MCM-41 Structure Research Articles

Synthesis and Luminescence of Eu Complex Incorporated MCM-41 Mesoporous Molecular Sieves

Rare earth complex Eu(phen)2Cl3·2H2O synthesized by precipitation method was incorporated into MCM-41 mesoporous molecular sieves which were synthesized via a hydrothermal method. Hybrid inorganic/organic mesoporous luminescent material Eu(phen)2/MCM-41 has been characterized by XRD, TEM, IR, UV-visible spectra and fluorescence spectra. Results indicated that the hybrid mesoporous material has typical structure of MCM-41 and retains the same pore structure as MCM-41 after the assembly process. The fluorescence spectra of these materials present a series of narrow lines assigned to 5D0 → 7F0,1,2,3,4 transitions. The high emission intensity observed is a promising property for application of the rare earth complex in technological luminescent devices.

Application of MCM-41 for dyes removal from wastewater

The adsorption of three basic dyes (Rhodamine B (RB), Crystal Violet (CV), and Methylene Green (MG)) and two acid dyes (Acid Red 1 (AR1) and Erioglaucine (EG)) onto MCM-41 was studied to examine the potential of MCM-41 for the removal of dyes from water solution. The revolution of pore structure and surface chemical characteristics of MCM-41 induced by dyes adsorption was characterized based on the analyses of XRD patterns, FTIR spectra, and nitrogen adsorption–desorption isotherms. The adsorption capacity of MCM-41 for the five dyes followed a decreasing order of RB > CV > MG > EG ∼ AR1. It was experimentally concluded that if the dyes adsorption did not introduce a serious disorder on the pore structure of MCM-41 (such as RB adsorption), MCM-41 might be a good adsorbent for the removal of basic dyes from water solution. The fitness of both Langmuir and Freundlich adsorption model on describing the equilibrium isotherms of three basic dyes was examined. The suitability of both pseudo-second-order kinetic model and the intraparticle diffusion model for the description of the kinetic data was investigated, from which the adsorption mechanism was examined.

Highly dispersed Mo–MCM-41 produced from silatrane and molybdenum glycolate precursors and its peroxidation activity

High surface area and high dispersion Mo/MCM-41 catalysts were successfully prepared using high-purity silatrane and molybdenum glycolate precursors. The precursors were synthesized using the Oxide One Pot Synthesis (OOPS) process. Mo was loaded onto MCM-41 by impregnation before and after heat treatment. After heat treatment, the catalysts were characterized using DRUV, XRD, FTIR, Laser Raman and BET. The %Mo dispersion was as high as 10 mol% or 0.265 g MoO 3/g SiO 2 while the structure of MCM-41 was still retained. Bulk MoO 3 was observed in the case of Mo-loaded onto the calcined support of MCM-41(c). The Mo-loaded uncalcined silicate MCM-41 support showed better catalytic activity during the peroxidative reaction.

Structural and catalytic properties of Ti–MCM-41 synthesised at room temperature up to high Ti content

A study of the pore structural properties and catalytic activity of MCM-41 containing titanium, prepared by direct synthesis at ambient temperature and pressure, using tetraethoxysilane, titanium alcoxides, cationic surfactants and ammonia is presented. The influence of different metal sources (titanium ethoxide, isopropoxide or n-butoxide), alcohols (ethanol or propan-2-ol), metal content (2 ⩽ Si/Ti ⩽ 100) and surfactants (tetra-, hexa- or octadecyltrimethylammonium bromide) is considered. The materials were characterised by X-ray diffraction, nitrogen adsorption at 77 K and diffuse reflectance UV–Vis spectroscopy. The catalytic activity was evaluated in the model reaction of oxidation of cyclohexene with anhydrous tert-butylhydroperoxide, for representative samples. It is concluded that this method allows the preparation in a short period of time of well structured and catalytically active Ti–MCM-41. Down to Si/Ti = 10 the materials have high pore volumes and very uniform pore size, with highest conversion and excellent selectivity demonstrated in the compositional range 30 ⩽ Si/Ti ⩽ 100, corresponding to materials containing predominantly isolated and tetracoordinated Ti. The increase in Ti content to Si/Ti ratios of 10 and 5 implied a reduction both in conversion and selectivity, although the MCM-41 structure was still observed for Si/Ti = 2.

