Periodic Mesoporous Silica Research Articles

Hard Templates for Soft Materials: Creating Nanostructured Organic Materials

This review describes the possibilities of using hard templates to create nanostructured “soft” materials, for example, polymer networks, carbon nitrides, or carbonaceous materials, that is, materials which are still organic in their nature. Examples are given for the whole range of hard templates described in the literature, starting from silica nanoparticles, zeolites, and periodic mesoporous silicas to aluminum oxide membranes and colloidal crystals.

An alternate approach to the preparation of versatile sulfonic acid functionalized periodic mesoporous silicas with superior catalytic applications

A novel approach for the synthesis of sulfonic acid functionalized periodic mesoporous silicas via covalent attachment of 2-(3,4 epoxycyclohexyl)ethyltrimethoxysilane or 3-glycidoxypropyltrimethoxysilane followed by reaction with sulfite ions and mild hydrochloric acid is presented. The materials demonstrated superior catalytic performance in esterification, acylation, and condensation reactions. The hydrophobic nature along with the crystalline pore wall structure of these materials are advantageous in increasing reaction rates by altering local water concentration and helpful in adjusting the modest active sites and surface properties. The materials also showed the higher stability and easily recyclable.

Sequential transformations of organic nitrogen functionalities in periodic mesoporous silica

New methods for the sequential organic functionalisation of periodic mesoporous silica materials have been established. In a first step, organic-inorganic hybrid materials were synthesized by direct substitution at the silicon atoms of the channel structure with metalorganic reagents such as lithiated aromatics. The resulting nitrogen-containing nanostructured hybrid materials were subsequently converted in a second step by reactions such as azo-coupling and the Meerwein–Schuster reaction, respectively. These efficient and economic reactions lead to the formation of complex organic-inorganic hybrid materials with new functionalities, including quaternized ammonium salts, aromatic imines and peptides, as well as azo-compounds and stilbenes. The new nanostructured materials were characterized with a comprehensive set of structural and spectroscopic methods, including X-ray diffraction, thermoanalysis, nitrogen sorption, as well as Raman, IR, UV-vis and solid-state NMR spectroscopy.

Molecular Simulation of Silica/Surfactant Self-Assembly in the Synthesis of Periodic Mesoporous Silicas

Understanding the synthesis of periodic mesoporous silica (PMS) is crucial for a more efficient use of these materials and is a necessary first step toward a rational design strategy for the templated synthesis of porous solids. In this paper, the early stages of the synthesis process of PMS materials are simulated directly by molecular dynamics, using realistic atomistic models. It is the first time that such computationally demanding calculations have been attempted. By comparing the self-assembly of cationic surfactants in the presence and absence of silicates, we are able to show that silica promotes the formation of larger aggregates than in a simple surfactant/water solution. The formation of these larger micelles is explained by a strong interaction of the silicate molecules with the surfactant head groups. This strong interaction increases the local concentration of silica at the surface of the micelles, which induces the formation of more condensed silicate species. The surfactant/silica structures observed here are potentially important intermediates in PMS synthesis.

Optimization of Reaction Conditions for the Metalorganic Modification of MCM-41

The modification of periodic mesoporous silica with organic groups following an optimized metalorganic route is described. Organolithium and Grignard reagents (n-butyllithium and allyl-magnesium bromide) were used to directly substitute silicon atoms with organic moieties in the channel walls of the silica host. A detailed study of several representative reactions with respect to reaction parameters, that is, reaction temperature, reaction time, and reagent concentration, shows that the degree of surface substitution can be controlled within a broad range and up to very high levels while still maintaining the integrity of the ordered silica host. The functionalized mesoporous materials can also retain high surface areas and pore volumes. Furthermore, the reactions can be performed at low temperatures compared to the more traditional grafting techniques on the basis of organosilanes.

