Cellulose–Silica Nanocomposite Aerogels by In Situ Formation of Silica in Cellulose Gel

Abstract

Aerogels with their low density (0.004-0.500 g cm À3 ), large internal surface area, and large open pores are promising candidates for various advanced applications.[1] The utilization of inorganic aerogels, however, has been hampered by their poor mechanical properties. A prominent example is silica aerogel, which is prepared by an organic sol-gel process, [2] and has unique features, such as ultralow density (the lightest silica aerogel has a density that is similar to the density of air, which is 0.00129 g cm À3 ), near transparency, and low thermal conductivity. However, the extreme fragility of this aerogel necessitates its reinforcement for practical uses. A typical method is hybridization with organic polymers, such as polyurea, polyurethane, poly(methyl methacrylate), polyacrylonitrile, and polystyrene. [3] Other candidates for the reinforcement of inorganic aerogels are insoluble polysaccharides, which are abundantly available and show wide varieties in structure and properties.[4] The useful features of these compounds are hydrophilicity, biocompatibility, hydroxy reactivity, and reasonable thermal and mechanical stabilities.[5] For example, nanofibrillar bacterial cellulose and microfibrillated cellulose gel have been proposed as templates for cobalt ferrite nanoparticles and titanium dioxide. [6] While in the above-mentioned work native cellulose with cellulose I crystallinity was used, cellulose can be prepared as a hydrogel with cellulose II crystallinity through dissolution and coagulation. Some of the resulting aerogels have remarkable mechanical strength and light transmittance.[7] They have high porosity with open structures and thus provide an effective substrate for the synthesis of metallic nanoparticles.[8] To further utilize the regenerated cellulose gel, we herein attempted in situ synthesis of silica in cellulose gels.While a similar attempt has been reported, in which the cellulose gel was obtained from solution in N-methylmorpholine-N-oxide monohydrate, [9] the development of the nanostructure (nitrogen BET surface area of 220-290 m 2 g À1 ) and the level of silica loading (less than 13 % w/w) were rather limited. By using the aqueous alkali-based solvent, we obtained the cellulose aerogel with a surface area of 356 m 2 g À1 , and a silica loading of more than 60 % w/w resulted in surface areas that exceeded 600 m 2 g À1 . We used the sol-gel synthesis method toward nanostructured silica, which typically starts from tetraethyl orthosilicate (TEOS). The resulting composite gels were dried with supercritical CO 2 to give cellulose-silica aerogels with low density, moderate light transmittance, a large surface area, high mechanical integrity, and excellent heat insulation. This method can also lead to fabrication of silica-only aerogels through the removal of cellulose by calcination, that is, the use of cellulose aerogel as sacrificial template. Figure 1 shows the preparation of the aerogel. The cellulose hydrogel is a transparent material that has a water content of 92 % and a poro...

Strong, Thermally Superinsulating Biopolymer–Silica Aerogel Hybrids by Cogelation of Silicic Acid with Pectin

“…[23] Pure pectin aerogels display an etwork of polymer "strands" or "nanofibers" with diameters of af ew tens of nanometers primarily with mesopores and small macropores. [19] The hybrids gelled at pH 1.5 do not show evidence of visible pectin "fibers" at all studied pectin concentrations (Figure 1a,d,g). At this pH, pectin most probably did not completely gel within the preparation time.H ybrid aerogels prepared at pH 3a nd pH 5s how ac oarser microstructure than aerogels prepared at pH 1.5 for ag iven pectin loading.…”

Comparative Study of Aerogels Obtained from Differently Prepared Nanocellulose Fibers

“…Among them, NCF aerogels are obtained from globally abundant and renewable sources. [28][29][30] The long and entangled NCFs of cellulose I crystal type can This article describes the fabrication of nanocellulose fibers (NCFs) with different morphologies and surface properties from biomass resources as well as their self-aggregation into lightweight aerogels. By carefully modulating the nanofibrillation process, four types of NCFs could be readily fabricated, including long aggregated nanofiber bundles, long individualized nanofibers with surface C 6 -carboxylate groups, short aggregated nanofibers, and short individualized nanofibers with surface sulfate groups.…”

Fabrication of cellulose aerogel from wheat straw with strong absorptive capacity

“…However, a suitable method to dissolve cellulose owing to its high crystallinity and polymerization (DP) as well as ultra-strong inter-and intramolecular hydrogen bonds is pivotal for fabricating an environmentally sustainable aerogel [17,18]. Many effective cellulose solvents have been proposed such asionic liquid [19][20][21], N-methylmorpholine-N-oxide (NMMO) [22], lithium chloride/N,N-dimethylacetamide (LiCl/ DMAC) [23], NaOH/urea [24,25] and NH 3 /NH 4 SCN [26]. Nevertheless, most of these are limited by their high cost, toxicity or volatility.…”