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The tert-butyl cation on zeolite Y: A theoretical and experimental study
Chemical Physics Letters 485 (2010) 124–128
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
The tert-butyl cation on zeolite Y: A theoretical and experimental study Nilton Rosenbach Jr., Alex P.A. dos Santos, Marcelo Franco, Claudio J.A. Mota * Universidade Federal do Rio de Janeiro, Instituto de Química, Av. Athos da Silveira Ramos 149, CT Bloco A, Cidade Universitária, 21941-909 Rio de Janeiro, Brazil INCT de Energia e Ambiente, UFRJ, 21941-909 Rio de Janeiro, Brazil
a r t i c l e
i n f o
Article history: Received 23 October 2009 In final form 1 December 2009 Available online 3 December 2009
a b s t r a c t The structure and energy of the tert-butyl cation on zeolite Y were calculated at ONIOM(MP2(FULL)/631G(d,p):MNDO) level. The results indicated that the tert-butyl cation is a minimum and lies between 40 and 51 kJ mol1 above in energy to the tert-butoxide, depending on the level of calculation. Both species are stabilized through hydrogen bonding interactions with the framework oxygen atoms. Experimental data of nucleophilic substitution of tert-butylchloride and bromide over NaY impregnated with NaCl or NaBr give additional support for the formation of the tert-butyl cation as a discrete intermediate on zeolite Y, in agreement with the calculations. Ó 2009 Published by Elsevier B.V.
1. Introduction Acid zeolites are widely used in petrochemistry due to its activity and selectivity in several important processes such as cracking, isomerization and alkylation [1]. The common key step in these reactions is the formation of adsorbed carbocations [2]. Although numerous efforts have been made to elucidate the nature of these species on the zeolite surface, there is no definite experimental evidence that simple alkyl carbocations are discrete intermediate in zeolite-catalyzed reactions [3–5]. Instead, most of the studies indicated that covalent species, named alkoxides (Chart 1), are usually thermodynamically more stable than simple alkyl carbocations and observed as persistent intermediates on the zeolite surface. Consequently, many authors believe that alkoxides should be the real intermediates in zeolite-catalyzed hydrocarbon reactions, whereas carbocations should be just transition states [6–8]. Most of the results on the role of carbocations and alkoxides in zeolite-catalyzed reactions arise from studies involving olefins [9– 16] or alcohols [17] over protonic zeolites. These procedures may lead to oligomerization, impairing a precise characterization of the reaction intermediates. We have been studying the carbocation/alkoxide system employing metal-exchanged zeolite and alkylhalides [18]. The metal cation acts as a Lewis acid site, coordinating with the alkylhalide to form a metal-halide species and a carbocation/alkoxide bonded to the zeolite structure. This strategy has shown to be efficient to characterize the different butoxides over zeolite Y by infrared spectroscopy [18], under controlled conditions. We have also employed this procedure in Friedel–Crafts reactions [19,20] and isobutane/2-butene alkylation [21]. * Corresponding author. E-mail address: [email protected] (C.J.A. Mota). 0009-2614/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.cplett.2009.12.003
Recently, we were able to show [22] the rearrangement of cyclopropylcarbinyl chloride over NaY at room temperature. The results were interpreted in terms of formation of the bicyclobutonium cation ðC4 Hþ 7 Þ, which may be nucleophilic attacked in three different positions, giving rise to the cyclobutyl and allylcarbinyl chlorides, as well as the parent cyclopropylcarbinyl chloride. Impregnation of sodium halide (NaCl or NaBr) inside the zeolite cavity led to nucleophilic substitution, indicating that the rearrangement is not concerted. These results strongly support the formation of simple alkyl carbocations as discrete intermediates on the zeolite Y pore structure. Calculations also revealed that the bicyclobutonium cation is a minimum on the potential energy surface and is stabilized by hydrogen bonding with the framework oxygen atoms. The tert-butyl system on zeolites remains one of the most studied cases. Spectroscopic studies, using infrared [11,18] and MAS NMR [16], neither characterized the tert-butoxide nor the ionic tert-butyl cation as long-lived species over the zeolite surface. In many cases, dimerization and oligomerization is observed [11– 15,18], which points out for the high reactivity of the system. From a theoretical standpoint, the tert-butyl cation was characterized as a discrete intermediate on the potential energy surface of isobutene protonation over Mordenite, Chabazite and Ferrierite zeolites [23–26]. These studies showed that the energy difference between the tert-butyl cation and the tert-butoxide depends, mainly, on short-range effects and local steric constraints. The use of hybrid techniques has been successfully applied in studies with zeolites. In this approach, the atoms of the active site, normally a T3 cluster (T = Si or Al), can be described with a more sophisticated theoretical level and the rest of the zeolite structure is treated with a less computer demanding method. The ONIOM scheme [27] is one of the most popular hybrid methods and it
N. Rosenbach Jr. et al. / Chemical Physics Letters 485 (2010) 124–128
R O Si
O Al
Si
Chart 1. Pictorial representation of the alkoxide on the zeolite surface.
