Supercritical Carbon Dioxide Drying Research Articles

Preparation and thermal insulation properties of lightweight high-strength polyimide aerogels containing benzoazole structures

Polyimide (PI) aerogels, serving as thermal insulation materials in extreme environments, stand out for their exceptional high temperature stability and superb thermal insulation properties, rendering them ideal for aerospace, transportation, and specialized equipment industries. However, conventionally synthesized PI aerogels suffer from significant drawbacks in mechanical properties and thermal stability, limiting their practical applications. To overcome this challenge, we successfully synthesized polyimide aerogels using biphenyl-3,3,4,4′-tetracarboxylic dianhydride (BPDA), 5-amino-2-(4-aminophenyl)benzimidazole (PABZ), and 5-amino-2-(4-aminophenyl)benzoxazole (APBO) followed by supercritical carbon dioxide drying. These synthesized PIs exhibit remarkable thermal stability, with high thermal weight loss temperatures (Td5 %=595–611℃) and high glass transition temperatures. Additionally, these novel PI aerogels demonstrate exceptional thermal insulation properties, high specific surface area, and extremely small pore size. Furthermore, boasting a low dielectric constant (ε=1.72), these newly developed PI aerogels offer along with excellent mechanical properties, holding substantial promise for applications in the forthcoming generation of advanced aerospace and specialized equipment materials.

Regulating pore structure of aramid nanofiber (ANF) separators for lithium–sulfur (Li–S) batteries

Excellent ionic conductivity and mechanical robustness are significant for separators of Li–S batteries. Aramid nanofiber (ANF) has been widely used in separators due to their excellent mechanical properties and high-temperature resistance. However, pure ANF separator possesses a dense pore structure resulting from the closely intertwined nanofibrous network, leading to inferior ionic conductivity. Herein, we propose a strategy of inhibiting of hydrogen bonding (IHB) among nanofibers to regulate the pore structure of ANF separators via employing pore-forming agent, solvation, and differentiated drying methods. Notably, a graph theoretical methodology (structural GT) is introduced to analyze the percolating network of ANF separator, revealing that the higher average nodal connectivity, the more abundant and homogeneous porous structure and higher conductivity. Excitingly, the pore size and the ionic conductivity of ANF separator by supercritical carbon dioxide drying (S-ANFs) is 44 nm and 0.171 mS/cm, which is 5 times and 1.9 times higher than pure ANF separator, respectively. Moreover, the ANF separator is dimensionally stable under 200 °C, demonstrating its desirable security under extreme conditions. Finally, the half-cell equipped resultant S-ANFs exhibits outstanding cycling stability (566 mAh/g after 200 cycles at 0.5 C) and Coulombic efficiency (99.25%). This work provides an efficient strategy to regulate the pore structure of ANF separator.

In vitro digestion of starch and protein aerogels generated from defatted rice bran via supercritical carbon dioxide drying

This study investigated the in vitro digestibility of starch and protein aerogels produced from defatted rice bran (DRB), an underutilized rice processing byproduct, using supercritical carbon dioxide (SC-CO2) drying. The extracted starch (i.e., purified starch), crude starch, and proteins were used for the aerogel formation at 15% (w/w) concentration and further characterized. All aerogels exhibited three-dimensional open porous structures with high surface areas of 36–47 m2/g, densities lower than 0.3 g/cm3, and porosities higher than 84%. The starch hydrolyses in starch and crude starch aerogels were 86 and 73%, respectively, while the protein hydrolysis in protein aerogels reached up to 82% after sequential oral, gastric, and intestinal digestion. Thus, the hydrolysis rates achieved in simulated digestions suggest that the developed aerogels from DRB have the potential to serve as vehicles for delivering bioactive compounds and add value to the underutilized DRB.

