Cascade dehydrative amination of glycerol to oxazoline

Abstract

Transformation of biomass into valuable nitrogen-containing compounds is highly desired, yet less explored. Here, we report a simple and efficient method for the direct conversion of bioglycerol to oxazoline involving glycerol dehydration to acetol followed by its amination using an aqueous solution of ammonia. For the two-step strategy a non-noble metal Cu–Zr catalyst was developed, giving a glycerol to acetol conversion of 78% followed by amination separately with 95% selectivity to oxazoline. Moreover, we have demonstrated a single-pot oxazoline synthesis using Ru/C as the most stable catalyst to achieve 95% selectivity to oxazoline without any leaching. XPS studies revealed the co-existence of multivalent Ru species in different percentages depending on the nature and structure of the support. These multivalent species (RuO₂ and RuO₃) have a synergistic effect on the activation of the carbonyl group, whereas Ru⁰ is an active site for ammonia dissociation. NH₃-TPD and Py-IR spectroscopy results also suggest that the presence of relatively moderate acid sites and a higher Brønsted/Lewis acid ratio in the catalyst promote the selective production of oxazoline. The reaction pathway involves first glycerol dehydration to acetol. In the subsequent step, NH₃ is dissociatively adsorbed on the catalyst surface and the imine thus formed is condensed with a second molecule of acetol to obtain oxazoline. The studied catalyst could be recycled successfully without any significant loss of catalytic activity.