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Partial Oxidation of Methane on the SiO2 SurfacesA Quantum Chemical Study Sukru Ozturk,† Isik Onal,‡ and Selim Senkan*,† Department of Chemical Engineering, University of California, Los Angeles, California, 90095, and Department of Chemical Engineering, Middle East Technical University, Turkey
Reaction pathways for methane partial oxidation (MPO) on silica were theoretically investigated using the semiempirical MOPAC-PM3 molecular orbital method. The surface of SiO2 was modeled by a helical Si6O18H12 molecular cluster that also exhibits a strained siloxane bridge defect. First, a bond energy analysis was performed on the silica cluster with isolated 3- and 4-coordinated Si surface atoms. Calculated bond dissociation energies for Si-H, SiO-H, and Si-OH were comparable to H-CH3, H-OH, and O-O. In the second phase, elementary reactions around the bridge structure were studied.The facile ring-opening reaction with water, which reconstitutes a pair of vicinal hydroxyls, was found both thermodynamically and kinetically favored, in good agreement with the experiment and other theoretical methods. Activation of methane by the lattice bridge oxygen was thermodynamically unfavorable with high activation energy. On the other hand, the computational results also confirmed the important role adsorbed or “activated” oxygen plays in an MPO reaction, and indicated the likely formation of methanol as an intermediate in formaldehyde production. Introduction Methane is the most abundant component of natural gas, usually over 90% of the hydrocarbon fraction; thus, it is a useful raw material for the synthesis of more valuable hydrocarbon products such as methanol and formaldehyde. Catalytic methane partial oxidation (MPO) to C1 oxygenates (particularly methanol and formaldehyde) in a single catalytic step have been intensively studied experimentally for the last 15 years by many research groups worldwide as reviewed by Pitchai and Klier1 and Amenomiya et al.2 Most studies have focused on increasing the overall formaldehyde yield and selectivity, while very few efforts have been placed toward developing a better mechanistic understanding of the underlying reactions. Isotopic-labeling techniques have been used to probe the origin of the oxygen incorporated into reaction products.3,4 However, such techniques did not provide definitive proof of the participation of the lattice or gas-phase oxygen in product formation because reaction products also undergo fast isotopic-exchange reactions. Silica has been widely used as a support for metal oxide catalysts for MPO, and a number of papers have shown that silica itself shows discernible activity for formaldehyde formation.5-11 Sun et al.8 have attempted to explain the working mechanism of the MPO reaction over a bare SiO2 surface by invoking the existence of surface siloxane defect sites generated during dehydroxylation of the silica at high temperatures. It has been suggested that different catalytic activities exhibited by various silicas could be associated with different concentrations of such defects, that is, “strained siloxane bridges” (SSB), on the surface. SSBs can be produced upon the condensation of neighboring vicinal silanol groups on the silica surface via heating. Several groups also reported evidence about the pres* To whom correspondence should be addressed. † University of California. ‡ Middle East Technical University.
