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Characterization of Lewis Acid Sites in Metal-Organic Frameworks Using Density Functional Theory Dahuan Liu and Chongli Zhong* Laboratory of Computational Chemistry, Department of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China
ABSTRACT The properties of Lewis acid sites in two MOFs, Cu-BTC and Cu-MIPT, were studied by using density functional theory (DFT). The strengths of these sites were investigated through exploring the geometry parameters, the natural bond orbital (NBO) charge, and the vibrational frequency as well as the adsorption energy of the probe CO molecule. The results show that both MOFs have Lewis acid sites, and the strength of the Lewis acid sites in Cu-BTC is stronger than that in Cu-MIPT. In addition, proposals for enhancing the acid strength of MOFs are discussed; increasing the number of atoms with high electronegativity in organic linkers and modification of the metal cation by the one with more empty d orbits are suggested as the two possible ways. SECTION Surfaces, Interfaces, Catalysis
etal-organic frameworks (MOFs) have received widespread interest due to the extraordinary combination of chemical tunability and high internal surface area.1-4 One of the earliest proposed applications for these materials is the possible use in heterogeneous catalysis.5,6 Nevertheless, only recently, MOF-based catalysis has been investigated experimentally.7-11 Thanks to the highly crystalline nature and the high metal content, MOFs show great advantages in catalysis that all active sites are identical. The coordinatively unsaturated metals exposed on the surface of the frameworks might serve as Lewis acid sites,12-16 which have shown high catalytic activity in several important reactions, such as the cyanosilylation reaction12,15,16 and the Mukaiyama-aldol reaction.16 Unfortunately, the catalysis mechanism,16 for which the computational techniques can provide useful information, is not quite clear. However, reports of catalytic studies in MOFs using computational techniques are very scarce up to date, compared with the extensive exploration in zeolites.17,18 Therefore, in this work, the investigations of the properties of Lewis acid sites in MOFs were performed by using density functional theory (DFT), which is anticipated to pave the way for the future design of novel MOFs and the study of the catalysis mechanism of MOFs from the computational point of view. The adsorption behavior of CO on the Lewis sites of Cu-BTC was first investigated. Figure 1 displays the local structures of the optimized Cu-BTC cluster interacting with the CO molecule at a commonly accepted active site, a coordinatively unsaturated metal Cu site. It is observed that CO can be adsorbed through both the C (Figure 1a) and O atom (Figure 1b), depending on the initial structural arrangement of the optimization process. The adsorbed CO molecule is almost aligned along the axis of the Cu-Cu atoms in the cluster, and no flat adsorption was found. The adsorption
M
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energy of CO was calculated by the following equation E ads ¼ -½E CO-Cu-BTC -ECO -E Cu-BTC
ð1Þ
where ECO-Cu-BTC, ECO, and ECu-BTC denote the energies of the CO-Cu-BTC complex, the isolated CO, and cluster of Cu-BTC, respectively. It should be noted that the values of energy contain the zero-point energy (ZPE) correction. The value of Eads is 6.63 kcal/mol when the CO molecule is adsorbed through the C atom (Figure 1a), while for O adsorption (Figure 1b), the value of Eads is 3.14 kcal/mol, implying that CO molecules tend to adsorb on Cu-BTC through C adsorption. This is consistent with the observation in zeolites.17 Therefore, in the following sections, only C adsorption for CO is discussed. The geometry parameters listed in Table 1 indicate that the structures of the CO molecule and Cu-BTC cluster are distorted slightly after CO adsorption. The bond length of the adsorbed CO is smaller than that of the isolated CO molecule, similar to the situation in zeolites.17 The interaction between probe molecules and the Cu-BTC cluster was further studied by the vibrational frequency of the adsorbed CO, which is a powerful tool routinely used to characterize the strength of active sites in zeolites. The changes in the frequency are primarily associated with undercoordination, localization, or charging of the active sites interacting with CO.19 It is found that the higher the perturbed frequency of CO, the greater their acidity.17,20 In this work, the calculated stretching frequency of CO is 2168 cm-1, agreeing well with the experimental value of 2123 cm-1 at low CO coverage.13 In order to analyze the structure of the Cu-BTC cluster with adsorbed CO comprehensively, the natural bond Received Date: September 28, 2009 Accepted Date: October 27, 2009 Published on Web Date: November 06, 2009
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Figure 1. The structures of the CO molecule adsorbed on the cluster of Cu-BTC: (a) C adsorption; (b) O adsorption. (Cu, green; O, red; C, gray; H, white.) Table 1. Geometry Parameters of Cu-BTC, CO, and the CO-CuBTC Complex (units of Å) parameters
Cu-BTC
CCO-OCO
CO 1.135
CO-Cu1a,c
complex (C adsorption) 1.132 2.438
Cu1-O1b,c
1.952
1.992
a
The shortest distance between the CO and Cu atom. b The average distance between bonded Cu1 and O1 atoms in Cu-BTC. c The locations of Cu1 and O1 atoms are shown in Figure 1.
