Microporous Materials最新文献

MCM-48 with Al incorporated into the walls constructing its mesopores (Al-MCM-48) was synthesized by a hydrothermal method. Al-MCM-48 possesses almost the same surface area, thermal stability and pore size as MCM-48, and has an acidity equivalent to Al-MCM-41. However, in the isopropylation of naphthalene, Al-MCM-48 exhibits a higher activity than Al-MCM-41. This is attributed to its larger pore size and three-dimensional pore system, which is more advantageous for molecular diffusion than the relatively narrow and one-dimensional pore system of Al-MCM-41. Compared with the yields of 2,6-diisopropylnaphthalene on large-pore zeolites, those on A1-M41S (Al-MCM-41 and Al-MCM-48) are not high because of their fewer acid sites. Al-MCM-48 also shows a higher activity than Al-MCM-41 in the isopropylation of pyrene, and the activities on the mesoporous M41S materials were much higher than that on zeolite Y. Isopropylation of pyrene occurs inside the mesopores of M41S, while pyrene derivatives cannot be produced inside the micropores of zeolite Y. However, coke formation and adsorption of the reactant and/or product proceeded in the mesoporous channels and caused catalyst deactivation. The product distributions, especially the ratio of 2-isopropylpyrene/1-isopropylpyrene in pyrene isopropylation, indicate that shape selectivity occurs inside the regular mesopores of M41S.

The adsorption and desorption behaviour of benzene, acetonitrile (CD₃CN) and pyridine for large crystals of SAPO-5 and GaAPO-5 is studied using FTIR microscopy with polarised IR radiation. From the analysis of the polarised adsorption bands it is concluded that the adsorbates are ordered in the molecular sieves at 373 K. The orientation of molecules interacting with Brönsted or Lewis acid sites and of physisorbed molecules are discussed. Benzene adsorbed at Brönsted sites is oriented with the aromatic ring parallel to the crystallographic c-axis of the molecular sieve, whereas physisorbed benzene is oriented with the aromatic ring parallel to the crystallographic a,bplane. Acetonitrile can adsorb on top of the Brönsted sites, resulting in an orientation with the CN-axis in the a,bplane. Physisorbed acetonitrile has the CN-axis parallel to the c-axis. Pyridinium ions have their aromatic rings parallel to the a,b-plane. Pyridine molecules at Lewis sites are oriented parallel to the a,b-plane in SAPO-5 and perpendicular to this plane in GaAPO-5. Physisorbed pyridine is oriented with its C₂-axis parallel to the c-axis.

Pure silica MCM-41 materials were synthesized at room temperature at different pH values ranging from 8.5 to 12. The solids were characterized by XRD, ²⁹Si-MAS-NMR spectroscopy and N₂ and Ar adsorption measurements. When the synthesis pH is too high, the degree of polycondensation of the silicate species is too low and, after removal of the organic species by calcination, amorphization occurs. For an intermediate pH value (11), the N₂ and Ar adsorption isotherms at 77 K, determined on the corresponding solid, are ‘type I-b like’ and are characteristic of the presence of primary and secondary micropores (d<2 nm). At a low pH value (8.5), a typical MCM-41 material with a mesopore diameter of ~2.6nm is obtained. The adsorption isotherms are of type IV. The influence of the thermal treatment on the as-synthesized samples was studied, and an effect was observed in the material containing micropores. Whereas a short calcination time at 873 K leads to a decrease in the intensity of the XRD peaks, a prolonged calcination time at this temperature leads to an improvement in the degree of organization and to an increase in the total microporous volume of about 18%. This increase can be explained by the greater homogeneity of the micropore surface after such a thermal treatment.

We present a detailed study on the preparation of highly-ordered MCM-41 molecular sieves based on a new delayed neutralization process. Products synthesized from cationic surfactants with different carbon chain lengths (alkyltrimethylammonium salt), counterions and head groups gave almost constant wall thickness (about 1.7nm), small lattice contraction after calcination, and sharp pore size distribution. However, the structural order decreased with the decrease of the carbon chain length. Adding a proper amount of alcohols as cosurfactants would improve the XRD patterns of the surfactants with carbon chain lengths less than 14. A head group of larger size would shrink the pore size and damage somewhat the structural order of MCM-41 materials. The rate of acidification and the source of the acid did not have much effect on the XRD patterns of MCM-41, but would affect its morphology. The formation process and the nature of the MCM-41 product based on octadecyltrimethylammonium bromide (C₁₈TMAB) are dependent on the synthetic temperature.

