Microporous Materials最新文献

The hexagonally ordered siliceous mesoporous molecular sieve MCM-41 and the disordered mesoporous molecular sieve designated as KIT-1 were studied by means of the X-ray diffraction (XRD), transmission electron microscopy and low temperature nitrogen adsorption over a wide pressure range. It was shown that the combination of these techniques allows for a thorough characterization of novel mesoporous materials. A repeated pH adjustment happens to be crucial in the synthesis of MCM-41 samples of high pore size uniformity. An additional influence of the salt effect (i.e. the addition of Na₄EDTA during the hydrothermal crystallization) allows one to obtain MCM-41 materials of excellent porous structures and remarkably high thermal and hydrothermal stability. The latter features may be a result of the presence of relatively thick pore walls for those mesoporous molecular sieves.

A disordered character of KIT-1, which was synthesized with an addition of Na₄EDTA to the initial synthesis gel, was supported by the fact that the sample exhibits a high pore size uniformity but yet a rather featureless XRD pattern. It was shown that comparative methods, such as the α_s-plot, are very useful for analysis of mesoporous molecular sieves, allowing for an accurate calculation of the specific surface area of the samples, which exhibit condensation in mesopores in the pressure range used in the standard BET method. A method to calculate the pore diameter for MCM-41 samples from XRD and adsorption data was shown to be convenient and useful for an accurate pore size assessment.

In continuation of our earlier work on silica aerogels, the experimental results on monolithicity, bulk density and Vickers hardness of glycerol modified silica aerogels as a function of sintering temperatures are reported. The molar ratios of methanol (MeOH) solvent, glycerol (G) additive, ammonia (NH₄OH) catalyst and water (H₂O) to the tetramethoxysilane (TMOS) precursor were found to be best in between the ranges: 6–12, 0.3–0.83, 3.6 × 10⁻²–1.0 × 10⁻¹ and 4–8, respectively, for the production of monolithic silica aerogels. The best quality silica aerogels in terms of monolithicity, transparency and low density have been obtained with the molar ratio of 1TMOS:0.83G:12MeOH:4H₂O:3.6 × 10⁻³ NH₄OH, respectively. The aerogels of this molar ratio have been characterized by density, Vickers hardness measurements and scanning electron microscopic observations as a function of sintering temperatures. The pore size distribution was found to shift towards the narrow pore radii as the temperature increased above 260°C. The results have been supported by the particle and pore sizes observed using scanning electron microscopy technique and BET surface analyses.

MCM-22 is a medium-pore high-silica-containing zeolite. Its unique structure contains two noninterconnected channel systems with potential for transition metal-based heterogeneous catalysis. Paramagnetic Cu(II) cation is ionexchanged into MCM-22 to act as a probe for electron spin resonance and electron spin echo modulation spectroscopic methods to verify the location of the ion-exchangeable sites. The electron spin resonance spectra of dehydrated Cu-MCM-22 are strikingly well-resolved and exhibit features related to both ⁶³Cu and ⁶⁵Cu nuclei. Interactions with water, ammonia, methanol, pyridine, benzene, cyclohexylamine and triethylamine adsorbed on Cu-MCM-22 are studied. Three types of ion-exchangeable extra framework site are identified.

Aluminum, gallium and antimony cations were incorporated into the framework of highly siliceous mordenite to modify the acid strength of Bronsted acid sites by the ‘atom-planting method’ with the corresponding metal chloride vapors at elevated temperatures. The framework incorporation of metal cations was confirmed by IR, MAS NMR and catalytic reactions. The optimum temperature to achieve a maximum incorporation of metal cations into the mordenite framework was demonstrated to be 873 K. Tetrahedral Ga cations exhibited an IR band owing to acidic bridging Si(OH)Ga groups at 3620cm⁻¹ and a characteristic band at 159 ppm in the ⁷¹Ga MAS NMR spectrum. The IR band owing to bridging Si(OH)Sb groups was observed at 3663 cm⁻¹. The acid strength of Si(OH)Me (Me = Al, Ga and Sb) groups estimated from their IR frequency, IR spectra of adsorbed pyridine and the activity for toluene disproportionation and cumene alkylation with 2-propanol was in the order Si(OH)Al>Si(OH)Ga> > Si(OH)Sb. The weakest acid sites, Si(OH)Sb, inactive for the toluene disproportionation, showed a significant activity for the cumene alkylation with higher para-selectivity.

