Zeolites最新文献

Three novel metal-substituted gallium phosphates, (C₃N₂H₅)₈[Me₈Ga₁₆P₂₄O₉₆] (Me = Co, Fe, Mn), have been synthesised under solvothermal conditions and their structures determined using single-crystal X-ray diffraction. They are isostructural, with monoclinic symmetry, space group C 2/c and unit-cell dimensions ca. 15 × 13 × 15 Å, β ca. 111°. The materials all have frameworks with the laumontite topology (structure type code LAU), constructed from alternating PO₄ and MO₄ tetrahedra (M = Me and Ga) with imidazole cations occupying sites within the channels of the framework. Magnetic susceptibility measurements show that the materials exhibit Curie-Weiss behaviour over wide ranges of temperature and that the transition metals are present as divalent cations.

Crystal structure refinements of mordenites indicate that partial (Si,Al) order in the tetrahedral sites regularly recurs within the observed range (6–9) Al/unit cell. Consequently, for a given Si/Al ratio, it is possible to determine the maximum number of OH groups that can be allocated on each framework oxygen. Assuming that only one proton can be bonded at the same time to the oxygens of a tetrahedron, structural data indicate that it is highly probable that Brønsted sites are on framework oxygens 02 and 07, heading toward the center of the 12-ring, and on 09, heading toward the center of the 8-ring, each with about the same population. The intensities of the i.r. stretching bands (around 3,600 cm⁻¹ wavenumbers) of the hydroxyls Si(OH)Al agree well with this result.

The stretching vibration of hydroxyl groups on H-ZSM-5 samples with different crystallite size and Al content was investigated by diffuse reflectance i.r. spectroscopy. Good linear relations for the intensity of the 3,740-cm⁻¹ for the region versus the external surface area and the intensity of the 3,610-cm⁻¹ for the region versus Al content were obtained. It is proven that the absorbance at 3,740 cm⁻¹ is attributed to the terminal (Si-) O-H groups, which are located completely on the external surface, and an absorbance at 3,610 cm⁻¹ is ascribed to (Al-)O-H groups, which are distributed on both the external and internal surface of H-ZSM-5.

Interaction of lanthanum(III) chloride with hydrogen forms of ferrierite, mordenite, zeolite L, and faujasite, under completely anhydrous conditions, is described. The course of the interaction has been studied by in situ i.r. and XRD techniques. No evidence of interaction between LaCl₃ and small-pore H-ferrierite was found. The other three structures investigated do interact with LaCl₃ even under the complete exclusion of water or water vapor from the surroundings. Plausible mechanisms of the solid-state ion exchange in zeolites are also discussed.

A novel GeAPO-11 material has been synthesized and characterized. It is observed that Ge is incorporated in the molecular sieve framework, creating a new Brønsted acid site [Ge(OH)Al]. This catalyst is effective for skeletal isomerization of n-butene.

The states of aluminum in zeolites β were investigated in detail with FTIR spectroscopy. Three kinds of aluminum—framework, nonframework, and transient-state aluminum—were detected in template-removed zeolite β. The states of aluminum in our samples intensely depends on the nature of compensating cation, consistent with the conclusion of Brougeat-Lami et al. But some new assignments of aluminum species and new opinions about the states of aluminum and their transformation are proposed. From the experimental results, we suggest that, because the framework of zeolite β is deformable, the distortion and the tension of zeolite lattice occur when protons, which possess high electron affinity, are located at cationic sites, leading to the breakage of Al-O bond and the removal of aluminum. Therefore, zeolite β is subjected to dealumination in acidic medium. On the other hand, the high deformability also results in the relaxation of zeolite structure when H⁺ is replaced by other cations (e.g., Na⁺) so that transient-state aluminum and even nonframework aluminum can go back into the framework. Owing to ion-exchange, the cationic nonframework aluminum species enter the solution; moreover, it can also reinsert into the framework after hydroxylation in basic medium.

Mutinaite is the third new zeolite from Ferrar dolerites at Mt. Adamson (Northern Victoria Land, Antarctica). The mineral occurs as subspherical aggregates of tiny radiating lath-like fibres or as aggregates of transparent, colourless to pale-milky, tiny tabular crystals; it has vitreous luster, white streak and good 100 cleavage. Mutinaite is brittle with d_meas = 2.14(3) and d_calc = 2.17 g/cm³. Optically, it is biaxial negative with α = 1.485(2), β = 1.487(2) and γ = 1.488(2). The orientation is X = b, Y = a, Z = c. Mutinaite is orthorhombic with a = 20.223(7), b = 20.052(8), c = 13.491 (5)Å, space group Pnma. The strongest powder X-ray diffraction lines are (d(Å), I, hkl): 11.20, 84, 101, 011; 9.98, 35, 200,020; 3.85, 100, 501, 051; 3.75, 98, 303; 3.67, 27, 133; 3.00, 32, 503. The framework topology is that of the synthetic zeolite ZSM-5. The chemical formula is: (Na_2.76K_0.11Mg_0.21Ca_3.78)_{Σ = 6.86}(Al_11.20Si_84.91)_Σ=96.11O₁₉₂ · 60H₂O. The Si/Al ratio, equal to 7.6, is the highest found in a natural zeolite. Thermal stability and rehydration capacity are very high. The name is from Mutina, the ancient Latin name of the city of Modena.

HSAPO-5 with various Si concentrations was used as a catalyst for the alkylation of toluene with methanol to investigate weak acid sites. On HSAPO-5 with Si/(Al + P + Si) ≦ 0.026, the conversion of toluene increased with time on stream for a certain period, whereas the conversion decreased monotonously on HSAPO-5 with higher Si concentrations. Pretreatment with methanol on HSAPO-5 with Si/(Al + P + Si) = 0.018 increased the initial conversion of toluene. From the measurement of the amount of coke deposited on HSAPO-5, it is suggested that during the initial stage of the reaction coke is mainly produced by the reaction of methanol by itself at the acid sites on the external surface of HSAPO-5 with low Si concentrations. A small amount of coke thus formed would gradually retard the further self-conversion of methanol into coke. Therefore, the fraction of methanol used for the alkylation could increase, resulting in the increase in toluene conversion. It is suggested that the acid sites on the external surface are stronger than those formed by the substitution of Si for P but are weaker than those formed in aluminosilicate regions existing in HSAPO-5 with high Si concentrations.

High-field ⁷¹Ga MAS n.m.r. spectroscopy shows that gallium occupies preferentially the framework positions in Ga-MFI structures prepared by direct synthesis from a gel. However, modifications of boralite with Ga³⁺ in autoclave experiments provide structures with extraframework gallium species and only partially occupied boron vacancies in framework positions. Evidence of gallium oxohydrates in the modified samples was given by X-ray analysis. Extraframework gallium species with a quadrupole coupling constant of 8.8 MHz could be detected, but part of the extraframework gallium remains n.m.r. invisible.

Cycle sequences are presented as topological invariants of zeolites. The sequences are calculated for 374 topologically inequivalent atoms in 105 zeolite frameworks. All sequences are found to be different. The first nonzero terms of the sequence indicate the length of the smallest rings, which occur in the given framework and show which of the atoms belong to these rings.