Catalytic formation of acetic anhydride over tungstophosphoric acid-impregnated SBA-15 mesoporous materials

Studies in Surface Science and Catalysis 156 M. Jaroniec and A. Sayari (Editors) 9 2005 ElsevierB.V. All rights reserved

859

Catalytic formation of acetic anhydride over tungstophosphoric acid-impregnated SBA-15 mesoporous materials Nongyue He, a' b, c. Chun Yang, d Chang-Soo Woo, b Hyeon-Gook Kim b and Ho-ln Lee b

aLaboratory of NanoBio Medicines and Devices; Key Laboratory for Packaging and Printing New Technology, Zhuzhou Institute of Technology, Zhuzhou 412008, China. Email: [email protected] bCatalyst and Surface Chemistry Laboratory; School of Chemical Engineering, Seoul National University, Seoul 151-744, South Korea. CChien-Shiung Wu Laboratory, Southeast University, Nanjing 210096, China. dCollege of Chemistry and Environmental Science, Nanjing Normal University, Nanjing 210097, China. Tungstophosphoric acid H3PWI2040 (TPA) was impregnated on mesoporous materials SBA-15 with pore sizes 91.9 and 63.9 A at different loadings. The samples were characterized by means of X-ray powder diffraction (XRD), Fourier transform framework vibration infrared spectra (FTIR), Transmission electron micrographs (TEM), ICP and N2 adsorption. It was shown that TPA species was finely dispersed. TPA IR spectrum was not detected for loading up to 1:5 (TPA:SBA-15, w:w), and became obvious with the increase in loading. The XRD diffraction peaks of TPA were not found for samples with loading up to 1:4 and were very weak for the samples with loading 2:3, while the TPA XRD diffraction peaks of physical mixtures of TPA and mesoporous (1:10, w:w) were very strong. Results demonstrated that the TPA supported SBA-15 catalyst could catalyze the reaction at a relatively low temperature (823 K) with a high selectivity (82%) of acetic anhydride. Generally, it was revealed that the finely dispersed and chemically bound TPA species and mesoporous structure were responsible for the enhanced activity and selectivity in our systems, while the pure bulk TPA showed low activity and no selectivity for acetic anhydride catalytically formed from acetic acid under the investigation. 1. INTRODUCTION Acetic anhydride is a very important chemical raw material for the production of many fine chemicals. In industry process, acetic anhydride is usually synthesized by condensation of acetic acid at high temperature (higher than 973 K) or by the oxidation of CH3CHO followed

860 by condensation. The disadvantage of the former is high power consuming. The latter gave some side products. Therefore, to develop a new solid acid catalyst to perform the process at lower temperature is of significance. As we know, the conventional heteropoly acids (HPAs) are complex proton acids that incorporate polyoxometalate anions (heteropolyanions) having metal-oxygen octahedra as the basic structural units. Because HPAs have several advantages as catalysts that make them economically and environmentally attractive, they have been applied to catalyze a wide variety of reactions in homogeneous liquid phase, offering strong options for more efficient and cleaner processing compared to conventional mineral acids [1-5]. Recently, HPAs have found applications in industrial practice both in electrophilic catalysis and in oxidation reactions [ l, 6-8]. In this contribution, we prepared the solid acid catalysts by impregnating mesoporous material SBA-15 with different amount of tungstophosphoric acid H3PWI2040 (TPA) and investigated the catalytic formation of acetic anhydride over tungstophosphoric acid-impregnated SBA- 15 mesoporous materials. 2. M A T E R I A L S AND METHODS

