Covalent attachment of 3,4,9,10-perylenediimides onto the walls of mesoporous molecular sieves MCM-41 and SBA-15

This work describes the covalent grafting of 3,4,9,10-perylenediimides (PDI), which are fluorescent dyes with very interesting optical properties, onto the walls of mesoporous molecular sieves MCM-41 and SBA-15. The mesoporous materials were first treated with 3-aminopropyltriethoxysilane (APTES) in anhydrous toluene, generating amine-containing surfaces. The amine-containing materials were then reacted with 3,4,9,10-perylenetetracarboxylic dianhydride (PTCA), generating surface-grafted PDI. Infrared spectra of the materials showed that the reaction with amino groups took place at both anhydride ends of the PTCA molecule, resulting in surface attached diimides. No sign of unreacted anhydride groups were found. The new materials, designated as MCMN2PDI and SBAN(2)PDI, presented absorption and emission spectra corresponding to weakly coupled PDI chromophores, in contrast to the strongly coupled rings usually found in solid PDI samples. The materials showed a red fluorescence, which could be observed by the naked eye under UV irradiation or with a fluorescence microscope. The PDI-modified mesoporous materials showed electrical conductivity when pressed into a pellet. The results presented here show that the new materials are potentially useful in the design of nanowires. (C) 2007 Elsevier Inc. All rights reserved.

A new insight into growth mechanism and kinetics of mesoporous silica nanoparticles by in situ small angle x-ray scattering

The growth mechanism and kinetics of mesoporous silica nanoparticles (MSNs) were investigated for the first time by using a synchrotron time-resolved small-angle X-ray scattering (SAXS) analysis. The synchrotron SAXS offers unsurpassed time resolution and the ability to detect structural changes of nanometer sized objects, which are beneficial for the understanding of the growth mechanism of small MSNs (∼20 nm). The Porod invariant was used to quantify the conversion of tetraethyl orthosilicate (TEOS) in silica during MSN formation, and the growth kinetics were investigated at different solution pH and temperature through calculating the scattering invariant as a function of reaction time. The growth of MSNs was found to be accelerated at high temperature and high pH, resulting in a higher rate of silica formation. Modeling SAXS data of micelles, where a well-defined electrostatic interaction is assumed, determines the size and shape of hexadecyltrimethylammonium bromide (CTAB) micelles before and after the addition of TEOS. The results suggested that the micelle size increases and the micelle shape changes from ellipsoid to spherical, which might be attributed to the solubilization of TEOS in the hydrophobic core of CTAB micelles. A new "swelling-shrinking" mechanism is proposed. The mechanism provides new insights into understanding MSN growth for the formation of functional mesoporous materials exhibiting controlled morphologies. The SAXS analyses were correlated to the structure of CTAB micelles and chemical reaction of TEOS. This study has provided critical information to an understanding of the growth kinetics and mechanism of MSNs.

Evaluation of the zinox and zeolite materials as adsorbents to remove H2S from natural gas

MELO, Dulce Maria de Araújo et al. Evaluation of the Zinox and Zeolite materials as adsorbents to remove H2S from natural gas. Colloids and Surfaces. A, Physicochemical and Engineering Aspects, Estados Unidos, v. 272, p. 32-36, 2006.

Rhodamine 6G encapsuled mesoporous silica channels

We report the effect of solvent on the rhodamine 6G encapsuled into channels of mesoporous silica, synthesized by two-step process that gives intermediary stable hybrid micelles. Mesoporous materials have been obtained by the method that involves surfactant micelles (mainly cationic) and inorganic precursor of the structure to be obtained. MSU-X type mesoporous silica has been synthesized with polyethylene oxide surfactant as the directing-structure agent and tetraethyl orthosilicate Si(OEt)(4) as the silica source. The influence of the solvent on the encapsulation of rhodamine dye was systematically explored, specially its influence on the luminescence properties. Rhodamine 6G encapsuled into mesoporous silica channel was characterized by UV-Vis and luminescence spectroscopies, scanning electron microscopy, small angle x ray scattering and N(2) sorption-desorption. The pore size and the solvent effects into luminescence dye encapsuled into mesoporous silica channels are observed in the visible absorption and emission spectra of rhodamine 6G. The intense photo luminescence band of rhodamine 6G dye is in 500 to 600 nm region. The observed shift of the absorption and emission bands can be assigned to the effect of the solvents dielectric constant and pore size of mesoporous silica.

