Supporting Information

Transcription

1 Supporting Information Roles of Water Molecules in Modulating the Reactivity of Dioxygen-bound - ZSM-5 toward Methane: A Theoretical Prediction Takashi Yumura,,* Yuuki Hirose, Takashi Wakasugi, Yasushige Kuroda, and Hisayoshi Kobayashi Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto, , Japan Department of Chemistry, Graduate School of Natural Science and Technology, kayama University, Tsushima, Kita-ku, kayama , Japan * To whom all correspondence should be addressed. yumura@kit.ac.jp (Takashi Yumura) S1. Model of ZSM-5 zeolite: (Figure S1) S2. Calculated Total energies of ptimized Geometries for 2-bound ZSM- 5 with or without water molecule in Figure 1: (Table S1) S3. Properties of 2 bound Dicopper te inside a ZSM-5 Cavity, Compared with Species: (Figure S2) S4. Spin Densities on Reactive Intermediates during the Dioxygen Activation by ZSM-5 in the Presence of ne Water Molecule: (Figure S3) S5. Activation of a Methane C H Bond by the Species inside a ZSM- 5 Cavity: (Figure S4) S6. Influence of ne Water Molecule in the Reactivity of 2-bound Trinuclear Copper te inside a ZSM-5 Cavity: (Figure S5) S7. Dioxygen Activation by ZSM-5 in the Presence of Two Water Molecules (Figures S6 and S7) 1

2 S1. Model of ZSM-5 zeolite As shown in Figure S1, we constructed an aluminum-free ZSM-5 model (silicalite- 1) including a 10-membered ring (10-MR) from periodic silicalite-1 structure (Figure S1(a)) implemented in Cerius 2. The model has chemical formula of 92151H66 (Figure S1(b)). In Figure S1(c), we magnified ten-membered rings of the 92151H66 model. To construct ZSM-5 model, we substituted some atoms for atoms of 92151H66 resulting in H66. In this study, we considered the pair being third-nearest neighbor within a ten-membered ring of the ZSM-5 mode, schematically given in Figure S1(d). The H66 structure has a negative charge, and then the two copper cations are encapsulated inside a ZSM-5 cavity to neutralize the systems in this study. By using the -ZSM-5 model, we optimized a dinuclear copper structure inside a ten-membered ring of the ZSM-5 model with the third nearest neighbor pair, as shown in Figure S1(e). See details in Ref. 41. S2. Calculated Total energies of ptimized Geometries for 2-bound ZSM- 5 with or without water molecules in Figure 1 Table S1. Total energies of optimized geometries for 2-bound ZSM-5 with or without water molecules in Figure 1. Values are given in Hartree. Number of water molecules Label a Total energy (Hartree) 0 a b c d e f a The optimized structures for 2-bound ZSM-5 with or without water molecules can be found in Figure 1. S3. Properties of 2 bound Dicopper te inside a ZSM-5 Cavity, Compared with Species 2

3 (a) -free ZSM-5 (b) -free ZSM-5 model: H 66 H (e) optimized dinuclear copper structure inside the ZSM-5 model with pair being a third nearest neighbor within a tenmembered ring ( ZSM-5 (TNN)) (d) schematic view of orientation of pair within a ten-membered ring of the H 66 model, and position of a dinuclear copper site near the pair being a third nearest neighbor. Third-nearest-neighbor pair (TNN) (c) magnification of ten-membered rings of the H 66 model Figure S1. Modeling of ZSM-5 zeolite. (a) periodic structure of -free ZSM-5 (b) -free ZSM-5 model: H 66 (c) magnification of ten-membered rings of the H 66 model (d) Schematic view of orientation of pair being a third nearest neighbor within a ten-membered ring of the H 66 model (e) optimized dinuclear copper structure inside the ZSM-5 model with pair being a third nearet neighbor within a ten-membered ring ( ZSM-5 (TNN)).

4 2 bound dicopper site inside a ZSM-5 cavity ( ) in Figure 1 has a peroxo species (2 2 ). In contrast, a species, which was proposed in Ref. 7, has an oxo species ( 2 ). Therefore, the species is less reactive than the species, which can be confirmed from the magnitude of their spin densities on oxygen atoms. In fact, the calculated spin density on the oxygen atom of the species is ~0.6 (Figure S2), while those on oxygen atoms in the species are ~0.4 (Figure 4). Reflecting from the different behaviors of oxygen atoms between the two species, their reactivities toward water molecules are completely different. More reactive species, due to the presence of the radical oxygen atom in a ZSM-5 cavity, efficiently reacts with water molecules. In fact, DFT calculations found that reaction between water and the species can proceed easily from a viewpoint of the energetics, as shown in Figure S2. In the initial step, one water molecule near the species binds into a copper cation to form a H2 intermediate. After that, the H2 intermediate converts into a H H intermediate that does not have radical oxygen atoms. The H H intermediate is the final product, because further H atom transfers are impossible. The final product is the most stable energetically among the intermediates considered. The result indicates that the species under water environment tends to convert to an intermediate without radical oxygen atoms. In other words, the species under water environment loses the reactivity toward methane due to disappearance of the radical oxygen atom, which is in agreement with one of the referee s comment. In contrast, the species (Figure 1) is stable under water environment due to the absence of reactive oxygen atoms. In fact, the species in the presence of one attached water molecule ( H2) is the most 3

