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A natureza dos carbocátions adsorvidos na superfície da zeólita é discutida neste artigo, destacando estudos experimentais e teóricos. A adsorção de halogenetos de alquila sobre zeólitas trocadas com metais tem sido usada para estudar o equilíbrio entre alcóxidos, que são espécies covalentes, e carbocátions, que têm natureza iônica. Cálculos teóricos indicam que os carbocátions são mínimos (intermediários) na superfície de energia potencial e estabilizados por ligações hidrogênio com os átomos de oxigênio da estrutura zeolítica. Os resultados indicam que zeólitas se comportam como solventes sólidos, estabilizando a formação de espécies iônicas.The nature of carbocations on the zeolite surface is discussed in this account, highlighting experimental and theoretical studies. The adsorption of alkylhalides over metal-exchanged zeolites has been used to study the equilibrium between covalent alkoxides and ionic carbocations. Theoretical calculations indicated that the carbocations are minima (intermediates) on the potential energy surface and stabilized by hydrogen bonds with the framework oxygen atoms. The results indicate that zeolites behave like solid solvents, stabilizing the formation of ionic species.

Section: Alkylhalides As Source Of Carbocations On Zeolitesmentioning

confidence: 99%

Carbocations on zeolites: quo vadis?

Mota

Rosenbach

2011

“…It is not completely clear however, that ionic reactions on zeolites take place directly through the involvement of alkoxides, or there occurs a rapid equilibrium between alkoxides and free carbocations. We were able to show 13 that they can catalyze Friedel-Crafts reactions and we wish to present now, preliminary results indicating that this approach can be used in isobutane/2-butene alkylation, with a superior performance than a protonic catalyst.…”

mentioning

confidence: 96%

Isobutane/2-butene alkylation with zeolite Y without Br<FONT FACE=Symbol>Æ</FONT>nsted acidity

Rosenbach¹,

Mota²

2005

Foi estudada a alquilação de isobutano com 2-buteno catalisada por zeólitas Y trocadas com cátions metálicos, usando um cloreto de alquila com iniciador de carbocátions. Os resultados indicaram que, comparavelmente a uma zeólita protônica, a desativação foi significativamente mais lenta e a produção de tri-metilpentanos foi maior nas zeólitas trocadas com metais. Os cátions agem como sítios ácidos de Lewis, interagindo com o cloreto e iniciando as reações de carbocátions. Os resultados suportam a hipótese de que os sítios protônicos favorecem a adsorção das olefinas, aumentando a velocidade de oligomerização.Alkylation of isobutane with 2-butene was performed on metal-exchanged Y zeolites, using an alkylchloride as carbocation initiator. The results indicated that, compared with a protonic zeolite, the deactivation was significantly slower and the production of trimethylpentanes was higher on the metal-exchanged zeolites. The metal cations act as Lewis acids, interacting with the chloride initiating the carbocationic reactions. The results also support the view that protonic sites on the catalyst favor adsorption of the olefin, thus increasing oligomerization rate. Keywords: alkylation, zeolite, Br∅nsted acidity, gasoline IntroductionThe alkylation of isobutane with 2-butene is a reaction normally catalyzed by Br∅nsted acids. The primary function of the acid system is to protonate the hydrocarbons to generate small concentrations of carbocations. Nevertheless, as the reaction begins it becomes autocatalytic 1-3 due to a hydride transfer step as shown in Scheme 1. Alkylation has been used since the 1930's to produce high octane gasoline. 4,5 However, due to environmental concerns, it will grow in importance in oil refining in the next years. The alkylate, predominantly with eight carbon atoms, is a valuable blending for the adjustment of the gasoline composition to attain the environmental requirements.3 Alkylation is normally catalyzed by H 2 SO 4 or HF. However, these acid systems present severe environmental problems, especially HF, whose use in new alkylation plants is forbidden in the USA. 6 Today, these acids account for about 30% of the refining catalyst market, but this share tends to increase, as alkylation grows in importance. 3The search for a solid catalyst to replace HF and H 2 SO 4 in alkylation has been a long-standing challenge for chemists.6-8 Many materials are active for alkylation, but they show rapid deactivation, due to oligomerization, 9 impairing the development of a commercial solid catalyst for this process. Some studies tend to overcome this problem, by working on process engineering solutions. 10According to some authors, the rapid deactivation of solid acid catalysts is due to a preferred adsorption of the olefins on the surface. 3,11 This favors successive alkylation reactions, forming the oligomers, which do not desorb from the catalyst surface.Zeolites are among the best solid catalysts tested in alkylation. One of the main features of the zeolites is their ability to catalyze hyd...

“…Iron-exchanged zeolites have been reported 12,l3 to be good catalysts for benzylation of aromatic hydrocarbons. We have also shown 14 that FeY is an active catalyst for tert-butylation of ethylbenzene with tert-butylchloride, yielding a selectivity of 98% to monoalkylation with 96% conversion (scheme 1).…”

Section: Introductionmentioning

confidence: 79%

Mechanistic aspects of Friedel-Crafts alkylation over FeY zeolite

Bidart¹,

Borges²,

Chagas³

et al. 2006

Self Cite

Foram estudados os efeitos da razão molar aromático/cloreto de alquila, adição de base, solvente e agente alquilante, na alquilação do etilbenzeno catalisada pela zeólita FeY. Ficou caracterizada a alquilação ocorrendo no interior dos poros da zeólita, uma vez que ela é pouco afetada pela razão molar dos reagentes. A adição de bases e solventes oxigenados, capazes de complexar com o sítio ativo, inibe a reação. Foi proposto um mecanismo envolvendo transferência de um elétron entre as espécies de ferro ativas e o cloreto de alquila, para explicar os resultados.The effect of aromatic/alkylchloride molar ratio, base addition, solvent and alkylating agent on the ethylbenzene alkylation over FeY zeolite was investigated. It was shown that alkylation occurs inside the pores and it is little affected by the reactants molar ratio. On the other hand, addition of bases or oxygenated compounds, capable of complexing with the active site, blocks the reaction. A mechanism involving single electron transfer between the active iron species and the alkylchloride was propose to account for the results.