Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 1(8), 18-23, Nov. (2011) Res.J.Chem.Sci. International Science Congress Association 18 Theoretical Study of the Hydrogenation of Cyclopentene without Catalyst and in the Presence of Molybdenum Disulfide Atohoun Y.G.S., Kuevi U.A., Kpotin G., Kpota A. H. and Mensah J. B. Lab. de Chimie Théorique et de Spectroscopie Moléculaire (LACTHESMO) Université d’Abomey-Calavi, BENINAvailable online at: www.isca.in (Received 16th August 2011, revised 23rd August 2011, accepted 23rd September 2011)Abstract A study Hartree Fock of the hydrogenation of cyclopentene was carried out at the temperature of 298.15 Kelvin, under atmospheric pressure, without catalyst, on the one hand, and in the presence of molybdenum disulfide, catalyst of hydrotreating modeled by a catalytic site of MoS type, on the other hand. The results of calculations showed that, without catalyst, the hydrogenation of the molecule leads to the dislocation of its carbon skeleton by giving propane and acetylene. Inthe presence of molybdenum disulfide used as catalyst, one observes a simple reaction of addition on the cycle. The product of reaction is then cyclopentane. This result highlights the double hydrogenating and selective character of catalyst. Keywords: cyclopentene, cyclopentane, hydrogenation, hydrotreating catalyst, cyclic carbon skeleton, atomic rearrangement, Hartree Fock, LanL2DZ base set, disulphur of molybdenum, propane, acetylene, MoS site. Introduction Cyclopentene (C) is a cyclic olefin, normally being able to undergo a reaction of addition such as hydrogenation. In the presence of a catalyst of hydrotreating having a good hydrogenating activity and under adapted experimental conditions of temperature and pressure, the hydrogenation of such a molecule should occur without rearrangement of its carbon skeleton1, 2. The disturbances related to the secondary reactions possibly being able to intervene during the process of hydrogenation are then avoided. In practice, such a reaction is used as criterion of evaluation of the hydrogenating activity of catalysts of hydrotreating3,4. Moreover, the selectivity of such catalysts in hydrodesulphurization compared to the hydrogenation of olefins must be controlled and if possible improved in order to limit the loss of octane number of gasoline. With an aim of appreciating hydrogenating character of molybdenum disulfide MoS (usual catalyst of hydrotreating), we carried out the modeling of the hydrogenation of cyclopentene, at the temperature of 298.15 Kelvin and under the atmospheric pressure, without catalyst, on the one hand, and in presence of catalyst, on the other hand. Material and Methods Calculation methods and drawings of chemical systems: The calculations were performed using the program Gaussian-03W, by quantum methods HF, in the basis Lanl2DZ. Data obtained are geometry, multiplicity and charge of studied systems 6–9. The drawings of chemical systems studied were produced with ChemDraw Ultra 6.0, Chem3D Ultra 6.0 and GaussView 3.09. The work was done in the “Laboratoire de Chimie Théorique et de Spectroscopie Moléculaire” of “Université d’Abomey-Calavi” in Republic of Benin, on a HP Pentium 4 microcomputer. Reaction Modeling: Hydrogenation is an addition reaction of hydrogen molecule (H) on an unsaturated reactive molecule. The modeling of the hydrogenation of cyclopentene without catalyst was carried out by progressive approach of the two reactive molecules (H and C). The approach consisted in decreasing distance H14 between the atom of cyclopentene and the H14 atom of hydrogen molecule until that the system reaches its lowest energy state, figure-1a. The reaction in the presence of molybdenum sulfide was modeled in a same way, but in two stages: the adsorption of cyclopentene molecule on the catalyst (MoS) modeled by the catalytic site MoS, and simultaneous approach of two hydrogen molecules of the complex formed by cyclopentene and catalyst with the stage of adsorption. During the adsorption stage, the coordinates of reaction is the Mo14distance and during hydrogenation stage, they are H21 and 23 distances, figure-1b. Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X Vol. 1(8), 18-23, Nov. (2011) Res.J.Chem.SciInternational Science Congress Association 19 a: Without catalyst b: In presence of MoS 3 H 3 + Figure - 1 Modeling of hydrogenation of cyclopentene Results and Discussion Reaction without catalyst: One hydrogen molecule was gradually to bring closer to a molecule of cyclopentene. Beginning at the end of reaction, while passing by an intermediate state (IS) 10, the geometrical parameters (interatomic distances and angles), the atomic charges and the energy of the formed chemical system, varied according to the coordinate of reaction H14. The curve of figure-2 represents the variation of the energy of the system according to the coordinate of reaction. It shows that the system is more stable at the end of the reaction than at the beginning. This observation is well in agreement with the computed values of the enthalpy (Hr = -626,463 kJ/mol) and of the free energy (Gr = -600,494 kJ/mol) of the reaction. The negative signs of these energies indicate respectively that the reaction is exothermic and thermodynamically favorable. Figure – 2 Path of the cyclopentene hydrogenation without catalyst Step 1 Step 2 Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X Vol. 1(8), 18-23, Nov. (2011) Res.J.Chem.SciInternational Science Congress Association 20 Table - 1 Interatomic distances and angles calculated during hydrogenation without catalyst Calculated parameters Initial state Intermediate state Final state Distances (Å) 111115141415 1.556 1.519 1.333 1.087 2.204 1.087 2.156 9.737 10.00 0.732 4.964 1.419 1.413 4.859 1.097 2.702 1.076 2.702 1.090 0.820 7.397 1.202 4.094 8.473 1.085 2.174 1.086 1.086 1.086 2.494 Angles (°) 135 104 122.640 112.399 118.955 110.709 179.955 179.971 The computed values of geometrical parameters of the studied chemical system are presented in table-1, with various stages of the process. These results reveal that during the process, the system underwent important rearrangements. Interatomic distances knew significant variations, from the beginning at the end of the process: C passes from 1.556 Å to 7.397 Å, C from 1.333 Å to 4.094 Å, C11 from 1.087 Å to 8.473 Å, Cfrom 1.087 Å to 2.174 Å and H1415 from 0.732 Å to 2.494 Å. All these important increases in interatomic distances indicate the rupture of the corresponding chemical bonds. In particular, the ruptures of C and C bonds indicate that the cyclic molecule of cyclopentene is dislocated to give two aliphatic molecules, figure-3. In addition, new chemical bonds were formed as the variations of the following interatomic distances indicate it: 11 passes from 2.204 Å to 1.085 Å, C from 2.156 Å to 1.086 Å, C15 from 9.737 Å to 1.086 Å and C14 from 10.00 Å to 1.086 Å. The hybridization state of C atom passed from sp to sp. This allows the formation of new chemical bonds between the C atom and the H14 and Hatoms. The C and C atoms passed from the sp2 and sp3 hybridization states to sp respectively. That results in the transformation of simple covalent bond C into a triple bond C, figure-3. According to what precedes, the two reactive molecules were dislocated during the reaction to give two products of reaction: propane and acetylene, figure-3c. The computed values of the geometrical parameters of these products are close to the standard and experimental values published in the literature for these parameters 11 – 12. Reaction in the presence of catalyst: The modeling of the reaction was carried out while approaching simultaneously and uniformly two hydrogen molecules H2122 and H2324 of the complex formed by cyclopentene and the catalytic site MoS with adsorption, figure-4a. a: Complex (C-MoS + ) with adsorption b: Transition state c: Final state Figure – 4 Drawings of the chemical system with various stages of reaction Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X Vol. 1(8), 18-23, Nov. (2011) Res.J.Chem.SciInternational Science Congress Association 21 Beginning at the end of the reaction while passing by the transition state (TS), the geometrical parameters, the atomic charges and the energy of the chemical system, varied according to the coordinate of reaction H23 (or H21). The transition state (TS) of the reaction was determined by QST2 process. It was obtained at distance H23 equal to 2.030 Å and characterized by only one vibration of negative frequency (- 649.834 cm-1) among all its modes of vibration. The curve of figure-5, presents the variations of the energy of the system according to the reaction coordinate H23. In this case the reaction is also exothermic (Hr = -25,803 kJ/mol) and thermodynamically favorable (Gr = -3,109 