www.elsevier.com/locate/apcata

Applied Catalysis A: General 320 (2007) 159–165

Preparation of H5PMo10V2O40 (PMo10V2) catalyst immobilized on

nitrogen-containing mesoporous carbon (N-MC) and its

application to the methacrolein oxidation

Heesoo Kim a, Ji Chul Jung a, Dong Ryul Park a, Sung-Hyeon Baeck b, In Kyu Song a,*a School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Shinlim-dong,

Kwanak-ku, Seoul 151-744, South Koreab Department of Chemical Engineering, Inha University, Incheon 402-751, South Korea

Received 6 November 2006; received in revised form 9 January 2007; accepted 15 January 2007

Available online 20 January 2007

Abstract

Nitrogen-containing mesoporous carbon (N-MC) with high surface area (=938 m2/g) and large pore volume (0.99 cm3/g) was synthesized

by a templating method using SBA-15 and polypyrrole as a templating agent and a carbon precursor, respectively. The N-MC was then

modified to have a positive charge, and thus, to provide sites for the immobilization of the H5PMo10V2O40 (PMo10V2) catalyst. By taking

advantage of the overall negative charge of [PMo10V2O40]5�, the PMo10V2 catalyst was immobilized on the N-MC support as a charge

matching component. The prepared PMo10V2/N-MC catalyst was applied to the vapor-phase oxidation of methacrolein (a model surface-type

reaction). It was found that the [PMo10V2O40]5� species were finely and chemically immobilized on the N-MC support as charge matching

species. In the vapor-phase oxidation of methacrolein, the PMo10V2/N-MC catalyst showed a higher conversion of methacrolein and a higher

yield for methacrylic acid than the unsupported PMo10V2 catalyst. Furthermore, the PMo10V2/N-MC catalyst also showed a higher conversion

of methacrolein and a higher yield for methacrylic acid than the PMo10V2/SBA-15 catalyst prepared by an impregnation method. The enhanced

catalytic performance of PMo10V2/N-MC in the model surface-type reaction was due to the fine dispersion of PMo10V2 species formed via

chemical immobilization.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Heteropolyacid catalyst; Nitrogen-containing mesoporous carbon; Chemical immobilization; Methacrolein; Oxidation

1. Introduction

Heteropolyacids (HPAs) have been widely employed as

homogeneous and heterogeneous catalysts for acid–base and

oxidation reactions [1–6]. The vapor-phase oxidation of

methacrolein to methacrylic acid is a typical commercialized

process, in which HPA is utilized as a heterogeneous catalyst

[7–9]. One of the great advantages of HPA catalysts is that their

oxidation catalytic properties can be tuned in a systematic way

by changing the identity of counter-cation, central heteroatom,

and framework polyatom [2–5,10–12]. However, one of the

disadvantages of HPA catalysts is that their surface area is very

low (<10 m2/g). To overcome the low surface area, HPA

* Corresponding author. Tel.: +82 2 880 9227; fax: +82 2 889 7415.

E-mail address: inksong@snu.ac.kr (I.K. Song).

0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2007.01.034

catalysts have been supported on inorganic mesoporous

materials by a conventional impregnation method [13–15].

Another promising approach for enlarging the surface area

of HPA catalysts is to take advantage of the overall negative

charge of heteropolyanions. By this method, HPAs have been

immobilized on conjugated conducting polymers such as

polyaniline and polypyrrole [16–19] or ion-exchanged resins

such as poly-4-vinylpyridine [20]. Although such an attempt

utilizing inorganic supporting materials has been restricted due

to the difficulty in forming a positive charge on the inorganic

supporting materials, considerable progress for obtaining

molecularly dispersed HPA catalysts has been made recently

by successfully forming a positive charge on the mesoporous

supporting materials through a surface modification process

[21,22]. These examples include HPA catalysts immobilized on

surface-modified mesoporous carbon [21] and surface-mod-

ified mesostructured cellular foam silica [22].

