Theoretical and Experimental Chemistry最新文献

New carbene complex catalysts based on palladium chloride, including PEPPSI-complexes (PEPPSI = Pyridine-Enhanced Precatalyst Preparation Stabilization and Initiation), have been synthesized. These catalysts show high efficiency in Buchwald–Hartwig C–N cross-coupling between aryl chlorides/bromides and amines, including sterically shielded reagents, and high efficiency in the Suzuki–Miyaura C–C cross-coupling reaction, where their use results in the formation of several tetra-ortho-substituted biaryls at very low amounts of catalyst under mild conditions. Modification of the catalyst structure to the PEPPSI complex leads to an unexpected decrease in activity in the Buchwald–Hartwig amination, in contrast to the Suzuki–Miyaura reaction, in which the PEPPSI complex showed greater activity in some cases.

The catalytic effect of alkali metal carbonates, in situ generated carbene, and anionite in the transesterification reaction of alkyl benzoates with methanol, ethanol, benzyl alcohol, and cyclohexanol, as well as vegetable oils with methanol and ethanol was studied. Catalytic reactions with methanol occur at room temperature with yields close to quantitative. The reactions of the vegetable oils in the presence of potassium carbonate with small amounts of methanol or ethanol (3.2 eq) at room temperature lead to high yields of biodiesel fuel and are suitable for industrial use.

The influence of sulfur on the morphology, spectral, and photocatalytic properties of bulk (S-g-C₃N₄) and crystalline carbon nitride (S-CGCN), obtained by the pyrolysis of melamine–S₈ mixtures and additional thermal treatment of S-g-C₃N₄ in the eutectic melt of KCl and LiCl salts, has been investigated. It is shown that crystalline carbon nitride obtained in the presence of 5 wt.% of S₈ (5S-CGCN) demonstrate the highest photocatalytic activity in the hydrogen evolution process from water–ethanol solutions under the effect of visible light. The effective quantum yield of H₂ formation with the participation of 5S-CGCN and metal co-catalysts (palladium and gold) is Φ ≈ 100% at λ = 405 nm, which is almost twice as much as with the participation of unmodified CGCN. The high activity of 5S-CGCN can be associated with more intensive absorption of light, better separation of photogenerated charges, and suppression of their recombination.

The kinetics of the reaction of glycerol with acetic acid in the presence of natural aluminosilicates treated with nitric acid has been investigated. The influence of a molar ratio of the reagents, the reaction temperature and the catalyst mass fraction on glycerol conversion, as well as a rate of the accumulation of the products and their selectivities has been studied. It is established that in presence of clinoptilolite and trepel, monoacetin is the main product. In the case of bentonite, with the increase in the molar fraction of acetic acid and the catalyst mass fraction, the selectivity of monoacetin decreases due to a diacetin accumulation.

The catalytic activity of rhodium-containing nanocomposites with carbon nanodots and Si, Al and Zr oxides in high-temperature water shift reaction has been established. The role of support lies in regulation of the surface concentration of the active component (Rh) and its localization. The use of the support with a low specific surface promote the localization of the active component on the external surface that leads to the decrease of the distance between supported Rh particles, which is important for the surface migration of intermediate products of the reaction.

It is shown that the initial reaction pathways of the processes of steam reforming of ethanol (SRE) or ethanol to propylene (ETP) conversion can be the same and include dehydrogenation and condensation reactions with the formation of acetaldehyde and acetone. The equilibrium yields of propylene and hydrogen in the ETP process, in contrast to SRE, depend little on both temperature and ethanol concentration. An equation for the determination of hydrogen yield as the function of selectivity of carbon-containing compounds has been proposed, based on which the hydrogen concentration in products of the SRE and ETP processes can be calculated.

Hierarchical CsAlTi-BEA zeolites, due to the presence of a high concentration of Lewis acid sites formed by tetrahedrally coordinated Ti⁴⁺ ions in the zeolite structure and the neutralization of Brønsted acid sites by Cs⁺ cations, exhibit high catalytic activity in the tandem reaction of cyclic carbonate formation from styrene in the presence of tert-butyl hydroperoxide under CO₂ pressure. The conversion of styrene into epoxide and further into carbonate increases with the content of titanium in the samples, while the selectivity for the cyclic carbonate increases with the content of cesium.

The ways of targeted design of effective nanocomposite catalysts based on metal oxide systems and zeolites, in particular BEA type, for the processes of dry/super-dry conversion of methane/biogas for the obtaining of hydrogen and syngas with adjustable H₂/CO ratio as well as oxidative dehydrogenation of C₃-C₄ alkanes with the participation of CO₂ producing valuable olefins and dienes are considered. Isolated metal cations/small clusters exhibit higher activity, selectivity, and stability when compared to bulk structures in alkane dehydrogenation reactions since their activation involves the participation of coordinatively unsaturated metal cations on anionic vacancies. This can serve as a scientific basis for the targeted development of highly active and stable catalysts for such processes.

Dynamic light scattering (DLS), electron paramagnetic resonance (EPR), and gas chromatography-mass spectrometry (GC-MS) methods were used to study interaction products of catalytic system components consisting of organoaluminum compounds (OAC), WCl6, and ethyl alcohol. Organic radicals (g = 2,0037, ∆H = 1,7mT) and W⁵⁺ complexes (g_z = 1,746, g_y = 1,820, g_x = 1,842, g₁ = 1,9957, g₂ = 1,9540, g₃ = 1,9465 at 77 K, and g₀ = 1.9583 at 300 K) with an additional hyperfine structure from two chlorine nuclei ^35,37 Cl have been identified by the EPR method. The formation of such compounds is accompanied by ethane and ethylene release. It is shown that a Cl₆/Et₃Al (Et₂AlCl)/EtOH-based catalytic system of olefin metathesis is a colloidal solution at the initial stages of the interaction of components. The further action of OAC leads to the formation of a molecular solution.

A series of Mn²⁺ -doped hybrid perovskites MAPb_1-xMn_xHal₃ (MA = CH₃NH₃, Hal = Cl and Br in different ratios, x = 0.33, 0.5, 0.67, 0.8) was obtained for the first time by mechanochemical synthesis. It is shown that materials obtained retain the perovskite structure even at the highest content of manganese ions (x = 0.8), and their structural and optical characteristics are mainly determined by the Cl/Br ratio. It is established that dispersions in toluene and nanocomposite films in polystyrene exhibit the intense photoluminescence (PL) due to the formation of pseudo-2D nanoparticles as a result of the ultrasonic disintegration of powders in an organic solvent. It is shown that the PL spectra of dispersions and nanocomposite films based on MAPb_1–xMn_xCl₃ are characterized by the appearance of an additional emission band from Mn²⁺ ions in the red region of the spectrum, due to which it is possible to increase the PL quantum yield from 0.2% (for the undoped sample) to 6.3% (films) and 10.2% (dispersions).