“Extracting” the Key Fragment in ETS‐10 Crystallization and Its Application in AM‐6 Assembly

The mechanism of crystallization of microporous titanosilicate ETS‐10 was investigated by Raman spectroscopy combined with ²⁹Si magic‐angle spinning (MAS) NMR spectroscopy, DFT calculations, and SEM imaging. The formation of three‐membered ring species is shown to be the key step in the hydrothermal synthesis of ETS‐10. They are formed by means of a complex process that involves the interaction of silicate species in the reaction mixture, which promotes the dissolution of TiO₂ particles. These insights into the mechanism of ETS‐10 growth led to the successful development of a new synthesis route to the vanadosilicate AM‐6 that involves the use of intermediates that contain three‐membered ring species as an initiator.     

稀土负载钛-硅沸石 ETS-10 的制备及其光催化性质(英文)

以水玻璃和四氯化钛为原料, 在不使用有机模板剂、氟离子和晶种的条件下, 用水热法合成了钛-硅沸石 ETS-10, 将 La、Ce、Pr 和 Nd 四种稀土元素负载到合成的 ETS-10 上. 通过 X 射线粉末衍射、N2 吸附-脱附、29Si 魔角旋转核磁共振、紫外漫反射光谱、X 射线荧光光谱等表征手段对负载前后的 ETS-10 进行了表征. 以有机染料甲基橙为底物, 考察了负载各种稀土及氢氟酸腐蚀对 ETS-10 的光催化活性的影响. 结果表明, 四种稀土元素的引入均可有效提高 ETS-10 的光催化活性. 反应活性提高的程度与稀土元素负载量有关. 对 ETS-10 同时进行氢氟酸腐蚀和稀土元素的负载, 可以将 ETS-10 的光催化活性提高近一倍, 与锐钛矿相 TiO2 相当, 但前者更易分离.     

Mechanism of noncatalytic synthesis of tris(hydroxymethyl)phosphine. Nonempirical quantum-chemical approach

The gas-phase reaction of the synthesis of tris(hydroxymethyl)phosphine from phosphine and formaldehyde was studied using a calculation scheme based on the density functional theory with hybrid exchange-correlation functional B3LYP in the 6–311++G** basis. The reaction was shown to proceed with the participation of unstable intermediates containing three-membered ring. The transformation into final products includes opening of the three-membered ring and intramolecular proton transfer. The results can be useful at selecting catalysts and for explaining the mechanism of the catalytic reaction of tris(hydroxymethyl)phosphine synthesis.     

Pentacoordinate 1H-phosphirenes: reactivity, bonding properties, and substituent effects on their structures and thermal stability

The synthesis, reactivity, and bonding properties of several pentacoordinate P-phenyl-substituted 1H-phosphirenes are discussed. X-ray crystallographic analysis of one of them reveals a highly distorted square pyramidal (SP) arrangement around the phosphorus. NMR studies confirm that they retain the SP structure in solution and demonstrate that the endocyclic P-C bonds in the three-membered ring have a very high degree of p character, which results from their being both basal bonds in the SP structure and endocyclic bonds of the three-membered ring. Structural parameters of the three-membered ring of the pentacoordinate phosphirenes obtained by experiment and theoretical calculations are very close to those of a tetracoordinate phosphirenium cation. Thus, by analogy with tetracoordinate phosphirenium cations, it can be considered that a sigma-pi interaction between the sigma orbital of the apical bond and the pi orbital of the C=C bond in the three-membered ring is operative in pentacoordinate phosphirenes. The sigma-pi interaction is found to lower the reactivity of the C=C bond of the three-membered ring. The reactivities of the pentacoordinate phosphirenes are also affected by the substituent on the carbon atom in the three-membered ring.     

