Supercritical Carbon Dioxide Drying Research Articles

Rapid and facile synthesis of a low-cost monolithic polyamide aerogel via sol–gel technology

We report the first synthesis of a good-formability polyamide aerogel using melamine and isophthaloyl chloride via sol–gel technology followed by carbon dioxide supercritical drying. The chemistry employed is straightforward and simple, relying on no catalysts and no nitrogen-based protective atmosphere. The influence of an important factor, temperature, on the aerogels’ structure is briefly discussed and the optimal temperature is 75°C. The aerogels’ structural properties are characterized by the field emission scanning electron microscopy (FESEM), the high resolution transmission electron microscopy (HRTEM) and the Brunauer–Emmett–Teller methods (BET). The results indicate that the aerogel has a typical three-dimensional porous structure with pore diameter about 20nm and the specific surface area can be as high as 282m2/g. Due to the good flame resistance of melamine, the aerogel has potential for use in fireproofing building materials.

View cell investigation of silica aerogels during supercritical drying: Analysis of size variation and mass transfer mechanisms

We report the synthesis and supercritical drying of silica aerogels made via a sol–gel process. Tetramethylortosilicate has been used as precursor. Hydrolysis and poly-condensation steps were followed by carbon dioxide supercritical drying (T=45°C; P=10.5MPa). The complete supercritical drying step was video recorded in order to study the evolution of the size of the gels, concluding that a noticeable shrinkage only takes place during the decompression of CO2 at the end of the drying process, being the total shrinkage of 3–4%. The mass transfer mechanisms during drying have also been studied through analysis of the evolution transparency of the aerogels along the supercritical drying process. The mass transfer processing during drying was observed to be dominated by convection in the earliest stages, where a direct relationship between drying rate and CO2 flow were found. In the later stages, diffusion of the remaining organic solvent through the alcogel determined the mass transfer process.

Starch aerogel beads obtained from inclusion complexes prepared from high amylose starch and sodium palmitate

Starch aerogels are a class of low density, highly porous, renewable materials currently prepared exclusively from retrograded starch gels and are of interest due to their high surface area, porosity, biocompatibility, and biodegradability. Recently, we have reported the preparation and properties of amylose-fatty acid salt helical inclusion complexes prepared by steam jet-cooking amylose-containing starches and then blending the resulting dispersions with sodium palmitate. Dispersions of these amylose-sodium palmitate complexes prevent amylose from retrograding and possess polyelectrolyte properties due to the anionic charge of the sodium palmitate. Stable aqueous dispersions of these complexes form hydrogels when added to acid to lower the pH, and these hydrogels were used to prepare their corresponding xerogels, cryogels, and aerogels. Solvent exchange with ethanol and supercritical carbon dioxide (SC-CO2) drying of the starch gels preserved the structure of the gels and yielded aerogels having a macroporous nanoparticulate internal structure. Depressurization rates during SC-CO2 drying were examined to optimize aerogel properties, and starch aerogels with densities between 0.120–0.185 g cm−3 and BET surface areas ranging between 313–362 m2 g−1 were obtained. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, and scanning electron microscopy (SEM), were used to characterize the aerogels. The corresponding xerogels and cryogels had inferior properties. This method provides an alternative method for the preparation of starch aerogels and eliminates many difficulties associated with starch gelatinization and retrogradation procedures that are currently used to prepare the prerequisite starch gels.

3D PLLA/Ibuprofen composite scaffolds obtained by a supercritical fluids assisted process

The emerging next generation of engineered tissues is based on the development of loaded scaffolds containing bioactive molecules in order to control the cellular function or to interact on the surrounding tissues. Indeed, implantation of engineered biomaterials might cause local inflammation because of the host's immune response; thereby, the use of anti-inflammatory agents, whether steroidal or nonsteroidal is required. One of the most important stages of tissue engineering is the design and the generation of a porous 3D structure, with high porosity, high interconnectivity and homogenous morphology. Various techniques have been reported in the literature for the fabrication of biodegradable scaffolds, but they suffer several limitations. In this study, for the first time, the possibility of generating 3D polymeric scaffolds loaded with an active compound by supercritical freeze extraction process is evaluated; this innovative process combines the advantages of the thermally induced phase separation process and of the supercritical carbon dioxide drying. Poly-L-lactid acid/ibuprofen composite scaffolds characterized by a 3D geometry, micrometric cellular structures and wrinkled pores walls have been obtained; moreover, homogeneous drug distribution and controlled release of the active principle have been assured.