5 (a) ZSM-5( 2 ) with the species Spin density on the oxygen atom of the moiety: 0.6 (b) H 2 inside ZSM-5( 2 ) with the species H 2 H H 2 H 2 H H 0 kcal/mol 20.9 kcal/mol 22.6 kcal/mol Figure S2. Reaction between H 2 and the species inside ZSM-5 caivty with a pair is a third nearest neighbor. Potential energy surface in the triplet spin state was considered. We obtained optimized geometries for inside a ZSM-5 cavity where H 2 is weakly bound, H 2, and H H. Bond lengths are given in Å, and relative energies are in kcal/mol.

6 stable among reaction intermediates generated from the H2 species in Figure 3. Note that two reaction intermediates (H H and H H ), generated by the H atom migration within the hydroperoxo species to cleave the bond, have a radical oxygen atom. Measuring from the dissociation limit toward -ZSM-5, water and dioxygen, the two intermediates are more stable in energy. Therefore, the H2 addition to less reactive species can assist to form radical-oxygen containing intermediates in ZSM-5, being responsible for cleaving a methane C H bond. S4. Spin Densities on Reactive Intermediates during the Dioxygen Activation by ZSM-5 in the Presence of ne Water Molecule Changes in spin densities on reactive intermediates during the dioxygen activation by ZSM-5 in the presence of one water molecule (Figure 4) can be roughly understood in Figures S3(a) and S3(b). A comparison between Figures S3(a) and S3(b) indicates that one unpaired electron on one (II) cation transfers to the σ* orbital of 2. The electron transfer results in the disappearance of spin densities on one copper cation. At the same time, the single occupant in the 2 σ* orbital makes the bond become completely cleaved to generate an oxygen atom with radical characteristics. The radical characteristics can be observed in the H H and H H species, where significant spin density was found on the bridged and end oxygen atoms, respectively (Figure S3(c)). Therefore, the DFT results indicate that the bond activation by ZSM-5 in the presence of one water molecule near the active site directly links to the formation of a radical oxygen atom. 4

7 (a) H 2 H H activation (b) (I) H H (II) H H 3d 3d 3d 3d σ σ (II) π (II) (II) π (III) dioxygen π dioxygen π (c) (I) H H (II) H H optimized geometry Spin density distributions optimized geometry Spin density distributions Figure S3. Changes in electronic properties of 2 -bound ZSM-5 in the presence of one water molecule. (a) two (II) cations exist in intermediates formed before cleaving dioxygen bond by ZSM-5. (b) both (II) and (III) cations exist in intermediates formed after the cleavage. (c) spin density distributions of H H and H H intermediats inside a ZSM-5 cavity. pha and beta spin density distributions are given by the colors blue and red, respectively.

8 S5. Activation of a Methane C H Bond by the Species inside a ZSM- 5 Cavity Here, we investigated reaction mechanisms for the C H bond activation of methane by the species inside a ZSM-5 cavity, which was proposed in Ref. 7. We considered that a ten-membered ring of ZSM-5 contains one pair being the third nearest neighbor. Figure S4(a) displays the energetics of the methane activation by the inner species in the triplet spin state. nce the species has a radical oxygen atom bridged by the dinuclear copper site from the discussion in Supporting Information (S3), a C H methane bond is cleaved in a homolytic fashion. We obtained the transition state for the C H bond activation by the species inside a ZSM-5 cavity by using the QST approach. The calculated barrier for the C H bond activation is 30.1 kcal/mol measured from the first complex (methane complex). After that, a methyl radical is formed in the next intermediate (radical intermediate), similar to those in Figures 5 and 6. In addition, we searched same chemical reactions by using two types of smaller models; the medium model is the species bound to 21820H42 (Figure S4(b)), and the smallest one is the species bound to 2420H14 (Figure S4(c)). The smallest model is comparable to that in Ref. 7. During the optimization in the small model systems, all atoms are constrained at the original coordination defined crystallographically. The 6-311G* basis sets were used for the species, methane, and framework oxygen atoms attached to copper cations, and the 6-31G* basis sets were used for the other atoms. By using the smaller models, we obtained reaction intermediates formed during the cleavage of a methane C-H bond by the species bound to a zeolite framework in Figure S4(b) and S4(c). The DFT calculations found similar activation energies for the methane C-H bond cleavage 5

