The promoting effect of alkali metal and H2O on Mn-MOF derivatives for toluene oxidation: A combined experimental and theoretical investigation
Graphical abstract
Introduction
With the further development of industrialization and urbanization, industrial emissions and the lives of residents have generated many volatile organic compounds (VOCs), which were the precursor of air pollution problem such as photochemical smog and haze [1]. Photocatalytic degradation [2], [3], adsorption [4], [5], biological treatment [6], catalytic oxidation [7], [8] and photothermal catalysis technologies [9] could be used to control VOC emissions. Compared with other control technologies, the catalytic oxidation method could reduce the reaction temperature, energy consumption and secondary pollution, which was suitable for the degradation of VOCs derived from dyes and petrochemicals [10]. Toluene is a representative component of VOCs of great concern because of its carcinogenicity, mutagenicity, and toxicity [3]. Additionally, toluene, as a typical industrial waste gas, usually contains water vapor during industrial discharge and water vapor reduces the catalytic activity because it competes with toluene for adsorption sites [11]. Therefore, it was very necessary to synthesize a catalyst with high water resistance and high activity for toluene oxidation.
Based on previous reports, transition metal catalysts with adjustable metal oxides as active components were low-cost and anti-poisoning [7]. Among these transition metal catalysts, Mn-based catalysts were one of the most promising catalysts for catalytic oxidation of VOCs due to the variable valence of Mn (2+, 3+ and 4+) and low cost [12], [13], [14]. Compared with other Mn-based catalysts, alkali-metal doped Mn-based catalysts are beneficial to the synthesis of ammonia [15], conversion of CO2 to gasoline-range (C5–C11) hydrocarbons [16], production of olefin [17] and catalytic oxidation of VOCs, including toluene [13]. There were many methods for doping alkali metals in Mn-based catalysts. Zhu et al. hydrothermally prepared K-doped α-MnO2 by adjusting the hydrothermal time, which had the highest oxygen vacancy concentration [18]. Dong et al. reported that Na+ was introduced to MnO2 by the cation-exchange method, which enhanced toluene oxidation activity via the exposure of acidic sites and abundance of oxygen vacancies [19]. Hong et al. reported that the introduction of Na+ into the OMS-2 tunnel through a simple solid-state reaction method reduced the formation energy of oxygen vacancies and adsorption energy of ozone [20]. Zhu et al. found that KOH was considered to be the best precursor for alkali metal modification of α-MnO2 through simple posttreatment, which could enhance the activity and content of lattice oxygen [21]. Therefore, the doping of alkali metal on Mn-based catalysts synthesized by different methods has been widely used in the field of catalytic oxidation.
Recently, alkali metal doping catalysts by using metal–organic frameworks (MOFs) as precursors have been widely studied [22]. MOFs, which have the advantages of adjustable pore size, high specific surface area and easy surface functionalization, are widely used in adsorption [4], electrocatalysis [23], photocatalysis [3], photothermal catalysis [24] and catalysis [25]. Furthermore, in recent studies, MOFs have emerged as a promising self-sacrificial template for the synthesis of some metal nanoparticles (NPs), oxides or their composites by simple pyrolysis [26], [27], [28]. By tuning the selection of appropriate MOFs precursors and applying of control strategies, MOFs-derived nanostructures with diverse morphologies could be rationally designed. These MOFs-derived nanostructures showed great potential for applications such as gas storage and separation [29], catalysis [30], [31], energy storage and conversion [32]. In our previous work, MOFs-derived MnCeOx catalysts were synthesized by different calcination temperatures. A series of characterization results proved that the calcination temperature of pyrolyzing MOFs precursor affected the metal chemical state, adsorbed oxygen, surface oxygen vacancy concentration and low-temperature reduction ability of the catalyst [7]. These findings highlight the advantages of rational template modification in synthesizing finer nanoparticles, which can open up many new avenues for designing MOF-derived nanomaterials. Generally, the conversion of MOFs into catalysts with excellent performance by calcination is considered to be a common strategy [33]. However, the calcination method was energy-consuming and complicated to operation. It has been reported that direct alkali treatment method of MOF precursors might be an effective and steerable strategy to realize the ex situ conversion of MOFs to alkali metal-doped metal oxides with excellent catalytic oxidation performance [34]. In fact, despite the excellent performance of MOFs, the cost of MOFs as precursors for preparing catalysts is relatively high. In this study, more attention was paid to the catalytic oxidation of VOCs by catalysts, and the oxidation mechanism of VOCs on MOF-derived catalysts was studied in depth. Therefore, the purpose of this study is to prepare alkali metal-doped Mn-based catalysts with high activity and water resistance for toluene oxidation by using MOFs as precursors through a simple synthetic method.