The commercially available tungstophosphoric acid H3PW12040 (TPA) with purity of 99% was purchased from Aldrich. Tetraethoxysilane (TEOS) was also purchased from Aldrich. The Nonionic Triblock template Pluronic 123 (May = 5800), EOEoPO70EO20, was purchased from BASF. The reason to choose TPA is that the synthesis of acetic anhydride from condensation of acetic acid at relatively low temperature should be catalyzed with strong acid like H2SO4, while TPA is the strongest among all the heteropoly acids [ 1]. The SBA-15 samples were synthesized by reference to the procedure described by Zhao et al. [9-10]. For the preparation of SBA-15-1, 8.023 g of Pluronic P123 was dissolved in 250.75 g of water and 49.30 g of concentrated HC1 (35 wt.%) solution with stirring at 323 K for 6 h. Then 17 g of TEOS was added into that solution with stirring at 323 K for 20 h. The mixture was aged at 368 K for 36 h without stirring. The solid product was recovered, washed, and air-dried at RT, followed by refluxing twice with ethanol at a ratio of 1/150 (g/mL) of solid sample to ethanol) to extract template. Then the solid sample was recovered again by filtration and dried in an oven at 383 K for 2 h. The dried sample was calcined at 823 K for 6 h in air to remove the remnant template. Because the pore size of SBA-15 mesoporous materials can be greatly adjusted by changing the reaction and aging time and temperature or by adding cosolvent organic molecules [9-10], we repeated the above preparation procedure, except for adding the TEOS and stirring at 313 K instead of 323 K, and aging the mixture at 353 K instead of 368 K and for a shorter time (24 h). The obtained calcined sample is called SBA-15-2 with a smaller pore size. To prepare the TPA-impregnated SBA-15 samples, the desired amount of TPA was dissolved in 2.0 ml of distilled water and the solution was mixed with SBA-15-1 or SBA-15-2 sample (0.5 g). The suspension was shaken under ultrasonic input for 1 h and kept at ambient temperature overnight. The product (TPA/MCM-15) was mildly dried at 353 K for 4 h, then dried at 383 K for 2 h more, and finally calcined for 2 h at 443 K. The calcined samples were stored and kept dry in a desiccator before being used for characterization or catalytic testing.

861 The ratio in weight of TPA to both SBA-15 substrates were arranged at 1:100, 1:20, 1:10, 1:6, 1:4, 1:2 and 2:3, the samples were called TPA/SBA-15-1(I:100), TPA/SBA-15-2(I:100), TPA/SBA- 15-1 (1:20), TPA/SBA- 15-2(1:20), and so on. To investigate the hydrothermal stability of the parent samples, two blank comparisons were simultaneously prepared under the same impregnation procedure without loading TPA and were designated as TPA/SBA- 15-1 (h) and TPA/SBA-15-2(h). Moreover, for comparison, physical mixtures of TPA and SBA-15 were prepared at the ratio in mass of TPA to SBA-15 1:10 and were designated as TPA/SBA- 15-1-m (1:10), TPA/SBA- 15-2-m (1:10). Small angle X-ray powder diffraction patterns were taken on a Bruker Small-Angle X-Ray Scattering (SAXS) with General Area Detector Diffraction (GADDS) using Cu Ka radiation at a step 0.01 ~ The wide range X-ray powder diffraction patterns were obtained on a MQC Science M18XHF22-SRA X-ray diffractometer using Cu K~t radiation at a step 0.02 ~ The nitrogen adsorption and desorption isotherms at 77 K were measured using a Micromeritics ASAP 2010 system. The data were analyzed by the BJH (Barrett-Joyner-Halenda) method using the Halsey equation for multilayer thickness. The pore size distribution curves were calculated from the analysis of the adsorption branches of the isotherms. The pore volume were taken at P/Po = 0.97 single point. The Fourier transform framework vibration infrared spectra (FTIR) were recorded on a MIDAC Prospect IR spectrometer using KBr pellets at a resolution 4 cm 1. The samples were mixed with KBr in the ratio 1:150 and ground in a mortar by hand with a pestle, then were pressed into pellets under the pressure 4 ton. The ICP analysis was carried out using a Shimadzu ICPS-1000IV instrument. All samples were analyzed three times and averaged. Transmission electron micrographs (TEM) were taken on a JEM-2000 EXII electron microscope operating at 200 kV. The formation of acetic anhydride by catalytic dehydration of acetic acid was performed using quartz tube reactor at 823 K for 1 hour. Helium was used as a carrier gas and acetic acid was injected to catalyst layer at the rate of 1 cc/h. The amount of all the catalysts was fixed to 50 mg. Products were analyzed by gas chromatograph equipped with HP 20M column and flame ionization detector. 3. RESULTS AND DISCUSSION