was used to correctly reproduce the properties of the Brønsted acid sites in ZSM-5 zeolite [28]. Other studies show that the ONIOM method can also discriminate different crystallographic acid sites, indicating that lattice effects have a significant role [29]. In this study we used the ONIOM method to study the protonation of isobutene over zeolite Y to characterize the minima on the potential energy surface and the energy profile. We have also performed experimental studies of tert-butyl halides over NaY impregnated with sodium halide, aiming to observe nucleophilic substitution reactions that would support the formation of the tert-butyl cation as a discrete intermediate, similarly to solution chemistry. 2. Computational details A cluster model (Fig. 1) of the zeolite Y comprising 288 atoms (Si84O132H72), corresponding to two coupled supercavities, was obtained from the crystallographic coordinates available in the literature [30]. In order to avoid dangling bonds, the free valences of the border silicon atoms were saturated with hydrogen atoms, located at 1.09 Å distance and in the same plane of the Si–O bond. The position of the hydrogen atoms was kept fixed during the optimization steps, to avoid topological distortion of the model compared to the original zeolite Y structure. The aluminium atom and the organic moiety were inserted afterwards. We choose the O1 position to link the alkoxide, because this position is one of the most preferred positions for the proton, according to neutron scattering studies [31] and theoretical calculations [32]. Thus, it is reasonable to assume that other covalent groups would be preferentially located at this position. All calculations were done using the ONIOM method available in GAUSSIAN 98 package [33]. In the optimization steps, the system was divided in two layers (high and low layers) and the atoms of the T3 cluster model and the organic moiety (high layer) were treated at the MP2(FULL)/6-31(d,p) level, whereas the rest of the zeolite cavity (low layer) was treated by the semiempirical MNDO method. We performed single point calculations at ONIOM(MP2(FULL)/6-31(d,p):PBE1PBE/6-31G(d,p)) and at PBE1PBE le-
Fig. 1. Structure of the cluster model of the zeolite Y (Si84O132H72) used in the calculations, computed at ONIOM(MP2(FULL)/6-31G(d,p):MNDO) level.
125
vel to evaluate contributions from medium/long range effects to the energy profile. In the later case, the single point calculations included the whole system (high and low layers). Vibrational analysis in the harmonic approximation (HO) was performed for all calculated structures at ONIOM(PBE1PBE/6-31(d,p):MNDO) level and frequencies scaled by 0.96, to correct for the zero-point energy (ZPE) and thermal effects (298.15 K). This scale factor is appropriate to reproduce fundamental frequencies computed from HO approximation at DFT level [34]. Small imaginary frequencies, related to the fixed hydrogen atoms, were not considered in these calculations. All energy data refer to the enthalpic term at 298.15 K and 1 atm. 3. Experimental section The adsorption and ionization of the tert-butylhalides were studied on a NaY zeolite (Si/Al = 2.6 and surface area of 704 m2 g1) impregnated with NaBr or NaCl. The impregnation procedure was reported elsewhere [22]. The reactions were carried out in a flow unit (fixed bed) at room temperature and atmospheric pressure. About 200 mg of the zeolite was initially pretreated at 300 °C (2.5 °C min1), under N2 atmosphere (40 mL min1), for 30 min. The reactor was cooled to room temperature and 0.5 mL of the tert-butylhalide (chloride or bromide) was injected in the N2 flow with the use of a syringe and passed over the zeolite bed. The products were collected at the reactor outlet using a trap immersed in ice bath. The product arisen from nucleophilic substitution was identified in a gas chromatograph coupled to a mass quadrupole spectrometer, using electron impact ionization (70 eV). 4. Results and discussion 4.1. Theoretical calculations Figs. 2–4 show the optimized structures of the p-complex, the tert-butoxide and the tert-butyl cation on the zeolite Y cluster at ONIOM(MP2(FULL)/6-31G(d,p):MNDO) level. The adsorption of isobutene results in a small stretching (0.01 Å) of the O–H bond and the proton is located nearer to the primary carbon atom of
Fig. 2. Structure of the isobutene p-complex adsorbed on zeolite Y cluster, computed at ONIOM(MP2(FULL)/6-31G(d,p):MNDO) level.
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N. Rosenbach Jr. et al. / Chemical Physics Letters 485 (2010) 124–128 Table 1 Enthalpy difference of the p-complex, alkoxide and carbocation adsorbed on zeolite Y, at different level of theory (values in kJ mol1). Species
ONIOM(MP2:MNDO)
ONIOM(MP2:PBE1PBE)
PBE1PBE
p-Complex
0 38 78
0 10 62
0 18 62
tert-Butoxide tert-Butyl cation
Fig. 3. Structure of the tert-butoxide adsorbed on zeolite Y cluster, computed at ONIOM(MP2(FULL)/6-31G(d,p):MNDO) level.
