Toluene adsorption on VSbO4(110): a study of an electronic structure

Toluene adsorption on VSbO₄(110): a study of an electronic structure

B.L.Irigoyen^I; A. Juan^II; S.A.Larrondo^I; N.E.Amadeo^I

^ILaboratorio de Procesos Catalíticos, Pabellón de Industrias, FIUBA, Ciudad Universitaria 1428, Buenos Aires, Argentina

^IIDepto. de Física, UNS, Avenida Alem 1253, 8000 Bahía Blanca, Argentina

^{Address to correspondence}

ABSTRACT

The objective of this work is to electronically analyze toluene adsorption reactions on VSbO₄(110). Thus, perpendicular and parallel toluene interactions on the different active sites of the oxide surface (O, Sb and V ions) were studied. Adsorption energy was calculated using the ASED-MO theory, while the electronic analysis was performed with the YAEHMOP code. The electronic density of states (DOS) of the VSbO₄ cluster, modeled with a trirutile-type tetragonal supercell, resembles that of 3D solids with a rutile structure. However, due to the presence of vanadium, small peaks appear above the Fermi level. The DOS of toluene has several peaks resulting from the interaction of the aromatic ring with the methyl fragment, which changes when the adsorbate interacts with the oxide surface. The C-H bonds in the methyl fragment as well as the C_methyl–C_phenyl bond weaken when some electronic density is removed. Also, hydrocarbon oxidation could weaken the p system of the aromatic ring. For toluene perpendicular adsorption (on the V site) the calculations show a hybridization of those orbitals coming from the methyl fragment and the phenyl-methyl interaction energy region. After toluene parallel adsorption on Sb-V sites, the DOS shows an important broadening of some of the methyl and phenyl fragment orbitals. In addition, a study of the overlap population suggests that one of the H atoms of the methyl group can be abstracted with the participation of the Sb cation.

Keywords: toluene oxidation, VSbO₄, DOS.

INTRODUCTION

To describe toluene oxidation reactions from a microscopic point of view, we need some information about the geometric and electronic structure of the molecule and the catalyst. The heterogeneous oxidation of toluene involves three different mechanisms: (a) oxidation of the side chain, without affecting the p-electron system of the aromatic ring, which proceeds along the nucleophilic route and yields products such as benzaldehyde and benzoic acid, (b) electrophilic oxidation of the p-electron system, involving the destruction of the aromatic ring, which forms maleic anhydride, and (c) combustion to CO₂ and H₂O. These reaction mechanisms have been confirmed from theoretical studies of toluene interactions on V₂O₅ (Haber et al. 1994; Witko et al. 1993). The most frequently studied catalysts for toluene partial oxidation are based on transition metal oxides, which are either simple or in binary or ternary mixtures. The prevailing compositions are based on V₂O₅ supported on TiO₂ or SiO₂, but heavily loaded with Sb, Nb and Cs (Grasselli, 1999). Particularly, the rutile-type VSbO₄ phase has been postulated to be a more selective catalyst (Grasselli, 1997, 1999). Also, the experimental results obtained in our laboratory for the oxidation reactions of toluene on V-Sb oxides show a selectivity for benzaldehyde of about 30% (Barbaro et al., 2000).

VSbO₄ synthesis was initially reported by Birchall and Sleight (1976), who observed a tetragonal structure of a rutile type in which Sb and V ions are present as Sb⁺⁵ and V⁺³. Moreover, in powder diffraction studies Hansen et al. (1993) reported different probable combinations of metal and oxygen. On the other hand, for FeSbO₄, an isostructural catalyst with VSbO₄, a trirutile structure was reported in electron diffraction studies (Berry et al., 1987). Thus, the VSbO₄ catalyst has been modeled with a tetragonal trirutile like superstructure, which has the most probable combinations of metal and oxygen reported by Hansen et al. (1993). Taking into account the results of toluene oxidation on V oxide reported by Haber et al. (1994), our theoretical study involves two main types of toluene approaches to the (110) surface of VSbO₄ along perpendicular and parallel reaction pathways.

