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Extent of monomolecular and bimolecular mechanism in n-butene skeletal isomerization to isobutene over molecular sieves
Applied Catalysis A: General 179 (1999) 217±222
Extent of monomolecular and bimolecular mechanism in n-butene skeletal isomerization to isobutene over molecular sieves JirÏÂõ CÏejkaa,*, Blanka WichterlovaÂa, Priit Sarvb a
J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, DolejsÏkova 3, CZ 182 23 Prague 8, Czech Republic b Institute of Chemical Physics and Biophysics, Akadeemia 23, EE 00 26 Tallinn, Estonia Received 10 April 1998; received in revised form 14 September 1998; accepted 15 September 1998
Abstract Mechanism of skeletal isomerization of n-butene to isobutene over CoAlPO-11 and H-ferrierite molecular sieves was investigated using 13 C labeled 1-butene. It was con®rmed that a high selectivity to isobutene can be reached exclusively via a monomolecular reaction pathway. The bimolecular mechanism is responsible not only for the formation of propylene, pentenes, and other by-products but also for a substantial amount of isobutene. At least 30% of isobutene formed over ferrierite at 620 K was produced via dimerization of butenes followed by isomerization and cracking. # 1999 Elsevier Science B.V. All rights reserved. Keywords: n-Butene; Skeletal isomerization; Bimolecular mechanism; Molecular sieve; Monomolecular Mechanism
1. Introduction Currently, particular interest is devoted to skeletal isomerization of linear butenes to isobutene as isobutene represents a starting material for production of MTBE, an octane-enhancer to gasoline. Moreover, it is used for butyl rubber production or can be converted into isoprene. Various catalysts differing in the number and nature of their active sites have been investigated in this reaction [1] and it has been recognized that especially molecular sieves possessing 10-member rings (MeAlPO-11, ferrierite/ZSM-35, ZSM-22, ZSM-23, MCM-22) exhibit high selectivity to isobutene mainly due to their shape selective properties [2±6]. *Corresponding author.
Despite a high number of papers dealing with skeletal isomerization of linear butenes mechanism of isobutene formation has not yet been unequivocally explained. A bimolecular reaction pathway with simultaneous formation of large amount of C3 and C5 ole®ns was at ®rst suggested by Mooiweer et al. [2]. The so-called pseudomonomolecular mechanism of isobutene formation including a participation of coke deposits was assumed by Guisnet et al. [7]. Using 13 C labeled 1-butene Meriaudeau et al. [8] proposed that a bimolecular mechanism operates on the fresh, non-selective ferrierite catalyst while a monomolecular reaction proceeds on the used, selective catalyst. This suggestion was based on the different content of isobutene molecules in the products containing two 13 C, one 13 C and no 13 C atoms. A substantial improvement of isobutene selectivity with increasing dilution
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00320-2
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Ï ejka et al. / Applied Catalysis A: General 179 (1999) 217±222 J. C
of protonic sites found on MCM-22 also supports the monomolecular isomerization of n-butenes into isobutene [5]. The purpose of this contribution is to give an evidence on the prevailing mechanism of skeletal isomerization of 1-butene using 13 C 1-butene over highly selective, represented by CoAlPO-11, and non-selective catalyst, H-ferrierite, producing large amount of by-products. 