Siliceous Mesoporous Molecular Sieves Research Articles

Synthesis of mesoporous silicate molecular sieves by the aerosol-assisted method for loading and release of drug.

The mesoporous silicate molecular sieves were synthesized with polyether F127 as the template by the aerosol-assisted method for loading and release of ibuprofen (IBU). The synthesized samples were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and N2 adsorption–desorption isotherms. The drug IBU was applied as a model drug to investigate the drug release behaviour by ultraviolet spectrophotometry measurements. The investigation results demonstrate that mesoporous silicate molecular sieves by the aerosol-assisted method are spherical with a core–shell structure. As the drug carrier, it has good structural stability and can achieve drug controlled release which is expected. It exhibits safety to a certain degree. Therefore, the aerosol-assisted synthesis method provides a new idea for the synthesis of sustained-release drug carriers.

Structure and Porosity of Silicon Carbide Produced by Matrix Method

Silicon carbide in the $$ \beta $$ modification with a developed surface and a large pore size was obtained by carbothermal reduction using mesoporous siliceous molecular sieves (SBA-15, KIT-6, MCF, SBA-3). The formation of silicon carbide with high porosity parameters is favored by the use of the KIT-6 mesoporous molecular sieve (MMS) with a three-dimensional structure and a mutually intersecting system of mesopores as matrix, by the close to the stoichiometric C/SiO2 ratio, and also by the higher level of filling with carbon in the pores of the siliceous MMS.

Ru-Based Complexes with Quaternary Ammonium Tags Immobilized on Mesoporous Silica as Olefin Metathesis Catalysts

Ruthenium olefin metathesis catalysts bearing a polar quaternary ammonium group in N-heterocyclic ligand were immobilized on silica and siliceous mesoporous molecular sieves with different pore sizes (SBA-15 and MCM-41). The activity of the heterogeneous catalysts was found to increase with an increase in pore size of the support used, with the best results observed for SBA-15-supported catalyst. The influence of reaction conditions (temperature, solvent, catalyst, and substrate concentration) on the efficiency of new heterogeneous catalysts was established. A significant influence of the counterion present in the ruthenium complex on the activity of immobilized catalysts was also found: those derived from chloride containing ion exhibited the highest activity. High activity in ring-closing metathesis of substrates as citronellene, 1,7-octadiene, and diallyl compounds as well as in cross-metathesis of unsaturated aliphatic compounds with methyl acrylate was observed under optimized conditions. In some cas...

Adsorption Behaviour of Si-mcm-41 for Co2: Effect of Pressure and Temperature on Adsorption

Adsorption process is an alternative process for CO2 removal which could offer solutions to the existing problems associated with absorption process. A suitable adsorbent which has high adsorption capacity and selectivity for CO2 is required in adsorption process. Highly ordered siliceous mesoporous molecular sieve, Si-MCM-41 which has high surface area and large pore volume is an attractive candidate for use as adsorbent in the adsorption separation. Si-MCM-41 material was synthesized by hydrothermal process at 100 °C for 8 days then dried at 120 °C for 24 hours and finally calcined at 550 °C for 6 hours. Calcined Si-MCM-41 was characterized by XRD, FTIR, HRTEM and N2 physisorption. All the characterization results confirm that the synthesized material is Si-MCM-41. Adsorption behaviour of Si-MCM-41 for CO2 was investigated at temperatures between 25 - 100 °C and pressures from 0 - 20 bar using Rubothermgravimetric-densimetric apparatus. Si-MCM-41 showed low adsorption capacity of 27.43 mg-CO2/g- adsorbent at 1 bar and 25 °C. This low adsorption of CO2 on Si-MCM-41 could be attributed to the weak interaction between the siliceous material and CO2. Results show that adsorption of CO2 increases with increasing pressure and at it reaches to 296.48 mg CO2/g-adsorbent at 20 bar which can be explained asthe CO2 molecules are forced into the deep inside the pores of Si-MCM-41 at high pressure. The Si-MCM- 41 material also shows an increased in CO2 adsorption with increased temperature with 335.77 mg-CO2/g- adsorbent adsorbed at 50 °C. However, further increase in temperature to 75 °C and 100 °C decreases theadsorption of CO2 to 295.07 mg-CO2/g-adsorbent and 269.86 mg-CO2/g-adsorbent, respectively. This decrease in adsorption is suggested due to weakening of the Van der Waals forces between Si-MCM-41 and CO2. Overall results show that Si-MCM-41 has high potential to be used as adsorbent for CO2 removalat pressures up to 20 bar and temperatures up to 75 °C.

