Structures and Mechanisms of the Carbonyl-ene Reaction between

Jun 26, 2008 - Department of Chemistry and Center of Nanotechnology, Kasetsart University, Bangkok 10900, Thailand, NANOTEC Center of Excellence, ...
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J. Phys. Chem. C 2008, 112, 10855–10861

10855

Structures and Mechanisms of the Carbonyl-ene Reaction between MOF-11 Encapsulated Formaldehyde and Propylene: An ONIOM Study S. Choomwattana,†,‡ T. Maihom,†,‡ P. Khongpracha,†,‡ M. Probst,§ and J. Limtrakul*,†,‡ Department of Chemistry and Center of Nanotechnology, Kasetsart UniVersity, Bangkok 10900, Thailand, NANOTEC Center of Excellence, National Nanotechnology Center, Kasetsart UniVersity, Bangkok 10900, Thailand, Institute of Ion Physics and Applied Physics, UniVersity of Innsbruck, Innsbruck, Austria ReceiVed: March 12, 2008; ReVised Manuscript ReceiVed: April 9, 2008

Metal-organic framework structures containing formaldehyde (HCHO@MOF-11) and its interactions with propylene were investigated by density-functional (B3LYP/6-31G(d,p)) and ONIOM (B3LYP/6-31G(d,p): UFF) calculations. For comparison, the carbonyl-ene reaction of propylene and formaldehyde was also studied with Cu+ alone as a catalyst. It was found that the metal-organic framework leads to an energy barrier of the reaction ∆Eact of 24.1 kcal/mol. This compares to values of ∆Eact of 34.4 kcal/mol for the uncatalyzed system and 6.4 kcal/mol if the reaction takes place with the Cu+ alone. The carbonyl-ene reaction of propylene using HCHO@MOF-11 takes place in a single concerted reaction step. The ∆Eact value for MOF-11 is similar to that obtained for the zeolite catalyzed reaction (∆Eact ) 25.1 kcal/mol, HCHO@Na-faujasite/CH3CH)CH2). 1. Introduction Various hydrocarbons rearrangements have been very versatile in chemical synthesis. Part of this “toolbox” is the ene reaction, the indirect substituting addition of a compound with a double bond (enophile) to an olefin with an allylic hydrogen (ene). This reaction is less known than the related Diels-Alder addition but is still used in applications from industrial to biosynthetic processes.1 The reaction normally requires a strongly electrophilic carbonyl compound and is catalyzed by Lewis acids.2 The carbonyl-ene reaction is the enantioselective reaction between all carba-ene components and heteroenophiles.3–6 The production of 3-buten-1-ol from formaldehyde and propylene is an example of a carbonyl-ene reaction. 3-buten1-ol is acquired as a monomer in polymerization reactions and as an intermediate for tetrahydrofuran (THF) synthesis.7 Because formaldehyde has the disadvantage of a low boiling point of -19.5 °C and the tendency to self-polymerize to solid paraformaldehyde and trioxane, the commonly used carbon electrophiles need thermal or Lewis acid treatment prior to use. This pretreatment unfortunately involves problems concerning corrosion, handling, and toxic waste. Environmentally friendly porous materials such as zeolites were found to be candidates for the storage of formaldehyde.8 Subsequently, the production of 3-buten-1-ol has been studied. More recently, the carbonyl-ene reaction between formaldehyde encapsulated in Na-faujasite and propylene has been investigated theoretically.9 For the reaction with larger molecules, porous materials like metal-organic frameworks (MOFs) become more advantageous because of the accessible variation of pore dimension and chemistry inside the cavity. MOFs are known as crystalline porous materials with remarkable properties. They consist of three-dimensional clusters of metal oxide held together by organic linkers forming a * Corresponding author. E-mail: [email protected]. † Department of Chemistry and Center of Nanotechnology, Kasetsart University. ‡ NANOTEC Center of Excellence, National Nanotechnology Center, Kasetsart University. § Institute of Ion Physics and Applied Physics, University of Innsbruck.

