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Developing an economic and efficient approach for the production of adipic acid is a grand challenge in the chemical industry. Toward this goal, we re...
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Supported Ni−Al−VPO/MCM-41 As Efficient and Stable Catalysts for Highly Selective Preparation of Adipic Acid from Cyclohexane with NO2 Jian Jian,† Kuiyi You,*,†,‡ Qing Luo,† Hongxu Gao,† Fangfang Zhao,† Pingle Liu,†,‡ Qiuhong Ai,†,‡ and He’an Luo*,†,‡ †

School of Chemical Engineering, Xiangtan University, Xiangtan 411105, P.R. China National & Local United Engineering Research Center for Chemical Process Simulation and Intensification, Xiangtan University, Xiangtan 411105, P. R. China

‡

S Supporting Information *

ABSTRACT: Developing an economic and efficient approach for the production of adipic acid is a grand challenge in the chemical industry. Toward this goal, we report a simple method for selective synthesis of adipic acid from cyclohexane with NO2 by using Ni−Al−VPO/MCM-41 as catalyst. The physical-chemical properties of supported Ni−Al−VPO/MCM-4 catalysts were characterized, the reaction conditions were optimized, and the reusability of the catalyst was examined. The results showed that the supported 30%Ni−Al−VPO/MCM-41 catalyst exhibited excellent catalytic performance with 65.1% of cyclohexane conversion and 85.3% of selectivity toward adipic acid, especially, its catalytic performance was still stable after five runs. Maybe this developed method has potential industrial application prospects for production of adipic acid. or ozone and so on.16−20 Among these approaches, one-step conversion of cyclohexane into AA is considered as one of the most simple, economic and promising processes due to cyclohexane is a cheap and readily available organic compound. Therefore, from the industrial application demands, there is an exigent need to develop an efficient method toward such process for highly selective one-step production of AA. Vanadium phosphorus oxide (VPO) is a high-efficient catalyst in the selective oxidation of n-butane to maleic anhydride.21,22 In our previous work, it was found that VPO composites also showed high catalytic performance on the nitrosation of cyclohexane with NOHSO4 to afford caprolactam,23 and the vapor phase or liquid phase nitration-oxidation of cyclohexane with NO2 to obtain AA.24−26 These results

1. INTRODUCTION Adipic acid (AA) is a very important commodity chemical,1,2 it has been mainly used in manufacturing nylon-66, polyurethanes, and plasticizers, etc.3 Currently, the major industrial production of AA from cyclohexane undergoes a two-step process,4 which involves the oxidation of cyclohexane with air to KA oil by using either homogeneous cobalt-based catalyst or no catalyst, and the oxidation of KA oil to AA via the use of 50−65% nitric acid as oxidant.5,6 However, in this process, not only the efficiency is low, but also a large amounts of undesirable nitrous oxide (N2O) (about 300 kg of N2O per tonne of AA) were released, which shows an global greenhouse effect and ozone depletion.7,8 Hence, it is urgent to develop an economic, efficient and environment-friendly method for production of AA. Recently, several alternative routes have been reported for clean production of AA. Such as bis-hydroformylation of butadiene,9,10 synthesis from D-gulcose or mucic acid,11,12 direct conversion of cyclohexene with hydrogen peroxide as oxidant,13−15 one-step oxidation of cyclohexane by dioxygen © 2016 American Chemical Society

Received: Revised: Accepted: Published: 3729

January 2, 2016 March 13, 2016 March 16, 2016 March 16, 2016 DOI: 10.1021/acs.iecr.6b00008 Ind. Eng. Chem. Res. 2016, 55, 3729−3735

