Theoretical study of the carbonyl-ene reaction between formaldehyde and propylene on the MgY zeolite

Computational and Theoretical Chemistry 982 (2012) 51–57

Contents lists available at SciVerse ScienceDirect

Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc

Theoretical study of the carbonyl-ene reaction between formaldehyde and propylene on the MgY zeolite Hui Fu a,b,⇑, Shouwen Xie b, Aiping Fu c, Tianxu Ye b a

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Shandong, Qingdao 266555, People’s Republic of China College of Science, China University of Petroleum, Shandong, Qingdao 266555, People’s Republic of China c Institute for Computational Science & Engineering, Qingdao University, Shandong, Qingdao 266071, People’s Republic of China b

a r t i c l e

i n f o

Article history: Received 26 September 2011 Received in revised form 8 December 2011 Accepted 9 December 2011 Available online 23 December 2011 Keywords: Formaldehyde Carbonyl-ene reaction MgY zeolite ONIOM2

a b s t r a c t Alkaline earth-metal cation-exchanged faujasite zeolites have received widely attention, especially in catalyzing the reaction of oxygen-containing organic compounds. As a common one-carbon electrophile in the organic synthesis, preservation and reaction of formaldehyde were limited by its low boiling point and polymerization tendency. In this paper, a classical carbonyl-ene reaction between formaldehyde and propylene was examined on the MgY zeolite catalyzed system and the bare system by ONIOM2 (B3LYP/631G(d,p):UFF) method and density-functional (B3LYP/6-31G(d,p)) calculation, respectively. It was found that reaction energy barrier on the MgY zeolite was 15.4 kcal/mol, lower than the bare system (26.1 kcal/ mol). And the adsorption energy of formaldehyde on the MgY zeolite have reached 27.5 kcal/mol which illustrate that formaldehyde can be successfully preserved by MgY zeolite in a monomer at the ambient temperature. In addition, some thermodynamic constants (DH and DG) and rate constants (k) were also analyzed. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction In organic synthesis, the carbonyl-ene reaction is not only an efficient method for CAC bond formation but also a valuable route for homoallylic alcohols, which mainly occurs between an alkene with an allylic hydrogen (the ene) and a carbonyl compound (the enophile) [1–4]. Since the carbonyl-ene reaction involves the breaking of a CAH r bond, it usually has a much higher activation energy than the analogous Diels–Alder reaction, and so it is often carried out with Lewis acids as promoters [5,6]. The product of 3-buten-1-ol from formaldehyde and propylene is an example of a carbonyl-ene reaction. 3-Buten-1-ol is acquired as a monomer in polymerization reactions and as an intermediate for tetrahydrofuran (THF) synthesis. It is well known that formaldehyde is a common one-carbon electrophile in the orgnic synthesis [7–10]. However, its synthetic utility is often limited by its disadvantage of the low boiling point (19.5 °C) and the tendency to rapidly polymerize to solid paraformaldehyde and trioxane. In 1990, Yamamoto et al. [9] synthesized a stable formaldehydebulky organoaluminum complex which enabled the carbonyl-ene reaction with olefins. But this complex was not stable for a long time at 0 °C. Thus, it was recognized that formaldehyde is difficult ⇑ Corresponding author at: College of Science, China University of Petroleum, Shandong, Qingdao 266555, People’s Republic of China. E-mail address: [email protected] (H. Fu). 2210-271X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.comptc.2011.12.010

to preserve in a pure monomer form under ambient conditions. Recently, Okachi and Onaka [10] reported a successful storage of formaldehyde using NaY zeolite which not only suppress self-polymerization and decomposition of formaldehyde but also effectively catalyze the reaction of formaldehyde with various olefins. As a result, the carbonyl-ene reaction between formaldehyde and olefins has progressively moved toward zeolite-based processes. Nowadays, zeolite catalysts, as environmentally friendly solid acid catalysts, have been extensively applied in the chemical and petroleum industries [11–13]. The extra-framework cations present in zeolites play a significant role in determining their adsorption and catalytic properties. Framework oxygen atoms are known to exhibit basic properties [14]. The cations serve as Lewis acids and create strong electric fields [15]. Due to the specific balance of acidic and basic function, alkali-metal and alkaline earthmetal cation-exchanged zeolites have been found to be potential catalysts in many reactions [16–20]. In early stage, alkaline-earth exchanged zeolites, in particular BaY, have been reported as effective constrained media for photoassisted selective oxidation of hydrocarbons with O2 under irradiation with visible light [16]. Recently, Jasra et al. have reported solvent free aldol condensation of propanal to 2-methylpentenal with alkali ion-exchanged zeolites [17]. Furthermore, Pirnguber et al. studied the role of the extra-framework cations (Li, Na, K, Cs) in the adsorption of CO2 on faujasite Y [20]. Particularly, Tsukamoto et al. [19] investigated the effects of metal cations (alkali and alkaline earth cation) on the

