Theoretical Study on Structures and Reaction Mechanisms of Ethylene

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Theoretical Study on Structures and Reaction Mechanisms of Ethylene Oxide Hydration over H-ZSM-5: Ethylene Glycol Formation Thana Maihom,†,‡ Supawadee Namuangruk,§ Tanin Nanok,†,‡ and Jumras Limtrakul*,†,‡ Department of Chemistry, Faculty of Science, Kasetsart UniVersity, Bangkok 10900, Thailand, Center of Nanotechnology and NANOTEC Center of Excellence, Kasetsart UniVersity, Bangkok 10900, Thailand, and National Nanotechnology Center (NANOTEC), NSTDA, Khongluang, Pathumthani 12120, Thailand ReceiVed: April 22, 2008; ReVised Manuscript ReceiVed: June 10, 2008

Stepwise and concerted mechanisms of the ethylene oxide hydration reaction within a cluster model covering the nanocavity, where the straight and zigzag channels intersect, of H-ZSM-5 have been proposed and investigated by means of the ONIOM(B3LYP/6-31G(d,p):UFF) and embedded-ONIOM(B3LYP/6-31G(d,p): UFF) methods. For the stepwise mechanism, the hydration reaction of ethylene oxide starts from the ring opening of ethylene oxide by breaking of the C-O bond to form the alkoxide intermediate followed by hydration of the alkoxide intermediate to produce ethylene glycol as the product of the reaction. The calculated activation energies are computed to be 34.4 and 14.5 kcal/mol for the ring-opening and hydration steps, respectively. For the concerted mechanism, the ring opening and hydration take place simultaneously with a small activation barrier of 13.3 kcal/mol. This reaction is suggested as a more economical alternative to the present noncatalytic hydration of ethylene oxide via the concerted mechanism for production of ethylene glycol, a versatile intermediate in a wide range of applications, and should be of particular interest to industry. 1. Introduction Widespread industrial production of ethylene glycol began in 1937 when ethylene oxide, a component in its synthesis, became cheaply available. As an adaptable intermediate, it has now become increasingly important, particularly in the plastics industry, in a wide range of applications, for example, polyester textiles, plasticizers, consumer packaging, and lubricants. In addition, ethylene glycol capabilities as an antifreeze coolant make it a prominent constituent of vitrification. It can be produced by a well-known process: hydration of ethylene oxide via the ring-opening reaction. Industrially, the noncatalytic hydration is the main process for production of ethylene glycol. However, the high conversion and selective production requires a large excess of water, which results in costly excessive energy consumption for the product evaporation process. To solve this problem, a number of catalysts have been tested over the years for the effective catalyzation in the hydration of ethylene oxide, for example, ion-exchange resin,1–3 quaternary phosphonium halides,4 supported metal oxides,5–8 polymeric organosilane ammonium salt,9 macrocyclic chelating compounds,10 and zeolite.6 These catalysts, however, still have some drawbacks for the economic catalyzation in terms of both conversion and selectivity. In the field of catalysis, zeolite is considered to be one of the best catalysts because it has many advantageous and diverse properties, such as easy product separation, it can be reactivated, and it is reusable. In addition, zeolite has increasing benefits from nanopores size and shape selectivity. Recently, an acidic ZSM-5 zeolite was found to be an active catalyst for hydration of ethylene oxide.6 Under a reaction temperature of 150 °C and a water to ethylene oxide * To whom correspondence should be addressed. Phone: +66-2-5625555 ext 2159. E-mail: [email protected]. † Department of Chemistry, Kasetsart University. ‡ Center of Nanotechnology and NANOTEC Center of Excellence, Kasetsart University. § National Nanotechnology Center (NANOTEC).

