Direct carboxylation of alcohols

The direct carboxylation of alcohols (6.6) and the reaction of alcohols with urea (used as an active form of CO2) (6.7) represent appealing alternatives. The former has some kinetic and thermodynamic barriers, the latter raises NH3 recovery issues and product separation problems. 

Because of their Gibbs free energy (see below), reaction (6.6) is shifted to the left much more than reaction (6.7). As a matter of fact, the literature shows that the equilibrium concentration of DMC in reaction (6.6) (R = Me) is less than 1 %, depending on the reaction temperature [20], whereas in reaction (6.7) it can easily exceed 20 %. Other alcohols, such as ethanol and allyl alcohol [21], show an analogous behavior. Unfavorable thermodynamics is not the only drawback of reaction (6.6) the water formed in the reaction pushes the equilibrium to the left and affects the catalyst structure and activity [20, 22,23] in both homogeneous and heterogeneous processes water traps have been used, either chemical- or process-based, and are discussed in Sect. 6.2.2.3. 

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Reaction Mechanisms in the Direct Carboxylation of Alcohols, Polyols, Cyclic Ethers, and Cyclic Amines to Afford Monomeric Compounds and Polymeric Materials... 

Reaction Mechanisms in the Direct Carboxylation of Alcohols, Polyols. 

In a catalyst-free process, the direct carboxylation of alcohols requires, a priori, two distinct steps as categorized in Scheme 6.1. In (/) an alcohol molecule is activated by a base and an alkoxo-moiety is generated which interacts (//) with CO2 affording the hemi-carbonate, R0-C(0)0. The acid activation of the second molecule of alcohol Hi) produces the alkyl moiety which reacts with the hemi-carbonate and... 

Scheme 6.2 Nb-hemicarbonate as active species in the direct carboxylation of alcohols with Nb-alkoxo catalysts. Adapted with permission from [34]. Copyright (2014) American Chemical Society...

Niobium-Based Catalysts Other soluble alkoxides such as those of titanium (IV) [27, 35, 36] and Group 5 metals [21, 28, 37] are active catalysts in the direct carboxylation of alcohols. The reaction mechanism has been elucidated with penta-alkoxo species of Group 5 elements [21, 28, 37] combining DFT and experimental studies. The overall reaction mechanism is depicted in Scheme 6.2. 

The unfavorable thermodynamics of the direct carboxylation of alcohols has pushed efforts to find alternative routes for the conversion of alcohols into the relevant carbonates. An interesting possibility is represented by the use of urea which is formed from CO2 and NH3. Urea can be considered an active form of CO2 (Fig. 6.12). Its formation AG is some 200 kJ mol less negative than that of CO2. However, in principle, its reactions with alcohols would have less negative thermodynamics than the reaction of CO2 and, consequently, the equilibrium positions should be shifted towards the right, reaching higher conversion at the equilibrium. 

Dibenedetto A, Pastore C, Aresta M (2006) Direct carboxylation of alcohols to organic carbonates comparison of the Group 5 element alkoxides catalytic activity. Catal Today 115 88-94... 

Aresta M, Dibenedetto A, Angelini A, Papal I (2014) Reaction mechanisms in the direct carboxylation of alcohols for the synthesis of acyclic carbonates. Top Catal 58(1) 2-14... 

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