Principle of supercritical fluid anti-solvent (SAS) technology
SAS (Supercritical Fluid Anti-Solvent) technology has the advantages of easy solvent selection and low anti-solvent operating pressure.
Since Gallagers et al. proposed the concept of supercritical fluid antisolvent in 1989, the research and application of this method has been developed rapidly and has been applied to many fields.
At present, the SAS method is mainly used in explosives, catalysts, pharmaceuticals, dyes and superconductors, etc.
Application of SAS method in heat-sensitive explosives
SAS was originally used for the micronization of heat-sensitive explosives, because the small particle size explosives can accelerate the combustion process and release higher energy.
Gallagher et al used SAS to produce nitroguanidine, and investigated the effect of antisolvent on product morphology, particle size and particle size distribution.
- The faster the anti-solvent addition rate, the smaller the particles and the narrower the particle size distribution.
- The particle size can vary from 1 μm to hundreds of microns.
- Different liquid organic solvents have different expansion paths and have different effects on the products.
- Under the same conditions, when cyclohexylamine is used as a solvent, the precipitated particles are smaller, the particle size distribution is narrower, and the shape is more uniform.
Application of SAS method in the processing of polymer microspheres
The processing of polymer microspheres is an important aspect of the application of SAS technology.
It can be used to prepare chromatographic stationary phases, adsorbents, catalyst supports, and drug sustained-release systems.
Influence of process parameters
Bleich and others used dichloromethane as a solvent to carry out biodegradable materials such as polylactic acid and PDA.
In the study of micronization, within the range of experimental conditions studied, the obtained polylactic acid particles are spherical, and the particle size of 50% of the particles is concentrated between 1 and 10 m; while PDA does not appear to precipitate, which shows that the influence of process parameters is extremely important.
Influence of operating pressure
Randolph et al found that during the preparation of polylactic acid, when the operating pressure of the process is not enough to makeWhen the droplets disappear completely, an irregular film is formed; when the concentration of polylactic acid is greater than 4%, long fibers are generated; when the concentration is between 0.3% and 1%, spherical particles are generated. The particle size increases with the increase of pressure and temperature, and the average particle size is between 0.6~1.4m.
Formation of polyaramid
Yeo et al used dimethyl sulfoxide (DMSO), dimethyl formamide (DMF) and dimethyl acetylAmine (DMA) was used as a solvent to study the formation of polyaramid. In the case of intermittent operation and low expansion rate, polycrystalline microspheres of 1~10pm were obtained. In continuous operation, fibers with a diameter of 0.1~1m were obtained.
Influence of concentration of the solution
Dixon and others successfully precipitated polystyrene particles from toluene.
When using the GAS method, when the temperatureFrom 10 ℃ to 40 ℃, the particle size increases obviously, accompanied by agglomeration, and the particle cluster size is between 10 and 20 m.
When the concentration of the solution increases from 1% to 5%, fibers are generated. When the diffusion rate of SC-CO2 to the center of the droplet exceeds the volatilization rate of toluene, hollow microspheres are generated.
Luna et al precipitated polyacrylonitrile from toluene, and the resulting fiber morphology changed with increasing flowChange, also has a lot to do with the concentration of the solution
Chen et al. used biodegradable polymers as carriers to try to prepare drug-releasing capsules using an anti-solvent processWhen the concentration of the polymer is low, only part of the drug is encapsulated into a capsule; if the concentration is too high, aggregation will occur.
Application of SAS method in medicine preparation
The micronization of drugs is another important field of SAS applications.
The particles with special particle size can not only improve the biological activity of the drug, but also reduce the dose of the drug or change the delivery method of the drug.
Precipitated insulin and catalase
Debenedetti yeo success fully precipitated insulin and catalase from the system of 90% ethanol and 10% water.
Catalase is about 1m spherical powder, insulin is about 5m microspheres or needles. The difference in precipitation time results in the appearance of multiple particle morphologies.
When precipitating insulin from dimethyl sulfoxide and dimethylformamide, the morphology of insulin particles is not sensitive to process parameters and solvent characteristics within the scope of the investigation.
Particles with a particle size of less than 4m account for about 90%, less than 1pm particles account for about 10%.
Raman spectroscopy studies have shown that although the secondary structure of the drug has changed, it can still restore all biological activities.
When using three coaxial channels for the preparation of anti-solvent process of enzymes (lysozyme and pancreatin), lysozyme with an average particle size of 0.78m and pancreatin with an average particle size of 1.53m can be obtained. The biological activity of lysozyme reached 95%, while the activity of pancreatin was less than 40%.
Precipitated prednisolone acetate and hydrocortisone
Schimtt et al used CO2 as an anti-solvent, from tetrahydrofuran (THF) and dimethyl formamide saturated solution successfully precipitated prednisolone acetate and hydrocortisone acetate particles. They found that the use of lower temperature and dilute solution is conducive to the formation of small particles, while pressure has little effect on the characteristics of the final product, and the solvent mainly affects the size and morphology of the particles.