molar ratio of 22, ethylene oxide conversion and ethylene glycol selectivity can reach 98% and 76%, respectively, over H-ZSM-5 catalyst. Zeolites are microporous aluminosilicates with a large number of atoms in the unit cell. In theoretical studies, small clusters are usually used for representing models of catalytic zeolite. However, these cluster models do not incorporate the framework effect, which has an important role for reactions inside zeolites.11–13 In order to account for the framework effect on the zeolite model, various techniques such as the large quantum clusters and periodic ab initio calculations are performed. While these techniques are able to produce accurate results, they are not practical for the large unit cell of zeolites since they are computationally too demanding and expensive. In research endeavors to overcome these limitations, researchers discovered various hybrid methods, i.e., the embedded cluster model,14–19 combined quantum mechanics/molecular mechanics (QM/ MM),11,20–22 as well as the more general ONIOM (our-OwnN-layered Integrated molecular Orbital + molecular Mechanics).23–34 Recently, the ONIOM method has been successfully used to investigate the adsorptions27–29,34,35 and reactions of hydrocarbons over zeolites.23–25,30,31 To the best of our knowledge, details of the reaction mechanism of ethylene oxide hydration over zeolite have not previously been reported in the literature. This is undoubtedly due mainly to its complicated three-dimensional porous structure that limits the ability to observe it experimentally. From an industrial point of view, understanding the reaction mechanism on a molecular scale could help in optimizing the reaction conditions, which will result in increasing product selectivity. To gain insight into the molecular mechanisms, we present in this study a theoretical investigation of the reaction mechanisms of ethylene oxide hydration over H-ZSM-5 zeolite using the ONIOM method. The structures and energetics of the reaction coordinates and reaction mechanisms are discussed.

10.1021/jp8034677 CCC: $40.75  2008 American Chemical Society Published on Web 07/31/2008

Ethylene Oxide Hydration over H-ZSM-5

J. Phys. Chem. C, Vol. 112, No. 33, 2008 12915 theory is applied for the 12T cluster model, covering the 10membered ring window of the zigzag channel of the zeolite, which is considered as the active region. The 128T extended framework is treated with the well-calibrated universal force field (UFF) to represent the confinement effect37–39 of the zeolite nanopore structure. This force field has been reported to give a good description of the short-range van der Waals interactions. All calculations were performed using the Gaussian03 code.40 During optimization, only the 5T cluster of the active region, [(tSiO)3Al(OH)Sit], and the probe molecules are allowed to relax. The vibrational spectra were performed at the same level of theory to ensure that each transition structure has only one imaginary frequency which corresponds to the expected motion of atoms for each transition structure. This calculation confirmed that the transition-state formed involved the required reaction coordinate. In order to include the long-range effects of the electrostatic Madelung potential from the infinite zeolite lattice, all optimized ONIOM geometries are embedded in the two sets of point charges and performed with the single-point calculations. The first set is represented by the fixed partial atomic charges (-1.0 e for O and +2.0 e for Si) which are placed in the zeolite lattice positions at 13-17 Å from the 12T quantum cluster, accounting for the medium effect of the electrostatic potential. Another set is placed at the extended lattice framework up to 38 Å from the 12T quantum cluster. These point charges are optimized to reproduce the long-range Madelung potential from the infinite lattice of the zeolite structure. Such method is called “the embedded ONIOM” method shown in Figure 1b and has been performed in our previous calculations, such as the pyridine adsorption in zeolites33,41 and the reaction mechanisms of benzene alkylation with propylene in newly synthesized H-ITQ24 zeolite.31 3. Results and Discussion

Figure 1. ONIOM (a) and embedded ONIOM (b) models of the 128T cluster of the H-ZSM-5 zeolite. Atoms belonging to the quantum region are drawn as balls and sticks, while the lines represent the universal force field (UFF). The dots graphics in the embedded ONIOM2 scheme illustrate the Madelung point charges.

2. Models and Methods The structure of the 128T cluster model encompasses the intersection region of the straight and zigzag channels, as shown in Figure 1a, and is used for representing the lattice structure of an acidic H-ZSM-5 zeolite.36 For computational efficiency, the two-layered ONIOM scheme is selected. In this ONIOM model the active region of zeolite and the adsorbate molecules are treated with the high-accuracy method, while the rest of the 128T cluster model is treated with the less-accurate method to reduce computational time. The B3LYP/6-31G(d,p) level of

