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Conversion of syn-gas to lower alkenes over Fe-TiO2-ZnO-K2O catalyst system
Applied Catalysis A: General 220 (2001) 153–164
Conversion of syn-gas to lower alkenes over Fe-TiO2 -ZnO-K2 O catalyst system Subhash Ch. Roy a,1 , H.L. Prasad a , P. Dutta a , A. Bhattacharya a , B. Singh a , S. Kumar a , S. Maharaj b , V.K. Kaushik c , S. Muthukumaru Pillai c,∗ , M. Ravindranathan c a c
Central Fuel Research Institute, P.O. Box FRI-828108, Dhanbad, Jharkhand, India b RRL, Jammu, India Research Centre, Indian Petrochemicals Corporation Limited, Baroda 391346, India
Received 21 March 2001; received in revised form 19 June 2001; accepted 19 June 2001
Abstract Conversion of syn-gas to lower alkenes is studied over a multicomponent catalyst system, Fe-TiO2 -ZnO-K2 O. Various reaction variables affecting the conversion were studied. At H2 :CO = 1, 2.5 kg/cm2 pressure, 250◦ C temperature and GHSV = 960 h−1 , maximum selectivity of 68% to alkenes was obtained at 45% conversion of the feed. The alkenes consist of a mixture of propylene (65%) and ethylene (35%). The catalyst is active even after 200 h of use. The catalyst samples were characterised by BET surface area measurements, TPR and TPD. XRD and ESCA studies reveal the presence of Fe2 O3 phase in the fresh catalyst. This phase is ably promoted by Zn and K. On reaction with CO + H2 , electron-rich species are formed on catalyst surface, which are the most likely active species. In addition to this, there is an improvement in dispersion of the active phase. These factors may contribute toward better performance of the catalyst in the conversion of syn-gas to lower alkenes. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Syn-gas; Alkenes; Multicomponent catalyst; Fe2 O3 ; Promoters; Dispersion
1. Introduction Conversion of syn-gas to alkenes is a well-known industrial process. The synthol process by SASOL in South Africa produces hydrocarbons by iron-based catalyst both by low temperature Fischer–Tropsch (LTFT) and high temperature Fischer–Tropsch (HTFT) routes [1]. Iron-based catalyst containing alkali metal and promoters is used as extrudates or ∗
Corresponding author. Fax: +91-265-272098. E-mail addresses: [email protected] (S.Ch. Roy), sm [email protected], [email protected] (S. Muthukumaru Pillai). 1 Fax: +91-326-364350.
in powder form [2,3]. C1 –C4 fraction of products consist of 55% alkenes [4]. LTFT process produces 64% alkenes in the C5 –C12 range and 50% alkenes in C13 –C18 range with ␣-selectivity of 96%. On the other hand, HTFT process produces C5 –C10 fraction containing 70% alkenes and C11 –C14 fraction with 60% alkenes. Here, ␣-selectivity is 55–60%. Kolbel and Ralek [5] have shown that in liquid phase, Fischer–Tropsch process C2 –C4 olefins are formed with 40–60 g/N m3 consumption of syn-gas. Here the selectivity of olefins is 70%. Blanchard and Vanhove [6] reported that in a slurry reactor using complex and cluster derived cobalt catalyst, syn-gas can be converted yielding up to 75% lower alkenes (C2 –C6 range). There are several possibilities for modifying
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the classical Fischer–Tropsch process to yield predominantly alkenes. Production of alkanes and subsequent steam cracking to lower alkenes is one option for modified Fischer–Tropsch process [7]. Upgrading of Fischer–Tropsch liquids for lower alkenes is another option [8,9]. Modification of Fischer–Tropsch catalyst to achieve higher selectivity for alkenes formation is yet another option [10]. Solid acid catalysts like zeolite can catalyse the conversion of syn-gas into methanol which is subsequently converted into alkenes [11]. The catalyst system is chosen in such a way that certain additives or promoters are added with a view to increase the selectivity for alkenes or oxygenates. For example, when alumina is used, more oxygenates and higher alkenes are produced [12,13]. Through appropriate choice of process conditions and catalyst formulation and by morphology of catalyst, a product mixture with higher content of lower alkenes can be obtained. For example, the higher amounts of alkenes obtained with SASOL LTFT process are due to smaller catalyst particles which allow the alkenes to escape from the catalyst particles before being hydrogenated to paraffins [14].
The purpose of the present work is to develop an active, selective and stable catalyst for conversion of syn-gas to lower alkenes and to study the effect of different reaction parameters on the conversion and selectivity. The study is also aimed at understanding the electronic changes taking place on the catalyst surface with a view to ascertain the factors responsible for improved activity of the promoted catalyst.
2. Experimental The reaction was conducted in stainless steel reactor tubing 40 cm of length, outer diameter 30 mm and inside diameter 20 mm (Fig. 1). A cylindrical furnace was installed outside the reactor to ensure the desired temperature of reaction. Catalyst is loaded in the middle position of the reactor. The temperature of the catalyst bed is measured using a Cr–Al thermocouple. Syn-gas is metered through the mass flow controller. The reactions were carried out at 1–10 atm pressure of syn-gas. Pressure gauges were kept at inlet and outlet of the reactor to monitor the pressure drops of the
Fig. 1. High pressure stainless steel reactor assembly.