Reverchon et al successfully precipitated tetracycline from nitromethylpyrrolidone and obtained nano-sized cis-particles.
Kitamura et al dissolved antibiotic sulfathiazole in ethanol and successfully prepared ultrafine powder using supercritical CO2 antisolvent.
Application of SAS method in the manufacture of dyes and superconductors
Ultrafine particles also have many applications in other fields. Submicron dye powder can greatly improve the dyeing strength; the nano-sized particle catalyst has a small particle size, large specific surface area, and strong catalytic activity.
In the manufacture of superconductor SAS, the generation of ultrafine superconductor particles can effectively prevent non-superconducting materials Mixing people, thus avoiding hindering the movement of magnetic streamlines, significantly improves the performance of superconductors.
Gao et al conducted a SAS preparation study on the precipitation of pigment-precipitated red C, yellow I, and blue 15 from acetoneIt was found that heating and depressurizing helps to generate smaller ultrafine particles.
Reverchon et al. applied for a number of patents for the production of superconductor precursor nanoparticles by the SAS method.
If SC-CO2 is used as an anti-solvent, the precipitation of barium, copper, yttrium, samarium, and rubidium acetate from dimethyl sulfoxide, whether precipitated separately or co-precipitated with several compounds, can obtain a particle size of about 100m and a particle size Amorphous particles with narrow distribution.
Application of SAS method in catalyst preparation
The SAS method has been extensively studied in the preparation of catalysts.
First, catalyst precursor particles are prepared by the SAS method, and then the catalyst is obtained by calcination.
The method has simple steps, and the prepared catalyst has small particle size, uniform distribution of active components, and both solvent and anti-solvent CO2 can be recovered. It is a green catalyst preparation method.
So far, researchers have prepared many catalysts or catalyst carriers using the SAS method.
At present, the research on the preparation of catalysts using the SAS method includes the Reeverchon research group of Salerno University in Italy, the Hutchings research group of Cardif University in the United Kingdom, and the Zhang Minhua research group of Tianjin University in China.
Reverchon research group
Reverchon and others were the first scientists to use SAS to prepare catalysts, but they are currently not their main research direction.
- They successfully prepared zinc oxide precursors-zinc acetate ultrafine particles with a concentration of 8mg/mL zinc acetate in DMSO.
At 15MPa and 40℃, the average particle size is 50m, and the smallest is 30nm50.
Due to the existence of a large number of internal pores, the specific surface area of the particles can reach 175m2/g. After firing at 300℃ for 1h, the specific surface area decreased to 55m2/g.
- In addition, they used the SAS method to precipitate strontium acetate (SrAc) particles 5 from DMSO. At 15MPa, 40℃, and the precursor concentration was 5mg/mL, the average particle size of the precursor particles was 144m. The average particle size of the SrO2 particles obtained after calcination is 100nm, and the specific surface area is only 8m2/g. The specific surface area of strontium oxide (SrO2) particles obtained by calcining SrAc particles prepared without SAS treatment is as high as 80m2/g.
The application of the above two SrO2 products to the catalytic oxidative dehydrogenation of ethane found that with the same conversion rate, the ethylene yield obtained on the SrO2 catalyst treated by the SAS method was higher.
Hutchings research group
The Hutchings research group prepared a variety of catalyst supports using the SAS method, and measured the reactivity of the catalysts prepared from them, and achieved good results.
Loading with metal Au
Tang et al used methanol as the solvent and precipitated the precursor in a cerium acetylacetonate solution with a concentration of 13.33 mg/mL at 15 MPa and 40 °C to obtain nanoparticles with a particle size ranging from 45 to 76 m, which was calcined at 400 °C for 2 h After that, CeO2 particles with a specific surface area of 31m2/g were obtained.
After loading with metal Au, it showed good dispersion performance for Au. The catalytic activity of CO oxidation was evaluated. It was found that the catalytic effect per unit mass of the catalyst was higher than that of any Au/ reported previously. Both CO2 catalysts are good. Miedziak et al. supported PdAu bimetal on CeO2 carrier prepared by SAs method and used it to catalyze the oxidation reaction of benzyl alcohol. Compared with CeO2 supported catalyst prepared without SAS treatment, the amount of benzyl alcohol converted on the metal per unit of substance And the amount of benzaldehyde produced is higher
Titanium titanium acetylacetonate
Tang et al success fully prepared TiO2 particles using SAS with titanium acetylacetonate as a precursor. inUnder the conditions of 1MPa and 40℃, the specific surface area of the precursor particles is 160m2/g, and the specific surface area of the TiO2 particles obtained after calcination is 35m2/g.
The TiO2 is used as a carrier to catalyze the oxidation of CO after loading Au, and its catalytic performance is much higher than that of the TO2 supported Au catalyst that has not been treated by the SAS method.
Lu et al tried to further control the morphology of TO2 precursor particles by changing the operating conditions of SAS, and then control the morphology of the catalyst particles, and obtained nanoparticles with a particle size ranging from 25 to 78 mm. The particle size has little effect, and the use of low flow rate and high concentration of precursor solution can cause the particle size to become larger.