ence of SSB dimer rings and less reactive SiO trimer ring defects on various forms of silica.12-15 It was claimed that ring strain increases the reactivity of the siloxane bonds, especially in dimer ring (SSB) structures. Although the unique and fundamental role of the silica surface in the MPO reaction toward the formation of formaldehyde has been shown experimentally,5-10,16-18 we are not aware of any theoretical investigations in which the heterogeneous reaction mechanisms of MPO on the bare silica surface with a SSB defect structure have been studied. A number of theoretical studies can be found in the literature with regard to the interaction of the silica surface with various adsorbents.19 However, few studies focused on the SSB structure as the site responsible for chemisorption. In addition to the work of Ferrari et al.20 which will be discussed below, Bendale and Hench21 studied the cyclo disilisilic and tricyclo tetra silisilic acids as model clusters containing SSB structures via semiempirical MOPAC-AM1 and PM3 calculations. They concluded that the PM3 model results were in good agreement with similar ring systems observed on the silica surface in terms of equilibrium structure predictions. In related studies, ab initio computations have been undertaken to model the etching reactions of silica surfaces.22 Blaszkowsky and Van Santen23 reported ab initio studies of chemisorption and activation of methanol in a protonic zeolite lattice. Van Santen and Kramer24 also gave a review of the chemicalcluster approach using ab initio methods in zeolite catalysis in particular. In this paper the energetics of the adsorption of water and the catalytic partial oxidation of methane process on the silica surface have been explored using the semiempirical MOPAC-PM3 formalism25,26 The thermochemistry and kinetics of the catalytic methane activation reactions were also investigated, and mechanisms for the formation of C2 hydrocarbons and MPO products were identified. Another, and equally impor-
10.1021/ie990252c CCC: $19.00 © 2000 American Chemical Society Published on Web 01/15/2000
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the total enthalpy of formation of the reactant molecule + Si6O18H12 complex at any interatomic distance and the sum of the enthalpies of formation of the free Si6O18H12 cluster and the approaching reactant molecule at infinite separation. Results and Discussion
Figure 1. Si6O18H12 cluster with siloxane bridge.
tant objective of the present study was to demonstrate the utility of semiempirical quantum chemical methods as fast and reliable screening tools for the discovery and optimization of catalytic materials. Surface Model and Calculation Method In this study the SSB fragment on the silica surface has been considered to be the site responsible for methane activation. The SiO2 catalyst surface was modeled as a helical Si6O18H12 molecular cluster that exhibits such an SSB defect. This cluster size is significantly larger than the molecular sizes of CH4, O2, and MPO products, so that the chemisorption properties of the surfaces should be minimally altered. We also considered a cluster that was twice the size of Si6O18H12.. This study led to the determination of virtually the same adsorption energetics, albeit at a significantly higher computational cost. Consequently, we decided to use the smaller cluster size to speed up the calculations. Computing times using the smaller SiO2 cluster were of the order of minutes to fractions of hours, demonstrating the utility of semiempirical methods as attractive screening tools to evaluate large combinations of materials for catalytic properties. The Si6O18H12 cluster was assumed to exhibit a lowquartz crystal structure of SiO2 with a Si-O bond length of 1.61 Å, taken to be the same value as that for the bulk lattice.27 In this crystal structure one silicon atom has four nearest oxygen neighbors placed at four corners of a tetrahedron. The tetrahedra are connected through the corner oxygen atoms to form the particular crystalline phase. Figure 1 illustrates a sketch of the Si6O18H12 cluster with an SSB defect used in the simulations. As a general computational procedure, the energies of the reactants, intermediates, and the products were calculated by fixing some parts of the crystal geometry of the cluster and by optimizing the remaining structural parameters of the system. MOPAC-PM3 formalism with the key word PM3 was used for all computations.25,26 The resulting relative energies for the cluster and reactant molecule complex are plotted in kcal/mol (1 kcal/mol ) 4186.8 J/mol) versus the reaction coordinate interatomic distance in Angstrom (1 Å ) 10-10 m). The relative energy is defined to be the difference between
First, a bond dissociation energy (BDE) analysis was performed to establish the overall energetics (stability) of hydrated silica relative to CH4, H2O, and O2. The Si-H bond dissociation energy from a Si4c (4-coordinated Si) site was calculated to be 176 kJ/mol, and 435 kJ/mol for Si-OH and 444 kJ/mol for SiO-H. Bond dissociation energies calculated around Si3c, however, were 339 kJ/mol for Si-OH and 343 kJ/mol for SiO-H bonds. Similarly, the BDE were calculated to be 389 kJ/ mol for H-CH3, 412 kJ/mol for H-OH, and 486 kJ/mol for O-O bonds. These results indicate that the formation of low-coordinated vacant Si surface species and the dissociation of CH4 are energetically more favorable processes in accordance with the experimental data reported in the literature.28 The next stage of computations was conducted in three parts: (1) Reconstruction energetics for SSB and silanol sites, (2) direct activation