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. 2017 Aug 30;2(8):5179–5186. doi: 10.1021/acsomega.7b00592

Efficient and Environmentally Friendly Synthesis of AlFe-PILC-Supported MnCe Catalysts for Benzene Combustion

Shufeng Zuo †,*, Peng Yang †, Xianqin Wang ‡,*
PMCID: PMC6641775  PMID: 31457791

Abstract

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An efficient and environmentally friendly synthesis of AlFe-pillared clay (AlFe-PILC)-supported MnCe catalysts was explored. Mixed AlFe pillaring agents were prepared by a one-step method using Locron L and ferric nitrate solutions at a high temperature and high pressure. Montmorillonite was treated with the AlFe pillaring agents to synthesize AlFe-PILC. MnOx and CeO2 with different Mn/Ce atomic ratios were loaded onto the AlFe-PILC support by an impregnation method. The catalysts were characterized using X-ray diffraction, N2 adsorption, and high-resolution transmission electron microscopy–energy dispersive spectrometry and were tested for the catalytic combustion of benzene and temperature-programmed surface reaction using a microreactor. Compared to conventional methods, this method is simpler and less costly and results in a larger specific surface area, pore volume, and basal spacing, with the ability to control the structure of the catalytic materials. MnCe(6:1)/AlFe-PILC has the highest catalytic activity and can completely degrade benzene (600 ppm in air) at 250 °C. The activity of the catalyst is stable, and no obvious deactivation is observed at 230 °C after 1000 continuous hours. The catalyst is resistant to water and Cl-poisoning. The amount of CeO2 added is critical to the dispersion of MnOx on the support and the creation of optimum number of oxygen vacancy defect sites for the benzene oxidation reaction. The AlFe-PILC-supported MnCe catalyst is a promising porous material; the support structure, proper dispersion of active species, and addition of Ce are essential for achieving complete degradation of organic toxic chemicals at relatively low temperatures.

1. Introduction

Clay minerals, such as montmorillonite (Mt), kaolinite, hydrotalcite, and so on, have recently received considerable attention because of their abundance, low cost, and environmental compatibility. Mt belongs to the smectite clay mineral group and is the most popular type of clay applied in pillaring processes.1−4 Because of their excellent properties and versatility, clay minerals are excellent materials for use as heterogeneous catalysts. Clay pillaring is a procedure that converts layered clay minerals into highly porous structures.5 The process involves exchanging the interlayer cations with large inorganic polymeric oxyhydroxy cationic species, which are converted into oxides that are strongly bonded to minerals (“pillars”) after calcination processes.6 In common pillared clay (PILC) synthesis, metal ions, such as Al3+, Zr4+ and Fe3+, are mixed with a basic solution (NaOH or NaCO3) and stirred at a certain temperature for 3–12 h, followed by 6–24 h of aging at 60–80 °C to obtain polycations, which are pillaring agents. The main drawback of this process is that it requires the preparation and consumption of large amount of solutions.7−9 Thus the preparation is expensive, unstable, and difficult to reproduce because of the effects of the container, mixing equipment, temperature, and humidity. It is also prone to contamination when exposed to air, so it is unfavorable for a wide range of applications.

Using a variety of elements to synthesize composite pillaring agents for the preparation of PILCs can greatly improve the specific surface area, pore volume, and thermal stability. Vicente et al. have described the wide uses of mixed pillaring agents over the past 30 years.5 The authors have listed more than 10 types of mixed pillaring agents that contain Al as a component and have mentioned that Al-free mixed pillaring solutions have also been employed.4 However, disadvantages in the preparation process remain. In this study, we use a high-temperature, high-pressure, one-step method to synthesize composite AlFe pillaring agents for the first time. This method has the following advantages: (a) It offers simple preparation, minimal equipment requirements, and highly reproducible results. (b) It is inexpensive and makes use of readily available materials in the preparation process. For example, Locron L (commercially available as an antiperspirant) replaces NaOH and AlCl3 solutions, reducing production costs and environmental pollution. (c) It involves the synthesis of AlFe-PILC, which has a large specific surface area, pore volume, and basal spacing as well as a controllable structure.

