Supercritical Isopropanol Research Articles

Efficient synthesis of γ-valerolactone from ethyl levulinate via catalytic transfer hydrogenation in supercritical isopropanol over nickel aluminum oxide nanosheets

This study reports a novel method for γ-valerolactone (GVL) production from ethyl levulinate (EL) utilizing non-noble metal catalysts within a supercritical isopropanol (IPA) system. Nickel-loaded nickel–aluminum oxide nanosheets (NiXAl-R) were fabricated via hydrothermal treatment of Ni–Al layered double hydroxide precursors, followed by hydrogen reduction at 550 °C for 1.5 h. Using NiXAl-R as the catalyst and IPA as the hydrogen donor, we achieved notable results: 82.7% EL conversion, 73.6% GVL yield, and 1.178 molGVL g−1 h−1 reaction efficiency after 1.5 h in supercritical IPA. The catalytic activity of NiXAl-R remained stable over five catalytic cycles. The surface of NiXAl-R exhibits abundant oxygen vacancies and unsaturated Al species (Alx+), which promote the initial transfer hydrogenation of EL to form the ethyl 4-hydroxypentanoate (4-HPE) intermediate. Simultaneously, the supercritical IPA environment overcomes mass transfer limitations at the solid–liquid interface between 4-HPE and the catalyst, thereby facilitating the Alx+-catalyzed intramolecular esterification of 4-HPE into GVL.

Ni Nanoparticles on the Reduced Graphene Oxide Surface Synthesized in Supercritical Isopropanol.

Nanocomposites based on ferromagnetic nickel nanoparticles and graphene-related materials are actively used in various practical applications such as catalysis, sensors, sorption, etc. Therefore, maintaining their dispersity and homogeneity during deposition onto the reduced graphene oxide substrate surface is of crucial importance to provide the required product characteristics. This paper demonstrates a new, reproducible method for preparing a tailored composite based on nickel nanoparticles on the reduced graphene oxide surface using supercritical isopropanol treatment. It has been shown that when a graphene oxide film with previously incorporated Ni2+ salt is treated with isopropanol at supercritical conditions, nickel (2+) is reduced to Ni (0), with simultaneous deoxygenation of the graphene oxide substrate. The resulting composite is a solid film exhibiting magnetic properties. XRD, FTIR, Raman, TEM, and HRTEM methods were used to study all the obtained materials. It was shown that nickel nanoparticles on the surface of the reduced graphene oxide had an average diameter of 27 nm and were gradually distributed on the surface of reduced graphene oxide sheets. The data obtained allowed us to conduct a reconnaissance discussion of the mechanism of composite fabrication in supercritical isopropanol.

Получение частиц диоксида титана (рутила) на поверхности восстановленного оксида графена в сверхкритическом изопропаноле

Sequential synthesis of rutile modification titanium oxide particles on reduced graphene oxide in supercritical isopropanol is described. In this case, only graphene oxide was reduced to reduced graphene oxide. A one-stage method (one-pot) was also developed for the preparation of rutile particles on reduced graphene oxide, where supercritical isopropanol was the graphene oxide reducing agent and the reaction medium. The resulting composites were studied using X-ray phase analysis, transmission electron microscopy, and atomic force spectroscopy methods.

Dry Reforming of Methane over 5%Ni/Ce1-xTixO2 Catalysts Obtained via Synthesis in Supercritical Isopropanol.

A series of 5%Ni/Ce1-xTixO2 catalysts was prepared with nickel impregnation of mixed Ce-Ti oxides obtained via synthesis in supercritical isopropanol. All oxides have a cubic fluorite phase structure. Ti is incorporated into the fluorite structure. Small amounts of impurities of TiO2 or mixed Ce-Ti oxides appear with Ti introduction. Supported Ni is presented as the NiO or NiTiO3 perovskite phase. Ti introduction increases total samples reducibility and results in stronger interaction of supported Ni with the oxide support. The fraction of rapidly replaced oxygen and the average tracer diffusion coefficient also increase. The number of metallic nickel sites decreased with increasing Ti content. All catalysts except Ni-CeTi0.45 demonstrate close activity in tests of dry reforming of methane. The lower activity of Ni-CeTi0.45 can be connected to Ni decoration with species of the oxide support. The incorporation of Ti prevents detachment of Ni particles from the surface and their sintering during dry reforming of methane.

