Direct gas-phase epoxidation of propylene to propylene oxide through radical reactions: A theoretical study

Chemical Physics Letters 487 (2010) 183–189

Contents lists available at ScienceDirect

Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett

Direct gas-phase epoxidation of propylene to propylene oxide through radical reactions: A theoretical study Ali Can Kizilkaya a, Mehmet Ferdi Fellah a,b, Isik Onal a,* a b

Department of Chemical Engineering, Middle East Technical University, Ankara 06531, Turkey Department of Chemical Engineering, Yuzuncu Yil University, Van 65080, Turkey

a r t i c l e

i n f o

Article history: Received 1 October 2009 In final form 18 January 2010 Available online 21 January 2010

a b s t r a c t The gas-phase radical chain reactions which utilize O2 as the oxidant to produce propylene oxide (PO) are investigated through theoretical calculations. The transition states and energy profiles were obtained for each path. The rate constants were also calculated. The energetics for the competing pathways indicate that PO can be formed selectively due to its relatively low activation barrier (9.3 kcal/mol) which is in a good agreement with the experimental value (11 kcal/mol) of gas-phase propylene epoxidation. The formation of the acrolein and combustion products have relatively high activation barriers and are not favored. These results also support the recent experimental findings. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Propylene oxide (PO) is one of the most important intermediates used in chemical industry. The current technology allows the production of PO through either the chlorohydrin process, using chlorine and water, or the hydroperoxide process where organic hydroperoxides are used. While the chlorohydrin process causes environmental problems, the hydroperoxide process produces equal amount of styrene or t-butyl co-products [1]. Thus, it is desirable to design a new catalytic production route for PO using molecular oxygen that is economic and environmentally friendly. Although, such a direct route has been developed for the production of ethylene oxide using a silver catalyst, the selectivity of silver is around 5% for propylene epoxidation [2]. The reason behind the difficulty of PO formation is surmised to be the abstraction of allylic hydrogen atoms of propylene which results in combustion. Considerable effort has been put to study the mechanism of silver-catalyzed ethylene epoxidation [3–5], while there have been very little studies regarding the mechanism of PO formation [6,7]. Catalytic systems consisting of various catalysts and oxidants were proposed for propylene epoxidation. In 1998, Haruta et al. reported a titania supported gold catalyst that is selective for epoxidation provided that hydrogen is co-feeded [8]. In 2000, copper was shown to be a more selective catalyst for propylene epoxidation than silver [9]. Later, NaCl-modified VCexCu1x mixed oxide catalysts were reported to be effective for PO formation using molecular oxygen as the oxidant and hydrogen co-feeding was * Corresponding author. Fax: +90 312 210 2600. E-mail address: [email protected] (I. Onal). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.01.036

claimed to increase selectivity [10]. Vaughan and co-workers offered copper as a selective catalyst for propylene epoxidation, which would give PO selectivity in the same order of the previously reported catalysts without the need of hydrogen co-feeding [11]. Torres et al. theoretically investigated propylene epoxidation on copper comparing to ethylene epoxidation, proposing the lower basicity of the oxygen atoms adsorbed on the surface as the reason for the higher selectivity of copper [6]. There are also other studies that utilize silica as support material, iron or bimetallic catalysts, and nitrous oxide as the oxidant [12–14]. Mantashyan et al. [15,16] studied experimentally gas-phase propylene oxidation at 633 K under static conditions. Recently, Mimura and co-workers [17] proposed a new catalytic system that composed of a catalytic bed and an empty post-catalytic bed volume that would selectively epoxidize propylene to propylene oxide. They reported that PO was the main oxidation product instead of the allylic oxidation product acrolein. It was also observed that the selectivity to combustion products decreased after the post-catalytic volume was introduced. Due to the significant increase in PO selectivity induced by the empty volume, they concluded that the function of the catalytic bed was to generate radicals and PO was produced mainly in the post-catalytic bed through gas-phase radical chain reactions [18]. The likely mechanism for the gas-phase propylene epoxidation at 573 K was proposed in the experimental literature as in part a of Scheme 1 [17]. The aim of this study is to identify the validity of the proposed radical chain reaction mechanism for the formation of propylene oxide through Density Functional Theory (DFT) and Coupled Cluster Theory calculations, B3LYP and CCSD(T), respectively. The reason behind the selectivity to propylene oxide and lack of

