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Theoretical evaluation of the deactivation mechanism of substituted dibenzothiophenes

 

Evaluación teórica del mecanismo de desactivación de dibenzotiofenos sustituidos

 

G. Ramírez-Galicia*, M. Poisot and H. Martínez-Pacheco

 

Universidad del Papalopan, División de Estudios de Posgrado. Circuito Central 200, Parque Industrial, Tuxtepec, Oaxaca, 68301. * Corresponding author. E-mail: gramirez@unpa.edu.mx; memorgal@gmail.com

 

Received 12 of June 2012
Accepted 31 of October 2012

 

Abstract

The petrol and diesel combustion in motors produces sulfur and nitrogen oxides, they are the precursors of acid rain. Unfortunately, the Mexican fuel shows high-sulfur content; this concentration should be reduced to 50 ppmw or lower in order to achieve the international regulations. The oxidative desulfurization (OD) is an alternative to obtain it. In this work, we propose a theoretical reaction mechanism for the OD; this pathway is carried out through a combination of the molibdate anion, the hydrogen peroxide and several dibenzothiophenes by Density Functional Theory (DFT) using the B3LYP functional and the DGDZVP double zeta basis. The results show that the energy of the determinant step of the reaction is lower than the experimental amount obtained wishout catalyst. Finally, the environment provided by water molecules is important for decreasing the reaction mechanism energy.

Keywords: oxidative desulfurization, DFT, B3LYP, reaction mechanism, dibenzothiophene.

 

Resumen

La combustión de gasolina y diesel en automotores produce, entre otros, óxidos de azufre y nitrógeno, los cuales forman parte de los precursores de la lluvia ácida. Desafortunadamente, los combustibles mexicanos presentan un alto contenido de azufre, el cual debe de ser reducido a menos de 50 ppm para cumplir con las normas internacionales. Una forma de llegar a esta concentración es por medio de la desulfuración oxidativa (DO). Este trabajo presenta una propuesta teórica del mecanismo de reacción para la DO utilizando como catalizador una combinación de molibdato y peróxido de hidrógeno y diferentes derivados de dibenzotiofeno, utilizando la Teoría de los Funcionales de la Densidad (TFD) con el funcional B3LYP y la base doble zeta DGDZVP. La etapa determinante de lo reacción presenta meyor energía que el valor experimental sin el uso de catalizador. Finalmente, la presencia de moléculas de agua es importante para disminuir la energía del mecanismo de reacción.

Palabras clave: desulfuración oxidativa, TDF, B3LYP, mecanismo de reacción, dsbenzotiofeno.

 

1 Introduction

Recently, the fuel production free of contaminants is a very important topic. For example, diesel is a complex mixture of lineal, branched and cyclic alkanes and several organosulfur compounds are obtained from the oil separation; after combustion, sulfur oxides are emitted to the environment, poisoning also the catalytic converter in the automobile (Zhu et al., 2008). In consequence, several countries have introduced sever regulations about the sulfur content limits in fuels (Campos-Martinez et al., 2004). However, to reach these concentrations it is necessary to improve several processes. Traditionally, the hydrodesulphurization process allows to remove thiols, sulfurs and disulfur compounds from the fuel mixture, however, the organosulfur compounds with sterical hindrance like the 4,6-dimethyldibenzothiophene (4,6DMDBT) requires hard conditions in order to be removed (Babich and Moulija, 2003), including high hydrogen concentrations. On the other hand, those compounds shorten the catalyst half life.

An alternative to obtain low sulfur concentrations in diesel is the oxidative desulfurization (OD), where the organosulfur compounds are oxidized to sulfone, then they are removed by extraction, adsorption, distillation or decomposition (Huela et al., 2001; Anisimov et al., 2003; Palomeque et al., 2002; Yazu et al., 2001; Djangkung et al., 2003; Shiraishi et al., 2003). This process is carried out in liquid phase under mild conditions.

