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Iron antimony oxide catalysts for the ammoxidation of propene to acrylonitrile: comments on the method of preparation of tellurium promoted catalysts
Applied Catalysis A: General 217 (2001) 33–39
Iron antimony oxide catalysts for the ammoxidation of propene to acrylonitrile: comments on the method of preparation of tellurium promoted catalysts Matthew D. Allen1 , Graham J. Hutchings∗ , Michael Bowker 2 Leverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, Liverpool L69 3BX, UK Received 4 December 2000; received in revised form 26 February 2001; accepted 4 March 2001
Abstract Three methods of catalyst preparation for tellurium promoted iron antimony oxide catalysts are investigated: (a) co-precipitation, (b) impregnation and (c) reaction of solid oxides. For all three methods, the addition of Te increases the selectivity for acrylonitrile significantly. For catalysts prepared by co-precipitation, this effect is only apparent at low Te concentrations whereas, for the other two preparation methods, the positive effect on selectivity is maintained at higher doping levels. The catalysts are characterised using X-ray diffraction and X-ray photoelectron spectroscopy. For the Fe30 Sb60 Te2 Ox formulations prepared using the three methods, a similar Te/Sb surface atomic ratio was observed which is consistent with the similar specific activity observed with all three catalysts, indicating that the three methods of preparation can prepare similar surfaces following calcination and catalyst testing. However, the co-precipitation method gives catalyst surface areas approximately three times higher than the other two methods and, consequently, gives the best catalyst performance. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Co-precipitation; Impregnation; Acrylonitrile; X-ray photoelectron spectroscopy; Catalyst promotion
1. Introduction The ammoxidation of propene to acrylonitrile represents a major commercial reaction that has been established on the basis of research by Grasselli and co-workers [1–4] using bismuth molybdate catalysts ∗ Corresponding author. Present address: Department of Chemistry, Cardiff University, Cardiff CF10 3TB, UK. E-mail addresses: [email protected] (G.J. Hutchings), [email protected] (M. Bowker). 1 Present address: NNC Ltd., Warrington Road, Risley, Warrington, Cheshire WA3 6BZ, UK. 2 Co-corresponding author. Present address: Department of Chemistry, University of Reading, Whiteknights Park, Reading RG6 2AD, UK.
and uranium antimonate catalysts. Other oxide systems have been found to be effective and these include tin antimony oxides [5] and iron antimony oxides [6–12]. Industrial catalysts, based on iron antimony oxide, contain a number of other components, e.g. Fe66 Cu2.4 Sb15 Te0.01 Mo0.3 W0.15 O45.5 (SiO2 )36 [13] which have been optimised to carry out the necessary steps of reactant activation and product formation. Of these promoters, Te is often incorporated as an additive in FeSb2 O4 catalysts [14] and, more recently, Te is also incorporated into catalyst formulations for propane ammoxidation [15–18]. In this paper, we compare three preparation methods for Te-promoted iron antimony oxide catalysts.
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 5 8 3 - X
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M.D. Allen et al. / Applied Catalysis A: General 217 (2001) 33–39
2. Experimental 2.1. Catalyst preparation 2.1.1. Co-precipitation Iron(III) nitrate (BDH, Analar) was heated to 60◦ C at which temperature it dissolved in the water of hydration. The temperature was raised to 80◦ C and antimony(III) oxide (Aldrich, 99%) was then added and the resulting slurry was stirred and the pH was adjusted to pH3 with the addition of ammonium hydroxide. A solution of the required amount of telluric acid (Aldrich) in water (10 ml) was added and the mixture was stirred at 80◦ C. The catalyst was then dried (120◦ C, 12 h, air), followed by stepwise calcination (200◦ C, 2 h; 500◦ C, 3 h; 795◦ C, 4 h). A range of formulations were prepared using this method, Fe30 Sb60 Tea Ox where a = 0, 1, 2, 4, 9 and 15. 2.1.2. Solid oxide method The method of Aso et al. [6] was used to prepare an iron antimony oxide with an Fe:Sb atomic ratio of 1:2. A slurry of iron nitrate and antimony oxide at pH3 was prepared as described above. Tellurium oxide (TeO2 , Aldrich, 99%) was then added and the slurry was stirred at 80◦ C until almost all the water had evaporated. The catalyst was then dried (120◦ C, 12 h, air), followed by calcination (900◦ C, 7 h). Two catalysts were prepared by this method, Fe30 Sb60 Tea Ox where a = 0, 2. 2.1.3. Impregnation Impregnated catalysts were prepared using the method of incipient wetness with FeSb2 O4 prepared by the solid oxide method described above. The required amount of telluric acid (H6 TeO6 , Aldrich) was dissolved in the minimum volume of water to just wet the calcined FeSb2 O4 . The solution was then added dropwise to the FeSb2 O4 and the catalyst was dried (120◦ C, 12 h) and calcined (480◦ C, 5 h). Three catalysts were prepared with this method, Fe30 Sb60 Tea Ox where a = 0, 2, 4. The Te-free catalyst was prepared in an analogous manner using water in place of the aqueous solution of telluric acid. 2.2. Catalyst characterisation Catalysts were characterised prior to, and following, catalyst testing using a range of techniques. Surface
areas were determined according to the BET method using nitrogen adsorption. Powder X-ray diffraction was carried out using a Philips PW 710 instrument using Cu K␣ radiation. X-ray photoelectron spectroscopy was carried out using a VG ESCA 3 spectrometer using Al K␣ radiation with an anode voltage of 10 keV, emission current of 20 mA and a pass energy of 20 eV. Pre- and post-reactor samples were mounted using double sided adhesive tape and data were analysed using Spectra5 software. Photoionisation cross-sections were obtained from Scofield [19] and corrected for the angular distribution of the photoelectron and asymmetry parameters using the method of Reilman et al. [20]. 2.3. Catalyst testing Catalyst testing was carried out in a standard laboratory microreactor using a fixed volume of catalyst (4 ml). A propene/ammonia mixture (2/3 by vol), air and nitrogen were fed to the reactor using calibrated mass flow controllers. The flow rates were maintained at C3 H6 /NH3 , 12 ml/min; O2 , 12 ml/min; N2 48 ml min−1 . The reactor tube was mounted vertically and the reactant gases flowed up through the reactor. The reactor temperature was controlled using a thermocouple positioned in the centre of the catalyst bed. The gas lines exiting the reactor were heated to ensure condensation of products did not occur and the products were analysed using on-line gas chromatography.
