Mechanism of the oxidative carbonylation of terminal alkynes at the ≡C-H bond in solutions of palladium complexes

The formation mechanism of the active catalyst in the oxidative carbonylation of terminal alkynes at the ≡C-H bond has been investigated for the catalytic system Pd(OAc)₂-PPh₃-p-benzoquinone (Q)-MeOH. It has been demonstrated by NMR spectroscopy, X-ray crystallography, and kinetic measurements that the catalytically active palladium is in the oxidation state 0 and is bound into complexes stabilized by p-benzoquinone (PdL₂Q, where L = PPh₃). A mechanism is suggested for the catalytic process, which includes the formation of the complex PdL₂Q, the oxidative addition of the alkyne to this complex at the ≡C-H bond, the insertion of CO into the Pd-C bond, and steps in which hydride hydrogen is intramolecularly transferred to the p-quinone.     

Methylpyrazine Ammoxidation over Binary Oxide Systems: V. Effect of Phosphorus Additives on the Physicochemical and Catalytic Properties of a Vanadium–Titanium Catalyst in Methylpyrazine Ammoxidation

Oxide vanadium–titanium catalysts modified by phosphorus additives (20V₂O₅–(80 –n)TiO₂–nP₂O₅, n = 1, 3, 5, 10, and 15 wt %) are studied in methylpyrazine ammoxidation. Two regions of compositions are found corresponding to radically different catalytic properties, namely, catalysts with a low (5 wt % P₂O₅) and high (10 wt % P₂O₅) concentration of the additive. In the first case, the introduction of phosphorus is accompanied by a gradual increase in the activity. In the second case, an increase in the additive concentration results in a decrease in the activity and selectivity to the target product, pyrazineamide, and a simultaneous increase in the selectivities to by-products, pyrazine and carbon oxides. The catalysts are characterized by X-ray diffraction analysis, differential dissolution, IR, and NMR spectroscopic data. As in the binary system, the active sites of the samples with a low concentration of phosphorus contain V⁵⁺ cations in a strongly distorted octahedral oxygen environment, which are strongly bound to a support due to the formation of V–O–Ti bonds. The catalytic properties of the samples containing 10 wt % P₂O₅ are due to the presence of the phase of a triple V–P–Ti compound with an atomic ratio V : P : Ti approximately equal to 1 : 1 : 1. The V⁵⁺ cations in this compound occur in a weakly distorted tetrahedral oxygen environment and are bound to the tetrahedral P⁵⁺ cations.     

The structure of zirconium-silicate fiberglasses and Pt-containing fiberglass catalysts as revealed by solid-state NMR spectroscopy

¹H, ²³Na and ²⁹Si solid-state NMR spectroscopy was used to investigate the formation of fiberglass zirconium-silicate supports and catalysts (glass → fiberglass → leached fiberglass → catalyst). The main emphasis was made on OH-H₂O domains in fiberglass and their role in catalyst preparation. The size of OH-H₂O domains and their accessibility to H-D exchange were determined. Platinum was shown to be anchored on the bridging hydroxyl groups. The quantum-chemical calculations demonstrated the activity of Pt⁰ clusters forming in fiberglass to depend on their nuclearity.     

Solid state NMR characterization of individual compounds and solid solutions formed in Sc(2)O(3)--V(2)O(5)--Nb(2)O(5)--Ta(2)O(5) system

In this study, (51)V, (45)Sc and (93)Nb MAS NMR combined with satellite transition spectroscopy analysis were used to characterize the complex solid mixtures: VNb(9(1-x))Ta(9x)O(25), ScNb((1-x))Ta(x)O(4) and ScNb(2(1-x))Ta(2x)VO(9) (x = 0, 0.3, 0.5, 0.7, 1.0). This led us to describe the structures of Sc and V sites. The conclusions were based on accurate values for (51)V quadrupole coupling and chemical shift tensors obtained with (51)V MAS NMR/SATRAS for VNb(9)O(25), VTa(9)O(25) and ScVO(4). The (45)Sc NMR parameters have been obtained for Sc(2)O(3), ScVO(4), ScNbO(4) and ScTaO(4). On the basis of (45)Sc NMR and data available from literature, the ranges of the (45)Sc chemical shift have been established for ScO(6) and ScO(8). The gradual change of the (45)Sc and (51)V NMR parameters with x confirms the formation of solid solutions in the process of synthesis of VNb(9(1-x))Ta(9x)O(25) and ScNb((1-x))Ta(x)O(4), in contrast to ScNb(2(1-x))Ta(2x)VO(9). The cation sublattice of ScNb((1-x))Ta(x)O(4) is found to be in octahedral coordination. The V sites in VNb(9(1-x))Ta(9x)O(25) are present in the form of slightly distorted tetrahedra. The (93)Nb NMR parameters have been obtained for VNb(9)O(25).     

Structure of С@Al<Subscript>2</Subscript>O by multinuclear solid-state NMR spectroscopy

The ²⁷Al, ¹H MAS NMR method is used to study initial nanosized metastable aluminum oxides of a pseudoboehmite series (γ- and δ-Al₂O₃) after being coated with graphene (С@Al₂O₃) and annealed in the air (С@Al₂O₃-Т). It is demonstrated that aluminum nanoparticles coated with graphene and annealed at high temperatures (to 750°C for γ and 1180°C for δ) preserve their phase composition but differ from initial oxides by a very low concentration of defects (ОН groups). After the annealing of the graphene coating the hydroxyl cover of oxides is reduced, however, the set of ОН groups differs greatly from that of the initial oxides. Only one type of terminal ОН groups with a ~0.2 ppm shift and one type of bridging μ²-ОН groups with a 1.8 ppm shift for γ-Al₂O₃ from OH-μ²-AlVAl_n and 2.1 ppm for δ-Al₂O₃ OH-μ²-AlIVAl_n are observed. The data obtained make it possible to characterize in detail the δ-Al₂O₃ pure phase.     

Ammoxidation of ethane on V-Mo-Nb oxide catalysts

Summary Catalytic and physicochemical properties of V-Mo-Nb oxide catalysts (V_0.3Mo₁Nb_x, where x = 0.05, 0.15, 0.22, and 0.27) have been studied in the ammoxidation of ethane. An increase in the Nb content in the samples is accompanied by an increase in the catalytic activity and selectivity to acetonitrile. It was established that a triple Mo₅O₁₄-like phase with a variable composition (V_{0.23 ±0.3}Mo₁Nb_x, where х = 0.2?0.37) acts as an active component in the catalyst.</o:p>     

Metatitanic acid: Synthesis and properties

Features of proton-for-lithium ion substitution in monoclinic and pseudocubic lithium titanate are studied by X-ray diffraction, NMR spectroscopy, and Raman spectroscopy. Proton incorporation into the lithium titanate structure decreases the parameter a _c of the pseudocubic unit cell from 8.28 Å in Li₂TiO₃ to =8.15 Å in H₂TiO₃. Metatitanic acid, like hydrous titania, has weak acid properties, but unlike titania, it sorbs hydrolyzable multicharged cations from aqueous solutions.