angles and bond distances of the zeolite active site are more symmetric with adsorption of the tert-butyl cation. We have already reported similar geometric modifications in studies at other level of calculation and cluster size [35,36]. Table 1 shows the energy difference among the calculated structures. The isobutene p-complex was the most stable species at all level of calculation. This might explain the spectroscopic results [11,16,18], which failed to characterize the tert-butoxide as a long-living species at the zeolite Y pore network. Nevertheless, the tert-butoxide is 40–51 kJ mol1 more stable than the tert-butyl cation on zeolite Y at all levels of theory used in this study. The stabilization of the tert-butyl cation is mostly due to short-range interactions, such as hydrogen bonding and electrostatic effects, because the energy difference relative to the respective alkoxide did not change much when performing single point calculation at PBE1PBE level. On the other hand, the alkoxide is more sensitive to dispersion interactions, since the energy gap increases at ONIOM(MP2:PBE1PBE), which better describes this type of interaction among the theoretical levels used in this work. In other studies, using small pore zeolites such as Chabazite, the relative stability between the tert-butoxide and the tert-butyl cation is mainly governed by the steric repulsions with the framework [26]. Thus, the covalent alkoxide is more affected than the ionic carbocation. The zeolite Y cavity is wide enough to minimize the steric repulsions between the methyl groups of the alkoxide and the framework. Thus, electrostatic interactions and hydrogen bonding are mainly responsible for the stabilization of the carbocation within the zeolite cavity. 4.2. Experimental studies
Fig. 4. Structure of the tert-butyl cation adsorbed on zeolite Y cluster, computed at ONIOM(MP2(FULL)/6-31G(d,p):MNDO) level.
the double bond. The attachment of the tert-butyl group to the T3 cluster, to yield the alkoxide, significantly modifies the geometry to minimize steric constraints between the methyl groups and the framework. This effect is more pronounced in the Al–O(C)–Si bond angle, which decreases about 13° when the tert-butoxy is attached instead of the proton. The tert-butyl cation structure is planar and both methyl groups interact with the zeolite framework through hydrogen bonds (1.88 and 1.97 Å). This interaction stretches both C–H bonds (1.12 Å) of the methyl groups compared with the distance in the tert-butoxide (about 1.08 Å). The alkoxide induces a local distortion of the zeolite framework, with a significant reduction of the SiO(C)Al bond angle and stretching of the Al–O(C) bond to accommodate the bulk tert-butyl group. By contrast, the bond
The theoretical calculations indicated that the tert-butyl cation can be an intermediate within the pores of zeolite Y. To support these results, we carried out an experimental study on adsorption and ionization of tert-butyl halides (bromide and chloride) over NaY impregnated with sodium halide (NaCl or NaBr) at 25 °C. Upon passing a flow of nitrogen enriched with tert-butylchloride over NaY impregnated with sodium bromide we were able to observe tert-butyl bromide in the reactor effluent. The same procedure with tert-butylbromide over NaY impregnated with NaCl yields tert-butylchloride in the reactor effluent. These are typical products of nucleophilic substitution of the tert-butylhalides over NaY impregnated with sodium halide. It is well known in physical organic chemistry that nucleophilic substitution in tertiary systems, like tert-butylchloride and bromide, can only occur through a SN1 type mechanism, involving ionization of the substrate to ion pairs and carbocations [37]. Thus, the nucleophilic substitution with the impregnated NaY must involve the ionization of the tert-butylchloride or bromide to an ion pair of the tert-butyl cation and the zeolite structure. Then, the carbocation can be attacked by the halide (chloride or bromide) impregnated inside the pores, yielding the nucleophilic substitution product. Characterization of the impregnated zeolite revealed [22] that most of the halide is dispersed inside the pores, probably as nanostructured sodium halide species. It should be stressed however, that besides nucleophilic attack of the impregnated halide to the tert-butyl cation, we were able to
N. Rosenbach Jr. et al. / Chemical Physics Letters 485 (2010) 124–128
Si
O
Al
We suggest that zeolites act as solid solvents, capable of ionizing substracts and stabilizing the formation of ionic species. The study with alkyl halides shows that many aspects of zeolite chemistry has resemblance with solution, suggesting that the key to improve zeolite activity and selectivity is the understanding of the carbocation formation and stabilization. The comparison of zeolites as solid solvents has already been proposed in the literature [41,42], but in terms of partition coefficients. We now suggest that besides this property, zeolites may be called solid solvents in terms of ionizing power and stabilization of ionic species.
