Jiří Rathouský

Mesoporous silica with controlled porous structure and regular morphology

Abstract A new procedure for the synthesis of mesoporous silica with controlled porous structure and regular morphology was developed. It is based on the precipitation from a homogeneous environment using cetyltrimethylammonium bromide as a structure directing agent. The decrease in pH, which causes the formation of solid particles, is achieved by the hydrolysis of ethyl acetate. The procedure enables to obtain not only the MCM-41 mesoporous molecular sieve with a very high degree of pore ordering and phase purity, but also materials of a new type, viz. bimodal silicas containing both the MCM-41 mesopore system with a pore size of about 3 nm and a system of larger mesopores with sizes ranging from 10 to 30 nm. Owing to their structural properties and regular worm-like morphology, bimodal silicas are promising materials for applications in separation processes or as supports for bulky molecules or nanoparticles.

Controlling of morphology and characterization of pore structure of ordered mesoporous silicas

A new procedure based on the precipitation of MCM-41 particles due to a gradual decrease in pH was developed, enabling control of the morphology of mesoporous molecular sieves. As the decrease in pH is achieved owing to the hydrolysis of ethylacetate, local variations of pH do not occur and the formation of a rigid structure due to silica polymerization is delayed. By a judicious adjustment of reaction conditions, materials with markedly different morphologies were obtained. An MCM-41 molecular sieve with a high degree of internal mesopore ordering is distinguished by an almost perfectly uniform particle shape and size. The mesopore structure of a bimodal mesoporous silica with a worm-like morphology consists of mesopores, whose size is typical for an MCM-41 material, and larger ones with a mean diameter of 18 nm. A tentative mechanism explaining how the morphogenesis is controlled by the synthesis strategy used is suggested. A new method based on nitrogen adsorption data was developed for the quantitative determination of the content of amorphous phase in calcined samples. The consistency of the structure parameters determined by physically differing methods was checked using a geometrical model of the honeycomb structure.

Adsorption on MCM-41 mesoporous molecular sieves. Part 2.—Cyclopentane isotherms and their temperature dependence

The effect of temperature on adsorption isotherms of cyclopentane on mesoporous MCM-41 molecular sieves with different pore size has been determined at 243–333 K. The occurrence of adsorption hysteresis upon capillary condensation in mesopores depends on the pore critical temperature, which is lower than the bulk critical temperature and decreases with decreasing pore diameter. If the pore critical temperature is within the range of experimental temperatures of adsorption, capillary condensation on a single sieve may occur both reversibly (hypercritical region) and irreversibly (subcritical region).

Nitrogen adsorption study of organised mesoporous alumina

As the arrangement of pores generally achieved in organised mesoporous alumina is not completely regular, gas adsorption methods appear important for characterisation of this material. One of the promising approaches to adsorption data analysis is based on the comparison of the adsorption isotherm of nitrogen at −196 °C on the porous solid under study with the standard adsorption isotherm of the same adsorbate on a reference non-porous solid. To obtain reliable standard nitrogen adsorption data for characterisation of the alumina porous structure, nitrogen adsorption isotherms on Degussa Aluminiumoxid C and α-alumina were carefully measured at −196 °C with ASAP 2010 and modified Accusorb 2100E instruments. Standard nitrogen adsorption data (adsorbed amount divided by surface area) were listed for 41 points in the relative pressure range 0.001 to 0.95. Their applicability was tested on organised mesoporous aluminas of various structural parameters using the comparison plot method. Although some samples of organised mesoporous alumina were calcined at relatively high temperatures, only the standard data on Degussa Aluminiumoxid C proved suitable for the comparative analysis of adsorption isotherms. The standard adsorption data on Degussa Aluminiumoxid C transformed to the statistical film thickness curve were used for calculation of Barrett–Joyner–Halenda (BJH) pore size distributions. The determined material parameters of the organised mesoporous aluminas were also supported by X-ray diffraction data and were consistent with the conditions of synthesis and heat treatment of these materials.

