Microporous Materials最新文献

Iron incorporation in VPI-5 occurs within a narrow range of gel composition and hydrothermal treatment conditions. Physico-chemical characterization was carried out by X-ray diffraction, SEM, thermogravimetry and solidstate NMR. The ESR studies showed that Fe³⁺ incorporation occurs into lattice sites only; no occluded Fe species are observed. A significant amount of framework iron was removed during the thermal transformation of Fe-VPI-5 to FAPO-8, forming aggregates of occluded iron species in the pores. During the process, part of the Fe³⁺ reduced to Fe²⁺ with concurrent observations of a signal near zero field.

Study of the reaction of zeolites A (LTA), and X and Y (FAU) with NH₄H₂PO₄ melts at 230°C for varying time periods (30 min to 36 h) found the final product formed in each case to have a powder X-ray diffraction pattern consistent with ammonium aluminum pyrophosphate, NH₄AlP₂O₇, the single crystal structure of which has not been reported. No intermediate crystalline phases were detected. ²⁹Si MAS NMR confirmed, however, that some Si was still present, and X-ray photoelectron spectroscopy revealed it to be located primarily at the crystallite surfaces. The crystallites were found to have an extremely low surface area (ca. 10m²g⁻¹), and hence not to be porous. MAS NMR studies found it to consist of six-coordinate aluminum and four-coordinate phosphorus in the aluminum pyrophosphate, and four-coordinate silicon in a highly siliceous environment. These results contradict the previous report of this reaction producing ‘phosphated aluminosilicates in which phosphorus substitutes into the zeolite framework sites thereby forming promising anion exchangers’ (A. Dyer, S.A. Malik, A. Araya, T.J. McConville, in: P.A. Williams, M.J. Hudson (Eds.), Recent Developments in Ion Exchange (Pap. Int. Conf. Ion Exch. Processes), 1987, pp. 257–263.).

The interaction of pyrrole with alkali cation exchanged FAU-type zeolites is studied using a combination of the electronegativity equalization method (EEM) and the Monte Carlo technique. Pyrrole adsorbs at S_II cations, the NH-group pointing to the most basic oxygen O4. The NH-bond polarization and the pyrrole framework interaction energy reflect the fundamental and well established trends of the intrinsic framework basicity: an increasing basicity with an increasing Al-content and, at a given Si:Al-ratio, an increasing basicity with increasing size of the exchangeable cations. In contrast to a recent interpretation of the IR-spectra of the NH-stretching vibration of pyrrole (D. Murphy, P. Massiani, R. Franck, D. Barthomeuf, J. Phys. Chem., 100 (1996) 6731; D. Murphy, P. Massiani, R. Franck, D. Barthomeuf, Stud. Surf. Sci. Catal., 105 (1997) 639). our Monte Carlo calculations show that there is only one adsorption site for pyrrole in the FAU-structure at low loadings. An alternative explanation for the observed heterogeneity of the IR-spectra of pyrrole is discussed on the basis of our calculations: the orientation of the pyrrole molecule with respect to the six-ring depends on the Si:Al-ratio of the tetrahedra forming the six-rings.

Magnesium containing molecular sieves, viz. MgAPO-44, MgAPO-34 (CHA) and MgAPO-5 (AFI), were prepared with cyclohexylamine (structures 44 and 5) and tetraethylammonium hydroxide (structure 34) as templating agents. The crystallization was performed at 473 K by means of a microwave power system. The samples were characterized by chemical analysis, thermal analysis, adsorption measurements, calorimetric measurement of the differential molar heat of NH₃ sorption, temperature-programmed NH₃ desorption, infrared spectroscopy and MAS NMR spectroscopy. For the as-synthesized samples, up to 7.7 wt% of MgO is incorporated into lattice positions of the CHA structure and substitutes aluminum.

The relative intensities of the deconvoluted ³¹P NMR signals of the as-synthesized chabazite-like samples (MgAPO-44 and MgAPO-34) depend on the type of templating molecules. The calcined samples (723 K), however, showed nearly identical results which could be interpreted in terms of different ³¹P(4 n Al, n Mg) environments. A random distribution of the Mg atoms is deduced from the NMR results.