Microporosity, pore anisotropy and surface properties of organized mesoporous silicates (OMSi) containing cobalt and cerium

Nanostructured, organized mesoporous silicate materials, containing cerium plus cobalt, were synthesized from a system based on polyacrylic acid (Pac), C16TAB and TEOS. The synthesis took place along a titration path from low pH (≈2) to high pH (≈10). Along this path the C16TAB species are attached to the Pac, and on the formed micelles, hydrolysis of TEOS (as well as of cobalt and cerium species) takes place. The initial atomic ratios of Ce(IV) : Co(II/III) were 1 : 9, 2.5 : 7.5 and 5 : 5. In each of those three cases, three different samples were isolated at pH = 5.5, 7.5 and 9.5 which, after drying and firing at 600 °C, were characterized by X-ray diffraction (XRD), nitrogen adsorption porosimetry, diffuse reflectance spectrospopy (DRS), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Cerium is already quantitatively incorporated into the solid at pH = 5.5. At this low pH value, only ≈20% of the cobalt is incorporated into the solid, but it is fully incorporated at the high pH of 9.5 The silicate species containing a low content of Ce(IV) and Co(II/III) (x : z = 1 : 9) show a self-organized morphology as previously observed in samples free of cerium. This self-organization is destroyed when the content of cerium is increased (x : z = 2.5 : 7.5 and 5 : 5). The samples with a low content of Ce(IV) and Co(II/III) possess an MCM-41 structure, which is gradually degenerated as the amount of the metals increases, and tiny crystallites of CeO2 and Co3O4 become apparent with XRD. The surface area of the solids was in the range 1230–770 m2 g−1, and it drops almost linearly as the content of the metals in the solids increases with the increase of pH. The I-point methodology was successfully applied to detect and distinguish distinct domains of microporosity and mesoporosity in the solids, as well as for the estimation of the percentage of micropore volumes (%Vmic), found in the range 15–30%, using the variation of the C parameter of the BET equation. The pore anisotropy, b, was also determined in the range 10 < b < 2300. A correlation of the form log b = log b0 − (%Vmic) was observed for the materials of the present study, in line with previous findings.

Influence of the structure characteristics of ordered mesoporous silica materials on their properties as supports for Ni-modified hydrogenation catalysts

The structure characteristics of Ni-modified (7 wt% Ni) mesoporous materials with cubic (Ia3d and Fm3m space groups) structure have been compared to that of the hexagonal MCM-41 material with uni- dimentional channel arrangement by X-ray diffraction and N2 adsorption–desorption measurements. Thermal gravimetric analysis and toluene hydrogenation have been used for characterization of the reducibility of the supported Ni phase and its catalytic activity in this test reaction, respectively. The results from N2 physisorption suggest that substantially higher pore blockage with Ni occurs when it is deposited on the 1D channel structure of MCM-41 compared to the cage-like pores of Fm3m and the interconnected pores of Ia3d material. It is proposed that the pore architecture determines the distribution of the oxide metal phase and its reducibility and strongly influences the hydrogenation activity of the catalysts.

Distribution and properties of catalytically active Cu2+ sites in mesoporous MCM-41 modified with Al, Zr, or W ions