Figure 1. MOF-11 structure generated from XRD Data. MOF-11 is composed of inorganic clusters of Cu2(CO2)4 paddlewheel units and adamantine organic linkers

systematic network with nanoscale periodic channels and cavities. The pore structure can be customized to satisfy various applications by using appropriate linkers. With their inorganic joints and assorted organic linkers, MOFs are promising catalysts, gas separators, molecular sensors, and so forth. MOF11 has an open metal site of Cu2(CO2)4 paddlewheel units and a spongy network of adamantine, illustrated in Figure 1. Similar to zeolites, its porosity can be shape-selective. Its open metal sites can play a central role in highly selective and specific molecular transformations, as well as transport and storage.10–13 We want to investigate if its open metal site can be used to catalyze the ene-reaction in a way similar to the Na cation in Na-FAU as described above. We approach the reaction mechanisms on a molecular level by means of quantum chemical calculations. Because both zeolites and MOFs are micromesoporous materials, the computational methods and schemes used for MOFs can be adopted from the ones used in the study of zeolites. Like in zeolites,14–17 only a small part of the framework affects the electronic

10.1021/jp8021437 CCC: $40.75  2008 American Chemical Society Published on Web 06/26/2008

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Figure 2. Model used in this work to represent the reaction on MOF-11. (a) The cavity dimensions (width, height, and depth) of the structure are 14.44 × 6.00 × 8.47Å3. (b) Layers of the ONIOM scheme applied to the model.

properties of the reactive site, thus facilitating modeling using quantum chemical methods. Alternatively, because of the role of the MOF framework in the adsorption of the reactants,18 it cannot be completely neglected. Hybrid methods such as embedded cluster or combined quantum mechanics/molecular mechanics (QM/MM) methods19–26 as well as the ONIOM scheme are well suited to such systems. We decided to use the ONIOM (Our-own-N-layer Integrated molecular Orbital + molecular Mechanics) method25,26 because it is frequently exploited to study extended systems27–31 and has also been applied to the adsorption of ethylene, benzene, and ethylbenzene over acidic and alkaline faujasite and ZSM-5 zeolites.30,32–34 In this work, we study the mechanism of the carbonyl-ene reaction between MOF-11 encapsulated formaldehyde and propylene. To our knowledge there is no experimental data for this reaction, but by comparison with the zeolite-based system we hope to predict the MOF-11 case correctly. 2. Theoretical Methods The MOF-11 structure was obtained from XRD data.35 MOF11 is based on a 3D channel system with a diameter of 6.0-6.5 Å. It consists of square-shaped Cu2(CO2)4 paddlewheel building units connected to 1,3,5,7-adamantane tetracarboxylate (ATC) linkers in PtS topology,35 where each admantane is bound to four Cu2O4C8 squares. The active paddlewheel unit is the effective part of the molecules of the system because formal-

dehyde can easily enter between the adamantine units. This region makes up the inner ONIOM layer and is treated on the B3LYP/6-31G(d,p) level of theory. The framework environment constitutes the outer ONIOM layer. In it, mostly van der Waals interactions due to confinement in the mesoporous materials play a role and are considered with the universal force field (UFF).36 Earlier comparisons between calculations using UFF and experimental results in organic-inorganic systems36–40 indicate that UFF is reliable for this purpose. In our calculations, the MOF framework was kept at the crystallographic geometry, whereas the upper part of the paddlewheel active site (CuO4C4, see Figure 2b) and the adsorbates (HCHO and CH3CHdCH2) were fully optimized. Normal-mode analyses were performed to confirm the transition state to have one imaginary frequency whose mode corresponds to the reaction coordinate. These data have been used to predict the rate constant for the reaction by using simple transitionstate theory (TST). The equation for calculating the reaction rate constant is

kr )

kBT qTS exp(-∆Ea ⁄ RT) h qInt

(1)

where ∆Ea is the activation energy, kB is Boltzmann’s constant, h is Plank’s constant, T is the temperature, R is the universal gas constant, and qTS and qInt are the total partition functions