Several oxidant agents were used in the OD process (Campos-Martin et al., 2004); however, the hydrogen peroxide (H₂O₂) is the most used due to its low cost, no-contaminant characteristics and oxidant properties. Unfortunately, without a catalyst, the oxidation velocity is very slow, but several catalysts have proven to increase its reaction velocity (Shiraishi et al., 2003; Ramirez-Verduzco et al., 2004). Other catalysts have been tested as Cu-supported in TiO₂ where the activity of this catalyst was attributed to its capacity to decompose the oxidant and the interaction Cu-support (Cedeño-Caero et al., 2005); Ag-supported in TiO₂ and Au-supported in TiO₂, for silver catalyst, the particles crystallized better and this catalyst is more active when the temperature of thermal treatment is increased whereas the gold catalyst improves the sulfone production at low temperature (Zanella et al., 2007); in the catalyst V₂O₅-supported in Al₂O₃ the temperature is a very important factor in the thermal decomposition of H₂O₂ (Navarro-Amador et al., 2006; Gómez-Bernal and Cedeño-Caero, 2006; Becerra-Hernandez et al., 2006). On the other hand, polyoxymetalic catalyst as tungstenate (Torres-Garcia et al., 2011) and molybdate (Garcia-Gutierrez et al., 2006, 2008) have shown excellent results in the oxidation of organosulfur compounds.

Catalysts of tungsten (Torres-García et al., 2011; Rodriguez-Gattorno et al., 2009; and Galano et al., 2008) and molybdenum (Vergara-Mendez et al., 2011) mixed with H₂O₂ have been studied by computational techniques. Torres-García etal. (2011) concluded that the reaction mechanism with tungstenate supported in zirconia is carried out by the addition reaction followed by the elimination reaction, where the last one is more energetic than the first one; whereas Vergara-Mendez et al. (2011) concluded that the water molecules take part in the complex formation of the catalyst and also play an important role in the energy minimization. On the other hand, the reaction mechanism in heterogeneous phase was studied by Density Functional Theory (DFT), for the following examples: the propene oxidation to acroline (Pudar et al., 2007) and the aminoxidation of propene (Jang et al., 2002).

This work proposes a theoretical reaction mechanism for the oxidation of the 4,6-dimethyldibenzothiophene compound present in the diesel fuel using a peroxomolybdate catalyst presented by Vergara-Mendez et al. (2011). In this mechanism, the monomer of molybdate was used as a catalyst model because this specie was found in alkaline aqueous solution as Srinivasan (2004) informed.

 

2 Methodology

All calculations were carried out by Gaussian 03W package (Frish et al., 2004). Geometry optimization and vibrational mode calculations of all molecules were performed by B3LYP functional (Becke, 1988, Lee et al., 1998) and DGDZVP double zeta base set.

The vibrational modes were calculated to confirm the behavior of stationary points (all positive frequencies for minimums of energy and only one imaginary frequency for transition state with internal coordinate in direction of the reaction trajectory). The vibrational frequencies were scaled by 0.9614, the standard factor proposed by Scott and Radom (1996) to consider the zero-point energy in B3LYP functional.

The solvent effect was simulated as explicit manner placing two water molecules surrounding the catalyst as Vergara-Mendez et al. (2011) proposed.

Additionally, the NBO analysis was carried out as Carpenter and Weinhold (1988), Foster and Weinhold (1980), Reed and Weinhold (1983), Reed et al. (1985) and Reed et al. (1988) recommend it for determining which hybrid orbitals and their energies take part in the electronic interactions of the oxidation reaction path. The NBO analysis represents correctly the bonding orbitals and their occupation, the core orbitals, the lone pair, the antibonding orbitals and the Rygberg vacancies.

 

3 Results and discussion

3.1 Oxidation mechanism

Torres-García et al. (2011) proposed that the sulfur oxidation into the DBT and its derivatives is reached in two steps: an addition followed by an elimination. Considering that the peroxotungstenate was used as a catalyst, the determinant step is the catalyst elimination. Whereas, Vergara-Mendez et al. (2011) studied the oxomolybdate catalyst formation, suggesting that the oxomoybdate intermediate could play a determinant role in the oxidation of the sulfur atom in aqueous solution considering water molecules in the environment.

