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Enhanced oxygen vacancies to improve ethyl acetate oxidation over MnOx-CeO2 catalyst derived from MOF template
Accepted Manuscript Enhanced Oxygen Vacancies to Improve Ethyl Acetate Oxidation Over MnOx-CeO2 Catalyst Derived from MOF Template Yiwen Jiang, Jingheng Gao, Qian Zhang, Ziyi Liu, Mingli Fu, Junliang Wu, Yun Hu, Daiqi Ye PII: DOI: Reference:
S1385-8947(19)30707-7 https://doi.org/10.1016/j.cej.2019.03.233 CEJ 21357
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
21 January 2019 22 March 2019 25 March 2019
Please cite this article as: Y. Jiang, J. Gao, Q. Zhang, Z. Liu, M. Fu, J. Wu, Y. Hu, D. Ye, Enhanced Oxygen Vacancies to Improve Ethyl Acetate Oxidation Over MnOx-CeO2 Catalyst Derived from MOF Template, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.03.233
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Enhanced Oxygen Vacancies to Improve Ethyl Acetate Oxidation Over MnOx-CeO2 Catalyst Derived from MOF Template
Yiwen Jiang a, Jingheng Gao a, Qian Zhang a, Ziyi Liu a, Mingli Fu a,b,c*, Junliang Wu a,b,c
, Yun Hu a,b,c, Daiqi Ye a,b,c
a
School of Environment and Energy, South China University of Technology,
Guangzhou 510006, China b
National Engineering Laboratory for VOCs Pollution Control Technology and
Equipment, Guangzhou 510006, China c
Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution
Control, Guangzhou 510006, China
Abstract1 Industrial volatile organic compounds (VOCs) are harmful to the environment and human health. Catalytic oxidation is the most effective method for end-of-pipe control. The challenges of VOC catalytic oxidation are the high space velocities and low oxidation temperatures. In this work, Mn cations were introduced into Ce-BTC supports by an impregnation method, after which the mixture was calcined. The as-synthesized CeO2-supported Mn catalyst MnOx-CeO2-s exhibited an outstanding * Corresponding author Email address: [email protected] (M. Fu). 1
Abbreviations F-OV = Frankel-type oxygen vacancy I-OV = Intrinsic oxygen vacancy 1
activity for ethyl acetate (EtOAc) catalytic oxidation, which reached complete conversion at 210°C (T99 = 210°C), even at a high space velocity (WHSV =60,000 ml g-1 h-1). With in situ UV Raman and other characterization methods, the Frankel oxygen vacancy (F-OV) was determined to be the crucial active site for EtOAc catalytic oxidation. The excellent low temperature activity of MnOx-CeO2-s benefited from the enhanced F-OVs concentration. The high temperature stability was attributed to the stable F-OVs recovery potential of the Ce-BTC-derived CeO2 support. The durability at a high space velocity was the result of the unique crossed channel structure inherited from Ce-BTC. These findings unraveled the crucial role of F-OVs in the EtOAc catalytic oxidation and the value of MOF-derived materials, which highlighted a feasible approach for the design of VOC-removal catalysts.
Keywords: Ethyl acetate oxidation; Sacrificial template method; Oxygen vacancy
1. Introduction VOCs are typical gaseous pollutants. Generally, VOCs are defined as organic compounds that start boiling at less than or equal to 250°C [1-3]. These varieties of VOCs can directly cause harm to human health and the environment. Moreover, they are also precursors of secondary pollutants generated by atmospheric photochemical reactions [4-6]. To prevent VOC pollutions, many techniques have been developed to control the emissions [7, 8]. Among these techniques, catalytic oxidation is the most effective and economical method of industrial VOC removal [9]. 2
In the wake of a series of environmentally friendly policies promoted by the Chinese government, EtOAc has been widely used to replace poisonous benzene thinners in chemical engineering industries. Although the replacement of benzenes is primarily regulated by the emissions and pollution of VOCs, a new problem arose. The large differences in structures between EtOAc and traditional thinners implied that the reaction mechanisms and critical active sites of catalytic oxidation could be vastly different [10-13]. For example, benzene ring opening was determined to be the decomposition step for toluene catalytic oxidation [14]. In contrast, the C=O bond is the strongest bond in the EtOAc molecule and similar oxygenated VOCs [15, 16]. The significant differences in structures between toluene and EtOAc implies that the reaction mechanisms and critical active sites of catalytic oxidation can be significantly different. Hence, a novel, bottom-up designed catalyst for EtOAc is necessary. Cerium is the second element in the lanthanide series, which possesses outstanding oxygen storage and release capacities [17]. Thus, cerium-based materials have been extensively studied and used as the supports of noble or transition metal catalysts. For example, Peng and co-workers have prepared Pt catalysts supported on CeO2 supports for toluene oxidation [18]. For industrial VOC removal, one of the greatest challenges is the high space velocity, which requires high specific surface areas (SSAs) [19]. However, the SSAs of conventional cerium-based supports are insufficient. Metal organic frameworks (MOFs) are porous crystalline materials with periodic multidimensional network structures, formed by coordination-bonded metal ions and 3
organic ligands [20]. In recent years, MOF derivatives, especially MOF-derived metal oxides, have attracted a large amount of attention because of their high SSAs and ample pore structures inherited from the MOF structure [21, 22]. Therefore, MOFs are being used as templates to synthesize metal oxides or complex composites, which could be a promising branch of MOF applications. [21] For instance, Chen and co-workers prepared a CeO2 catalyst through in situ pyrolysis of Ce-BTC for toluene combustion [23]. Mn cations in manganese oxides possesses multiple valance states. Lin and co-workers have reported that the loading of Mn oxides on CeO2 supports can improve the soot oxidation activity [24]. These Mnx+ cations can constitute Lewis acid-base pairs with O2- in the oxides, which is important for VOCs catalytic oxidation [25]. Therefore, Mn is suitable for loaded active components to improve the performance of CeO2 supports. The activities of VOC removal catalysts are primarily attributed to gaseous oxygen activation abilities [26]. Recently, surface oxygen vacancies (OVs) have been widely reported to play crucial roles in gaseous oxygen activation for CO and VOC catalytic oxidation. These defects generated on metal oxides capture oxygen from the ambient atmosphere and transform gaseous oxygen into more activated oxygen species [24]. However, detailed investigations of the surface oxygen vacancies in the system of EtOAc catalytic oxidation are highly challenging, the adsorption and oxidation behaviors of EtOAc and the critical effect of OVs must be further examined. Thus, we upgraded the EtOAc adsorption performances and catalytic activities of CeO2 catalysts in this study. Herein, we report a MnOx-CeO2-s catalyst derived from a 4
Ce-BTC MOF by a sacrificial template method. Manganese oxides were introduced into the Ce-BTC template via wet-impregnation and calcination. The EtOAc catalytic oxidation behavior was tested, and the surface OV concentration of MnOx-CeO2-s was calculated to investigate the crucial active sites of EtOAc catalytic oxidation.
2. Experimental 2.1. Catalyst preparation CeO2-h was synthesized by a hydrothermal method [27]. First, 5 mmol of cerium acetate was dissolved in 20 ml of deionized water, after which 55 ml of a NaOH solution (7 mol L−1) was added. This stock solution was stirred at room temperature for 30 min and transferred to a 100-ml Teflon-lined stainless steel autoclave. Hydrothermal treatment was carried out at 130°C for 5 h. After the treatment was completed, the obtained solid product was washed with deionized water and ethanol several times and dried at 70°C for 12 h. CeO2-s was synthesized by a sacrificial template method. First, 0.01 mol of cerium nitrate was dissolved in 50 ml of deionized water, and 0.01 mol of trimesic acid was dissolved in 150 ml of deionized water and 200 ml of ethanol. The two solutions were added to one beaker and stirred at room temperature for 30 min. The turbid liquid was washed with deionized water and ethanol three times sequentially using vacuum filtration and dried at 120°C for 12 h. The obtained floppy solid was Ce-BTC. CeO2-s was derived from Ce-BTC by calcination at 300°C for 6 h. MnOx-CeO2-h and MnOx-CeO2-s were synthesized by an impregnation method. First, 5
1 g of CeO2-h or 2 g of Ce-BTC was impregnated separately in a mixture of 0.595 mL of MnNO3 solution and 10 mL of ethanol. After stirring for 30 min at a rotation speed of 240 r/min, these two half-finished samples were treated in a shaker at 50°C until they were dry. Two solid samples were obtained after grinding the samples into powder and calcining them at 300°C for 6 h. They were labeled as MnOx-CeO2-h and MnOx-CeO2-s, respectively.