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TABLE 1: Optimized Geometric Parameters of Reactants, Transition State, and Product of the Carbonyl-ene Reaction between Formaldehyde and Propylene on MOF-11 using the ONIOM (B3LYP/6-31G(d,p):UFF) Method isolated molecule

formaldehyde adsorption

C-O C1-C2 C2-C3 C3-H C-C1 O-H Cu1-O Cu1-O1 Cu1-O2 Cu1-O3 Cu1-O4 Cu1-Cu2

1.207 1.333 1.501 1.097

1.215

1.936 1.936 1.936 1.936 2.506

2.318 1.940 1.941 1.959 1.958 2.591

∠O1-Cu1-O3 ∠O2-Cu1-O4

173.0 173.0

168.0 168.0

parameters

coadsorption complex

transition state

product

1.218 1.334 1.498 1.095 3.213 2.995 2.285 1.937 1.949 1.965 1.949 2.595

1.310 1.403 1.424 1.211 1.864 1.486 2.248 1.944 1.980 1.973 1.939 2.652

1.436 1.507 1.341 2.258 1.543 0.972 2.265 1.944 1.968 1.958 1.934 2.599

167.7 167.6

164.1 164.3

167.2 167.1

Distances (Å)

Angles (degrees)

for the transition state and intermediate complex, respectively, which include electronic, translation, rotational, and vibrational partition functions. Because of the immobility of the transition state and the intermediate complex, which are embedded into the MOF framework, only the vibrational frequency components were used in the calculation of the total partition functions. All of the rate constants were derived in the temperature range of 300 to 600 K in steps of 50 K and are used for explaining the reactions. The charge distribution in the complexes has been analyzed via the natural population analysis (NPA)41–45 partitioning scheme using the B3LYP/6-31G(d,p) densities. All quantum chemical calculations were performed with the Gaussian 03 code.46 3. Results and Discussion We separate the discussion into Sections 3.1 to 3.3. In Section 3.1, we discuss the structure of MOF-11 and the existence of encapsulated formaldehyde in MOF-11 (HCHO@MOF-11) in order to study the reactivity of this species. Then in 3.2, we report the study of their interactions with propylene using the ONIOM model. A symbol such as in M@S signifies that a molecule M adsorbed on an active site S of MOF. Finally in 3.3, we present the reaction rate constants (kr) and the equilibrium constants (Keq). 3.1. MOF-11 and MOF Encapsulated Formaldehyde (HCHO@MOF-11). As found previously for zeolites,32–34,47–53 current works18,54 show that the MOF interactions of hydrocarbons or aromatic adsorbates with the network play a vital role in the structure and energetics of the adsorption process. The overall MOF-11 structure and the model used in the study are illustrated in Figures 1 and 2, respectively. Key optimized geometrical parameters are listed in Table 1. Figure 3a exhibits the structure of formaldehyde stabilized in the MOF-11 framework. The Cu unit (Cu1-Cu2) is barely changed upon the adsorption of formaldehyde (0.085 Å and 5° for changes in Cu1-Cu2 bond distance and O-Cu-O bond angles, respectively), which is a smaller adjustment than that found in Na-Faujasite. The distance between formaldehyde and MOF11 is found to be 2.3 Å. The carbon-oxygen bond of formaldehyde is elongated from 1.207 to 1.215 Å. According to the interaction between the hydrogen atoms of formaldehyde and the oxygen atoms of the framework, the corresponding distance between the formaldehyde oxygen and the Cu atom of MOF-11 active site is 2.318 Å and the resulting adsorption

energy for the HCHO@MOF-11 complex is -12.34 kcal/mol. The C-O · · · Cu/MOF angle is 115.0°. 3.2. Carbonyl-ene Reaction between MOF-11 Encapsulated Formaldehyde and Propylene (HCHO@MOF-11/ CH3CHdCH2). For the carbonyl-ene reaction, a concerted mechanism was proposed in which the bond between the carbon atom of formaldehyde (C) and the propylene carbon (C1) is formed and the propylene proton (H) is transferred. Assuming the same mechanism for the reaction in Na-faujasite and the calculated energy profile, we propose the reaction in the following steps:

HCHO + MOF-11 f HCHO@MOF-11 CH3CH)CH2+ HCHO@MOF-11

(1a)

f CH2)CHCH2CH2OH@MOF-11 (2) CH2)CHCH2CH2OH@MOF-11 f CH2)CHCH2CH2OH + MOF-11 (3) In step 1, formaldehyde adsorbs over the paddlewheel active site of MOF-11 via lone pair electron interaction with an adsorption energy of -12.3 kcal/mol. Then, in step 2, the earlier encapsulated formaldehyde interacts with diffusing propylene via a π interaction with a coadsorption energy of -19.0 kcal/ mol, followed by the chemical reaction in order to produce 3-buten-1-ol. The required activation energy is 24.1 kcal/mol. The calculated transition state confirms the proposed concerted pathway (Figure 3c). Its imaginary frequency belongs to the mode in which the C-C bond is formed and H being transferred. The product formation is exothermic by -28.1 kcal/mol. The product needs 11.9 kcal/mol to desorb from the active site in the final step (3). The key geometrical parameters of the carbonyl-ene reaction between propylene and MOF-11 encapsulated formaldehyde from the geometric optimizations are presented in Table 1. The energetics of the reaction is illustrated in Figure 4. The Cu atom is slightly moved away from the lower plane toward the reacting molecules. Specifically, the following geometric parameters of the reactants are changed: The propylene C3-H bond length is stretched from 1.095 to 1.211 Å, and the distance between the propylene proton (H) and the formaldehyde oxygen (O) is shortened from 2.995 to 1.486 Å. The C-O double bond of formaldehyde is changed from 1.218 to 1.310 Å as the distance between the propylene carbon (C1)

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Figure 3. Structures of the HCHO@MOF-11/CH3CHdCH2 complexes: (a) HCHO@MOF-11, (b) coadsorption, (c) transition state, and (d) product.

Figure 4. Calculated energetic profiles (kcal/mol) for the carbonylene reaction between HCHO and CH3CHdCH2 in MOF-11.

and formaldehyde carbon (C) is contracted from 3.213 to 1.864 Å. The C1-C2 and C2-C3 bond lengths are stretched from 1.334 and 1.498 to 1.403 and 1.424 Å, respectively. Because of the steric effect from the paddlewheel framework, the overall activation energy is 5.1 kcal/mol. These results show that the electrostatic contribution from the Cu also stabilizes the transition-state structure inside MOF-11. The product formation

is illustrated in Figure 3d. The adsorbed 3-buten-1-ol product is subsequently desorbed endothermically. We also examined the atomic charges of the molecules involved in the reaction by means of the NPA method,41,42,45 as shown in Table 2. Cu in the upper plane in the MOF structure bears a positive charge of 1.29. As formaldehyde adsorbs over the Cu, it becomes slightly more positive. The adsorption also causes the O atom in formaldehyde to become more negative and its C atom becomes more positive. The subsequent coadsorption of propylene does not alter the charge significantly. At the transition-state structure, the negative charge over the H accepting O atom (-0.74) becomes comparable to the charge over C3 (-0.76). The increased negative charge of O facilitates the proton transfer. The Cu charge of 1.36 stabilizes the structure. In order to investigate the effect of the framework, consisting of four carboxylic groups connected to Cu atoms at the active site and their organic linkers, we also studied the naked Cu+ system. The data for this system are in Table 3 and Figures 5 and 6. Formaldehyde adsorbs on Cu+ with a Cu · · · O distance of 1.91 Å and the adsorption energy of -47.6 kcal/mol. As expected, both reacting molecules are held closer by the ion, with a C1 · · · C distance of 2.62 Å and an H · · · O distance of 2.91 Å. The coadsorption energy is -56.7 kcal/mol. In propylene, C1-C2 and C3-H bonds are slightly stretched to 1.35 and 1.10 Å, while there is a contraction of the C2-C3