Figure 1 shows the energetic profile of the oxidation mechanism for 4,6DMDBT and the oxomolybdate catalyst. This reaction mechanism shows three steps: the first one is the peroxomolybdate formation, whereas the second and third ones are the consecutive sulfur oxidation of 4,6DMDBT.

In the first step to form the oxomolybdate specie (Table 1 and Fig. 2) the transition state (TS01) is observed at 4.07 kcal/mol over the reagents, this energy is lower than the energy reported by Vergara-Mendez et al. (2011); for the oxomolybdate and dioxomolydate catalyst formation, that energy was 29.01 kcal/mol without explicit water molecules and around 22.00 kcal/mol with two water molecules. These data drive us to conclude that the peroxomolybdate specie is formed in situ, and this one is the active specie in the oxidation of the sulfur atom.

The sulfur oxidation is carried out in two consecutive steps, each one for an oxygen addition from the catalyst to the organosulfur compound (Fig. 3). The transition states (TS02 and TS03) show energies of 27.34 and 23.83 kcal/mol respectively, i.e.; the second oxidation is easier than the first one. These energy amounts are comparable with the elimination energy calculated by Torres-Garcia et al. (2011), they calculated the transition state using ZrO₂ as support and the transition energies obtained are 23.64 and 13.37 kcal/mol for the first and second elimination reaction respectively. Clearly, it is seen that the first elimination is again more complicated than the second one.

Geometrically, the interaction between the catalyst and the 4,6DMDBT in both oxidations is completely different. Table 2 shows some notable geometric data as distances, bond angles and dihedral angles of this interaction. Particularly, the direction of the peroxo group and the metallic center of the catalyst regarding to the rings forming the 4,6DMDBT; for the first oxidation, this direction is close to be parallel, whereas for the second oxidation, the direction is near to be perpendicular. This change is related to the sterical interaction observed in the sulfoxide group formation.

In the transition state, the distance between the sulfur and molybdenum atoms in the first oxidation is longer than in the second oxidation, allowing the oxygen addition (O_per1) from the peroxo group. On the other hand, the catalyst elimination occurs nearby the 4,6DMDBT rings plane. The first oxidation represents the elimination of the catalyst at the angle of 159.1°, whereas for the second oxidation this angle is 136.8°. Pettersson et al. (2003) proposed that the formation of peroxo- and diperoxo moieties in hepta and octamolybdate compounds can be observed on the terminal molybdenum atoms. Our results show that the monomer molybdate catalyst containing the peroxide groups reflects a positive effect on the success of the oxidation reaction.

García-Gutiérrez et al. (2008) suggested that the oxidation mechanism is driven by the nucleophilic attack of the sulfur atom of 4,6DMDBT to the oxygen of the peroxo group of the catalyst; such attack is activated by the high coordination number and the high oxidation state of the molybdenum atom. The molybdenum atom charge changed from 1.62 to 1.15 charge units when the catalyst acts as oxo form (reagent) or as peroxo form (Intermediate 1, I01). It is important to note that the oxidation state and electrical charge of the molybdenum atom are practically constant along the reaction trajectory and that the oxygen of oxo and peroxo groups shows similar behavior (Table 3).

The electrical charge of the peroxo group oxygens (around of -0.50 charge units) is smaller than that observed in the water oxygen (around -0.90 charge units). These results confirm the electrophilic characteristic of the peroxo group oxygens. On the other hand, the positive charge of the sulfur atom was increasing according to its oxidation state going from 0.41 to 0.78-0.88 units up to 1.13. These results indicate that the sulfur atom is not the nucleophilic center, and the reaction cannot be reached.