H. Kim et al. / Applied Catalysis A: General 320 (2007) 159–165160

Mesoporous materials have found successful applications

in many fields of science and engineering such as adsorption,

separation, and catalysis [23–25]. They have many advan-

tages as catalyst supports due to their unique textural

properties such as high surface area, large pore volume,

and uniform pore size distribution. If mesoporous materials

can be modified to have a positive charge, and thus, to provide

sites for the immobilization of heteropolyanions, they can

serve as an excellent support for the preparation of finely

dispersed HPA catalysts.

In this work, nitrogen-containing mesoporous carbon (N-

MC) was synthesized using SBA-15 and polypyrrole as a

templating agent and a carbon precursor, respectively. The

N-MC support was then modified to have a positive charge for

the chemical immobilization of [PMo10V2O40]5�. By taking

advantage of the overall negative charge of [PMo10V2O40]5�,

the H5PMo10V2O40 (PMo10V2) catalyst was immobilized on

the N-MC support as a charge matching component. The

prepared PMo10V2/N-MC catalyst was applied to the vapor-

phase oxidation of methacrolein. It is well known that the

catalytic oxidation of methacrolein to methacrylic acid over

HPA catalysts is a typical surface-type reaction [7–9], in which

an enhanced oxidation catalytic activity would be expected

over the highly dispersed HPA catalysts.

2. Experimental

2.1. Preparation of nitrogen-containing mesoporous

carbon (N-MC)

Highly ordered SBA-15 was synthesized [26] for use as a

templating material for nitrogen-containing mesoporous

carbon (N-MC). For the preparation of SBA-15, PEO–PPO–

PEO triblock copolymer (Pluronic 123, BASF) and tetraethyl

orthosilicate (TEOS, Fluka) were used as a structure directing

agent and a silica precursor, respectively [26]. The SBA-15 was

calcined at 550 8C for 5 h prior to the preparation of the

nitrogen-containing mesoporous carbon (N-MC). Nitrogen-

containing mesoporous carbon (N-MC) was prepared as

Fig. 1. Schematic procedures for the surface modification of N-MC and

follows. FeCl3 (1.0 g) dissolved in an HCl solution (1.5 ml,

1.0 M) was impregnated onto SBA-15 (1.0 g) by an incipient

wetness method. The yellow-colored slurry was dried in an

oven at 100 8C for 4 h. The resulting solid was reacted with

pyrrole monomer (0.5 g) at room temperature under vacuum for

the polymerization of pyrrole. The SBA-15/polypyrrole

composite was dried at 80 8C, and it was then carbonized at

900 8C for 5 h in a stream of nitrogen (40 ml/min). The silica

template was removed by the treatment with HF and HNO3.

After washing the solid with deionized water several times, the

resulting solid was finally dried at 100 8C in a convection oven

to yield the N-MC [27].

2.2. Immobilization of PMo10V2 on the N-MC support

Fig. 1 shows the schematic procedures for the surface

modification of N-MC and the subsequent immobilization of

PMo10V2 on the N-MC support. N-MC (1.0 g) was activated

by flowing hydrogen (10 ml/min) at 150 8C for 2 h to create

amine groups on the surface of the N-MC. The activated N-

MC was then treated with an aqueous HCl solution (pH < 4)

for 12 h to form a positive charge. The resulting N-MC

support was washed with deionized water several times, and

subsequently, dried overnight at 100 8C to yield the surface-

modified N-MC support. For the preparation of PMo10V2/N-

MC, PMo10V2 (1.0 g) and surface-modified N-MC (1.0 g)

were dissolved in acetonitrile (100 ml). The pH of the mixed

slurry was maintained below 2.0 using an aqueous HCl

solution. The slurry was stirred for 24 h at room temperature

for the immobilization of the PMo10V2 species on the surface-

modified N-MC support. After the solid product was

recovered by filtration, it was washed several times with

deionized water until the washing solvent became colorless.

