Sol–Gel Synthesis and Characterization of Microporous Silica Membranes I: SAXS Study on the Growth of Polymeric Structures

Abstract

series (11) will present the synthesis chemistry of silica sols Silica-based microporous membranes can be successfully pro-to make unsupported membranes with predictable pore size/ cessed by sol-gel techniques. However tailor making of such mem-porosity. This article aims to give a detailed account of small branes is still not possible because of the unavailability of system-angle X-ray scattering of silica sols and further calculations atic studies on membrane oriented sol processing and characteriza-of fractal dimensions and monomer/polymer sizes. Fractal alcohol condensation r GSi{OR / HO{SiG S GSi{O{SiG / ROH R alcoholysis INTRODUCTION[2] Microporous silica membranes are attractive materials for gas separation and membrane reactors. The synthesis and water condensation r GSi{OH / HO{SiG S GSi{O{SiG / H 2 O R hydrolysis characterization of such membranes and their precursor materials have been dealt with by many authors for the past few[3] years. Authors like Iler (1) and Scherer (2) have detailed descriptions of the whole sol-gel chemistry of silica and Yoldas (3) and Klein (4) have elaborated the sol-gel syn-Microporous structures are generally made available by acid thesis with emphasis to the wet gel characteristics. A number catalysis of these reactions. Hydrolysis will then take place of articles were also published (5-9) about typical sol-through a fast protonation of the alkoxide, followed by congel silica membranes with porosity/permeation/separation densation to siloxane bond. By controlling the synthesis conbehavior. Sol-gel silica binary systems for membrane syn-ditions and reactant concentrations the sol-gel morphology thesis were also dealt with (10). Even with all these studies, can be changed (2). This is essentially achieved by the there are no selection rules available for tailor making micro-control of rates of hydrolysis and condensation by changing porous membranes. the amount of water or catalyst used. The structure of the It is the aim of the present authors to present a more resulting polymers can change from more or less linear to detailed account of the sol-gel synthesis of silica along with weakly branched polymers. The morphology of the resulting characteristics of the resulting membranes, in a series of gels depends on the structure of these polymers and to a articles of which this is the first. The second part of this better extent the efficiency of packing.For the formation of microporous membranes, the ability of the polymers to interpenetrate must be considerable. Short 1 To whom correspondence should be addressed at Advanced Polymer branched linear polymers are the best in this regard. HowLaboratory, Japan High Polymer Center, Yanagibashi 2-22-13, Taito-ku, Tokyo 111, Japan.ever, the probability is higher to form a dense structure when From the fractal range, where the intensity has a power three, where the object is Euclidean. As the fractal dimension law relation, the dimension D f can be calculated as the negadecreases the structure will look sparser, giving essentially ...

Synthesis, characterisation and gas permeation studies on microporous silica and alumina-silica membranes for separation of propane and propylene

“…The present synthesis conditions are moderate leading to nearly linear fractal polymers at the end of the 3 h synthesis time. The fractal dimension of the polymer o WaS nearly 0.96 and the radius of gyration 17 A as reported elsewhere [17]. Controlled drying of this sol lead to gels with a microporosity of 20% as is shown in the nitrogen adsorption isotherm in Fig.…”

Microporous Niobia–Silica Membrane with Very Low CO2 Permeability

“…[17] The Porod plot shown in Figure 1 b represents a scattering curve typical of polymeric metal-oxide sols. [16] The section of the curve between q = 0.55 nm À1 and q = 2.09 nm À1 is linear. This implies that I(q) is a power function of q [15] [Eq.…”

“…This is of similar magnitude as that reported by Nair et al for acid-catalyzed TEOS-derived silica sols. [16] The fractal dimension D f is also an important parameter and should be controlled to obtain a material with micropores only. A microporous network can only be formed when the sol particles can interpenetrate each other during film formation.…”

Investigation of factors influencing the performance of open‐tubular stationary phases in capillary electrochromatography

“…Indeed, it is widely accepted that the H + -catalyzed sol gel reactions result in the formation of polymers of the short branched, linear type, which interpenetrate easily during drying to form dense structures with low surface areas [8]. However, it has also been postulated by Menon and coworkers [9] that in a two step sol-gel protocol, involving first a highly acidic synthesis environment followed by a basic treatment to drive the hydrolysis to completion, the polymers should be highly branched and not interpenetrate during drying.…”