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reactor. A cyclone separator and liquid collector after the reactor were maintained at ambient temperature. A wet gas meter and bubbled flow meter were calibrated for different flow rates at various H2 :CO ratio at the desired pressure. Gas samples were taken out at different intervals and injected directly to GC for analysis. 2.1. Catalysts The following catalysts were prepared and tested for the conversion of syn-gas to lower alkenes: 1. 2. 3. 4.
iron oxide (unpromoted) (Fe2 O3 ); iron-titania (unpromoted) (Fe-TiO2 ); iron-titania-zinc oxide (Fe-TiO2 -ZnO); promoted and calcined iron-titania (Fe-TiO2 -ZnOK2 O); 5. pillared clay-based promoted iron-titania; 6. zeolite-based promoted iron-titania. 2.2. Preparation of promoted and calcined iron-titania catalyst (from TiO2 )
The requisite amount of ferric nitrate was dissolved in distilled water. A known amount of titania was added to this solution and a 1:1 ammonia solution was added to this with continuous stirring till the pH reached 8–9. The gel was then filtered and washed with distilled water till free from nitrate ions. The residue was then dried in the air oven at 115◦ C for 5–6 h. Requisite amounts of zinc nitrate and potassium carbonate in water solution were impregnated on the above dry mass and this was dried on the water bath. The dried mass was heated in the air oven at 110–115◦ C for 4–5 h. This was then calcined in the muffle furnace at 900◦ C for 4–5 h. The catalyst was sized from −6 to +14 mesh BSS. The final catalyst composition is as follows: Fe2 O3 = 70%, TiO2 = 20%, ZnO = 5% and K2 O = 2%. Promoted and calcined Fe-TiO2 (with low titania content) catalyst was also prepared by the same procedure. 2.3. Preparation of promoted and calcined iron-titania catalyst (from TiCl4 ) The requisite amount of ferric nitrate was dissolved in distilled water and to this solution, a known amount of TiCl4 was added with constant stirring. Ammonia solution (1:1) was added slowly with constant stirring
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till the pH reached 9. The solution was kept for 72 h, then filtered and washed with distilled water till free from chloride and nitrate ions. The gel was then dried at 115◦ C in the air oven for 4 h. The dried gel was ground and soaked with the required amount of zinc nitrate and potassium carbonate in water solution. This mixture was kept for 4 h and then evaporated to dryness on a water bath, followed by calcination at 900◦ C for 4 h to obtain the catalyst. The final catalyst composition is as follows: Fe2 O3 = 70%, TiO2 = 20%, ZnO = 5% and K2 O = 2%. The catalyst was sized from −6 to +14 mesh BSS. 2.4. Preparation of pillared clay-based promoted iron-titania catalysts The requisite amount of iron nitrate was dissolved in hot distilled water. To this a known amount of titanium pillared clay (Montmorillonite clay) was added and the mixture was soaked for 1 h with occasional stirring. This was then dried on the water bath and the dried mass was powdered and soaked stepwise with the required amount of zinc acetylacetonate in acetylacetone and potassium carbonate in water solution; this was then evaporated to dryness on water bath. Dried mass was calcined at 450◦ C for 4 h. The catalyst was sized from −6 to +14 mesh BSS. Final catalyst composition is: Fe2 O3 = 50%, TiO2 = 40%, ZnO = 5% and K2 O = 2.5%. Pillared clay (from Indian Bentonite clay) -based promoted Fe-TiO2 catalyst was also prepared by the above procedure. 2.5. Preparation of promoted and calcined Co-TiO2 catalysts A known amount of cobalt nitrate was dissolved in distilled water. To this the required amount of TiO2 powder was added. To this mixture, ammonia solution (1:1) was added slowly with constant stirring till the pH reached 8–9. This was then filtered and washed to remove nitrate ions. The solid material was dried in the air oven at 110–115◦ C for 4 h. The dried mass was soaked with the required amount of zinc nitrate and potassium carbonate in water solution and then evaporated to dryness on water bath. This was then heated in the air oven at 110–115◦ C for 4 h, followed by calcination at 900◦ C for 4 h to obtain the catalyst. The catalyst was sized from −6 to +14 mesh BSS.
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The final catalyst composition is CoO = 70%, TiO2 = 20%, ZnO = 5% and K2 O = 2%. 2.6. XRD measurements Powder X-ray diffraction spectra were recorded at 25◦ C using Rich-Seiferts MZ III diffractometer with Ni filter and Cu K␣ radiation (40 kV, 30 mA). 2.7. Surface area measurements The surface area of catalysts was determined by using BET apparatus by N2 adsorption at liquid nitrogen temperature.
Fe-TiO2 catalyst (both calcined and fused) were tested for the conversion of syn-gas to lower alkenes (C2 –C4 range). The results are presented in Table 1. The influence of promoters for the iron-based catalyst is very crucial for the production of alkenes [17]. Among these catalysts promoted and calcined, Fe-TiO2 catalyst (Fe-TiO2 -ZnO-K2 O) showed highest activity and selectivity to C2 –C4 alkenes (57–68%). Particularly, the catalyst system derived from titania support (Fe2 O3 :TiO2 :ZnO:K2 O) is quite selective for C1 –C3 hydrocarbons with predominant formation of ethylene and propylene. Hence, this system was chosen for detailed study.
2.8. TPR/TPD studies of catalyst
3.2. Promoted and calcined Fe-TiO2 catalyst system
TPR of the catalyst using hydrogen and temperatureprogrammed desorption (TPD) of hydrogen from catalyst were conducted in a pulse flow system using a thermal conductivity detector (Institute of Isotopes, Budapest, Hungary).
Using the above catalyst system, the effect of the following reaction variables were studied for the conversion of syn-gas to lower alkenes:
2.9. XPS study of catalysts X-ray photoelectron spectra of different catalysts were recorded on VG Scientific’s ESCALAB MK II spectrometer interfacing with a DELL computer for acquisition and processing of data. The spectrometer was calibrated using the Ag 3d5/2 photoelectron line at 368.3 eV [15]. Catalyst samples were either dusted or placed on sample holder and kept overnight in the preparation chamber before being transferred to the analysis chamber where a vacuum better than 10−8 mbar was maintained. All spectra were recorded using Al K␣ X-ray source operating at 10 kV×10 mA. At least three different runs were taken for each measurement. Reported binding energies are an average of these values. O 1s photoelectron line at 530.3 eV reported in literature [16] for various oxides of iron was used as reference for binding energy measurements of Fe 2p photoelectron lines. 3. Results and discussion 3.1. Evaluation of catalysts A number of catalysts such as Fe-Al pillared clay, zeolite-based promoted iron catalyst and promoted
1. 2. 3. 4.
the effect of pressure; the effect of reaction temperature; the effect of gas hourly space velocity; the effect of syn-gas composition.