3. Results and discussion The catalytic performance of the Te-promoted FeSb2 O4 catalysts was determined for the ammoxidation of propene and the results are given in Table 1. For the co-precipitated catalysts, the addition of Te at low levels enhanced the selectivity for the formation of acrylonitrile significantly. This was primarily due to decreasing the selectivity to the minor non COx by-products: acetonitrile, acrolein and HCN. For the Fe30 Sb60 Te1 Ox formulation, compared with the unpromoted formulation, HCN and acrolein were formed in only trace amounts and, at 402◦ C, the only significant by-product was ca. 2% acetonitrile, together with trace levels of COx , HCN and acrolein. Higher levels
M.D. Allen et al. / Applied Catalysis A: General 217 (2001) 33–39
35
Table 1 Catalytic performance of Te-promoted FeSb2 Ox catalystsa Temperatureb (◦ C)
Catalyst
c
FeSb2 Ox Fe30 Sb60 Te1 Ox c Fe30 Sb60 Te2 Ox c Fe30 Sb60 Te4 Ox c Fe30 Sb60 Te9 Ox c Fe30 Sb60 Te15 Ox c FeSb2 Ox d Fe30 Sb60 Te2 Ox d Fe2 Sb2 Ox e,f Fe30 Sb60 Te2 Ox e Fe30 Sb60 Te4 Ox e
277
302
327
352
377
402
427
452
3 (4) 2 (5) 2 (8) 3 (4) 7 (2) 1 (10) 2 (80) 9 (5) 4 (0) 0.5 (0.1) 0.5 (0.1)
4 3 4 4 8 2 8 10 5 1 2
5 5 7 7 9 3 11 12 8 2 3
8 8 13 10 11 5 18 16 11 3 4
16 15 24 16 13 8 24 21 17 7 8
24 22 38 22 16 11 30 29 26 11 12
38 32 52 30 20 15 45 40 38 17 15
45 44 66 38 25 20 53 53 51 24 16
(22) (34) (42) (24) (10) (55) (72) (14) (2) (2) (5)
(55) (56) (65) (45) (23) (80) (69) (30) (12) (13) (30)
(75) (78) (86) (63) (38) (88) (60) (50) (24) (53) (70)
(84) (93) (89) (80) (52) (92) (60) (67) (35) (60) (89)
(90) (98) (87) (89) (63) (92) (61) (78) (45) (62) (92)
(90) (97) (88) (89) (75) (88) (62) (83) (51) (70) (81)
(90) (94) (90) (90) (78) (85) (62) (83) (52) (72) (73)
C3 H6 /NH3 /O2 /N2 = 1/2/3/12 mole ratio; GHSV = 1330 h−1 . Results quoted as conversion (acrylonitrile selectivity). c Co-precipitated. d Solid oxide method. e Impregnation. f Catalyst treated analogously as the Te-promoted impregnated catalyst using incipient wetness with water. a
b
of Te-doping did not lead to an enhancement in selectivity, but an increase in catalyst activity was apparent. The increase in selectivity for acrylonitrile was also apparent for the impregnated catalysts. However, it is apparent that treatment of the catalyst, in the absence of Te, with water leads to a significant decrease in acrylonitrile selectivity and a slight decrease in activity. The impregnation process does not decrease the surface area of the unpromoted FeSb2 O4 material, both catalysts had surface areas of 14 m2 /g following catalyst testing and, therefore, the reduction in selectivity is probably due to hydroxylation of the surface. However, addition of Te leads to a significant enhancement in acrylonitrile selectivity. For the materials prepared using the solid oxide preparation route, again the addition of Te gives a marked enhancement in the selectivity for acrylonitrile. It is, therefore, apparent that the enhancement in acrylonitrile selectivity is a general feature of Te-promotion at low levels. The effect is due to a decrease in the formation of acetonitrile indicating that the Te-doped catalyst surface has a decreased activity with respect to carbon–carbon bond scission, a process that has to be eliminated if high acrylonitrile selectivities are to be achieved. The addition of Te has a significant effect on the surface area of the promoted catalysts after