of methane by the SSB site, and (3) simulation of the catalytic MPO reaction by a sequential reaction pathway consisting of oxygen preadsorption on the SSB structure followed by reaction with methane. (1) Reconstruction Energetics for SSB and Silanol Sites. The first step in methane activation calculations was the study of the reconstruction energetics of SSB through dehydration (condensation reaction leading to the elimination of water) of vicinal silanol groups as shown in Figure 2. The availability of SSB sites on the silica structure is reported to increase with increasing temperature of the initial heat treatment.29 However, because the condensation reaction is a reversible one, the energetics of that conversion is more important. In this computation, the O2-Si1 distance was selected as the reaction coordinate, and dehydroxylation of one of the vicinal OH groups, namely, the OH group on the Si1 site (see Figure 2), was studied. The local structure consisting of the bridge atoms O1, O2, and O3 as well as the H1 and H2 atoms was relaxed in this calculation. During the dissociation process the O2H1 group interacted with the neighboring hydrogen, H2, which was originally on the O3 site, thereby forming an H2O molecule as illustrated in Figure 2a,b, and dissociated from the cluster as a water molecule. After dissociation, a reconstructed SSB was obtained by the optimization of the remaining silica structure as shown in Figure 2c. The optimized SSB structure required more energy compared to the unoptimized structure. Alternatively, by selection of the reaction coordinate as the H2-O2 distance, the formation of an intermediate H2O structure on the Si1 site, which is similar to the structure shown in Figure 2, led to the reconstruction of the original vicinal silanol structure. Because it is important to be able to compare at least qualitatively semiempirical computation results with basic experimental and theoretical information reported in the literature, it was decided for the next stage of computations to investigate the adsorption of H2O onto the SSB structure for which experimental and theoretical data exist. This provides general calibration for the semiempirical PM3 method and the appropriate geom-
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Figure 2. (a) and (b) H2O formation during the dissociation of the OH group from the Si1 site through the interaction with the H2 atom and (c) reconstructed siloxane bridge.
etry (e.g., cluster shape and size) used in subsequent computations of present work involving energetic and mechanistic investigations with regard to MPO reaction on silica. For this calculation, the HOH-O2 distance was chosen as the reaction coordinate. In this case, the SSB structure was destroyed and the vicinal silanol groups were reconstructed by the dissociative adsorption of an H to O2 and an OH to Si2 sites as can be seen in Figure 3. The reaction was highly exothermic at a value of 85.5 kJ/mol (20.4 kcal/mol) with no activation energy barrier. The transition-state structure was then located
by first calculating a saddle point (key word SADDLE) between the reactants (Si6O18H12 cluster and H2O) and the optimum product structure. The resulting structure was further refined by means of a transition-state calculation (key word TS). The structure obtained from this computation was confirmed through a FORCE calculation where the Hessian matrix met the requirement of having only one negative force constant. All of the cluster atoms were relaxed throughout these calculations. Figure 4 shows the transition-state structure obtained. Ferrari et al.20 studied the SSB interactions with H2O, CO2, H2, and N2 by use of IR spectroscopy as well as theoretical ab initio studies. In the latter case, they used a much smaller molecular Si2O2H4 cluster to simulate the SSB defect. They reported full agreement between experimental and theoretical studies and concluded that the reaction is facile only with H2O. Table 1 gives a comparison of experimental and theoretical (ab initio, density functional, and semiempirical) results in terms of structural (bond length and bond angle), thermodynamic (heat of formation), and spectroscopic (vibrational frequency) information obtained by various groups including the present work. Experimental thermodynamic and structural surface data for the SSB structure are not available. In Table 1 theoretical results are given at five levels of treatment: (i) the self-consistent field SCF (ab initio); (ii) MP2 with the frozen-core approximation (ab initio); (iii) DFT method using local S-VWN functionals; (iv) DFT method using gradientcorrected B-LYP functionals; and (v) MOPAC-PM3 semiempirical method used in this study. As shown in Table 1, for the H2O molecule SCF and PM3 both tend to underestimate whereas the ab initio and DFT methods overestimate the bond lengths. However, in the case of the Si-O bond length for the SSB structure, the values computed at the MP2 and PM3 levels are in excellent agreement with each other at 1.695 Å for both levels. The B-LYP value is also in close proximity at 1.708 Å. The ∠Si-O-Si angle shows the largest variations between different levels of computation and deviates most for the PM3 method. One has to bear in mind, however, that the ∠Si-O-Si angle is very sensitive to both basis set extension and level of computational treatment. This is because the force constant for the ∠Si-O-Si bending mode is very small. Recent work by Bar and Sauer30 indicates the energy difference between
Figure 3. Optimized structure for the reaction products between water and the silica cluster and the corresponding energy diagram.
Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 253 Table 1. Computed and Experimental Geometries and Formation Energies of H2O, Silica Cluster, and the Products of Reaction between the Silica Cluster and H2O (Bond Lengths in Angstroms (Å), Angles in Degrees, Heats of Formation in kJ/mol, and Vibrational Frequencies in cm-1) computational method molecule H2O (H2SiO2)2 SSB structure
(H2SiO2)2-H2O Si6O18H12 SSB structure Si6O18H12-H2O a
Defrees et al.40
molecular property
experimental
O-H bond length ∠H-O-H angle heat of formation Si-O bond length ∠O-Si-O angle ∠Si-O-Si angle O-H vibrational frequency
0.959a 103.9a -241.83d 1.61a (bulk crystal) 2760b
heat of formation (optimum structure) Si-O bond length ∠O-Si-O angle ∠Si-O-Si angle O-H vibrational frequency heat of formation (optimum structure) b
Ferrari et al.20
c
Present work.
SCF
S-VWN
B-LYP
0.944b 106.9b
0.963b 105.0b
MP2
0.972b 105.3b
0.976b 104.3b
1.665b 88.4b 91.6b 4256b uncorrected 2740b corrected -150b
1.695b 89.8b 90.2b
1.682b 90.1b 89.9b
1.708b 89.8b 90.2b
-136b
-163b
-135b
PM3 0.941c 107.7c -221.75c
1.695c 82.5c 98.0c 4226c uncorrected -85.5c d
Chase et al.41
Figure 4. Transition-state structure for the reaction between water and the silica structure.
the linear and bent configuration of ∠Si-O-Si to be as low as 150 cm-1. It can also be observed in Table 1 that
there is very good agreement between O-H vibrational frequency data computed by SCF and PM3 methods (4256 vs 4226 cm-1). This value is not available for the other methods. Ferrari et al.20 scale down the value computed by SCF for the known deficiencies of the SCF method by 0.92 and further by 0.70 for taking the isotopic shift into account, and their corrected value of 2740 cm-1 compares favorably with the experimental value of 2760 cm-1. Similar corrections may apply to PM3 as well. In terms of heat of formation results, the exothermic value computed for the H2O molecule is somewhat underestimated by the PM3 method (221.75 kJ/mol vs the experimental value 241.83 kJ/mol). The PM3 method essentially gives the same qualitative trend as the other theoretical methods for the spontaneous (no activation barrier) and exothermic reaction between the H2O molecule and the silica cluster containing the SSB structure (see Table 1). These reactivity results are also qualitatively consistent with what has been experimentally reported by other groups. The presence of SSB sites and their reactivity with slightly acidic H atoms (H atoms of water, ammonia, methanol, pyridine) was first
Figure 5. Oxygen adsorption to the Si2 site, optimized structure, FIXED bridge, and the corresponding energy diagram.
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Figure 6. Oxygen adsorption to the Si2 site, optimized structure, RELAXED bridge, and the corresponding energy diagram.