The effect of pressure on the conversion of syn-gas (H2 :CO = 1:1) to C2 –C3 alkenes is shown in Table 2. At 2.5 kg/cm2 pressure, the selectivity to C2 –C3 alkenes is maximum (68%) at 45% conversion based on the total hydrocarbons formed. With increase of pressure, the alkene selectivity decreases, presumably due to secondary reactions. Table 3 deals with the effect of reaction temperature on the conversion of syn-gas to C2 –C3 alkenes. Maximum selectivity of 68% to C2 –C3 alkenes was achieved at 250◦ C and then the selectivity gradually declined with increase of temperature. The influence of gas hourly space velocity on the conversion of syn-gas to lower alkenes is summarised in Table 4. It is observed that at 960 h−1 GHSV, the selectivity to C2 –C3 alkenes is maximum (68%). The effect of syn-gas composition on the selectivity to C2 –C3 alkenes from syn-gas is shown in Table 5. Under identical reaction conditions, an equimolar composition of syn-gas (H2 :CO = 1:1) gives the maximum selectivity to C2 –C3 alkenes (68%). The formation of C2 –C3 alkenes is decreased (22.7%)
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Table 2 Effect of pressure on the conversion of syn-gas to lower alkenesa Pressure (kg/cm2 )
Selectivity to C2 –C3 alkenes (wt.%)
1 2.5 5.0 10.0
30.0 68.0 52.2 42.8
Catalyst = Fe-TiO2 -ZnO-K2 O, H2 :CO = 1:1; temperature = 250◦ C; GHSV = 960 h−1 . a
Table 3 Effect of temperature on the conversion of syn-gas to lower alkenesa Reaction temperature (◦ C)
Selectivity to C2 –C3 alkenes (wt.%)
200 250 300 320 340
60.0 68.0 50.2 36.0 34.0 a
H2 :CO = 1:1; pressure = 2.5 kg/cm2 ; GHSV = 960 h−1 .
Table 4 Effect of gas hourly space velocity on the conversion of syn-gas to lower alkenesa Gas hourly space velocity (h−1 )
Selectivity to C2 –C3 alkenes (wt.%)
560 840 930 960 1050
34.4 47.5 55.0 68.0 41.7
a H :CO = 1:1; reaction temperature = 250◦ C; pressure = 2 2.5 kg/cm2 .
while that of C2 –C3 alkanes increased at higher partial pressure of hydrogen (H2 :CO = 2:1). Fig. 2 shows that, with increasing gas hourly space velocity, the yield of lower alkenes (g/N m3 of syn-gas consumed) increases and it is maximum at GHSV of 1060 h−1 . From Fig. 3 it is obvious that, with increasing reaction time, the selectivity to C2 –C3 alkenes
increases and reaches a maximum (68%) at about 50 h of reaction time and it becomes steady then onwards upto 200 h of reaction (65–68%). There is a strong synergism between iron, zinc, potassium and TiO2 components of the catalyst and the combination of all these produces a long stable catalyst. A typical distribution of C1 –C3 hydrocarbon products formed in the conversion of syn-gas to lower alkenes is shown in Fig. 4. The selectivity to propene is 44% whereas that to ethene is 24%. The selectivities to alkanes namely methane, ethane and propane are 19.7, 6.3 and 6%, respectively. So this catalyst system is quite selective to C2 –C3 alkenes. 3.3. Catalyst characterisation The surface area of the active promoted and calcined Fe-TiO2 is found to be 10 m2 /g. In order to study the adsorption/desorption of H2 over promoted and calcined Fe-TiO2 catalyst, the temperature-programmed reduction and temperature-programmed desorption studies have been carried out. From TPR of promoted and calcined Fe-TiO2 catalyst, it is observed that the H2 uptake starts at 360◦ C. It is known that reduction of iron oxides takes place at temperatures above 350◦ C [17,18]. The total uptake of H2 at the temperature range 360–700◦ C is estimated to be 4.5 mol/g of catalyst (Fig. 5). TPD of hydrogen from the above catalyst shows that the maximum desorption of hydrogen is at 400◦ C (Fig. 6) suggesting that adsorbed H2 during syn-gas reaction is available for conversion of CO into hydrocarbons at 200–350◦ C. The X-ray diffractograms of the promoted and calcined Fe-TiO2 show d-values at 1.45, 1.5, 1.53, 1.6, 1.62, 1.69, 1.75, 1.84, 1.88, 2.06, 2.44, 2.5, 2.69, 2.94, 3.21, 3.22, 3.65, 5.09, 7.89 Å. This profile reveals the presence of two major phases, one of which is hematite, ␣-Fe2 O3 . The second major phase is ␥-Fe2 O3 .
Table 5 Effect of syn-gas composition on the conversion of syn-gas to lower alkenesa Syn-gas composition (wt.%)
Selectivity to lower alkenes (C2 –C3 range) (wt.%)
Selectivity to C2 –C3 alkanes
H2 :CO = 1:1 H2 :CO = 2:1
68 22.7
12.3 23.9
a
Pressure = 2.5 kg/cm2 ; reaction temperature = 250◦ C; GHSV = 960 h−1 .
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Fig. 2. Effect of GHSV on the yield of lower alkenes from syn-gas. Catalyst: promoted and calcined Fe-TiO2 ; pressure: 2.5 kg/cm2 ; temperature: 250◦ C.
