Russian Journal of Physical Chemistry B最新文献

Hydrolysis lignin conversion in a supercritical n-hexane medium is conducted in two stages: (1) heat treatment of lignin in an autoclave in argon atmosphere at 250°C and 15 MPa and (2) the hydrogenation of the products in solution formed at the first stage at 250°C and 9.0 MPa in the presence of Ru/C catalyst. The products obtained at the first and second stages are analyzed by high-performance liquid chromatography, size exclusive chromatography, gas chromatography, chromatography–mass spectrometry, and elemental analysis. At the first stage, the degree of depolymerization is 17%; the mono- and oligomeric fragments of lignin that pass into solution due to depolymerization have a molecular mass distribution (MMD) in the range of 65 Da to 270 kDa. It is found that the composition of monomers in the first-stage depolymerization products is as follows: mono-, di-, tri-, and tetraalkyl benzene derivatives (2.8 wt %), guaiacol, and 4-alkyl guaiacol derivatives (0.6 wt %), in particular, coniferyl alcohol (0.06 wt %). The following processes occur at the second stage: (i) the depolymerization of oligomers to monomers, (ii) the hydrogenation of monomeric phenols to oxo- and alkyl cyclohexane derivatives, and (iii) the formation of gaseous products (mostly methane, 95 vol %) due to the catalytic hydrocracking of the solvent.

Composite aerogels were prepared from sodium alginate–chitosan binary hydrogels by including magnetite followed by drying in supercritical carbon dioxide. An optimum concentration of magnetite to be introduced in the sodium alginate–chitosan system and conditions for supercritical drying of the composite hydrogels in carbon dioxide were selected experimentally to obtain aerogel with a highly developed porous structure. The resulting aerogels were studied by low-temperature nitrogen adsorption and shown to have a developed mesoporous structure with open cylindrical pores, the maximum specific surface area being 300 m²/g. The experimental study showed that the developed magnetite-containing composite aerogels possess high bactericidal activity against S. aureus, E. coli, and B. subtilis. Due to this, they are very promising for creating various medical products based on them, including wound dressings, active drug carriers, and hemostatic or sorption materials.

The solubility of betulin in pure and modified supercritical carbon dioxide (SC-CO₂) at 313.15–333.15 K and 8.0–30.0 MPa is experimentally studied by the dynamic method. It is found that betulin swells in pure SC-CO₂ without being dissolved. The solubility of betulin increases upon the addition of 5% ethanol as a cosolvent. The pressures corresponding to the first and second crossover points are determined to be 7.7–8.3 and 27.1–27.6 MPa, respectively. The experimental data are described using a model based on the Peng–Robinson equation of state.

The degree of deagglomeration of carbon nanotubes (CNTs) was correlated with the number of treatments by rapid expansion of supercritical suspensions (RESS) under different conditions. Two RESS treatments make it possible to increase the degree of CNT deagglomeration more efficiently than one treatment. Three treatments in the case of the nitrogen solvent lead to a slight increase in the degree of dispersion compared to two treatments. When the CO₂ solvent is used, three treatments lead to a lower degree of dispersion compared to two treatments. When nitrogen is used as a dispersion medium, changes in the temperature and pressure do not lead to a change in the degree of dispersion of CNTs. In the case of CO₂, a decrease in the fluid density ensures higher efficiency of CNT processing. Thus, for efficient dispersion of CNTs by multiple RESS, it is reasonable to use either two treatments with nitrogen or treatment with low-density CO₂.

Two variants of modeling the solubility of palladium hexafluoroacetylacetonate in carbon dioxide (under sub- and supercritical conditions) were considered based on previously obtained experimental data. In the first variant, the Peng–Robinson equation of state was used, and the Chrastil equation was used as an alternative. The optimal parameters of the model equations were found. Both modeling variants made it possible to describe, with satisfactory accuracy, the solubility isotherms in the temperature range of 313–353 K and at mole fractions of the complex up to 0.8%. The standard deviations were 2.6 and 2.0 g/L for the Peng–Robinson and Chrastil equations, respectively.

