Periodic Mesoporous Silica Materials Research Articles

Effective One-Step Synthesis of Silica Supported 1,3-Dibutylimidazolium Acetate for Carbon Dioxide Capture

A new methodology has been developed to immobilize 1,3-dibutylimidazolium acetate on the surface of amino-propyl functionalized periodic mesoporous silica materials via an efficient one-pot synthesis. FTIR and solid state 13C NMR analysis confirmed the presence of the supported 1,3-dibutylimidazolium acetate compared to the separately prepared pure ionic liquid 1,3-dibutylimidazolium acetate. The resulting materials were investigated for use as possible capture reagents for carbon dioxide.

High 1H Relaxivity Gadolinium‐Grafted Mesoporous Silica Materials

AbstractSoft metal‐organic grafting of periodic mesoporous silica (PMS) has recently been demonstrated to give gadolinium‐loaded nanostructured hybrid materials with interesting 1H relaxometric properties. Here, the sterically demanding alkoxide and aryloxide complexes, [Ln(OCH2CMe3)3]4 and Ln(OC6H3tBu2‐2,6)3 (Ln = La, Gd), respectively, were grafted onto PMS materials MCM‐41, SBA‐15, and SBA‐1. Support materials with distinct topology and pore diameters were utilized in order to direct the metal precursors towards the PMSs internal or external surfaces. All hybrid materials were examined by N2 physisorption, Diffuse Reflectance Infrared Fourier Transform (DRIFT) and solid‐state NMR spectroscopy, as well as elemental analysis. The characterization data demonstrated the successful silica attachment of the alk(aryl)oxide complexes via the surface silanol groups. Careful design of the hybrid materials based on findings in our previous studies resulted in GdIII loaded materials with particularly high 1H longitudinal relaxivities at high magnetic field strength (400 MHz). The highest relaxivity of r1 = 31 mM–1s–1 was found for material [Gd(OCH2CMe3)3]4@SBA‐1 showing no intrapore metal loading. This value is amongst the highest relaxivities reported for GdIII loaded mesoporous nanomaterials so far at such high field strength. The metal loading, localization of metal centers (directed by the topology of the PMS), and the particular properties of the metal‐organic precursor are factors found to influence r1.

Synthetic Chemistry with Periodic Mesostructures at High Pressure

Over the last two decades, researchers have studied extensively the synthesis of mesostructured materials, which could be useful for drug delivery, catalytic cracking of petroleum, or reinforced plastics, among other applications. However, until very recently researchers used only temperature as a thermodynamic variable for synthesis, completely neglecting pressure. In this Account, we show how pressure can affect the synthetic chemistry of periodic mesoporous structures with desirable effects. In its simplest application, pressure can crystallize the pore walls of periodic mesoporous silicas, which are difficult to crystallize otherwise. The motivation for the synthesis of periodic mesoporous silica materials (with pore sizes from 2 to 50 nm) 20 years ago was to replace the microporous zeolites (which have pore sizes of <2 nm) in petroleum cracking applications, because the larger pore size of mesoporous materials allows for faster transport of larger molecules. However, these mesoporous materials could not replace zeolite materials because they showed lower hydrothermal stability and lower catalytic activity. This reduced performance has been attributed to the amorphous nature of the mesoporous materials' channel walls. To address this problem, we developed the concept of "nanocasting at high pressure". Through this approach, we produced hitherto-unavailable, periodic mesostructured silicas with crystalline pore walls. In nanocasting, we compress a periodic mesostructured composite (e.g. a periodic mesoporous silica with carbon-filled pores) and subsequently heat it to induce the selective crystallization of one of the two phases. We attain the necessary high pressure for synthesis using piston-cylinder and multianvil apparatuses. Using periodic mesostructured silica/carbon nanocomposites as starting material, we have produced periodic mesoporous coesite and periodic mesoporous quartz. The quartz material is highly stable under harsh hydrothermal conditions (800°C in pure steam), verifying that crystallinity in the channel walls of periodic mesoporous silicas increases their hydrothermal stability. Even without including the carbon phase in the silica pores, we could obtain mesoporous coesite materials. We found similar behavior for periodic mesoporous carbons, which convert into transparent, mesoporous, nanopolycrystalline diamond at high-pressure. We also show that periodic mesoporous materials can serve as precursors for nanocrystals of high-pressure phases. We obtained nearly monodisperse, discrete stishovite nanocrystals from periodic mesoporous silicas and coesite nanocrystals from periodic mesoporous organosilicas. The stishovite nanocrystals disperse in water and form colloidal solutions of individual stishovite nanocrystals. The stishovite nanocrystals could be useful for machining, drilling, and polishing. Overall, the results show that periodic mesoporous materials are suitable starting materials for the synthesis of nanoporous high-pressure phases and nanocrystals of high pressure phases. The substantially enhanced hydrothermal stability seen in periodic mesoporous silicas synthesized at high pressure demonstrates that high pressure can be a useful tool to produce porous materials with improved properties. We expect that synthesis using mesostructures at high pressure can be extended to many other materials beyond silicas and carbons. Presumably, this chemistry can also be extended from mesoporous to microporous and macroporous materials.

