Partially graphitized carbon filaments from as-synthesized silica/surfactant composite

The carbonization behavior of surfactants templated within mesoporous silica is studied in detail. Cetyltrimethylammonium bromide and poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P123) are used as the structure-directing agents for MCM-41 and SBA-15 synthesis, respectively. Thermal treating the as made silica/surfactant composites under argon flow at 900 °C produces partially graphitized carbon filaments as a result of the carbonization of the surfactants within the mesopores. Furthermore, the carbon materials derived from P123 in SBA-15 yield a more developed graphite structure than the carbon obtained from CTAB in MCM-41, as evidenced by the narrower X-ray Bragg reflections in the powder XRD and larger I_G/I_D ratio in the Raman spectra.     

Isothermal template removal from MCM-41 in hydrogen flow

Thermal evacuation of template surfactant from MCM-41 in hydrogen atmosphere was investigated. Micelle templated silica was prepared using hexadecyltrimethylammonium bromide. The mechanism of template removal appears to be completely different in hydrogen as compared to calcination in air. It seems that moderate heating of as-synthesized MCM-41 (up to about 250 °C) in hydrogen stream for about 15 h is effective and simple method of template removal leaving no pure carbon residues on silica surface. Adsorption properties of partially evacuated MCM-41 samples were tested using gas chromatography. Products of template degradation were analyzed using GC–MS technique. The main products of template degradation at 250 °C in hydrogen flow are hexadecene and hexadecane-N,N-dimethylamine. Concentration of surface silanols for MCM-41 calcined and thermally treated in hydrogen flow was investigated by NMR technique.     

Octadecyl grafted MCM-41 silica spheres using trifunctionalsilane precursors – preparation and characterization

MCM-41 silica spheres were prepared via the pseudomorphic route. Subsequent surface modification of the mesoporous silica spheres was achieved by two silylating agents, n-octadecyltrihydridosilane and n-octadecyltrimethoxysilane, which provided different surface coverages. The MCM-41 pore structure, surface properties and morphological features were examined by small angle X-ray scattering, nitrogen adsorption–desorption and scanning electron microscopy. The investigations revealed an influence of the silica source on the mesoporous structure, as reflected by a higher long-range order for the pores in MCM-41 spheres prepared from Kromasil silica. Surface modification is accompanied by a reduction of the surface area, pore diameter and pore volume of the MCM-41 materials, whereas the spherical morphology of the spheres is retained. The degree of grafting and cross-linking of the alkylsilanes was determined by ²⁹Si NMR spectroscopy. A higher degree of alkyl chain grafting was observed for the solvent extracted MCM-41 spheres and for samples prepared via surface polymerization.FTIR and ¹³C NMR spectroscopies were employed to study the conformational behaviour and mobility of the grafted octadecyl chains. The conformational order was found to strongly depend on the history of the MCM-41 supports (calcination, solvent extraction) and on the actual surface modification procedure. In general, a lower conformational order was observed for the present mesoporous alkyl modified silica spheres as compared to conventional C₁₈ modified silica gels which is mainly attributed to the lower surface coverage.     

Utilization of mesoporous molecular sieves synthesized from natural source rice husk silica to Chlorinated Volatile Organic Compounds (CVOCs) adsorption

The adsorption of trichloroethylene (TCE), tetrachloroethylene (PCE), and carbon tetrachloride was studied over our synthesized mesoporous material, MCM-41, from rice husk silica source, abbreviated as RH-MCM-41. More than 99% silica for RH-MCM-41 synthesis was extracted from rice husk under refluxing in HBr solution and then calcined at 873 K for 4 hours. RH-MCM-41 possessed surface area around 750-1,100 m²/g with a uniform pore size with an average diameter of 2.95 nm, narrow range of pore distribution and somewhat hexagonal structure, similar to properties of parent MCM-41. The adsorption of CC1₄ to RH-MCM-41 was stronger than that of TCE and PCE. The adsorption capacity of RH-MCM-41 for CVOCs (chlorinated volatile organic compounds) was higher than commercial mordenite and activated carbons.     

A new method for grafting functional groups onto mesoporous silica: an electrochemical approach

A novel method for the modification of mesoporous silica, MCM-41, using an electrochemical approach has been developed, and the process was monitored by cyclic voltammetery and spectrometric methods. The method was applied to the modification of mesoporous silica with new functional groups which are not accessible by conventional methods. Malononitrile-functionalized MCM-41 mesoporous silica was characterized by low-angle X-ray diffraction, Fourier transform infrared spectroscopy, mass spectrometry, thermal analysis, elemental analysis, high resolution transmission electron microscopy, and surface area measurement (S _BET). In addition, the application of malononitrile-functionalized MCM-41 as a sorbent for gold ions was demonstrated.     

