Loading of Bacterial Cellulose Aerogels with Bioactive Compounds by Antisolvent Precipitation with Supercritical Carbon Dioxide

Bacterial cellulose aerogels overcome the drawback of shrinking during preparation by drying with supercritical CO₂. Thus, the pore network of these gels is fully accessible. These materials can be fully rewetted to 100% of its initial water content, without collapsing of the structure due to surface tension of the rewetting solvent. This rehydration property and the high pore volume of these material rendered bacterial cellulose aerogels very interesting as controlled release matrices. Supercritical CO₂ drying, the method of choice for aerogel preparation, can simultaneously be used to precipitate solutes within the cellulose matrix and thus to load bacterial cellulose aerogels with active substances. This process, frequently termed supercritical antisolvent precipitation, is able to perform production of the actual aerogel and its loading in one single preparation step. In this work, the loading of a bacterial cellulose aerogel matrix with two model substances, namely dexpanthenol and L-ascorbic acid, and the release behavior from the matrix were studied. A mathematical release model was applied to model the interactions between the solutes and the cellulose matrix. The bacterial cellulose aerogels were easily equipped with the reagents by supercritical antisolvent precipitation. Loading isotherms as well as release kinetics indicated no specific interaction between matrix and loaded substances. Hence, loading and release can be controlled and predicted just by varying the thickness of the gel and the solute concentration in the loading bath.     

Dry,hydrophobic microfibrillated cellulose powder obtained in a simple procedure using alkyl ketene dimer

In order to produce dry and hydrophobic microfibrillated cellulose (MFC) in a simple procedure, its modification with alkyl ketene dimer (AKD) was performed. For this purpose, MFC was solvent-exchanged to ethyl acetate and mixed with AKD dissolved in the same solvent. Curing at 130 °C for 20 h under the catalysis of 1-methylimidazole yielded a dry powder. Scanning electron microscopy of the powder indicated loss in nanofibrillar structure due to aggregation, but discrete microfibrillar structures were still present. Water contact angle measurements of films produced from modified and unmodified MFC showed high hydrophobicity after AKD treatment, which persisted even after extraction with THF for 8 h. The hydrophobized MFC was characterized by Fourier transform infrared spectroscopy, nuclear magnetic resonance and X-ray analysis. In summary, strong indications for the presence of AKD on the surface of MFC before and after extraction with solvent were found, but only a very small amount of covalent β-ketoester linkages between the modification agent and cellulose was revealed.     

Nanofibrillation of alkyl ketene dimer (AKD)-treated cellulose in tetrahydrofuran

A hardwood bleached kraft pulp (HBKP) was reacted with alkyl ketene dimer (AKD) under solvent-free and heterogeneous conditions, and a fibrous chloroform-insoluble fraction with a degree of substitution of AKD β-ketoester groups of 0.75 was obtained at a weight ratio of 95 % of the total reaction product. When this AKD-treated and CHCl₃-insoluble HBKP was mechanically disintegrated in tetrahydrofuran, the suspension turned to a viscous and translucent gel. When an unfibrillated fraction was removed by filtration using a glass filter with 40 μm pore size, the weight recovery ratio of the filter-passed fraction was approximately 30 %. Scanning and transmission electron microphotographs of the glass filter-passed fraction showed that this fraction consisted of cellulose nanofibrils more than several micrometers in length, the surfaces of which probably had AKD β-ketoesters in high densities.     

Influence of supercritical drying fluids on structures and properties of low-density Cu-doped SiO2 composite aerogels

The Cu-doped SiO₂ composite aerogels were successfully prepared by sol–gel process and subsequently supercritical drying with ethanol and CO₂. The Cu-doped SiO₂ composite aerogels had porous texture, low density (<100 mg cm^?3) and high specific surface area (>800 m² g^?1), which were investigated by FESEM and nitrogen adsorption desorption porosimetry. The FTIR spectra of the aerogels showed that the ethanol-dried aerogels had been modified by ethyl while the corresponding CO₂-dried aerogels had more Si–OH groups. The phase structure and thermal stability were investigated by XRD and TGA, respectively. Due to the reducibility of ethanol, the copper was crystalline in ethanol-dried sample. The Cu-doped SiO₂ composite aerogels dried with supercritical ethanol had larger pore diameter and better thermal stability under 400 °C in comparison with CO₂-dried composite aerogels. The structures and properties of Cu-doped SiO₂ composite aerogels are obviously affected by supercritical drying conditions. The effect research could instruct the synthesis of different state of Cu in composite aerogels.     

