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Isomerization of 1-pentene over SAPO, CoAPO (AEL, AFI) molecular sieves and HZSM-5
Applied Catalysis A: General 207 (2001) 397–405
Isomerization of 1-pentene over SAPO, CoAPO (AEL, AFI) molecular sieves and HZSM-5 M. Höchtl, A. Jentys1 , H. Vinek∗ Institute of Physical and Theoretical Chemistry, University of Technology, Vienna, Veterinärplatz 1, A-1210 Vienna, Austria Received 18 February 2000; received in revised form 15 June 2000; accepted 15 June 2000
Abstract Skeletal isomerization of 1-pentene was carried out over SAPO and CoAPO molecular sieves with AEL and AFI type structure and for comparison over AlPO-11 and HZSM-5. The equilibrium distribution within the linear pentenes and iso-pentenes was established over all catalysts except at very low temperatures or conversions. The approach to the equilibrium distribution between the linear pentenes and iso-pentenes depends on the strength of the acid sites, the pore dimensions and the structure of the catalyst. Over AlPO4 -11, a material with very weak acid sites, the selectivity to skeletal isomerization was only 5%, while the double bond isomerization and the isomerization via methyl shift were in equilibrium. Over SAPO-11 and CoAPO-11, having medium strong acid sites and one-dimensional pores with a 10-membered ring-system, the distribution of linear and iso-pentenes was near the equilibrium. On SAPO-5 and CoAPO-5, materials with sites of similar strength and one-dimensional pores with a 12-membered ring-system, the equilibrium distribution was not achieved because of the rapid deactivation that diminished the accessibility of the acid sites inside the pores. Due to the presence of strong acid sites and a two-dimensional pore system with a 10-membered ring-system, the equilibrium distribution is achieved on HZSM-5, but this ‘pentene pool’ react further by dimerization and cracking. © 2001 Elsevier Science B.V. All rights reserved. Keywords: 1-Pentene; Isomerization; SAPO; CoAPO; AEL; AFI
1. Introduction Oxygenates, alcohols and especially tertiary ethers, have become important components in gasoline due to tightening legislation concerning fuels. In the case of gasoline, the clean air act (CAA) amendments in USA established, among others, the reduction of volatility, reduction of aromatics and the introduction of oxygenated compounds [1]. Oxygenates ∗ Corresponding author. E-mail address: [email protected] (H. Vinek). 1 Present address: Institute of Technical Chemistry II, Technical University of Munich, Lichtenbergstr. 4, D-85747 Garching, Germany.
improve the combustion of the fuels and thus reduce the exhaust emissions and simultaneously boost the gasoline octane number. The most frequently used oxygenate is methyl-tert-butyl-ether (MTBE), which is prepared from iso-butene and methanol. Among the higher alkenes, n-pentenes are the most interesting feedstock. They are used to manufacture the octane booster tert-amyl-methyl-ether (TAME), which exhibits a much better biodegradability than MTBE. However, the availability of iso-alkenes is a limiting factor in expanding the MTBE and TAME production. During 1-pentene isomerization, studied over HZSM-5 [2], only double bond isomerization was observed at low temperature, whereas at higher
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M. Höchtl et al. / Applied Catalysis A: General 207 (2001) 397–405
temperature the total isomerization equilibrium is obtained. Since the skeletal isomerization is sufficiently fast, an equilibrium distribution was maintained although pentenes are continuously withdrawn for the formation of dimerization and cracking products. Houzvicka and Ponec [3] studied the n-butene isomerization over phosphorus containing catalysts and HZSM-5. The latter showed a very low selectivity to iso-butene and a large amount of by-products. The product distribution obtained with SAPO-11 showed a high relative concentration of iso-butene, whereas SAPO-5 deactivated rapidly, and the relative concentration of iso-butene was low. It was found that medium pore zeolites (10MR windows) were significantly more selective than large pore ones (12MR) and even in 10MR zeolites the presence of large cavities or channel intersections in the structure led to a decrease in the iso-butene selectivity [4,5]. Ferrierite (FER) was found to be an excellent catalyst for the skeletal isomerization of n-butenes and n-pentenes [6,7]. Another active catalyst for skeletal isomerization of alkenes is ZSM-22 [8,9] which consists of a one-dimensional, 10MR pore system. The skeletal isomerization of alkenes proceeds on acid sites via formation of carbenium ions. These carbenium ions can isomerize via a protonated cyclopropane intermediate. Besides isomerization, dimerization and subsequently the cracking of the alkenes can occur. It is generally accepted that the acid strength required for these reactions decreases in the order: cracking ≈oligomerization>skeletal isomerization double-bond isomerization [10]. According to this, strong acid sites will promote the undesired oligomerization-cracking reactions, while weaker-acid sites will be more selective towards skeletal isomerization. However, when the acidity is too low, the activity of the catalyst is only sufficient for double-bond isomerization. Houzvicka et al. [11] found the highest selectivity in skeletal isomerization for catalysts of moderate acid strength. Besides the need of moderate acidic sites, the pore size is an important factor influencing the isoalkene selectivity in zeolite catalysts [12,13]. Medium pore molecular sieves with one-dimensional pore system show an increased selectivity to isomerization. The highest selectivity in 1-butene isomerization was ob-
tained with ZSM-22 and CoAPO-11 catalysts [14]. Both these catalysts are 10MR molecular sieves with a one-dimensional structure of channels. In this paper, the results of the isomerization of 1-pentene over aluminum phosphates of type AEL and AFI and HZSM-5 were reported.