52

H. Fu et al. / Computational and Theoretical Chemistry 982 (2012) 51–57

photochemical methyl ketone production and found that Mg2+exchanged zeolite Y could enhance the production of methyl ketone, with maintaining high selectivity. The same as ketone, formaldehyde is also the carbonyl compound. Therefore, we speculate if the MgY zeolite can preserve formaldehyde and catalyze carbonyl-ene reaction. Theoretical calculations based on quantum chemistry are a useful tool to analyze the reaction mechanism in deep detail. Consequently, several theoretical approaches have been proposed to study the reaction mechanism on zeolite catalysts, such as the bare cluster model, the periodic density functional theory, and the combined quantum mechanics/molecular mechanics (QM/MM). However, interactions between guest molecules and the extended zeolite framework are neglected when utilizing small clusters. Periodic calculations are too computationally expensive for zeolites that have large unit cells. While, the QM/MM approach employs minimal computational demand and is practical for calculating a large system. Recently, ONIOM methods have been widely used to investigate adsorption and reaction mechanism on zeolites [21–28]. The calculation results yielded adsorption energies close to the experimental estimates and could elucidate the reaction mechanisms. Thus, in this work, the ONIOM method was applied to study carbonyl-ene reaction on the MgY zeolite. Since Okachi and Onaka [10] discovered that NaY zeolite is able to store formaldehyde and catalyze the reaction of formaldehyde with various olefins. Subsequently, the group of Limtrakul has studied the carbonyl-ene reaction between formaldehyde and propylene in Na–FAU zeolite and MOF-11 by ONIOM method [21,22]. They verified that Na–FAU can preserve formaldehyde and catalyze the carbonyl-ene reaction. Their study also predicted that the MOF-11 could be used as an effective catalyst in the carbonylene reaction of formaldehyde. Then, in addition to Na–FAU zeolite and MOF-11, are there any other catalysts can preserve formaldehyde and activate the corresponding carbonyl-ene reaction and is it better than Na–FAU zeolite or MOF-11? In this work, we study the mechanism of the carbonyl-ene reaction between MgAY encapsulated formaldehyde and propylene. As far as know, there is no experimental data for this reaction. But we hope the elucidation of the reaction mechanism help to optimize the reaction conditions and design catalysts for industrial production.

2. Calculation details Faujasite zeolite exchanged with Mg2+ has been studied by many experiments [29–31]. The experimental results have shown that the majority of the alkaline earth metal cations are located at site II, which are in the center of a single six ring (S6R) and are considered to be the active sites of faujasite zeolite. According to the study of Snurr et al. [25,32], the BaY cluster is most stable when two Al atoms are in the single six ring (S6R) and (1,4) Al arrangement. The different Al arrangements were calculated in our work, and the calculated results also show that Mg2+ is most stabilized by the cluster with (1,4) Al arrangement. Therefore, the (1,4) Al arrangement was applied in this work. An ONIOM2 method was used to study the MgY zeolite catalyzed system. In this work, the computation cluster was modeled by 96T including the supercage covering the 12-membered ring channel taken from the lattice structure of Y zeolite [33]. To increase the computation efficiency and reduce computational expenses, the 96T cluster was divided into two layers which were treated using different basic sets. The inner layer was the active region, containing 12T, and was treated more accurately with the B3LYP method and the 6-31G(d,p) basis set, which has been applied by Fellah [34], while the outer layer was treated with the universal force filed (UFF) [35]. The dangling bonds in this model were

saturated by hydrogen atoms. All the SiAH bond lengths were fixed at 1.49 Å. The density-functional calculation with the same method (B3LYP) and basis set (6-31G(d,p)) was applied to the bare system. Transition state (TS) was characterized by vibration analysis, which was used to check whether the geometry of the TS has only one imaginary frequency. From the transition state, the reaction path was traced by the IRC to ensure the correctness of the transition state. For bare and MgY-catalyzed systems, additional single-point energy calculations with the 6-311++G(d,p) basis set have been carried out, and the basis set superposition error (BSSE) was estimated by the counterpoise (CP) method. Such calculated results are listed as Supporting information. The charge distribution in the complexes has been analyzed via the natural population analysis (NPA) [36–40] partitioning scheme using the B3LYP/631G(d,p) densities. The Gibbs free energies, enthalpies and energies in this system were also calculated and analyzed. In addition, standard enthalpies, Gibbs free energies changes of all structures in the rate determining step were calculated. All of these data were used to predict the rate for the rate determining step of the reaction by using simple transition state theory (TST) [41]. The rate constant is expressed as:

