Oxidative Dehydrogenation of n-butane over Tin Pyrophosphate Based Catalysts

Transcription

1 Prog. Catal, Vol. 10, No. 1-2, pp (2001) Prog. Catal. Oxidative Dehydrogenation of n-butane over Tin Pyrophosphate Based Catalysts Ioan-Cezar Marcu *a, Jean-Marc M. Millet b, Ioan Săndulescu a a University of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis b Institut de Recherches sur la Catalyse-CNRS associé à l Université Claude Bernard Lyon I, 2, Av. Albert Einstein, Villeurbanne Cedex, France Received ; accepted Abstract Two tin pyrophosphates synthesized using different methods were characterized and tested in the oxidative dehydrogenation of n-butane in the temperature range C, at atmospheric pressure. The catalytic properties have been shown to depend upon the crystallinity and surface area of the pyrophosphates, which appearedto be different depending on the method of preparation used and on the phase composition of the catalysts. Key words: tin pyrophosphates, oxidative dehydrogenation, n-butane 1. Introduction The oxidative dehydrogenation of alkanes could become an alternative reaction to produce olefins due to its potential advantages over dehydrogenation. The catalysts developped up to now are not efficient enough for an industrial application and the discovery of new active and selective catalysts is a major challenge of the petrochemical industry. Several phosphate-based catalysts have been tested for the oxidative dehydrogenation of propane [1,2] and isobutane [3-5] and revealed interesting catalytic properties. Our previous study on Ti, Zr, Ce and B phosphates showed that for the oxidative dehydrogenation of n-butane, the best catalysts were titanium pyrophosphate based catalysts [6]. In this article, we report the study of two tin pyrophosphates based catalysts for the oxidative dehydrogenation of n-butane. Although Sn has been studied as additive to phosphate based catalysts for light alkanes oxidation and tin phosphate was studied as support [7,8], it is the first time that pure phases are studied as catalysts. * Corresponding author: Univ. Bucharest, Fac. Chemistry, Dept. Chemical Technology and Catalysis, Bd. Regina Elisabeta, No. -12, Sector 3, Bucharest 7036, Romania, Fax , icmarcu@chem.unibuc.ro

2 72 I.C. Marcu et al. 2. Experimental Two methods have been used to prepare the catalysts. The first one (M1) consisted to react phosphoric acid (85%) with tin oxide. The two compounds were mixed until homogeneous slurry was formed. This was dried at 200 C for 3 h and calcined at 00 C for 1 h and at 900 C for 12 h. The second method (M2) was inspired from a method described by Ai [9], and based upon the reaction between a solution containing 580 ml of water, 0 ml of lactic acid (85%), 5ml of NH 3 (28%) and 7.39 g of SnCl 5H 2 O with a solution of phosphoric acid (106 g, 30%) added dropwise. The gel obtained was separated by centrifugation and dried between 80 and 200 C with a gradual increase of the temperature over 6 h. The obtained solid was calcined under air at 300, 50 and 600 C for 6 h at each temperature. The metal contents of the catalysts were quantitatively determined by atomic absorption (ICP) and their specific surface areas measured by the BET method using nitrogen adsorption. Powder X-ray diffraction (XRD) patterns were obtained using a Bruker D5005 diffractometer and Cu Kα radiation. They were recorded with 0.02 (2θ) steps over the 3-80 angular range with 1 s counting time per step. XPS measurements were performed with a VG ESCALAB 200 R. Charging of the samples was corrected by setting the binding energy of adventitious carbon (C1s) at 28.5 ev. The experimental precision on XPS quantitative measurements was considered to be around 20%. Mössbauer spectra have been recorded at room temperature and the chemical shifts δ were quoted with respect to BaSnO 3. The spectra were taken on a velocity scale between 6 and +6 mm/s. Scanning electron microscopy (SEM) was performed on a Hitachi S-800 scanning electron microscope in order to study the morphology of the catalysts. The samples were sputtered coated with gold before obtaining the micrographs. The oxidative dehydrogenation of n-butane was carried out in a fixed bed quartz tube down-flow reactor operated at atmospheric pressure. The conditions of the catalytic test were described elsewhere [6]. A gas feed with an air to n-butane ratio equal to 5 and a VVH of 1000 h -1 with respect to n-butane were used. The major products formed under these reaction conditions were 1- and 2-butenes, butadiene, CO, CO 2, and cracking products (methane, ethane, ethylene and propylene). Characterization of the samples has been performed before and after catalytic test. After catalytic test, the catalysts were recovered by cooling them down in the flow of reactants. 3. Results and discussion 3.1. Characterization of the catalysts The physico-chemical characteristics of the catalysts are presented in Table 1. The P:Sn ratio obtained from chemical analysis was close to the theoretical one for SnP 2 O 7 -M1, but smaller for SnP 2 O 7 -M2. This result may be interpreted by the fact that two phases, SnP 2 O 7 and SnO 2, should be present in the SnP 2 O 7 -M2 catalyst. It is

