Article pubs.acs.org/IC
Computational Study on M1/POM Single-Atom Catalysts (M = Cu, Zn, Ag, and Au; POM = [PW12O40]3−): Metal−Support Interactions and Catalytic Cycle for Alkene Epoxidation Chun-Guang Liu,*,† Meng-Xu Jiang,† and Zhong-Min Su*,‡,† †
College of Chemical Engineering, Northeast Electric Power University, Jilin City 132012, P. R. China Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun City 130024, P. R. China
Downloaded via FORDHAM UNIV on June 29, 2018 at 20:58:00 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Geometrical structures, metal−support interactions, and infrared (IR) spectroscopy of a series of M1/POM (M = Cu, Zn, Ag, and Au; POM = [PW12O40]3−) single-atom catalysts (SACs), and catalytic cycle for alkene epoxidation catalyzed by M1/ POM SACs were studied using density functional theory (DFT) calculations. The calculations demonstrate that the most probable anchoring sties for the isolated single atoms studied here in the M1/POM SACs are the fourfold hollow sites on the surface of POM support. The bonding interaction between single metal atom and surface of POM support comes from the molecular orbitals with a mixture of d atomic orbital of metal and 2p group orbital of surface oxygen atoms of POM cage. The calculated adsorption energy of isolated metal atoms in these M1/POM SACs indicates that the early transition metals (Cu and Zn) have high thermal stability. The DFT-derived IR spectra show that the four characteristic peaks of free Keggin-type POM structure split into six because of introduction of isolated metal atom. Compared with other metal atoms, the Zn1/POM SAC has the high reactivity for activity of dioxygen molecule, because the dioxygen moiety in Zn1/POM SAC displays O2−· radical feature with [POM4−·Zn2+O2−·]3− configuration. Finally, a catalytic cycle for ethylene epoxidation by O2 catalyzed by Zn1/POM SAC was proposed based on our DFT calculations. Supported noble-metal SACs are among the most important catalysts currently. However, noble metals are expensive and of limited supply. Development of non-noble-metal SACs is of essential importance. Therefore, the reported Zn1/POM SAC would be very useful to guide the search for SACs into non-noble metals.
1. INTRODUCTION Dispersion of metal atoms on metal oxide surface is currently a popular method for preparation of single-atom catalysts (SACs) in the field of heterogeneous catalysis.1−4 Compared with the metal cluster- or nanoparticle-based catalysts, SACs have distinct geometrical and electronic structures, especially the valence-shell electron configuration of metal atoms, and thus provide various novel and unique physicochemical properties and excellent catalytic activity. For example, the reported Pt1/ FeOx SAC is 2−3 times more active than the sub-nanometersized counterpart for the reactions of CO oxidation and preferential oxidation of CO1 and has a very high turnover frequency of 1500 h−1 and selectivity of ∼99% for the hydrogenation reaction of 3-nitrosyrene,5 which is the highest value for the reported Pt group metal catalyst. Density functional theory (DFT) calculations indicated that the exceptional catalytic activity of Pt1/FeOx SAC arises from a charge transfer from the Pt single atom to FeOx,1 which leads to positively charged Pt single atoms anchored on the FeOx surfaces without Pt−Pt bonds. The partially vacant 5d orbitals of the positively charged, high-valent Pt single atom efficiently reduce both the CO adsorption energy and the activation © 2017 American Chemical Society
barriers for CO oxidation. These results clearly demonstrate how the electronic configuration of the metal single atom in supported metal SACs affects their catalytic properties. Because of a very high surface energy, the metal single atom tends to form aggregates during synthetic procedures and catalytic reaction. Reducing the metal loading to a very low level to avoid agglomeration is a common synthetic strategy for preparation of SACs. However, this would reduce active site. Recently, several efficient synthetic strategies have been developed to enhance sintering-resistant behavior of SACs and increase metal loading. Datye et al. have successfully prepared a thermally stable Pt1/CeO2 SAC by using hightemperature vapor transport method.6 This SAC has been tested for the CO oxidation reaction, indicating the sinteringresistant behavior at high temperatures. They proposed that different exposed surface facets of the ceria powders is closely associated with their thermal stability. Polyhedral ceria and nanorods were more effective than ceria cubes at anchoring the platinum. Zheng et al. reported an ultrastable Pd1/TiO2 SAC Received: June 13, 2017 Published: August 18, 2017 10496
DOI: 10.1021/acs.inorgchem.7b01480 Inorg. Chem. 2017, 56, 10496−10504
Article
Inorganic Chemistry with a Pd loading as high as 1.5 wt %.7 The Pd single atoms were dispersed on ethylene glycolate-stabilized ultrathin TiO2 nanosheets via a photochemical synthetic approach. The reported Pd1/TiO2 SAC displays a high catalytic activity in hydrogenation of CC bonds relevant to the commercial Pd catalysts. A 55-fold catalytic enhancement has been obtained in the hydrogenation reaction of aldehydes. The UV light-induced formation of ethylene glycolate radicals on TiO2 nanosheets is critical to the successful synthesis of the Pd1/TiO2 SAC. Currently, stabilization of isolated metal atoms on a special support with high load ratio without compromising catalytic activity and reaction selectivity is still a challenge for preparation of SACs. Polyoxometalates (POMs) are early transition-metal oxoanion clusters. Unlike the network structure of metal oxides, solid POMs possess a discrete anionic structure, which is more like a discrete fragment of the solid metal oxides.8−13 POMs also have been considered as catalyst-support materials.14−23 POMs-supported catalytic materials have the following advantages: (i) POMs are an isolated and controlled anionic structure, which can be unequivocally characterized at the molecular