Article

Industrial & Engineering Chemistry Research

deionized water was evaporated and the resulting solid was dried at 373 K for 10 h. Finally, the samples were calcined in dry air at 773 K for 5 h. All samples were dried before use. 2.3. Characterization. XRD patterns of catalysts were examined by a Rigaku D/Max-2550VB+ diffractometer, operated at 40 kV and 30 mA using Cu Kα radiation. The BET surface areas were recorded with a Quantachrome Instruments NOVA-2200e analyzer, using N2 adsorption at 77 K. The morphology of samples was observed with TEM on a JEM-2100F (the accelerating voltage is 200 kV). FT-IR spectra were collected on a Nicolet 380 spectrometer using a potassium bromide disk. UV−vis DRS were performed with UV-2550 spectrophotometry using barium sulfate as the reflectance standard. The valence states of elements were analyzed by an XPS (K-Alpha 1063), operated at 12 kV and 6 mA using Al Kα radiation. Binding energies (BEs) for each element were referred to the C 1s signal (284.8 eV). H2-TPR was performed in Quantachrome Instruments CHEMBET 3000. The sample (100 mg) was reduced with a 5 vol % H2 in Ar (30 mL•min−1) from 333 to 1273 K at a rate of 10 K•min−1. 2.4. Typical Experimental Procedures. The catalytic oxidation reaction of cyclohexane with NO2 was performed in a 150 mL stainless steel reactor with a Teflon lining. First, a certain amount of catalyst, cyclohexane and cold liquid NO2 (99.9%) were purchased from Shanghai Chemical and Dalian reagent factory, respectively. MCM-41 was used as purchased, and CBrCl3 was purchased from Aladdin Industrial Corporation. Stainless steel reactor was purchased from apparatus factory of Yantai. Gas chromatography (GC) (Shimadzu, GC-2010Plus) and liquid chromatography (LC) (Agilent 1260) were used as quantitative analysis of products, and Mass spectra (MS) (Shimadzu, GCMS-QP2010Plus) was used as qualitative analysis of products. 2.2. Catalyst Preparation. The Ni−Al−VPO composite was prepared according to our previous procedure.24 The supported Ni−Al−VPO/MCM-41 composite catalysts were prepared by impregnation method. First, the Ni−Al−VPO composite and mesoporous MCM-41 were immersed in deionized water with 1 g solid per 30 mL water, then the suspension was stirred at 298 K for 24 h. After that, the

conversion of cyclohexane(%) =

the mole of cyclohexane reacted × 100% the mole of cyclohexane added

(1)

Figure 1. N2 adsorption−desorption isotherms (A) and pore size distribution curves (B) of MCM-41 and Ni−Al−VPO/MCM-41 catalysts with different loadings. 3730

DOI: 10.1021/acs.iecr.6b00008 Ind. Eng. Chem. Res. 2016, 55, 3729−3735

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Industrial & Engineering Chemistry Research selectivity of adipic acid(%) =

the mole of adipic acid × 100% the mole of cyclohexane reacted

is also can be observed from the pore size distribution in Figure 1B. The pore diameter of 50%Ni−Al−VPO/MCM-41 is significantly smaller than the other low loading samples. The possible reason is that the support MCM-41 pores are plugged by the excess Ni−Al−VPO components, and resulting in the reduction of the pore diameter and pore volume. In addition, the textural properties of several typical samples are summarized in Table 1. Apparently, the specific surface areas of supported Ni−Al−VPO catalysts are remarkable increase compare to the bulk Ni−Al−VPO composite. Moreover, the loading of Ni−Al−VPO has a significant effect on the surface area and pore volume, which decreases with enhanced Ni−Al− VPO loading. The small-angle and wide-angle XRD patterns of all samples are shown in Figure 2. Figure 2A show the diffraction peaks at 2θ = 2.08°, 3.55° and 4.10°, which are characteristic peaks of mesoporous structure of MCM-41.30,31 The Ni−Al−VPO loading has a significant effect on the pore structure of MCM-41 support. The intensity of characteristic peaks is reduced notably with increased Ni−Al−VPO loading. This indicates that the MCM-41 mesopores is partly covered by Ni− Al−VPO species and decreased the ordered of hexagonal mesophase.32 According to the wide-angle XRD patterns in Figure 2B, all of supported Ni−Al−VPO/MCM-41samples show the obvious characteristic diffraction peaks at 2θ = 21.6°, 35.1° and 43.2° corresponding to (VO)2P2O7 phase (JCPDS: 41−0697), and the peaks at 2θ = 23.0°, 26.2° and 30.1°corresponding to β-VOPO4 phase (JCPDS: 27−0948). These results reveal that the Ni−Al−VPO particles were successfully loaded on the MCM-41 surface. The TEM images of MCM-41 and Ni−Al−VPO/MCM-41 catalysts with different loadings, refer to Figure S1 in the Supporting Information (SI). For the samples with low Ni− Al−VPO loading (≤30%), the hexagonal channels can still be clearly observed, which indicates that the active components Ni−Al−VPO particles are well dispersed on the mesoporous MCM-41. However, as the loading increases to 50%, it is clearly found that excess Ni−Al−VPO particles are aggregated around the pore mouths and formed local accumulation, which results in the reduction of BET surface areas and pore diameter. These results are well consistent with the N2 adsorption−desorption data. Therefore, a suitable loading of Ni−Al−VPO composite is important to maintain textural properties of MCM-41. In addition, the other physical-chemical properties of these samples are also characterized by FT-IR (Figure S2 in the SI), UV−vis DRS (Figure S3 in the SI), XPS (Figure S4 in the

