Density functional theory studies on the skeletal isomerization of 1-butene catalyzed by HZSM-23 and HZSM-48 zeolites †

The reaction mechanism of the skeletal isomerization of 1-butene to isobutene on 10-membered ring zeolites HZSM-23 and HZSM-48 was investigated using the ONIOM(B3LYP/6-31G(d,p):UFF) method. It is demonstrated that the skeletal isomerization follows a monomolecular process, which involves the formation of two important intermediates: 2-butoxide and butoxide. The active centers on both zeolites are identi ﬁ ed to involve two Brønsted acid sites and three exposed vertex O atoms of the aluminum – oxygen tetrahedron on the pore surface. We further ﬁ nd that the pore size exhibits a signi ﬁ cant con ﬁ nement e ﬀ ect that a ﬀ ects the energetics of each intermediate's formation on both zeolites. Considering the free energy pathways at 700 K, the rate-determining steps are found to be the transformation of 2-butoxide to butoxide on HZSM-23 and the formation of 2-butoxide on HZSM-48, respectively. Our work provides mechanistic insights on the elementary processes of skeletal isomerization on zeolites.


Introduction
Isobutene, an isomer of butene, is a hydrocarbon of industrial signicance.It has been used as an important intermediate to produce a variety of chemical products such as methyl tert-butyl ether (MTBE), 1 ethyl tert-butyl ether (ETBE), 2 neohexene, 3 methacrolein 4,5 and pivalic acid. 6Therefore, the skeletal isomerization of 1-butene, a catalytic process to produce isobutene, has attracted extensive industrial and scientic attention.Many experimental studies [7][8][9] have reported that ferrierite (FER) zeolite is the most efficient catalyst for this isomerization and its high selectivity is attributed to the connement effect of the unique pore structure, which contains intersecting 10membered ring (4.2 Â 5.4 Å) and 8-membered ring (3.5 Â 4.8 Å) channels.Three possible mechanisms have been proposed for the skeletal isomerization of 1-butene: monomolecular, 10-12 bimolecular [13][14][15] and pseudo-monomolecular mechanisms. 16,17n monomolecular mechanism, a single butene molecule is transformed to isobutene via the transition of carbenium ions and butoxide intermediates. 18,19In bimolecular mechanism, isobutene and byproducts were produced by the processes of butene dimerization, isomerization and cracking. 20,21In pseudo-monomolecular mechanism, an active carbonaceous species in coke deposit were considered to be the catalyst for the n-butene isomerization to isobutene. 17,22Among these three mechanisms, the monomolecular mechanism was generally considered to the predominantly mechanism for the isomerization, whereas the bimolecular route was mainly responsible for the formation of byproducts. 23,246][27][28] Though HZSM-23, HZSM-48 and FER have a similar 10-membered rings, the differences in channel systems such as pore size and shape is believed to have an inuence on the reactivity of isomerization. 27,29,30Nevertheless, the mechanism of 1-butene isomerization on HZSM-23 and HZSM-48 and the effect of the channel systems on the stability of reactive intermediates were rarely mentioned in the literature.For instance, the skeleton isomerization of 1-butene over HZSM-23 has also been believed to follow the monomolecular mechanism by experiment study. 25It was reported that isobutene was formed from n-butene via a methyl cyclopropane carbenium intermediate, but whether the tertiary butyl cation is an intermediate or not is still unclear.Moreover, some experimental studies have indicated that the steric interaction between intermediate and the pore wall can strongly affect the types of products. 25However, the origin of this steric interaction is not clear to explain the inuence of channel systems of zeolites on reactions.
In the present study, the reaction mechanism of the skeletal isomerization of 1-butene to isobutene on HZSM-23 and HZSM-48 zeolites were investigated using a quantum chemical ONIOM (B3LYP/6-31G(d,p):UFF) method.Considering the unique shape selectivity of zeolites in the overall reactivity, a complete 10membered ring channel was contained in all calculation models.In order to investigate the monomolecular mechanism of skeletal isomerization of 1-butene in the presence of environment effects of the zeolite framework, 80T and 96T cluster models for HZSM-23 and HZSM-48 were applied, respectively.The structures of active centre for skeletal isomerization of 1-butene on HZSM-23 and HZSM-48 were proposed and the effects of different channel systems (pore size and shape) on the activation energies required in the intermediates and the reaction elementary steps were discussed.