Cellulose–silica composite aerogels from “one-pot” synthesis

Cellulose–silica composite aerogels were prepared via “one-pot” process: aqueous solutions of cellulose–8 wt% NaOH and sodium silicate were mixed, coagulated and dried with supercritical CO₂. The system was studied both in the fluid and solid (dry) states. Cellulose and sodium silicate solutions were mixed at different temperatures and concentrations; mixture properties were monitored using dynamic rheology. The gelation time of the mixture was strongly reduced as compared to that of cellulose–NaOH solutions; we interpret this phenomenon as cellulose self-aggregation inducing partial coagulation due to competition for the solvent with sodium silicate. The gelled cellulose/sodium silicate samples were placed in aqueous acid solution which completed cellulose coagulation and led to in situ formation of sub-micronic silica particles trapped in a porous cellulose matrix. After drying with supercritical CO₂, an organic–inorganic aerogel composite was formed. The densities obtained were in the range of 0.10–0.25 g/cm³ and the specific surface area was between 100 and 200 m²/g. The silica phase was shown to have a reinforcing effect on the cellulose aerogel, increasing its Young’s modulus.     

Sequestration of CO2 using Cu nanoparticles supported on spherical and rod-shape mesoporous silica

The CO₂ sequestration is one of the most promising solutions to tackle global warming. In this study, spherical mesoporous silica particles (MPS-S) and rod-shaped mesoporous silica particles (MPS-R) loaded with Cu nanoparticles were selectively prepared and employed for CO₂ adsorption. For the first time uniform Cu nanoparticles were incorporated into the rod-shaped mesoporous silica particles by post-synthesis modification using both N-[3-(trimethoxysilyl)propyl]ethylenediamine (PEDA) and ethylenediamine (EDA) as coupling agents. The physiochemical properties of the mesoporous and copper grifted silica composites were investigated by CHN elemental analysis, FTIR spectroscopy, thermogravimetric analysis, X-ray diffraction, energy dispersive X-ray spectroscopy (EDX), surface area analysis, scanning, transmission electron microscopy and gas analysis system (GSD 320, TERMO). The mesoporous silica shows highly ordered mesoporous structures, with the rod-shaped particles having a higher surface area than the spherical ones. Copper nanoparticles with an average diameter of 6.0 nm were uniformly incorporated into the MPS-S and MPS-R. Moreover, Cu-loaded mesoporous silica exhibits up to 40% higher CO₂ adsorption capacity than the bare MPS. The MPS-R modified with Cu nanoparticles showed a maximum CO₂ adsorption capacity of 0.62 mmol/g and the humidity showed a slight negative effect on CO₂ uptake process. The enhancement of CO₂ adsorption onto transition metal/mesoporous substrates provides basis for imminent CO₂ sequestration.     

The Importance of Precursors and Modification Groups of Aerogels in CO2 Capture

The rapid growth of CO₂ emissions in the atmosphere has attracted great attention due to the influence of the greenhouse effect. Aerogels’ application for capturing CO₂ is quite promising owing to their numerous advantages, such as high porosity (~95%); these are predominantly mesoporous (20–50 nm) materials with very high surface area (>800 m²∙g⁻¹). To increase the CO₂ level of aerogels’ uptake capacity and selectivity, active materials have been investigated, such as potassium carbonate, K₂CO₃, amines, and ionic-liquid amino-acid moieties loaded onto the surface of aerogels. The flexibility of the composition and surface chemistry of aerogels can be modified intentionally—indeed, manipulated—for CO₂ capture. Up to now, most research has focused mainly on the synthesis of amine-modified silica aerogels and the evaluation of their CO₂-sorption properties. However, there is no comprehensive study focusing on the effect of different types of aerogels and modification groups on the adsorption of CO₂. In this review, we present, in broad terms, the use of different precursors, as well as modification of synthesis parameters. The present review aims to consider which kind of precursors and modification groups can serve as potentially attractive molecular-design characteristics in promising materials for capturing CO₂.     

