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).

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Quantitative 13 C NMR spectroscopy was used to investigate the complex chemical equilibria in ternary liquid mixtures of formaldehyde-water-methanol at temperatures between 298 and 383 K. In these mixtures, formaldehyde is predominantly bound in methylene glycol, poly-(oxymethylene) glycols, hemiformal, and poly(oxymethylene) hemiformals, which are formed by a series of oligomerization reactions. The present study is the first in which data on the species distribution in the studied technically important ternary system is reported. It complements previous work in which the focus was on the binary systems formaldehyde-water and formaldehyde-methanol. It also extends the temperature range in which data are available, which was previously limited by the fact that all experiments were carried out in batch cells under ambient pressure. In the present experimental study, a pressurized NMR flow cell was used for the first time. The results from the present study were obtained independently in two different laboratories with different NMR techniques and instruments. Details on the experimental procedures are presented. A comparison of the two data sets shows excellent agreement. The experimental results are compared to predictions from a recently published physicochemical model that aims at the description of vapor-liquid equilibria in the studied mixtures. The results suggest that some model parameters should be revised if the model is to be applied to quantitatively predict liquid-phase compositions (e.g., the distribution of formaldehyde to different species) in the studied ternary system.