Fe-containing Materials Research Articles

Modification of MCM-48-, SBA-15-, MCF-, and MSU-type Mesoporous Silicas with Transition Metal Oxides Using the Molecular Designed Dispersion Method

Four various mesoporous silicas (MCM-48, SBA-15, MCF, and MSU) were modified by the molecular designed dispersion method using Fe(acac)3, Cr(acac)3, and Cu(acac)2 complexes. The deposition was performed at the same concentration of the metal acetylacetonate (acac) complex in a toluene solution. All as-synthesized samples were investigated by diffuse reflectance infrared Fourier transform spectroscopy, Fourier transform infrared photoacoustic spectroscopy, and thermogravimetric analysis. The calcined materials were studied with respect to their textural properties (Brunauer-Emmett-Teller adsorption isotherm) and chemical composition (electron microprobe analysis). It allowed elucidation of the mechanism of interaction between the acac complex and the silanol groups. For the MCM-48, SBA-15, and MCF materials, the formation of hydrogen bonding was found for the chromium- and copper-modified samples, whereas the Fe-containing materials showed the ligand exchange mechanism. The strong interaction of the MSU support and the different acetylacetonate complexes, resulting in a loss of at least one acac ligand, was observed. The mesoporous silicas modified with transition metal oxides were studied by UV-vis-DR spectroscopy. The different metal dispersions were found for the samples containing various transition metal oxides.

Preparation and Application of Nanocatalysts via Surface Functionalization of Mesoporous Materials.

Surface immobilization of active species onto mesoporous materials is gaining importance, especially in the design of functionalized mesoporous materials as a nanocatalyst through heterogenization of homogeneous catalytic systems. This article summarizes recent work on the synthesis, characterization and catalytic performance of the functionalized mesoporous catalysts performed by the present authors. A cationic rhenium(I) complex was encapsulated into mesoporous Al-MCM-41 molecular sieve using a ion-exchange method, yielding a new photocatalyst to be active for photocatalytic reduction of CO2. Surface functionalization of mesoporous silica SBA-15 with sulfonic acid groups was investigated to give a solid acid catalyst. The chemically modified Fe-containing mesoporous materials, which are active for hydroxylation of phenol, were prepared by a surface-grafting method that iron salts are immobilized onto mesoporous Si-MCM-41 with the help of 3-aminopropyltrimethoxysilane as a linker. A cobalt(III) complex was heterogenized onto mesoporous silica SBA-15 containing carboxylic groups in order to utilize as a solid catalyst for the liquid-phase oxidation of aromatic hydrocarbons.

Template synthesis and characterisation of MCM-41 mesoporous molecular sieves containing various transition metal elements—TME (Cu, Fe, Nb, V, Mo)

A series of metallosilicates (transition metal elements—TME) MCM-41 (TME=Fe, Cu, Nb, V, Mo) mesoporous molecular sieves with variable Si/TME ratios have been synthesized and characterised by low temperature N 2 adsorption/desorption, XRD, XPS, H 2-TPR, FTIR combined with NO+NO 2 adsorption, diffuse reflectance UV–Vis spectroscopy, and ESR study. All the materials exhibit hexagonal arrangement of uniform mesopores (with exception of CuMCM-41). Defect holes amid the nanochanels besides well-ordered mesopores characterise mainly Fe-containing materials, in which the highest TME loading was reached. Similar but smaller defects take place in NbMCM-41. The amount of TME included into MCM-41 structure under the preparation conditions used in this work changes in the order: Fe>Nb>Cu≫V≫Mo. This sequence is not related to the oxidation state of metals which was estimated in calcined materials as Fe 3+, Nb 5+, Cu 2+, V 5+, Mo 6+. It does not also correlate with cation sizes in a simple way. The possibility of forming tetrahedral coordination seems to limit the TME incorporation into the MCM-41 skeleton if free metal cations are used in the gel (Cu 2+, Fe 3+, Nb 5+). Al in the gel makes the isomorphously substitution of silicon by copper easier, but part of Cu occupies extra framework cationic positions in the final material.

