Microporous Materials最新文献

The shrinkage of the opaque portion of zeolite reaction mixtures during hydrothermal treatment was studied. Experimental results for unstirred, batch zeolite A reaction systems showed that the non-(solid gel) fraction changed during the course of the reaction. The results also showed that the sudden shrinkage of the opaque portion during the reaction did not correlate with the sudden appearance of crystals in the mixture, but was due to crystal growth. Experiments also were performed in which nutrient materials, both with and without a nucleation control agent (triethanolamine), were added to reacting mixtures at various times during reaction. Nutrient additions without nucleation control resulted in only a small increase in size of the zeolite A product. The largest crystal sizes were obtained, however, when the nucleation control agent was present in the original mixture from the beginning of the reaction.

A meaningful solubility product of a zeolite should remain constant as composition of zeolite mother liquors varies at a given temperature. We identify a solubility product of zeolite A, ∏_s=[Si(OH)₄][Al(OH⁻₄][Na⁺]=1.2×10⁻⁸M³. from experimental data in highly alkaline aqueous solutions under conditions typical for zeolite A synthesis (1 M ≤ [Na] ≤ 4 M, 353 K ≤ T ≤ 363 K). To accomplish this we develop a thermodynamic model of dilute sodium aluminosilicate solutions representing mother liquors in equilibrium with zeolite A. Proper modeling of the liquid phase adjacent to a growing zeolite is necessary to obtain meaningful supersaturation values and solubility products. Further quantitative modeling of solution mediated zeolite growth and dissolution should include a physically relevant model of aluminosilicate speciation in the liquid phase.

Syntheses were performed in the system 10SiO₂:1Al₂O₃:xNa₂O:[y wt.%organic compound + (100-y)wt.%H₂O] at 150°C. In the presence of dioxane, for Na₂O contents between 2 and 3, primarily mazzite-type products were obtained with Si/Al ratios ranging from 3.3 to 4.1. The dioxane content (y=5–85) influences both the availability of the dioxane-Na⁺ complex, which is essential for MAZ crystallization, and the effective alkalinity. As such, well-crystallized MAZ samples with high and low Si/Al ratios could be obtained at low (y=5–10) and high dioxane content (y=85), respectively, while at intermediate content (y=50), denser phases crystallized. For Na₂O contents lower than 2, sodalite phases were formed with the neutral dioxane molecule acting as a templating agent, while above 3, mainly gismondine and analcime were produced. For syntheses with high dioxane content, the cyclic ether was partially replaced by other organic solvents (ethers and alcohols) to study the effect of the solvent properties on zeolite crystallization. The solubility of the organic compound in the aqueous phase and its solvating properties toward anions and cations are of key importance with respect to the rate and the type of zeolite synthesis in organic medium.

In order to design magnetic carriers for medical drugs, carbon-supported iron samples with an iron content of 23, 29 and 43 wt.% have been prepared by impregnation of synthetic carbon with aqueous solution of iron oxalate followed by calcination and by reduction in hydrogen. The samples obtained after reduction times of 15, 30, 45 and 60 min have been characterized by Mössbauer spectroscopy, X-ray diffraction (XRD), and magnetization as well as adsorption measurements. Analysis of the Mössbauer and XRD data shows the presence of both oxidic and metallic iron particles with sizes in the 3–30 nm range. The saturation magnetization of the carbon loaded with 23, 29, and 43 wt.% iron has been determined to be 20, 30 and 80 emu per gram of solid, respectively. The hydrogen treatment induces in addition an increase of the meso- and macropore volumes of the iron-loaded carbon by a factor of about 1.4 with respect to the unloaded carbon. This fact is shown to lead to a better adsorption capacity for macro-molecular drugs like streptomycin and vitamin B12.

Titanium-containing MCM-41 was prepared from the synthesis gel derived from Ti(OC₄H₉)₄ and several Si sources. By using tetraethyl orthosilicate (TEOS), which is free from Al and Na, as a silica source, we were able to incorporate titanium into the framework of MCM-41 without the formation of anatase. Ti-MCM-41 was applicable to the oxidation of bulky alkenes with H₂O₂ or t-butyl hydroperoxide (TBHP) as an oxidant. The stability of Ti-MCM-41 against water was low compared to pure-silica MCM-41.

A procedure for the preparation of low-density silica described in a patent filed in 1969 [V. Chiola, J.E. Ritsko, C.D. Vanderpool, US Patent 3 556 725, 1971] leads to the formation of solids having all the properties of MCM-41, the ordered mesoporous silica disclosed in 1991 [J.S. Beck, C.T.-W. Chu, I.D. Johnson, C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.W. Vartuli, WO Patent 91/11390, 1991].

