Article Cite This: ACS Appl. Nano Mater. 2018, 1, 6701−6710 www.acsanm.org Aqueous Synthesis of Compressible and Thermally Stable Cellulose fi − Nano bril Silica Aerogel for CO2 Adsorption † Feng Jiang, Sixiao Hu, and You-lo Hsieh* Fiber and Polymer Science, University of California, Davis, California 95616, United States *S Supporting Information ABSTRACT: Cellulose nanofibrils (CNF)−silica aerogels have been facilely synthesized via a one-step in situ aqueous sol−gel process of polymerizing and aging the silica precursor in the presence of CNFs to encompass the superior dry compressive strength and flexibility of CNF aerogels and the thermal stability of silica aerogels. Sodium silicate (Na2SiO3) was hydrolyzed and polymerized in the presence of CNFs at varied ratios to synthesize hydrogels whose storage and loss modulus confirmed CNFs to function as the structural skeleton. At the optimal 8:2 CNFs/Na2SiO3 composition, the hydrogels with homogeneously dispersed silica and CNF can be freeze-dried into hierarchically mesoporous aerogels with ultralow density of 7.7 mg/cm3, high specific surface of 342 m2/g, and pore volume of 0.86 cm3/g. This robust sol−gel approach employs naturally abundant silica and cellulose in aqueous system to generate improved CNF−silica aerogels that had much higher compressive strength and modulus of up to 28.5 and 177 kPa and structural flexibility than silica aerogel and enhanced thermal stability and specific surface over CNF aerogel. Further − functionalization of CNF silica aerogels via organosilane reaction introduced primary amine groups capable of capturing CO2 with an adsorption capacity of 1.49 mmol/g. fi − KEYWORDS: cellulose nano bril, silica, aerogel, sol gel synthesis, CO2 adsorption ■ INTRODUCTION dissolving cellulose in N-methylmorpholine N-oxide,22 LiOH− 20,24 − 23 − 25 Aerogels are ultralight and highly porous materials that can be urea, NaOH thiourea, NaOH ZnO, or ionic liquid (1-ethyl-3-methylimidazolium acetate)−dimethyl sulfoxide made up of varied organic or inorganic materials and are often 21 characterized by low thermal and acoustic conductivity owing (DMSO), then regenerating in a nonsolvent. The disadvan- 1,2 − to the over 99% trapped air. Silica and cellulose, the most tages of this gelation impregnation method include the abundant inorganic and polymer on earth respectively, have copious organic solvents needed for both cellulose dissolution − been extensively investigated for aerogel preparation.3 8 Silica and regeneration, and the loss of native cellulose I crystal and cellulose aerogels, albeit sharing many of the same general structure from dissolution. aerogel characteristics, differ due to their intrinsic material More recently, the fabrication of cellulose aerogels from ff Downloaded via UNIV OF CALIFORNIA DAVIS on March 20, 2019 at 16:59:00 (UTC). property discrepancy, most notably di erent in their thermal aqueous nanocellulose suspension has been shown to be more − and mechanical properties. Silica aerogel is highly thermally promising and green.12,13,26 31 Similarly, nanocellulose−silica See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. stable but extremely brittle to collapse under gentle pressure or aerogel can also be prepared by immersing nanocellulose 9,10 even capillary forces. In contrast, cellulose aerogels are aerogels cross-linked by Kymene resin,32,33 bacterial cellulose ductile and flexible, capable of withstanding compressive 34,35 fi 11 hydrogel 3D network, and silylated nano brillated deformation of up to 80% and even more resilient in wet cellulose bioscaffold36 in silica precursor. In contrast to the compressive strength12,13 butdecomposesatmoderately − ° two-step gelation impregnation process, a one-step homoge- elevated temperatures of 300 C or less. neous dispersion and simultaneous gelation of nanocellulose To overcome the intrinsic brittleness of silica aerogels, it is and silica precursor are more attractive. However, current most common to incorporate flexible organic skeletons of practice mostly employs organic silica precursors, such as synthetic polymers, such as poly(hexamethylene diisocya- 37,38 39 9,14 15 16 17 TEOS and methyltrimethoxysilane, which requires re- nate), epoxy, polybenzobisthiazole, polystyrene, poly- 39 acrylonitrile,18 and resorcinol-formaldehyde19 to form organ- dispersing nanocellulose into a common DMSO or alcohol − solvent37,38 and therefore is more expensive and environ- ic inorganic composite aerogels. Cellulose has also been 40 investigated in reinforcing silica aerogel by impregnating mentally unfavorable. preformed cellulose hydrogel/aerogel with silica precursors − such as tetraethyl orthosilicate (TEOS)20 23 or sodium silicate Received: August 30, 2018 solution24 in a two-step gelation−impregnation procedure. In Accepted: December 6, 2018 these practices, cellulose hydrogel is generally preformed by Published: December 6, 2018 © 2018 American Chemical Society 6701 DOI: 10.1021/acsanm.8b01515 ACS Appl. Nano Mater. 