Br
Cl O
O
Si
Si
Cl
Al
O
Si
Br
H Si
O
Al
O
π-complex
Si
Si
O
Al
O
Si
127
Si
O
Al
O
5. Conclusions Si
alkoxide
Scheme 1. Possible pathways for the ionization of tert-butylchloride over zeolite NaY impregnated with NaBr.
observe olefins, mainly isobutene coming from elimination, as well as C8 products, arisen from alkylation of the formed olefins. These results support the formation of the tert-butyl cation as a discrete intermediate inside the pores of the zeolite Y, in agreement with the calculations. From the theoretical and experimental results we may conclude that the tert-butyl cation is a discrete intermediate on the zeolite Y surface. Nevertheless, calculations indicated that the tert-butoxide and the p-complex are lower in energy than the tert-butyl cation. Thus, the key to increase zeolite activity and selectivity in hydrocarbon reactions is the stabilization of the carbocation relative to the alkoxide. We believe that there is a parallel between zeolites and solution chemistry. In many solvents, like water and alcohols, carbocations are surrounded by the solvent molecules or dissolved nucleophiles. As soon as they are formed, they are rapidly transformed into stable compounds (ethers, alcohols). Stable, long-lived, carbocations can be spectroscopically observed only in very poor nucleophilic media, such as superacid solutions [38]. Then, it is not surprising that on the highly nucleophilic zeolite environment, simple alkyl carbocations cannot be spectroscopically observed. On zeolites, the oxygen atoms of the structure act as nucleophilic centers. Thus, they may attack a formed carbocation to yield an alkoxide, in a typical SN1 type reaction. The nucleophilic substitution of tert-butyl halides on NaY impregnated with sodium halides cannot directly occur through the tert-butoxide, because in tertiary systems the SN1 type reaction always prevails [37]. Thus, a discrete ion pair between the tert-butyl cation and the zeolite structure must be formed and might undergo, at least, three different pathways (Scheme 1): elimination to afford the p-complex, nucleophilic substitution with the impregnated halide, or nucleophilic attack by the framework oxygen atoms to afford the alkoxide. The last pathway does not lead to a stable isolated product and subsequent ionizations occur, which may shift the equilibrium to olefin formation and dimerization, as observed in other studies [11–16,18]. In fact, this behaviour is also observed in solution [39,40], when the nucleophilic attack by the solvent does not lead to a stable isolated product. This is the case of 1,4-dioxane–water mixtures in nucleophilic substitution of tertiary halides. There may occur nucleophilic attack of the dioxane in the ion pair to form an oxonium ion. This species is highly reactive and cannot be, normally, isolated from the medium. Thus, consecutive ionization of the formed oxonium ion yields the carbocation, which now can be nucleophilic attacked by water to form an alcohol. This chemistry has been widely studied and characterized in solution and is similar to what may be happening on the zeolite surface. The alkoxide is electronically similar to the oxonium ion and, once formed, might undergo consecutive ionization to the carbocation, which subsequently leads to other products.
Calculations at ONIOM(MP2(FULL)/6-31G(d,p):MNDO) level of theory showed that the tert-butyl cation is a minimum on the zeolite Y surface and it is stabilized by hydrogen bonding with the framework oxygen atoms. At all levels of calculations the p-complex is the most stable species. The energy difference between the tert-butyl cation and the tert-butoxide is in the range of 40–51 kJ mol1, depending on the level of calculation. Experimental data of nucleophilic substitution of tert-butylchloride and bromide over NaY impregnated with sodium halide gives additional support for the formation of the tert-butyl cation as a discrete intermediate on zeolite Y, in agreement with the calculations. Acknowledgements Authors thank financial support from CNPq, CAPES, FAPERJ and PRH-ANP. References [1] A. Corma, Chem. Rev. 95 (1995) 559. [2] H.V. Bekkum, J.C. Jansen, E.M. Flanigen (Eds.), Introduction to Zeolite Science and Practice, Elsevier, Amsterdam, 1991. [3] J.F. Haw, J.B. Nicholas, T. Xu, L.W. Beck, D.B. Ferguson, Acc. Chem. Res. 29 (1996) 259. [4] J.B. Nicholas, T. Xu, J.F. Haw, Topics Catal. 