Oxidation of Methanol to Formaldehyde on Silica-Supported

J. Phys. Chem. C 2010, 114, 2967–2979

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Oxidation of Methanol to Formaldehyde on Silica-Supported Molybdena: Density Functional Theory Study on Models of Mononuclear Sites Laurence J. Gregoriades,* Jens Do¨bler, and Joachim Sauer Institut fu¨r Chemie, Humboldt-UniVersita¨t zu Berlin, Unter den Linden 6, D-10099 Berlin, Germany ReceiVed: September 6, 2009; ReVised Manuscript ReceiVed: December 19, 2009

Isolated molybdenum centers bearing either one (oxomolybdenum system) or two (dioxomolybdenum system) terminal oxo ligands are considered, which are modeled by appropriate mononuclear oxomolybdenum methoxides and oxomolybdasilsesquioxanes. Although the oxidation process in both systems is characterized by the same fundamental steps, that is, dissociative addition of methanol followed by rate-determining hydrogen abstraction from the methoxy group, the mechanism of the oxidation reaction differs in each case. In the oxomolybdenum system, the first step leads to cleavage of a bond in a Mo-O-Si sequence and the formation of a surface molybdenum methoxide species. Hydrogen is then abstracted from the methoxide ligand by a terminal oxo ligand in a process entailing a closed-shell transition structure. In contrast, the preferred mechanism in the dioxomolybdenum system involves a hydroxomolybdenum methoxide intermediate formed without cleavage of a bond in a Mo-O-Si sequence. Furthermore, the hydrogen abstraction in the second step is effected by the hydroxide ligand formed in the first step and proceeds via an open-shell singlet transition structure. 1. Introduction Formaldehyde (CH2O) inconspicuously pervades everyday life in the form of plastics, adhesives, paints, treated textile and paper products, as well as specialty chemicals.1 Currently, the synthesis of CH2O on an industrial scale is achieved by the catalytic dehydrogenation of methanol (CH3OH) in a heterogeneous process, in which the catalyst is either ferric molybdate or silver. Although both catalysts are used essentially to the same extent in the global production of CH2O, most new plants utilize the ferric molybdate technology.2,3 The catalyst in the ferric molybdate process is employed in a fixed-bed reactor and is thus susceptible to the occurrence of unwanted temperature gradients and so-called hot spots, which can lead to incomplete reactant oxidation and product decomposition.2 These negative features can be remedied by making use of a fluidized-bed reactor, but the poor mechanical properties of ferric molybdate do not permit the application of such a process.2 Hence, researchers are seeking alternative materials with improved mechanical properties to bring about the conversion of CH3OH to CH2O. One such candidate is molybdena (MoO3) supported on silica (SiO2), since it is known that MoO3 catalyzes the oxidation of CH3OH to CH2O4 and that SiO2 is an affordable, chemically inert, and mechanically robust material.5 The oxidation of CH3OH to CH2O on MoO3-based materials is believed to follow a Mars-van Krevelen mechanism,6 in which two steps are presumed. In the first step, a hydrogen molecule is formally abstracted from CH3OH, leading to the formation of water and a reduced metal center via removal of a surface oxygen atom. In the second step, the reduced site is reoxidized. In the present contribution, only the first step is dealt with, which can be decomposed further into the following two * Corresponding author. Present address: Atotech Deutschland GmbH, Erasmusstraße 20, D-10553 Berlin, Germany. Phone: ++49 (0)30 34985 743. Fax: ++49 (0)30 34985 618. E-mail: laurence.gregoriades@ atotech.com.

Figure 1. Isolated molybdenum centers, which may exist on MoO3/ SiO2 catalyst surfaces.

steps: (a) dissociative addition of CH3OH to form a surface hydroxide and methoxide and (b) hydrogen abstraction from this surface methoxide, resulting in the formation of CH2O and in the reduction of the MoVI center to a MoIV center. The structure of the molybdenum-containing moieties on the surface of SiO2-supported MoO3 (MoO3/SiO2) is said to depend on the amount of molybdenum present in the catalyst and on the method of catalyst preparation.7,8 Thus, the surface may comprise isolated molybdenum centers bearing one terminal oxo ligand (oxomolybdenum moieties), isolated molybdenum centers bearing two terminal oxo ligands (dioxomolybdenum moieties), a combination of both, or polynuclear aggregates. In this Article, the results concerning isolated molybdenum centers, which are illustrated schematically in Figure 1, are described. Despite extensive experimental investigations, the reaction steps that constitute the CH3OH to CH2O oxidation process over MoO3-based materials are still insufficiently understood. Researchers involved in this field are therefore resorting to computational chemistry in their attempts to clarify this issue and a variety of theoretical methods and surface models have been applied. Allison and Goddard conducted one of the earliest theoretical studies addressing the issue of CH3OH oxidation on MoO3-based catalysts.9 The authors performed generalized valence bond (GVB) and configuration interaction (CI) calcula-

10.1021/jp908609s  2010 American Chemical Society Published on Web 01/28/2010

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Figure 2. Schematic representations of the models used by Chempath and Bell12 to study the oxidation of CH3OH at oxo- and dioxomolybdenum sites on MoO3/SiO2 catalyst surfaces ([Si] ) SiOH).

SCHEME 1: Oxidation of CH3OH to CH2O on MoO3/ SiO2 as Studied by Domen and Co-Workers11

Figure 3. MoO3/SiO2 catalyst surface models (gray, carbon; white, hydrogen; blue, molybdenum; red, oxygen; orange, silicon).

2. Computational Details

tions using oxomolybdenum tetrachloride (OMoCl4) and dioxomolybdenum dichloride (O2MoCl2) as models of oxomolybdenum and dioxomolybdenum surface species, respectively, in molybdate materials (eqs 1 and 2). They concluded that dioxomolybdenum sites are more effective than oxomolybdenum sites in activating CH3OH and that adjacent dioxomolybdenum moieties may be important catalytic sites.

The models chosen by Allison and Goddard exclude the possibility of investigating the significance of bridging oxo ligands in the oxidation of CH3OH.10 This aspect was neglected by Domen and co-workers in a theoretical study of the oxidation of CH3OH on MoO3/SiO2, in which the only methoxide ligand oxidation reaction considered was hydrogen abstraction by the molybdenum center (Scheme 1).11 The role of bridging oxo ligands was also not addressed in a density functional theory (DFT) study recently reported by Chempath and Bell,12 in which the models depicted schematically in Figure 2 were chosen to represent MoO3/SiO2 catalyst surfaces. In a previous theoretical study,13 during which the oxidation of CH3OH over vanadia (V2O5) supported on SiO2 (V2O5/SiO2) was examined, oxovanadium methoxides as well as models based on polyhedral oligomeric silsesquioxanes (POSSs) were used to describe the surface of V2O5/SiO2 catalysts. POSSs have been shown to adequately reproduce the electronic structure of SiO214–17 and POSS-based models have been used successfully in other studies of catalytic systems.18–20 Analogous models were therefore used to investigate the mechanism of the oxidation of CH3OH to CH2O over MoO3/SiO2.

The B3LYP hybrid functional21,22 was used along with the TZVP basis sets23 and a relativistic pseudopotential for the 28electron core of molybdenum24 in all calculations. Calculations pertaining to the structures of reactants, intermediates, and products were executed with the TURBOMOLE software25,26 and involved preliminary optimization using the BP86 functional.27–29 In order to accelerate these calculations, the Coulomb part was evaluated using the MARI-J method30,31 along with optimized TZVP auxiliary basis sets.32 The resulting structures were then optimized anew with the B3LYP hybrid functional. Unrestricted Kohn-Sham calculations were performed for triplet spin state systems. All optimized structures were characterized as minima on the potential energy surface by calculations of vibrational frequencies based on analytical second derivatives of the energy with respect to the nuclear coordinates.33,34 The quoted frequencies of the hydroxyl and methoxy group vibrations are scaled by a factor of 0.96887, which was used in previous work on V2O5/SiO2.13 Unless otherwise stated, the frequencies of the ModO stretching and bending vibrations of 1.1 and 4.1 (Figure 3) are scaled by a factor of 0.9717, derived from the calculated and experimental frequencies of OMoF435 and O2MoCl2.36 All other frequencies are unscaled. Transition structures were initially sought for in the simpler oxo- and dioxomolybdenum methoxide systems, which are not based on POSSs, by the quadratic synchronous transit (QST) method37,38 as implemented in the Gaussian 03 software.39 The structures that served as starting points were 3.1 (Figure 3) and 3.4 (Figure 6) in the case of the oxomolybdenum system and 6.1 (Figure 3) and 6.4 (Figure 10) in the case of the dioxomolybdenum system. All subsequent calculations were performed using the TURBOMOLE software. Second derivative calculations showed that the stationary points arising from the QST procedure were first-order saddle points and the corresponding structures were refined by trust radius image minimization using an analytical Hessian (statpt module of TURBOMOLE).40 These transition structures served as guides in locating the transition structures of the POSS-based systems. Thus, the structural parameters of the appropriate POSS-based models were altered accordingly and second derivatives were calculated for the resulting structures, which were then refined by trust radius image minimization. Following previous work on V2O5/SiO2,13 the broken-symmetry approach41 was employed in the treatment of open-shell singlet systems, within which the energy of the low spin state (Els) is obtained by spin projection from the energies of the broken-symmetry (Ebs) solution and the triplet (Etr) solution for

DFT Study on Models of Mononuclear Sites

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a given geometric structure according to eq 3.42 In this equation, 〈S2〉 is the expectation value of the total-spin operator for the broken-symmetry solution.

