G. Schulz-Ekloff

Chromophores in porous silicas and minerals: preparation and optical properties

Abstract Various synthesis strategies for the preparation of chromophores in porous silicas and minerals like zeolites, zeolite analogues and clays are described focusing on soft procedures like the sol–gel technique, microwave-assisted crystallization inclusion, and deposition in solution in combination with the Langmuir–Blodgett technique. The mild preparation methods protect sensitive dye molecules from chemical degradations. The influences (i) of the chemical properties of the mineral hosts, e.g., polarity and acidity, and (ii) of their spatial constraints on the guest molecules, i.e., their organized arrangement and photophysical properties, are demonstrated exemplarily.

Photo-oxidation of phenol and monochlorophenols in oxygen-saturated aqueous solutions by different photosensitizers

The photo-oxidation of phenol and 2-, 3- and 4-monochlorophenols by irradiation with visible light in aqueous alkaline solution in the presence of photosensitizers and oxygen is described. The reactions were carried out in the absence or presence of detergents. The photosensitizers used were Al(III), Zn(II) and Ga(III) complexes of 2,9,16,23-tetrasulphophthalocyanine (1a–1c), 5,10,15,20-tetrakis(4-carboxy-phenyl)porphyrin (2), a cationically charged perylene tetracarboxylic acid diimide (3), rose bengal (4) and methylene blue (5). The photosensitizers exhibit different activities and photo-oxidative stabilities which are strongly dependent on the pH, hydrolysis sensitivity and aggregation tendency. Singlet oxygen, obtained by photoinduced energy transfer from the excited photosensitizers, dominates the initial steps of photo-oxidation. A reaction sequence is proposed on the basis of the identification of carbon dioxide and maleic or fumaric acid as the products of photo-oxidation in the absence of detergent and the evaluated mass balances.

Synthesis of faujasite supported phthalocyanines of cobalt, nickel and copper

Abstract Faujasite supported phthalocyanines of cobalt, nickel and copper are prepared by the reaction of ion-exchanged zeolites with 1,2-dicyanobenzene. The fraction of encaged phthalocyanine decreases in the sequence CoPc-X > NiPc-X > CuPc-X, which is interpreted with a respective increase of the Bronsted acidity in the zeolite framework.

Electron microscopy study of the interaction of Ni, Pd and Pt with carbon: I. Nickel catalyzed graphitization of amorphous carbon☆

Abstract The aggregation of nickel supported on amorphous carbon during heating in vacuum and at how hydrogen pressures in the temperature range 1000 K was investigated by means of high resolution electron microscopy, electron diffraction and microdiffraction. It was established that the interaction of highly dispersed nickel with amorphous carbon substrates is strong enough to break away carbon atoms from the bulk at relatively low temperatures (∼ 700 K) Carbon atoms dissolved in nickel precipitate as graphite. As a result the aggregation of nickel is accompanied by the conversion of the amorphous carbon substrate to graphitic carbon.

PHOTOOXIDATION OF 2‐MERCAPTOETHANOL BY VARIOUS WATER‐SOLUBLE PHTHALOCYANINES IN AQUEOUS ALKALINE SOLUTION UNDER IRRADIATION WITH VISIBLE LIGHT

Abstract A selection of charged water‐soluble phthalocyanines was employed for the photocatalytic oxidation of 2‐mercaptoethanol in aqueous alkaline solution in the presence of oppositely charged detergents. Most efficient are Zn(II) and Al(III) phthalocyaninetetrasulfonic acids, which oxidize the thiolate to the sulfonic acid and sulfate, whereas Co(II)phthalocyaninetetrasulfonic acid does not exhibit a photoeffect. During the photooxidation reactions, decomposition (photobleaching) of the zinc phthalocyanine derivatives occurs, whereas the analogous alumina chelates are more stable. Covalent binding of Zn(II)phthalocyanine derivatives to silica carriers results in photocatalytic activities even in the absence of detergents, since the immobilization of the complexes preserves the required monomeric state. Moreover, the photodecomposition (bleaching) of the heterogenized complexes is strongly retarded. For the photo‐oxidations mainly the pathway via the sensitized formation of singlet oxygen with subsequent oxidation reactions is valid. Hydrogen peroxide as one reduction product of oxygen was found.

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.

Adsorption on MCM-41 mesoporous molecular sieves. Part 1.—Nitrogen isotherms and parameters of the porous structure

Nitrogen adsorption isotherms have been measured on a series of aluminosilicate and titanosilicate MCM-41 Molecular sieves, whose mean pore radius varied from ca. 0.9 to ca. 2 nm. By means of comparison plots the nature of the adsorption on these materials was found to depend strongly on their pore size. With pores of radius of around 1 nm multilayer coverage of the pore walls occurs. If the radius is increased to 1.5–1.8 nm the mechanism of adsorption changes into a two-stage one, i.e.. the multilayer coverage of the pore walls is suceeded by the spontaneous filling of the pore volume by capillary condensation without hysteresis. In even greater pores the usual capillary condensation with hysteresis occurs.The estimation of pore structure parameters was based on standard methods of adsorption isotherm processing, including the calculation of the pore size distribution from the desorption branch of the hysteresis loop. With the smaller pore materials, where the Kelvin equation does not hold, the estimation was based on the cylindrical pore model. A detailed knowledge of the pore structure of MCM-41 materials was thus obtained.

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.

Anchoring of Functional Dye Molecules in MCM-41 by Microwave-Assisted Hydrothermal Cocondensation.

Covalent anchoring of functional dyes in the pores of a mesoporous silicate host Si-MCM-41 (shown in the picture) is achieved with the microwave-assisted cocondensation presented here. The short reaction time (20 min) ensures that no dye degrades during the hydrothermal synthesis.

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).