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Article

The Variety of Carbon-Metal Bonds inside Cu-ZSM-5 Zeolites: A Density Functional Theory Study

by
Takashi Yumura
1,*,
Saki Hasegawa
1,
Atsushi Itadani
2,
Hisayoshi Kobayashi
1 and
Yasushige Kuroda
2
1
Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto, 606-8585, Japan
2
Department of Fundamental Material Science, Division of Molecular and Material Science, Graduate School of Natural Science and Technology, Okayama University, Tsushima, Kita-ku, Okayama 700-8530, Japan
*
Author to whom correspondence should be addressed.
Materials 2010, 3(4), 2516-2535; https://doi.org/10.3390/ma3042516
Submission received: 31 January 2010 / Revised: 29 March 2010 / Accepted: 1 April 2010 / Published: 5 April 2010
(This article belongs to the Special Issue Organometallic Compounds)
Browse Figures
Versions Notes

Abstract

:
Large-scale density functional theory calculations (DFT) found various types of binding of an unsaturated hydrocarbon (C2H2 and C2H4) to a ZSM-5 zeolite extraframework copper cation. We employed the DFT calculations based on the B3LYP functional to obtain local minima of an unsaturated hydrocarbon adsorbed on one or two copper cations embedded inside ZSM-5, and then compared their stabilization energies. The DFT results show that the stabilization energies are strongly dependent on the copper coordination environment as well as configurations of two copper cations. Consequently, the inner copper-carbon bonds are influenced substantially by a nanometer-scale cavity of ZSM-5.

1. Introduction

Interactions between a transition metal atom and a hydrocarbon have attracted many researchers [1,2,3,4,5,6,7,8,9], because the interactions may result in an activation of the hydrocarbon. The interactions weaken the C–H and C–C bonds of a hydrocarbon, and therefore facilitate the transformation of a hydrocarbon into a more valuable species. The activation of the C–H and C–C bonds is a key in catalytic reactions in heterogeneous and homogeneous systems. Thus we need to obtain detailed information on the activation processes for the purpose of constructing a promising catalyst. One of the well-known examples is that a transition metal atom binds coordinatively to an unsaturated hydrocarbon, such as alkenes and alkynes, called Dewar-Chatt-Duncanson models [10]. In these models, π and π* orbitals of an unsaturated hydrocarbon are responsible for the interactions with a transition metal cation, because these orbitals match d orbitals in terms of orbital symmetry. Such orbital interactions can result in electron transfers between the two. If π orbitals of an unsaturated hydrocarbon are depopulated through the interactions with a transition metal atom, or its π* orbitals are populated, the CC bonds of unsaturated hydrocarbons are activated.
Of course the electron transfers depend on types of transition metal atom as well as its coordination environment [10]. When a transition metal atom is embedded in a nanometer-scale cavity of a host, the interactions with a guest unsaturated hydrocarbon can be further affected by host confinement. The confinement effects on the inner bond formation have been well discussed in our recent theoretical studies [11,12,13,14,15,16]. In particular, we found various types of binding of a guest molecule into copper cations enclosed in the restricted environment of a ZSM-5 zeolite [14,15,16]. Thus it is intriguing to investigate whether the zeolite confinement can have an impact on its inner catalytic reactions. Along our previous studies, we have a special interest on how a guest molecule interacts with an extraframework copper cation of the ZSM-5 zeolite, because Cu–ZSM-5 exhibits unique catalytic behaviors [17]. With respect to chemical phenomena involving unsaturated hydrocarbons inside copper-containing zeolites, they can afford to catalyze the formation of diynes from alkyenes [18] and the partial oxidization of propylene into acrolein [19]. In these catalytic reactions, alkynes and alkenes are expected to coordinate to embedded copper cations. However, our knowledge how the zeolite confinement affects the inner copper–carbon bond formation is still lacking. The clarification will contribute to construct catalysts that can form selectively a desirable product.
In this direction, one of the promising tools is computer simulations based on quantum chemistry, in particular density functional theory (DFT) methods, because DFT results can provide atomic-scale view of the metal–carbon bond formation inside a zeolite cavity. Accordingly we employed DFT calculations to analyze how the restricted environment affects the inner coordination bonds. In this study, we focus on copper–carbon bonds formed inside a nanometer-sized cavity surrounded by a ten-membered ring of copper-exchanged ZSM-5 (Cu–ZSM-5) in Figure 1. In the present study, we will discuss two issues: (a) how an unsaturated hydrocarbon (acethylene or ethylene) interacts with an extraframework copper cation of ZSM-5, and (b) factors determining characters of copper–carbon bonds formed in the restricted environment of a ten-membered ring of ZSM-5.