Figure 7. Methane activation on preadsorbed oxygen at the Si2 site, FIXED bridge, leading to methanol formation, and the corresponding energy diagram.
reported by Morrow and co-workers.31-33 They also reported that the SSB structure was highly reactive and it was rapidly destroyed by the dissociative chemisorption of water, methanol, ammonia, and pyridine. However, molecular hydrogen, nitrogen, and carbon dioxide were reported to be inactive on SSB defect sites. Bunker et al.34 investigated the kinetics of reactions between water, ammonia, methanol, and methylamine on dehydroxylated silica containing SSB defects by use of FTIR spectroscopy. They reported that all of the gases undergo a dissociative chemisorption on the strained bonds of the SSB structure. Relative reactivities of unstrained and strained Si-O bonds indicated that bond strain promotes the bond rupture reactions. (2) Direct Activation of Methane by the SSB Site. Methane activation on the SSB structure was then studied by selecting the H3CH-O2 or O1 distance as the reaction coordinate.The resulting energetics were found to be both kinetically (high activation energy
barrier in the order of 200-250 kJ/mol) and thermodynamically (endothermic heat of reaction) unfavorable for a lattice SSB oxygen to activate methane in contrast to MgO-based catalysts where methyl radical generation by a lattice oxygen has been confirmed to be a favorable process by experiments as well as by quantum mechanical computations.35 A subsequent transition-state calculation using the key word TS confirmed this finding by locating a minimum energy transition-state structure with methane being far away from the silica cluster. These results may be considered somewhat in agreement with the experimentally reported activation energies of 239-285 kJ/mol (57-68 kcal/mol) over various types of silica catalysts for the formation of C2 products.8 Our calculations clearly support the fact that the formation of methyl radicals is a difficult process on the silica surface. Once methyl radicals are formed and desorbed from the surface, they can easily combine to form higher hydrocarbons. There are also other types
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Figure 8. Methane activation on preadsorbed oxygen at the Si2 site, RELAXED bridge, leading to methanol formation, and the corresponding energy diagram.
of strong experimental evidence supporting these calculations. For example, the continuous analysis of the reaction products with a quadrupole mass spectrometer as reported by Parmaliana and co-workers17,18,28 indicated the lack of participation of lattice oxygen in the MPO reaction. The isotopic oxygen-exchange capability of SiO2 has also been reported to be very low and does not start below 923 K.36 (3) Simulation of Catalytic MPO Reaction by a Sequential Pathway. Previous experimental studies have shown that the presence of gaseous oxygen facilitates the MPO activation of unloaded silica.11 The concept of partial oxidation of the methane molecule by “activated” oxygen has also been proposed and gained acceptance in experimental literature on silica catalysts.17,18,37,38 Therefore, to assess computationally the role adsorbed oxygen species plays in the MPO reaction mechanism, methane activation on adsorbed oxygen species for both relaxed and fixed siloxane bridges was studied using a sequential pathway similar to that of a method Onal and Senkan35 utilized in an earlier study on OCM catalysis involving MgO and Li/MgO clusters. For this, the energetics of oxygen adsorption on the siloxane bridge were first investigated by choosing the reaction coordinate as the distance between the approaching O atom of O2 and the Si2 site on the bridge structure (see Figures 5 and 6). Calculations gave highly exothermic heat of reaction values both for fixed and relaxed bridge structures, 352 kJ/mol (84 kcal/mol) and 452 kJ/mol (108 kcal/mol), respectively. For the fixed bridge structure where only the atoms of the adsorbed oxygen molecule are optimized, a cyclic adsorbed oxygen structure formed, and the final geometry of the cluster is shown in Figure 5. In the case of the relaxed bridge where the O atoms of the SSB structure are also being optimized, however, oxygen adsorption destroyed the SSB through stronger interactions with the O2 site of the original SSB and resulted in a more exothermic heat of reaction (Figure 6). Similar calculations in which the distance between O1 or O2 and the O atom of an approaching O2 molecule is the reaction coordinate gave
Figure 9. Optimized structure around a relaxed siloxane bridge after desorption of methanol.
