Elucidation of Changes in Cellulose Ultrastructure and Accessibility in Hardwood Fractionation Processes with Carbohydrate Binding Modules

Vera Novy; Fredrik Nielsen; Johanna Olsson; Kevin Aïssa; Jack N Saddler; Ola Wallberg; Mats Galbe

doi:10.1021/acssuschemeng.9b07589

Elucidation of Changes in Cellulose Ultrastructure and Accessibility in Hardwood Fractionation Processes with Carbohydrate Binding Modules

ACS Sustain Chem Eng. 2020 May 4;8(17):6767-6776. doi: 10.1021/acssuschemeng.9b07589. Epub 2020 Mar 31.

Authors

Vera Novy^{1

2}, Fredrik Nielsen^{1

2}, Johanna Olsson², Kevin Aïssa¹, Jack N Saddler¹, Ola Wallberg², Mats Galbe²

Affiliations

¹ Department of Wood Science, Faculty of Forestry, The University of British Columbia, 2424 Main Mall, Vancouver, British Columbia V6T 1Z4, Canada.
² Department of Chemical Engineering, Faculty of Engineering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden.

Abstract

We have recently presented a sequential treatment method, in which steam explosion (STEX) was followed by hydrotropic extraction (HEX), to selectively fractionate cellulose, hemicellulose, and lignin in hardwood into separate process streams. However, above a treatment severity threshold, the structural alterations in the cellulose-enriched fraction appeared to restrict the enzymatic hydrolyzability and delignification efficiency. To better understand the ultrastructural changes in the cellulose, hardwood chips were treated by single (STEX or HEX) and combined treatments (STEX and HEX), and the cellulose accessibility quantified with carbohydrate-binding modules (CBMs) that bind preferentially to crystalline (CBM2a) and paracrystalline cellulose (CBM17). Fluorescent-tagged versions of the CBMs were used to map the spatial distribution of cellulose substructures with confocal laser scanning microscopy. With increasing severities, STEX increased the apparent crystallinity (CBM2a/CBM17-ratio) and overall accessibility (CBM2aH6 + CBM17) of the cellulose, whereas HEX demonstrated the opposite trend. The respective effects could also be discerned in the combined treatments where increasing severities further resulted in higher hemicellulose dissolution and, although initially beneficial, in stagnating accessibility and hydrolyzability. This study suggests that balancing the severities in the two treatments is required to maximize the fractionation and simultaneously achieve a reactive and accessible cellulose that is readily hydrolyzable.