2. Experimental CoAlPO-11 was synthesized in our laboratory according to the procedure described in [9]. Ferrierite was purchased from TOSOH. XRD, SEM, and FTIR skeletal vibrations con®rmed good crystallinity of both samples. The number of acid sites was determined by the temperature programmed desorption of ammonia and FTIR spectra of preadsorbed d3-acetonitrile. It was found that ferrierite (Si/Al8.4) dehydrated at 670 K contained 1.50 mmol OH group/g and 0.07 mmol of Lewis sites/g. In CoAlPO-11 0.6 wt% of cobalt ions was localized in tetrahedral framework positions as con®rmed by UV±vis spectroscopy. Both molecular sieves were calcined prior to the reaction in a stream of a dry oxygen at 720 K for 2 h. 1-Butene isomerization was tested in a through-¯ow glass microreactor under atmospheric pressure, with 0.4 g of catalyst, 10 vol% of n-butene in nitrogen, WHSV 4.5 hÿ1, and at 620 K. Reaction products were analyzed using an ``on-line'' connected high resolution gas chromatograph (HP 6890) equipped with ¯ame ionization detector and capillary column (KCl/Al2O3). For experiments with labeled 1-butene 13 CH2 CH2 CH2 CH3 the same experimental setup was used; however, instead of continuous ¯ow of 1-butene the pulses of labeled 1-butene were used. 13 C 1-butene was obtained from CDN Isotopes and contained 99% of 13 C in position 1. Ten ml of pure labeled 1-butene under the pressure of 2.5 atm (0.065 g) was injected into the stream of nitrogen ¯ow through the reactor. Due to the difference in the pressures it can be expected that almost pure 1butene was introduced to the catalyst bed. The products were analyzed with the same column but using mass spectrometric detector (MSD 5971A). More-
over, the reaction products were condensed in a glass trap at 77 K, transferred into NMR tube and sealed. Before the NMR experiments the NMR tube was cooled down, the deuterated chloroform was added and the tube was sealed again. 13 C liquid NMR spectra with proton decoupling were recorded on Bruker AMX500 spectrometer at 125.7 MHz. Relaxation delay was 2 s. J-modulated spin-echo experiment for 13 C nuclei was used to determine the number of attached protons. The 13 C NMR peaks were assigned to the speci®c carbon sites in the molecules according to their chemical shift and the number of attached protons. Most of the peaks in the spectrum are accompanied by small satellites on both sides of the peak. These satellites correspond to the 13 C atoms attached to other 13 C atoms. Due to homonuclear J-coupling between the 13 Cÿ13 C pair the NMR line has splitted, revealing two lines on both sides of the central peak. The central peak corresponds to the lone 13 C atoms having no 13 C neighbors. In the case of the double bond the J-coupling is stronger than in the case of the single bond and the satellites are, respectively, more apart. The relative intensity of the satellites compared to the central line gives us the relative amount of the 13 Cÿ13 C pairs. 3. Results and discussion Table 1 summarizes data for 1-butene conversion, expressed as a conversion of all n-butene isomers (the double bond shift and cis±trans isomerization of nbutenes reach the equilibrium before the skeletal isomerization sets in [10]), and selectivity to all products after 5 and 1440 min of T-O-S. It is clearly seen that with CoAlPO-11 the selectivity to isobutene was higher than 97% from the beginning of the reaction (conversion 46%) and conversion and yield slowly decreased with T-O-S (Fig. 1). Under the same conditions with H-ferrierite a substantially higher initial conversion of n-butenes was obtained (61%), the selectivity to isobutene was only 49% and a high concentration of propylene (24%) and C5±C6 ole®ns (15.5%) was formed. After a signi®cant decrease in conversion of n-butenes the selectivity to isobutene increased up to 92% (T-O-S of 1440 min). The increase in selectivity is connected with the decrease in conversion of n-butenes but also can re¯ect narrow-
J. CÏejka et al. / Applied Catalysis A: General 179 (1999) 217±222
219
Table 1 The conversion and selectivity in skeletal isomerization of n-butenes over CoAlPO-11 and ferrierite CoAlPO-11 T-O-S (min) Conversion (%) Selectivity (vol%) Ethane Propane Propylene Isobutane Butane Isobutene P C P 5 P C6 P C7 C8 Yield a
Ferrierite
5 46.1
1440 39.2
0 0 1.0 0 0 97.4 0.7 0 0.4 0.2 44.9
0 0 0.6 0 0 98.5 0.3 0 0.3 0 38.6
a
Pulse 54.0
5 61.1
1440 42.1
0 0 7.4 0 0 86.0 6.6 0 0 0 46.4
2.3 1.7 24.0 0.6 3.8 49.4 12.7 2.8 2.6 0.2 30.2
0 0 3.7 0 0.6 92.1 2.4 0.7 0.5 0 38.8
Pulsea 56.3 2.1 2.9 17.0 3.2 4.1 50.6 16.0 4.1 0 0 28.5
Conversion and selectivities were calculated from integral areas of total ion chromatogram, reaction arrangement given in Section 2.
