Microporous silicas

Di Renzo, F., Cambon, H., and Dutarte, R. (1997) A 28-year-old synthesis of micelle-templated mesoporous silica. Microporous Mater., 10, 283-286 Chiola, V., Ritsko, J.E., and Vanderpool, C.D. (1971) Process for producing low-bulk density silica. US Patent 3,556,725, assigned to Sylvania Electric Products, Inc. 

Note Microporous silica, microporous glass and zeolites are common examples of aerogels. 

It has been shown that the alumina—free zeolites are hydrophobic and hence interact unfavorably with water. This renders them thermodynamically unstable in aqueous solution with respect to dense phases. Only when organic molecules are occluded does water penetration decrease and the favourable interaction of occluded molecules with the zeolitic silica micropore wall become dominant. This is a means whereby the alumina-free zeolites may become thermodynamically stable. 

Effect due to the nature of the microporous solid. Several experiments carried out with microporous supports of comparable surface but of a different nature have shown the reaction G value to be distinctly influenced by the nature of the surface. The two last results of Table II make this situation apparent in the case of alumina and silica microporous supports the reaction G value for alumina is 50% higher than for silica. An analogous effect has been systematically observed in the course of all our studies carried out on various systems. 

The main difference between the synthesis of MCM-41 mesoporous material and traditional synthesis of zeolite or silica molecular sieve is the use of different templates. An individual organic molecule or metal cation is used for the traditional synthesis of silica microporous molecular sieve. For example, the typical template for ZSM-5 synthesis is tetrapropylammonium ion the crystal is formed through the condensation of silicate species around the template molecule, while for the formation of MCM-41, the typical template is the assembly of large molecules containing one hydrophobic chain with more than 10 carbons. 

M.F. Ottaviani, A. Galarneau, D. Desplantier-Giscard, F. Di Renzo, and F. Fajula, EPR Investigations on the Formation of Micelle-templated Silica. Microporous Mesoporous Mater., 2001, 44, 1-8. 

Membrane systems with pore diameters in the micropore range (gas separation, nanofiltration) are not yet commercially available but are produced for development and marketing purposes by, e.g., Velterop B.V. (Enschede, Netherlands) and Media and Process Technology Inc. (Pittsburgh, USA). These systems have an a-alumina support combined with multilayered y-alumina (mesoporous) layers and a silica (microporous) separation layer. 

Silica microporous membranes combining high separation factors and high permeation values were first reported by Uhlhorn et al. [28,58] and were further developed and analysed by de Lange et al. [59-63]. More recently silica membranes made by a CVD process with similar qualities were reported by Lin et al. [67] and by Wu et al. [68]. 

Ottaviani, M.F. et al., EPR investigations on the formation of micelle-templated silica, Microporous Mesoporous Mater., 44, 1, 2001. 

Hunnis, M., Rufmska A. and Maier W. F. (1999) Selective surface adsorption versus imprinting in amorphous microporous silicas, Micropor. 

Pure silica microporous solids show no such strong affinity for polar molecules, and are of interest to separate hydrocarbon molecules of different shapes. In particular, the separation of xylene isomers is of great industrial significance, and since the relative diffusivities of the para isomer are known to be much higher than those of the ortho (and meta) isomers within silicalite, the preparation of silicalite membranes is an attractive target. If the aluminium content can be made vanishingly small, the membranes can be used for hydrocarbon separation at elevated temperatures without the effects of coke formation due to catalytic reaction. 

Silica membranes prepared by the sol-gel process, the pore size of which can be controlled by the colloidal size, have been applied for CO2/N2 separations [58] and olefin/paraffin separations [59]. For example, silica microporous membranes showed a separation factor of propylene over propane as high as 75 with an approximate permeance of 0.2 x 10 m m s kPa at 35°C. However, it was pointed out that silica membranes needs to be improved with respect to their stability in humid air, because microporous silica becomes densified, resulting in a decrease in permeance [56]. 

Qu F, Zhu G, Huang S, Li S, Zhang JSD, Qiu S (2006) Controlled release of Captopril by regulating the pore size and morphology of ordered Mesoporous silica. Micropor Mesopor Mater 92 1-9 RamQa A, Munoz B, Perez-Pariente J, Vallet-Regi M (2003) Mesoporous MCM-41 as drug host system. J Sol-Gel Sd Technol 26(1) 1199-1202... 

Hua L., Tan SJST. Capillary electrophoresis with an integrated on-capillary tabular detector based on a carbon sol-gel-derived platform. Anal. Chan. 2000 72(20) 4821-4825 Hunnius M., Rufinska A., Maier W.F. Selective surface adsorption vosus imprinting in amorphous microporous silicas Micropor. Mesopor. Mater. 1999 29(3) 389-403 Kim M.A., Lee W.Y. Amperometric phenol biosensor based on sol-gel silica/Nafion composite film. Anal. Chim. Acta 2003 479(2) 143-150... 

Smitha, S., Shajesh, P., Aravind, P.R., Rajesh Kumar, S., Krishna Pillai, P., and Warrier, K.G.K. (2006) Effect of aging time and concentration of aging solution on the porosity characteristics of subcritically dried silica. Microporous Mesoporous Mater., 91, 286-292. 

These processes involve a multistep transformation from the carbohydrate fraction to the value-added products which makes most of them far from commercialization. Hence, intensive efforts are stiU required to enable scale up of synthetic protocols developed on a lab-scale into industrial processes. Some of the current drawbacks might be overcome by the one-pot transformation of lignocellulose carbohydrates in value-added chemicals without isolation of the intermediate platform molecules (Delidovich et al., 2014). Moreover, nanoporous materials, such as acidic, basic or metallic catalysts (zeolites, mesoporous silicas, microporous/mesoporous carbons, resins, metal oxides, etc.), wUl play a crucial role in this biomass transformation (Wang and Xiao, 2015). 

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