3 73 Oxidative Dehydrogenation of n-butane over Tin Pyrophosphate Based Catalysts possible that, during the preparation of M2 catalyst, phosphoric acid remain unreacted. Table 1. The physicochemical characteristics of the catalysts Catalyst BET surface area (m 2.g -1 ) P/M atomic ratio Before test After test Bulk (ICP) Surface (XPS) SnP 2 O 7 -M SnP 2 O 7 -M The specific surface areas of the catalysts have been measured before and after catalytic test (Table 1). The specific surface area of the solid prepared using method M1 was low compared to that of the solid prepared using method M2. The latter appeared to be uncommonly high for pyrophosphates. Although this surface area was very high, the pore volumes of this solid was small (0.26 cm 3.g -1 ), with pore size equal to that of mesopores. The adsorption isotherm of this solid showed a hysteresis loop (Fig. 1) classified as type IV by IUPAC [10]. After catalytic test, the specific surface area remained almost the same for SnP 2 O 7 -M1 but decreased substantially for SnP 2 O 7 -M2. Adsorbed-desorbed volume (cm 3.g -1 ) ,0 0,2 0, 0,6 0,8 1,0 P/P 0 a b theta (degres) Fig. 1: Adsorption isotherm of SnP 2 O 7 -M2. Fig. 2. X-ray diffraction patterns of the tin pyrophosphate catalysts before catalytic test: (a) SnP 2 O 7 -M1, (b) SnP 2 O 7 -M2. The XRD patterns of the prepared compounds are displayed in Fig. 2. The solid prepared using method M1 corresponded to a well-crystallized pure phase whereas that prepared using method M2 was amorphous. The XRD patterns of the catalysts recorded after catalytic test were not affected by the catalytic test. The surface composition of the catalysts was investigated using XPS (Table 1). For SnP 2 O 7 -M2, P:Sn surface ratio was lower than into the bulk and SnP 2 O 7 -M1 presented a P:Sn surface ratio equal to the bulk ratio. The total absence of remaining chlore at the surface of the catalyst prepared using method 2 has been checked.

4 7 I.C. Marcu et al. The results of the scanning electron microscopic investigation are shown in Fig. 3. They show that the morphology of the catalyst particles is different for the two pyrophosphates. SnP 2 O 7 -M2 particles were round-shaped and the SnP 2 O 7 -M1 particles had less uniform size and shape because of sintering of the sample. (a) (b) Fig. 3: Scanning electron micrographs of catalyst samples as prepared: (a) SnP 2 O 7 -M1, (b) SnP 2 O 7 -M2. Relative Transmission Relative transmission (a) Velocity (mm/s) (c) Velocity mm/s Relative transmission Relative transmission (b) Velocity mm/s Velocity mm/s Fig. : Mössbauer 119 Sn spectra of SnP 2 O 7 catalysts recorded before and after catalytic test: (a) SnP 2 O 7 -M1 before the test, (b) SnP 2 O 7 -M1 after the test, (c) SnP 2 O 7 -M2 before the test, (d) SnP 2 O 7 -M2 after the test (d)

5 75 Oxidative Dehydrogenation of n-butane over Tin Pyrophosphate Based Catalysts The Mössbauer spectra of the catalysts before and after the catalytic test are displayed in Fig. and the Mössbauer parameters calculated are listed in Table 2. All values are in mm/s. Table 2 Mössbauer 119 Sn parameters of SnP 2 O 7 before and after the catalytic test. Sample δ (mm/s) a (mm/s) b Γ (mm/s) c A (%) d Atribution SnP 2 O 7 -M1 before -0.09(2) 0.2(2) 0.88(1) 100 Sn IV SnP 2 O 7 -M1 after -0.12(1) 0.236(3) 0.85(2) 100 Sn IV SnP 2 O 7 -M2 before -0.33(1) 0.538(3) 0.899(6) 100 Sn IV SnP 2 O 7 -M2 after (1) 3.37(1) 0.511(2) 1.67(3) 0.858() 0.858() a chemical shift (quoted with respect to BaSnO 3 ). b quadrupole splitting. c line width. d contribution of site. The Mössbauer spectra taken before catalytic test shown, for both catalysts, only the signal coresponding to Sn IV. After the catalytic test, the Mössbauer spectrum of SnP 2 O 7 -M1 is unchanged, but that of SnP 2 O 7 -M2 shown a low intensity signal which correspond to Sn II species. This may be interpreted by a reduction of the SnP 2 O 7 -M2 catalyst during the reaction of n-butane Catalytic properties of the catalysts The catalytic properties of the pyrophosphates have been determined in the temperature range of C with an air:n-buatne ratio of 1:5 and a VVH = 1000 h -1. The results obtained at 90 and 530 C are presented in Table 3 with the selectivities to butenes, butadiene, total oxidation and cracking products. Table 3. Catalytic properties of the tested catalysts Selectivity (%) Catalyst Temp. Conv. ( C) (%) butenes butadiene TDS a CRA b CO x SnP 2 O 7 -M SnP 2 O 7 -M SnO a Total Dehydrogenation Selectivity. b Cracking products. The two pyrophosphates presented very different activities but were both selective towards oxidative dehydrogenation products. SnP 2 O 7 -M1 was mainly selective towards butenes and formed very few butadiene and CO x. SnP 2 O 7 -M2 was selective towards butadiene but produced large amounts of CO x. The oxido-reduction of the tin during the catalytic reaction evidenced by Mossbauer spectroscopy in the case of SnP 2 O 7 -M2, may be responsible for the deep oxidation of butenes into butadiene and carbon oxides. On the other hand, the characterization of the SnP 2 O 7 -M2 catalyst has Sn IV Sn II