The XRD of the calcined parent SBA-15-1 and SBA-15-2 samples are shown in Fig. 1. Both XRD patterns display three well-defined peaks at 20 values smaller than 2 that can be indexed as (100), (110), and (200) Bragg reflections, typical of hexagonal (p6mm) SBA-15 mesoporous silica [9-10]. The very strong (100) diffraction peaks at 20 -- 0.90 and 0.96, respectively, give the d~00 spacing values of 98 and 92 A. The cell parameters a0 are 113.2 and 106.2 A respectively for SBA-15-1 and SBA-15-2. The nitrogen adsorption and desorption isotherms for both samples show obvious steps at high P/P0 due to the filling of the mesopores (not shown here), corresponding to the pore sizes 91.9 and 63.9 A as listed in Table 1. TEM images in Fig. 2 shows regular mesoporous structure for both samples. The pore sizes from TEM images are consistent with those from the N2 adsorption pore size distribution curves. The wall thickness is 23.3 and 42.3 A for SBA-15-1 and SBA-15-2, respectively.

862

a

=o

0

! 2 3 2 Theta(degree)

4

Figure 1. XRD patterns of (a) SBA-15-1 (dashed line) and (b) SBA-15-2.

Figure 2. TEM images of parent samples (a) SBA-15-1 and (b) SBA-15-2.

From the XRD patterns over a wide 20 range for the TPA/SBA-15 impregnation samples, the physical mixtures of TPA and SBA-15 substrates clearly display the XRD patterns of TPA with Keggin structure. Take it for example, the XRD pattern for the TPA/SBA-15-2-m (1:10) was shown in Fig. 3 (f). Whereas no apparent TPA XRD pattern was detected for the TPA impregnated samples with loading ___ 20 % on both parent samples. When the loading increases from 30% to 40%, some main peaks accounting for TPA gradually occur. Actually, the situations occurred for both SBA-I 5 substrates are similar to each other and, therefore, only the XRD patterns for TPA/SBA-15-2 impregnation samples were shown here (Fig. 3). Because the XRD pattern is related to whether a separate detectable crystal phase exists or not, the above XRD data indicate that the impregnated TPA species is finely dispersed on the surfaces of both SBA-15 substrates at the TPA loading as high as 20%. If the loading is further increased, part of the impregnated TPA will aggregate into a separate crystal phase. This result is similar to the previous reports on TPA-supporting siliceous MCM-41 mesoporous materials with

t

[-

Ill

=~

=o

"" " - ~ -

5

15 25 35 45 2 Theta (de~ree~

55

Figure 3. XRD patterns ofTPA/SBA-15-2 with different impregnated TPA loading (TPA: SBA-15-2, w:w): (a) 0:1, (b) 1:10, (c) 1:4, (d) 3:7 and (e) 2:3; (f) TPA/SBA-15-2-m (1:10).

i

1200 1000 800 600 Wavenumbers (cm !) Figure 4. Framework IR spectra of (a)-(g): TPA/SBA-15-2 with different impregnated TPA loading (TPA:SBA-15-2, w:w): (a)0:1, (b) 1:20, (r 1:10, (d) 1:5, (e) 1:4, (f) 3:7 and (g) 2:3; (h) TPA/SBA-15-2-m (1:10)and (i) TPA.

