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.     

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.     

Tethering of Cu(II) Schiff base metal complex on mesoporous material MCM-41: catalyst for Ullmann-type coupling reactions

The siloxane-functioned Cu(II) complex derived from 3-aminopropyltrimethoxysilane, terephthaldehyde and 2-aminophenol was anchored on hexagonal mesoporous silica MCM-41. The MCM-41 and its Schiff base MCM-41 complex was characterized by FTIR, small-angle X-ray diffraction, SEM-EDX, N₂ adsorption and desorption analysis and TGA. The synthesized Cu-Schiff base MCM-41 catalyst has been successfully applied as a catalyst for Ullmann-type coupling reaction of the aryl halides with aryl halides, phenols, amines and N-heterocyclic amines. In the coupling reaction the yields of the products were good and the catalyst was recovered by simple filtration method and can be reused.     

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.     

Adsorption of phosphate and nitrate anions on ammonium-functionnalized mesoporous silicas

Surface modified mesostructured silica materials represent potential adsorbents offering an opportunity to remediate several important water pollutants. In the present work, ammonium-functionnalized MCM-41, MCM-48 and SBA-15 mesoporous silica materials were synthesized via post-synthesis grafting and co-condensation. Their efficiency to remove nitrate and phosphate anions in aqueous solutions was investigated. The adsorbent materials showed high adsorption capacities reaching 46.5 mg NO₃⁻/g and 55.9 mg H₂PO₄⁻/g under the operating conditions explored. The mesoporous silica materials functionalized via post-synthesis grafting method exhibited higher performances in terms of percentage pollutant removal and adsorption capacities if compared to their analogs synthesized according to the co-condensation strategy.     

Mesoporous silica supports for improved thermal stability in supported Au catalysts

In this work, we present a comprehensive review of our research on the role of mesoporous silica pore architecture, composition of the pore walls (addition of Co or Al), and silica surface chemistry (surface modification by TiO₂) to improve the hydrothermal stability of Au particles. We have found that mesoporous silica architecture plays an important role in improving Au stability, with three dimensional mesoporous architectures being less effective than one dimensional (1-D) pores. The tortuous 1-D pores in aerosol silica were found to be most effective at controlling Au particle size. Since Au particles continue to grow larger than the pore diameter, we conclude that Ostwald ripening must be the dominant sintering pathway for these Au catalysts. These catalysts are active for CO oxidation even after the Au particles have grown large enough to block the pores, suggesting that the thin walls of mesoporous silica provide easy access to gas phase molecules. Further improvements in Au stability and reactivity were obtained by surface modification of the aerosol and MCM-41 silica with TiO₂. After TiO₂ modification of the silica, the Au particles remained smaller than the pore size (< 3 nm) even after three cycles of CO oxidation at temperatures up to 400 °C.     

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.     

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.     

Synthesis, Characterization of Ag/MCM-41 and the Catalytic Performance for Liquid-phase Oxidation of Cyclohexane

Nano-scale silver supported mesoporous molecular sieve Ag/MCM-41 was directly prepared by one-pot synthesis method. The prepared sample was characterized by XRD, TEM, and N₂ sorption. The results showed that the sample of Ag/MCM-41 had no appreciable incorporation of silver into the mesoporous matrix of MCM-41 with good crystallinity, and silver nanoparticles were dispersed inside or outside of the channels in the mesoporous host. The catalytic performance of the sample for the cyclohexane liquid-phase oxidation into cyclohexanone and cyclohexanol by oxygen in the absence of solvents without inducing agents was investigated. The 83.4% selectivity to cyclohexanol and cyclohexanone at 10.7% conversion of cyclohexane was obtained over Ag/MCM-41 catalyst at 428 K for 3 h. The turn over numbers (TONs) of Ag/MCM-41 was up to 2946. The catalytic activity of Ag/MCM-41 was also compared with Ag/TS-1 as well as Ag/Al₂O₃. The results indicated that Ag/MCM-41 showed superior activity to both Ag/TS-1 and Ag/Al₂O₃. A calcined Ag/MCM-41 was found to be an efficient catalyst for the cyclohexane oxidation into cyclohexanol and cyclohexanone using oxygen as oxidant.     