The XPS survey scans of fresh and active promoted catalysts are shown in Fig. 7. It is evident from these scans that the dispersion of iron in the catalyst improves on exposure to reaction environment. This is also clear from the Fe/Ti atomic ratio estimated for some of these catalyst systems reported in Table 6. It
is worth mentioning here that improvement in surface iron dispersion for promoted Fe-TiO2 -ZnO-K2 O catalyst (D) is considerably higher compared to that for other active catalysts. In order to look for active state of iron in these catalysts, multiscan data in 703–733 eV binding
Fig. 3. Effect of reaction time on the product composition for the conversion of syn-gas to lower alkenes. Catalyst: promoted and calcined Fe-TiO2 ; pressure: 2.5 kg/cm2 ; temperature: 250◦ C.
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Fig. 4. Selectivity profile of various hydrocarbons in the product of syn-gas reaction.
energy range were collected and precise binding energy measurements were made for Fe 2p3/2 ; Fe 2p1/2 photoelectron lines. The results are shown in Table 6. A comparison of binding energies [19], satellite structure [20] and correlation of performance of these catalysts shown in Table 6 indicates that Fe2 O3 is an active phase of the catalyst in the conversion of syn-gas to alkenes. However, the presence of promoters (Ti, Zn, K) in reaction sphere improves the electronic environment of iron such that Fe2 O3 surface
becomes electron rich in nature. This electron-rich Fe2 O3 surface is responsible for improved performance of promoted catalyst in syn-gas conversion. Fig. 8 shows ∼0.8 eV binding energy shift in Fe 2p3/2 photoelectron line of fresh Fe-TiO2 -ZnO-K2 O (E) and active Fe-TiO2 -ZnO-K2 O (D) catalysts with satellite structure ∼8 eV away from Fe 2p3/2 photoelectron line [16]. When Fe 2p spectra in active Fe-TiO2 -ZnO-K2 O (D) and active Fe2 O3 (A) catalysts are compared, it is observed that the Fe 2p line in catalyst A is slightly broad due to the presence of lower oxidation state
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Fig. 5. TPR of Fe-TiO2 catalyst.
Fig. 6. TPD of Fe-TiO2 catalyst.
Table 6 XPS measurements of fresh and active catalystsa
Fe/Ti Fe 2p3/2 (eV) Satellite (eV) Fe 2p1/2 (eV) Total alkenes (%)
Active Fe2 O3 (A)
Active Fe-TiO2 (B)
Active Fe-TiO2 ZnO (C)
Active Fe-TiO2 -ZnOK2 O (D)
Fresh Fe-TiO2 ZnO-K2 O (E)
Fresh Fe-TiO2 ZnO (F)
– 710.7 – 724.3 51.10
5.1 711.1 719.5 724.7 35.47
4.4 710.6 718.3 724.2 53.35
8.7 710.4 718.2 724.1 68.00
1.1 711.2 719.2 724.9 –
1.5 710.9 719.1 724.5 –
a Active: catalyst after syn-gas reaction; fresh: catalyst just after preparation and not exposed to syn-gas. Estimated uncertainty in binding energy measurements ± 0.2 eV.
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Fig. 7. Survey scans of fresh (E) and active (D) Fe-TiO2 -ZnO-K2 O catalyst.
Fig. 8. Fe 2p3/2 photoelectron spectra of fresh (E) and active (D) Fe-TiO2 -ZnO-K2 O catalyst.
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Fig. 9. Fe 2p photoelectron spectra of active (A) Fe2 O3 and active (D) Fe-TiO2 -ZnO-K2 O catalysts.
species, which also results in the change of satellite structure of this photoelectron line compared to that for active Fe-TiO2 -ZnO-K2 O (D) shown in Fig. 9.
4. Conclusions Promoted and calcined iron-titania (Fe-TiO2 -ZnOK2 O) is an active catalyst for conversion of syn-gas to C2 –C3 alkenes. The maximum selectivity to alkenes on this catalyst is 68%, the yield of C2 –C3 alkenes achieved is 52 g/N m3 of syn-gas consumed at 45% conversion. The catalyst is active even after 200 h of use. ESCA study reveals the presence of Fe2 O3 phase in the fresh catalyst. This phase is ably promoted by zinc and potassium in CO + H2 reaction due to electronic modification of iron oxide (Fe2 O3 ) and its dispersion on TiO2 support is improved considerably. These two factors are responsible for better performance of promoted and calcined catalyst in the conversion of syn-gas to alkenes.
Acknowledgements The authors are grateful to Dr. P. Samuel, Head, Chemicals and Liquid Fuel Division, and Dr. S.K. Roy, CFRI, Dhanbad for their valuable suggestions and to Director, CFRI, Dhanbad for his interest in the work and permission to publish the paper. The authors thank the management of IPCL, Baroda for permission to carry out catalyst characterisation studies. Grateful thanks are due to Department of coal (CMPDIL), Ranchi, for funding the project.
References [1] G.P. Vander Laan, A.A.C.M. Beenackers, Stud. Surf. Sci. Catal. 119 (1998) 179. [2] G.P. Vander Laan, A.A.C.M. Beenackers, Ind. Eng. Chem. Res. 38 (1999) 1277. [3] B. Jager, R. Espinoza, Catal. Today 23 (1995) 17. [4] J.C. Hoogendoom, in: Proceedings of the 23rd Annual Meeting of AIME, New York (1974) 117.
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[5] H. Kolbel, M. Ralek, Catal. Rev. Sci. Eng. 21 (1980) 225. [6] M. Blanchard, D. Vanhove, J. Chem. Soc., Chem. Commun. (1980) 908. [7] C.B. Murchison, D.A. Murdick, South African Patent 79/1448, 27 March 1979. [8] P.D. Caesar, R.A. Morrison, US Patent, 4,083,888 (1978) to Mobil Oil Corporation. [9] P.D. Caesar, R.A. Morrison, US Patent, 4,083,889 (1978) to Mobil Oil Corporation. [10] R.B. Anderson, Catal. Rev. Sci. Eng. 21 (1980) 53. [11] H. Hagiwara, K. Kawamura, S. Ikai, F. Mizuno, H. Okado, Progress in C1 Chemistry in Japan, Elsevier, Amsterdam, 1989 (Chapter 7). [12] B.D. Dombek, Comments Inorg. Chem. 4 (1985) 241.
