Catalytic Aerobic Oxidation of Acetaldehyde over Keggin-type Molybdovanadophosphoric Acid/SBA-15...

Chinese Journal of Chemistry, 2006, 24, 1001—1005 Full Paper

* E-mail: [email protected], [email protected]; Fax: 86-21-65641740 Received September 29, 2005; revised March 3, 2006; accepted April 26, 2006. Project supported by the National Natural Science Foundation Committee of China (Nos. 20371013, 20273017, 20421303) and the Major State Basic

Research Development Program of China (No. 2003CB615807).

© 2006 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Catalytic Aerobic Oxidation of Acetaldehyde over Keggin- type Molybdovanadophosphoric Acid/SBA-15 under

Ambient Condition

ZHOU, Yan(周琰) YUE, Bin*(岳斌) BAO, Ren-Lie(包任烈) LIU, Shi-Xi(刘世熙) HE, He-Yong*(贺鹤勇)

Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, China

Keggin-type molybdovanadophosphoric acids (HPA), H4PMo11VO40 (1), H5PMo10V2O40 (2) and H6PMo9V3O40 (3) were anchored onto γ-aminopropyltriethoxysilane (APTS) aminosilylated silica mesoporous SBA-15 through acid-base neutralization and the resulting HPA/APTS/SBA-15 were characterized by BET, TEM, XRD, ICP, FTIR and 31P MAS NMR. The characterization results indicate that the Keggin-structure of these HPAs is preserved within the mesoporous silica host. The samples were tested for catalytic aerobic oxidation of acetaldehyde hetero-geneously in liquid phase under ambient condition. The electrostatic force between heteropoly acid and amino groups grafted on the silica channel surface leads to strong immobilization of HPA inside SBA-15 which is against the leaching during the reaction. The good catalytic performance and easy recycle of these catalysts make them as potential environmental friendly catalysts for elimination of indoor air pollutants.

Keywords Keggin-type molybdovanadophosphoric acid, aminosilylation, mesoporous SBA-15, acetaldehyde, oxidation, ambient condition

Introduction

Air pollutants such as acetaldehyde, formaldehyde, hydrogen sulfide, methyl mercaptan, ammonia, trimethylamine, and nicotine, are harmful to human health. Several systems have been designed to eliminate the air pollutants by physisorption, photocatalytic deg- radation or oxidation reaction.1-3 For the most widely used physical adsorption systems, the adsorbents would be saturated with the molecules adsorbed and the sec- ondary pollution is inevitable in the process for regen- eration of the adsorbents. And for photocatalytic degra- dation, UV light with high energy is necessary. However, the application of UV-light leads to high cost and is in- applicable for common indoor environment. Recently, the development of novel and effective catalysts for the destruction of the air pollutants is of great interest.4-7 In view of application, the method of catalytic aerobic oxidation under ambient condition will certainly be po- tentially useful for air purification.

Polyoxometalates have been extensively studied as oxidative and acidic catalysts in homogeneous and heterogeneous systems.8-11 The immobilization of polyoxometalates on different supports has made them the effective catalysts in environmental purification.12-14 Aldehydes are among the most deleterious and foul-

smelling volatile organic compounds in indoor air. Hill et al.7 reported that polyoxometalate-modified fabrics can be used as heterogeneous catalyst for oxidation of acetaldehyde by oxygen under ambient condition. In previous studies, we found an approach to obtain nanowires of metal oxides using APTS aminosilylated silica mesoporous SBA-15 as template and polyoxometalate as precursor.15-17 Here, we report the immobilization of three molybdovanadophosphoric acids inside aminosilylated SBA-15 channels, which are employed as catalyst in the aerobic oxidation of acetaldehyde by molecular oxygen under ambient condition.

Experimental

Preparation

Three Keggin-type molybdovanadophosphoric acids (HPA), H4PMo11VO40 (1), H5PMo10V2O40 (2) and H6PMo9V3O40 (3), were prepared according to the lit- erature methods.19 SBA-15 was synthesized using P123 as template and tetraethyl orthosilicate (TEOS) as silica source under acidic condition.20,21 Details of aminosilylation of SBA-15 and immobilization procedure have been reported elsewhere.16,17

1002 Chin. J. Chem., 2006, Vol. 24, No. 8 ZHOU et al.


Characterization

Element analysis was performed by using a Thermo Elemental IRIS Intrepid ICP-AES spectrometer. 31P MAS NMR measurements were carried out on a Bruker Advance DSX 300 spectrometer. IR spectra were re- corded on an AVATAR-360 fourier transform infrared instrument. TEM characterization was performed with a Jeol JEM 2011 electron microscope. N2 adsorption ex- periments were performed on a Micromeritics Tristar 3000 apparatus.