level. Several X-ray single-crystal structures of POMssupported organometallic compounds have been reported. (ii) POMs can be dissolved in both organic solvent and water to become an isolated anionic structure and thus can act as both homogeneous and heterogeneous catalysts. (iii) Because of the isolated and intact structure of POMs both in solution or solid, catalytic performance of POMs-supported catalysts is easily monitored by using conventional technologies. Thus, POMssupported catalytic materials are an ideal model system and provide the unique opportunity for mechanistic studies at the molecular levels. Taking into account this point, POMssupported SACs maybe have a high loading meanwhile avoiding agglomeration because of their isolated anionic structure. As a pioneer, Yan and co-workers reported a stable Pt1/POM SAC with a high Pt loading (close to 1 wt %) on Keggin-type POMs surface.23 According to their experimental studies and DFT calculations, they proposed that Pt atom is stabilized by four oxygen atoms in a distorted square-planar geometry of Keggin-type POMs with positive charge. This Pt1/ POMs SAC exhibits excellent performance in the hydrogenation of nitrobenzene and cyclohexanone. These results indicate that POMs-supported single-atom system has broad prospects and great potential for development of SACs. Selective oxidation of alkane, olefin, arene, alcohol, cellulose, etc. to target products, using POMs as the catalyst,23−28 has attracted considerable attention in the past one-half century. The environmentally friendly, low cost, and easily available oxygen and hydrogen peroxide are often employed as oxidizing agent in these selective oxidization reactions. However, many problems in this field need be solved, because the combination of POM-based catalysts with O2 or H2O2 often generates radicals.29 The existence of radical always results in the indiscriminate attack against reactants and overoxidation of products. To the best of our knowledge, the efficient O2- or H2O2-based catalytic oxidation systems for selective oxidation arises from effective coordination of O2 or H2O2 to transitionmetal center to avoid radical reaction pathways,30−35 which means that the transition-metal center needs to be arranged into a low-coordination environment and thus an unsaturated metal atom. As mentioned above, anchoring metal atom onto the POM surface to form SACs always gives rise to an
unsaturated transition-metal atom and thus provides a key structural basis for the selective oxidation reactions. Keggin-type POMs, the typical and important structure in POM chemistry, possess desirable surface structure and provide the favorable adsorption site for metal atom. There are essentially various possible locations for metal atom to adsorb on the surface of Keggin-type POMs. In the present paper, the geometrical structures, metal−support interactions, infrared (IR) spectroscopy, and potential catalytic performances for epoxidation of alkene in M1/POM SACs (M = Cu, Zn, Ag, Au, POM = [PW12O40]4−) were systematically studied based on DFT calculations. The results indicate that the Zn1/POM SAC displays excellent catalytic activation for the epoxidation of alkene, because the coordinated dioxygen moiety in Zn1/POM SAC displays O2−· radical feature.
2. COMPUTATIONAL DETAILS All calculations were performed using the Gaussian 09 Revision D.01 software36 suite and the local meta-GGA exchange correlation functional M06L.37 The M06L functional is based on the M05 correlation functional,38 which is developed from the work of Perdew and Wang, Becke, Perdew, and Stoll et al. Previous theoretical investigations have demonstrated the good performance of M06L functional in calculations of the main group and transition-metal element thermochemistry, thermochemical kinetics, noncovalent interactions, and vibrational frequencies.37 Meanwhile, the accuracy of the M06L functional in prediction of geometry, vibrational frequency, thermochemistry, etc. of POMs was validated in our previous work.39 Geometry optimizations and evaluation of harmonic frequencies were performed using M06L functional with the 6-31G(d) basis sets for main-group atom and the pseudopotential basis sets LANL2DZ40−42 for the transition-metal atoms. All of the structures discussed in the present work are minima or transition states on the corresponding potential energy surfaces, as confirmed by the correct number of imaginary frequencies. The single-point calculations on optimized geometries were performed using the same functional with the polarization and diffusion basis sets 6-31+G(d) for main-group atoms and the relativistic energy-consistent pseudopotential basis set SDD for transition-metal atoms to further refine the electronic energy. All geometry optimizations were performed without any symmetry constraints. Bulk solvent effect of tetrahydrofuran media was taken into account via the self-consistent reaction field (SCRF) method, using the integral equation formalism polarizable continuum model (IEFPCM)43 solvent model. The SMD solvation model44 proposed by Truhlar and co-workers was performed for computing ΔG of solvation. Natural bond orbital (NBO)45 analysis at the M06L/6-31G(d) level was performed to assign the atomic charges and Wiberg bond indices (WBI) (LANL2DZ basis sets on the metal atom). 3. RESULTS AND DISCUSSION 3.1. Anchoring Sites of Keggin-Type POM Surface. Understanding and designing of surface structure and anchoring sites of catalytic supports are the most critical for the development of supported SACs. It is well-known that Keggin-type POM anions are composed of 12 MO 6 octahedrons sharing their corners or edges with a central XO4 tetrahedron. The surface oxygen atoms in such structure can be divided into three sets according to whether they are in 10497
DOI: 10.1021/acs.inorgchem.7b01480 Inorg. Chem. 2017, 56, 10496−10504
Article
Inorganic Chemistry
Figure 1. Structural diagrams for the M1/POM SACs (M = Cu, Zn, Ag, Au, POM = [PW12O40]4−) (a) and their dioxygen-adsorption complexes (b).
