Synthesis,characterization and catalytic properties of mesoporous MCM-48 containing zeolite secondary building units

Mesoporous aluminosilicate MCM-48 containing zeolite secondary building units in the pore wall has been synthesized in alkaline media with a two-step procedure. The aluminosilicate precursors comprising zeolite secondary building units were first synthesized by carefully controlling reaction conditions and then were assembled using cotemplates of gemini surfactant [C₁₈H₃₇N(CH₃)₂(CH₂)₃N(CH₃)₂C₁₈H₃₇]²⁺ (18-3-18) and triethanolamine (TEA). X-ray Diffraction (XRD) patterns of the as-made samples indicated that highly ordered mesostructured MCM-48 was formed. Transmission Electron Microscopy (TEM) images further verified the formation of MCM-48 with uniform cubic pore channel system having the pore opening diameter of about 25 Å. Compared with the conventionally synthesized MCM-48, the as-synthesized MCM-48 sample showed an adsorption band at 520 600 cm^-1 in its FT-IR spectrum, which was assigned to five-membered ring vibration from zeolite structure. This suggested the presence of zeolite building units in the pore wall. N2 adsorption data showed that the material had a much higher specific surface area (1 200 m₂/g) than the conventional MCM-48(1 100 m₂/g). Finally, the catalytic performance of the as-made MCM-48 was evaluated by hydrogenation dealkylation reaction of heavy aromatic hydrocarbons. Catalytic results showed that the as-made MCM-48 catalyst exhibited higher conversion than the conventional MCM-48 catalyst. The as-made mesostructured MCM-48 may have a potential catalytic application in the conversion of bulky molecules.     

Preparation self-bonded zeolite MCM-22 bodies by vapor-phase transport method

Self-bonded bodies of zeolite MCM-22 were prepared by vapor-phase transport method. The resultant materials were characterized by means of XRD, SEM, NH₃-TPD, BET, ²⁷Al MAS NMR, Pascal 140 and Pascal 240. It was found that the bodies, prepared by aluminosilicate gel, had been transformed into zeolite MCM-22. The zeolite bodies that were composed of MCM-22 crystal chips avoided binder accession. The total acid sites, the specific surface area, mechanical resistance and the average pore radius of the zeolite bodies reached 1.01 × 10⁻³ mol/g, 486.82 m²/g, 126 N/cm and 57.93 nm, respectively.     

A high CO2 permselective mesoporous silica/carbon composite membrane for CO2 separation

Ordered mesoporous silica/carbon composite membranes with a high CO₂ permeability and selectivity were designed and prepared by incorporating SBA-15 or MCM-48 particles into polymeric precursors followed by heat treatment. The as-made composite membranes were characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and N₂ adsorption, of which the gas separation performance in terms of gas permeability and selectivity were evaluated using the single gas (CO₂, N₂, CH₄) and gas mixtures (CO₂/N₂ and CO₂/CH₄, 50/50 mol.%). In comparison to the pure carbon membranes and microporous zeolite/C composite membranes, the as-made mesoporous silica/C composite membranes, and the MCM-48/C composite membrane in particular, exhibit an outstanding CO₂ gas permeability and selectivity for the separation of CO₂/CH₄ and CO₂/N₂ gas pairs owing to the smaller gas diffusive resistance through the membrane and additional gas permeation channels created by the incorporation of mesoporous silicas in carbon membrane matrix. The channel shape and dimension of mesoporous silicas are key parameters for governing the gas permeability of the as-made composite membranes. The gas separation mechanism and the functions of porous materials incorporated inside the composite membranes are addressed.     

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.     

Comparative Study of the Acidic and Catalytic Properties of the Mesoporous Material H-MCM-41 and Zeolite H-Y

The objective of this work is to give a comparative characterization of aluminosilicate MCM-41 and zeolite Y, in particular with respect to acidity and catalytic properties in hydrocarbon cracking. These studies are compared to a well investigated amorphous aluminosilicate, based on the fact that the new mesoporous MCM-41 materials are a mixture of ordered material with amorphous pore walls. Characterization of differently treated MCM-41 materials and zeolite Y for comparison by N₂ sorption, XRD, TPAD (Temperature Programmed Ammonia Desorption) with in situ FT-IR in combination with catalytic testing by MAT (Micro Activity Test) is discussed. Combination of the characterization data and the catalytic testing gives an interesting explanation of the surface properties especially in comparison with zeolite Y.     