PILCs have uniform pore distributions, large specific surface areas, and high thermal stability and are therefore excellent catalytic materials and supports. PILCs can be considered as promising alternative supports because they have larger pore size than zeolites,10 thus making pore blockage by carbon deposition more difficult. A recent popular research area has been the preparation of PILC-supported catalysts for environmentally friendly reactions.11−15 The destruction by complete oxidation of hazardous gaseous pollutants is an interesting application of PILC catalysts.16−19 Pollutants are usually present in air streams at low concentrations, requiring highly efficient technologies for total oxidation, avoiding the formation of harmful byproducts.20,21

Volatile organic compounds (VOCs) include all organic compounds that exist in the gaseous state in ambient air. To reduce the environmental impact of VOCs, the existing legislation has to be more stringent.22 The catalytic combustion of VOCs can occur at lower temperatures than that without a catalyst. It is more energy efficient, and therefore has a distinct economic advantage.23,24 The most commonly used catalysts in the complete oxidation of VOCs are noble metals and metal oxides.25−28 Noble metals that can be used in catalytic combustion include Pt and Pd. Supported noble metal catalysts exhibit excellent initial activity, as manifested mainly by lower light-off temperatures and minimal differences between the complete combustion temperature and the light-off temperature; that is, their activity increases rapidly with increasing reaction temperature.29,30 However, noble metals also possess several disadvantages because of their rarity, high cost, and poor toxic tolerance. Therefore, researchers are searching for alternatives to noble metals. Transition metal oxide catalysts have suitable catalytic activity for the catalytic combustion of VOCs and have strong antipoisoning capability. The most commonly used active ingredients are Cr, Co, Mn, and Cu oxides.21,27,28,31−35 Rare earth metals (predominantly in the form of oxides) in catalysts play a critical role in the catalytic combustion of industrial organic waste gases and can be added to improve the thermal stability, activity, and reaction half-life of the catalyst.19,29,34,36−39

Here, a high-temperature, high-pressure, one-step method is used to synthesize composite AlFe pillaring agents, and subsequently AlFe-PILC. Compared to Na–Mt and Al-PILC, this material has larger specific surface area and pore volume as well as an adjustable pore structure. Given these excellent structural characteristics, AlFe-PILC can successfully act as a catalytic material. Our previous work with cerium in precious metal and transition metal catalysts showed that cerium plays an important role in combustion reactions.40−43 Here, we used the transition metals Mn and Ce on AlFe-PILC to prepare composite oxide catalysts with our efficient and environmentally friendly method and investigated their activities in the catalytic degradation of benzene.

2. Experimental Section

2.1. Preparation of the Support and Catalyst

2.1.1. Materials

The starting material for the pillaring procedure was the sodium form of montmorillonite (Na–Mt) (>100 mesh, Baotou, China). Al-pillaring agents aged at 60 °C for 8 h were synthesized, and Al-PILC was prepared following a procedure similar to that described in our previous work.19,40,41

2.1.2. Preparation of AlFe Pillaring Agents

AlFe pillaring agents were prepared as follows: Locron L [Al2(OH)5Cl·2–3H2O], a basic aluminum chloride-based liquid (Clariant, Switzerland), which is commercially available as an antiperspirant, was used as the aluminum source. Locron L (10 mL, Al ion concentration was 6.0 mol/L) and the desired volume of ferric nitrate solution (6.0 mol/L) were added to an autoclave at an atomic ratio of Al/Fe = 5/1, and then deionized water was added into the autoclave to further dilute the solution until the concentration of Al ions reached 2.5 mol/L. The autoclave was then placed in an oven at 120 °C for 12 h and was then removed and placed in cold water to cool. The solution was removed from the autoclave once the temperature dropped to room temperature and was then diluted to 600 mL to adjust the Al ion concentration to 0.1 mol/L.41,44 The solution was used in subsequent steps as an AlFe pillaring agent. The preparation of AlFe-PILC is as follows: AlFe pillaring agents were added dropwise into Na–Mt (2 wt %) at a ratio of 20 mmol of Al per gram of Na–Mt and stirred at 60 °C for 3 h, followed by centrifugation and washing with deionized water to remove Cl–; the product was then dried at 110 °C in an oven and calcined in a muffle furnace at 550 °C for 4 h to obtain AlFe-PILC.