2. Computational Section

In order to investigate interactions between an unsaturated hydrocarbon (acethylene or ethylene) and an extraframework copper cation of ZSM-5, we employed a hybrid Hartree–Fock/DFT method (B3LYP) [20,21,22,23,24], in the Gaussian 03 program package [25]. ZSM-5 zeolite consists of 5- and 6-membered rings (MRs) on channel walls, and 10-MRs in the straight and sinusoidal channels, as shown in Figure 1. Note that the 10-MRs are on the order of nanometers in terms of separation between diametrically-opposed Si atoms. In this study we adopt Si3O4H8 and Si92O151H66 clusters as models of aluminum-free ZSM-5 (silicalite) [26], as shown in the right-hand side of Figure 1. The Si92O151H66 model, whose terminal Si atoms are bound by H atoms, corresponds to the red part of the ZSM-5 framework [26], and contains ZSM-5 ten-MRs explicitly. The B3LYP calculation shows that the model has purple ten-MR cavities whose diametrically-opposed Si atoms are ~9.4 Å apart [14,15]. The cavity sizes are essentially identical to those observed experimentally. Thus the model is realistic to represent a 10-MR cavity of ZSM-5.
Materials 03 02516 g001 1024
Figure 1. (a) The structures of ZSM-5 zeolite, models of aluminum-free ZSM-5, (b) Si3O4H8 and (c) Si92O151H66. The Si92O151H66 model corresponds to the red part in ZSM-5, and terminal atoms are saturated by H atoms.
Figure 1. (a) The structures of ZSM-5 zeolite, models of aluminum-free ZSM-5, (b) Si3O4H8 and (c) Si92O151H66. The Si92O151H66 model corresponds to the red part in ZSM-5, and terminal atoms are saturated by H atoms.
Materials 03 02516 g001
Using the realistic Si92O151H66 model in Figure 1, we constructed a Cun–ZSM-5 model (n = 1 or 2) where a copper cation is located in the vicinity of an aluminum atom substituted for a silicon atom within the purple ten-MR. To obtain optimized geometries for an unsaturated hydrocarbon adsorbed on a copper active center inside the ZSM-5 model, we used the 6-311G* basis set for the adsorbing molecule and the Cu+ cations [27,28,29], and the 6-31G* basis set for substituted Al atom and the two O atoms bound to the substituted Al atom, and are usually coordinated by the cations [30,31,32], and the 3-21G basis set for other atoms in the zeolite framework [33,34,35]. After the optimizations for an unsaturated hydrocarbon adsorbed on Cun–ZSM-5, we estimated their stabilization energy Estabilization, defined as E(adsorbent–Cun–ZSM-5) – E(adsorbent) – E(Cun–ZSM-5), where E(adsorbent–Cun–ZSM-5) is the total energy of an optimized structure for an adsorbent bound to Cun–ZSM-5, E(adsorbent) is that of an optimized structure for an adsorbent, and E(Cun–ZSM-5) is that of an optimized structure for Cun–ZSM-5. In the estimation, the Estabilization values were corrected for basis set superposition errors (BSSEs) by using the counterpoise method [36]. Within the B3LYP calculations, the optimized separations between a copper cation and a framework oxygen atom fall in the range of 1.89 to 2.55 Å, being consistent with the previous DFT results [37,38,39,40,41,42,43,44]. The ranges of the calculated Cu–O separations are in good agreement with those obtained from the XRD [45] and EXAFS [46,47,48] analyses (1.98~2.56 Å). Thus the theoretical method of our choice is appropriate for the present study.

3. Results and Discussion

3.1. ZSM-5 containing monocopper cation (Cu1–ZSM-5)

3.1.1. The small monocopper zeolite model

First we investigate bindings of an unsaturated hydrocarbon into a monocopper cation bound to a small zeolite framework (AlSi2O4H8) for obtaining a baseline for comparison. In the small zeolite model, the monocopper cation is coordinated by two framework oxygen atoms. Using the small zeolite model, we obtained optimized geometries for the binding of an unsaturated hydrocarbon into the monocopper cation, as shown in Figure 2.
Materials 03 02516 g002 1024
Figure 2. (a) Optimized structures for the binding of an unsaturated hydrocarbon ((1) C2H2 or (2) C2H4) into the monocopper cation bound to the AlSi2O4H8 model, and (b) their key orbital interactions.
Figure 2. (a) Optimized structures for the binding of an unsaturated hydrocarbon ((1) C2H2 or (2) C2H4) into the monocopper cation bound to the AlSi2O4H8 model, and (b) their key orbital interactions.
Materials 03 02516 g002
Figure 2 shows that the unsaturated hydrocarbons bind into the monocopper cation in a η2-bridging fashion. The Cu–C bond lengths were optimized to be 1.925 and 1.967 Å in the acethylene and ethylene cases, respectively. The optimized structures lie 33 kcal/mol below their dissociation limits toward an unsaturated hydrocarbon and the small zeolite model. The Estabilization values are consistent with those reported in the previous theoretical studies [49,50,51,52]. The stabilization mainly comes from in-phase interactions between the dxz(Cu) orbital of the extraframework monocopper and a π* orbital of C2H2 or C2H4, as shown in Figure 2. Due to the orbital interactions, the Cu(I) cation can donate electrons to the empty π* orbital of an unstaturated hydrocarbon. Accordingly, their CC bonds are activated by the bindings: the optimized CC bond lengths in the acethylene (1.241 Å) and ethylene (1.388 Å) cases are longer than the unperturbed cases by 0.042 Å and 0.061 Å, respectively.
The adsorbed acethylene and ethylene cannot retain liner D∞h and planar D2h structures, respectively. The geometrical distortions change their vabrational structures. In fact, we see in Table 1 and Table 2 that calculated CC stretching vibrational frequencies in the adsorbed acethylene and ethylene are 1793.8 and 1529.4 cm-1, respectively [53]. The values are smaller than those of free acethylene and ethylene (2001.4 and 1645.2 cm-1, respectively). The significant decrease in the CC stretching vibrational frequencies is due to the CC bond activation. Furthermore, their bindings into the extraframework copper cation make CC stretching modes infrared (IR)-active, due to symmetry lowering. Note that CC stretching vibrational modes of free C2H4 and C2H2 span Ag and Σg+, respectively and thus the modes are IR-inactive [54,55]. Similar symmetry lowering can be seen in the symmetric CH stretching mode in the adsorbed C2H2, and thus a new IR peak appears around 3273 cm-1 after the C2H2 binding into the monocopper cation [56,57,58].
Table
Table 1. Calculated vibrational frequencies (cm-1) of CC and CH stretching modes of C2H2 before and after the binding into the small monocopper zeolite model.
Table 1. Calculated vibrational frequencies (cm-1) of CC and CH stretching modes of C2H2 before and after the binding into the small monocopper zeolite model.
Free C2H2C2H2 on Cu1-zeolite
SymmetryFrequencyIR intensity aFrequencyIR intensity a
C≡C stretchΣg+2001.40.001793.81.12
C–H stretchΣg+3400.00.003273.70.40
C–H stretchΣu+3300.513204.01
a IR intensities are given relative to that of the C–H stretching mode spanning Σu+.
Table
Table 2. Calculated vibrational frequencies (cm-1) of CC and CH stretching modes of C2H4 before and after the binding into the small monocopper zeolite model.
Table 2. Calculated vibrational frequencies (cm-1) of CC and CH stretching modes of C2H4 before and after the binding into the small monocopper zeolite model.
Free C2H4C2H4 on Cu1-zeolite
SymmetryFrequencyIR intensity aFrequencyIR intensity a
C≡C stretchAg1645.20.001529.40.32
C–H stretchAg3036.30.003023.90.00
C–H stretchB3g3089.80.003090.70.00
C–H stretchB1u3021.30.643017.70.98
C–H stretchB2u3117.513113.21
a IR intensities are given relative to that of the C–H stretching mode spanning B2u.

