Interaction of water vapour at 298 K with Al-MCM-41 materials synthesised at room temperature

Abstract The interaction of water vapour with Al-MCM-41, prepared by direct synthesis at ambient temperature and pressure, using tetraethoxysilane, aluminium sulfate, hexadecyltrimethylammonium bromide and ammonia, and its effect on the pore structure were studied in order to investigate the stability towards prolonged exposure to water vapour and the influence of the aluminium content. With this purpose two consecutive water adsorption isotherms were determined at 298 K on samples with Si/Al ratio between 15 and 100. The samples were characterised by X-ray diffraction and adsorption of nitrogen at 77 K and toluene at 298 K, prior to and after exposure to water vapour. Pore size distributions were calculated from nitrogen, toluene and water adsorption isotherms using, respectively, the NLDFT method, a recently developed hybrid MC-DBdB method and the DBdB macroscopic approximation. It was found that Al-MCM-41 samples are significantly stable and that the stability improves as the amount of aluminium increases. Upon prolonged exposure to water vapour, there is a small decrease in pore size (3-5%), pore volume (8-16%) and total surface area (3-7%). The structural changes are essentially a consequence of the surface hydroxylation that occurred and not a result of a partial collapse of the pore structure. Although the presence of some extraframework Al can contribute to the improvement of the stability by protecting the surface, it was concluded that tetracoordinated Al plays an important role. The stabilizing effect of the Al incorporated in the walls can result from a higher degree of condensation on the surface of the pore walls and from the mild acidity generated

Plugged hexagonal templated silica: a unique micro-and mesoporous composite material with internal silica nanocapsules †

We describe in this paper the development of plugged hexagonal templated silicas (PHTS) which are hexagonally ordered materials, with internal microporous silica nanocapsules; they have a combined micro-and mesoporosity and a tuneable amount of both open and encapsulated mesopores and are much more stable than other tested micellar templated structures. Researchers of Mobil published in 1992 a breakthrough report on the synthesis of ordered mesoporous silica materials. All synthesised materials exhibit the typical X-Ray diffraction patterns of the 2D hexagonal pore ordering in the p6mm space group. 2 † The 77 K nitrogen isotherm in High resolution TEM measurements confirm this model. § The rather thick walls (~4 nm) of the large cylindrical mesopores are perforated with micropores. Moreover, the cylindrical mesopores themselves are 'plugged' with amorphous silica nanocapsules, which are also microporous. These nanocapsules are created by the large excess of the silica source (TEOS) that is used in the synthesis and by rapid hydrolysis of the silicon alkoxide at the very low pH used. The micropores in the silica walls can be explained by the penetration of hydrophilic poly(ethyleneoxide) chains of the triblock copolymer in the silica wall, as already suggested by Kruk et al. 7 The microporosity of the plugs may have a different origin. It is known that Pluronic triblock copolymers are in fact polydisperse mixtures of several triblock copolymers with a wide range of molecular weights, and that they contain appreciable amounts of diblock copolymers and even free PO chains. Some † Electronic supplementary information (ESI) available

Plugged hexagonal templated silica: a unique micro-and mesoporous composite material with internal silica nanocapsules †

We describe in this paper the development of plugged hexagonal templated silicas (PHTS) which are hexagonally ordered materials, with internal microporous silica nanocapsules; they have a combined micro-and mesoporosity and a tuneable amount of both open and encapsulated mesopores and are much more stable than other tested micellar templated structures. Researchers of Mobil published in 1992 a breakthrough report on the synthesis of ordered mesoporous silica materials. All synthesised materials exhibit the typical X-Ray diffraction patterns of the 2D hexagonal pore ordering in the p6mm space group. 2 † The 77 K nitrogen isotherm in High resolution TEM measurements confirm this model. § The rather thick walls (~4 nm) of the large cylindrical mesopores are perforated with micropores. Moreover, the cylindrical mesopores themselves are 'plugged' with amorphous silica nanocapsules, which are also microporous. These nanocapsules are created by the large excess of the silica source (TEOS) that is used in the synthesis and by rapid hydrolysis of the silicon alkoxide at the very low pH used. The micropores in the silica walls can be explained by the penetration of hydrophilic poly(ethyleneoxide) chains of the triblock copolymer in the silica wall, as already suggested by Kruk et al. 7 The microporosity of the plugs may have a different origin. It is known that Pluronic triblock copolymers are in fact polydisperse mixtures of several triblock copolymers with a wide range of molecular weights, and that they contain appreciable amounts of diblock copolymers and even free PO chains. Some † Electronic supplementary information (ESI) available

Nanoscale Confinement and Fluorescence Effects of Bacterial Light Harvesting Complex LH2 in Mesoporous Silicas

Many key chemical and biochemical reactions, particularly in living cells, take place in confined space at the mesoscopic scale. Toward understanding of physicochemical nature of biomacromolecules confined in nanoscale space, in this work we have elucidated fluorescence effects of a light harvesting complex LH2 in nanoscale chemical environments. Mesoporous silicas (SBA-15 family) with different shapes and pore sizes were synthesized and used to create nanoscale biomimetic environments for molecular confinement of LH2. A combination of UV-vis absorption, wide-field fluorescence microscopy, and in situ ellipsometry supports that the LH2 complexes are located inside the silica nanopores. Systematic fluorescence effects were observed and depend on degree of space confinement. In particular, the temperature dependence of the steady-state fluorescence spectra was analyzed in detail using condensed matter band shape theories. Systematic electronic-vibrational coupling differences in the LH2 transitions between the free and confined states are found, most likely responsible for the fluorescence effects experimentally observed

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