Morphology and topography analysis of mesoporous titania templated by micrometric latex sphere arrays

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

A comparative study of glycerol dehydration catalyzed by micro/mesoporous MFI zeolites

The catalytic properties of monomodal microporous and bimodal micro-mesoporous zeolites were investigated in the gas-phase dehydration of glycerol. The desilication methodology used to produce the mesoporous zeolites minimized diffusion limitations and increased glycerol conversion in the catalytic reaction due to the hierarchical system of secondary pores created in the zeolite crystals. The chemical and structural properties of the catalyst were studied by X-ray diffraction, nitrogen adsorption-desorption isotherms, NH3-TPD and pyridine chemisorption followed by IR-spectroscopy. Although the aim was to desilicate to create mesoporosity in the zeolite crystals, the desilication promoted the formation of extra-framework aluminum species that affected the conversion of glycerol and the products distribution. The results clearly show that the mesoporous zeolites with designed mesopore structure allowed a rapid diffusion and consequently improved the reaction kinetics. However, especial attention must be given to the desilication procedure because the severity of the treatment negatively interfered on the Brønsted and Lewis acid sites relative concentration and, consequently, in the efficiency of the catalysis performed by these materials. On the other hand, during the catalytic reaction, the intracrystalline mesopores allowed carbonaceous compounds to be deposited herein, resulting in less blocked micropores and catalysts with higher long-term stability.

About the thermal stability and pore elimination in the ordered hexagonal mesoporous silica SBA-15

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Morphology and topography analysis of mesoporous titania templated by micrometric latex sphere arrays

In this work, mesoporous titania is prepared by templating latex sphere arrays with four different sphere diameters at the micrometric scale (phi > 1 mu m). The mesoporous titania homogeneously covers the latex spheres and substrate, forming a thin coating characterized by N-2 adsorption isotherm, small angle X-rays scattering, atomic force, field emission and transmission electronic microscopies. Mesoporous titania has been templated into different shapes such as hollow particles and monoliths according to the amount of sol used to fill the voids of the close packed latex spheres. Titania topography strongly depends on the adsorption of polymeric segments over latex spheres surface, which could be decreased by changing the dimensions of latex spheres (phi = 9.5 mu m) generating a lamellar architecture. Thus, micrometric latex sphere arrays can be used to achieve new surface patterns for mesoporous materials via a fast and inexpensive chemical route for construction of functional devices in different technological fields such as energy conversion, inclusion chemistry and biomaterials. (C) 2011 Elsevier Inc. All rights reserved.

Complex materials via colloidal crystallization

The work presented in this thesis deals with complex materials, which were obtained by self-assembly of monodisperse colloidal particles, also called colloidal crystallization. Two main fields of interest were investigated, the first dealing with the fabrication of colloidal monolayers and nanostructures, which derive there from. The second turned the focus on the phononic properties of colloidal particles, crystals, and glasses. For the fabrication of colloidal monolayers a method is introduced, which is based on the sparse distribution of dry colloidal particles on a parent substrate. In the ensuing floating step the colloidal monolayer assembles readily at the three-phase-contact line, giving a 2D hexagonally ordered film under the right conditions. The unique feature of this fabrication process is an anisotropic shrinkage, which occurs alongside with the floating step. This phenomenon is exploited for the tailored structuring of colloidal monolayers, leading to designed hetero-monolayers by inkjet printing. Furthermore, the mechanical stability of the floating monolayers allows the deposition on hydrophobic substrates, which enables the fabrication of ultraflat nanostructured surfaces. Densely packed arrays of crescent shaped nanoparticles have also been synthesized. It is possible to stack those arrays in a 3D manner allowing to mutually orientate the individual layers. In a step towards 3D mesoporous materials a methodology to synthesize hierarchically structured inverse opals is introduced. The deposition of colloidal particles in the free voids of a host inverse opal allows for the fabrication of composite inverse opals on two length scales. The phononic properties of colloidal crystals and films are characterized by Brillouin light scattering (BLS). At first the resonant modes of colloidal particles consisting of polystyrene, a copolymer of methylmethacrylate and butylacrylate, or of a silica core-PMMA shell topography are investigated, giving insight into their individual mechanical properties. The infiltration of colloidal films with an index matching liquid allows measuring the phonon dispersion relation. This leads to the assignment of band gaps to the material under investigation. Here, two band gaps could be found, one originating from the fcc order in the colloidal crystal (Bragg gap), the other stemming from the vibrational eigenmodes of the colloidal particles (hybridization gap).