3.1.2. Realistic Cu1–ZSM-5 model

In the previous section (3.1.1), we discussed how an unsaturated hydrocarbon binds into a two-coordinated copper cation by using the small zeolite model. Although we obtained a baseline of the Cu–C bindings, the information is not sufficient to fully understand inner Cu–C interactions in real Cu–ZSM-5 framework due to the variety of the copper coordination environment. The coordination environment should change d-splittings of an extraframework copper cation, and therefore the d–π* interactions are influenced by the copper sites. As a result, the coordination environment should determine the attraction forces operating between the copper cation and an unsaturated hydrocarbon.
Materials 03 02516 g003 1024
Figure 3. Local structures of three optimized structures for a monocopper cation sitting inside the AlSi91O151H66 ZSM-5 model. (a) the copper cation sits at the intersection between a straight and a zigzag channel, denoted by I. (b) the copper cation sits above a 5-membered ring of a wall along a straight channel, denoted by 5-MR. (c) the copper cation sits above a 6-membered ring of a wall along a straight channel, denoted by 6-MR.
Figure 3. Local structures of three optimized structures for a monocopper cation sitting inside the AlSi91O151H66 ZSM-5 model. (a) the copper cation sits at the intersection between a straight and a zigzag channel, denoted by I. (b) the copper cation sits above a 5-membered ring of a wall along a straight channel, denoted by 5-MR. (c) the copper cation sits above a 6-membered ring of a wall along a straight channel, denoted by 6-MR.
Materials 03 02516 g003
ZSM-5 zeolite has 12 distinguishable tetrahedral(T)-sites in the orthogonal structure. Ref. [37] shows there is no significant difference between the relative energies of ZSM-5 where one Al atom replaces one Si atom in different T-sites. In addition, Nachtigall and Bell separately investigated CO adsorption into Cu–ZSM-5 with one substituted Al atom in different T-sites. Their extensive studies show that the interaction energies between CO and Cu–ZSM-5 as well as CO stretching frequencies change significantly, depending on Cu site types. Also they indicated that the location of the Al atom does not have influence on the binding energies. Judging from the interaction energies, binding sites for an extraframework cation inside ZSM-5 are categorized into three subgroups in Figure 3: one is a cation site near an intersection between a straight and a sinusoidal channels, denoted by I, and the others are cations located above a 5-membered and 6-membered rings of a wall along a straight channel, denoted by 5-MR and 6-MR, respectively. Along their theoretical findings, we considered the three binding sites for the Cu cation inside ZSM-5, where the substituted Al atom is located near the cation.
Figure 3 shows that the copper coordination environment in the I configuration is similar to that in the small zeolite model: in the I configuration, the monocopper cation is bound to two framework oxygen atoms near the substituted Al atom. In contrast, the 5-MR and 6-MR configurations have the monocopper cations with a coordination number of 3 [16]. Due to the different copper coordination environment, their electronic configurations are different, as shown in Table 3. The coordination of a Cu(I) cation into ZSM-5 framework results in some degree of the 3d10→3d94s1 promotion [41]. In terms of the 3d10–3d94s1 promotion, there is a slight difference between the I and 5-MR (6-MR) configurations: the amount of the 4s electron in the I configuration (0.25e) is less significant than those in the 5-MR and 6-MR configurations (~0.4 e). Since we found the differences in the Cu coordination environments in the three configurations, it is interesting to investigate how the copper coordination environment affects the interactions with an unsaturated hydrocarbon.
Table
Table 3. Electronic configurations of Cu–ZSM-5 before and after the binding of an unsaturated hydrocarbon, based on natural atomic orbital analyses (NPA).
Table 3. Electronic configurations of Cu–ZSM-5 before and after the binding of an unsaturated hydrocarbon, based on natural atomic orbital analyses (NPA).
ConfigurationAdsorbentElectronic configuration
I–4s ( 0.25) 3d ( 9.84)
C2H24s ( 0.36) 3d ( 9.56) 4p ( 0.01)
C2H44s ( 0.37) 3d ( 9.57) 4p ( 0.01)
5-MR–4s ( 0.42) 3d ( 9.73)
C2H24s ( 0.36) 3d ( 9.57) 4p ( 0.01)
C2H44s ( 0.36) 3d ( 9.58) 4p ( 0.01)
6-MR–4s ( 0.44) 3d ( 9.71)
C2H24s ( 0.36) 3d ( 9.55) 4p ( 0.01)
C2H44s ( 0.37) 3d ( 9.57) 4p ( 0.01)
Optimized structures for unsaturated hydrocarbons adsorbed on a Cu1–ZSM-5 model are shown in Figure 4. Table 4 tabulates key parameters for all optimized geometries. We can see in Figure 4 the same types of bindings of the unsaturated hydrocarbons into the copper cation (η2-fashion), irrespective of different copper coordination environment as well as types of unsaturated hydrocarbon considered. Similar binding fashions have been already reported in Refs. [49,50,51,52]. In the η2-fashion of the C2H2 (C2H4) bindings, the optimized Cu–C and CC bond lengths are ~1.94 (~1.97) Å and ~1.24 (~1.38) Å, respectively. The CC bonds in the adsorbed unsaturated hydrocarbons are lengthened relative to the free unsaturated hydrocarbons, indicating that the Cu–C bond formation results in the CC bond activation. Interestingly the Cu–C bondings in the η2-fashion are completely identical to those of the small model. Note that the copper coordination environments in the three configurations are also same after the bindings, in contrast to those before the bindings. Reflecting the same Cu coordination environments, the three models have similar electron configurations of the Cu(I) cation in Table 3. In addition, the amounts of electron transferring (~0.2e) upon the bindings are similar among the three configurations. However, we see slight difference between the I and 5-MR (6-MR) configurations in terms of how the electrons transfer between an unsaturated hydrocarbon and Cu–ZSM-5. In the I configuration, the 4s electron densities increase, whereas the 3d electron densities decrease. Since the two Cu–O bond lengths remain almost unchanged during the binding (Figure 3 and Figure 4), the electron transfer is responsible for the Cu–C bond formation. On the other hand, we can see that both 3d and 4s orbitals are depopulated upon the bindings in the 5-MR and 6-MR configurations. Compared with the I configuration, the electron transfers in these configurations originate from not only the formation of the two Cu–O bonds but also significant changes in the copper coordination environments. In fact we see in Figure 3 and Figure 4 that the Cu coordination number changes from 3 to 2, upon the bindings of an unsaturated hydrocarbon into an extraframework copper at 5-MR or 6-MR configuration. In contract, such changes in the Cu coordination number are not seen in the I configuration.