Development and optimization of general drying models for cod (Gadus spp.) using supercritical carbon dioxide

This study aimed to investigate the supercritical carbon dioxide (SC-CO2) drying of cod (Gadus spp.) by developing and optimizing a general mathematical drying model. We applied a central composite rotatable design (CCRD) considering the time (180–360 min), temperature (35–45 °C), and CO2 flow rate (15–25 kg/h) as independent factors, aiming at the reduction of moisture and water activity (aw) of Gadus morhua. Time and flow rate (FR) were the main factors affecting the moisture and aw responses. The mathematical models developed were validated with the accuracy and bias factors and generalized using Gadus chalcogrammus samples, resulting in the acceptable range of 1.003–1.121. The optimal parameter values for drying performance were 312.38 min, 45.38 °C, and 27.4 kg/h (R2 = 0.999). Our results suggest a promising application of SC-CO2 as a fish drying technology, describing a generalist-validated and optimized SC-CO2 drying model for Gadus spp. Industrial applicationThe present study collaborates with understanding supercritical carbon dioxide (SC-CO2) as an innovative fish drying technology at a laboratory scale. The optimal variables that were effective in reducing cod fillets water content were 312.38 min, 45.38 °C, and 27.4 kg/h, in which 26.4% moisture and 0.815 aw were predicted. Therefore, SC-CO2 is suggested to be a promising non-thermal food drying technology applied to fish products.

Effect of different drying methods on the structure and properties of porous starch

In order to investigate the effects of different drying methods on the properties of porous starch. The present study used four drying methods, namely hot air drying (HD), spray drying (SPD), vacuum freeze drying (FD) and supercritical carbon dioxide drying (SCD) to prepare maize and kudzu porous starch. Findings indicated that the physicochemical properties (e.g., morphology, crystallinity, enthalpy value, porosity, surface area and water absorption capacity as well as dye absorption capacity, particle size) of porous starch were significantly affected by the drying method. Compared with other samples, SCD-treated porous starch exhibited the highest surface areas of the starch (2.943 and 3.139 m2/g corresponding to kudzu and maize, respectively), amylose content (22.02 % and 16.85 % corresponding to kudzu and maize, respectively), MB and NR absorption capacity (90.63 %, 100.26 % and 90.63 %, 100.26 %, corresponding to kudzu ad maize, respectively), and thermal stability, whereas HD-treated porous starch showed the highest water-absorption capacity (123.8 % and 131.31 % corresponding to kudzu and maize, respectively). The dye absorption of the maize and kudzu porous starch was positively correlated with surface area, according to Pearson's correlation analysis. Therefore, in this study, our aim was to explore the effects of different drying methods on the Structure and properties of porous starch, and provide reference for selecting the best drying method for its application in different fields.

Graphene aerogels: part 2 - derived from commercial graphene and chemically reduced graphene oxide via supercritical carbon dioxide drying

Graphene aerogels (GAs), the most important class of carbonaceous aerogels, have attracted attention of many researchers due to their superior physical and chemical properties. In this study, commercial graphene (GR) and chemically reduced graphene oxide (RGO) were used as graphene-based precursor materials, unlike graphene oxide (GO), which is widely used in the literature in GA synthesis. GAs were synthesized using the sol-gel technique and dried with supercritical carbon dioxide (SCCO2). In addition, graphene-based materials were used in different ratios and their distribution in the aerogel matrix and its effect on surface properties were investigated. In addition, the synthesized GAs were structurally compared with GR, RGO, and carbon aerogel (CA) without graphene-based materials. Physical characterizations (Brunauer, Emmett, and Teller (BET) analysis, scanning electron microscope-energy dispersive X-ray (SEM-EDX) analysis, micro-Raman spectroscopy, X-ray diffractometer (XRD) were made to examine the structural properties of GAs. In order to analyze the behavior of the surfaces of the synthesized materials against electrochemical corrosion, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analyses were performed. As a result of the electrochemical corrosion process of the synthesized materials, the change in their specific capacitance and the formation of pseudocapacitive charge on the surfaces were examined.

Graphene aerogels: part 1 - derived from graphene oxide and thermally reduced graphene oxide via supercritical carbon dioxide drying

