Fabrication of mesoporous lignocellulose aerogels from wood via cyclic liquid nitrogen freezing–thawing in ionic liquid solution

Abstract

Homogeneous mesoporous lignocellulose aerogels were prepared from hardwood using 1-allyl-3-methylimidazolium chloride (AMImCl) as an ionic liquid (IL) via cyclic liquid nitrogen freezing–thawing (NFT, from −196 °C to 20 °C) treatment processes. The obtained hydrogels after NFT treatment were solvent-exchanged by acetone, washed with liquid carbon dioxide and then dried by releasing the carbon dioxide at the critical temperature. It was observed that the obtained aerogels after five-cycles of NFT treatment had open-structured 3-dimensional (3D) fibril-like networks. The surface areas and pore size distributions could be adjusted by controlling the NFT treatment cycles. However, the samples treated by common freezing–thawing (FT, from −20 °C to 20 °C) displayed a film-like 2D structure and low surface area. This may be ascribed to the difference of the assembled ‘secondary units’ formed during the freezing process. For instance, in the NFT process, 3D-like ‘secondary assembly units’ were squeezed out by small IL crystals formed during the ultra low temperature freezing process. These ‘secondary assembly units’ were able to build a large 3D network through the connection and overlap effect with cyclic NFT processes. However, the assembled 2D-like ‘secondary assembly units’ were squeezed out by large IL crystals in the common FT process due to the slow freezing process and finally were developed to a film-like structure. These ‘secondary assembly units’ could be linked together in a slow thawing process to form compact aerogels. This study provides a new means to fabricate novel mesoporous lignocellulose aerogels, which is crucial to fully utilize abundant lignocellulose biomass. The homogeneous mesoporous aerogels can be used as highly insulating materials with low thermal conductivity and also have appealing performance in sound absorption and noise reduction properties.