Mesoporous glasses

Gryaznov, V. M., A. N. Karavanov, O. K. Knisil nikova, G. L. Chernysova and A. V. Patrikeev. 1981a. Palladium on mesoporous glass as a catalyst for the hydrogenation of unsaturated compounds. Izv. Akad. Nauk, SSSR, Ser. Khim, 7 1663-1666. 

In the current paper, we discuss some of the new approaches and results that have been developed and obtained recently within the context of such molecular modeling research, and in particular with the mean field and Monte Carlo studies of a lattice model. The next section describes the Gaussian random field method (Woo et al, 2001), which provides a computationally efficient route to generate realistic representations of the disordered mesoporous glasses. Application of the mean field theory, and Monte Carlo simulations are described in Secs. 3 and 4, respectively. 

A lattice model Hamiltonian that has been successfully used to model fluids in mesoporous glasses reads... 

Our overall conclusion, therefore, is that for mesoporous glasses adsorption, hysteresis is a dynamic phenomenon that is not simply related to a capillary vapor-liquid phase transition. Slow dynamics for long times makes the states accessible in experiments in the hysteresis loop appear equilibrated and quite reproducible. Mean field theory and Monte Carlo simulations in the grand ensemble provide a physically realistic description of these phenomena. 

The mercury porosimetry intrusion plot of a mesoporous glass membrane is given in Fig. 2. That curve and the isotherm shapes in Fig. 1 are indicators for a relatively narrow pore size distribution of the ultrathin porous glass membranes. 

Fig. 2 Mercury porosimetry intrusion curve of a mesoporous glass membrane Table 1...

The optical transparency of the porous glass membranes is interesting for sensor applications. It depends on the structural features of the membranes [13]. The UV-VIS spectra of micro- and mesoporous glass membranes are compared in Fig. 6. The membranes with an... 

The interaction between water molecules and silica substrate is described in the framework of the PN-TrAZ model [15] which has proven to model successfiilly the adsorption of simple adsorbates on various zeolites [20]. In this model, the pair potential decomposes in two parts a repulsion term Ae" due to electronic clouds and the attractive dispersion terms. The repulsive parameters (A,b) for silica atoms (Si, O, H) are those obtained from studies of adsorption of simple gazes on various zeolites [20] and mesoporous glass [21]. Those for water oxygen are chosen to fit the repulsive part of Lennard-Jones from SPC model in the range around equilibriiun distance, and those for water hydrogen are taken equal to the parameters for surface hydrogen of vycor. The cross repulsive parameters A and b are obtained by Bohm and Ahlrichs [22] combination mles. The dispersion terms are calculated from polarizabilities and effective niunber of electron Neff according to the PN-TrAZ model up to order r °. Values are listed in table 1. 

Mesoporous glass ("Vycor type") can be produced by a combined heat-treatment and leaching procedure as summarised by Burggraaf and Keizer [78] in an extended overview of synthesis methods. Modification of this process leads to microporous hollow fibres with interesting properties (see Chapter 9). Details of this modification process are not known to this author. 

Chemical Vapour Deposition (CVD) of microporous silica films with a thickness of about 1.5 pm onto mesoporous glass or y-alumina substrates are obtained by deposition from TEOS-oxygen mixtures at 300-700°C. Pore sizes are estimated to be 0.4-0.6 nm or are virtually absent. CVD techniques may also be useful for repairing residual defects and for pore narrowing. 

Mesoporous glass (Vycor type) can be produced by a combined heat-treatment and leaching procedure [9]. Modification of this process can lead to microporous hollow-fibre systems with interesting properties as discussed by Shelekhin, Ma et al [56]. For further discussion see Sections 9.4.2 to 9.4.4. 

The influence of metal oxide derived membrane material with regard to permeability and solute rejection was first reported by Vernon Ballou et al. [42,43] in the early 70s concerning mesoporous glass membranes. Filtration of sodium chloride and urea was studied with porous glass membranes in close-end capillary form, to determine the effect of pressure, temperature and concentration variations on lifetime rejection and flux characteristics. In this work experiments were considered as hyperfiltration (reverse osmosis) due to the high pressure applied to the membranes, 40 to 120 atm. In fact, results reproduced in Table 12.3 show that these membranes do not behave as h)qjerfiltra-tion membranes but as membranes with intermediate performances between ultra- and nanofiltration in which surface charge effect of metal oxide material plays an important role in solute rejection. 

Rejection of NaCl (58.5 g mole ) and Urea (56 g mole ) using mesoporous glass membranes over a range of solute concentration, from Ref. [42]... 

The inorganic support should present a high hydrophilicity and a very high specific surface area it is a case, for instance, for silica or mesoporous glass beads. A general scheme is given in Fig. 1. 

Other methods to prepare porous membranes include pyrolysis for carbon membranes, heat treatment and leaching for mesoporous glass membranes, and anodization for alumina membranes. The microporous carbon membranes are prepared by coating a polymeric precursor such as polyfurfuril alcohol and polycarbosilane on porous substrates, followed by controlled pyrolysis under N2 atmosphere [15]. The carbon membrane structure is determined by the fabrication variables, including the polymeric solution concentration, solvent extraction, heating rate, and pyrolysis temperature [16]. 

The facilities of high-pressure mercury porosimetry for the investigation of preloaded porous solids will be demonstrated using mesoporous glass and micro-porous activated carbon preloaded with n-decane or with water (ref. 1). 

The mesoporous glass (CPG-10 240 A from Fluka) should have a pore volume of 960 (Fluka) we measured 986 mm g" - (porosimeter up to 2000 bar),... 

Fig. 4 shows a plot of the experimental points of the pore volume as function of the n-decane load for mercury intrusion in the mesoporous glass. The extrusion (not shown in Fig. 4) shows a hysteresis in pressure but releases the intruded mercury almost completely. 

Fig. 4. Pore size distribution of mesoporous glass with different contents of n-decane...

Fig. 5. Pore volume of mesoporous glass as function of the content of n-decane 1 Intrusion (o) 2 Calculated from P,V,T-data, x extrusion...

Wang, H. and G.R. Gavalas, Mesoporous glass films sujjported on -AI2O3. Journal of Membrane Science, 2000.176(1) 75-85. 

Markovi, A.,D.Stoltenberg,D.Enke,E.U.Schlunder,andA.Seidel-Morgenstem, Gas permeation through porous glass membranes Part I. Mesoporous glasses— Effect of pore diameter and surface properties. Journal of Membrane Science, 2009.336(1-2) 17-31. 

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