Fig. 1. Time-on-stream dependence of conversion (&), isobutene selectivity (*) and isobutene yield (~) for CoAlPO-11.
ing of channels and their cross-sections by coke deposits (see Fig. 2). This might prevent formation of n-butene dimers (or from n-buteneisobutene) and particularly the isomerization of formed C8 ole®ns as a result of restricted transition state selectivity. The alternative explanation of the signi®cant increase in isobutene yield over ferrierite with coke deposits was proposed by Guisnet et al. [7], the so-called pseudomonomolecular mechanism including high molecular
weight deposits as active sites for isobutene formation. To exclude a potential role of these deposits in our experiments, the ``fresh'' molecular sieves without any carbon deposits were used not to in¯uence the product distribution and 13 C scrambling by the interaction of reactants and/or products with coke deposits. Pulse catalytic experiments with labeled 13 C 1-butene were used to estimate the extent of mono-
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Ï ejka et al. / Applied Catalysis A: General 179 (1999) 217±222 J. C
Fig. 2. Time-on-stream dependence of conversion (&), isobutene selectivity (*) and isobutene yield (~) for H-ferrierite.
and bimolecular reaction. Experiments were performed over ``fresh'' molecular sieves and at a similar conversion level (54% ± CoAlPO-11, 56% ± ferrierite) while the selectivities to isobutene differed substantially (86% ± CoAlPO-11, 50% ± ferrierite). Although comparable conversions in T-O-S and pulse experiments were achieved, the reaction conditions used were signi®cantly different. The obtained by-products in these experiments consisted mainly of propylene and pentenes; moreover, in the case of ferrierite as expected also ethane, n-butane, isobutane, and C6 ole®ns were found (Table 1). Formation of relatively high amount of propylene and pentenes took place probably mainly on the external surface of CoAlPO-11 and it can be supposed that these active sites were deactivated during the ®rst minutes of T-O-S. The distribution of isobutene molecules according to their molecular weight in the products was established from their mass spectra (after T-O-S of 45 s) and 13 C NMR data characterized the products condensed at liquid nitrogen temperature. Over both catalysts a mixture of isobutene molecules consisting of non-labeled isobutene and those labeled with one or two 13 C atoms (molecular weight 56, 57 and 58, respectively) was formed. The distribution of these three isobutene molecular weights was substantially different for CoAlPO-11 and ferrierite. On CoAlPO-11 the ratio of M58/M57 (isobutene with two 13 C atoms/one 13 C)
was 0.03, while for ferrierite was 0.15 and the same ratios were determined also for n-butene isomers. 13 C NMR data measured on CoAlPO-11 gave the same concentrations of n-butenes and isobutene with two 13 C atoms. From known concentrations of n-butenes and isobutene with two and one 13 C atoms and without 13 C it is possible to evaluate the extent of the bimolecular mechanism to the selectivity to isobutene. There exist three pathways how isobutene can be formed and probably neither of them can be fully omitted. At ®rst, the monomolecular mechanism operates which provides high selectivity to isobutene with only one 13 C atom in molecule. Secondly, dimerization (reaction of two n-butene molecules) or codimerization (n-buteneisobutene already formed) followed by isomerization and cracking can lead, except to various by-products which must be always present, to the formation of isobutene again with only one 13 C atom in the molecule. However, these isobutene molecules cannot be distinguished from those formed via monomolecular mechanism and one can only estimate the prevailing mechanism from the concentration of by-products formed in this reaction. Thirdly, the skeletal isomerization of dimers and codimers occurs in such a way that n-butenes or isobutene formed contain either two 13 C atoms or none labeled carbon.