184

A.C. Kizilkaya et al. / Chemical Physics Letters 487 (2010) 183–189

Scheme 1. (a) Mechanism for propylene oxide formation. (b) Reaction for acrolein formation.

observance of acrolein production, the likeliness of the combustion reactions in gas phase and the feasibility of each of the competing reaction pathways will be discussed through the light of the quantum chemical calculations.

2. Computational method All calculations in this study are based on DFT [19] as implemented in GAUSSIAN’03 suite of programs [20]. In order to take into account the effects of exchange and correlation, Becke’s [21,22] three-parameter hybrid method involving the Lee et al. [23] correlation functional (B3LYP) formalism and 6–31G(d, p) basis set are utilized. It has been demonstrated that hybrid B3LYP method is a high-quality density functional method certainly for this type of organic chemistry reactions [24]. All calculations were performed by using unrestricted spin method. The projected methodology was preferred to correct the spin contamination, which affects the values of the evaluated parameters inherent in the unrestricted calculation. Single point energies were also calculated at the CCSD(T) [25], triple excitations level of CCSD, with 6–31G(d, p) basis set by use of the optimized geometries obtained with B3LYP. It has been demonstrated [26–29] that CCSD(T) method produces reliable energy values of equilibrium and transition state geometries for gas-phase radical reactions including bi- and tri-radicals. All the atoms of the reactant and product molecules were fully relaxed. Energy profile, equilibrium geometry (EG), and transition state geometry (TS) calculations were performed for determination of activation barriers and relative energies. All energies and energy differences are calculated with zero point energy (ZPE) corrections at experimental temperature [17] of 573 K. Computed hS2i values confirmed that the spin contamination was very small (after annihilation) [30]. Vibrational analysis was performed for all transition states to confirm that they have only one imaginary mode of vibration. Convergence criterions which are gradients of maximum force, rms force, maximum displacement and rms displacement in GAUSSIAN’03 software are 0.000450, 0.000300, 0.001800 and 0.001200, respectively. Details of an example computational strategy employed in this study can be found in our very recent theoretical study [31]. Transition states have been calculated using the synchronous quasi-Newtonian method of optimization, QST3 [32]. The TST rate coefficients, kTST(T), are calculated by using the standard transition state theory as follows [26,27]:

Fig. 1. Equilibrium final geometry for Reaction 1.

kðTÞ ¼ j

  kB T 0 1m DG exp ðc Þ ¼ jkTST ðTÞ h RT

ð1Þ

where kB and h Boltzman and Planck constants, respectively. j is the transmission coefficient which is assumed to be 1, c0 (=P/RT) is the standard unit of concentration and m is the molecularity of the reaction. The standard Gibbs free energy (DG#) was calculated at experimental temperature [17], 573 K, for gas-phase propylene epoxidation by using B3LYP method. Pre-exponential factors were also computed by using Arrhenius equation. 3. Results and discussion After optimization of the allyl radical and the oxygen molecules, a reaction coordinate calculation is performed. The reaction coordinate is selected as the distance between the C1 atom of the allyl radical and O1 atom of the oxygen molecule for Reaction 1. The reaction is found to proceed exothermically with no activation barrier and the peroxy radical formed has a relative energy value of 18.6 kcal/mol. Spin contamination hS2i value for EG is calculated to be 0.75. The EG for the peroxy radical obtained is given in Fig. 1. The relative energy is also computed as 28.5 kcal/mol by using CCSD(T). Following this path, the peroxy radical can undergo transition either to a peroxodimer or a hydroperoxide. The relative energy profile for the hydroperoxide formation (Reaction 2) is shown in part c of Fig. 2 to illustrate the methodology in obtaining the relative energy profiles. The reaction coordinate is the distance between the allylic hydrogen atom (H6) of the propylene molecule and the terminal oxygen atom (O2) of the peroxy radical. The endo-

