N,N-Dimethylhydrazone as a Reversible Derivatization Agent to Promote the Hydroxymethylation of Furfural with Formaldehyde.

Abstract

We explore here the synthesis of HMF from concentrated feeds of two low cost and industrially abundant chemicals: furfural and formaldehyde. By adjusting the acidity of the solvent, we discovered an alternative mechanism in which the reaction selectivity stops to the hydroxymethylation step, in contrast to previously reported acid-catalyzed pathways leading to the formation of the bisfuranic dimer as major products. One of the keys of this study relies on the reversible derivation of the -CHO group of furfural with N,N-dimethylhydrazone which plays a dual role (1) it restores the nucleophilicity of the furan ring and (2) it reacts with HCHO to form in situ an electrophilic zwiterrionic species stabilized through hydrogen transfer. By means of experimental and theoretical investigations, we discovered that guaiacol can be used as a biobased and safe solvent. Under optimized conditions, the hydroxymethylation of the furan ring of furfural occurs with more than 94% selectivity, at only 50 °C and with a stoichiometric amount of HCHO. A concentrated feed of furfural as high as 25 wt% in guaiacol was employed without impacting the reaction selectivity leading to an improvement of the reactor productivity to about 25 kg/m3/h. The recycling of the N,N-dimehylhydrazone is also discussed.