kðTÞ ¼

  kB T DG– exp  h RT

ð1Þ

where DG–, kB, h, T and R denote the Gibbs free energy of activation, Boltzmann’s constant, Plank’s constant, the temperature and the universal gas constant, respectively. All calculations were performed using the Gaussian 03 program [42]. 3. Results and discussion 3.1. Structures and energies of the MgY zeolite and adsorption of the formaldehyde As shown in Fig. 1, the calculated structure for binding of Mg shows that Mg2+ is located almost in the plane of the single six ring (S6R) with an symmetrical position which is closer to the center of the hexagon (Fig. 1b). It can be seen there are four types of oxygen atoms in faujasite zeolite, e.g. O(1), O(2), O(3), and O(4), respectively [25]. It can be seen from Table 1, the mean distance of the Mg atom from the three O(2) in the S6R is 2.065 Å which is in agreement with the experimental value of 2.184 Å [29], while the mean distance of the Mg atom from the three O(4) in the S6R is 3.027 Å. The angle of O(2)AMgAO(2) is 119.9° which agrees with the value obtained by experimental study (115.2°) [29] and the calculated angle of O(2)AMgAO(4) is 67.3°. The optimized geometrical parameters are shown in the first and second column of Table 1 which describe the interaction between MgY zeolite and formaldehyde. When formaldehyde adsorbs on the zeolite (Figs. 1b and 2a), the Mg ion slightly moves away from the plane of the hexagonal ring toward the formaldehyde, which is reflected in the following section: The bond lengths of MgAO(2) and MgAO(4) increase (from 2.065 to 2.105 Å and from 3.027 to 3.042 Å, respectively) due to the interaction between the adsorbate and zeolite, while the angles of the O(2)AMgAO(2) and O(2)AMgAO(4) decrease (from 119.9 to 117.6° and from 67.3 to 67.0°). For formaldehyde, the bond length of carbonyl group is also slightly larger (1.223 Å) than the value of the isolated formaldehyde (1.207 Å) due to the interaction between the lone electron pair of carbonyl oxygen in formaldehyde and the Mg2+. And the strength of this interaction is mainly described by this bond. After adsorption, the distance between Mg2+ and oxygen of the carbonyl is 2.079 Å and the Mg2+  O@C adsorption complex is nonlinear, with the angle being 132.3°. Another interaction also exists

53

H. Fu et al. / Computational and Theoretical Chemistry 982 (2012) 51–57

Fig. 1. Top and side view of MgY zeolite cluster including a supercage of faujasite zeolite showing a S6R, where the Mg2+ ion is located. The location of two Al atoms are denoted as (1,4). O (red), Al (pink), Mg (yellow) and Si (gray). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 1 Geometric parameters of reactants, transition state, and product of the carbonyl-ene reaction between formaldehyde and propylene on MgY zeolite using the DFT (B3LYP/631G(d,p)) and ONIOM2 (B3LYP/6-31G(d,p):UFF) method: the distances (Å) and the angels (°). Isolated molecule RCAC1 RC1AC2 RC2AC3 RC3AH3 RH3AO RHAO(1) RHAO(2) RHAO(4) RCAO RMgAO RMgAO(2) RMgAO(4) AO(2)AMgAO(2) AO(2)AMgAO(4)

Formaldehyde adsorption

Co-adsorption complex

3.583 3.456 2.447 1.223 2.079 2.105 3.042 117.6 67.0

3.278 1.338 1.501 1.097 4.307 3.517 3.446 2.584 1.226 2.066 2.111 3.045 107.2 66.9

1.333 1.502

1.207 2.065 3.027 119.9 67.3

between the hydrogen atom of formaldehyde and oxygen atoms on the frame of zeolite. As shown in Table 1, the distances between H of formaldehyde and oxygen on the frame of zeolite (O(1), O(2) and O(4)) are 3.583, 3.456 and 2.447 Å, respectively, illustrating that hydrogen bond making the adsorption complex more stable. The calculated frequencies show that the mC@O stretching vibration of formaldehyde adsorbed on the MgY zeolite still exists and just shifts to lower wave-number (from 1845 to 1778 cm1). And the new vibration frequency was not found in this process. It illustrates that this process is physical adsorption, not chemical adsorption or reaction. In addition to geometrical parameters and the frequency calculation, the adsorption energy and adsorption Gibbs free energy were calculated at 298.15 K in this step. The adsorption energy is expressed as:

Eads ¼ EMZ  EZ  EM

ð2Þ

where the subscript ‘‘Z’’ and ‘‘M’’ denote the zeolite cluster and formaldehyde. The Eads, EM–Z, EZ and EM represent the adsorption energy, energy of zeolite cluster interacted with formaldehyde, energy of bare zeolite cluster and energy of isolated formaldehyde, respectively. And the adsorption Gibbs free energy is expressed as:

DG ¼ GZM  GZ  GM

ð3Þ

where the DG, GZ–M, GZ and GM denote adsorption Gibbs free energy, Gibbs free energy of the zeolite interacted with formaldehyde,

Transition state 1.705 1.428 1.413 1.211 1.590

1.345 1.965 2.138 3.073 115.4 66.3

Product 1.530 1.507 1.336 2.784 0.974 3.443 3.178 2.742 1.459 2.037 2.117 3.045 116.9 66.7

Gibbs free energy of bare zeolite and Gibbs free energy of isolated formaldehyde, respectively. The calculated results show that the formaldehyde adsorption on the MgY zeolite is an exothermic process with adsorption energy of 27.5 kcal/mol. The value is greater than it on the Na–FAU zeolite (18.4 kcal/mol) [21] and MOF-11 (12.3 kcal/mol) [22]. It illustrates that this adsorption process is easier and stronger than on the Na–FAU zeolite and the MOF-11. The adsorption Gibbs free energy of formaldehyde on the zeolite is 20.6 kcal/mol at 298.15 K which means it is a spontaneous process. These results indicate the MgY catalyst can be a good storage of formaldehyde in a monomeric form under ambient conditions as we initially envisioned. 3.2. Carbonyl-ene reaction between formaldehyde adsorbed on the MgY zeolite and propylene As for carbonyl-ene reaction, Limtrakul et al. have suggested the following steps [21]: Firstly, isolated formaldehyde adsorbs on the zeolite; then the adsorbed formaldehyde reacts with propylene producing the 3-buten-1-ol; finally, 3-buten-1-ol desorbs from the zeolite. According to the reaction mechanism, formaldehyde adsorbs over the active site of MgY via lone pair electron interaction with 27.5 kal/mol in step 1 (Figs. 2a and 3), which is detailedly described in the first part.