2.2. Activity measurement EtOAc oxidation experiments were performed in a fixed-bed quartz microreactor with a continuous flow of 100 mL min-1, and the concentration was 500 ppm EtOAc + 20% O2/N2 balance. Next, 0.2 g of catalyst was used in the low space velocity (WHSV =30,000 mL g-1 h-1) experiment, and 0.1 g catalyst was used in the high space velocity (WHSV =60,000 mL g-1 h-1) EtOAc conversion, which was analyzed when it reached steady state using online gas chromatography (GC) with a flame ionization detector (A91, PANNA, China). The conversion rate CREtOAc (%) was analyzed using the inlet and outlet concentrations, which were calculated based on the following equation: CREtOAc (%) = 100 × (Cin-Cout) /Cin, where CEtOAc denotes the EtOAc conversion at a certain temperature, and Cin (ppm) and Cout (ppm) denote the concentrations of EtOAc in the inlet and outlet gases, respectively.
6
2.3. Catalyst characterization Nitrogen adsorption-desorption isotherms were collected at 77 K (ASAP 2020, Micromeritics, USA), and the specific surface areas were calculated with the Brunauer-Emmett-Teller (BET) equation. The pore volumes were determined at a relative pressure (P/P0) value of 0.995, and the mean pore diameters were calculated using the Barrett-Joyner-Halenda (BJH) equation from the desorption branches of the isotherms. The loading capacities of manganese in MnOx-CeO2-h and MnOx-CeO2-s were quantified by ICP-MS on an Agilent 7500ce (Agilent, USA). TGA was performed on an STA 449 system (Netzsch, Germany) under dry air flow with a heating rate of 5°C min-1 from room temperature to 600°C. Scanning electron microscopy (SEM) was used with a scanning electron microscope (Merlin, Zeiss, Germany). Transmission electron microscopy (TEM) was utilized with a microscope with an operating voltage of 100 kV (Tecnai G2 f20 s-twin, FEI, USA). X-ray diffraction (XRD) patterns were collected using Cu Kα radiation (λ =0.1542 nm) operating at 40 kV and 200 mA (Empyrea, PANalytical B.V., Holland). The patterns were obtained from 10° to 80° at a scan rate of 10° min-1. X-ray photoelectron spectroscopy (XPS) was carried out with Al Kα excitation source at a constant pass energy (hv = 1486.6 eV), and all the results were adjusted to the binding energy of C1s (284.6 eV) (ESCALAB250, Thermo, USA). UV Raman was obtained in a LabRAM HR Evolution Laser Raman Spectrometer 7
(HORIBA, France) with a CCD detector and a He-Cd laser source (325 nm). The spectrum was recorded two times for 180 s subsequent laser exposure periods.
3. Results and discussion 3.1. Characteristics of Ce-BTC-derived CeO2 support 3.1.1. Activity for EtOAc catalytic oxidation Figure 1a shows the catalytic activity of EtOAc conversion over MnOx-CeO2-h and MnOx-CeO2-s at low space velocity of 30,000 ml g-1 h-1 and temperature from 30°C to 300°C. MnOx-CeO2-s exhibited a better activity than MnOx-CeO2-h, while 99% EtOAc conversion was achieved at 210°C for MnOx-CeO2-s and 230°C for MnOx-CeO2-h, respectively. Furthermore, the activities of CeO2-h, CeO2-s, MnOx-CeO2-h, and MnOx-CeO2-s were tested at a higher space velocity (WHSV =60,000 ml g-1 h-1). The comparison is shown in Figure 1b and Table 1. At the higher space velocity, MnOx-CeO2-s still exhibited the best EtOAc conversion activity among the four catalysts, with the 99% EtOAc conversion temperature (T99) of 210°C, 50% EtOAc conversion temperature (T50) of 188°C, and 10% EtOAc conversion temperature (T10) of 149°C respectively. The activities of the four catalysts were ranked as follows: MnOx-CeO2-s > MnOx-CeO2-h > CeO2-s > CeO2-h. Although MnOx-CeO2-h performed well at a higher space velocity, the 10/50/99% conversion temperature increased by 20°C, while MnOx-CeO2-s preserved its activity despite quadrupling of the space velocity. This stability demonstrated the outstanding performance of the MOFs-derived CeO2 support. 8
To explore the reusability of MnOx-CeO2-s, three consecutive cycles from 30–210°C were carried out, and the results are shown in Figure 1. The EtOAc conversion curve for the first run showed different tendencies than the second and third runs before 180°C. Thus, the property of MnOx-CeO2-s changed after reacting during the first run. However, the curves of all three runs overlapped after 180°C and finally reached complete conversion at the same temperature, indicating that this kind of change did not affect the activity of MnOx-CeO2-s. Additionally, the long-term stability test of MnOx-CeO2-s was also evaluated. As shown in Figure 1, the EtOAc conversion successfully maintained 97% during the 64 h reaction process (reaction conditions: 200 mg catalyst, 500 ppm EtOAc, temperature at 210°C), exhibiting an excellent stability. The reusability and stability test results revealed that the MnOx-CeO2-s prepared by the sacrificial template method displayed stable activities for EtOAc oxidation.
3.1.2. Morphology and structure SEM characterization was employed to exhibit the exterior morphology of CeO2-h, MnOx-CeO2-h, CeO2-s, MnOx-CeO2-s, and Ce-BTC. Figure S1 displays the morphologies of CeO2-h and MnOx-CeO2-h. The hydrothermally synthesized catalysts exhibited agglomerated block structures consisting of CeO2 nanocrystals with diameters of about 10–20 nm. The shape and size of MnOx-CeO2-h were similar to those of CeO2-h, and it can be inferred that low content loading of manganese did not affect the morphology of the CeO2-h. Figure 2 shows the morphologies of CeO2-s 9
and MnOx-CeO2-s, and the sacrificial-template-synthesized catalysts exhibited anomalous rod morphologies and relatively rough surfaces compared to the original Ce-BTC. The widths of CeO2-s and MnOx-CeO2-s were distributed in the range of 100–200 nm. However, the length of MnOx-CeO2-s was in the range of 2–4 µm, which was significantly shorter than the 4–10 µm size range of CeO2-s. The shortening of MnOx-CeO2-s was attributed to the 30-min stirring in manganese. Figure S2 shows SEM images of the original Ce-BTC, which exhibited a microrod structure with a length of 5–10 µm and a width of 200–500 nm. Under 10,000x magnification, the Ce-BTC rods exhibited ordered rectangular morphologies with smooth surfaces, and neither cracks nor fissures were observed. The dispersions of manganese on MnOx-CeO2-h and MnOx-CeO2-s were characterized using energy dispersive X-ray spectroscopy (EDX) elemental mappings. As shown in Figure S3, the spots of Ce, O, and Mn species were uniformly scattered over the samples, demonstrating the good dispersion of Mn. TEM characterization was used to explore the interior structures of CeO2-h, MnOx-CeO2-h, CeO2-s, and MnOx-CeO2-s. As demonstrated in Figure S4, MnOx-CeO2-h and MnOx-CeO2-s exhibited nearly the same internal textures as those of CeO2-h and CeO2-s. No particles or crystals of manganese were observed on the Mn-loaded catalysts, which indicated that manganese was successfully dispersed among the catalysts. The average diameters of the CeO2-h and MnOx-CeO2-h nanocrystals were calculated from several different samples, and both were 15.2 nm. Most of the CeO2-h and MnOx-CeO2-h nanocrystals aggregated together and clung to 10
each other tightly. In contrast, although CeO2-s and MnOx-CeO2-s possessed rod-like shapes, and it can be clearly observed in Figure 3 that these rods were composed of loosely spaced CeO2 nanocrystals. Compared to the smooth structure of Ce-BTC shown in Figure S3, the formation of nanocrystals was the result of calcination, accompanied by the decomposing of Ce-BTC. Meanwhile, interspaces were formed among the CeO2 nanocrystals, which generated penetrated pore channels throughout the microrods. The average diameters of the CeO2-s and MnOx-CeO2-s nanocrystals were both 5.1–5.2 nm, which were three times smaller than the hydrothermally synthesized catalysts, and the average width of the channels was 2.3 nm. The exposed lattice facet sizes were calculated from the lattice fringes of the high resolution TEM images. By measuring the representative interplanar distances of the lattice fringes and matching with the standard cards of the cerium and manganese phases, the primary exposed facets were assigned to CeO2 (220), (222) for CeO2-h; CeO2 (220), (222); Mn2O3 (400), Mn3O4 (112) for MnOx-CeO2-h; CeO2 (220), (222) for CeO2-s; CeO2 (222) and Mn2O3 (400), and Mn3O4 (112) for MnOx-CeO2-s. In conclusion, the catalysts derived from Ce-BTC were properly synthesized using the sacrificial template method. The unique inner channels endowed the MOF-derived catalysts with higher SSAs than the hydrothermally synthesized catalysts, which enhanced the VOC catalytic oxidation activities.