2.1.3. Preparation of the Supported MnCe Catalysts

Particles of Mn/Na–Mt, Mn/Al-PILC, Mn/AlFe-PILC, and MnCe/AlFe-PILC were obtained by impregnating equal volumes of the support as well as Na–Mt, Al-PILC, and AlFe-PILC onto the precursor overnight to a Mn and Ce nitrate solution, followed by drying, calcining the samples at 500 °C for 2 h, and passing the solution through a #40–60 sieve. The total loading amount of Mn or MnCe for all of the catalysts was 10 wt %, and the Mn/Ce atomic ratios were 3:1, 6:1, 9:1, and 12:1.41

2.2. Characterization

X-ray diffraction (XRD) patterns of the samples were obtained using a PANalytical Empyrean powder diffractometer operated with a Cu Kα source (λ = 0.15406 nm), a tube voltage of 40 kV, and a tube current of 40 mA. The powder technique was used, and the oriented specimens were prepared by spreading the sample on a glass slide. The divergence slit was 1/8° and the antiscatter slit was 1/4°. The intensity data were collected in the 2θ range of 3–15° (80°) with a scan step-size of 0.0263° and a scan speed of 2°/min.

The specific surface area (SBET) and pore volume (Vp) of each clay material and the catalyst were measured using a TriStar II 3020 apparatus (Micromeritics Company, USA) at liquid nitrogen temperature (77 K). The samples were pretreated at 250 °C in vacuum for 4 h. SBET was calculated using the Brunauer–Emmett–Teller (BET) equation, whereas Vp was evaluated from the nitrogen uptake at a relative N2 pressure of P/P0 = 0.99. The t-plot method was used to determine the mesoporous surface area (Ames) and micropore volume (Vmic). The pore-size distribution was calculated using the Barrett–Joyner–Halenda (BJH) method.

High-resolution transmission electron microscopy (HRTEM) was conducted using a JEOL-2010F (HR) to obtain the morphologies of Na–Mt and AlFe-PILC. The working voltage was 200 kV. The samples were embedded in epoxy resin and then microtomed into sub-100 nm ultrathin films at room temperature. These thin-film samples that floated on water or other solvents were collected by a copper mesh with a polymer microgrid for HRTEM imaging and elemental mapping. The particle morphologies of MnCe/AlFe-PILC were observed using a JEOL-2100F (HR). The processing was the same as that given above. The chemical composition of the catalyst was determined with energy dispersive spectroscopy (EDS) using an Oxford INCA instrument (Oxford Instruments, UK).

The manganese and cerium contents of 10% MnCe(6:1)/AlFe-PILC were determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a Leeman ICP-AES Prodigy XP (Leeman Labs, USA) after the powder was dissolved in a mixture of fuming nitric acid and concentrated hydrochloric acid (volume ratio: 1/1).

X-ray photoelectron spectroscopy (XPS) spectra (Ce 3d) of 10% MnCe(6:1)/AlFe-PILC were collected on a Thermo K-Alpha spectrometer using Al–K radiation at 1486.6 eV/12.5 kV, and all spectra were calibrated based on the binding energy of C1s (284.6 eV). The Mn and Ce contents of the catalyst were approximately 6.52 and 2.73 wt %, respectively, and the ratio of Mn/Ce was about 6.08.

2.3. Catalytic Activity Tests

The activity of the catalyst (200 mg) was evaluated in a WFS-3010 microreactor (Xianquan, Tianjin, China) at a benzene concentration of 600 ppm and a space velocity of 30 000 h–1. The online measurement was conducted using a Shimadzu GC-14C (flame ionization detector) gas chromatograph (operating conditions: vaporizer temperature of 170 °C and column temperature of 120 °C), and the data were recorded and analyzed using an N2000 chromatography data workstation. The activity of the catalysts was examined by analyzing three consecutive combustion tests between 100 and 500 °C. The combustion experiments consisted of increasing the temperature in steps of 20 °C. The T50 parameter (temperature needed to attain 50% conversion) was used as the criterion for evaluating the catalytic performance. The conversion data were calculated by the difference between the inlet and outlet concentrations. The degradation products were detected by mass spectrometry (MS). No byproducts other than H2O and CO2 were detected. Thus, the conversion was calculated based on benzene consumption.