Soy protein–nanocellulose composite aerogels

Organic aerogels based on two important and widely abundant renewable resources, soy proteins (SP) and nanofibrillar cellulose (NFC) are developed from precursor aqueous dispersions and a facile method conducive of channel- and defect-free systems after cooling and freeze-drying cycles that yielded apparent densities on the order of 0.1 g/cm³. NFC loading drives the internal morphology of the composite aerogels to transition from network- to fibrillar-like, with high density of interconnected cells. Composite aerogels with SP loadings as high as ca. 70 % display a compression modulus of 4.4 MPa very close to that obtained from reference, pure NFC aerogels. Thus, the high compression modulus of the composite system is not compromised as long as a relatively low amount of reinforcing NFC is present. The composite materials gain moisture (up to 5 %) in equilibrium with 50 % RH air, independent of SP content. Furthermore, their physical integrity is unchanged upon immersion in polar and non-polar solvents. Fast liquid sorption rates are observed in the case of composite aerogels in contact with hexane. In contrast, water sorption is modulated by the chemical composition of the aerogel, with an important contribution from swelling. The potential functionalities of the newly developed SP–NFC composite green materials can benefit from the reduced material cost and the chemical features brought about the amino acids present in SPs.     

Investigation of the cellulose ethers effect on the Portland cement hydration by thermal analysis

Cellulose ethers (CE) are introduced in almost all cement-based dry mortars in order to retain water in mortar mass avoiding losing it too quickly by substrate absorption or water evaporation. In this way the workability of the fresh material, the adherence to the substrate and internal-strength characteristics of mortar, render or tile adhesive are improved. One of the side effects of cellulose ethers is the Portland cement hydration delaying. The influence of six commercial cellulose ethers, hydroxyethylmethyl cellulose (HEMC) type, on the hydration of Portland cement CEM I 42.5 R, was followed by thermal analysis (TG and DTA curves). Three of these cellulose ethers are unmodified, and have different viscosities, while three of them have the same viscosity but differ in the degree of modification (unmodified, one with medium modification and one with high modification). The interest of dry mortars producers for the effects of these cellulose ethers, is generated by the wide offer available on the market and by the absence of systematic data on the effect of different viscosities and degrees of modification on dry mortars properties. In order to quantify the effect of the CE on the cement hydration, the surface area of the endothermic effect corresponding to the dehydration of portlandite (Ca(OH)₂), formed after 1, 3, and 7 days of hydration, was defined. It was noted that the proportion of Ca(OH)₂ in samples containing CE after 1 day was 30–40 % lower than in reference sample. After 3 and 7 days of hydration the proportion of Ca(OH)₂ in samples containing CE approaches that of reference sample (10–20 % less). For the same period of hydration, the different viscosity, and different degree of modification of cellulose ethers cause variations in narrow limits of the proportion of Ca(OH)₂, and the degree of cement hydration, respectively.     

Preparation of piperazine-grafted amine-functionalized UiO-66 metal organic framework and its application for CO<Subscript>2</Subscript> over CH<Subscript>4</Subscript> separation

UiO-66 amine functionalized was synthesized by solvothermal method. Post-synthetic modification of UiO-66-NH₂ with piperazine, a known promoter to enhance the chemisorption rate of CO₂ uptake, was carried out and analyzed to understand its crystalline structure, morphology and porous structure. Results show that piperazine is an effective agent for enhancing the capacity of absorption of CO₂. This porous product exhibits an improved CO₂ uptake at pressures up to 3000 kPa via physisorption and chemisorption mechanisms. The CH₄ adsorption and desorption isotherms on UiO-66, UiO-66-NH₂ and pip-UiO-66-NH₂ at temperature of 298.15 K and pressures ranging from 0 to 5000 kPa were carried out. IAS theory for a mixture of 0.05 bar CO₂, 0.85 bar CH₄ and 0.1 bar other gas revealed a selectivity factor of 19.09 for CO₂/CH₄ from pip-UiO-66-NH₂. Results show that these materials are effective adsorbents for CO₂ and CH₄ uptakes.     