As-synthesized MCM-41 silica: new adsorbent for perchlorate

Here we demonstrate that the as-prepared MCM-41 mesoporous silica material, which is synthesized using cetyltrimethylammonium bromide as a cationic surfactant exhibits very high capacity for perchlorate uptake from solutions. Thus we discover a new function for the as-synthesized mesoporous materials containing cationic surfactants. These materials are shown to have better capacity than the currently used activated carbon, which is preloaded with cationic surfactant. As-synthesized MCM-41 has a higher removal of perchlorate with 0.378 ± 0.038 meq/g than the surfactant modified activated carbon sample, which removed 0.304 ± 0.005 meq/g i.e., MCM-41 has a 24 % higher capacity than the surfactant modified activated carbon sample for perchlorate uptake because of higher surfactant content and higher positive charge in the former. The residual positive charge on the cationic surfactant micelles trapped in mesopores of silica is responsible for the high perchlorate uptake. The excess positive charge on the micelles in the as-synthesized MCM-41 silica is balanced by bromide ions and these ions are involved in exchange with perchlorate ions.     

Surfactant chain length effect on the hexagonal-to-cubic phase transition in mesoporous silica synthesis

In this study, silica-based mesoporous materials (the M41S family mesoporous molecular sieves) are synthesized using alkyltrimethylammonium bromide with different chain lengths (C_nH_2n+1N(CH₃)₃Br, n = 10, 12, 14, 16) as templates. The resulting silica structures are characterized by X-ray diffraction and are found to exhibit the phase transformation from the hexagonal mesophase MCM-41 to the cubic mesophase MCM-48 (with the space group of Ia3d). The structural phase transition in our study is controlled by the alkyl chain length of the surfactant: with an increase in the surfactant chain length (from C10 to C16), the structure goes from MCM-41 (synthesized by C10), through an intermediate structure (synthesized by C12), to MCM-48 (synthesized by C14 and C16). The amount of ethanol, which is used as a cosolvent, affects the pore size of the structured mesoporous silica, but only to a small extent. In the mean time, the autoclaving time has some effect, though not distinctively, on the structure integrity as well. With increased surfactant to silica ratio, the phase transformation can be shifted to longer chain template.     

Preparation and characterization of Pd/Si-MCM-41 with high hydrogenation activity

The pure silica mesoporous molecular sieve MCM-41 was synthesized under hydrothermal conditions. Pd/Si-MCM-41 was prepared by the incipient wetness impregnation of pure silica MCM-41 with PdCl₂ as precursor. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen adsorption–desorption isotherms at 77 K, inductively coupled plasma (ICP) spectroscopy measurements. The formation of Pd particles reduced the crystalline character of Si-MCM-41, but the structure of Si-MCM-41 framework was retained. The designed Pd/Si-MCM-41 mesoporous material was used as catalysts for hydrogenation of rosin, and showed excellent catalytic performance. Such outstanding catalytic performance should be attributed to the proper size of Pd particles and its high dispersion.     

Photodecomposition of polyphenols in <Emphasis Type="Italic">E. camaldulensis</Emphasis> leaves in the presence of hybrid catalyst of titania and MCM-41 synthesized from rice husk silica

Photocatalytic degradation (PCD) of polyphenols (gallic acid) from E. camaldulensis leaves on TiO₂/MCM-41 was investigated in order to get rid of substances harmful to aquatic life. The TiO₂/MCM-41 catalysts with titania loading of 2–40% were synthesized by hydrothermal method using rice husk silica and tetraethyl orthotitanate as silica and titania sources, respectively. The obtained catalysts were characterized by XRD, TEM, Zeta potential analyzer, N₂ adsorption-desorption and diffuse reflectance UV spectroscopy. Hexagonal array of MCM-41 was confirmed, but its crystallinity decreased dramatically with titania loading. Zeta potential of TiO₂/MCM-41s surface varied from 2.11 to 6.00 with the increase of TiO₂ from 0 to 100 wt%. Band gap energy of TiO₂ shifted from 394.1 to 425.1 nm after adding 60%MCM-41 (40%TiO₂/MCM-41), facilitating the ease of OH establishment. Gallic acid - a weak acid solution (pK_a=4.0) around 27 ppm was favorable to dissolve in water. PCD of gallic acid was carried out on irradiating of 400W of mercury lamp. The results showed gallic acid solution about 10 wt% properly adsorbed on 10%TiO₂/MCM-41 and effectively degraded at pH solution of 9.0. PCD completed at 60 minutes of irradiation time through catalyst concentration of 0.17 g/L and obeyed pseudo-first order. Intermediate products were formic, oxalic, pyruvic, malanic and maleic acids that finally mineralized to CO₂ and H₂O as downstream products.     

