D. Méhn

Mechanical and chemical breaking of multiwalled carbon nanotubes

Catalytically prepared multiwall carbon nanotubes (MWNTs) were cut and functionalised by mechanical and/or chemical methods. Products were characterised by transmission electron microscopy, infrared spectroscopy and BET method. It can be concluded that physical and chemical breaking procedures complete each other very well. With certain MWNT samples containing surface oxides preliminary investigations were done for testing them as catalyst support.

Production of carbon nanotubes inside the pores of mesoporous silicates

Abstract Multiwall carbon nanotubes (MWNTs) were synthesized by a novel method of mesoporous MCM-41 and MCM-48 silicates in the absence of any metal traces. A transmission electron microscopy (TEM) study shows that the diameter distribution of the obtained carbon nanotubes is very narrow – 5–6 nm in diameter and 200 nm in length – compared to nanotube samples produced by other catalytic chemical vapor deposition (CCVD) techniques.

“Wash and go”: sodium chloride as an easily removable catalyst support for the synthesis of carbon nanotubes

A sodium chloride supported cobalt catalyst was found to be active in the synthesis of carbon nanotubes by a CCVD method.

Structural consequences of mild oxidative template removal in the synthesis of modified MCM-41 silicates

Abstract Results concerning the structural consequences of template removal from MCM-41 mesoporous materials are described using ozone, N 2 O and NO 2 as oxidants in comparison with the conventional method applying oxygen. Si, TiSi-, VSi and ZrSi-MCM-41 samples were synthesized by the usual methods. For characterization of the as-synthesized and treated samples XRD, nitrogen adsorption, 29 Si MAS NMR-, IR- and UV–Vis spectroscopic methods were used. The catalytic activity of the samples was tested in the Friedel–Crafts alkylation of toluene by benzyl chloride. The comparison of template removal agents showed that ozone was the most active at low temperature (423 K), and the treatment was less destructive than burning off the template in oxygen. Nitrogen oxide treatment (NO 2 and N 2 O) resulted in template removal at relatively low temperature (573–623 K), and structure deterioration was small. Si NMR spectroscopic data and IR spectra taken in the framework vibration range revealed that more original –SiOH groups remained as hydroxyl nests, furthermore, the heteroatom remained in tetrahedral coordination after ozone and nitrogen oxide treatment compared to burning off the template by oxygen. The results proved the advantages of ozone or nitrogen oxide treatments: (i) gentler to heteroatoms situated in the framework of the materials, probably leaves them intact, (ii) does not result in the formation of secondary micropores, which would decrease the uniform arrangement of the original pore systems, (iii) by preserving the active centers in their original coordination more uniform product distribution may be expected in catalytic reactions.

Destruction of chlorobenzenes by catalytic oxidation over transition metal containing ZSM-5 and Y(FAU) zeolites

A potential method for oxidative destruction of environmentally harmful chlorobenzenes has been studied in a fixed-bed laboratory reactor using several tran- sition metal containing ZSM-5 and Y(FAU) zeolite catalysts prepared by conventional (denoted L) and solid state (denoted S) ion-exchange procedures. The catalysts used for 10 hours in chlorobenzene (or dichlorobenzenes) oxidation have been destroyed in different degrees. The ZSM-5 structure proved to be more stable than that of Y(FAU). Considering the conversion and destruction efficiency of chlorobenzene oxidation at 450 °C as test reaction, activities of the ZSM-5 samples can be listed as follows: Na < CoL < CoS < MnS < MnL < FeL < FeS < CuS < H < CrL < CrS. In the side reactions polychlorinated derivatives (i.e. dioxines and PCB-s as undesired environmental pollution compounds) may be formed by the participation of elemental chlorine produced by the Deacon process. The activity series for the “coupling” reactions is as follows: CrS, CrL < H < FeL, FeS < CoS, CoL < MnS < MnL < CuS. Using CrZSM-5(S) catalyst at 550–600 °C with relatively high residence time and stoichiometric air, the conversion of chlorobenzene exceeds 90% with a decomposition efficiency (formation of HC1 and Cl2 from organic chlorine) of about 85–90%.

UV–vis diffuse reflectance spectroscopic study of transition-metal (V, Ti) containing catalysts

Abstract The oxidation state, coordination and distribution of V or Ti in the silicate framework is very sensitive to the method of synthesis. The reflection mode of UV–vis spectroscopy proved to be the simplest but powerful method for characterizing these materials. Upon burning the template off, the coordination of V and Ti changes. This treatment seems to determine the catalytic activity of these materials. Using low-temperature template removal by ozone, minor changes were observed in the coordination of V and Ti. This template removal technique may be suggested as general calcinations method.

Anchoring copper–amino acid complexes on silica or in montmorillonite - an FT-IR study

Abstract Cu(amino acid) complexes were immobilised in montmorillonite or on silica gel by two methods (i) cation exchange first followed by ligation (two-step method) or (ii) depositing the ligands onto/into the support, then introducing the metal ion and, finally, filling the empty coordination sites of the central ions by added ligands (three-step method). The amino acids were l -tyrosine, l -histidine or l -aspartic acid. Immobilisation was followed by infrared spectroscopy. Host–guest substances were formed in every Cu2+–amino acid– montmorillonite system, however, significant anchoring occurred on silica gel only when tyrosine was the ligand. The heat stabilities of these host–guest substances were appreciable, complete decomposition only occurred at 673 K.

Flexibility of the MCM-41 structure: Pore expansion and wall-thickening in MCM-41 derivatives

Abstract Utilizing the flexibility of the structure of MCM-41 type mesoporous metal silicates pore size modification was successfully performed for some MCM-41 samples including materials which contain Al or Ti heteroatoms in the wall or organic modifiers covalently bounded to the wall. Changing, i.e. expanding or narrowing the pore diameter was made by post-synthesis treatments. The products of these procedures were characterized by XRD, BET, IR and 29 Si MAS NMR techniques. For the expanded samples, a well characteristic thinning while for narrowed ones a definite thickening of pore walls, were established which were also accompanied by changes in the BET areas. The modified MCM-41 specimens retained their semi-crystalline character and the modification did not result in an increase of the concentration of structural imperfections.

Selective oxidation of cyclic hydrocarbons with H2O2 over Ti- and V-containing zeolites and mesoporous catalysts

Ti- and V-containing MFI, MEL and MCM-41 catalysts were studied in the oxidation of cyclohexane, cyclohexene, naphthalene, tetralin, decalin and phenol with H2O2. Although TS-1 and TS-2 exhibited the highest activity and selectivity in the oxidation of n-hexane and 1-hexene, cyclohexene could only be oxidized effectively on the MCM-41 silicates. Since the oxidation of condensed aromatic systems over Ti- and V-containing MFI and MEL zeolites runs to difficulties, MCM-41 type catalysts may be offered for these reactions.

Modeling superoxide dismutase: Immobilizing a Cu−Zn complex in porous matrices and activity testing in H2O2 decomposition

A bimetallic bridged complex, the Cu(II)-diethylene-triamino- μ -imidazolato-Zn(II)- tris -aminoethylamine triperchlorato complex, was prepared and immobilized in A1MCM-41 and montmorillonite. The host-guest materials were intended to be superoxide dismutase mimics, however, only the complex intercalated in montmorillonite proved to be active in oxygen transfer reaction. The inability of the complex immobilized in A1MCM-41 to decompose an aqueous solution of H 2 O 2 might be due to diffusional constraints exerted on the reactants and/or the products by the relatively more regulated environment of molecular sieve than that of the layered clay.