2.4. Temperature-Programmed Surface Reaction (TPSR) of Benzene

The in situ TPSR of benzene on the catalytic materials was also conducted. The catalyst (200 mg) was pretreated in a 20% O2/Ar flow (99.99%, 60 mL min–1) at 300 °C for 30 min. After cooling to 50 °C, 1000 ppm benzene was injected into the reaction system. After reaching the adsorption–desorption equilibrium, the catalyst was heated from 50 to 500 °C in increments of 7.5 °C min–1 in 600 ppm benzene; the balance gas was 20% O2/Ar at 60 mL min–1. The concentrations of benzene, any byproduct, and the final product (CO2) were measured online by MS (QGA, Hiden, UK).

3. Results and Discussion

3.1. XRD Analysis

Figure 1 shows the XRD spectra of Na–Mt, Al-PILC, and AlFe-PILC. The spectra show that the 2θ value of the PILCs shifted and decreased. The d001 value of Al-PILC is 1.77 nm, which is much larger than that of Na–Mt (1.23 nm). The d001 value of the composite AlFe-PILC is 1.91 nm, which is larger than that of Al-PILC. This increase in the basal spacing occurred because the Al/Fe polycations that intercalated between the layers are larger than the Al polycations (Keggin ions) in size, thereby creating larger layers. These results illustrate that the synthesis of the PILCs was successful. The thermal stability of AlFe-PILC was evaluated based on these results: the layered structure remained stable at 650–700 °C, but the diffraction peak at 2θ = 4.64° completely disappeared at approximately 800 °C, indicating the destruction of the layered structure (these spectra are not included).

Figure 1.

Figure 1

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XRD spectra of the supports: (a) Na–Mt; (b) Al-PILC; and (c) AlFe-PILC.

Figure 2 presents the XRD spectra (2θ: 3°–80°) of the catalysts, and the reflections at 19.8° and 26.7° (2θ) belong to cristobalite and quartz, respectively. After MnCe loading on the supports, the d001 peak still exists and there is no change in the d001 value, which indicates that the structure of the supports are very stable. The diffraction peaks of MnOx and Fe2O3 are present in the spectra of the catalysts. No CeO2 is observed, possibly because of the low loading amount and its high dispersion on the supports. For Mn/Na–Mt, Mn2O3 is observed, whereas no MnO2 is detected. By contrast, for Mn/AlFe-PILC, MnO2 is observed, whereas no Mn2O3 is detected. Neither MnO2 nor Mn2O3 is detected on MnCe(6:1)/AlFe-PILC, indicating that the addition of Ce dispersed MnOx, resulting in a crystalline size that was too small to be detected by the in-house XRD. Therefore, more manganese oxide species exist in higher oxidation states on AlFe-PILC-based catalysts, and the dispersion of MnO2 is improved by adding CeO2.

Figure 2.

Figure 2

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XRD spectra of the catalysts: (a) Mn/Na–Mt; (b) Mn/AlFe-PILC; and (c) MnCe(6:1)/AlFe-PILC.

3.2. N2 Adsorption–Desorption

Figure 3 presents the adsorption–desorption isotherms and pore-size distributions of various types of samples. The adsorption–desorption isotherms of all of the materials are the standard IV type, whereas the H3-type adsorption–desorption hysteresis appears above a relative pressure of 0.45, indicating the presence of slit pores formed by the layerlike structures in the material. The N2 adsorption capacity of Na–Mt is small, as reflected by the small pore volume of this material,44 which is confirmed by the data presented in Table 1. The improved pore structures of Al-PILC considerably increased their N2 adsorption capacities; moreover, the N2 adsorption capacity of AlFe-PILC is significantly larger than the mononuclear PILC. The same trend is observed for SBET and Vp. This indicates that the composite AlFe polycations that formed were larger than the Al polycations (Keggin ions); therefore, the clay layers were more separated and more porous structures were formed. The addition of Mn and Ce to Na–Mt, Al-PILC, and AlFe-PILC resulted in a decrease in the N2 adsorption capacity, reflecting a reduction in the pore volume. The loss of pore volume indicates that the doped cations entered the pores of the supports.

