Acid Catalytic Effects of Hot Compressed Water and Water–Alcohol Mixtures, and Their Applications as Tunable and Catalyst-Free Solvents

This paper provides a comprehensive overview of research findings concerning the acid catalytic effect (ACE) of hot compressed water and water–alcohol mixtures, along with the applications of these solvents. The ACE observed during reactions can be categorized into three types: inherent, associated, and interfering. These ACE types originate from the solvent, solutes, and reactor, respectively. Distinguishing and evaluating these ACEs is crucial for elucidating reaction mechanisms and developing reaction models. Water exhibits inherent ACE in both its dissociated and undissociated forms under hot compressed conditions. Hot compressed water–alcohol mixtures possess the capability to tune the characteristics of solvents, including ACE, through their composition. The application of hot compressed water and water–alcohol is prevalent in a variety of fields, including the conversion of biomass and biomass-derived materials, extraction, biodiesel production, organic synthesis reactions, recycling via the decomposition of polymers, and inorganic material synthesis. In these applications, the utilization of water–alcohol mixtures resulted in a higher yield of target products and/or superior properties of products compared to the use of pure solvents, such as water alone or alcohol alone. The observed results can be attributed to the optimization of the roles of water and alcohol in the reaction through mixing them.