Gradual hydrophobic surface functionalization of dry silica aerogels by reaction with silane precursors dissolved in supercritical carbon dioxide

We report the synthesis and the surface modification of hydrophilic and hydrophobic silica, aerogels obtained via a sol–gel process. Tetramethyl orthosilicate or a mix of the first one with, trimethylethoxysilane have been used as precursors. The hydrolysis and poly-condensation steps were, followed by carbon dioxide supercritical drying (T=45°C; P=105bar). The resulting dry hydrophilic, aerogels were subjected to a hydrophobic surface functionalization using supercritical carbon dioxide, as the solvent for different silane functionalization reactants: trimethylethoxysilane, octyltrimethoxysilane and chlorotrimethylsilane. Effects of the working pressure and reagent concentration on the functionalization were analyzed using FT-IR spectroscopy and exposing the treated aerogels to saturated moisture conditions in order to study the mass increment during the humidification. Nitrogen adsorption measurements show a considerable drop on the specific areas (13–17%) and on the pore volumes which were reduced by 50%. By modification of the operating pressure and variation of the functionalization agent employed, the degree of functionalization could be gradually increased up to the values of the aerogels synthesized as hydrophobic in the sol–gel phase.

Synthesis and Properties of P<sub>2</sub>O<sub>5</sub>-SnO<sub>2</sub>-ZnO Penetrated into Silica Aerogel Matrix by Sol-Gel Technique

This work demonstrated the synthesis of SiO2-P2O5-SnO2-ZnO quaternary porous aerogel by sol-gel followed supercritical carbon dioxide drying, starting from tetraethoxysilane (TEOS) as precursor for SiO2and triethyl phosphate, tin (iv) chloride pentahydrate and zinc nitrate hexahydrate as precursor for P2O5-SnO2-ZnO. It has been recorded that prepared P2O5-SnO2-ZnO sol was added to silica sol, then formed the quaternary aerogel by base catalyst. The microstructure, morphology and properties of the quaternary aerogel were studied by TG/DTA, FTIR, solid-state29Si NMR, BET, scanning electron microscopy (SEM) measurement and BJH nitrogen gas adsorption. The silica aerogels tetrahedral subunit structure and the influence of the loaded oxide have been observed. The results indicated that the quaternary aerogel exhibited average pore diameter of 14.15 nm, cumulative pore volume of 2.31 ml/g, the specific surface area as high as 796.29 m2/g and bulk density of 0.275g/cm3. There were four types of Si (O1/2)4tetrahedral unit structure in the quaternary aerogel. From this study, different chemical bond of P2O5-SnO2-ZnO penetrated into the silica network structure.

Preparation of dye sensitized solar cell by using supercritical carbon dioxide drying

Dye-sensitized solar cells (DSSC) derived from TiO2 aerogel film electrodes were fabricated. TiO2 aerogels were obtained by using sol–gel method and supercritical carbon dioxide (sc-CO2) drying. First, TiO2 wet gels were obtained by sol-gel method. Then, the solvents in the TiO2 wet gels were replaced by acetone. The TiO2 aerogels were obtained by using sc-CO2 drying from the TiO2 wet gels. The conditions of sc-CO2 drying were at 313, 323K and 7.8–15.5MPa. The electrodes with TiO2 aerogel films were obtained by deposition of the aerogels on glass substrates. The electrodes with TiO2 aerogel films and a commercial particle film of various thickness were obtained by repetitive coatings and calcinations. The amount of dye adsorbed on the TiO2 films with sc-CO2 drying was higher than that of commercial particle film. The amount of dye adsorbed on the TiO2 films increased with increasing surface area of the TiO2 film. DSSCs were assembled by using the TiO2 aerogel film electrodes and their current–voltage performance was measured. The power performance of DSSC made by supercritical drying was higher than that of commercial particles. The DSSC with the film electrode made at 313K and 15.5MPa showed the best power performance (Jsc=7.30mA/cm2, Voc=772mV, η=3.28%).