Graphene aerogels have become promising materials in many areas of industry, especially in energy applications due to their superior physical and electrochemical properties. Generally, graphene oxide (GO)-derived aerogels (A) are synthesized by using the hydrothermal method. In this study, both GO and reduced graphene oxide (RGO)-derived aerogels were synthesized by using the sol-gel method coupled with the supercritical carbon dioxide (SCCO2) drying process. It aims to examine the changes in the structure of the final aerogel by changing the amount (0.25–0.5–1% wt.) and type of graphene-based precursor materials used in the synthesis. Physical characterizations of graphene aerogels were conducted using Brunauer-Emmett-Teller (BET) analysis, scanning electron microscope-energy dispersive X-ray (SEM-EDX) analysis, transmission electron microscopy (TEM), micro-Raman spectroscopy, X-ray diffractometer (XRD) to highlight their structural properties. Additionally, X-ray photoelectron spectroscopy (XPS) analyses were performed to determine the oxidation levels on the surface of the RGO-1 aerogel. The cyclic voltammetry (CV) method was used to examine the electrochemical behavior of the graphene aerogels against corrosion. Specific capacitance values of the synthesized materials were calculated before and after corrosion. Furthermore, the surface charge changes that occur after corrosion were examined. GOAs displayed the highest specific capacitance value among graphene aerogels. Notably, the RGOA-1 aerogel exhibited the highest corrosion resistance. The pseudo-capacitive charge ratio of RGOA-1 after corrosion was measured at 0.5 mC cm–2.

Formation of nanoporous aerogels from defatted rice bran via supercritical carbon dioxide drying

This study converts underutilized defatted rice bran into high-value nanoporous starch and protein aerogels using supercritical carbon dioxide drying. The generated aerogels can be used for the development of functional foods.

Superlight macroporous aerogels produced from cold-set egg white protein hydrogels show superior oil structuring capacity

In this study, a novel green cold gelation approach was developed to fabricate superlight macroporous aerogels from egg white protein (EWP). The cold-set hydrogels were prepared through preheating of EWP solution at high alkaline pH and then the gelation was induced by a food-grade crosslinker, glucose-d-lactone (GDL). The cold-set hydrogels prepared by this method showed mechanical strength comparable to the heat-set hydrogels obtained by direct heating of EWP solution, but with much less biopolymer (protein) content. The hydrogels were then used to produce porous EWP aerogels by supercritical carbon dioxide (SC–CO2) drying. While aerogels obtained from the heat-set hydrogels showed a compact structure with high density and low macroporosity, the aerogels obtained from the cold-set hydrogels had 3-5 times the lower density and significantly higher macroporous volume, due to the lower biopolymer concentration and less shrinkage of their precursor hydrogels. These aerogel scaffolds were then employed as templates for structuring a liquid soybean oil. Compared to the aerogels obtained from the heat-set hydrogels, the aerogels from the cold-set hydrogels showed excellent oil structuring capacity, where solid-like, plastic oleogels with an elastic module of more than 2.0 × 105 Pa were developed. This work suggests that the gelation method had profound effects on the density and porosity of the EWP aerogels and the oil structuring capacity of the aerogels.

Fabrication of boehmite nanofiber aerogels by a phosphate gelation process for optical applications

A transparent wet gel was obtained in a few minutes at room temperature by adding an aqueous phosphoric acid solution of appropriate concentration to boehmite nanofiber sol. After room temperature aging and supercritical carbon dioxide drying, low bulk density aerogels with visible light transmittance of over 90% at 10 mm thickness were obtained. These aerogels exhibited high Young’s modulus and visible light transmittance, while having the same bulk density as the samples obtained by a conventional gelation process using a base. The high optical transmittance of the aerogel were hardly lost even at high humidity because the phosphorylation of the skeletal surface reduced the percentage of hydroxyl groups. The three-dimensional imaging inspection of the exterior and interior of the aerogel was carried out. The various developments reported in this paper make aerogels with ultralow bulk density (<0.01 g cm−3) and high visible light transmission even more promising for applications in the physical field. Graphical abstract

Developing dual nano/macroporous starch bioaerogels via emulsion templating and supercritical carbon dioxide drying

In this study, emulsified oil droplets were employed as a temporary porogen to obtain dual nano/macroporous starch aerogels by supercritical carbon dioxide (SC-CO2) drying. This method took advantage of the solubility of the oil droplet porogens in acetone, and the insolubility of corn starch in this solvent, so this process could be integrated into the typical aerogel processing method. The effect of porogen content and starch concentration on physical and mechanical properties and the internal morphology of the obtained aerogels were studied. While the neat starch aerogel showed a compact structure in macroscale size with interconnected nanopores, the sacrificing oil droplet porogens induced macropores in the emulsion-templated aerogels. Furthermore, the nanoporous structures of starch aerogels were also well-preserved in which the macropores were surrounded by fine and interconnected nanofibrous networks. It resulted in aerogels that exhibited internal morphology in two scales (macropores and nanopores) with a high surface area (156–190 m2/g).