activation and catalyst testing. For the series of catalysts prepared using co-precipitation, Fe30 Sb60 Tea Ox , the surface areas (all ±1 m2 /g) were found to be a = 0, 23 m2 /g; a = 1, 21 m2 /g; a = 2, 24 m2 /g; a = 4, 20 m2 /g; a = 9, 13 m2 /g; a = 15, 7 m2 /g. Hence, at low levels of doping, a = 1–4, there is no significant effect on surface area but, at higher levels, significant sintering is observed. The co-precipitated catalysts give much higher surface areas when compared with the catalysts prepared using the solid oxide methods or impregnation (FeSb2 O4 solid oxide method = 14 m2 /g). For the impregnated catalysts, the addition of Te does lead to significant sintering even at low Te-levels (Fe30 Sb60 Te4 Ox , impregnated 8 m2 /g). It is, therefore, important to consider the catalyst activity of these materials in terms of the specific activity. The addition of low levels of Te leads to a significant enhancement in the specific activity for the formation of acrylonitrile (i.e. mol acrylonitrile/m2 /h), with the highest activity observed with Fe30 Sb60 Te2 Ox prepared using the co-precipitation and impregnation methods. Characterisation of the catalysts before and after catalyst testing was carried out using powder X-ray diffraction and the results are shown in Fig. 1 for the co-precipitated catalysts and Fig. 2 for the impregnated catalysts. There are two significant observations.
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M.D. Allen et al. / Applied Catalysis A: General 217 (2001) 33–39
Fig. 1. Powder X-ray diffraction patterns of catalysts prepared by co-precipitation before and after catalyst testing. Key: FeSbO4 (䊉); ␣-Sb2 O4 (䊏); (a) FeSb2 O4 after catalyst testing; (b) FeSb2 O4 fresh; (c) Fe30 Sb60 Te1 Ox after catalyst testing; (d) Fe30 Sb60 Te1 Ox fresh; (e) Fe30 Sb60 Te15 Ox after catalyst testing; (f) Fe30 Sb60 Te15 Ox fresh.
First, there are no separate tellurium-containing phases observable in the promoted catalyst by either preparation method. The phases observed being FeSbO4 and ␣-Sb2 O4 . Second, for both sets of catalysts, the reflections for ␣-Sb2 O4 appear to diminish in intensity on Fe-doping, particularly in the fresh calcined catalysts. Further characterisation was carried out using X-ray photoelectron spectroscopy (Table 2). Te 3d3/2 binding energy for all the promoted catalysts indicates that it is 586.5 ± 0.5 eV. Comparison with reference samples (TeO2 , Te4+ , 3d3/2 = 586.4 eV; H6 TeO6 , Te6+ , 3d3/2 = 587.8 eV) indicates that, for all these catalysts, tellurium is present on the surface as Te4+ . The
Fe/Sb surface ratio was not significantly affected by the addition of Te for either the fresh or used catalysts. All samples showed significant enhancement of the surface concentration of both Sb and Te above the levels present in the bulk of the catalyst. The surface enrichment of Te after catalyst testing increases with the level of Te-doping, but is always higher than the concentration expected from the stoichiometric bulk composition. The Te/Sb surface ratios for the Fe30 Sb60 Te2 Ox catalysts are not too dissimilar, although the impregnation method, as expected, does lead to a slightly higher level of Te-enrichment [Te/Sb surface ratio: 0.074 (co-precipitated), 0.084 (solid
M.D. Allen et al. / Applied Catalysis A: General 217 (2001) 33–39
37
Fig. 2. Powder X-ray diffraction patterns of catalysts prepared using impregnation before and after catalyst testing. Key: FeSbO4 (䊉); ␣-Sb2 O4 (䊏); (a) Fe30 Sb60 Te2 Ox after catalyst testing; (b) Fe30 Sb60 Te2 Ox fresh; (c) FeSb2 O4 following incipient wetness with water, after used; (d) FeSb2 O4 following incipient wetness with water, fresh.