9 (a) Large ZSM-5( 2 ) containing species CH methane complex TS Radical intermediate 0 kcal/mol 30.1 kcal/mol 22.4 kcal/mol (b) Medium ZSM-5( 2 ) model containing species CH methane complex TS Radical intermediate 0 kcal/mol 28.4 kcal/mol 22.4 kcal/mol (c) Small ZSM-5( 2 ) model containing species CH methane complex TS Radical intermediate 0 kcal/mol 32.2 kcal/mol (22.4 kcal/mol) 12.7 kcal/mol Figure S4. (a) The methane C H bond activation by inside a ZSM-5 cavity with a pair is a third nearest neighbor. ptimized structures for the methane complex, radical intermediate, and the transition state (TS) for the methane C H bond activation by. Potential energy surface in the triplet spin state was considered. milar reaction pathways were obtained by using a medium model (b) and a small model (c). The medium model is H 42, and the small model is H 14. In the small model, zero-point vibrational energy correction was included in the activation energy, whose value is given in the parenthesis. Bond lengths are given in Å, and relative energies are in kcal/mol.

10 (28.4 kcal/mol for the medium model and 32.2 kcal/mol for the smallest model). We also obtained the activation energy by using the broken symmetry state (32.0 kcal/mol for the large model, 31.0 kcal/mol for the medium model and 34.6 kcal/mol for the small model), and found that the different spin states do not significantly affect the activation energy values. Furthermore, the smallest model calculations can include zero-point vibrational energy (ZPVE) corrections in the activation energy by using the single-point calculations with the G* basis sets. As a result, the ZPVE corrected activation energy was calculated to be 22.4 kcal/mol for the triplet state and 22.3 kcal/mol for the broken symmetry state, which is slightly larger than that obtained in Ref. 7 (18.5 kcal/mol). The deviation would come from different optimized geometries for the methane complex. ur DFT calculations obtained the optimized geometry, where a methane H atom is close to the radical oxygen atom (H--- separation: 2.66 Å), and at the same time two methane H atoms are close to one copper cation (H--- separations: 2.83 and 3.04 Å). These separations are basically consistent with those in the medium and large model calculations. However, Ref. 7 obtained a short H--- separation (1.91 Å) in the methane complex. S6. Influence of ne Water Molecule in the Reactivity of 2-bound Trinuclear Copper te inside a ZSM-5 Cavity Recently, Grundner proposed that a trinuclear copper site in mordeneite is responsible for the formation of an active site for the direct methane to methanol conversion. 18 ong the recent proposal, DFT calculations were further performed to investigate whether important roles of one water molecule can be found to enhance the reactivity of 2-bound ZSM-5 containing a trinuclear copper site (inner 3 6

11 species) or not. In a ten-membered ring of ZSM-5, two or three atoms were substituted for atoms within a ten-membered ring, which are denoted by ZSM-5(2) or ZSM-5(3) respectively. First, we obtained optimized structures for ZSM- 5(n) with a trinuclear copper site (inner 3 species), 2-bound ZSM-5(n) (2 ), as well as 2-bound ZSM-5(n) in the presence of one water molecule near its active site (2 H2), which are depicted in Figure S5. After the formation of the 2 H2 species, an H atom of H2 migrates to an oxygen atom of attached dioxygen to form an intermediate containing a species plus two attached H groups ( H H). The H H species is the most stable in energy within all optimized structures formed during the reaction. More interestingly, the H H species has a significant spin density on the bridged oxygen atom in the moiety (0.8 and 0.6 for the ZSM- 5(2) and ZSM-5(3) cases, respectively), whose spin density distributions are seen in Figure S5. The magnitude of the spin density on the bridged oxygen atom in the H H species is comparable to that in the corresponding intermediate in the dinuclear system (H H inside ZSM-5(2)). See Figure 4 and Figure S3. Accordingly, the H H species has a radical oxygen atom that can activate effectively a methane C H bond, being similar to the dinuclear system. The DFT findings indicate that the reactivity of 2-bound trinuclear copper site inside a ZSM-5 cavity is enhanced by the presence of one water molecule, being similar to the dinuclear case. S7. Dioxygen Activation by ZSM-5 in the Presence of Two Water Molecules 7

12 (a) ZSM-5( 3 ) H H 2 Inner kcal/mol 51.0 kcal/mol H H 2 H H 77.7 kcal/mol 97.3 kcal/mol (b) ZSM-5( 2 ) Spin density distribution of H H spin density on the oxygen atom of the moiety: H 2 + H Inner kcal/mol kcal/mol H 2 H H kcal/mol kcal/mol Spin density distribution of H H spin density on the oxygen atom of the moiety: 0.6 Figure S5. ptimized structures for reaction intermediates and transition states during dioxygen activation by ZSM-5 containing a trinuclear copper active site in the presence of one water molecule. We considered two types of trinuclear copper site containing ZSM-5 where (a) three or (b) two atoms are contained. ptimized bond lengths are given in Å. Energies relative to the dissociation limit toward ZSM-5, dioxygen, and one water molecule are given in kcal/mol. Spin density distributions are gien in the final intermediate consisting of H H.