Effects of the spacer length of gemini surfactants on the ordered pore of silica

A family of designed gemini surfactants C₁₄H₂₉(CH₃)₂N⁺-(CH₂)_S–N⁺(CH₃)₂C₁₄H₂₉·2Br⁻ (designated as C_14-S-14, S = 4, 6, 8, 10) was utilized as structure-directing agent to prepare ordered porous MCM-41 silica. The samples were characterized by small angle X-ray diffraction, high-resolution transmission electron microscopy and N₂ adsorption/desorption analysis. The results showed that the obtained materials possessed 2D-hexagonal periodic structure (space group 2D-p6mm), but the pore diameter obviously decreased as the length of the spacer group of the gemini surfactants increased. Two types mesoporosity, that is, framework-confined mesoporosity (primary pores) and voids between particles (textural mesoporosity) existed in the samples. In addition, the self-assembly ability of C_14-S-14 was stronger than that of tetradecyltrimethylammonium bromide (TTAB, which was regarded as the corresponding monomer of gemini surfactant C_14-S-14) in controlling the orderly pore structure and pore size of the porous materials.     

Synthesis of highly ordered supermicroporous silica using short-chain cationic trimeric surfactant as structure-directing agent

Highly ordered supermicroporous silica was synthesized by short chains cationic trimeric surfactant [C₁₀H₂₁N⁺(CH₃)₂(CH₂)₂N⁺(CH₃)(C₁₀H₂₁) (CH₂)₂N⁺(CH₃)₂C₁₀H₂₁] · 3Br⁻ (denoted C_10-2-10-2-10) with a short spacer group (s = 2) as the structure-directing agent and tetraethyl orthosilicate as the precursor. The obtained samples were characterized by small-angle X-ray diffraction, high resolution transmission electron microscopy, and N₂ adsorption–desorption. The results showed that the pore structure of the resulting samples belonged to the two-dimensional hexagonal structure (space group 2D-p6mm) with a pore size from 1.92 to 2.16 nm, which was within the supermicroporous range. The high-quality supermicroporous silica was formed at a low molar ratio of C_10-2-10-2-10 to tetraethyl orthosilicate (0.08:1), which indicated that the self-assembly ability of C_10-2-10-2-10 was stronger than that of corresponding monovalent surfactants. We strictly compared the methods of calculating surface area and pore size of supermicroporous materials, and the surface area was found to be in the range of 910–1,135 m² g⁻¹ by the α_s plot method. With the increase of hydrothermal temperature, the ordering of the supermicroporous structure increased first then decreased, at the same time the pore size was enlarged.     

Oligomerization of propene on an alumoxane-grafted MCM-41 host with bis(cyclopentadienyl)zirconium dimethyl (Cp2Zr(CH3)2)

The anchoring of alumoxane, synthesized by the in situ hydrolysis of trimethylaluminum, on the internal pore walls of a mesoporous MCM-41 support generates a highly active and selective host for bis(cyclopentadienyl)zirconium dimethyl (Cp₂Zr(CH₃)₂) in the oligomerization of propene. The regioselective preference of the immobilized metallocene is preserved and a typical Flory–Schulz distribution for the propene oligomers is obtained. This revised version was published online in July 2006 with corrections to the Cover Date.     

Characterization of (Fe,Al)FER synthesized in presence of ethylene glycol and ethylene diamine

(Fe,Al)Ferrierite type zeolite was hydrothermally crystallized in a gel system of sodium aluminosilicate with ethylene glycol a nitrogen-free organic compound, as well as, ethylene diamine. The zeolite crystals obtained were characterized by X-ray diffractometry (XRD), thermal analysis (TGA–DTA), electron scanning microscopy (SEM), chemical analysis, N₂ adsorption/desorption at 77 K and ²⁹Si–, ²⁷Al–, ¹³C–, and ¹H NMR spectroscopy. Fe atoms essentially occupy framework positions and rather high amounts of SiOH defect groups are present in the as-made samples.     

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.     

含L沸石次级结构单元的介孔分子筛的合成

以极低浓度n(CTAB)∶n(SiO₂)=0.06的阳离子表面活性剂十六烷基三甲基溴化铵（CTAB）为模板,以具有L沸石次级结构单元的溶胶为前驱体,在碱性条件下合成介孔分子筛L-MCM-41。样品的XRD、FT-IR、N₂吸附脱附和NH₃-TPD等表征结果表明,合成出的介孔分子筛是孔壁含有L沸石次级结构单元的六方晶相结构,该介孔分子筛的孔壁厚于常规水热方法合成的MCM-41的孔壁,具有较高的水热稳定性和较强的酸性。     