3.1.3. Crystal structure The XRD patterns of CeO2-h, MnOx-CeO2-h, CeO2-s, and MnOx-CeO2-s are shown in 11
Figure 4. The diffraction peaks of all four samples agreed well with the face-centered cubic phase of cerium dioxide (JCPDS No. 34-0394) without exhibiting any signals of Mn species or Ce-BTC, suggesting the high dispersion of manganese and complete decomposition of MOF precursors. The XRD peaks appeared at 2θ = 28.55°, 33.08°, 47.48°, 56.33°, 59.09°, 69.40°, 76.70°, 79.07°, and 88.41°, corresponding to the (111), (200), (220), (311), (222), (400), (331), (420), and (422) planes, respectively. Additionally, no peak position shifting of the Mn-loaded catalysts were observed compared to the Mn-free catalysts, which suggested that manganese oxides were distributed on the surfaces rather than being embedded inside the catalysts. Moreover, the peaks of the MOF-derived catalysts visibly broadened in contrast to the hydrothermally synthesized catalysts. This broadening suggested that the lattice structures were more significantly distorted and the number of defects was greater for the MOF-derived catalysts compared to those of the hydrothermally synthesized catalysts [28].
3.1.4. Thermal stability TGA curves of Ce-BTC and MnOx-CeO2-s are shown in Figure S5. There were two major weight losses of Ce-BTC between ambient temperature and 600°C. The first stage below 170°C was attributed to a desolvation process, while the second sharp weight loss above 350°C corresponded to the collapse of the Ce-BTC framework. For MnOx-CeO2-s, there was no obvious weight loss stage during the test, indicating the completely removal of the Ce-BTC framework. 12
3.1.5. Specific surface area and pore distribution N2 sorption measurements were used to measure the specific surface areas and pore size distributions of CeO2-h, MnOx-CeO2-h, CeO2-s, and MnOx-CeO2-s. As shown in Figure 5 and Figure S6, all the catalysts exhibited typical type ΙV isotherms with H3 hysteresis loops, indicating mesoporous structures. The BET surface areas of CeO2-h, MnOx-CeO2-h, CeO2-s, and MnOx-CeO2-s were 54.70, 52.99, 158.67, 155.93 m2 g-1, respectively. Although the SSAs of the MOF-derived catalysts decreased sharply after the calcination process, they were still threefold higher than those of the hydrothermally synthesized catalysts. The high SSAs of the MOF-derived catalysts can be attributed to the penetrated pore channels. Specifically, after loading with manganese, the SSAs of the catalysts slightly declined by about 2 m2 g-1, which suggested that the loaded Mn did not significantly block the channels or pores in the MOF-derived catalysts. The pore size distribution of CeO2-h, MnOx-CeO2-h, CeO2-s, and MnOx-CeO2-s were calculated using the DFT method. The hydrothermally synthesized catalysts were all mesoporous with irregular sizes distributed from 8.8 to 13.1 nm. As for the MOF-derived catalysts, a combination of micropores (1.4 nm) and mesopores (36.8 nm) were observed, verifying the existence of the hierarchically meso- and microporous structures.
3.2. Effect of OVs on EtOAc oxidation 3.2.1. Crucial active sites of EtOAc oxidation on catalysts 13
OVs play an important role in VOC catalytic oxidation over metallic oxide catalysts.[18, 29] For example, the adsorption and reaction processes of benzenes comply with Mars-van Krevelen mechanism, which can be summarized as follows: (1) the adsorption of a toluene molecule, (2) the adsorption and activation of gaseous oxygen, and (3) the reaction of the adsorbed toluene molecule and active oxygen [30-32]. It has been widely accepted that gaseous oxygen activation is the rate-determining step and corresponds to OVs [33, 34]. However, for EtOAc oxidation, it was reported that OVs mainly act as EtOAc adsorption sites rather than oxygen activation sites [25]. Hence, in situ UV Raman was employed to explore the specific role of OVs in EtOAc oxidation. Commonly, OVs are classified as F-OVs and intrinsic OVs (I-OVs) [35]. In our previous work, F-OVs and I-OVs were found to serve as oxygen suppliers and activators in catalytic oxidation, respectively. F-OVs can provide active interstitial oxygen to the surface of the catalyst and react with the reactant, while I-OVs capture gaseous oxygen from the ambient atmosphere and transforms it into interstitial oxygen [24, 36]. The relative concentrations of F-OVs and I-OVs can be calculated using the ratios of the peak areas of the UV Raman spectra. In this experiment, the relative concentrations of the F-OVs and I-OVs of MnOx-CeO2-s in two different atmospheres were calculated. Initially, pure N2 was introduced to the in situ reactor, after which 300 ppm of EtOAc with an N2 balance was introduced. Data were recorded after the Raman signal was stable. The results are shown in Figure 6a. Compared to pure N2 conditions, the relative F-OVs concentration of MnOx-CeO2-s increased by 72.43% after EtOAc was introduced to 14
the reactor. The increase in the F-OVs can be ascribed to the transfer of Frankel oxygen to the active oxygen species, which generated F-OVs on the catalysts. In the pure N2 atmosphere, the concentration of F-OVs showed no obvious change, indicating that the oxygen species were stable. In the presence of EtOAc, a large number of Frankel oxygens were activated, implying that EtOAc had adsorbed on the surface of the catalyst and reacted with activated Frankel oxygens, which led to the formation of F-OVs. Additionally, the I-OV concentration also increased by 46.44% accompany the F-OVs. The increase in the F-OVs and I-OVs can be explained by the well-known Langmuir-Hinshelwood mechanism (Figure 6b): (1) EtOAc adsorbed on the surfaces of the catalysts, (2) interstitial oxygen reacted with the EtOAc and F-OVs generated at the vacancy sites, (3) lattice oxygen transferred to the F-OVs and were activated to active interstitial oxygen, while the I-OVs were generated at the sites of the original lattice oxygens; and (4) I-OVs replenished gaseous oxygen from the ambient atmosphere (in this experiment, I-OVs could not be replenished in the O2-free atmosphere) [37-39]. In conclusion, Frankel oxygen atoms on the catalysts were determined to be the vital reactants of EtOAc oxidation. As the factory of Frankel oxygen atoms, F-OVs could deeply affect the catalytic activity of EtOAc oxidation.
3.2.2. Surface F-OV concentration and chemical state UV Raman was utilized to calculate the OV concentration of CeO2-h, MnOx-CeO2-h, CeO2-s, and MnOx-CeO2-s. As mentioned above, the OV concentration could be 15
elucidated by the ratios of the peaks areas. In the typical UV Raman spectra of CeO2, three distinct peaks corresponding to OV could be clearly observed. The peak at 465 cm-1 was ascribed to the symmetric stretching vibration mode (F2g) of the cerium fluorite phase. The peak at 600 cm-1 was the defect-induced mode (D) related to F-OVs. The peak at 1165 cm-1 could be attributed to the second-order longitudinal phonon peak [40-42]. The relative concentration of F-OVs and I-OVs could be calculated by the following equations [35]: (1) CF OV A600 / A465 , (2) CI OV ( A1165 A600) / A465 , Where Cx is the concentration of x and Ay is the peak area at the position of y cm-1. The UV Raman spectra and the relative F-OV concentration derived from the equation are shown in Figure 7(A) and (B) and Table 2. The order of the relative F-OV concentration was: MnOx-CeO2-s > MnOx-CeO2-h > CeO2-s > CeO2-h, which was in good agreement with the EtOAc catalytic oxidation activity results, confirming that the F-OVs were crucial sites in the reaction. The Mn-loaded catalysts exhibited a distinctly higher F-OV concentration than those of the Mn-free catalysts, indicating that the surface manganese oxides were highly dispersed and thereby the F-OV concentration of the Ce-based catalysts increased. Additionally, the MOF-derived catalysts also displayed higher F-OV concentrations compared to the hydrothermally synthesized catalysts, which was ascribed to the further progressed lattice distortion and higher SSA inherited from the unique Ce-BTC MOF structure. XPS was used to further investigate the OV concentration, surface composition, and 16
chemical states. Figure 7(C) showed that the Ce 3d XPS spectra of CeO2-h, MnOx-CeO2-h, CeO2-s, and MnOx-CeO2-s. Generally, the Ce 3d spectra could be deconvoluted into ten peaks, where the labels V and U were used to denote five pairs of spin-orbit components assigned to 3d5/2 and 3d3/2. The six peaks denoted V, V′′, V′′′, U, U′′, and U′′′ were ascribed to Ce4+, while the other four peaks labelled V0, V′, U0, and U′ were characteristic peaks of Ce3+ [43, 44]. Remarkably, the conversion of Ce4+ into Ce3+ indicated the effusion of lattice oxygen. Thus, OVs were formed due to effusion, and the ratio of Ce3+/ (Ce3++ Ce4+) can be used to estimate the surface OV concentration. The values calculated by the peak areas are listed in Table 2, where MnOx-CeO2-s represents the highest OV concentration (23.14%), followed by MnOx-CeO2-h (20.28%), CeO2-s (16.20%), and CeO2-h (14.81%). The sequence of the OV concentration agreed with the UV-Raman results, providing further evidence of the L-H mechanism about the F-OVs. Figure 7(D) shows the XPS spectra of Mn 2p. The peak at 641.25eV was attributed to Mn2+/Mn3+, while the other two peaks at 643.57 and 653.26 eV were attributed to Mn4+ [45]. The relative concentration ratio of Mn2+/Mn3+ to Mn4+, deduced by the peak areas, was 1.08, indicating that the surface Mn oxides mainly existed in the formation of MnO, MnO2, and Mn2O3.