[13] B.D. Dombek, Adv. Catal. 32 (1983) 325. [14] E. Iglena, S.C. Reyes, S.L. Soled, in: E.R. Becker, C.J. Pereira (Eds.), Computer Aided Design of Catalysis, Marcel Dekker, New York, 1993, p. 199. [15] V.K. Kaushik, J. Electron. Spectrosc. 56 (1991) 273. [16] C.R. Brundle, T.J. Chuang, K. Wandelt, Surf. Sci. 68 (1977) 459. [17] M.E. Dry, in: J.R. Anderson, M. Boudart (Eds.), Catalysis Science and Technology, Vol. 1, Springer, New York, 1981, p. 159. [18] A. Basinska, W.K. Jozwiak, J. Goralski, F. Domka, Appl. Catal. A: Gen. 190 (2000) 107. [19] R. Holm, S. Storp, Appl. Phys. 9 (1976) 217. [20] K. Huokawa, M. Oku, Talanta 26 (1979) 855.
Conversion of syn-gas to lower alkenes over Fe-TiO2 -ZnO-K2 O catalyst system Subhash Ch. Roy a,1 , H.L. Prasad a , P. Dutta a , A. Bhattacharya a , B. Singh a , S. Kumar a , S. Maharaj b , V.K. Kaushik c , S. Muthukumaru Pillai c,∗ , M. Ravindranathan c a c
Central Fuel Research Institute, P.O. Box FRI-828108, Dhanbad, Jharkhand, India b RRL, Jammu, India Research Centre, Indian Petrochemicals Corporation Limited, Baroda 391346, India
Received 21 March 2001; received in revised form 19 June 2001; accepted 19 June 2001
Abstract Conversion of syn-gas to lower alkenes is studied over a multicomponent catalyst system, Fe-TiO2 -ZnO-K2 O. Various reaction variables affecting the conversion were studied. At H2 :CO = 1, 2.5 kg/cm2 pressure, 250◦ C temperature and GHSV = 960 h−1 , maximum selectivity of 68% to alkenes was obtained at 45% conversion of the feed. The alkenes consist of a mixture of propylene (65%) and ethylene (35%). The catalyst is active even after 200 h of use. The catalyst samples were characterised by BET surface area measurements, TPR and TPD. XRD and ESCA studies reveal the presence of Fe2 O3 phase in the fresh catalyst. This phase is ably promoted by Zn and K. On reaction with CO + H2 , electron-rich species are formed on catalyst surface, which are the most likely active species. In addition to this, there is an improvement in dispersion of the active phase. These factors may contribute toward better performance of the catalyst in the conversion of syn-gas to lower alkenes. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Syn-gas; Alkenes; Multicomponent catalyst; Fe2 O3 ; Promoters; Dispersion
1. Introduction Conversion of syn-gas to alkenes is a well-known industrial process. The synthol process by SASOL in South Africa produces hydrocarbons by iron-based catalyst both by low temperature Fischer–Tropsch (LTFT) and high temperature Fischer–Tropsch (HTFT) routes [1]. Iron-based catalyst containing alkali metal and promoters is used as extrudates or ∗
Corresponding author. Fax: +91-265-272098. E-mail addresses: [email protected] (S.Ch. Roy), sm [email protected], [email protected] (S. Muthukumaru Pillai). 1 Fax: +91-326-364350.
in powder form [2,3]. C1 –C4 fraction of products consist of 55% alkenes [4]. LTFT process produces 64% alkenes in the C5 –C12 range and 50% alkenes in C13 –C18 range with ␣-selectivity of 96%. On the other hand, HTFT process produces C5 –C10 fraction containing 70% alkenes and C11 –C14 fraction with 60% alkenes. Here, ␣-selectivity is 55–60%. Kolbel and Ralek [5] have shown that in liquid phase, Fischer–Tropsch process C2 –C4 olefins are formed with 40–60 g/N m3 consumption of syn-gas. Here the selectivity of olefins is 70%. Blanchard and Vanhove [6] reported that in a slurry reactor using complex and cluster derived cobalt catalyst, syn-gas can be converted yielding up to 75% lower alkenes (C2 –C6 range). There are several possibilities for modifying
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the classical Fischer–Tropsch process to yield predominantly alkenes. Production of alkanes and subsequent steam cracking to lower alkenes is one option for modified Fischer–Tropsch process [7]. Upgrading of Fischer–Tropsch liquids for lower alkenes is another option [8,9]. Modification of Fischer–Tropsch catalyst to achieve higher selectivity for alkenes formation is yet another option [10]. Solid acid catalysts like zeolite can catalyse the conversion of syn-gas into methanol which is subsequently converted into alkenes [11]. The catalyst system is chosen in such a way that certain additives or promoters are added with a view to increase the selectivity for alkenes or oxygenates. For example, when alumina is used, more oxygenates and higher alkenes are produced [12,13]. Through appropriate choice of process conditions and catalyst formulation and by morphology of catalyst, a product mixture with higher content of lower alkenes can be obtained. For example, the higher amounts of alkenes obtained with SASOL LTFT process are due to smaller catalyst particles which allow the alkenes to escape from the catalyst particles before being hydrogenated to paraffins [14].
The purpose of the present work is to develop an active, selective and stable catalyst for conversion of syn-gas to lower alkenes and to study the effect of different reaction parameters on the conversion and selectivity. The study is also aimed at understanding the electronic changes taking place on the catalyst surface with a view to ascertain the factors responsible for improved activity of the promoted catalyst.
