IMPRIMIR VOLTAR
A. Ciências Exatas e da Terra - 4. Química - 1. Físico-Química

A THEORETICAL STUDY OF CARBOCATIONICS SPECIES ON Y ZEOLITE

Francisléia Vieira Vidal 1
Nilton Rosenbach Jr. 2
Claudio J. A. Mota 2
(1. Grupo de Pesquisa em Física/UFJF; 2. Laboratório de Reatividade de Hidrocarbonetos e Catálise Orgânica/UFRJ)
INTRODUÇÃO:
Zeolites are the main catalyst of the petrochemical industries. The importance of this aluminosilicates is due their capacity to promote many reactions. According to initial suggestions, carbocations should be key intermediates in these reactions. However, these species are seldom observed on the zeolite surface as persistent intermediates at time scale of the spectroscopy techniques available. In these experimental investigations, ones observe only covalent species, named alkoxides. Therefore, many studies suggest that the alkoxides should be the key intermediates, whereas the carbocations should be only transition states in the reactions on zeolite surface. However, recent studies show halogen derivates can give carbocationics reactions on Y zeolite surface. On the other hand, the bicyclobutonium/cyclopropylcarbinyl system plays a key role in solvolysis of cyclopropylcarbinyl and cyclobutyl derivates. In this case, there is equilibrium between the two cabocations (bicyclobutonium and cyclopropylcarbinyl) in solution. On zeolite surface, there are not studies of these species. In order investigate in more detail the mechanism of halogen derivates reactions on zeolite surface; we carried out quantum mechanical calculations to determine the possible intermediates involved in reaction of cyclopropylcarbinyl and cyclobutyl derivates on Y zeolite surface. In this sense, the ONIOM approach can be of great utility because it allows us to study complex and large molecular system.
METODOLOGIA:

In this study, we have used a molecular system with 161 atoms (AlSi45O69H46), which represents a supercavity of the Y zeolite. This zeolite type is the most common in the catalysts process of the petrochemical industries. The crystalline structure of the Y zeolite is formed by association of the aluminum and silicon tetrahedrons, linked by oxygen atoms. The free valences of aluminum and silicon atoms were saturated with hydrogen. The calculations were performed using the ONIOM method available in GAUSSIAN 98 package. In this approach, the molecular system was divided into layers. The atoms of the active site of the Y zeolite (high layer) were treated by the B3LYP functional with 6-31G(p, d) orbital basis set, whereas the other atoms (low layer) were treated by the semiempirical MNDO method. Among the possible intermediates (minima on the potential energy surface), we have calculated only that of primary interest: carbocations (bicyclobutonium and cyclopropylcarbinyl) and alkoxides (cyclobutyl, cyclopropylcarbinyl and homoallyl). Geometries of all species investigated were fully optimized, and they were characterized as intermediates on the potential energy surface by the absence of imaginary frequencies, after vibrational analysis of the optimized geometries. Zero-point energies (ZPE) and thermal correction were calculated utilizing the frequencies calculated at the same level. Relative energies were computed and refer to enthalpy differences at 298K.

RESULTADOS:
The calculations showed the bicyclobutonium carbocation as a minimum in the potential energy surface. The cyclopropylcarbinyl carbocation is higher energy by 1.96 kcal.mol-1. This order of stability is observed in solution and gas phase as showed in previous calculations. The three alkoxides are the most stable than all carbocations. The relative energies for all species investigated show the following order of stability: homoallyl (0.00 kcal.mol-1) > cyclobutyl (4.49 kcal.mol-1) > cyclopropylcarbinyl (4.72 kcal.mol-1) > bicyclobutonium (36.27 kcal.mol-1) > metylcyclopropyl (38.23 kcal.mol-1). Analysis of the geometries shows that all carbocations are located on the active site, near the aluminum atom. This preferential localization is responsible by the neutralization of the negative charge results from of the tetracoodination of the aluminum atom in the crystalline structure of the Y zeolite. The alkoxides are linked to oxygen atom near the aluminum atom, forming a covalent carbon-oxygen bond. For this reason, the alkoxide are most stable than all cabocations. The analysis of the carbon-oxygen bond distance shows the following order: cyclobutyl (1.48Å) = cyclopropylcarbinyl (1.48Å) > homoallyl (1.47Å). This order reflects the steric repulsion between the hydrocarbon and the zeolite structure.  The increase of carbon-oxygen distance results in a minor stability of alkoxide. In fact, the relative energy of the alkoxides show that homoallyl is the most stable of the alkoxides.
CONCLUSÕES:

The theoretical results show by the first time a carbocation as stable specie on Y zeolite surface (a minimum on the potential energy surface). Based on our calculations, the reactions of the hydrocarbons and their derivates on zeolite surface involve carbocationic species. In these reactions, the carbocations will be intermediates and not transition state as suggest some previous mechanistic proposals. Like in the solvolysis of cyclopropylcarbinyl and cyclobutyl derivates, on zeolite surface there are also equilibrium between the two carbocationics species: bicyclobutonium and cyclopropylcarbinyl. Our calculations also showed alkoxides as the most stable species on zolite surface.  This result is according with previous experimental investigations. In summary, based on insights gained via quantum mechanical calculations for the reactions of cyclopropylcarbinyl and cyclobutyl derivates on Y zeolite surface, we suggest equilibrium between alkoxides and carbocations, since both species are intermediates or minimum in the potential energy surface.

Instituição de fomento: CAPES
 
Palavras-chave: Zeolite; Carbocation; ONIOM.
Anais da 58ª Reunião Anual da SBPC - Florianópolis, SC - Julho/2006