Kinetic study on disproportionations of C1 aldehydes in supercritical water: methanol from formaldehyde and formic acid.
The reaction pathways and kinetics of C1 aldehydes, formaldehyde (HCHO) and formic acid (HCOOH=HOCHO), are studied at 400 degrees C in neat condition and in supercritical water over a wide range of water density, 0.1-0.6 g/cm3. Formaldehyde exhibits four reactions: (i) the self-disproportionation of formaldehyde generating methanol and formic acid, (ii) the cross-disproportionation between formaldehyde and formic acid generating methanol and carbon dioxide, (iii) the water-independent self-disproportionation of formaldehyde generating methanol and carbon monoxide, and (iv) the decarbonylation of formaldehyde generating hydrogen and carbon monoxide. The self- and cross-disproportionations overwhelm the water-independent self-disproportionation and the formaldehyde decarbonylation. The rate constants of the self- and cross-disproportionations are determined in the water density range of 0.1-0.6 g/cm3. The rate constant of the cross-disproportionation is 2-3 orders of magnitude larger than that of the self-disproportionation, which indicates that formic acid is a stronger reductant than formaldehyde. Combining the kinetic results with our former computational study on the equilibrium constants of the self- and cross-disproportionations, the reaction mechanisms of these disproportionations are discussed within the framework of transition-state theory. The reaction path for methanol production can be controlled by tuning the water density and reactant concentrations. The methanol yield of approximately 80% is achieved by mixing formaldehyde with formic acid in the ratio of 1:2 at the water density of 0.4 g/cm3.
Morooka
Saiko
S
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan.
Matubayasi
Nobuyuki
N
Nakahara
Masaru
M
D005557:Formaldehyde / Q000737:chemistry*; D005561:Formates / Q000737:chemistry*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy / Q000379:methods / Q000592:standards; D000432:Methanol / Q000138:chemical synthesis*; D008956:Models, Chemical; D011312:Pressure; D013696:Temperature; D013997:Time Factors; D014867:Water / Q000737:chemistry
D005561:Formates; D014867:Water; D005557:Formaldehyde; D000432:Methanol