ВОССТАНОВЛЕНИЕ КОМПОЗИЦИОННОГО МАТЕРИАЛА НА ОСНОВЕ ОКСИДА ГРАФЕНА И СОЛЕЙ МЕТАЛЛОВ

The results of reduction of a composite based on graphene oxide with metal salts introduced into its structure are described. The composite is an extended dense film. Metal cations are bound to the surface of graphene oxide through oxygen functional groups. The material was further reduced chemically and thermally, and also treated in supercritical isopropanol. Processing under supercritical conditions leads to the reduction of graphene oxide, while metal salts are reduced to metal nanoparticles.

Получение наночастиц оксида титана на поверхности восстановленного оксида графена в сверхкритическом изопропаноле

Sequential synthesis of anatase modification titanium oxide nanoparticles on reduced graphene oxide in supercritical isopropanol is described. In this case, only graphene oxide was reduced to reduced graphene oxide. A one-stage method (one-pot) was also developed for the preparation of titanium oxide nanoparticles on reduced graphene oxide, where supercritical isopropanol was the graphene oxide reducing agent and the reaction medium. The resulting nanocomposites were studied using X-ray phase analysis, transmission electron microscopy, and atomic force spectroscopy methods.

PHYSICO-CHEMICAL AND SORPTION PROPERTIES OF NANOCOMPOSITE AEROGELS BASED ON MODIFIED CARBON NANOTUBES AND GRAPHENE

The paper presents the technology for obtaining nanocomposite aerogels based on oxidized carbon nanotubes (o-CNTs) and reduced graphene oxide (r-GO) modified with polyaniline and phenoldehyde resin. Aerogel was obtained in high-pressure autoclave among supercritical fluid - isopropanol. The complex diagnostic of synthesized nanocomposite was carried out by the methods of scanning and transmission electron microscopy, infrared spectroscopy, X-ray diffractometry, Raman spectroscopy. The parameters of the porous space were evaluated by adsorption of nitrogen. The resulting electron images indicate that the CNTs was as structural formers, concentrating on their surface graphene sheets. It should also be noted that in the samples, particles of spherical polyaniline were found. It was established that nanocomposite aerogel is a mesoporous material with a specific surface of 289 m2/g. A comparison of infrared and Raman spectra, as well as X-ray diffractograms of the initial materials with aerogel spectra was carried out. According to the results, the nanocomposite contains combined peaks of all source materials. After drying in the supercritical isopropanol media, the ordered structure of the carbon frame is preserved. Electronodononic oxygen and nitrogen-containing groups, which are identified according to the IR spectroscopy as part of the composite, can be active centers for adsorption of heavy metals due to the possibility of the formation of coordination bonds. The nanocomposite sorption activity was evaluated by the example of the heavy metals sorption, namely lead, from model aqueous solutions. To determine the contact time and the absorption mechanism, kinetic studies of adsorption were carried out in a limited volume. It was found that 99% of the pollutant was sorbed in the first 15 min with an achievement of the adsorption capacity of 350 mg/g. Using of pseudo-first and second-order models, the Elovich model and intra-particle diffusion models the adsorption mechanism were offered. For citation: Kuznetsova T.S., Burakov A.E., Pasko T.V., Burakova I.V., Dyachkova T.P., Memetova A.E. Physico-chemical and sorption properties of nanocomposite aerogels based on modified carbon nanotubes and graphene. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2023. V. 66. N 3. P. 66-76. DOI: 10.6060/ivkkt.20236603.6726.

Carbon Formation during Methane Dry Reforming over Ni-Containing Ceria-Zirconia Catalysts.

Two series of Ni/Ce(Ti/Nb)ZrO2 catalysts were prepared using citrate route and original solvothermal continuous flow synthesis in supercritical isopropanol and studied in dry reforming of methane (DRM). TEM, XPS and FTIRS of adsorbed CO confirm influence of support composition and preparation method on the catalysts’ morphology and surface features. The oxygen mobility was studied by isotope heteroexchange with C18O2. After testing in DRM, carbon deposits after catalysts’ testing in DRM were investigated by temperature-programmed oxidation with thermo-gravimetric analysis. The lowest amounts of carbon deposits were obtained for unmodified Ni-CeZr and Ni-CeNbZr compositions. Ti addition lead to an increased amount of carbon, which was removed at higher temperatures. The use of supercritical supports also resulted in the formation of a higher amount of coke. Catalysts prepared by the supercritical synthesis were tested in DRM for 25 h. The highest activity drop was observed in the first three hours. For all compositions, close values of carbon deposits were revealed.

