Mesoporous Silica-Supported Manganese Oxides for Complete

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Kinetics, Catalysis, and Reaction Engineering

Mesoporous Silica-supported Manganese Oxides for Complete Oxidation of VOCs: the Influence of Mesostructure, Redox Properties and Hydrocarbon Dimension Zhen Wang, Yi Qin, Feng Pan, Zhuo Li, Weidong Zhang, Feng Wu, Dong Chen, Weijia Wen, and Jinjun Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00630 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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Industrial & Engineering Chemistry Research

Mesoporous Silica-supported Manganese Oxides for Complete Oxidation of VOCs: the Influence of Mesostructure, Redox Properties and Hydrocarbon Dimension

Zhen Wang, Yi Qin, Feng Pan, Zhuo Li, Weidong Zhang, Feng Wu, Dong Chen, Weijia Wen, and Jinjun Li*

School of Resources and Environmental Sciences, Wuhan University, Wuhan 430079, China

*Corresponding author.

*Email: [email protected]. Tel./Fax: +86-27-68778936.

1

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

Manganese oxides supported on ordered mesoporous silicas and non-ordered commercial silicas were comparatively studied in terms of physical properties and catalytic behavior in the complete oxidation of propane, n-decane, toluene and p-cymene. Manganese oxides are well-dispersed on large-pore SBA-15, KIT-6, and commercial silicas, whereas agglomerate on small-pore HMS and MCM-41. Mn/SBA-15 contains oxide species with the best reducibility, but its two-dimensional mesopores are significantly blocked. The alkane removal efficiency depends on the amount of accessible manganese oxides; therefore, the three-dimensional Mn/KIT-6 shows better performance than the pore-blocked Mn/SBA-15. In contrast, the ignition of aromatic hydrocarbons, which needs higher temperatures than alkanes, seems more strongly dependent on the reducibility, and Mn/SBA-15 shows the lowest ignition temperatures. Despite of their much smaller surface areas, non-ordered commercial silicas present significant advantages in the catalytic removal of large-molecule hydrocarbons due to the favorable mass transfer in the short mesopores within their thin particles.

Keywords: Volatile organic compounds, Catalytic oxidation, Mesoporous silica, Manganese oxide, Pore topology, hydrocarbon dimension.

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1. INTRODUCTION Volatile organic compounds (VOCs) are a large category of air pollutants, which are harmful 1−3

due to their involvement in the formation of tropospheric ozone and secondary organic aerosol.

4,5

Catalytic oxidation is a widely used method for the elimination of industrial VOCs emission,

and tremendous efforts have been devoted to optimizing catalysts for catalytic oxidation processes. Ordered mesoporous silicas (OMSs) with uniform pore sizes and large surface areas, have been considered as ideal catalyst supports. 11,12

MCM-41

6−8

9

8,10

Typical mesoporous silicas, such as SBA-15, KIT-6,

13

and HMS, have been extensively used to support noble metals and transition metal

oxides for the complete oxidation of VOCs. Generally, SBA-15 and KIT-6 have a mean pore diameter of about 5-10 nm with a well-ordered two-dimensional hexagonal and three-dimensional cubic mesopore system, respectively.

14−16

MCM-41 has a smaller mesopore diameter, of about

2−6 nm, with a well-ordered two-dimensional hexagonal mesopore system, less ordered worm-like mesopores that are much shorter in length.

17,18

while HMS has

19,20

The high specific surface areas and large pore sizes of the ordered mesoporous silicas favor the dispersion of the active phases; besides, the wide pore sizes and special pore topologies of the mesoporous silicas enhance mass transfer during catalytic reactions. According to previous research, the activity of Pd towards the catalytic oxidation of benzene, toluene, and ethyl acetate is 21

higher when the metal is supported on SBA-15 rather than on MCM-41. 21

attributed to the higher dispersion of Pd on the larger-pore SBA-15.

This difference was

Three-dimensional cubic

KIT-6 appeared to be a more favorable support for manganese oxide and copper oxide in the catalytic oxidation of ethyl acetate than two-dimensional SBA-15. 3


22,23

. The cubic KIT-6 ensured


better metal exposure to the reactants than hexagonal SBA-15,24 and the former allowed a higher metal loading satisfying good dispersion and accessibility.25 Pd/HMS showed better performance than Pd/MCM-41, which was attributed to the better accessibility to Pd associated with the 20

wormhole framework and the high textural porosity of HMS.

In addition to the properties of the

catalyst support, the dimensions of the VOCs could also influence their mass transfer in the pore channels of different pore-structured materials.