54

H. Fu et al. / Computational and Theoretical Chemistry 982 (2012) 51–57

Fig. 2. Structures of intermediate, transition state, and product of the carbonyl-ene reaction: (a) formaldehyde adsorbed on the zeolite; (b) co-adsorption complex; (c) transition state; (d) product in this reaction.

24.2

Relative energy/kcal/mol

10 0

Table 2 Energies, enthalpies and Gibbs free energies (in kcal/mol) for the carbonyl-ene reaction in bare system and MgY zeolite catalyzed system at 298.15 K.

c

20

26.1 0.0 (1)

TS TS-cat

-1.9 -14.6

-20

-40

(2)

-23.3

-27.5 a and propylene

c -38.7

15.4

b d

-50

DH

DG

26.1 15.4

24.08 17.49

30.16 24.47

b

-10

-30

DE

-53.0

Fig. 3. Calculated energy profile of the carbonyl-ene reaction on the MgY zeolite system (black line) and bare system (red line). (1): formaldehyde, propylene and MgY zeolite; a: formaldehyde adsorbed on the zeolite; b: co-adsorption complex; c transition state; d: product in this reaction on the MgY system; (2): 3-buten-1-ol and zeolite. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Then, step 2 can be divided into two parts. Firstly, the earlier adsorbed formaldehyde interacts with diffusing propylene via a p electron (interaction between carbonyl and carbon–carbon double bond in the propylene) forming the co-adsorption complex (Figs.

2b and 3) with the energy being 11.2 kcal/mol lower than formaldehyde adsorption. The formation of co-adsorption complex makes some geometric parameters change. The double bond length of C1@C2 changes from 1.333 to 1.338 Å and the C@O length changes from 1.223 to 1.226 Å. In Table 1, it is shown that the weak hydrogen bonds exist between the complex and the frame of zeolite which can be illustrated by the bond lengths between hydrogen in the complex and oxygen in the frame of zeolite (the HAO(1) is 3.517 Å, the HAO(2) is 3.446 Å and the HAO(4) is 2.584 Å). While the mC@O stretching vibration of formaldehyde and mC@C stretching vibration of propylene are also clearly observed (1764 cm1 and 1714 cm1, respectively), which shift to lower wave-number (from 1778 to 1764 cm1and from 1734 to 1714 cm1) compared with the adsorbed formaldehyde and isolated propylene. However, new vibration mode is not observed in this process. These results illustrate that propylene and formaldehyde does not react, just with the formation of co-adsorption complex and without any new bond formation in this part. Secondly, the co-adsorption complex converts into 3-buten-1ol  MgY zeolite (Fig. 2d) via six-membered ring transition state

55

H. Fu et al. / Computational and Theoretical Chemistry 982 (2012) 51–57 Table 3 Atomic charge distribution calculated from the natural population analysis (NPA) for bare system, MgY zeolite and Na–FAU zeolite [21] catalyzed reactions. Isolated molecule

Bare system Co-ads

a

q(M) q(O) q(C) q(C1) q(C2) q(C3) q(H) a b

1.79 0.49 0.22 0.45 0.21 0.71 0.24

0.51 0.22 0.46 0.21 0.72 0.26

b

MgY zeolite system TS 0.66 0.02 0.53 0.09 0.73 0.38

Product 0.76 0.10 0.53 0.23 0.45 0.45

Ads-HCHO

Co-ads

1.77 0.65 0.31

1.77 0.67 0.31 0.47 0.22 0.72 0.26

Na–FAU zeolite system b

TS

Product

Ads-HCHO

Co-adsb

TS

Product

1.77 0.91 0.04 0.55 0.09 0.75 0.36

1.77 0.89 0.10 0.53 0.27 0.41 0.54

0.91 0.61 0.26

0.91 0.61 0.26 0.47 0.22 0.72 0.24

0.91 0.79 0.02 0.55 0.01 0.74 0.36

0.92 0.84 0.10 0.54 0.25 0.44 0.51

Metal atoms (Mg or Na). Co-adsorption complex of formaldehyde and propylene.