Hoveyda–Grubbs first generation type catalyst immobilized on mesoporous molecular sieves

New heterogeneous, reusable catalysts for olefin metathesis have been prepared by immobilizing the Hoveyda–Grubbs first generation type alkylidene (commercially available as the Zhan catalyst-1C) on siliceous mesoporous molecular sieves with different pore sizes and architectures (SBA-15, SBA-16, MCM-41 and MCM-48) having the surface modified with molecules bearing dicyclohexylphosphine (PCy2) end groups. Resulting novel heterogenized catalysts proved high activity in ring-closing metathesis (RCM) of 1,7-octadiene, diethyl diallylmalonate, tert-butyl N,N-diallylcarbamate and N,N-diallyl-2,2,2-trifluoroacetamide, in homometathesis of 1-decene, 5-hexenyl acetate and methyl 10-undecenoate, and in cross-metathesis (CM) of allylbenzene with cis-1,4-diacetoxy-2-butene. The catalytic activity decreased with decreasing pore size of supports in the order SBA-15>MCM-48>SBA-16 and MCM-41. Filtration test suggested that the solid catalyst was responsible for the catalytic activity. Catalysts were easily separated from the reaction mixtures and products of low levels of Ru concentration were obtained.

Short-range interactions between surfactants, silica species and EDTA4− salt during self-assembly of siliceous mesoporous molecular sieve: A UV Raman study

The effects of surfactants, counterions and additive salts on the formation of siliceous mesoporous molecular sieves during self-assembly process were investigated by UV Raman spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. The surfactant molecules experience the rearrangement after adding the silica species and adjusting the pH value. The obvious change of the Raman bands related to the surfactants supports a cooperative interaction between surfactant and inorganic species during self-assembly process. The addition of EDTANa4 to the system induces the interaction between the COO− groups of EDTA4− and silanol groups of silica and a strong interaction between the EDTA4− and the N+(CH3)3 groups of the surfactant. The above interactions may be the main reason for the salt effect. The new information from the change of the chemical bonds allows for a further analysis to the interactions of different salts between surfactants and silica species at molecular level.

Effect of Synthesis Duration on the Physicochemical Properties of Siliceous Mesoporous Molecular Sieve (Si-MMS)

Pure silica mesoporous molecular sieve (MMS) solid has been synthesized at 100°C by hydrothermal process. The effect of synthesis duration from 2 to 10 days has been investigated on the physicochemical properties of mesoporous molecular sieve. Samples were characterized by low angle XRD, N2adsorption-desorption and HRTEM analysis. XRD patterns of the as-synthesized samples showed four well-defined diffraction peaks corresponding to 100, 110, 200 and 210 planes. These peaks are the fingerprint characteristics of MCM-41 mesoporous material. The high intensity diffraction peaks were observed in 8-days sample that define the high ordering of the pores and long range order. N2adsorption-desorption results showed that all samples possessed a type IV isotherm having hysteresis loop of typeH1which is an identification of mesoporous material. Calcined samples exhibited high surface area i.e., 984-1036 m2g-1, pore volume i.e., 1.00-1.13 cm3g-1and average pore diameter i.e., 3.04-3.30 nm. A hexagonal pore structure was found in the synthesized materials by HRTEM analysis, which confirms that the synthesized materials are MCM-41. HRTEM analysis showed the effect of synthesis duration on the materials and found that 8-days sample exhibited highly ordered hexagonal pore structure like honeycomb structure. All the samples were calcined at 550°C to remove the template and to study the changes in the mesoporous framework. The results showed that the mesoporous structure remained intact after calcination at 550°C, indicating that the mesoporous materials exhibit high thermal stability.

RuCl 2( p-cymene)(PCy 3) immobilized on mesoporous molecular sieves as catalyst for ROMP of norbornene and its derivatives

New heterogeneous catalysts for ring opening metathesis polymerization (ROMP) have been prepared by immobilization of RuCl 2( p-cymene)(PCy 3) (Cy = cyclohexyl) on silica and siliceous mesoporous molecular sieves SBA-15 and MCM-41. Activity of these catalysts was investigated in ROMP of norbornene (NBE) and its derivatives (dicyclopentadiene, 5-norbornene-2-yl acetate). High molecular weight polymers ( M w up to 400 000) were prepared in yields up to 80% using catalysts based on mesoporous sieves. In contrast, with catalyst based on conventional silica the yield did not exceed 28% (ROMP of NBE). Filtration test proved that the catalytic activity was bound to the solid phase. Catalysts could be easily separated from the reaction mixture in contrast to the corresponding homogeneous system. Therefore, polymers with reduced amounts of catalyst residues were obtained.