26

In previously published papers concerning catalytic oxidation of VOCs on mesoporous silica-supported catalysts, OMSs were often undisputedly regarded as good catalyst supports. Some reports were focused on the comparative study of catalysts supported on ordered mesostructured silicas with different pore topologies.14,20−25 However, the practical effects of mesostructural ordering on catalytic behavior were seldom reported.22 In fact, most commercial silicas also possess mesoporous structures but lack mesostructural ordering. The differences in catalytic behavior of catalysts supported on ordered and non-ordered silicas are still ill-defined. Although the mass transfer of reactants in mesopores could depend on their molecular dimension, the model pollutants typically used in studies are popular solvents, such as benzene, toluene and hexane, which do not show significant difference in sizes relative to the mesopore sizes. In contrast, larger-molecular-weight organics, which are less volatile but could also contribute significantly to the formation of secondary organic aerosols, were seldom tested; therefore, the available information is still far from sufficient to draw conclusions regarding the relations between the mesostructures, molecule dimensions and catalytic performance. The aim of this work was to find out whether ordered silicas are more efficient than non-ordered 4


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ones in the catalytic oxidation of VOCs, and to reveal the effect of pore size and pore topology of silica supports, and of molecular dimensions of VOCs on catalytic performances. Manganese oxide was selected as the active phase; four different types of OMSs, including SBA-15, KIT-6, MCM-41 and HMS, and two types of commercial silicas, were used as catalyst supports. Hydrocarbons of different dimensions, namely propane, n-decane, toluene, p-cymene, were tested as model pollutants. 2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Manganese nitrate (50 wt.% aqueous solution), ethanol, n-butyl alcohol, aqueous ammonia, hydrochloric acid, toluene, n-decane, cetyltrimethyl ammonium bromide (CTAB), tetraethyl orthosilicate (TEOS), and octadecylamine (ODA) were supplied by Sinopharm Chemical Reagent Co. Ltd. (China). The Pluronic triblock-co-polymer P123 (EO20PO70EO20) was obtained from Sigma-Aldrich Co., Ltd (USA). p-Cymene was purchased from Energy Chemical Co. Ltd (China). Two commercial silicas, referred to as Silica-B and Silica-O, were supplied by Qingdao Bangkai Hi-tech Materials Co. Ltd. (China) and Qingdao Ocean Chemical Co. Ltd. (China), respectively. Propane (1000 ppm) balanced by air are supplied by Foshan Huate Gas Co. Ltd (China). 2.2 Synthesis of mesoporous silicas and the supported manganese oxide catalysts. Ordered mesoporous silicas, including SBA-15, KIT-6, MCM-41, and HMS, were synthesized using methods reported in literature.

15,16,27,28

The details of the synthesis procedures are reported in the

supporting information. Manganese oxides were loaded onto both the ordered mesoporous silicas and the commercial 5



silicas by the incipient wetness impregnation method using an aqueous manganese nitrate solution. The impregnated solids were finally calcined in a muffle furnace at 450°C for 3 h, obtaining catalysts referred to as Mn/SBA-15, Mn/KIT-6, Mn/MCM-41, Mn/HMS, Mn/Silica-O and Mn/Silica-B, respectively. The manganese loading (weight ratio of manganese to silica) in all the catalysts was 11.8%. 2.3 Catalyst characterization. Powder X-ray diffraction (XRD) patterns were recorded on an X’Pert Pro X-ray diffraction diffractometer (PANalytical, Almelo, the Netherlands) using Cu Kα radiation at a generator voltage of 40 kV and a tube current of 40 mA. Transmission electron microscopy (TEM) images were taken on a JEM-2100 (HR) transmission electron microscope (JEOL, Tokyo, Japan) at an acceleration voltage of 200 kV. The nitrogen adsorption-desorption isotherms of the samples were obtained on a V-Sorb 2800P Surface Area and Porosimetry Analyzer (Gold App, Beijing, China) at liquid nitrogen temperature. Prior to determining the isotherm, the samples were degassed at 150°C in vacuum for 2 h. The Brunauer-Emmett-Teller (BET) specific surface area was calculated using the adsorption data for relative pressures (P/P0) ranging from 0.05 to 0.25. The pore-size distribution curve was deduced based on the adsorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) method. Fourier transform infrared spectra (FTIR) were recorded using a Nicolet 5700 Fourier infrared spectrometer (Nicolet, Wisconsin, USA). X-ray photoelectron spectra (XPS) were recorded on an Escalab 250Xi spectrometer (Thermo Fisher, Massachusetts, USA) using Al Kα radiation (1253.6 eV). The electron binding energy (BE) calibration was based on the C1 s spectrum (Eb = 284.8 eV). 6


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The

temperature-programmed

reduction

in

hydrogen

(H2-TPR)

and

the

temperature-programmed desorption of oxygen (O2-TPD) were tested on a DAS-7000 chemisorption analyzer (Huasi, Changsha, China), using and H2-N2 gas mixture containing 5 vol.% H2, and high-purity He, respectively. In each test, 50 mg of catalyst were placed in a quartz-tube reactor and heated at a rate of 10°C/min under a gas flow rate of 30 mL/min. 2.4 Catalytic activity evaluation. The catalyst activities were evaluated for the oxidation of propane, n-decane, toluene and p-cymene. In each test, 0.1 g of catalyst was placed in a U-tube reactor with an inner diameter of 4 mm. For the oxidation of propane, compressed gas was used as gas source; the inlet pollutant concentration was 1,000 ppm, and the flow rate was controlled at 100 mL/min using a mass flow meter. For the oxidation of the other pollutants, the air flows containing the gaseous pollutants were generated by a micro-injection pump, as reported previously.