Els ) Etr + 2(Ebs - Etr)/(2 - 〈S2〉)

TABLE 1: Oxomolybdenum to Dioxomolybdenum Transformation Reactions and Their Associated Gibbs Reaction Energies (kJ mol-1) at 0 (∆G0 ) ∆E0a) and 523 K (∆G523)b

(3)

All reported energies are electronic and do not include zeropoint vibrational or dispersion energy contributions unless otherwise stated. Dispersion energy contributions were calculated using the DFT-D method.43 Thermodynamic data were determined with the freeh module of TURBOMOLE.

reaction

∆G0 ) ∆E0

1.1 + H2O f 7.1 2.1 + H2O f 5.1

-4 -106

∆E0 ) change in electronic energy vibrational contributions. b Only vibrational chemical potential are taken into account models. For H2O, translational and rotational included. a

∆G523 53

-56

including zero-point contributions to the for the POSS-based contributions are also

3. Models The models selected as representatives of the surface of a MoO3/SiO2 catalyst are depicted in Figure 3. Isolated oxomolybdenum sites were modeled using the C4V-symmetric oxomolybdasilsesquioxanes (OMo-POSSs) H12MoO21Si12 (1.1) and H4MoO9Si4 (2.1) and the C2-symmetric compound oxomolybdenum tetramethoxide (OMo(OCH3)4 (3.1)). Isolated dioxomolybdenum sites were represented by two incompletely condensed dioxomolybdasilsesquioxanes (O2Mo-POSSs), that is, the C1symmetric H8MoO14Si7 (4.1) and the Cs-symmetric H5MoO10Si4 (5.1), as well as by the C2-symmetric compound dioxomolybdenum dimethoxide (O2Mo(OCH3)2 (6.1)). The models 2.1 and 5.1 were constructed so that the effect of confining the molybdenum atoms to six-membered rings, rather than eightmembered rings as in 1.1 and 4.1, could be explored. The structure of the C1-symmetric O2Mo-POSS H14MoO22Si12 (7.1), which was devised so that the transformation of 1.1 to a dioxomolybdenum species in the presence of water could be studied, is also shown in Figure 3. The molybdenum atoms in 1.1-3.1 exhibit a tetragonal pyramidal coordination mode, and each model contains an oxomolybdenum moiety. In 1.1 and 2.1, this moiety is anchored to the POSS via four bridging oxo ligands, whereas, in 3.1, it is connected to four methoxide ligands. Compound 3.1, which has been characterized by mass spectrometry, infrared spectroscopy, and X-ray crystallography,44 served as a simple model for the preliminary investigation of the oxidation of surface methoxide species formed after the dissociative addition of CH3OH at an oxomolybdenum site. In 4.1-7.1, the molybdenum atoms are tetrahedrally coordinated and each model comprises a dioxomolybdenum moiety. In 4.1, 5.1, and 7.1, this moiety is attached to the POSS via two bridging oxo ligands, while, in 6.1, it is bound to two methoxide ligands. The model compound 4.1 closely resembles an incompletely condensed POSS isolated and crystallographically characterized by Feher and co-workers.45 Such POSSs are the subject of intense investigation due to their potential in a variety of catalytic processes.46 In order to generate 4.1 from Feher’s POSS, the cyclopentyl groups and trimethylsiloxy group attached to the silicon atoms were replaced with hydrogen atoms and a hydroxy group, respectively, and the pyridine ligand was removed from the molybdenum center. Compound 6.1, which has been isolated and spectroscopically characterized by DeKock and co-workers,47 was chosen as a model for the preliminary investigation of the oxidation of surface methoxide species formed after the dissociative addition of CH3OH at a dioxomolybdenum site. 4. Results 4.1. Relative Stability of Oxo- and Dioxomolybdenum Models. Since it has been claimed that isolated oxo- and dioxomolybdenum moieties may coexist on the surface of

Figure 4. Products resulting from the dissociative addition of CH3OH to 1.1. Reaction energies (kJ mol-1) are given in parentheses.

MoO3/SiO2 catalysts,8 the interconversion of these two moieties was studied according to the chemical equations listed in Table 1. The overall trend observed for these reactions is that the conversion of the OMo-POSSs to O2Mo-POSSs becomes more endergonic at higher temperature due to the loss of translational and rotational freedom of the water molecule chemisorbed from the gas phase. The conversion of 2.1 to 5.1 remains exergonic at higher temperature, albeit to a lesser degree. This suggests that the confinement of the molybdenum atom to six-membered rings rather than eight-membered rings, as in 1.1, induces significant strain in the structure of 2.1. 4.2. Oxomolybdenum Site. DissociatiWe Addition of CH3OH. The dissociative addition of CH3OH onto MoO3/SiO2 catalyst surfaces leads to the formation of surface methoxide species.48–50 However, it is unclear which oxo ligand (bridging or terminal) accepts the proton originating from the hydroxyl group of CH3OH. Furthermore, it cannot be said with certainty whether a molybdenum methoxide or a methoxysilyl moiety or a combination thereof is formed. Five products might be reasonably expected following the dissociative addition of CH3OH to 1.1, and these are illustrated in Figure 4. Two of these products are molybdenum methoxide species (1.2, 1.3), while the other three are methoxysilyl species (1.4-1.6). The energetically least favored product, the hydroxomolybdenum methoxide 1.2, is formed in an endothermic process involving proton transfer from CH3OH to the terminal oxo ligand of 1.1 and does not entail cleavage of any of its bonds. The coordination number of the molybdenum atom increases from five to six during the addition reaction. The other four products, the formation of which involves cleavage of a bond in a Mo-O-Si or “surface” Si-O-Si sequence of 1.1, are clearly preferred over 1.2. The oxomolybdenum methoxide 1.3 and the methoxysilyl species 1.4 are formed in mildly exother-

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Figure 5. Molybdenum methoxide products resulting from the dissociative addition of CH3OH to 2.1. Reaction energies (kJ mol-1) are given in parentheses.

Figure 6. Products resulting from methoxide ligand oxidation in 1.2 (1.10), 1.3 (1.7-1.9), and 3.1 (3.2-3.4). The products 1.7 and 3.2 are closed-shell species, while all others are open-shell triplet species. Reaction energies (kJ mol-1) are given in parentheses. Energies relative to 1.1 + CH3OH are given in curly brackets where appropriate.

mic processes by appropriate cleavage of a bond in a Mo-O-Si sequence. The rupture of a bond in a Si-O-Si sequence, which is energetically just as likely as that in a Mo-O-Si sequence, can yield the methoxysilyl species 1.5 or 1.6. These results therefore suggest that it is a bridging oxo ligand, rather than a terminal oxo ligand, which accepts the proton from CH3OH. Moreover, since the compounds 1.3-1.6 are within about 5 kJ mol-1 of each other, it is not unlikely that a mixture of surface oxomolybdenum methoxide and methoxysilyl moieties exists on the catalyst surface following the dissociative addition step. In order to establish whether the size of the ring within which the molybdenum atom is contained affects reactivity, the formation of the molybdenum methoxide species 2.2 and 2.3 (Figure 5), which result from the dissociative addition of CH3OH to 2.1, was examined. As with 1.1, the product resulting from addition without cleavage of any of the bonds in 2.1 (2.2) is less stable than that formed by the reaction involving bond rupture (2.3). However, the reaction yielding 2.3 is about 75 kJ mol-1 more exothermic than that leading to the analogous compound 1.3, which implies that the structure of 2.1 is appreciably more strained than that of 1.1. Methoxide Ligand Oxidation to CH2O. The investigation of this reaction began by first exploring how it might proceed in the much simpler model 3.1. A similar approach was followed in the previous study on V2O5/SiO213 and found to be of great assistance in the investigation of the larger systems. One can thus imagine the oxidation of CH3OH on a MoO3/SiO2 catalyst surface being represented by the appropriate intramolecular reaction in 3.1. This reaction involves abstraction of hydrogen from a methoxide ligand and three products seem likely (Figure 6). According to Holstein and Machiels, a molybdenum hydride species may be a relevant intermediate in the oxidation process.51