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TABLE 2: Atomic Partial Charges Calculated from Natural Population Analysis (NPA) for MOF-11 and Cu+ Catalyzed Reactions isolated molecule q(Cu) q(O) q(C) q(C1) q(C2) q(C3) q(H)

1.292 -0.494 0.221 -0.448 -0.213 -0.714 0.242

q(Cu) q(O) q(C) q(C1) q(C2) q(C3) q(H)

1.000 -0.494 0.221 -0.448 -0.213 -0.714 0.242

ads-HCHO MOF-11 System 1.324 -0.557 0.264

Cu+ System 0.903 -0.662 0.334

TABLE 3: Optimized Geometric Parameters of Reactants, Transition State, and Products of the Carbonyl-ene Reaction between Formaldehyde and Propylene on Cu+ (B3LYP/ 6-31G(d,p) Method, Distances are in Å) distances isolated formaldehyde coadsorption transition (Å) molecule adsorption complex state product C-O C1-C2 C2-C3 C3-H C-C1 O-H

1.207 1.333 1.501 1.097

1.231

1.250 1.349 1.493 1.098 2.617 2.907

1.374 1.439 1.417 1.188 1.656 1.658

1.479 1.509 1.340 2.363 1.530 0.985

bond to be 1.50 Å. The C-O bond is found to be elongated from 1.21 to 1.25 Å. The transition-state structure is again confirmed by possessing exactly one imaginary frequency. The bare Cu+ cation, without the shielding effect of surrounding O atoms, causes larger electrostatic field than that in the MOF, which in turn leads to a much lower activation of 6.4 kcal/mol. As shown in Figure 6, the activation energy of Cu+ (6.4 kcal/ mol) is lower than those of MOF-11 (24.1 and 5.1 kcal/mol) and the uncatalyzed reactions (34.4 kcal/mol).

Figure 5. Structures of HCHO@Cu+/CH3CHdCH2 complexes: (a) HCHO@Cu+, (b) coadsorption, (c) transition state, and (d) product.

coads

TS

product

1.330 -0.594 0.282 -0.472 -0.208 -0.727 0.245

1.363 -0.739 0.000 -0.550 -0.037 -0.760 0.252

1.315 -0.814 -0.096 -0.539 -0.217 -0.480 0.224

0.858 -0.707 0.235 -0.463 -0.154 -0.735 0.242

0.807 -0.875 -0.066 -0.564 -0.154 -0.759 0.370

0.867 -0.883 -0.101 -0.537 -0.258 -0.430 0.544

The results indicate that MOF-11 can be used as a catalyst in the carbonyl-ene reaction and that it stabilizes all species in the carbonyl-ene reaction systems. The apparent activation energy of 5.05 kcal/mol is higher than that in Na-faujasite, but lower than that in the noncatalyzed reaction (31.1 kcal/mol). Because the activation energy for this transition state is 24.08 kcal/mol, which is comparable to the reaction on Na-faujasite (25.1 kcal/mol), it is likely that the open site Cu atom can catalyze the reaction in a similar way to that of Na(I) in the faujasite. Both Na@FAU and MOF-11 catalysts can lower the activation energy compared to the uncatalyzed reaction (34.4 kcal/mol). The advantage of MOF over FAU is the adjustable size of the active MOF cavity by using different organic linkers to form the network structure of the framework. 3.3. Rate Constants. The reaction rate constants (kr) and the equilibrium constants (Keq) for the carbonyl-ene reaction in MOF-11 are given in Table 4. To obtain the rate constants, energies and modes of the coadsorption complex of formaldehyde with propylene and of the transition state for 3-buten-1-ol formation are used as input for the TST equation given in the introduction. Kr and Keq were calculated in the range of 300-600 K. The calculated rate constants are small values at low temperatures but increased to large values as the temperature rose. At 300-500 K, the forward reaction for 3-buten-1-ol

Figure 6. Calculated energy profiles (kcal/mol) for the carbonyl-ene reaction between HCHO and CH3CHdCH2 in the MOF-11 system (solid line), the naked Cu+ system (dotted line), and the bare system (dashed line).