Synthesis and Characterization of Titanium Mesoporous Silicate

In this project, surfactant hegzadesil three-methyl ammonium chloride was used as a structural informant or mould for synthesis of Mesoporous silicate molecular sieves. Many of the Mesoporous titanium silicates were synthesized by adding titanium compounds to the mixture, but considerable ratio of titanium centers placed in the frame are not accessible. This disadvantage was compensated by titanium butoxide as a titanous compound after being linked to the interior surfaces of host Mesoporous. The Mesoporous silicate was synthesized by, S+I− method, and then titanium was linked into the cavities. The structure of synthesized compound was characterized and studied by using XRD, XRF, TGA, SEM, IR techniques, and surface adsorption of nitrogen. The results showed the formation of meso structure with hexagonal cavities, high surface area, isomorphic cavities, spherical morphology, functional groups, and the amount of titanium input into the structure.

Mesoporous Molecular Sieves as Advanced Supports for Olefin Metathesis Catalysts

AbstractSummary: Siliceous mesoporous molecular sieves MCM‐41, MCM‐48 and SBA‐15 and organised mesoporous alumina represent progressive supports for new heterogeneous catalysts for olefin metathesis and metathesis polymerization. In combination with Mo and Re oxides they provide catalysts of considerably higher activity in comparison with those based on conventional silica and alumina. Immobilization of Mo and Ru alkylidenes on these materials led to the highly active and selective catalysts with negligible leaching of transition metal.

Optimising catalytic properties of supported sulfonic acid catalysts

Siliceous mesoporous molecular sieves (SBA-15) have been functionalised with propylsulfonic acid groups by both co-condensing 3-mercaptopropyltrimethoxysilane with the solid at the synthesis (sol–gel) stage and by grafting the same compound to pre-prepared SBA-15, followed, in both cases, by oxidation to sulfonic acid. The acidic and catalytic properties of the supported sulfonic acids prepared in the two ways have been compared, using ammonia adsorption calorimetry and the benzylation reaction between benzyl alcohol and toluene. Using a combination of X-ray photoelectron spectroscopy and other analytical techniques, the level of functionalisation and the extent of subsequent oxidation of tethered thiol to sulfonic acid, both in the bulk and close to the surface of SBA-15 particles, have been assessed. The research shows that the co-condensing route leads to higher levels of functionalisation than the grafting route. The extent of oxidation of added thiol to acid groups is similar using the two routes, about 70% near the surface and only 50% in the bulk. Comparison is made with polymer supported sulfonic acid catalysts, Amberlysts 15 and 35, and Nafion. Nafion shows the highest acid strength and the highest specific catalytic activity of all materials studied. Amongst the other materials, average acid strengths are broadly similar but there appears to be a relationship between the concentration of acid sites on the catalysts and their specific activity in the benzylation reaction. A model is proposed to explain this, in which clustering of sulfonic acid groups, even to a small extent, leads to disproportionately enhanced catalytic activity.

Effect of organosilane/polymer ratio on adsorption properties of periodic mesoporous ethane-silica

Remarkable achievements in the area of pure siliceous mesoporous molecular sieves such as MCM-41, SBA-15, SBA-16 and FDU-1 paved the way for the discovery of periodic mesoporous organosilicas (PMOs). This work reports the effect of organosilane/polymer ratio on the synthesis of PMO with ethane bridging groups inside the SBA-15-type framework. This ethane-silica was obtained by self-assembly of 1,2-bis(triethoxysilyl)ethane (BTESE) precursor and poly(ethylene oxide)- block-poly(propylene oxide)- block-poly(ethylene oxide) triblock copolymer Pluronic P123 (EO 20PO 70EO 20) as structure directing agent under low acidic conditions. The amount of triblock copolymer P123 was increased progressively from 0.5 and 3 g, while the mass of BTESE in the synthesis gel mixture was kept constant. The polymeric template was removed via extraction with acidified ethanolic solution. The resulting PMO materials were characterized by powder X-ray diffraction, thermogravimetry and nitrogen adsorption at −196 °C. The latter was used to evaluate the specific surface area, mesopore width, total pore volume and the volume of complementary pores. The polymer/BTESE weight ratios of 0.73 and 1.46 favoured the formation of additional amorphous silica inside channel-like mesopores, which resulted in plugged hexagonal templated silicas (PHTS) with low mesopore volume, small mesopore size, wide pore size distribution (PSD), thick pore walls and low mesostructural ordering. A further increase in the polymer/BTESE ratio (2.2) led to a material with large pore diameter, narrow PSD, uniform pore openings and high structural ordering. On the other hand, high polymer/BTESE ratios (from 2.93 to 4.39) afforded PMOs with very broad PSD and high non-uniformity of pore entrances as well as low ordering. Furthermore, a complete removal of ethane bridging groups at elevated temperatures in air did not lead to the disappearance of mesostructural ordering in the OMS studied.