2. Experimental The reaction was conducted in stainless steel reactor tubing 40 cm of length, outer diameter 30 mm and inside diameter 20 mm (Fig. 1). A cylindrical furnace was installed outside the reactor to ensure the desired temperature of reaction. Catalyst is loaded in the middle position of the reactor. The temperature of the catalyst bed is measured using a Cr–Al thermocouple. Syn-gas is metered through the mass flow controller. The reactions were carried out at 1–10 atm pressure of syn-gas. Pressure gauges were kept at inlet and outlet of the reactor to monitor the pressure drops of the
Fig. 1. High pressure stainless steel reactor assembly.
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reactor. A cyclone separator and liquid collector after the reactor were maintained at ambient temperature. A wet gas meter and bubbled flow meter were calibrated for different flow rates at various H2 :CO ratio at the desired pressure. Gas samples were taken out at different intervals and injected directly to GC for analysis. 2.1. Catalysts The following catalysts were prepared and tested for the conversion of syn-gas to lower alkenes: 1. 2. 3. 4.
iron oxide (unpromoted) (Fe2 O3 ); iron-titania (unpromoted) (Fe-TiO2 ); iron-titania-zinc oxide (Fe-TiO2 -ZnO); promoted and calcined iron-titania (Fe-TiO2 -ZnOK2 O); 5. pillared clay-based promoted iron-titania; 6. zeolite-based promoted iron-titania. 2.2. Preparation of promoted and calcined iron-titania catalyst (from TiO2 )
The requisite amount of ferric nitrate was dissolved in distilled water. A known amount of titania was added to this solution and a 1:1 ammonia solution was added to this with continuous stirring till the pH reached 8–9. The gel was then filtered and washed with distilled water till free from nitrate ions. The residue was then dried in the air oven at 115◦ C for 5–6 h. Requisite amounts of zinc nitrate and potassium carbonate in water solution were impregnated on the above dry mass and this was dried on the water bath. The dried mass was heated in the air oven at 110–115◦ C for 4–5 h. This was then calcined in the muffle furnace at 900◦ C for 4–5 h. The catalyst was sized from −6 to +14 mesh BSS. The final catalyst composition is as follows: Fe2 O3 = 70%, TiO2 = 20%, ZnO = 5% and K2 O = 2%. Promoted and calcined Fe-TiO2 (with low titania content) catalyst was also prepared by the same procedure. 2.3. Preparation of promoted and calcined iron-titania catalyst (from TiCl4 ) The requisite amount of ferric nitrate was dissolved in distilled water and to this solution, a known amount of TiCl4 was added with constant stirring. Ammonia solution (1:1) was added slowly with constant stirring
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till the pH reached 9. The solution was kept for 72 h, then filtered and washed with distilled water till free from chloride and nitrate ions. The gel was then dried at 115◦ C in the air oven for 4 h. The dried gel was ground and soaked with the required amount of zinc nitrate and potassium carbonate in water solution. This mixture was kept for 4 h and then evaporated to dryness on a water bath, followed by calcination at 900◦ C for 4 h to obtain the catalyst. The final catalyst composition is as follows: Fe2 O3 = 70%, TiO2 = 20%, ZnO = 5% and K2 O = 2%. The catalyst was sized from −6 to +14 mesh BSS. 2.4. Preparation of pillared clay-based promoted iron-titania catalysts The requisite amount of iron nitrate was dissolved in hot distilled water. To this a known amount of titanium pillared clay (Montmorillonite clay) was added and the mixture was soaked for 1 h with occasional stirring. This was then dried on the water bath and the dried mass was powdered and soaked stepwise with the required amount of zinc acetylacetonate in acetylacetone and potassium carbonate in water solution; this was then evaporated to dryness on water bath. Dried mass was calcined at 450◦ C for 4 h. The catalyst was sized from −6 to +14 mesh BSS. Final catalyst composition is: Fe2 O3 = 50%, TiO2 = 40%, ZnO = 5% and K2 O = 2.5%. Pillared clay (from Indian Bentonite clay) -based promoted Fe-TiO2 catalyst was also prepared by the above procedure. 2.5. Preparation of promoted and calcined Co-TiO2 catalysts A known amount of cobalt nitrate was dissolved in distilled water. To this the required amount of TiO2 powder was added. To this mixture, ammonia solution (1:1) was added slowly with constant stirring till the pH reached 8–9. This was then filtered and washed to remove nitrate ions. The solid material was dried in the air oven at 110–115◦ C for 4 h. The dried mass was soaked with the required amount of zinc nitrate and potassium carbonate in water solution and then evaporated to dryness on water bath. This was then heated in the air oven at 110–115◦ C for 4 h, followed by calcination at 900◦ C for 4 h to obtain the catalyst. The catalyst was sized from −6 to +14 mesh BSS.
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The final catalyst composition is CoO = 70%, TiO2 = 20%, ZnO = 5% and K2 O = 2%. 2.6. XRD measurements Powder X-ray diffraction spectra were recorded at 25◦ C using Rich-Seiferts MZ III diffractometer with Ni filter and Cu K␣ radiation (40 kV, 30 mA). 2.7. Surface area measurements The surface area of catalysts was determined by using BET apparatus by N2 adsorption at liquid nitrogen temperature.
Fe-TiO2 catalyst (both calcined and fused) were tested for the conversion of syn-gas to lower alkenes (C2 –C4 range). The results are presented in Table 1. The influence of promoters for the iron-based catalyst is very crucial for the production of alkenes [17]. Among these catalysts promoted and calcined, Fe-TiO2 catalyst (Fe-TiO2 -ZnO-K2 O) showed highest activity and selectivity to C2 –C4 alkenes (57–68%). Particularly, the catalyst system derived from titania support (Fe2 O3 :TiO2 :ZnO:K2 O) is quite selective for C1 –C3 hydrocarbons with predominant formation of ethylene and propylene. Hence, this system was chosen for detailed study.
