. According to the increase of Mn component, the surface area of Mn/NZ decreases to 138.34 m³/g from 164 m³/g for NZ. On the other hand, the pore volume of Mn/NZ shows slightly higher compared with that of NZ, 0.18 cm³/g. Among them, the pore volume of 10 wt% Mn/NZ shows the larger value 0.1961 cm³/g and the 3 wt% Mn/NZ catalyst is the next. As for the Mn/RM, both of surface area and pore volume are slightly decreased compared with the pristine RM. Especially, the surface areas of RM and Mn/RM, 36.525 and 33.627 m²/g, respectively are clearly lower than those of NZ and Mn/NZ in the range of 138~164 m²/g.

In Figure 1, the distributions of the acid sites on the NZ and MN/NZ catalysts are described according to the temperatures. Compared to the NZ catalyst, the acid sites of the Mn-loaded NZ increase in the range of temperatures 100~650 °C. There are three different temperature regions, a low temperature region less than 300 °C corresponding to weak acid sites, a medium temperature region in the range of 100~500 °C, and a high temperature region more than 500 °C corresponding to strong acid sites. The strength and the proportion of acid sites of catalyst can play the pivotal role for the destruction of organic pollutants[28]. Based on the result of acid sites distribution of NH₃-TPD in Figure 1, the 3 wt% Mn/NZ would show the best activity under the medium temperature range and the high temperature region.

In Figure 2, the XRD patterns of NZ and Mn-loaded NZ (1, 3, 5, 7, and 10 wt% Mn/NZ) are described. It might be confirmed that there is no significant difference in phase transition by the addition of Mn and the loaded Mn components are spread evenly into the natural zeolite. In addition, in Figure 3, the XRD results of the RM and the 3 wt% Mn/RM catalysts are depicted. The XRD pattern of Mn/RM doesn’t deviate from the pristine RM because the added content of Mn is too small to appear notable difference.

In Figure 4, the XPS results of Mn 2p binding energies of Mn³⁺ and Mn⁴⁺ species for the NZ and the Mn/NZ catalysts are described. The double splitting peaks of Mn 2p, Mn 2p_1/2 and Mn 2p_3/2, are assigned in the range of 660~650 eV and 650~638 eV, respectively. The binding energies of Mn 2p_3/2 for Mn³⁺ and Mn⁴⁺ for all the Mn-loaded catalysts are assigned to the peaks in the range of 641.5~642 eV and 644.5~645.7 eV, respectively. The Mn 2p peaks for the pristine catalysts of NZ and RM could be detected. In Table 2, the binding energies of Mn 2p_3/2 for Mn³⁺ and Mn⁴⁺ for the Mn-loaded catalysts, 1, 3, 5, 7, and 10 wt% Mn/NZ and 3 wt% Mn/RM are listed. Because the Mn³⁺ species can play a pivotal role for the removal of organic pollutants[29], the ratios of [Mn³⁺]/([Mn³⁺] + [Mn⁴⁺]) for each catalyst are estimated in Table 2 as well. Mostly, the values of [Mn³⁺]/([Mn³⁺] + [Mn⁴⁺]) for Mn-loaded catalysts are estimated in the range of 0.84 – 0.85 if the Mn content is more than 3 wt%.

In Table 3, the binding energies of lattice oxygen (O_lattice)[30] and surface adsorbed oxygen (O_adsorbed)[31] were estimated from the XPS results of O 1s of the catalysts. The distributions of O_lattice and O_adsorbed species of the NZ-based catalysts appeared in the range of 531.5~532.5 eV and 530~531 eV, respectively. The estimated ratios of the two species, O_adsorbed/O_lattice are also listed in Table 3. The ratios of two oxygen species tend to increase linearly by the addition of Mn except the 7 wt% Mn/NZ. On the other hand, compared with the NZ-based catalysts, the binding energies of O_lattice and O_adsorbed for the RM-based catalyst, RM and 3 wt% Mn/RM, appeared in a little bit lower energy region, 531~531.5 eV and around 529.4 eV, respectively. For the reason of the greater values of oxygen ratio of the RM-based catalysts, the absolute area corresponding to O_lattice of O 1s XPS data of RM is smaller than that of NZ. However, the area of O_adsorbed of the pristine RM is greater than that of NZ. In Figure 5, the O 1s XPS spectra are described for the 3 wt% Mn/ZN and the 3 wt% Mn/RM. In addition, it could be confirmed that the addition of Mn to RM led to increasing the distribution of O_adsorbed species as show in Figure 6.

