Probable mechanism for alkane oxidation with H2O2 over VS-2

Metallosilicate zeolites containing Ti and V, TS-2 and VS-2, have been synthesized using tetrabutylammonium cation as the template. Although both Ti and V exist in zeolite framework in a highly dispersed state, V easily leaches by K₂CO₃ or H₂O₂ treatments in contrast to Ti. Spin trapping experiments and relative reactivity of linear/cyclic alkanes indicate that the oxidation of alkanes over TS-2 and VS-2 proceed by different mechanisms; it is conceivable that the oxidation occurs by way of the attack of ·OH species produced from the V species in the zeolite framework with H₂O₂.     

An investigation of the relationship between microstructure and permeation properties of ZSM-5 membranes

The influence of membrane microstructure on the transport properties of ZSM-5 membranes was investigated. Two zeolite membranes with (1 0 1)- and (0 0 2)-orientations were grown layer-by-layer onto seeded alumina support. The membrane morphology was kept constant as well as the shape of the individual crystal grains that made up the polycrystalline zeolite membrane layer. The membrane microstructure were characterized and quantified using six microstructural parameters that include membrane thickness (τ), grain size (d), grain morphology (M), zeolite population (N), crystal intergrowth (I_c) and film orientation. Eight different gases including He, H₂, N₂, Ar, CH₄, n-C₄H₁₀, i-C₄H₁₀ and SF₆ were used as molecular probes to investigate the transport processes through the membrane of different thicknesses. By maintaining a comparable non-zeolite flow, it was demonstrated that the (1 0 1)- and (0 0 2)-oriented ZSM-5 membranes have comparable transport resistance. Also, the results of the multi-thickness comparison using the different sized molecular probes indicate a strong similarity in the transport mechanism and diffusion pathway through these two membranes. The experiment suggests that the grain boundary is the main non-zeolite diffusion pathway in the membrane and their elimination through grain growth can result in better membrane performance.     

Preparation and catalytic activity of titanium silicalite-1 zeolite membrane with TPABr as template

The preparation of the supported titanium silicalite-1 (TS-1) zeolite membrane with inexpensive tetrapropylammonium bromide (TPABr)/weak base synthesis system was studied by three methods, and the catalytic activity of the obtained TS-1 zeolite membrane was evaluated with the oxidation of 2-propanol (IPA) under pervaporation condition. It was found that TS-1 zeolite membrane could be successfully prepared with “seeding” or “seeding in situ” method, but could not be achieved with “in situ” method. Adding a little amount of promoter ions of PO₄³⁻ into the synthesis gel was of benefit to the catalytic activity of the prepared TS-1 zeolite membrane, but had no obvious effect on the membrane layer formation on the mullite porous support. For “seeding” method, the membrane prepared with the synthesis gel having molar composition of SiO₂:0.1TPABr:0.9Et₂NH:0.03TiO₂:80H₂O:0.06H₃PO₄ at 150 °C for 48 h showed the highest oxidation conversion of IPA of 72% accompanied by a flux of 0.35 kg/m² h. Further more, much higher IPA oxidation conversion of 76% accompanied by a flux of 0.65 kg/m² h was obtained for the TS-1 zeolite membrane prepared with the same synthesis gel by “seeding in situ” method at 150 °C for 72 h.     

金属对钛硅分子筛TS-1催化丁二烯环氧化性能的影响

制备了不同金属改性的钛硅分子筛样品M-TS-1 (M = V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, La, 负载量为1%), 并研究了其催化丁二烯环氧化反应的性能. 结果表明, V, Cr, Mn的加入导致H₂O₂无效分解, 因而在丁二烯环氧化反应中表现出较高的H₂O₂转化率和较低有效利用率; Fe, Co, Ni以及稀土金属La均在一定程度上促进了TS-1对H₂O₂的有效利用; Cu, Zn抑制了H₂O₂的转化, 使得H₂O₂转化率和有效利用率都较低; Cd有效提高了TS-1的催化活性, H₂O₂转化率和有效利用率均接近100%. 采用X射线衍射、N₂吸附-脱附、X射线光电子能谱、傅里叶变换红外光谱等手段表征了上述各金属的改性对TS-1骨架结构以及Ti活性中心周围电子环境的影响. 结果表明, 各M-TS-1样品的骨架结构都仍保持原有的MFI构型, 但是TS-1中Ti活性中心周围的电子环境受到来自各种金属的不同程度影响, 关联催化活性时没有特定的规律性.     

