Microporous Materials最新文献

Several spectroscopic techniques have been used to investigate the acidic properties of a set of galliosilicate molecular sieves with the offretite structure. IR spectra in the OH stretching region contain well-defined bands at or near 3735, 3640, 3600 and 3550 cm⁻¹ that can be attributed to OH groups associated with SiOH, and with M(IV)OH groups located in secondary building units, in the main channels (or in the gmelinite cages), and with M(IV)OH groups associated with oxygens shared by the main channels and the cancrinite cages; M = Al,Ga. Spectra of chemisorbed pyridine show that all crystals contain both Bronstedt (B) and Lewis (L) acid sites, and that the presence of Ga seems to decrease B-acid strength and to increase the L-sites' density and strength. Microcalorimetry experiments with ammonia and pyridine as probe molecules show that Ga insertion into the aluminosilicate framework increases the strength of the strongest acid sites and that the presence of Ga generate a heterogeneous distribution of acid site strengths absent in the parent Al-analogs. Moreover, in H,Ga-offretites, there is a drastic reduction in acid site density while the initial acid site strength remains high. NMR results suggest that the observed effects can be attributed, in part, to the presence of extra-framework Al and Ga atoms. Levels of Al(VI) and Ga(VI), as well as partial lattice degradation, strongly depend on the NH₄-exchange and calcination procedure used to activate these offretite crystals.

Iron(III) oxide pillared clays have low micropore volumes (0.04 cm³ g⁻¹) and surface areas (95 m² g⁻¹). In order to increase the porosity and thereby the adsorption capacity, two different modification procedures were carried out. The first modification is based on the preadsorption of butylammonium ions between the clay sheets prior to the pillaring procedure with the iron precursors. This gives rise to a decrease of the Fe pillar density which results in an increased porosity (Fe-BuA-PILC). Preadsorption of butylammonium ions doubles the adsorption capacity of an Fe-PILC. A second modification is the incorporation of Zr into the Fe precursors to create mixed Fe-Zr pillars with entirely new properties (pillar charge, pillar symmetry, etc.) creating a pillared clay with a different enhanced porosity. These pillared clays were tested with respect to their adsorption and desorption capacities towards CCl₄, CHCl₃, CH₂Cl₂ and CH₄ at 0 °C. These experiments indicate that the adsorption capacity of the mixed Fe-Zr-PILC is four times higher than pure Fe-PILC. Not only does the capacity differ, but the isotherm type also changes, indicating that adsorbents with different properties can be created by means of simple modification techniques.

The photocatalytic activities of water splitting by small particles (0.1–2 μm) of K₄Nb₆O₁₇ were studied. The milled catalysts were prepared by two kinds of ball-milling methods from the powder whose particles were several tens of micrometers in diameter. The surface area of the milled catalyst measured by N₂ adsorption at 77 K increased by an order of magnitude. The crystal structure was almost retained as K₄Nb₆O₁₇, as evidenced by the results of X-ray diffraction and scanning electron micrography. The photocatalytic activity for an overall decomposition of water of the Ni-loaded K₄Nb₆O₁₇ was twice as high as that for the original catalyst.

In the microwave synthesis of zeolite NaA ageing of the synthesis mixture is a prerequisite for fast crystallization. The rapid heating in microwave synthesis requires better preparation of the synthesis mixture compared to conventional synthesis methods. Sufficiently aged mixtures can yield NaA, with crystal sizes ranging from 0.1 to 0.3 μm, after l min in the microwave. It was also found that the synthesis of an unaged mixture can be improved dramatically by adding aged synthesis mixture to it. This suggests that the rearrangement of the synthesis mixture to yield nuclei is the bottleneck in a microwave synthesis.