kJ/mol). Figure-5 Path of the cyclopentene hydrogenation on catalytic site MoS+ The interatomic distances and angles calculated with various stages of the process are given in table-2. These results show that during the process, certain chemical bonds broke; others changed and of new chemical bonds also made themselves, table-2. Indeed, distance C passes from 1.344 Å to 1.555 Å, angles H13 and H12 pass from 124.841° to 106.295° and from 124.661° to 112.388° respectively. Bond C=C is thus transformed into a bond CC and the C and C atoms pass from the hybridization sp to sp. Distances H2122 and H2324 pass from 0.732 Å to 4.181 Å and 3.194 Å respectively, H21 and H23 pass from 10.00 Å to 1.082 Å and 1.078 Å respectively and H2224 pass from 4.986 Å to 0.732 Å. It results from these variations of interatomic distances that the two hydrogen molecules were dislocated and new chemical bonds, like H21, H23 and 2224, were formed. Distance H2224 obtained at the end of the reaction indicates that it was formed a new hydrogen molecule with hydrogen atoms coming from the two initial hydrogen molecules, figure-4c. From all that proceeds, it comes out that the product of the cyclopentene hydrogenation in the presence of molybdenum sulfide (represented by the catalytic site MoS) is cyclopentane. The computed values of the geometrical parameters of the obtained product are close to the values standards and experimental published in the literature on cyclopentane 12. Influence catalyst on the reaction: Without catalyst, the reaction of hydrogenation of cyclopentene gives Propane and Acetylene. During the reaction, the two reactive molecules (C and H) are dislocated with atomic rearrangements. Only one hydrogen molecule reacts with the cyclopentene molecule, scheme-1. Table – 2 Interatomic distances and angles calculated during hydrogenation in presence of MoS catalyst Calculated parameters Initial state Transition state Final state Distances (Å) 2122 2324213 234 22241.383 0.732 0.732 10.00 10.00 5.704 1.536 1.359 0.851 1.131 2.030 0.863 1.555 4.181 3.194 1.082 1.078 0.732 Angles (°) 133 124 120.736 123.705 118.635 114.307 106.295 112.388 Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X Vol. 1(8), 18-23, Nov. (2011) Res.J.Chem.SciInternational Science Congress Association 22 CH CH HCCH Scheme – 1 Scheme of cyclopentene hydrogenation without catalystIn the presence of catalyst, the product of the reaction is cyclopentane. The carbon skeleton of the cyclopentene molecule remains cyclic during the reaction, which implies two hydrogen molecules for one molecule of cyclopentene, scheme 2. There was no rearrangement of the atoms of cyclopentene molecule. The reaction, was only a simple addition of two hydrogen atoms on the cyclopentene molecule, on the level of its unsaturated chemical bond C=C. MoS / 2H + HScheme – 2 Scheme of cyclopentene hydrogenation in presence of MoS catalyst In addition, the system obtained at the end of the reaction in the presence of catalyst, is more stable than that obtained at the conclusion of the reaction without catalyst. It is what energies of the system at the end of the reaction reveal: E (with catalyst) = -294,193 Hartree E (without catalyst) = -195,028 Hartree. Also, the energy barrier observed during the reaction without catalyst (322,852 kJ/mol) is definitely higher than that presented by the reaction in the presence of catalyst (154,146 kJ/mo) (Figures 2 and 5). Thus, among the two studied reactions, the reaction carried out in the presence of catalyst is easiest, which is quite right since, on the one hand, a reaction is all the more fast as its energy of activation is weaker, and on the other hand, the main role of a catalyst is to accelerate or support a chemical reaction 13–14. The reaction in the presence of molybdenum disulfide being produced without rearrangement of the carbon skeleton of the cyclopentene molecule, one can say that the molybdenum disulfide is a catalyst which has a good hydrogenating activity. This conclusion is in agreement with experimental results published in the literature . Conclusion The hydrogenation of cyclopentene, modeled at the temperature of 298.15 Kelvin, under atmospheric pressure, without catalyst, on the one hand, and in the presence of molybdenum disulfide, on the other hand, gives various products of reaction of one case to the other. 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