863 relatively large BET surface areas of 988 [11] or 939 m2/g [12] and on KIT-1 mesoporous materials with surface area of 750 cm2/g [13]. In contrast, when loading TPA on amorphous silica (SBET200-300 m2/g), the TPA crystal phase already appeared at ca. 20 TPA crystal phase already appeared at ca. 20 wt% TPA loading level [ 14-15]. The form and microcosmic chemical environment therein of the detected species are more related to a framework IR spectrum resulting from bond vibrations than to the phase in which the detected molecule or material exists. IR is thus a suitable method for the structural characterization of polyanions. We found that the framework IR spectra of TPA/SBA-15 impregnation samples for both SBA-15 substrates are similar to each other and, therefore, only the spectra for TPA/SBA-15-2 impregnation samples were shown here (Fig. 4). The spectra of bulk TPA in Fig. 4 (curve a) is in good agreement with the spectra reported elsewhere [ 16]: 1082 cm -~ band for P-O, 981 cm -! band for W=O, both 898 cm -I and 813 cm ~ bands for W-O-W. Owing to the overlapping of TPA bands at 813 and 1082 cm -~ with those from SBA-15 therein, only the TPA bands at 898 and 981 cm -l are discernable in the spectra for the mixture of TPA with SBA-15-2 (curves h in Fig. 4). From Fig. 4, we can find that the intensities of TPA bands gradually increase with the loading. However, no obvious TPA bands are shown for loadings 1:20 and 1:10; the latter possesses the same TPA content as that in the mixtures of TPA and SBA-15-2. This means that the impregnated TPA species in these samples do not retain the Keggin structure, maybe due to the TPA interacting with the substrate silanol groups [17-19]. Upon further increasing the TPA loading, a weak shoulder band a t - 9 0 0 cm -~ appears for loading 1:5, this band is enhanced for loading 1:4 (20 wt.%), accompanied by the appearance of the weak shoulder band at about 980 cm -~, somewhat similar to the spectra of the 1:10 mixtures. However, as shown in Fig. 3, the mixture displays strong TPA XRD patterns, whereas the loading 1:4 sample shows no TPA XRD patterns, indicating that some impregnated TPA species that are not interacting with the substrate exist in a discrete state and in an intact Keggin structure. This TPA species in discrete state and intact Keggin structure became a separate phase when the loading was increased to 3:7 (30 wt.%) and 2:3 (40 wt.%). Therefore, some main TPA diffraction peaks were detected in Fig. 3 (curves d and e). However, it is worth noting again that these discernable XRD peaks only show very much weak intensity. Taking into consideration that the 1:10 mixtures possess only about 10 wt.% TPA and show weak TPA IR bands and strong TPA XRD diffraction peaks, while the 30 or 40 wt.% loading samples show much more obvious TPA IR spectra and much weaker TPA XRD diffraction peaks than the (1:10) mixture, we conclude that the impregnated TPA is also high dispersed on the 30 or 40 wt.% loading samples. Summarized in Table 1 are the catalytic characterization data for all the samples. It was clearly demonstrated that two pure siliceous mesoporous substrates, SBA- 15-1 and SBA- 15-2, before or after the same impregnation procedure without loading TPA, show the similar activity. However, the former gives a better selectivity of acetic anhydride; it appears that the larger the pore size, the better the selectivity for the pure substrate SBA-15. Taking into consideration that the active sites Si-OH in both pure substrates should be similar, one can conclude that a larger pore size favors the formation of acetic anhydride. This is because the formed acetic anhydride diffuses and escapes out from the catalyst with a larger pore size more easily than from one with a smaller pore size. It will otherwise probably further degrade

864

Table 1 Conversion and selectivity for the formation of acetic anhydride from acetic acid at 823 K over samples dloo spacing Sample (A) SBA-15-1 98.3 TPA/SBA-15-1(h) 98.5 TPA/SBA-15-1(1-100) TPA/SBA-15-1 (1:20) TPA/SBA-15-1(I" 10) TPA/SBA- 15-1(1:5) TPA/SBA-15-1(1:4) TPA/SBA-15-1 (3:7) TPA/SBA-15-1 (2:3) -

aoa (A) 113.5 113.3 -

Surface area (m!g-l) 824 820 811 755 724 720 669 437 356

Pore volume (cm3g-l) 1.02 1.01 0.99 0.92 0.94 0.90 0.83 0.58 0.46

SBA-I 5-2 TPA/SBA- 15-2(h) TPA/SBA-15-2(I: 100) TPA/SBA-15-2(1:20) TPA/SBA-15-2(1:10) TPA/SBA- 15-2(1:5) TPA/SBA-I 5-2(1:4)

106.5 106.7 -

840 664 690 674 672 625 622

1.05 0.79 0.79 0.77 0.77 0.72 0.71

92.2 92.0 -

TPA < 10c a Calculated from N2 adsorption isotherms. b Selectivity of acetic anhydride. c Cited from literature [23].