Figure 2. Metal-based molecular orbitals, which qualitatively represents the bonding molecular orbital of the adsorbed Zn atom and Keggin-type POM cages.
terminal (Ot), bridging two metal atoms (Ob), and at the corners of the Keggin structure (Oc). Combination of these surface oxygen atoms of Keggin-type structure could generate single-, two-, three-, and fourfold anchoring sites for isolated metal atom (see Figure S1). In the present paper, catalytically important metal atoms Cu, Zn, Ag, and Au were introduced into our studied system to analyze the nature of these anchoring sites for isolated single metal atoms studied here. Starting from all possible models of anchoring sites on Keggin-type POM surfaces for isolated metal atom studied here, we performed the geometric optimization for these models without any restriction at M06L/6-31G(d) levels (LANL2DZ basis sets on metal atoms). For the Cu atom, out DFT-M06L calculations obtained four anchoring models. For the Zn atom, the optimized calculations obtained five
anchoring models. For the Ag and Au atoms, our DFT-M06L calculations obtained four anchoring models, respectively. On the basis of these optimized geometries, we calculated the free energy of these models; the calculated free energy indicates that the most stable species for all metal studied here comes from anchoring isolated single atoms onto the fourfold hollow sites of Keggin-type POM surface, and thus all the discussions in the following would focus on this structure (see Figure 1a). Yan et al. also showed that the most probable sites for Pt single atoms in the Pt1/POM SACs are the fourfold hollow sites on the surface of Keggin-type POM,23 where each Pt atom is coordinated by four surface oxygen atom according to their DFT calculations. As shown in Figure 1a, metal atom in these M1/POM SACs is stabilized by four oxygen atoms in a distorted rectangular 10498
DOI: 10.1021/acs.inorgchem.7b01480 Inorg. Chem. 2017, 56, 10496−10504
Article
Inorganic Chemistry
Table 1. Calculated and Experimental Vibrational Frequencies and Assigned Bands (cm−1), Adsorption Energy of Metal Atom (kcal mol−1), NBO Partial Charges of Metal Atom (Q(M), in a.u.) for the M1/POM SACs Studied Here Keggin
Cu-POM
Zn-POM
Ag-POM
Au-POM
cal
cal
IR absorption bands parameters ν1 ν2 ν3 ν4 ν5 ν6 Ead Q(M)
exp
cal
1080 987
1079 986
890 802
900 828
cal
cal
1066 1063 1069 977 976 974 894 887 896 826 823 830 763 765 770 664 681 684 adsorption energies and partial charges −48.55 −23.11 −20.02 0.750 1.072 0.639
pyramid geometry. Compared with the free Keggin structure, adsorption of metal atom does not largely affect the molecular geometry of Keggin cage because of its rigid metal−oxo framework. A slight structural reorganization comes from the angle of W−O−W corresponding to the four coordinated oxygen atoms. 3.2. Metal−Support Interactions and Charge Transfer. The electronic structure can provide qualitative insight into the bonding picture. We employed Zn1/POM system as an example to analyze the metal−support interaction between metal atom and surface oxygen atoms of Keggin-type POM cage based on its electronic structure. According to simple molecular orbital (MO) theory, the nature of a coordination bond is mainly determined by (i) interacting atomic orbital, which governs its shapes and sizes, and (ii) charge transfer, which governs its relative energy. It is well-known that the Zn atom has the 3d104s2 configuration with a closed shell structure. The six metal-based MO diagram obtained from the optimized structures of Zn1/POM system using M06L functional are listed in Figure 2. It can be found that the five MOs (Nos. 135−139) are metal-based d orbitals and responsible for the bonding interaction between Zn atom and four surface oxygen atoms of Keggin-type POM cage, which are made from an overlap of the 2p orbitals of the surface oxygen atoms with the d orbitals of the Zn atom, and each orbitals contains the large metal character (>90% according to our DFT-M06L calculations). The large contribution from metal atom in the five metal-based d orbitals would lead to a charge transfer from the Zn atom to POM cage. We also examine the electronic structure of those M1/POM SACs by using NBO analysis in this work. The calculated NBO partial charges of the series of metal atoms (Q) are listed in Table 1. It can be found that the charge of the adsorbed metal atom in these M1/POM SACs is positive. Compared with the neutral free metal atom, the positive charge on the adsorbed metal atom indicates the charge reorganization or transfer from the transition-metal center to the Keggin-type POM cage, which are well in agreement with the MO prediction. For the highest occupied molecular orbital (HOMO; No. 259) of the Zn−POM system, it accounts for the nonbonding orbital, which is made from 4s orbital of adsorbed Zn atom and d orbitals of W atom of Keggin-type POM cage. All these metal-based d orbitals are doubly occupied and provide the MO basis for the understanding of the bonding interaction between the adsorbed metal atom and Keggin-type POM cage.
assignment ν(P−Oa) ν(W−Ot) ν(W−Ob/Oc−W) ν(W−Ob−W) ν(W−Oc−W) ν(W−Ob/Oc−W)
1078 976 898 820
−8.013 0.013
The adsorption energy of isolated metal atoms is defined as the energy difference between M1/POM SAC systems and an isolated metal atom and free POM clusters. Ead = EMetal − POMs − (EMetal + EPOMs)