Preparation and thermal treatment of titania aerogels

TiO2 aerogels were prepared by sol-gel process and supercritical carbon dioxide drying. The influence of thermal treatment temperature on the crystalline structures and pore structures of TiO2 aerogels was investigated by X-ray diffraction and N2 adsorption-desorption. The results showed that the fresh prepared TiO2 aerogels were amorphous. After thermal treatment, the samples still possessed high surface areas and pore volumes, and the amorphous TiO2 gradually transforms to anatase and further to rutile. The crystallite sizes presented a persistent increase with the rise of temperature. The light absorption and the band gap of samples treated at different temperatures were also surveyed by diffuse reflection ultraviolet-visible (UV-vis) spectroscopy.

Shaped mesoporous materials from fresh macroalgae

Reproducible highly mesoporous monolithic materials (0.5–0.7 cm3 g−1, 9–15 nm) are prepared from a variety of fresh shaped abundant macroalgae using a simple green approach without the necessity for supercritical carbon dioxide (scCO2) drying. This opens up the possibility for low cost and sustainable structured materials for chromatography, catalyst supports and drug delivery systems.

Dynamics and model for supercritical carbon dioxide drying of tilapia fillet

Dynamics and model for supercritical carbon dioxide drying of tilapia fillet

Strong, low density, hexylene- and phenylene-bridged polysilsesquioxane aerogel–polycyanoacrylate composites

Nanocomposite aerogels were prepared by chemical vapor deposition and polymerization of cyanoacrylate on the surface of bridged polysilsesquioxane aerogels. Phenylene- and hexylene-bridged aerogels were prepared by sol–gel polymerizations and supercritical carbon dioxide drying. Hydrophobic organic bridging groups in the polysilsesquioxane aerogels reduced the amount of adsorbed water available for initiating polymerizations and led to higher molecular weight polycyanoacrylate than was observed with silica aerogels. Densities increased as much as 65% due to the addition of the organic polymer, but the nanocomposite aerogels remained highly porous with surface areas between 440 and 750 m2/g. Polycyanoacrylate–phenylene-bridged aerogel composites were the strongest with flexural strengths up to 780 kPa or 16-fold stronger than the untreated phenylene-bridged aerogels and fivefold stronger than a silica aerogel of the same density. The strongest polycyanoacrylate–hexylene-bridged aerogel composites had flexural strength of 285 kPa or ninefold stronger than the untreated hexylene-bridged aerogels and twice as strong as a silica aerogel of comparable density. The greater strength of the new composites is, in part, due to the greater strength of the bridged aerogels. However, higher molecular weight polycyanoacrylate, due to less surface water on the hydrophobic bridged aerogels, also contributes to the greater nanocomposite strengths.

Hierarchically Nanostructured Zeolites of Tunable Porosities with Aerogel Templating

ABSTRACTWe present the synthesis of resorcinol-formaldehyde aerogels and carbon aerogels of different nanoporosities, emphasizing on the recent developments in fabrication pathways of lower cost. Recent results showed a simple way to the production of highly nanoporous carbon xerogels. While using an approach combined colloidal silica nanocasting and carbon dioxide supercritical drying, hydrophilicity-controlled carbon aerogels with high mesoporosity were synthesized. Then, we demonstrate the functions of these aerogels for template synthesis of hierarchically nanostructured zeolites having micropores and mesopores.