Benzylation of benzene by benzyl chloride over iron mesoporous molecular sieves materials

The alkylation of benzene with benzyl chloride has been investigated over a series of iron-containing mesoporous silicas with different Fe contents. These materials (Fe-HMS- n) have been prepared at room temperature using a route based on hydrogen bonding and self-assembly between neutral primary amine micelles (S 0) and neutral inorganic precursors (I 0). They have been characterized by chemical analysis, BET, XRD, and UV–vis spectroscopy. The mesoporous Fe-containing materials were very active benzylation catalysts with almost 100% selectivity to monoalkylated product and showed excellent stability. The kinetics of the benzene benzylation over this catalyst have been thoroughly investigated. A mechanism in which the reaction could be initiated by an oxidation of benzyl chloride with formation of a charge transfer complex is proposed.

Conductivity and Permittivity Studies in the Diluted Perovskite System [(NH3)(CH2)6(NH3)]FexZn1-xCl4, x=1, 0.8, 0.5, and 0

The dielectric permittivity and AC conductivity of the perovskite-like system [(NH3)(CH2)6(NH3)]FexZn1-xCl4 (HDAFxZ1-x), where x=1, 0.8, 0.5 and 0, were measured at different frequencies in the temperature range 100 K<T<430 K. At temperatures below 273 K, for x=1 the material exhibits a transition at (245±1) K, while for x=0 transitions at (155±5) K, (220±4) K and (255±2) K were found. A rotational-type transition in the range 295 - 305 K was found for the Fe-containing materials. Ferroelectric transitions were observed in the high temperature region for all four concentrations. Differential thermal scanning confirmed the existence of the phase transitions above room temperature. The conductivity decreases with Zn addition, an the conduction mechanism varies with the temperature and concentration. Extrinsic conduction prevails for T<150 K for all concentrations. At intermediate temperatures an Arrhenius relation with frequency dependent activation energy (ΔE=0.46 - 0.06 eV) is observed for Fecontaining materials. The frequency dependent conductivity for all materials has a linear response following the power law: σac=A(T) ·ωS(T) with the exponent s varying with temperature and composition. At high temperatures, for Zn-rich materials series type conduction with s ∼ 0.6±0.1 is identified, while for Fe-rich materials band type conduction prevails. In the low temperature region ionic hopping prevails.

Characterization of surface layers on M-50 steel exposed to perfluoropolyalkyethers at elevated temperatures

Complex surface layers have been identified on M-50 steel samples exposed to a linear perfluoropolyalkyether (PFPAE) in the presence of air at 260°C. An understanding of the mechanisms that drive the formation of these layers is believed to be crucial for efforts to develop PFPAEs as effective lubricants for advanced high temperature turbine engines. Work presented here shows that the extreme surface region contains physisorbed PFPAE and Fe fluoride under which an Fe3O4 layer is observed followed by an FeF2 layer which is adjacent to the substrate. Similar layered structures have been observed for Fe-containing materials exposed to HF but have not been reported for PFPAE systems. X-ray photoelectron spectroscopy (XPS) and conversion electron Mössbauer spectroscopy (CEMS) were used to unambiguously characterize these layers.

Fe-containing mesoporous molecular sieves materials: very active Friedel-Crafts alkylation catalysts

Benzylation of benzene has been conducted over Fe-containing zeolite molecular sieve materials prepared by different methods and exhibiting different pore sizes. It was shown that the mesoporous Fe-containing materials were very active benzylation catalysts with a monoalkylated product selectivity of 100%, and had better stability than the previously reported iron pillared clays (FePILCs). Temperature and pore size were two critical factors influencing the activities of the investigated catalysts. A redox mechanism can explain the observed experimental results.

Room-temperature synthesis of an Fe-containing mesoporous molecular sieve

An Fe-containing hexagonal mesoporous silica molecular sieve material was synthesized at room temperature for the first time. X-ray powder diffraction, infrared spectroscopy, electron spin resonance (ESR) and Mossbauer spectroscopy confirm the incorporation of Fe(III) in the structural framework. The tetrahedrally coordinated Fe(III) in this material is thermally more stable compared to that in another kind of hexagonal mesoporous molecular sieve containing Fe. It also exhibits better hydrothermal stability than Fe-free and Al-containing hexagonal molecular sieves.