Processing of cellulosic biomass with ionic liquids: insights from structural characterization

Abstract

The processing of lignocellulosic fibrous biomass under environmentally-friendly conditions towards novel technological materials is a key in contributing to the efficient valorization of available side-streams. In particular, the creation of nanoscale components allows to erase the heterogeneity of the original biomass and add additional functionality to the resulting materials. The microfluidic processing of cellulose fibers results in the disruption of the cellulose structure into a micro- to nanofibrillated fiber network with excellent mechanical properties. However, the high energy input needed for processing makes this route industrially irrelevant and different pretreatment methods have to be designed. The pre-swelling of cellulose fibers in presence of ionic liquids is an attractive ‘green route’ to weaken the hydrogen bonding network. In this study, the effect of different ionic liquid types on the morphological and structural properties of cellulose has been studied, as an optimum pretreatment step should only weaken the molecular fiber interactions without causing fiber degradation. Two different ionic liquids have been evaluated, namely 1-butyl-3-methylimidazolium chloride and 1-ethyl-3-methylimidazolium chloride, in order to monitor the influence of alkyl chain length on swelling efficiency and resulting cellulose properties after different processing time. The thermal stability of the treated fibers observed during thermogravimetric analysis has been correlated to significant changes in microscopic morphology and crystallinity as quantified through XRD analysis in combination with FTIR and Raman spectroscopy. From these correlations, a mild regime of pretreatment parameters has been identified while significantly contributing to the initiation of the fibrillation process. As a first step towards upscaling the process into a microfluidizer, the rheological characterization of the pre-treated fiber slurries in a homogenizer indicates a decrease and stabilization in viscosity as a function of longer fibrillation times.