13 After discussing on 2-bound ZSM-5 in the presence of one water molecule (one-water system), let us discuss whether the presence of additional water molecule affects reactivity of 2-bound ZSM-5 toward methane (two-water system). When two H2 molecules approach to a dinuclear copper site of an 2-bound ZSM- 5, the H2 H2 species is formed (Figures 1 and S6). Note that the H2 H2 species is 1.3 kcal/mol unstable relative to H2 H2. As discussed in the main text, the H2 H2 intermediate has dioxygen attached to the two copper cations in a cis end-on fashion, being in contrast to that in ZSM-5 in the presence of one water molecule. After the formation of this intermediate, an H atom of a water molecule migrates to the bound dioxygen (TS1). The activation energy was calculated to be 15.4 kcal/mol, measured from H2 H2. This migration results in the formation of a hydroperoxo species bridged the two copper cations plus one H group and one water (H H H2). Interestingly, an H atom of the remaining water is weakly bound to a newly-formed H group through hydrogen bondings, whose separation is 1.42 Å. The next step is the intra H-atom migration within the hydroperoxo moiety (TS2). This process, which requires the activation energy of 39.3 kcal/mol, corresponds to the rate-limiting step. In this event, the hydroperoxo bond is kept in the two-water system, being in contrast to the one-water system. Actually, we found a slightly lengthening bond in the transition state for the intra H-atom migration (1.57 Å), as well as a newly formed H2 H H species (1.45 Å). For keeping the hydroperoxo bond, the H-atom migration from H2 to a newly-formed H group (inter H-atom migration) is responsible: an H atom of the water molecule moves to the end H group in a direction opposite to the intra H-atom migration. f course, the inter H atom migration between water and H groups cannot be seen in the one-water system. 8

14 H 2 H H 2 H 2 TS1 H H H kcal/mol kcal/mol kcal/mol intra H-atom migration within hydroperoxo TS2 H 2 H H 87.4 kcal/mol kcal/mol Figure S6. ptimized structures for reaction intermediates and transition states during dioxygen activation by ZSM-5 in the presence of two water molecules. ptimized bond lengths are given in Å. Energies relative to the dissociation limit toward ZSM-5, dioxygen, and two water molecules are given in kcal/mol.

15 To find out roles of the inter H-atom migration in keeping the hydroperoxo bond, we will focus on the spin densities in the reaction intermediates in Figure S7. As shown in Figure S7, spin densities on copper cations remain almost unchanged during the conversion from the H H H2 to H2 H H species. Calculated spin densities on copper cations in H H H2 are 0.71 and 0.68, and those in H2 H H are 0.64 and These results indicated that the copper cations have electronic charges of ~ +2, independent of the two intermediates. The similarities in the spin densities on copper cations are understandable, because both intermediates have commonly one copper cation coordinated by one water and atom of H group, and the other cation coordinated by H group and H moiety of H group. Judging from the calculated spin densities on copper cations, the electron transfers, which are responsible for cleaving the bond, do not occur in 2-bound ZSM-5 in the presence of two water molecules. As a result, keeping the hydroperoxo bond is characteristic in the two-water system. As discussed above, the reaction intermediates during the dioxygen activation by ZSM-5 in the presence of two water molecules are quite different for those in the one-water system. In the one-water system of 2-bound ZSM-5, a radical oxygen atom is contained in two reaction intermediates, H H and H H. Such radical-oxygen containing intermediates were not found in the two-water system. At this point, the one-water and two-water systems can be distinguished by whether dioxygen bridged by the dicopper site is cleaved completely or not. The existence of one water near the active site of 2-bound ZSM-5 facilitates to cleave its bond leading to a radical oxygen atom that can activate a methane C H bond. In contrast, keeping the bond was found in 2-bound ZSM-5 in the presence of two water molecules, and therefore the two-water system does not exhibit reactivity toward methane. Therefore, DFT findings clearly elucidated the importance of number 9

16 H 2 H 2 I I H 0.71 H H H 0.18 H H H H 0.68 Figure S7. Changes in calculated spin densities on reaction intermediates during the dioxygen activation by ZSM-5 in the presence of two water molecules.

17 of water molecules near an active site of 2-bound ZSM-5 in modulating reactivity toward methane activation. 10