2.8. TPR/TPD studies of catalyst
3.2. Promoted and calcined Fe-TiO2 catalyst system
TPR of the catalyst using hydrogen and temperatureprogrammed desorption (TPD) of hydrogen from catalyst were conducted in a pulse flow system using a thermal conductivity detector (Institute of Isotopes, Budapest, Hungary).
Using the above catalyst system, the effect of the following reaction variables were studied for the conversion of syn-gas to lower alkenes:
2.9. XPS study of catalysts X-ray photoelectron spectra of different catalysts were recorded on VG Scientific’s ESCALAB MK II spectrometer interfacing with a DELL computer for acquisition and processing of data. The spectrometer was calibrated using the Ag 3d5/2 photoelectron line at 368.3 eV [15]. Catalyst samples were either dusted or placed on sample holder and kept overnight in the preparation chamber before being transferred to the analysis chamber where a vacuum better than 10−8 mbar was maintained. All spectra were recorded using Al K␣ X-ray source operating at 10 kV×10 mA. At least three different runs were taken for each measurement. Reported binding energies are an average of these values. O 1s photoelectron line at 530.3 eV reported in literature [16] for various oxides of iron was used as reference for binding energy measurements of Fe 2p photoelectron lines. 3. Results and discussion 3.1. Evaluation of catalysts A number of catalysts such as Fe-Al pillared clay, zeolite-based promoted iron catalyst and promoted
1. 2. 3. 4.
the effect of pressure; the effect of reaction temperature; the effect of gas hourly space velocity; the effect of syn-gas composition.
The effect of pressure on the conversion of syn-gas (H2 :CO = 1:1) to C2 –C3 alkenes is shown in Table 2. At 2.5 kg/cm2 pressure, the selectivity to C2 –C3 alkenes is maximum (68%) at 45% conversion based on the total hydrocarbons formed. With increase of pressure, the alkene selectivity decreases, presumably due to secondary reactions. Table 3 deals with the effect of reaction temperature on the conversion of syn-gas to C2 –C3 alkenes. Maximum selectivity of 68% to C2 –C3 alkenes was achieved at 250◦ C and then the selectivity gradually declined with increase of temperature. The influence of gas hourly space velocity on the conversion of syn-gas to lower alkenes is summarised in Table 4. It is observed that at 960 h−1 GHSV, the selectivity to C2 –C3 alkenes is maximum (68%). The effect of syn-gas composition on the selectivity to C2 –C3 alkenes from syn-gas is shown in Table 5. Under identical reaction conditions, an equimolar composition of syn-gas (H2 :CO = 1:1) gives the maximum selectivity to C2 –C3 alkenes (68%). The formation of C2 –C3 alkenes is decreased (22.7%)
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Table 2 Effect of pressure on the conversion of syn-gas to lower alkenesa Pressure (kg/cm2 )
Selectivity to C2 –C3 alkenes (wt.%)
1 2.5 5.0 10.0
30.0 68.0 52.2 42.8
Catalyst = Fe-TiO2 -ZnO-K2 O, H2 :CO = 1:1; temperature = 250◦ C; GHSV = 960 h−1 . a
Table 3 Effect of temperature on the conversion of syn-gas to lower alkenesa Reaction temperature (◦ C)
Selectivity to C2 –C3 alkenes (wt.%)
200 250 300 320 340
60.0 68.0 50.2 36.0 34.0 a
H2 :CO = 1:1; pressure = 2.5 kg/cm2 ; GHSV = 960 h−1 .
Table 4 Effect of gas hourly space velocity on the conversion of syn-gas to lower alkenesa Gas hourly space velocity (h−1 )
Selectivity to C2 –C3 alkenes (wt.%)
560 840 930 960 1050
34.4 47.5 55.0 68.0 41.7
a H :CO = 1:1; reaction temperature = 250◦ C; pressure = 2 2.5 kg/cm2 .
while that of C2 –C3 alkanes increased at higher partial pressure of hydrogen (H2 :CO = 2:1). Fig. 2 shows that, with increasing gas hourly space velocity, the yield of lower alkenes (g/N m3 of syn-gas consumed) increases and it is maximum at GHSV of 1060 h−1 . From Fig. 3 it is obvious that, with increasing reaction time, the selectivity to C2 –C3 alkenes
increases and reaches a maximum (68%) at about 50 h of reaction time and it becomes steady then onwards upto 200 h of reaction (65–68%). There is a strong synergism between iron, zinc, potassium and TiO2 components of the catalyst and the combination of all these produces a long stable catalyst. A typical distribution of C1 –C3 hydrocarbon products formed in the conversion of syn-gas to lower alkenes is shown in Fig. 4. The selectivity to propene is 44% whereas that to ethene is 24%. The selectivities to alkanes namely methane, ethane and propane are 19.7, 6.3 and 6%, respectively. So this catalyst system is quite selective to C2 –C3 alkenes. 3.3. Catalyst characterisation The surface area of the active promoted and calcined Fe-TiO2 is found to be 10 m2 /g. In order to study the adsorption/desorption of H2 over promoted and calcined Fe-TiO2 catalyst, the temperature-programmed reduction and temperature-programmed desorption studies have been carried out. From TPR of promoted and calcined Fe-TiO2 catalyst, it is observed that the H2 uptake starts at 360◦ C. It is known that reduction of iron oxides takes place at temperatures above 350◦ C [17,18]. The total uptake of H2 at the temperature range 360–700◦ C is estimated to be 4.5 mol/g of catalyst (Fig. 5). TPD of hydrogen from the above catalyst shows that the maximum desorption of hydrogen is at 400◦ C (Fig. 6) suggesting that adsorbed H2 during syn-gas reaction is available for conversion of CO into hydrocarbons at 200–350◦ C. The X-ray diffractograms of the promoted and calcined Fe-TiO2 show d-values at 1.45, 1.5, 1.53, 1.6, 1.62, 1.69, 1.75, 1.84, 1.88, 2.06, 2.44, 2.5, 2.69, 2.94, 3.21, 3.22, 3.65, 5.09, 7.89 Å. This profile reveals the presence of two major phases, one of which is hematite, ␣-Fe2 O3 . The second major phase is ␥-Fe2 O3 .