The incorporation of Mg creates isolated bridging hydroxyls. The temperature of the structure collapse decreases with increasing Mg content.

The effect of framework aluminum on the dissolution behavior of ZSM-5 zeolite crystal was systematically studied using an interferometric technique. We succeeded in the three-dimensional measurement of the dissolution rates of aluminum-containing ZSM-5 zeolite crystal, namely the (010) face of ZSM-5 zeolite crystal as well as the (100) and (001) faces. The dissolution rate was dependent strongly upon the amount of framework aluminum of the zeolite crystal and the concentration of NaOH aqueous solution used. It was also found that the aluminum-containing ZSM-5 zeolite crystal is convex and that the inclination of the (010) face at the edge of the ZSM-5 zeolite crystal is steeper than that of silicalite crystal.

An isosteric method has been used to determine the adsorption equilibrium of hexane in silicalite-1 from 333 K to 423 K, and loadings of 2.8 to 5.5 molecules per unit cell. The isosteric heats of adsorption, q_st, and the differential molar entropies of adsorption, ΔS∘, have been calculated from the isosteres. Isotherms were also obtained from the isosteres. All isotherms show an inflection at a coverage of ~4 molecules per unit cell.

The bulk and surface properties of a series of titanium-rich Al₂O₃-TiO₂ mixed oxides catalytic supports have been determined by different techniques (S_BET, pore size distribution, XRD, DTA, FTIR, FT-Raman, UV-vis diffuse reflectance and FTIR of adsorbed pivalonitrile and pyridine). The analysis of the results indicates that the prepared solids were fairly homogeneous, with titania showing the anatase structure and no segregation of alumina particles. Also, the addition of small amounts of alumina to titania enhances significantly its thermal stability. The UV, FTIR, Raman and XRD results suggest that part of the alumina forms a solid solution with TiO₂ and that two types of aluminum are present, one due to surface monolayer-type species and other due to octahedrally coordinated bulk species. The FT-Raman and the FTIR spectra of adsorbed probe molecules support the idea that the anatase particles are largely covered by surface alumina partial monolayers.

A new synthesis method for a rapid crystallization of metallosilicates called the in situ seeding method was applied to the MFI titanosilicate (TS-1) synthesis. This method, which involves the preheating of the silicate gel and the consequent generation in situ of MFI silicate (silicalite-1) seeds before the addition of Ti source, dramatically accelerated the crystallization of TS-1 in methylamine medium. Ti ions were readily incorporated into 1he zeolite framework as confirmed by IR and the unit cell expansion measured by XRD. In comparison, the classical seeding method achieved under identical conditions yielded Ti-free silicalite-1 crystals. The optimal crystallimty of the preheated silicate gel for the in situ seeding method was evaluated to ca. 5%; a lower amount of silicalite-1 seeds led to a slower crystallization of TS-1, while crystallinity over 5% resulted in the continuation of the seed growth, leading eventually to crystalline silicalite-1.

The hydrothermal stability of high-silica Y zeolites dealuminated by Al/Si substitution may be increased by alumination of the crystal surface. The externally introduced non-framework aluminum species — alkali aluminosilicates or alumina — cover the surface where the water molecules attack the terminal OH groups and energy-rich Si-O-Si bonds. An alkaline and combined acid/alkaline pathway of alumination are suggested. A reinsertion of aluminum into the zeolite lattice could not be observed.

Two series of samples were prepared by means of oxidation of a parent activated carbon with either HNO₃ or H₂O₂. An important effect of the HNO₃ treatment temperature on the textural properties of the resulting samples was found, since below 333 K only the mesoporous region was affected, whereas at 363 K a generation of micropores and a loss of mesopores took place. The textural properties were strongly affected when reflux conditions were applied. The effect of the H₂O₂ treatment concentration on the textural properties was low as compared with the effect of the HNO₃ treatment temperature. In both treatments, modifications on the adsorption distribution potentials obtained by the Dubinin-Astakhov formalism were observed.