Table 2 XPS Data for Te-promoted catalysts Catalyst
Te 3d3/2 (eV)
a
Fe30 Sb60 Te1 Ox Fe30 Sb60 Te2 Ox a Fe30 Sb60 Te4 Ox a Fe30 Sb60 Te9 Ox a Fe30 Sb60 Te15 Ox a Fe30 Sb60 Te2 Ox b Fe30 Sb60 Te2 Ox c Fe30 Sb60 Te4 Ox c
Te/Sb
Fresh
Used
Fresh
Used
Fresh
Used
587.2 586.9 586.7 586.6 586.7 585.5 586.4 586.4
586.4 587.0 586.6 586.8 586.4 586.6 586.0 586.0
0.046 0.084 0.133 0.346 0.375 0.068 0.12 0.20
0.054 0.074 0.219 0.272 0.329 0.084 0.10 0.18
0.15 0.16 0.16 0.19 0.18 0.13 0.17 0.16
0.17 0.19 0.17 0.17 0.19 0.17 0.17 0.16
Co-precipitated, unpromoted Fe/Sb used = 0.17. Solid oxide method, umpromoted Fe/Sb used = 0.12. c Impregnation. a
b
Fe/Sb
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M.D. Allen et al. / Applied Catalysis A: General 217 (2001) 33–39
Fig. 3. Variation in specific activity (452◦ C, 10−5 mol acrylonitrile/m2 /h) and surface Te/Sb atomic ratio as a function of bulk Te/Sb atomic ratio. Key: (䊊): co-precipitated; (䊐): solid oxide method; (): impregnated; open symbols: specific activity; closed symbols: surface Te/Sb ratio.
oxide) and 0.1 (impregnated)]. Since this level of Te doping gives the highest enhancement in the specific activity, we consider that this surface Te/Sb ratio is optimal for propene activation. The specific activity (mol acrylonitrile m−2 h−1 ) and surface Te/Sb ratio are shown as a function of the bulk Te/Sb atom ratio in Fig. 3. It is apparent that the specific activity shows a significant enhancement with low Te doping levels. However, it is apparent that the data for three types of catalysts (i.e. prepared by co-precipitation, solid oxide method or impregnation) within experimental error are similar for the same bulk Te/Sb ratio. These results confirm that similar catalyst surfaces are being formed, and this is not affected markedly by the method of preparation. However, the co-precipitated catalysts exhibit the highest surface areas, being approximately three times higher than the other catalysts for the Fe30 Sb60 Te2 Ox formulation, and hence the best catalytic performance is observed with this method. It is clear from this study, comparing catalysts prepared by different methods, that Te-addition enhances both the selectivity for acrylonitrile and the specific activity. However, the optimal effects are observed at different loadings. For selectivity, the optimal
enhancement is observed with Fe30 Sb60 Te1 Ox with a surface Te/Sb ratio of 0.05 whereas, for the enhancement of specific activity, the optimal effects are observed with Fe30 Sb60 Te2 Ox with a surface Te/Sb ratio of 0.07–0.10. This may indicate that the effects have different origins since the enhancement in selectivity may be due to the blocking of non-selective oxidation sites, e.g. removal of ␣-Sb2 O4 from the catalyst as observed by powder X-ray diffraction analysis. The enhancement in activity may be due to electronic factors with Te4+ aiding the control or maintenance of the surface oxidation state of iron. However, with the solid oxide and impregnation method of catalyst preparation, there is the conflicting effect that Te addition leads to catalyst sintering even at low Te concentrations. It is only with the co-precipitation method that the high surface areas achieved with this method are maintained with the low levels of Te doping, and the sintering effect is only observed at higher Te concentrations. In conclusion, Te-doping of FeSb2 O4 leads to the observation of three effects (a) an enhancement in the selectivity to acrylonitrile due to the poisoning of carbon-carbon bond scission reactions, (b) an enhancement in the specific activity of acrylonitrile formation, particularly for Fe30 Sb60 Te2 Ox , and (c) a decrease in surface area of the catalyst with increasing Te concentration, particularly for the solid oxide and impregnation methods of preparation. The effects on selectivity and activity may result from different surface structural features since the optimal effects are observed at different surface Te/Sb atomic ratios. However, the three preparation methods investigated in this study produce similar surfaces and give similar specific activities and, hence, the effects are not considered to be dependent on the preparation method.
Acknowledgements We thank BASF and the EPSRC for financial support.
References [1] J.D. Burrington, C.T. Karisek, R.K. Grasselli, J. Catal. 63 (1980) 235.
M.D. Allen et al. / Applied Catalysis A: General 217 (2001) 33–39 [2] J.D. Burrington, C.T. Kartisek, R.K. Grasselli, J. Catal. 75 (1982) 225. [3] J.D. Burrington, C.T. Kartisek, R.K. Grasselli, J. Catal. 81 (1983) 489. [4] R.K. Grasselli, L. Callahan, J. Catal. 14 (1969) 93. [5] F.J. Berry, Adv. Catal. 30 (1981) 97. [6] I. Aso, S. Furakawa, N. Yamazoe, T. Seiyama, J. Catal. 64 (1980) 29. [7] N. Yamazoe, I. Aso, T. Amamoto, T. Seiyama, in: Proceedings of the 7th International Congress on Catalysis, 1980, p. 1239. [8] V.V. Malakho, F.G. Abdikov, Kinet. Katal. 13 (1972) 168. [9] N. Burriesci, F. Garbassi, F. Petera, M. Petrini, J. Chem. Soc., Farad. Trans. 1 78 (1982) 817. [10] V. Fattore, Z.A. Fuhrman, G. Manara, B. Notari, J. Catal. 37 (1975) 223.