6 (1998) 141. [5] J.F. Haw, Phys. Chem. Chem. Phys. 4 (2002) 5431. [6] V.B. Kazansky, Acc. Chem. Res. 24 (1991) 379. [7] M.V. Frash, V.B. Kazansky, A.M. Rigby, R.A. van Santen, J. Phys. Chem. B 101 (1997) 5346. [8] M. Boronat, P. Viruela, A. Corma, J. Phys. Chem. A 102 (1998) 982. [9] K. Domen, J.N. Kondo, F. Wakabayashi, Res. Chem. Intermed. 24 (1998) 411. [10] J.F. Haw, B.R. Richardson, I.S. Oshiro, N.D. Lazo, J.A. Speed, J. Am. Chem. Soc. 111 (1989) 2052. [11] H. Ishikawa, E. Yoda, J.N. Kondo, F. Wakabayashi, K. Domen, J. Phys. Chem. B 103 (1999) 5681. [12] J.N. Kondo, H. Ishikawa, E. Yoda, F. Wakabayashi, K. Domen, J. Phys. Chem. B 103 (1999) 8538. [13] J.N. Kondo, S. Liqun, F. Wakabayashi, K. Domen, Catal. Lett. 47 (1997) 129. [14] J.N. Kondo, F. Wakabayashi, K. Domen, J. Phys. Chem. B 102 (1998) 2259. [15] J.N. Kondo, F. Wakabayashi, K. Domen, Catal. Lett. 53 (1998) 215. [16] N.D. Lazo, B.R. Richardson, P.D. Schettler, J.L. White, E.J. Munson, J.F. Haw, J. Phys. Chem. 95 (1991) 9420. [17] M.T. Aronson, R.J. Gorte, W.E. Farneth, D. White, J. Am. Chem. Soc. 111 (1989) 840. [18] R.J. Correa, C.J.A. Mota, Phys. Chem. Chem. Phys. 4 (2002) 4268. [19] A.M.F. Bidart, A.P.S. Borges, H.C. Chagas, L. Nogueira, E.R. Lachter, C.J.A. Mota, J. Braz. Chem. Soc. 17 (2006) 758. [20] A.M.F. Bidart, A.P.S. Borges, L. Nogueira, E.R. Lachter, C.J.A. Mota, Catal. Lett. 75 (2001) 155. [21] N. Rosenbach, C.J.A. Mota, J. Braz. Chem. Soc. 16 (2005) 691. [22] M. Franco, N. Rosenbach, G.B. Ferreira, A.C.O. Guerra, W.B. Kover, C.C. Turci, C.J.A. Mota, J. Am. Chem. Soc. 130 (2008) 1592. [23] M. Boronat, P.M. Viruela, A. Corma, J. Am. Chem. Soc. 126 (2004) 3300. [24] X. Rozanska, R.A. van Santen, T. Demuth, F. Hutschka, J. Hafner, J. Phys. Chem. B 107 (2003) 1309. [25] C. Tuma, J. Sauer, Angew. Chem., Int. Ed. 44 (2005) 4769. [26] M. Boronat, A. Corma, Appl. Catal. A 336 (2008) 2. [27] S. Dapprich, I. Komaromi, K.S. Byun, K. Morokuma, M.J. Frisch, J. Mol. Struct. (Theochem) 462 (1999) 1. [28] K. Sillar, P. Burk, J. Phys. Chem. B 108 (2004) 9893. [29] K. Sillar, P. Burk, J. Mol. Struct. (Theochem) 589 (2002) 281.
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[30] C.H.M. Baerlocher, W.M. Meier, D.H. Olson, Atlas of Zeolite Framework Types, Elsevier, Amsterdam, 2001. [31] Z. Jirak, S. Vratislav, V. Bosacek, J. Phys. Chem. Solids 41 (1980) 1089. [32] J.R. Hill, C.R. Freeman, B. Delley, J. Phys. Chem. A 103 (1999) 3772. [33] M.J. Frisch et al., GAUSSIAN 98, Revision A.11, Gaussian, Inc., Pittsburgh, PA, 1998. [34] L.A. Curtiss, K. Raghavachari, P.C. Redfern, J.A. Pople, Chem. Phys. Lett. 270 (1997) 419. [35] R.J. Correa, C.J.A. Mota, Phys. Chem. Chem. Phys. 4 (2002) 375. [36] C.J.A. Mota, P.M. Esteves, Mauro B. De Amorim, J. Phys. Chem. 100 (1996) 12418.
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
The tert-butyl cation on zeolite Y: A theoretical and experimental study Nilton Rosenbach Jr., Alex P.A. dos Santos, Marcelo Franco, Claudio J.A. Mota * Universidade Federal do Rio de Janeiro, Instituto de Química, Av. Athos da Silveira Ramos 149, CT Bloco A, Cidade Universitária, 21941-909 Rio de Janeiro, Brazil INCT de Energia e Ambiente, UFRJ, 21941-909 Rio de Janeiro, Brazil
a r t i c l e
i n f o
Article history: Received 23 October 2009 In final form 1 December 2009 Available online 3 December 2009
a b s t r a c t The structure and energy of the tert-butyl cation on zeolite Y were calculated at ONIOM(MP2(FULL)/631G(d,p):MNDO) level. The results indicated that the tert-butyl cation is a minimum and lies between 40 and 51 kJ mol1 above in energy to the tert-butoxide, depending on the level of calculation. Both species are stabilized through hydrogen bonding interactions with the framework oxygen atoms. Experimental data of nucleophilic substitution of tert-butylchloride and bromide over NaY impregnated with NaCl or NaBr give additional support for the formation of the tert-butyl cation as a discrete intermediate on zeolite Y, in agreement with the calculations. Ó 2009 Published by Elsevier B.V.