Table 5 Effect of syn-gas composition on the conversion of syn-gas to lower alkenesa Syn-gas composition (wt.%)
Selectivity to lower alkenes (C2 –C3 range) (wt.%)
Selectivity to C2 –C3 alkanes
H2 :CO = 1:1 H2 :CO = 2:1
68 22.7
12.3 23.9
a
Pressure = 2.5 kg/cm2 ; reaction temperature = 250◦ C; GHSV = 960 h−1 .
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Fig. 2. Effect of GHSV on the yield of lower alkenes from syn-gas. Catalyst: promoted and calcined Fe-TiO2 ; pressure: 2.5 kg/cm2 ; temperature: 250◦ C.
The XPS survey scans of fresh and active promoted catalysts are shown in Fig. 7. It is evident from these scans that the dispersion of iron in the catalyst improves on exposure to reaction environment. This is also clear from the Fe/Ti atomic ratio estimated for some of these catalyst systems reported in Table 6. It
is worth mentioning here that improvement in surface iron dispersion for promoted Fe-TiO2 -ZnO-K2 O catalyst (D) is considerably higher compared to that for other active catalysts. In order to look for active state of iron in these catalysts, multiscan data in 703–733 eV binding
Fig. 3. Effect of reaction time on the product composition for the conversion of syn-gas to lower alkenes. Catalyst: promoted and calcined Fe-TiO2 ; pressure: 2.5 kg/cm2 ; temperature: 250◦ C.
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Fig. 4. Selectivity profile of various hydrocarbons in the product of syn-gas reaction.
energy range were collected and precise binding energy measurements were made for Fe 2p3/2 ; Fe 2p1/2 photoelectron lines. The results are shown in Table 6. A comparison of binding energies [19], satellite structure [20] and correlation of performance of these catalysts shown in Table 6 indicates that Fe2 O3 is an active phase of the catalyst in the conversion of syn-gas to alkenes. However, the presence of promoters (Ti, Zn, K) in reaction sphere improves the electronic environment of iron such that Fe2 O3 surface
becomes electron rich in nature. This electron-rich Fe2 O3 surface is responsible for improved performance of promoted catalyst in syn-gas conversion. Fig. 8 shows ∼0.8 eV binding energy shift in Fe 2p3/2 photoelectron line of fresh Fe-TiO2 -ZnO-K2 O (E) and active Fe-TiO2 -ZnO-K2 O (D) catalysts with satellite structure ∼8 eV away from Fe 2p3/2 photoelectron line [16]. When Fe 2p spectra in active Fe-TiO2 -ZnO-K2 O (D) and active Fe2 O3 (A) catalysts are compared, it is observed that the Fe 2p line in catalyst A is slightly broad due to the presence of lower oxidation state
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Fig. 5. TPR of Fe-TiO2 catalyst.
Fig. 6. TPD of Fe-TiO2 catalyst.
Table 6 XPS measurements of fresh and active catalystsa
Fe/Ti Fe 2p3/2 (eV) Satellite (eV) Fe 2p1/2 (eV) Total alkenes (%)
Active Fe2 O3 (A)
Active Fe-TiO2 (B)
Active Fe-TiO2 ZnO (C)
Active Fe-TiO2 -ZnOK2 O (D)
Fresh Fe-TiO2 ZnO-K2 O (E)
Fresh Fe-TiO2 ZnO (F)
– 710.7 – 724.3 51.10
5.1 711.1 719.5 724.7 35.47
4.4 710.6 718.3 724.2 53.35
8.7 710.4 718.2 724.1 68.00
1.1 711.2 719.2 724.9 –
1.5 710.9 719.1 724.5 –
a Active: catalyst after syn-gas reaction; fresh: catalyst just after preparation and not exposed to syn-gas. Estimated uncertainty in binding energy measurements ± 0.2 eV.
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Fig. 7. Survey scans of fresh (E) and active (D) Fe-TiO2 -ZnO-K2 O catalyst.
Fig. 8. Fe 2p3/2 photoelectron spectra of fresh (E) and active (D) Fe-TiO2 -ZnO-K2 O catalyst.
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Fig. 9. Fe 2p photoelectron spectra of active (A) Fe2 O3 and active (D) Fe-TiO2 -ZnO-K2 O catalysts.
species, which also results in the change of satellite structure of this photoelectron line compared to that for active Fe-TiO2 -ZnO-K2 O (D) shown in Fig. 9.
4. Conclusions Promoted and calcined iron-titania (Fe-TiO2 -ZnOK2 O) is an active catalyst for conversion of syn-gas to C2 –C3 alkenes. The maximum selectivity to alkenes on this catalyst is 68%, the yield of C2 –C3 alkenes achieved is 52 g/N m3 of syn-gas consumed at 45% conversion. The catalyst is active even after 200 h of use. ESCA study reveals the presence of Fe2 O3 phase in the fresh catalyst. This phase is ably promoted by zinc and potassium in CO + H2 reaction due to electronic modification of iron oxide (Fe2 O3 ) and its dispersion on TiO2 support is improved considerably. These two factors are responsible for better performance of promoted and calcined catalyst in the conversion of syn-gas to alkenes.
Acknowledgements The authors are grateful to Dr. P. Samuel, Head, Chemicals and Liquid Fuel Division, and Dr. S.K. Roy, CFRI, Dhanbad for their valuable suggestions and to Director, CFRI, Dhanbad for his interest in the work and permission to publish the paper. The authors thank the management of IPCL, Baroda for permission to carry out catalyst characterisation studies. Grateful thanks are due to Department of coal (CMPDIL), Ranchi, for funding the project.
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