2. Experimental The aluminum phosphate molecular sieves were obtained by hydrothermal synthesis as described in earlier papers [15,16]. HSZM-5 with a Si/Al ratio of 26 was provided by Union Carbide. The BET surface area was determined from N2 -adsorption isotherms measured at 77 K. The number of Brønsted acid sites was determined by NH3 -TPD. The experiments were carried out in vacuum using a mass spectrometer for the detection of the desorbed species. FTIR-spectroscopy (Bruker IFS 28, MCT-Detector) after adsorption of benzene was applied to estimate the acid strength of the bridged hydroxyl groups (Brønsted acid sites). The shift of the corresponding IR stretching vibration was taken as measure for the Brønsted acidity of the catalysts. The samples were prepared as self-supporting wafers inside a ring shaped furnace in a vacuum cell. Infrared spectra were recorded in transmission mode with a resolution of 4 cm−1 . The FTIR in situ experiments were carried out in a flow cell with CSTR (continuous stirred tank reactor) characteristic. The conditions resembled those in the plug flow reactor in order to obtain comparable results. In the reaction cell temperatures between 250 and 400◦ C were applied. The deactivation of the catalysts was investigated at 400◦ C. For the kinetic experiments, a downstream plug flow reactor was used with an inner diameter of 4 mm. Typically 10–50 mg of the catalyst were fixed between two layers of quartz glass wool. The catalyst was used in the calcined state without further modifications and mixed with quartz sand. The pentene partial pressure was kept constant at 10 mbar in helium. The total flow was varied between 5 and 100 ml min−1 and the total pressure was 1200 mbar at temperatures ranging from 250 to 550◦ C. During the reaction the pressure drop over the catalyst bed was always below 10 mbar.
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Table 1 Catalyst properties
SAPO-5 CoAPO-5 SAPO-11 CoAPO-11 AlPO4 -11 ZSM-5
Brønsted acid sites (mmol/g)
Acid site strength 1ν OH (benzene)
BET (m2 /g)
Mean crystallite size (mm)
0.24 0.14 0.23 0.16 – 0.38
280 315 310 345 – 360
339 277 184 207 195 357
3–7 3–10 2–5 3–5 1.5–2 5–7
3. Results and discussion The conversion of 1-pentene was carried out over SAPO and CoAPO molecular sieves from type AEL and AFI and compared to the results obtained over ZSM-5. The properties of the catalysts are given in Table 1. ZSM-5 exhibited the highest acid strength of the catalysts used. The crystallite size was in the same range for all materials studied. Based on the product distributions, a reaction network was suggested (Fig. 1). This scheme comprises five main reactions as defined in Table 2: (i) double bond isomerization (DbI), (ii) skeletal isomerization (SkI), (iii) direct cracking of pentene (Cr), (iv) dimerization (and oligomerization) (Dim) and (v) hydride transfer (HT). From the conversion/yield plots in Fig. 2 it can be deduced, that over SAPO-11 the linear pentenes act like one reactant while the branched pentene isomers appear as primary products. Only for low conversions, even the double bond isomerization of 1-pentene was not complete and thus the linear pentenes were not in the thermodynamic equilibrium. At higher conversions, double bond isomerization of
Fig. 1. Reaction network of pentene-isomers.
1-pentene was near the thermodynamic equilibrium (calculated from the enthalpies of formation [17]) over all catalysts (Fig. 3). Thus, in the first step double bond isomerization occurred and subsequently the linear pentenes acted as ‘linear pentene pool’. Consequently, all reactions were calculated relative to the sum of the linear isomers and the conversion of 1-pentene was assumed as conversion of the linear pentene-isomers. Conversion = 1 −
= C5linear × 100[%] = C5inlet
The main reaction pathways of linear pentenes and the method used for the calculation of the selectivities are given in Table 2. Since ethene can only result from direct cracking of the pentenes, it was directly taken as measure for this pathway. Dimerization is calculated from all products higher pentene, butenes and from the excess of propene, assuming that direct cracking
Fig. 2. Conversion/yield plot of pentene isomers over SAPO-11.
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Table 2 Reaction pathways of linear pentenes Reaction pathway
Index in Fig. 1
Double bond isomerization Skeletal isomerization Hydride transfer Cracking Dimerization
(DbI) (SkI) (HT) (Cr) (Dim)
Calculation of the selectivities P = SDbI = C5linear P = SSkI = C5branched P SHT = PC5o SCr = 2 C2 P P P P SDim = C6 + C4 + C 3 − C2
= Fig. 3. Distribution of linear pentenes (6(C5linear ))=100%), T=400◦ C.
leads to equimolar amounts of ethene and propene. To obtain the hydride transfer selectivity, the amounts of saturated compounds were summarized. Products >C6 were not present in the product stream of the investigated reactions. = ) versus time on The conversion of the 6(C5linear stream (TOS) is shown in Fig. 4. During the initial period, the reaction was accompanied by a deactivation phase, which was significantly stronger over the larger pore AFI molecular sieves compared to the small pore catalysts with AEL structure. The loss of activity after 10 h was 1.25% of the initial value (determined from activity between 5 and 10 min TOS) for CoAPO-11 and 2% for SAPO-11, while for SAPO-5 the deactivation was 30% and for CoAPO-5, 80%. For ZSM-5, the decrease of activity was comparable to that of the AEL catalysts (below 2%). Yang et al. [18,19] found a high activity and selectivity in butene and 1-pentene isomerization over
Fig. 4. Deactivation of the catalysts during the reaction of 1-pentene, T=400◦ C.