A.C. Kizilkaya et al. / Chemical Physics Letters 487 (2010) 183–189

185

Fig. 3. (a) Transition state geometry. (b) Equilibrium final geometry for Reaction 3.

dinate is the distance between the C4 atom of the propylene molecule and oxygen atom (O2) of the peroxy radical. From the comparison of the activation barrier of the two competing reac-

Fig. 2. (a) Transition state geometry. (b) Equilibrium final geometry. (c) Relative energy profile for Reaction 2.

thermic reaction that results in the formation of hydroperoxide is found to have an activation barrier of 14.4 kcal/mol and the final geometry has a relative energy value of 3.8 kcal/mol. The transition state was identified as having one imaginary vibrational frequency of 1768 cm1 for the O–H stretching. The corresponding distance for the transition state is 1.219 Å. The TS geometry and the EG of the product are represented in parts a and b of Fig. 2, respectively. Spin contamination hS2i values for TS and EG are calculated as 0.756 and 0.753, respectively. The activation barrier and relative energy for this reaction were obtained to be 20.4 kcal/mol and 4.2 kcal/mol, respectively by using CCSD(T). The transition state rate constant, kTST, and pre-exponential factor for this reaction were calculated to be 4.86  1025 and 1.50  1019 cm3 mol1 s1, respectively. The competing reaction (Reaction 3) of the peroxy radical, i.e. the transition to the peroxodimer yielded an activation barrier of 9.3 kcal/mol and the reaction is slightly exothermic with a relative energy value of 3.4 kcal/mol. For this reaction, the reaction coor-

Fig. 4. (a) Transition state geometry. (b) Equilibrium final geometry for Reaction 4.

186

A.C. Kizilkaya et al. / Chemical Physics Letters 487 (2010) 183–189

Fig. 5. A summary energy diagram showing a comparison of Reactions 1–4 for gas-phase propylene epoxidation reactions.

Table 1 Mulliken atomic charges and Mulliken atomic spin densities for Reactions 1–4. Reaction