The surface of pure mesoporous SiO2 with an MCM-41 structure has been modified by introducing Al, Zr, or W ions (1 mmol/g). The original and modified materials have been loaded with Cu2+ ions. The distribution, properties, and thermal stability of different Cu2+ sites have been studied by EPR and IR spectroscopy. The resulting catalysts have been tested for activity in ethane oxidation. The modification of original MCM-41 exerts a very strong effect on the stability of isolated Cu2+ ions on the support surface. Among the modified supports, Al-MCM-41 affords the highest thermal stability and degree of dispersion (70–80%) of the copper-containing phase. There is no correlation between the total number of surface Cu2+ sites and the catalytic activity. The specific catalytic activity (per Cu2+ ion accessible to the reactants) depends strongly on the local structure of the sites. The isolated pentacoordinated Cu2+ sites stabilized by the Al-MCM-41 surface show a comparatively high activity in the sample calcined at 520°C. The heat treatment of Cu/Al-MCM-41 at 650–750°C reduces the specific activity of the catalytic sites by a factor of ∼20 without sintering the copper phase, as in the case of CuHZSM-5 zeolite. The least dispersed copper phase, which is observed in the original MCM-41 and likely consists of aggregates of weakly interacting Cu2+ ions, exhibits the highest specific activity and thermal stability. In the case of Cu/W-MCM-41, heat treatment causes both the sintering of copper particles and a decrease in the specific activity of the surface Cu2+ ions.

Structural characteristics and reactivity properties of the tantalum modified mesoporous silicalite (MCM-41) catalysts

The structural characteristics of the Ta–MCM-41 catalyst have been investigated by BET measurement, small angle X-ray diffraction, UV–vis–NIR diffuse reflectance spectroscopy (DRS) and Raman spectroscopy techniques. Three types of tantalum oxide species: an isolated TaO4 species in the MCM-41 framework, an isolated surface TaO4 species on the MCM-41 surface, and bulk Ta2O5, can be present individually or coexist on the Ta–MCM-41 catalysts, and its relatively intensity is dependent on the Ta concentration. The local structure of the Ta atoms in the Ta–MCM-41 catalyst forms an isolated active site with the bridging Ta–O–Si bonds on the surface and in the frame structure of MCM-41. The catalytic properties of the Ta–MCM-41 catalysts were chemically probed with propane oxidative dehydrogenation (ODH) and methanol oxidation reactions in order to distinguish the different surface active sites present on the catalyst. Consequently, the well-dispersed isolated active sites exhibit high redox catalytic properties to produce formaldehyde (HCHO) and methyl formate (MF), and bulk Ta2O5 only possesses acid sites to form dimethyl ether (DME) The presence of Ta–O–Si bonds in the catalyst is responsible for the high reactivity/selectivity of the oxidation reactions.

Mesoporous carbons prepared by nano-casting with meso- or non-porous silica nanoparticles

Mesoporous carbons were obtained by sequential nano-casting using the following hard templates: (i) mesoporous silica nanospheres with MCM-41 or MCM-48 analogous structure; (ii) non-porous silica nanospheres and (iii) pyrogenic silica (Aerosil 200). These silica templates (patrix) were impregnated with sucrose, carbonized at 800 oC under nitrogen atmosphere and finally the silica dissolved with hydrofluoric acid. It was observed that the specific surface area of the carbon matrixes can be enhanced reducing the diameter of the silica nanospheres or the distance between them by agglomeration prior to the impregnation of the silica patrix with sucrose. Mesoporous carbons with specific surface areas higher than 500 m2 g-1 were obtained using mesoporous silica spheres. In this case, the carbon matrixes contain mesopores with a narrow pore size distribution and with diameters in the order of the wall thickness of the mesoporous silica used as hard template.

In situ generation of Pd nanoparticles in MCM-41 and catalytic applications in liquid-phase alkyne hydrogenations

