Hydrothermal Carbon−Carbon Bond Formation and Disproportionations of C1 Aldehydes: Formaldehyde and Formic Acid

Abstract

Hydrothermal reaction pathways and kinetics of C1 (carbon-one) aldehydes, formaldehyde (HCHO) and formic acid (HCOOH = HOCHO), are studied at 225 °C without and with hydrochloric acid (HCl) up to 0.6 M (mol dm^-3). Reactions unveiled are the following: (i) the self-disproportionation forming methanol and formic acid, a redox reaction between two formaldehydes, (ii) the cross-disproportionation forming methanol and carbonic acid, a redox reaction between formaldehyde and formic acid, and (iii) the acid-catalyzed C−C bond formation producing glycolic acid (HOCH₂COOH) as a precursor of the simplest amino acid, glycine. Reaction iii is a hydrothermally induced chemical evolution step from C1 aldehydes, formaldehyde and formic acid. Disproportionations i and ii are found to proceed even without base catalysts unlike the classical Cannizzaro reaction. Acid catalyzes the self-disproportionation (i) and the C−C bond formation (iii), but retards the cross-disproportionation (ii). The rate constants of noncatalyzed and acid/base-catalyzed paths for reactions i, ii, and iii are given additively as 2 × 10^-4 + (2 × 10^-3)[H⁺], 10^-4 + 10³[OH^-], and (2 × 10^-3)[H⁺] M^-1 s^-1, respectively; the concentrations of proton [H⁺] and hydroxide ion [OH^-] are expressed in M. The rate constant of the noncatalytic (neutral) cross-disproportionation is 1 order of magnitude larger than that of the self-disproportionation. The reaction pathways are controlled on the basis of the kinetic analysis to make the glycolic acid and methanol productions dominant by tuning the concentrations of formaldehyde, formic acid, and HCl. The conversion to glycolic acid reaches ∼90% when formaldehyde, HCl, and formic acid are mixed in the ratio of 1:2:17. The conversion of formaldehyde to methanol reaches ∼80% when formic acid is added in excess to formaldehyde.