Unusual type of adsorption isotherm describing capillary condensation without hysteresis

Adsorption isotherms of cyclopentane and nitrogen on a new porous crystalline aluminosilicate from the family MCM-41 are characterized by the volume filling of channels by capillary condensation without hysteresis, which commences in the region prior to the lower closure point at the hysteresis loop.

Transparent Conducting Films of Antimony‐Doped Tin Oxide with Uniform Mesostructure Assembled from Preformed Nanocrystals

Transparent conducting films of antimony-doped tin oxide (ATO) with a uniform 3D-mesostructure are prepared by self-assembly of crystalline ATO nanoparticles with various antimony content directed by commercially available Pluronic copolymers. The mesostructure of the films calcined at 300 – 500 C exhibits periodicity of 14 nm, surface area of 200 300 m/cm and a well-developed accessible porosity of 45 55 %. The high crystallinity of the nanoparticles serving as building blocks enables to obtain the fully crystalline inorganic frameworks with sufficient electric conductivity already at temperatures as low as 300 C. Ferrocene molecules covalently immobilized in the conducting mesoporous matrix show significantly enhanced electrochemical response proportional to the electrode surface area. The high electric conductivity and the uniform accessible mesoporosity combined with a simple and generally applicable preparation procedure make the developed ATO films attractive as the nanostructured transparent electrodes for various optoelectronic applications.

Synthesis of titanium-containing MCM-41 mesoporous molecular sieves in the presence of zinc phthalocyanine and rhodamine B

Abstract The synthesis of titanium-containing MCM-41 mesoporous molecular sieves in the presence of dye molecules results in a well developed hexagonal structure if the powder dye is dissolved in a surfactant solution. These materials contain titanium species in tetrahedral and octahedral coordinations simultaneously, indicated (i) by absorption bands at 960 cm−1 in the infrared and Raman spectra, which are assigned to titanium in a silica network, and (ii) by bands in the Raman spectra which are characteristic for rutile. The flat slope of the red flank of the UV bands below 300 nm points also to the presence of fourfold and sixfold coordinated titanium. Zinc phthalocyanine is proposed to be embedded in the micelles as a monomer predominantly, whereas rhodamine B is assumed to be exclusively located outside the micelles or at the external surface as aggregates.

High-temperature transformations of organised mesoporous alumina

The high-temperature behaviour of organised mesoporous alumina was investigated using X-ray powder diffraction, nitrogen adsorption isotherms and transmission electron microscopy. Based on the combination of these methods it has been shown that organised mesoporous alumina synthesised using long-chain carboxylic acids is transformed at temperatures between 600 and 1000 °C into a material which consists of a packing of δ-alumina particles.

Comparison of the adsorption of humic acids from aqueous solutions on active carbon and activated charcoal cloths

Abstract Batch experiments were conducted to determine the adsorption of humic acids from aqueous solutions by active carbon, activated charcoal cloth, and textile. The adsorption isotherms and adsorption kinetics curves of humic acids on these adsorbents at 25°C have been determined. The adsorption capacities of the ACC samples are much lower than that of the active carbon. Effective intraparticle diffusion coefficients of humic acids of 1.2 × 10−7 and 2.5 × 10−10cm2/s have been calculated for active carbon and activated charcoal cloth, respectively, from the kinetic curves. The adsorption behavior and different diffusion coefficients are consequences of their various textural characteristics. Only the surface area corresponding to mesopores and macropores is effective, the micropores are inaccessible to humic acids.

MCM-41 as-synthesized and calcined materials: temporal development of X-ray reflection intensity and pore volume

Abstract The comparison of the temporal development of the X-ray reflection intensities of as-synthesized and calcined MCM-41 materials demonstrates that 90% of the hexagonal channel arrangement is formed after 5–6 h whereas the fully developed mesopore structure requires 72 h. The progress of the reaction can be quantified by reference to the maximum pore volume that can be obtained from adsorption isotherms or from model calculations based on the packing density of the surfactant and the pore wall thickness of ca. 1 nm. For low critical micelle concentrations the maximum amount of MCM-41 formed can be calculated and used as a reference in order to follow quantitatively the progress of the MCM-41 formation.