Simple Approach to the Fabrication of Lanthanum Orthoniobates and Nanocomposites with Ni, Cu, and Co Metal Nanoparticles Using Supercritical Isopropanol

Orthoniobates of rare-earth elements are a promising group of materials attractive for the design of nanocomposite hydrogen separation membranes owing to a perspective type of proton conductivity, good mechanical properties, and high stability in H2O- and CO2-containing atmospheres. In general, the promising method involves the synthesis of nanocomposites with transition metals (Cu, Ni, and Cu-Ni alloys) and their oxides with high electronic conductivity. For the first time, lanthanum orthoniobates and nanocomposites with NiCu and NiCo nanoparticles were synthesized using alcohol solutions of salts of the corresponding metals by the solvothermal method in a flow reactor in a supercritical isopropanol medium. This method made it possible to obtain single-phase La0.99Ca0.01NbO4–δ oxide. The introduction of doping titanium cations did not allow obtaining a single-phase La0.99Ca0.01Nb0.98Ti0.02O4–δ sample, as impurities in lanthanum methaniobate and La2Ti2O7 were found. Calcined powders and gastight pellets of orthoniobates and nanocomposites were characterized by X-ray diffraction analysis as well as scanning and transmission electron microscopy. Transport characteristics were investigated by the Van der Pauw technique, varying measurement temperatures in a wet H2 atmosphere. The oxygen mobility was estimated by the oxygen isotope heteroexchange with C18O2.

Immobilization of In2O3 nanoparticles on the surface of reduced graphene oxide

This communication describes a new method for immobilizing indium oxide nanoparticles (∼20 nm) on the surface of reduced graphene oxide. Dispersion of graphene oxide with added In2O3 nanoparticles was treated in supercritical isopropanol, both a reducing agent of graphene oxide and a reaction medium. The resulting nanocomposite was characterized by different methods of physical and chemical analysis.

NANOPARTICLES OF NOBLE METALS ON THE SURFACE OF GRAPHENE FLAKES

Carbon is a spread element that has many different reaction combinations. Obtaining new composite materials based on nanoparticles is a very actual and perspective topic because nanoparticles possess unique properties. These properties are retained and even amplified when nanoparticles are located in various matrixes. Furthermore, nowadays, the creation of graphene-based composites and graphene-related structures is a promising area of synthesis of composite nanomaterials. Previous research has determined that graphene has a unique set of electrophysical, thermal, optical, and mechanical properties. In this study, the synthesis of nanocomposites representing nanoparticles of noble metals (Au, Pd, Rh) on the surface of graphene flakes were carried out, and the study of their composition, structure, physical and chemical properties, and possible applications in catalysis. The immobilization of nanoparticles on the surface of graphene oxide and graphene was developed, and the original method of synthesis of nanocomposite noble metal nanoparticles on the graphene flakes surface using supercritical isopropanol as a reduction agent for the transformation of graphene oxide into graphene was created. The study of physical and chemical properties of the obtained nanocomposites and results of the study of obtained nanocomposites as catalysts for model organic reactions of cross-coupling and hydroformylation showed that it is possible to create the graphene-based nanostructures as effective functional nanomaterials. Research on the synthesis of graphene compounds and its unique physical properties form a promising direction in the chemistry and physics of new inorganic functional materials. The resulting nanocomposites can be used in such branches as electrodes for LEDs and solar cells, field-effect transistors, supercapacitors, sensors, fuel cells.

Structural, Textural, and Catalytic Properties of Ni-CexZr1−xO2 Catalysts for Methane Dry Reforming Prepared by Continuous Synthesis in Supercritical Isopropanol

A series of 5%Ni-CexZr1−xO2 (x = 0.3, 0.5, 0.7) catalysts has been prepared via one-pot solvothermal continuous synthesis in supercritical isopropanol and incipient wetness impregnation of CexZr1−xO2 obtained by the same route. The textural, structural, red-ox, and catalytic properties in methane dry reforming (MDR) of Ni-modified Ce-Zr oxides synthesized by two routes have been compared. It was shown by XRD, TEM, and Raman spectroscopy that the method of Ni introduction does not affect the phase composition of the catalysts, but determines the dispersion of NiO. Despite a high dispersion of NiO and near-uniform distribution of Ni within Ce-Zr particles observed for the one-pot catalysts, they have shown a lower activity and stability in MDR as compared with impregnated ones. This is a result of a low Ni concentration in the surface layer due to segregation of Ce and decoration of nickel nanoparticles with support species.