Table 2. Calculated NBO Charges on Different Atoms of Cu-BTC, CO, and the CO-Cu-BTC Complex (units of |e|) atoms
CO
CCO
0.506
OCO Cu1a C1a
complex (C adsorption)
Figure 2. The optimized structure of the CO molecule adsorbed on the cluster of Cu-MIPT through C adsorption. (Cu, green; O, red; C, gray; H, white).
0.620
-0.506
O1a a
Cu-BTC
0.989
-0.418 0.746
-0.676
-0.662
0.865
0.869
materials has an effect on the value of transferred electrons. In addition, there is also electron transfer from the other atoms in the framework of Cu-BTC to Cu2þ, which is indicated by the change of the charge of O1 (from -0.676 to -0.662 |e|). The σ-bond was formed with the electron transferred, and the d orbital of the Cu2þ cation was combined with the empty LUMO of the CO molecule to form π-back-donation. In order to draw a comparison with Cu-BTC, the behavior of CO adsorbed on Cu-MIPT (MIPT=5-methylisophthalate) was also investigated. As shown in Figure 2, Cu-MIPT has the same Cu cluster as in Cu-BTC, while the organic linker is different. Using a similar method, the structure of CO adsorbed on the Cu-MIPT cluster was optimized (Figure 2), and the geometry parameters are listed in Table 3. The bond length of CO and Cu1-O1 are optimized at 1.133 and 1.990 Å, respectively, indicating the slight distortion of the CO and Cu-MIPT cluster. However, the changes of bond distance are smaller than those in Cu-BTC, as shown in Table 1. At the same time, the shortest distance between the adsorbed CO molecule and the Cu atom in the CO-Cu-MIPT complex is 2.442 Å, which is larger than that in the CO-Cu-BTC complex (2.438 Å); these parameters imply that the strength of the CO bonding to the Lewis acid site in Cu-MIPT is weaker than that in Cu-BTC.
The locations of Cu1, O1, and C1 atoms are shown in Figure 1.
orbital (NBO) charges were calculated since the strength of Lewis acid sites is related to the capability of accepting an electron. The values are shown in Table 2. The NBO charges on CCO and OCO atoms are 0.620 and -0.418 |e|, respectively, in the complex with CO through C adsorption, that is, the CO molecule bears a positive charge (0.202 |e|) when it is adsorbed on Cu-BTC. At the same time, the NBO charge on Cu2þ changes from 0.989 to 0.746 |e| with the formation of the CO-Cu-BTC complex; this indicates that electrons are transferred from the CO molecule to the Cu2þ when CO is adsorbed on Cu-BTC, showing that the Cu2þ in Cu-BTC actually serves as the Lewis acid site. It is interesting that the value of transferred electrons in Cu-BTC is larger than that in ZSM-5 zeolite.21 This may be mainly attributed to the different coordinative environment of the metal atoms; for Cu-BTC, the metal atom is connected with four O atoms, while only two O atoms are present in the cluster of ZSM-5. Also, the difference in the structure of the frameworks in these two
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strong Lewis acids, replacing the metal cluster may be an efficient way. Combined with the discussion on the NBO charges, the metal cations with more empty d orbitals, such as Mn2þ, are anticipated as good candidates to increase the π-back-donation. In fact, this kind of modification based on Cu-BTC is ongoing in our group, using both computational and experimental methods. In conclusion, the Lewis acid sites in two MOFs, Cu-BTC and Cu-MIPT, were investigated using density functional theory in this work. Through the comparison of the optimized geometrical properties, the vibrational frequency of the adsorbed CO molecule, the NBO charges, and the adsorption energy, it can be found that the strength of Lewis acid site in Cu-BTC is stronger than that in Cu-MIPT. In addition, increasing the number of atoms with high electronegativity in the organic linker as well as replacing the metal cluster may be the possible ways to modulate the character of Lewis acid sites in MOFs, and the latter is suggested as the more efficient one. On the basis of these, novel MOFs for catalysis will be designed, and the catalysis mechanism of MOFs for some important reactions will be explored in the future. CO is a good candidate for the evaluation of the properties of the Lewis sites in zeolites,17 which has also been commonly used as a suitable probe molecule to investigate the Lewis acid sites in MOFs experimentally13,14,20 since it is very sensitive to the electronic properties of the adsorption site. Therefore, in this study, we present a detailed study on the interaction of CO molecules with the frameworks of MOFs. Figure 3 shows the atomic structures of the two MOFs studied in this work, constructed from the corresponding experimental single-crystal Xray diffraction (XRD) data14,26 using the Materials Studio Visualizer.27 Cu-BTC is a well-studied MOF first synthesized by Chui et al.26 and has a three-dimensional channel structure connecting a system of tetrahedral-shaped cages with small triangular windows. It is one of the first studied MOFs for catalytic properties12 and has served as a Lewis acid catalyst.12,13 