6 76 I.C. Marcu et al. shown that it presented bulk and surface P/Sn ratios lower than 2, which was interpreted by the fact that two phases, SnP 2 O 7 and SnO 2, are present in this catalyst even if the latter phase has not been detected by XRD because it is probably under the limit of detection of this technique. We suppose that this SnO 2 phase is responsible for the large amounts of CO x produced with SnP 2 O 7 -M2 catalyst. For confirming this, the catalytic properties of SnO 2 have been determined and are also presented for comparison in Table 3. We observe that this phase is non-selective for oxidative dehydrogenation, leading mainly to carbon oxides. The activation energies corresponding to the transformation of n-butane on the different catalysts have been calculated. The activation energy on SnP 2 O 7 -M1 was equal to 32 kcal.mol -1 and for SnP 2 O 7 -M2 was lower, namely 21 kcal.mol -1. The activation energy for SnO 2 was only 3.6 kcal.mol -1. The value obtained for the activation energy on SnP 2 O 7 -M2 catalyst, lower than that on SnP 2 O 7 -M1 catalyst, confirm once more that SnO 2 phase is present in this catalyst. Finally, it is interesting to note that the catalytic performances of SnP 2 O 7 -M1 are comparable with those of the best catalysts in this reaction reported in the literature [11], which are summarized in Fig. 5. All data points fall below the line corresponding to the case of k A = k E of equation: k alkane + [O] A k alkene + [O] E degradation products which indicate thate none of the oxide catalysts known is more active for n-butane activation than for butene and butadiene activation. Dehydrogen. Select. (%) Conversion (%) Fig. 5. Selectivity for oxidative dehydrogenation of n-butane to butenes and butadiene on the best catalysts reported in the literature ( SnP 2 O 7 -M1). ( - data taken from literature [11]) Solide line denotes selectivity-conversion relationship for k A = k E.

7 77 Oxidative Dehydrogenation of n-butane over Tin Pyrophosphate Based Catalysts. Conclusions The crystallinity, composition, surface area and atomic P/Ti surface ratio of the two tin pyrophosphate based catalysts were different depending upon the method used to prepare them. Consequently, the catalytic properties of the two catalysts were different: SnP 2 O 7 -M1 was mainly selective towards butenes and formed very few butadiene and CO x, and SnP 2 O 7 -M2 was selective towards butadiene but produced large amounts of CO x. A reduction of the tin during the catalytic reaction was evidenced in the case of SnP 2 O 7 -M2. SnP 2 O 7 -M1 catalyst is comparable with the best catalysts in this reaction reported in the litterature. Acknowledgements The authors are gratefull to Dr. Jean-Claude Jumas for her expert technical assistance with Mössbauer spectra recording. References 1. Takita, Y., Yamashita, H., and Moritaka, K., Chem. Lett., 1733 (1989). 2. Savary, L., Saussey, J., Costentin, G., Bettahar, M.M., Gubelman-Bonneau, M., and Lavalley, J.C., Catal. Today, 32, 57 (1996). 3. Takita, Y., Kurosaki, K., Mizuhara,Y., and Ishihara, T., Chem. Lett., 335 (1993).. Takita, Y., Sano, K., Kurosaki, K., Kawata, N., Nishiguchi, H., Ito, M., and Ishihara, T., Appl. Catal., 167, 9 (1998). 5. Al-Zahrani, S.M., Elbashir, N.O., Abasaeed, A.E., and Abdulwahed, M., Catal. Lett., 69, 65 (2000). 6. Marcu, I.C., Millet, J.M.M., and Sandulescu, I., Rev. Roum. Chim., in press. 7. Blum, P.R., and Pepera, M.A., EU Patent Appl. (1992) EP Sadykov, V.A., Pavlova, S.N., Saputina, N.F., Zolatarskii, I.A., Pakhomov, N.A., Moroz, E.M., Kuzmin, V.A., and Kalinkin, A.V., Catal. Today, 61(1-), 93 (2000). 9. Ai, M., Appl. Catal., 8, 51 (1989). 10. IUPAC Reporting physisorption data for gas/solid system, Pure Appl. Chem., 57, 603 (1985). 11. Kung, H.H., Adv. Catal., 0, 1 (199).