Synthesis of porphyrin-bridged periodic mesoporous organosilica and their catalytic applications

Periodic mesoporous organosilicas (PMOs) were prepared by cooperative assembly with corresponding organosilane precursors in the presence of surfactants. Recently, approaches for the preparation of a new class of porphrin-bridged PMOs have been developed. Porphyrin-bridged PMOs were synthesized by direct co-condensation using a thermal sol–gel method or a rapid microwave-assisted method with tetrakis(carboxyphenyl)porphyrin(TCPP)-silsesquioxane (TCPPS) and various silica sources in the presence of templates. These porphyrin PMO exhibited high catalytic activities and selectivity and could be used repeatedly in many kinds of applications owing to easy accessibility, rapid diffusion, and favorable mass transfer for substrates into and out of the mesopores. In addition, the TCPPS incorporated into the PMO walls could effectively defend damage of the ordered structure and also inhibit the leaching of active sites. The current review deals with recent development in the synthesis, characterization, and applications such as hydrogenation, photocatalysis, chiral catalysis, and Baeyer–Villiger oxidation of each of these types of porphyrin-bridged periodic mesoporous silica materials.

Iron Silylamide-Grafted Periodic Mesoporous Silica

The surface chemistry of a series of well-defined metalorganic ferrous and ferric iron complexes on periodic mesoporous silica (PMS) was investigated. In addition to literature known Fe(II)[N(SiMe(3))(2)](2)(THF), Fe(II)[N(SiPh(2)Me(2))(2)](2), and Fe(III)[N(SiMe(3))(2)](2)Cl(THF), the new complexes [Fe(II){N(SiHMe(2))(2)}(2)](2) and Fe(III)[N(SiHMe(2))(2)](3)(μ-Cl)Li(THF)(3) were employed as grafting precursors. Selection criteria for the molecular precursors were the molecular size (monoiron versus diiron species), the oxidation state of the iron center (II versus III), and the functionality of the silylamido ligand (e.g., built-in spectroscopic probes). Hexagonal channel-like MCM-41 and cubic cage-like SBA-1 were chosen as two distinct PMS materials. The highest iron load (12.8 wt %) was obtained for hybrid material [Fe(II){N(SiHMe(2))(2)}(2)](2)@MCM-41 upon stirring the reaction mixture iron silylamide/PMS/n-hexane for 18 h at ambient temperature. Size-selective grafting and concomitantly extensive surface silylation were found to be prominent for cage-like SBA-1. Here, the surface metalation is governed by the type of iron precursor, the pore size, the reaction time, and the solvent. The formation of surface-attached iron-ligand species is discussed on the basis of diffuse reflectance infrared Fourier transform (DRIFT) and electron paramagnetic resonance (EPR) spectroscopy, nitrogen physisorption, and elemental analysis.

Periodic mesoporous silica and organosilica with controlled morphologies as carriers for drug release

In this paper, we report the effects of morphology, wall composition of mesoporous materials and different buffer solutions on drug delivery profiles. Hollow spheres of periodic mesoporous organosilica (PMO) were prepared and used as drug carriers which exhibited higher loading capacity and slower release rate compared to the conventional periodic mesoporous silica (PMS) spheres and solid spheres of PMO. This hollow PMO showed promising properties as a reservoir to encapsulate and store larger quantities of guest molecules within its “empty” core. Moreover, its organic reactive sites allowed stronger interactions to the hydrophobic guest molecules, in contrast to inorganic wall possessed by PMS materials. Antibiotic tetracycline was used as a model drug to study the effect of framework difference between PMO and PMS materials on the loading and release processes. Two kinds of release medium, simulated body fluid (SBF) solution (pH 7.4) and phosphate buffer (PB) solution (pH 1.5) were used in this study, which revealed very different release profiles. A slower delivery rate was observed in SBF solution, attributed to the different ionic interactions between the guest molecule and the host material in the two different pH solutions. Overall, hollow PMO shows the lowest release rate and the highest loading amount compared to the other two materials studied herein. The kinetic study reveals that drug release from host material follows the second order kinetic model better than the first order mass transfer model.

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.

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.