The solid product was dried at 100 8C overnight and calcined

at 200 8C for 3 h to yield the PMo10V2/N-MC. For the

comparison of catalytic performance, PMo10V2 catalyst

supported on SBA-15 was also prepared by an impregnation

method. The loading of PMo10V2 in the PMo10V2/SBA-15

was 13.0 wt%.

the subsequent immobilization of PMo10V2 on the N-MC support.

H. Kim et al. / Applied Catalysis A: General 320 (2007) 159–165 161

2.3. Characterization

N2 adsorption–desorption isotherms of support and sup-

ported catalyst were obtained with an ASAP-2010 instrument

(Micromeritics). Pore structures of support and supported

catalyst were examined by TEM (Jeol, JEM-2000EXII).

Infrared spectra of support and supported catalyst were

obtained with a FT-IR spectrometer (Nicolet, Impact 410).

Thermal stabilities of support and supported catalyst were

confirmed by TGA analyses (Pheometric Scientific, TGA-100).

Support and supported catalyst were further characterized by

Raman spectroscopy (Horiaba Jobin Yvon, T64000), ICP-AES

(Shimadzu, ICPS-1000IV), and XRD (Mac Science, M18XHF)

measurements.

2.4. Vapor-phase oxidation of methacrolein

The vapor-phase oxidation of methacrolein to methacrylic

acid was carried out in a continuous flow fixed-bed reactor at

atmospheric pressure. Unsupported PMo10V2 (700 mg on

PMo10V2 basis), PMo10V2/SBA-15 (200 mg on PMo10V2

basis) or PMo10V2/N-MC (200 mg on PMo10V2 basis) was

charged into a tubular quartz reactor, and then the catalyst was

pretreated with a mixed stream of nitrogen (40 ml/min) and

oxygen (3 ml/min) at 365 8C for 1 h. Methacrolein

(1.8 � 10�3 mol/h) was sufficiently vaporized by passing

through a pre-heating zone and was continuously fed into

the reactor together with oxygen, water vapor, and nitrogen

carrier (40 ml/min). The molar feed composition was fixed at

oxygen (1.0):nitrogen (13.4):water vapor (3.17):methacrolein

(0.51). The catalytic reaction was carried out at 350 8C for 5 h.

Reaction products were periodically sampled and analyzed

with a gas chromatograph (HP 5890 II). Conversion of

methacrolein and yield for methacrylic acid were calculated on

the basis of carbon balance. The catalytic performance of

PMo10V2/N-MC was further examined with time on stream and

with respect to reaction temperature.

3. Results and discussion

3.1. Chemical compositions and textural properties of

support and supported catalyst

Chemical compositions and textural properties of PMo10V2,

N-MC, and PMo10V2/N-MC are summarized in Table 1. The

amounts of nitrogen in the N-MC and PMo10V2/N-MC were

determined by CHN elemental analyses. The nitrogen content

Table 1

Chemical compositions and textural properties of PMo10V2, N-MC, and

PMo10V2/N-MC

Nitrogen

content

(wt%)

PMo10V2

content

(wt%)

Surface

area

(m2/g)

Pore

volume

(cm3/g)

Average

pore size

(nm)

PMo10V2 – – 4 – –

N-MC 2.0 – 938 0.99 3.8

PMo10V2/N-MC 1.4 13.6 720 0.77 3.8

of N-MC was 2.0 wt%, while that of PMo10V2/N-MC was

determined to be 1.4 wt%. The decreased nitrogen content of

the PMo10V2/N-MC was due to the loading of PMo10V2 onto

the N-MC support. The amount of PMo10V2 immobilized on

the N-MC support was found to be 13.6 wt%. We also

attempted to immobilize the PMo10V2 on CMK-3, the pore

structure of which is very similar to that of N-MC. In this case,

however, the PMo10V2 species were totally dissolved out

during the washing step due to the absence of anchoring sites on

the CMK-3 for PMo10V2, as observed in a previous work [21]

reporting the immobilization of [PMo12O40]3� on the CMK-3.

