Kinetics and Mechanism of Cyclohexanol Dehydration in High-Temperature Water

Abstract

We examined cyclohexanol dehydration in pure water at temperatures of 250, 275, 300, 350, and 380 °C with water densities ranging from 0.08 to 0.81 g/cm 3 . Under these conditions, cyclohexanol dehydrates readily in the absence of added catalysts to form cyclohexene as the major product. The most abundant minor products are 1-and 3-methyl cyclopentenes. The reaction rate and product distribution at 380 °C show a remarkable sensitivity to the water density. At low densities, the reaction is slow, and cyclohexene is the only product. At high densities, the reaction is nearly complete, and methyl cyclopentenes appear along with cyclohexene. The experimental results implied a reaction mechanism that comprises two pathways: (1) reversible cyclohexanol dehydration to form cyclohexene through an E2 mechanism, and (2) subsequent cyclohexene protonation to form the cyclohexyl cation, which rapidly rearranges to form methyl cyclopentyl cations, which then lose a proton to form methyl cyclopentenes. A kinetics model based on the proposed mechanism was able to predict the striking effect of the water density on the product yields at 380 °C and, thereby, to demonstrate that the proposed mechanism captures the trends in the experimental data. An analysis of mechanistic issues regarding cyclohexanol dehydration in high-temperature water (HTW) revealed three roles for water. Water participates in elementary reaction steps as a reactant and as a product, water is the source of the acid catalyst (H 3 O + ), and water also drives the mechanism toward E2 by favoring, through solvation, the oxonium ion rather than the carbocation as the reaction intermediate. This study provides further evidence that acid-catalyzed reactions can be accomplished readily in HTW in the absence of added acid and that HTW has potential applications in environmentally benign industrial chemistry.

Conversion of Cellulose to 5-Hydroxymethylfurfural using Inorganic Acidic Catalysts in the Presence of Pressurized Water Steam

“…However, during these treatments, the HMF yield was only 2 mol% for (b), (c) and (e). This result suggested that the crystallinity of cellulose was not affected by the steam processing and that the hydrolysis took place at the crystalline surface of the cellulose (Akiya and Savage 2001). When the KH2PO4 catalyst was used ((d) and (f) in Fig.…”

Selective hydrodeoxygenation of lignin-related 4-propylphenol into n-propylbenzene in water by Pt-Re/ZrO2 catalysts

“…The direct C−O bond hydrogenolysis may be difficult because of the high bond dissociation energy of C aromatic -OH bond (468 kJ/mol) [40]. In contrast, we [29,30] and other groups [40][41][42] reported that alcohols were dehydrated to the corresponding alkenes by in-situ generated protons from high-temperature water (pK w = 12 at 300 °C) [44]. In addition, we also found that the HDO of 1 into 2 by Ni-Re/ZrO 2 under similar conditions involved the dehydration of 5 to 4 and subsequent dehydrogenation of 4 to 2 [28].…”

Synthesis of benzimidazoles in high-temperature waterThis work was presented at the Green Solvents for Catalysis Meeting held in Bruchsal, Germany, 13–16th October 2002.Electronic supplementary information (ESI) available: analytical data for compounds 3a–f and 5g–j. See http://www.rsc.org/suppdata/gc/b2/b212394k/

“…13 This property has allowed acid-and base-catalysed reactions to be performed at this near-critical temperature without adding an acid or base, e.g. in the acid-catalysed Beckmann and pinacol rearrangements 14 and dehydration of alcohols 15,16 and in the base-catalysed aldehyde disproportionation 17 and ester hydrolysis. 18 The majority of constructive organic chemistry in hot water has been conducted at near-critical temperatures 19 where the ionic product is favourable, the dielectric constant comparable to organic solvents and the rate of decomposition of organic molecules less than at even harsher conditions.…”