A Neutral Three-Membered 2π Aromatic Disilaborirane and the Unique Conversion into a Four-Membered BSi2N-Ring

We report the design, synthesis, structure, bonding, and reaction of a neutral 2π aromatic three-membered disilaborirane. The disilaborirane is synthesized by a facile one-pot reductive dehalogenation of amidinato-silylene chloride and dibromoarylborane with potassium graphite. Despite the tetravalent arrangement of atoms around silicon, the three-membered silicon-boron-silicon ring is aromatic, as evidenced by NMR spectroscopy, nucleus independent chemical shift calculations, first-principles electronic structure studies using density functional theory (DFT) and natural bond orbital (NBO) based bonding analysis. Trimethylsilylnitrene, generated in situ, inserts in the Si−Si bond of disilaborirane to obtain a four-membered heterocycle 1-aza-2,3-disila-4-boretidine derivative. Both the heterocycles are fully characterized by X-ray crystallography.     

A study of heat-treatment induced framework contraction in strontium-ETS-4 by powder neutron diffraction and vibrational spectroscopy.

The effects of heat-treatment on the structure of the strontium ion-exchanged titanosilicate ETS-4 have been studied by Rietveld analysis of powder neutron diffraction data and by FT-Raman spectroscopy. Hydrous Sr-ETS-4 (space group Cmmm), upon heat-treatment under inert atmosphere at temperatures between 423 and 573 K, exhibits framework contraction as evinced by the decrease in the unit cell dimensions. The effects of heat-treatment on the dimensions of the transport-controlling eight-membered ring (8MR) are elucidated by Rietveld analysis. It is also found that during heat-treatment: (a) the double three membered rings (D3MRs) in ETS-4 are sites of structural instability, (b) the titania chains running along [010] exhibit a large degree of disorder in the bridging oxygen atoms, and (c) significant relocations of the strontium cations take place, which may affect the separation properties of the heat-treated materials. Raman spectra of heat-treated ETS-4 crystals exhibit strong cation-framework interaction effects. Vibrational modes involving the atoms in the titania chains show progressive frequency shifts and loss of intensity with increasing heat-treatment temperature, in a manner consistent with the crystallographic results. The study indicates the potential for continuously varying the effective pore dimension of ETS-4 by combining heat-treatment with appropriate ion-exchange procedures.     

Pd-catalyzed ring-opening copolymerization of 2-aryl-1-methylenecyclopropanes with CO to afford polyketones via alternating insertion of the two monomers and C [bond] bond C activation of the three-membered ring

Pd-bpy complexes catalyze the ring-opening copolymerization of 2-aryl-1-methylenecyclopropanes with CO, affording the new polyketones composed of ring-opened structural units only. Kinetic and isotope-labeling studies revealed the mechanism of the reaction, which involves 1,2-insertion of the cyclic monomer followed by beta-alkyl elimination of the three-membered ring of the polymer chain.     

A computational study to elucidate the extraordinary reactivity of three-membered heterocycles in nucleophilic substitution reactions

The accelerated rates of small-membered heterocycles relative to acyclic analogues are typically rationalized solely in terms of relief of ring strain. The relative rates of attack of ammonia on oxirane, oxetane, thiirane, and thietane were determined computationally in the gas phase at the MP2(Full)/6-31+G(d) level with respect to the model acyclic compounds methoxyethane and thiomethylethane. Because the cyclic ether and thioether pairs have very similar strain energies, they should react at similar rates by the S(N)2 mechanism if the degree of strain energy release in the transition state is approximately equal. The reactivity of the four-membered rings could be explained almost entirely by relief of strain. The three-membered rings reacted at rates at least 10(6) times faster than calculated from ring strain considerations alone. The electronic distribution of the transition states was determined using AIM methodology and found to indicate that bond cleavage was virtually complete, while bond formation was incomplete. Calculation of atomic charges by the Mulliken, AIM, CHELPG, and NBO methods indicated that positive charge at the reaction center was significantly lower for the three-membered rings than other members of the series. A simple electrostatic model identified differences in energy sufficient to account for the observed rate acceleration. The unique topological features of a three-membered ring make it possible for the partially negatively charged oxygen or sulfur to reduce the positive charge on the reaction center.     

