Microporous Materials最新文献

Various crystallographic forms of AlPO₄-16 were prepared from F⁻ containing media in the presence of five different organic templates. Heptamethyleneimine and 3-azabicyclo [3.2.2] nonane lead to the cubic form, whereas hexamethyleneimine and quinuclidine lead to the tetragonal form. Using cobalticinium cations, the symmetry is monoclinic. The products were characterized using several techniques such as SEM, XRD, elemental analysis, thermal analysis and solid-state NMR spectroscopy. The change in symmetry could be related to the value of the site occupancy factor of the D4R [4⁶] units by the fluoride ions.

This study investigates the effects of the variation in reagents on the synthesis of zeolite A. The synthesis was attempted under autogenic conditions and at temperatures of 100 (±1)°C and 70(±1)°C, using various aluminium and silicon sources. Preparation of the gels used silica and aluminium to form mixed systems of organic and inorganic reagent sources. Products formed were analysed and characterised instrumentally using X-ray diffraction, X-ray fluorescence, Fourier transform infrared spectroscopy, thermogravimetric analysis and scanning electron microscopy. Systems using aluminium isopropoxide consistently produced uniformly sized crystals, each having deep, chamfered edges. Sodium reagent sources produced sharp edged crystals. The system using tetraethyl-orthosilicate with aluminium powder produced a hexagonal morphology. The change in the hydroxide concentrations, relative to the systems using sodium silicate and sodium aluminate sources was also monitored, and while it had a marked effect on crystallinity and stability, the crystal morphology remained consistent.

A new simulation protocol, based on molecular mechanics with systematic sampling, was developed to calculate efficiently the adsorption structure of sorbates on zeolite without long-time simulations. Using this technique in combination with the force field parameters appropriately chosen, the interaction energies between zeolite framework and sorbates were calculated. They could reproduce the tendency of the experimental heat of adsorption of ammonia, pyridine, 2-methylpyridine, and isopropylamine on mordenite, ZSM-5, and Y. The higher heat of adsorption could be explained from the favorable framework–sorbate interaction, the importance of which should be recognized. The simulation procedure established here allows the selection of the best types of zeolite from a large number of candidates for its simplicity by ranking them according to their acidity or activity, and would assist catalyst development in industry by providing a research guideline.

Zeolitic membranes were synthesized on a porous alumina support by the vapor-phase transport method using one of two alumina sources: aluminum sulfate and sodium aluminate. By choosing the appropriate period and temperature for drying the aluminosilicate gel on the alumina support, MOR, FER and MFI could be synthesized. MOR was formed when the water content in the dry gel was high. As the water content decreased, FER and MFI were formed sequentially. Preferential formation of MFI was achieved by treating the dry gel at 373 K. The temperature at which the support was dipped in the parent gel also influenced the type of zeolite formed, although the effect of drying conditions was more pronounced. A similar trend was found for both aluminum sulfate and sodium aluminate.

When the alumina source was sodium aluminate and the aluminosilicate gel was treated at 373 K, a preferentially oriented MFI membrane could be synthesized, as the (a00) and (0b0) crystal faces were oriented in parallel to the surface of porous alumina support. The uniform and dense morphology of the dry gel seems to account for the appearance of the orientation.

A model for the crystallization mechanism of VPI-8 from lithium zincosilicate reaction mixtures of typical composition Si:Zn=10 and Li:Zn=2 is presented. When the reaction mixture is heated to temperatures above 135°C, an amorphous gel is formed which contains a homogeneous distribution of silicon and zinc; Si:Zn=8. With heating time, zinc migrates to the superficial region of the gel particles creating a zinc-depleted core region where the nucleation of VPI-8 occurs. Simultaneous with the nucleation of VPI-8 in the core region, two other zinc-rich phases form in the superficial region. As the heating time is increased, one of the zinc-rich phases is dissolved and the other remains while the yield of VPI-8 with a framework Si:Zn of around 27 increases. Ultimately, VPI-8 with a small amount of a contaminating phase with Si:Zn=1 is obtained. The model presented for the crystallization mechanism of VPI-8 is consistent with all reported synthesis and physicochemical data.

