Chitin depolymerization in sub and supercritical water by ultrafast reactors

Abstract

Chitin [β-1,4-poly(n-acetyl-D-glucosamine)] is the second most abundant biopolymer after cellulose, and it is produced by crustaceans, mollusks, insects, and fungi. Chitin has high interest as a biocompatible and biodegradable material, but also as a source of biologically active oligosaccharides and nanoparticles. The recalcitrant structure of chitin makes traditional processes use harsh acidic conditions to generate these products. In this work, the use of water at high temperature (270 to 400ºC) and pressure (20MPa) was studied to produce oligosaccharides and nanoparticles. The physicochemical properties of water (density, viscosity, diffusivity, ionic product, and dielectric constant) change dramatically below and above of the critical point (374 °C and 22 MPa) providing a tunable reaction medium, remarkably at subcritical medium (SubCW) ionic reactions are promoted due to the high ionic product while the low the concentration of [H+] and [OH-] at supercritical conditions (SCW) favor radical reactions. Furthermore, control on residence time is critical in such conditions: according to literature, formation of solid (char) and liquid (5-hidroxy methyl furfural) degradation compounds have even prevailed working in batch-type systems even at short times up to 1 minute. In this work, residence times as short as 0.1 to 8 s are explored thanks to a Press-Tech group designed facility working in continuous mode: heating and cooling down are achieved almost instantaneously by mixing an aqueous suspension of chitin at room temperature with hot pressurized water in “T” piece just before the micro-reactor (1.2-25 mL) and, afterwards, is cooled down by a sudden expansion valve at the outlet.