3.2.3. F-OV recovery potential The average reaction rates of MnOx-CeO2-h and MnOx-CeO2-s at different temperatures are shown in Figure 8. Notably, MnOx-CeO2-h started to decline at T32 (185°C), while MnOx-CeO2-s maintained a high reaction rate until T75 (195°C), which 17
indicated the durability of MnOx-CeO2-s to high temperatures. To further explore the nature of the durability, in situ Raman was introduced to observe the evolution of the F-OVs, which were the crucial sites for the EtOAc catalytic oxidation activity. The experiment was conducted in an enclosed in situ reactor at 500 ppm EtOAc with a dry air balance. Initially, the UV Raman spectra of MnOx-CeO2-s were recorded under an ambient temperature (25°C) after the signal was stable. Afterward, the reactor was heated to the complete conversion temperature (210°C), and UV Raman spectra were also recorded. MnOx-CeO2-h was tested under the same conditions. The spectra and calculated relative F-OV concentration are shown in Figure 8 and Table 3, respectively. Figure 8 and Table 3 show that the peak area at 465 cm-1 of MnOx-CeO2-s and MnOx-CeO2-h did not fluctuate significantly, while the peak at 600 cm-1 diminished distinctly, indicating a reduction of the F-OVs. The reduction can be attributed to the thermal expansion and mode softening under high temperatures [46]. The F-OV reduction rates of both catalysts were calculated and are listed in Table 3. Notably, the F-OV concentration of MnOx-CeO2-s only reduced by 15% when the temperature increased from 25 to 210°C, while MnOx-CeO2-h decreased by 27%. According to the L-H mechanism, the F-OVs served as transfer intermediates of O2- species, which were generated and vanished sequentially in the reaction. Thus, the difference in the F-OV reduction rate was the difference of the F-OV recovery potential between the MOF-derived and hydrothermally synthesized catalysts. Under heated conditions, the unique structures of the MOF-derived catalysts could resist the thermal effects. Thus, 18
the Frankel sites could recover to F-OVs. The abundant F-OVs of the MOF-derived catalysts ultimately led to the higher activity in the final stage of the reaction.
4. Conclusion This study focused on the synthesis of high-performance MnOx-CeO2-s catalysts based on Ce-MOFs and the relationship between the catalytic EtOAc oxidation activity and surface F-OV concentrations of the catalysts. To satisfy the requirement of the high space velocity condition, a sacrificial template method was used to synthesize the CeO2 support with higher BET SSAs. To enrich the active sites, 7% manganese oxide was introduced to Ce-BTC by impregnation and calcination in air. The BET SSA of the as-synthesized MnOx-CeO2-s was drastically enhanced (155.9 m2 g-1), which was threefold higher than those of the conventional catalysts (MnOx-CeO2-h, 53.0 m2 g-1; CeO2-h, 54.7 m2 g-1). The increase in the SSA led to the superior activity in the high space velocity (60,000 mL min-1 g-1). The loading of Mn only slightly decreased the SSA, but greatly enhanced the activity. The T99 values of the Mn-loaded catalysts (MnOx-CeO2-s, 210°C; MnOx-CeO2-h, 250°C) showed 50–70°C downgrades of the bare supports (CeO2-s, 280°C; CeO2-h, 300°C). EtOAc catalytic oxidation on the cerium-based catalysts followed the L-H mechanism, and F-OVs were found to be crucial sites in the reaction. The unique crossed channel structure of the Ce-BTC-derived support led to its high SSA and F-OVs recovery potential at high temperatures. The loading of manganese oxides could effectively promote the generation of surface F-OVs, which led to the promotion of the EtOAc 19
catalytic oxidation activity. In summary, a high performance catalyst for EtOAc catalytic oxidation was successfully synthesized, which might be a potential candidate for industrial EtOAc removal. Furthermore, the crucial active site was determined, which could serve as a reference for the design of similar catalysts.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 51578245, 51878293, 21777047), the National Key Research and Development Plan (No. 2018YFB0605200), Natural Science Foundation of Guangdong Province, China (Grant No. 2016A030311003) and the Scientific Research Project of Guangzhou City (No. 201804020026). We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.
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Figure 1. Conversion of ethyl acetate over MnOx-CeO2-s and MnOx-CeO2-h. Catalyst: 200 mg; ethyl acetate concentration: 500 ppm. (A) WHSV: 30,000 mL g-1 h-1. (B) WHSV: 60,000 mL g-1 h-1. (C) Reusability test for MnOx-CeO2-s. (D) Stability test for MnOx-CeO2-s.
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Figure 2. SEM images of (A) and (B) CeO2-s and (C) and (D) MnOx-CeO2-s.
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Figure 3a. TEM images of CeO2-s. Subfigure (E) the microrod structure of CeO2-s. (F) The detailed morphology of CeO2-s. (G) The mean size of the nanocrystals in the CeO2-s microrods. (H) The lattice fringe of CeO2-s.
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Figure 3b. TEM images of MnOx-CeO2-s. Subfigure (I) the microrod structure of MnOx-CeO2-s. (J) The detailed morphology of MnOx-CeO2-s. (K) The mean size of the nanocrystals in the MnOx-CeO2-s microrods. (L) The lattice fringe of MnOx-CeO2-s.
31
(111)
(a)
(200)
(220)
(311) (222)
(400)
(331)
(422) (420)
(b) (c) (d)
Figure 4. XRD patterns of (a) CeO2-h, (b) MnOx-CeO2-h, (c) CeO2-s, and (d) MnOx-CeO2-s.
32
Figure 5. (A) BET nitrogen adsorption-desorption isotherms for CeO2-s and MnOx-CeO2-s. (B) Pore width distributions of CeO2-s and MnOx-CeO2-s.
33
Figure 6a. In situ UV Raman spectra for MnOx-CeO2-s in an atmosphere of 500 ppm EtOAc + N2 balance and pure N2.
34
Figure 6b. Supposed Langmuir-Hinshelwood mechanism for EtOAc catalytic oxidation reaction on cerium-based catalysts.
35
Figure 7. (A) UV Raman spectra. (B) Relative F-OV concentration. (C) Ce 3d XPS spectra for (a) CeO2-h, (b) CeO2-s, (c) MnOx-CeO2-h, and (d) MnOx-CeO2-s. (D) Mn 2p XPS spectra for (a) MnOx-CeO2-h and (b) MnOx-CeO2-s.
36
Figure 8. Average reaction rates versus temperature of MnOx-CeO2-h and MnOx-CeO2-s. The average reaction rates were calculated using the difference of the EtOAc conversions divided by the differences of the corresponding temperatures. For instance, average reaction rate at 195°C = (C200-C190)/ (200-190).
37
Figure 9. (A) In situ UV Raman spectra for MnOx-CeO2-s and MnOx-CeO2-h in an atmosphere of 500 ppm EtOAc + air balance at 30 and 210°C. (B) F-OV concentration reduction rate for MnOx-CeO2-s and MnOx-CeO2-h at 30 and 210°C.
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Table 1. Catalytic activities for all catalysts. Tx represent the temperature when the EtOAc conversion rate reached x%. Catalyst: 100 mg; EtOAc concentration: 500 ppm; WHSV: 60,000 mL g-1 h-1 Catalysts
T10(°C)
T50(°C)
T90(°C)
T99(°C)
CeO2-h
220
268
289
300
CeO2-s
214
246
268
280
MnOx-CeO2-h
167
203
228
250
MnOx-CeO2-s
149
188
205
210
39
Table 2. OVs concentration calculated from Raman and XPS for all catalysts. F-OV
OV
A600/A645×102
Ce3+/(Ce3++Ce4+)×100%
MnOx-CeO2-s
112.09
23.14%
MnOx-CeO2-h
96.33
20.28%
CeO2-s
68.94
16.20%
CeO2-h
25.26
14.81%
Catalysts
40
Table 3. F-OVs concentration evolution from 30°C to 210°C calculated from in situ Raman. Catalysts
F-OV (30°C)
F-OV (210°C)
A600/A645×102
A600/A645×102
MnOx-CeO2-h
86.43
63.52
27%
MnOx-CeO2-s
100.36
85.49
15%
41
Reduction rate
42
Highlights 1. MnOx-CeO2 catalyst was successfully synthesized by sacrificial template method. 2. MOF-derived catalyst exhibited the best ethyl acetate catalytic activity. 3. Frankel-type oxygen vacancies are the active sites in ethyl acetate oxidation. 4. MOF-derived catalyst has higher oxygen vacancies recovery potential.