Materials 03 02516 g004 1024
Figure 4. Local structures of three optimized structures for the binding of an unsaturated hydrocarbon ((1) C2H2 or (2) C2H4) into a monocopper cation embedded inside the AlSi91O151H66 ZSM-5 model. (a) the copper cation sits at the intersection between a straight and a zigzag channel, denoted by I. (b) the copper cation sits above a 5-membered ring of a wall along a straight channel, denoted by 5-MR. (c) the copper cation sits above a 6-membered ring of a wall along a straight channel, denoted by 6-MR. Optimized bond lengths are in Å. The Estabilization values are in kcal/mol.
Figure 4. Local structures of three optimized structures for the binding of an unsaturated hydrocarbon ((1) C2H2 or (2) C2H4) into a monocopper cation embedded inside the AlSi91O151H66 ZSM-5 model. (a) the copper cation sits at the intersection between a straight and a zigzag channel, denoted by I. (b) the copper cation sits above a 5-membered ring of a wall along a straight channel, denoted by 5-MR. (c) the copper cation sits above a 6-membered ring of a wall along a straight channel, denoted by 6-MR. Optimized bond lengths are in Å. The Estabilization values are in kcal/mol.
Materials 03 02516 g004
Despite the same binding types in the three configurations, the stabilization energies depend on their copper coordination environment: the calculated Estabilization values in the I configuration (–34.4 (C2H2) and –32.4 (C2H4) kcal/mol) are more pronounced than those in the 5-MR configuration (–14.6 (C2H2) and –14.3 (C2H4) kcal/mol) and the 6-MR configuration (–14.4 (C2H2) and –14.7 (C2H4) kcal/mol). The stabilization energies in the I configuration are almost the same as those in the small zeolite models, whereas those in the 5-MR and 6-MR configurations are different. The different Estabilization values are also associated with the changes in the copper coordination environments upon the bindings. Here we consider quantitatively why the copper coordination environment plays an essential role in determining the stabilization energies. In general, an extraframework cation shifts from its original position by approaching of an adsorbent to the cation. The cation shifts destabilize a zeolite structure itself. The destabilization by the cation shifts counteracts direct attractive interactions between an extraframework cation and an adsorbent. Thus the balance between the destabilization by the cation shifts and the stabilization by the direct interactions determines a stable conformation of an adsorbent inside a zeolite. The importance of the balance has been already discussed by Nachtigall and coworkers [59,60]. We can see dependences of the destabilization on the copper coordination environment in Table 4, where the destabilization energies E(deform) are listed. The E(deform) values are defined by E(deformed Cu–ZSM-5) – E(adsorbent-free Cu–ZSM-5). Here E(deformed Cu–ZSM-5) is the single-point-energy of a deformed Cu–ZSM-5 taken from an optimized adsorbent–Cu–ZSM-5 complex, and E(adsorbent-free Cu–ZSM-5) is the total energy of an optimized adsorbent-free Cu–ZSM-5 structure. Positive E(deform) values indicate that a Cu–ZSM-5 itself is destabilized by the inner coordination bond formation. Table 4 indicates strong site-dependencies of the E(deform) values.
Table
Table 4. Key parameters of the optimized structures for an unsaturated hydrocarbon adsorbed on a dicopper active center embedded inside a realistic ZSM-5 model.
Table 4. Key parameters of the optimized structures for an unsaturated hydrocarbon adsorbed on a dicopper active center embedded inside a realistic ZSM-5 model.
Configuration aAdsorbentBinding modeCu–CbCCbEstabilizationcE(deform) d
IC2H2η21.934, 1.9351.239–34.42.6
5MRC2H2η21.934, 1.9601.238–14.620.4
6MRC2H2η21.937, 1.9381.240–14.422.5
IC2H4η21.969, 1.9741.386–32.42.3
5MRC2H4η21.974, 1.9791.385–14.317.8
6MRC2H4η21.970, 1.9721.386–14.720.3
a Configuration: I is the intersection site, 5MR and 6MR are the 5- and 6-membered sites.; b Bond lengths in Å.; c Estabilization in kcal/mol.; d E(deform) in kcal/mol.
In fact the E(deform) values in the I configuration (2.6 (C2H2) and 2.3 (C2H4) kcal/mol) are negligible relative to those in the 5-MR configuration (20.4 (C2H2) and 17.8 (C2H4) kcal/mol) and the 6-MR configuration (22.5 (C2H2) and 20.3 (C2H4) kcal/mol). The site-dependent E(deform) values are reasonable, because decreasing the copper coordination number in the 5-MR and 6-MR configurations loses attractive Cu–O interactions at some extent. Taking the different E(deform) values into account, we can understand that bindings of an unsaturated hydrocarbon to the monocopper cation in the I configurations are energetically favorable over those in the 5-MR and 6-MR configurations. Note that the deformation energies are more significant than those in the interaction with NO molecule (the E(deform) values are 1 and 8 kcal/mol for intersection and channel wall sites, respectively [41]). The larger E(deform) values suggest that the η2-bindings require larger displacement of copper cations rather than the η1-bindings. In this situation we demonstrated from DFT calculations that the copper coordination environment is a key factor determining the bindings of an unsaturated hydrocarbon into an extraframework monocopper cation of ZSM-5.