Hydrocracking of petroleum gas oil over NiW/MCM-48-USY composite catalyst

MCM-48-USY composite materials were prepared by coating USY zeolite by a layer of MCM-48 mesoporous material at different meso/microporous ratios (SiO₂/USY ratios of 0.1, 0.2, 0.3, 0.4, 0.5) and used as support for nickel and tungsten. The NiW/MCM-48-USY catalysts were prepared using the incipient wetness method. The prepared catalysts were characterized by TPD-TGA acidity, TGA thermal stability, BET surface area, pore volume, pore size, XRD, SEM and TEM and then tested for hydrocracking of petroleum gas oil at reaction temperature of 450 °C, contact time of 90 min and catalyst to gas oil ratio of 0.04. In all prepared samples, the catalyst activity and properties were improved with increasing SiO₂/USY ratio and found that maximum values of a total conversion and liquid product (total distillate fuels) were obtained at SiO₂/USY ratio of 0.5. Finally, the obtained results from hydrocracking of gas oil over composite MCM-48-USY catalysts were compared with those obtained over physically mixed USY and MCM-48 catalysts.     

System Approach to Analysis of the Role of the Synthesis Components and Stability of the MCM-41 Mesostructured Silicate Material

The formation of a mesostructured silicate material of the MCM-41 type is studied. The influence of the replacement of components in a reaction medium is investigated experimentally. Analysis of the results obtained and the data available in the literature suggests that the process under investigation is based on the approximate stoichiometric supramolecular interaction between [Si₄O_{4 + x} (OH)_{9 − x} ]^{−(1 + x)} silicate polyanions and C₁₆H₃₃(CH₃)₃N⁺ cetyltrimethylammonium cations with the formation of supramolecular aggregates, which condense to a mesostructured organosilicate composite. A further evolution of the product involves hydrolysis of the inner surface and the polymerization of the inorganic component. It is demonstrated that the properties of the product are determined, to a large extent, by the components of the reaction medium, which control the relative reaction rates in the process. The inference is made that an alcohol-ammonia medium is the most optimum for alkaline synthesis. This medium provides good preparation of the initial components for the reaction and the high rate of hydrolysis of pore walls, minimizes the osmotic effects during hydrothermal treatment, and, eventually, favors the formation of a highly structured hydrothermally stable material.Original Russian Text Copyright © 2005 by Fizika i Khimiya Stekla, Kirik, Belousov, Parfenov, Vershinina.     

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.     

Synthesis of layered MCM-22(P) in the presence of hexamethonium cations and its transformation into EUO- and MWW-type zeolites

Layered MCM-22(P) was synthesized in the presence of hexamethonium (HM) cations. Compared to zeolite EU-1 (EUO-type structure), which crystallizes in similar conditions, its formation is favored at high HM concentrations (HM/SiO₂ ≥ 0.3) and in the absence of Na₂O. HM-containing MCM-22(P) was used as starting material for zeolite formation. Upon hydrothermal treatment, HM-MCM-22(P) transforms into zeolite EU-1 and upon calcination into a MWW-type zeolite. Transformation mechanisms were studied by standard characterization techniques such as XRD, SEM and TEM. Catalytic properties of the MWW-type zeolite obtained from this precursor were evaluated in a m-xylene isomerization reaction. Compared to zeolite MCM-22 prepared with hexamethyleneimine, a higher catalytic activity and an increased isomerization selectivity were observed and discussed.     

One-step synthesis of hydrothermally stable cubic mesoporous aluminosilicates with a novel particle structure

Hydrothermally stable and highly ordered cubic mesoporous aluminosilicate has been successfully prepared by using a one-step synthesis route. It consists of introducing the zeolite primary building units into the walls of the material. The synthesized material was analyzed by XRD, N₂ adsorption–desorption, FT-IR, HRTEM and other techniques. The characterization results showed that the hydrothermal stability of the material was greatly improved, while the pore system remained highly ordered. The hydrothermal stability is believed to be related with a novel “core-shell” particle structure that formed during the assembling of aluminosilicate precursors with the surfactant molecules together with most zeolite primary building units into the shell.     

Gas transport behavior of DMDCS modified MCM-48/polysulfone mixed matrix membrane coated by PDMS

Mesoporous MCM-48 silica was synthesized by templating method and the structure of particles was characterized by XRD, TEM and N₂ adsorption techniques. The surface modification of particles in order for introducing into PSF matrix was performed by dimethyldichlorosilane (DMDCS) silylation agent. SEM images of as-synthesized and modified MCM-48/PSF MMMs indicate that in the modified MCM-48 silica particles adhered well to the PSF matrix and that the synthesized MMMs were defect free. The incorporation of MCM-48 particles in to the PSF matrix and also surface coating of these MMMs by polydimethylsiloxane (PDMS) were performed. The quality of surface coating was investigated by SEM images and permeability tests. For all gases tested (N₂, CO₂, CH₄ and O₂), the permeabilities increased in proportion to the weight percent of MCM-48 present in the film and the calculated CO₂/CH₄ and O₂/N₂ selectivities of PDMS coated membranes showed enhancement in ideal and actual selectivities both.     