After the ethylene oxide gas molecule is diffused into the zeolite pore, it was then adsorbed on the Brønsted acid site, at which stage the ethylene oxide-zeolite adduct is formed via hydrogen bonding. This adduct can be transformed to ethylene glycol by two different types of reaction pathways: stepwise and concerted. In this study, the mechanisms for hydration of ethylene oxide are investigated using the ONIOM method. In addition, the effects of the infinite lattice structure of zeolite on the reaction pathways are studied by evaluation of the embedded ONIOM approach. 3.1. Stepwise Reaction Pathway. The stepwise mechanism commences with ethylene oxide being adsorbed on the Brønsted acid site of H-ZSM-5 zeolite (see Figure 2a) via hydrogen bonding to form the ethylene oxide-zeolite adsorption complex (Ads1_S). The selected geometric parameters are listed in Table 1. The adsorption energy is calculated to be -23.3 kcal/mol, which is lower than that for propene oxide (-30.8 kcal/mol) due to there being no electron-donating group substituted on the epoxide ring.23 In this adsorption complex, the hydrogenbonding interaction between the Oe of ethylene oxide and the acidic proton of zeolite (H1) induces the lengthening of the O1-H1 bond distance from 0.97 to 1.02 Å. In addition, electron transfer from the Oe to the H1 results in a slight elongation of the C1-Oe and C2-Oe bond lengths by about 0.03 and 0.02 Å, respectively. Following this, the C1-Oe bond, which is weaker than the C2-Oe bond, ruptures to open the epoxide ring and then form the surface ethoxide intermediate (Int_S) via the primary carbenium-like ion transition state (TS1_S). At the transition structure (cf. Figure 2b), the acidic proton of the

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Figure 2. Optimized structures of the (a) ethylene oxide adsorption complex (Ads1_S), (b) first transition state (TS1_S), and (c) alkoxide intermediate (Int_S) for the ring-opening step of the stepwise mechanism of ethylene oxide hydration calculated at the ONIOM(B3LYP/6-31G(d,p):UFF) level of theory.

TABLE 1: Optimized Geometrical Parameters and Mulliken Charge of Organic Fragments of Adsorption Complexes for the Ethylene Oxide Ring-Opening Step of the Stepwise Mechanism Using the ONIOM2 Methoda

parameter distances O1-H1 Oe-H1 C1-Oe C1-C2 C2-Oe C1-O2 C2-O2 angles ∠Si1O1Al ∠Si2O2Al ∠O1H1Oe charges (q)

isolated cluster 0.97 1.43 1.47 1.43

128.8 129.6

ethylene oxide adsorption (Ads1_S)

first transition state (TS1_S)

alkoxide intermediate (Int_S)

1.02 1.56 1.46 1.47 1.45 3.25 3.35

1.61 1.01 2.10 1.47 1.43 2.33 3.27

3.42 0.98 2.46 1.52 1.40 1.53 2.53

127.0 131.4 173.9

126.4 132.5 162.6

127.1 124.8 104.2

+0.550

+0.704

+0.440

a

Distances are in Ångstroms, angles are in degrees, and charges are in electrons.

zeolite is transferred to the adsorbed ethylene oxide, the C1-Oe bond is broken, and the hybridization of C1 changes from tetrahedral (sp3) to planar (sp2). The transition state is confirmed by the frequency calculation with one imaginary frequency at -365.1 cm-1, which is related to movement of the acidic proton of zeolite (H1) to the ethylene oxide oxygen (Oe) and breaking of the C1-Oe bond. This step is called the ring-opening step. The calculated activation energy barrier is 34.4 kcal/mol, and the apparent activation energy is 11.1 kcal/mol. The result of this is that the ethoxide intermediate (Int_S) is formed at the O2 of the zeolite structure and stabilized by the zeolite framework (Figure 2c). The adsorption energy of this ethoxide species is computed to be -21.5 kcal/mol, which is slightly less stable than that for the ethylene oxide adsorption complex

