Effect of support on performance of MoVTeO catalyst for selective oxidation of propane to acrolein

Studies in Surface Science and Catalysis, volume 147 X. Bao and Y. Xu (Editors) {)2004 Elsevier B.V. All rights reserved.

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Effect of support on performance of MoVTeO catalyst for selective oxidation of propane to acrolein C. J. Huang, W. Guo, X. D. Yi, W. Z. Weng and H. L. Wan* State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen, 361005, China

ABSTRACT Selective oxidation of propane to acrolein over supported MoV0.2Te0.1Ox catalysts has been investigated. It was found that supports affected the structure, reducibility and O2-desorption behaviour of the catalysts, which were closely related to catalyst performance. Among the catalysts studied, MoVo.2Teo.~/SiO2 showed the best performance for oxidation of propane to acrolein. 1. INTRODUCTION Selective transformation of propane to acrolein is a challenging task in both economic and scientific terms. Most catalysts for this reaction reported in the literature are unsupported multi-component oxides. Among them, AgBiVMoO catalyst shows the best catalytic performance under conditions where, however, primary gas-phase dehydrogenation of propane occurred [1]. The modification of this catalyst by coprecipitating the main components in the presence of support led to an increase in propane conversion, but no improvement in acrolein yield [2]. Recently, K-modified Fe/SiO2 was reported active for oxygenates formation from propane oxidation [3]. Over this catalyst, acrolein selectivity was 22% with a propane conversion of 10% at 748 K. However, the systematic studies on the effect of supports have been scarcely reported. In this paper, we report an investigation on the effects of the supports (CaO, CeO2, SiO2, ZrO2 and y-A1203) on the catalytic performance of MoV0.zTe0.1Ox catalyst for selective oxidation of propane to acrolein.

2. EXPERIMENTAL 2.1 Catalyst preparation Supported MoV0.2Te0.~Ox catalysts with * Corresponding author

MoO3

content of 11 wt% were

662 Table 1 Surface areas and XRD phases of catalysts Catalysts Code MoV02Te0.1Ox M MoV0.2Te0.1/CaO MCa MoV0.2Te0.1/CeO2 MCe MoV0.2Te0.1/SiO2 MSi MoV02Te0 ~/ZrO2 MZr MoV0.2Te0.1/A1203 MA1

7.53 19.12 17.34 60.16 6.42 139.81

Phases observed MoO3 CaMoO4, CaO CeO2 MoO3, SiO2 ZrMo208, ZrO2 y-A1203

prepared by incipient wet impregnation of commercial oxides (i.e., CaO, CeO2, SiO2, ZrO2 and 7-A1203) with an aqueous solution (pH ~ 6) of heptamolybdate, metavanadate and tellurium acid. The resulting precursors were dried at 110 ~ overnight and then calcined in air at 600 ~ for 5 h. For the unsupported catalyst, the above solution was evaporated and the resulting solid was dried and calcined under the same conditions as the supported samples. In all cases, the atomic ratio of Mo/V/Te = 1.0/0.2/0.1 was fixed. The catalyst samples, along with their notations and some physico-chemical properties such as specific areas and phases, are shown in Table 1.

2.2 Catalytic tests Catalytic tests were carried out in a fixed bed flow reactor (quartz tube, 5 mm i.d.) under the following conditions: TR, 500 ~ P p,, 1 bar; Wc~t, 0.27 g. The reactant gas mixture with a molar composition of C3H8 " 02 " He = 1.2 9 1.0 9 1.2 was passed through the catalyst bed at a space velocity of 3900 mlog-cat~oh ~. The products were analysed by two gas chromatographs (an FID-GC with GDX103 column and a TCD-GC with squalane/Al203 and carbosieve columns). The data were collected after 0.5 h on stream.