The aim of this study is to distinguish the effect on the chemical shift of the anisotropy of sample magnetism (shape, chemical composition, external magnetic field B₀) from the local effect (binding electron, neighbouring nuclei) for NMR experiments. We calculate the former for an angle θ=54°44″ (magic angle) between the B₀ direction and the axis of the rotor. For a small adsorption rate of p-fluoroacetophenone on MFI and FAU-type zeolites, this effect on ¹⁹F chemical shift is not negligible, and it depends on the zeolite nature and structure type. The δ_obs correction varies from +1.8 ppm for MFI with Si/Al ≥35.5 to about +1.4 ppm for FAU zeolites with Si/Al=2.9 and 21.

Sorption isotherms for n-pentane, 2-methylbutane and cyclopentane in the aluminophosphate molecular sieve AlPO₄-5 are reported at 303, 313, 323 and 333 K. The sorption data were fitted into Langmuir, Sips and Dubinin–Radushkevich isotherms. The heats of sorption at zero coverage determined from the data in the Henry region are in the order 2-methylbutane>cyclopentane>n-pentane. Free energy and entropy changes and heats of sorption for these sorbates at various sorption coverages were also determined from the sorption data. The heat of sorption for n-pentane varies only marginally with sorption coverage. However, the heats of sorption for 2-methylbutane and cyclopentane increase significantly with the coverage indicating an increase in sorbate–sorbate interactions. The dependence of thermodynamic properties like the free energy of sorption (ΔG) and the entropy change (ΔS) for 2-methylbutane and cyclopentane on coverage show a sharp decrease and increase at sorption coverages 0.1–0.2 mmol g⁻¹ suggesting the rearrangement of molecular packing in the sorbed phase. These observations have been explained in terms of molecular interactions between the AlPO₄-5 channels and sorbate molecules.

10 wt.% vanadia, zirconia and chromia impregnated silica and montmorillonite K10 catalysts have been prepared. They were characterised by surface area measurements, XRD and ESR. Acidity was determined by stepwise ammonia desorption at three temperature ranges (A= 353–423 K, B=423–573 K, C=573–723 K). Aniline alkylation was carried out at 673 K, 12 cm³ h⁻¹ g⁻¹ (feed rate) and atmospheric pressure. The catalytic conversion is explained on the basis of acidity differences.

The potential of ¹³C-NMR spectroscopy of stationary samples for the study of mechanisms of anisotropic molecular motion of Mo(CO)₆ adsorbed in zeolites has been examined. Free diffusional rotation or jump processes about the various symmetry axes of the octahedral complex cause motional averaging of ¹³C chemical shift anisotropy, allowing for the characterization of the different motional mechanisms. At 110 K a large portion of the Mo(CO)₆ molecules is firmly fixed in zeolites NaY and HY as indicated by a broad (ca. 410 ppm) chemical shift powder pattern. A narrow 13C-NMR component is observed in both zeolites for molecules undergoing isotropic motion or rotation about a three-fold axis. An additional line, which is inverted in shape and reduced in width by a factor of −1/2 compared to the rigid case, is only observed for NaY. This line is assigned to molecules undergoing free rotation about the four-fold axis, but it may also be due to Mo(CO)₆ jumping about a two-fold axis. The changes in the distribution of molecules in different motional states was explored between 110 K and ambient temperature. Molecules undergoing rotations or jumps about the four- or two-fold axes only exist in NaY at lower temperatures when stationary Mo(CO)₆ molecules are also present. At 180 K and higher temperatures, rotation about the three-fold axis or isotropic motion are the sole types of molecular motion.

Several organic structure-directing agents (SDAs) that are variations of a 4,4′-trimethylenebis(N-methyl, N-R₁-piperidinium) moiety were synthesized and used for the synthesis of pure-silica molecular sieves. Pure-silica MTW, MFI, BEA and the MCM-41-type mesoporous materials could be prepared by changing only the R₁ group. When R₁ was pentyl or hexyl, MFI was crystallized, suggesting that each SDA molecule spans two channel intersections. For R₁ groups smaller than pentyl, MTW was formed. Pure-silica BEA could easily be prepared when R₁ was benzyl or cyclohexylmethyl. Numerous variations of the organics used to crystallize BEA were explored and the features necessary to structure direct the formation of BEA elucidated. When R₁ was either octyl or nonyl, the MCM-41-type mesoporous materials were formed, indicating that these organics can act as surfactants. The results presented provide further evidence that organic molecules can serve as structure-directing agents for the synthesis of pure-silica molecular sieves.