(Ads1_S). The evaluated energy profile of all species involved in this ring-opening step is illustrated in Figure 5a. The next step is the hydration reaction of the surface ethoxide intermediate and the water molecule to form ethylene glycol. The selected geometric parameters for this are listed in Table 2. After forming the stable ethoxide intermediate, the nucleophilic water molecule adsorbs next to the active ethoxide species to form the ethoxide-water coadsorption complex (Ads2_S) with an adsorption energy of -33.6 kcal/mol. This coadsorption complex is more stable than the alkoxide intermediate as 12.1 kcal/mol, which is involved in a weak interaction of the water molecule on the alkoxide intermediate. This is confirmed by the slight lengthening of the C1-O2 covalent bond from 1.53 to 1.54 Å. The transition state (TS2_S) of this step, shown in Figure 3b, is a SN2 reaction where the water oxygen atom (Ow) attacks the carbon of the ethoxide intermediate (C1). The activation energy for this step is rather small; it is computed to be 14.5 kcal/mol. Normal mode analysis reveals one imaginary frequency at -619.0 cm-1 associated with the transition state, which corresponds to movement along the reaction coordinate in which the O2-C1 bond breaks and the C1-Ow bond forms simultaneously. It is noted that there is no mode of the proton transferring to the zeolite framework. Thus, the protonated ethylene glycol intermediate which might form in the nanocavity of zeolite is optimized to find the local minimum. However, we found from the theoretical calculation that this protonated structure is an unstable species; it is readily transformed to the ethylene glycol product without requiring a reaction barrier. This may explain why there is no experimental report for the finding of this labile structure. The product of the stepwise reaction (Prod_S) is the gauche conformer of ethylene glycol adsorbed on the active site of zeolite due to the transition structure control (Figure 3c). The reaction energy for this reaction pathway is calculated to be -59.2 kcal/mol. Finally, the adsorbed ethylene oxide is desorbed endothermically and requires an energy of 30.5 kcal/mol.

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Figure 3. Optimized structures of the (a) water and alkoxide adsorption complex (Ads2_S), (b) second transition state (TS2_S), and (c) gaucheethylene glycol conformer adsorption complex (Prod_S) for the hydration step of the stepwise mechanism of ethylene oxide hydration calculated at the ONIOM(B3LYP/6-31G(d,p):UFF) level of theory.

TABLE 2: Optimized Geometrical Parameters and Mulliken Charge of Organic Fragments of the Adsorption Complexes for the Ethylene Oxide Hydration Step of the Stepwise Mechanism Using the ONIOM2 Methoda water coadsorption (Ads2_S)

second transition state (TS2_S)

product (Prod_S)

distances C1-Oe C1-C2 C2-Oe C1-O2 C1-Ow Ow-H2 O1-H2

2.46 1.52 1.41 1.54 3.11 0.98 4.49

2.44 1.49 1.40 2.20 2.12 0.97 3.64

2.47 1.53 1.41 3.25 1.45 1.48 1.05

angles ∠Si1O1Al ∠Si2O2Al ∠O2C1Ow

128.0 124.8 152.1

128.6 130.5 155.8

126.2 133.1 72.9

dihedral OwC1C2O2

-106.0

-84.7

-48.8

charges (q)

+0.470

+0.728

+0.577

parameter

a Distances are in Ångstroms, angles are in degrees, and charges are in electrons.

3.2. Concerted Reaction Pathway. In contrast to the stepwise reaction pathway, ethylene oxide hydration can be considered to proceed in a concerted reaction, the energy profile for which is shown in Figure 5b, and selected geometrical parameters are listed in Table 3. The initial step starts with adsorption of an ethylene oxide on the Brønsted acid site of zeolite with the hydrogen-bonding interaction. Then, the water molecule is weakly coadsorbed next to the adsorbed ethylene oxide to form the coadsorption complex (Ads_C), as shown in Figure 4a. At the Ads_C, the C1-Ow and O2-Ow distances are calculated to be 3.22 and 3.79 Å, respectively. The weak interaction of the water molecule on adsorbed ethylene oxide results in a small binding energy of -8.3 kcal/mol, which is lower than the binding energy of the water molecule adsorbed on the acidic zeolite of -23.9 kcal/mol. Subsequently, the