Models and methods
The one-dimensional 10-membered ring channel topologies of the zeolites and the basic structural properties are shown in Fig. 1.The structures of the HZSM-23 and HZSM-48 zeolites were obtained from X-ray diffraction data in IZA structural database. 31Based on previous studies on the stability of Brønsted acid sites, 32,33 the most stable sites Al4-O4-Si5 on HZSM-23, and Al2-O4-Si1 and Al2-O7-Si3 on HZSM-48 were chosen as the active centre for isobutene formation (see Fig. 1).According to the previous study by Boronat et al., 34 the long range effect of the zeolite lattice is necessary to accurately describe the local isomerization of 1-butene in the channel of zeolite.To ensure the reliability of our model, specic channels consisting of the complete rings were cut from the crystalline structures of the HZSM-23 and HZSM-48 zeolites as 80T and 96T clusters for calculation, respectively.Since the computational model is rather large, we do not perform high level quantum calculations on the entire region of the model.1][42][43][44][45] Therefore, two-layer ONIOM methodologies 10,11,46 combined with QM/MM calculations were applied in this study. 10,36,47cording to the ONIOM scheme, a minimum of two layers are dened: the "high" level is treated by quantum mechanical methods whereas the "low" level is described either by classical mechanics or a less rigorous quantum mechanical level.In this study, B3LYP/6-31G(d,p) method was employed on the highlevel atoms, including 4T cluster model (depicted in balls and sticks).The similar size of the cluster model has also been chosen to be the quantum region in previous studies. 44,45,48For the low-level atoms (depicted in wires), the universal force eld (UFF) method, 49 was employed to account for the dispersion effects as well as the connement effects of the zeolites. 50,51The combined B3LYP:UFF method has been widely used to explore the transition state structure and reaction mechanism over zeolites. 44,48or all the cluster models, the dangling silicon atoms were terminated by hydrogen atoms along the bond direction of the next lattice oxygen atoms with a distance of 1.47 Å.The atoms of the reactant and the three inner most coordination spheres were relaxed during the optimization procedure, including three Si atoms, four O atoms, one Al atom and an acidic proton.The remaining structures of the zeolites were xed in their crystallographic positions to retain the zeolite structures.All of the calculations were performed using the GAUSSIAN 09 soware package. 52The optimization of transition states (TS) was calculated using the synchronous quasi-Newtonian method, QST3. 53Frequency analyses were performed to make sure the TS has only one imaginary frequency.

Adsorption of 1-butene and isobutene
Prior to investigating the catalytic process, we rst consider the adsorption of the reactant and production Brønsted acids.It is found that the acidic proton can exhibit a weak p-interaction with the C-C double bond.The optimized structures and the selected bond distances are listed in Fig. 2 and Table 1.Aer interacting with the acidic proton, the C]C bond 1.34 Å for both 1-butene and isobutene.Meanwhile, the length of O a -H z1 slightly elongates from 0.96 to 0.98-0.99Å.The O a -Al-O b angle Fig. 1 The cluster models used in this study.The portions depicted in balls and sticks are the high layers treated with the B3LYP/6-31G(d,p) level, and the portions depicted in wire frame are the low layers treated with the UFF potential.decreases by $3.0 and the Si-O a -Al angle only increases by $1.0 .These results suggest that the adsorption process doesn't signicantly change the structures of both adsorbents and zeolites, compared to the isolated structures. 54Due to the less steric hindrance for the terminal C atom, the length of C1-H z1 (2.52 Å for HZSM-23 and 2.45 Å for HZSM-48) is much closer than that of C2-H z1 (2.59 Å for HZSM-23 and 2.46 Å for HZSM-48).Moreover, the hydrogen is further away from the C]C bond of isobutene than the C]C bond of 1-butene.
To further consider the interaction of C-C double bond with Brønsted acids, we calculate the adsorption energies, listed in Table 2.For 1-butene, the adsorption energies show little difference between HZSM-23 and HZSM-48.This is probably attributed that the pore size in both zeolites is larger enough and has little effect on the linear 1-butene molecule.However, for the isobutene molecule, it has a branched-chain structure, leading to a larger steric repulsion with the walls of HZSM-48 than with the walls of HZSM-23.As a result, it exhibits significantly less adsorption energy on HZSM-48 than on HZSM-23.Interestingly, the adsorption energy values of 1-butene is less than that of isobutene on HZSM-23, the trend of adsorption energies is dissimilar to the trend of the intermolecular distances.This result implies that the drop-shaped pore framework in HZSM-23 has an important effect on the adsorption of isobutene.The branched isobutene locates at the place where the hindrance is the smallest due to the widest aperture.Moreover, the hydroxyl group offsets toward the carbon end of isobutene, leading to the formation of two hydrogen bonds, which signicantly stabilize isobutene on the Brønsted acid site.These results indicate that the drop-shaped pore framework of HZSM-23 enhance the adsorption of isobutene compared to other zeolites.In addition, our results also reveal that the adsorption energy of isobutene decreases as the pore size increases due to the lower steric hindrances from the pore wall.