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S1385-8947(19)30707-7 https://doi.org/10.1016/j.cej.2019.03.233 CEJ 21357
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
21 January 2019 22 March 2019 25 March 2019
Please cite this article as: Y. Jiang, J. Gao, Q. Zhang, Z. Liu, M. Fu, J. Wu, Y. Hu, D. Ye, Enhanced Oxygen Vacancies to Improve Ethyl Acetate Oxidation Over MnOx-CeO2 Catalyst Derived from MOF Template, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.03.233
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Enhanced Oxygen Vacancies to Improve Ethyl Acetate Oxidation Over MnOx-CeO2 Catalyst Derived from MOF Template
Yiwen Jiang a, Jingheng Gao a, Qian Zhang a, Ziyi Liu a, Mingli Fu a,b,c*, Junliang Wu a,b,c
, Yun Hu a,b,c, Daiqi Ye a,b,c
a
School of Environment and Energy, South China University of Technology,
Guangzhou 510006, China b
National Engineering Laboratory for VOCs Pollution Control Technology and
Equipment, Guangzhou 510006, China c
Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution
Control, Guangzhou 510006, China
Abstract1 Industrial volatile organic compounds (VOCs) are harmful to the environment and human health. Catalytic oxidation is the most effective method for end-of-pipe control. The challenges of VOC catalytic oxidation are the high space velocities and low oxidation temperatures. In this work, Mn cations were introduced into Ce-BTC supports by an impregnation method, after which the mixture was calcined. The as-synthesized CeO2-supported Mn catalyst MnOx-CeO2-s exhibited an outstanding * Corresponding author Email address: [email protected] (M. Fu). 1
Abbreviations F-OV = Frankel-type oxygen vacancy I-OV = Intrinsic oxygen vacancy 1
activity for ethyl acetate (EtOAc) catalytic oxidation, which reached complete conversion at 210°C (T99 = 210°C), even at a high space velocity (WHSV =60,000 ml g-1 h-1). With in situ UV Raman and other characterization methods, the Frankel oxygen vacancy (F-OV) was determined to be the crucial active site for EtOAc catalytic oxidation. The excellent low temperature activity of MnOx-CeO2-s benefited from the enhanced F-OVs concentration. The high temperature stability was attributed to the stable F-OVs recovery potential of the Ce-BTC-derived CeO2 support. The durability at a high space velocity was the result of the unique crossed channel structure inherited from Ce-BTC. These findings unraveled the crucial role of F-OVs in the EtOAc catalytic oxidation and the value of MOF-derived materials, which highlighted a feasible approach for the design of VOC-removal catalysts.
Keywords: Ethyl acetate oxidation; Sacrificial template method; Oxygen vacancy
1. Introduction VOCs are typical gaseous pollutants. Generally, VOCs are defined as organic compounds that start boiling at less than or equal to 250°C [1-3]. These varieties of VOCs can directly cause harm to human health and the environment. Moreover, they are also precursors of secondary pollutants generated by atmospheric photochemical reactions [4-6]. To prevent VOC pollutions, many techniques have been developed to control the emissions [7, 8]. Among these techniques, catalytic oxidation is the most effective and economical method of industrial VOC removal [9]. 2
In the wake of a series of environmentally friendly policies promoted by the Chinese government, EtOAc has been widely used to replace poisonous benzene thinners in chemical engineering industries. Although the replacement of benzenes is primarily regulated by the emissions and pollution of VOCs, a new problem arose. The large differences in structures between EtOAc and traditional thinners implied that the reaction mechanisms and critical active sites of catalytic oxidation could be vastly different [10-13]. For example, benzene ring opening was determined to be the decomposition step for toluene catalytic oxidation [14]. In contrast, the C=O bond is the strongest bond in the EtOAc molecule and similar oxygenated VOCs [15, 16]. The significant differences in structures between toluene and EtOAc implies that the reaction mechanisms and critical active sites of catalytic oxidation can be significantly different. Hence, a novel, bottom-up designed catalyst for EtOAc is necessary. Cerium is the second element in the lanthanide series, which possesses outstanding oxygen storage and release capacities [17]. Thus, cerium-based materials have been extensively studied and used as the supports of noble or transition metal catalysts. For example, Peng and co-workers have prepared Pt catalysts supported on CeO2 supports for toluene oxidation [18]. For industrial VOC removal, one of the greatest challenges is the high space velocity, which requires high specific surface areas (SSAs) [19]. However, the SSAs of conventional cerium-based supports are insufficient. Metal organic frameworks (MOFs) are porous crystalline materials with periodic multidimensional network structures, formed by coordination-bonded metal ions and 3
organic ligands [20]. In recent years, MOF derivatives, especially MOF-derived metal oxides, have attracted a large amount of attention because of their high SSAs and ample pore structures inherited from the MOF structure [21, 22]. Therefore, MOFs are being used as templates to synthesize metal oxides or complex composites, which could be a promising branch of MOF applications. [21] For instance, Chen and co-workers prepared a CeO2 catalyst through in situ pyrolysis of Ce-BTC for toluene combustion [23]. Mn cations in manganese oxides possesses multiple valance states. Lin and co-workers have reported that the loading of Mn oxides on CeO2 supports can improve the soot oxidation activity [24]. These Mnx+ cations can constitute Lewis acid-base pairs with O2- in the oxides, which is important for VOCs catalytic oxidation [25]. Therefore, Mn is suitable for loaded active components to improve the performance of CeO2 supports. The activities of VOC removal catalysts are primarily attributed to gaseous oxygen activation abilities [26]. Recently, surface oxygen vacancies (OVs) have been widely reported to play crucial roles in gaseous oxygen activation for CO and VOC catalytic oxidation. These defects generated on metal oxides capture oxygen from the ambient atmosphere and transform gaseous oxygen into more activated oxygen species [24]. However, detailed investigations of the surface oxygen vacancies in the system of EtOAc catalytic oxidation are highly challenging, the adsorption and oxidation behaviors of EtOAc and the critical effect of OVs must be further examined. Thus, we upgraded the EtOAc adsorption performances and catalytic activities of CeO2 catalysts in this study. Herein, we report a MnOx-CeO2-s catalyst derived from a 4
Ce-BTC MOF by a sacrificial template method. Manganese oxides were introduced into the Ce-BTC template via wet-impregnation and calcination. The EtOAc catalytic oxidation behavior was tested, and the surface OV concentration of MnOx-CeO2-s was calculated to investigate the crucial active sites of EtOAc catalytic oxidation.
2. Experimental 2.1. Catalyst preparation CeO2-h was synthesized by a hydrothermal method [27]. First, 5 mmol of cerium acetate was dissolved in 20 ml of deionized water, after which 55 ml of a NaOH solution (7 mol L−1) was added. This stock solution was stirred at room temperature for 30 min and transferred to a 100-ml Teflon-lined stainless steel autoclave. Hydrothermal treatment was carried out at 130°C for 5 h. After the treatment was completed, the obtained solid product was washed with deionized water and ethanol several times and dried at 70°C for 12 h. CeO2-s was synthesized by a sacrificial template method. First, 0.01 mol of cerium nitrate was dissolved in 50 ml of deionized water, and 0.01 mol of trimesic acid was dissolved in 150 ml of deionized water and 200 ml of ethanol. The two solutions were added to one beaker and stirred at room temperature for 30 min. The turbid liquid was washed with deionized water and ethanol three times sequentially using vacuum filtration and dried at 120°C for 12 h. The obtained floppy solid was Ce-BTC. CeO2-s was derived from Ce-BTC by calcination at 300°C for 6 h. MnOx-CeO2-h and MnOx-CeO2-s were synthesized by an impregnation method. First, 5
1 g of CeO2-h or 2 g of Ce-BTC was impregnated separately in a mixture of 0.595 mL of MnNO3 solution and 10 mL of ethanol. After stirring for 30 min at a rotation speed of 240 r/min, these two half-finished samples were treated in a shaker at 50°C until they were dry. Two solid samples were obtained after grinding the samples into powder and calcining them at 300°C for 6 h. They were labeled as MnOx-CeO2-h and MnOx-CeO2-s, respectively.
2.2. Activity measurement EtOAc oxidation experiments were performed in a fixed-bed quartz microreactor with a continuous flow of 100 mL min-1, and the concentration was 500 ppm EtOAc + 20% O2/N2 balance. Next, 0.2 g of catalyst was used in the low space velocity (WHSV =30,000 mL g-1 h-1) experiment, and 0.1 g catalyst was used in the high space velocity (WHSV =60,000 mL g-1 h-1) EtOAc conversion, which was analyzed when it reached steady state using online gas chromatography (GC) with a flame ionization detector (A91, PANNA, China). The conversion rate CREtOAc (%) was analyzed using the inlet and outlet concentrations, which were calculated based on the following equation: CREtOAc (%) = 100 × (Cin-Cout) /Cin, where CEtOAc denotes the EtOAc conversion at a certain temperature, and Cin (ppm) and Cout (ppm) denote the concentrations of EtOAc in the inlet and outlet gases, respectively.
