Method for producing ditrimethylolpropane

A method for producing ditrimethylolpropane (di-TMP) by recovering the di-TMP from a reaction solution containing trimethylolpropane (TMP), the di-TMP, tritrimethylolpropane (tri-TMP) and bistrimethylolpropane (bis-TMP), which is produced by reacting n-butyl aldehyde and formaldehyde in the presence of a base catalyst, said method comprising:

• (I) reacting n-butyl aldehyde and formaldehyde in the presence of the base catalyst to obtain the reaction solution (A1) containing the TMP, the di-TMP, and the bis-TMP and/or the tri-TMP as by-products;
• (II) distilling the reaction solution (A1) to remove unreacted formaldehyde therefrom and obtain the TMP-containing solution (A2);
• (III) subjecting the TMP-containing solution (A2) to extraction with the organic solvent (Y1) to separate a reaction product-containing extract therefrom;
• (IV) distilling the extract obtained in (III) to remove the organic solvent (Y1) therefrom and obtain the crude TMP (A3);
• (V) distilling the crude TMP (A3) to separate the purified TMP (A4) therefrom and obtain a di-TMP-containing solution (B1) from the distillation still residue;
• (VI) further distilling the di-TMP-containing solution (B1) to separate a di-TMP-containing solution (B2) as a distillate therefrom;
• and
• (VII) subjecting the di-TMP-containing solution (B2) to crystallization with the organic solvent (Y2) containing an aliphatic ketone to obtain di-TMP (B3);
• said method being carried out under the following conditions (1) and (2):
• (1) the di-TMP-containing solution (B2) obtained in (VI) contains the bis-TMP and the tri-TMP in amounts of 10 parts by mass or less and 5 parts by mass or less, respectively, on the basis of 100 parts by mass of the di-TMP;
• and
• (2) in (VII), the crystallization is initiated under pressure at a temperature exceeding a boiling point of the organic solvent (Y2) as measured under normal pressures, and the resulting crystallization product solution is cooled at a temperature drop rate of 2° C./min or less.