Elsevier

Journal of Rare Earths

Volume 40, Issue 10, October 2022, Pages 1564-1572
Journal of Rare Earths

Acid modified carrier on catalytic oxidation of dichloromethane over CeO2/HZSM-5 catalysts

https://doi.org/10.1016/j.jre.2021.08.018Get rights and content

Abstract

In this study, a series of Hydrogen-Zeolite Socony Mobil-5-X (HZSM-5-X) catalysts were prepared by acid modification, then Ce/HZSM-5-X (X = 0, 0.2, 0.4, 1.0, 2.0) catalysts were prepared by impregnation method. The catalytic performance of the catalysts on dichloromethane (DCM) oxidation was investigated. Through different characterizations, HZSM-5-X exhibit high specific surface area, good redox ability, rich acidity and much suitable acidic site distribution after acid treatment. Among them, Ce/HZSM-5-0.4 shows better catalytic activity with the lowest by-product and the best CO2 yield. Its T90 is 302 °C and the CO2 yield of T90 is more than 80 wt%, which demonstrates that the acid modification of carrier plays the positive effect on the catalytic capacity for DCM oxidation.

Graphical abstract

In this study, HZSM-5 was modified by sulfuric acid which increases the specific surface area and redox capacity of HZSM-5, and optimized the distribution of acidic sites on the surface. After loading active component CeO2, Ce/HZSM-5-0.4 shows good catalytic activity and deep oxidation ability for dichloromethane, which has high research value.

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Introduction

Chlorinated volatile organic compounds (CVOCs) are considered to be important precursors of ozone and smog, which are harmful to the environment and human health.1, 2, 3 As one of the typical CVOCs, dichloromethane (DCM) is widely used in industrial production. As the most effective DCM degradation technology, catalytic oxidation technology has the advantages of low energy consumption, high efficiency and no secondary pollution, which can convert DCM into harmless CO2, H2O and HCl.

The core of the catalytic oxidation technology of CVOCs is developing high performance catalysts. Taralunga et al.4 studied the catalytic degradation of chlorobenzene by Pt catalysts. It was found that Pt catalysts were beneficial to the chlorination of intermediate products to form dichlorobenzene, which led to the increase of poly chlorination by-products in the oxidation process. Miranda et al.5 studied the catalytic oxidation performance of Al2O3–Pd supported catalyst for CVOCs. It was found that Pt-based catalysts were prone to carbon deposition, chlorine poisoning and poor stability. Dai et al.6 prepared a series of Ru doped CeO2 catalysts. Through the investigation of the catalytic oxidation performance of each catalyst to chlorobenzene, it was found that Ru doped CeO2 catalysts exhibited high catalytic activity and stability for the catalytic oxidation of chlorobenzene, and RuO2 could quickly convert Cl adsorbed by the catalysts to Cl2, making the catalysts obtain obvious improvement in the toxicity of anti-chlorine.

At present, due to high cost, low thermal stability and easy reaction with chlorine species, precious metal catalysts are difficult to be used in industry. Therefore, with remarkable redox performance, thermal stability and resistance to Cl poisoning, the transition metal oxides have attracted much attention. Cao et al.1 prepared Ce–Ti oxide catalyst by hydrothermal impregnation method and tested its catalytic performance for DCM. The results showed that Ce3+ generated by doping Ce played an important role in removing Cl and promoted the further oxidation of by-products. Liu et al.7 found that a series of spinel-type CoCr2O4 oxides showed high activity and selectivity for the catalytic combustion of methylene chloride. Analysis results showed that under high temperature calcination, Cr3+ and Cr6+ could partially replace spinel-type oxide Co3+ at the octahedral position, thereby enhancing the reducibility and surface acidity of the oxide, and synergistically controlling the catalytic behavior. Sinquin et al.8 studied the catalytic performance of CH2Cl2 eliminated by LaMnO3. It was found that CH2Cl2 could be completely degraded at a temperature lower than 550 °C on the LaMnO3 catalyst, and maintained better resistant Cl poisoning and higher selectivity.

In general, the catalyst surface modification has a certain promotion effect on the catalytic oxidation performance, which is mainly divided into the modification of the active component and the modification of the support. The acid treatment of the metal oxide catalysts could induce the formation of abundant active oxygen, improve the structural characteristics of the catalysts, increase oxygen vacancies, raise the mobility of surface oxygen and the low-temperature reducibility of the catalysts, which makes the catalysts exhibit excellent catalytic oxidation performance. Yao et al.9 studied the propane catalytic combustion performance of LaCoO3 catalysts which were treated with acidic H2O2 solution. The results showed that the modification of LaCoO3 catalysts with acidic H2O2 solution could improve the porosity and increase the oxygen vacancy concentration, resulting in excellent performance, good catalytic activity and high stability of the catalysts. Jiang et al.10 treated MnO2 with H2SO4, HNO3 and HAc. The results showed that the performance of MnO2 was further improved by acid treatment, and acid treatment could effectively enhance the catalyst surface oxygen species, reducibility and acid sites, and improve the catalytic combustion activity. Yang et al.11 prepared the acid-treated MnO2 and studied the effect of acid treatment on the catalytic behavior of toluene. The results showed that the acid treatment catalysts could not only induce the formation of abundant active oxygen, but also improve the mobility of surface oxygen and the low-temperature reducibility of the catalysts, thereby exhibiting excellent catalytic performance. Li et al.12 synthesized a mesoporous Co3O4 catalyst by precipitation method, then treated it with dilute HNO3 solution, and studied the catalytic performance of toluene oxidation. The results showed that the acid-treated Co3O4 catalysts possessed good catalytic performance, which had a high specific surface area and a large number of weakly acidic sites.