Catalytic reaction

In a typical catalytic test, 0.1 g of powdered catalyst and 20 mL of 1,2-dichloroethane were placed in a 25-mL flask sealed with a septum stopper and linked with a 500-mL O2 balloon through a screw cap. While the suspension was stirring, 1 mL of acetaldehyde was added into the mixture to initiate the reaction. At the end of the reaction, the flask was cooled in an ice bath before the aliquots were extracted and analyzed by GC. The mineralization rate of acetaldehyde is calculated according to the following formulae:

Mineralization rate＝[Acetaldehyde] [Org]

[Acetaldehyde]i f

i

－

×100%

where [Acetaldehyde]i represents the initial concentra- tion of acetaldehyde in the solvent, [Org]f the final con- centrations of organics other than the solvent remaining in the solution. The mineralization rate also equals theo- retically half of the yield of carbon dioxide.

Results and discussion

Characterization of the catalysts

As shown in Figure 1, the typical nitrogen sorption isotherms of 2/APTS/SBA-15 belong to the type IV, which is typical for mesoporous materials. After aminosilylation and immobilization of 2, no change in the isotherm type is observed, proving the conservation of the host framework, whereas the change from H1 to H2 hysteresis indicates a partial filling of the pores. This leads to the decrease in the mean pore size, BET surface area and pore volume of SBA-15 from 7.78 nm, 674 m2/g and 1.25 cm3/g, respectively, to 5.99 nm, 195 m2/g and 0.343 cm3/g of 2/APTS/SBA-15. The 70% reduc- tion of the pore volume indicates that the aminosilylation and immobilization of 2 inside the pore channels of SBA-15.17,22 The same results can be drawn from the other two samples, 1/APTS/SBA-15 and 3/APTS/SBA- 15, and the N2 sorption data are listed in Table 1. In Figure 2, the exemplified TEM images of the 2/APTS/ SBA-15 further demonstrate that the hexagonal ordered channel structure of SBA-15 has no significant change after aminosilylation of SBA-15 and immobilization of 2 into the channels. The polyoxometalates filled in the pores of SBA-15 are shown by the highly contrast lines in Figure 2(a) in comparison with pure SBA-15.

Figure 1 N2 adsorption-desorption isotherm of 2/APTS/SBA- 15. Insert: pore size distribution.

Figure 2 TEM images of 2/APTS/SBA-15 perpendicular (a) and parallel (b) to the channel direction of SBA-15.

Figure 3 shows the XRD patterns of SBA-15, APTS/SBA-15, and 2/APTS/SBA-15 in the 2θ range of 0.6°—10°, All samples exhibit three peaks that can be indexed as characteristic (100), (110) and (200) diffractions of hexagonal mesoporous SBA-15, which indicates that these samples consist of well-ordered chan-nels. Therefore, the primary structure of SBA-15 is maintained after aminosilylation and immobilization of 2. The immobilization of 2 inside SBA-15 channels re-sults in a rather significant decrease in the intensity of all diffractions, which is probably attributed to the pore

Keggin-type molybdovanadophosphoric acid Chin. J. Chem., 2006 Vol. 23 No. 8 1003


Table 1 Physicochemical properties of SBA-15, APTS/SBA-15 and HPA/APTS/SBA-15

Mo/V molar ratio Sample Surface area/(m2•g－1) Pore diameter/nm Pore volume/(cm3•g－1)

Anal. Calc. HPA loading/wt%

SBA-15 674 7.78 1.250

APTS/SBA-15 333 6.72 0.639

H4PMo11VO40/APTS/SBA-15 198 5.92 0.444 12.20 11 15.4

H5PMo10V2O40/APTS/SBA-15 195 5.99 0.343 4.94 5 17.3

H6PMo9V3O40/APTS/SBA-15 196 6.03 0.355 3.04 3 12.3

filling of the host material with 2 and higher absorption factors of V and Mo atoms for X-ray. In addition, the peaks of (100), (110), and (200) diffractions of APTS/SBA-15 and 2/APTS/SBA-15 slightly shift to higher angle in comparison with those for SBA-15, which accounts for the contraction of the host frame-work during the surface modification and immobiliza-tion of 2 inside the channels. The physical parameters of SBA-15 after immobilization of HPA are also shown in Table 1.

Figure 3 Low-angle XRD patterns of SBA-15 (a), APTS/SBA- 15 (b), and 2/APTS/SBA-15 (c).

The molar ratio of Mo∶V detected by ICP remains unchanged after the immobilization of HPA and the loading of HPA is in the range of 12—17 wt% (Table 1). After the immobilization of HPA, the corresponding polyoxoacid remaining in the solution was recrystallized and the crystals still have the Keggin structure with the same molar ratio as the starting species, indicating that no decomposition or change of HPA occurred during the immobilization course. Moreover, IR spectra of 2/ APTS/SBA-15 exhibit characteristic vibration bands at 1060, 960, 850, 800 cm－1, which are similar to that of bulk 2, indicating that no observable change of the structure and the composition of 2 during the immobilization. IR spectra of 1/APTS/SBA-15 and 3/APTS/ SBA-15 exhibit characteristic vibration bands at 1061, 959, 861, 774 cm－1 and 1060, 960, 849, 811 cm－1, re-spectively, which are also unvaried after immobilization.