Analysis on the diffusion and mechanical properties of eucalyptus dried via supercritical carbon dioxide

Using the molecular dynamics software Materials Studio, a micro-level carbon dioxide – cellulose model was established to study the supercritical carbon dioxide drying of eucalyptus wood. The change of the primary components of the eucalyptus wood cellulose were also studied, by simulating various pressures, i.e., 10, 15, 20, 25, and 30 Pa, and simulating a temperature of 323 K. Results showed that the diffusion coefficient of carbon dioxide decreases as the pressure increases, and it reaches the maximum at 20 Pa, which was confirmed by the number of hydrogen bonds in the carbon dioxide cellulose model. Combined with the comprehensive analysis of the mechanical parameters, the Poisson’s ratio γ and the ratio of bulk modulus to shear modulus (K/G) values of cellulose first increased and then decreased as the pressure increased, and the Young’s modulus increased as the pressure increased. From a microscopic point of view, the study shows that eucalyptus cellulose has good mechanical properties when dried by supercritical carbon dioxide under a pressure of 20 Pa. The simulation results of the dynamic model agreed well with the measured results, and the simulation results support the previous experimental data and the practical application results in production.

Environment-Friendly Catalytic Mineralization of Phenol and Chlorophenols with Cu- and Fe- Tetrakis(4-aminophenyl)-porphyrin—Silica Hybrid Aerogels

Fenton reactions with metal complexes of substituted porphyrins and hydrogen peroxide are useful tools for the mineralization of environmentally dangerous substances. In the homogeneous phase, autooxidation of the prophyrin ring may also occur. Covalent binding of porphyrins to a solid support may increase the lifetime of the catalysts and might change its activity. In this study, highly water-insoluble copper and iron complexes of 5,10,15,20-tetrakis(4-aminophenyl)porphyrin were synthesized and bonded covalently to a very hydrophilic silica aerogel matrix prepared by co-gelation of the propyl triethoxysilyl-functionalized porphyrin complex precursors with tetramethoxysilane, followed by a supercritical carbon dioxide drying. In contrast to the insoluble nature of the porphyrin complexes, the as-prepared aerogel catalysts were highly compatible with the aqueous phase. Their catalytic activities were tested in the mineralization reaction of phenol, 3-chlorophenol, and 2,4-dichlorophenol with hydrogen peroxide. The results show that both aerogels catalyzed the oxidation of phenol and chlorophenols to harmless short-chained carboxylic acids under neutral conditions. In batch experiments, and also in a miniature continuous-flow tubular reactor, the aerogel catalysts gradually reduced their activity, due to the slow oxidation of the porphyrin ring. However, the rate and extent of the degradation was moderate and did not exclude the possibility that the as-prepared catalysts, as well as their more stable derivatives, might find practical applications in environment protection.

Canola protein aerogels via salt-induced gelation and supercritical carbon dioxide drying

Canola protein is used to produce novel, mesoporous, plant-based aerogels through a salt-induced gelation technique and supercritical carbon dioxide drying. The use of calcium chloride in formulation permits the preservation of gel monoliths during solvent exchange processing, customarily a major challenge in aerogel preparation. The gels could be successfully converted to aerogels by supercritical drying using accessible, off-the-shelf instrumentation, rather than the purpose-built systems commonly used in aerogel research. The use of calcium chloride represents a novel approach to canola protein gelation, achieving crosslinking via an environmentally-friendly approach. The first-ever supercritically-dried canola aerogels are shown to be mesoporous with an approximate density and specific surface area of 0.2 g/cm3 and 113 m2/g, respectively. The typical low-densities (0.03 – 0.5 g/cm3) and high surface areas (50 – 600 m2/g) of bioaerogels lend themselves to novel pharma- and nutraceutical applications, with canola aerogels now another addition to this family of advanced materials.