Methoxylation of α-pinene over mesoporous carbons and microporous carbons: A comparative study

A biomass derived carbon, a commercial microporous carbon and a xerogel mesoporous carbon catalysts were used in the study of α-pinene methoxilation reaction and the influence of textural and physical–chemical properties of the carbons was evaluated. Biomass carbon presented the higher activity, whereas the commercial one is the less active in the conditions studied. The main product of the reaction was α-terpinyl methyl ether and good values of selectivity were obtained over all the catalysts. A kinetic model was developed assuming that the α-pinene is consumed according to the parallel reaction network. The kinetic model presents high quality fittings to the experimental concentration profiles. These results show that it is possible to activate a waste residue using H3PO4 and convert it to high added value product such as acid catalyst.

Mesoporous Metal Complex–Silica Aerogels for Environmentally Friendly Amination of Allylic Alcohols

Two series of mesoporous hybrid iron(III) complex–silica aerogels were prepared in one-pot synthesis by using the sol–gel coordination chemistry approach. The use of the ligands 3-(2-aminoethylamino)propyltrimethoxysilane and 2-(diphenylphosphino)ethyltriethoxysilane, both with terminal triethoxysilyl groups, were used to incorporate metal complexes in situ into the framework of silica, through their co-condensation with a silicon alkoxide during the aerogel formation. This methodology yielded optically translucent hybrid mesoporous gels with homogeneous metal incorporation and excellent textural properties. The catalytic performance of these materials was tested in the direct amination of allylic alcohols in water as a target reaction, with activities comparable or even higher than those corresponding to the homogeneous iron(III) complex. Furthermore, these catalysts were stable and maintained their catalytic activity after six reaction cycles.

Modeling nitrogen adsorption in spherical pores of siliceous materials by density functional theory

Adsorption of nitrogen in spherical pores of FDU-1 silica at 77 K is considered by means of a nonlocal density functional theory (NLDFT) accounting for a disordered structure of pore walls. Pore size distribution analysis of various FDU-1 samples subject to different temperatures of calcination revealed three distinct groups of pores. The principal group of pores is identified as ordered spherical mesopores connected with each other by smaller interconnecting pores and irregular micropores present in the mesopore walls. To account for the entrances (connecting pores) into spherical mesopores, a concept of solid mass distribution with respect to the apparent density was introduced. It is shown that the introduction of the aforementioned distribution was sufficient to quantitatively describe experimental adsorption isotherms over the entire range of relative pressures spanning six decades.

Effect of pore packing defects in 2-D ordered mesoporous carbons on ionic transport

Ordered mesoporous materials show great importance in energy, environmental, and chemical engineering. The diffusion of guest species in mesoporous networks plays an important role in these applications, especially for energy storage, such as supercapacitors based on ordered mesoporous carbons ( OMCs). The ion diffusion behavior in two different 2-D hexagonal OMCs was investigated by using cyclic voltametry and electrochemical impedance spectroscopy. In addition, transmission electron microscopy, small-angle X-ray diffraction, and nitrogen cryosorption methods were used to study the pore structure variations of these two OMCs. It was found that, for the OMC with defective pore channels ( termed as pore packing defects), the gravimetric capacitance was greatly decayed when the voltage scan rate was increased. The experimental results suggest that, for the ion diffusion in 2-D hexagonal OMCs with similar mesopore size distribution, the pore packing defect is a dominant dynamic factor.