J. CÏejka et al. / Applied Catalysis A: General 179 (1999) 217±222
On CoAlPO-11 we found 3% of n-butenes and isobutene with two 13 C atoms in molecules which means that also 3% of n-butenes and isobutene without 13 C should be formed. Thus, on this catalyst unambiguously at least 6% (from 86%) of isobutene produced was formed via a bimolecular reaction. The remaining part of isobutene molecules with only one 13 C atom in the molecule was formed either via monomolecular or bimolecular reaction pathway without mixing of the 13 C atoms between original reactants. With fresh ferrierite possessing substantially lower selectivity (Table 1) almost 30% of isobutene (from 50.6%) was unequivocally formed via bimolecular mechanism. It means that at least 65% of products was formed via cracking of C8 ole®ns assuming that all isobutene molecules possessing only one 13 C atom were formed via monomolecular mechanism. However, it cannot be excluded that a part of isobutene molecules with one 13 C was also formed via bimolecular mechanism. This would increase the extent of isobutene formed by the bimolecular reaction. These results are in contradiction to those published by HouzÏvicÏka and Ponec [11,12] who supposed that only by-products are formed via cracking of octene isomers. 13 C NMR measurements provided information that labeled 13 C atoms were located in all positions of isobutene molecules with the distribution CH3:CH2:C 1.5:1:1. This means that 13 C atoms are almost evenly distributed in all positions of isobutene molecules without any preference. This could result from the monomolecular mechanism of n-butene transformation taking place via methyl±cyclopropyl carbocation or just as the result of scrambling. In 1-butene the second labeled 13 C atom was also evenly distributed in all positions, but in 2-butene (both cis and trans) the 3C position was not occupied. This can be seen from the 13 C spectrum, where the carbon with double bond (125.8 ppm) has only one pair of satellites, corresponding to the CH3±CH= pair, and the satellites corresponding to the CH=CH pair are missing. Combining the results obtained in the reaction T-O-S and pulse experiments it is clear that for CoAlPO-11 a very selective monomolecular formation of isobutene takes place. With ``fresh'' ferrierite a substantial amount of isobutene is formed via non-selective bimolecular mechanism. From a comparison of these
221
results it is evident that n-butene isomerization to isobutene over fresh CoAlPO-11 and ferrierite is controlled by the restricted transition state selectivity. In the case of monodimensional elliptic channels of Ê the CoAlPO-11 with channel dimension 3.96.3 A monomolecular mechanism operates while the threedimensional channel system of ferrierite (4.25.4 and Ê ), in addition to monomolecular mechan3.54.8 A ism, enables also formation of dimers and codimers, their isomerization and cracking leading to a spectrum of products including isobutene. It can be assumed that the bimolecular reaction followed by isomerization and cracking proceeds mainly in the channel intersections of ferrierite crystal structure which are able to accommodate these dimers or codimers. In addition, we assume that branched octenes are too bulky to ®t into monodimensional channels of CoAlPO-11, thus, they cannot be formed there and monomolecular mechanism strongly prevails. It is necessary to admit that although we have estimated the minimum extent of the bimolecular reaction connected with the formation of non-labeled isobutene and isobutene with two 13 C atoms we were not able to distinguish between isobutene molecules formed via monomolecular or bimolecular mechanism with one 13 C atom in the molecule. Acknowledgements Financial support from EC-Copernicus project (CIPA-CT94-0184) and Grant Agency of Academy of Sciences of the Czech Republic (No. A4040707) is highly acknowledged. PS also acknowledges support from the Estonian Science Foundation under Grant No. 2302.
References [1] J. HouzÏvicÏka, V. Ponec, Catal. Rev.-Sci. Eng. 39 (1997) 319. [2] H.H. Mooiweer, K.P. de Jong, B. Kraushaar-Czarnetzki, W.H.J. Stork, S.C. Krutzen, Stud. Surf. Sci. Catal. 84 (1994) 2327. [3] L.H. Gielgens, I.H.E. Veenstra, V. Ponec, M.J. Haanepen, J.H.C. van Hooff, Catal. Lett. 32 (1995) 195. [4] M. Guisnet, P. Andy, N.S. Gnep, E. Benazzi, C. Travers, J. Catal. 158 (1996) 551. [5] M. Asensi, A. Corma, A. Martinez, J. Catal. 158 (1996) 561.