(Fig. 2c) which involves two new bond forming, one old bond breaking and three bond type converting. The propylene could give one proton (H3) to the carbonyl oxygen (O) of formaldehyde corresponding to the C3AH3 bond breaking and OAH3 bond forming while carbonyl C@O bond and C1@C2 bond changing to single bond and C2AC3 bond changing to double bond. At the same time, new CAC1 bond forms between formaldehyde and propylene. This transformation process needs activation energy about 15.4 kcal/ mol. And the calculated transition state confirms the proposed concerted pathway (Fig. 2c). The only imaginary frequency belongs to the vibration mode in which the CAC1 bond stretching and H3 rocking between C3 and O. In this part, the Mg ion is also further slightly moved away from the plane of the hexagonal ring in the zeolite toward the co-adsorption complex and the distance between Mg atom and O atom is shortened from 2.066 to 1.965 Å. Specifically, from b to c, the geometric parameters of the intermediate change as following: The C3AH3 bond length of the propylene is stretched from 1.097 to 1.211 Å, and the distance between the propylene proton (H3) and the formaldehyde oxygen (O) is shortened from 4.307 to 1.590 Å. The C@O double bond of formaldehyde is elongated from 1.226 to 1.345 Å as the distance between the propylene carbon (C1) and formaldehyde carbon (C) is contracted from 3.278 to 1.705 Å. And the C1AC2 and C2AC3 bond lengths are changed from 1.338 to 1.428 Å and from 1.501 to 1.413 Å, respectively. According to our calculation, the mC@O stretching vibration of formaldehyde is not found in the structure d. But the obvious new stretching vibration in d is observed at 3703 cm1 of the mOAH. It illustrates the formation of OAH bond and new substance in the second part of step 2. This new substance is alcohol which is consistent with the predicted product. From Table 1, it is shown that the following geometric parameters from b to d via transition state c are changed: The CAO and C1AC2 bonds elongated from 1.226 to 1.459 Å and from 1.338 to 1.507 Å with the bond converting from double bond to single bond. The length of CAC1 bond is changed from 3.278 to 1.530 Å with the single bond formation of CAC1. The bond length of C2AC3 is shortened from 1.501 Å (single bond) to 1.336 Å (double bond) in this process. The distance between C3 and H3 elongated from 1.097 Å to 2.784 Å. At the same time, the distance between H3 and O decreased from 4.307 Å to 0.974 Å. The product d is 3-buten-1-ol adsorbed on the MgY zeolite (Fig. 2d) and the bond length of MgAO (O atom of 3-buten-1-ol) is 2.037 Å (Table 1). The weak hydrogen bond exists between 3-buten-1-ol and the frame of zeolite which mainly reflect the interaction between hydrogen of 3-buten-1-ol and oxygen atom of zeolite: The bond lengths of HAO(1), HAO(2) and HAO(4) are 3.443, 3.178 and 2.742 Å, respectively. The energy diagram for the calculated reaction path is given in Fig. 3. In order to compare the reaction with the catalyzed system, we also calculated the reaction of the bare system. While in bare system, propylene and formaldehyde firstly formed a more stable

complex with energy loss about 1.9 kcal/mol (Fig. 3). Then, with activation energy of 26.1 kcal/mol, 3-buten-1-ol formed. The whole reaction is exothermic with 14.6 kcal/mol. To be noted, the calculated activation energy (26.1 kcal/mol) and the reaction energy (14.6 kcal/mol) are very close to the experimental data (26.4 kcal/mol and 13.5 kcal/mol) [43]. From Fig. 3, we observe that the reaction on MgY zeolite is exothermic with 53 kcal/mol, while the same reaction is exothermic by 39.7 kcal/mol on Na–FAU [21] and 28.1 kcal/mol on the MOF11 [22]. For the carbonyl-ene reaction, it is initiated by co-adsorption of propylene and the adsorbed formaldehyde at the active site of the MgY zeolite. The co-adsorption energy is 38.7 kcal/mol, which is lower than that on the Na–FAU and MOF-11 (28.3 and 19.0 kcal/mol) [21,22]. But the lower co-adsorption energy is mainly due to the formaldehyde adsorption. It can be found that the energy difference between a added propylene and b is 11.2 kcal/mol on MgY which is close to the value of 9.9 kcal/mol on Na–FAU. That is to say, the propylene adsorption changes little on different catalysts in this reaction. For MgY catalyzed system, it maybe imply that such stable co-adsorption complex could not react easily in the next step. However, from Fig. 3, it can be seen that the required energy barrier is only 15.4 kcal/mol to generate the 3buten-1-ol from the co-adsorption complex, lower than the energy barrier of the bare system (26.1 kcal/mol) (Fig. 3), the Na–FAU system (25.1 kcal/mol) [21] and the MOF-11 system (24.1 kcal/mol) [22]. This is due to the electrostatic field generated by the Mg2+ and the stabilizing role of the weak hydrogen bond between 3-buten-1-ol and the oxygen atom in the zeolite frame surrounding the cation (Table 1). In this reaction system, the adsorbed 3-buten-1-ol product would endothermically desorb from the active acid site of the MgY zeolite in step 3, which requires 38.4 kcal/mol in this process. For such high desorption energy, reduction of the catalyst load might be prevented by the 3-buten-1-ol over the active site of MgAY zeolite. Nevertheless, the whole reaction is exothermic although the reaction temperature is not high-energy compensation may counterbalance such high energy consumption. To be noted, the additional calculations of single-point energy calculation with large basis set (Supporting information) are consistent with the above discussion. We also examined the atomic charges of the molecules involved in the reaction by means of the NPA method, as shown in Table 3. For comparison, the atomic charge of Na–FAU [21] system also listed in Table 3. It can be seen, Mg in the plane of the single six ring (S6R) on the Y zeolite bears a positive charge of 1.79, which compensated by the surrounding oxygen atoms in the S6R. As formaldehyde adsorbs over the Mg, it becomes slightly more negative (1.77) because its partial positive charge is dispersed by the carbonyl of formaldehyde. At the same time, the interaction between Mg and formaldehyde make electron cloud of the carbonyl deviate from C atom. Thus the carbon atom becomes more positive (0.22 ? 0.31) and the oxygen atom of formaldehyde to become

