Carboranes: the strongest Brønsted acids in alcohol dehydration

Abstract

Alcohol dehydration mechanisms identified through the slopes of activation energies vs. carbenium ion stability of alcohols.

Genesis of Strong Brønsted Acid Sites in WZr-KIT-6 Catalysts and Enhancement of Ethanol Dehydration Activity

“…The kinetic C 4 H 8 isomer selectivities differed significantly from the predicted equilibrium distribution (Table ). The significant formation of cis -2-C 4 H 8 suggests that n -butanol dehydration occurs possibly via an E 1 mechanism, where relatively strong Brønsted sites are required to form carbocations, rather than via the E 2 mechanism, which is subject to significant steric hindrance. , The presence of Brønsted acid sites in the W x Zr y KIT-6 catalysts renders the E 2 mechanism less likely.…”

“…The significant formation of cis-2-C 4 H 8 suggests that n-butanol dehydration occurs possibly via an E 1 mechanism, where relatively strong Brønsted sites are required to form carbocations, rather than via the E 2 mechanism, which is subject to significant steric hindrance. 34,35 The presence of Brønsted acid sites in the W x Zr y KIT-6 catalysts renders the E 2 mechanism less likely. Temporal ethanol conversion profiles of WZr-KIT-6 materials with various W and Zr loadings showed a deactivating trend between 4 and 18 h on stream (Figure 6).…”

Mechanism of Isobutylene Polymerization: Quantum Chemical Insight into AlCl3/H2O-Catalyzed Reactions

“…We note that the bridging of a proton between two chlorine atoms in 1 is very similar to the proton coordination observed in halogenated carborane superacids, which have been shown to be extremely strong Brønsted acids. , The structural similarity between 1 and the known carborane catalysts is indirect support for the plausibility of the existence of our calculated structure. Thus, we found that the combination of two Lewis acids with water can form a Brønsted superacid capable of protonating IB in nonpolar environments.…”

Icosahedral Carborane Superacids and their Conjugate Bases Comprising H, F, Cl, and CN Substituents: A Theoretical Investigation of Monomeric and Dimeric Cages

“…The properties of various carborane acids (including the H(CHB 11 Cl 11 ) acid considered the strongest isolable acid both in gas phase [21] and solution [46] ) are the subject of ongoing theoretical and experimental studies, [21,[44][45][46][47][48][49][50] however, it seems likely that many of the systems whose structure involves the icosahedral carborane core are yet to be discovered. Recent experimental and theoretical investigations concerning functionalized carborane acids include, among others, the electronic structure studies of variously substituted carborane acids and their conjugate bases, [51,52] the discussion of stabilities of tetravalent oxygen and sulfur centers (e. g., in various dications such as H(CH 3 ) 3 O 2 + ) in complexes with H(CHB 11 F 11 ), [53] demonstration of the catalytic activity of various carborane acids (fluorinated, chlorinated, and brominated) in alcohol dehydration reactions, [54] and the experimental demonstration that even weak bases such as CO 2 or N 2 might be protonated by certain carborane acids (under the condition of acid monomerization). [55] The main goal of our studies was to investigate the highsymmetry quasi-icosahedral carborane superacids utilizing electronegative F, Cl, and CN substituents and predict their acid strength as well as the electronic stabilities (manifested by the values of vertical excess electron detachment energies) of their conjugate bases (i. e., the anions resulting from deprotonation thereof).…”