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Piotr Michorczyk, Jan Ogonowski, Jolanta Sroka CATALYTIC ACTIVITY OF MCM-41-SUPPORTED VANADIUM OXIDE CATALYSTS IN THE DEHYDROGENATION OF PROPANE IN THE PRESENCE AND ABSENCE OF CO2 (Institute of Organic Chemistry and Technology, Cracow University of Technology) E-mail: [email protected] The dehydrogenation of propane to propene in the presence and absence of CO2 over MCM-41-supported vanadium oxide catalysts has been studied. The effect of vanadium content and feed composition on the dehydrogenation of propane has been investigated. It has been found that these materials exhibit a moderate catalytic activity and an excellent selectivity in dehydrogenation of propane. Moreover, it has been found that the introduction of CO2 to the reaction zone promotes the dehydrogenation of propane over the catalysts with V content above 5% of V2O5. INTRODUCTION

Vanadium oxide supported on siliceous materials (SiO2 and mesoporous materials: MCM-41, SBA-15) have been considered as promising catalysts for oxidation of methane to formaldehyde [1,2] and in the selective oxidative dehydrogenation of alkanes to alkenes by oxygen [3,4] or by mild oxidants like: N2O or CO2 [5, 6]. Recent reports have shown that the catalytic activity of V-containing materials depends strongly on the content of vanadium load. The isolated tetrahedral V5+ species containing terminal V=O groups have been suggested as the most selective ones in the oxidation reactions [1-3]. These species predominate at low vanadium content, while at higher loading, polymeric and crystalline V2O5 species are formed also on the surface of supports. The latter species contain V-O-V groups, which are responsible for over oxidation reactions. Taking into account above, in order to prepare more active and selective vanadium oxide based catalysts for the dehydrogenation of propane in the presence of CO2, a high dispersion of vanadium oxide at vanadium loading as high as possible should be achieved. Therefore, in the present paper great attention has been dedicated to siliceous mesoporous supports which possess high surface area. We have used siliceous mesoporous MCM-41 material (SBET=1085 m2g-1 and with 2.4 nm pore diameter) as support for dispersing vanadium oxide species. The V/MCM-41 catalysts with V2O5 loading in the range of 1-20 wt% were prepared by impregnation, characterized by H2TPR technique and tested in the dehydrogenation of propane to propene in the presence and absence of CO2. EXPERIMENTAL

MCM-41 was prepared under basic conditions using n-hexadecyltrimethylammonium chloride (HTMACl) as the surfactant and tetraethyl orthosilicate (TEOS) as the silica source at room temperature.

A typical synthesis procedure was as follows: Solution A was prepared by mixing 45.3 cm3 HTMACl (25wt% in aqueous solution, Aldrich), with 523 cm3 of distilled water and 44.0 cm3 of aqueous ammonia (25wt%, Polish Chemical Reagent) was added to the surfactant solution. The solution was then stirred for 10 min and 48.6 cm3 TEOS (purum 98%, Aldrich) was added under vigorous stirring (Solution B). Stirring was continued for 1 h at room temperature. The resulting solid was recovered by filtration, washed with distilled water and dried at 80°C for 6 h. The template was removed by calcinations at 550°C for 12 h in air. MCM-41 supported vanadia catalysts were prepared by using the impregnation method. An aqueous solution of ammonium metavanadate (Polish Chemical Reagent) was contacted with MCM-41 and water was rotaevaporated until completely dryness. The samples were calcined at 600°C for 3 h in air. The catalysts are designated as Vx/MCM-41, where x expresses the total V content in wt% of V2O5. The catalytic tests were carried out in a fixedbed quartz tabular flow reactor packed with 0.5 g of catalyst (grain size 0.2-0.3). The feed was a mixture of C3H8:CO2(He) at the molar ratio 1:5. The total flow rate of the feed was 30 cm3min-1. First the catalysts were preheated in dry He stream at 600°C for 30 min. The products and unreacted substrates were analyzed using two gas chromatographs. One of them was equipped with a glass column (3mx3mm) packed with Porapak Q and a flame ionization detector. It was used to analyse of hydrocarbons. The second chromatograph was equipped with a stainless steel column (3m´3mm) packed with Carboxen 1000 and a thermal conductivity detector (for H2, CO and CO2 analyses). Propane conversion, hydrocarbons yields and selectivities were calculated as shown below (Eqs. 1-3). Yi (%) =