3.2. ZSM-5 containing dicopper active center (Cu2–ZSM-5)

In this section we will focus on an unsaturated hydrocarbon bound to a dicopper active center embedded in ZSM-5. In this situation, configurations of the two copper cations within a ZSM-5 cavity may be responsible for the unsaturated hydrocarbon bindings, in addition to their copper coordination environment. Experimentally the presence of Cu pairs in ZSM-5 was demonstrated by using extended X-ray absorption fine structure (EXAFS) [61,62,63] spectroscopy and X-ray power diffraction [45] studies. To fully understand behaviors of an unsaturated hydrocarbon inside a ZSM-5 cavity, it is indispensable to clarify how configurations of the two copper cations inside ZSM-5 affect the properties of adsorbed unsaturated hydrocarbons.

3.2.1. The small dicopper zeolite model

First we use a small zeolite model containing the dicopper active center ([Cu–AlSi2O4H8]2) to increase our preliminary understanding of the interactions with an unsaturated hydrocarbon. Using the small dicopper zeolite model, we optimized an unsaturated hydrocarbon adsorbed on the dicopper active center, except for the Cu•••Cu separation (SCu•••Cu). Potential energy surfaces of the approaching of an unsaturated hydrocarbon into the dicopper active center are seen in Figure 5 as a function of SCu•••Cu. Figure 5 shows their stabilization energies Estabilization’ defined as – E(adsorbent) – E(Cu2–zeolite[SCu•••Cu]) + E(adsorbent–Cu2–zeolite[SCu•••Cu]), where E(adsorbent) is the total energy of the optimized structure for an adsorbent, E(adsorbent–Cu2–zeolite[SCu•••Cu]) is that of an optimized structure for an adsorbent bound to the small dicopper zeolite at a certain SCu•••Cu value, and E(Cu2–zeolite[SCu•••Cu]) is that of a Cu2–zeolite taken from the optimized adsorbent–Cu2–zeolite[SCu•••Cu] structure. When SCu•••Cu is 6 Å, C2H2 or C2H4 binds into one copper cation bound to the AlSi2O4H8 zeolite model. Then the optimized structures for the C2H2 and C2H4 bindings are, respectively, 35.6 and 37.2 kcal/mol stable relative to those in the dissociation limits toward an unsaturated hydrocarbon and the free small zeolite model with the two copper cations apart by 6 Å. Of course, the Estabilization’ values are close to those obtained in the small monocopper zeolite.
Decreasing the Cu•••Cu separation, two Cu–C bonds are newly generated. As a result, an unsaturated hydrocarbon binds into both copper cations in a μ-η2: η2 fashion. Because of the new Cu–C bond formation, the Estabilization’ values lower significantly until a certain SCu•••Cu value. In fact we see in Figure 5 that the potential energy surface of the C2H2–dicopper (C2H4–dicopper) complex has one local minimum at SCu•••Cu of 2.888 (3.735) Å.
Materials 03 02516 g005 1024
Figure 5. (a) Potential energy surfaces of approaching of an unsaturated hydrocarbon (C2H2 or C2H4) into the smaller dicopper zeolite model ([Cu–Al1Si3O4H8]2). Estabilization’ is plotted as a function of the Cu•••Cu separation. (b) optimized structures for the binding of an unsaturated hydrocarbon ((1) C2H2 or (2) C2H4) into the small dicopper zeolite model.
Figure 5. (a) Potential energy surfaces of approaching of an unsaturated hydrocarbon (C2H2 or C2H4) into the smaller dicopper zeolite model ([Cu–Al1Si3O4H8]2). Estabilization’ is plotted as a function of the Cu•••Cu separation. (b) optimized structures for the binding of an unsaturated hydrocarbon ((1) C2H2 or (2) C2H4) into the small dicopper zeolite model.
Materials 03 02516 g005
As shown in Figure 5 and Table 5 the optimized C2H2– and C2H4–dicopper complexes lie, respectively, 34.0 and 21.4 kcal/mol below the structures with the SCu•••Cu values of ~6 Å. These DFT results clearly show that the interactions of acethylene with the dicopper active center are more significant than the ethylene case. The differences between the acethylene and ethylene additions are unique in the dicopper cases, which cannot be seen in the monocopper cases. Moreover, we can see some discrepancy in the optimized structures between the acathylene and ethylene cases: the optimized C2H2–Cu2–zeolite contains a μ-η2: η2 Cu2C2 core with a butterfly form, whereas the C2H4–Cu2–zeolite contains a planar μ-η2: η2 core. Reflecting the structural differences, the C2H2–Cu2–zeolite has a smaller SCu•••Cu value than that in C2H4–Cu2–zeolite.
Table
Table 5. Key parameters of the optimized structures of an unsaturated hydrocarbon adsorbed onto a dicopper active center bound to a small zeolite model.
Table 5. Key parameters of the optimized structures of an unsaturated hydrocarbon adsorbed onto a dicopper active center bound to a small zeolite model.
AdsorbentBinding modeCu–CaCCaSCu•••CubEstabilization’c
C2H2μ−η2: η21.932, 1.933
1.933, 1.934		1.2972.888–69.6
C2H4μ−η2: η22.003, 2.003,
2.003, 2.003		1.4493.735–58.6
a Bond lengths in Å.; bSCu•••Cu is the Cu•••Cu separation in Å.; c Estabilization’ in kcal/mol.
Whether the Cu2C2 core has a planar or a butterfly structure can be confirmed by their IR spectra, especially their CC stretching vibrational modes (Table 6 and Table 7). We see in Table 6 and Table 7 that CC stretching vibrational frequencies were calculated to be 1557.1 and 1469.4 cm-1 in C2H2 and C2H4 adsorbed on the dicopper active center, respectively. Lower CC stretching frequencies than those in the monocopper model are ascribed to more significant CC bond activation by the dicopper active center: the optimized CC bond lengths in the acethylene and ethylene additions are 1.297 and 1.449 Å, respectively (see Table 5). More importantly, the CC stretching vibrational mode in the acethylene addition is IR-active, whereas that in the ethylene addition is IR-inactive due to the planarity of its Cu2C2 core. The calculated IR data will help to determine how an unsaturated hydrocarbon binds into a dicopper active center embedded inside ZSM-5.
Table
Table 6. Calculated vibrational frequencies (cm-1) of CC and CH stretching modes of C2H2 before and after the binding into the small dicopper zeolite model.
Table 6. Calculated vibrational frequencies (cm-1) of CC and CH stretching modes of C2H2 before and after the binding into the small dicopper zeolite model.
Free C2H2C2H2 on Cu2-zeolite
SymmetryFrequencyIR intensityaFrequencyIR intensitya
C≡C stretchΣg+2001.40.001557.11.44
C–H stretchΣg+3400.00.003170.00.90
C–H stretchΣu+3300.513127.81
a IR intensities are given relative to that of the C–H stretching mode spanning Σu+.
Table
Table 7. Calculated vibrational frequencies (cm-1) of CC and CH stretching modes of C2H4 before and after the binding into the small dicopper zeolite model.
Table 7. Calculated vibrational frequencies (cm-1) of CC and CH stretching modes of C2H4 before and after the binding into the small dicopper zeolite model.
Free C2H4C2H4 on Cu2-zeolite
Symmetry FrequencyIR intensity aFrequencyIR intensity a
C≡C stretchAg1645.20.001469.40.00
C–H stretchAg3036.30.003004.90.00
C–H stretchB3g3089.80.003085.40.00
C–H stretchB1u3021.30.643001.817.10
C–H stretchB2u3117.513100.51
a IR intensities are given relative to that of the C–H stretching mode spanning Β2u.