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Fig. 5. In situ FTIR-spectra of SAPO-5 (a) and SAPO-11 (b) after reaction of 1-pentene and He purge at 400◦ C.
SAPO-11 and a decay over SAPO-5. Gielgens et al. [20] reported that the adsorption of 1-butene at 400◦ C can block the pores of CoAPO-5 irreversibly. The high conversion for both AFI catalysts can be explained by the high initial rate of dimerization/cracking, that shows a strong decrease after 60 min TOS. The enhanced deactivation was mainly caused by the formation of coke in the large pores of the AFI materials, where bimolecular reactions are less hindered than in the AEL pores. Note that in AEL catalysts the double bond shift reaction and skeletal rearrangements is favored by the shape selectivity. In situ FTIR spectroscopy showed different levels of coking on SAPO-5 and SAPO-11. The difference IR spectra of the O–H, C–H and C–C stretching vibrations of SAPO-5 (a) and SAPO-11 (b) after 10 h time on stream at 400◦ C and subsequent purging with He for 30 min are shown in Fig. 5. New bands appeared around 1350 and 1580 cm−1 in the spectra of both catalysts. Additionally, a weak signal of C–H vibrations of unsaturated species was found at 3050 cm−1 over SAPO-5. Lavalley et al. [21] assigned bands between 1200 and 1600 cm−1 to vibrations of irreversibly adsorbed species after reaction of n-hexane over H-Mordenite. A band at 1600 cm−1 was attributed to conjugated polyenes, while bands at lower wavenumbers were suggested to stem from aromatic compounds. In the same region bands at 1478, 1458 and 1385 cm−1 were attributed by Kondo et al. [22]
401
to vibrations of dimerized butenes. On the catalysts investigated, multiple unsaturated and cyclic compounds were formed during the reaction. The amount of coke formed, estimated from the C–C vibrations, was much higher on SAPO-5 compared to SAPO-11, which was in accordance with the difference in the decrease of activity. In the OH-stretching region only a slight decrease of the OH bands on SAPO-5 can be observed, thus deactivation is more likely due to coke formation at the entrance of the molecular sieve pores, that causes a decrease of the number of accessible sites. The active sites of SAPO-5 and SAPO-11 possess medium acid strength, the difference in catalytic stability must be primarily related to the structure of the catalysts. The increased formation of coke over the large pore catalysts is reflected in a higher selectivity to dimerization and hydride transfer. In the pores of the AFI molecular sieves two or more pentene molecules can undergo dimerization and oligomerization, leading to the formation of coke precursors. These reactions seem to be suppressed in the smaller pores of AEL. The most favored reaction over all SAPO and CoAPO catalysts was the skeletal isomerization (Fig. 6), however, the isomerization selectivity was lower over the AFI compared to AEL catalysts. Over HZSM-5 dimerization predominated. The selectivity to dimerization was 9% over SAPO-5 and 14% over CoAPO-5, while it was only about 1.5% over the AEL materials. The enhanced dimerization reaction is accompanied by a hydride transfer, which showed a somewhat higher selectivity over AFI than over AEL. Fig. 7 shows that a correlation between the number of intermolecular reactions, dimerization and hydride transfer, and the deactivation exists. The distribution of the branched pentenes corresponds to the thermodynamic equilibrium over all catalysts (Fig. 8). However, the extent of the skeletal isomerization is different over the catalysts studied. Over ZSM-5 the ratio between linear pentenes and branched pentenes corresponds to the thermodynamic equilibrium values, whereas over the aluminum phosphates this ratio decreased (Fig. 9). Over AlPO4 -11, as the least active catalyst, only 5% branched pentenes existed at 400◦ C. Due to the absence of strong acid sites [16] on AlPO-11 the main reaction is the double bond isomerization. Over SAPO-5 and CoAPO-5,
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Fig. 6. Selectivities of the different reaction pathways for 1-pentene at 400◦ C.
which possess medium strong acid sites [16], the minor amount of skeletal isomerization results from the loss of accessibility of active sites caused by the strong deactivation. For this reason, the catalysts with the lowest level of deactivation, i.e. ZSM-5 with strong acid sites and SAPO-11 and CoAPO-11 with medium strong acid sites, closely approach the pentene equilibrium distribution. However, the selectivity of the other reaction pathways over ZSM-5 exhibited great differences compared to the aluminum phosphate catalysts. At
Fig. 7. Dependence of the deactivation on the dimerization and the hydride transfer-selectivity, T=400◦ C.