In this paper we analyze toluene interactions on VSbO₄(110), studying the changes in the electronic structure by means of the YAEHMOP code (Landrum, 1997). The adiabatic total energies of these reactions were obtained with the semiempirical ASED-MO theory (Andersson, 1975).

ACTIVE SITES AND ADSORPTION MODEL

The VSbO₄ oxide was modeled with a trirutile-type tetragonal supercell (see Fig. 1, right). The lattice parameters are a = b = 4.636 Å and c = 3 x 3.048 Å (Berry et al., 1996). The VSbO₄-(110) cluster used in our calculations (see Fig. 1, left) has 228 atoms. The (110) plane was chosen because it appears to be one of the most stable crystal faces of rutile oxides and results from breaking the smallest number of metal-oxygen bonds.

The extra plane of O atoms was retained in such a way that the catalytic surface, having V sites separated by Sb ions, also contains two fold coordinated oxygen atoms.

The interactions of toluene on VSbO₄(110) were analyzed by calculating adsorption energy for two main adsorption routes: with the aromatic ring perpendicular to the catalyst surface (on V, Sb and O sites; see Fig. 2, left) and with it parallel (on Sb-Sb, O-O, V-Sb and Sb-V sites; see Fig. 2, right). To find the local energy minimum the toluene-surface distance, measured from the C atom of the methyl group to the corresponding site, was varied by 0.1 Å steps.

RESULTS AND DISCUSSION

While toluene perpendicular end-on adsorption (see Fig 2, left) on O or Sb atoms is of a physical nature, on the V cation it is more favorable and the system achieves a total energy value of –1.78 eV at a C-V distance of 2.11 Å (see Fig. 3).

On the other hand, during toluene parallel adsorption the molecule, with its aromatic ring plane parallel to the cluster surface, was brought close to the VSbO₄(110) surface along the z axis (see Fig 2, right). Although toluene parallel interaction on O-O sites is physisorptive, on the other sites it is chemisorptive.

During toluene parallel adsorption on Sb-Sb sites, a minimum energy value of DE_total = -1.49 eV is attained at a C-Sb distance of 2.53 Å. Also for that on V-Sb sites, the optimum of DE_total = -2.01 eV is located at the same C cation distance. In addition, toluene interaction on Sb-V sites, with the methyl group placed over a Sb cation and the phenyl centered on a V cation, is the most exothermic. A minimum energy value of DE_total = -3.22 eV is obtained at a C-Sb distance of 2.39 Å (see Fig. 3).

Analysis of Toluene-Surface Bonding

We will now analyze the more stable adsorption sites in the parallel or perpendicular approach.

In Table 1 we can see that during toluene perpendicular adsorption on the V cation the electron transfer (0.181 e^-) is mainly due to the methyl fragment (0.155 e^-). On the other hand, during parallel adsorption on Sb-V sites the transfer from methyl (0.303 e^-) is about 50% of the total. This is accompanied by an important weakening of one of the methyl C-H bonds and the beginning of an H-Sb bonding interaction. Due to methyl interaction on the Sb cation, the electrons are drained to the Sb-coordinated O atoms. However, in methyl and phenyl interactions on V sites these metal cations become strongly oxidized.

As mentioned above, during toluene perpendicular adsorption on the V cation site electronic transfer is mainly due to the methyl fragment. Thus, to study the influence of the aromatic ring we compare this transfer with that of methane, placed at the same position as toluene (C-V distance of 2.11 Å). The changes in the fragment charge of methyl are similar in toluene and methane (0.155 and 0.130, respectively) as are those of the V cation sites (1.768 and 1.795, respectively). Thus, during perpendicular adsorption toluene seems to behave like a methane molecule with one of its H atoms substituted by the phenyl fragment, which improves the electron transfer.