Synthesis and Evaluation of Adsorption Properties of Reduced Graphene Oxide Hydro- and Aerogels Modified by Iron Oxide Nanoparticles

The synthesis of a nanocomposite material based on graphene modified with iron oxide nanoparticles, which is an effective sorbent of organic pollutants contained in aqueous media, has been considered. The production of graphene by chemical reduction of graphene oxide (GO) is the most flexible and popular method for the synthesis of composite materials of various structures based on graphene. Chemical reduction of graphene oxide was carried out using gamma-lactone-2,3-dehydro-L-gulonic acid (ascorbic acid). The method of obtaining the composite under study includes processing the hydrogel obtained after the GO reduction in supercritical isopropanol, which makes it possible to obtain graphene aerogel containing nanoparticles of iron(II, III) oxide. The analysis of the specific surface and porosity of the obtained aerogel showed that the specific surface area of the aerogel was ~7 times higher as compared to the specific surface of the xerogel obtained by drying of the hydrogel under ordinary conditions in air. The diffractometric analysis of xerogel and aerogel samples suggests that, during supercritical drying in the organic fluid, the processes of iron hydroxide reduction to iron(II, III) oxide proceed. The sorption capacity of the obtained hydro- and aerogel was studied by the extraction of methylene blue organic dye from aqueous solutions in a static mode (batch method). The results of the experiments showed that the maximum values of the sorption capacity for the methylene blue dye were 1370 and 1326 mg/g for the hydro- and aerogel, respectively.

Particularities of pure and Al-doped ZnO nanostructures aerogels elaborated in supercritical isopropanol

Undoped and Al-doped ZnO nanoparticles are synthesized in supercritical isopropanol using zinc acetate di-hydrate, aluminum nitrates and methanol as precursor, dopant source and solvent, respectively. The atomic ratio [Al]/[Zn] is fixed to 3%. X-ray diffraction (XRD) results indicate that the synthesized ZnO nanoparticles possess hexagonal wurtzite structure. Al-doping of ZnO aerogel leads to the improvement of the crystalline quality due to the occupation of Zn2+ sites by Al3+ ions. The crystallites size is found to be 32 nm in pure ZnO and 50 nm in the doped aerogel. Fourier-transform infrared spectroscopy spectra reveal that the intensity of the absorption bands in ZnO decreases and the Zn-O vibration band position shifts towards higher wave numbers upon Al-doping. UV–Visible measurements show a reduction in the band gap and a shift towards long wavelength side of the absorbance with Al-doping. Scanning electron microscopy images reveal that aerogel grains are quasi-spherical in pure ZnO and have different and random forms of morphologies in the doped sample. Room temperature photoluminescence (PL) studies put into evidence that the PL emission peaks are influenced by the Al-doping, in particular, a decrease in the green emission at 535 nm is observed.

Synthesis and Study of Bismuth Nanoparticles on Graphene

Two methods to synthesize Bi/Gr composites of Bi nanoparticles on graphene were developed: a three-step one (method I) and a two-step one (method II). By these methods, the composites are synthesized in supercritical isopropanol (SCI) through bismuth oxide on graphene oxide (GO): Bi2O3/GO. A distinguishing feature of this work is the fact that SCI is both a reducer, and a reaction medium, and it reduces both GO, and Bi2O3 nanoparticles. The obtained nanocomposites were studied by X-ray powder diffraction analysis, transmission electron microscopy, atomic force microscopy, and Raman spectroscopy. Methods I and II produced Bi/Gr nanocomposites with average bismuth nanoparticle sizes of ∼4 and 40–80 nm, respectively.

Processing of a Diamond Blend in Supercritical Isopropanol

Abstract—Samples of a diamond blend (DB) are processed in supercritical isopropanol using a specially designed unit, and the IR spectra, Raman spectra, and X-ray diffraction data before and after processing are analyzed. It is shown that a noticeable decrease in the intensity of the IR absorption bands of the oxygen-containing bonds is observed as a result of this process, which is indicative of the effective etching of the shell of diamond crystallites during processing. A noticeable increase is found in the half-width of the diamond line in the Raman spectra of the samples subjected to processing, which is determined by the decrease in the dimensions of diamond nanocrystallites due to the etching of the surface defect layer. The proposed method has significant advantages over a traditional acid-based technology of purification of a diamond blend and indicates the prospect of practical application for this process.