3.2.2. Realistic Cu2–ZSM-5 model

In section 3.2.1, we used the small dicopper zeolite model, and found differences between the dicopper and monocopper cations in terms of the interactions with an unsaturated hydrocarbon. Next we turn to dicopper active centers located in a 10-MR cavity of the realistic ZSM-5 model Al2Si90O151H60. Since copper cations usually sit near the Al substituted positions, the locations of the double Si→Al substitution within the ZSM-5 framework control the configurations of a dicopper active center. Here we consider four locations of the double substitution in Figure 6: the 2NN, 3NN, 4NN, and 5NN configurations contain the Al pairs being, respectively second, third, fourth, and fifth nearest-neighbors with respect to tetrahedral sites contained in the 10-MR. Using the different configurations of the Al pair, their initial geometries were constructed by placing each Cu apart by ~2.0 Å from two oxygen atoms bound to a substituted Al atom. After the B3LYP optimization, four configurations of the dicopper center inside ZSM-5 were obtained, as shown in Figure 6. The Cu•••Cu separation in Cu2–ZSM-5 decreases in the order 5NN (6.442 Å) > 4NN (6.370 Å) > 3NN (2.561 Å) > 2NN (2.372 Å). The Cu•••Cu separations in the 2NN and 3NN configurations are close to those obtained by EXAFS analyses (2.47–3.13 Å). In the optimized Cu2–ZSM-5 structures, each Cu cation coordinates into two or three framework oxygen atoms. The Cu•••Cu separations are out of the range of a suitable span between the two copper cations into which an unsaturated hydrocarbon preferentially binds in a μ-η2: η2-fashion (Figure 5). Note that the formation of Cu pairs in the 3NN and 2NN configurations is consistent with the Spuhler’s findings [38] by means of a combined quantum mechanics/interatomic potential function technique(QM-pot).
Taking varying Cu•••Cu separations by locations of the double Si→Al substitution into account, we discuss how an unsaturated hydrocarbon binds into a dicopper active center. Figure 7 shows the optimized structures for an unsaturated hydrocarbon adsorbed on a dicopper active center embedded in a ZSM-5 model, whose key parameters are listed in Table 8. We can see in Figure 7 and Table 8 two types of binding of C2H2 into a dicopper active center. In the 2NN configuration, C2H2 binds into the dicopper active center in a μ-η1: η1 fashion, whereas the 3NN, 4NN, and 5NN configurations have a μ-η2: η2 Cu2C2 core. In the 3NN, 4NN, and 5NN configurations, the Cu•••Cu separations were optimized to be 3.047, 3.236, and 3.315 Å, respectively. These SCu•••Cu values are close to the separation between the two copper cations into which C2H2 binds in a μ-η2: η2 fashion in Figure 5. The significant differences in the SCu•••Cu values between before and after the C2H2 binding (ΔSCu•••Cu) indicate that two copper cations significantly shift after the binding. In contrast, the 2NN configuration does not have a room, and thus the two copper cations cannot move to positions suitable for the C2H2 binding in a μ-η2: η2 fashion. Instead, the 2NN configuration adopts the μ-η1:η1 binding fashion with a stabilization energy of –33.7 kcal/mol. Surprisingly the Estabilization value is comparable to that of the 3NN configuration (–35.8 kcal/mol), despite the different C2H2 binding fashions.
Materials 03 02516 g006 1024
Figure 6. Local structures of four optimized Cu2–ZSM-5 structures where the ZSM-5 framework was modeled as a Al2Si90O151H66 cluster. The four types of optimized structures can be distinguished by positions of double Si→Al substitution: the two substituted Al atoms within a ten-membered ring are second nearest neighbor (2NN), third nearest neighbor (3NN), fourth nearest neighbor (4NN), and fifth nearest neighbor (5NN). Optimized bond lengths are given in Å.
Figure 6. Local structures of four optimized Cu2–ZSM-5 structures where the ZSM-5 framework was modeled as a Al2Si90O151H66 cluster. The four types of optimized structures can be distinguished by positions of double Si→Al substitution: the two substituted Al atoms within a ten-membered ring are second nearest neighbor (2NN), third nearest neighbor (3NN), fourth nearest neighbor (4NN), and fifth nearest neighbor (5NN). Optimized bond lengths are given in Å.
Materials 03 02516 g006
Materials 03 02516 g007 1024
Figure 7. Local structures of four optimized structures for the binding of an unsaturated hydrocarbon ((1) C2H2 or (2) C2H4) into a dicopper active center embedded inside the Al2Si90O151H66 ZSM-5 model. The four types of optimized structure can be distinguished by positions of double Si→Al substitution: the two substituted Al atoms within a ten-membered ring are (a) second nearest neighbor (2NN), (b) third nearest neighbor (3NN), (c) fourth nearest neighbor (4NN), and fifth nearest neighbor (5NN). Optimized bond lengths together with the Cu•••Cu separations are given in Å. The Estabilization values are in kcal/mol.