Geometry

Mulliken atomic charge/Mulliken atomic spin density

Atoms of geometry C1

C2

C3

O1

O2

H5

C4

C5

C6

H6

1

EG

Charge Density

0.005 0.0197

0.029 0.016

0.220 0.0067

0.175 0.2916

0.166 0.7073

0.127 0.00003

– 

– –

– –

– –

2

EG

Charge Density Charge Density

0.043 0.0012 0.035 0.0025

0.017 0.00005 0.020 0.0056

0.234 0.00001 0.233 0.0072

0.310 0.0004 0.258 0.1027

0.341 0.0062 0.310 0.3811

0.113 0.00001 0.115 0.0011

0.269 0.6713 0.403 0.4139

0.037 0.2642 0.010 0.1620

0.234 0.6996 0.215 0.3586

0.333 0.0001 0.335 0.0435

Charge Density Charge Density

0.043 0.00012 0.040 0.00565

0.034 0.00015 0.040 0.00412

0.232 0.00003 0.235 0.00055

0.297 0.00093 0.258 0.08239

0.295 0.03927 0.245 0.47885

0.114 0.00091 0.110 0.00186

0.057 0.04637 0.134 0.17582

0.076 1.00528 0.007 0.61022

0.337 0.07801 0.347 0.04510

0.110 0.03235 0.116 0.01019

Charge Density Charge Density

0.007 0.0441 0.056 0.02477

0.051 0.0092 0.032 0.00312

0.236 0.0255 0.245 0.00056

0.344 0.8466 0.351 0.49729

0.460 0.0084 0.354 0.17259

0.132 0.0668 0.112 0.03660

0.029 0.0029 0.104 0.03936

0.136 0.0008 0.036 0.70227

0.341 0.0052 0.341 0.05285

0.136 0.0017 0.095 0.00287

TS 3

EG TS

4

EG TS

tions, it can be concluded that the formation of the peroxodimer will be favored over the formation of the hydroperoxide, which is indeed a prerequisite for the chain reaction to result in the final product of PO. The transition state was identified as having one imaginary vibrational frequency of 515 cm1 for the O–O stretching. The corresponding distance for the transition state is 1.911 Å. Spin contamination hS2i values for TS and EG are calculated to be 0.756 and 0.75, respectively. The TS geometry and the EG of the product are represented in parts a and b of Fig. 3, respectively. The activation barrier and relative energy for this reaction were also obtained to be 12.4 kcal/mol and 6.2 kcal/mol respectively by using CCSD(T). The transition state rate constant, kTST, and pre-exponential factor for this reaction were calculated to be 2.08  1020 and 7.58  1017 cm3 mol1 s1, respectively. Reaction 4, where the reaction coordinate is the distance between the O2 atom and C5 atom, results in the formation of propylene oxide from the more probable peroxodimer pathway. Here, an activation barrier of 9.3 kcal/mol was found, and the propylene

oxide molecule formed has a relative higher exothermic energy value of 26.2 kcal/mol. The transition state was identified as having one imaginary vibrational frequency of 704 cm1 for the C–O stretching. The corresponding distance for the transition state is 1.966 Å. Spin contamination hS2i values for TS and EG are computed as 0.757 and 0.75, respectively. The TS geometry and the EG of the product are represented in parts a and b of Fig. 4, respectively. The activation barrier and relative energy for this reaction were also calculated to be 10.6 and 25.3 kcal/mol, respectively by using CCSD(T). The transition state rate constant, kTST, and pre-exponential factor for this reaction were computed to be 1.07  1012 and 3.66  109 s1, respectively. The selectivity of this reaction is nearly 100% calculated on the basis of these rate constants. The calculated activation barrier value of 9.3 kcal/mol for propylene oxide formation is in a good agreement with an earlier experimental value (11 kcal/mol) of gas-phase propylene epoxidation reported by Mantashyan et al. [15,16]. The overall energy diagram summarizing Reactions 1–4 is given in Fig. 5. Mulliken

A.C. Kizilkaya et al. / Chemical Physics Letters 487 (2010) 183–189

187

Scheme 2. (a) Mechanism for the combustion reactions of the peroxy radical. (b) Mechanism for the combustion reactions of the hydroperoxide radical. (c) Mechanism for the combustion reactions of the alkoxy radical.

atomic charge and Mulliken atomic spin density values for Reactions 1–4 are reported in Table 1. The coordinate driving calculations are also performed to investigate acrolein formation starting from the common peroxodimer intermediate. The reaction resulting in acrolein formation is given in part b of Scheme 1. The activation barrier for acrolein formation was obtained as 51.7 kcal/mole (59.7 kcal/mol computed by use of CCSD(T)). The relatively high difference between the activation

energies of PO and acrolein formation indicates that PO formation is favored extensively over acrolein formation. Thus, the reason behind the statement in Ref. [17] that almost no selectivity to acrolein formation contrary to high PO selectivity can be explained in the concept obtained in our results of the respective activation energies of the two competing reactions. The transition state rate constant, kTST, and pre-exponential factor for this reaction were calculated to be 2.31  1029 and 1.28  109 s1, respectively.

188

A.C. Kizilkaya et al. / Chemical Physics Letters 487 (2010) 183–189

Table 2 Calculated activation barriers and relative energies in unit of kcal/mol for reactions of propylene oxide and acrolein formations and combustion reactions of peroxy, hydroperoxide and alkoxy radicals for B3LYP and CCSD(T) methods. Reaction