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[11] R.G. Teller, J.F. Brazdil, R.K. Grasselli, J. Chem. Soc., Farad. Trans. 1 81 (1985) 1693. [12] G.I. Straguzzi, K.B. Bischoff, T.A. Koch, G.C. Shuit, J. Catal. 103 (1987) 357. [13] Y. Sasaki, H. Utsumi, Y. Nakamura, K. Mori, European Patent 0,281,280 (1980). [14] R.K. Grasselli, Appl. Catal. 57 (1990) 149. [15] H.W. Zanthoff, NATO ASI Ser. 3 44 (1998) 435. [16] M. Vaarkamp, U. Takashi, Appl. Catal. A a174 (1998) 99. [17] A. Kaddouri, C. Mazzocchia, E. Tempesti, Appl. Catal. A 180 (1999) 271. [18] H.W. Zanthoff, S. Schaefer, G.-U. Wolf, Appl. Catal. A 164 (1997) 105. [19] J.H. Scofield, J. Elect. Spec. Rel. Phen. 9 (1976) 29. [20] R. Reilman, A. Msecane, S.T. Manson, J. Elect. Spec. Rel. Phen. 8 (1976) 389.
Iron antimony oxide catalysts for the ammoxidation of propene to acrylonitrile: comments on the method of preparation of tellurium promoted catalysts Matthew D. Allen1 , Graham J. Hutchings∗ , Michael Bowker 2 Leverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, Liverpool L69 3BX, UK Received 4 December 2000; received in revised form 26 February 2001; accepted 4 March 2001
Abstract Three methods of catalyst preparation for tellurium promoted iron antimony oxide catalysts are investigated: (a) co-precipitation, (b) impregnation and (c) reaction of solid oxides. For all three methods, the addition of Te increases the selectivity for acrylonitrile significantly. For catalysts prepared by co-precipitation, this effect is only apparent at low Te concentrations whereas, for the other two preparation methods, the positive effect on selectivity is maintained at higher doping levels. The catalysts are characterised using X-ray diffraction and X-ray photoelectron spectroscopy. For the Fe30 Sb60 Te2 Ox formulations prepared using the three methods, a similar Te/Sb surface atomic ratio was observed which is consistent with the similar specific activity observed with all three catalysts, indicating that the three methods of preparation can prepare similar surfaces following calcination and catalyst testing. However, the co-precipitation method gives catalyst surface areas approximately three times higher than the other two methods and, consequently, gives the best catalyst performance. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Co-precipitation; Impregnation; Acrylonitrile; X-ray photoelectron spectroscopy; Catalyst promotion
1. Introduction The ammoxidation of propene to acrylonitrile represents a major commercial reaction that has been established on the basis of research by Grasselli and co-workers [1–4] using bismuth molybdate catalysts ∗ Corresponding author. Present address: Department of Chemistry, Cardiff University, Cardiff CF10 3TB, UK. E-mail addresses: [email protected] (G.J. Hutchings), [email protected] (M. Bowker). 1 Present address: NNC Ltd., Warrington Road, Risley, Warrington, Cheshire WA3 6BZ, UK. 2 Co-corresponding author. Present address: Department of Chemistry, University of Reading, Whiteknights Park, Reading RG6 2AD, UK.
and uranium antimonate catalysts. Other oxide systems have been found to be effective and these include tin antimony oxides [5] and iron antimony oxides [6–12]. Industrial catalysts, based on iron antimony oxide, contain a number of other components, e.g. Fe66 Cu2.4 Sb15 Te0.01 Mo0.3 W0.15 O45.5 (SiO2 )36 [13] which have been optimised to carry out the necessary steps of reactant activation and product formation. Of these promoters, Te is often incorporated as an additive in FeSb2 O4 catalysts [14] and, more recently, Te is also incorporated into catalyst formulations for propane ammoxidation [15–18]. In this paper, we compare three preparation methods for Te-promoted iron antimony oxide catalysts.