Figure 3.

Figure 3

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Characteristics of the samples: (a) N2 adsorption–desorption isotherms and (b) pore-size distribution.

Table 1. Characteristics of the Samples: Surface Area and Pore Volume.

samples SBET (m2/g) Amesa (m2/g) Vpb (cm3/g) Vmic (cm3/g)
Na–Mt 51 41 0.076 0.0043
Al-PILC 242 94 0.171 0.050
AlFe-PILC 317 168 0.195 0.077
Mn/Na–Mt 26 25 0.058 0.0032
Mn/AlFe-PILC 200 93 0.147 0.040
MnCe(6:1)/AlFe-PILC 160 82 0.143 0.035
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a

Mesoporous surface area estimated from the Horvath and Kawazoe (HK) model.

b

Pore volumes of the samples were estimated through N2 isotherms at a relative pressure of P/P0 = 0.99.

The average mesopore diameters of the AlFe-PILC materials are approximately 3.96 nm, which is wider than the pore diameters of Al-PILC and Na–Mt; this indicates that the pore size increased during pillaring, leading to a layered mesoporous structure. The addition of Mn and Ce to AlFe-PILC decreased the pore volume. The average pore size of Mn/AlFe-PILC and MnCe(6:1)/AlFe-PILC is approximately 3.89 nm. The mesoporous structure of the AlFe-PILC support was not destroyed, illustrating the excellent performance of AlFe-PILC as a support.

Table 1 summarizes the textural data of the samples. The optimal synthesis conditions were an Al/Fe atomic ratio of 5, pillaring-agent aging time of 12 h, and aging temperature of 120 °C. Under these conditions, the SBET, Vp, and d001 values of AlFe-PILC are 318 m2/g, 0.195 cm3/g, and 1.91 nm, respectively. Compared with the Na–Mt and AlFe-PILC supports, the supported Mn or MnCe catalysts exhibit lower SBET and Vp values. This decrease can be attributed to the migration of some of the Mn and Ce ions into the pores and clay layers, which blocked some of the pores. Meanwhile, SBET and Vp decreased when the catalyst was doped with Ce, possibly because the radius of a Ce ion is much larger than that of a Mn ion, and thus the Ce ion can block more pores.39 Abundant micro-mesoporosity in the AlFe-PILC supports is favorable for the even dispersion of active species and rapid diffusion of reactants, which could greatly enhance their catalytic performance during various reactions.

3.3. HRTEM Analysis

Figure 4 presents the HRTEM images and EDS spectra of the samples. A two-dimensional layered structure with a small basal space is shown in the image of Na–Mt. A regular two-dimensional porous structure is shown in the image of AlFe-PILC. Compared with Na–Mt, the clay layers in AlFe-PILC were widely separated and had larger pore structures, indicating that the regular mesoporous structure was retained during the high-temperature calcination process in the AlFe-PILC preparation. The layered structure of MnCe/AlFe-PILC was not damaged by the loading of the active ingredients, and the active particles were uniformly distributed throughout the support and did not aggregate. The HRTEM images clearly reveal the stable layered structure and the well-dispersed active ingredients in AlFe-PILC. Al, Fe, Mn, Ce, O, and other elements were identified in the EDS spectra, confirming the successful loading of the active species (Mn and Ce) on the surface of AlFe-PILC. These results indicate that AlFe-PILC is an excellent support and that the active species of the catalysts are well-dispersed without aggregation on the support. All of these properties contribute to the excellent catalytic performance of the catalyst in benzene degradation.

Figure 4.

Figure 4

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HRTEM picture of (a) Na–Mt; (b) AlFe-PILC; and (c) MnCe(6:1)/AlFe-PILC and the EDS spectra.

