1,3,5-Triazinenitrolic acids. Synthesis and NO-releasing properties

Reactions of salts of 1,3,5-triazine dinitromethyl derivatives with nitrogen dioxide that result mainly in 1,3,5-triazinenitrolic acids are investigated. The behavior of nitrolic acid in electroreduction at a mercury drop electrode at various pH was studied; oxygen was shown to accelerate the process. The promoting effect of nitrolic acids on the activity of soluble guanylate cyclase of human platelets was shown. The studied nitrolic acids are active donors of nitric oxide, a universal and important regulator of cell metabolism functions. NO-releasing properties of furoxans are investigated.     

Preparation of Co- or Mn-substituted LTL zeolites and their catalytic properties in cyclohexane oxidation

Co-or Mn-substituted LTL zeolites were hydrothermally synthesized by a novel organic-ligand-assisted method.XRD,UVVis DRS,XPS,and EPR techniques verified that the Co and Mn ions were incorporated into the zeolite framework.No organic species were retained in as-synthesized Co or Mn-LTL zeolites,which resulted in porous materials without calcination.In the oxidation of cyclohexane with tert-butyl hydroperoxide(TBHP),the Co-LTL and Mn-LTL gave a 40%–48%KA oil(cyclohexanone and cyclohexanol)yield as well as nearly 100%TBHP conversion under mild reaction conditions.The reactions were confirmed to be heterogeneous and to have proceeded catalytically.No loss of catalytic activity or leaching of metal active sites ions were observed during 4 reuses.     

Thermal properties of Y-type zeolites

The thermal and structural properties of two parent NaY zeolites and of those modified by ion exchange (ReNaY, HNaY, FeNaY) were investigated by simultaneous thermal analysis (TG-DTA-DTG) and by X-ray diffraction spectroscopy. Both the intracrystalline water and the zeolite framework were in our attention. The impurities (Fe) located in the lattice as well as the ions which entered by ion exchange (Re, H, Fe) influence the properties of the zeolites. The values of the activation energy of the dehydration process prove that the water molecules are more strongly bonded in all modified samples than in the parent ones. As compared to the NaY zeolites, an increased thermal stability, of about 100°C was revealed for ReNaY or of about 180°C for HNaY, and a decreased stability, of about 50°C, for FeNaY samples. The temperature at which the lattice break-down beginsT _amf, estimated by following the X-ray diffraction patterns for samples heated in air at temperatures from 300 to 1100°C, is the temperature which may be related to the structural characteristics of the zeolites, i.e., to the lattice constant of the uncalcined materials. The XRD studies reveal the heterogeneity of the crystallites constituting the zeolite material from both the point of view of the lattice constant values and the thermal stability. As the temperatureT _amf, generally, does not coincide with the temperature of the first exothermic peak,T ₁, of the DTA curve, we suggest the temperatureT _amf to be taken as an unambiguous measure of the thermal stability.     

Regulation of the Si/Al ratios and Al distributions of zeolites and their impact on properties

Zeolites are typically a class of crystalline microporous aluminosilicates that are constructed by SiO₄ and AlO₄ tetrahedra. Because of their unique porous structures, strong Brönsted acidity, molecular-level shape selectivity, exchangeable cations, and high thermal/hydrothermal stability, zeolites are widely used as catalysts, adsorbents, and ion-exchangers in industry. The activity, selectivity, and stability/durability of zeolites in applications are closely related to their Si/Al ratios and Al distributions in the framework. In this review, we discussed the basic principles and the state-of-the-art methodologies for regulating the Si/Al ratios and Al distributions of zeolites, including seed-assisted recipe modification, interzeolite transformation, fluoride media, and usage of organic structure-directing agents (OSDAs), etc. The conventional and newly developed characterization methods for determining the Si/Al ratios and Al distributions were summarized, which include X-ray fluorescence spectroscopy (XRF), solid state ²⁹Si/²⁷Al magic-angle-spinning nuclear magnetic resonance spectroscopy (²⁹Si/²⁷Al MAS NMR), Fourier-transform infrared spectroscopy (FT-IR), etc. The impact of Si/Al ratios and Al distributions on the catalysis, adsorption/separation, and ion-exchange performance of zeolites were subsequently demonstrated. Finally, we presented a perspective on the precise control of the Si/Al ratios and Al distributions of zeolites and the corresponding challenges.

