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Several literature reports describe the role of aqueous solutions of N-methylmorpholine-N-oxide monohydrate (NMMO) as a suitable medium for the generation of transition metal (nano)particles in or on cellulosic materials and further elaborate its role as a co-reactant of the transition metal salts that are reduced to the elemental metal. However, this would assign NMMO the role of a reductant, which is in contradiction of its obvious oxidative nature. In the present study, the exemplary cases of silver, gold, and platinum salts as the precursors of the respective metal (nano)particles in aqueous NMMO/cellulose mixtures were investigated. Naturally, NMMO did not act as a reducing agent in any case—this role was taken over by the frequently used NMMO stabilizer propyl gallate, or by cellulose itself, into which carbonyl and carboxyl groups were introduced. Also, hypochlorite—produced intermediately from chloride ions and subsequently undergoing disproportionation into chloride and chlorate—or transient N-methylene(morpholinium) ions generated from NMMO, which are in turn oxidized to formyl morpholide, can act as the corresponding reductants while the metal ions are reduced, depending on the reaction conditions. Apart from providing interesting mechanistic insights, the study points to the importance of a precise description of the composition of the chemical systems used, as well as the importance of seemingly inert auxiliaries, which turned out to be essential co-reactants in the metal (nano)particle generation.Graphic abstract

Recently, a new member of the nanocellulose family was introduced, a cellulose II gel consisting of nanostructured and spherical particles. In this study, we compared two different drying techniques to obtain highly porous powders from this gel with preserved meso- and macroporous nanostructure: first, freeze-drying after solvent exchange to t BuOH and second, supercritical drying of the respective EtOH alcogel. The approaches yielded aerogel powders with surface areas of 298 and 423 m 2 /g, respectively. Both powders are amphiphilic and possess energetically heterogeneous surfaces with dominating dispersive term of the surface energy in the range of 50–52 mJ/m 2 , as determined by a combination of physicochemical surface characterization techniques, such as iGC, BET and SEM. Despite the lower surface area, the cheaper and more widespread method, freeze-drying, yields a more polar and reactive cryogel.

The effective and straight-forward modification of nanostructured celluloses under aqueous conditions or as “never-dried” materials is challenging. We report a silanization protocol in water using catalytic amounts of hydrogen chloride and then sodium hydroxide in a two-step protocol. The acidic step hydrolyzes the alkoxysilane to obtain water-soluble silanols and the subsequent addition of catalytic amounts of NaOH induces a covalent reaction between cellulose surficial hydroxyl groups and the respective silanols. The developed protocol enables the incorporation of vinyl, thiol, and azido groups onto cellulose fibers and cellulose nanofibrils. In contrast to conventional methods, no curing or solvent-exchange is necessary, thereby the functionalized celluloses remain never-dried, and no agglomeration or hornification occurs in the process. The successful modification was proven by solid state NMR, ATR-IR, and EDX spectroscopy. In addition, the covalent nature of this bonding was shown by gel permeation chromatography of polyethylene glycol grafted nanofibrils. By varying the amount of silane agents or the reaction time, the silane loading could be tuned up to an amount of 1.2 mmol/g. Multifunctional materials were obtained either by prior carboxymethylation and subsequent silanization; or by simultaneously incorporating both vinyl and azido groups. The protocol reported here is an easy, general, and straight-forward avenue for introduction of anchor groups onto the surface of never-dried celluloses, ready for click chemistry post-modification, to obtain multifunctional cellulose substrates for high-value applications.

The agglomeration of cellulosic materials upon drying, often called hornification, causes a reduction of water retention, among other undesired effects. It is one of the main issues in industrial cellulose processing, especially with regard to nanocelluloses. As a consequence, high transportation and storage costs arise since nanocelluloses need to remain in aqueous suspensions unless trade-offs in reactivity, redispersibility and surface properties are accepted. In this study, different drying strategies for TENCEL ® gel, a nanostructured gel derived from the Lyocell process consisting of spherical particles, are compared and evaluated. First, freeze-drying with consideration of the influence of freezing temperature and the use of tert -butanol as cryo-protectant, and second, simple oven-drying at 60 °C. Surprisingly, oven-dried xerogels showed higher water retention values and also better colloidal stability than the cryogels. This is in stark contrast to cellulose nanofibrils for which freeze-drying has been shown to be significantly superior to oven drying in terms of redispersibility. For the TENCEL ® gel, oven-drying was thus selected and the influence of additives on the redispersibility of the cellulose II gel was studied by means of the common water retention value, particle size, colloidal stability, appearance of the redispersed gel and viscosity. The addition of the polysaccharides carboxymethyl cellulose or xanthan showed the most promising results with regard to redispersibility. Also sucrose and ammonium bicarbonate provided higher colloidal stabilities than that of the untreated TENCEL ® gel. The redispersibility of the cellulose II xerogels could thus be significantly improved by simple and cost-efficient mixing with additives prior to drying.

Environmentally friendly insulation materials based on renewable resources come more and more to the fore, replacing less sustainable materials, such as mineral wool or polystyrene foams. In this study, we present a straight-forward avenue to a highly porous cellulose II powder for application as sound absorber. The material was prepared from a lyocell-derived cellulose II gel by a freeze-drying approach. The powder has a porosity of 98% and a specific surface area of 177 m 2 /g. It featured a sound absorption maximum in the frequency range of one-third-octave bands of 400–800 Hz (65 mm thickness) and was significantly more efficient as acoustic insulation material than commercial alternatives. The high performance makes the cellulose II sound absorber especially attractive for a use in loudspeakers for small electronics. Graphical Abstract