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.

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.

Ca ion-exchanged zeolites have been characterized by electron spin resonance (e.s.r.), ²⁷Al magic-angle spinning nuclear magnetic resonance (MAS n.m.r.), ²⁹Si MAS n.m.r., and diffuse reflectance Fourier transform infrared spectroscopy (i.r.). The presence of the radical species that are assigned to solid-state defects was confirmed in CaA zeolites, and the amount of the radical was dependent on the thermal treatment temperature in air. The present experimental results suggest that the formation of extralattice aluminum in the CaA sodalite cage and the change of calcium species by thermal treatment relate to the formation and disappearance of radical species.

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.

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.

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.

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.

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.

Samples obtained from thermal dealumination (in the presence and absence of water vapour) of three offretite zeolites with, respectively, 4.4, 3.4, and 2.9 aluminium atoms per unit cell, were investigated by i.r. spectroscopy. X-ray diffraction, and nitrogen adsorption. The percentage of crystallinity was then evaluated by two different methods: the standard XRD technique and a new method based on nitrogen adsorption data analysed through the α_s-method. In this case the volume of the ultramicropores (width less than 0.7 nm) was considered. Differences between the two estimates, as well as the loss of crystallinity during dealumination, which is expressed by the representation of both crystallinity evaluations, C_XRD or C_ADS, versus ν₁ (the frequency of the stretching asymmetric T-O band), were analysed for self-steamed and steamed samples. The results obtained agree within 20%, and often better. For self-steamed samples the crystallinity evaluated from XRD patterns is always smaller than that estimated from adsorption data, whereas for steamed samples the opposite situation is generally observed.

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.