This indicates that nitrogen in the N-MC played an important

role in forming a nitrogen-derived functional group (amine

group) for the immobilization of the PMo10V2 catalyst.

Theoretically, a single [PMo10V2O40]5� molecule should

interact with five [NH3+] groups. However, simple calculation

based on the chemical composition data in Table 1 revealed that

a single [PMo10V2O40]5� molecule occupies 18.2 nitrogen-

containing species. This indicates that all the nitrogen species

on the N-MC were not modified into the nitrogen-derived

functional groups (amine groups). It also implies that the

immobilized [PMo10V2O40]5� species were molecularly dis-

persed on the N-MC support.

Fig. 2 shows the nitrogen adsorption–desorption isotherms

and pore size distributions of N-MC and PMo10V2/N-MC. Both

N-MC and PMo10V2/N-MC exhibited type IV isotherms and

type H2 hysteresis loops. The average pore size of N-MC and

PMo10V2/N-MC was found to be ca. 3.8 nm (Table 1). These

results indicate that the pore structure of N-MC was still

maintained even after the immobilization of PMo10V2. Table 1

also shows that surface area (938 m2/g) and pore volume

(0.99 cm3/g) of N-MC decreased slightly upon immobilization

of the PMo10V2. However, the PMo10V2/N-MC still retained

relatively large surface area (720 m2/g) and large pore volume

(0.77 cm3/g).

Fig. 2. Nitrogen adsorption–desorption isotherms and pore size distributions

(inset) of N-MC and PMo10V2/N-MC.

Fig. 3. TEM images of (a) N-MC and (b) PMo10V2/N-MC.

H. Kim et al. / Applied Catalysis A: General 320 (2007) 159–165162

Fig. 3 shows the TEM images of N-MC and PMo10V2/N-

MC. Two-dimensional pore arrays can clearly be observed in

both N-MC and PMo10V2/N-MC. One again, this result

supports the conclusion that the pore structure of N-MC was

still maintained after immobilization of the PMo10V2 onto the

surface of the N-MC. This result is in good agreement with

previous observations reporting that mesoporous carbon [21]

and mesostructured cellular foam silica [22] retained their

own pore structures even after the immobilization of

H3PMo12O40. Furthermore, no visible evidence representing

PMo10V2 species was found in the PMo10V2/N-MC, indicat-

ing that PMo10V2 species were finely dispersed on the N-MC

support.

Fig. 4 shows the Raman spectrum of N-MC. Two distinct

bands were observed at around 1350 cm�1 (D-band) and

1580 cm�1 (G-band). The D-band is closely related to the

disorder-induced scattering resulting from the imperfections or

the loss of hexagonal symmetry of the graphite structure, while

the G-band is known to appear in both amorphous carbon and

graphite [28–30]. Therefore, the area ratio between the two

Fig. 4. Raman spectrum of N-MC.

bands (AD/AG) can be used as an index for the degree of

graphitization [29,30]. The AD/AG value of N-MC was

determined to be 0.76, a smaller value than that of Vulcan

XC-72 carbon (1.65) [31]. This indicates that the N-MC support

with a highly graphitic structure was successfully prepared in

this work.

Fig. 5. FT-IR spectra of unsupported PMo10V2, N-MC, and PMo10V2/N-MC.

Fig. 6. XRD patterns of unsupported PMo10V2, N-MC, and PMo10V2/N-MC. Fig. 8. Catalytic performance of unsupported PMo10V2 (700 mg on PMo10V2

basis), PMo10V2/SBA-15 (200 mg on PMo10V2 basis), and PMo10V2/N-MC

(200 mg on PMo10V2 basis) catalysts in the vapor-phase oxidation of metha-

crolein obtained at 350 8C after 5 h-reaction.