Table 2 shows that, due to toluene interactions, the population overlap between methyl and phenyl fragments is reinforced. Moreover, analysis of the overlap between each of these fragments and the corresponding metal-cation site shows that the strongest of the methyl bonding interactions occurs with a Sb cation (OP(CH₃-Sb) = 0.36). Looking at the phenyl fragment we can see that its interaction on a V atom is almost antibonding. In addition, by analysis of the population overlap between toluene atoms one of the C_methyl-H bonds is found to be very weakened (~ 18%). This is accompanied by the beginning of a H-Sb bonding interaction (OP(H-Sb) = 0.2). Simultaneously, all C-H bonds as well as the C_methyl-C_phenyl bond are reinforced in the aromatic ring. In the oxide, the corresponding Sb-O bonds are debilitated while the V-O bonds are reinforced.

Our calculations suggest that during toluene parallel adsorption on Sb-V sites one of the H atoms of the methyl group can be abstracted with the participation of the Sb cation. The importance of the Sb sites was postulated by Grasselli (1999) during the hydrogen abstraction of propylene on V-Sb oxide surfaces. Moreover, the resulting H₅C₆-CH₂ species desorbs easily due to the nonbonding phenyl-V interactions and then reacts on other surface sites. After the second toluene dehydrogenation, the H₅C₆-CH species interacts on the two fold coordinated O atoms of the oxygen extra plane, forming a C-O bond (OP(C-O) = 0.90). Simultaneously, the HC-C_phenyl bond is reinforced indicating the formation of a possible aldehyde precursor. Finally, benzaldehyde desorption increases the energy of the system to -4.32 eV (Irigoyen et al., 2001).

Analysis of the Electronic Properties of Toluene-VSbO₄ Interactions

The DOS of the VSbO₄ cluster, is similar to that of 3D solids with a rutile structure and exhibits small peaks above the Fermi level (E_F = -10.26 eV) due to the presence of vanadium. On the other hand, the toluene DOS (see Fig. 4a) has several peaks resulting from the interaction of the aromatic ring with the methyl fragment (-27 eV to -24 eV). Next to –20 eV as well as -17.2 eV and -16.3 eV the peaks mainly come from the phenyl group. Next to –15 eV there are also methyl-phenyl interactions. In this last region the main changes may be produced when the adsorbate interacts with the oxide surface. After toluene parallel adsorption the DOS changes significantly in the (–19 eV, -12 eV) region (see Fig. 4b). Peaks at -15.7 eV are due to methyl interactions on the Sb site, while at -13 eV they are due to phenyl interactions on the V cation. Besides, the corresponding V_3dx2-y2 orbitals show depopulation in the –10 eV region. Also, the toluene unoccupied orbitals at -5 eV and -8 eV are broader. This region matches in energy that of Sb (at –7.6 eV), in which the electronic density decreases.

On the other hand it can be seen that, after toluene perpendicular adsorption on the V site, hybridization in the (–16 eV, -13 eV) region is much lower than that in the parallel adsorption (see Fig 4c). These changes correspond to toluene orbitals resulting from the phenyl -methyl interactions (at -13.eV) and the methyl fragment (at -15 eV and -15.74 eV).

CONCLUSIONS

The results indicate that toluene adsorption on VSbO₄(110) occurs preferentially during its parallel interaction on Sb-V sites (methyl fragment on Sb and phenyl on V). Analysis of the orbital interactions reveals that the methyl and phenyl fragments decrease their electronic population as does the V cation. Most of the changes on the Sb cation come from the unoccupied orbitals above the Fermi level. In addition, the overlap population indicates that one of the methyl H atoms interacts with the Sb cation.

ACKNOWLEDGMENTS

Our work was supported by Universidad de Buenos Aires, Departamento de Física-Universidad Nacional del Sur and Fundación Antorchas-ANPCYT (PICT 1203576). N. E. Amadeo and A. Juan are members of CONICET.

Received: March 5, 2002

Accepted: August 22, 2002

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Publication Dates

• Publication in this collection
  19 Mar 2003
• Date of issue
  Mar 2003

History

• Received
  05 Mar 2002
• Accepted
  22 Aug 2002