Fluorescent cellulose aerogels containing covalently immobilized (ZnS)x(CuInS2)1−x/ZnS (core/shell) quantum dots

Photoluminiscent (PL) cellulose aerogels of variable shape containing homogeneously dispersed and surface-immobilized alloyed (ZnS)_x(CuInS₂)_1?x/ZnS (core/shell) quantum dots (QD) have been obtained by (1) dissolution of hardwood prehydrolysis kraft pulp in the ionic liquid 1-hexyl-3-methyl-1H-imidazolium chloride, (2) addition of a homogenous dispersion of quantum dots in the same solvent, (3) molding, (4) coagulation of cellulose using ethanol as antisolvent, and (5) scCO₂ drying of the resulting composite aerogels. Both compatibilization with the cellulose solvent and covalent attachment of the quantum dots onto the cellulose surface was achieved through replacement of 1-mercaptododecyl ligands typically used in synthesis of (ZnS)_x(CuInS₂)_1?x/ZnS (core–shell) QDs by 1-mercapto-3-(trimethoxysilyl)-propyl ligands. The obtained cellulose—quantum dot hybrid aerogels have apparent densities of 37.9–57.2 mg cm^?3. Their BET surface areas range from 296 to 686 m² g^?1 comparable with non-luminiscent cellulose aerogels obtained via the NMMO, TBAF/DMSO or Ca(SCN)₂ route. Depending mainly on the ratio of QD core constituents and to a minor extent on the cellulose/QD ratio, the emission wavelength of the novel aerogels can be controlled within a wide range of the visible light spectrum. Whereas higher QD contents lead to bathochromic PL shifts, hypsochromism is observed when increasing the amount of cellulose at constant QD content. Reinforcement of the cellulose aerogels and hence significantly reduced shrinkage during scCO₂ drying is a beneficial side effect when using α-mercapto-ω-(trialkoxysilyl) alkyl ligands for QD capping and covalent QD immobilization onto the cellulose surface.     

Abatement of Gaseous Xylene Using Double Dielectric Barrier Discharge Plasma with In Situ UV Light: Operating Parameters and Byproduct Analysis

Ultraviolet (UV) light with a wavelength of 254 nm was applied to a double dielectric barrier discharge (DDBD) system to decompose of gaseous xylene. The results show that a significantly synergistic effect can be achieved with the introduction of UV light into the DDBD system. When UV light is applied, the system show a 21.8 % increase in its removal efficiency for xylene at 35 kV with an ozone concentration close to 971 ppmv. The CO _x (x = CO₂ and CO) selectivity of outlet gas rises from 6.54 to 76.2 %. The optimal synergetic effect between UV light and DDBD can be obtained at a peak voltage of 30 kV. The system is robust for humidity, which only slightly reduces the xylene removal efficiency at a high peak voltage (30–35 kV). With the increase of gas flow rate, the removal efficiency for xylene decreases due to a reduced residence time. In addition, the products of xylene degradation were also analyzed. The major products of the degradation were found to be CO₂ and H₂O while byproducts such as O₃ and HCOOH were observed as well.     

A Petrochemical-Free Route to Superelastic Hierarchical Cellulose Aerogel

Cellulose aerogels are plagued by intermolecular hydrogen bond-induced structural plasticity, otherwise rely on chemicals modification to extend service life. Here, we demonstrate a petrochemical-free strategy to fabricate superelastic cellulose aerogels by designing hierarchical structures at multi scales. Oriented channels consolidate the whole architecture. Porous walls of dehydrated cellulose derived from thermal etching not only exhibit decreased rigidity and stickiness, but also guide the microscopic deformation and mitigate localized large strain, preventing structural collapse. The aerogels show exceptional stability, including temperature-invariant elasticity, fatigue resistance (∼5 % plastic deformation after 10⁵ cycles), high angular recovery speed (1475.4° s⁻¹), outperforming most cellulose-based aerogels. This benign strategy retains the biosafety of biomass and provides an alternative filter material for health-related applications, such as face masks and air purification.     