Pd–MCM-41 materials were synthetized by using the cationic surfactant tetradecyltrimethylammonium bromide (C 14TABr) for both the stabilization of the Pd particles and the construction of the mesoporous structure of MCM-41. Two Pd–MCM-41 samples, for which the Pd particles were generated before and after formation of the MCM-41 framework (Pd-A and Pd-B, respectively), were investigated. Structural characterization of the samples was carried out by ICP-AES, N 2 sorption, XRD and TEM measurements. It was established that the highly ordered structure of MCM-41 was not appreciably affected by the formation of the Pd particles. Further, a similar particle size control was achieved for both Pd–MCM samples. However, both the location and the size distribution of the Pd particles were found to depend strongly on the preparation procedure. For Pd-A, the Pd nanoparticles were essentially situated on the external surface of MCM-41, whereas for Pd-B, the particles were found to be encapsulated inside the mesopores. For the liquid-phase hydrogenations of alkynes, the catalytic activity of Pd-A clearly surpassed that of Pd-B, indicating that the external Pd crystallites were more readily accessible for the reactants than those incorporated in the MCM-41 framework. The limited activity observed for Pd-B was attributed to mass transport limitations due to diffusion of the reactants into the mesopores of the MCM-41 host.

An Analytical Approach to Determine the Pore Shape and Size of MCM-41 Materials from X-ray Diffraction Data

The pore shape and size of MCM-41 were studied analytically by comparing the observed powder X-ray diffraction intensities with that derived from the MCM-41 crystal structure models, with two different pore shapes, a hexagon and a circle. The powder diffraction patterns from the as-synthesized and the calcined MCM-41 were measured by a synchrotron radiation at SPring-8, Japan. The MCM-41 structure with circular and hexagonal pore shapes explains well for the as-synthesized and the calcined MCM-41 crystals, respectively. The pore size and boundary obtained by this approach agree with those obtained from an N2 gas adsorption measurement combined with the Fourier synthesized density map.

Oxidative dehydrogenation of propane over Co, Ni and Mo mixed oxides/MCM-41 catalysts: Effects of intra- and extra-framework locations of metals on product distributions

A series of mesoporous MCM-41-based catalysts containing single and binary metal oxides (Mo, Co, Ni, NiMo and CoMo) has been prepared with binaries having one metal incorporated into MCM-41 structure while the other impregnated on the walls. A sample of the catalysts was characterized by temperature programmed reduction (TPR), X-ray diffraction (XRD) and BET surface area. The catalysts performances depended on the location of the active component – intra or extra – in MCM-41 framework. At similar level of propane conversions (14%), Mo- and Ni-incorporated MCM-41 exhibited selectivities to olefins (ethylene, propylene, 1-butylene, i-butylene and 1,3-butadiene) of 47.7% and 28.6% respectively. NiMCM-41, in addition exhibited selectivity to 1-pentylene of 12.2%. A catalyst produced by Mo incorporation followed by Ni impregnation (Ni-MoMCM-1) showed selectivity to propylene of 41.7% with no other olefins. When the sequence was reversed (Mo-NiMCM-41), the propylene selectivity was maintained and 10% ethylene and 14% C4-olefins were additionally produced, at similar propane of conversions. The highest olefins yield of 13.4% was obtained on Mo-NiMCM-41. The sequence of incorporation and impregnation of Mo- and Co- showed similar but less significant effects on the catalysts performance than in the case of Mo- and Ni-MCM-41. Thus, the location of the metals in the catalyst framework is a key parameter in determining the products distributions.

Vapor-phase alkylation of toluene by benzyl alcohol on H 3PO 4-modified MCM-41 mesoporous silicas

Several mesoporous aluminosilicate molecular sieves with the MCM-41 structure (SiO 2/Al 2O 3 = 20–200) have been synthesized using different aluminum sources and modifying several synthesis parameters during the preparation process, such as the temperature and the content of water and sulfuric acid in the gel mixture. All samples were characterized by element chemical analysis, X-ray diffraction, N 2 physisorption, thermal analyses, and electron microscopy. These Al-MCM-41 materials have BET surface areas up to 940 m 2 g −1. The catalytic properties of their H 3PO 4-treated derivatives for Friedel–Crafts alkylation of toluene with benzyl alcohol have been evaluated. Toluene benzylation preferentially gave p-benzyl-toluene. The conversion of toluene to monoalkylated products increased with increasing the H 3PO 4 content in the catalysts and reached a maximum value for a H 3PO 4 loading of 25%, a further increase in H 3PO 4 leading to a decrease in the activity for alkylation. The influence of H 3PO 4 loading and of the operating parameters on the performance of the catalysts was also investigated.