H. Kim et al. / Applied Catalysis A: General 320 (2007) 159–165 163

3.2. Chemical immobilization of PMo10V2 on the N-MC

support

Chemical immobilization of PMo10V2 on the N-MC

support was confirmed by FT-IR analyses. Fig. 5 shows the

FT-IR spectra of unsupported PMo10V2, N-MC, and PMo10V2/

N-MC. The primary structure of unsupported PMo10V2 could

be identified by the four characteristic IR bands appearing in

the range of 700–1100 cm�1. The characteristic IR bands of

PMo10V2 appeared at 1063, 964, 873, and 794 cm�1, which

are assigned to P–O, M (addenda atom) O, interoctahedral

M–O–M, and intraoctahedral M–O–M bands, respectively [2].

It was very difficult to obtain an IR spectrum of N-MC because

of the strong absorbance of the carbon material by the infrared

beam. FT-IR analysis revealed that no characteristic IR bands

of N-MC were observed in the range of 700–1100 cm�1. It is

noteworthy that the characteristic IR bands of PMo10V2 in the

PMo10V2/N-MC catalyst were clearly observed at 1056 (P–O

band), 887 (interoctahedral M–O–M band), and 798 (intraoc-

tahedral M–O–M band) cm�1, except for M O band.

Furthermore, the characteristic IR bands of PMo10V2 in the

PMo10V2/N-MC appeared at slightly shifted positions

Fig. 7. TGA profiles of (a) unsupported PMo10V2, and (b)

compared to those of the unsupported PMo10V2. This indicates

that PMo10V2 was successfully immobilized on the N-MC

support via strong chemical interaction between two

components.

3.3. Fine dispersion of PMo10V2 on the N-MC

Fig. 6 shows the XRD patterns of unsupported PMo10V2, N-

MC, and PMo10V2/N-MC. Unsupported PMo10V2 showed the

characteristic XRD peaks of the HPA. On the other hand, N-MC

showed the characteristic diffraction peaks for (0 0 2) and

(1 0 1) planes at around 2u = 26.38 and 438, respectively,

indicating that the N-MC retained a graphic structure. However,

PMo10V2/N-MC catalyst exhibited no characteristic XRD

pattern of PMo10V2 but showed almost the same XRD pattern

as N-MC, even though 13.6 wt% PMo10V2 was loaded in the

PMo10V2/N-MC catalyst. This result means that PMo10V2

species were not in a crystal state but in an amorphous-like

state, demonstrating that PMo10V2 species were finely

dispersed on the N-MC support.

N-MC and PMo10V2/N-MC (heating rate = 10 8C/min).

Fig. 9. Catalytic performance of PMo10V2/N-MC (200 mg on PMo10V2 basis) catalyst in the vapor-phase oxidation of methacrolein: (a) with time on stream at

350 8C and (b) with respect to reaction temperature after 5 h-reaction.

H. Kim et al. / Applied Catalysis A: General 320 (2007) 159–165164

3.4. Thermal stability of PMo10V2, N-MC, and PMo10V2/

SM-N-MC

Fig. 7 shows the TGA profiles of unsupported PMo10V2, N-

MC, and PMo10V2/N-MC. Thermal scanning was performed at

temperatures ranging from 50 to 700 8C in a stream of air. The

bulk PMo10V2 catalyst experienced significant weight loss in

the low temperature region due to the removal of crystalline

water molecules, and then finally decomposed at around

395 8C. The N-MC support was thermally stable up to ca.

470 8C. The decomposition temperature of PMo10V2/N-MC

was almost identical to that of unsupported PMo10V2. This

indicates that the sudden weight loss at around 395 8C observed

in the PMo10V2/N-MC was attributed to the thermal decom-

position of PMo10V2 component. It is expected that PMo10V2/

N-MC catalyst with finely dispersed PMo10V2 species would

show an excellent catalytic performance in the surface-type

reactions performed below 395 8C.