Catalytic Activity of MCM-41–TBD in the Selective Preparation of Carbamates and Unsymmetrical Alkyl Carbonates from Diethyl Carbonate

The synthesis of carbamates 3 and unsymmetrical alkyl carbonates 5 by reaction of diethyl carbonate with aliphatic amines or alcohols has been realized by using as heterogeneous catalyst a hybrid organic–inorganic material prepared by anchoring TBD to MCM-41 silica. Products are obtained in high yield and very good selectivity and the solid catalyst can be recovered simply by filtration and reused for different cycles without apparent lowering of activity. A supported N-carbethoxyguanidinium active intermediate is proposed, and some spectroscopic data are shown to support the mechanistic hypothesis.     

Investigations on MCM-41 as a fume catalytic incinerator: a heptane case study

This study shows that mesoporous MCM-41 compounds, that have hexagonal-close-packed channels, are good candidates as micro-reactors for fume catalytic incineration reactions. MCM-41 is synthesized and used as a catalyst to incinerate normal heptane (nC7) at different temperatures and equivalence ratios (ϕ). Destruction and removal efficiency (DRE, 99.99%) of heptane is achieved at 500 °C and ϕ=0.85 using MCM-41 as a catalyst compared to 73.45% under the same conditions without the catalyst. Even 99.9% DRE of heptane is achieved at 450 °C. The number and quantity of some analyzed products of incomplete combustion (PIC) are dramatically decreased when using MCM-41 material. No coke formation occurs.Under fuel-rich conditions, virtually complete consumption of oxygen molecules occurred at 450 °C when using MCM-41 compared to 75% in thermal incineration at the same conditions.While MCM-41 improves DRE at high temperature, e.g. 450 °C, it worsens it dramatically at lower ones, e.g. 300 °C. This inhibition of destruction effect depends also on the equivalence ratio of the mixture. It seems that diffusion properties and the chromatographic effect of MCM-41 channels become important at low temperatures.The enhanced DRE of heptane, upon using MCM-41, at relatively low temperatures, 450 °C, suggests that MCM-41 channels act as nano-vessels for free radical incineration reactions rather than a meter-scale incineration chamber as in the case in thermal incineration processes.     

Fe(III)-salim anchored MCM-41: synthesis,characterization and catalytic activity towards liquid phase cyclohexane oxidation

Abstract  

A novel organic–inorganic hybrid catalyst [MCM-41-Fe^III(salim)] was synthesized by covalently anchoring Fe^III(salim) complex into the pore channels of MCM-41. The material was synthesized by the co-condensation of tetraethyl orthosilicate (TEOS) and the precursor of salicylaldehyde modified with 3-aminopropyl triethoxysilane in the presence of cetyltrimethyl ammonium bromide (CTAB). The Fe(III)-salicylideneimine MCM-41 mesoporous silica was generated (in situ) by taking MCM-41-salimH (I) and FeCl₃ in ethanol solution. The immobilization of the complex on the functionalized silica was confirmed by small angle X-ray diffraction (XRD), N₂ adsorption–desorption, electron paramagnetic resonance (EPR), Fourier transform infrared spectroscopy (FT-IR) and diffuse reflectance UV–vis spectroscopy. Solid state CP-MAS NMR spectroscopy of ²⁹Si (²⁹Si CP-MAS NMR) showed a highly condensed siloxane network. Catalytic activity of the supported catalyst was examined by the oxidation of cyclohexane. Cyclohexane was successfully oxidized in good conversion (68%) to cyclohexanone, as a major product with 72% selectivity using tert. butylhydroperoxide as oxidant and acetonitrile as solvent. The catalysts can be reused up to three cycles without losing much of its activity.     