Micellization of CTAB in the presence of silicate anions and the exchange between bromide and silicate at the micelle surface: A step to understand the formation of mesoporous molecular sieves at extremely low surfactant and silicate concentrations

The effect of silicate anions, from dilute aqueous tetramethylammonium silicate (TMASi) solutions (0–3.0 mmol L −1 in silicon), on the formation of hexadecyltrimethylammonium bromide (CTAB) micelles was investigated by means of a series of simple conductivity experiments. These two compounds are used in the preparation of mesoporous silicate molecular sieves. An increase in the monovalent silicate anion concentration decreases the critical micelle concentration (cmc) of CTAB, as might be expected from the decreased repulsive forces between the polar heads of the surfactant molecules. However, the decrease in cmc values is less pronounced than that observed in the presence of bromide ions, suggesting that Br − binds more strongly than Si(OH) 3O − at the micelle surface. Through the ion-exchange formalism, a selectivity coefficient for Si(OH) 3O −/Br − exchange of 0.30 ± 0.03 was estimated from the conductivity data. This value compares well with that of 0.4 ± 0.1 also determined in this work by the pyrene fluorescence quenching method. The experimental results were used to rationalize the formation of a surfactant supramolecular-templated mesoporous molecular sieve at extremely low surfactant (0.63 mmol L −1) and silicate (4.00 mmol L −1) concentrations.

Amino acid adsorption onto mesoporous silica molecular sieves

Mesoporous molecular sieves are promising as adsorbents for purification of biological molecules, such as amino acids, due to their tuneable mesopore sizes and high surface area. In this study, the adsorption of the basic amino acid, lysine, onto MCM-41, a siliceous mesoporous molecular sieve, has been investigated under a range of solution conditions. It was found to adsorb according to a Langmuir-type isotherm with a maximum capacity at pH 6 of 0.21 mmol/g. The extent of adsorption depends strongly on the pH and ionic strength of the adsorbate solution, due to a combination of ion exchange and electrostatic interactions governing the adsorption process.

In situ FTIR spectroscopic investigation of active sites and adsorbate interactions in mesoporous aluminosilicate SBA-15 molecular sieves

Siliceous mesoporous molecular sieve SBA-15 was synthesized, and incorporated with aluminum via post-synthesis procedures. The type and concentration of hydroxyl groups in siliceous and aluminosilicate SBA-15 as a function of the aluminum content were investigated by in situ Fourier-transform infrared spectroscopy (FTIR) following adsorption of various probe molecules, such as pyridine, 1-hexene, benzene, and crotonaldehyde. The data show that isolated silanol groups on the pore surface of siliceous SBA-15 are the primary sites for aluminum incorporation. While different acid sites were identified in aluminosilicate SBA-15, a strong dependence of acidity/hydrocarbon adsorption capacity on the aluminum content was not observed, primarily due to the fact that the amorphous SBA-15 mesoporous walls lack well-defined tetrahedral symmetry for aluminum as found in crystalline zeolite materials. However, the aluminosilicate SBA-15 still showed activity toward adsorbate adsorption and transformation. At elevated temperatures up to 450 °C, a structural transformation of crotonaldehyde to carbonaceous matter (coke) was observed.

Rh(cod)Cl]2 complex immobilized on mesoporous molecular sieves MCM-41-a new hybrid catalyst for polymerization of phenylacetylene

Abstract A new hybrid catalyst affording polymerization of phenylacetylene (PhA) into high-molecular-weight (Mw = 180 000) high-cis-polyphenylacetylene (PPhA) was prepared via immobilization of [Rh(cod)Cl]2 (cod=η4-cycloocta-1,5-diene) on siliceous mesoporous molecular sieves MCM-41 modified by 3-aminopropyltrimethoxysilane. The polymerization was confirmed to proceed on anchored Rh-species and PPhA formed was continuously released into liquid phase, from which it was easily isolated. In comparison with related homogeneous catalysts, the hybrid catalyst provided comparable or even higher PPhA yields, considerably higher molecular weight of PPhA and reduced amounts of oligomeric by-products. The catalyst was found to be resistant to Rh leaching; however, chemical transformation of anchored Rh-species occurring during polymerization strongly reduced the possibility of catalyst re-using.