ХИМИЯ И ХИМИЧЕСКАЯ ТЕХНОЛОГИЯ 2008 том 51 вып. 7

ai ni 3 nC2 H 8 + nC2 H 6

2 2 1 + nC2 H 6 + nC2 H 4 + nCH 4 3 3 3

× 100

(Eq.1)

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S i (%) = nC 2 H 6

ai ni 3 × 100 2 2 1 + nC 2 H 6 + n C2 H 4 + nCH 4 3 3 3

C p (%) =

Wp Sp

× 100 ,

(Eq.2)

(Eq.3)

low vanadium loading isolated V5+ species are predominated. Moreover, the progressive shift of the reduction maximum to higher temperature with vanadium loading suggests a progressive formation of less reducible polymeric vanadium.

where: Yi(%) – yield, Si(%) – selectivity, Cp(%) conversion, ai is the number of carbon atoms in compound i. H2-TPR analysis of the catalysts were performed using Ar/H2 gas mixture (90/10 vol. %). The flow rate of the carrier gas was 30 cm3min-1. The sample (100 mg) was first treated in dry helium at 600°C for 30 min and next it was heated at a rate 10°C·min-1 to the final temperature of 750 °C. RESULTS AND DISCUSSION

Temperature programmed reduction (TPR) studies were performed for the fresh MCM-41supported vanadium oxide catalysts (Fig.1). The single reduction peak is observed for the samples with vanadium loading below 3 wt% of V2O5. In the samples with higher vanadium loading the low temperature peak is shifted to higher temperature and a shoulder broad peak appears with the reduction maximum above 600°C. By analogy with previous studies on the reducibility of V-containing meso-porous materials [7], the low temperature peak is ascribed to reduction of dispersed tetrahedral vana-dium species, whereas the high broad temperature reduction peak is related to the reduction of polymeric V5+ species. This reveals that over the catalysts with

Fig. 1. H2-TPR profiles of MCM-41-supported vanadium oxide catalysts

Table summarizes the catalytic results obtain in the dehydrogenation of propane in the presence and absence of CO2. It is clear that the deposition of vanadium oxide on MCM-41 enhances the conversion of propane. With increasing vanadium loading the conversion of propane increased.

Table. Dehydrogenation of propane over MCM-41-supported vanadium oxide catalysts at 600°C.a Catalyst V1/MCM-41 V3/MCM-41 V5/MCM-41 V10/MCM-41 V20/MCM-41 a

Feed gas C3H8/CO2 C3H8/He C3H8/CO2 C3H8/He C3H8/CO2 C3H8/He C3H8/CO2 C3H8/He C3H8/CO2 C3H8/He

Conversion (%)

Selectivity (%)

C3H8

CO2

C3H6

C2H6

C2H4

CH4

3.4 3.6 9.2 9.6 14.5 15.2 24.4 22.7 25.8 23.3

0.5 1.7 2.8 4.6 6.2 -

94.6 95.4 96.1 96.8 94.8 95.8 95.4 96.5 94.7 95.7

0.8 0.9 0.6 0.8 1.0 1.1 1.2 1.1 2.1 1.4

3.2 2.4 2.0 1.6 2.6 2.0 2.3 1.6 2.1 1.8

1.4 1.3 1.3 0.8 1.6 1.0 1.1 0.7 1.1 1.1

Reaction conditions: molar ratio of CO2(He)/C3H8 = 5/1, total flow rate = 30 cm3min-1, catalyst weight = 500 mg, the results after 10 min-on-stream.

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Propene was the main hydrocarbons product during the dehydrogenation of propane in the presence and absence of CO2. In both processes, the selectivity to propene is changed in the narrow range between 94 to 96 %, which clearly suggests that the MCM-41 vanadium oxide materials are very selective catalysts for the dehydrogenation of propane. In addition, hydrocarbons byproducts attributed to cracking such, as ethene, ethane and methane are de-tected in the gaseous products flow. The influence of vanadium loading on an initial yield of propene at reaction temperature of 600°C is shown in Figure 2. It can be seen that the initial yield of propene increases almost linearly with vanadium loading up to 10 wt% of V2O5 and then it only slightly increases at higher V loading. Moreover, a comparison of the initial propene yield obtained in CO2 to those in an inert gas atmosphere reveals that CO2 promotes propene formation over the MCM-41-supported vanadium oxide catalysts with high V loading. In a contrast, over the V/MCM-41 catalysts with low vanadium loading CO2 exerts an adverse effect.