Laser ignited combustion of mesoporous nitrocellulose/graphene oxide composite aerogel with strong photo-thermal sensitivity and high flame propagation rate

In this paper, mesoporous nitrocellulose (NC)/graphene oxide (GO) composite aerogel is manufactured by a sol-gel method followed by supercritical carbon dioxide drying. The effects of GO on the microstructure, decomposition, and thermal conductivity are studied. The laser absorption, laser ignition property, and combustion performance of NC aerogel and NC/aerogel are also investigated. The results indicate that NC/GO aerogel is established by the combination of NC nano-fibers and GO sheets, and the aerogels are mesoporous with average pore size at 40 nm. Moreover, the thermal stability of aerogels is decreased by the nano-structure of NC, and the laser absorption is significantly increased by the nano-porous structure and the adoption of GO. In addition, the laser ignition and combustion efficiency are also greatly improved by the nano-porous aerogel structure and the incorporation of evenly uniformly dispersed GO sheets with high photothermal conversion efficiency and thermal conductivity. Laser ignition times reduced by 73.7% and 99.6% are obtained, and flame propagation rates up to 4.7 and 12.3 times are observed for NC aerogel and NC/5 wt. % GO aerogel, respectively.

Novel Flyweight Three-Dimensional Self-Assembled Graphene Oxide/Octa(Aminophenyl) Polyhedral Oligomeric Silsesquioxane Hybrid Aerogels

Herein, flyweight organic-inorganic hybrid scaffolds were fabricated by self-assembly and reduction of graphene oxide via covalent reaction of octa(aminophenyl) polyhedral oligomeric silsesquioxane with graphene oxide. Octa(aminophenyl) polyhedral oligomeric silsesquioxane created a decorative coating on the graphene oxide surface. It acts as a nano-crosslinker, especially on the overlapped-zone of graphene oxide platelets to bind them close-fitting. The resulting hybrid hydrogel was transformed into aerogel by the solvent exchange process with liquid carbon dioxide, followed by liquid carbon dioxide supercritical drying. Different concentrations of graphene oxide and octa(aminophenyl) polyhedral oligomeric silsesquioxane were prepared and the structure-property relationship of obtained aerogels is elucidated. The bulk density and porosity of the aerogels are located between the super-low values of 2.7 to 5.9 mg cm-3 and beyond 99.5 %, respectively. According to the adsorption-desorption isotherms of BET-technique, the surface area of obtained aerogels was between 250 to 713 m2/g. The findings remark the potential application of obtained aerogels in the production of supercapacitors, lithium-ion batteries, solar cells, etc. in energy storage and conversion devices, electrode materials, sensors, gas/oil/dye adsorbents, and high-temperature insulators in the aerospace industry.

Green Synthesis, Spectroscopic Characterization and Biomedical Applications of Carbon Nanotubes.

Carbon nanotubes are nano-sized cylindrical chicken wire-like structures made of carbon atoms. Carbon nanotubes have applications in electronics, energy storage, electromagnetic devices, environmental remediation and medicine as well. The biomedical applications of carbon nanotubes can be owed to features like low toxicity, non-immunogenicity, high in vivo stability and rapid cell entry. Carbon nanotubes have a great prospect in the treatment of diseases through diagnostic as well as therapeutic approaches. These nanostructures are interesting carriers for delivery and translocation of therapeutic molecules e.g. proteins, peptides, nucleic acids, drugs, etc. to various organs like the brain, lungs, liver, and pancreas. Commonly used methods to synthesize carbon nanotubes are arc discharge, chemical vapor deposition, pyrolysis, laser ablation etc. These methods have many disadvantages such as operation at high temperature, use of chemical catalysts, prolonged synthesis time and inclusion of toxic metallic particles in the final product requiring additional purification processes. In order to avoid these setbacks, various green chemistry-based synthetic methods have been devised, e.g., those involving interfacial polymerization, supercritical carbon dioxide drying, plant extract assisted synthesis, water- assisted synthesis, etc. This review will provide a thorough outlook of the eco-friendly synthesis of carbon nanotubes reported in the literature and their biomedical applications. Besides, the most commonly used spectroscopic techniques used for the characterization of carbon nanotubes are also discussed.