In Figure 4(a) and 4(c) 31P MAS NMR spectra of 1 and 3 showed peaks at δ －3.5, －4.1 and －1.8, －2.0, respectively, before and after immobilization. 31P MAS NMR spectrum of bulk 2 shows five peaks with chemi-cal shifts of δ －2.1, －2.6, －2.8, －3.6, and －4.3,

which reveals that there are at least five geometrical isomers of divanadium-substituted Keggin heteropolya-cid. As the bulk 2 was prepared according to the litera-ture method, it may contain five geometrical isomers of divanadium atoms statistically distributed α-Keggin heteropolyacid and two geometrical isomers of β-Keggin heteropolyacid referring to the 31P NMR re-sults in aqueous solution.23 Although it is difficult to assign each peak to the specific geometrical isomer, the spectrum in Figure 4(b) proves the existence of isomers of bulk 2 in solid state and shows the similarity to the case in aqueous solution to some extent. However, 2/APTS/SBA-15 gives only one peak with chemical shift of δ －4.3, which is possible due to the aminosily-lated SBA-15 somewhat preferentially anchor one of isomers of 2 in the process of immobilization. The low loading of other isomers in the mesopores may account for the missing of their signals.

Catalytic aerobic oxidation of acetaldehyde

Table 2 summarized the activities of HPA/APTS/ SBA-15 in comparison with blank, SBA-15, APTS/ SBA-15, bulk HPA and 2/SBA-15 as catalysts for the oxidation of acetaldehyde. The catalyst, 2/SBA-15, containing the same content of 2 as that in 2/APTS/ SBA-15, was prepared by normal impregnation method. The blank system shows the similar conversion as that of SBA-15 and APTS/SBA-15 but much lower miner-alization rate. In addition, carbon dioxide with minor carbon monoxide was detected as the end prod- ucts in the blank system. 2/APTS/SBA-15 shows the best cata-lytic activity of all three immobilized catalyst. In the systems containing 2, the conversions of acetaldehyde increase at least three times from those without 2. When bulk 2 or 2/SBA-15 was used as catalyst, the dissolution of bulk 2 or the leaching of 2 in 2/SBA-15 is significant because of water produced in the reaction. At the end of the reactions, nearly half amount of bulk 2 or 20% of 2 in 2/SBA-15 leaves in the solutions after solid-liquid separation. However, 2/APTS/SBA-15 shows high sta-bility in polar solvent, where the leaching of 2 is negli gible and the catalytic activity of the fifth recycled cata-lyst remains unchanged, indicating that the strong inter-action between 2 and amino-groups on the silica surface prevents leaching of active species of 2. The similar re-sults are observed in the system containing 1/APTS/ SBA-15 and 3/APTS/SBA-15, however, the activity of

1004 Chin. J. Chem., 2006, Vol. 24, No. 8 ZHOU et al.


Figure 4 31P MAS NMR spectra of H4PMo11VO40 (a), H5PMo10V2O40 (b), H6PMo9V3O40 (c) before and after immobilization and poly-hedral model of α-Keggin structure (d).

Table 2 Catalytic aerobic oxidation of acetaldehydea

Catalyst CH3CHO conversion/% Mineralization rate/%

No catalyst 22.5 1.44

SBA-15 23.9 13.48

APTS/SBA-15 20.2 11.78

1 68.5 53.98

2 75.3 64.53

3 70.1 57.90

2/SBA-15 67.1 51.06

2/APTS/SBA-15 73.0 50.52

2/APTS/SBA-15b 71.7 49.54

2/APTS/SBA-15c 70.5 49.14

1/APTS/SBA-15 64.9 44.46

3/APTS/SBA-15 68.2 48.15 a Reaction conditions: 1 mL (0.848 mol•L－1) of CH3CHO and 0.1 g of catalyst were stirred in 20 mL of 1,2-dichloroethane un-der 500 mL of O2 at 20 ℃ for 24 h; b

2/APTS/SBA-15 recycled after the first run. HPA loading is 17.0% detected by ICP; c 2/ APTS/SBA-15 recycled after the fifth run, HPA loading is 16.7% detected by ICP.

2/APTS/SBA-15 is more notable. In conclusion, the catalyst HPA/APTS/SBA-15

combines catalytic activity of molybdovanadophosphoric acid and high surface area of SBA-15. Such a kind of material can be used as environmentally benign

heterogeneous catalyst for the aerobic (air or O2-based) oxidation reaction of acetaldehyde in the liquid phase under ambient condition.

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Catalytic Aerobic Oxidation of Acetaldehyde over Keggin-type Molybdovanadophosphoric Acid/SBA-15...

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