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–(OCH2)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 1,4-butynediol. 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 Kx,n≥2R–OH, which is independent of the substructure (R) of the solvent. This holds for the oligomerization reactions leading to R–(OCH2)n–OH with n ≥ 2. The chemical equilibrium constant Kx,1R–OH of 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 Kx,n≥2R–OH was tested for the reactions of formaldehyde with ethanol and 1-propanol, for which neither Kx,1R–OH nor Kx,nR–OH was previously available. The corresponding equilibria were studied by 13C NMR spectroscopy and the equilibrium constants were determined. A novel method was developed and used in these studies to obtain data on Kx,1R–OH by NMR spectroscopy, which is difficult because of 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)

“…They found that the chemical equilibria of the reactions leading to formaldehyde oligomers with a minimum of two CH 2 O segments, for example reactions (II) and (IV) of this work, can be described using a generalized mole fraction-based chemical equilibrium constant that is scribed very well. This shows that the generalized mole fraction-based chemical equilibrium constant from Kircher et al 29 for the oligomerization reactions of formaldehyde, leading to oligomers with a minimum of two CH 2 O segments, also holds for isoprenol. The result for K x,(III) fits perfectly in the trend observed by Kircher et al 29 for the other alcohols, see The bars are labeled with the respective alcohol.…”

“…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.…”

“…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.…”

“…Recently, Kircher et al 29 analyzed the entire available data on oligomerization reactions of formaldehyde with water and alcohols. Data on the oligomerization reactions of formaldehyde and isoprenol were not available at the time Kircher et al 29 carried out their study.…”

“…NMR spectroscopy was successfully used before to study the chemical equilibria in systems containing formaldehyde. 16,18,20,[27][28][29][30][31][32][33][34][35][36][37][38] The new experimental data were used together with data that was available in the litera-ture to extend the physico-chemical model of the system (formaldehyde + water) of Kuhnert et al 12 to include isoprenol.…”