1. Introduction Acid zeolites are widely used in petrochemistry due to its activity and selectivity in several important processes such as cracking, isomerization and alkylation [1]. The common key step in these reactions is the formation of adsorbed carbocations [2]. Although numerous efforts have been made to elucidate the nature of these species on the zeolite surface, there is no definite experimental evidence that simple alkyl carbocations are discrete intermediate in zeolite-catalyzed reactions [3–5]. Instead, most of the studies indicated that covalent species, named alkoxides (Chart 1), are usually thermodynamically more stable than simple alkyl carbocations and observed as persistent intermediates on the zeolite surface. Consequently, many authors believe that alkoxides should be the real intermediates in zeolite-catalyzed hydrocarbon reactions, whereas carbocations should be just transition states [6–8]. Most of the results on the role of carbocations and alkoxides in zeolite-catalyzed reactions arise from studies involving olefins [9– 16] or alcohols [17] over protonic zeolites. These procedures may lead to oligomerization, impairing a precise characterization of the reaction intermediates. We have been studying the carbocation/alkoxide system employing metal-exchanged zeolite and alkylhalides [18]. The metal cation acts as a Lewis acid site, coordinating with the alkylhalide to form a metal-halide species and a carbocation/alkoxide bonded to the zeolite structure. This strategy has shown to be efficient to characterize the different butoxides over zeolite Y by infrared spectroscopy [18], under controlled conditions. We have also employed this procedure in Friedel–Crafts reactions [19,20] and isobutane/2-butene alkylation [21]. * Corresponding author. E-mail address: [email protected] (C.J.A. Mota). 0009-2614/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.cplett.2009.12.003
Recently, we were able to show [22] the rearrangement of cyclopropylcarbinyl chloride over NaY at room temperature. The results were interpreted in terms of formation of the bicyclobutonium cation ðC4 Hþ 7 Þ, which may be nucleophilic attacked in three different positions, giving rise to the cyclobutyl and allylcarbinyl chlorides, as well as the parent cyclopropylcarbinyl chloride. Impregnation of sodium halide (NaCl or NaBr) inside the zeolite cavity led to nucleophilic substitution, indicating that the rearrangement is not concerted. These results strongly support the formation of simple alkyl carbocations as discrete intermediates on the zeolite Y pore structure. Calculations also revealed that the bicyclobutonium cation is a minimum on the potential energy surface and is stabilized by hydrogen bonding with the framework oxygen atoms. The tert-butyl system on zeolites remains one of the most studied cases. Spectroscopic studies, using infrared [11,18] and MAS NMR [16], neither characterized the tert-butoxide nor the ionic tert-butyl cation as long-lived species over the zeolite surface. In many cases, dimerization and oligomerization is observed [11– 15,18], which points out for the high reactivity of the system. From a theoretical standpoint, the tert-butyl cation was characterized as a discrete intermediate on the potential energy surface of isobutene protonation over Mordenite, Chabazite and Ferrierite zeolites [23–26]. These studies showed that the energy difference between the tert-butyl cation and the tert-butoxide depends, mainly, on short-range effects and local steric constraints. The use of hybrid techniques has been successfully applied in studies with zeolites. In this approach, the atoms of the active site, normally a T3 cluster (T = Si or Al), can be described with a more sophisticated theoretical level and the rest of the zeolite structure is treated with a less computer demanding method. The ONIOM scheme [27] is one of the most popular hybrid methods and it
N. Rosenbach Jr. et al. / Chemical Physics Letters 485 (2010) 124–128
R O Si
O Al
Si
Chart 1. Pictorial representation of the alkoxide on the zeolite surface.
was used to correctly reproduce the properties of the Brønsted acid sites in ZSM-5 zeolite [28]. Other studies show that the ONIOM method can also discriminate different crystallographic acid sites, indicating that lattice effects have a significant role [29]. In this study we used the ONIOM method to study the protonation of isobutene over zeolite Y to characterize the minima on the potential energy surface and the energy profile. We have also performed experimental studies of tert-butyl halides over NaY impregnated with sodium halide, aiming to observe nucleophilic substitution reactions that would support the formation of the tert-butyl cation as a discrete intermediate, similarly to solution chemistry. 2. Computational details A cluster model (Fig. 1) of the zeolite Y comprising 288 atoms (Si84O132H72), corresponding to two coupled supercavities, was obtained from the crystallographic coordinates available in the literature [30]. In order to avoid dangling bonds, the free valences of the border silicon atoms were saturated with hydrogen atoms, located at 1.09 Å distance and in the same plane of the Si–O bond. The position of the hydrogen atoms was kept fixed during the optimization steps, to avoid topological distortion of the model compared to the original zeolite Y structure. The aluminium atom and the organic moiety were inserted afterwards. We choose the O1 position to link the alkoxide, because this position is one of the most preferred positions for the proton, according to neutron scattering studies [31] and theoretical calculations [32]. Thus, it is reasonable to assume that other covalent groups would be preferentially located at this position. All calculations were done using the ONIOM method available in GAUSSIAN 98 package [33]. In the optimization steps, the system was divided in two layers (high and low layers) and the atoms of the T3 cluster model and the organic moiety (high layer) were treated at the MP2(FULL)/6-31(d,p) level, whereas the rest of the zeolite cavity (low layer) was treated by the semiempirical MNDO method. We performed single point calculations at ONIOM(MP2(FULL)/6-31(d,p):PBE1PBE/6-31G(d,p)) and at PBE1PBE le-
Fig. 1. Structure of the cluster model of the zeolite Y (Si84O132H72) used in the calculations, computed at ONIOM(MP2(FULL)/6-31G(d,p):MNDO) level.