3.4. Catalytic Activity and Stability Tests

Figure 5 presents the conversion of benzene using various catalysts. The Na–Mt-supported Mn catalyst exhibited poor activity, and complete conversion of the benzene did not occur until 480 °C. The Al-PILC-supported Mn catalyst can completely degrade benzene at 380 °C, approximately 100 °C lower than the temperature required for Mn/Na–Mt. The AlFe-PILC-supported Mn catalyst exhibits a significantly improved activity and completely degrades benzene at 330 °C. The addition of various amounts of Ce further improves the catalytic activity of the Mn/AlFe-PILC catalysts. Additionally, the Mn/Ce atomic ratios affect the catalytic activity. MnCe(6:1)/AlFe-PILC exhibits the highest activity and can completely degrade benzene at approximately 250 °C. Compared with the noble metal catalysts or other transition metal oxide catalysts, this catalyst has excellent performance in benzene combustion, as shown in Table S1.39,45−48

Figure 5.

Figure 5

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Light-off curves of benzene combustion during the third reaction cycle. Benzene concentration: 600 ppm; GHSV: 30 000 h–1; and catalyst amount: 200 mg.

Figure 6 shows the T50 values corresponding to three consecutive reaction cycles performed for Mn/Na–Mt, Mn/Al-PILC, Mn/AlFe-PILC, and MnCe(6:1)/AlFe-PILC. Generally, T50 decreases after consecutive cycles for all of the catalysts, and this results in an increase in the catalytic activity. The largest decrease in T50 occurs between the first and second cycles, whereas T50 remains relatively stable after the second cycle. These results indicate the activation of the chemical sites of the catalysts during the first cycle, which may occur during the heating process. The Ce 3d spectra of 10% MnCe(6:1)/AlFe-PILC (fresh and after the first cycle) were detected by XPS, as shown in Figure S1. The u′ and v′ represented the two characteristic peaks of Ce3+, and the other six characteristic peaks were attributed to Ce4+.49,50 The change in the atomic ratio of Ce4+/Ce3+ for the fresh catalysts, (Ce4+/Ce3+ = 4.75) and (Ce4+/Ce3+ = 3.59), for the first cycle indicated that there was a redox reaction that occurred between Ce4+ and Ce3+. A higher Ce3+ concentration can produce more oxygen vacancies on catalysts. The spectra indicate that the used catalyst after the first cycle contains more oxygen vacancies. It is known that CeO2 has been studied because of its ability to store and release oxygen via the redox shift between Ce4+ and Ce3+. In this study, the lattice oxygen ([O]) came from the valence state changes of Ce and Mn, which can be described as 2CeO2 → Ce2O3 + [O] and 2MnO2 → Mn2O3 + [O]; the transfer of O2 from the adsorbed oxygen (O*) to lattice oxygen in metal oxides is O2 → 2O*; Ce2O3 + O* → CeO2; and Mn2O3 + O* → MnO2.51 Considering the results from our studies, the ratio of CeO2/Ce2O3 on the surface of the catalysts plays an important role in determining the activity. Interestingly, the addition of ceria to the MnOx catalyst leads to an increase in the activity of the mixed oxides. MnCe(6:1)/AlFe-PILC is the most active and stable catalyst, which may be attributed to CeO2 because of its high oxygen storage capacity and a facile Ce4+/Ce3+ redox cycle as well as the interaction between CeO2 and MnOx.52−54

Figure 6.

Figure 6

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T50 temperature for three consecutive benzene catalytic combustion cycles.

Figure 7 shows the result of the lifetime test for the most active catalyst (MnCe(6:1)/AlFe-PILC) at 230 °C for 1000 h. The conversion of benzene remained at approximately 90%, with no obvious deactivation, which indicates that the catalyst is stable for benzene degradation. In practice, water and chlorinated VOCs exist in waste gases. As shown in Figure 7, the activity of the catalyst decreases slightly during the first continuous 100 h reaction in the presence of 100 ppm chlorobenzene because of competitive adsorption and oxidation on the active sites. When 10 000 ppm water is introduced, the activity decreases further because of competitive adsorption. The catalytic activity recovers to the initial level after water and chlorobenzene are removed, indicating that the presence of water may promote the removal of coke and Cl species from the catalyst surface. These results indicate that MnCe(6:1)/AlFe-PILC has promise for adoption by industry because of its high catalytic performance for the combustion of both nonchlorinated and chlorinated VOCs and its excellent resistance to water and Cl adsorption.