Gregoriades et al. The formation of the oxomolybdenum hydride 3.2 (closed-shell singlet ground state) and CH2O was therefore considered and found to be the least favored process. The oxomolybdenum species 3.3 and the hydroxomolybdenum species 3.4 (both triplet ground state species), obtained by hydrogen transfer to a neighboring methoxide ligand and to the oxo ligand, respectively, are the preferred products, the latter being the most stable one. It should be noted that, compared to the reactant 3.1, the oxidation state of the molybdenum center in the hydride 3.2 is unchanged (MoVI), while that of the molybdenum centers in the triplet products 3.3 and 3.4 is reduced (MoIV). The products resulting from the analogous reactions of the OMo-POSS methoxide derivative 1.3 are also shown in Figure 6. As with the simpler model, the hydride product 1.7 (closedshell singlet ground state) is the least stable one. Hydrogen transfer to a bridging oxo ligand, which in the simpler model was represented by transfer to a neighboring methoxide ligand, results in the formation of 1.8 (triplet ground state). The most stable product, 1.9 (triplet ground state), is in fact obtained by hydrogen transfer to the terminal oxo ligand, as also shown for the simpler model. Figure 6 depicts the aqua complex 1.10 (triplet ground state) as well, which is derived from the least stable methoxide intermediate 1.2 and is similar to the aqua complex considered in the oxomolybdenum system of Chempath and Bell.12 However, the molybdenum atom in 1.10 is contained in eightmembered rather than six-membered rings. Relative to 1.1 + CH3OH, 1.10 is more stable than 1.7 and 1.8 but less stable than 1.9. The formation of products based on the smaller OMo-POSS 2.1 (Figure 3), which are analogous to 1.9 and 1.10, was also examined. Relative to 2.1 + CH3OH, the product analogous to 1.9 (open-shell triplet ground state) is more stable by 26 kJ mol-1 while the 1.10 analogue (open-shell triplet ground state) is 102 kJ mol-1 less stable. As summarized in Figure 4, the simultaneous existence of oxomolybdenum methoxide and methoxysilyl moieties on the surface of a MoO3/SiO2 catalyst is highly likely, and therefore, the oxidation of the methoxy groups in the methoxysilyl models 1.4-1.6 was also examined. However, these oxidation reactions were found to be significantly more endothermic than the conversion of 1.3 to 1.9 by at least 70 kJ mol-1 (Table 2). The results therefore suggest that the oxidation of CH3OH to CH2O on a MoO3/SiO2 catalyst should involve a surface methoxide intermediate and a species bearing CH2O as a ligand, the structures of which should resemble those of 1.3 and 1.9, respectively. With the energies of the possible products of the model systems at hand, the relevant energy barriers may now be considered. As illustrated in Figure 6, the hydroxomolybdenum species 3.4 is the most favored product derived from 3.1. The transition structure 3.TS (closed-shell singlet ground state; Figure 7) located for this system lies 143 kJ mol-1 above the reactant 3.1. Similarly, 1.9 is the most stable product derived from 1.3 (Figure 3) and the transition structure 1.TS1 (closedshell singlet ground state; Figure 7) found for this reaction lies 149 kJ mol-1 above the reactant. The product 1.10 is obtained via the transition structure 1.TS2 (open-shell singlet ground state, 〈S2〉 ) 0.14; Figure 7), which lies 130 kJ mol-1 (124 kJ mol-1 based on the projected low spin state energy) above the corresponding reactant 1.2. Thus, the intrinsic energy barrier to the formation of 1.10 is lower than that of 1.9. However, the energy of 1.TS2 relative to 1.1 + CH3OH (174 kJ mol-1; 168 kJ mol-1 based on the projected



TABLE 2: Reaction Energies (kJ mol-1) for the Oxidation of the Methoxy Groups in the Methoxysilyl Species 1.4-1.6 and 4.5a methoxysilyl model 1.4 1.5 1.6 4.5 a

hydrogen transfer to terminal oxo ligand

hydrogen transfer to bridging oxo ligand

hydrogen transfer to hydroxo ligand

187 191 199 211

173 141 170 301

117 {109}

{179} {185} {190} {183}

{165} {135} {161} {273}

263 {235}

Energies relative to 1.1 + CH3OH (1.4-1.6) and 4.1 + CH3OH (4.5) are given in curly brackets.

SCHEME 2: Removal of CH2O from 1.9 and Formation of Watera

a

Figure 7. The transition structures 1.TS1, 1.TS2, and 3.TS compared to the relevant reactants and products (interatomic distances in pm). The corresponding energy barriers (kJ mol-1) are given in parentheses below each transition structure. In the case of the POSS-based systems, the apparent energy barriers (energies relative to 1.1 + CH3OH) are given in curly brackets. For 1.TS2, the intrinsic barrier obtained using the projected low spin state energy is given in brackets and the apparent barrier is derived from that value.

low spin state energy) is higher than that of 1.TS1 (145 kJ mol-1) and therefore 1.9 seems to be the favored product. The oxidation of the methoxide ligand in 2.3 (Figure 5), the smaller analogue of 1.3 derived from 2.1, involves a closedshell transition structure similar to 1.TS1, which lies 153 kJ mol-1 above 2.3 (75 kJ mol-1 relative to 2.1 + CH3OH), 8 kJ mol-1 below the methoxide intermediate 2.2 (Figure 5), and 27 kJ mol-1 below the product analogous to 1.10 (Figure 6). Thus, a route via which a bond in a Mo-O-Si sequence is cleaved appears to be preferred in the oxidation of CH3OH by 2.1. In order to complete the CH3OH oxidation reaction in the system based on 1.1, the CH2O ligand must be removed from 1.9. This is an endothermic process requiring less than 40 kJ mol-1, which leads to the intermediate 1.11 (Scheme 2). Formally, the Mars-van Krevelen mechanism6 also dictates the production of water, which may be achieved by the conversion of 1.11 to 1.12 in a process requiring just over 50 kJ mol-1 (Scheme 2).

Reaction energies (kJ mol-1) are given above the arrows.

Figure 8. Products resulting from the dissociative addition of CH3OH to 4.1. Reaction energies (kJ mol-1) are given in parentheses.

4.3. Dioxomolybdenum Site. DissociatiWe Addition of CH3OH. As mentioned above, the identity of the methoxy species formed following the dissociative addition of CH3OH onto MoO3/SiO2 catalyst surfaces cannot be determined unambiguously. The possibilities considered for the dissociative addition of CH3OH to 4.1 involved the rupture of a bond in either a Mo-O-Si sequence or a Si-O-Si sequence, as well as addition without bond cleavage. The eight products deemed reasonable for this reaction are illustrated in Figure 8. In parallel with the OMo-POSS system, addition without bond cleavage yields the least stable product. This may be 4.2 or 4.3, depending on which terminal oxo ligand in the O2Mo-POSS 4.1 accepts the proton originating from the CH3OH hydroxyl group. The coordination number of the molybdenum atom increases from four to five during the addition reaction and hence the resulting structures are not as strained as that of 1.2, obtained from the comparable reaction of the OMo-POSS 1.1. This in turn explains why the reaction leading to 4.2 or 4.3 is only marginally endothermic, if at all, compared to the reaction yielding 1.2. Rupture of a bond in a Mo-O-Si sequence appears to be preferred and may lead either to the dioxomolybdenum methoxide 4.4 or the methoxysilyl species 4.5. The dissociative addition of CH3OH with cleavage of a bond in a “surface” Si-O-Si sequence leads to the methoxysilyl species 4.6-4.9, depending on which bond in which Si-O-Si

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Figure 9. Molybdenum methoxide products resulting from the dissociative addition of CH3OH to 5.1. Reaction energies (kJ mol-1) are given in parentheses.

sequence is cleaved. As with the OMo-POSS system, the simultaneous existence of molybdenum methoxide and methoxysilyl moieties on the catalyst surface is highly likely. However, since the rupture of a bond in a Mo-O-Si sequence is distinctly more favorable than that of a bond in a Si-O-Si sequence, the proportion of molybdenum methoxide to methoxysilyl moieties on a catalyst consisting of dioxomolybdenum sites should be higher than that of a catalyst consisting of oxomolybdenum sites. These results suggest that the proton of CH3OH is accepted by a bridging oxo ligand rather than a terminal oxo ligand, as also shown for the oxomolybdenum system. Furthermore, there seems to be a preference for attachment to the bridging oxo ligand in a Mo-O-Si rather than a Si-O-Si sequence. The effect of confining the molybdenum atom to a sixmembered rather than an eight-membered ring was also determined for the O2Mo-POSS system. This was achieved by studying the formation of the molybdenum methoxide species 5.2 and 5.3 (Figure 9), which are afforded by the dissociative addition of CH3OH to 5.1. As with 4.1, the product obtained by addition without rupture of any of the bonds in 5.1 (5.2) is less stable than that obtained by addition involving bond cleavage (5.3). However, the reaction leading to 5.3 is about 25 kJ mol-1 more exothermic than that yielding the analogous compound 4.4, which suggests that the structure of 5.1 is somewhat more strained than that of 4.1. Methoxide Ligand Oxidation to CH2O. The investigation of this reaction was approached by first examining the intramolecular oxidation of a methoxide ligand in 6.1. As with the oxomolybdenum system, three products appeared reasonable (Figure 10). Hydrogen transfer directly to the metal center yields the hydridomolybdenum compound 6.2 (closed-shell singlet ground state), while transfer from one methoxide ligand to a neighboring one (“bridging” oxo ligand) yields the dioxomolybdenum species 6.3 (triplet ground state), which is the least stable product. The most stable product, the hydroxomolybdenum compound 6.4 (open-shell singlet ground state, 〈S2〉 ) 0.17), is formed by hydrogen transfer to a terminal oxo ligand. The products resulting from the oxidation of the methoxide ligand in the O2Mo-POSS 4.4 are also illustrated in Figure 10. Hydrogen abstraction by the molybdenum center yields the hydride product 4.10 (closed-shell singlet ground state), whereas abstraction by the bridging oxo ligand of the Mo-O-Si sequence affords 4.11 (closed-shell singlet ground state; the value of 〈S2〉 after optimization as an open-shell singlet species is only 0.02), which is the least stable product. The hydroxomolybdenum species 4.12 (closed-shell singlet ground state) is obtained by hydrogen transfer to the terminal oxo ligand not involved in hydrogen bonding with the silanol groups. The most stable product, the hydroxomolybdenum species 4.13 (closedshell singlet ground state), is obtained by hydrogen transfer to the terminal oxo ligand involved in hydrogen bonding with the two silanol groups.