Viscosity reduction of cellulose + 1-butyl-3-methylimidazolium acetate in the presence of CO2

Viscosities of microcrystalline cellulose + 1-butyl-3-methylimidazolium acetate ([bmIm][Ac]) solutions (0.6–1.2 wt%) in contact with CO₂ were measured at 312 K with a resonant vibrational viscometer. At 4 MPa and 312 K, the CO₂ could reduce the viscosity of 1.2 wt% cellulose + [bmIm][Ac] solution by about 80 %, whereas N₂ at the same conditions gave less than a 10 % reduction in viscosity. The viscosity-averaged degree of polymerization and IR spectrum showed that cellulose did not decompose during experiments and that [bmIm][Ac] acted as a non-derivatizing solvent during the dissolution and viscosity reduction process. Further, although CO₂ does react with [bmIm][Ac] to form 1-butyl-3-methylimidazolium-2-carboxylate, the reaction seems to be reversible and it does not affect the cellulose. Thus, [bmIm][Ac] with CO₂ provides an effective solvent for cellulose and the solvent system can probably be recycled or reused.     

Enzymatic Hydrolysis and Succinic Acid Fermentation from Steam-Exploded Corn Stalk at High Solid Concentration by Recombinant Escherichia coli

Steam-exploded corn stalk biomass was used as the substrate for succinic acid production via lignocellulose enzymatic hydrolysis and fermentation. Succinic acid fermentation was investigated in Escherichia coli strains overexpressing cyanobacterium Anabaena sp. 7120 ecaA gene encoding carbonic anhydrase (CA). For the washed steam-exploded corn stalk at 30 % substrate concentration, i.e., 30 % water-insoluble solids (WIS), enzymatic hydrolysis yielded 97.5 g/l glucose solution and a cellulose conversion of 73.6 %, thus a high succinic acid level up to 38.6 g/l. With the unwashed steam-exploded corn stalk, though a cellulose conversion of 71.2 % was obtained in hydrolysis at 30 % solid concentration (27.9 % WIS), its hydrolysate did not ferment at all, and the hydrolysate of 25 % solid loading containing 3.8 g/l acetic acid and 168.2 mg/l furfural exerted a strong inhibition on succinic acid production.     

Utilization of a biphasic oil/aqueous cellulose nanofiber membrane bioreactor with immobilized lipase for continuous hydrolysis of olive oil

A simple method is proposed to fabricate a biphasic lipase-immobilized cellulose membrane bioreactor with high enzyme loading and activity retention. This bioreactor was assembled with electrospun cellulose nanofiber membranes that were fixed in a spiral form and wound to increase their specific surface area. To improve the catalytic efficiency of the immobilized enzymes, the supports went through alkaline hydrolysis, NaIO₄ oxidation and pentaethylenehexamine modification before covalently binding the lipase. Enzyme loading could reach 28.9 mg/g with the highest activity retention of 44.3 % for the immobilized lipases. The effects of the operational variables, namely the organic phase flow rate, aqueous phase flow rate and substrate concentration, on the performance of this bioreactor were investigated with continuous hydrolysis of olive oil. It was found that under optimum operational conditions, 100 % hydrolysis conversion of olive oil was achieved after 9 organic phase circulations at 10.5 mL/min organic phase flow rate, 600 mL/min aqueous phase flow rate and using a substrate of pure olive oil. Nanofiber membrane bioreactors offer potential as applications for various lipase-catalyzing reactions in industrial productions.     