Size-Selective Surface Organometallic Chemistry on Cagelike Mesoporous Silica

Periodic mesoporous silicas SBA-1, SBA-2, and SBA-16 feature a unique cagelike pore topology consisting of large supercages interconnected by smaller windows. Compared to channel-like mesoporous silica MCM-48, these cagelike silicas display a distinct reactivity toward magnesium silylamides {Mg[N(SiHMe2)2]2}2 and {Mg[N(SiMe3)2]2}2. All surface silanol groups ⋮Si−OH of MCM-48 can be derivatized with the magnesium grafting precursors, forming stable silicon−oxygen−magnesium linkages. The completeness of the reaction is evidenced by a dramatic loss of specific pore volume (0.92 → 0.27 and 0.30 cm3 g-1), disappearance of the OH stretching vibration in the IR spectrum and the magnesium content (3.5 and 4.0 wt %) of the hybrid materials Mg[N(SiHMe2)2]x@MCM-48 and Mg[N(SiMe3)2]x@MCM-48. In contrast, the amount of magnesium that can be grafted onto the surface of cagelike silica SBA-1 does not exceed 0.3 wt %. The specific pore volume of SBA-1 hybrid materials is only slightly decreased compared to the MCM-48 materials and the majority of the SiOH groups is still visible in the IR spectrum. We assume that the surface reaction is limited to the external surface of the SBA-1 silica particles. The remaining silanol functional groups located at the internal surface of SBA-1 are prone to further derivatization with small silylating reagents or small metalorganic compounds such as HN(SiHMe2)2 or Ti(NMe2)4 yielding organic/metalorganic or heterobimetallic hybrid materials, respectively.

Electrochemiluminescence of a Periodic Mesoporous Organosilica Containing 9,10-Diarylanthracene Units

A 9,10-diarylanthracene derivative having two terminal trimethoxysilyl groups has been synthesized and used in combination with tetraethyl orthosilicate as a silicon source for the preparation of two periodic mesoporous silicas with MCM-41 and SBA-15 structures having about 15 wt % of the organic component. The resulting solids exhibit electrochemiluminescence arising from the organic moieties. The benefit for the electrochemiluminescence activity of having a large surface area solid with periodic porous structure is revealed by the fact that an analogous amorphous material prepared with the same precursor does not exhibit any measurable electrochemiluminescence activity under the same conditions.

Bridging the gap between micropores and mesopores by the controlled transformation of bifunctional periodic mesoporous silicas

Novel bifunctional periodic mesoporous silicas having high content of organic groups in the pore channels and chromium in the frame wall has been synthesized by direct co-condensation method, using organo trialkoxysilanes (chloro propyl, vinyl, methyl), chromium nitrate and tetraethyl orthosilicate, in presence of cetyltrimethyl ammonium surfactants. The obtained materials show one main peak in the XRD patterns up to a molar ratio of 1:1 between TEOS and organosilane in the synthesis gel and N 2 adsorption–desorption analysis shows that the mesopore structure remains intact after the surfactant removal process. Among the organosilanes, chloro propyl and vinyl pendant chromium samples shows a shift in the main peak to higher 2-theta values with a corresponding decrease in pore volume and pore diameter, with the percentage of organosilane functionalization. Small-pore silicas and chromium silicas can be prepared from the large pore mesoporous MCM-41, by effectively removing the organic groups by calcination. By this method, the pore size of the material can be tailored into the super-microporous region, without changing the chain length of the surfactant used in the assembly process. The shrinkage in pore size is dependent on the nature and percentage of the organic pendant groups that the chloro propyl and vinyl pendant mesoporous materials show the maximum pore size shrinkage than the small chain length methyl units. Because of the tailorable pore size and with better textural characteristics, chromium-silica materials find applications in the field of shape selective heterogeneous catalysis, as demonstrated in the oxidation reaction of cyclohexane, than the conventional metal-containing mesoporous materials.