The calculated adsorption energy of metal atom for these M1/ POM SACs is listed in Table 1. It can be found that the calculated adsorption energy of these M1/POM SACs increases in the following order: Cu−POM (−48.55) < Zn−POM (−23.11) < Ag−POM (−20.02) < Au−POM (−8.013), which indicates that the late transition metals (Ag and Au) have low tendency for the Keggin-type POM support studied here. This is mainly due to larger energy gaps between metal d orbitals of these late transition metal and 2p orbitals of surface oxygen atoms of Keggin-type POM cage than that of early transition metals. 3.3. Infrared Spectroscopy. Isolated single atoms in SACs are the center of the catalytically active sites. Therefore, conforming the existence of the only isolated single metal atoms becomes most important for development of SACs. The most convincing and intuitive experimental approach for evaluating the existence of single metal atoms includes scanning tunneling microscopy techniques, X-ray absorption near edge spectroscopy, extended X-ray absorption fine structure spectroscopy, and infrared (IR) spectroscopy. Compared with metal oxide support, Keggin-type POM possesses distinguishing IR features.46−48 IR spectroscopy is one of the most widely used technologies for confirming the Keggin-type POM structure. In the present paper, we employed the DFT-M06L method to calculate the IR spectra of the series of these M1/POM SACs. For the free Keggin structure, α-[PW12O40]3−, the four infrared characteristic absorption peaks appear at 1080 cm−1 corresponding to asymmetric stretch vibration of P−Oa bonds (Oa corresponds to oxygen atom of tetrahedral phosphate group), 987 cm−1 relevant to asymmetric stretch vibration of W = Ot bonds, 890 cm−1 relative to bending vibration of W−Ob−W bonds, and 802 cm−1 for bending vibration of W−Oc−W bonds according to experimental measures. To check the reliability of our DFT calculations, we first calculated IR spectra of α[PW12O40]3− by using M06L functional at 6-31G(d) levels (LANL2DZ basis sets on metal atoms). It can be found that DFT-M06L calculations well reproduce these frequencies. The largest difference between calculated and experimental frequencies is found to be the IR band at 802 cm−1 (ν5) corresponding to the bending vibration of W−Oc−W bonds (802 cm−1 (exp) vs 828 cm−1 (cal)). 10499
DOI: 10.1021/acs.inorgchem.7b01480 Inorg. Chem. 2017, 56, 10496−10504
Article
Inorganic Chemistry Because of anchoring of the metal atoms onto the surface of Keggin-type POMs, the W−Ob−W and W−Oc−W bending vibration (∼890 and 802 cm−1) of free Keggin structure (α[PW12O10]3−) mix each other and generate four new IR bands (ν3, ν4, ν5, and ν6) for our studied system (see Table 1). Meanwhile, the W = Ot and P−Oa vibrations (ν1 and ν2) are not significantly shifted when compared with that of the intact Keggin structure. Therefore, we proposed that the six characteristic absorption peaks are easily employed to identify the structure of the M1/POM SACs. Currently, in the whole research process of the SACs, characterizing composition and structure, especially conforming the existence of the isolated single metal atoms, often becomes the slowest and most difficult step. The proposed distinct infrared signatures (six characteristic absorption peaks) in this work may be a useful tool for probing structure of the Keggin-type POM-supported SACs based on our DFT-M06L calculations. For DFT-derived IR spectra of the M1/POM SACs (M = Cu, Zn, and Ag), analysis of the six infrared characteristic absorption peaks indicates no obvious differences, which indicates that the IR spectra of these M1/POM SACs are not sensitive to the metal substituted effects. It is, surprisingly, the DFT-derived IR spectra of the POM-supported Au system. There are only four infrared characteristic absorption peaks, which are very similar to that of α-[PW12O40]3−. This is mainly due to the very weak interaction between Au atom and surface oxygen atom of Keggin-type POMs (Ead = −8.03 kcal mol−1; see Table 1). 3.4. Activation of Dioxygen Molecule. The catalytically active centers in the supported metal SACs are the isolated single metal atoms that anchor at the surface of the supports. The catalytic performances would be closely associated with the interaction between the single metal atoms and their supports. For our studied M1/POM SACs, the dioxygen molecule has been employed as a probe to test the catalytic nature of these single metal atom centers. According to our DFT optimization calculations, coordination of dioxygen molecule to metal atom generates η2 complexes for all our studied systems (see Figure 1b). Very recently, a Rh1/POM SACs also has been reported by Yan group.49 According to X-ray absorption near-edge structure spectra, they proposed that the chemically adsorbed O2 molecule on the Rh1/POM catalyst also forms an η2 complex, where the isolated Rh atom coordinates to six oxygen atoms. In the present paper, the calculated adsorption energy of dioxygen, partial charges, and selected structural parameters for the dioxygen-adsorption complexes studied here are summarized in Table 2. Comparing the structure of M1/POM SACs and the dioxygen-adsorption complexes, the adsorption of dioxygen molecule over the metal atoms does not largely affect the geometries of M1/POM SACs. The interaction between dioxygen molecule and the metal atoms is closely associated with the nature of M−O bonds (M− O5 and M−O6 in these η2 complexes, see Figure 1b and Table 2). It is well-known that the M−O bond distance depends on both the interaction of two atoms and the metal radius. And thus, we examine the bonding nature of those POM complexes by using NBO analysis. The calculated WBI value is listed in Table 2. It can be found that the WBI values of M−O5/O6 bond decrease in the order: Cu (0.28/0.28) < Zn (0.23/0.23) < Ag (0.18/0.10) < Au (0.03/0.03), indicating the weak single bonding interactions and the relevant strong interaction for the Cu and Zn systems when compared with the Ag and Au
Table 2. Geometric Parameters (bond length in Å), Adsorption Energy (kcal mol−1), Calculated Vibrational Frequencies of Dioxygen Moiety (cm−1), WBI Values, and Partial Charges (a.u.) of the Dioxygen-Adsorption Complexes parameters M−O5 WBI(M−O5) M−O6 WBI(M−O6) O5−O6 ν(O2) WBI(O5−O6) Ead(O2) M O5 O6 total charge
Cu-POM
Zn-POM
Ag-POM