Figure 7.

Figure 7

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Lifetime test performed for MnCe(6:1)/AlFe-PILC at 230 °C.

3.5. Benzene-TPSR

The catalytic process is a dynamic in situ surface reaction. Thus, to investigate the oxidative performance of the catalysts under dynamic conditions and to obtain additional information on the adsorption–desorption and oxidizing properties of the catalysts during benzene combustion, the evolution of any possible organic byproduct and final product (CO2) on the catalyst surface was evaluated by in situ TPSR. As shown in Figure 8, from 50 to 180 °C and most noticeably at approximately 90–110 °C, the signal of benzene is observed in waste gas, whereas the signal of CO2 is absent, indicating that only the desorption of benzene occurred. Na–Mt, Al-PILC, and AlFe-PILC-supported catalysts differed greatly in their benzene desorption capacity, and the capacity increases in the order of Mn/Na–Mt < Mn/AlFe-PILC < Mn/AlFe-PILC ≈ MnCe(6:1)/AlFe-PILC. These results indicate that the significantly increased pore volume and basal spacing of AlFe-PILC compared with Na–Mt and Al-PILC caused a significant increase in the benzene desorption capacity. The desorption peak of benzene is smaller for Mn/Na–Mt than that for AlFe-PILC-supported catalysts, implying that its adsorption ability is weaker, which is not beneficial for benzene oxidation.44 As the reaction temperature increases, the oxidation of benzene becomes apparent because of the detection of CO2. The benzene adsorbed on the catalyst surface at low temperatures is destroyed at elevated temperatures. The temperature at which the benzene signal disappears increases in the order of MnCe(6:1)/AlFe-PILC < Mn/AlFe-PILC < Mn/Al-PILC < Mn/Na–Mt, which agrees with the order of catalytic activity (shown in Figure 5). The evolution of CO2 is accompanied by the oxidation of benzene. The temperature for the appearance of the CO2 signal increases in the order of MnCe(6:1)/AlFe-PILC < Mn/AlFe-PILC < Mn/Al-PILC < Mn/Na–Mt, which is consistent with the order of the benzene signal disappearance. These results show that the promoted support structure and addition of Ce can significantly promote the catalytic combustion of benzene.

Figure 8.

Figure 8

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Results of TPSR characterization for benzene combustion over the catalysts.

4. Conclusions

In this study, AlFe-pillaring agents were synthesized using a one-step, high-temperature, high-pressure hydrothermal method, followed by the synthesis of AlFe-PILC through ion-exchange. The structure and properties of these materials were characterized. The results indicate that the specific surface area, pore volume, and basal spacing of AlFe-PILC are much larger than those of Na–Mt and that the AlFe-PILC pore structure is superior to Al-PILC, demonstrating that the AlFe-PILC synthesized in this study is a porous material with excellent properties. Preliminary experiments were conducted to investigate the performance of the optimum AlFe-PILC sample for the catalytic combustion of benzene. The results demonstrate that after calcination at 550 °C, the pore structure of the catalyst support remained stable and the supported active ingredients were uniformly dispersed without aggregation. MnCe(6:1)/AlFe-PILC completely degrades a low concentration of benzene at approximately 250 °C, and the conversion remained stable for 1000 h, demonstrating the excellent performance and durability of the catalyst for the catalytic combustion of benzene.

Acknowledgments

We gratefully acknowledge the support provided by the National Natural Science Foundation of China (no. 21577094).

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00592.

  • Main data of reported literatures on the catalytic combustion of benzene over the catalysts, additional information about determination of the change in the atomic ratio of Ce4+/Ce3+, and Ce 3d spectra of 10% MnCe(6:1)/AlFe-PILC (fresh and after the first cycle) (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao7b00592_si_001.pdf (127.3KB, pdf)

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