A new synthesis of hydrotalcite-like materials having a composition [M²⁺_1−xAl³⁺_x](OH)₂ × A⁻_x × mH₂O, where M²⁺ is magnesium, nickel or cobalt and A⁻ is a monocarboxylic anion, is described. The synthesis involves reacting a source of the divalent cation with pseudoboehmite and a monocarboxylic acid in an aqueous slurry at 95°C for 6 h. The reactive source of magnesium is either magnesium oxide or hydroxide and the reactive source for nickel and cobalt are the respective hydroxides. This synthesis method results in the formation of hydrotalcites with a unique sheet-like morphology with the sheets having breadth to thickness ratio in excess of 100, in contrast to the previously known hexagonal hydrotalcites which have a breadth to thickness ratio between 5 and 10. The phrase ‘sheet-like’ in this work describes the macroscopic morphology of the hydrotalcite crystallites. A key finding was that the presence of monocarboxylic acid was necessary for the formation of sheet morphology. It was also demonstrated that sheet and hexagonal morphologies readily interconvert via a common mixed oxide postcursor.

Syntheses were performed in the system 10SiO₂:yAl₂O₃:(1−y)Ga₂O₃:zP₂O₅:(2.4−4.1)Na₂O:0.4(18C-6 or 15C-5):(140−180)H₂O. For the Si-Al system, the presence of P₂O₅ allows the crystallization of EMT at high alkalinity, due to the complexation of Al(OH)4/− by phosphate anions. Remarkably, this complexation is less efficient when 18C-6 is replaced by 15C-5, giving FAU phases with low Si/Al ratio. The addition of small amounts of Ga (y=0.05–0.1) leads to intergrowth formation of EMT and FAU. This is ascribed to the preferential location of Ga in the double six-rings which connect the successive FAU sheets. In the absence of P₂O₅, Ga is less reactive than Al and will be more homogeneously distributed over the framework. Under these conditions, up to 25% of Al can be substituted by Ga in the EMT structure.

A synthesis route to obtain high-silica MOR using cyclic amines as structure-directing agents is described. As far as we know, the use of these structure-directing agents for the synthesis of high silica MOR is not described elsewhere.

The pore size of the synthesised products (zeolite MOR, FER and mixtures of zeolite MOR and FER), was related to the ring size of the organic structure-directing agent in the reaction mixture. Various saturated cyclic amines were used. Pure high-silica MOR of high crystallinity was obtained via direct synthesis using hexa- and heptamethyleneimine. In addition, high-silica FER was prepared among others with pyrrolidine. A mixture of MOR and FER was synthesised in the presence of piperidine. The materials were characterised and the catalytic performance of the high silica MOR was evaluated.

Catalysts prepared with direct synthesis of high-silica MOR as base material were active in the isomerisation of pentane and hexane. Presumably the direct-synthesis high-silica MOR samples were somewhat less active than conventional MOR (dealuminated by leaching to the same Si/Al ratio SAR), because of a lower accessibility of acid sites. The lower accessibility of the direct synthesis mordenite may be due to amorphous material. Other explanations are that extra-framework aluminum species were formed during calcination and preferential aluminium siting in these MOR crystals.

This study reports on the physico-chemical characterisation of an aluminium-containing microporous titanosilicate ETS-10 material in its protonic form in connection with catalytic testing. ²⁷Al MAS NMR spectra reveal the successful incorporation of aluminium into the lattice structure. XRD patterns indicate that the investigated ETAS-10 material in its protonic form is relatively stable thermally, an indispensable requirement for the application as an industrial catalyst material. Nevertheless, for all samples, except parent ETAS-10, XRD and ²⁹Si MAS NMR measurements indicate that an increasing amount of amorphous silica is present. Acidity investigations with combined FT-IR spectroscopy and temperature-programmed ammonia desorption and with ¹H MAS NMR revealed that dehydroxylation of bridging hydroxyl groups, which are assumed to be of the type Ti-(OH)-Si, occur between 350 and 550°C. An increase in both the number and strength of catalytically active bridging hydroxyl groups seems to be limited by a conventional protonic ion-exchange. The acid properties corroborate excellently with the catalytic activity and selectivity in n-hexadecane and 1,3,5-triisopropylbenzene cracking. The results show that electron acceptor sites, which may originate from titaniumoxide impurities or defect sites especially in titanium octahedra, could be involved in the catalytic reactions forming radical precursors.