Multi-scale processes of beech wood disintegration and pretreatment with 1-ethyl-3-methylimidazolium acetate/water mixtures

Abstract

Background: The valorization of biomass for chemicals and fuels requires efficient pretreatment. One effective strategy involves the pretreatment with ionic liquids which enables enzymatic saccharification of wood within a few hours under mild conditions. This pretreatment strategy is, however, limited by water and the ionic liquids are rather expensive. The scarce understanding of the involved effects, however, challenges the design of alternative pretreatment concepts. This work investigates the multi length-scale effects of pretreatment of wood in 1-ethyl-3-methylimidazolium acetate (EMIMAc) in mixtures with water using spectroscopy, X-ray and neutron scattering.

Results: The structure of beech wood is disintegrated in EMIMAc/water mixtures with a water content up to 8.6 wt%. Above 10.7 wt%, the pretreated wood is not disintegrated, but still much better digested enzymatically compared to native wood. In both regimes, component analysis of the solid after pretreatment shows an extraction of few percent of lignin and hemicellulose. In concentrated EMIMAc, xylan is extracted more efficiently and lignin is defunctionalized. Corresponding to the disintegration at macroscopic scale, SANS and XRD show isotropy and a loss of crystallinity in the pretreated wood, but without distinct reflections of type II cellulose. Hence, the microfibril assembly is decrystallized into rather amorphous cellulose within the cell wall.

Conclusions: The molecular and structural changes elucidate the processes of wood pretreatment in EMIMAc/water mixtures. In the aqueous regime with >10.7 wt% water in EMIMAc, xyloglucan and lignin moieties are extracted, which leads to coalescence of fibrillary cellulose structures. Dilute EMIMAc/water mixtures thus resemble established aqueous pretreatment concepts. In concentrated EMIMAc, the swelling due to decrystallinization of cellulose, dissolution of cross-linking xylan, and defunctionalization of lignin releases the mechanical stress to result in macroscopic disintegration of cells. The remaining cell wall constituents of lignin and hemicellulose, however, limit a recrystallization of the solvated cellulose. These pretreatment mechanisms are beyond common pretreatment concepts and pave the way for a formulation of mechanistic requirements of pretreatment with simpler pretreatment liquors.

Keywords: Crystallinity; Disintegration; EMIMAc; Ionic liquid; Lignocellulose; Microfibrils; Pretreatment; SANS; XRD.

Fig. 1

Morphology of beech chips pretreated in EMIMAc-water mixtures. The water mass fraction increases from left to right as indicated above the samples. While the material is completely disintegrated at low water content, it retains the original morphology at highest water content

Fig. 2

Sugars obtained after enzymatic hydrolysis of wood chips pretreated with varying water concentrations in EMIMAc (72 h, 45 °C using 1 ml buffer and 20 mg pretreated wood). Data points are the average of two hydrolysis experiments with deviations of ~1 wt%

Fig. 3

Recovered mass fraction after pretreatment of beech wood in EMIMAc/water mixtures. The dashed line indicates the transition center at 8.6 wt% water content in EMIMAc. The numbers depict the amount of extracted material at the two plateaus

Fig. 4

Semi-quantitative concentration of functional groups from Raman spectra. The areas were calculated to depict the relative change of functional groups The dashed line shows the average value obtained with native beech wood

Fig. 5

X-ray diffraction patterns (left), equatorial intensity distributions (middle), and light microscopic images of the regions of sample used for scattering experiments. The fiber axis is approximately horizontal in those patterns exhibiting orientation (i.e., with water content of 10.7 wt% and greater). The intensity distributions were derived by scanning in the radial direction across the two intense Bragg peaks and are plotted as a function of reciprocal coordinate R = 1/d, where d is the Bragg spacing [Å]

Fig. 6

Crystallinity index (top) and spacing of the (200) diffraction of the samples pretreated in EMIMAc/water mixtures. Both the crystallinity and the spacing change when the material is disintegrated

Fig. 7

Radially averaged SANS profiles of native and pretreated wood samples in D₂O. While native wood shows a characteristic nanostructure of the cell wall, the data indicate the formation of larger structures due to pretreatment in EMIMAc/water

Fig. 8

Radii of gyration (R _g) of cellulose fibrils as a function of the water content of EMIMAc during the disintegration process. The radii are larger in the disintegrated samples than those in the sample at 12.6 wt% water content during pretreatment and seem to decrease after processing in concentrated EMIMAc