Figure 10. Products resulting from methoxide ligand oxidation in 4.3 (4.14), 4.4 (4.10-4.13), and 6.1 (6.2-6.4). 4.10-4.14 and 6.2 are closed-shell species, while 6.4 is an open-shell singlet species. 6.3 is an open-shell triplet species. Reaction energies (kJ mol-1) are given in parentheses. Where appropriate, reaction energies obtained using projected low spin state energies and energies relative to 4.1 + CH3OH are given in brackets and curly brackets, respectively.

Oxidation of the methoxide ligand in 4.3 yields the aqua complex 4.14 (closed-shell singlet ground state; Figure 10). Although this model bears some resemblance to the aqua complex treated in the dioxomolybdenum system of Chempath and Bell,12 the molybdenum atom in 4.14 is contained in an eight-membered rather than a six-membered ring. Relative to 4.1 + CH3OH, 4.14 is essentially as stable as 4.13. Products based on the smaller O2Mo-POSS 5.1 (Figure 3), which are analogous to 4.13 and 4.14, were also modeled. Relative to 5.1 + CH3OH, the product analogous to 4.13 (openshell singlet ground state, 〈S2〉 ) 0.12) is less stable by 109 kJ mol-1 (103 kJ mol-1 based on projected low spin state energy), while the 4.14 analogue (closed-shell singlet ground state) is 48 kJ mol-1 less stable. The oxidation of the methoxy group in 4.5, the most stable methoxysilyl species derived from 4.1, was also studied. However, the products arising from 4.5 (Table 2) are less stable than 4.13 by at least 127 kJ mol-1. The energy barrier that must be overcome to arrive at the hydroxomolybdenum product 6.4 amounts to 189 kJ mol-1 and involves the transition state structure 6.TS (closed-shell singlet ground state; Figure 11). The analogous reaction of the corresponding O2Mo-POSS 4.4 entails the transition structure 4.TS1 (closed-shell singlet ground state; Figure 11) and a barrier of comparable magnitude (202 kJ mol-1; 167 kJ mol-1 relative to 4.1 + CH3OH). Although the barrier to the formation of 4.12 is identical to that of 4.13, the preferred product is the latter due to the increased stability imparted to the structure by the additional hydrogen-bonding interaction. The barrier to the formation of the hydrido product 6.2 was also evaluated and found to be appreciably higher (224 kJ mol-1) than that for the formation of 6.4. The formation of 4.14 involves the transition structure 4.TS2 (open-shell singlet ground state, 〈S2〉 ) 0.26; Figure 11) and a barrier of 130 kJ mol-1 (119 kJ mol-1 based on the projected low spin state energy; identical energy relative to 4.1 + CH3OH), which is considerably lower than the barrier that must be surmounted to arrive at 4.13. Therefore, it appears that 4.14 may in fact be the preferred product. The oxidation of the methoxide ligand in 5.3 (Figure 9), the smaller analogue of 4.13 based on 5.1, involves a closed-shell transition structure similar to 4.TS1, which lies 200 kJ mol-1


J. Phys. Chem. C, Vol. 114, No. 7, 2010 2973 TABLE 3: Unscaled Vibrational Frequencies (cm-1) of the POSS-Based Model Catalysts mode

1.1

2.1

MoO5 moiety “breathing” ModO stretch

1086 1018

1041

ModO stretch OdModO bend

Figure 11. The transition structures 4.TS1, 4.TS2, and 6.TS compared to the relevant reactants and products (interatomic distances in pm). The corresponding energy barriers (kJ mol-1) are given in parentheses below each transition structure. In the case of the POSS-based systems, the apparent energy barriers (energies relative to 4.1 + CH3OH) are given in curly brackets. For 4.TS2, the intrinsic barrier obtained using the projected low spin state energy is given in brackets and the apparent barrier is derived from that value.

SCHEME 3: Removal of CH2O from 4.13 and 4.14 and Formation of Watera

a

Reaction energies (kJ mol-1) are given next to the arrows.

above 5.3 (144 kJ mol-1 relative to 5.1 + CH3OH), 168 kJ mol-1 above the methoxide intermediate 5.2 (Figure 9), and 120 kJ mol-1 above the product analogous to 4.14 (Figure 10). Depending on which route is followed, the CH3OH oxidation process in the system based on 4.1 is completed by detachment of the CH2O ligand from either 4.13 or 4.14 (Scheme 3). In the case of 4.13, this step may be preceded by intramolecular transfer of two hydrogen atoms to yield 4.14 in a marginally endothermic process, from which loss of CH2O then leads to the complex 4.15 (triplet ground state). Thus, the abstraction of the CH2O ligand is an endothermic process requiring about 60 kJ mol-1. 4.4. Vibrational Frequencies. The unscaled vibrational modes of interest involving molybdenum calculated for the POSS-based model catalysts 1.1, 2.1, 4.1, and 5.1 are compared in Table 3. The ModO stretching vibration of the OMo-POSS 1.1 is predicted to occur at a lower frequency than the in-phase OdModO stretching mode of the O2Mo-POSS 4.1. This trend is reversed in the smaller models. Furthermore, an interesting result arising from the calculations on 1.1 is the prediction of a

4.1

5.1

1034 (in-phase) 993 (out-of-phase) 335

1009 (in-phase) 965 (out-of-phase) 322

Raman- and infrared-active mode, albeit of low infrared intensity, corresponding to the “breathing” vibration of the MoO5 moiety. Table 4 contains the vibrational frequencies of interest calculated for selected molybdenum methoxide and methoxysilyl species derived from the models 1.1 and 4.1. It also includes frequencies of the adducts 1.13 and 4.16 (Figure 12), which are formed by the coordination of CH3OH to the molybdenum centers of 1.1 and 4.1, respectively. Calculations were conducted on these adducts in an attempt to model the spectroscopic properties of Lewis-bound CH3OH, which may be a relevant species in the oxidation process.49,50 As expected, the hydroxomolybdenum, silanol, and Lewisbound CH3OH O-H stretching vibrations give rise to the modes with the highest frequencies and span the range between 3120 and 3760 cm-1. The hydroxomolybdenum O-H stretching modes are found either at the upper or lower end of this range, depending on whether hydrogen-bonding interactions are absent or present. The silanol O-H stretching modes occur between 3410 and 3760 cm-1. Again, hydrogen-bonding interactions lead to red-shifted frequencies. The C-H stretching vibrations fall in the range between 2895 and 3075 cm-1, and the two out-of-phase modes in each species are separated by 6-58 cm-1. The in-phase C-H stretching vibrations are separated from the corresponding out-of-phase modes of lower frequency by 57-75 cm-1. The frequencies of the Lewis-bound CH3OH species are higher than those of the methoxysilyl species, which in turn tend to be higher than those of the molybdenum methoxide species. The H-C-H deformation modes are found between 1400 and 1465 cm-1, and the difference between the two out-of-phase vibrations in each species amounts to 2-10 cm-1. The in-phase vibrations are separated from the corresponding out-of-phase modes of lower frequency by 8-28 cm-1. The frequencies of the Lewis-bound CH3OH and methoxysilyl species are similar and tend to be higher than those of the molybdenum methoxide species. A trend is also apparent in the frequencies of the O-H bending vibrations. The highest frequencies are yielded by Lewis-bound CH3OH and are followed by those of the silanol groups. The hydroxomolybdenum groups tend to present the lowest frequencies, with the exception of 4.5. This deviation from the trend may be attributable to the hydrogen-bonding interaction of the hydroxomolybdenum group with the methoxy group. The frequencies of the ModO stretching and OdModO deformation vibrations do not seem to present any significant trends, except in the oxomolybdenum system. The frequencies of the ModO stretching modes of the methoxysilyl species 1.4-1.6 are higher than that of the molybdenum methoxide 1.3, which is in turn higher than that of the Lewis adduct 1.13.