56

H. Fu et al. / Computational and Theoretical Chemistry 982 (2012) 51–57

more negative (0.49 ? 0.65). The concentration of negative charge over O can stabilize the adsorption of formaldehyde. The same trend could be seen from Na–FAU system in Table 3. As we know, the adsorption of formaldehyde over MgY zeolite (27.5 kcal/mol) is stronger than it on Na–FAU zeolite (18.4 kcal/ mol). Here, it could be explained by the charge difference of O atom. The charge of O atom in adsorbed formaldehyde on MgY zeolite is 0.65 while it is 0.61 in Na–FAU zeolite. The more negative charge of O atom leads to the greater adsorption energy on MgY than on Na–FAU. The subsequent co-adsorption of propylene does not alter the charge significantly. At the transition-state structure, the charge over O atom (0.91) becomes more negative than the charge over C3 (0.75). The increased negative charge of O facilitates the proton transfer. Compared the TS of MgY catalyzed system and Na–FAU catalyzed system, the former has the more negative charge of O atom (0.91) with 0.12 difference than the latter (0.79). This could explain why the active energy on MgY (15.4 kcal/mol) is much lower than on Na–FAU (25.1 kcal/mol). We could also understand the poor activity of the bare system with the charge of O atom in TS being 0.66. As for product, in like manner, the negative charge of O atom (0.89) on MgY is greater than it on Na–FAU (0.84). The increased negative charge over oxygen enhances the interaction between 3-buten-1-ol and zeolite. Therefore, the desorption of product on MgY becomes difficult compared with it on Na–FAU. In Table 2, the enthalpies, Gibbs free energies and energies relative to the transition state are given for two different systems, e.g. the bare system and the MgY-catalyzed system, respectively. As shown in Table 2, compared with the bare system, DH and DG both reduce about 7 kcal/mol on MgY zeolite, while DE reduces about 10 kcal/mol. In the same system, the differences between DH and DE are no more than 2 kcal/mol. However, the difference between DG and DE is more than 4 kcal/mol, higher than the above. This difference is mainly accounted for by differences in translational and rotational entropy [44]. To obtain the reaction rate constant (k) at 298.15 K, the Gibbs free energies of the coadsorption complex of formaldehyde with propylene and the transition state are discussed in this reaction. And these results are used as input for the TST equation given in the calculation details. The calculated results show that the rate constant is 7.23  106 at 298.15 K in the catalyzed system which is greater than the bare system (4.86  1010). It illustrates that the MgY zeolite catalyst can accelerate this reaction. In order to better examine the effect of this catalyst for this reaction and the impact of temperature on the catalyst, the rate constants and equilibrium constants were calculated at 300– 650 K. The calculated results are listed in the Table 4. Over the temperature range from 300 to 650 K, the forward reaction for 3butl-1-ol is faster than the reverse reaction at 300–650 K, while the rate constants for both forward and reverse reaction increase with temperature being higher. At the same time, the equilibrium

constants decrease. Nevertheless, the MgY zeolite can still well catalyze this reaction at 650 K. No related experiment on this reaction over the MgY zeolite has been reported and so we provide only a qualitative discussion of the results. It should be noted that when the temperature is higher than 450 K, the equilibrium constants almost remain unchanged, although the rate constants is still increasing. Therefore, in order to balance the temperature and the catalytic properties, the reaction temperature is preferable no more than 450 K in the experiment. The above discussion indicates that MgY zeolite can be used as a new catalyst in carbonyl-ene reaction and that they stabilize all species in the carbonyl-ene reaction systems. MgY especially can successfully preserve formaldehyde in a monomeric form at ambient temperature and can also activate it sufficiently to promote its reaction with various olefins.

Table 4 Equilibrium constants (Keq) and reaction rate constants (k) of the carbonyl-ene reaction of formaldehyde with propylene on MgY zeolite for the temperatures range 300–650 K.

References

T (K)

300 350 400 450 500 550 600 650

k (s1)