Aluminum-containing MCM-41 (AlMCM-41) was prepared by a method of gel equilibrium adjustment, in which the pH of the gel was adjusted to 11 or 10 using acetic acid during the synthesis in order to shift the equilibrium into the formation of mesoporous materials, and its structure, solid acidity, and catalytic activity were measured. The material thus obtained had a clear hexagonal array of mesopores with a Si/Al₂ molar ratio of 17 to 16; the maximum concentration of aluminum. More than 70% of aluminum atoms had a tetrahedral configuration. Test reaction for the acidity, TPD of ammonia and IR of adsorbed pyridine showed that it had an acidity almost similar to usual silica-alumina. The AlMCM-41 showed a high and durable activity in the isopropylation of 2-isopropylnaphthalene, which was not observed in the HY zeolite and the usual silica-alumina, but no shape-selectivity for the product formation was observed.

The synthesis of SAPO-34 in the presence of a new templating agent, namely piperidine, was investigated. The effects of the gel composition, the template concentration, the time and temperature of crystallization on the degree of crystallinity and the purity of the crystalline phase were examined. It was found that SAPO-34 and SAPO-20 are competing phases at high piperidine concentrations, whereas SAPO-34, SAPO-17 and SAPO-35 are the phases present at low template concentrations. Pure SAPO-34 with high crystallinity was obtained at a piperidine/Al₂O₃ ratio of 1.1:1.0. An optimal synthesis procedure giving an homogeneous SAPO-34 framework without Si islands, consists of heating a gel of composition 1.0Al₂O₃/0.6P₂O₅/1.1Pip./0.8SiO₂/100H₂O at 200 °C for 4–8 days. Such a reproducible procedure gives rise to an acidic catalyst active in the aldol condensation of acetaldehyde with formaldehyde.

We have synthesized monolithic, surfactant-templated particulate gels that have pore volumes and surface areas comparable to silica xerogels and aerogels. The gels have a complex microstructure with micro-, meso- and macroscopic features that emerge over five orders of magnitude in length (1) amorphous silica walls (characterized by a broad distribution of Si-Si spacings of ≈0.4 nm), (2) periodic hexagonal arrays of 1-d channels within each particle (≈3 nm channel diameter), (3) a feature in the gas adsorption measurements that indicates a second class of ≈10–50 nm diameter mesopores, (4) particles that are ≈ 150 to ≈500 nm in diameter, (5) interparticle pores that are on the order of the particle size, and (6) fractal domains larger than the particle size (>10 000nm). The microstructure can be controlled by the varying the initial silica content, the template size, the drying conditions, or the calcination conditions. The wet gel monoliths exhibit calculated densities as low as ≈0.02 g/cm³; the dried and calcined gels have bulk densities that range from 0.3 to 0.5 g/cm³. The materials possess large interparticle (1.1–2.2 cm³/g) and intraparticle (0.3–0.6 cm³/g) porosities.

The nature and characteristics of the catalytic surface of supported MoO₃ catalysts were studied. Changes that occurred on oxidizing alcohols in air over carbon and silica supported MoO₃ were examined. Structural data were combined with electron microscopy and photoelectron spectroscopy to demonstrate that the carbon support promotes segregation and fragmentation of MoO₃, whereas sintering occurs on silica. Results indicated that this may be correlated with a synergism between the carbonaceous material and metal oxide which provides a reoxidation pathway for reduced Mo, thus preventing formation of extended zones of MoO₂, which is inactive for the oxidation of alcohols.

Co(II)saloph (N, N-bis(salicylidene)-1,2-phenylenediamino-cobalt (II)) was encapsulated into VPI-5 and AlPO₄-8. Aluminophosphate-hosted cobalt complexes were obtained by impregnating VPI-5 with Co(saloph). Co(saloph) encapsulated into AlPO₄-8 was obtained by the structural transformation of VPI-5 occluding Co(saloph) into AlPO₄-8. These complexes were characterized by XRD, and IR, ³¹P NMR and UV-VIS-NIR spectroscopy. The encapsulated complexes do not undergo any significant structural distortions in either case. As the amount of loaded Co(saloph) increases, five-coordinated complexes are formed in VPI-5, but not in AlPO₄-8.

In this paper we have focused on the surface and porosity properties of [Mg-Al-CO₃] and [Mg-Al-Cl] obtained by the coprecipitation method. Different synthetic routes are used as well as post-synthesis treatments in order to measure how far textural modifications can be induced in these materials. For example, by performing the direct synthesis of [Mg-Al-CO₃] in mixtures composed of water and an organic solvent, or by regenerating calcined samples in similar mixtures, the surface areas, pore volumes and pore sizes are shown to be greatly affected by some solvents: synthesis in ethylene glycol causes an increase of 81% in the surface area, while in glycerol it results in the induction of microporosity (30%). On the other hand, we have also investigated the influence of these organic solvents on the textural properties during anion exchange reactions. Through analysis of their BET nitrogen adsorption/desorption isotherms, unpillared [Mg-Al-CO₃] proved to be mostly mesoporous, while [Mg-Al] hydrotalcite pillared with hexacyanoferrate (III) exhibited high microporosity, this characteristic being greatly influenced when the exchange reactions were carried out in organic media. SEM images provided evidence of the microtextural modifications undergone by the products throughout these different treatments.