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TABLE 4: Calculated Vibrational Frequencies (cm-1) of Selected Species Derived from 1.1 and 4.1a mode OsH stretch (CH3OH) OsH stretch (SiOH) OsH stretch (MoOH) CsH stretch out-of-phase CsH stretch in-phase HsCsH deformation out-of-phase HsCsH deformation in-phase OsH bend (CH3OH)

1.2

ModO stretch

1.4

1.5

1.6

3468

3460

3033 2994 2930 1461 1452 1444

3033 2989 2928 1461 1453 1445

976

976

1.13

4.3

4.4

4.5

4.16

3566 3692 2996 2954 2897 1443 1438 1418

3496 3414

3501 3123 3033 3002 2935 1460 1453 1444

3649 3753 3700 3003 2993 2920 1437 1429 1404

OsH bend (SiOH) OsH bend (MoOH)

1.3

3005 2978 2914 1439 1437 1409

3245 3030 2993 2930 1458 1454 1442

898 781 759 741

3071 3013 2945 1462 1455 1428 1328 1076

934

887 998

1017

3008 3002 2927 1447 1442 1431 949 947 918

783 782 1015

1013

OdModO deformation

975

1026 996

949

3674 3523 3058 3019 2948 1460 1450 1433 1313 1054 963

1038 1021 1008 948 336 323

1022 992 336

1039 967 344

a All frequencies, with the exception of those of the ModO stretching and OdModO deformation vibrations, are scaled with a scaling factor of 0.96887; see ref 13.

TABLE 5: Comparison of Experimentally Observeda and Calculated (Scaled)b Vibrational Frequencies (cm-1)c

Figure 12. The adducts 1.13 and 4.16, obtained by coordination of CH3OH to 1.1 and 4.1, respectively. Reaction energies (kJ mol-1) are given in parentheses.

5. Discussion 5.1. Comparison of the Oxomolybdenum and Dioxomolybdenum Models. A comparison of the data in Figures 4 and 8 reveals that the dissociative addition of CH3OH appears to occur more readily at dioxomolybdenum rather than at oxomolybdenum sites. The formation of the O2Mo-POSS 4.4 is more than 8 times as exothermic as the formation of the OMo-POSS 1.3. This is most likely a result of the greater relief in strain brought about by the ring-opening reaction as well as the increase in the number of hydrogen-bonding interactions that the relevant terminal oxo ligand is subjected to. Furthermore, cleavage of a bond in a Mo-O-Si sequence is preferred in the case of the dioxomolybdenum model, while in the oxomolybdenum model rupture of a bond in a Mo-O-Si sequence is essentially as likely as that in a Si-O-Si sequence. Thus, following the dissociative addition reaction, the proportion of molybdenum methoxide to methoxysilyl moieties on a MoO3/ SiO2 catalyst surface might be expected to be considerably lower in the oxomolybdenum catalyst system than in the dioxomolybdenum one. Another interesting point is the reaction energy associated with the dissociative addition without bond cleavage. For both systems, it leads to the least stable product, but it is significantly more endothermic for the oxomolybdenum system. The increase in coordination number at the molybdenum center from five to six that occurs when 1.1 is converted to 1.2 probably inflicts significant strain on the POSS, since the molybdenum center is quite rigidly anchored to the cage via four bridging oxo ligands. On the other hand, the dioxomolybdenum system

observeda

assignment

calculated

1020 976-988 968 364

OdModO stretch in-phase ModO stretch OdModO stretch out-of-phase OdModO bend

1005 (4.1) 989 (1.1) 965 (4.1) 326 (4.1)

a MoO3/SiO2 catalysts with a MoO3 content of 1-8% by weight; see ref 52. b Scaling factor ) 0.9717. c The model to which each of the calculated frequencies belongs is given in parentheses.

demonstrates greater structural flexibility, since the tetrahedrally coordinated molybdenum center in 4.1 is anchored to the POSS only by two bridging oxo ligands and is hence more capable of accommodating a further ligand without significant structural rearrangement. As might be expected of compounds containing MoIV (d2) centers,5 the oxomolybdenum derivatives 1.9 and 3.4 are triplet ground state products. Single point calculations on the triplet structures of 1.9 and 3.4 reveal that the open-shell singlet states are both considerably less stable by about 100 kJ mol-1. Optimization of 1.9 and 3.4 as closed-shell singlet species yields structures that are 63 and 35 kJ mol-1 less stable, respectively. In contrast, the dioxomolybdenum derivatives 4.13 and 6.4 are closed-shell singlet and open-shell singlet products, respectively, with both of the corresponding triplet structures lying about 33 kJ mol-1 above the singlet structures. 5.2. Comparison with Experimental Results for MoO3/ SiO2. Vibrational Spectra Prior to CH3OH Addition. The vibrational frequencies of the various modes of interest involving molybdenum calculated for the POSSs 1.1 and 4.1 are compared with the relevant frequencies obtained experimentally for MoO3/ SiO2 materials with a MoO3 content of 1-8% by weight52 in Table 5. The data of samples with a MoO3 content greater than 8% are excluded because bands due to crystalline MoO3 may be present. There are two frequency ranges which are of particular interest when characterizing the structure of the surface species of MoO3-based materials.53–56 The first spans the range between 950 and 1050 cm-1 and is the most significant from a practical



TABLE 6: Experimental Vibrational Frequencies (cm-1) of MoO3/SiO2 Catalysts Measured after the Addition and during the Oxidation of CH3OH, and Their Assignments as Deduced from the Present Calculations 1% MoO3/SiO2a

1% MoO3/SiO2b

5% MoO3/SiO2a

5% MoO3/SiO2c

2995 2957

2999 2957

2995 2957

2857

2857

2857

2996 2959 2927 2857 1473 1463

1464 1451 1405 970 844 775 a

assignment C-H C-H C-H C-H

stretch stretch stretch stretch

(out-of-phase)d (out-of-phase)d (out-of-phase)e (in-phase)d

H-C-H deformation (out-of-phase)d H-C-H deformation (out-of-phased or in-phasee) H-C-H deformation (in-phase)e

894 842 768

Reference 48. b Reference 50. c Reference 49. d Methoxysilyl. e Oxomolybdenum methoxide.

point of view, since it is the range in which the various stretching modes of the ModO bonds are expected. The experimental spectra display three bands in this region between 965 and 1025 cm-1. In the present calculations, ModO bond stretching frequencies of 989 cm-1 for 1.1 and 965 and 1005 cm-1 for 4.1 are predicted, which suggests that the MoO3/SiO2 samples prepared by Lee and Wachs52 may contain a mixture of oxoand dioxomolybdenum sites. Furthermore, the in-phase OdModO stretching vibration of 4.1 is predicted to occur at a higher frequency than that of the ModO stretching vibration of 1.1. This is independent of the scaling and in contrast to the predictions of previous calculations on smaller models,8,57 which Lee and Wachs52 used to assign the frequency at 1020 cm-1 to oxomolybdenum ModO stretching vibrations, those between 976 and 988 cm-1 to in-phase dioxomolybdenum ModO stretching vibrations, and that at 968 cm-1 to out-of-phase dioxomolybdenum ModO stretching vibrations. The present calculations therefore suggest that it might be more appropriate to reverse the first two assignments. The second frequency range of interest is the one below 400 cm-1 and includes the various bending modes of the ModO bonds. The experimental spectra display an absorption at 364 cm-1 in this region, which Lee and Wachs52 assigned to OsMosO deformation. The only relevant vibration predicted between 300 and 400 cm-1 is the OdModO bending vibration of 4.1. Stretching modes associated with oxygen atoms doubly or triply bridging molybdenum atoms are expected in the range between 400 and 900 cm-1. However, in the case of MoO3/ SiO2, this range can at best be used to verify the presence of crystalline MoO3, since SiO2 absorbs strongly in this region and any stretching modes associated with the aforementioned oxygen atoms at MoO3 contents below that giving rise to crystalline MoO3 would be masked. An interesting feature arising from the present calculations is the prediction of a mode at 1086 cm-1 (unscaled) corresponding to the “breathing” vibration of the MoO5 moiety in 1.1, which is both infrared- and Raman-active. The calculated infrared intensity of this mode is low, and it is therefore likely that its detection by infrared spectroscopy, which would be helpful in confirming the presence of isolated oxomolybdenum sites on MoO3/SiO2 catalyst surfaces, may be challenging. Vibrational Spectra Following CH3OH Addition. The experimental vibrational frequencies of various MoO3/SiO2 systems measured after introduction of CH3OH48–50 are listed in Table 6 along with the assignments deduced from the present calculations.