the same optimized final structures and energy values as noted above. Next, the energetics for methane adsorption were determined by choosing the reaction coordinate as the distance between a preadsorbed oxygen (i.e., O3 and/or O4) on Si2 and H of CH4. In both cases methane strongly interacted with the preadsorbed oxygen species and formed methanol which remained adsorbed on the surface. The final optimized structures together with relative energy changes as a function of the reaction coordinates are presented in Figures 7 and 8 for fixed and relaxed SSB cases, respectively. In the case of the fixed siloxane bridge an activation energy barrier of 172 kJ/mol (41 kcal/mol) was calculated, while the heat of reaction was exothermic at 117 kJ/mol (28 kcal/mol). Interestingly, the desorption of the methanol moiety from the surface was also exothermic at a value of 134 kJ/mol (32 kcal/mol). Evidently the gaseous CH3OH was the preferred state under the conditions investigated. It is significant to note that methane interactions with
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Figure 10. Methane activation on preadsorbed oxygen (O4) on a RELAXED bridge and the corresponding energy diagram.
Figure 11. Methanol dissociation to formaldehyde on a siloxane bridge and the corresponding energy diagram.
both preadsorbed oxygen atoms (i.e., O3 and O4) gave results similar to those described above. In the case of the relaxed bridge, however, the activation energy for methanol formation on O3 increased to 234 kJ/mol(57 kcal/mol) while the exothermicity of the overall reaction decreased to 46 kJ/mol (11 kcal/mol) as presented in Figure 8. Desorption of the methanol molecule was also found to be less exothermic at 46 kJ/mol (11 kcal/mol) compared to that of the fixed SSB case. Similarly, methane interaction with O2 led to methanol formation with an activation energy barrier height of 188 kJ/mol (45 kcal/mol). However, the reaction was more exothermic at 209 kJ/mol (50 kcal/mol) compared to that with O3. Desorption of methanol was also found to be exothermic at 17 kJ/mol (4 kcal/mol). The optimized structure of the cluster after desorption of methanol is shown in Figure 9. Another interesting feature of the relaxed bridge calculations was the observation of a different activation mechanism on preadsorbed oxygen atoms which formed a 5-membered ring structure together with O1, Si1, and
Si2. In this case, methane activation on O4 resulted in the dissociative adsorption of methane on Si2 and O4 (see Figure 10) with a significantly lower energy barrier of 155 kJ/mol (37 kcal/mol). The reaction was endothermic by 134 kJ/mol (32 kcal/mol), also given in Figure 10. Desorption of the methyl radical from Si2 required only 4.2 kJ/mol (1 kcal/mol). Methanol activation calculations on adsorbed oxygen species did not give any further reaction products, but methanol remained adsorbed on oxygen species. To complete the catalytic cycle, the activation of methanol on SSB was also studied. For this, the distance between O1 and one of the hydrogens connected to the carbon atom of CH3OH was selected as the reaction coordinate. The resulting interaction led to the destruction of SSB, concomitant with the formation of gaseous formaldehyde, CH2O. The remaining two hydrogen atoms terminated the dangling bonds of Si2 and O1 atoms, as illustrated in Figure 11. This reaction was highly exothermic at 402 kJ/mol (96 kcal/mol) and exhibited a low activation energy barrier of 54 kJ/mol