Prediction of Supercritical Carbon Dioxide Drying of Food Products in Packed Beds

Drying assisted by supercritical carbon dioxide is foreseen to become a promising technology for sensitive food products. In this contribution, a mathematical model is derived to describe the changes in water concentration in both a solid food matrix and a fluid carrier during drying. Finite different element method is used to solve the set of mass balance equations. A remarkable agreement between simulated and experimental data was obtained. Moreover, the simulated changes in water concentration in the solid and fluid carrier gave a coherent description of the process. This model can be used as a tool for optimizing the operating conditions and process scale-up in supercritical carbon dioxide assisted drying.

Drying processes for preparation of titania aerogel using supercritical carbon dioxide

Supercritical carbon dioxide drying was performed for the preparation of titania aerogels from sol–gel routes. The conditions of supercritical carbon dioxide drying were 313–323 K and 7.8–15.5 MPa. The solvents in titania wet gels obtained from the sol–gel routes were replaced by acetone. The titania aerogels obtained from supercritical carbon dioxide drying form needle-like structures. In supercritical carbon dioxide drying, the extraction rates of acetone from the wet gels were measured by using an on-line Fourier transform infrared spectroscope. It was found that the titania aerogels with lower cohesion were induced from the formations of homogenous phase for carbon dioxide + acetone system and the lower extraction rates of acetone. Furthermore, titania films were prepared by the depositions of the titania aerogels on ITO-coated PET substrates. The needle-like aerogels with lower cohesion derive the titania film with high surface area.

Supercritical-CO 2 drying of foodstuffs in packed beds: Experimental validation of a mathematical model and sensitive analysis

In this contribution, a mathematical model is built to predict the changes in water concentration in both a solid food matrix and a fluid carrier during supercritical carbon dioxide (SC–CO 2) drying. The mass balance equations of the model involve five dimensionless parameters: Peclet number modified Sherwood number, Fourier number, mass ratio and equilibrium constant. The differential equations were discretized using the finite explicit difference method. The resulting model was implemented and solved in Matlab/Simulink using an explicit Runge–Kutta solver. A very good agreement ( ARD = 7.2%) between experimental data, obtained by an independent group, and the present model was observed. The axial dispersion diffusion coefficient seems not to play a significant role during the drying process. A sensitivity analysis revealed that the predictions are relatively more sensitive to the equilibrium constant and the mass ratio than to Peclet and modified Sherwood numbers. Furthermore, in the case of Peclet and modified Sherwood numbers, the sensitivity and the uncertainty of the output are function of the final moisture content. The present model could be used as an optimization tool for kinetic studies to investigate the effects of different operation conditions on the performance and design of the supercritical drying technology.

Supercritical Carbon Dioxide Drying of Methanol−Zinc Borate Mixtures

Supercritical carbon dioxide (CO2) drying of zinc borate species was investigated to evaluate possible chemical alterations in the product during the drying. Methanol-wetted zinc borates produced either from borax decahydrate and zinc nitrate hexahydrate (2ZnO·3B2O3·7H2O) or from zinc oxide and boric acid (2ZnO·3B2O3·3H2O) were dried by both conventional and supercritical carbon dioxide drying methods. Zinc borate samples dried by both techniques were characterized using analytical titration, X-ray powder diffraction (XRD), thermo gravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, elemental analysis, and scanning electron microscopy (SEM). It was found that while zinc borate obtained from zinc oxide and boric acid did not have any chemical interaction with CO2, carbonates were formed on the surface of zinc borate obtained from borax decahydrate and zinc nitrate hexahydrate. The main factor for the carbonate formation during supercritical CO2 drying is anticipated as the structural differences of zinc borate species. CO2 is a nonpolar solvent, and it does not usually react with polar substances unless water is present in the medium. While 2ZnO·3B2O3·3H2O had three bound OH groups, 2ZnO·3B2O3·7H2O had five bound OH groups and one mole of water of crystallization. It is proposed that the water of crystallization reacts with CO2 forming carbonic acid. Then, carbonic acid, which is stronger than boric acid, substitutes borate ions from their zinc salts.