6
2.3. Catalyst characterization Nitrogen adsorption-desorption isotherms were collected at 77 K (ASAP 2020, Micromeritics, USA), and the specific surface areas were calculated with the Brunauer-Emmett-Teller (BET) equation. The pore volumes were determined at a relative pressure (P/P0) value of 0.995, and the mean pore diameters were calculated using the Barrett-Joyner-Halenda (BJH) equation from the desorption branches of the isotherms. The loading capacities of manganese in MnOx-CeO2-h and MnOx-CeO2-s were quantified by ICP-MS on an Agilent 7500ce (Agilent, USA). TGA was performed on an STA 449 system (Netzsch, Germany) under dry air flow with a heating rate of 5°C min-1 from room temperature to 600°C. Scanning electron microscopy (SEM) was used with a scanning electron microscope (Merlin, Zeiss, Germany). Transmission electron microscopy (TEM) was utilized with a microscope with an operating voltage of 100 kV (Tecnai G2 f20 s-twin, FEI, USA). X-ray diffraction (XRD) patterns were collected using Cu Kα radiation (λ =0.1542 nm) operating at 40 kV and 200 mA (Empyrea, PANalytical B.V., Holland). The patterns were obtained from 10° to 80° at a scan rate of 10° min-1. X-ray photoelectron spectroscopy (XPS) was carried out with Al Kα excitation source at a constant pass energy (hv = 1486.6 eV), and all the results were adjusted to the binding energy of C1s (284.6 eV) (ESCALAB250, Thermo, USA). UV Raman was obtained in a LabRAM HR Evolution Laser Raman Spectrometer 7
(HORIBA, France) with a CCD detector and a He-Cd laser source (325 nm). The spectrum was recorded two times for 180 s subsequent laser exposure periods.
3. Results and discussion 3.1. Characteristics of Ce-BTC-derived CeO2 support 3.1.1. Activity for EtOAc catalytic oxidation Figure 1a shows the catalytic activity of EtOAc conversion over MnOx-CeO2-h and MnOx-CeO2-s at low space velocity of 30,000 ml g-1 h-1 and temperature from 30°C to 300°C. MnOx-CeO2-s exhibited a better activity than MnOx-CeO2-h, while 99% EtOAc conversion was achieved at 210°C for MnOx-CeO2-s and 230°C for MnOx-CeO2-h, respectively. Furthermore, the activities of CeO2-h, CeO2-s, MnOx-CeO2-h, and MnOx-CeO2-s were tested at a higher space velocity (WHSV =60,000 ml g-1 h-1). The comparison is shown in Figure 1b and Table 1. At the higher space velocity, MnOx-CeO2-s still exhibited the best EtOAc conversion activity among the four catalysts, with the 99% EtOAc conversion temperature (T99) of 210°C, 50% EtOAc conversion temperature (T50) of 188°C, and 10% EtOAc conversion temperature (T10) of 149°C respectively. The activities of the four catalysts were ranked as follows: MnOx-CeO2-s > MnOx-CeO2-h > CeO2-s > CeO2-h. Although MnOx-CeO2-h performed well at a higher space velocity, the 10/50/99% conversion temperature increased by 20°C, while MnOx-CeO2-s preserved its activity despite quadrupling of the space velocity. This stability demonstrated the outstanding performance of the MOFs-derived CeO2 support. 8
To explore the reusability of MnOx-CeO2-s, three consecutive cycles from 30–210°C were carried out, and the results are shown in Figure 1. The EtOAc conversion curve for the first run showed different tendencies than the second and third runs before 180°C. Thus, the property of MnOx-CeO2-s changed after reacting during the first run. However, the curves of all three runs overlapped after 180°C and finally reached complete conversion at the same temperature, indicating that this kind of change did not affect the activity of MnOx-CeO2-s. Additionally, the long-term stability test of MnOx-CeO2-s was also evaluated. As shown in Figure 1, the EtOAc conversion successfully maintained 97% during the 64 h reaction process (reaction conditions: 200 mg catalyst, 500 ppm EtOAc, temperature at 210°C), exhibiting an excellent stability. The reusability and stability test results revealed that the MnOx-CeO2-s prepared by the sacrificial template method displayed stable activities for EtOAc oxidation.
3.1.2. Morphology and structure SEM characterization was employed to exhibit the exterior morphology of CeO2-h, MnOx-CeO2-h, CeO2-s, MnOx-CeO2-s, and Ce-BTC. Figure S1 displays the morphologies of CeO2-h and MnOx-CeO2-h. The hydrothermally synthesized catalysts exhibited agglomerated block structures consisting of CeO2 nanocrystals with diameters of about 10–20 nm. The shape and size of MnOx-CeO2-h were similar to those of CeO2-h, and it can be inferred that low content loading of manganese did not affect the morphology of the CeO2-h. Figure 2 shows the morphologies of CeO2-s 9
and MnOx-CeO2-s, and the sacrificial-template-synthesized catalysts exhibited anomalous rod morphologies and relatively rough surfaces compared to the original Ce-BTC. The widths of CeO2-s and MnOx-CeO2-s were distributed in the range of 100–200 nm. However, the length of MnOx-CeO2-s was in the range of 2–4 µm, which was significantly shorter than the 4–10 µm size range of CeO2-s. The shortening of MnOx-CeO2-s was attributed to the 30-min stirring in manganese. Figure S2 shows SEM images of the original Ce-BTC, which exhibited a microrod structure with a length of 5–10 µm and a width of 200–500 nm. Under 10,000x magnification, the Ce-BTC rods exhibited ordered rectangular morphologies with smooth surfaces, and neither cracks nor fissures were observed. The dispersions of manganese on MnOx-CeO2-h and MnOx-CeO2-s were characterized using energy dispersive X-ray spectroscopy (EDX) elemental mappings. As shown in Figure S3, the spots of Ce, O, and Mn species were uniformly scattered over the samples, demonstrating the good dispersion of Mn. TEM characterization was used to explore the interior structures of CeO2-h, MnOx-CeO2-h, CeO2-s, and MnOx-CeO2-s. As demonstrated in Figure S4, MnOx-CeO2-h and MnOx-CeO2-s exhibited nearly the same internal textures as those of CeO2-h and CeO2-s. No particles or crystals of manganese were observed on the Mn-loaded catalysts, which indicated that manganese was successfully dispersed among the catalysts. The average diameters of the CeO2-h and MnOx-CeO2-h nanocrystals were calculated from several different samples, and both were 15.2 nm. Most of the CeO2-h and MnOx-CeO2-h nanocrystals aggregated together and clung to 10
each other tightly. In contrast, although CeO2-s and MnOx-CeO2-s possessed rod-like shapes, and it can be clearly observed in Figure 3 that these rods were composed of loosely spaced CeO2 nanocrystals. Compared to the smooth structure of Ce-BTC shown in Figure S3, the formation of nanocrystals was the result of calcination, accompanied by the decomposing of Ce-BTC. Meanwhile, interspaces were formed among the CeO2 nanocrystals, which generated penetrated pore channels throughout the microrods. The average diameters of the CeO2-s and MnOx-CeO2-s nanocrystals were both 5.1–5.2 nm, which were three times smaller than the hydrothermally synthesized catalysts, and the average width of the channels was 2.3 nm. The exposed lattice facet sizes were calculated from the lattice fringes of the high resolution TEM images. By measuring the representative interplanar distances of the lattice fringes and matching with the standard cards of the cerium and manganese phases, the primary exposed facets were assigned to CeO2 (220), (222) for CeO2-h; CeO2 (220), (222); Mn2O3 (400), Mn3O4 (112) for MnOx-CeO2-h; CeO2 (220), (222) for CeO2-s; CeO2 (222) and Mn2O3 (400), and Mn3O4 (112) for MnOx-CeO2-s. In conclusion, the catalysts derived from Ce-BTC were properly synthesized using the sacrificial template method. The unique inner channels endowed the MOF-derived catalysts with higher SSAs than the hydrothermally synthesized catalysts, which enhanced the VOC catalytic oxidation activities.