Periodic mesoporous silica having covalently attached tris(bipyridine)ruthenium complex: synthesis, photovoltaic and electrochemiluminescent properties

A tris(bipyridine)ruthenium derivative with two terminal triethoxysilyl groups attached to one of the bipyridine ligands has been synthesized and used in combination with TEOS for the preparation of a tris(bipyridine)ruthenium-containing mesoporous silica, using CTABr as a structure-directing agent, under basic conditions at 100 °C. The resulting solids exhibited XRD patterns, isothermal gas adsorption and electron-microscopy images characteristic of hexagonal mesoporous silica, while the presence of tris(bipyridine)ruthenium complexes at the walls is manifested by the orange coloration of the solids and the optical spectra of the material corresponding to tris(bipyridine)ruthenium(II). The removal of the structure-directing agent without damaging the complex or the mesoporous structure of the pores was done by exhaustive solid–liquid extraction. As-synthesized mpRu exhibits photovoltaic (VOC = 140 mV, ISC = 2.6 µA) and electrochemiluminescence activity (λmax = 610 nm), the presence of CTABr plays a positive role in the activity by promoting intraparticle charge migration. In contrast, for an analogous material prepared under the same conditions but in the absence of the structure-directing agent and having a low surface area, the amorphous structure did not exhibit significant a photovoltaic or electrochemiluminescence response. This result exemplifies the benefits of having porosity and accessibility to the photoactive metallic complexes.

2H-solid state NMR and DSC study of isobutyric acid in mesoporous silica materials

Solid state deuterium NMR has been used to study the molecular motion of d(6)-isobutyric acid (d(6)-iBA) in the pure (unconfined) state and confined in the cylindrical pores of two periodic mesoporous silica materials (MCM-41, pore size 3.3 nm and SBA-15, pore size 8 nm), and in a controlled pore glass (CPG-10-75, pore size ca. 10 nm). The line shape analysis of the spectra at different temperatures revealed three rotational states of the iBA molecules: liquid (fast anisotropic reorientation of the molecule), solid I (rotation of the methyl group) and solid II (no rotational motion on the time scale of the experiment). Transition temperatures between these states were determined from the temperature dependence of the fraction of molecules in these states. Whereas the solid I-solid II transition temperature is not affected by confinement, a significant lowering of the liquid-solid I transition temperature in the pores relative to the bulk acid was found for the three matrix materials, exhibiting an unusual dependence on pore size and pore morphology. Complementary DSC measurements on the same systems show that the rotational melting (solid I-liquid) of d(6)-iBA in the pores occurs at a temperature 20-45 K below the thermodynamic melting point. This finding indicated that the decoupling of rotational and translational degrees of freedom in phase transitions in confined systems previously found for benzene is not restricted to molecules with non-specific interactions, but represents a more general phenomenon.

Influence of cationic phosphorus dendrimers on the surfactant-induced synthesis of mesostructured nanoporous silica

The concomitant use of polycationic dendrimers and cationic surfactants for the elaboration of periodic mesoporous silica of type MCM-41 is described. The hexagonal structure is preserved up to about 26% in weight of dendrimer included in the silica. The cationic surfactant can be selectively removed to liberate the pores, while preserving the non-covalently incorporated dendrimers. These dendrimers included in the mesoporous silica are fully accessible through the mesoporous volume to small molecules such as HCl and tetrahydrofuran.

Advanced Surface Functionalization of Periodic Mesoporous Silica: Kinetic Control by Trisilazane Reagents