Hoveyda-Grubbs type metathesis catalyst immobilized on mesoporous molecular sieves—The influence of pore size on the catalyst activity

New hybrid olefin metathesis catalysts were prepared by immobilization of Hoveyda-Grubbs type catalyst (commercially available as Zhan catalyst-1B) on the surface of mesoporous molecular sieves differing in pore size and architecture (MCM-41, MCM-48, and SBA-15) and conventional silica for a comparison. The activity of these catalysts was tested in RCM of (−)-β-citronellene, metathesis of 1-decene, ADMET of 1,9-decadiene, and in ROMP of cyclooctene and was found to increase significantly with the increasing pore size of the supports used. In all reactions, the activity of hybrid catalysts based on mesoporous molecular sieves was higher than that of catalyst using conventional silica as a support. In ROMP of cyclooctene, high molecular weight polymer (M_w = 300,000) in high yield (70-80%) was obtained with catalysts based on mesoporous supports, however, only 40% polymer yield was obtained using catalyst based on conventional silica.     

Preparation of sulfated alumina supported on mesoporous MCM-41 silica and its application in esterification

New types of mesoporous SA/MCM-41 solid acid catalysts were prepared by loading sulfated alumina (SA) on MCM-41. The prepared catalysts were characterized by XRD, IR, N₂ physisorption, elemental analysis, FT-IR of adsorbed pyridine and NH₃-TPD. The esterification of acetic acid with n-butanol and citric acid with n-butanol were used as model reactions to test the catalytic activities and reusability of the SA/MCM-41 solid acid catalysts. Compared with SA catalyst, SA/MCM-41 catalysts exhibited higher catalytic performances, which were attributed to their high BET surface area and large pore volume. Moreover, 20SA/MCM-41 solid acid catalyst showed excellent reusability in both esterifications.     

Synthesis, characterization, and catalytic properties of a hydrothermally stable Beta/MCM-41 composite from well-crystallized zeolite Beta

A Beta/MCM-41 composite has been synthesized with a new method by using well-crystallized zeolite Beta as silica and aluminum source. The prepared composite was characterized by XRD, FTIR, N₂ adsorption/desorption at 77 K, FE-SEM, DTG, ²⁹Si MAS NMR spectral techniques. It was shown that the composite consisted of a highly ordered mesoporous MCM-41 phase and a zeolite Beta phase. Its hexagonal mesoporous structure was still retained after statically treated for 120 h in boiling water. In contrast, the structure of generally synthesized Al-MCM-41 nearly completely collapsed. This might be attributed to the assembly of the dissolved fragments such as the first and/or secondary structural units of zeolite Beta into the mesoporous structure around surfactant micelles. This is supported by the catalytic result that the prepared composite showed higher activity and selectivity for medium fraction in hydrocracking of Daqing vacuum residue than the parent zeolite Beta, the Al-MCM-41 as well as the mechanical mixture of these two materials.     

Selective removal of sulfur compounds in city-gas by adsorbents

Silver nitrate impregnated on beta zeolite (BEA), mesoporous silica MCM-41 and SBA-15 (AgNO₃/BEA, AgNO₃/MCM-41, AgNO₃/SBA-15) were prepared to remove sulfur compounds selectively in city-gas, which contains tetrahydrothiophene and tert-butylmercaptane. Sulfur adsorption capacity was determined when the sulfur concentration of effluent gas in breakthrough test reached 0.1 ppm, which is acceptable sulfur concentration for hydrogen production in a reformer for fuel cells. As the AgNO₃ concentration in AgNO₃/BEA, AgNO₃/MCM-41, and AgNO₃/SBA-15 increased, their sulfur adsorption capacities also increased. Although microporous zeolite BEA has smaller pore volume and lower surface area than those of mesoporous silica MCM-41 and SBA-15, the sulfur adsorption capacity of AgNO₃/BEA was higher than those of AgNO₃/MCM-41 and AgNO₃/SBA-15. Adsorbed sulfur molecules per impregnated silver nitrate and the shape change of breakthrough curves depending on the adsorption temperature showed that not only chemisorption but also physisorption was involved in sulfur compounds adsorption on AgNO₃ impregnated adsorbent. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Near-infrared luminescent mesoporous MCM-41 materials covalently bonded with ternary thulium complexes

