Aerogel is a highly porous material with unique properties such as low density, thermal conductivity, and high water/oil retention capacity. It is mainly fabricated from synthesized polymers or cross-linked by additional chemical agents. With the increasing demand for green materials and sustainable processes, more attention has been drawn to developing aerogels directly from natural and renewable materials such as lignocellulose.<br/>Molten salt hydrate is an aqueous solution with a high salt concentration where the molar ratio of water to salt is near to the coordination number of the salt cation. Because of the similar properties of molten salt hydrates and ionic liquids, molten salt hydrates showed their capabilities to dissolve cellulose. Zinc chloride is a well-known cellulose swelling and dissolving agent with inexpensive price and low toxicity. Dissolved cellulose in zinc chloride salt hydrate has been reported to form hydrogel and films while cross-linking is necessary to provide sufficient strength.<br/>Waste fines (i.e., end-of-life paper products) and food packaging paperboards are typical unrecyclable biomass. Waste fines are rejected fibers in the paper recycling process because the short fibers are unable to provide sufficient strength for paper. Food packages have challenges in their recycling as they contain food residues like food dyes and oils. Hence, waste biomass and food packages are mostly landfilled or incinerated, which causes increasing economic and environmental issues. However, these are potential resources for cellulose-based aerogel synthesis. Also, calcium carbonate in the waste fines can generate calcium ions, which are well-known as cross-linking agents for hydrogel formation in the proposed salt hydrate system.<br/>Herein, we report a facile route to fabricate cellulose aerogel directly from waste biomass using zinc chloride salt hydrate, where the role of calcium carbonate changes from impurities to cross-linkers to provide strength. The process is simple and under mild conditions. The aerogels showed high mechanical strength and lightweight with good thermal insulation performance. This study could pave a new way to valorize the waste biomass.

Fabrication of aerogels directly from waste biomass using zinc chloride salt hydrate

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