Eco-friendly synthesis for MCM-41 nanoporous materials using the non-reacted reagents in mother liquor

Nanoporous materials such as Mobil composite material number 41 (MCM-41) are attractive for applications such as catalysis, adsorption, supports, and carriers. Green synthesis of MCM-41 is particularly appealing because the chemical reagents are useful and valuable. We report on the eco-friendly synthesis of MCM-41 nanoporous materials via multi-cycle approach by re-using the non-reacted reagents in supernatant as mother liquor after separating the solid product. This approach was achieved via minimal requirement of chemical compensation where additional fresh reactants were added into the mother liquor followed by pH adjustment after each cycle of synthesis. The solid product of each successive batch was collected and characterized while the non-reacted reagents in supernatant can be recovered and re-used to produce subsequent cycle of MCM-41. The multi-cycle synthesis is demonstrated up to three times in this research. This approach suggests a low cost and eco-friendly synthesis of nanoporous material since less waste is discarded after the product has been collected, and in addition, product yield can be maintained at the high level.     

Direct Electrochemistry and Electrocatalysis of Hemoglobin on Bimetallic Au–Pt Inorganic–Organic Nanofiber Hybrid Nanocomposite and Mesoporous Molecular Sieve MCM-41

A glassy carbon electrode modified with MCM-41 and bimetallic inorganic–organic nanofiber hybrid nanocomposite was prepared and used for determination of trace levels of hydrogen peroxide (H₂O₂). The direct electron transfer (DET) and electrocatalysis of hemoglobin (Hb) entrapped in the MCM-41 modified Au–Pt inorganic–organic nanofiber hybrid nanocomposite electrode (Au–PtNP/NF/GCE) were investigated by using cyclic voltammetry in 0.1 M pH 7.0 phosphate buffer solution. Due to its uniform pore structure, high surface area and good biocompatibility, the mesoporous silica sieve MCM-41 provided a suitable matrix for immobilization of biomolecules. The MCM-41 modified Au–Pt inorganic–organic nanofiber hybrid nanocomposite electrode showed significant promotion to DET of Hb, which exhibited a pair of well-defined and quasi-reversible peaks for heme Fe(III)/Fe(II) with a formal potential of ?0.535 V (vs. Ag/AgCl). Additionally, the Hb immobilized on the MCM-41 modified electrode showed excellent electrocatalytic activity toward H₂O₂ reduction.     

Efficient degradation of <Emphasis Type="Italic">para</Emphasis>-chlorobenzoic acid in water by catalytic ozonation with La–Ce–MCM-41

La–Ce–MCM-41 was directly synthesized by a hydrothermal method and applied as heterogeneous catalyst for the ozonation process of para chlorobenzoic-acid (pCBA). La³⁺ and Ce³⁺ were successfully incorporated into the framework of MCM-41 and the formation of degradation products (p-chlorophenol, p-dihydroxybenzene, maleic acid and oxalic acid) were monitored qualitatively using gas chromatography–mass spectrometer and high performance liquid chromatography. Due to the synergy of bimetal and the fast degradation of accumulated intermediates, total organic carbon (TOC) removal efficiency was significantly improved (92 %) in La–Ce–MCM-41/O₃ process compared with ozonation (40 %) at identical reaction condition. The presence of tert-butanol (TBA) in La–Ce–MCM-41/O₃ process indicated that the oxidation of pCBA was mainly due to the function of hydroxyl radicals in the liquid bulk, and a plausible degradation pathway was proposed. TOC removal slightly decreased from 90 to 86 % after La–Ce–MCM-41 being re-utilized three times, which illustrated that La–Ce–MCM-41 was an efficient of promising catalyst for ozonation of pCBA.     

Preparation of Cr–Ti Binary Oxide Anchored Mesoporous Silica by CVD Method and Their Photocatalytic Activities

Cr–Ti binary oxide anchored mesoporous silica (Cr–Ti/MCM-41) was prepared by stepwise CVD treatments of MCM-41 with TiCl₄ and CrO₂Cl₂. Cr–Ti/MCM-41 exhibited higher efficiency for the photocatalytic polymerization of ethylene as well as the oxidation of CO into CO₂ than those on single component Cr⁶⁺-oxide anchored MCM-41 under UV and visible light.     

Immobilization of a Mo,V-polyoxometalate on cationically modified mesoporous silica: Synthesis and characterization studies

This paper presents studies on the immobilization of the polyoxometalate [PV₂Mo₁₀O₄₀]⁻⁵ (referred to as “POM”) on modified mesoporous MCM-41. The MCM-41 host material was made cationic by functionalization of the surface with [(MeO)₃Si(CH₂)₃N⁺(CH₃)₃]Cl. In polar solvents, POM is deprotonated and could be easily immobilized by wet impregnation of the modified silica using MeOH as the solvent. The physical properties of the samples were examined using XRD, FTIR, DR UV–Vis spectroscopy, ³¹P MAS-NMR, N₂ physisorption, and TEM. These techniques indicated that the POM is intact on the surface after impregnation. High loadings of POM caused a decrease in the surface area and pore volume of the solid, presumably due to both pore blockage and restructuring of the silica during wet impregnation. The texture and structure of the MCM-41 was studied as a function of POM loading.     