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[6] R. Byggninsbacka, L.E. Lindfors, N. Kumar, Ind. Eng. Chem. Res. 36 (1997) 2990. [7] M. Guisnet, P. Andy, N.S. Gnep, C. Travers, E. Benazzi, J. Chem. Soc., Chem. Commun. 1685 (1995). [8] P. Meriaudeau, R. Bacaud, L.N. Hung, A.T. Vu, J. Mol. Catal. A 110 (1996) L177.
Ï ejka, B. WichterlovaÂ, Appl. Catal. B 15 [9] J. DeÏdecek, J. C (1998) 233. [10] Z.-X. Cheng, V. Ponec, J. Catal. 148 (1994) 607. [11] J. HouzÏvicÏka, V. Ponec, Ind. Eng. Chem. Res. 36 (1997) 1424. [12] J. HouzÏvicÏka, Ph.D. Thesis, Leiden University, 1998.
Extent of monomolecular and bimolecular mechanism in n-butene skeletal isomerization to isobutene over molecular sieves JirÏÂõ CÏejkaa,*, Blanka WichterlovaÂa, Priit Sarvb a
J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, DolejsÏkova 3, CZ 182 23 Prague 8, Czech Republic b Institute of Chemical Physics and Biophysics, Akadeemia 23, EE 00 26 Tallinn, Estonia Received 10 April 1998; received in revised form 14 September 1998; accepted 15 September 1998
Abstract Mechanism of skeletal isomerization of n-butene to isobutene over CoAlPO-11 and H-ferrierite molecular sieves was investigated using 13 C labeled 1-butene. It was con®rmed that a high selectivity to isobutene can be reached exclusively via a monomolecular reaction pathway. The bimolecular mechanism is responsible not only for the formation of propylene, pentenes, and other by-products but also for a substantial amount of isobutene. At least 30% of isobutene formed over ferrierite at 620 K was produced via dimerization of butenes followed by isomerization and cracking. # 1999 Elsevier Science B.V. All rights reserved. Keywords: n-Butene; Skeletal isomerization; Bimolecular mechanism; Molecular sieve; Monomolecular Mechanism
1. Introduction Currently, particular interest is devoted to skeletal isomerization of linear butenes to isobutene as isobutene represents a starting material for production of MTBE, an octane-enhancer to gasoline. Moreover, it is used for butyl rubber production or can be converted into isoprene. Various catalysts differing in the number and nature of their active sites have been investigated in this reaction [1] and it has been recognized that especially molecular sieves possessing 10-member rings (MeAlPO-11, ferrierite/ZSM-35, ZSM-22, ZSM-23, MCM-22) exhibit high selectivity to isobutene mainly due to their shape selective properties [2±6]. *Corresponding author.
Despite a high number of papers dealing with skeletal isomerization of linear butenes mechanism of isobutene formation has not yet been unequivocally explained. A bimolecular reaction pathway with simultaneous formation of large amount of C3 and C5 ole®ns was at ®rst suggested by Mooiweer et al. [2]. The so-called pseudomonomolecular mechanism of isobutene formation including a participation of coke deposits was assumed by Guisnet et al. [7]. Using 13 C labeled 1-butene Meriaudeau et al. [8] proposed that a bimolecular mechanism operates on the fresh, non-selective ferrierite catalyst while a monomolecular reaction proceeds on the used, selective catalyst. This suggestion was based on the different content of isobutene molecules in the products containing two 13 C, one 13 C and no 13 C atoms. A substantial improvement of isobutene selectivity with increasing dilution
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00320-2
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Ï ejka et al. / Applied Catalysis A: General 179 (1999) 217±222 J. C
of protonic sites found on MCM-22 also supports the monomolecular isomerization of n-butenes into isobutene [5]. The purpose of this contribution is to give an evidence on the prevailing mechanism of skeletal isomerization of 1-butene using 13 C 1-butene over highly selective, represented by CoAlPO-11, and non-selective catalyst, H-ferrierite, producing large amount of by-products. 