geometrical parameters 2.045 2.067 2.269 0.281 0.233 0.182 2.046 2.070 2.857 0.280 0.231 0.102 1.293 1.366 1.247 1293 1138 1439 1.303 1.246 1.392 −66.83 −78.00 −48.63 partial charge 0.882 1.369 0.810 −0.155 −0.363 −0.101 −0.155 −0.362 −0.058 −0.310 −0.725 −0.159
Au-POM 3.427 0.030 3.452 0.030 1.219 1614 1.498 −41.92 0.005 −0.001 0.000 −0.001
systems. The calculated adsorption energy of dioxygen atoms in these dioxygen-adsorption complexes increases in the following order: Zn−POM (−78.00) < Cu−POM (−66.83) < Ag−POM (−48.63) < Au−POM (−41.92). Zn and Cu systems provided a considerable adsorption energy of −78.00 and −66.83 kcal mol−1 and thus a relevant strong interaction, which is well in agreement with the prediction of bonding feature. The calculated NBO partial charges of these dioxygenadsorbed complexes were listed in Table 2. The NBO-derived partial charge of the dioxygen moiety increases in the order of Zn−POM (−0.725) < Cu−POM (−0.310) < Ag−POM (−0.159) < Au−POM (−0.001). The calculated NBO partial charges of the dioxygen moiety is negative for all systems studied here, compared with the neutral free dioxygen molecule, which indicates that a charge transfer from metal atom to the coordinated dioxygen molecule. This charge transfer would lead to elongation of O−O bond. As shown in Table 2, the optimized O5−O6 bond distance increases in the following order: Au−POM (1.219 Å) < Ag−POM (1.247 Å) < Cu−POM (1.293 Å) < Zn−POM (1.366 Å). And the WBI values of O5−O6 bond increase in the order of Zn−POM (1.246) < Cu−POM (1.303) < Ag−POM (1.392) < Au−POM (1.498), confirming the weakened O−O double bond nature in these dioxygen-adsorption complexes. All results indicate that the Zn1/POM system has the longest O−O distance and the smallest WBI value because of a significant charge transfer relative to other metal atoms. The Mulliken spin population analysis shows that the spin density of dioxygen-adsorption Zn−POM complex in its triplet ground state is mainly localized on the dioxygen moiety (1.06), and part is localized on adjacent W atom of the POM ligand, but nearly no spin density resides on the Zn atom (−0.03), combination of the NBO partial charge on the dioxygen moiety (−0.73) and Zn atom (1.37); thus, the entire complex can be briefly represented as [POM4−·Zn2+O2−·]3−, the coordinated dioxygen moiety can be viewed as an O2−· radical, and the Keggin-type POM cage can be viewed as an electron acceptor. It is well-known that the special ability of POMs is relative to acceptor one or several electrons with minimal structural change. Thus, Zn1/POM SAC system may have high stability relevant to organic or organometallic compounds. 10500
DOI: 10.1021/acs.inorgchem.7b01480 Inorg. Chem. 2017, 56, 10496−10504
Article
Inorganic Chemistry
Figure 3. Calculated energy profile for the epoxidation of ethylene obtained by M06L/6-31+G(d) calculations (metal atoms for SDD basis sets) in tetrahydrofuran and gas phase (in brackets).
3.5. Epoxidation of Alkene on Zn1/POM SAC. In the present paper, the Zn1/POM system was considered for the epoxidation of the alkene because of the O2−· radical feature in activation of dioxygen. Meanwhile, ethylene was employed as a model molecule to probe the possible reaction mechanism. For the epoxidation of alkene catalyzed by Zn1/POM system, it involves the mechanism of three reaction steps as following: Step1
molecule and a Zn−oxo complex ([Zn]O). The calculated reaction energy profile for the step 2 in triplet state is listed in Figure 3. It consists of an oxygen transfer from the dioxygenadsorption Zn complex to first C2H4 molecule. At the reaction complex precursor CP, the C2H4 molecule is located on the two oxygen atoms in a triangle structure with a C1−O5−O6 angle of ∼81.7°. The C1···O5 and C1···O6 distances are 3.608 and 3.670 Å, respectively, and the C1C2 double bond of C2H4 molecule slightly elongates from 1.328 to 1.330 Å. At the transition state (TS1), the C1−O5−O6 angle changes from 81.7° to 111.26°, and the C1···O5 and C1···O6 distances are contracted to 1.802 and 2.651 Å, respectively. Meanwhile, the bond length of O5−O6 increases from 1.366 to 1.397 Å. TS1 has a barrier of 17 kcal mol−1 in tetrahydrofuran with one imaginary frequency of 576i cm−1, which is relative to the C1− O5 bond formation and the O5−O6 bond scission. These results indicate an oxygen transfer trend from the dioxygen-adsorption Zn complex to C2H4 molecule. The formation of new C1−O5 bond (1.424 Å) gives rise to an intermediate IM1. At the IM1, the C1−C2 bond of C2H4 molecule elongates to 1.480 Å, indicating a significant single bond feature. And the O5−O6 distance increases to 1.498 Å, which indicates an O5−O6 bond scission. The formation of IM1 requires an energy of ∼10.5 kcal mol−1, indicating IM1 is not a thermodynamically stable intermediate. The Mulliken spin population analysis shows that the spin density of IM1 with triplet state is mainly localized on the C2 atom (1.11) and the adjacent W atoms of the POM ligand; nearly no spin density resides on the Zn (0.05), C2 (−0.06), O5 (0.05), and O6 (0.05) atoms. These results indicate that the IM1 can be briefly represented as [POM4−·Zn2+−O···OCH
[Zn] + O2 → [Zn]O2
Step2
[Zn]O2 + 1st C2H4 → [Zn]O−O···C2H4 → [Zn]O + C2H4O
Step3
[Zn]O + 2ndC2H4 → [Zn]O···C2H4 → [Zn] + C2H4O
In step 1, it is about generation of dioxygen-adsorption complex, which was discussed earlier (see Table 2). Because of the high reactivity of the isolated single Zn1 atom, it may interact with the coexisting C2H4 molecule directly (Step 1′, [Zn] + C2H4 → [Zn] C2H4) and thus gives rise to a mechanism of competing reaction for activation of dioxygen. We calculated the free energy of both element reactions corresponding to steps 1 and 1′ at room temperature and 1 bar pressure. The calculated ΔG298 K (kcal mol−1) shows that both reactions are thermodynamically allowed. Reactions of Zn1/ POM system with dioxygen (ΔG298 K = ca. −27.2 kcal mol−1) are readily accessible relevant to the reaction of Zn1/POM system with C2H4 (ΔG298 K = ca. −3.72 kcal mol−1). These results suggest that adsorption of dioxygen molecule may be a feasible route thermodynamically for the Zn1/POM system studied here. In step 2, the dioxygen-adsorption Zn complex interacts with the first C2H4 molecule forming an ethylene oxide (C2H4O) 10501