On the other hand, Abdullah et al.13 reported the performance of Cr or Cu catalysts supported by HZSM-5 modified with silicon tetrachloride. It was found that treating catalysts with silicon tetrachloride can improve the chemical resistance of hydrogen-Zeolite Socony Mobil-5 (HZSM-5) to HCl. Sara et al.14 used citric acid to modify the HZSM-5 support, which effectively increased the surface area and surface acidity of the catalysts, so that the catalysts maintained high catalytic activity and low by-product yield. Relevant studies showed that the modification of the carrier had a significant influence on the improvement of the catalytic effect of the catalysts. Deng et al.15 modified cordierite with nitric acid, which effectively increased the specific surface area and active sites of the carrier. After supporting Zr–Ti oxide, the catalytic activity and stability of the catalysts were significantly improved. Wu et al.16 induced the formation of Al2O3 nanosheets by phosphate, and then loaded Pd to make a Pd/Al2O3 nanosheet catalyst, and tested its catalytic combustion activity for propane. The results showed that the presence of phosphate increased the thermal stability of the Al2O3 nanosheet and made the Pd distribution on the Al2O3 nanosheet more uniform.

The C–Cl bond in the CVOCs molecule has a strong polarity, which is easy to be dissociated, adsorbed and activated at the acidic sites on the catalyst surface. HZSM-5 zeolite has abundant surface acid sites, which can promote the rapid adsorption and activation of reactants. As a catalyst carrier, HZSM-5 zeolite is conducive to the fracture of C–Cl in CVOCs and its special pore structure has a certain shape-selective catalytic effect on intermediates products and final products. In order to improve the apparent combustion activity of HZSM-5, modification by various metals (Cr-Cu,13 Ce-Cr,17,18 Mn-Ce,2,19 Ce-Zr-Cu20) is one of the most effective methods. However, metal modification is easy to produce by-products in the process of practical application due to the incompatibility between surface acidity and redox ability. Recent studies have shown that zeolite structure and acidity are the two most important factors affecting catalyst activity, product distribution and coke formation. Acid treatment can induce the formation of abundant active oxygen, improve the structural characteristics of the catalysts, increase oxygen vacancies, increase the mobility of surface oxygen and the low-temperature reducibility of the catalysts, and more importantly, it can change the acidity of the catalyst surface. In the catalytic oxidation of CVOCs, the substrate molecules are first dechlorinated at the acidic site of the catalyst surface. The change of the acidic site can improve the dechlorination performance of the catalyst, adjust the surface acid property of the molecular sieve, improve the deep oxidation ability of the catalyst to the intermediate product and reduce by-products.

In summary, HZSM-5 treated by acid can promote the catalytic oxidation of CVOCs. Some people21, 22, 23, 24 have changed the specific surface area, acidity and pore volume of HZSM-5 by acid treatment, which proves that acid modification is an effective treatment method. CeO2-based catalysts possess high oxygen storage capacity and Ce4+/Ce3+ cyclicity,25,26 which have good catalytic performance for CVOCs.

In this work, CeO2/HZSM-5 catalyst was prepared with surfic acid modifying carrier, and its catalytic oxidation performance for DCM was studied. The effects of acid treatment on the physicochemical properties, such as structure, morphology, porosity, redox ability, surface acidity and surface active substances were revealed by various characterization methods.

Section snippets

Preparation of acid-treated zeolite

HZSM-5 (Si/Al = 18, purchased from Tianjin Nanhua Catalyst Company, Ltd.) was calcined in a muffle furnace at 550 °C for 6 h. After cooling, 4.0 g of HZSM-5 was added to 20 mL of various concentrations of H2SO4 solution (0.2, 0.4, 1.0 and 2.0 mol/L). The mixture was stirred at 30 °C for 2 h. The acid-treated zeolite was obtained by filtration, and then washed with deionized water until neutrality was reached. After washing, the zeolite was dried at 110 °C for 10 h, which is respectively

Catalytic activity and generation of products

If the catalysts lack sufficient adsorption capacity and redox sites, deep oxidation of CVOCs will not be achieved, and more toxic by-products may be generated when CVOCs are cracked.30 Therefore, the catalytic activity and the by-product CH3Cl were detected by GC, and the CO2 concentration at the outlet was also detected to calculate the CO2 yield to analyze the degree of oxidation of DCM. The detection results are shown in Fig. 1, Fig. 2. First, comparing the apparent catalytic activity of

Conclusions

Compared with the Ce/HZSM-5-0, the catalytic activity of the catalysts with sulfuric acid modified carrier is improved, and especially Ce/HZSM-5-0.4 shows better catalytic activity, the lowest by-product yield and the highest CO2 yield. Ce/HZSM-5-0.4 was characterized with larger specific surface area, abundant acid sites and more surface adsorbed oxygen owing to the acid treatment, which are responsible for better catalytic capacity. Furthermore, Ce/HZSM-5-0.4 catalyst also shows good thermal

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    Foundation item: Project supported by Science and Technology Department of Jiangsu Province (BE2016769).

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