2. Experimental CoAlPO-11 was synthesized in our laboratory according to the procedure described in [9]. Ferrierite was purchased from TOSOH. XRD, SEM, and FTIR skeletal vibrations con®rmed good crystallinity of both samples. The number of acid sites was determined by the temperature programmed desorption of ammonia and FTIR spectra of preadsorbed d3-acetonitrile. It was found that ferrierite (Si/Al8.4) dehydrated at 670 K contained 1.50 mmol OH group/g and 0.07 mmol of Lewis sites/g. In CoAlPO-11 0.6 wt% of cobalt ions was localized in tetrahedral framework positions as con®rmed by UV±vis spectroscopy. Both molecular sieves were calcined prior to the reaction in a stream of a dry oxygen at 720 K for 2 h. 1-Butene isomerization was tested in a through-¯ow glass microreactor under atmospheric pressure, with 0.4 g of catalyst, 10 vol% of n-butene in nitrogen, WHSV 4.5 hÿ1, and at 620 K. Reaction products were analyzed using an ``on-line'' connected high resolution gas chromatograph (HP 6890) equipped with ¯ame ionization detector and capillary column (KCl/Al2O3). For experiments with labeled 1-butene 13 CH2 CH2 CH2 CH3 the same experimental setup was used; however, instead of continuous ¯ow of 1-butene the pulses of labeled 1-butene were used. 13 C 1-butene was obtained from CDN Isotopes and contained 99% of 13 C in position 1. Ten ml of pure labeled 1-butene under the pressure of 2.5 atm (0.065 g) was injected into the stream of nitrogen ¯ow through the reactor. Due to the difference in the pressures it can be expected that almost pure 1butene was introduced to the catalyst bed. The products were analyzed with the same column but using mass spectrometric detector (MSD 5971A). More-
over, the reaction products were condensed in a glass trap at 77 K, transferred into NMR tube and sealed. Before the NMR experiments the NMR tube was cooled down, the deuterated chloroform was added and the tube was sealed again. 13 C liquid NMR spectra with proton decoupling were recorded on Bruker AMX500 spectrometer at 125.7 MHz. Relaxation delay was 2 s. J-modulated spin-echo experiment for 13 C nuclei was used to determine the number of attached protons. The 13 C NMR peaks were assigned to the speci®c carbon sites in the molecules according to their chemical shift and the number of attached protons. Most of the peaks in the spectrum are accompanied by small satellites on both sides of the peak. These satellites correspond to the 13 C atoms attached to other 13 C atoms. Due to homonuclear J-coupling between the 13 Cÿ13 C pair the NMR line has splitted, revealing two lines on both sides of the central peak. The central peak corresponds to the lone 13 C atoms having no 13 C neighbors. In the case of the double bond the J-coupling is stronger than in the case of the single bond and the satellites are, respectively, more apart. The relative intensity of the satellites compared to the central line gives us the relative amount of the 13 Cÿ13 C pairs. 3. Results and discussion Table 1 summarizes data for 1-butene conversion, expressed as a conversion of all n-butene isomers (the double bond shift and cis±trans isomerization of nbutenes reach the equilibrium before the skeletal isomerization sets in [10]), and selectivity to all products after 5 and 1440 min of T-O-S. It is clearly seen that with CoAlPO-11 the selectivity to isobutene was higher than 97% from the beginning of the reaction (conversion 46%) and conversion and yield slowly decreased with T-O-S (Fig. 1). Under the same conditions with H-ferrierite a substantially higher initial conversion of n-butenes was obtained (61%), the selectivity to isobutene was only 49% and a high concentration of propylene (24%) and C5±C6 ole®ns (15.5%) was formed. After a signi®cant decrease in conversion of n-butenes the selectivity to isobutene increased up to 92% (T-O-S of 1440 min). The increase in selectivity is connected with the decrease in conversion of n-butenes but also can re¯ect narrow-
J. CÏejka et al. / Applied Catalysis A: General 179 (1999) 217±222
219
Table 1 The conversion and selectivity in skeletal isomerization of n-butenes over CoAlPO-11 and ferrierite CoAlPO-11 T-O-S (min) Conversion (%) Selectivity (vol%) Ethane Propane Propylene Isobutane Butane Isobutene P C P 5 P C6 P C7 C8 Yield a
Ferrierite
5 46.1
1440 39.2
0 0 1.0 0 0 97.4 0.7 0 0.4 0.2 44.9
0 0 0.6 0 0 98.5 0.3 0 0.3 0 38.6
a
Pulse 54.0
5 61.1
1440 42.1
0 0 7.4 0 0 86.0 6.6 0 0 0 46.4
2.3 1.7 24.0 0.6 3.8 49.4 12.7 2.8 2.6 0.2 30.2
0 0 3.7 0 0.6 92.1 2.4 0.7 0.5 0 38.8
Pulsea 56.3 2.1 2.9 17.0 3.2 4.1 50.6 16.0 4.1 0 0 28.5
Conversion and selectivities were calculated from integral areas of total ion chromatogram, reaction arrangement given in Section 2.