Hybrid Catalysts for Acetylenes Polymerization Prepared by Anchoring [Rh(cod)Cl]2 on MCM-41, MCM-48 and SBA-15 Mesoporous Molecular Sieves - The Effect of Support Structure on Catalytic Activity in Polymerization of Phenylacetylene and 4-Ethynyl-N-{4-[(trimethylsilyl)ethynyl]benzylidene}aniline

Hybrid catalysts for polymerization of acetylenes were prepared by anchoring, via (3-aminopropyl)trimethoxysilane linker, the [Rh(cod)Cl]2 complex on siliceous mesoporous molecular sieves differing in the pore size and architecture (MCM-41, MCM-48 and SBA-15). In comparison with [Rh(cod)Cl]2 used as homogeneous catalyst, all hybrid catalysts exhibited comparable or even higher catalytic activity in the polymerization of phenylacetylene and 4-ethynyl-N-{4-[(trimethylsilyl)ethynyl]benzylidene}aniline. The initial polymerization rate increased with increasing accessibility of mesoporous surface of catalysts in the order: MCM-41 < MCM-48 < SBA-15. Molecular weights of the prepared polymers increased in reverse order suggesting suppression of the chain growth termination reactions by space limitations in the pores. No effect of catalyst support on the microstructure of formed polymers was found.

Morphogenesis of surface patterns and incorporation of redox‐active metals in mesoporous silicate molecular sieves

AbstractHierarchically ordered mesoporous metallosilicate MCM‐41 molecular sieves have been synthesized. The redox‐active metals Ti, V, Cr and Mn were incorporated in the framework, as characterized by various spectroscopic techniques. These materials have extraordinary morphologies in terms of their diverse and remarkable shapes and impressive surface patterns, such as centipede‐like, rope, vesicle and gyroidal spheroid, as revealed by transmission electron microscopy (TEM) images. The spatial relationship between the channel axis and the morphologies of the mesoporous silicates was investigated. The morphology formation of metallosilicates could be affected by the properties of the incorporated transition metal species. Copyright © 2001 John Wiley & Sons, Ltd.

Capillary condensation in MMS and pore structure characterization

Phenomena of capillary condensation and desorption in siliceous mesoporous molecular sieves (MMS) with cylindrical channels are studied by means of the non-local density functional theory (NLDFT). The results are compared with macroscopic thermodynamic approaches based on Kelvin–Cohan (KC) and Derjaguin–Broekhoff–de Boer (DBdB) equations. We show that: The KC equations, which constitute the basis of the traditional BJH method for the pore size distribution analysis, are in error even in pores as large as 20 nm. The DBdB equations with consistently determined thickness of the adsorbed layer (disjoining pressure isotherm) can be justified for pores wider than ≈7 nm in diameter. As the pore size decreases, the macroscopic arguments become less accurate, and the NLDFT and DBdB results differ significantly in pores smaller than ≈4 nm. The adsorption–desorption isotherms predicted by NLDFT are found to be in quantitative agreement with the experimental nitrogen (77 K) and argon (87 K) isotherms on MCM-41 type materials with pores larger than 5 nm. Therewith, the experimental desorption branch corresponds to the equilibrium capillary condensation/evaporation transition. The experimental adsorption branch corresponds to the spontaneous spinodal condensation, which occurs at the limit of stability of adsorption films. The NLDFT method has been developed for the calculation of pore size distributions from both the adsorption and desorption isotherms.

Siliceous mesoporous molecular sieves derived from crown ether surfactants

New surfactants derived from crown ethers were synthesized and used as templates in the preparation of inorganic mesostructures. These surfactants having a crown ether group as hydrophilic head yielded silica mesoporous molecular sieves of the M41S family which were transformed with subsequent thermal treatment into a material of particular unprecedented morphology. The crown ether derived surfactants were characterized by 1H NMR and mass spectroscopy and the mesoporous materials by X-ray diffraction (XRD), nitrogen physisorption, transmission electron microscopy (TEM) and solid state 29Si NMR spectroscopy. The TEM technique was used to assess the different phases present in the calcined samples. The morphology of these materials is quite different from that of MCM-41 and shows lamellar stackings of silica sheets pillared by minor amounts of hexagonal phase. Using molecular simulations of the surfactants and their cation complexed forms, some detailed conclusions could be drawn regarding the effect of various cations on pore size and shape.