Fig. 2. Variation of initial propene yield with the catalyst V content during the dehydrogenation of propane in the presence and absence of CO2 at 600°C

Fig. 3. Variation of the selectivity to propene and the conversion of propane with time-on-stream at 600°C. Symbols: (□, ■) conversion of propane; (○, ●) selectivity to propene. Open symbols: reaction in the absence of CO2. Close symbols: reaction in the presence of CO2

Base on the catalytic tests and the H2-TPR measurements it could be concluded that the presence

of dispersed V5+ species of type (SiO)3≡V=O is crucial for high catalytic activity in the dehydrogenation.


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On the other hand, the promoting effect of CO2 is due to the presence of the polymeric V5+ species, V2O5like. It is widely known that the V2O5-like species could be reduced to a greater extent than isolated tetrahedral vanadium sites [2, 7]. This leads us to conclusion that the higher yield of propene observed over the catalysts with V loading above 5% of V2O5 in the dehydrogenation with CO2 may be due to its oxidative abilities. Quite recently, it has been proposed that in the dehydrogenation of hydrocarbons CO2 serves as an oxidant agent in the redox cycle (1 and 2).

R + VO x CO 2

+

VO x-1

R' + VO x-1 + H2O

(1)

CO + VO x

(2)

where: R - saturated hydrocarbon or H2, R’- unsaturated hydrocarbon (absence in the case of H2). Such redox mechanism was proposed for explanation of promoting role of CO2 observed in the dehydrogenation of ethylbenzene, isobutane and propane over V-containing catalysts [5, 8, 9]. Serious problem in the dehydrogenation of light alkanes is rapid catalysts deactivation by the coke formation. In the present study this unfavorable effect has revealed as a decreasing of the propane conversion vs. time-on-stream (Fig.3). The deactivation is observed over all of the tested MCM-supported vanadium oxide catalysts. Moreover, there is no significant difference in the deactivation rate between process carried out with and without of CO2, indicating that its presence does not

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suppress deactivation, as has been observed in the process carried out over SiO2-supported chromium oxide catalysts [10]. Presented results reveal that the MCMsupported vanadium oxide materials are active and very selective catalysts for the dehydrogenation of propane in the presence and absence of CO2. The catalytic activity of these materials is connected with presence of dispersed V5+ species, which predominates at low V loading. AKNOWLEDGMENTS. The researches were supported in part by the Polish State Committee for Scientific Research, under Project: C2/331/BW/2007. REFE RENCES 1.

Vornes V., Lopez C., Lopez H., Marinez A., Appl. Catal. A: General 2003. N 249. P. 345. 2. Berndt H., Martin A., Bruckner, A., Schreier E., Muller D., Kosslick H., Wolf G-U., Lucke B., J. Catal. 2000. V. 191 P. 384. 3. Solsona B., Blasco T., Lopez Nieto J., Pena M., Rey F., Vidal-Moya V., J. Catal. 2001. V. 203. P.443. 4. Kondratenko E., Cherian M., Bearns M., Su D., Schlogl R., Wang X., Wachs I., J. Catal. 2005. V. 234. P. 131. 5. Takahara I., Saito M., Inaba M., Murata K., Catal. Lett. 2005. V. 102. P. 201. 6. Gajek T., Michorczyk P., Ogonowski J., Przemysł Chemiczny (in press). 7. Liu Y-M., Cao Y., Yi N., Feng W-L., Dai W-L., Yan S-R., He H-Y., Fan K-N. J. Catal. 2004. V. 224. P. 417. 8. Sakurai Y., Suzuki T., Nakagawa K., Ikenaga N., Aota H., Suzuki T., 2002. V. 209. P. 16. 9. Ogonowski J., Skrzynska E., Chim. Chim. Techn. 2007. V. 50. P 48. 10. Takahara I., Chang W., Mimura N., Saito M., Catal. Today 1998. V. 45. P. 55.