In addition, H2O contained in the exhaust gas emitted from the actual production had a significant impact on the structure of the catalyst and catalytic VOC degradation performance. Therefore, the effect of H2O on catalytic VOC performance has been extensively studied [35], [36]. For example, the δ-MnO2 catalyst showed only a slight decrease in catalytic efficiency at 5 vol% H2O and exhibited good water resistance [37]. Some studies have shown that the role of water in the catalytic system might dissociate at the active site to generate enriched hydroxyl groups to promote the formation of other products [38]. Our previous studies showed that H2O had no effect on the catalytic performance of Pd-loaded UiO-66 catalysts under the condition of 10–20 vol% H2O [39], and H2O could facilitate CO oxidation on Mn2O3 catalysts derived from Mn-MOFs [40]. The previous studies only considered H2O as an influencing factor of the catalytic performance. To date, there is no systematic study on the adsorption and activation of intermediates by H2O introduction in combination with density functional theory (DFT) calculations and experiments.
Herein, alkali metal-doped MnCO3 with a small amount of Mn3O4 catalysts obtained from Mn-MOF through ex situ post-treatment method was used for catalytic oxidation of toluene. Firstly, multiple characterization methods were used to evaluate the difference in physical and chemical properties after doping with alkali metals. Secondly, catalytic testing of toluene, intermediates detection technologies were used to study the catalytic performance and reaction mechanism of doped alkali metal catalysts. Thirdly, the promotion effect of alkali metal was demonstrated by DFT calculation. The reasons of H2O promotion and the intermediates of catalytic oxidation of toluene in the presence of H2O was researched thoroughly. Finally, we summarized the reasons for the catalytic oxidation of toluene by alkali metals and H2O on Na-doped Mn-MOF derivative. This work provided new ideas for developing high-efficiency and water-promotion Mn-based materials for applications of VOCs control.
Section snippets
Materials
All chemicals are of analytical grade and can be used directly. More information about these chemicals can be found in the supplementary information.
Synthesis of the Mn-MIL-100
Mn-MIL-100 was synthesized by the solvothermal method, according to our previous work [41]. 0.50 g (0.002 mol) Mn(NO3)2·4H2O and 0.39 g (0.002 mol) C6H3(COOH)3 were dissolved by 18 mL CH3OH with stirring. The reaction mixture was stirred until completely dissolved at normal temperature, and then placed in a 150 mL Teflon-lined stainless-steel
Phase compositions and textural properties
The crystal phase structure of Mn-MIL-100 and alkali metal-doped MOF derivatives was identified by XRD technique, and the results are shown in the Fig. 1(a). The XRD spectra proved that Mn-MIL-100 was successfully synthesized, which was in accordance with the reported works [43]. Samples treated with NaHCO3 or KHCO3 were converted into MnCO3, and the peak of manganese oxide was not found. The peaks at 24.2°, 31.36°, 37.5°, 41.4°, 45.18° and 51.4° corresponded to (0 1 2), (1 0 4), (1 1 0), (1 1 3), (2 0
Conclusions
In the present study, different alkali metal-doped Mn-MOF derivative materials were obtained through ex situ posttreatment method. The effect of alkali metals species on the catalytic performance of Mn-MOF derivative materials for the oxidation of toluene was investigated. Considering the existence of water in the actual VOCs emission, the effect of water in the reaction atmosphere on the catalytic performance was also explored. In summary, the introduction of the alkali metal in the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is sponsored financially by the National Natural Science Foundation of China (No. 21906104 and No. 12175145), and the Shanghai Rising-Star Program (21QA1406600).
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