The calculated C-H stretching frequencies of the model compounds in Table 4 span the range between 2895 and 3075 cm-1. The relevant experimental frequencies are in the range from 2855 to 3000 cm-1, and the feature common to all experimental spectra in this region is the presence of three main bands.48–50 The first is between 2990 and 3000 cm-1, the second between 2955 and 2960 cm-1, and the third at 2857 cm-1. Following the introduction of CH3OH, Domen, Oyama, and coworkers observed bands at 2999, 2957, and 2857 cm-1 in the spectra of their pure SiO2 system as well as their MoO3/SiO2 system.50 Hence, it is most likely that these three bands are due to the C-H stretching vibrations of methoxysilyl groups. According to the present calculations, the two higher frequency bands can be assigned to C-H out-of-phase stretching modes, while the lower frequency band can be assigned to the C-H in-phase stretching mode. Wachs and co-workers observed a band at 2927 cm-1 in their MoO3/SiO2 system,49 the frequency of which is about 30 cm-1 lower than that of the band assigned herein to the methoxysilyl C-H out-of-phase stretching mode of lower frequency. The authors assigned this band to the C-H in-phase stretching mode of a molybdenum methoxide moiety. According to the calculations on the molybdenum methoxide species 1.2, 1.3, 4.3, and 4.4, this mode should, in the case of 1.2, 1.3, and 4.3, be found at a lower frequency than or, in the case of 4.4, coincide with the C-H in-phase stretching modes of the methoxysilyl species. Furthermore, the present calculations suggest that it might be more appropriate to assign the band at 2927 cm-1 to the C-H out-of-phase stretching mode of lower frequency of oxomolybdenum methoxide moieties in structures resembling 1.3 and 4.3. The calculated H-C-H deformation frequencies cover a range of 65 cm-1 (1400-1465 cm-1), and as with the C-H stretching frequencies, each of the species in Table 4 displays three modes (two out-of-phase, one in-phase). In practice, however, the two out-of-phase modes would be expected to be indistinguishable and therefore two bands would be anticipated in the relevant region of the experimental spectrum (1400-1500 cm-1). Wachs and co-workers detected bands at 1473 and 1463 cm-1 for their MoO3/SiO2 system in this region, which they assigned to H-C-H out-of-phase and in-phase deformation vibrations, respectively.49 On the other hand, Domen, Oyama, and co-workers observed absorptions at 1464, 1451, and 1405 cm-1 for their systems, the band at 1464 cm-1 being a common feature of the pure SiO2 and MoO3/SiO2 systems.50 These authors chose the following band assignments: methoxy group H-C-H out-of-phase deformation (1464 cm-1), adsorbed CH3OH H-C-H in-phase deformation (1451 cm-1), and

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TABLE 7: Adsorption Energies and Activation Barriers (kJ mol-1) Calculated for the Systems Based on 1.1 and 4.1a catalyst

∆E0,ads

∆Hads

∆Eq0

∆Hq + RT

q ∆E0,app

q ∆Happ + RT

1.1 (via 1.TS1) 1.1 (via 1.TS2) 4.1 (via 4.TS1) 4.1 (via 4.TS2) MoO3/SiO2 (experimental)b V2O5/SiO2 (calculated)

-4 (-17) 49 (13) -30 (-47) 4 (-27)

3 (-10) 50 (14) -28 (-45) 6 (-25) -19 ( 13 -50c,d

138 (132) 110 (103) 188 (193) 106 (101)

137 (131) 111 (105) 190 (195) 107 (103) 108 ( 13 144c,d

134 (115) 158 (116) 157 (146) 110 (74)

140 (121) 161 (119) 162 (151) 114 (78) 89 ( 13 121c,d

-40c -65e

154c 167e

114c 102e

V2O5/SiO2 (experimental)

82 ( 10f 99 ( 4g

a ∆H values related to 1.1 and 4.1 calculated at 523 K. The values in parentheses include dispersion energy contributions. The values related to 1.TS2 and 4.TS2 were calculated using the projected low spin state energies of the transition structures. Experimental values for MoO3/SiO2 (MoO3 content 1% by weight, see ref 50) and V2O5/SiO258,59 and values calculated for V2O5/SiO213,60 are included for comparison. b MoO3 content 1% by weight, see ref 50. c Reference 13. d Calculated at 503 K. e Reference 60. f Reference 58. g Reference 59.

adsorbed CH3OH O-H bending (1405 cm-1). Since the spectra of these authors and Wachs and co-workers display bands at around 1465 cm-1, it is most likely that these particular bands are due to methoxysilyl species, but it is difficult to determine whether they arise from an out-of-phase or in-phase H-C-H deformation mode. Moreover, the assignments of Domen, Oyama, and co-workers concerning the bands below 1460 cm-1 may be misleading. The calculations on 1.13 and 4.16 suggest that the H-C-H in-phase deformation mode of adsorbed CH3OH should not be readily distinguishable from the corresponding mode of methoxysilyl groups. In addition, the unscaled O-H bending modes of adsorbed CH3OH would be expected at frequencies more than 100 cm-1 lower than the unscaled H-C-H in-phase deformation modes of methoxysilyl groups. Given that the experimentally observed bands at 1405 and 1451 cm-1 only appear in the MoO3/SiO2 system and that the calculated frequency for the H-C-H in-phase deformation modes of the oxomolybdenum methoxides 1.2 and 1.3 are about 40 cm-1 lower than the frequencies of the corresponding modes of the methoxysilyl models, it is proposed that the band at 1405 cm-1 be assigned to the H-C-H in-phase deformation mode of oxomolybdenum methoxide moieties. Furthermore, it is suggested that the band at 1451 cm-1 arises from the overlap of the methoxysilyl group H-C-H in-phase deformation and the oxomolybdenum methoxide H-C-H out-of-phase deformation modes. This in turn implies that the bands around 1465 cm-1 are due to the H-C-H out-of-phase deformation mode of methoxysilyl groups. The region below 1100 cm-1 is of limited use during CH3OH oxidation, since the calculations suggest that ModO stretching and OsH bending modes may coincide in this region. This would hamper any attempts at structural elucidation. Domen, Oyama, and co-workers also observed bands at 3740, 3467, and 3310 cm-1, which they assigned to the O-H stretching vibrations of silanol groups, of CH3OH adsorbed on SiO2, and of CH3OH adsorbed on MoO3/SiO2, respectively.50 There is no doubt about the assignment of the first band, but the assignment of the latter two bands deserves some consideration. On increasing the temperature above 40 °C for the pure SiO2 surface, not only does the band at 3467 cm-1 disappear but also the silanol band at 3740 cm-1 gains intensity.50 In agreement with this observation, the present calculations show that the bands at 3467 and 3310 cm-1 may be due to adsorbed CH3OH but with CH3OH acting as an acceptor in hydrogen bonds with silanol and hydroxomolybdenum groups. This view is further supported by the calculated O-H stretching frequencies for silanol groups engaged in hydrogen bonds with methoxy species in 1.5 (3468 cm-1) and 1.6 (3460 cm-1). This shows at the same time that it would be difficult to distinguish between

physisorbed methanol and methoxy groups as the origin of these bands. In addition, since the O-H stretching vibrations of the Lewis-bound CH3OH molecules in the adducts 1.13 and 4.16 are at 3649 and 3674 cm-1, respectively, and exhibit a bathochromic shift of only 50 and 25 cm-1, respectively, compared to the O-H stretching vibration of gas phase CH3OH (3699 cm-1; unscaled frequency 3818 cm-1), it is unlikely that the corresponding vibration of CH3OH physisorbed on SiO2 will deviate significantly from these values. The absorption at 3310 cm-1 could also be due to adsorbed CH3OH acting as a hydrogen bond acceptor, but the larger shift requires a hydrogen bond donor with a weaker O-H bond. The present calculations show that hydroxomolybdenum groups have indeed lower O-H stretching frequencies (3700 cm-1, model 1.2) than silanol groups. When interacting with physisorbed methanol or chemisorbed methoxy groups, they undergo a larger shift (455 cm-1 for model 1.4, O-H stretching vibration at 3245 cm-1) than silanol groups (290 cm-1, models 1.5 and 1.6). q ActiWation Barriers. The apparent energy barrier, ∆E0,app , is q the sum of ∆E0 and ∆E0,ads (eq 4), which are the intrinsic energy barrier and the energy of adsorption, respectively, at 0 K including zero-point vibrational energy contributions. The Arrhenius activation energy, Ea, is given by eq 5, in which ∆Hq is the enthalpy of activation. The apparent Arrhenius activation energy, Ea,app, can be expressed in a similar manner (eq 6) in q , which is terms of the apparent enthalpy of activation, ∆Happ q the sum of ∆H and the enthalpy of adsorption, ∆Hads (eq 7). q ∆E0,app ) ∆Eq0 + ∆E0,ads

(4)

Ea ) ∆Hq + RT

(5)

Ea,app ) ∆Hqapp + RT

(6)

∆Hqapp ) ∆Hq + ∆Hads

(7)

