Microporous Materials最新文献

The discovery of M41S periodic mesoporous silicates in 1991–1992 based on the use of surfactant supramolecular templates had an immediate strong impact on the area of mesostructured inorganic materials. A variety of related synthesis strategies have been developed while a great diversity of materials in terms of both composition and structure has been achieved. This review deals with non-silica mesostructured materials. Supramolecular templating techniques based on electrostatic interactions (ionic bonding), van der Waals interactions (hydrogen bonding) and direct covalent bonding between organic and inorganic species are presented, discussed and evaluated. Important issues, particularly regarding thermal stability of the materials, have been identified for future research.

A new method for determining the pore sizes of zeolites is introduced. Data are obtained spectrophotometrically for the uptake, from an isooctane solution, of alkane diazodicarboxylate chromophores. The size of the alkane groups was varied, thereby introducing differences in the sizes of the probe molecules being adsorbed. The method demonstrates no uptake of chromophore for small pore zeolites, somewhat hindered uptake for intermediate pore zeolites, and variable uptake rates for large-pore zeolites because the solvent can compete. Crystallite size, pore system dimensionality, and hydrophobicity (in the case of the large pore zeolites) are shown to affect the uptake data. Determination of products residing in the pores after 1000 min of solution circulating over the zeolite bed was determined by TGA/MS analysis. The method was applied to several high silica zeolites with known structures, as well as to unknown structures like SSZ-25 and 35. The study confirmed that the unusual zeolite SSZ-25 has pore openings larger than silicalite or ZSM-5 but smaller than a number of so-called large pore zeolites. Limitations, and future prospects for the use of this relatively simple system, are discussed.

Temperature programmed desorption (TPD) of n-alkanes, ranging from n-hexane to n-hexadecane, from silicalite-1, was conducted between ambient temperature and 500°C on a thermogravimetric (TG) balance at heating rates of 3, 6 and 10 C° min⁻¹. The derivative TG curves all showed two distinctive peaks. An inflection point in the TG profiles was found at about four molecules per unit cell in all cases except hexadecane, which occurred at 1.92 molecules per unit cell. The saturation adsorption volumes of all sorbates was close to the theoretical value of ∼0.19 ml g⁻¹.

The adsorption of nitrogen and argon on laboratory steam dealuminated and equilibrium commercial FCC catalysts (Y zeolite plus matrix) of various types, covering a wide range of catalyst properties, was studied. Different approaches were used to estimate micropore volumes. Micropore size distributions were assessed based on the area-averaged cylindrical micropore model. With both nitrogen and argon adsorption isotherms, it was possible to locate the peak indicating the pore size in the fresh catalysts at the expected value of 0.74 nm if proper values for the physical parameters in the model were adopted. On the dealuminated samples, nitrogen and argon yielded significantly different results. Ar-based micropore size distributions suggested that the physical parameters involved in the model change with the variations in the chemical composition accompanying dealumination. N₂-based pore size distributions are more sensitive to variations in the overall adsorption energy due to the growing influence of specific contributions (quadrupolar interactions). The magnitude of the minimum of potential energy in the nitrogen adsorption process decreases with dealumination up to a stable value for Si/Al ratios over about 8.

The products obtained during the controlled heating of potassium-exchanged zeolite A were characterized by different techniques such as X-ray diffractometry, Fourier transform infrared spectroscopy, differential thermogravimetric analysis, differential scanning calorimetry, scanning electron microscopy, determination of the particle size distribution and determination of the specific surface area. Based on the results of this characterization, it was found that controlled heating causes a phase transformation in the sequence (Na,K)-zeolite A→amorphous (Na,K)-aluminosilicate→kalsilite + kaliophilite. In contrast to the structural changes, the particulate properties (particle shape, particle size, particle size distribution, specific surface area) of the amorphous aluminosilicate and the mixture of kalsilite and kaliophilite are, curiously enough, similar, or even the same as the particulate properties of the starting crystalline material (potassium-exchanged zeolite A).

The clustering of cobalt in CoAPO-5 is studied by means of quantitative ³¹P MAS NMR. To explain the amount of “NMR-invisible” P as function of the cobalt content, clusters consisting of at least five cobalt atoms have to be present inside the framework. Probably, cobalt atoms belonging to one cluster are situated in the wall of one and the same pore.

Large-port (LP) and small-port (SP) mordenites with different particle size and shape have been submitted under similar conditions to known dealumination procedures in view of comparing their respective behavior, and establishing the similarities and discrepancies among the dealuminated solids. The dealuminated zeolites have been characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), acidity measurements with ammonia and X-ray photoelectron spectroscopy (XPS). The particle size of the starting mordenite markedly influenced the extraction of Al when dealumination was performed by direct acid attack or dry thermal treatment. Admission of steam during calcination noticeably improved the removal of Al from SP mordenite compared with other methods. Elimination of the extra-framework Al species produced at the calcination step required acid concentration as high as 6 M. (Hydro)thermally dealuminated LP mordenites exhibited similar surface and bulk chemical composition. Conversely, dealuminated SP mordenites showed surface enrichment in Al up to Si/Al ratios of about 40, and depletion in Al at higher ratios. These surface/bulk heterogeneous distributions were suppressed when dealumination combined preliminary acid leaching and steaming. Dealuminated small- and large-port mordenites exhibited differences in their textural characteristics, and for both, a contraction of the unit cell volume and the development of meso- and secondary microporosity.

MFI-type zeolites are synthesized with N-trimethoxysilylpropyl-N,N,N-trialkylammonium (TAASi) salts using the composition SiO₂:0.1TAASiBr:0.25NaOH:40 H₂O (molar ratios). Prehydrolysis of the organosilane precursor is necessary for zeolite formation. Quantitative NMR spectra show that the majority of the organsilicon carbon-silicon bonds are cleaved in the as-synthesized zeolite.

Zeolite A was the first commercial zeolite, and remains in use as a sorbent, molecular sieve and ion exchanger. Because of its unique structure, there has been increasing effort to understand the framework vibrations of zeolite A on a theoretical basis. Although the vibrational spectra of zeolite A have been calculated by many workers, most of the calculations include a pure silica framework to avoid the complications presented by charge-compensating cations. However, these calculations cannot be verified directly because no experimental spectra of completely siliceous zeolite A have been reported due to the fact that it is very difficult to produce. In this paper, we report, for the first time the IR and Raman spectra of completely siliceous zeolite A.

In this work, a multiple-process equilibrium model was used to analyze gas-solid adsorption equilibria in the porous carbonaceous material, BPL. Methane was used as a non-polar gaseous probe molecule to study the adsorption process. This analysis provided adsorption capacities, n_i, along with the associated equilibrium constants, K_i, for the three processes needed to fit the isotherms. The temperature dependence of the K_i allowed determination of the enthalpies of the adsorption processes, ΔH_i. These enthalpies were greater than the heat of vaporization for methane but less than that usually associated with chemisorption processes. Simulated adsorption data were generated based on the methane analysis to determine the effect of pore size distribution on the analysis. It is concluded that, given a continuous distribution of pore sizes, a large number of adsorption processes is necessary to describe accurately the total adsorption. For heterogeneous distributions, as is observed in activated carbon materials, it is possible to resolve discontinuities in the distribution.