Pore sizea (/~)

TPA (wt.%)

91.9

-

90.5 90.4 90.2 91.8 90.6 88.5 91.3 93.3

0 0.99 4.76 9.09 16.67 20.0 30.0 40.0

63.9 64.0 64.0 64.0 63.9 64.0 64.1

0 0.99 4.76 9.09 16.67 20.0 -

100

Conversion (%) 51 52 52 51 47 49 53 46 29

Selectivityb (%) 64 65 65 74 82 56 19 8 1

48 49 48 53 48 49 47

27 29 55 55 55 63 25

27

0

,,

to methane and acetone and, therefore, the selectivity decreases. Let us now consider the effect of the nature of the impregnated TPA species on the catalysis. From Table 1, it is very interesting and worth noting that the impregnated TPA species can significantly enhance the selectivity. When the TPA loading increases from 1:100 to 1:4, the activities for both samples remain at the level of about 50%, but the selectivity changes greatly. For the SBA-15-2 sample, the selectivity abruptly increases from 29% (TPA/SBA-15-2(h)) to 55% even if o n l y - 1 wt.% (1:100) of TPA was loaded, then remains at the same level and increases again for the 1:5 loading sample, and finally decreases at the level of 1:4 loading. The case for SBA-15-1 sample is different from that for SBA-15-2 sample. As compared with the pure SBA-15-1 substrate, the selectivity changes little at 1:100 loading, and gradually increases with the loading increasing, reaching the highest selectivity (82%) at 1:10 loading. Then it gradually decreases with the increase in loading. As shown in the IR and XRD spectra for the samples (Figs. 3-4) and as discussed above, the TPA species in the TPA impregnated SBA-15 sample with loading 1:5 exists in finely distributed form. Therefore, it is proposed that the finely distributed TPA species should be responsible for the enhancement of acetic anhydride. According to the previous reports [12, 17, 20] and our results presented here, the highly distributed TPA species should exist in either pure discrete surface TPA molecules with intact Keggin structure or state interacting with substrates. Which kind of the distributed TPA enhances the selectivity? From Figs. 3-4, we have found that the

865 1:4 loading sample displays framework IR spectra accounting for TPA Keggin structure but does not show the TPA XRD pattern, correspondingly the selectivity for both 1:4 loading samples is poor (see Table 1). While we have found that all the samples with loading _< 1:5 present neither TPA Keggin IR spectra nor TPA XRD pattern, these results suggest that the interaction between finely distributed TPA species and surface Si-OH group somewhat alters the TPA Keggin structure and thus both IR spectrum and XRD pattern typical of TPA Keggin structure are lost. This clearly demonstrates that the interacted TPA species with substrate Si-OH is responsible for the enhancement of selectivity. Actually, even if the detailed interaction mechanism is still not very clear, owing to that the alteration of TPA itself or the interaction of it with substrate is complicated under various conditions [12, 17-21], and it is often difficult to explain the catalytic behavior of TPA in some reactions, still the interaction of TPA with mesoporous MCM-41 has been detected [12, 17]. The interacted TPA species with MCM-41 greatly enhanced the conversion of 1,3,5-triisopropylbenzene [12] and the liquid-phase trans-de-t-butylation of 2,6-di-t-butyl-4-methylphenol [ 17] on TPA impregnated MCM-41 mesoporous materials. The selectivity of different products from the conversion of 1,3,5-triisopropylbenzene changed with the increase in TPA loading [12]. Based on our experimental results, it is reasonable to ascribe the enhanced selectivity to the interacted TPA species. Why the finely distributed and chemically bound TPA species can enhance the activity of the TPA supported mesoporous materials? A possible explanation is that the acid strength of the surface TPA species interacting with Si-OH groups gives rise to the enhanced acidity of the investigated TPA supported MCM-41 mesoporous systems [12, 17, 22]. However, this needs further investigation and will be reported elsewhere. 4. CONCLUSIONS TPA supported SBA-15 catalyst could catalyze the formation acetic anhydride from acetic acid at a relatively low temperature (823 K) with a high selectivity (82%) of acetic anhydride. The finely dispersed and chemically bound TPA species and mesoporous structure were responsible for the enhanced activity and selectivity in our systems. ACKNOWLEDGEMENT This research is supported by the National Natural Science Foundation of China and the BK 21 Program of South Korea. REFERENCES

