Chiral nematic mesoporous carbon derived from nanocrystalline cellulose.

Template synthesis based on the self-assembly of lyotropic liquid crystals offers access to mesoporous solids with high specific surface areas and periodic structures. Incorporating mesopores (i.e., pores ranging from 2 to 50 nm in diameter) into carbonaceous materials may be advantageous for certain applications, including the adsorption of large molecules, electrochemical double-layer capacitors, lithium ion batteries, catalyst supports, and field-effect transistors. Ordered mesoporous carbon materials were first synthesized by using ordered mesoporous silica as a hard template. In the hardtemplating (also referred to as nanocasting) approach, mesoporous silica is repeatedly infiltrated with a suitable carbon precursor (e.g., sucrose) that is carbonized within the pores of the silica at elevated temperature. After sufficient pore loading and etching of the silica, mesoporous carbon with a structure that is the inverse of the original silica template is obtained. Despite the potential benefits of using mesoporous carbon over traditional activated carbon, the cost of making these materials may be prohibitive. Finding more economical synthetic routes, both in terms of the number of steps involved and precursors used, is important if mesoporous carbon is to be implemented in new technologies. Direct surfactant-templating approaches (soft templating) have also been developed for the synthesis of mesoporous carbon by condensing polymerizable carbon precursors (e.g., phenolic resins) around block copolymer templates. Soft templating requires fewer synthetic steps than hard templating and offers improved control over the morphology of the mesoporous carbon products. For example, free-standing mesoporous carbon membranes have been synthesized through evaporation-induced self-assembly coupled with soft templating. The specific surface areas of these films, however, are considerably less than those of mesoporous carbons produced by hard templating. The use of both hardand soft-templating approaches has enabled mesoporous carbon to be synthesized with cubic and hexagonal pore systems that are ultimately derived from the self-assembly of surfactants into ordered mesophases. The synthesis of mesoporous carbon templated by other liquid-crystal phases, for example nematic and chiral nematic phases, has been virtually unexplored. In particular, the incorporation of chiral organization into mesoporous carbon could open the door for applications that involve enantioselective adsorption. Chiral nematic liquid crystals, which consist of mesogens organized into a long-range helical assembly, exhibit unique properties, such as the selective reflection of circularly polarized light. The incorporation of chiral nematic organization into solid-state materials could give rise to novel properties. Kyotani and co-workers have synthesized graphitic carbon with chiral nematic ordering by first polymerizing polyacetylene within a thermotropic chiral nematic liquid crystal followed by doping with iodine and pyrolysis. It is expected that these materials will display interesting electromagnetic properties. As the major constituent of plant cell walls, cellulose is the most abundant biological material on the planet. Recently, there has been significant interest in the study of cellulose fibrils with nanometer dimensions that have high surface area and can behave as lyotropic liquid crystals. Stable suspensions of nanocrystalline cellulose (NCC) can be obtained through hydrolysis of bulk cellulosic material with sulfuric acid. In water, suspensions of NCC organize into a chiral nematic phase that can be preserved upon slow evaporation, thereby resulting in chiral nematic films. The unique physical properties and natural abundance of NCC make it attractive as a potential template for porous materials. Although bulk cellulosic materials are commonly used to generate activated carbon, to date there have been no studies on the use of NCC as a template for mesoporous carbon. Recently, our research group reported that evaporationinduced self-assembly of NCC with different silica precursors can result in composite films with chiral nematic structures, and that the removal of NCC from these films generates chiral nematic mesoporous silica. Herein we report that NCC– silica composite films may also be used to generate mesoporous carbon with a high specific surface area and excellent retention of the chiral nematic organization. This provides the first example of using nanocrystalline cellulose as a template for mesoporous carbon as well as the first demonstration of a mesoporous carbon with chiral nematic ordering. We demonstrate that the use of silica is necessary for both the introduction of mesoporosity and the preservation of the [*] K. E. Shopsowitz, Prof. Dr. M. J. MacLachlan Department of Chemistry, University of British Columbia 2036 Main Mall, Vancouver, BC, V6T 1Z1 (Canada) E-mail: [email protected]