A theoretical study of the hydride transfer between 1-Adamantyl cation and isopentane

Abstract

Foi realizado um estudo teórico da transferência de hidreto entre o cátion 1-adamantila e isopentano, usando a teoria do funcional da densidade em nível PBE1PBE/6-31G (d,p). Os resultados indicaram que há formação de um íon carbônio como intermediário, que é mais estável que os complexos de van der Waals entre os íons carbênios e os alcanos. Os fatores entrópicos são bastante importantes e sobrepujam o termo entálpico à temperatura ambiente.A density functional teory study of the hydride transfer between 1-adamantyl cation and isopentane has been performed at the PBE1PBE/6-31G(d,p) theoretical level. The results indicate that a carbonium ion is involved as intermediate and is more stable than the van der Waals complexes between the carbenium ions and the alkanes. Entropic factors are important and overcome the enthalpic term at room temperature. Keywords: hydride transfer, carbocations, adamantane, isopentane IntroductionIntermolecular or bimolecular hydride transfer (HT) is a crucial step in acid-catalyzed hydrocarbon transformations. This reaction is involved in the propagation of the catalytic cycle of cracking 1,2 and alkylation 3 of hydrocarbons, as well as in the formation of aromatic hydrocarbons 4 in gasoline and coke, 5,6 which are carbonaceous materials deposit over the catalyst surface that may lead to loss of activity. Conceptually, HT involves the bimolecular transfer of a hydride from an alkane to a carbenium ion (Scheme 1). R⊕ + R'-H → R-H + R'⊕ Scheme 1. Schematic representation of hydride transfer reactions.In the gas phase, HT is much slower than proton transfer 7 and shows a negative temperature dependence. [8][9][10] The reaction is believed to occur via formation of a "tight" complex, 11,12 whose structure may resemble a carbonium ion, 13 formed by the interaction of the empty p-orbital of a carbenium ion with the C-H bond of the alkane. The reaction is used 14,15 to assess the carbenium ion stability in the gas phase.In condensed phase, especially on zeolites and other solid acid catalysts, there are few studies regarding hydride transfer. 16,17 Product distribution obtained from bimolecular alkane reactions is potentially interesting for the evaluation of the HT capability of solid acids. However, the validity of the results depends on the system chosen as model reaction. Indeed, variation of the hydride acceptor seems to affect the rates of hydride transfer to a considerably greater extent, than an equal change of the thermodynamic driving force caused by variation of the hydride donor. 18 In addition, some systems involve simultaneous reactions competing with HT, and may significantly affect the interpretation of the results.The 1-adamantyl cation is a stable species 19 and can be synthesized from the 1-adamantanol and fluorosulfonic acid-antimony pentafluoride. 20 On the zeolite surface, this species can be also generated from its halide. 21 In addition de Oliveira et al. 2181 Vol. 21, No. 12, 2010 to the stabilization by sC-C bond hyperconjugation, the 1-adamantyl cation has a cage ...

Carbocation Stability in H-ZSM5 at High Temperature

“…The four-carbon species were also examined, including the butyl (Figure f), sec -butyl (Figure g), and tert -butyl (Figure h) species. Unlike previous studies, − neither embedding nor pw-PBC could find a minimum for the primary carbocation. The butyl compound has been shown to form cyclic structures, , but these species were not found in the constrained zeolite channel used in this study.…”

“…However, four carbon species carbocations have not been experimentally observed as persistent intermediates in zeolites. − ,− It is unclear if this is due to the absence of these species in the reaction pathways or to short lifetimes in the zeolite. Some theoretical studies have proposed that these short-chain (4 carbons or fewer) carbocations exist as reaction intermediates in the recent literature. − Alternatively, other species, alkoxides, have been observed as persistent intermediates in zeolites for short chain species, i.e., fewer than five carbon atoms. − ,− ,− The mechanisms that form alkoxides would follow a stepwise pathway, as illustrated in Scheme c. In this mechanism, shown for isopropanol, dehydration occurs while forming a carbon–oxygen bond to the zeolite framework, , in this case an ethyl group bound to the oxygen site in the framework.…”

“…Previous first-principle calculations have shown that alkoxides are more stable than carbocations, with the exception of the tert -butyl cation both using 0 K conditions and using entropic corrections for 298 K. − ,− ,− These studies have employed models consisting of small clusters treated with density functional theory (DFT), embedding methods using DFT with semiempirical methods or molecular mechanics, ,,,,− periodic DFT, ,, and the hybrid MP2:DFT method at 298 K. ,, With the exception of the lattermost, these model–method combinations exhibit significant approximations for carbocations relative to alkoxides. Specifically, cluster models do not capture the steric and long-range electrostatic effects present in zeolites.…”

Adamantane-Based Cation and [MFn]− Anion Synergistically Enhanced Catalytic Performance of Sulfuric Acid for Isobutane Alkylation

“…Therefore, series of cation, anion, or neutral surfactants, such as sulfolane quaternary ammonium salt, n -dodecylamine, and aminobenzoic acid, have been developed to accelerate the alkylation reaction kinetics and improve the alkylate quality by improving the interfacial properties. In addition, the intermolecular hydride-transfer reaction is rate-determining for isobutane alkylation catalyzed by Brønsted acid, which refers to the H – transferred from isobutane to C 8 + to form trimethylpentane (Scheme ) . , Some researchers have improved reaction performances by increasing the rate of the hydride-transfer step using hydride donors such as adamantane, pinene, and their derivatives with at least one unoccupied bridgehead carbon. , Moreover, it is well accepted that the acidity and composition of the catalyst affect the protonation of the olefin and the rate of hydride transfer. − The easy reduction of sulfuric acid’s acidity, which is due to poisoning by the side products and dilution by inner impurities entrained from the feeds, causes the short lifetime of sulfuric acid and a great deal of spent acid during reaction process. Therefore, Huang et al introduced the ionic liquid [Bmim]­[SbF 6 ] as a buffer agent to inhibit the acidity reduction and tune the microstructure and composition of the catalyst system, which increased the lifetime of sulfuric acid used alone by a factor of at least 2.…”

“…Because of their unique properties, ionic liquids have an enormous range of applications in many fields such as catalysis, separations, materials science, and electrochemistry. − Their outstanding advantages lie in ion designability, diversity, and cooperativity. Ionic liquids offer the opportunity to design cocatalysts from single functions to multiple and cooperative functionalities for isobutane alkylation.…”