Fig. 1. Time-on-stream dependence of conversion (&), isobutene selectivity (*) and isobutene yield (~) for CoAlPO-11.
ing of channels and their cross-sections by coke deposits (see Fig. 2). This might prevent formation of n-butene dimers (or from n-buteneisobutene) and particularly the isomerization of formed C8 ole®ns as a result of restricted transition state selectivity. The alternative explanation of the signi®cant increase in isobutene yield over ferrierite with coke deposits was proposed by Guisnet et al. [7], the so-called pseudomonomolecular mechanism including high molecular
weight deposits as active sites for isobutene formation. To exclude a potential role of these deposits in our experiments, the ``fresh'' molecular sieves without any carbon deposits were used not to in¯uence the product distribution and 13 C scrambling by the interaction of reactants and/or products with coke deposits. Pulse catalytic experiments with labeled 13 C 1-butene were used to estimate the extent of mono-
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Ï ejka et al. / Applied Catalysis A: General 179 (1999) 217±222 J. C
Fig. 2. Time-on-stream dependence of conversion (&), isobutene selectivity (*) and isobutene yield (~) for H-ferrierite.
and bimolecular reaction. Experiments were performed over ``fresh'' molecular sieves and at a similar conversion level (54% ± CoAlPO-11, 56% ± ferrierite) while the selectivities to isobutene differed substantially (86% ± CoAlPO-11, 50% ± ferrierite). Although comparable conversions in T-O-S and pulse experiments were achieved, the reaction conditions used were signi®cantly different. The obtained by-products in these experiments consisted mainly of propylene and pentenes; moreover, in the case of ferrierite as expected also ethane, n-butane, isobutane, and C6 ole®ns were found (Table 1). Formation of relatively high amount of propylene and pentenes took place probably mainly on the external surface of CoAlPO-11 and it can be supposed that these active sites were deactivated during the ®rst minutes of T-O-S. The distribution of isobutene molecules according to their molecular weight in the products was established from their mass spectra (after T-O-S of 45 s) and 13 C NMR data characterized the products condensed at liquid nitrogen temperature. Over both catalysts a mixture of isobutene molecules consisting of non-labeled isobutene and those labeled with one or two 13 C atoms (molecular weight 56, 57 and 58, respectively) was formed. The distribution of these three isobutene molecular weights was substantially different for CoAlPO-11 and ferrierite. On CoAlPO-11 the ratio of M58/M57 (isobutene with two 13 C atoms/one 13 C)
was 0.03, while for ferrierite was 0.15 and the same ratios were determined also for n-butene isomers. 13 C NMR data measured on CoAlPO-11 gave the same concentrations of n-butenes and isobutene with two 13 C atoms. From known concentrations of n-butenes and isobutene with two and one 13 C atoms and without 13 C it is possible to evaluate the extent of the bimolecular mechanism to the selectivity to isobutene. There exist three pathways how isobutene can be formed and probably neither of them can be fully omitted. At ®rst, the monomolecular mechanism operates which provides high selectivity to isobutene with only one 13 C atom in molecule. Secondly, dimerization (reaction of two n-butene molecules) or codimerization (n-buteneisobutene already formed) followed by isomerization and cracking can lead, except to various by-products which must be always present, to the formation of isobutene again with only one 13 C atom in the molecule. However, these isobutene molecules cannot be distinguished from those formed via monomolecular mechanism and one can only estimate the prevailing mechanism from the concentration of by-products formed in this reaction. Thirdly, the skeletal isomerization of dimers and codimers occurs in such a way that n-butenes or isobutene formed contain either two 13 C atoms or none labeled carbon.