The NBO analysis was performed on the intermediates (I01 and I03) and on the transition states (TS02 and TS03). This analysis shows that the lone pairs of the sulfur atoms (LP1S or LP2S) are the nucleophilic sites, whereas the sigma bond and sigma antibond orbitals of the peroxo oxygens in the catalyst are the electrophylic sites.

Figure 4 shows the energy localization of the main orbital that interacts in the first oxidation, the lone pairs of the sulfur atom represented at -0.44 and -0.05 hartrees; it is clear that there are different interactions and energies between the LP1S and LP2S. The LP2S is the first nucleophilic center, whereas the sigma antibond orbital of the peroxo group (σ_Oper1-Oper2) is localized at 0.32 hartrees.

The main interaction for the initial oxidation is between PL2S and the σ_Oper1-Oper2. This interaction is confirmed in the transition state TS02 because these orbitals are transformed to σs_-Oper2, σs_-Oper2 and LP_Oper1. Clearly, it is seen that the peroxo group in the catalyst is not completely broken in the TS02; its distance is 1.9 Å (Table 2). On the other hand, the geometry of this transition state could be named an anti-Hammond belated transition state as Thorton (1967) proposed.

Table 4 shows the electronic occupations of the orbitals directly involved in the oxidation reaction. In the TS02, σs_-Oper2occupation is 1.8606 electrons, occupation minor than a single bond, whereas the σs_-Oper2 occupation is 0.4556 electrons, these results permit to establish the electronic transference to be carried out in this transition state. On the other hand, the peroxo bond broken originates a new lone pair on the oxygen 1 with an electronic occupation of 1.3617 electrons. In TS02, the global electronic charge transference is around 0.4 electrons from the catalyst to the 4,6DMDBT, this result is observed in the charge changes from I01 to TS02.

 

Figure 5 shows the energy localization of the main orbital that interacts in the second oxidation reaction; in this case, there is one LP1S from the sulfoxide molecule of 4,6DMDBT to be considered. This orbital is localized at -0.35 hartrees and it behaves as a nucleophilic center, on the other hand, the σ_Oper1-Oper2 is again the electrophilic center. The interaction between these orbitals in the transition state decreased the energy gap by 0.4813 hartrees. It is important to note that the symmetry of the hybrid orbitals did not change from I03 to TS03, this transition state is classified as an early Hammond transition state as Hammond (1955) proposed.

 

In this case, the LP1S transfers electronic density to the σ*_Oper1-Oper2 in the TS03 and the orbital occupation is 1.6241 and 0.3878 respectively. The global charge changes from the sulfoxide to the catalyst molecules respectively, i.e., the sulfoxide group is a positive molecule and the catalyst is a negative molecule (Table 4).

 

Conclusions

The catalyst transformation from oxo form to peroxo form was calculated by B3LYP functional theory; and the DGVZVP double zeta basis requiring low transition energy was only 4 kcal/mol. This result suggests that the peroxomolybdate molecule is the active specie in the oxidation reaction, and it is expected to appear in situ.

The oxidation reaction is carried out for two consecutive additions of oxygens from the catalyst to the sulfur atom of 4,6DMDBT; the first one is the determinant step of reaction, because this transition energy is higher than the transition energy of the second oxidation. Geometrically, the reaction mechanism of the first oxidation is carried out following a parallel direction between the catalyst and the organosulfur compound, whereas in the second oxidation, the direction is perpendicular.

Finally, the geometry of the transition states was classified as belated and early, respectively, for the two oxidation reactions on the sulfur atom. The lone pairs of the sulfur atom in the 4,6DMDBT are the nucleophillic centers, whereas the σ*_Oper1-Oper2orbital of the catalyst is the electrophilic center in these reactions.

 

 

Acknowledgement

GRG acknowledges to Programa de Mejoramiento del Profesorado (PROMEP) for the economical support to the project UNPA-PTC-048, likewise to Mr. A. A. García-Gomez for technical support in the finished of this project. The English was kindly reviewer by Miss Dessiree Argott.

 

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