125
vel to evaluate contributions from medium/long range effects to the energy profile. In the later case, the single point calculations included the whole system (high and low layers). Vibrational analysis in the harmonic approximation (HO) was performed for all calculated structures at ONIOM(PBE1PBE/6-31(d,p):MNDO) level and frequencies scaled by 0.96, to correct for the zero-point energy (ZPE) and thermal effects (298.15 K). This scale factor is appropriate to reproduce fundamental frequencies computed from HO approximation at DFT level [34]. Small imaginary frequencies, related to the fixed hydrogen atoms, were not considered in these calculations. All energy data refer to the enthalpic term at 298.15 K and 1 atm. 3. Experimental section The adsorption and ionization of the tert-butylhalides were studied on a NaY zeolite (Si/Al = 2.6 and surface area of 704 m2 g1) impregnated with NaBr or NaCl. The impregnation procedure was reported elsewhere [22]. The reactions were carried out in a flow unit (fixed bed) at room temperature and atmospheric pressure. About 200 mg of the zeolite was initially pretreated at 300 °C (2.5 °C min1), under N2 atmosphere (40 mL min1), for 30 min. The reactor was cooled to room temperature and 0.5 mL of the tert-butylhalide (chloride or bromide) was injected in the N2 flow with the use of a syringe and passed over the zeolite bed. The products were collected at the reactor outlet using a trap immersed in ice bath. The product arisen from nucleophilic substitution was identified in a gas chromatograph coupled to a mass quadrupole spectrometer, using electron impact ionization (70 eV). 4. Results and discussion 4.1. Theoretical calculations Figs. 2–4 show the optimized structures of the p-complex, the tert-butoxide and the tert-butyl cation on the zeolite Y cluster at ONIOM(MP2(FULL)/6-31G(d,p):MNDO) level. The adsorption of isobutene results in a small stretching (0.01 Å) of the O–H bond and the proton is located nearer to the primary carbon atom of
Fig. 2. Structure of the isobutene p-complex adsorbed on zeolite Y cluster, computed at ONIOM(MP2(FULL)/6-31G(d,p):MNDO) level.
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N. Rosenbach Jr. et al. / Chemical Physics Letters 485 (2010) 124–128 Table 1 Enthalpy difference of the p-complex, alkoxide and carbocation adsorbed on zeolite Y, at different level of theory (values in kJ mol1). Species
ONIOM(MP2:MNDO)
ONIOM(MP2:PBE1PBE)
PBE1PBE
p-Complex
0 38 78
0 10 62
0 18 62
tert-Butoxide tert-Butyl cation
Fig. 3. Structure of the tert-butoxide adsorbed on zeolite Y cluster, computed at ONIOM(MP2(FULL)/6-31G(d,p):MNDO) level.
angles and bond distances of the zeolite active site are more symmetric with adsorption of the tert-butyl cation. We have already reported similar geometric modifications in studies at other level of calculation and cluster size [35,36]. Table 1 shows the energy difference among the calculated structures. The isobutene p-complex was the most stable species at all level of calculation. This might explain the spectroscopic results [11,16,18], which failed to characterize the tert-butoxide as a long-living species at the zeolite Y pore network. Nevertheless, the tert-butoxide is 40–51 kJ mol1 more stable than the tert-butyl cation on zeolite Y at all levels of theory used in this study. The stabilization of the tert-butyl cation is mostly due to short-range interactions, such as hydrogen bonding and electrostatic effects, because the energy difference relative to the respective alkoxide did not change much when performing single point calculation at PBE1PBE level. On the other hand, the alkoxide is more sensitive to dispersion interactions, since the energy gap increases at ONIOM(MP2:PBE1PBE), which better describes this type of interaction among the theoretical levels used in this work. In other studies, using small pore zeolites such as Chabazite, the relative stability between the tert-butoxide and the tert-butyl cation is mainly governed by the steric repulsions with the framework [26]. Thus, the covalent alkoxide is more affected than the ionic carbocation. The zeolite Y cavity is wide enough to minimize the steric repulsions between the methyl groups of the alkoxide and the framework. Thus, electrostatic interactions and hydrogen bonding are mainly responsible for the stabilization of the carbocation within the zeolite cavity. 4.2. Experimental studies
Fig. 4. Structure of the tert-butyl cation adsorbed on zeolite Y cluster, computed at ONIOM(MP2(FULL)/6-31G(d,p):MNDO) level.