Keq

Forward rate

Reverse rate

9.38  106 3.85  103 3.57  101 1.23  101 2.10  102 2.17  103 1.54  104 8.04  104

2.05  1012 7.52  109 3.61  106 4.78  104 2.12  102 5.06  101 7.15 6.78  101

4.57  106 5.12  105 9.89  104 2.57  104 9.90  103 4.29  103 2.14  103 1.19  103

4. Conclusions Density-functional theory (B3LYP/6-31G(d,p)) and the ONIOM2 method (B3LYP/6-31G(d,p):UFF) are used to investigate the carbonyl-ene reaction between formaldehyde and propylene on the MgY zeolite. And two systems are studied: the bare system and the system of MgY zeolite as catalyst. Mg2+ as the active acid site of the zeolite can catalyze the carbonyl-ene reaction. The frame of the MgY zeolite can stabilize the co-adsorption complex between formaldehyde and propylene and reduce the reaction barrier compared with the bare system. The reaction energy barrier on the MgY zeolite is 15.4 kcal/mol, which is lower than the bare system (26.1 kcal/mol) and the Na–FAU [21] catalyzed system (25.1 kcal/mol). And the adsorption energy of formaldehyde on the MgY zeolite is 27.5 kcal/mol. The calculated rate constants in the two systems illustrate that the MgY zeolite as catalyst can accelerate this reaction. All results indicate that MgY zeolite can successfully preserve formaldehyde in a monomeric form under the ambient conditions and catalyze the carbonyl-ene reaction as new catalyst. Acknowledgements This work is supported by the Fundamental Research Funds for the Central Universities (10CX04020A), the National important Scientific Foundation (2008ZX05026-004-05), the National Natural Science Foundation of China (No. 21103096) and the Natural Science Foundation of Shandong Province (ZR2010BM024). We also thank the Project of Shandong Province Higher Educational Science and Technology Program, China (No. J10LB06). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.comptc.2011.12.010.

[1] B.B. Snider, Lewis-acid-catalyzed ene reactions, Acc. Chem. Res. 13 (1980) 426. [2] K. Mikami, M. Shimizu, Asymmetric ene reactions in organic synthesis, Chem. Rev. 92 (1992) 1021. [3] B.B. Snider, The prins and carbonyl ene reactions, Comp. Org. Syn. 2 (1991) 527. [4] M.L. Clarke, M.B. France, The carbonyl-ene reaction, Tetrahedron 64 (2008) 9003. [5] Y. Yamamoto, Acyclic stereocontrol via allylic organometallic compounds, Acc. Chem. Res. 20 (1987) 243. [6] U. Pinaur, G. Luts, C. Otto, Acceleration and selectivity enhancement of Diels– Alder reactions by special and catalytic methods, Chem. Rev. 93 (1993) 741. [7] T. Okachi, K. Fujimoto, M. Onaka, Practical carbonyl-ene reactions of amethylstyrenes with paraformaldehyde promoted by a combined system of boron trifluoride and molecular sieves 4A, Org. Lett. 4 (2002) 1667. [8] K. Maruoka, B.C. Arne1, N. Murase, M. Oishi, N. Hirayama, H. Yamamoto, Stabilization of reactive aldehydes by complexation with methylaluminum

H. Fu et al. / Computational and Theoretical Chemistry 982 (2012) 51–57

[9]

[10]

[11] [12] [13]

[14]

[15]

[16] [17]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

bis(2,6-diphenylphenoxide) and their synthetic application, J. Am. Chem. Soc. 115 (1993) 3943. K. Maruoka, B.C. Arne1, N. Hirayama, H. Yamamoto, Generation of a stable formaldehyde-organoaluminum complex and its synthetic utility, J. Am. Chem. Soc. 112 (1990) 7422. T. Okachi, M. Onaka, Formaldehyde encapsulated in zeolite: a long-lived, highly activated one-carbon electrophile to carbonyl-ene reactions, J. Am. Chem. Soc. 126 (2004) 2306. A. Corma, Inorganic solid acids and their use in acid-catalyzed hydrocarbon reactions, Chem. Rev. 95 (1995) 559. W.E. Farneth, R.J. Gorte, Methods for characterizing zeolite acidity, Chem. Rev. 95 (1995) 615. D.G. Fleming, D.J. Arseneau, M.Y. Shelley, B. Beck, H. Dilger, E. Roduner, lSR studies of hyperfine couplings and molecular interactions of the Mucyclohexadienyl radical in Y-zeolites and in solid bulk benzene, J. Phys. Chem. C 115 (2011) 11177. J. Xu, B.L. Mojet, J.G. van Ommen, L. Leffers, Propane selective oxidation on alkaline earth exchanged zeolite Y: room temperature in situ IR study, Phys. Chem. Chem. Phys. 5 (2003) 4407. Y.H. Yeom, B. Wen, W.M.H. Sachtler, NOx reduction from diesel emissions over a nontransition metal zeolite catalyst: a mechanistic study using FTIR spectroscopy, J. Phys. Chem. B 108 (2004) 5386. F. Blatter, H. Sun, H. Frei, Double oxygen atom centered rhodium–gold clusters, Angew. Chem. Int. Ed. Engl. 35 (1996) 635. S.K. Sharma, P.A. Parikh, R.V. Jasra, Solvent free aldol condensation of propanal to 2-methylpentenal using solid base catalysts, J. Mol. Catal. A: Chem. 278 (2007) 135. D.R. Fernandes, R.J. Nilton, C.J.A. Mota, Catalytic conversion of chloromethane to methanol and dimethyl ether over metal-exchanged zeolite Y, Appl. Catal. A: Gen. 367 (2009) 108. D. Tsukamoto, Y. Shiraishi, T. Hirai, Highly efficient methyl ketone synthesis with photoactivated acetone and olefins assisted by Mg(II)-exchanged zeolite Y, J. Org. Chem. 75 (2010) 1450. G. D Pirnguber, P. Raybaud, Y. Belmabkhout, J. Cejka, A. Zukal, The role of the extra-framework cations in the adsorption of CO2 on faujasite Y, Phys. Chem. Chem. Phys. 12 (2010) 13534. W. Sangthong, M. Probst, J. Limtrakul, Computational study of the carbonylene reaction of encapsulated formaldehyde in Na–FAU zeolite, J. Mol. Struct. 748 (2005) 119. S. Choomwattana, T. Maihom, P. Khongpracha, M. Probst, J. Limtrakul, Structures and mechanisms of the carbonyl-ene reaction between MOF-11 encapsulated formaldehyde and propylene: an ONIOM study, J. Phys. Chem. C 112 (2008) 10855. M. Vandichel, D. Lesthaeghe, J. Van der Mynsbrugge, M. Waroquier, V. Van Speybroeck, Assembly of cyclic hydrocarbons from ethene and propene in acid zeolite catalysis to produce active catalytic sites for MTO conversion, J. Catal. 271 (2010) 67. Z.F. Ke, A. Satoshi, U. Takafumi, M. Keiji, Rh-catalyzed polymerization of phenylacetylene: theoretical studies of the reaction mechanism, regioselectivity, and stereoregularity, J. Am. Chem. Soc. 133 (2011) 7926.