Figure 7. Local structures of four optimized structures for the binding of an unsaturated hydrocarbon ((1) C2H2 or (2) C2H4) into a dicopper active center embedded inside the Al2Si90O151H66 ZSM-5 model. The four types of optimized structure can be distinguished by positions of double Si→Al substitution: the two substituted Al atoms within a ten-membered ring are (a) second nearest neighbor (2NN), (b) third nearest neighbor (3NN), (c) fourth nearest neighbor (4NN), and fifth nearest neighbor (5NN). Optimized bond lengths together with the Cu•••Cu separations are given in Å. The Estabilization values are in kcal/mol.
Materials 03 02516 g007
In the above discussion, we found that whether the two copper cations smoothly shift determines a preferable C2H2 binding fashion in a ZSM-5 cavity. Although the importance of the copper shifts can be also seen in the monocopper case, the E(deform) values in the dicopper cases (Table 8) are more pronounced than those in the monocopper cases. In addition, we found from Table 8 strong dependences of the E(deform) values on the dicopper configurations: the E(deform) values decrease in the following order: 5NN ≈ 4NN > 3NN > 2NN. The decrease in the E(deform) values has a close relation to the ΔSCu•••Cu values. Table 8 shows that the ΔSCu•••Cu values decrease in the order 4NN ≈ 5NN > 3NN > 2NN, suggesting that the C2H2 bindings in the 5NN and 4NN configurations cause two copper cations to shift significantly, compared with those in the 3NN and 2NN configurations. Therefore, the dependent E(deform) values are understandable. Reflecting the E(deform) values, the stabilization in the 4NN and 5NN configurations is less significant than that in the 3NN configuration, because the destabilization by the cation shifts diminishes the attraction by the direct interactions. Thus the variety of the Cu–C bonding characters is unique in the restricted environment of ZSM-5.
Table
Table 8. Key parameters of the optimized structures of an unsaturated hydrocarbon adsorbed onto a dicopper active center embedded inside a realistic ZSM-5 model.
Table 8. Key parameters of the optimized structures of an unsaturated hydrocarbon adsorbed onto a dicopper active center embedded inside a realistic ZSM-5 model.
Configuration aAdsorbentBinding modeCu–C bCC bSCu•••Cu c
(ΔSCu•••Cud)		Estabilization eE(deform) f
2NNC2H2μ−η1: η11.896, 1,8961.2662.289
(–0.083)		–33.719.0
3NNC2H2μ−η2: η21.915, 1.923,
1.987, 2.014		1.2853.047
(0.486)		–35.824.7
4NNC2H2μ−η2: η21.934, 1.939,
1.956, 1.990		1.2863.236
(–3.134)		–19.045.7
5NNC2H2μ−η2: η21.958, 1.960,
1.967, 1.974 		1.2843.315
(–3.127)		–19.147.2
2NNC2H4μ−η1: η11.946, 1.9431.4212.253
(–0.119)		–30.118.0
3NNC2H4μ−η1: η21.938, 2.034,
2.097		1.3993.223
(0.662)		–16.425.5
4NNC2H4μ−η2: η21.982, 1.987,
2.024, 2.038		1.4423.730
(–2.640)		–13.641.9
5NNC2H4μ−η2: η22.015, 2.028,
2.029, 2.034		1.4413.787
(–2.655)		–16.643.1
a Configuration: The 2NN, 3NN, and 4NN configurations have Al pairs being second-, third-, and fourth nearest neighbors within a ten-membered ring of the ZSM-5 model, respectively.; b Bond lengths in Å.; c SCu•••Cu is the Cu•••Cu separation in Å.; d ΔSCu•••Cu is the difference in the Cu•••Cu separation between before and after the binding of an unsaturate hydrocarbon.; e Estabilization’ in kcal/mol.; f E(deform) in kcal/mol.
Compared with the C2H2 bindings, more complicated binding modes are found in the C2H4 bindings into a dicopper center in Figure 7: the 2NN configuration has a μ-η1:η1 form, the 3NN configuration has a μ-η1:η2 form, and the 4NN and 5NN configurations have a μ-η2:η2 form. The calculated Estabilization values are close to those obtained experimentally (–15.4 and –21.4 kcal/mol) [64]. The μ-η2:η2 binding fashions in the 4NN and 5NN configurations are similar to that obtained in the small zeolite model: in both cases, the Cu and C atoms virtually lie in a plane. The Cu•••Cu separations in the 4NN and 5NN configurations (3.730 and 3.787 Å, respectively) are essentially identical to the equilibrium separation between C2H4 and the small zeolite model (3.735 Å in Figure 5), and thus the similarity in the binding fashion is reasonable. In contrast, the optimized SCu•••Cu values in the 2NN and 3NN configurations are 1.482 and 0.512 Å smaller than the equilibrium separation in the small dicopper zeolite model. Accordingly these configurations cannot adopt a μ-η2:η2 binding fashion. Note that the μ-η1:η2 binding fashion (3NN) is an intermediate between the μ-η1:η1 (2NN) and μ-η2:η2 (4NN and 5NN) fashions. How C2H4 binds into a dicopper active center inside ZSM-5 is also followed by the balance rule. As shown in Table 8, destabilization of the Cu2–ZSM-5 by the C2H4 binding is similar to that by the C2H2 binding form a viewpoint of energetics. However, direct interactions by the C2H4 binding are 11.0 kcal/mol weaker than those by the C2H2 binding (see Table 5). Compared with the C2H2 bindings, the importance of the destabilization by the cation shifts to determine a preferable C2H4 binding fashion is more effective rather than the direct attractive C2H4–dicopper interactions.