DOI: 10.1021/acs.inorgchem.7b01480 Inorg. Chem. 2017, 56, 10496−10504
Article
Inorganic Chemistry C−·H2]3−, a radical complex, which provides the good structural foundation for the ring closure of the epoxide ring via formation of the C2−O5 bond. The ring closure of the epoxide ring takes place though a transition state TS2 at a barrier of 14.7 kcal mol−1 in tetrahydrofuran. Finally, the C2H4O is easily desorbed from the dioxygen-adsorption Zn complex with an exothermic process (9.1 kcal mol−1) and generates a POM-supported Zn−oxo complex, which is one of the reactants for the epoxidation of the second C2H4 molecule. The overall reaction starting from the dioxygen-adsorption Zn complex and C2H4 molecule is exothermic by 16.6 kcal mol−1. This suggests the thermodynamic driving force for the epoxidation reaction studied here. In step 3, the Zn−oxo complex directly interacts with the second C2H4 molecule generating another C2H4O molecule. The Mulliken spin population analysis shows that the spin density of the Zn−oxo complex with triplet state is mainly localized on the O6 atom (1.01) and the adjacent W atoms of the POM ligand; nearly no spin density resides on the Zn (0.05) atom, indicating a radical complex with [POM4−·Zn2+− O−·]3−. According to our DFT-M06L calculations, the formation of C2H4O molecule occurs in this step without any energy barrier. We calculated the free energy of the reaction of step 3 at room temperature and 1 bar pressure. The calculated ΔG298 K (kcal mol−1) is ca. −6.9 kcal mol−1, indicating this reaction is thermodynamically allowed. The overall energy profile of the epoxidation process of ethylene was calculated at M06L/6-31G(d) levels (LANL2DZ basis sets on metal atoms) and shown in Figure 3. These features are similar to those in previous theoretical studies.50−53 The overall mechanism can be divided into two steps. The first step consists of the first C−O bond formation, generating a radical complex, which undergoes the subsequent second C−O bond formation. Finally, the ring closure of the epoxide ring leads to the formation of the epoxide product. According to our DFT-derived energy profiles, a catalytic cycle for epoxidation of ethylene by O2 catalyzed by the Zn1/ POM SAC is proposed. The reactants of the catalytic cycle are O2 and C2H4O molecules, and the product of the catalytic cycle is C2H4O molecule. This mechanism suggests that the adsorbed single Zn atom is the catalytically active center. The role of supported Keggin-type POM cluster is the stabilization of the Zn atom and acceptor electron in the whole catalytic process. On the whole, the two energy barriers in this catalytic cycle is moderate (∼17 kcal mol−1 for TS1 and ∼14.7 kcal mol−1 for TS2). Thus, the studied Zn1/POM SAC possesses potential catalytic activity for the epoxidation of alkene.
POM and Cu1/POM SACs display the high thermal stability according to the calculated adsorption energy of isolated metal atoms on the POM support. DFT-derived IR spectroscopy indicates that the four characteristic peaks of free Keggin-type POM structure split into six because of anchoring single metal atom on the POM surface. The six characteristic absorption peaks may be useful and easy available spectroscopic information to identify the existence of the isolated single metal atoms in M1/POM SACs. Because of the dioxygenadsorbed complex in Zn1/POM SAC has a [POM4−· Zn2+O2−·]3− configuration, where the coordinated dioxygen moiety displays O2−· radical feature, Zn1/POM SAC has the high reactivity for activation of dioxygen molecule among all metal atoms studied here. The catalytic cycle about ethylene epoxidation by O2 catalyzed by Zn1/POM SAC has been probed based on our DFT calculations. The Zn1/POM SAC possesses potential catalytic activity for the epoxidation of alkene because of the moderate energy barriers in this catalytic cycle.
■
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01480. Structural diagrams for possible anchoring sites on the surface of Keggin-type POM support for isolated metal atom, xyz coordinates for most relevant structures reported in this paper (PDF)
■
AUTHOR INFORMATION
Corresponding Authors
*Phone: 86 0432 64606919. Fax: 86 0432 64606919. E-mail:
[email protected] or
[email protected]. (C.G.L.) *E-mail:
[email protected]. (Z.M.S.) ORCID
Chun-Guang Liu: 0000-0002-1220-5236 Zhong-Min Su: 0000-0002-3342-1966 Notes
The authors declare no competing financial interest.
■ ■
ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21373043). REFERENCES
(1) Qiao, B.; Wang, A.; Yang, X.; Allard, L. F.; Jiang, Z.; Cui, Y.; Liu, J.; Li, J.; Zhang, T. Single-atom catalysis of CO oxidation using Pt1/ FeOx. Nat. Chem. 2011, 3, 634−641. (2) Yang, X.-F.; Wang, A.; Qiao, B.; Li, J.; Liu, J.; Zhang, T. Singleatom catalysts: a new frontier in heterogeneous catalysis. Acc. Chem. Res. 2013, 46, 1740−1748. (3) Thomas, J. M. Catalysis: Tens of thousands of atoms replaced by one. Nature 2015, 525, 325−326. (4) Liu, J. Catalysis by supported single metal atoms. ACS Catal. 2017, 7, 34−59. (5) Wei, H.; Liu, X. Y.; Wang, A.; Zhang, L.; Qiao, B. T.; Yang, Y. F.; Huang, Y. Q.; Miao, S.; Liu, J.; Zhang, T. FeOx-supported platinum single-atom and pseudo-single-atom catalysts for chemoselective hydrogenation of functionalized nitroarenes. Nat. Commun. 2014, 5, 5634. (6) Jones, J.; Xiong, H.; DeLaRiva, A. T.; Peterson, E. J.; Pham, H.; Challa, S. R.; Qi, G.; Oh, S.; Wiebenga, M. H.; Pereira Hernandez, X.