Meerwein–Ponndorf–Verley–Oppenauer Non-catalytic Reaction of Monoterpenoids in Supercritical Fluids

Abstract—The Meerwein–Ponndorf–Verley–Oppenauer non-catalytic reaction of cyclic monoterpenic ketones and alcohols (menthone, camphor, menthol, and borneol) was studied under supercritical conditions. It has been showed that the ketones can be reduced in supercritical isopropanol with high selectivity (89–98%) without any catalyst or base. At the same time, the conversion of menthone was 63% at 350°C, and that of camphor was 11% at 300°C for 5 h reaction. The conversions of menthol and borneol in the reverse dehydrogenation reaction in supercritical acetone were 34 and 22%, respectively, and the selectivities were 95 and 88%. Thermal dehydration is a main side reaction for all the reactions.

Production of renewable fuels by blending bio-oil with alcohols and upgrading under supercritical conditions

The work studied a non-catalytic upgrading of fast pyrolysis bio-oil by blending under supercritical conditions using methanol, ethanol and isopropanol as solvent and hydrogen donor. Characterisation of the bio-oil and the upgraded bio-oils was carried out including moisture content, elemental content, pH, heating value, gas chromatography-mass spectrometry (GCMS), Fourier transform infrared radiation, 13C nuclear magnetic resonance spectroscopy, and thermogravimetric analysis to evaluate the effects of blending and supercritical reactions. The GCMS analysis indicated that the supercritical methanol reaction removed the acids in the bio-oil consequently the pH increased from 2.39 in the crude bio-oil to 4.04 after the supercritical methanol reaction. The ester contents increased by 87.49% after the supercritical methanol reaction indicating ester formation could be the major deacidification mechanism for reducing the acidity of the bio-oil and improving its pH value. Simply blending crude bio-oil with isopropanol was effective in increasing the C and H content, reducing the O content and increasing the heating value to 27.55 from 17.51 MJ·kg−1 in the crude bio-oil. After the supercritical isopropanol reaction, the heating value of the liquid product slightly further increased to 28.85 MJ·kg−1.

Rapid determination of 13 ultraviolet absorbents in plastic food contact materials by ultra-performance convergence chromatography

A rapid method based on ultra-performance convergence chromatography (UPC2) was developed for the analysis of 13 ultraviolet (UV) absorbents in plastic food contact materials. The UV absorbents were extracted from plastic food contact materials by supersonic extraction with methanol, purified by C18 solid phase extraction column, and analyzed via UPC2 before filtration with an organic filtration membrane (0.22 μm). An ACQUTY UPC2 HSS C18 SB chromatographic column (150 mm×3.0 mm, 1.8 μm) was used for the detection of the UV absorbents. An effective separation was achieved within 4 min under the optimized conditions. The mobile phases were supercritical carbon dioxide and isopropanol as a modifier. The results showed that the 13 UV absorbents exhibited good linear relationships in the respective linear ranges with the correlation coefficients no less than 0.9985. The limits of detection (S/N=3) were in the range of 0.05-0.15 mg/kg. The recoveries were from 86.8% to 115.7%, and the relative standard deviations were between 0.73% and 5.61%. This method is rapid, convenient, accurate and reliable, and can be used for the rapid determination of the 13 UV absorbents in plastic food contact materials.

Highly selective reduction of nitroarenes by sc-isopropanol in the presence of zirconia in a flow reactor

Reduction of nitroarenes using supercritical isopropanol as a source of hydrogen was studied in a flow reactor in the presence of zirconia at temperatures of 515 − 630 K and with residence time about 10 min. Zirconia was shown for the first time to be an effective and selective catalyst for transfer hydrogenation of nitroarenes. The use of complex supercritical fluid iPrOH/CO2 provided 100% selectivity for aniline at 95% conversion of nitrobenzene. In the absence of CO2, the selectivity was reduced due to formation of alkoxylation and N-alkylation by-products. The system can also be used for highly selective reduction of bromonitrobenzene avoiding undesired hydrodehalogenation.