4. Conclusions

We found from large-scale DFT calculations that characters of copper–carbon bonds formed inside ZSM-5 change significantly, depending on its copper coordination environment. Actually attractive interactions of an unsaturated hydrocarbon with a two-coordinated extraframework copper cation are significant relative those with a higher-coordinated copper cation. The dependences of the interactions are related with shifts of a copper cation accompanied by the bindings: the shift of a cation with a higher coordination number costs energy a lot. Thus site-preferences of two-coordinated copper cations as the unsaturated-hydrocarbon binding site are reasonable. When an unsaturated hydrocarbon binds into an embedded dicopper active center, configurations of the two copper cations are important to determine the bindings, in addition to its coordination environment. Because of the different interactions between an unsaturated hydrocarbon and a mono- or dicopper copper active center, various binding fashions (η2, μ-η1:η1, μ-η1:η2, and μ-η2:η2 fashions) are expected in Cu–ZSM-5. The variety of characters of the copper–carbon bonds is unique in the restricted environment of a zeolite.

Acknowledgements

Support by Grants-in Aid for Scientific Research from the Ministry of Education, Cluture, Sports, Science, and Technology of Japan (No. 21655021).

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MDPI and ACS Style

Yumura, T.; Hasegawa, S.; Itadani, A.; Kobayashi, H.; Kuroda, Y. The Variety of Carbon-Metal Bonds inside Cu-ZSM-5 Zeolites: A Density Functional Theory Study. Materials 2010, 3, 2516-2535. https://doi.org/10.3390/ma3042516

AMA Style

Yumura T, Hasegawa S, Itadani A, Kobayashi H, Kuroda Y. The Variety of Carbon-Metal Bonds inside Cu-ZSM-5 Zeolites: A Density Functional Theory Study. Materials. 2010; 3(4):2516-2535. https://doi.org/10.3390/ma3042516

Chicago/Turabian Style

Yumura, Takashi, Saki Hasegawa, Atsushi Itadani, Hisayoshi Kobayashi, and Yasushige Kuroda. 2010. "The Variety of Carbon-Metal Bonds inside Cu-ZSM-5 Zeolites: A Density Functional Theory Study" Materials 3, no. 4: 2516-2535. https://doi.org/10.3390/ma3042516

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