Monomolecular mechanism of the skeletal isomerization
The monomolecular mechanism of the skeletal isomerization is decomposed into three elementary steps, as is shown in Fig. 3 3 and 4. The imaginary frequency for transition states on zeolites are showed in Table 5.Based on the proposed reaction mechanism, the active center for skeletal isomerization of 1butene to isobutene is identied to exhibit a special Al-O-Si structure where the three vertex O atoms of the aluminiumoxygen tetrahedron are exposed on the surface of the pores.The mechanism of the isomerization of 1-butene on HZSM-23.In the rst step, the carbon end (C1) of the adsorbed 1butene (REAC) is protonated by the acid proton of the zeolite via a transition of a secondary carbenium (TS1).The primary carbenium is hard to be formed, due to the steric hindrance that make the proton inaccessible to the C2 atom of 1-butene.The calculated imaginary frequency is 98i cm À1 , corresponding to the movement of the H z1 atom from the O a to C1 atom (see Fig. 4(b)).The activation energy (E a1 ) for this step is calculated to be 11.2 kcal mol À1 .During this step, the O a -H z1 bond distance is increased from 0.99 Å to 1.51 Å and the C1-H z1 bond is formed with a bond length of 1.21 Å. Simultaneously, the C2-O b distance is shorten to 1.53 Å, indicating that there is a strong covalent bond between C2 and O b , forming a 2-butoxide intermediate (INT1).In the second step, the methyl group (C4) is transferred from C3 to C2 via a transition of a triangular protonated cyclopropyl cation.At the transition state, the distance of C3-C4 bond and C2-C4 bond are 1.83 Å and 1.86 Å, respectively.Aer the transition, C4 is bound to C2 with a bond length of 1.55 Å and the C4-C3 bond is broken.Simultaneously, C3 becomes bound to the lattice O b with a bond length of 1.52 Å, forming an isobutoxide intermediate (INT2).This process need to overcome a high activation barrier of 35.1 kcal mol À1 .Finally, the isobutoxide intermediate is converted into isobutene by transferring the proton H z2 to the lattice O b .The energy barrier for this step is 28.9 kcal mol À1 .The transition state of this step is a primary isobutyl carbenium (TS3), which is similar to the previous experimental and theoretical observation over the theta-1 zeolite. 33,55While Wattanakit 11 reported that the last step still need to experience a tert-butyl carbenium intermediate that would be rapidly deprotonated to form isobutene.However, based on our vibrational analysis for the transition state (TS3), the vibrational mode of the imaginary frequency (560i cm À1 ) demonstrates that H z2 is inclining to bind with the lattice O b when it moves towards C3.Therefore, we inferred that isobutene could be formed directly via a primary isobutyl carbenium.
The mechanism of the isomerization of 1-butene on HZSM-48.The isomerization mechanism on HZSM-48 is similar to that on HZSM-23.The adsorbed 1-butene (REAC) is rst protonated by the acid proton of the zeolite.Since the connement effect of zeolite is reported to not only affect the reaction enthalpy but also exhibit a signicant role in the reaction entropy, 56,57 we further consider the free energetics along the reaction path by including the harmonic entropy contribution.Note that anharmonic entropy contribution is not considered in this work, though it is found important in recent studies. 58For gas molecules, rotational, translational and vibrational entropies were all considered in the free energy estimation while for zeolites and adsorbed intermediates only the vibrational entropy was included. 59The temperature used for the entropy estimation is chosen to be 700 K, consistent with the reaction temperature of 1-butene isomerization in recent experimental reports. 60,61he whole free energy reaction paths on both HZSM-23 and HZSM-48 are shown in Fig. S1 † and the comparison of energy barriers with or without entropy contribution are listed in Table 6.The entropy effect does not change the relative   comparison for each elementary step between both zeolites.For example, the barrier of 2-butoxide formation (E a1 or DG a1 ) on HZSM-23 is always lower than that on HZSM-48 regardless of whether entropy is included; so do isobutoxide formation (E a2 or DG a2 ) and isobutene formation (E a3 or DG a3 ).However, it is found that the entropy effect strongly affects the energetics for the formation of each intermediate.For the formation of 2butoxide, it increase the energy barrier by $9 kcal mol À1 on both zeolites; for isobutoxide formation and isobutene formation, it, conversely, decreases the energy barriers by $3 kcal mol À1 on HZSM-23 and by $6 kcal mol À1 on HZSM-48.This leads to the signicant shrink of the difference between DG a1 and DG a2 .Specially, the rate-determine step on HZSM-48 is changed from isobutoxide formation to 2-butoxide formation when the entropy contribution is included.Finally, the free energy barrier of the rate-determine step on HZSM-23 is 32.3 kcal mol À1 , slightly higher than that on HZSM-48 (i.e.27.5 kcal mol À1 ).This suggests that HZSM-23 is a little less reactive for butene isomerization than HZSM-48.