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(13 kcal/mol). These calculations are in agreement with the literature where the most effective route leading to the formation of formaldehyde has been suggested to proceed via the participation of gas-phase oxygen, while the involvement of bulk lattice ions has been suggested to result in COx formation.11 However, computational results at this time rule out the direct formation of formaldehyde from methane on the silica surface. To our knowledge, this type of detailed and sitespecific reaction mechanism study has not been reported previously. Earlier mechanisms proposed in the experimental literature involved only a simplistic accounting of surface-activated oxygen molecules reacting with methane to produce first a methoxide ion and then formaldehyde without the formation of methanol as an intermediate. The reaction mechanism involving oxidation of methanol to formaldehyde is, however, presented for molybdena and vanadia on silica catalysts as a viable route.39 It should also be pointed out that under the relatively high temperature conditions of the MPO reaction, it is very probable that the methanol formed can also react to form total oxidation products such as CO or CO2, thereby causing lower formaldehyde selectivity. In summary, good agreement has been found between the semiempirical MOPAC-PM3 method and the experimental data regarding the MPO reaction. In particular, strained siloxane bridge (SSB) defect sites have been identified to be likely reaction centers on silica surfaces. The facile ring-opening reaction with water, which reconstitutes a pair of vicinal hydroxyls, has been found both thermodynamically and kinetically favored. The computational results also confirmed the important role adsorbed or “activated” oxygen plays in MPO reaction and indicated the likely formation of methanol as an intermediate in formaldehyde production. Because semiempirical methods have a distinct computational speed advantage over more rigorous ab initio wave function and density functional methods, they can be exploited as a useful screening tool in catalyst research and development. Semiempirical methods can be used to establish the relative reactivity trends of a large number of clusters with modest computational resources, thereby steering the experiments as well as the more rigorous computational methods toward the most promising directions in catalyst discovery and development. Acknowledgment This work was supported, in part, by funds from the National Science Foundation, the U.S. Environmental Protection Agency, UCLA, Middle East Technical University, and the Scientific and Technical Research Council of Turkey. Literature Cited (1) Pitchai, R.; Klier, K. Partial Oxidation of Methane. Catal. Rev.-Sci. Eng. 1986, 28, 13. (2) Amenomiya, Y.; Birss, V. I.; Goledzinowski, M.; Galuszka, J.; Sanger, A. R. Conversion of Methane by Oxidative Coupling. Catal. Rev.-Sci. Eng. 1990, 32, 163. (3) Banares, M. A.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A.; Fierro, J. L. G.; Hodnett, B. R.; Parmaliana, A.; Klier, K.; Ozkan, U. S.; Krylov, O. V.; Moffat, J. B.; Li, C.; Wachs, I. E. Mechanistic Aspects of the Selective Oxidation of Methane to C1-Oygenates over MoO3/SiO2 Catalysts in a Single Catalytic Step. Stud. Surf. Sci. Catal. 1993, 75B, 1131.