B3LYP method

CCSD(T) method

Activation barrier

Relative energy

Activation barrier

Relative energy

Propylene oxide production

1 2 3 4

0 14.4 9.3 9.3

18.6 3.8 3.4 26.2

0 20.4 12.4 10.6

28.5 4.2 6.2 25.3

Acrolein production

5

51.7

56.2

59.7

55.5

Peroxy

6 7 8 9 10

24.5 28.2 58.2 75.3 23.3

39.4 26.2 38.2 75.3 10.1

38.8 36.9 65.4 54.4 29.7

25.5 31.5 41.2 66.7 13.0

Hydroperoxide

11 12 13 14 15

29.4 32.3 58.2 54.5 29.1

29.2 32.8 58.2 54.5 23.8

37.3 38.1 65.1 61.1 44.0

33.2 35.8 65.1 61.1 32.4

Alkoxy

16 17 18 19 20

19.2 21.6 48.9 45.9 5.4

20.2 21.8 48.9 45.9 42.9

24.5 25.4 65.4 62.5 14.4

28.5 26.4 65.4 60.5 34.6

To investigate the feasibility of the combustion reactions in gas phase, the attack of the oxygen molecule on various sites of the three radical molecules that are produced in the reaction scheme given in part a of Scheme 1 are investigated. The proposed combustion reactions taking place are given in Scheme 2. Relative energies and activation barriers of these reactions are different for each conformer as seen in Table 2. From the activation barriers summarized in Table 2, it can be seen that the combustion reactions of the peroxy radical (Reactions 6–10) and of the hydroperoxide radical (Reactions 11–16) have relatively high activation barriers compared to the reactions that take part in part a of Scheme 1. In fact, among all investigated reactions, abstraction of the terminal hydrogen atom from alkoxy radical (Reaction 20) is the only reaction that has a low barrier of 5.4 kcal/mol. This reaction is also stated to be a low barrier process in the experimental literature [33]. However, from the examination of part a of Scheme 1, it can be seen that peroxy and hydroperoxide radicals are produced before the production of PO, while the alkoxy radical is produced as a side product of Reaction 4 that produces PO. Thus, it can be concluded that the combustion reactions are unlikely to occur from the peroxy and hydroperoxide radicals, which take part in the production process of PO because of their high activation barriers. The combustion products would originate only from the alkoxy radical. Since alkoxy radical is produced as a side product of Reaction 4 (PO formation), the low activation barrier for the combustion of this radical would not affect the high PO selectivity which is calculated as nearly 50%. From this picture, two points could be made. Firstly, it can be inferred that PO production would increase in the gas phase, since the mechanism proposed in part a of Scheme 1 is energetically feasible. Secondly, the amount of combustion products would decrease in the gas phase, since some of the radicals do not undergo combustion reactions but instead take place in the PO production mechanism. Thus, the results of the DFT calculations are in agreement with the experimental result that after the introduction of the post-combustion reactor, the selectivity to COx decreased while the selectivity to PO was increased [17]. Consequently, the perspective which is experimen-

tally proposed in Ref. [17] is supported calculations computed in this study.