4. CONCLUSIONS DFT/M06L calculations have been employed to investigate geometrical structures, metal−support interactions, IR spectroscopy, and potential catalytic activity for alkene epoxidation of the series of M1/POM SACs. The results indicate that anchoring isolated single metal atoms onto the fourfold hollow sites of Keggin-type POM surface gives the most stable species among all possible structures studied here. The metal−supports interactions arise from chemical bonding interactions. The MO theory predicts that the bonding MO comes from a mixture of d atomic orbitals of metal and 2p group orbitals of surface oxygen atom of POM cage. Because of the large contribution from metal atom in these bonding orbitals, a charge transfer from the metal atom to POM cage has been obtained. Zn1/ 10502
DOI: 10.1021/acs.inorgchem.7b01480 Inorg. Chem. 2017, 56, 10496−10504
Article
Inorganic Chemistry I.; Wang, Y.; Datye, A. Thermally stable single-atom platinum-on-ceria catalysts via atom trapping. Science 2016, 353, 150−154. (7) Liu, P.; Zhao, Y.; Qin, R.; Mo, S.; Chen, G.; Gu, L.; Chevrier, D. M.; Zhang, P.; Guo, Q.; Zang, D.; Wu, B.; Fu, G.; Zheng, N. Photochemical route for synthesizing atomically dispersed palladium catalysts. Science 2016, 352, 797−800. (8) Katsoulis, D. E. A Survey of applications of polyoxometalates. Chem. Rev. 1998, 98, 359−388. (9) Dolbecq, A.; Dumas, E.; Mayer, C. R.; Mialane, P. Hybrid organic−inorganic polyoxometalate compounds: From structural diversity to applications. Chem. Rev. 2010, 110, 6009−6048. (10) Hill, C. L. Introduction: Polyoxometalatess-multicomponent molecular vehicles to probe fundamental issues and practical problems. Chem. Rev. 1998, 98, 1−2. (11) Izarova, N. V.; Pope, M. T.; Kortz, U. Noble metals in polyoxometalates. Angew. Chem., Int. Ed. 2012, 51, 2−21. (12) Sun, M.; Zhang, Z.; Putaj, P.; Caps, V.; Lefebvre, F.; Pelletier, J.; Basset, J.−M. Catalytic oxidation of light alkanes (C1−C4) by heteropoly compounds. Chem. Rev. 2014, 114, 981−1019. (13) Zheng, S.−T.; Yang, G.−Y. Recent advances in paramagneticTM-substituted polyoxometalates (TM = Mn, Fe, Co, Ni, Cu). Chem. Soc. Rev. 2012, 41, 7623−7646. (14) Coperet, C.; Comas-Vives, A.; Conley, M. P.; Estes, D. P.; Fedorov, A.; Mougel, V.; Nagae, H.; Nunez-Zarur, F.; Zhizhko, P. A. Surface organometallic and coordination chemistry toward single-site heterogeneous catalysts: Strategies, methods, structures, and activities. Chem. Rev. 2016, 116, 323−421. (15) Brandts, J. A. M.; Berben, P. H. Application of immobilized rhodium catalyst precursors in enantio- and chemoselective hydrogenation reactions. Org. Process Res. Dev. 2003, 7, 393−398. (16) Torviso, M. R.; Blanco, M. N.; Caceres, C. V.; Fraile, J. M.; Mayoral, J. A. Supported heteropolyanions as solid counterions for the electrostatic immobilization of chiral copper complexes. J. Catal. 2010, 275, 70−77. (17) Tanielyan, S. K.; Augustine, R. L.; Marin, N.; Alvez, G. Anchored wilkinson catalyst. ACS Catal. 2011, 1, 159−169. (18) Augustine, R.; Tanielyan, S.; Anderson, S.; Yang, H. A new technique for anchoring homogeneous catalysts. Chem. Commun. 1999, 1257−1258. (19) Augustine, R. L.; Tanielyan, S. K.; Mahata, N.; Gao, Y.; Zsigmond, A.; Yang, H. Anchored homogeneous catalysts: the role of the heteropoly acid anchoring agent. Appl. Catal., A 2003, 256, 69−76. (20) Ahn, S.−H.; Choi, M.−S.; Im, J.−S.; Sheikh, R.; Park, Y.−H. Improved method for immobilization of a chiral complex on PTA/ alumina for asymmetric hydrogenation of a β-ketoester. J. Mol. Catal. A: Chem. 2013, 373, 55−60. (21) He, P.; Xu, B.; Xu, X.; Song, L.; Wang, X. Surfactant encapsulated palladium-polyoxometalates: controlled assembly and their application as single-atom catalysts. Chem. Sci. 2016, 7, 1011− 1015. (22) Tanielyan, S.; Biunno, N.; Bhagat, R.; Augustine, R. Anchored wilkinson catalyst: hydrogenation of β pinene. Top. Catal. 2014, 57, 1564−1569. (23) Zhang, B.; Asakura, H.; Zhang, J.; Zhang, J.; De, S.; Yan, N. Stabilizing a platinum1 single-atom catalyst on supported phosphomolybdic acid without compromising hydrogenation activity. Angew. Chem., Int. Ed. 2016, 55, 8319−8323. (24) Mallat, T.; Baiker, A. Oxidation of alcohols with molecular oxygen on solid catalysts. Chem. Rev. 2004, 104, 3037−3058. (25) Gelalcha, F. G. Heterocyclic hydroperoxides in selective oxidations. Chem. Rev. 2007, 107, 3338−3361. (26) Roithova, J.; Schroder, D. Selective activation of alkanes by gasphase metal ions. Chem. Rev. 2010, 110, 1170−1211. (27) Afewerki, A.; Cordova, A. Combinations of aminocatalysts and metal catalysts: a powerful cooperative approach in selective organic synthesis. Chem. Rev. 2016, 116, 13512−13570. (28) Wang, V. C.-C.; Maji, S.; Chen, P. P.-Y.; Lee, H. K.; Yu, S. S.-F.; Chan, S. I. Alkane oxidation: Methane monooxygenases, related enzymes, and their biomimetics. Chem. Rev. 2017, 117, 8574.