Synthesis and characterization by FTIR spectroscopy of silica aerogels prepared using several Si(OR) 4 and R′′Si(OR′) 3 precursors

We report the synthesis and Fourier Transform Infrared spectroscopy characterization results dealing with the surface modification of silica aerogels obtained via a two-step sol–gel process where various silicon precursors and co-precursors were used. The hydrolysis and poly-condensation steps were followed by carbon dioxide supercritical drying ( T c = 31.1 °C; P c = 73.7 bar). The silicon precursors contain four identical hydrolysable alkoxy groups (methoxy or ethoxy), while in the co-precursors, one of the alkoxy groups is substituted by a non-hydrolysable alkyl group (methyl, ethyl, n-propyl, iso-butyl, n-octyl, vinyl or phenyl). Identically, surface-functionalized silica aerogels were obtained from various silicon precursor/co-precursor combinations and their chemical structures were compared. The infrared spectroscopy revealed the existence of chemically comparable solid networks with some differences due to the nature of the silicon precursors.

Preparation of nanostructured carbon materials

The low-density organic aerogels formed by the supercritical carbon dioxide drying of 5-methylresorcinol- formaldehyde gels are a good source material for the preparation of low-density carbon aerogels with a homogeneous structure. I n our research the supercritical drying process was optimized so that the resulting aerogels would not significantly shrink durin g the process. The density of the resulting 5-methylresorcinol-formaldehyde organic aerogel was as low as 0.1 g/cm 3 , its specific surface area being more than 350 m 2 /g. Also, the pyrolysis of the organic aerogel to get carbon material with a proper structure was optimized in relation to the low rate of the evolution of pyrolysis products during the process. The carbon material obtain ed had a uniform structure, consisting of sparsely packed particles with a narrow size distribution. The density of carbon aerogel s obtained was 0.2 g/cm 3 , their specific surface area being over 700 m 2 /g; the shrinkage was up to 30%. It was also found that the porosity of carbon aerogels could be varied by changing the cond itions of synthesis. The aerogels obtained were examined using scanning electron microscopy, infrared spectroscopy, and nitrogen adsorption-desorption analysis.

Synthesis and characterization of bacterial cellulose/alginate blend membranes

AbstractBacterial cellulose and alginate in an aqueous NaOH/urea solution were used as substrate materials for the fabrication of a novel blend membrane. The blend solution was cast onto a Teflon plate, coagulated in a 5 wt % CaCl2 aqueous solution, and then treated with a 1% HCl solution. Supercritical carbon dioxide drying was then applied for the formation of a nanoporous structure. The physical properties and morphology of the regenerated bacterial cellulose and blend membranes were characterized. The blend membrane with 80% bacterial cellulose/20 wt % alginate displayed a homogeneous structure and exhibited a better water adsorption capacity and water vapor transmission rate. However, the tensile strength and elongation at break of the film with a thickness of 0.09 mm slightly decreased to 3.38 MPa and 31.60%, respectively. The average pore size of the blend membrane was 10.60 Å with a 19.50 m2/g surface area. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Novel nanoporous membranes from regenerated bacterial cellulose

AbstractBacterial cellulose (BC) in an NaOH/urea aqueous solution was used as a substrate material for thefabrication of a novel regenerated cellulose membrane. The dissolution of BC involved swelling BC in a 4 wt % NaOH/3 wt % urea solution followed by a freeze–thaw process. The BC solution was cast onto a Teflon plate, coagulated in a 5 wt % CaCl2 aqueous solution, and then treated with a 1 wt % HCl solution. Supercritical carbon dioxide drying was then applied to the formation of a nanoporous structure. The physical properties and morphology of the regenerated bacterial cellulose (RBC) films were characterized. The tensile strength, elongation at break, and water absorption of the RBC membranes were 4.32 MPa, 35.20%, and 49.67%, respectively. The average pore size of the RBC membrane was 1.26 nm with a 17.57 m2/g surface area. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008