400◦ C, the high dimerization selectivity was overlaid by a high activity for cracking, that evidently avoids the formation of bulkier molecules that act as coke precursors. Consequently, the hydride transfer selectivity is comparable to that measured over the AEL catalysts (2%). By-products are mainly formed via a bimolecular mechanism on the strong acid sites [23]. This mechanism, however, is suppressed in pores of 10-membered ring molecular sieves, since the pore dimensions are too small for accommodation of branched molecules. Note that ZSM-5, although a medium pore zeolite (with high acid strength) has channel intersections, which offer enough space for an almost unhindered oligomerization. In Fig. 10 the different reaction pathways of the 1-pentene isomerization as functions of temperature for ZSM-5 and CoAPO-11 are shown. Evidently the skeletal isomerization over ZSM-5 is suppressed by dimerization at low temperatures and by cracking at high temperatures. CoAPO-11 shows almost exclusively skeletal isomerization in the observed temperature range. As expected from the thermodynamic equilibrium data, the iso-alkene yield decreased with increasing temperature. All parallel reactions exhibit very low yields, although a maximum for dimerization at 400◦ C and an increase of cracking at higher temperatures can be observed. The activity for the latter reaction is approximately 100 times higher on ZSM-5 compared to CoAPO-11 (Fig. 10).
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= Fig. 8. Distribution of branched pentenes (6 (C5branched )=100%), T=400◦ C.
The data reported above show that the catalytic activity of the materials in the conversion of 1-pentene could be correlated with the scale of their Brønsted acid strength, as determined by NH3 -TPD and by IR spectroscopy of adsorbed benzene [16]. The reaction is catalyzed by the Brønsted acid sites, which protonate the 1-pentene to produce the sec-pentyl cation that can form a protonated cyclopropane in-
termediate [24]. Cleavage of C–C bond and proton elimination give rise to methyl-butenes according to the monomolecular mechanism. Over ZSM-5 dimerization to higher alkenes and cracking are the main reactions. Only at temperatures lower than 270◦ C, skeletal isomerization prevail. The proton elimination step from the tert-pentyl carbenium ion to give iso-pentene can be assumed to be an acid–base re-
Fig. 9. Selectivity to linear and branched pentenes (6(C5= )=100%), T=400◦ C.
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Fig. 10. Temperature dependence of the reaction pathways over CoAPO-11 and ZSM-5.
action, needing a basic site on the catalyst to locate the proton. The latter step is more favored on weakly acidic than on strong acidic catalysts. Aluminum phosphates are described as materials with a milder acidity than zeolites [25]. In the alkene reactions, this lower acidity is reflected in a distinct preference of the isomerization pathway over the SAPO and CoAPO catalysts, while over HZSM-5 the iso-pentene yield is lowered by competing reactions. Connected to the differences in the selectivities, a change in the conversion yield dependencies is found (Fig. 11). Over the SAPO and CoAPO materials, the products from the skeletal isomerization appear to be primary products, while all other products result from secondary reactions. On the contrary, over ZSM-5, cracking appears as primary reaction, while the yield for branched pentene is decreasing with increasing total conversion at 400◦ C. Hence, the skeletal isomerization seems to be almost as rapidly approaching the thermodynamic equilibrium as the double bond isomerization.
= Fig. 11. Conversion/yield plots of the reaction pathways over aluminum phosphate catalysts (conversion (400◦ C)=conversion 6(C5linear )) and ZSM-5 (conversion (500◦ C)=conversion 6(C5= )). Symbols correspond to Fig. 10.
M. Höchtl et al. / Applied Catalysis A: General 207 (2001) 397–405
4. Conclusions • Acidic OH groups are the active (and selective) sites on the catalysts for skeletal isomerization. • The strength of the acid sites increases in the order: AlPO-11