The adsorption energies and activation barriers calculated for the systems based on 1.1 and 4.1 are listed in Table 7. Table 7 also includes experimental values determined for MoO3/SiO250 and V2O5/SiO258,59 catalysts and values calculated for V2O5/ SiO2 systems13,60 for comparison. A comparison of the ∆E0,ads values calculated for V2O5/ SiO2 with those calculated for MoO3/SiO2 which do not include dispersion energy contributions reveals that the dissociative addition of CH3OH is more exothermic in the



TABLE 8: Comparison of Calculated and Experimentala Kinetic Data of the Oxidation of CH3OH on MoO3/SiO2 Catalysts at 523 Kb Kads (atm-1)

catalyst 1.1 (via 1.TS1) 1.1 (via 1.TS1)d 1.1 (via 1.TS2) 1.1 (via 1.TS2)d 4.1 (via 4.TS1) 4.1 (via 4.TS1)d 4.1 (via 4.TS2) 4.1 (via 4.TS2)d experimentala

1.93 5.07 2.07 1.35 2.37 1.28 1.74 2.38 7.46

× × × × × × × × ×

10-6 (3.49 × 10-6) 10-5 (4.80 × 10-6) 10-15 (1.97 × 10-10) 10-11 (3.57 × 10-10) 10-5 (4.02 × 10-8) 10-3 (4.60 × 10-8) 10-10 (7.33 × 10-10) 10-7 (7.58 × 10-10) 10 (9.44 × 10-1)

kox (s-1) 1.95 8.76 4.06 1.43 2.35 1.01 4.84 1.45 5.48

kappc (atm-1 s-1)

× 10-4 (9.24 × 109) × 10-4 (1.01 × 1010) (5.29 × 1011) × 10 (4.46 × 1011) × 10-6 (2.24 × 1013) × 10-5 (3.19 × 1014) × 10 (2.64 × 1012) × 102 (2.72 × 1012) × 10-3 (3.35 × 108)

3.80 × 10-10 (3.26 × 104) 4.44 × 10-8 (4.87 × 104) 8.49 × 10-15 (1.05 × 102) 1.91 × 10-10 (1.58 × 102) 2.69 × 10-11 (4.34 × 105) 4.55 × 10-10 (5.15 × 105) 8.36 × 10-9 (1.91 × 103) 3.47 × 10-5 (2.06 × 103) 0.340 (2.63 × 108)e 0.080 (6.19 × 107)f 0.240 (1.86 × 108)g 0.200 (1.55 × 108)h

a MoO3/SiO2 catalyst with a MoO3 content of 1% by weight; see ref 50. b The pre-exponential factors (the pre-exponential factors for Kads, kox, and kapp are those derived from eqs 13, 11, and 9, respectively) are given in parentheses. c Experimental values obtained by dividing the turnover frequencies (TOFs) by the CH3OH partial pressure. d Dispersion energy contribution included. e 5% CH3OH, 5% H2O (overall TOF). f 5% CH3OH, 5% H2O (selective TOF of CH3OH to CH2O). g 8.5% CH3OH (overall TOF). h 8.5% CH3OH (selective TOF of CH3OH to CH2O).

SCHEME 4: Oxidation of CH3OH at an Oxomolybdenum Sitea

a Gibbs reaction energies (kJ mol-1) at 523 K (∆G523) are given below the arrows. Only vibrational contributions to the chemical potential are taken into account for all species, with the exception of CH3OH, for which translational and rotational contributions are also included. The values in parentheses include dispersion energy contributions.

case of V2O5/SiO2. In contrast, the intrinsic activation barriers calculated for V2O5/SiO2 are more endothermic than those of MoO3/SiO2, with the exception of the dioxomolybdenum system involving 4.TS1. The experimental Ea,app values determined for both V2O5/SiO2 and MoO3/SiO2 are essentially the same. The calculated values suggest that Ea,app is generally higher for MoO3/SiO2, with the exception of the dioxomolybdenum system involving 4.TS2, the value of which is comparable to those calculated for V2O5/ SiO2. Rate Constants. Before advancing to a comparison of experimental and calculated kinetic data, it should be stressed that an adsorption equilibrium precedes the rate-determining hydrogen abstraction reaction. Provided that the pressure is low enough, the amount of CH3OH adsorbed is proportional to the CH3OH pressure and the apparent reaction rate, kapp, q , as shown by eq 8. In can be expressed in terms of ∆E0,app this equation, q is the partition function of the activated complex (q), the CH3OH in the gas phase (meth), and the

MoO3/SiO2 catalyst (cat). Only vibrational contributions are considered for the components in the solid state (qcat, qq), whereas the rotational and translational degrees of freedom are also included for CH3OH in the gas phase. The volume work on adsorption of CH3OH (pV ) RT (ideal gas)) is accounted for by the additional e term. Equation 8 may be q and rearranged so that kapp is expressed in terms of ∆Happ q , as shown in the apparent entropy of activation, ∆Sapp eq 9.

kapp ) (kBT/h)[qq /(qmethqcat)]ee-∆E0,app/RT

(8)

kapp ) (kBT/h)e2e∆Sapp/Re-(∆Happ+RT)/RT

(9)

q

q

q

The rate of methoxide ligand oxidation, kox, can be determined according to eq 10, where qint is the partition function of the methoxide intermediate. Equation 10 may also be written in

2978


Gregoriades et al.

SCHEME 5: Oxidation of CH3OH at a Dioxomolybdenum Sitea

Gibbs reaction energies (kJ mol-1) at 523 K (∆G523) are given below the arrows. Only vibrational contributions to the chemical potential are taken into account for all species, with the exception of CH3OH, for which translational and rotational contributions are also included. The values in parentheses include dispersion energy contributions. a

such a way that kox is expressed in terms of ∆Hq and the entropy of activation, ∆Sq (eq 11).

kox ) (kBT/h)(qq /qint)e-∆E0/RT

(10)

kox ) (kBT/h)ee∆S /Re-(∆H +RT)/RT

(11)

q

q

q

The adsorption equilibrium constant, Kads, can be evaluated using eq 12, which can be rearranged into eq 13, where ∆Sads is the entropy of adsorption.

Kads ) [qint /(qmethqcat)]ee-∆E0,ads/RT

(12)

Kads ) ee∆Sads/Re-∆Hads/RT

(13)

Selected experimental and calculated rate constants are collected in Table 8. The associated Gibbs reaction energies (∆G) can be found in Schemes 4 (1.1) and 5 (4.1). According to the calculated apparent rate constants in Table 8, the mechanism of the oxidation of CH3OH appears to be different in each MoO3/SiO2 system, which is confirmed by the ∆G values given in Schemes 4 and 5. In the oxomolybdenum system (Scheme 4), the preferred route entails proton transfer from CH3OH to a bridging oxo ligand and, therefore, cleavage of a bond in a Mo-O-Si sequence followed by hydrogen abstraction from the methoxide ligand by the terminal oxo ligand. In contrast, the favored path in the dioxomolybdenum system (Scheme 5) does not include cleavage of a bond in a Mo-O-Si sequence, even though the methoxide intermediate resulting from such a bond cleavage reaction is more stable: it rather involves proton transfer from CH3OH to a terminal oxo ligand, and the resulting hydroxide ligand then abstracts a hydrogen atom from the methoxide ligand. The apparent and intrinsic energy barriers calculated for the systems involving the transition structures 1.TS1, 1.TS2, and 4.TS2 are within -11/29 kJ mol-1 of the experimental values

(Table 7). The calculated Kads and kapp values are several orders of magnitude lower than the experimental values. This lack of agreement may be due to the fact that the number of methoxy groups detected experimentally in MoO3/SiO2 systems noticeably exceeds the number of molybdenum atoms.49,50 The deviation of the calculated kox values, however, is not so severe, especially in the case of the oxomolybdenum model, the value of which is only 1 order of magnitude lower than the experimental value. 6. Conclusions The ModO stretch vibrational frequency of the oxomolybdenum surface species is predicted to fall in between the inphase and out-of-phase OdModO stretch frequency of the dioxomolybdenum surface species. This is in contrast to previous assignments and may alter conclusions about which types of species are present on a given surface. The oxidation of CH3OH on MoO3/SiO2 surfaces begins with its dissociative addition, which involves the rupture of a Mo-O-Si and/or Si-O-Si sequence on the catalyst surface, both in the oxo- and dioxomolybdenum systems. Although molybdenum methoxide and methoxysilyl moieties may be present on the catalyst surface following the dissociative addition reaction, the rate-determining step, which entails hydrogen abstraction from a methoxy group, is overcome much more readily in the case of a molybdenum methoxide moiety. The oxidation reaction proceeds by a different mechanism in each system. In the oxomolybdenum system, a molybdenum methoxide intermediate, formed by cleavage of a Mo-O-Si sequence, is involved, in which the hydrogen abstraction of the second step is brought about by a terminal oxo ligand. Although the dissociative addition of CH3OH on catalysts comprising dioxomolybdenum sites also leads to the formation of molybdenum methoxide and methoxysilyl surface species, the second step is surmounted with greater ease if the molybdenum methoxide intermediate is formed without cleavage of a bond in a Mo-O-Si sequence, even though this intermediate is not the most stable one. The hydrogen abstraction is effected by the hydroxide ligand in this intermediate.