MCM-41介孔材料的合成条件和特性对吸附镉离子的影响

以气相氧化硅为硅源,十六烷基三甲基溴化铵(cetyl trimethyl ammonium bromide,CTAB)为模板剂,分别在碱性[氢氧化钠(NaOH),四乙基氢氧化铵,tetraethyl ammonium hydroxide,(C2Hs)4NOH(TEAOH)]和酸性介质条件[盐酸(HCl)]T水热合成了MCM-41有序介孔材料MCM-41-N,MCM-41-T和MCM-41-H.用X射线衍射、氮气吸附-脱附等手段对比分析了合成的3种MCM-41介孔材料的物相、比表面积、孔径、孔体积等,发现酸性介质中合成的介孔材料的孔径最大.在此基础上,利用MCM-41介孔材料对比研究了处理含镉离子(Cd2 )废水的效果和机理,确定了不同介孔材料用量、不同初始pH值条件下MCM-41介孔材料对水中Cd2 的吸附率和吸附量.结果表明:介孔材料用量相同时,溶液pH值的增大有利于提高3种MCM-41介孔材料对水中Cd2 的处理效果.在pH值从7.0到8.0的过程中,其吸附率有1个突变,MCM-41-T的Cd2 吸附率从35.65%提高到62.15%;MCM-41-N的从38.80%提高到69.40%;MCM-41-H的从50.22%提高到73.47%.孔径最大的MCM-41-H对Cd2 的吸附效果最佳,最大吸附率为89.56%,最大吸附容量为8.57 mg/g.吸附溶液pH值的大小和介孔材料的孔径尺寸是决定吸附量大小的关键因素,因此,重点应通过优化合成工艺提高介孔材料的孔径.     

Bifunctional mesoporous Cu–Al–MCM-41 materials for the simultaneous catalytic abatement of NOx and VOCs

This study explored the possibility of using waste organic solvent as the source of volatile organic compound (VOC) and it served as a reducing agent of selective catalytic reduction (SCR) deNO_x process, in which the VOC itself can be catalytically oxidized on the mesoporous Cu and/or Al substituted MCM-41 catalysts. The synthesized Cu–Al–MCM-41 catalysts were extensively characterized by powder low-angle X-ray diffraction (XRD), N₂ adsorption–desorption measurements, transmission electron microscopy (TEM), UV–Visible diffuse reflectance spectroscopy (UV–Vis DRS), ²⁷Al magic angle spinning-nuclear magnetic resonance spectroscopy (MAS-NMR), electron paramagnetic resonance spectroscopy (EPR) and inductively coupled plasma–mass spectrometer (ICP–MS) analysis. The XRD, TEM and N₂ adsorption–desorption studies clearly demonstrated the presence of a well ordered long range hexagonal array with uniform mesostructures. The Cu–Al–MCM-41 materials showed a better long-term-stability than that of copper ion-exchanged H–ZSM-5 (Cu–ZSM-5) zeolite. The Cu–Al–MCM-41 material was found to be an efficient catalyst than that of Cu–MCM-41 without aluminum for the simultaneous catalytic abatement of NO_x and VOCs, which was attributed to the presence of well dispersed and isolated Cu²⁺ ions on the Cu–Al–MCM-41 catalyst as observed by UV–Vis DRS and EPR spectroscopic studies. And the presence of aluminum (Al³⁺ ions) within the framework of Cu–Al–MCM-41 stabilized the isolated Cu²⁺ ions thus it led to higher and stabilized activity in terms of NO_x reduction.     

Tailoring the Surface of Aluminum-Enriched Diatom Biosilica

Aluminosilicates are widely used as sorbent materials, ion exchangers, and catalysts. While they are often synthesized by hydrothermal methods, microorganisms may open “green” synthesis ways. Diatoms can incorporate aluminum into their micro- and nanostructured silica-based cell walls. Thus, diatoms create intricately structured aluminosilicate materials. The present study investigates not only possible morphological changes during the in vivo Al-enrichment of the diatom species Thalassiosira pseudonana, but also the increase of the specific surface area of Al-enriched biosilica in vitro by etching with alkaline solutions.