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[4] M.A. Asensi, A. Corma, A. Mart´ınez, J. Catal. 158 (1996) 561. [5] C.-L. O’Young, R.J. Pellet, D.G. Casey, J.R. Ugolini, R.A. Sawicki, J. Catal. 151 (1995) 467. [6] P. Grandvallet, K.P. DeJong, H.H. Mooiweer, A.G.T.G. Kortbeek, B. Kraushaar-Czarnetzki, Eur. Patent EP501577 (1992). [7] E.J. Kuhlmann, J.R. Pascoe, C. J. Thom, US Patent 5,463,160 (1995). [8] R. Byggningsbacka, L.E. Lindfors, N. Kumar, Ind. Eng. Chem. Res. 36 (1997) 2990. [9] M.A. Asensi, A. Corma, A. Mart´ınez, M. Derewinski, J. Krysciak, S.S. Tamhankar, Appl. Catal. A 174 (1998) 163. [10] A. Corma, B.W. Wojciechowski, Catal. Rev.-Sci. Eng. 24 (1982) 1. [11] J. Houzvicka, J.G. Nienhuis, V. Ponec, Appl. Catal. A 174 (1998) 207. [12] P. Mériaudeau, A. Vu Tuan, N. Le Hung, G. Szabo, Catal. Lett. 47 (1997) 71. [13] J. Houzvicka, S. Hansildaar, V. Ponec, J. Catal. 167 (1997) 273. [14] J. Houzvicka, S. Hansildaar, J.G. Nienhuis, V. Ponec, Appl. Catal. A 176 (1999) 83. [15] M. Höchtl, A. Jentys, H. Vinek, Stud. Surf. Sci. Catal. 125 (1999) 425. [16] M. Höchtl, A. Jentys, H. Vinek, Microporous Mesoporous Mater. 31 (1999) 271. [17] D.R. Stull, E.F. Westrum, The Chemical Thermodynamics of Organic Compounds, Wiley, London, 1969. [18] S.M. Yang, S.Y. Liu, in: Proceedings of Ninth International Zeolite Conference, Montreal, 1992, p. 623. [19] S.M. Yang, D.H. Guo, J.S. Lin, G.T. Wang, Stud. Surf. Sci. Catal. 84 (1994) 1677. [20] L.H. Gielgens, I.H.E. Veenstra, V. Ponec, M.J. Haanepen, J.H.C. van Hooff, Catal. Lett. 32 (1995) 195. [21] J-C. Lavalley, M. Maache, J. Saussey, SPIE Vol. 1341, Infrared Technology XVI, 1990, p. 244. [22] J.N. Kondo, F. Wakabayashi, K. Domen, Catal. Lett. 53 (1998) 215. [23] J. Houzvicka, O. Diefenbach, V. Ponec, J. Catal. 164 (1996) 288. [24] J.A. Martens, P.A. Jacobs, in: J.B. Moffat (Ed.), Theoretical Aspects of Heterogeneous Catalysis, Van Nostrand Reinhold, New York, 1990, p. 69. [25] J.A. Martens, P.J. Grobet, P.A. Jacobs, J. Catal. 126 (1990) 299.
Isomerization of 1-pentene over SAPO, CoAPO (AEL, AFI) molecular sieves and HZSM-5 M. Höchtl, A. Jentys1 , H. Vinek∗ Institute of Physical and Theoretical Chemistry, University of Technology, Vienna, Veterinärplatz 1, A-1210 Vienna, Austria Received 18 February 2000; received in revised form 15 June 2000; accepted 15 June 2000
Abstract Skeletal isomerization of 1-pentene was carried out over SAPO and CoAPO molecular sieves with AEL and AFI type structure and for comparison over AlPO-11 and HZSM-5. The equilibrium distribution within the linear pentenes and iso-pentenes was established over all catalysts except at very low temperatures or conversions. The approach to the equilibrium distribution between the linear pentenes and iso-pentenes depends on the strength of the acid sites, the pore dimensions and the structure of the catalyst. Over AlPO4 -11, a material with very weak acid sites, the selectivity to skeletal isomerization was only 5%, while the double bond isomerization and the isomerization via methyl shift were in equilibrium. Over SAPO-11 and CoAPO-11, having medium strong acid sites and one-dimensional pores with a 10-membered ring-system, the distribution of linear and iso-pentenes was near the equilibrium. On SAPO-5 and CoAPO-5, materials with sites of similar strength and one-dimensional pores with a 12-membered ring-system, the equilibrium distribution was not achieved because of the rapid deactivation that diminished the accessibility of the acid sites inside the pores. Due to the presence of strong acid sites and a two-dimensional pore system with a 10-membered ring-system, the equilibrium distribution is achieved on HZSM-5, but this ‘pentene pool’ react further by dimerization and cracking. © 2001 Elsevier Science B.V. All rights reserved. Keywords: 1-Pentene; Isomerization; SAPO; CoAPO; AEL; AFI
1. Introduction Oxygenates, alcohols and especially tertiary ethers, have become important components in gasoline due to tightening legislation concerning fuels. In the case of gasoline, the clean air act (CAA) amendments in USA established, among others, the reduction of volatility, reduction of aromatics and the introduction of oxygenated compounds [1]. Oxygenates ∗ Corresponding author. E-mail address: [email protected] (H. Vinek). 1 Present address: Institute of Technical Chemistry II, Technical University of Munich, Lichtenbergstr. 4, D-85747 Garching, Germany.