The surface reactions of mesoporous silica MCM-41 with a series of new trisilylamines (trisilazanes) (SiHMe2)2NSiMe2R and (SiMe2Vin)2NSiMe2R (R = indenyl, norpinanyl, chloropropyl, 3-(N-morpholin)propyl; Vin = vinyl), disilylalkylamine (SiHMe2)iPrNSiMe2(CH2)3Cl, and monosilyldialkylamines Me2NSiMe2R (R = indenyl, chloropropyl, 3-(N-morpholin)propyl) were investigated. 1H, 13C, and 29Si MAS NMR spectroscopy, nitrogen adsorption/desorption, infrared spectroscopy, and model reactions with calix[4]arene as a mimic for an oxo surface were used to clarify the chemical nature of surface-bonded silyl groups. The trisilylamines exhibited a comparatively slow surface reaction, which allowed for the adjustment of the amount of silylated and nonreacted SiOH groups and led to a stoichiometric distribution of surface functionalities. The 2:1 integral ratio of SiHMe2 and SiMe2R moieties of such trisilazanes was found to be preserved on the silica surface as indicated by microanalytical as well as 13C and 29Si MAS NMR spectroscopic data of the hybrid materials. For example, the reaction of MCM-41 with (SiHMe2)2NSiMe2(CH2)3Cl, (SiHMe2)iPrNSiMe2(CH2)3Cl, and Me2NSiMe2(CH2)3Cl provided bi- and monofunctional hybrid materials with one-third, one-half, or all chemically accessible silanol groups derivatized by chloropropyl groups, respectively. Thus, a molecular precursor strategy was developed to efficiently control the relative amount of three different surface species, SiHMe2 (or SiVinMe2), SiMe2R, and SiOH, in a single reaction step. The reaction behavior of indenyl-substituted monosilazanes and trisilazanes (R = Ind) with calix[4]arene proved that the indenyl substituent can act as a leaving group forming a dimethylsilyl species, which is anchored bipodally on the silica surface, that is, via two Si-O bonds.

Physisorbed films in periodic mesoporous silica studied byin-situsynchrotron small-angle diffraction

Physisorbed films in periodic mesoporous silica studied by<i>in-situ</i>synchrotron small-angle diffraction

Preparation and characterization for a new sulfonic acid-functionalized mesoporous silica stationary phase

A novel strategy for functionalized mesoporous silica used as chromatographic packing material is reported. Periodic mesoporous silica containing sulfonic acid bearing groups were synthesized using tetramethyloxysilicate (TMOS) and mercaptopropyltrimethoxysilane (MPTMS) as framework precursors under acidic conditions in the presence of Pluronic P123 as surfactant and cetyltrimethylammonium bromide (CTAB) as cosurfactant. Conversion of the mercaptopropyl groups into sulfonic acid moieties was achieved via oxidation using hydrogen peroxide. Wormhole mesostructure with high surface area (up to 718.6 m 2/g) and narrow pore size distribution (up to 4.0 nm) were obtained. The effect of the synthesized mesoporous silica microspheres as a HPLC packing material was examined.

Assembly of hydrothermally stable ethane-bridged periodic mesoporous organosilicas with spherical and wormlike structures

Hydrothermally stable periodic mesoporous organosilicas with spherical as well as wormlike frame structures were synthesised by two different synthesis routes using the silsesquioxane precursor, 1,2-bis (triethoxy silyl) ethane (BTEE) and surfactant octadecyl trimethyl ammonium (ODTA), under basic conditions. The materials were characterized by PXRD, SEM, TEM, N 2 adsorption–desorption, FT-IR, 29Si MAS NMR, 13C CP MAS NMR and TG-DTA techniques. Characterization techniques revealed that the morphology of the hybrid material prepared from pure silsesquioxane precursor resulted in spherical particles while a combination of tetra ethyl orthosilicate and 1,2-bis (triethoxy silyl) ethane (90:10 ratio) resulted in materials having wormlike structures. Hydrothermal stability of the hybrid material was evaluated by refluxing the sample under boiling water for various time periods and was compared with that of a periodic mesoporous silica material, Si-MCM-41. Results showed that the hybrid material is stable up to 100 h in boiling water, with the existence of d 100 peak, while the mesopore structure of Si-MCM-41 get destroyed even after a 12 h reflux time. The improved hydrothermal stability of the hybrid material is related to the more hydrophobic environment induced inside the pore channels, due to the presence of integrating ethane groups in the wall channels.