by

theoretical

4. Conclusions The gas-phase radical chain reactions starting with the allyl radical, utilizing molecular oxygen as the oxidant and propylene molecule and finally resulting with the formation of propylene oxide are investigated employing theoretical calculations of Density Functional Theory and Coupled Cluster Theory, B3LYP and CCSD(T) methods, respectively. From the overall energy diagram of the Reactions 1–4, it can be concluded that PO formation through radical reactions is plausible as stated by Mimura et al. [17], which is indicated by the relatively low activation barrier (9.3 kcal/mol by B3LYP method and 10.6 by CCSD(T) method) of the PO formation reaction and also the relatively low activation energies of the successive pathways resulting in PO formation. It is found that the calculated activation barrier value for propylene oxide formation in gas phase is in a good agreement with an earlier experimental value (11 kcal/mol) reported by Mantashyan et al. [15,16]. It is also found that the formation of the allylic oxidation product acrolein is not favored in gas phase due to its much higher activation barrier than PO formation, which explains why acrolein is not produced at all with the system proposed in [17]. In addition, the high activation barriers obtained for combustion reactions of radicals that take part in the PO production mechanism in gas phase point out that the amount of radicals that undergo combustion reactions decrease in the gas phase and help explain why the selectivity to COx decreased after the post-catalytic empty volume was used [17]. Acknowledgments We would like to thank the Scientific and Technical Research Council of Turkey (TUBITAK Project No.108T378) for financial support. References [1] T.A. Nijhuis, M. Makkee, J.A. Moulijn, B.M. Weckhuysen, Ind. Eng. Chem. Res. 45 (2006) 3447. [2] M. Akimoto, K. Ichikawa, E.J. Echigoya, Catalysis 76 (1982) 333. [3] X.E. Verykios, F.P. Stein, R.W. Coughlin, Catal. Rev. Sci. Eng. 22 (1980) 197. [4] W.M. Sachtler, H.C. Backx, R.A. van Santen, Catal. Rev. Sci. Eng. 23 (1981) 127. [5] R.A. van Santen, H.P.C.E. Kuipers, Adv. Catal. 35 (1987) 265. [6] D. Torres, N. Lopez, F. Illas, R.M. Lambert, Angew. Chem. Int. Ed. 46 (2007) 2055. [7] H. Kobayashi, Y. Shimodaira, J. Mol. Struc.-Theochem. 762 (2006) 57. [8] T. Hayashi, K. Tanaka, M. Haruta, J. Catal. 178 (1998) 566. [9] J.J. Cowell, A.K. Santra, R.M. Lambert, J. Am. Chem. Soc. 122 (2000) 2381. [10] J. Lu, M. Luo, H. Lei, X. Bao, C. Li, J. Catal. 211 (2002) 552. [11] O.P.H. Vaughan, G. Kyriakou, N. Macleod, M. Tikhov, R.M. Lambert, J. Catal. 236 (2005) 401. [12] W. Zhu, Q. Zhang, Y. Wang, J. Phys. Chem. C 112 (2008) 7731. [13] S. Yang, W. Zhu, Q. Zhang, Y. Wang, J. Catal. 254 (2008) 251. [14] J. Llorca et al., J. Catal. 258 (2008) 187. [15] A.A. Mantashyan, S.D. Arsentiev, R.R. Grigoryan, React. Kinet. Catal. Lett. 21 (1982) 347. [16] A.A. Mantashyan, S.D. Arsentiev, R.R. Grigoryan, Khim. Fiz. 4 (1985) 75. [17] N. Mimura, S. Tsubota, K. Murata, K.K. Bando, J.J. Bravo-Suarez, M. Haruta, S.T. Oyama, Catal. Lett. 110 (2006) 47. [18] Z. Song, N. Mimura, S. Tsubota, T. Fujitani, S.T. Oyama, Catal. Lett. 121 (2008) 33. [19] W. Kohn, L. Sham, J. Phys. Rev. 140 (1965) A1133. [20] M.J. Frisch et al., GAUSSIAN 03, Revision D.01, Gaussian, Inc., Wallingford, CT, 2004. [21] A.D. Becke, Phys. Rev. A 38 (1988) 3098. [22] A.D. Becke, M.R. Roussel, Phys. Rev. A 39 (1989) 3761. [23] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [24] J. Baker, M. Muir, J. Andzelm, A. Scheiner, ACS Symp. Ser. 629 (1996) 342. [25] J.A. Pople, M. Head-Gordon, K. Raghavachari, J. Chem. Phys. 87 (1987) 5968. [26] M.C.L. Scaldaferri, A.S. Pimentel, Chem. Phys. Lett. 470 (2009) 203. [27] D.J. Henry, C.J. Parkinson, L. Radom, J. Phys. Chem. A 106 (2002) 7927. [28] Y.Y. Chuang, E.L. Coitin, D.G. Truhlar, J. Phys. Chem. A 104 (2000) 104. [29] A.B. Ryzhkov, P.A. Ariya, Phys. Chem. Chem. Phys. 6 (2004) 5042.

A.C. Kizilkaya et al. / Chemical Physics Letters 487 (2010) 183–189 [30] D.C. Young, Computational Chemistry, John Wiley & Sons, Inc., 2001 (Chapter 27). [31] M.F. Fellah, R.A. van Santen, I. Onal. J. Phys. Chem. C 113 (2009) 15307.

[32] C. Peng, H.B. Schlegel, Israel J. Chem. 33 (1993) 449. [33] J.J. Orlando, G.S. Tyndall, T.J. Wallington, Chem. Rev. 103 (2003) 4657.

189