This review highlights the basic principles and the state-of-the-art methodologies for regulating the Si/Al ratios and Al distributions as well as their impact on performances. A perspective on their precise control is presented.     

The unified method for computing thermodynamic properties of natural zeolites based on their crystallochemical formulas

The standard thermodynamic potentials of natural zeolites in universal stoichiometric representation are computed. The enthalpies are estimated using linear deconvolution into chemical elements for calibration minerals. The entropies are computed using the additive scheme for oxide components; the Gibbs free energies are computed based on the calculated standard thermodynamic functions and entropies of thermochemically simple compounds.     

Chemical composition and ion exchange properties of zeolites

Series of exchange selectivity for alkali cations in zeolites change in a regular manner in dependence on the ratio (z) between the number of particles capable of solvating cations in the frameworks of zeolites (molecules of zeolite water, one-fourth of the framework oxygen ions) and the number of zeolite cations. For z=5–6 zeolites have maximum selectivity for large cations, for z<3–4 they selectively absorb sodium, while if z<2 they selectively absorb lithium. An analysis is given of the reasons of this dependence of the ion exchange selectivity of zeolites on the chemical composition of their frameworks. It is shown that the free energy of solvation of cations in zeolites is less than the free energy of their hydration. The effect of transformation of the selectivity series of zeolites can be connected with the contribution, on the part of the effect of redistribution of water between the solid and liquid phases of the ion exchange system, to the free energy of ion exchange reactions, which differs in dependence on the composition and structure of the crystals.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 3, pp. 378–382, May–June, 1990.     

Study of physical and thermochemical properties of modified zeolites

The treatment of zeolite with a solution of NaOH at different concentrations creates modified forms of natural zeolites.These modified zeolites exhibit an increased uptake of caesium and a satisfactory uptake of cobalt. The structural changes in the modified zeolites were studied by XRD analysis, which reveals the mineral compositions of the studied materials. The thermodynamic properties of the modified zeolites were studied by means of DTA and thermogravimetry, and the chemical composition too was determined.     

On the acidic properties of H-SABO zeolites

Computer handling of TPAD data reveals a decreasing strength of Brönsted sites and forced diffusion limitations caused by incorporation of boron at pentasil type zeolite lattice positions.

---

, , , .

     

Thermal and structural properties of rare earth exchanged zeolites

A series of rare earth zeolites of types X and Y were prepared by cation exchange. Thermal analysis curves (TG/DTA) were employed to estimate the structural changes in the zeolite framework. Analogous investigations were carried out by independent methods such as XRD and IR spectroscopy. The results indicate increased thermal stability on the replacement of sodium by rare earth. Partial replacement of rare earth by ammonium/hydrogen enhances the thermal stability. The type Y zeolites are more stable than those of X type.

---

Zusammenfassung Eine Reihe von Ionen der Seltenen Erdmetalle enthaltenden Zeolithen des Typs X und Y wurden durch Ionenaustausch hergestellt. Thermoanalytische Kurven (TG/DTA) wurden zur Beurteilung der strukturellen Veränderungen des Zeolithgitters herangezogen. Analoge Untersuchungen wurden mit unabhängigen Methoden (XRD und IR-Spektroskopie) ausgeführt. Es wurde festgestellt, daß ein Ersatz der Natriumionen durch Ionen der Seltenen Erdmetalle zu einer Erhöhung der thermischen Stabilität führt. Ein teilweiser Ersatz der Seltenen Erdmetallionen durch Ammonium-/Wasserstoffionen erhöht die thermische Stabilität. Zeolithe des Typs Y sind stabiler als die des X-Typs.

---

X . (/) . - . . . X .

---

---

NCL Communication No: 2186     

Electron distribution of small Ni particles and Ce ions in X and Y zeolites related to their catalytic properties

Catalytic behaviour of nickel particles smaller than 1.3 nm, encaged in X and Y zeolites, in the presence of Ce³⁺, is related to their electronic properties. In general the rates of reaction are lower than those of a standard catalyst made of 2.5 nm nickel particles on SiO₂; this is attributed to the strong localization of the Ni 3d states due to the size effect. However a high rate, more than twice the standard, is found for Ni in X zeolite, in relation with a Ni 3d ¹⁰ configuration. On the other hand the fragmentation of butane is deeper with Ni in Y support than in X support and this corresponds to a high density of Nid states at the Fermi level.     