(29) Kamata, K.; Yonehara, K.; Nakagawa, Y.; Uehara, K.; Mizuno, N. Efficient stereo- and regioselective hydroxylation of alkanes catalysed by a bulky polyoxometalate. Nat. Chem. 2010, 2, 478−483. (30) Kim, C.; Chen, K.; Kim, J.; Que, L., Jr. Stereospecific alkane hydroxylation with H2O2 catalyzed by an iron(II)−tris (2-pyridylmethyl) amine complex. J. Am. Chem. Soc. 1997, 119, 5964−5965. (31) Roelfes, G.; Lubben, M.; Hage, R.; Que, L., Jr; Feringa, B. L. Catalytic Oxidation with a non-heme iron complex that generates a low-spin FeIIIOOH intermediate. Chem. - Eur. J. 2000, 6, 2152−2159. (32) Chen, M. S.; White, M. C. A Predictably selective aliphatic C−H oxidation reaction for complex molecule synthesis. Science 2007, 318, 783−787. (33) Gómez, L.; Garcia-Bosch, I.; Company, A.; Benet-Buchholz, J.; Polo, A.; Sala, X.; Ribas, X.; Costas, M. Stereospecific CH oxidation with H2O2 catalyzed by a chemically robust site-isolated iron catalyst. Angew. Chem., Int. Ed. 2009, 48, 5720−5723. (34) Yiu, S.-M.; Man, W.-L.; Lau, T.-C. Efficient catalytic oxidation of alkanes by Lewis acid [OsVI(N)Cl4]− using peroxides as terminal oxidants. Evidence for a metal-based active intermediate. J. Am. Chem. Soc. 2008, 130, 10821−10827. (35) Das, S.; Incarvito, C. D.; Crabtree, R. H.; Brudvig, G. W. Molecular recognition in the selective oxygenation of saturated C-H bonds by a dimanganese catalyst. Science 2006, 312, 1941−1943. (36) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2009. (37) Zhao, Y.; Truhlar, D. G. A new local density functional for maingroup thermochemistry, transition metal bonding, thermochemical kinetics, and noncovalent interactions. J. Chem. Phys. 2006, 125, 1−18. (38) Zhao, Y.; Schultz, N. E.; Truhlar, D. G. Exchange-correlation functional with broad accuracy for metallic and nonmetallic compounds, kinetics, and noncovalent interactions. J. Chem. Phys. 2005, 123, 3−4. (39) Liu, C.−G.; Liu, S.; Zheng, T. Computational study of metal− dinitrogen Keggin-type polyoxometalate complexes [PW11O39MIIN2)]5− (M = Ru, Os, Re, Ir): Bonding nature and dinitrogen splitting. Inorg. Chem. 2015, 54, 7929−7935. (40) Hay, P. J.; Wadt, W. R. Ab initio effective core potentials for molecular calculations. Potentials for the transition metal atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270−283. (41) Wadt, W. R.; Hay, P. J. Ab initio effective core potentials for molecular calculations. Potentials for main group elements Na to Bi. J. Chem. Phys. 1985, 82, 284−293. (42) Hay, P. J.; Wadt, W. R. Ab initio effective core potentials for molecular calculations. Potentials for K to Au including the outermost core orbitals. J. Chem. Phys. 1985, 82, 299−310. (43) Tomasi, J.; Mennucci, B.; Cammi, R. Quantum mechanical continuum solvation models. Chem. Rev. 2005, 105, 2999−3093. (44) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. Universal solvation model based on solute electron density and on a continuum model of the solvent defined by the bulk dielectric constant and atomic surface tensions. J. Phys. Chem. B 2009, 113, 6378−6396. (45) Glendening, A. E.; Carpenter, J. E.; Weinhold, F. NBO, Version 3.1.; University of Wisconsin, 2017 (46) Ho, R.; Klemperer, W. Polyoxoanion supported organometallics: synthesis and characterization of α-[(η 5 -C 5 H 5 )Ti(PW11O39)]4‑. J. Am. Chem. Soc. 1978, 100, 6772−6774. (47) Dablemont, C.; Proust, A.; Thouvenot, R.; Afonso, C.; Fournier, F.; Tabet, J. C. Functionalization of Polyoxometalates: From Lindqvist to Keggin derivatives. 1. synthesis, solution studies, and spectroscopic and ESI mass spectrometry characterization of the rhenium phenylimido tungstophosphate [PW11O39{ReNC6H5}]4‑. Inorg. Chem. 2004, 43, 3514−3520. (48) Duhacek, J. C.; Duncan, C. D. Phenylimido functionalization of α-[PW12O40]3‑. Inorg. Chem. 2007, 46, 7253−7255. (49) Zhang, B.; Asakura, H.; Yan, N. Atomically dispersed rhodium on self-assembled phosphotungstic acid: Structural features and 10503
DOI: 10.1021/acs.inorgchem.7b01480 Inorg. Chem. 2017, 56, 10496−10504
Article
Inorganic Chemistry catalytic CO oxidation properties. Ind. Eng. Chem. Res. 2017, 56, 3578−3587. (50) Khenkin, A. M.; Kumar, D.; Shaik, S.; Neumann, R. Characterization of manganese(V)−oxo polyoxometalate intermediates and their properties in oxygen-transfer reactions. J. Am. Chem. Soc. 2006, 128, 15451−15460. (51) Shaik, S.; Kumar, D.; de Visser, S. P.; Altun, A.; Thiel, W. Theoretical perspective on the structure and mechanism of cytochrome P450 enzymes. Chem. Rev. 2005, 105, 2279−2328. (52) Shaik, S.; Cohen, S.; Wang, Y.; Chen, H.; Kumar, D.; Thiel, W. P450 enzymes: Their structure, reactivity, and selectivitymodeled by QM/MM calculations. Chem. Rev. 2010, 110, 949−1017. (53) Meunier, B.; de Visser, S. P.; Shaik, S. Mechanism of oxidation reactions catalyzed by cytochrome P450 enzymes. Chem. Rev. 2004, 104, 3947−3980.
10504
DOI: 10.1021/acs.inorgchem.7b01480 Inorg. Chem. 2017, 56, 10496−10504