Mesoporous silica–titania composed materials

Titania mesosized particles were obtained by TiCl 4 hydrolysis in Aerosol OT/water/ n-hexane microemulsion. These particles were incorporated in surfactant templated silica mesoporous materials of MCM-41 and MCM-50 structures. Results depended on the surfactant: hexadecyltrimethylammonium bromide templated materials retained the honeycomb structure with small modifications in their characteristics. The dodecyltrimethylammonium bromide templated material changed from honeycomb to lamellar structure when the titania particles were included, with dramatic changes in the structure characteristics. The didodecyldimethylammonium bromide templated lamellar structure was retained after TIO 2 inclusion, with a slight increase in the specific area, pore diameter and pore walls thickness.

The effect of MCM-41 structure on the chemical properties of grafted vanadium oxide species

A solid state ion exchange method is described that introduces vanadium into zeolite MCM-41. Two types of vanadium species (VO 2+ complex in a silanol nest and vanadium ions in cationic position) are identified when the amount of V 2O 5 is equivalent to sample acidity. The vanadium MCM-41 samples are characterized by the methods of FTIR, UV–vis spectroscopies and temperature programmed reduction/oxidation cycles (TPR/TPO). The effect of the grafted vanadium ions and the efficiency of vanadium centres have been investigated in liquid phase benzene hydroxylation and complete ethylacetate oxidation.

Synthesis of Multiwall Carbon Nanotubes in the Pore System and/or on the Outer Surface of Mesoporous MCM-41 Structures of Various Morphology

Multiwall carbon nanotubes were synthesized either on the outer surface of iron containing mesoporous silicates using catalytic chemical vapor deposition (CCVD) or in the pore system of morphologically different mesoporous materials (hexagonal or spherical shapes) with graphitization of the template molecules. Transmission electron microscopy (TEM) study shows that the CCVD method resulted in long, bent and well graphitized carbon nanotubes on impregnated samples irrespective to the morphology of the silicate. Isomorphously substituted spherical MCM-41 with low Si/Fe ratio was found to be active catalysts for carbon nanotube production in CCVD as well. Synthesis of MWNTs with graphitization of template molecules in the pores of MCM-41 was successful in hexagonal MCM-41 samples irrespective that they contain or not iron in the silicate framework. Carbon nanotube formation was not observed in spherical derivatives of these samples during the graphitization process.

Adsorption of basic dyes onto MCM-41

The adsorption of two basic dyes, Basic Green 5 (BG5) and Basic Violet 10 (BV10), onto MCM-41 was studied to examine the possible effect of interactions between large adsorbates and MCM-41 on the pore structure stability of MCM-41 and the potential of MCM-41 for the removal of basic dyes from wastewater. The revolutions of surface characteristics and pore structure of MCM-41 induced by dyes adsorption were characterized based on the analyses of the nitrogen isotherms, the XRD patterns, and the FTIR spectra. It was experimentally concluded that when the effect of interactions between large dyes (such as BV10) and MCM-41 on the pore structure stability of MCM-41 was insignificant, MCM-41 might be a good adsorbent for the removal of basic dyes from wastewater. The adsorption of BV10 on MCM-41 with respect to contact time, pH, and temperature was then measured to provide more information about the adsorption characteristics of MCM-41. Both Langmuir and Freundlich adsorption models were applied to describe the equilibrium isotherms and the pseudo-second-order kinetic model was used to describe the kinetic data, from which some adsorption thermodynamic parameters were also evaluated.