3.1.3. Crystal structure The XRD patterns of CeO2-h, MnOx-CeO2-h, CeO2-s, and MnOx-CeO2-s are shown in 11
Figure 4. The diffraction peaks of all four samples agreed well with the face-centered cubic phase of cerium dioxide (JCPDS No. 34-0394) without exhibiting any signals of Mn species or Ce-BTC, suggesting the high dispersion of manganese and complete decomposition of MOF precursors. The XRD peaks appeared at 2θ = 28.55°, 33.08°, 47.48°, 56.33°, 59.09°, 69.40°, 76.70°, 79.07°, and 88.41°, corresponding to the (111), (200), (220), (311), (222), (400), (331), (420), and (422) planes, respectively. Additionally, no peak position shifting of the Mn-loaded catalysts were observed compared to the Mn-free catalysts, which suggested that manganese oxides were distributed on the surfaces rather than being embedded inside the catalysts. Moreover, the peaks of the MOF-derived catalysts visibly broadened in contrast to the hydrothermally synthesized catalysts. This broadening suggested that the lattice structures were more significantly distorted and the number of defects was greater for the MOF-derived catalysts compared to those of the hydrothermally synthesized catalysts [28].
3.1.4. Thermal stability TGA curves of Ce-BTC and MnOx-CeO2-s are shown in Figure S5. There were two major weight losses of Ce-BTC between ambient temperature and 600°C. The first stage below 170°C was attributed to a desolvation process, while the second sharp weight loss above 350°C corresponded to the collapse of the Ce-BTC framework. For MnOx-CeO2-s, there was no obvious weight loss stage during the test, indicating the completely removal of the Ce-BTC framework. 12
3.1.5. Specific surface area and pore distribution N2 sorption measurements were used to measure the specific surface areas and pore size distributions of CeO2-h, MnOx-CeO2-h, CeO2-s, and MnOx-CeO2-s. As shown in Figure 5 and Figure S6, all the catalysts exhibited typical type ΙV isotherms with H3 hysteresis loops, indicating mesoporous structures. The BET surface areas of CeO2-h, MnOx-CeO2-h, CeO2-s, and MnOx-CeO2-s were 54.70, 52.99, 158.67, 155.93 m2 g-1, respectively. Although the SSAs of the MOF-derived catalysts decreased sharply after the calcination process, they were still threefold higher than those of the hydrothermally synthesized catalysts. The high SSAs of the MOF-derived catalysts can be attributed to the penetrated pore channels. Specifically, after loading with manganese, the SSAs of the catalysts slightly declined by about 2 m2 g-1, which suggested that the loaded Mn did not significantly block the channels or pores in the MOF-derived catalysts. The pore size distribution of CeO2-h, MnOx-CeO2-h, CeO2-s, and MnOx-CeO2-s were calculated using the DFT method. The hydrothermally synthesized catalysts were all mesoporous with irregular sizes distributed from 8.8 to 13.1 nm. As for the MOF-derived catalysts, a combination of micropores (1.4 nm) and mesopores (36.8 nm) were observed, verifying the existence of the hierarchically meso- and microporous structures.
3.2. Effect of OVs on EtOAc oxidation 3.2.1. Crucial active sites of EtOAc oxidation on catalysts 13
OVs play an important role in VOC catalytic oxidation over metallic oxide catalysts.[18, 29] For example, the adsorption and reaction processes of benzenes comply with Mars-van Krevelen mechanism, which can be summarized as follows: (1) the adsorption of a toluene molecule, (2) the adsorption and activation of gaseous oxygen, and (3) the reaction of the adsorbed toluene molecule and active oxygen [30-32]. It has been widely accepted that gaseous oxygen activation is the rate-determining step and corresponds to OVs [33, 34]. However, for EtOAc oxidation, it was reported that OVs mainly act as EtOAc adsorption sites rather than oxygen activation sites [25]. Hence, in situ UV Raman was employed to explore the specific role of OVs in EtOAc oxidation. Commonly, OVs are classified as F-OVs and intrinsic OVs (I-OVs) [35]. In our previous work, F-OVs and I-OVs were found to serve as oxygen suppliers and activators in catalytic oxidation, respectively. F-OVs can provide active interstitial oxygen to the surface of the catalyst and react with the reactant, while I-OVs capture gaseous oxygen from the ambient atmosphere and transforms it into interstitial oxygen [24, 36]. The relative concentrations of F-OVs and I-OVs can be calculated using the ratios of the peak areas of the UV Raman spectra. In this experiment, the relative concentrations of the F-OVs and I-OVs of MnOx-CeO2-s in two different atmospheres were calculated. Initially, pure N2 was introduced to the in situ reactor, after which 300 ppm of EtOAc with an N2 balance was introduced. Data were recorded after the Raman signal was stable. The results are shown in Figure 6a. Compared to pure N2 conditions, the relative F-OVs concentration of MnOx-CeO2-s increased by 72.43% after EtOAc was introduced to 14
the reactor. The increase in the F-OVs can be ascribed to the transfer of Frankel oxygen to the active oxygen species, which generated F-OVs on the catalysts. In the pure N2 atmosphere, the concentration of F-OVs showed no obvious change, indicating that the oxygen species were stable. In the presence of EtOAc, a large number of Frankel oxygens were activated, implying that EtOAc had adsorbed on the surface of the catalyst and reacted with activated Frankel oxygens, which led to the formation of F-OVs. Additionally, the I-OV concentration also increased by 46.44% accompany the F-OVs. The increase in the F-OVs and I-OVs can be explained by the well-known Langmuir-Hinshelwood mechanism (Figure 6b): (1) EtOAc adsorbed on the surfaces of the catalysts, (2) interstitial oxygen reacted with the EtOAc and F-OVs generated at the vacancy sites, (3) lattice oxygen transferred to the F-OVs and were activated to active interstitial oxygen, while the I-OVs were generated at the sites of the original lattice oxygens; and (4) I-OVs replenished gaseous oxygen from the ambient atmosphere (in this experiment, I-OVs could not be replenished in the O2-free atmosphere) [37-39]. In conclusion, Frankel oxygen atoms on the catalysts were determined to be the vital reactants of EtOAc oxidation. As the factory of Frankel oxygen atoms, F-OVs could deeply affect the catalytic activity of EtOAc oxidation.
3.2.2. Surface F-OV concentration and chemical state UV Raman was utilized to calculate the OV concentration of CeO2-h, MnOx-CeO2-h, CeO2-s, and MnOx-CeO2-s. As mentioned above, the OV concentration could be 15
elucidated by the ratios of the peaks areas. In the typical UV Raman spectra of CeO2, three distinct peaks corresponding to OV could be clearly observed. The peak at 465 cm-1 was ascribed to the symmetric stretching vibration mode (F2g) of the cerium fluorite phase. The peak at 600 cm-1 was the defect-induced mode (D) related to F-OVs. The peak at 1165 cm-1 could be attributed to the second-order longitudinal phonon peak [40-42]. The relative concentration of F-OVs and I-OVs could be calculated by the following equations [35]: (1) CF OV A600 / A465 , (2) CI OV ( A1165 A600) / A465 , Where Cx is the concentration of x and Ay is the peak area at the position of y cm-1. The UV Raman spectra and the relative F-OV concentration derived from the equation are shown in Figure 7(A) and (B) and Table 2. The order of the relative F-OV concentration was: MnOx-CeO2-s > MnOx-CeO2-h > CeO2-s > CeO2-h, which was in good agreement with the EtOAc catalytic oxidation activity results, confirming that the F-OVs were crucial sites in the reaction. The Mn-loaded catalysts exhibited a distinctly higher F-OV concentration than those of the Mn-free catalysts, indicating that the surface manganese oxides were highly dispersed and thereby the F-OV concentration of the Ce-based catalysts increased. Additionally, the MOF-derived catalysts also displayed higher F-OV concentrations compared to the hydrothermally synthesized catalysts, which was ascribed to the further progressed lattice distortion and higher SSA inherited from the unique Ce-BTC MOF structure. XPS was used to further investigate the OV concentration, surface composition, and 16
chemical states. Figure 7(C) showed that the Ce 3d XPS spectra of CeO2-h, MnOx-CeO2-h, CeO2-s, and MnOx-CeO2-s. Generally, the Ce 3d spectra could be deconvoluted into ten peaks, where the labels V and U were used to denote five pairs of spin-orbit components assigned to 3d5/2 and 3d3/2. The six peaks denoted V, V′′, V′′′, U, U′′, and U′′′ were ascribed to Ce4+, while the other four peaks labelled V0, V′, U0, and U′ were characteristic peaks of Ce3+ [43, 44]. Remarkably, the conversion of Ce4+ into Ce3+ indicated the effusion of lattice oxygen. Thus, OVs were formed due to effusion, and the ratio of Ce3+/ (Ce3++ Ce4+) can be used to estimate the surface OV concentration. The values calculated by the peak areas are listed in Table 2, where MnOx-CeO2-s represents the highest OV concentration (23.14%), followed by MnOx-CeO2-h (20.28%), CeO2-s (16.20%), and CeO2-h (14.81%). The sequence of the OV concentration agreed with the UV-Raman results, providing further evidence of the L-H mechanism about the F-OVs. Figure 7(D) shows the XPS spectra of Mn 2p. The peak at 641.25eV was attributed to Mn2+/Mn3+, while the other two peaks at 643.57 and 653.26 eV were attributed to Mn4+ [45]. The relative concentration ratio of Mn2+/Mn3+ to Mn4+, deduced by the peak areas, was 1.08, indicating that the surface Mn oxides mainly existed in the formation of MnO, MnO2, and Mn2O3.