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TABLE 4: Reaction Rate Constants (kr) and Equilibrium Constants (Keq) of the Carbonyl-ene Reaction of Formaldehyde with Propylene In MOF-11 for the Temperatures Range 300-600 K kr (s-1) T (K)

forward rate

reverse rate

300 350 400 450 500 550 600

5.19 × 7.96 × 10-5 3.22 × 10-3 5.43 × 10-2 5.00 × 10-1 2.98 1.29 × 101

4.38 × 5.92 × 10-7 1.32 × 10-4 8.95 × 10-3 2.62 × 10-1 4.15 4.16 × 101

10-7

10-10

Keq 1.18 × 103 1.35 × 102 2.43 × 101 6.07 1.91 7.18 × 10-1 3.09 × 10-1

formation is faster than the reverse reaction. At higher temperatures (550-600 K), this is reversed and Keq decreases. 4. Conclusions Density-functional theory and the ONIOM approach are used for investigating the metal organic framework structures interacting with formaldehyde and their reaction with propylene, which was studied by two different models: MOF11 and naked Cu+. The reaction mechanism is proposed to be intermediate-free concerted, consisting of proton transfer and carbon-carbon bond formation. The energy barrier and the apparent activation energy for the MOF-11 system are 24.1 and 5.1 kcal/mol, respectively. The Lewis acid role as a catalyst is studied from the Cu+ system. It was found that inclusion of the extended metal-organic framework has an effect on the structure and energetics of the adsorption complexes and leads to a lower energy barrier (∆Eact ) 24.1 kcal/mol) of the reaction as compared to the bare model system (∆Eact ) 34.4 kcal/mol). If the naked Cu+ interacts with the HCHO/CH3CHdCH2 complex, then the energy barrier of the system is even lower than that for HCHO@MOF11/CH3CHdCH2 because of the large electrostatic field of the Cu+ cation (∆Eact ) 6.4 kcal/mol). The result derived for MOF-11 is similar to that for the zeolite catalyst (∆Eact ) 25.1 kcal/mol obtained for faujasite zeolite in HCHO@NaFAU/CH3CHdCH2), suggesting that MOF-11 might be a good candidate material for use in catalysis. Acknowledgment. This work was supported in part by grants from the Thailand Research Fund (TRF Senior Research Scholar to J.L.) and the Kasetsart University Research and Development Institute (KURDI), as well as the Ministry of University Affairs under the Science and Technology Higher Education Development Project (MUAADB funds). Support from the National Nanotechnology Center under the National Science and Technology Development Agency is also acknowledged. References and Notes (1) Hoffmann, H. M. R. Angew. Chem., Int. Ed. Engl. 1969, 8, 556. (2) Yamanaka, M.; Mikami, K. HelV. Chim. Acta 2002, 85, 4264. (3) Maruoka, K.; Hoshino, Y.; Shirasaka, T.; Yamamoto, H. Tetrahedron Lett. 1988, 29, 3967. (4) Mikami, K.; Terada, M.; Nakai, T J. Am. Chem. Soc. 1989, 111, 1940. (5) Carreira, E. M.; Lee, W.; Singer, R. A. J. Am. Chem. Soc. 1995, 117, 3649. (6) Johannsen, M.; Joergensen, K. A. J. Org. Chem. 1995, 60, 5757. (7) Squire, E. N. Synthesis of Tetrahydrofuran; du Pont de Nemours, E. I., and Co.; Application: US, 1981; 4 pp. (8) Okachi, T.; Onaka, M. J. Am. Chem. Soc. 2004, 126, 2306. (9) Sangthong, W.; Probst, M.; Limtrakul, J. J. Mol. Struct. 2005, 748, 119.

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