(2) selectivity of nitrocyclohexane(%) the mole of nitrocyclohexane = × 100% the mole of cyclohexane reacted

carbon balance(%) =

(3)

total carbon of all products except CO2 total carbon of cyclohexane reacted

× 100%

(4)

In the present reaction system, the liquid phase products are nitrocyclohexane (NCH) with a small amount of cyclohexanol, cyclohexanone, and cyclohexyl nitrate (CHN). The solid phase products are AA with small amount of glutaric acid (GA) and succinic acid (SA). The tail-gases includes NO2, NO and N2 with small amount of CO2 and N2O. CO2 was produced from the formation process of GA and SA.

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. The N2 adsorption− desorption isotherms of all samples are shown in Figure 1A, and the pore size distribution calculated by the BJH method from desorption isotherm are depicted in Figure 1B. The BET surface area, pore volume and mode pore diameter of bulk Ni− Al−VPO, MCM-41 and Ni−Al−VPO/MCM-41 catalysts with different loadings are given in Table 1. As shown in Figure 1A, Table 1. Textural Properties of Bulk Ni−Al−VPO, MCM-41 and Ni−Al−VPO/MCM-41 catalysts with different loadings sample bulk Ni−Al−VPO MCM-41 10% Ni−Al−VPO/ MCM-41 30% Ni−Al−VPO/ MCM-41 50% Ni−Al−VPO/ MCM-41

surface area (m2 g−1)

pore volume (cm3 g−1)

pore diameter (nm)

6.4 937.5 756.0

0.98 0.89

2.75 2.75

490.8

0.51

2.74

277.7

0.20

1.91

all of samples are similar to type IV isotherm. The isotherms reveal a hysteresis loop at P/P0 = 0.3−0.4, which is the characteristic of mesoporous structure materials.27−29 However, this hysteresis loop is almost disappeared for 50%Ni−Al− VPO/MCM-41 sample, which indicates the mesoporous structure of MCM-41 is damaged seriously. This phenomenon

Figure 2. Small-angle (A) and wide-angle (B) XRD patterns of MCM-41 and Ni−Al−VPO/MCM-41 catalysts with different loadings. 3731

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Industrial & Engineering Chemistry Research Table 2. Catalytic Performance of Different Catalysts in the Oxidation of Cyclohexane with NO2a selectivity (%) catalyst

amount (g)

conv. (%)

AA

NCH

SA

GA

CHN

carbon balance

none MCM-41 Bulk Ni−Al−VPO 10%Ni−Al−VPO/MCM-41 30%Ni−Al−VPO/MCM-41 50%Ni−Al−VPO/MCM-41 Ni−Al−VPO+MCM-41b

0.050 0.015 0.150 0.050 0.030 0.050

40.4 41.0 54.9 60.2 65.1 57.8 55.9

78.2 78.9 83.1 84.2 85.3 84.0 82.6

14.8 13.7 12.3 11.3 10.0 11.5 12.4

4.3 4.7 2.9 2.7 3.0 2.8 3.1

1.3 1.6 1.2 1.4 1.3 1.2 1.4

0.2 0.4 0.2 0.1 0.1 0.2 0.1

97.1 97.5 98.4 98.6 98.5 98.8 98.3

a

Reaction conditions: reaction temperature 353 K. reaction time 24 h, cyclohexane 5.04 g and NO2 13.80 g (the molar ratio is 0.2:1). b0.015 g Ni− Al−VPO and 0.035 g MCM-41 were physically mixed.