Selective reduction of nitric oxide by methane on cobalt-ion-exchanged synthetic clinoptilolite zeolite in oxygen-rich atmosphere

Selective reduction of NO by CH₄ in the presence of excess O₂ has been studied on cobalt-ion-exchanged synthetic clinoptilolite (Co-CLI) zeolite. The catalytic results are compared with those obtained from Co-ZSM-5 and Co-FER, the two most widely studied catalysts for this reaction. At T ≥ 500 oC, Co-CLI is much more active for NO reduction than Co-ZSM-5, but it is less active than Co-FER. However, it is found that the selectivity for CH₄ toward reacting with NO at high temperatures (T ≥ 450 oC) is higher on Co-CLI than on Co-ZSM-5 or Co-FER. On the basic of the overall catalytic results, the importance of zeolite pore size as a structural parameter influencing the activity of intrazeolitic Co²⁺ ions for NO reduction by CH₄ is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of the nature of the aluminum source on the acidic properties of some mesostructured materials

Mesostructured aluminosilicates have been synthesized using gels prepared by reacting colloidal silica (Ludox AS) with Al(OH)₃, aluminum isopropoxide (Al(iPrO)₃) or NaAlO₂ in the presence of a surfactant. The hydrothermal transformation at 110°C of these gels produced solids with the hexagonal structure typical of MCM-41-type materials. These crystals have been characterized by X-ray diffraction, thermal analysis (TG/DTA), N₂ sorption and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The type, strength and density of acid sites have been studied using microcalorimetry and infrared (IR) spectroscopy.As a general trend, pore volume and average pore size decreased as the gel SiO₂/Al₂O₃ ratio decreased from 32–8, while the pore wall thickness remained in the 1.0–1.5 nm range. Except for one sample, Al was incorporated in tetrahedral coordination inside the pristine crystals. However, as expected, dealumination occurred upon calcination at 600°C/12 h, yielding materials having both tetrahedral and octahedral Al-species. Fourier transform infrared (FTIR) experiments with pyridine have indicated that these mesostructured aluminosilicates contain both Brönsted and Lewis acid sites and that acidity is strongest in samples prepared with NaAlO₂.Microcalorimetry experiments with ammonia as the probe molecule have shown that Al insertion into the mesoporous silicate framework affects acid site strength and distribution in a manner controlled by synthesis condition. Samples prepared with Al(OH)₃ contain a wide distribution of acid site strengths, indicating the absence of preferred locations of Si-O-Al groups within the pore walls. In contrast, distinct populations of acid sites with strengths in the 130–140 kJ mol⁻¹ range or near 150 kJ mol⁻¹, appear in materials prepared with Al(iPrO)₃ or with NaAlO₂, respectively. Al sites may be located at the surface of the pore wall, where they interact directly with the basic probe molecule. They may also be sandwiched between silica layers within the pore wall (3–4 Si layers thick), giving acid sites with a strength comparable with that of smectites. Finally, they may also be present in the pore (or within the pore walls) as extraframework Al(VI)-species. The nature, size, concentration, ease of hydrolysis and condensation of the aluminum precursors during synthesis control aluminum incorporation, distribution and location within the structure and with it the acidity of the resulting mesoporous aluminosilicate.     

孔壁中含有13X沸石基本结构单元的铝硅酸盐介孔分子筛的合成与结构表征

以天然矿物微斜长石为主要原料,以13X沸石前驱体为晶核剂,结合介孔分子筛MCM-41与13X沸石合成工艺,合成了孔壁中含有13X沸石基本结构单元的铝硅酸盐介孔分子筛.在合成过程中,Na2CO3与微斜长石粉体以摩尔比为1:1.05混合,先在1 093 K温度焙烧2.5 h,焙烧后的矿粉与十六烷基三甲基溴化铵模板剂溶液和13X沸石晶核剂混合,并在378K温度晶化48h.对所得粉体在823K温度焙烧5h制得铝硅酸盐介孔分子筛.用X射线衍射、小角X射线散射、红外光谱和核磁共振等测试手段对样品进行了表征.结果表明:合成的分子筛具有类似于MCM-41的六方平行密堆的孔道结构,平均孔径为6nm;样品的孔壁中含有13X沸石基本结构单元双六元环;样品孔壁中的硅酸盐结构单元主要由Si(OSi)4(71%,摩尔分数,下同)和Si(OSi)3(OH)(29%)构成.该样品在高压沸水中具有很好的水热稳定性.