3.2. Removal of MEK and ozone

In Figure 7(a), the removal efficiencies of MEK and ozone using the NZ-based catalysts are compared. Compared to the pristine NZ catalyst, the Mn-loaded catalysts show an improved activity and among the Mn/NZ catalysts. Tthe 3 wt% Mn/NZ is confirmed to be the best catalyst for ozone and MEK removal while the Mn/NZ catalysts with more than 3 wt % Mn content show less activity. In addition, the 1 wt% Mn/NZ catalyst is also inferior to the 3 wt% Mn/NZ. Among the NZ-based catalysts, the 3 wt% Mn content seems to be the best optimized for MEK and ozone removal at the room temperatures. The concentrations of CO and CO₂ produced from MEK decomposition are depicted in Figure 7(b) and show a similar pattern with the MEK removal efficiencies in Figure 7(a). Particularly for the NZ catalyst which has no addition of Mn, there is no detection of CO and CO₂ for the product analysis as shown in Figure 7(b). It might be explained by the physical adsorption of MEK by the NZ rather than chemical decomposition.

On the other hand, based on the result of the NZ-based catalysts, the 3 wt% Mn/RM was prepared and performed for the removal of MEK and ozone. As the same as the NZ-based catalysts, the catalytic activity of the 3 wt% Mn/RM was improved noticeably. The removal efficiencies of the RM and the 3 wt% Mn/RM for the removal of MEK and ozone were described in Figures 8(a) and 9(a), respectively. In addition, the produced CO and CO₂ concentrations were estimated in Figures 8(b) and 9(b), respectively. As the reaction time increases, the decomposition rates of MEK by the RM-based catalysts increase for 10 min, and then decrease. The removal rate of MEK by the RM-based catalysts is similar with that of ozone.

After the reaction of decomposition of MEK and ozone, the intermediates and products attached on the 3 wt% Mn/NZ and the 3 wt% Mn/RM were analyzed by a GC/MSD system and described in Figures 10(a) and 10(b), respectively. As intermediates aldehyde was confirmed to attach on the catalyst and some extent of MEK was found on the GC/MSD graph for both the Mn/NZ and the Mn/RM with 3 wt% Mn content.

The carbon conversion into CO_x [CO+CO₂] and the CO₂ selectivity derived from the oxidation of MEK by the catalytic ozonation using the 3 wt% Mn/NZ and the 3 wt% Mn/RM were estimated and plotted in Figure 11(a) and 11(b), respectively. The conversion of CO_x by the 3 wt% Mn/NZ shows increasing tendency as the increase of reaction time while the conversion by the 3 wt% Mn/RM decreases after 10 min. The CO₂ selectivity was the same as the CO_x conversion for both catalysts. As for the 3 wt% Mn/NZ the CO_x conversion of MEK for an hour was slightly lower than 20% and the CO₂ selectivity reached out around 10%. However, as for the 3 wt% Mn/RM the CO_x conversion of MEK for an hour was less than 3% and the CO₂ selectivity was very small less than 1%.

4. Conclusions

The activity of natural zeolites and red mud for the catalytic ozonation of MEK to decompose through oxidation process were determined to be improved by the addition of Mn component. Among the Mn/NZ catalysts, the 3wt% Mn/NZ shows the best activity for the removal of MEK and ozone at the room temperatures because of the high distribution of acid sites of the catalyst. Although the catalytic activity of the RM-based catalysts is inferior to that of the NZ-based catalysts, the appropriate pretreatments such as neutralization or modifications might be expected to improve the catalytic activity for the removal of volatile organic materials in the presence of ozone.

Acknowledgement

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2021R1I1A1A01059154).

Figures

Figure 1.NH₃-TPD curves of NZ and Mn/NZ (1, 3, 5, 7, and 10 wt% Mn/NZ).

Figure 2.XRD patterns of NZ and Mn loaded NZ (1, 3, 5, 7, and 10 wt% Mn/NZ).

Figure 3.XRD patterns of RM and 3 wt% Mn/RM.

Figure 4.XPS results of Mn2p_3/2 spectra of Mn³⁺ and Mn⁴⁺ species for the Mn/NZ catalysts (1, 3, 5, 7, and 10 wt% Mn/NZ).

Figure 5.XPS O1s spectra of 3 wt% Mn/NZ and 3 wt% Mn/RM.

Figure 6.XPS O1s spectra of RM and 3 wt% Mn/RM.

Figure 7.MEK and ozone removal efficiencies of NZ-based catalysts (a), and CO and CO₂ concentrations measured from the outlet gas (b).

Figure 8.Removal efficiencies for MEK and ozone (a) and CO and CO₂ concentrations (b) using RM.

Figure 9.Removal efficiencies for MEK and ozone (a), and CO and CO₂ concentrations (b) using 3 wt% Mn/RM.

Figure 10.Analysis of materials attached on the 3 wt% Mn/NZ (a) and the 3 wt% Mn/RM (b) by GC/MSD measurements.

Figure 11.COX (CO and CO₂) conversion (a) and CO₂ selectivity (b) from the catalytic ozonation of MEK.

Tables

Table 1.BET Surface Area and Pore Volume of NZ and RM Catalysts

Table 2.The Binding Energies (eV) of Mn 2p_3/2 and the Ratios of Mn³⁺/(Mn³⁺ + Mn⁴⁺) for Mn-loaded Catalysts

Table 3.Binding Energies (eV) and the Oxygen Ratio for O_lattice and O_adsorbed for the Catalysts

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