improve the combustion of the fuels and thus reduce the exhaust emissions and simultaneously boost the gasoline octane number. The most frequently used oxygenate is methyl-tert-butyl-ether (MTBE), which is prepared from iso-butene and methanol. Among the higher alkenes, n-pentenes are the most interesting feedstock. They are used to manufacture the octane booster tert-amyl-methyl-ether (TAME), which exhibits a much better biodegradability than MTBE. However, the availability of iso-alkenes is a limiting factor in expanding the MTBE and TAME production. During 1-pentene isomerization, studied over HZSM-5 [2], only double bond isomerization was observed at low temperature, whereas at higher
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temperature the total isomerization equilibrium is obtained. Since the skeletal isomerization is sufficiently fast, an equilibrium distribution was maintained although pentenes are continuously withdrawn for the formation of dimerization and cracking products. Houzvicka and Ponec [3] studied the n-butene isomerization over phosphorus containing catalysts and HZSM-5. The latter showed a very low selectivity to iso-butene and a large amount of by-products. The product distribution obtained with SAPO-11 showed a high relative concentration of iso-butene, whereas SAPO-5 deactivated rapidly, and the relative concentration of iso-butene was low. It was found that medium pore zeolites (10MR windows) were significantly more selective than large pore ones (12MR) and even in 10MR zeolites the presence of large cavities or channel intersections in the structure led to a decrease in the iso-butene selectivity [4,5]. Ferrierite (FER) was found to be an excellent catalyst for the skeletal isomerization of n-butenes and n-pentenes [6,7]. Another active catalyst for skeletal isomerization of alkenes is ZSM-22 [8,9] which consists of a one-dimensional, 10MR pore system. The skeletal isomerization of alkenes proceeds on acid sites via formation of carbenium ions. These carbenium ions can isomerize via a protonated cyclopropane intermediate. Besides isomerization, dimerization and subsequently the cracking of the alkenes can occur. It is generally accepted that the acid strength required for these reactions decreases in the order: cracking ≈oligomerization>skeletal isomerization double-bond isomerization [10]. According to this, strong acid sites will promote the undesired oligomerization-cracking reactions, while weaker-acid sites will be more selective towards skeletal isomerization. However, when the acidity is too low, the activity of the catalyst is only sufficient for double-bond isomerization. Houzvicka et al. [11] found the highest selectivity in skeletal isomerization for catalysts of moderate acid strength. Besides the need of moderate acidic sites, the pore size is an important factor influencing the isoalkene selectivity in zeolite catalysts [12,13]. Medium pore molecular sieves with one-dimensional pore system show an increased selectivity to isomerization. The highest selectivity in 1-butene isomerization was ob-
tained with ZSM-22 and CoAPO-11 catalysts [14]. Both these catalysts are 10MR molecular sieves with a one-dimensional structure of channels. In this paper, the results of the isomerization of 1-pentene over aluminum phosphates of type AEL and AFI and HZSM-5 were reported.
2. Experimental The aluminum phosphate molecular sieves were obtained by hydrothermal synthesis as described in earlier papers [15,16]. HSZM-5 with a Si/Al ratio of 26 was provided by Union Carbide. The BET surface area was determined from N2 -adsorption isotherms measured at 77 K. The number of Brønsted acid sites was determined by NH3 -TPD. The experiments were carried out in vacuum using a mass spectrometer for the detection of the desorbed species. FTIR-spectroscopy (Bruker IFS 28, MCT-Detector) after adsorption of benzene was applied to estimate the acid strength of the bridged hydroxyl groups (Brønsted acid sites). The shift of the corresponding IR stretching vibration was taken as measure for the Brønsted acidity of the catalysts. The samples were prepared as self-supporting wafers inside a ring shaped furnace in a vacuum cell. Infrared spectra were recorded in transmission mode with a resolution of 4 cm−1 . The FTIR in situ experiments were carried out in a flow cell with CSTR (continuous stirred tank reactor) characteristic. The conditions resembled those in the plug flow reactor in order to obtain comparable results. In the reaction cell temperatures between 250 and 400◦ C were applied. The deactivation of the catalysts was investigated at 400◦ C. For the kinetic experiments, a downstream plug flow reactor was used with an inner diameter of 4 mm. Typically 10–50 mg of the catalyst were fixed between two layers of quartz glass wool. The catalyst was used in the calcined state without further modifications and mixed with quartz sand. The pentene partial pressure was kept constant at 10 mbar in helium. The total flow was varied between 5 and 100 ml min−1 and the total pressure was 1200 mbar at temperatures ranging from 250 to 550◦ C. During the reaction the pressure drop over the catalyst bed was always below 10 mbar.
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Table 1 Catalyst properties
SAPO-5 CoAPO-5 SAPO-11 CoAPO-11 AlPO4 -11 ZSM-5
Brønsted acid sites (mmol/g)
Acid site strength 1ν OH (benzene)
BET (m2 /g)
Mean crystallite size (mm)
0.24 0.14 0.23 0.16 – 0.38
280 315 310 345 – 360
339 277 184 207 195 357
3–7 3–10 2–5 3–5 1.5–2 5–7
3. Results and discussion The conversion of 1-pentene was carried out over SAPO and CoAPO molecular sieves from type AEL and AFI and compared to the results obtained over ZSM-5. The properties of the catalysts are given in Table 1. ZSM-5 exhibited the highest acid strength of the catalysts used. The crystallite size was in the same range for all materials studied. Based on the product distributions, a reaction network was suggested (Fig. 1). This scheme comprises five main reactions as defined in Table 2: (i) double bond isomerization (DbI), (ii) skeletal isomerization (SkI), (iii) direct cracking of pentene (Cr), (iv) dimerization (and oligomerization) (Dim) and (v) hydride transfer (HT). From the conversion/yield plots in Fig. 2 it can be deduced, that over SAPO-11 the linear pentenes act like one reactant while the branched pentene isomers appear as primary products. Only for low conversions, even the double bond isomerization of 1-pentene was not complete and thus the linear pentenes were not in the thermodynamic equilibrium. At higher conversions, double bond isomerization of
Fig. 1. Reaction network of pentene-isomers.