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 5 8 3 - X
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M.D. Allen et al. / Applied Catalysis A: General 217 (2001) 33–39
2. Experimental 2.1. Catalyst preparation 2.1.1. Co-precipitation Iron(III) nitrate (BDH, Analar) was heated to 60◦ C at which temperature it dissolved in the water of hydration. The temperature was raised to 80◦ C and antimony(III) oxide (Aldrich, 99%) was then added and the resulting slurry was stirred and the pH was adjusted to pH3 with the addition of ammonium hydroxide. A solution of the required amount of telluric acid (Aldrich) in water (10 ml) was added and the mixture was stirred at 80◦ C. The catalyst was then dried (120◦ C, 12 h, air), followed by stepwise calcination (200◦ C, 2 h; 500◦ C, 3 h; 795◦ C, 4 h). A range of formulations were prepared using this method, Fe30 Sb60 Tea Ox where a = 0, 1, 2, 4, 9 and 15. 2.1.2. Solid oxide method The method of Aso et al. [6] was used to prepare an iron antimony oxide with an Fe:Sb atomic ratio of 1:2. A slurry of iron nitrate and antimony oxide at pH3 was prepared as described above. Tellurium oxide (TeO2 , Aldrich, 99%) was then added and the slurry was stirred at 80◦ C until almost all the water had evaporated. The catalyst was then dried (120◦ C, 12 h, air), followed by calcination (900◦ C, 7 h). Two catalysts were prepared by this method, Fe30 Sb60 Tea Ox where a = 0, 2. 2.1.3. Impregnation Impregnated catalysts were prepared using the method of incipient wetness with FeSb2 O4 prepared by the solid oxide method described above. The required amount of telluric acid (H6 TeO6 , Aldrich) was dissolved in the minimum volume of water to just wet the calcined FeSb2 O4 . The solution was then added dropwise to the FeSb2 O4 and the catalyst was dried (120◦ C, 12 h) and calcined (480◦ C, 5 h). Three catalysts were prepared with this method, Fe30 Sb60 Tea Ox where a = 0, 2, 4. The Te-free catalyst was prepared in an analogous manner using water in place of the aqueous solution of telluric acid. 2.2. Catalyst characterisation Catalysts were characterised prior to, and following, catalyst testing using a range of techniques. Surface
areas were determined according to the BET method using nitrogen adsorption. Powder X-ray diffraction was carried out using a Philips PW 710 instrument using Cu K␣ radiation. X-ray photoelectron spectroscopy was carried out using a VG ESCA 3 spectrometer using Al K␣ radiation with an anode voltage of 10 keV, emission current of 20 mA and a pass energy of 20 eV. Pre- and post-reactor samples were mounted using double sided adhesive tape and data were analysed using Spectra5 software. Photoionisation cross-sections were obtained from Scofield [19] and corrected for the angular distribution of the photoelectron and asymmetry parameters using the method of Reilman et al. [20]. 2.3. Catalyst testing Catalyst testing was carried out in a standard laboratory microreactor using a fixed volume of catalyst (4 ml). A propene/ammonia mixture (2/3 by vol), air and nitrogen were fed to the reactor using calibrated mass flow controllers. The flow rates were maintained at C3 H6 /NH3 , 12 ml/min; O2 , 12 ml/min; N2 48 ml min−1 . The reactor tube was mounted vertically and the reactant gases flowed up through the reactor. The reactor temperature was controlled using a thermocouple positioned in the centre of the catalyst bed. The gas lines exiting the reactor were heated to ensure condensation of products did not occur and the products were analysed using on-line gas chromatography.
3. Results and discussion The catalytic performance of the Te-promoted FeSb2 O4 catalysts was determined for the ammoxidation of propene and the results are given in Table 1. For the co-precipitated catalysts, the addition of Te at low levels enhanced the selectivity for the formation of acrylonitrile significantly. This was primarily due to decreasing the selectivity to the minor non COx by-products: acetonitrile, acrolein and HCN. For the Fe30 Sb60 Te1 Ox formulation, compared with the unpromoted formulation, HCN and acrolein were formed in only trace amounts and, at 402◦ C, the only significant by-product was ca. 2% acetonitrile, together with trace levels of COx , HCN and acrolein. Higher levels
M.D. Allen et al. / Applied Catalysis A: General 217 (2001) 33–39
35
Table 1 Catalytic performance of Te-promoted FeSb2 Ox catalystsa Temperatureb (◦ C)
Catalyst
c
FeSb2 Ox Fe30 Sb60 Te1 Ox c Fe30 Sb60 Te2 Ox c Fe30 Sb60 Te4 Ox c Fe30 Sb60 Te9 Ox c Fe30 Sb60 Te15 Ox c FeSb2 Ox d Fe30 Sb60 Te2 Ox d Fe2 Sb2 Ox e,f Fe30 Sb60 Te2 Ox e Fe30 Sb60 Te4 Ox e
277
302
327
352
377
402
427
452
3 (4) 2 (5) 2 (8) 3 (4) 7 (2) 1 (10) 2 (80) 9 (5) 4 (0) 0.5 (0.1) 0.5 (0.1)
4 3 4 4 8 2 8 10 5 1 2
5 5 7 7 9 3 11 12 8 2 3
8 8 13 10 11 5 18 16 11 3 4
16 15 24 16 13 8 24 21 17 7 8
24 22 38 22 16 11 30 29 26 11 12
38 32 52 30 20 15 45 40 38 17 15
45 44 66 38 25 20 53 53 51 24 16
(22) (34) (42) (24) (10) (55) (72) (14) (2) (2) (5)
(55) (56) (65) (45) (23) (80) (69) (30) (12) (13) (30)
(75) (78) (86) (63) (38) (88) (60) (50) (24) (53) (70)
(84) (93) (89) (80) (52) (92) (60) (67) (35) (60) (89)
(90) (98) (87) (89) (63) (92) (61) (78) (45) (62) (92)
(90) (97) (88) (89) (75) (88) (62) (83) (51) (70) (81)
(90) (94) (90) (90) (78) (85) (62) (83) (52) (72) (73)