(4) Mauti, R.; Mims, C. A. Oxygen Pathways in Methane Selective Oxidation over Silica-Supported Molybdena. Catal. Lett. 1993, 21, 201. (5) Spencer, N. D. Partial Oxidation of Methane to Formaldehyde by Means of Molecular Oxygen. J. Catal. 1988, 109, 187. (6) Kasztelan, S.; Moffat, J. B. Partial Oxidation of Methane by Oxygen over Silica. J. Chem. Soc., Chem. Commun. 1987, (21), 1663. (7) Kastanas, G. N.; Tsignidos, G. A.; Schwank, J. Selective Oxidation of Methane over Vycor Glass, Quartz Glass, Magnesia, and Alumina Surfaces. Appl. Catal. 1988, 44, 33. (8) Sun, Q.; Herman, R. G.; Klier, K. Selective Oxidation of Methane with Air over Silica Catalysts. Catal. Lett. 1992, 16, 251. (9) Vikulov, K.; Martra, G.; Coluccia, S.; Miceli, D.; Arena, F.; Parmaliana A.; Paukshtis, E. FTIR Spectroscopic Investigation of the Active Sites on Different Types of Silica Catalysyts for Methane Partial Oxidation to Formaldehyde. Catal. Lett. 1996, 37, 235. (10) Parmaliana, A.; Arena, F. Working Mechanism of Oxide Catalysts in the Partial Oxidation of Methane to Formaldehyde. 1. Catalytic Behaviour of SiO2, MoO3/SiO2, V2O5/SiO2, TiO2, and V2O5/TiO2 Systems. J. Catal. 1997, 167, 57. (11) Arena, F.; Giordano, N.; Parmaliana, A. Working Mechanism of Oxide Catalysts in the Partial Oxidation of Methane to Formaldehyde. 2. Redox Properties and Reactivity of SiO2, MoO3/ SiO2, V2O5/SiO2, TiO2, and V2O5/TiO2 Systems. J. Catal. 1997, 167, 66. (12) Low, M. J. D. Reactive Silica VI Evidence for the Existence of a Dual Reaction Center Involving Si-O Pairs. J. Catal. 1974, 32, 103. (13) Wright, A. C.; Desa, J. A. E. Phys. Chem. Glasses 1978, 19, 140. (14) Brinker, C. J.; Brow, R. K.; Tallant, D. R.; Kirkpatrick, R. J. Surface Structure and Chemistry of High Surface Area Silica Gels. J. Non-Cryst. Sol. 1990, 120, 26. (15) Grabbe, A.; Michalske, T. A.; Smith, W. L. Strained Siloxane Rings on the Surface on SilicasTheir Reaction with Organosiloxanes, Organosilanes, and Water. J. Phys. Chem. 1995, 99, 4648. (16) Parmaliana, A.; Frusteri, F.; Miceli, D.; Mezzapica, A.; Scurrell, M. S.; Giordano, N. Factors Controlling the Reactivity of the Silica Surface in Methane Partial Oxidation. Appl. Catal. 1991, 78, L7. (17) Parmaliana, A.; Frusteri, F.; Mezzapica, A.; Scurrell, M. S.; Giordano, N. Novel High Activity Catalysts for Partial Oxidation of Methane to Formaldehyde. J. Chem. Soc., Chem. Commun. 1993, 751. (18) Parmaliana, A.; Sokolovski, V.; Miceli, D.; Arena, F.; Giordano, N. Silica Supported MoO3 and V2O5 Catalysts in Partial Oxidation of Methane to FormaldehydesFactors Controlling Reactivity. ACS Symp. Ser. 1993, 523, 43. (19) Sauer, J.; Ugliengo, P.; Garrone, E.; Saunders, V. R. Theoretical Study of Van der Waals Complexes at Surface Sites in Comparison with the Experiment. Chem. Rev. 1994, 94, 2095. (20) Ferrari, A. M.; Garrone, E.; Spoto, G.; Ugliengo, P.; Zecchina, A. Reactions of Silica Strained RingssAn Ab-initio Study. Surf. Sci. 1995, 323, 151. (21) Bendale, R. D.; Hench, L. L. Molecular Orbital Models of Strained Tetrahedral Active Sites on Dehydroxylated SilicasAn AM1 and PM3 Study. Surf. Sci. 1995, 338, 322. (22) Jenichen, A. Modelling of Etching ReactionssAb-initio Calculations of the Reactions of CFM+ (M)1-3) and NFN (N)1,2) with Local Models of SiO2 Surface Structure. J. Phys. Chem. 1996, 100, 9820. (23) Blaszkowski, S. R.; van Santen, R. A. Density Functional Theory Calculations of the Activation of Methanol by a Bronsted Zeolitic Proton. J. Phys. Chem. 1995, 99, 11728. (24) van Santen, R. A.; Kramer, G. Reactivity Theory of Zeolitic Bronsted Acidic Sites. Chem. Rev. 1995, 95, 637. (25) Stewart, J. J. P. Optimization of Parameters for Semiempirical Methods. I. Method. J. Comput. Chem. 1989, 10, 209. (26) Stewart, J. J. P. Optimization of Parameters for Semiempirical Methods. 3. Extension of PM3 to Mg, Zn, Ga, Ge, As, Se, Cd, In, Sn, Sb, Te, Hg, Tl, Pb, and Bi. J. Comput. Chem. 1991, 12, 320. (27) Sosman, R. B. The Phases of Silica; Rutgers University Press: New Brunswick, NJ, 1965.
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Received for review April 6, 1999 Revised manuscript received December 2, 1999 Accepted December 19, 1999 IE990252C