2018, 1, 6701−6710 ACS Applied Nano Materials Article Scheme 1. Synthesis of Aminated CNF/Silica Aerogel via Sodium Silicate Hydrolysis and Condensation the Presence of CNF To Form Hydrogel and Amination of Aerogel Toward a more streamlined integration and green process, a purification. All water used was purified using a Milli-Q plus water facile one-step sol−gel process involving all aqueous purification system (Millipore Corporate, Billerica, MA). dispersible precursors was developed to fabricate cellulose Fabrication of CNF−Silica Aerogel. CNF−silica aerogel was fi − synthesized via a sol−gel process using TEMPO oxidized cellulose nano brils (CNFs) silica aerogel. The synergistic gelation of fi CNF and sodium silicate in aqueous suspension is conceptually nano brils and aqueous sodium silicate as precursors (Scheme 1). different from prior works that involved multiple steps or Aqueous sodium silicate was hydrolyzed into silicic acid, which polymerizes into polysilicic acid and then condenses to silica (Scheme organic silica precursors as discussed in previous sections. 1).43 Aqueous sodium silicate (0.6 wt %) with pH adjusted to 5 by Silica aerogels have been produced from sodium silicate, an adding 1 mol/L HCl was freshly prepared and mixed with 0.6 wt % inexpensive and water-soluble silicon source41 alone or in 25 CNFs at 9:1, 8:2, 6:4, and 4:6 (by volume) CNF/sodium silicate mixture with cellulose dissolved in NaOH. The fact that ratios. CNF−silica hydrogel was formed by aging each mixture (10 aqueous sodium silicate readily gels makes it ideal for mL) for 48 h at ambient condition, then washing with 0.2 mol/L HCl. hybridizing with cellulose nanofibrils in aqueous suspension. These CNF−silica hydrogels were solvent exchanged to tert-butanol, The gelation of CNFs with sodium silicate was probed by their frozen (−20 °C), and then freeze-dried (−50 °C, 0.05 mbar) in a viscoelastic behaviors over time to elucidate the interaction freeze-drier (FreeZone 1.0 L benchtop freeze-dry system, Labconco, between CNF and silica as well as the hydrogel formation. The Kansas City, MO). For surface amination, the 4:6 CNF−silica gradual growth of silica nanoparticles on CNFs is observed hydrogel (10 mL) was solvent exchanged with anhydrous ethanol into − under atomic force microscope (AFM), and the derived alcogel, saturated with APTES in 10 30 wt % APTES/ethanol (10 aerogel was systematically characterized for the morphologies, mL) for 24 h, and then reacted with water in freshly prepared ethanol (10 mL) containing 0.6−1.8 mL of water (with respect to 10−30 wt chemical structure and composition, specific surface area and % APTES) for 24 h. The amine-functionalized alcogel was then pore size, mechanical properties, and thermal stability. In solvent exchanged to tert-butanol and freeze-dried as previously addition, the silica in the aerogel introduces silanol groups to described. facilitate reaction with organic silane to add novel function- Characterization. The dynamic rheological properties of CNF− alities to the aerogels, which has been demonstrated to react silica hydrogels were measured at 25 °C using a AR1000-N rheometer with (3-aminopropyl)triethoxysilane (APTES) to introduce (TA Instruments) equipped with two parallel acrylic plates (20 mm amine groups for CO2 adsorption. While CO2 adsorption by diameter) separated by 2 mm. The oscillatory time sweep of each APTES is a typical method, the preparation of dry resilient fresh CNF and sodium silicate mixture was carried out under a CNF/silica aerogel using this new and sustainable method is constant 2 Pa oscillation stress and 1 Hz frequency. The oscillatory novel, and the CNF scaffold could offer the typically brittle frequency sweep of cured CNF−silica hydrogel was conducted under pure silica gel dry strength and yet very low density from a constant oscillation stress of 1 Pa. sodium silicate precursor. The progressive growth of silica nanoparticles onto CNFs during static aging was observed by atomic force microscopy (AFM, Asylum- Research MFP-3D) at 5, 24, and 48 h. Each respective gel (10 μL) ■ EXPERIMENTAL SECTION was diluted in 10 mL of water and sonicated for 5 min (Branson 2510, μ Materials. Cellulose nanofibrils (CNFs) were derived from Branson Ultrasonics Corp.), and then 10 L of the dispersed hydrogel cellulose isolated from rice straw by 2,2,6,6-tetramethylpyperidine-1- was deposited onto a freshly cleaved mica surface and air-dried. AFM oxyl (TEMPO) mediated oxidation employing 5 mmol/g NaClO/ was scanned