Two β-diketones 4,4,4-trifluoro-1-2-thenoyl-1,3-butanedione (Htta) and 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butanedione (Htfnb), which contain trifluoroalkyl chain, were selected as the main sensitizer for synthesizing Tm(L)₃phen (L = tta, tfnb) complexes. The two near-infrared (NIR) luminescent thulium complexes have been covalently bonded to the ordered mesoporous material MCM-41 via a functionalized 1,10-phenanthroline (phen) group 5-(N,N-bis-3-(triethoxysilyl)propyl)ureyl-1,10-phenanthroline (phen-Si) [The resultant mesoporous materials are denoted as Tm(L)₃phen–MCM-41 (L = tta, tfnb)]. The Tm(L)₃phen–MCM-41 (L = tta, tfnb) mesoporous materials were characterized by small-angle X-ray diffraction (XRD) and N₂ adsorption/desorption, and they show characteristic mesoporous structure of MCM-41. Luminescence spectra of the Tm(L)₃phen–MCM-41 (L = tta, tfnb) mesoporous materials were recorded and the corresponding luminescence decay curves were obtained. After ligand-mediated excitation, the emission spectra of the Tm(L)₃phen–MCM-41 (L = tta, tfnb) mesoporous materials show the characteristic NIR-luminescence of the Tm³⁺ ion. The full width at half maximum (fwhm) of the 1474-nm emission band are 96 and 100 nm for Tm(tta)₃phen–MCM-41 and Tm(tfnb)₃phen–MCM-41, respectively. The good luminescent performances enable these NIR-luminescent mesoporous materials to have potential applications in optical amplification [broadening amplification band from C band (1530–1560 nm) to S⁺ band (1450–1500 nm)]. Furthermore, the comparison of the luminescence behavior for Tm(tta)₃phen–MCM-41 and Tm(tfnb)₃phen–MCM-41 mesoporous materials was investigated. It shows that Tm(tfnb)₃phen–MCM-41 is somewhat superior to Tm(tta)₃phen–MCM-41 as optical amplifier.     

Mesoporous silicas containing carboxylic acid: Preparation,thermal degradation,and catalytic performance

The calcination and thermal degradation behaviors of surfactants in mesoporous silicas SBA-15 and MCM-41 were investigated by FT-IR, ¹³C CP/MAS NMR, TG/DTA, and GPC. It was found that carboxylic acid-containing products were generated as active components in the mesopores of SBA-15 and MCM-41 from the triblock copolymer (PEO)₂₀(PPO)₇₀(PEO)₂₀ and cetyltrimethylammonium bromide (CTAB), respectively; the latter materials were used as templates. The carboxylic acid-containing mesoporous silica obtained showed a catalytic activity for hydrolysis of sucrose. The acidity was evaluated by means of NaOH titration. The acidity sensitively depended on both the calcination temperature and the atmosphere; the maximum appeared at 150 °C in air for SBA-15 where the highest activity was observed. However, the product in MCM-41 showed a lower catalytic activity than that in SBA-15. The SBA-15 product was easily leached from the mesopores of SBA-15 into the solution, but the degree of leaching for MCM-41 was considerably smaller than that for SBA-15.     

MCM-41介孔分子筛合成与改性的研究进展

简要介绍了MCM-41介孔分子筛的特点,应用和改性原因.综述了MCM-41介孔分子筛的合成方法,主要包括水热合成法,室温合成法,微波合成法等,列出了每种合成方法的优缺点和合成过程中的影响因素,pH值、晶化时间、晶化温度、模板剂的种类及用量等都会对MCM-41介孔分子筛的结构和孔径产生很大影响.阐述了MCM-41介孔分子筛的改性方法,包括金属杂原子取代法,如主族金属、过渡金属、稀土金属等,有机修饰或功能化法,负载型改性法,如负载金属氧化物、无机酸、杂多酸、有机碱、金属的配合物等.最后就MCM-41介孔分子筛的应用前景做了展望.     