the double bond. The attachment of the tert-butyl group to the T3 cluster, to yield the alkoxide, significantly modifies the geometry to minimize steric constraints between the methyl groups and the framework. This effect is more pronounced in the Al–O(C)–Si bond angle, which decreases about 13° when the tert-butoxy is attached instead of the proton. The tert-butyl cation structure is planar and both methyl groups interact with the zeolite framework through hydrogen bonds (1.88 and 1.97 Å). This interaction stretches both C–H bonds (1.12 Å) of the methyl groups compared with the distance in the tert-butoxide (about 1.08 Å). The alkoxide induces a local distortion of the zeolite framework, with a significant reduction of the SiO(C)Al bond angle and stretching of the Al–O(C) bond to accommodate the bulk tert-butyl group. By contrast, the bond
The theoretical calculations indicated that the tert-butyl cation can be an intermediate within the pores of zeolite Y. To support these results, we carried out an experimental study on adsorption and ionization of tert-butyl halides (bromide and chloride) over NaY impregnated with sodium halide (NaCl or NaBr) at 25 °C. Upon passing a flow of nitrogen enriched with tert-butylchloride over NaY impregnated with sodium bromide we were able to observe tert-butyl bromide in the reactor effluent. The same procedure with tert-butylbromide over NaY impregnated with NaCl yields tert-butylchloride in the reactor effluent. These are typical products of nucleophilic substitution of the tert-butylhalides over NaY impregnated with sodium halide. It is well known in physical organic chemistry that nucleophilic substitution in tertiary systems, like tert-butylchloride and bromide, can only occur through a SN1 type mechanism, involving ionization of the substrate to ion pairs and carbocations [37]. Thus, the nucleophilic substitution with the impregnated NaY must involve the ionization of the tert-butylchloride or bromide to an ion pair of the tert-butyl cation and the zeolite structure. Then, the carbocation can be attacked by the halide (chloride or bromide) impregnated inside the pores, yielding the nucleophilic substitution product. Characterization of the impregnated zeolite revealed [22] that most of the halide is dispersed inside the pores, probably as nanostructured sodium halide species. It should be stressed however, that besides nucleophilic attack of the impregnated halide to the tert-butyl cation, we were able to
N. Rosenbach Jr. et al. / Chemical Physics Letters 485 (2010) 124–128
Si
O
Al
We suggest that zeolites act as solid solvents, capable of ionizing substracts and stabilizing the formation of ionic species. The study with alkyl halides shows that many aspects of zeolite chemistry has resemblance with solution, suggesting that the key to improve zeolite activity and selectivity is the understanding of the carbocation formation and stabilization. The comparison of zeolites as solid solvents has already been proposed in the literature [41,42], but in terms of partition coefficients. We now suggest that besides this property, zeolites may be called solid solvents in terms of ionizing power and stabilization of ionic species.
Br
Cl O
O
Si
Si
Cl
Al
O
Si
Br
H Si
O
Al
O
π-complex
Si
Si
O
Al
O
Si
127
Si
O
Al
O
5. Conclusions Si
alkoxide
Scheme 1. Possible pathways for the ionization of tert-butylchloride over zeolite NaY impregnated with NaBr.
observe olefins, mainly isobutene coming from elimination, as well as C8 products, arisen from alkylation of the formed olefins. These results support the formation of the tert-butyl cation as a discrete intermediate inside the pores of the zeolite Y, in agreement with the calculations. From the theoretical and experimental results we may conclude that the tert-butyl cation is a discrete intermediate on the zeolite Y surface. Nevertheless, calculations indicated that the tert-butoxide and the p-complex are lower in energy than the tert-butyl cation. Thus, the key to increase zeolite activity and selectivity in hydrocarbon reactions is the stabilization of the carbocation relative to the alkoxide. We believe that there is a parallel between zeolites and solution chemistry. In many solvents, like water and alcohols, carbocations are surrounded by the solvent molecules or dissolved nucleophiles. As soon as they are formed, they are rapidly transformed into stable compounds (ethers, alcohols). Stable, long-lived, carbocations can be spectroscopically observed only in very poor nucleophilic media, such as superacid solutions [38]. Then, it is not surprising that on the highly nucleophilic zeolite environment, simple alkyl carbocations cannot be spectroscopically observed. On zeolites, the oxygen atoms of the structure act as nucleophilic centers. Thus, they may attack a formed carbocation to yield an alkoxide, in a typical SN1 type reaction. The nucleophilic substitution of tert-butyl halides on NaY impregnated with sodium halides cannot directly occur through the tert-butoxide, because in tertiary systems the SN1 type reaction always prevails [37]. Thus, a discrete ion pair between the tert-butyl cation and the zeolite structure must be formed and might undergo, at least, three different pathways (Scheme 1): elimination to afford the p-complex, nucleophilic substitution with the impregnated halide, or nucleophilic attack by the framework oxygen atoms to afford the alkoxide. The last pathway does not lead to a stable isolated product and subsequent ionizations occur, which may shift the equilibrium to olefin formation and dimerization, as observed in other studies [11–16,18]. In fact, this behaviour is also observed in solution [39,40], when the nucleophilic attack by the solvent does not lead to a stable isolated product. This is the case of 1,4-dioxane–water mixtures in nucleophilic substitution of tertiary halides. There may occur nucleophilic attack of the dioxane in the ion pair to form an oxonium ion. This species is highly reactive and cannot be, normally, isolated from the medium. Thus, consecutive ionization of the formed oxonium ion yields the carbocation, which now can be nucleophilic attacked by water to form an alcohol. This chemistry has been widely studied and characterized in solution and is similar to what may be happening on the zeolite surface. The alkoxide is electronically similar to the oxonium ion and, once formed, might undergo consecutive ionization to the carbocation, which subsequently leads to other products.
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