Supercritical carbon dioxide drying of municipal sewage sludge – Novel waste-to-energy valorization pathway

Eighty percent of the world's wastewater is discharged back into nature without reuse and further treatment and solid byproduct of wastewater is either spread on land or landfilled. The valorization of municipal sewage sludge for energy use is a major opportunity for sustainable energy applications. This includes effective dewatering and drying of the sludge in addition to producing biogas during the anaerobic digestion. Supercritical CO 2 extraction process was investigated for the purpose of drying municipal sewage sludge. Remarkably our results showed that after sCO2 processing the higher heating value (HHV) of sludge at wet basis moisture content of 52% was 14 MJ/kg and thus higher than the HHV of brown coal with similar moisture level (12 MJ/kg). Our research also shows that under specific conditions sCO2 extraction reduces the moisture content of the municipal sludge from 89% to 53% in 15 min. Although, more research is needed this dewatering process could be considerably less energy intensive than conventional thermal and other drying processes. Another benefit is extraction of micro pollutants by sCO2 treatment and thus making the dried sludge more useable in energy applications as per regulations. Our results show an alternative and novel pathway for highly effective dewatering and drying of municipal and other types of sludge.

Removal of anionic hexavalent chromium and methyl orange pollutants by using imidazolium-based mesoporous poly(ionic liquid)s as efficient adsorbents in column

Poly(ionic liquid)s (PILs) are attractive for their various applications, but the use of porous PILs have rarely been reported in anionic pollutants removal via ion-exchange by column. Herein, we report a serial of crosslinked imidazolium-based mesoporous PILs with Cl− and Br− as anions for hexavalent chromium (Cr(VI)) and methyl orange (MO) removal. Among them, PDVIm-Cl-SCD, from the free-radical polymerization of a dicationic monomer (N,N'-methylene-bis(1-(3-vinylimidazolium)) chloride, DVIm-Cl) and further supercritical carbon dioxide drying (SCD), displayed a very high sorption capacity (328.2 mg g−1 at 25 °C) and excellent utilization of adsorption sites (UOA, 86.2%) towards Cr(VI), and an unprecedentedly high sorption capacity (1615.0 mg g−1 at 25 °C) with a UOA of 67.4% to MO. Moreover, PDVIm-Cl-SCD also exhibited a broad pH range, excellent regeneration and remarkable reusability. Regarding to Cr(VI) removal, the volume of saturated KCl aqueous used for regenerating the Cr(VI) saturated PDVIm-Cl-SCD column (7.5–9.5 mL) was much less than the volume of treated Cr(VI) solution (160–200 mL). For MO removal, the volume of saturated NaCl solution used for regenerating the MO saturated PDVIm-Cl-SCD column (10.5–13.5 mL) was also much less than the volume of treated MO solution (220–235 mL), implying the great potential of PDVIm-Cl-SCD in sustainable wastewater treatment.

Ultrafast and complete drying of ecamsule solution using supercritical carbon dioxide with fluctuating pressure technique

Supercritical carbon dioxide (scCO2) drying, a green and energy-efficient technology, was investigated for the extraction of ethanol from an ecamsule-ethanol solution and complete drying of the ecamsule powder. Two experimental designs, a constant pressure and a fluctuating pressure scCO2 drying technique, were tested. In the constant pressure scCO2 drying technique, the effects of varying the drying parameters, namely, the temperature (40 °C–60 °C), pressure (8 MPa–16 MPa), CO2 flow rate (20 mL min–1–40 mL min–1), and time (15 min–180 min), on the efficiency of ethanol removal were investigated. The scCO2 drying parameters were further analyzed using response surface methodology (RSM); based on the results of the RSM analysis, the optimum conditions for the constant pressure scCO2 drying technique were 60 °C, 14.2 MPa, and 71.7 min, at which the minimal amount (0.23 wt%) of residual ethanol remained in the dried ecamsule powder. In contrast, near-complete removal of ethanol was achieved using the fluctuating pressure scCO2 drying technique within only 37 min at 60 °C and 10 MPa–14 MPa with 60 pressure fluctuation cycles and a CO2 flow rate of 40 mL min–1. The physicochemical properties of the ecamsule powder dried using the fluctuating pressure CO2-technique were similar to those of powder obtained using conventional oven drying. An energy analysis of the ecamsule drying process suggested that the energy required for scCO2 drying was approximately half that of oven drying.