Competitive adsorption of Pb2+, Cu2+, and Cd2+ ions on microporous titanosilicate ETS-10

In the present study, the competitive adsorption characteristics of binary and ternary heavy metal ions Pb2+, Cu2+, and Cd2+ on microporous titanosilicate ETS-10 were investigated in batch systems. Pure microporous titanosilicate ETS-10 was synthesized with P25 as the Ti source and characterized by the techniques of X-ray diffraction (XRD), field emission-scanning electron microscope (FESEM), nitrogen adsorption, and zeta-potential. Equilibrium and kinetic adsorption data showed that ETS-10 displays a high selectivity toward one metal in a two-component or a three-component system with an affinity order of Pb2+ > Cd2+ > Cu2+. The equilibrium behaviors of heavy metals species with stronger affinity toward ETS-10 can be described by the Langmuir equation while the adsorption kinetics of the metals can be well fitted to a pseudo-second-order (PSO) model.     

Enhancement of the ETS-10 titanosilicate activity in the shape-selective photocatalytic degradation of large aromatic molecules by controlled defect production

In recent times, it has been shown that the microporous ETS-10 titanosilicate can be used as a shape-selective photocatalyst for the decomposition of aromatic molecules (Chem. Commun. 2001, 2131). Its actual use on practical grounds is however discouraged by its too low activity, when compared with that of TiO(2) photocatalysts. In the present work, we show how an ad hoc mild treatment with HF enhances the activity of ETS-10 toward the photodegradation of large aromatic molecules that are unable to penetrate inside the zeolitic pores, such as 2,5-dichlorophenol, 2,4,5-trichlorophenol, 1,3,5-trihydroxybenzene, and 2,3-dihydroxynaphthalene (DHN). The photoactivity of the acid-treated materials is comparable or even greater than that of the nonselective TiO(2) catalyst. Moreover, the enhancement of the photoactivity is accompanied by a remarkable parallel increase of the shape selectivity, particularly toward DHN (k(DHN)/k(P) = 127, where P = phenol). A complete characterization (by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, ultraviolet-visible spectroscopy, and X-ray aborption spectroscopy techniques) of a set of ETS-10 samples which have undergone a progressively severe HF treatment allows us to propose an explanation of the photocatalytic activity and selectivity of the modified materials.     

Cyclization and Isomerization Reactionsin Silylhydrazine Chemistry

 This review describes the synthesis and isomerization reactions of cyclic silylhydrazines. Topics of discussion are the ring expansion of the three-membered Si(SiN₂) to the four-membered (SiN)₂ ring by lithiation of the (SiN₂) ring and by thermal silyl group insertion into the N*N bond, the ring expansion of a three-membered (SiN₂) to a five-membered (CSi₂N₂) ring by SiCH₂ insertion into the Si*N bond, the formation of isomeric four- and six-membered (SiN₂)₂ rings, the synthesis of five- and six-membered silylhydrazine rings, and the expansion of a five-membered (N₂Si₂N)N ring to the isomeric six-membered (SiNN)₂ ring. The mechanisms of the isomerizations are explained by quantum chemical calculations, and the results are verified by crystal structure determinations.     

Heterocyclic Analogues of Methylenecyclopropanes

Heterocyclic three-membered rings with an exocyclic double bond merit special interest in view of their ring strain and their fascinating chemistry. Their synthesis, thermal decomposition, ring opening, and cycloaddition reactions will be discussed in this review. α-Lactams, alkylideneaziridines, diaziridinones, and diaziridinimines belong to the best known classes, but the chemistry of alkylideneoxiranes and -thiiranes, α-lactones, α-thiolactones, thiiranimines, and aziridinimines has also been studied to some extent. Two elusive three-membered rings are oxiranimines and thiaziridinimines; the former were postulated as reactive intermediates in the thermal decomposition of α-lactams, whereas the latter were trapped efficiently with double and triple bonds during thermolysis of N-sulfonyliminothiatriazolines. In spite of many attempts to prepare thione derivatives of the title compounds, they are still unknown. Included in this review are also some open-chain dipolar species which are isomeric with the heterocyclic three-membered rings.     

