Generalized Chemical Equilibrium Constant of Formaldehyde Oligomerization

Abstract

Formaldehyde reacts with solvents that contain hydroxyl groups (R-OH) in oligomerization reactions to oxymethylene oligomers (R-(OCH 2 ) n -OH). The chemical equilibria of these reactions have been studied in the literature for water, for the mono-alcohols methanol, ethanol, and 1-butanol, as well as for the diols ethylene glycol and 1,4butynediol. In the present work, the collective data were analyzed. It was found that the prolongation of the oxymethylene chains by the addition of formaldehyde can be described very well with a generalized chemical equilibrium constant K R-OHx,n≥2 , which is independent of the substructure (R) of the solvent. This holds for the oligomerization reactions leading to R-(OCH 2 ) n -OH with n ≥ 2. The chemical equilibrium constantof the reaction of formaldehyde with the solvent R-OH depends on the solvent, but simple trends are observed. The hypotheses of the existence of a generalized chemical equilibrium constant K R-OH x,n≥2 was tested for the reactions of formaldehyde with ethanol and 1-propanol, for which neither K R-OHx,1 nor K R-OHx,n were previously available.The corresponding equilibria were studied by 13 C NMR spectroscopy and the equilibrium constants were determined. A novel method was developed and used in these studies to obtain data on K R-OHx,1 by NMR spectroscopy, which is difficult due to the low amount of molecular formaldehyde. It was found that the generalized equilibrium constant is even valid for the acid-catalyzed formation of poly(oxymethylene) dimethyl ethers (OME).

Vapor–Liquid Equilibria and Chemical Equilibria in the System (Formaldehyde + Water + Isoprenol)

“…For formaldehyde oligomerization reactions, this was already demonstrated. 18,27,29,51,52 K x,j for a reaction j is defined according to eq 13 K x…”

“…Due to the small concentrations of monomeric formaldehyde in the studied chemical equilibria, a direct determination of the chemical equilibrium constants of the oligomerization reactions of formaldehyde and isoprenol from quantitative data obtained from the NMR spectra on mixtures of (formaldehyde + isoprenol) alone was not possible. Instead, in the present work, a method was used that is similar to those used by Berje et al 27 and Kircher et al 29 It is based on the knowledge of the chemical equilibria of the oligomerization reactions of formaldehyde and water as described in more detail by Kircher et al 29 Therefore, besides formaldehyde and isoprenol, some of the mixtures studied in this work had to contain water to some extent. Additional data on the (water-free) system (formaldehyde + isoprenol) were also used, as they yield better information on the long-chain oligomers of formaldehyde and isoprenol.…”

“…Note that the peaks of MG 2 (1) and MG 2 (2) were always superimposed in the spectra due to the symmetry of the molecule. Following Kircher et al, 29 the approach was validated in preliminary experiments with gravimetrically prepared samples of (isoprenol + methylal) by comparison of the gravimetrically and the NMR-spectroscopically determined mole fraction of methylal. The relative deviation of the mole fraction of methylal as determined by the two different methods was always less than 2%.…”

Density of solutions of formaldehyde in water and alcohols

“…The model can be extended to also include other alcohols than methanol, but this requires knowledge of the true species distribution in the solution, that is, the concentrations of all components including the formaldehyde oligomers in the chemical equilibrium must be known. This information is available for many practically relevant formaldehyde‐containing solutions, 18,19 but by far not for all. Other model parameters can be estimated using group‐contribution methods.…”

“…The alcohols that are presently included in the model are methanol, 1‐propanol, and isoprenol, but the model can easily be extended to also include other alcohols. 1‐propanol and isoprenol were chosen here besides methanol, as a chemical equilibrium model of the oligomerization reactions of formaldehyde with these alcohols is available, 18,19 which is needed for applying the model proposed by Ott 14…”