Conclusions
The reaction mechanisms of skeletal isomerization of 1-butene to isobutene was investigated over one-dimensional 10membered ring zeolites: HZSM-23 (MTT) and HZSM-48 (MRE) using the ONIOM (B3LYP/6-31G(d,p):UFF) method.The 80T and 96T cluster models were employed to represent the connement effect from the HZSM-23 and HZSM-48 zeolites, respectively.A monomolecular mechanism for the skeletal isomerisation was proposed, including the following three steps: (1) protonation of 1-butene to form a 2-butoxide intermediate, (2) transformation of 2-butoxide intermediate into an isobutoxide and (3) decomposition of the isobutoxide intermediate into the isobutene.The active center for skeletal isomerization is iden-tied to have a special Al-O-Si structure, where three vertex O atoms of the aluminium-oxygen tetrahedron need to be exposed on the surface of the pores.It was further found that the pore size exhibits distinct connement effect on the reaction energetic along the reaction path on both zeolites.Considering the entropy effect at 700 K, the rate-determining steps are found to be the transformation of 2-butoxide to the isobutoxide on HZSM-23 and the formation of 2-butoxide on HZSM-48, respectively.Our work implies a signicant role of the conferment effect in the chemistry of zeolites.
. First, the adsorbed 1-butene (REAC) is protonated to form a 2butoxide intermediate (INT1).In this step, the C1 is protonated by the proton from O a -H z1 and C2 is further bonded to the lattice O b .Second, the 2-butoxide intermediate is transformed into an isobutoxide intermediate (INT2) via a triangular protonated cyclopropyl cation.In this process, the methyl group (C4) shis from C3 to C2 and C3 is further bonded to the lattice O b .Finally, the isobutoxide intermediate is converted into isobutene.During this step, the C3-O b bond is broken and H z2 is transferred to the lattice O b , recovering the Brønsted acid site.Fig. 4(a), 5(a), 6(a) and 7 show the optimized structures of elementary steps on the HZSM-23 and HZSM-48 zeolites.The activation energies and the geometrical parameters of the transition states and intermediates involved the isomerization of 1-butene on HZSM-23 and HZSM-48 zeolites are listed in Tables

Fig. 5
Fig. 5 The structures of TS2 on the HZSM-23 and HZSM-48 zeolites.(a) The complete pore of zeolite.(b) The pore of the relaxed part of the zeolite and the corresponding vibrations of the imaginary frequencies of TS2.

Fig. 6
Fig. 6 The structures of TS3 on the HZSM-23 and HZSM-48 zeolites.(a) The complete pore of zeolite.(b) The pore of the relaxed part of the zeolite and the corresponding vibrations of the imaginary frequencies of TS3.

Fig. 8
Fig. 8 Reaction energy profiles of the skeletal isomerization of 1butene over HZSM-23 and HZSM-48 zeolites.The energies in italic are activation energies (E an , n ¼ 1, 2, 3), whereas the energies in bold are energy barriers.

Table 1
Selected bond distances of the adsorption complexes for 1butene and isobutene on the HZSM-23 and HZSM-48 zeolites (Å)

Table 6
Comparison of calculated energy barriers with or without harmonic entropy correction (kcal mol À1 )