Figure 3. Influences of molar ratio of reactants (A), reaction temperature (B) and reaction time (C), (D) on the catalytic oxidation reaction.

SI), and H2-TPR (Figure S5 in the SI). These characterization results indicate that vanadium species containing VOPO4 phase (V5+) and (VO)2P2O7 phase (V4+) are present in the supported Ni−Al−VPO/MCM-41catalysts. 3.2. Comparison on Catalytic Performance of Various Catalysts. The catalytic performances of MCM-41, bulk Ni− Al−VPO and Ni−Al−VPO/MCM-41 with different loading catalysts were examined in the selective oxidation of cyclohexane with NO2, and results were listed in Table 2. Compared to noncatalytic reaction, cyclohexane conversion and AA selectivity were hardly changed over pure MCM-41, which indicated that the support MCM-41 had no any catalytic activity in the oxidation of cyclohexane with NO2. However, the cyclohexane conversion was obviously enhanced as the bulk Ni−Al−VPO catalyst was introduced to reaction system. Moreover, compared to the bulk Ni−Al−VPO at the same amount of Ni−Al−VPO components, the cyclohexane conversion was further improved over the supported Ni−Al− VPO/MCM-4 catalyst, which showed that the catalytic

performance of active Ni−Al−VPO components could be improved efficiently by means of loading. This is because the Ni−Al−VPO composites as active components were well dispersed on MCM-41 support, which resulted in the full exposure of active sites. Therefore, the catalytic performance of supported catalyst was obviously enhanced. Furthermore, catalyst loading is also a very important factor for the catalytic activity. The cyclohexane conversion was increased from 60.2% to 65.1% as the active component Ni−Al−VPO loading was elevated from 10% to 30%. This is due to increasing the loading is equal to the enhancement of active components on MCM41. However, further elevating the active component Ni−Al− VPO loading (e.g., 50%) has not improved the cyclohexane conversion. Maybe the possible reason is that the excess Ni− Al−VPO particles would be aggregated, and the dispersity on MCM-41 was decreased, thus the catalytic performance was not further improved. In addition, the catalytic performance was similar to 0.015 g sole bulk Ni−Al−VPO as 0.015 g bulk Ni− Al−VPO and 0.035 g MCM-41 by physical mixing was added 3732

DOI: 10.1021/acs.iecr.6b00008 Ind. Eng. Chem. Res. 2016, 55, 3729−3735

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Industrial & Engineering Chemistry Research

and the results indicated that the optimal amount of 30%Ni− Al−VPO/MCM-41 is 0.05 g. 3.4. Recycle of 30%Ni−Al−VPO/MCM-41 Catalyst. The recycle and reuse of 30%Ni−Al−VPO/MCM-41 catalyst were tested in this heterogeneous-catalyzed reaction. First, the spent catalyst was recovered by filtration, then washed with distilled water and dried at 383 K. The obtained catalyst was reused in the next run under the same conditions. The catalytic performance of recovered catalyst were shown in Figure 4. It