2.3 Catalyst characterization XRD patterns were recorded on a Rigaku Rotflex D/max-C X-ray powder diffractometer (30 kV, 20 mA) scanning at 8 ~min "1 using Cu K~radiation. Laser Raman spectra were collected under ambient conditions using Renishaw UV-Vis Raman 1000 System equipped with a CCD detector and a Leica DMLM microscope. A green laser ()~ = 514.5 nm) was used for excitation. TPR experiments were performed on a flow apparatus using a 5% Hz/Ar mixture flowing at 20 ml/min. The heating rate was 10~ Hydrogen consumption was monitored by a TCD detector after removing the water formed. 02 -TPD was carried out in a flow system connected to a quadruple mass spectrometer (Balzers QMS 200 Omnistar). In each experiment, 0.2g catalyst was used. The sample was pretreated by heating under flowing air at 600~ for 30 min then cooling to room temperature. After purging with helium (20 ml/min) for 30 min, the sample was heated at 20~ from room temperature to 800~

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3. RESULTS and DISCUSSION 3.1 Catalytic performance From Table 2, significent influence of support on the activity and selectivity of MoV0.2Te0.~Ox catalyst is observed. All of the supported catalysts, except MCa, show higher activities than the unsupported ones. The oxidative conversion of propane decreases in the following order: MA1 > MZr > MSi-~ MCe >> M ~ MCa. The improvement in activity cannot be simply attributed to the increase in catalyst surface area (S.A.). For example, both M and MZr have almost the same S.A. (see Table 1), however, their activities are quite different. In comparison with unsupported catalyst, MCa shows a lower selectivity to COx and a higher selectivity to propylene, while MCe, MZr and MA1 display higher selectivities to CO~ and lower selectivities to propylene and acrolein. Among these catalysts only MSi exhibits a marked functionality towards the formation of acrolein (30%). The products over other catalysts are mainly C3H6(16-60%), COx (39-82%) and a small amounts (0.6-5.7%) of CH4, C2H4 and oxygenates. 3.2 XRD results Fig. 1 shows XRD patterns of the catalysts. Phases identified by XRD are shown in Table 1. Both M and MSi catalysts present XRD peaks assignable to MOO3, although the peaks of the latter are very weak and accompanied by a broad peak due to amorphous SiO2. In the case of MCa and MZr catalysts, the strong interactions between supports and Mo species lead to the formation of new phases, i.e. CaMoO4 and ZrMo208, respectively. For MCe and MA1 catalysts, only peaks from the oxide supports are observed, indicating that the loaded oxides highly disperse over the supports. In all the catalysts, no phase containing V or Te is detected. 3.3 Laser Raman spectra Fig. 2 shows the Raman spectra of unsupported and supported MoV0.2Te0.~Ox catalysts. MCa presents the bands from CaMoO4 at 793,848 and 878 cm 1 [4]. Table 2 Catalytic performance of the catalysts C3H8conversion Catalyst (%) C3H6 Blank - 0 M 3.9 56.0 MCa 2.4 60.7 MCe 21.7 16.0 MSi 22.1 25.5 MZr 25.0 28.6 MAI 32.4 34.6 * C2H4+ C2H40 +C3H60 + CH4

Selectivity (%) COx Acr 39.0 33.6 81.9 42.2 68.5 59.4

4.4 1.1 0.5 30.1 1.2 0.1

Others*

Acrolein yield (%)

