Properties and deactivation of the active sites of an MoZSM-5 catalyst for methane dehydroaromatization: Electron microscopic and EPR studies

The MoZSM-5 (4.0 wt % Mo) catalyst has been characterized by high-resolution transmission electron microscopy, EDXA, and EPR. Two types of molybdenum-containing particles are stabilized in the catalyst in the course of nonoxidative methane conversion at 750°C. These are 2-to 10-nm molybdenum carbide particles on the zeolite surface and clusters smaller than 1 nm in zeolite channels. According to EPR data, these clusters contain the oxidized molybdenum form Mo⁵⁺. The surface Mo₂C particles are deactivated at the early stages of the reaction because of graphite condensation on their surface. Methane is mainly activated on oxidized molybdenum clusters located in the open molecular pores of the zeolite. The catalyst is deactivated after the 420-min-long operation because of coke buildup on the zeolite surface and in the zeolite pores.     

Partial Synthesis and Characterization of the Mono-and Diepoxides of β-Cryptoxanthin

β-Cryptoxanthin ( 1 ) was acetylated and then epoxidized with monoperoxyphthalic acid. After hydrolysis, repeated chromatography, and crystallization, (3S,5R,6S)-5,6-epoxy-β-cryptoxanthin ( 3 ), (3S,5S,6R)-5,6-epoxy-β-cryptoxanthin ( 4 ), (3R,5′R,6′R)-5′,6′-epoxy-β-cryptoxanthin ( 5 ), (3S,5R,6S,5′R,6′S)-5,6:5′,6′-diepoxy-β-cryp-toxanthin ( 6 ), and (3S,5S,6R,5′S,6′R)-5,6:5′,6′-diepoxy-β-cryptoxanthin ( 7 ) were isolated as main products and characterized by their UV/VIS, CD, ¹H- and ¹³C-NMR, and mass spectra. The comparison of the carotenoid isolated from yellow, tomato-shaped paprika (Capsicum annuum var. lycopersiciforme flavum) with 3–5 strongly supports the structure of 3 for the natural product.     

Isolation and characterization of 3,5,6-trihydroxy-carotenoids from petals ofLilium tigrinum

Summary Reinvestigation by HPLC of the petals ofLilium tigrinum and the isolation of some minor compounds is reported. Using HPLC-controlled, preparative-column chromatography, 5,6-diakarpoxanthin (6), 6-epikarpoxanthin (2), 5,6-diacapsokarpoxanthin (8), and 9Z-antheraxanthin (9Z-13) were isolated and characterized. Based on spectroscopic data the absolute configurations of6 and8 were identical with those originating from paprika, thus the 5,6-diakarpoxanthin (6) and 5,6-diacapsokarpoxanthin (8) have the 3S, 5S, 6S configuration and 6-epikarpoxanthin (2) has the 3S, 5R, 6S configuration. Presented at: Balaton Symposium on High-Performance Separation Methods, Siófok, Hungary, September 3–5, 1997.     

Synthesis of mesoporous bimetallic Pt-Sn catalytic coatings from polynuclear precursors for fine organic synthesis processes