ethylene glycol is formed through the transition state (TS_C), which occurs simultaneously with both the ring-opening and hydration processes (Figure 4b) without forming an alkoxide intermediate. This transition state (TS_C) is considered as the SN2 attack. The C1-Oe bond is broken by the nucleophilic attack of the water molecule on the C1 of ethylene oxide with a OeC1Ow angle of 157.5°, and a new C1-Ow bond is formed with protonation of ethylene oxide at the Oe atom. The C1-Oe bond is lengthened from 1.46 to 1.87 Å, while the C1sOw distance is decreased from 3.22 to 2.05 Å. At the transitionstate configuration, the sp3 hybridization of the ethylene oxide C1 atom is altered to sp2 hybridization. The transition state is verified by frequency calculation with one imaginary frequency at -309.5 cm-1. The calculated activation energy is evaluated to be 13.3 kcal/mol. After forming the transition state (cf. Figure 4c), the proton (H2) of the water molecule transfers from Ow toward O2 to restore the acid site of zeolite and form the antiethylene glycol adsorbed on the active site of the zeolite with an adsorption energy of -48.5 kcal/mol. Finally, anti-ethylene glycol is desorbed from the active site by requiring an energy of 23.1 kcal/mol. The reaction energy for the concerted pathway is exothermic by 25.4 kcal/mol. In order to include the effect of long-range electrostatic interactions of the extended zeolite lattice beyond the ONIOM model, the embedded ONIOM method is employed. For this method the energy of the system is contributed by the electrostatic interactions, which primarily affect the ionic species, especially the transition states. In pervious studies,31 the intermediate and transition states were stabilized by the electrostatic interactions of the extended zeolite framework. Moreover, the activation energy of the ionic system was reduced due to the ionicity of the transition state organic fragment (cf. Tables 1–3). In this work, the embedded ONIOM method is performed on the ONIOM-optimized geometry by single-point energy calculation. With reference to the ONIOM energies, the results show that the adsorption and intermediate complexes are mostly stabilized in the range of 2-4 kcal/mol (see Figure 5a and 5b). It is noted that nonionicity, like Ads2_S in the

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Figure 4. Optimized structures of the (a) ethylene oxide and water molecule coadsorption complex (Ads_C), (b) transition state (TS_C), and (c) anti-ethylene glycol adsorption complex (Prod_C) for the concerted mechanism of ethylene oxide hydration calculated at the ONIOM(B3LYP/631G(d,p):UFF) level of theory.

TABLE 3: Optimized Geometrical Parameters and Mulliken Charge of Organic Fragments of the Adsorption Complexes for the Concerted Ethylene Oxide Hydration Mechanism Using the ONIOM2 Methoda

parameter

ethylene oxidewater coadsorption (Ads_C)

transition state (TS_C)

product (Prod_C)

distances O1-H1 Oe-H1 C1-Oe C1-C2 C2-Oe Ow-C1 Ow-C2 Ow-H2 H2-O2

1.02 1.64 1.46 1.47 1.45 3.22 3.79 0.97 2.63

1.65 1.01 1.87 1.46 1.45 2.05 2.87 0.97 3.21

4.15 0.97 2.42 1.53 1.42 1.46 2.44 1.40 1.07

angles ∠Si1O1Al ∠Si2O2Al ∠OeC1Ow

127.4 132.3 103.6

126.5 134.2 157.5

124.5 131.0 142.5

charges (q)

+0.534

+0.794

+0.605

a

Distances are in Ångstroms, angles are in degrees, and charges are in electrons.

stepwise mechanism, is not affected by the electrostatic interaction, whereas in the transition states, the activation energies are decreased by 3.6, 3.9, and 4.1 kcal/mol, which are in proportion to the ionicity of the organic fragments of the TS1_S, TS2_S, and TS_C, respectively. The corrected activation energies accounted from the electrostatic interaction are 30.8 and 10.6 kcal/mol for the ring opening and hydration of the stepwise mechanism, respectively. The concerted mechanism, in which the transition state is most stabilized from the Madelung potential, is evaluated to be 9.2 kcal/mol. The complete energetic profiles for the stepwise and concerted reaction pathways are illustrated in Figure 5. For the stepwise mechanism, the ring-opening process is found to be the ratedetermining step with an activation energy of 34.4 kcal/mol. This is a similar trend to that computed from the ONIOM method of 30.8 kcal/mol. The ethoxide intermediate is desta-