57

[25] C.Y. Sung, J. Linda, R.Q. Snurr, QM/MM study of the effect of local environment on dissociative adsorption in BaY zeolites, J. Phys. Chem. C 113 (2009) 15643. [26] Y.X. Sun, J. Yang, L.F. Zhao, J.X. Dai, H. Sun, A two-layer ONIOM study on initial reactions of catalytic cracking of 1-butene to produce propene and ethene over HZSM-5 and HFAU zeolites, J. Phys. Chem. C 114 (2010) 5975. [27] D. Roy, J.J. Dannenberg, The effects of regularly spaced glutamine substitutions on alpha-helical peptide structures: a DFT/ONIOM study, Chem. Phys. Lett. 512 (2011) 255. [28] Y.V. Joshi, K.T. Thomson, Embedded cluster (QM/MM) investigation of C6 diene cyclization in HZSM-5, J. Catal. 230 (2005) 440. [29] Y.H. Yeom, S.B. Jang, Y. Kim, S.H. Song, K. Seff, Three crystal structures of vacuum-dehydrated zeolite X, M46Si100Al92O384 M = Mg2+, Ca2+, and Ba2+, J. Phys. Chem. B 101 (1997) 6914. [30] T. Frising, P. Leflaive, Extraframework cation distributions in X and Y faujasite zeolites: a review, Microporous Mesoporous Mater. 114 (2008) 27. [31] G. Martra, R. Ocule, L. Marchese, G. Centi, S. Coluccia, Alkali and alkaline-earth exchanged faujasites: strength of Lewis base and acid centres and cation site occupancy in Na- and BaY and Na- and BaX zeolites, Catal. Today 73 (2002) 83. [32] C.Y. Sung, J.B. Linda, R.Q. Snurr, A DFT study of adsorption of intermediates in the NOx reduction pathway over BaNaY zeolites, Catal. Today 136 (2008) 64. [33] J.A. Hriljac, M.M. Eddy, A.K. Cheetham, J.A. Donohue, G.J. Ray, Powder neutron diffraction and 29Si MAS NMR studies of siliceous zeolite-Y, J. Solid. State. Chem. 106 (1993) 66. [34] M.F. Fellah, CO and NO adsorptions on different iron sites of Fe-ZSM-5 clusters: a density functional theory study, J. Phys. Chem. C 115 (2011) 1940. [35] A.K. Rappé, C.J. Casewit, K.S. Colwell, W.A. Goddard, W.M. Skiff, UFF, a full periodic table force field for molecular mechanics and molecular dynamics simulations, J. Am. Chem. Soc. 114 (1992) 10024. [36] A.E. Reed, L.A. Curtiss, F. Weinhold, Intermolecular interactions from a natural bond orbital, donor–acceptor viewpoint, Chem. Rev. 88 (1988) 899. [37] J.E. Carpenter, F. Weinhold, Analysis of the geometry of the hydroxymethyl radical by the ‘‘different hybrids for different spins’’ natural bond orbital procedure, J. Mol. Struct. (THEOCHEM) 169 (1988) 41. [38] J.P. Foster, F. Weinhold, Natural hybrid orbitals, J. Am. Chem. Soc. 102 (1980) 7211. [39] A.E. Reed, F. Weinhold, Natural bond orbital analysis of near-Hartree–Fock water dimer, J. Chem. Phys. 78 (1983) 4066. [40] A.E. Reed, R.B. Weinstock, F. Weinhold, Natural population analysis@fa@f), J. Chem. Phys. 83 (1985) 735. [41] D.G. Truhlar, B.C. Garrett, S.J. Klippenstein, Current status of transition-state theory, J. Phys. Chem. 100 (1996) 12771. [42] M.J. Frish, et al. Gaussian 03, Gaussian, Inc.: Wallingford, CT, 2004. [43] S.W. Benson, H.E. O’Neal, Kinetic Data on Gas Phase Unimolecular Reactions, National Standard and Reference Data Series, 1970. [44] F. Grein, A.C. Chen, D. Edwards, M.C. Cathleen, Theoretical and experimental studies on the Baeyer–Villiger oxidation of ketones and the effect of a-halo substituents, J. Org. Chem. 71 (2006) 861.