Membrane separation of multicomponent mixture of alkanes C1–C4

The membrane separation of the four-component mixture of gaseous alkanes C₁–C₄ is studied. Homogeneous films based on two high-permeable polymers, namely, addition-type poly[3-(trimethylsilyl)tricyclononene-7] and poly[3,4-bis(trimethylsilyl)tricyclononene-7], are used as membranes. Separation of the multicomponent mixture of hydrocarbons on these polymers follows the same trends as separation of binary mixtures CH₄-C₄H₁₀ on polyacetylenes. In the presence of higher hydrocarbons, the permeability coefficients of methane decrease and the permeates become enriched with higher hydrocarbons. During separation of the multicomponent mixture, permeability coefficients P(C₄H₁₀) attain high values (up to 12000 Barrers).     

Rh(OH)x/TS-1@KCC-1双功能催化剂上一锅反应合成伯酰胺

在微乳液体系中成功地将花瓣状介孔氧化硅微球(KCC-1)包覆在钛硅分子筛TS-1表面, 得到了一种新型微孔/介孔复合核壳结构材料TS-1@KCC-1. 详细考察了TS-1@KCC-1的合成条件及可能的形成机理. 适宜的合成温度及时间分别为373-393 K和4 h. 壳层厚度可方便地通过改变硅源正硅酸乙酯和TS-1的比例在25-80 nmm间进行调控. 以TS-1@KCC-1为载体负载Rh(OH)_x后即得到双功能催化材料Rh(OH)_x/TS-1@KCC-1, 该材料同时具有Ti和Rh(OH)_x活性中心, 可以高效催化醛、氨水和过氧化氢经一锅串联反应直接制备得到伯酰胺.     

Impact of a polymer modifier on directing the non-classical crystallization pathway of TS-1 zeolite: accelerating nucleation and enriching active sites

The crystallization process directly affects the physicochemical properties and active centers of zeolites; however, controllable tuning of the zeolite crystallization process remains a challenge. Herein, we utilized a polymer (polyacrylamide, PAM) to control the precursor structure evolution of TS-1 zeolite through a two-step crystallization process, so that the crystallization path was switched from a classical to a non-classical mechanism, which greatly accelerated nucleation and enriched active Ti sites. The TS-1 crystallization process was investigated by means of various advanced characterization techniques. It was found that specific interactions between PAM and Si/Ti species promoted the assembly of colloidal precursors containing ordered structural fragments and stabilized Ti species in the precursors, leading to a 1.5-fold shortened crystallization time and enriched Ti content in TS-1 (Si/Ti = 29). The PAM-regulated TS-1 zeolite exhibited enhanced catalytic performance in oxidative reactions compared to conventional samples.

Polyacrylamide (PAM) was utilized to switch the crystallization pathway of TS-1 zeolite from classical to non-classical by virtue of the specific interaction between PAM and Si/Ti species, thus accelerating nucleation and enriching active sites.     

Titanium Defective Sites in TS-1: Structural Insights by Combining Spectroscopy and Simulation

Ti silicates, and in particular, titanium silicalite-1 (TS-1), are nowadays important catalysts for several partial oxidation reactions in the presence of aqueous H₂O₂ as an oxidant. Despite the numerous studies dealing with this material, some fundamental aspects are still unclear. In particular, the structure and the catalytic role of defective Ti sites, other than perfect tetrahedral sites recognized as the main active species, has not been quantitatively discussed in the literature. We assess the structural features of defective Ti sites on the basis of outcomes of electronic spectroscopies, as interpreted through quantum mechanical simulation. Strong evidence is disclosed to support the fact that the most common defective Ti sites, often reported in the TS-1 literature, are monomeric Ti centers that are embedded in the zeolite framework, and which have a distorted octahedral local symmetry.     