C3 H6 /NH3 /O2 /N2 = 1/2/3/12 mole ratio; GHSV = 1330 h−1 . Results quoted as conversion (acrylonitrile selectivity). c Co-precipitated. d Solid oxide method. e Impregnation. f Catalyst treated analogously as the Te-promoted impregnated catalyst using incipient wetness with water. a
b
of Te-doping did not lead to an enhancement in selectivity, but an increase in catalyst activity was apparent. The increase in selectivity for acrylonitrile was also apparent for the impregnated catalysts. However, it is apparent that treatment of the catalyst, in the absence of Te, with water leads to a significant decrease in acrylonitrile selectivity and a slight decrease in activity. The impregnation process does not decrease the surface area of the unpromoted FeSb2 O4 material, both catalysts had surface areas of 14 m2 /g following catalyst testing and, therefore, the reduction in selectivity is probably due to hydroxylation of the surface. However, addition of Te leads to a significant enhancement in acrylonitrile selectivity. For the materials prepared using the solid oxide preparation route, again the addition of Te gives a marked enhancement in the selectivity for acrylonitrile. It is, therefore, apparent that the enhancement in acrylonitrile selectivity is a general feature of Te-promotion at low levels. The effect is due to a decrease in the formation of acetonitrile indicating that the Te-doped catalyst surface has a decreased activity with respect to carbon–carbon bond scission, a process that has to be eliminated if high acrylonitrile selectivities are to be achieved. The addition of Te has a significant effect on the surface area of the promoted catalysts after
activation and catalyst testing. For the series of catalysts prepared using co-precipitation, Fe30 Sb60 Tea Ox , the surface areas (all ±1 m2 /g) were found to be a = 0, 23 m2 /g; a = 1, 21 m2 /g; a = 2, 24 m2 /g; a = 4, 20 m2 /g; a = 9, 13 m2 /g; a = 15, 7 m2 /g. Hence, at low levels of doping, a = 1–4, there is no significant effect on surface area but, at higher levels, significant sintering is observed. The co-precipitated catalysts give much higher surface areas when compared with the catalysts prepared using the solid oxide methods or impregnation (FeSb2 O4 solid oxide method = 14 m2 /g). For the impregnated catalysts, the addition of Te does lead to significant sintering even at low Te-levels (Fe30 Sb60 Te4 Ox , impregnated 8 m2 /g). It is, therefore, important to consider the catalyst activity of these materials in terms of the specific activity. The addition of low levels of Te leads to a significant enhancement in the specific activity for the formation of acrylonitrile (i.e. mol acrylonitrile/m2 /h), with the highest activity observed with Fe30 Sb60 Te2 Ox prepared using the co-precipitation and impregnation methods. Characterisation of the catalysts before and after catalyst testing was carried out using powder X-ray diffraction and the results are shown in Fig. 1 for the co-precipitated catalysts and Fig. 2 for the impregnated catalysts. There are two significant observations.
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M.D. Allen et al. / Applied Catalysis A: General 217 (2001) 33–39
Fig. 1. Powder X-ray diffraction patterns of catalysts prepared by co-precipitation before and after catalyst testing. Key: FeSbO4 (䊉); ␣-Sb2 O4 (䊏); (a) FeSb2 O4 after catalyst testing; (b) FeSb2 O4 fresh; (c) Fe30 Sb60 Te1 Ox after catalyst testing; (d) Fe30 Sb60 Te1 Ox fresh; (e) Fe30 Sb60 Te15 Ox after catalyst testing; (f) Fe30 Sb60 Te15 Ox fresh.
First, there are no separate tellurium-containing phases observable in the promoted catalyst by either preparation method. The phases observed being FeSbO4 and ␣-Sb2 O4 . Second, for both sets of catalysts, the reflections for ␣-Sb2 O4 appear to diminish in intensity on Fe-doping, particularly in the fresh calcined catalysts. Further characterisation was carried out using X-ray photoelectron spectroscopy (Table 2). Te 3d3/2 binding energy for all the promoted catalysts indicates that it is 586.5 ± 0.5 eV. Comparison with reference samples (TeO2 , Te4+ , 3d3/2 = 586.4 eV; H6 TeO6 , Te6+ , 3d3/2 = 587.8 eV) indicates that, for all these catalysts, tellurium is present on the surface as Te4+ . The
Fe/Sb surface ratio was not significantly affected by the addition of Te for either the fresh or used catalysts. All samples showed significant enhancement of the surface concentration of both Sb and Te above the levels present in the bulk of the catalyst. The surface enrichment of Te after catalyst testing increases with the level of Te-doping, but is always higher than the concentration expected from the stoichiometric bulk composition. The Te/Sb surface ratios for the Fe30 Sb60 Te2 Ox catalysts are not too dissimilar, although the impregnation method, as expected, does lead to a slightly higher level of Te-enrichment [Te/Sb surface ratio: 0.074 (co-precipitated), 0.084 (solid
M.D. Allen et al. / Applied Catalysis A: General 217 (2001) 33–39
37
Fig. 2. Powder X-ray diffraction patterns of catalysts prepared using impregnation before and after catalyst testing. Key: FeSbO4 (䊉); ␣-Sb2 O4 (䊏); (a) Fe30 Sb60 Te2 Ox after catalyst testing; (b) Fe30 Sb60 Te2 Ox fresh; (c) FeSb2 O4 following incipient wetness with water, after used; (d) FeSb2 O4 following incipient wetness with water, fresh.