Preparation and characterization of ZrCO/C composite aerogels

Zr-containing organic aerogels were synthesized by ligand substitution reaction of polyzirconoxone and 2, 4-dihydroxybenzoic acid, followed by polymerization with formaldehyde, and then supercritical drying using CO₂. After carbonization and carbothermal reduction under an argon atmosphere, ZrCO/C composite aerogels with controllable zirconium content (47.8–78.6 wt%) were obtained. The carbothermal reduction was substantially completed at 1,500 °C, and the obtained ZrCO/C composite aerogels exhibit low oxygen contents (9.4–6.7 wt%) and high surface areas (589–147 m²/g). Pore morphologies of the ZrCO/C composite aerogels were investigated in detail by nitrogen sorption measurements, scanning electron microscopy and its associated energy-dispersive X-ray microanalysis measurements. The results show that the aerogels are composed of carbon framework and Zr-conglomerations, and the surface area of aerogel is severely affected by its zirconium content. The presence of reductive ZrC crystals can greatly enhance the oxidation resistance ability of amorphous carbon framework and prevent collapse.     

Transparent and flame retardant cellulose/aluminum hydroxide nanocomposite aerogels

Cellulose-based nanocomposite aerogels were prepared by incorporation of aluminum hydroxide (AH) nanoparticles into cellulose gels via in-situ sol-gel synthesis and following supercritical CO₂ drying. The structure and properties of cellulose/AH nanocomposite aerogels were investigated by Fourier transform infrared spectroscopy, scanning electron microscopy, ultraviolet-visible spectrometry, N₂ adsorption, thermogravimetric analysis, and micro-scale combustion calorimetry. The results indicated that the AH nanoparticles were homogeneously distributed within matrix, and the presence of AH nanoparticles did not affect the homogeneous nanoporous structure and morphology of regenerated cellulose aerogels prepared from 1-allyl-3-methylimidazolium chloride solution. The resultant nanocomposite aerogels exhibited good transparency and excellent mechanical properties. Moreover, the incorporation of AH was found to significantly decrease the flammability of cellulose aerogels. Therefore, this work provides a facile method to prepare transparent and flame retardant cellulose-based nanocomposite aerogels, which may have great potential in the application of building materials.     

Hydrothermal saccharification of cotton cellulose in dilute aqueous formic acid solution

Cotton cellulose subjected to a dilute aqueous formic acid solution, at acid concentrations up to 1% (w/w), under hydrothermal conditions in a semi-batch reactor was converted into glucose and oligomers with lower degrees of polymerizations (DP). After heating at 250 °C for 60 min in 0.1% (w/w) aqueous formic acid solution, yields of glucose and total sugar with DP = 1 to 9 were 36.6 and 83.8% (100 × gC/gC of initial cotton sample), respectively, and 5-hydroxymethylfurfural was almost as low as 1%. The yields of glucose and oligomers were significantly improved by adding the acid. The reaction was represented by first-order reaction kinetics with regard to (1 ?C x) where x is the conversion based on the total sugar or glucose yield. At 250 °C, the differences in the rate constants (k ? k _water) were proportional to the square root of formic acid concentration.     

Controlled incorporation of deuterium into bacterial cellulose

Isotopic enrichment has been widely used for investigating the structural and dynamic properties of biomacromolecules to provide information that cannot be carried out with molecules composed of natural abundance isotopes. A media formulation for controlled incorporation of deuterium in bacterial cellulose synthesized by Gluconacetobacter xylinus subsp. sucrofermentans is reported. The purified cellulose was characterized using Fourier Transform Infra-Red spectrophotometry and mass spectrometry which revealed that the level of deuterium incorporation in the perdeuterated cellulose was greater than 90 %. Small-angle neutron scattering analysis demonstrated that the overall structure of the cellulose was unaffected by the substitution of deuterium for hydrogen. In addition, by varying the amount of D-glycerol in the media it was possible to vary the scattering length density of the deuterated cellulose. A large disk model was used to fit the curves of bacterial cellulose grown using 0 and 100 % D-Glycerol yielding a lower bound to the disk radii, R _min = 1,132 ± 6 and 1,154 ± 3 Å and disk thickness, T = 128 ± 1 and 83 ± 1 Å for the protiated and deuterated forms of the bacterial cellulose, respectively. This agrees well with the scanning electron microscopy analysis which revealed stacked sheets in the cellulose pellicles. Controlled incorporation of deuterium into cellulose will enable new types of experiments using techniques such as neutron scattering to reveal information about the structure and dynamics of cellulose and its interactions with proteins and other (bio) polymers.