The effect of the method used to synthesize Ln–Al (Ln = La, Ce, Pr) mixed oxide systems with an Ln : Al atomic ratio of 1 : 1 on the formation of their phase composition and their catalytic properties in the oxidative coupling of methane (OCM) is studied. The precursors are prepared by impregnating ashless filter paper with mixed solutions of nitrates of the respective metals by the incipient wetness impregnation method and subsequent drying and combustion of the resulting mass in air. Further treatment is conducted by combining calcination at 600 and 900°C with a treatment in a water fluid (WF) or water–ammonia fluid (WAF) medium. The laws governing the transformation of the amorphous precursor of Pr–Al oxides during treatment in WF and WAF media and high-temperature synthesis are similar to those observed for the La–Al system. In both cases, the amorphous precursor in a water-containing fluid is transformed into LnAlO₃ with a cubic perovskite structure with an admixture of AlO(OH) (boehmite) and basic REE carbonate phases. The subsequent treatment in air at 900°C leads to the formation of a mixture containing LnAlO₃ aluminates and free La₂O₃ or PrO₂ oxides. Single-phase samples containing exclusively lanthanum and praseodymium aluminates are synthesized by heating amorphous precursors in air at 900°C. The treatment of the Ce–Al system in a WF or WAF leads to the formation of a well-crystallized CeO₂ oxide instead of aluminate or Ce-containing hydroxides, while the Al-containing component remains X-ray amorphous. Cerium aluminate CeAlO₃ is synthesized by treating a mixture of cerium and aluminum oxide precursors in a hydrogen stream. It is found that, due to differences in the 4th ionization potential (IP4) values of the La, Ce, and Pr atoms (49.9, 36.7, and 39.0 eV, respectively), completely different synthesis conditions are required to form LnAlO₃ aluminates with a perovskite structure that contain REE in the (3+) oxidation state. The catalytic properties of the synthesized samples in the OCM are studied. The efficiency and stability of isostructural LnAlO₃ aluminates in the OCM decreases in the following order: La > Pr > Ce. Despite the fact that PrAlO₃ is the most active of these aluminates, LaAlO₃ exhibits the highest selectivity for OCM products (ethane + ethylene).

The adsorption deposition of palladium hexafluoroacetylacetonate Pd(hfa)₂ on porous supports (γ-Al₂O₃ and hypercrosslinked polystyrene, HCP) in supercritical CO₂ is considered. The conditions for a transition of the Pd(hfa)₂/CO₂ system to a monophase region have been studied. Due to its reasonably high solubility in CO₂, the palladium compound can be deposited at relatively low pressures (~8.5 MPa) in the system. The adsorption of Pd(hfa)₂ on the given supports is well described by the Langmuir equation. An increase in the pressure in the system under isothermal conditions negatively affects the amount of adsorbed precursor because of the adsorption of CO₂ and distribution offset in the system. The limiting adsorption increases at elevated temperatures in the system under isochoric conditions.

The dynamics of femtosecond laser impact on water was experimentally studied and reconstructed using numerical modeling based on the classical molecular dynamics method in combination with the two-temperature model and dynamical rate equations. This process occurs in several stages. Initially, a femtosecond laser pulse interacts with the electron subsystem, generating plasma due to multiphoton, tunnel, and impact ionization. The energy transfer from plasma electrons to atoms, as shown using the two-temperature model, leads to ultrafast heating of the substance to a temperature of ~10 000 K, and the pressures achieved in the irradiated area are ~15 GPa, which leads to the generation of a shock wave. The temperatures and pressures exceeding the critical values, combined with high density fluctuations and clustering, indicate the transition of the substance to a supercritical state. The pressures and temperatures exceeding the critical values are achieved in a region slightly exceeding the cavitation zone, and this region experiences oscillations with a period close to the period of oscillations of the cavitation bubble. In the case of the femtosecond laser impact, the experimentally measured deposited energy density can be used as an initial condition under assumtion of unstantaneous heating of the medium, which significantly simplifies numerical modeling. Both the pressures achieved at the shock wave front and the dynamics of cavitation bubbles are successfully reconstructed within the framework of this approach.

In this paper, the “fluctuation theorem” was solved for the first time in a direct way on the mesoscopic length scale. An analytical expression was obtained for the pair correlation function of the critical state of the fluid; the parameter of the function is the density variance, which is easily reconstructed from optical experiments. Comparison of the obtained result with the Ornstein–Zernike pair correlation function made it possible to determine the Fisher correction η = 0.25, which increases the convergence. It was shown that the obtained results provide a reliable theoretical basis for optical diagnostics of the statistical state of critical fluids.

Multiple scattering of laser radiation by the structure of forming porous matrices gives rise to spatiotemporal intensity fluctuations in the field of scattered radiation (dynamic speckle modulation). Dynamic scattering of laser radiation in the volume of evolving foams was used to study the process of formation of highly porous polylactide matrices during their synthesis by the method of supercritical fluid foaming in a carbon dioxide medium. The average lifetime τ_lt of dynamic speckles, which were recorded by a high-speed CMOS camera, was taken as a parameter carrying information on the ensemble-averaged mobility of scattering centers (boundaries of developing pores). A phenomenological model was developed, which established the relationship between the current values of τ_lt, macroscopic parameters of the foam (volume and the rate of its change), and the ensemble-averaged microscopic mobility of interfaces (the first derivative of the average pore size with respect to time). The developed model was used to interpret experimental data on the dynamics of polylactide foam expansion during fast (0.03 MPa/s) and slow (0.006 MPa/s) depressurization of supercritical CO₂ in the reactor. It was determined that in the case of fast depressurization, structural rearrangements in the foam volume continue even after the foam reaches the maximum value of the expansion factor. This feature was qualitatively interpreted based on the concepts of thermodynamic nonequilibrium of porous matrices during their formation.