3.5. Performance of PMo10V2/N-MC in the vapor-phase

oxidation of methacrolein

Fig. 8 shows the catalytic performance of unsupported

PMo10V2, PMo10V2/SBA-15, and PMo10V2/N-MC catalysts in

the vapor-phase oxidation of methacrolein obtained at 350 8Cafter 5 h-reaction. To check the catalytic activity of these

catalysts within a comparable range, a large amount of

unsupported PMo10V2 (700 mg on PMo10V2 basis) was used

compared to PMo10V2/SBA-15 (200 mg on PMo10V2 basis)

and PMo10V2/N-MC (200 mg on PMo10V2 basis). In the

catalytic reaction, small amounts of CO, CO2, acetone, and

acetic acid were produced as by-products. Although a large

amount of bulk PMo10V2 was used, the PMo10V2/N-MC

catalyst showed a higher conversion of methacrolein and a

higher yield for methacrylic acid than the unsupported

PMo10V2 catalyst. Furthermore, the PMo10V2/N-MC catalyst

also showed a higher conversion of methacrolein and a higher

yield for methacrylic acid than the PMo10V2/SBA-15 catalyst.

The conversion of methacrolein and yield for methacrylic acid

decreased in the order of PMo10V2/N-MC > PMo10V2/SBA-

15 > unsupported PMo10V2. The enhanced catalytic perfor-

mance of PMo10V2/N-MC catalyst in the model surface-type

reaction was attributed to the fine dispersion of PMo10V2

species on the surface of N-MC support formed via chemical

immobilization.

Fig. 9 shows the catalytic performance of PMo10V2/N-MC

in the vapor-phase oxidation of methacrolein with time on

stream at 350 8C and with respect to reaction temperature. As

shown in Fig. 9(a), the PMo10V2/N-MC showed a stable

catalytic performance with a slight deactivation during the

reaction time extending over 12 h. Fig. 9(b) shows the catalytic

performance of PMo10V2/N-MC in the vapor-phase oxidation

of methacrolein with respect to reaction temperature obtained

after 5 h-reaction. The conversion of methacrolein, selectivity

for methacrylic acid, and yield for methacrylic acid over the

PMo10V2/N-MC catalyst monotonically increased with

increasing reaction temperature within the temperature window

of 320–360 8C.

4. Conclusions

Nitrogen-containing mesoporous carbon (N-MC) with high

surface area and large pore volume was synthesized by a

templating method. The PMo10V2 catalyst was then immobilized

on the N-MC support as a charge matching component, by taking

advantage of the overall negative charge of [PMo10V2]5�. The

nitrogen in the N-MC played an important role in forming a

nitrogen-derived functional group (amine group) for the

immobilization of the PMo10V2 catalyst. The PMo10V2 species

were finely dispersed on the N-MC support via strong chemical

immobilization. Furthermore, the pore structure of N-MC was

still maintained even after the immobilization of PMo10V2. In the

vapor-phase oxidation of methacrolein (a typical surface-type

reaction), the PMo10V2/N-MC catalyst showed a higher

conversion of methacrolein and a higher yield for methacrylic

acid than the unsupported PMo10V2 and the PMo10V2/SBA-15

H. Kim et al. / Applied Catalysis A: General 320 (2007) 159–165 165

catalysts. The conversion of methacrolein and yield for

methacrylic acid decreased in the order of PMo10V2/N-

MC > PMo10V2/SBA-15 > unsupported PMo10V2. The

enhanced catalytic performance of PMo10V2/N-MC in the

model surface-type reaction was attributed to the fine dispersion

of PMo10V2 species formed via chemical immobilization.

Acknowledgement

This work was supported by the Korea Research Foundation

Grant funded by the Korean Government (MOEHRD) (KRF-

2005-041-D00204).

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