Kinetics and Mechanisms of Hydrothermal Dehydration of Cyclic 1,2- and 1,4-Diols

Abstract

Hydrothermal dehydration is an attractive method for deoxygenation and upgrading of biofuels because it requires no reagents or catalysts other than superheated water. Although mono-alcohols cleanly deoxygenate via dehydration under many conditions, polyols such as those derived from saccharides and related structures are known to be recalcitrant with respect to dehydration. Here, we describe detailed mechanistic and kinetic studies of hydrothermal dehydration of 1,2- and 1,4-cyclohexanediols as model compounds to investigate how interactions between the hydroxyls can control the reaction. The diols generally dehydrate more slowly and have more complex reaction pathways than simple cyclohexanol. Although hydrogen bonding between hydroxyls is an important feature of the diol reactions, hydrogen bonding on its own does not explain the reduced reactivity. Rather, it is the way that hydrogen bonding influences the balance between the E1 and E2 elimination mechanisms. We also describe the reaction pathways and follow-up secondary reactions for the slower-dehydrating diols.

Acid-Catalytic Effect of Alcohol Dissociation on Pinacol Rearrangement in Hot Compressed Water

“…This reaction involves the rearrangement of CH 3 with the dehydration of dihydric alcohol. Many alcohols, whether monohydric or dihydric, are dehydrated in hot compressed water, 19,28,29 and since this reaction also involves dehydration, its reactivity is likely similar to that of other alcohols. To proceed with this reaction, a strong acid catalyst and harsh reaction conditions are required, such as boiling pinacol in 25% H 2 SO 4 for 3 h. 33 In hot compressed water, however, pinacol rearrangement proceeds without any acid catalyst at above 300 °C.…”

“…19,20 Although the acid-catalytic effect of alcohols has been suggested, it has rarely been considered. Alcohols can be the reactants or products of many organic reactions in hot compressed water, such as the hydrolysis of esters, 21 ethers, 22−26 and acetals 7,27 ; dehydration of alcohols 28,29 ; and addition 30 and alkylation reactions. 31,32 Therefore, investigating and quantitatively evaluating the acid-catalytic effect of alcohols are important.…”

“…For example, for the dehydration of t -butyl alcohol, a reaction mechanism incorporating H + formation from alcohol dissociation has been proposed and the acid-catalytic effect of t -butyl alcohol suggested. , Although the acid-catalytic effect of alcohols has been suggested, it has rarely been considered. Alcohols can be the reactants or products of many organic reactions in hot compressed water, such as the hydrolysis of esters, ethers, − and acetals , ; dehydration of alcohols , ; and addition and alkylation reactions. , Therefore, investigating and quantitatively evaluating the acid-catalytic effect of alcohols are important.…”

Intramolecular Dehydration of Cyclic Alcohols to Cyclic Alkenes via Cation- and Anion-Confined Catalysis over Ionic Liquids

“…To meet the requirements for green and sustainable development, it becomes more urgent to pursue energy and commodity chemicals from renewable resources. , Cyclic alcohols can be readily accessible from biomass-derived phenols via selective hydrogenation, which are regarded as biomass platform compounds to produce more valuable chemicals. Therefore, intramolecular dehydration of cyclic alcohols provides a green and sustainable strategy for the synthesis of cyclic alkenes, which has been widely investigated in recent years. − To date, various multifunctional catalytic systems (Scheme S1) including high-temperature water (>375 °C), , metal catalysts, , mineral acids ( e.g. , H 3 PO 4 , − H 2 SO 4 , , and HCl), solid acids, − organic acids, and metal oxides ( e.g.…”