Titanocene(IV) and vanadocene(IV) complexes of dicyanomethanidobenzoate

The new titanocene and vanadocene complexes of the non-linear pseudohalides Cp₂Ti(dcmb)₂, Cp₂VCl(dcmb), (η⁵-C₅H₄Me)₂VCl(dcmb) and Cp₂V(dcmb)₂ were prepared by reaction of titanocene dichloride (Cp₂TiCl₂) and vanadocene dichlorides (Cp₂VCl₂, (η⁵-C₅H₄Me)₂VCl₂) with dicyanomethanidobenzoic acid (dcmbH, PhC(OH)C(CN)₂). These reactions have proven that the dcmb ligand could be coordinated to the central metal by oxygen or nitrogen donor atoms. The bonding mode of the dcmb ligand reflects properties of the central metal. The strongly oxophilic titanium(IV) shows the bonding through oxygen atom while bonding through nitrogen atom was observed for less oxophilic vanadium(IV). The bonding fashion of the dcmb ligands was determined by spectroscopic methods. X-ray diffraction analysis was used for the structure determination of the compounds dcmbH·H₂O, Cp₂Ti(dcmb)₂·CH₂Cl₂, (η⁵-C₅H₄Me)₂VCl(dcmb) and [Cp₂V(OC(Ph)C(CN)C(OMe)NH)][dcmb].     

C2-Bridged sandwich and half-sandwich entities with Ti(IV) and Cr(0) centers

The intense purple colored bi- and trimetallic complexes {Ti}(CH₂SiMe₃)[CC(η⁶-C₆H₅)Cr(CO)₃] (3) ({Ti}=(η⁵-C₅H₅)₂Ti) and [Ti][CC(η⁶-C₆H₅)Cr(CO)₃]₂ (5) {[Ti]=(η⁵-C₅H₄SiMe₃)₂Ti}, in which next to a Ti(IV) center a Cr(0) atom is present, are accessible by the reaction of Li[CC(η⁶-C₆H₅)Cr(CO)₃] (2) with {Ti}(CH₂SiMe₃)Cl (1) or [Ti]Cl₂ (4) in a 1:1 or 2:1 molar ratio. The chemical and electrochemical properties of 3, 5, {Ti}(CH₂SiMe₃)(CCFc) [Fc=(η⁵-C₅H₅)Fe(η⁵-C₅H₄)] and [Ti][(CC)_nMc][(CC)_mM′c] [n, m=1, 2; n=m; n≠m; Mc=(η⁵-C₅H₅)Fe(η⁵-C₅H₄); M′c=(η⁵-C₅H₅)Ru(η⁵-C₅H₄); Mc=M′c; Mc≠M′c] will be comparatively discussed.     

Zeolite micromembranes

Free-standing silicalite-1 (Sil-1) zeolite micromembranes have been successfully fabricated onto silicon substrate. Gas permeation test using permanent gases (i.e., helium, hydrogen, argon and nitrogen) and hydrocarbons (i.e., methane and n-butane) indicates that the micromembranes have excellent permeance flux and high permselectivity.     

He(I) and He(II) photoelectron spectra of some mixed sandwich compounds of titanium,zirconium and hafnium

The He(I) and He(II) photoelectron spectra are reported for two series of transition metal mixed sandwich complexes of general formula L_aML_b (M = Ti, L_a = η⁵-C₅H₅, η⁵-CH₃C₅H₄, η⁵-C₅(CH₃)₅; M = Zr, Hf, L_a = η⁵-C₅(CH₃)₅; L_b = η⁷-C₇H₇ (series I); M = Ti, L_a = η⁵-CH₃C₅H₄, η⁵-C₅(CH₃)₅; M = Zr, L_a = η⁵-C₅(CH₃)₅; L_b = η⁸-C₈H₈ (series II)). Assignments were made of the metal d, cyclopentadienyl and carbocyclic π orbitals on the basis of He(I)/He(II) intensity ratios and shift effects and by comparison with UP data for related compounds. For series I no influence of the central metal upon the IEs of the highest occupied molecular orbital e₂ was observed. The IE of the non-bonding metal d_z2 orbital of Ti or Zr (5.28 and 4.70 eV, respectively) in the complexes of series II (L_a = η⁵-C₅(CH₃)₅) is very low.     