Physisorbed films in periodic mesoporous silica studied byin situsynchrotron small-angle diffraction

Adsorption and capillary condensation of an organic fluid in a periodic mesoporous silica (SBA-15) are studied by in situ synchrotron diffraction. Powder diffraction patterns resulting from the two-dimensional hexagonal packing of the cylindrical pores of SBA-15 are collected as a function of vapor pressure during continuous adsorption and desorption of the fluid (perfluoropentane ${\mathrm{C}}_{5}{\mathrm{F}}_{12}$), using a specially designed sorption cell. Seven diffraction peaks with systematic changes of the intensity are resolved as the adsorbed film thickness increases along the adsorption isotherm. The integrated intensities of the diffraction peaks are analyzed with a structural model involving four levels of electron density (dense silica matrix, microporous corona around the pores, adsorbed film, and core space of the pores). The model provides quantitative information about the structure of the evacuated specimen, the filling of the corona, and the growing thickness of the liquid film with increasing pressure. A very good fit of the data by this model is found for relative pressures up to $p∕{p}_{0}\ensuremath{\approx}0.5$, but the fit of the high-indexed diffraction peaks becomes poor close to the capillary condensation pressure $(p∕{p}_{0}\ensuremath{\approx}0.68)$. Tentatively, this fact may be attributed to a deviation of the liquid film structure from the simple flat geometry close to the phase transformation, presumably caused by density fluctuations.

Design of Hybrid Organic/Inorganic Adsorbents Based on Periodic Mesoporous Silica

We are interested in the design of hybrid organic−inorganic adsorbents (HOIAs) for gas separation and storage. In this paper, we propose a methodology for the design of HOIAs based on periodic mesoporous silicas. We use a method based on kinetic Monte Carlo simulation to generate realistic model adsorbents and then carry out Grand Canonical Monte Carlo simulations of adsorption in these model materials. Good agreement is obtained between the adsorption behavior of the model adsorbents and that of the corresponding real materials, giving confidence that we can identify optimal structures for particular gas-separation applications. We demonstrate this capability using, as an example, the pressure-swing adsorption of carbon dioxide from flue gas, using a HOIA based on MCM-41.

A novel way to synthesize high regular and periodic organic mesoporous silicas

Organic mesoporous silicas (OMSs) were synthesized in the presence of urea via one-pot synthesis method, in which tetraethyl orthosilicate (TEOS) and 3-aminopropyltriethoxysilica (APTES) were used as the silica resources, non-ionic surfactant was used as the template. XRD results showed that the average periodic mesopore sizes of OMSs in the presence of urea were larger than those in the absence of urea. It was also found that the pore sizes of the products in the presence of urea distributed more narrowly than those in the absence of urea, and the contents of organosiloxane incorporated into OMSs, the pore wall thicknesses, the pore volumes and the surface areas of the products all increased with the use of urea. This shows a novel way to synthesize high regular and periodic organic mesoporous silicas.

Metalorganic modification of periodic mesoporous silica: aromatic nitrogen functionalities

The channel walls of periodic mesoporous silica were modified with nitrogen-containing aromatic molecules through a metalorganic route, utilizing the lithiated aromatic core under mild reaction conditions and short reaction times. The functionalities grafted to the silica walls include primary and tertiary amines, as well as pyridine, pyrimidine and nitrobenzene. The new hybrid materials were characterized with X-ray diffraction, electron microscopy, thermogravimetry, nitrogen sorption, as well as Raman and UV-vis spectroscopies. The metalorganic modification of mesoporous molecular sieves with functional aromatics proves to be a fast, simple and economic method for the design of novel mesoporous inorganic–organic hybrid systems.

Heats of Adsorption and Adsorption Heterogeneity for Methane, Ethane, and Carbon Dioxide in MCM-41

We report the adsorption isotherms and the isosteric heats of adsorption of pure methane, ethane, and CO2 and a mixture of methane and CO2 in the periodic mesoporous silica MCM-41 using a multicomponent adsorption calorimeter of the Tian-Calvet type, looking in particular at the degree of heterogeneity in the adsorption of these species. The adsorption of methane and ethane in MCM-41 was found to be essentially homogeneous, while the adsorption of pure CO2 and of CO2 from a CO2/methane mixture was found to be significantly heterogeneous, reflecting the electrostatic interactions between CO2 and the adsorbent.