Zeolite hexagonal Y was successfully synthesized in the presence of both 18-crown-6 and the Gd(III) complexes of 18-crown-6 as templates. The crystallization times as well as the gadolinium content in the hexagonal Y were found to increase with increasing ratios of Gd³⁺ -18-crown-6/SiO₂ in the initial gel compositions. The resulting hexagonal Y zeolites were characterized by XRD, FT-IR and SEM. Partial evidence for the template role and encapsulation of Gd³⁺ -18-crown-6 are described. The room temperature proton relaxivities of aqueous suspensions of hexagonal Y entrapped Gd³⁺-18-crown-6 were measured. The pH stability of the hexagonal Y samples was also determined. Based on these results the potential application of hexagonal Y synthesized with Gd³⁺-18-crown-6 as oral MRI contrast agent is discussed.

The state of the triethylamine (TEA) template in AlPO₄-5 and SAPO-5 and the characteristics of the framework structure of the molecular sieves were studied by means of IR and Raman spectroscopy. Decomposition of TEA and the formation of OH groups were investigated by DRIFT measurements carried out within the temperature range from 300 to 633 K. It has been found that TEA exists as triethylammonium ions in the as-synthesized samples, and its decomposition occurs via abstraction of ethylene and the stepwise formation of diethyl- and ethylammonium cations. The decomposition mode under a nitrogen atmosphere is different from that under air and requires another temperature. A thermal treatment of both molecular sieves at above 970 K results in a transformation of the structure.

A new synthesis route leading to a rapid crystallization of boro-titano MFI type zeolites from methylamine media has been explored. The principal advantage of this synthesis method is that the use of costly ingredients such as TPAOH and Ti- and Si-alkoxides can be avoided. Ti and B are readily incorporated into the MFI framework as confirmed by spot EDX analysis, IR, ¹¹B-NMR and XPS. Preheating the borosilicate gel before mixing with titanium tetrachloride improves the (B,Ti)-MPI crystallization rate and prevents the formation of extraframework titanium dioxides.

Zeolite β (with a Si/Al ratio of 15.2) was aluminated with NaAlO₂ solution. The basicities of the aluminated samples were investigated by FTIR spectroscopy with pyrrole as a probe molecule. It was found that the NH-stretching frequency of pyrrole adsorbed on the samples shifts towards lower wavenumbers as the Si/Al ratio is decreased and correlates well with the negative charge on the framework oxygen calculated from the Sanderson electronegativity equalization principle. These results reveal that zeolite β possesses a stronger overall basicity after alumination; hence, the alumination is an effective method to improve the basicity. It is furthermore found that the strength of the basic sites is heterogenous and that stronger basic sites are created by the alumination procedure.

Kevlar^® (poly(p-phenylene terephthalamide), PPD-T) pulp was used for the first time as a feedstock to prepare activated carbon fibers (ACFs). These were obtained by pyrolysis to 1173 K under inert (Ar) atmosphere and subsequent activation with carbon dioxide at two different temperatures (1023 and 1073 K). The resulting ACFs were characterized by adsorption of N₂ (77 K) and CO₂ (273 K). The surface area and pore volume increased after pyrolysis and especially after CO₂ activation, reaching maxima around 60% burn-off. It is concluded that Kevlar pulp constitutes a suitable feedstock for preparing microporous ACFs having a narrow pore size distribution, which is closely associated with the crystalline character of the precursor.

The external surface acidity of zeolite Beta can be selectively passivated by means of an amorphous silica layer. This coating is permeable, thus leaving the inner pore system accessible to reactants. For the coating to be effective, a Beta preparation giving relatively large, well-formed crystallites needs to be used, the microcrystalline mass afforded by the Wadlinger procedure giving poor results. The catalytic properties of the silica-coated Beta macrocrystals were investigated for two different reactions. The coated zeolite Beta material can be regenerated several times without destroying the inert shell.