The hapticity of cyclooctatetraene in its first row mononuclear transition metal carbonyl complexes: Several examples of octahapto coordination

The two cyclooctatetraene metal carbonyls that have been synthesized are the tetrahapto derivative (η⁴-C₈H₈)Fe(CO)₃ and the hexahapto derivative (η⁶-C₈H₈)Cr(CO)₃ using the reactions of cyclooctatetraene with Fe(CO)₅ and with fac-(CH₃CN)₃Cr(CO)₃, respectively. Related C₈H₈M(CO)_n (M = Ti, V, Cr, Mn, Fe, Co, Ni; n = 4, 3, 2, 1) species have now been investigated by density functional theory in order to explore the scope of cyclooctatetraene metal carbonyl chemistry. In this connection, the existence of octahapto (η⁸-C₈H₈)M(CO)_n species is predicted as long as the central metal M does not exceed the 18-electron configuration by receiving eight electrons from the η⁸-C₈H₈ ring. Thus the lowest energy structures (η⁸-C₈H₈)Ti(CO)_n (n = 3, 2, 1), (η⁸-C₈H₈)M(CO)_n (M = V, Cr; n = 2, 1), and (η⁸-C₈H₈)Mn(CO) all have octahapto η⁸-C₈H₈ rings. An exception is (η⁶-C₈H₈)Fe(CO), with a hexahapto η⁶-C₈H₈ ring and thus only a 16-electron configuration for the iron atom. Hexahapto (η⁶-C₈H₈)M(CO)_n structures are predicted for the known (η⁶-C₈H₈)Cr(CO)₃ as well as the unknown (η⁶-C₈H₈)Ti(CO)₄, (η⁶-C₈H₈)V(CO)₃, (η⁶-C₈H₈)Mn(CO)₂, and (η⁶-C₈H₈)Fe(CO)₂ with 18, 18, 17, 17, and 18 electron configurations, respectively, for the central metal atoms. There are two types of tetrahapto C₈H₈M(CO)_n complexes. In the 1,2,3,4-tetrahapto (η⁴-C₈H₈)M(CO)_n complexes two adjacent CC double bonds, forming a 1,3-diene unit similar to butadiene, are bonded to the metal atom. In the 1,2,5,6-tetrahapto (η^2,2-C₈H₈)M(CO)₃ derivatives two non-adjacent CC double bonds of the C₈H₈ ring are bonded to the metal atom. The known (η⁴-C₈H₈)Fe(CO)₃ is a 1,2,3,4-tetrahapto complex. The unknown isomeric 1,2,5,6-tetrahapto complex (η^2,2-C₈H₈)Fe(CO)₃ is predicted to lie ∼15 kcal/mol above (η⁴-C₈H₈)Fe(CO)₃. The related 1,2,5,6-tetrahapto complexes (η^2,2-C₈H₈)Cr(CO)₄, (η^2,2-C₈H₈)Mn(CO)₄, [(η^2,2-C₈H₈)Mn(CO)₃]⁻, (η^2,2-C₈H₈)Co(CO)₂, and (η^2,2-C₈H₈)Ni(CO)₂ are all predicted to be low-energy structures.     