29

The flow rates were set to 150 mL/min, and the VOC concentrations were controlled

at 1,000 ppm. An online GC7806 gas chromatograph (Wenling Instrument, Beijing, China) equipped with a flame ionization detector coupled with a methanizer was used to measure the concentrations of VOCs and carbon dioxide in the feed and effluent gases. 3. RESULTS and DISCUSSION 3.1 Characterization of the silicas. The small-angle XRD patterns of the prepared mesoporous 13,15,30,31

silicas (Figure S1) show diffraction peaks that are in agreement with previous reports,

suggesting that they possess ordered mesostructures. This result is confirmed by the TEM images (Figure 1). SBA-15 and KIT-6 have pore diameters of about 8 nm, whereas MCM-41 and HMS have much smaller pore diameters. In contrast, Silica-B and Silica-O seem to be composed of tiny 7



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primary particles, and their aggregation produces abundant disordered inter-particle porosities, with pore channels that are much shorter than those present in the ordered silicas. Furthermore, comparing the images of the OMSs and of the commercial silicas, it is found that the aggregates of OMSs are significantly thicker than those of the commercial silicas. During the self-assembly synthesis of OMSs, the supermolecular organic templates generally exist in long-range ordered stacking, and their strong interactions with silica oligomers induce silica accumulation and 32,33

promote polymerization, leading to the formation of big aggregates.

The nitrogen sorption isotherms and the deduced BJH pore-size distributions of the silicas are shown in Figure S2. Each isotherm shows an apparent increase of nitrogen uptake at relative pressures above 0.25, which corresponds to nitrogen capillary condensation taking place in the mesopores; the hysteresis loop on the nitrogen adsorption-desorption isotherm is also characteristic of mesoporous materials.

34,35

The derived BJH pore size distribution profiles

suggests that the prepared silicas have narrow pore size distributions. In contrast, the pore diameters of commercial silicas are distributed in a wide range, from 3 to 20 nm. The data regarding the textural properties are listed in Table 1. 3.2 Characterizations of the silica-supported manganese oxide catalysts. Figure 2 shows the wide-angle XRD patterns of the supported catalysts. It suggests that the crystal phase of the manganese oxides is primarily formed by tetragonal pyrolusite MnO2 (JCPDS PDF 24-0735). In additon, the FTIR spectra (Figure S3) of the silica-supported manganese oxides show an 36

adsorption band at 588 cm-1, which is also assigned to the Mn-O stretching vibration of MnO2.

The MnO2 crystallite sizes were estimated based on their 101 diffraction peak (37.5°) using the 8


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Scherrer equation, and the corresponding data are shown in Table 2. Overall, the manganese oxides supported on large-pore silicas have smaller crystallite sizes than those supported on small-pore silicas. The reason can be that the larger mesopores favor the infiltration of manganese nitrate into the inner pores, allowing better oxide dispersion. The oxide crystallites sizes of Mn/SBA-15 and Mn/KIT-6 were ~ 8.8 nm and 10.2 nm, respectively, which are similar to their pore diameters, suggesting that the oxide particles were confined in the mesopores, thus limiting aggregation. In contrast, the oxide crystallite sizes of Mn/MCM-41 and Mn/HMS are much larger than their pore sizes, suggesting that many oxide particles are deposited on the outer surfaces. Interestingly, the manganese oxides supported on the commercial silicas feature apparently smaller crystallite sizes, smaller than 8 nm, in comparison to those supported on the OMSs, although the former have much smaller specific surface areas (Table 1). As revealed by the TEM images (Figure 1), the commercial silicas present thinner aggregates and shorter pore channels compared to the OMSs, therefore, it can be inferred that their porosity can be more open to the external environment and their inner surfaces can be more accessible to the impregnation solutions, resulting in better dispersion of the deposited manganese oxides. Figure 3 shows the TEM images of the supported manganese oxide catalysts. For commercial silicas, the loading of manganese oxide does not bring about significant heterogeneity on the surfaces, which might suggest that the manganese oxides are well dispersed; whereas big particles are observed on the external surface of small-pore OMSs. This is in good agreement with the XRD results. Manganese oxide also appears to be well dispersed on SBA-15; nevertheless, the filling of manganese oxides seemed to cause pore blockage. 9