J. CÏejka et al. / Applied Catalysis A: General 179 (1999) 217±222
On CoAlPO-11 we found 3% of n-butenes and isobutene with two 13 C atoms in molecules which means that also 3% of n-butenes and isobutene without 13 C should be formed. Thus, on this catalyst unambiguously at least 6% (from 86%) of isobutene produced was formed via a bimolecular reaction. The remaining part of isobutene molecules with only one 13 C atom in the molecule was formed either via monomolecular or bimolecular reaction pathway without mixing of the 13 C atoms between original reactants. With fresh ferrierite possessing substantially lower selectivity (Table 1) almost 30% of isobutene (from 50.6%) was unequivocally formed via bimolecular mechanism. It means that at least 65% of products was formed via cracking of C8 ole®ns assuming that all isobutene molecules possessing only one 13 C atom were formed via monomolecular mechanism. However, it cannot be excluded that a part of isobutene molecules with one 13 C was also formed via bimolecular mechanism. This would increase the extent of isobutene formed by the bimolecular reaction. These results are in contradiction to those published by HouzÏvicÏka and Ponec [11,12] who supposed that only by-products are formed via cracking of octene isomers. 13 C NMR measurements provided information that labeled 13 C atoms were located in all positions of isobutene molecules with the distribution CH3:CH2:C 1.5:1:1. This means that 13 C atoms are almost evenly distributed in all positions of isobutene molecules without any preference. This could result from the monomolecular mechanism of n-butene transformation taking place via methyl±cyclopropyl carbocation or just as the result of scrambling. In 1-butene the second labeled 13 C atom was also evenly distributed in all positions, but in 2-butene (both cis and trans) the 3C position was not occupied. This can be seen from the 13 C spectrum, where the carbon with double bond (125.8 ppm) has only one pair of satellites, corresponding to the CH3±CH= pair, and the satellites corresponding to the CH=CH pair are missing. Combining the results obtained in the reaction T-O-S and pulse experiments it is clear that for CoAlPO-11 a very selective monomolecular formation of isobutene takes place. With ``fresh'' ferrierite a substantial amount of isobutene is formed via non-selective bimolecular mechanism. From a comparison of these
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results it is evident that n-butene isomerization to isobutene over fresh CoAlPO-11 and ferrierite is controlled by the restricted transition state selectivity. In the case of monodimensional elliptic channels of Ê the CoAlPO-11 with channel dimension 3.96.3 A monomolecular mechanism operates while the threedimensional channel system of ferrierite (4.25.4 and Ê ), in addition to monomolecular mechan3.54.8 A ism, enables also formation of dimers and codimers, their isomerization and cracking leading to a spectrum of products including isobutene. It can be assumed that the bimolecular reaction followed by isomerization and cracking proceeds mainly in the channel intersections of ferrierite crystal structure which are able to accommodate these dimers or codimers. In addition, we assume that branched octenes are too bulky to ®t into monodimensional channels of CoAlPO-11, thus, they cannot be formed there and monomolecular mechanism strongly prevails. It is necessary to admit that although we have estimated the minimum extent of the bimolecular reaction connected with the formation of non-labeled isobutene and isobutene with two 13 C atoms we were not able to distinguish between isobutene molecules formed via monomolecular or bimolecular mechanism with one 13 C atom in the molecule. Acknowledgements Financial support from EC-Copernicus project (CIPA-CT94-0184) and Grant Agency of Academy of Sciences of the Czech Republic (No. A4040707) is highly acknowledged. PS also acknowledges support from the Estonian Science Foundation under Grant No. 2302.
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