A spectroscopic study on the adsorption of cationic dyes into mesoporous AlMCM-41 materials

Mesoporous materials loaded with dyes are of interest with respect to novel optical applications. The spectral behaviors of some dyes in these materials are considerably affected by the presence of surfactants. A comparative study has been carried out on the adsorption of the methylene blue, rhodamine 6G and thionine dyes into a surfactant-free and surfactant-containing mesoporous AlMCM-41. The ion exchange method has been employed for incorporation of the dye molecules into the structure of MCM-41. The exchangeable sites in the MCM-41 were generated prior to dye loading by isomorphous substitution of aluminum for silicon in the structure of the mesoporous material during the synthesis procedure. Diffuse reflectance measurements indicate adsorption of methylene blue and rhodamine 6G dye molecules into AlMCM-41 taken place via ion exchange at room temperature. The spectra show presence of monomer and dimer aggregates of the dyes established into the pores and surface of AlMCM-41. The ratio of dimer to monomer forms of rhodamine 6G incorporated in the surfactant-free and surfactant-containing AlMCM-41 is about one which is higher than those observed in aqueous solutions. The surfactant-containing AlMCM-41 induced destabilization and decomposition of methylene blue while uploading the dye. Degradation of this dye produces intermediate species identified as demethylated forms of methylene blue adsorbed on AlMCM-41. Our experiments revealed that kinetics of ion exchange for thionine dye is very slow at room temperature, but it become faster at higher temperatures. The spectral properties of thionine in AlMCM-41 are close to that of thionine in an aqueous solution, possibly due to high pore space in a mesoporous molecular sieve. No adsorption of thionine was observed for surfactant-containing AlMCM-41 even at higher temperatures.

Methanol synthesis over Pd/SiO 2 with narrow Pd size distribution prepared by using MCM-41 as a support precursor

All silicious MCM-41 was investigated as a support or a support precursor for Pd/SiO 2 and prepared catalysts were tested for methanol synthesis from CO and H 2. The methods of Pd loading on the MCM-41 were impregnation, seed impregnation and chemical vapor deposition (CVD). For both impregnations, most Pd existed outside of the pore as large particles, and only a small part of Pd was inserted into the pore of MCM-41 retaining the initial structure. On the contrary, in the catalyst prepared by CVD method, the MCM-41 structure was completely destroyed to become amorphous SiO 2. Yet the average Pd particle size in this catalyst was smaller and its distribution was narrower than those of the catalysts prepared by impregnation methods. In the methanol synthesis from CO hydrogenation the catalyst prepared by CVD showed higher methanol selectivity than other MCM-41-derived catalysts. This result was considered to be due to the more uniform distribution of the Pd particle size.

Investigation of structural order and morphology of MCM-41 mesoporous silica using an experimental design methodology

A 24 factorial experiment program was carried out to study the main and interaction effects of four factors (mixture CTMABr/SiO2, H2O/SiO2, and EthAc/SiO2, and reaction time) on pore ordering, hexagonal unit cell parameter a0, and morphology of MCM-41. The MCM-41 was synthesized from a sodium silicate solution using cetyltrimethylammonium bromide (CTMABr) surfactant and ethyl acetate (EthAc) pH modifier. None of the factors acted independently to determine pore ordering, in contrast to earlier limited literature data, which suggested a higher CTMABr/SiO2 disturbs the assembly of the MCM-41 structure. However, there is no contradiction between these results considering that the poorly ordered product was obtained previously from a reaction mixture with the higher EthAc/SiO2 and lower H2O/SiO2, which are shown to hinder pore ordering. A combination of these factors, resulting in a higher concentration of acetic acid (hydrolysis of EthAc), and thus, in a lower mixture alkalinity, implies that the pH affects pore ordering in MCM-41. This is consistent with extensive literature data on this mesoporous material. A small (up to ∼5%) variation of a0 due to the reaction composition and time variation was insignificant compared to the reported doubling of a0 caused by the effects of varying the surfactant alkyl chain length, addition of swelling organic compounds, or hydrothermal restructuring. Particle morphology (hexagonal platelets, gyroids, and crescent-like or worm-shaped particles) depended on the combination of mixture CTMABr/SiO2, H2O/SiO2, and EthAc/SiO2. This is consistent with the literature evidence that morphogenesis of hexagonally ordered silica is a complex phenomenon involving a variety of reaction variables.