1-pentene was near the thermodynamic equilibrium (calculated from the enthalpies of formation [17]) over all catalysts (Fig. 3). Thus, in the first step double bond isomerization occurred and subsequently the linear pentenes acted as ‘linear pentene pool’. Consequently, all reactions were calculated relative to the sum of the linear isomers and the conversion of 1-pentene was assumed as conversion of the linear pentene-isomers. Conversion = 1 −
= C5linear × 100[%] = C5inlet
The main reaction pathways of linear pentenes and the method used for the calculation of the selectivities are given in Table 2. Since ethene can only result from direct cracking of the pentenes, it was directly taken as measure for this pathway. Dimerization is calculated from all products higher pentene, butenes and from the excess of propene, assuming that direct cracking
Fig. 2. Conversion/yield plot of pentene isomers over SAPO-11.
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Table 2 Reaction pathways of linear pentenes Reaction pathway
Index in Fig. 1
Double bond isomerization Skeletal isomerization Hydride transfer Cracking Dimerization
(DbI) (SkI) (HT) (Cr) (Dim)
Calculation of the selectivities P = SDbI = C5linear P = SSkI = C5branched P SHT = PC5o SCr = 2 C2 P P P P SDim = C6 + C4 + C 3 − C2
= Fig. 3. Distribution of linear pentenes (6(C5linear ))=100%), T=400◦ C.
leads to equimolar amounts of ethene and propene. To obtain the hydride transfer selectivity, the amounts of saturated compounds were summarized. Products >C6 were not present in the product stream of the investigated reactions. = ) versus time on The conversion of the 6(C5linear stream (TOS) is shown in Fig. 4. During the initial period, the reaction was accompanied by a deactivation phase, which was significantly stronger over the larger pore AFI molecular sieves compared to the small pore catalysts with AEL structure. The loss of activity after 10 h was 1.25% of the initial value (determined from activity between 5 and 10 min TOS) for CoAPO-11 and 2% for SAPO-11, while for SAPO-5 the deactivation was 30% and for CoAPO-5, 80%. For ZSM-5, the decrease of activity was comparable to that of the AEL catalysts (below 2%). Yang et al. [18,19] found a high activity and selectivity in butene and 1-pentene isomerization over
Fig. 4. Deactivation of the catalysts during the reaction of 1-pentene, T=400◦ C.
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Fig. 5. In situ FTIR-spectra of SAPO-5 (a) and SAPO-11 (b) after reaction of 1-pentene and He purge at 400◦ C.
SAPO-11 and a decay over SAPO-5. Gielgens et al. [20] reported that the adsorption of 1-butene at 400◦ C can block the pores of CoAPO-5 irreversibly. The high conversion for both AFI catalysts can be explained by the high initial rate of dimerization/cracking, that shows a strong decrease after 60 min TOS. The enhanced deactivation was mainly caused by the formation of coke in the large pores of the AFI materials, where bimolecular reactions are less hindered than in the AEL pores. Note that in AEL catalysts the double bond shift reaction and skeletal rearrangements is favored by the shape selectivity. In situ FTIR spectroscopy showed different levels of coking on SAPO-5 and SAPO-11. The difference IR spectra of the O–H, C–H and C–C stretching vibrations of SAPO-5 (a) and SAPO-11 (b) after 10 h time on stream at 400◦ C and subsequent purging with He for 30 min are shown in Fig. 5. New bands appeared around 1350 and 1580 cm−1 in the spectra of both catalysts. Additionally, a weak signal of C–H vibrations of unsaturated species was found at 3050 cm−1 over SAPO-5. Lavalley et al. [21] assigned bands between 1200 and 1600 cm−1 to vibrations of irreversibly adsorbed species after reaction of n-hexane over H-Mordenite. A band at 1600 cm−1 was attributed to conjugated polyenes, while bands at lower wavenumbers were suggested to stem from aromatic compounds. In the same region bands at 1478, 1458 and 1385 cm−1 were attributed by Kondo et al. [22]
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to vibrations of dimerized butenes. On the catalysts investigated, multiple unsaturated and cyclic compounds were formed during the reaction. The amount of coke formed, estimated from the C–C vibrations, was much higher on SAPO-5 compared to SAPO-11, which was in accordance with the difference in the decrease of activity. In the OH-stretching region only a slight decrease of the OH bands on SAPO-5 can be observed, thus deactivation is more likely due to coke formation at the entrance of the molecular sieve pores, that causes a decrease of the number of accessible sites. The active sites of SAPO-5 and SAPO-11 possess medium acid strength, the difference in catalytic stability must be primarily related to the structure of the catalysts. The increased formation of coke over the large pore catalysts is reflected in a higher selectivity to dimerization and hydride transfer. In the pores of the AFI molecular sieves two or more pentene molecules can undergo dimerization and oligomerization, leading to the formation of coke precursors. These reactions seem to be suppressed in the smaller pores of AEL. The most favored reaction over all SAPO and CoAPO catalysts was the skeletal isomerization (Fig. 6), however, the isomerization selectivity was lower over the AFI compared to AEL catalysts. Over HZSM-5 dimerization predominated. The selectivity to dimerization was 9% over SAPO-5 and 14% over CoAPO-5, while it was only about 1.5% over the AEL materials. The enhanced dimerization reaction is accompanied by a hydride transfer, which showed a somewhat higher selectivity over AFI than over AEL. Fig. 7 shows that a correlation between the number of intermolecular reactions, dimerization and hydride transfer, and the deactivation exists. The distribution of the branched pentenes corresponds to the thermodynamic equilibrium over all catalysts (Fig. 8). However, the extent of the skeletal isomerization is different over the catalysts studied. Over ZSM-5 the ratio between linear pentenes and branched pentenes corresponds to the thermodynamic equilibrium values, whereas over the aluminum phosphates this ratio decreased (Fig. 9). Over AlPO4 -11, as the least active catalyst, only 5% branched pentenes existed at 400◦ C. Due to the absence of strong acid sites [16] on AlPO-11 the main reaction is the double bond isomerization. Over SAPO-5 and CoAPO-5,
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Fig. 6. Selectivities of the different reaction pathways for 1-pentene at 400◦ C.