bilized by electrostatic interactions from the zeolite lattice and thus active for further reaction, which is the ethoxide hydration. This step is more facile than the ring-opening step and has a small activation barrier of 14.5 kcal/mol for the ONIOM method and 10.6 kcal/mol for the embedded ONIOM method. For the concerted mechanism, the activation barrier is calculated to be 13.3 kcal/mol (9.2 kcal/mol for the embedded ONIOM method), which is significantly lower than the energy barrier of the ratedetermining step in the stepwise mechanism. Moreover, the relative energy of the transition state for the concerted mechanism (TS_C) is lower than that for both transition states for the stepwise mechanism. The long-range electrostatic interaction included by the embedded ONIOM method affects the stability of the adsorbed species inside the zeolite and, most importantly, significantly alters the stability of the ionic species like transition states, which appears to be a key point for this reaction. Therefore, from the energetics of the reactions and relative stability of the transition states, it can be concluded that formation of ethylene glycol by hydration of ethylene oxide over H-ZSM-5 zeolite preferably occurs via the concerted mechanism. Moreover, the obtained ethylene glycol products will be gauche or anti conformers depending on its reaction mechanism, which is controlled by the transition structure. The gauche-ethylene glycol is formed by the stepwise mechanism, while the anti-ethylene glycol is formed by the concerted mechanism. The adsorbed gauche-ethylene glycol is more stable than the adsorbed anti-ethylene glycol, which is due to the role of the confinement effects of the zeolite pore on these adsorbed molecules. The similar trend of interaction energies for the cis/ trans-but-2-ene adsorbed on H-ZSM-535 has been observed. Furthermore, the anti conformer can be desorbed more easily than the gauche conformer by 6 kcal/mol. Thus, the adsorbed anti-ethylene glycol product produced from the concerted mechanism is more easily separated from the nanocavity of zeolite than the gauche-ethylene glycol produced from the stepwise mechanism. 4. Conclusion This study elucidates the reaction mechanism for hydration of ethylene oxide to ethylene glycol over the nanoporous

Ethylene Oxide Hydration over H-ZSM-5

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Figure 5. Energetic profile for (a) stepwise and (b) concerted mechanisms of the ethylene oxide hydration calculated at the ONIOM(B3LYP/631G(d,p):UFF) level of theory. The values in parentheses are derived from the embedded ONIOM(B3LYP/6-31G(d,p):UFF) calculations (energies are in kcal/mol).

H-ZSM-5 zeolite with the aim of providing a procedure to reduce the large excess of water which, in the present noncatalytic hydration process, results in costly, excessive energy consumption. The ONIOM and embedded ONIOM methods were used to investigate this reaction mechanism with two different pathways: the stepwise and concerted mechanisms being considered. For the stepwise mechanism, the hydration reaction proceeds via the ring opening of ethylene oxide to form the alkoxide intermediate with the energy barrier of 34.4 kcal/ mol. Consequently, the alkoxide intermediate is hydrated by a water molecule nearby to form ethylene glycol as the product. The activation energy of this step is calculated to be 14.5 kcal/ mol. For the concerted mechanism, the hydration reaction occurs in a single step of simultaneous ring opening and hydration. The calculated activation energy barrier is only 13.3 kcal/mol. The results from the embedded ONIOM method indicate that the electrostatics accounted from the infinite lattice of zeolite play an important role in the stabilization of the transition structures and adsorption complexes. These result in lowering the activation barriers for whole reaction pathways for both the stepwise and concerted mechanisms. For the stepwise mechanism, the calculated activation energies are reduced to 30.8 and

10.6 kcal/mol for the ring-opening and hydration steps, respectively. For the concerted mechanism, the activation barrier is even lower, 9.2 kcal/mol. These results show a similar trend to the computations from the ONIOM method. Therefore, in this study it can be concluded that the hydration reaction of ethylene oxide to ethylene glycol over H-ZSM-5 zeolite preferably proceeds via the concerted mechanism. Acknowledgment. This work was supported in part by grants from the National Nanotechnology Center (NANOTEC Center of Excellence and Computational Nanoscience Consortium), the Thailand Research Fund, Kasetsart University Research and Development Institute (KURDI), and the Commission of Higher Education, Ministry of Education under Postgraduate Education and Research Programs in Petroleum and Petrochemicals and Advanced Materials. References and Notes (1) Strickler, G. R.; Landon, V. G.; Lee, G. S.; Rievert, J.; William, J. WO Patent 31033, 1999. (2) Iwakura, T.; Miyagi, H. JP Patent 11012206, 1999.

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