Effect of water and alkali modifications on ETS-10 for the cycloaddition of CO2 to propylene oxide

Sodium oxide (NaOx) impregnated Engelhard Titanosilicate-10 (ETS-10) molecular sieve catalysts were prepared to enhance the basicity associated with ETS-10 and subsequently investigated for the cycloaddition of carbon dioxide to propylene oxide to produce propylene carbonate. For dry NaOx-modified ETS-10 catalysts that contained no adsorbed water, a maximum yield of propylene carbonate was achieved at a loading of 2.0 excess NaOx species per unit cell. However, the greatest enhancements in the rate of reaction were observed when small amounts of water were adsorbed onto the unmodified ETS-10 catalyst immediately prior to reaction. Surface-bound water appears to enhance the surface Bronsted acidity of the unmodified ETS-10 catalyst via the formation of surface -OH groups at lower water loadings, producing a surface of better-tuned acid-base bifunctional characteristics for the cycloaddition reaction. At levels of hydration greater than 12.5% by mass, the yield of propylene carbonate was further enhanced, but at a smaller rate than that observed at lower rehydration levels, which is more indicative of an enhanced transport effect. Adsorption microcalorimetry of carbon dioxide indicated that, at loadings less than 2.0 NaOx per unit cell, the total uptake of the CO2 adsorption sites required for the reaction were less than in the parent ETS-10 material. However, at higher levels of NaOx occlusion, where the total uptake and strength of the adsorption sites exceeded those observed for the as-received ETS-10 material, the cycloaddition activity of this catalyst suffered due to the reduced pore volume and surface area. It appears that precise tuning of both the surface acidity and basicity is crucial in creating an effective acid-base bifunctional ETS-10 catalyst for the cycloaddition reaction investigated.     

Cyclization and Isomerization Reactionsin Silylhydrazine Chemistry

Summary.  This review describes the synthesis and isomerization reactions of cyclic silylhydrazines. Topics of discussion are the ring expansion of the three-membered Si(SiN₂) to the four-membered (SiN)₂ ring by lithiation of the (SiN₂) ring and by thermal silyl group insertion into the N*N bond, the ring expansion of a three-membered (SiN₂) to a five-membered (CSi₂N₂) ring by SiCH₂ insertion into the Si*N bond, the formation of isomeric four- and six-membered (SiN₂)₂ rings, the synthesis of five- and six-membered silylhydrazine rings, and the expansion of a five-membered (N₂Si₂N)N ring to the isomeric six-membered (SiNN)₂ ring. The mechanisms of the isomerizations are explained by quantum chemical calculations, and the results are verified by crystal structure determinations. Received February 23, 2001. Accepted (revised) April 24, 2001     

Isomeric structures and visible electronic spectrum of the C7H3 radicals

The 2-(buta-1,3-diynyl)cycloprop-2-yl-1-ylidene radical, a new three-membered ring chain with Cs symmetry, has been detected by electronic spectroscopy in the gas phase. The experimental investigation used a mass selective resonant two color two photon ionization technique coupled to a supersonic plasma source. Structures and relative stability energies of eight isomers of the C7H3 radical have been calculated. Based on the rotational analysis and the theoretical calculations, the observed spectrum is assigned as an 2A" <-- X2A' electronic transition of this exotic chemical species. This result shows that such a plasma source is a powerful tool to investigate intermediates involved in hydrocarbon chemistry as in flames.     

Computer simulations of adsorption and diffusion for binary mixtures of methane and hydrogen in titanosilicates

Equilibrium molecular dynamics (MD) simulations of equimolar mixtures of hydrogen and methane were performed in three different titanosilicates: naturally occurring zorite and two synthetic titanosilicates, ETS-4 and ETS-10. In addition, single-component MD simulations and adsorption isotherms generated using grand canonical Monte Carlo simulations were performed to support the mixture simulations. The goal of this study was to determine the best membrane material to carry out hydrogen/methane separations. ETS-10 has a three-dimensional pore network. ETS-4 and zorite have two-dimensional pore networks. The simulations carried out in this study show that the increased porosity of ETS-10 results in self-diffusion coefficients for both hydrogen and methane that are higher in ETS-10 than in either ETS-4 or zorite. Methane only showed appreciable displacement in ETS-10. The ability of the methane molecules to move in all three directions in ETS-10 was demonstrated by the high degree of isotropy shown in the values of the x, y, and z components of the self-diffusion coefficient for methane in ETS-10. From our simulations we conclude that ETS-10 would be better suited for fast industrial separations of hydrogen and methane. However, the separation would not result in a pure hydrogen stream. In contrast, ETS-4 and zorite would act as true molecular sieves for separations of hydrogen and methane, as the methane would not move through membranes made of these materials. This was indicated by the near-zero self-diffusion coefficient of methane in ETS-4 and zorite.     