Synthesis of a NO-releasing prodrug of rofecoxib

[reaction: see text] A newly developed synthesis of a NO-releasing prodrug of rofecoxib is described. The highly productive process consists of five chemical steps and produces prodrug 1 in an overall 64% yield from commercially available 3-phenyl-2-propyn-1-ol (4). The synthesis is highlighted by the carbometalation reaction of propargyl alcohol 4 to generate the tetrasubstituted olefin core, sulfone acid 2. Additionally, two alternate end-game strategies to prepare NO-COXIB 1 from this intermediate were explored and developed: (1) a convergent synthesis where a bromonitrate side chain is introduced in one step and (2) a two-step sequence that first installs the requisite six-carbon ester side chain followed by chemoselective nitration.     

The synthesis and physicochemical and catalytic properties of SHS zeolites

Superhigh-silica ZSM-type zeolites with silica moduli of 100–220 were synthesized using hexamethylenediamine as a structure-forming additive. Physicochemical methods were used to identify them as ZSM-5-type zeolites, determine their degree of crystallinity, specific surface areas, and morphology of crystals, and study the phase and elemental composition. The strength and concentration of acid centers were found to decrease as the silica modulus grew. The zeolites obtained can be used as the main component of catalysts for refining low-octane gasoline fractions and methanol conversion into hydrocarbons.     

Surface organometallic chemistry on zeolites: A tool for modifying the sorption properties of zeolites

The shape selective properties of zeolites can be modified by reaction of their surface functional groups with hydride, alkyl or alkoxy complexes of main group elements. The different methods are reviewed and their effects analyzed.     

Physicochemical properties of alkali-metal exchanged type X zeolites

The changes in the physicochemical properties of a series of faujasite type X zeolites cation exchanged with K⁺, Rb⁺ and Cs⁺ have been studied by XRD, IR, thermoanalytical methods and sorption measurements. As a consequence of the enhanced scattering of X-rays by larger alkali metal cations, the percent relative intensity of the XRD peaks of cation exchanged zeolites was found to have decreased considerably. The framework IR spectra also showed analogous changes. The alkali metal exchange was found to enhance the thermal stability of the parent zeolite. The available void volume and specific surface area (obtained by low temperature nitrogen sorption) also decreased with the increase in the degree of exchange and cationic size. Equilibrium sorption capacities (298 K andP/P ₀=0.5) for water,n-hexane, cyclohexane and 1,3,5-trimethylbenzene also exhibited the same trend.NCL communication no. 6056.     

Stabilized zeolites of Y-type. Their adsorption and catalytic properties

The stabilized decationized form of a Y-type zeolite has been prepared by the self-steaming calcination method. The temperature of the stabilization process influenced the nature of the interaction with ethylene.

---

Y . .

     

The properties of methylene- and amine-substituted zeolites from first principles

All-silica zeolite frameworks doped with methylene and amine groups are studied using density functional theory-based electron structure calculations. Strain energies are calculated in a novel way, by comparing zeolite energies with appropriate polymer reference systems. The modified zeolites are found to be mechanically stable structures with surprisingly little strain. Distortions due to impurities result in broadened Si-O-Si angle distributions in the lattice surrounding defects. Our results suggest that zeolites can accommodate both methylene and amine groups at high concentrations with minimal strain. The amine-doped zeolites are strong Lewis bases suggesting novel applications in base catalysis.     

Catalytic properties of zeolites in hydrogenation of acetaldehyde and isobutyraldehyde

Conclusions Zeolites of various types (A, faujasite, chabazite, erionite, and mordenite with Na, Ca, and Nd cations) are highly active and selective catalysts for the hydrogenation of acetaldehyde and isobutyraldehyde in a 200–350°C interval and an H₂ pressure of 30–50 atm, while zeolites NaM and NaA possess the maximum hydrogenating activity.Translated from Izvestiya Akademiya Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1847–1850, August, 1978.