A new method is developed to obtain nanosized catalytic Pt-Sn/TiO₂ coatings on the inner surface of a capillary microreactor during adsorption of polynuclear carbonyl Pt-Sn complexes on mesoporous TiO₂. Titanium oxide sol prepared in the presence of template (Pluronic F127 surfactant) is supported in dynamic mode. Pt-Sn bimetallic catalysts with an average particle size of 1.5–2 nm are synthesized by adsorption of the bimetallic [Pt₃(CO)₃(SnCl₃)₂(SnCl₂·H₂O)] _n ^2n? complex followed by thermal treatment. Physicochemical properties of samples (thickness, structure and morphology, chemical composition of the material, electronic state, specific surface area, pore volume and size distribution) are characterized by a set of methods (HR TEM, SEM, powder XRD, XRF, XPS, low-temperature nitrogen adsorption). Conditions to prepare the uniform non-peelable Pt-Sn/TiO₂ coating on the inner surface of a silica capillary with good adhesion are determined. To increase the TiO₂ thickness, multilayered TiO₂ films are synthesized by layerby layer deposition. The coating thickness is found to increase with an increase in the capillary diameter. The coating of a capillary with a diameter of 0.55 mm after 14-fold deposition is characterized by a thickness of 2 μm and an average pore size of 5.4 nm. The solvent effect on the adsorption of Pt-Sn carbonyl complexes into the TiO₂ support is studied. The amount of the adsorbed complex increases in the following order: ethanol < acetone ～ tetrahydrofuran. The physicochemical properties of the active component (surface concentration, dispersion, and composition) can be fine-tuned by varying the deposition method, precursor concentration in the initial solution, and temperature conditions of activation treatment. The catalyst activity in citral hydrogenation was 0.06–0.54 min^?1, with the selectivity with respect to unsaturated alcohols reaching 90% at citral conversion above 95%.     

Ethylene Induction of Non-Enzymatic Metabolic Antioxidants in Matricaria chamomilla

Phytochemical investigations of Matricaria chamomilla L. (Asteraceae) stated the presence of several compounds with an established therapeutic and antioxidant potential. The chamomile non-enzymatic antioxidant system includes low molecular mass compounds, mainly polyphenols such as cinnamic, hydroxybenzoic and chlorogenic acids, flavonoids and coumarins. The objective of this work was to evaluate the role of the non-enzymatic antioxidant system after stimulation by ethylene in tetraploid chamomile plants. Seven days of ethylene treatment significantly increased the activity of phenylalanine ammonia-lyase, which influenced the biosynthesis of protective polyphenols in the first step of their biosynthetic pathway. Subsequently, considerable enhanced levels of phenolic metabolites with a substantial antioxidant effect (syringic, vanillic and caffeic acid, 1,5-dicaffeoylquinic acid, quercetin, luteolin, daphnin, and herniarin) were determined by HPLC-DAD-MS. The minimal information on the chlorogenic acids function in chamomile led to the isolation and identification of 5-O-feruloylquinic acid. It is accumulated during normal conditions, but after the excessive effect of abiotic stress, its level significantly decreases and levels of other caffeoylquinic acids enhance. Our results suggest that ethephon may act as a stimulant of the production of pharmaceutically important non-enzymatic antioxidants in chamomile leaves and thus, lead to an overall change in phytochemical content and therapeutic effects of chamomile plants, as well.     

Stability of difference schemes in terms of Riemann invariants for a polytropic gas

The monotonicity and stability of a finite difference scheme with respect to initial data in the supremum norm are analyzed as applied to the polytropic gas equations written in terms of Rie-mann invariants for subsonic flows with 1 < γ < 3. Conditions on the initial and boundary data are obtained under which subsonic flows with no shock waves develop in the medium. The theoretical conclusions are supported by numerical results.     

Distributions of the implanted impurity and energy release in high-energy ion implantation

A new model for high-energy ion implantation is proposed, based on the use of the Fokker-Planck equation. An efficient algorithm of the adaptive type is devised for the numerical solution of the problem. A simulation of the implantation of boron in silicon is carried out for energies ranging from 10 to 100 MeV. Good agreement is obtained with the experimental data and the results of Monte Carlo simulations. Zh. Tekh. Fiz. 67, 61–67 (January 1997)     

Difference Schemes on Nonuniform Grids for the Two-Dimensional Convection–Diffusion Equation

New second-order accurate monotone difference schemes on nonuniform spatial grids for two-dimensional stationary and nonstationary convection–diffusion equations are proposed. The monotonicity and stability of the solutions of the computational methods with respect to the boundary conditions, the initial condition, and the right-hand side are proved. Two-sided and corresponding a priori estimates are obtained in the grid norm of C. The convergence of the proposed algorithms to the solution of the original differential problem with the second order is proved.