CuCl2 Immobilized on Amino-Functionalized MCM-41 and MCM-48 as Efficient Heterogeneous Catalysts for Dimethyl Carbonate Synthesis by Vapor-Phase Oxidative Carbonylation of Methanol

Pure silica mesoporous molecular sieves MCM-41 and MCM-48 organofunctionalized with 3-aminopropyltrimethoxysilane and N-[(3-trimethoxysilyl)propyl]ethylene-diamine, respectively, were served as supports to immobilize CuCl₂ with conventional impregnation method. The supported copper catalysts showed a considerable enhancement in the reaction rate in heterogeneous vapor-phase oxidative carbonylation of methanol to produce dimethyl carbonate in comparison with that obtained by CuCl₂-supported nonfunctionalized mesoporous silicas under identical conditions. The electronic donation of the amino groups, the accessibility and dischargeability of reactants in the regular mesopores probably account for the good catalytic performance as evidenced by the characteristic studies with XRD, BET, FTIR, TG-DTA, and XPS.     

Regenerable solid CO2 sorbents prepared by supercritical grafting of aminoalkoxysilane into low-cost mesoporous silica

The present work examines the functionalization of silica supports via supercritical CO₂ grafting of aminosilanes, which is an important step in the preparation of materials used as solid sorbents in CO₂ capture. Four materials have been considered as solid supports: two commercially available silica gels (4.1 and 8.8 nm pore diameter), the mesoporous silica MCM-41 (3.8 nm pore diameter) and a microporous faujasite of the Y type. Mono- and di-aminotrialkoxysilane were chosen for this study. The optimal operating conditions required to have free aminosilane in solution were first evaluated by studying the phase behavior of the system scCO₂/aminosilane at different pressures and temperatures. FTIR spectroscopy was used to determine the chemical structure of the grafted species. Aminosilane uptake was estimated by thermogravimetric and elemental analysis. Densities up to 3–4 molecules of monoaminosilane per nm⁻² were reached by using a small amount of a cosolvent together with the supercritical CO₂. The samples were characterized in regards of thermal stability, showing that aminosilane groups were covalently attached to the amorphous silica surface in the mesoporous supports, but not in the microporous zeolite. Low temperature N₂ and ambient temperature CO₂ isotherms were recorded to establish the adsorptive behavior of the prepared hybrid materials. The amine functionalized MCM-41 and the 8.8 nm silica gel exhibited a significant higher uptake of CO₂ at low pressures compared with the bare supports. On the contrary, for the 4 nm silica gel and the zeolite the adsorption decreased after impregnation.     

Synthesis and Characterization of Highly Dispersed Molybdenum Carbides in Mesoporous Silica

Highly dispersed molybdenum carbides in MCM-41 mesoporous silica are synthesized by temperature-programmed carburization and are characterized. Two methods of preparation are examined: (i) insertion of molybdenum during the synthesis of the MCM-41 silica and (ii) postsynthesis incorporation into a MCM-41 silica matrix by the incipient wetness method. Propene transformation (hydrogenation and metathesis) was used as a probe reaction; the observed catalytic behavior can be explained as a result of the preparation method, i.e., of the relative strength of interaction between the molybdenum oxide precursor and the support.     

Effect of pH in hydrothermal treatment of as-synthesized MCM-41 on wetting stability of resulting materials in aqueous solution

Effect of pH in aqueous media used in the hydrothermal treatment of as-synthesized MCM-41 on the structure and the wetting stability of the resulting materials was studied using XRD, TG-DTA and N₂ adsorption. The long-range structural order and the wetting stability of the mesoporous MCM-41 were improved through hydrothermal treatment of as-synthesized sample at around pH 7 without salt addition. The pH adjustment of aqueous media toward neutral is thought to shift the polymerization equilibrium of silica species around external surface of the micelles to a higher degree of condensation, and to reduce the electrostatic repulsion between the silicate–micelle composites. This leads to the further formation of Si–O–Si network within the silica walls of the mesoporous materials, resulting in the enhancement of the structural stability of MCM-41 samples.