Table 2 XPS Data for Te-promoted catalysts Catalyst
Te 3d3/2 (eV)
a
Fe30 Sb60 Te1 Ox Fe30 Sb60 Te2 Ox a Fe30 Sb60 Te4 Ox a Fe30 Sb60 Te9 Ox a Fe30 Sb60 Te15 Ox a Fe30 Sb60 Te2 Ox b Fe30 Sb60 Te2 Ox c Fe30 Sb60 Te4 Ox c
Te/Sb
Fresh
Used
Fresh
Used
Fresh
Used
587.2 586.9 586.7 586.6 586.7 585.5 586.4 586.4
586.4 587.0 586.6 586.8 586.4 586.6 586.0 586.0
0.046 0.084 0.133 0.346 0.375 0.068 0.12 0.20
0.054 0.074 0.219 0.272 0.329 0.084 0.10 0.18
0.15 0.16 0.16 0.19 0.18 0.13 0.17 0.16
0.17 0.19 0.17 0.17 0.19 0.17 0.17 0.16
Co-precipitated, unpromoted Fe/Sb used = 0.17. Solid oxide method, umpromoted Fe/Sb used = 0.12. c Impregnation. a
b
Fe/Sb
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M.D. Allen et al. / Applied Catalysis A: General 217 (2001) 33–39
Fig. 3. Variation in specific activity (452◦ C, 10−5 mol acrylonitrile/m2 /h) and surface Te/Sb atomic ratio as a function of bulk Te/Sb atomic ratio. Key: (䊊): co-precipitated; (䊐): solid oxide method; (): impregnated; open symbols: specific activity; closed symbols: surface Te/Sb ratio.
oxide) and 0.1 (impregnated)]. Since this level of Te doping gives the highest enhancement in the specific activity, we consider that this surface Te/Sb ratio is optimal for propene activation. The specific activity (mol acrylonitrile m−2 h−1 ) and surface Te/Sb ratio are shown as a function of the bulk Te/Sb atom ratio in Fig. 3. It is apparent that the specific activity shows a significant enhancement with low Te doping levels. However, it is apparent that the data for three types of catalysts (i.e. prepared by co-precipitation, solid oxide method or impregnation) within experimental error are similar for the same bulk Te/Sb ratio. These results confirm that similar catalyst surfaces are being formed, and this is not affected markedly by the method of preparation. However, the co-precipitated catalysts exhibit the highest surface areas, being approximately three times higher than the other catalysts for the Fe30 Sb60 Te2 Ox formulation, and hence the best catalytic performance is observed with this method. It is clear from this study, comparing catalysts prepared by different methods, that Te-addition enhances both the selectivity for acrylonitrile and the specific activity. However, the optimal effects are observed at different loadings. For selectivity, the optimal
enhancement is observed with Fe30 Sb60 Te1 Ox with a surface Te/Sb ratio of 0.05 whereas, for the enhancement of specific activity, the optimal effects are observed with Fe30 Sb60 Te2 Ox with a surface Te/Sb ratio of 0.07–0.10. This may indicate that the effects have different origins since the enhancement in selectivity may be due to the blocking of non-selective oxidation sites, e.g. removal of ␣-Sb2 O4 from the catalyst as observed by powder X-ray diffraction analysis. The enhancement in activity may be due to electronic factors with Te4+ aiding the control or maintenance of the surface oxidation state of iron. However, with the solid oxide and impregnation method of catalyst preparation, there is the conflicting effect that Te addition leads to catalyst sintering even at low Te concentrations. It is only with the co-precipitation method that the high surface areas achieved with this method are maintained with the low levels of Te doping, and the sintering effect is only observed at higher Te concentrations. In conclusion, Te-doping of FeSb2 O4 leads to the observation of three effects (a) an enhancement in the selectivity to acrylonitrile due to the poisoning of carbon-carbon bond scission reactions, (b) an enhancement in the specific activity of acrylonitrile formation, particularly for Fe30 Sb60 Te2 Ox , and (c) a decrease in surface area of the catalyst with increasing Te concentration, particularly for the solid oxide and impregnation methods of preparation. The effects on selectivity and activity may result from different surface structural features since the optimal effects are observed at different surface Te/Sb atomic ratios. However, the three preparation methods investigated in this study produce similar surfaces and give similar specific activities and, hence, the effects are not considered to be dependent on the preparation method.
Acknowledgements We thank BASF and the EPSRC for financial support.
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