The synthesis of zeolites from a mixture of a non-aqueous glycerol aluminosilicate gel and a controlled quantity of water has been studied. It was found that a minimum water content in the synthesis mixture is required to achieve a measurable crystallization rate. A further increase of the water content may facilitate the crystallization process of omega and then switch the crystal structure of the product from omega to analcime. At the Si/Al ratio of 6.95 in the synthesis mixture an increase of the NaOH content leads to a product sequence of amorphous → omega → analcime → analcime and sodalite. An increase of the Si/Al ratio in the synthesis mixture leads to a product sequence of sodalite → omega(+analcime) → ferrierite → magadiite. At the Si/Al ratio of 6.95 in the synthesis mixture a change of the alkali cations in a sequence of Li⁺ +Na⁺ → K⁺ + Na⁺→ Rb⁺ +Na⁺ → Cs⁺ + Na⁺ corresponds to a product sequence of omega (+ analcime) → offretite → offretite → analcime. With the new synthesis system, crystalline microporous materials including omega, offretite, analcime, and ferrierite can be readily obtained. The behaviors of the new synthesis system are discussed and compared with those of the normal hydrothermal synthesis system for zeolites.

Sorption of organics, such as 2,4-pentanedione and toluene, cause distinct and reversible changes in the ²⁹Si MAS NMR spectrum of highly siliceous MCM-22 that are characteristic of perturbations in local geometry. These subtle spectral features further substantiate the presence of the modified dodecasil-1H cage in the MCM-22 framework.

The polymeric silica present in clear tetrapropylammonium (TPA) silicate solutions, with the molar composition 9TPAOH:25SiO₂:480H₂O:100ethanol and from which discrete colloidal crystals of TPA-silicalite-1 may be synthesized, has been characterized with respect to particle size using a modified method in which the reaction rate of monomeric silica with molybdic acid was recorded. The estimated particle size of 2.8 nm obtained with this method agrees well with previous studies of the sub-colloidal fraction wherein cryo-TEM and high effect laser light scattering techniques were employed.

B-MFI and MTN-type materials were synthesized via the fluoride route from dry powders in the complete absence of a solution phase. Amorphous precursors obtained by drying SiO₂ · Al₂O₃ gels at 700°C were transformed into MTN-type material in the presence of dried NH₄F and TMACl. Amorphous precursors obtained by drying SiO₂ gels at 700°C were transformed into B-MFI in the presence of dried NH₄F, B₂O₃ and TPABr. Water is formed as a reaction product. However, the water vapor pressure is appreciably below the water vapor saturation pressure under the given reaction conditions. In addition, syntheses using seed crystals were carried out successfully.

The basic zeolites X which contain alkalis (K, Rb, Cs) in excess of the ion-exchange capacity (alkali-added zeolite X) showed an activity for 1-butene isomerization at 273 K, whereas alkali ion-exchanged zeolite X showed almost no such activity. The activities of the different alkali-added zeolites were in the following order: Cs-added zeolite X > Rb-added zeolite X > K-added zeolite X ≫ Na-added zeolite X. Temperature-programmed desorption (TPD) of adsorbed CO₂ showed that the amounts of adsorbed CO₂ were several times larger for alkali-added zeolites X than for alkali ion-exchanged zeolites X.

The poisoning experiment with CO₂ indicated that the active sites of alkali-added zeolites X are so basic that they retain CO₂ against evacuation at 573 K. The state of CO₂ adsorbed on the active sites was identified to be bidentate carbonate by IR study.

Ammonia TPD analyses of ZSM-5 zeolites of three Si/Al ratios (15, 30, 43) agglomerated with sodium montmorillonite revealed a noticeable modification on the acid properties of the bound zeolites as compared with the raw materials. An initial increase of the proportion of the weak acidity was found on the agglomerated zeolites, though the total and strong acidity decreases. The higher the Si/Al ratio of the zeolite, the higher is this increase. After several steps of ion exchange of the bound zeolites, the acidity reaches the expected values again, and the total and strong acidities are enhanced at the expense of the weak acidity. These results were related to a solid-state ion exchange between protons of the zeolite and cations of the clay during the calcination subsequent to the agglomeration.