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The nitrogen sorption isotherms and the pore size distributions of the supported manganese oxide catalysts were also obtained (Figure S4), and the derived textural properties are shown in Table 1. After manganese oxide loading, the specific surface area, pore size and pore volume diminish to different extents depending on the types of silicas. In the case of large-pore OMSs, SBA-15 lost more than a half of its specific surface and about 40% of its pore volume, which can be explained by the severe blockage of its two-dimensional mesopores; in contrast, KIT-6 showed a much lower loss of surface areas and pore volume, which can be due to the open environment of three-dimensional mesopores. In the case of small-pore OMSs, the porosity of MCM-41 was reduced significantly due to easy pore blockage; whereas the shorter pore channels of HMS, which can prevent severe pore blockage, maintained the majority of the surface area. Significantly, commercial silicas preserved their porosity better than the OMSs, probably because of their open pore structures. Figure 4 shows the Mn 2p XPS spectra of Mn 2p3/2 and Mn 2p1/2, with binding energies (BEs) of 641.5~642.5 and 653.3~653.9 eV, respectively; the Mn2p3/2 spectra deconvolution revealed two peaks; the peaks at 642.6~643.2 eV are generally assigned to Mn4+, while those at 641.4~642.0 37−39

could be assigned to Mn3+.

22,40

Mn2+ should have a shake-up satellite peak at ~647 eV,

which

does not appear in any of these catalyst. This suggests that the valence states of the surface manganese are mainly Mn3+ and Mn4+. In addition, the Mn 2p BEs are shown in Table 2. The results demonstrate that the Mn 2p BEs of the large-pore silica-supported catalysts are higher than those of the small-pore silica-supported counterparts. This could be attributed to the higher 41

manganese dispersion on the former, which lead to stronger electrostatic fields. 10


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H2-TPR profiles are shown in Figure 5. The reduction is divided into two steps; the peak for low-temperature reduction (LTR) occurring below 350°C is often assigned to the conversion of MnO2 to either Mn2O3 or Mn3O4, while the one for high-temperature reduction (HTR) is attributed 42−44

MnO is stable and cannot be reduced to

45

however, since the observed hydrogen

to the conversion of Mn2O3 or Mn3O4 to MnO.

elemental manganese below 1000°C using H2;

consumption deviated greatly from the calculated value, it was suggested that the variation in the manganese valence state alone is not sufficient to understand the stepwise reduction.

46,47

Nonetheless, the reduction behavior can reflect the oxide reducibility, which is often related to the catalytic oxidation performances. In literature, shoulder peaks were observed on the left side of the LTR peaks, which represent the hydrogen consumption by surface adsorbed oxygen species.48 In this work, the broad leading parts on the left sides of the LTR peaks could be attributed to the labile surface adsorbed oxygen, which are active in hydrocarbon oxidation.49 The wide temperature ranges of the LTR peaks imply the nonuniform surface Mn-O bond energy.49 The hydrogen consumption occurred at lower temperature generally corresponds to oxide species with better reducibility and reactivity. The onset reduction temperatures of the catalysts are marked in Figure 5. The reduction of Mn/SBA-15 begins at 169°C, which is lower than the other catalysts, suggesting that Mn/SBA-15 contains the most reducible manganese oxide species. The hydrogen consumption obtained from the H2-TPR tests is shown in Table 2. Apparently, manganese oxides supported on large-pore-size silicas consume more hydrogen molecules than those supported on small-pore-size HMS and MCM-41, especially during the LTR stage, suggesting that the former have larger amounts of reducible 11



oxides than the latter within the test temperature range, despite the fact that they have the same manganese loading. Based on the manganese loading in this work, the theoretical hydrogen consumptions related to the reduction of MnO2 to MnO is 1.92 mmol/g, which is significantly higher than any of the measured values. It is known that there is a strong interaction between the dispersed oxides and silica supports, with the formation of surface metal silicate species that are quite difficult to 50−52

reduce.

These surface species are not detected by either XRD or TEM. MCM-41 and HMS

have higher specific surface areas, and therefore they could capture more manganese atoms to form surface silicate-like species, resulting in lower hydrogen consumptions by their supported catalysts. It should be mentioned that the onset reduction temperature of Mn/MCM-41 is only slightly higher than the one of Mn/SBA-15, indicating that both catalysts contain highly reducible species, although they are present in lower amount in Mn/MCM-41. O2-TPD profiles are shown in Figure 6, and the derived desorbed oxygen amounts are reported in Table 2. The evolution of oxygen observed at low temperature (

Mesoporous Silica-Supported Manganese Oxides for Complete

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