3.2.3. F-OV recovery potential The average reaction rates of MnOx-CeO2-h and MnOx-CeO2-s at different temperatures are shown in Figure 8. Notably, MnOx-CeO2-h started to decline at T32 (185°C), while MnOx-CeO2-s maintained a high reaction rate until T75 (195°C), which 17
indicated the durability of MnOx-CeO2-s to high temperatures. To further explore the nature of the durability, in situ Raman was introduced to observe the evolution of the F-OVs, which were the crucial sites for the EtOAc catalytic oxidation activity. The experiment was conducted in an enclosed in situ reactor at 500 ppm EtOAc with a dry air balance. Initially, the UV Raman spectra of MnOx-CeO2-s were recorded under an ambient temperature (25°C) after the signal was stable. Afterward, the reactor was heated to the complete conversion temperature (210°C), and UV Raman spectra were also recorded. MnOx-CeO2-h was tested under the same conditions. The spectra and calculated relative F-OV concentration are shown in Figure 8 and Table 3, respectively. Figure 8 and Table 3 show that the peak area at 465 cm-1 of MnOx-CeO2-s and MnOx-CeO2-h did not fluctuate significantly, while the peak at 600 cm-1 diminished distinctly, indicating a reduction of the F-OVs. The reduction can be attributed to the thermal expansion and mode softening under high temperatures [46]. The F-OV reduction rates of both catalysts were calculated and are listed in Table 3. Notably, the F-OV concentration of MnOx-CeO2-s only reduced by 15% when the temperature increased from 25 to 210°C, while MnOx-CeO2-h decreased by 27%. According to the L-H mechanism, the F-OVs served as transfer intermediates of O2- species, which were generated and vanished sequentially in the reaction. Thus, the difference in the F-OV reduction rate was the difference of the F-OV recovery potential between the MOF-derived and hydrothermally synthesized catalysts. Under heated conditions, the unique structures of the MOF-derived catalysts could resist the thermal effects. Thus, 18
the Frankel sites could recover to F-OVs. The abundant F-OVs of the MOF-derived catalysts ultimately led to the higher activity in the final stage of the reaction.
4. Conclusion This study focused on the synthesis of high-performance MnOx-CeO2-s catalysts based on Ce-MOFs and the relationship between the catalytic EtOAc oxidation activity and surface F-OV concentrations of the catalysts. To satisfy the requirement of the high space velocity condition, a sacrificial template method was used to synthesize the CeO2 support with higher BET SSAs. To enrich the active sites, 7% manganese oxide was introduced to Ce-BTC by impregnation and calcination in air. The BET SSA of the as-synthesized MnOx-CeO2-s was drastically enhanced (155.9 m2 g-1), which was threefold higher than those of the conventional catalysts (MnOx-CeO2-h, 53.0 m2 g-1; CeO2-h, 54.7 m2 g-1). The increase in the SSA led to the superior activity in the high space velocity (60,000 mL min-1 g-1). The loading of Mn only slightly decreased the SSA, but greatly enhanced the activity. The T99 values of the Mn-loaded catalysts (MnOx-CeO2-s, 210°C; MnOx-CeO2-h, 250°C) showed 50–70°C downgrades of the bare supports (CeO2-s, 280°C; CeO2-h, 300°C). EtOAc catalytic oxidation on the cerium-based catalysts followed the L-H mechanism, and F-OVs were found to be crucial sites in the reaction. The unique crossed channel structure of the Ce-BTC-derived support led to its high SSA and F-OVs recovery potential at high temperatures. The loading of manganese oxides could effectively promote the generation of surface F-OVs, which led to the promotion of the EtOAc 19
catalytic oxidation activity. In summary, a high performance catalyst for EtOAc catalytic oxidation was successfully synthesized, which might be a potential candidate for industrial EtOAc removal. Furthermore, the crucial active site was determined, which could serve as a reference for the design of similar catalysts.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 51578245, 51878293, 21777047), the National Key Research and Development Plan (No. 2018YFB0605200), Natural Science Foundation of Guangdong Province, China (Grant No. 2016A030311003) and the Scientific Research Project of Guangzhou City (No. 201804020026). We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.
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Figure 1. Conversion of ethyl acetate over MnOx-CeO2-s and MnOx-CeO2-h. Catalyst: 200 mg; ethyl acetate concentration: 500 ppm. (A) WHSV: 30,000 mL g-1 h-1. (B) WHSV: 60,000 mL g-1 h-1. (C) Reusability test for MnOx-CeO2-s. (D) Stability test for MnOx-CeO2-s.
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Figure 2. SEM images of (A) and (B) CeO2-s and (C) and (D) MnOx-CeO2-s.
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Figure 3a. TEM images of CeO2-s. Subfigure (E) the microrod structure of CeO2-s. (F) The detailed morphology of CeO2-s. (G) The mean size of the nanocrystals in the CeO2-s microrods. (H) The lattice fringe of CeO2-s.
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Figure 3b. TEM images of MnOx-CeO2-s. Subfigure (I) the microrod structure of MnOx-CeO2-s. (J) The detailed morphology of MnOx-CeO2-s. (K) The mean size of the nanocrystals in the MnOx-CeO2-s microrods. (L) The lattice fringe of MnOx-CeO2-s.
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(111)
(a)
(200)
(220)
(311) (222)
(400)
(331)
(422) (420)
(b) (c) (d)
Figure 4. XRD patterns of (a) CeO2-h, (b) MnOx-CeO2-h, (c) CeO2-s, and (d) MnOx-CeO2-s.
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Figure 5. (A) BET nitrogen adsorption-desorption isotherms for CeO2-s and MnOx-CeO2-s. (B) Pore width distributions of CeO2-s and MnOx-CeO2-s.
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Figure 6a. In situ UV Raman spectra for MnOx-CeO2-s in an atmosphere of 500 ppm EtOAc + N2 balance and pure N2.
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Figure 6b. Supposed Langmuir-Hinshelwood mechanism for EtOAc catalytic oxidation reaction on cerium-based catalysts.
35
Figure 7. (A) UV Raman spectra. (B) Relative F-OV concentration. (C) Ce 3d XPS spectra for (a) CeO2-h, (b) CeO2-s, (c) MnOx-CeO2-h, and (d) MnOx-CeO2-s. (D) Mn 2p XPS spectra for (a) MnOx-CeO2-h and (b) MnOx-CeO2-s.
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Figure 8. Average reaction rates versus temperature of MnOx-CeO2-h and MnOx-CeO2-s. The average reaction rates were calculated using the difference of the EtOAc conversions divided by the differences of the corresponding temperatures. For instance, average reaction rate at 195°C = (C200-C190)/ (200-190).
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Figure 9. (A) In situ UV Raman spectra for MnOx-CeO2-s and MnOx-CeO2-h in an atmosphere of 500 ppm EtOAc + air balance at 30 and 210°C. (B) F-OV concentration reduction rate for MnOx-CeO2-s and MnOx-CeO2-h at 30 and 210°C.
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Table 1. Catalytic activities for all catalysts. Tx represent the temperature when the EtOAc conversion rate reached x%. Catalyst: 100 mg; EtOAc concentration: 500 ppm; WHSV: 60,000 mL g-1 h-1 Catalysts
T10(°C)
T50(°C)
T90(°C)
T99(°C)
CeO2-h
220
268
289
300
CeO2-s
214
246
268
280
MnOx-CeO2-h
167
203
228
250
MnOx-CeO2-s
149
188
205
210
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Table 2. OVs concentration calculated from Raman and XPS for all catalysts. F-OV
OV
A600/A645×102
Ce3+/(Ce3++Ce4+)×100%
MnOx-CeO2-s
112.09
23.14%
MnOx-CeO2-h
96.33
20.28%
CeO2-s
68.94
16.20%
CeO2-h
25.26
14.81%
Catalysts
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Table 3. F-OVs concentration evolution from 30°C to 210°C calculated from in situ Raman. Catalysts
F-OV (30°C)
F-OV (210°C)
A600/A645×102
A600/A645×102
MnOx-CeO2-h
86.43
63.52
27%
MnOx-CeO2-s
100.36
85.49
15%
41
Reduction rate
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Highlights 1. MnOx-CeO2 catalyst was successfully synthesized by sacrificial template method. 2. MOF-derived catalyst exhibited the best ethyl acetate catalytic activity. 3. Frankel-type oxygen vacancies are the active sites in ethyl acetate oxidation. 4. MOF-derived catalyst has higher oxygen vacancies recovery potential.
43