to the reaction system. These results fully testify that there is a loading effect between Ni−Al−VPO and MCM-41. Among these catalysts, 30%Ni−Al−VPO/MCM-41 exhibited the best catalytic performance with 65.1% of cyclohexane conversion and 85.3% of AA selectivity. 3.3. Optimization of Reaction Conditions. The reaction conditions for selective oxidation of cyclohexane with NO2 over 30%Ni−Al−VPO/MCM-41 were optimized, and the results were shown in Figure 3. Figure 3A depicted the influence of the molar ratio of reactants (cyclohexane: NO2) on the conversion and selectivity at 353 K for 24 h. The cyclohexane conversion increased remarkably from 21.9% to 78.2% as the molar ratio was reduced from 1:1 to 0.1:1. The selectivity to AA increased gradually, while the selectivity to NCH and CHN showed the decrease trend with the reduced molar ratio. Moreover, the selectivity of GA and SA sharply increased as the molar ratio reached to 0.1:1. In this catalytic reaction process, the nitration and oxidation of cyclohexane with NO2 over Ni−Al−VPO should happen simultaneously, the formed HNO2 maybe plays a critical role for forming AA in the oxidation reaction. This means increasing the concentration of NO2 would enhance the concentration of HNO2, which is helpful to promote the oxidation of activated cyclohexane to AA. However, the higher concentration of HNO2 may lead to produce more GA and SA. Therefore, selecting an appropriate ratio of reactants was important to regulate the product selectivity. In the present reaction conditions, the suitable molar ratio is 0.2:1. The effect of reaction temperature on the conversion and selectivity in the reaction time of 3 h was depicted in Figure 3B. The conversion of cychohexane rapidly increased from 3.8% at 333 K to 57.7% at 373 K, while the selectivity to AA was first increasing and then decreasing, and reached a maximum at 353 K. The possible reason is that the active components Ni−Al− VPO (V5+) are more easy to activate cyclohexane at higher temperature, so the cyclohexane conversion is improved significantly with raising the reaction temperature. Furthermore, higher cyclohexane conversion would lead to the higher concentration of HNO2, which is benefit to promote the oxidation of activated cyclohexane to AA, at the same time, too high temperature would lead to high yields of byproducts SA and GA. Therefore, from the point of view of selectivity to AA, the suitable reaction temperature was selected at 353 K. Figure 3C and D showed the influence of reaction time on the oxidation reaction over 30%Ni−Al−VPO/MCM-41 at 353 K. It is well observed that the cyclohexane conversion and yields of AA and NCH increase gradually with the prolonged reaction time. Obviously, the selectivity to AA increased gradually while the selectivity to NCH decreased slowly with the reaction time within 24 h. Continue to extend the reaction time to 36 h, the increased trend in conversion of cyclohexane was not obvious, but the selectivity to AA showed a decrease trend. Meanwhile, the amounts of byproducts SA and GA were increased clearly in the later reaction stage. These results may be still related to HNO2 concentration in this catalytic oxidation reaction. The AA selectivity is somewhat lower because of a lower HNO2 concentration in the earlier stage. The concentration of HNO 2 gradually increases with prolonging the reaction time, the selectivity to AA is also improved gradually. However, too high concentration of HNO2 also resulted in forming of more byproducts SA and GA. Therefore, from the points of view of selectivity to AA, the favorable reaction time should be 24 h. In addition, for the study on catalyst amount of catalyst, refer to Table S1 in the SI,

Figure 4. Recycle of 30%Ni−Al−VPO/MCM-41 catalysts in the oxidation reaction. Reaction conditions: reaction temperature 353 K, reaction time 24 h, cyclohexane 5.04 g, NO2 13.80 g, catalyst 0.05 g. The other products include SA, GA, and small amount of CHN.

is clearly seen that the catalytic performance is stable in the five runs, the selectivity to AA is maintained ca. 85% with 65% of cyclohexane conversion. The results demonstrated that the supported 30%Ni−Al−VPO/MCM-41 catalyst exhibited remarkable stability and reusability in the oxidation reaction. 3.5. Tail-gases Analysis under the Different Reaction Conditions. In this work, the generated tail-gases in this oxidation process under the different conditions were analyzed by GC with TCD detector. The obtained results were summarized in Table S2 in the SI. To our delight, the yield of N2O in this oxidation reaction is only 0.03 t/t AA, which is evidently lower than the traditional nitric acid oxidation method (0.3 t/t AA).4 Moreover, the yield of N2O would be less by reducing temperature or shortening reaction time. It is worth noting that the yield of N2 was significantly reduced as the molecular oxygen was added this reaction system. The possible reason is that the reduction reaction of NO to N2 was inhibited in the presence of oxygen and NO was easy to be converted to NO2. Furthermore, we had demonstrated that N2 was originated from the reduction of NO by the oxidation reaction experiment of cyclohexane and NO. 3.6. Possible Catalytic Reaction Pathway over Ni−Al− VPO/MCM-41 Catalyst. In this work, the possible reaction pathway for the oxidation of cyclohexane with NO2 over Ni− Al−VPO/MCM-41 catalyst was suggested in Scheme Scheme 2 according to our previous work.24,25 First, cyclohexane was activated by the V5+ species in the Ni−Al−VPO/MCM-41 catalyst, maybe cyclohexyl radical could be generated in this process. It is reported that V5+ species maybe can activate C−H bond from alkanes.33,34 Then the V4+ species contained hydrogen was oxidized to V5+ species by NO2 again, which resulted in the formation of nitrous acid (HNO2). At the same 3733