0.6 4.7 1.5 2.1 1.7 5.7

0.2 0.3 0.1 6.7 0.3 -0

664 On MZr, three distinct bands at 748, 944 and 998 cm -] are observed. Based on the results of XRD analysis, the bands are tentatively attributed to ZrMo2Os. On the other hand, MA1 displays only one broad band at around 960 cm 1, which has been assigned to polymolybdate by many authors [5]. Generally, the Raman bands in the region of 870 ~ 960 cm -~ are characteristic of amorphous phase such as surface molybdate and polymolybdate species [5]. The bands due to the amorphous phase are also observed in the spectra of MSi (877, 960 cm l ) and MCe (898, 960 cm -~) catalysts. For MCe, the weak band at 818 cm -~ is ascribed to a trace amount of MoO~ [6]. These observations suggest that the supports in the MA1, MCe and MSi catalysts are responsible for the high dispersion of Mo species, especially in the case of MA1. Among the catalysts investigated, only M and MS i present intense bands due to crystalline MoO~ at 666, 818 and 994 cm -~. However, their Raman spectra are actually different. The most striking difference is the intensity ratio of the 818cm ~ band to the 994-cm ~ band, which is higher for MSi than for M. This observation is significant since the 818-cm ~ band and 994-cm ~ band have been attributed to the Mo-O-Mo and Mo=O stretching vibrations, respectively [7]. It was proposed that over transition metal oxide surface the bridging oxygen species are more active than the terminal ones [8], which is consistent with the result obtained over MoO~ [9]. For MoO~ crystal, Bruckman et al. [10] found that the (010) face, on which Mo-O-Mo sites are

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preferably located [11], is responsible for oxygen insertion of allyl to form acrolein. Based on the above analyses, it is suggested that the unique catalytic performance of MSi in the selective oxidation of propane to acrolein is probably related to its abundant Mo-O-Mo sites over the crystalline MOO3.

3.4 H+-TPR results The H2-TPR profiles of various catalysts and related oxide supports are presented in Fig. 3, in which To.red indicates the onset temperature of reduction and Tm+• the temperature of the main reduction peak. CeO2 shows a reduction profile with a weak peak at about 510 ~ and a peak at temperatures higher than 750 ~ Other supports present TPR profiles with no reduction peak up to 750 ~ as expected. However, a pronounced effect of support on the reduction behaviour of the catalysts is found for all the supported ones. In the case of M, the reducibility is very weak at temperature lower than 700 ~ When M is loaded on the supports, To,red and Tm+• shift to lower temperatures. This may be related to the increased dispersion and/or the structure modification of the loaded oxides caused by the supports. Based on the T0,r+a and the Tm,• values shown in Fig. 3, the reducibility sequence of various catalysts can be given as follows: MZr > MA1 > MSi + MCe > MCa > M. The sequence is almost the same as that of activity of the catalysts except MA1. This reveals that the oxidative conversion of propane is closely related to the reducibility of the catalysts. Compared with MZr, the higher activity of MA1 may be attributed to its much larger SBETvalue (Table 1) and higher dispersion, as evidenced by the results of XRD and Raman. ~i

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3.5 O2-TPD measurements Fig. 4 shows the O2-TPD profiles of supported MoV0.2Te0.1Ox catalysts. Based on the onset temperature of O2 desorption (To, dep), the mobility scale of oxygen species over the catalysts can be evaluated as follows" MCe > MZr > MSi > MA1 >MCa. This sequence is almost the same as that of the CO• selectivity as shown in Table 1. Among the catalysts investigated, MCe catalyst shows the strongest peak with the lowest To, dep, which could explain its highest selectivity to COx formation. In contrast, MCa catalyst displays two very weak peaks at temperatures higher than 700 ~ On this catalyst, not only fewer CO~ but also little oxygenates are produced. The selectivity of C3H6, which is generally assumed as an intermediate in the conversion of propane to acrolein, is as high as 60%, while the selectivity of acrolein is fairly low. This may be due to the poor insertion ability of oxygen species on catalysts for transformation of C3H6 to oxygenates. On the other hand, MSi exhibits an intermediate behaviour of O2 desorption, as evidenced by its TPD profile with To,dep higher than that of MZr but lower than that of MCa. The moderate inserting function of the oxygen species on MSi would be beneficial to the formation of acrolein. In conclusion, the selective oxidation of propane over MoV0.2Te0.1Ox catalysts is profoundly affected by the nature of supports. The supports influence the structure, reducibility and O2-desorption behaviour of the catalysts, which are closely related to their catalytic performance. Among the catalysts investigated, MoV02Ye0 ~/SiO2 shows the best performance for the oxidation of propane to acrolein. ACKNOWLEDGEMENT This project is supported by the Ministry of Science and Technology of China (grant No. G 1999022408)

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