Cu-MIPT contains the paddle-wheel Cu2 clusters, which are interconnected by bent MIPT linkers to form an undulating twodimensional net with 44 topology.14 It shows active Lewis acid sites on channel walls and stable catalytic activity for CO oxidation.14 The interaction of CO molecules with the two MOFs were investigated by DFT calculations on the basis of the fragmental clusters illustrated in Figure 4 in this work, which is the common method widely used in zeolites.17,18 The Becke three-parameter hybrid method combined with the LYP correlation function (B3LYP) was used for the DFT calculations.28,29 For heavy atoms, the effective core potential (ECP) is often chosen in ab inito calculations to reduce the amount of necessary computation, and LANL2DZ is a collection of double-ζ basis sets that is one of the most common ECP basis sets for complexes containing transition-metal atoms.30,31 This basis set combined with the B3LYP method was considered as the accurate and the most economical level of theory in calculation of vibrational spectra for copper ions.32 Thus, the LANL2DZ basis set was used for Cu atoms, while 631G(d) was used for the other atoms by balancing the precision and computational time.33 With these basis sets, the calculated stretching frequency of CO in the free gas phase is 2125 cm-1,
Table 3. Geometry Parameters of Cu-MIPT, CO, and the CO-CuMIPT Complex (units of Å) parameters
Cu-MIPT
CO
CCO-OCO
1.135
CO-Cu1a,c
complex (C adsorption) 1.133 2.442
Cu1-O1b,c
1.955
1.990
a
The shortest distance between CO and the Cu atom. b The average distance between bonded Cu and O atoms in Cu-MIPT. c The locations of the Cu1 and O1 atoms are shown in Figure 2.
Table 4. Calculated NBO Charges on Different Atoms of Cu-MIPT, CO, and the CO-Cu-MIPT Complex (units of |e|) atoms
CO
Cu-MIPT
complex (C adsorption)
CCO
0.506
0.617
OCO
-0.506
-0.438
Cu1a O1a C1a a
0.986 -0.673
0.763 -0.669
0.862
0.866
The locations of Cu1, O1, and C1 atoms are shown in Figure 2.
In addition, the vibrational frequency of the adsorbed CO on the cluster of Cu-MIPT was calculated. In this work, the calculated C-O stretching frequency shifts to 2160 cm-1, which is in reasonable agreement with the experimental result (2113 cm-1) of Zou et al.14 This value is slightly lower than that of CO adsorbed on Cu-BTC. In order to further compare the strength of the Lewis acid site, the NBO charges were also calculated, and the values are listed in Table 4. It is obvious that there is 0.179 |e| transferred from the adsorbed CO molecule to the Cu-MIPT cluster, which is smaller than that in Cu-BTC. The above results demonstrate that there is also a Lewis acid site in Cu-MIPT, and it is weaker compared with Cu-BTC. From the analysis of the NBO charges, it could be seen that the π-back-donation between Cu2þ and the CO molecule is also weaker in Cu-MIPT. The reason may be that there are more O atoms in the organic linker in Cu-BTC and the electronegativity of O atoms is high, inducing more electron transfer to the framework. Therefore, increasing the number of atoms with high electronegativity in the organic linker, such as O, Cl, and F, may be the possible strategy to enhance the ability of electron transfer in MOFs, which may lead to stronger Lewis acid sites. The adsorption energy Eads of the CO molecule on the Cu-MIPT cluster is 3.49 kcal/mol, smaller than that in Cu-BTC, further illustrating that the strength of the Lewis acid sites in the former is weaker than that in the latter. However, these adsorption energies are all lower than those in most zeolites, such as mordenite,17 ZSM-5,21,22 FER,23 and Cu-Y zeolite,24 although they are close to FAU.25 Though the modification of the organic linker in the framework may enhance the adsorption energy of CO, the effect is not evident, and the increment is still not large compared with that in the zeolite. A similar conclusion can also be obtained in the analyses of the optimized geometry information, the NBO charges, and the stretching frequencies of CO. Therefore, in order to design new MOFs with catalysis for reactions requiring
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Figure 3. Crystal structures of the MOFs studied in this work: (a) Cu-BTC; (b) Cu-MIPT. (Cu, green; O, red; C, gray; H, white.)
Figure 4. Fragmented clusters used in the DFT calculations for the MOFs: (a) Cu-BTC; (b) Cu-MIPT. (Cu, green; O, red; C, gray; H, white.)
which agrees well with the experimental value of 2140 cm-1. All of the thermodynamic data were obtained at this computational level using the GAUSSIAN 03 suite of programs.34
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AUTHOR INFORMATION
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Corresponding Author: *To whom correspondence should be addressed. Tel: þ86-1064419862. E-mail:
[email protected].
ACKNOWLEDGMENT The financial support of the National
Natural Science Foundation of China (Grants 20906002, 20725622, 20821004) is greatly appreciated.
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