Carboxylation of a polypentenamer and preparation of its hydrogenated derivatives

Carboxylated three-membered ring derivatives of a polypentenamer (PP) that contained 82% trans and 17% cis double bonds were prepared by carbene addition of ethyldiazoacetate with a copper catalyst to the double bonds and subsequent hydrogenation of the residual unsaturation. In this way derivatives that contained approximately either 5 or 10 mole % three-membered rings with ester side groups were obtained. These side groups were further reacted by hydrolysis or neutralization to form carboxylic acid and cesium salts of carboxylic acid. Reaction conditions were chosen so that no backbone degradation occurred and side reactions that led to crosslinking were avoided. The derivatives were characterized by gel permeation chromatography (GPC), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), infrared (IR) spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy. It was found that an increase in substituent concentration decreased the melting point (T_m) in hydrogenated derivatives and increased the glass transition temperature (T_g) in unhydrogenated derivatives. The cesium salts of carboxylic acid were the least thermally stable among those studied and the hydrogenated derivatives were generally more stable than the unhydrogenated.     

Detection of OH on photolysis of styrene oxide at 193 nm in gas phase

Photodissociation of styrene oxide at 193 nm in gas phase generates OH, as detected by laser-induced fluorescence technique. Under similar conditions, OH was not observed from ethylene and propylene oxides, primarily because of their low absorption cross-sections at 193 nm. Mechanism of OH formation involves first opening of the three-membered ring from the ground electronic state via cleavage of either of two CO bonds, followed by isomerization to enolic forms of phenylacetaldehyde and acetophenone, and finally scission of the COH bond of enols. Ab initio molecular orbital calculations support the proposed mechanism.     

Selective photocatalytic transformations on microporous titanosilicate ETS-10 driven by size and polarity of molecules

Photocatalytic activity of microporous titanosilicate ETS-10 has been studied in water. The photoactivated ETS-10 shows catalytic activity driven by size and polarity of substrates. ETS-10 efficiently catalyzes a conversion of substrates with a size larger than the pore diameter of ETS-10. In contrast, the reactivity of small substrates depends strongly on substrate polarity; less polar substrates show higher reactivity on ETS-10. Electron spin resonance analysis reveals that large substrates or less polar substrates scarcely diffuse inside the highly polarized micropores of ETS-10 and, hence, react efficiently with hydroxyl radicals (*OH) formed on titanol (Ti-OH) groups exposed on the external surface of ETS-10. In contrast, small polar substrates diffuse easily inside the micropores of ETS-10 and scarcely react with *OH, resulting in low reactivity. The photocatalytic activity of ETS-10 is successfully applicable to selective transformations of large reactants or less polar reactants to small polar products, enabling highly selective dehalogenation and hydroxylation of aromatics.     

Solid state morphology and band gap studies of ETS-10 supported CdS nanoparticles

Engelhard titanosilicate (ETS-10) supported cadmium sulphide (CdS) nanoparticles were synthesized and characterized by various solid state techniques including: XRD, DR UV-Vis, TEM and FESEM. The effect of different synthesis routes of CdS nanoparticles on its physicochemical character was studied. It was observed that CdS nanoparticles prepared by both in situ sulphur reduction (CdS-IS) and reverse micelle (CdS-RM) methods showed similar roperties. However, CdS-IS nanoparticles are more feasible and economically practical. The reflectance measurements of the as-synthesized CdS nanoparticles are apparently blue-shifted compared to bulk CdS. This phenomenon of blue-shifted absorption edge has been ascribed to an increase in bandgap energy with a decrease in particle sizes. The bandgap of the as-synthesized CdS samples was calculated from the linear correlation of [F(R) hν]² and hν. The bandgap of CdS in ETS-10 was noticeably slightly reduced when compared with the as-synthesized CdS (8 nm) due to the formation of cluster arrays on the pores of ETS-10.