Synthesis,characterization and separation properties of a composite mordenite/ZSM-5/chabazite hydrophilic membrane

Composite mordenite/ZSM5/chabazite membranes were prepared on α-alumina tubular supports by in situ liquid phase hydrothermal synthesis. The membranes obtained were approximately 10 μm thick and were characterized by XRD, SEM and EPMA, as well as permeation of single gases (N₂ and n-butane). The membranes were then used to separate ternary mixtures, containing water, alcohol (methanol, ethanol or propanol), and a permanent gas (O₂). Water permeated faster because of the hydrophilic character of the composite membrane, with water/propanol selectivities as high as 149. The influence of operating conditions (temperature, pressure and feed composition) on the separation performance was analyzed. Also, the behavior of this composite zeolite membrane was compared with that of pure silicalite and ZSM5 membranes, and the differences observed are discussed in terms of relative organophilicity/hydrophilicity of the zeolites involved.     

Adsorption-Induced Deformation of Adsorbents

Adsorption-induced deformation of AR-V and AUK carbon adsorbents and NaX zeolite has been studied upon adsorption of n-С₅Н₁₂, n-С₆Н₁₈, n-С₇Н₁₆, and CO₂ at temperatures of 193?423 K. It has been shown that adsorption-induced deformation is positive upon the physical adsorption of gases and vapors on the surface of a nonporous (macroporous) solid when the excess adsorption is positive. When calculating the adsorption-induced deformation in the region of the capillary-condensation filling of mesopores, the additional pressure in capillaries, which is negative (contraction of an adsorbent), must be taken into account in the case of wetting a solid surface with a liquid adsorbate. The compressibility of AUK microporous carbon adsorbent as a porous solid is almost independent of the temperature and the properties of an adsorbate, and, for adsorption of n-C₅H₁₀ and n-C₇H₁₆ hydrocarbons and CO₂, it is γ_а = (5.6 ± 0.6) × 10^?6 bar^?1. The compressibility of AUK adsorbent appears to be 87% higher than that of nonporous graphite.     

Configuration of MFI-type zeolite interacting with the probe molecule P(CH<Subscript>3</Subscript>)<Subscript>3</Subscript> – a theoretical study

With P(CH₃)₃ as the probe molecule adsorbed on titanium silicalite (TS-1) zeolite, the special and important role of T12 site in MFI-type zeolite was clearly elucidated. There are altogether three active sites present in TS-1 zeolite with Ti at the T12 site. Owing to the preferential adsorption of probe molecules on the first Brönsted acidic site, the Ti12 center will probably fail to show Lewis acidity. The ionic [HP(CH₃)₃]⁺ species can be stabilized by the first or second Brönsted acidic site, with the former energetically favored. The latter was formed through the transfer of the ionic [HP(CH₃)₃]⁺ species from the first to the second Brönsted acidic site.     

含氟体系中高性能T型分子筛膜的合成

含氟体系中,在负载晶种的大孔莫来石支撑体表面快速合成了高性能且取向生长的T型分子筛膜。采用XRD、SEM和MAS NMR等手段对分子筛膜层和粉末进行表征。讨论了添加物、氟硅比、合成温度和合成时间等条件对膜生长与分离性能的影响,并阐述了含氟体系中T型分子筛膜快速晶化的机理。碱金属氟盐的加入促进了T型分子筛晶体层的晶化速率,并对晶体层形貌产生了一定的影响。膜应用于75℃、水/异丙醇(10∶90,w/w)体系的平均渗透通量和分离因子分别为(4.91±0.18)kg·m-2·h-1和7 060±1 130。     

Effect of Treatment with Carbon Dioxide of the Zn Form of the TsVK Zeolite on Its Acid and Catalytic Properties in the Conversion of Methanol to Hydrocarbons

It is shown that it is possible to increase the yield of liquid hydrocarbons of the benzene fraction in the Mobil process by use of a catalyst modified by treatment with CO₂ of the Zn form of the TsVK zeolite. It is established that on treatment with CO₂ of the Zn-TzVK zeolite the concentration of strongly acidic -centers is increased. As a result of alkylation of C₂-C₄ olefins by methanol at these sites more of the high molecular C₅-C₈ aliphatic hydrocarbons are formed. Selectivity of conversion of methanol to liquid C₅-C₁₀ hydrocarbons of the benzene fraction is increased, but selectivity with respect to the light C₂-C₄ fractions is decreased.     

Synthesis and gas permeation properties of silicalite-1 zeolite membrane

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