which possess medium strong acid sites [16], the minor amount of skeletal isomerization results from the loss of accessibility of active sites caused by the strong deactivation. For this reason, the catalysts with the lowest level of deactivation, i.e. ZSM-5 with strong acid sites and SAPO-11 and CoAPO-11 with medium strong acid sites, closely approach the pentene equilibrium distribution. However, the selectivity of the other reaction pathways over ZSM-5 exhibited great differences compared to the aluminum phosphate catalysts. At
Fig. 7. Dependence of the deactivation on the dimerization and the hydride transfer-selectivity, T=400◦ C.
400◦ C, the high dimerization selectivity was overlaid by a high activity for cracking, that evidently avoids the formation of bulkier molecules that act as coke precursors. Consequently, the hydride transfer selectivity is comparable to that measured over the AEL catalysts (2%). By-products are mainly formed via a bimolecular mechanism on the strong acid sites [23]. This mechanism, however, is suppressed in pores of 10-membered ring molecular sieves, since the pore dimensions are too small for accommodation of branched molecules. Note that ZSM-5, although a medium pore zeolite (with high acid strength) has channel intersections, which offer enough space for an almost unhindered oligomerization. In Fig. 10 the different reaction pathways of the 1-pentene isomerization as functions of temperature for ZSM-5 and CoAPO-11 are shown. Evidently the skeletal isomerization over ZSM-5 is suppressed by dimerization at low temperatures and by cracking at high temperatures. CoAPO-11 shows almost exclusively skeletal isomerization in the observed temperature range. As expected from the thermodynamic equilibrium data, the iso-alkene yield decreased with increasing temperature. All parallel reactions exhibit very low yields, although a maximum for dimerization at 400◦ C and an increase of cracking at higher temperatures can be observed. The activity for the latter reaction is approximately 100 times higher on ZSM-5 compared to CoAPO-11 (Fig. 10).
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= Fig. 8. Distribution of branched pentenes (6 (C5branched )=100%), T=400◦ C.
The data reported above show that the catalytic activity of the materials in the conversion of 1-pentene could be correlated with the scale of their Brønsted acid strength, as determined by NH3 -TPD and by IR spectroscopy of adsorbed benzene [16]. The reaction is catalyzed by the Brønsted acid sites, which protonate the 1-pentene to produce the sec-pentyl cation that can form a protonated cyclopropane in-
termediate [24]. Cleavage of C–C bond and proton elimination give rise to methyl-butenes according to the monomolecular mechanism. Over ZSM-5 dimerization to higher alkenes and cracking are the main reactions. Only at temperatures lower than 270◦ C, skeletal isomerization prevail. The proton elimination step from the tert-pentyl carbenium ion to give iso-pentene can be assumed to be an acid–base re-
Fig. 9. Selectivity to linear and branched pentenes (6(C5= )=100%), T=400◦ C.
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Fig. 10. Temperature dependence of the reaction pathways over CoAPO-11 and ZSM-5.
action, needing a basic site on the catalyst to locate the proton. The latter step is more favored on weakly acidic than on strong acidic catalysts. Aluminum phosphates are described as materials with a milder acidity than zeolites [25]. In the alkene reactions, this lower acidity is reflected in a distinct preference of the isomerization pathway over the SAPO and CoAPO catalysts, while over HZSM-5 the iso-pentene yield is lowered by competing reactions. Connected to the differences in the selectivities, a change in the conversion yield dependencies is found (Fig. 11). Over the SAPO and CoAPO materials, the products from the skeletal isomerization appear to be primary products, while all other products result from secondary reactions. On the contrary, over ZSM-5, cracking appears as primary reaction, while the yield for branched pentene is decreasing with increasing total conversion at 400◦ C. Hence, the skeletal isomerization seems to be almost as rapidly approaching the thermodynamic equilibrium as the double bond isomerization.
= Fig. 11. Conversion/yield plots of the reaction pathways over aluminum phosphate catalysts (conversion (400◦ C)=conversion 6(C5linear )) and ZSM-5 (conversion (500◦ C)=conversion 6(C5= )). Symbols correspond to Fig. 10.
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4. Conclusions • Acidic OH groups are the active (and selective) sites on the catalysts for skeletal isomerization. • The strength of the acid sites increases in the order: AlPO-11
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