DFT Study on Models of Mononuclear Sites Given the limited accuracy of DFT, the level of agreement reached between calculated and observed kinetic parameters, for example, calculated energy barriers are within -11/+29 kJ mol-1 of the experimental values, shows that the mechanistic details presented are significant. Acknowledgment. The authors gratefully acknowledge the Deutsche Forschungsgemeinschaft (SFB546) and the Fonds der Chemischen Industrie for financial support. L.J.G. is particularly indebted to the Deutsche Forschungsgemeinschaft for a research stipend (Forschungsstipendium der DFG). Supporting Information Available: Table of the electronic energies of the species used in this study. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Reuss, G.; Disteldorf, W.; Gamer, A. O.; Hilt, A. In Ullmann’s Encyclopedia of Industrial Chemistry, 6th ed.; Wiley-VCH: Weinheim, Germany, 2003; Vol. 15, p 1. (2) Soares, A. P. V.; Portela, M. F.; Kiennemann, A. Catal. ReV. 2005, 47, 125. (3) Bowker, M.; Holroyd, R.; Elliott, A.; Morrall, P.; Alouche, A.; Entwistle, C.; Toerncrona, A. Catal. Lett. 2002, 83, 165. (4) Haber, J. In Studies in Inorganic Chemistry 19: Molybdenum- an Outline of its Chemistry and Uses; Braithwaite, E. R., Haber, J., Eds.; Elsevier: Amsterdam, The Netherlands, 1994; p 477. (5) Wiberg, N. Holleman-Wiberg Lehrbuch der Anorganischen Chemie; 102nd ed.; Walter de Gruyter: Berlin, Germany, 2007. (6) Mars, P.; van Krevelen, D. W. Chem. Eng. Sci. Spec. Suppl. 1954, 3, 41. (7) Briand, L. E.; Farneth, W. E.; Wachs, I. E. Catal. Today 2000, 62, 219. (8) Radhakrishnan, R.; Reed, C.; Oyama, S. T.; Seman, M.; Kondo, J. N.; Domen, K.; Ohminami, Y.; Asakura, K. J. Phys. Chem. B 2001, 105, 8519. (9) Allison, J. N.; Goddard, W. A. J. Catal. 1985, 92, 127. (10) OMoCl4 reacts with CH3OH to yield OMoCl3(OCH3) via HCl elimination, whereas O2MoCl2 apparently does not react with CH3OH: Schierloh, E. M.; Ault, B. S. J. Phys. Chem. A 2003, 107, 2629. (11) Seman, M.; Kondo, J. N.; Domen, K.; Reed, C.; Oyama, S. T. J. Phys. Chem. B 2004, 108, 3231. (12) Chempath, S.; Bell, A. T. J. Catal. 2007, 247, 119. (13) Do¨bler, J.; Pritzsche, M.; Sauer, J. J. Am. Chem. Soc. 2005, 127, 10861. (14) Calzaferri, G.; Hoffmann, R. J. Chem. Soc., Dalton Trans. 1991, 917. (15) Sauer, J.; Hill, J. R. Chem. Phys. Lett. 1994, 218, 333. (16) Bieniok, A. M.; Bu¨rgi, H. B. J. Phys. Chem. 1994, 98, 10735. (17) Civalleri, B.; Garrone, E.; Ugliengo, P. Chem. Phys. Lett. 1998, 294, 103. (18) Magg, N.; Immaraporn, B.; Giorgi, J. B.; Schroeder, T.; Ba¨umer, M.; Do¨bler, J.; Wu, Z. L.; Kondratenko, E.; Cherian, M.; Baerns, M.; Stair, P. C.; Sauer, J.; Freund, H.-J. J. Catal. 2004, 226, 88. (19) Rozanska, X.; Fortrie, R.; Sauer, J. J. Phys. Chem. C 2007, 111, 6041. (20) Do¨bler, J.; Pritzsche, M.; Sauer, J. J. Phys. Chem. C 2009, 113, 12454. (21) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (22) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (23) Scha¨fer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829. (24) Andrae, D.; Häußermann, U.; Dolg, M.; Stoll, H.; Preuß, H. Theor. Chim. Acta 1990, 77, 123. (25) Ahlrichs, R.; Ba¨r, M.; Ha¨ser, M.; Horn, H.; Ko¨lmel, C. Chem. Phys. Lett. 1989, 162, 165. (26) Treutler, O.; Ahlrichs, R. J. Chem. Phys. 1995, 102, 346. (27) Becke, A. D. Phys. ReV. A 1988, 38, 3098.

J. Phys. Chem. C, Vol. 114, No. 7, 2010 2979 (28) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (29) Perdew, J. P. Phys. ReV. B 1986, 33, 8822. Erratum: Perdew, J. P. Phys. ReV. B 1986, 34, 7406. ¨ hm, H.; Ha¨ser, M.; Ahlrichs, R. Chem. (30) Eichkorn, K.; Treutler, O.; O Phys. Lett. 1995, 242, 652. (31) Sierka, M.; Hogekamp, A.; Ahlrichs, R. J. Chem. Phys. 2003, 118, 9136. (32) Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R. Theor. Chem. Acc. 1997, 97, 119. (33) Deglmann, P.; Furche, F.; Ahlrichs, R. Chem. Phys. Lett. 2002, 362, 511. (34) Deglmann, P.; Furche, F. J. Chem. Phys. 2002, 117, 9535. (35) Alexander, L. E.; Beattie, I. R.; Bukovszky, A.; Jones, P. J.; Marsden, C. J.; Van Schalkwyk, G. J. J. Chem. Soc., Dalton Trans. 1974, 81. (36) Neikirk, D. L.; Fagerli, J. C.; Smith, M. L.; Mosman, D.; Devore, T. C. J. Mol. Struct. 1991, 244, 165. (37) Peng, C. Y.; Ayala, P. Y.; Schlegel, H. B.; Frisch, M. J. J. Comput. Chem. 1996, 17, 49. (38) Peng, C. Y.; Schlegel, H. B. Isr. J. Chem. 1993, 33, 449. (39) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N. ; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03; revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (40) Sierka, M.; Sauer, J. J. Chem. Phys. 2000, 112, 6983. (41) Noodleman, L. J. Chem. Phys. 1981, 74, 5737. (42) Caballol, R.; Castell, O.; Illas, F.; Moreira, I. d. P. R.; Malrieu, J. P. J. Phys. Chem. A 1997, 101, 7860. (43) Grimme, S. J. Comput. Chem. 2006, 27, 1787. (44) Kessler, V. G.; Mironov, A. V.; Turova, N. Y.; Yanovsky, A. I.; Struchkov, Y. T. Polyhedron 1993, 12, 1573. (45) Feher, F. J.; Rahimian, K.; Budzichowski, T. A.; Ziller, J. W. Organometallics 1995, 14, 3920. (46) Duchateau, R. Chem. ReV. 2002, 102, 3525. (47) Kim, G. S.; Huffman, D.; DeKock, C. W. Inorg. Chem. 1989, 28, 1279. (48) Jehng, J. M.; Hu, H. C.; Gao, X. T.; Wachs, I. E. Catal. Today 1996, 28, 335. (49) Burcham, L. J.; Briand, L. E.; Wachs, I. E. Langmuir 2001, 17, 6164. (50) Seman, M.; Kondo, J. N.; Domen, K.; Radhakrishnan, R.; Oyama, S. T. J. Phys. Chem. B 2002, 106, 12965. (51) Holstein, W. L.; Machiels, C. J. J. Catal. 1996, 162, 118. (52) Lee, E. L.; Wachs, I. E. J. Phys. Chem. C 2007, 111, 14410. (53) Loo, B. H.; Yao, J. N.; Coble, H. D.; Hashimoto, K.; Fujishima, A. Appl. Surf. Sci. 1994, 81, 175. (54) Mestl, G.; Srinivasan, T. K. K.; Kno¨zinger, H. Langmuir 1995, 11, 3795. (55) Seguin, L.; Figlarz, M.; Cavagnat, R.; Lasse`gues, J. C. Spectrochim. Acta, Part A 1995, 51, 1323. (56) McEvoy, T. M.; Stevenson, K. J. Langmuir 2005, 21, 3521. (57) Chempath, S.; Zhang, Y. H.; Bell, A. T. J. Phys. Chem. C 2007, 111, 1291. (58) Deo, G.; Wachs, I. E. J. Catal. 1994, 146, 323. (59) Hess, C.; Drake, I. J.; Hoefelmeyer, J. D.; Tilley, T. D.; Bell, A. T. Catal. Lett. 2005, 105, 1. (60) Goodrow, A.; Bell, A. T. J. Phys. Chem. C 2007, 111, 14753.

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