DOI: 10.1021/acs.iecr.6b00008 Ind. Eng. Chem. Res. 2016, 55, 3729−3735

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Industrial & Engineering Chemistry Research Scheme 2. Schematic Reaction Pathway for the Formation of AA

time, cyclohexanol would be formed by the generated cyclohexyl radical reacted with HNO2, and was multistep oxidized to AA. In order to confirm whether the oxidation of cyclohexane with NO2 is also a radical process, we have performed the radical capture experiment by using CBrCl3 as the radical capture reagent.35 The results are shown in Table S3 in the SI. From the Entry 1, it is clearly found that cyclohexane is hard to react with CBrCl3 in the absence of catalyst under the present reaction conditions. However, the bromo-cyclohexane and chloro-cyclohexane can be obtained in the presence of NO2 (see Entry 2), that is to say, cyclohexyl radical would be generated in this oxidation process of cyclohexane with NO2, which is trapped by CBrCl3 immediately. This result maybe can prove the radical mechanism of cyclohexane oxidation with NO2 is possible. Furthermore, we have explored the function of Ni−Al−VPO/MCM-41 catalyst, and the result shows that the high yields of bromo-cyclohexane and chloro-cyclohexane are also obtained in the absence of NO2 (see Entry 3), which indicates that cyclohexane may be activated by Ni−Al−VPO/ MCM-41 catalyst to form cyclohexyl radical. This maybe can explain the cyclohexane conversion is obviously improved as Ni−Al−VPO/MCM-41 catalyst is introduced to this oxidation process. The reaction mechanism will be further studied in the future work.

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AUTHOR INFORMATION

Corresponding Authors

*(K.Y.) Phone: +86-731-58293545; fax: +86-731-58298267; email: [email protected] . *(H.L.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21276216), Specialized Research Fund for the Doctoral Program of Higher Education (20124301130001), Project of Technological Innovation & Entrepreneurship Platform for Hunan Youth (2014) and Hunan Provincial Innovation Foundation for Postgraduate (CX2014B268).

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4. CONCLUSIONS In conclusion, we have developed an efficient method for direct synthesis of AA from the selective oxidation of cyclohexane with NO2 by using supported Ni−Al−VPO/MCM-41 as catalyst in this work. The physical-chemical properties of supported Ni−Al−VPO/MCM-4 catalysts were characterized by BET, XRD, FT-IR, UV−vis DRS, XPS, and H2-TPR. The results showed that the active components of catalyst were well dispersed on the MCM-41 support. Under the present reaction conditions, 65.1% of cyclohexane conversion and 85.3% of selectivity toward adipic acid were achieved over 30%Ni−Al− VPO/MCM-41, and its catalytic performance was still stable after five runs. The catalytic efficiency of supported catalyst was better than our previous bulk Ni−Al−VPO composites catalyst. Maybe the present method has potential industrial application prospects for production of adipic acid.

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S4) and H2-TPR profiles (Figure S5) of MCM-41 and Ni−Al−VPO/MCM-41 catalysts with different loadings; Effects of the amount of catalyst on the catalytic oxidation reaction (Table S1), tail-gases analysis under the different reaction condition (Table S2) and the results of radical capture experiment (Table S3) (PDF)

REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00008. TEM images (Figure S1), FT-IR spectra (Figure S2), UV−vis DRS spectra (Figure S3), XPS spectra (Figure 3734

DOI: 10.1021/acs.iecr.6b00008 Ind. Eng. Chem. Res. 2016, 55, 3729−3735

Article

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DOI: 10.1021/acs.iecr.6b00008 Ind. Eng. Chem. Res. 2016, 55, 3729−3735