CuMn composite by SAS method
In addition to a single oxide, Tang et al also studied the preparation of CuMn composite by SAS method Oxide particles.
Using DMSO as the solvent and selecting acetate as the solute, the mixed precursor particles of manganese acetate and copper acetate were successfully prepared at 11 MPa and 40°C, and their specific surface area was greater than 300 m2/g.
Between 20~50m2/g, its morphology is branched chain CuMn2O4, length 160~200nm, diameter 40nm.
Its activity for catalyzing CO oxidation is much greater than that of CuMn2O4 catalyst prepared by co-precipitation method.
Prepared amorphous vanadium phosphate particles
The Hutchings research group also prepared amorphous vanadium phosphate particles, which is a catalyst for partially oxidizing alkanes. 3 It was found that the particle size of the precursor particles prepared at 1 MPa and 60 ℃ was 75 mm~ Between 5m, the specific surface area is 40m2/g, but the specific surface of the catalyst particles obtained after activation of the precursor particles is only 6m2/g.
The activity of this amorphous vanadium phosphate catalyst is comparable to that of traditional crystalline vanadium phosphate catalysts.
Zhang Minhua’s research group
The research on the preparation of catalysts by the SAS method also started earlier in Tianjin University.
Prepared manganese dioxide precursor
Gu Xueqian et al. successfully prepared manganese dioxide precursor-manganese acetate ultrafine particles using DMSO as solvent.
At 16MPa and 48℃, the particle size of the manganese acetate particles produced is the smallest. When the precursor concentration is 1.5% (mass fraction), the average particle size of the obtained manganese acetate is 70m.
Prepared ALO3 precursor
He Chunyan et al. successfully prepared ALO3 precursor-aluminum nitrate nanoparticles using SAS method at 16MPa using ethanol as the solvent At 48 ℃, the precursor concentration is 2% (mass fraction), the average particle size of the particles is 70nm.
The characterization of ALO3 supported Ni catalyst found that nano Al2O3 particles as a carrier have good dispersion properties for the metal active component Ni. In addition, they has successfully prepared A(NO3)3-Zr(NO3)4 mixed precursor particles 6).At 20MPa and 48℃, the solution concentration is 2% (mass fraction), the average particle size of the prepared particles is also The nano-composite oxide catalyst carrier obtained after calcination at 70nma has better dispersion performance for the active component Ni than the Al2O3ZrO2 carrier prepared by the impregnation precipitation method, and has good application prospects.
Increasing pollutant emissions from automobiles have become a very serious problem that endangers the ecological environment and human health. The cerium-zirconium composite oxide used for automobile exhaust purification has become a new research hotspot in recent years.
Prepare CeO2 precursor particles
Zhang Jinyan et al. successfully prepared CeO2 precursor particles with an average particle size of 50-70mm by SAS method, and systematically studied the preparation process conditions by orthogonal test method. The research results show that, compared with the catalyst prepared by the coprecipitation method, the nano-cerium-zirconium composite oxide catalyst prepared by the SAS method has the advantages of small crystallite uniformly dispersed particles, small size, regular shape, high oxygen storage, etc.; catalyst reduction performance test results It shows that the catalyst prepared by the SAS method has a single reduction peak and a higher total hydrogen consumption; the thermal stability experiment results show that the catalyst prepared by the SAS method has a higher thermal stability.
Perium-zirconium oxide solid solution prepared
Jiang Haoxi and others improved the specific surface area of the cerium-zirconium oxide solid solution prepared by the original process from about 10m2/g by about 35m2/g by adjusting the feed liquid concentration and the feed flow ratio of CO2, and the reduction peak temperature of the TPR test was reduced by 80°C. Oxygen storage increased from the original 55μmolg to 506mol/g.
The best preparation process conditions finally determined were: CO2 flow rate 45g/mn, solution flow rate 1mL/min, operating temperature 45℃, operating pressure 15MPa, solution concentration 0.5% (mass fraction).
Prepare cerium-zirconium oxide solid solution nanoparticles
The author also prepared cerium-zirconium composite oxide solid solution particles with a hollow nanosphere structure with a particle size ranging from 30 to 50 mm by using the SAS method, and explored the mechanism of the cerium-zirconium oxide solid solution nanoparticles prepared by the SAS method.
Sun Huanhua et al prepared ternary composite oxide nanoparticles by doping Cu in cerium-zirconium composite oxide solid solution by SAS method to further improve the redox performance of the catalyst.
The calcined precursor obtained a particle size range of 30-50nm Of copper-cerium-zirconium ternary composite oxide nano-catalyst particles. The characterization results show that Cu2+ has entered the CeO2 lattice and formed a CuO-CeO2 solid solution.
Its oxygen storage capacity can be increased from 506mo/g of the binary catalyst to 636.9mol/g, and the reduction peak of the ternary composite oxide particle catalyst The temperature can be significantly reduced from about 600℃ to only 120~240℃.