NMR Spectroscopic and Densimetric Study of Reaction Kinetics of Formaldehyde Polymer Formation in Water, Deuterium Oxide, and Methanol

Abstract

In industrial processes, formaldehyde is mainly handled in aqueous solutions, which often contain methanol. In these solutions, formaldehyde forms predominantly adducts with the solvents.In aqueous solutions, methylene glycol and poly(oxymethylene) glycols are formed, in methanolic solutions hemiformal and poly(oxymethylene) hemiformals. As both the formation of poly-(oxymethylene) glycol and of poly(oxymethylene) hemiformal are slow compared to typical residence times in separation equipment, reliable information on kinetics of these reactions is essential for process design. Two independent methods were applied to obtain this information: NMR spectroscopy and high-resolution densimetry. The experiments were carried out at temperatures between 273 and 334 K and pH between 2 and 9. Both for poly(oxymethylene) glycol formation and poly(oxymethylene) hemiformal formation, the minimal reaction rate occurs between pH 3 and 5. At 293 K, the inverse rate constant 1/k at this minimum is about 6 min for poly(oxymethylene) glycol formation and about 110 h for poly(oxymethylene) hemiformal formation. The rate constants determined with NMR spectroscopy and densimetry generally agree well. Previously reported discrepancies between results from both methods are explained by the fact that rate constants of poly(oxymethylene) glycol formation depend strongly on the solvent water or deuterium oxide. Reaction kinetics of poly(oxymethylene) glycol and poly-(oxymethylene) hemiformal formation in the mixed-solvent system with water and methanol predicted from results obtained in the single-solvent systems are in good agreement with experimental data. is large Ocmg¿cfa % 650 at 293 K), the inverse reaction, the degradation of methylene glycol, is slow (at room temperature typically 1/&*mg « 1 min).

Kinetics and chemical equilibrium of the hydration of formaldehyde

“…The research group of Maurer recently studied the kinetics and X Deceased. chemical equilibria of the polyoxymethylene glycol formation (Hasse & Maurer, 1991;Hahnenstein, Hasse, Kreitler, & Maurer, 1994b;Hahnenstein, Albert, Hasse, Kreitler, & Maurer, 1994a).…”

Vapor–liquid equilibrium of aqueous solutions of formaldehyde and methanol

“…1992 . Knowledge of the chemical equilibrium reaction constants in the liquid phase has also been extended by NMR Ž spectroscopy Kogan, 1979a;Hahnenstein et al, 1994aHahnenstein et al, , 1995. Balashov et al, 1996 .…”

Vapor‐liquid and chemical equilibria of formaldehyde‐water mixtures

“…Ž . 1994a andBalashov et al 1996 ; b on reaction kinetics Ž .x see Hahnenstein et al 1995 . However, spectroscopic investigations give only information on species concentrations, whereas in thermodynamic calculations the activities or activity coefficients have to be known.…”