[ 1] [2] [3] [4] [5] [6]

I.V. Kozhevnikov, Chem. Rev., 98 (1998) 171. I.V. Kozhevnikov, K. I. Matveev, Appl. Catal., 5 (1983) 135. I.V. Kozhevnikov, Russ. Chem. Rev., 56 (1987) 811. I.V. Kozhevnikov, Stud. Surf. Sci. Catal., 90 (1994) 21. I.V. Kozhevnikov, Catal. Rev. Sci. Eng., 37 (1995), 311. M. Misono and N. Nojiri, Appl. Catal., 64 (1990) 1.

866 [7] T. Okuhara, N. Mizuno and M. Misono, Adv. Catal., 41 (1996) 113. [8] N. Nojiri and M. Misono, Appl. Catal., 93 (1993) 103. [9] D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. E Chmelka and G. D. Stucky, Science, 279 (1998) 548. [ 10] D. Zhao, Q. Huo, J. Feng, B. F. Chmelka and G. D. Stucky, J. Am. Chem. Soc., 120 (1998) 6024. [11] W. Yang, J. Billy, Y. Ben Ta~xit, J.C. VSdrine and N. Essayern, Catalysis Today, 73 (2002) 153. [12] A. Ghanbari-Siahkali, A. Philippou, J. Dwyer and M. W. Anderson, Appl. Catal. A: General, 192 (2000) 57. [13] W. G. Kim, M. W. Kim, J. H. Kim and G~ Seo, Microporous Mesoporous Mater., 57 (2003) 113. [14] Y. Izumi, R. Hasebe and K. Urabe, J. Catal., 84 (1983) 402. [15] S. M. Kulikov, M. N. Timofeeva, I. V. Kozhevnikov, V. I. Zaikovsky, L. M. Plyasova and I. A. Ovsyanikova, Izv. Akad. Nauk SSSR Ser. Khim., (1991) 763. [16] C. Rocchiccioli-Deltcheff, M. Fournier, R. Frank and R. Thouvenot, Inorg. Chem., 22 (1983) 207. [17] I. V. Kozhevnikov, K. R. Kloetstra, A. Sinnema, H. W. Zandbergen and H. van Bekkum, J. Mol. Catal. A: Chemical, 114 (1996) 287. [ 18] F. Lefebvre, J. Chem. Sot., Chem. Commun., (1992) 756. [19] T.-H. Chang, J. Chem. Soc. Faraday Trans., 91 (1995) 375. [20] I. V. Kozhevnikov, A. Sinnema, R. J. J. Jansen, K. Pamin and H. van Bekkum, Catal. Lett., 30 (1995) 241. [21] P. A. Jalil, M. Faiz, N. Tabet, N. M. Hamdan and Z. Hussain, J. Catal., 217 (2003) 292. [22] S. Sato, C. Sakurai, H. Furuta, T. Sodesawa and F. Nozaki, J. Chem. Sot., Chem. Commun., (1991) 1327. [23] N. Mizuno and M. Misono, Chem. Rev., 98 (1998) 199.