Analysis and theory of gas transport in microporous sol-gel derived ceramic membranes

Abstract

Sol-gel modification of mesoporous alumina membranes is a very successful technique to improve gas separation performance. Due to the formed microporous top layer, the membranes show activated transport and molecular sieve-like separation factors. This paper concentrates on the mechanism of activated transport (also often referred to as micropore diffusion or molecular sieving). Based on a theoretical analysis, results from permeation and separation experiments with H2, CO2, 02, N2, CH4 and iso-CaHlo on microporous sol-gel modified supported ceramic membranes are integrated with sorption data. Gas permeation through these membranes is activated, and for defect-free membranes the activation energies are in the order of 13-15 kJ.mol-~ and 5-6 kJ.mol-1 for H2 and CO2 respectively. Representative permeation values are in the order of 6 × 10-7 mol.m-2.s-t.Pa-~ and 20 × 10-7 mol.m-2.s-~.Pa-~ for H2 at 25°C and 200°C, respectively. Separation factors for H2/CH4 and H2/iso-butane are in the order of 30 and 200 at 200°C, respectively, for high quality membranes. Processes which strongly determine gas transport through microporous materials are sorption and micropore diffusion. Consequently, the activation energy for permeation is an apparent one, consisting of a contribution from the isosteric heat of adsorption and the activation energy for micropore diffusion. An extensive model is given to analyse these contributions. For the experimental conditions studied, the analysis of the gas transport mechanism shows that interface processes are not rate determining. The calculated activation energies for micropore diffusion are 21 kJ.mol-1 and 32 kJ.mol-t for H2 and CO2, respectively. Comparison with zeolite diffusion data shows that these activation energies are higher than for zeolite 4A (dpo~ = 4 /~), indicating that the average pore size of the sol-gel derived membranes is probably smaller.

Microporous Niobia–Silica Membrane with Very Low CO2 Permeability

“…Table 1 lists the activation energies calculated from Figure 5, together with some data on pure and metal-oxide-doped silica membranes that have been reported elsewhere. [20][21][22][23][24][25][26] The values measured in the present study are close to those reported by Nair et al, who found activation energies for helium permeance of 23 kJ mol À1 in silica. [21] Asaeda and co-workers reported an increase in apparent activation energy of H 2 permeance from 3.4 to 44 kJ mol À1 when the Zr dopant loading in silica was increased from 10 mol % to 90 mol %.…”

“…Such a highly negative value suggests a high enthalpy of sorption. The literature values in Table 1 indicate either positive activation energies for CO 2 permeance [22,23] or slightly negative values. [20] Apart from data in a study in which activation energies of À9.2 to À11.2 kJ mol À1 were measured, [24] none of the reported values is close to (À14.2 AE 0.6) kJ mol À1 as measured on the NS membrane.…”

Preparation of particulate/polymeric sol–gel derived microporous silica membranes and determination of their gas permeation properties

“…After the addition was completed, the reaction mixture was heated to 60 • C for 3 h under constant stirring in a glove box. The standard silica sol molar TEOS:HNO 3 :H 2 O:EtOH ratio was 1:0.085:6.4:3.8 [13,14].…”

“…In contrast to sp5-220 nm and sp10-220 nm membranes, the N 2 and O 2 permeances through sp10-220 nm membrane consolidated from ethylene glycol diluted sol increased up to 350 K and stayed constant up to 400 K but the CO 2 permeance remained almost constant with increasing temperature. The increase in permeance values with increasing temperature leads to positive activation energy indicative of activated transport mechanism through the microporous selective layer [14,25]. The positive activation energy might be caused by either the formation of organically bound species due to utilizing ethylene glycol as a dilution medium that prevents the formation of microcracks on membrane surface or lower drying stress caused by slow drying in the course of ethylene glycol evaporation that might result in microcrack free membrane surface.…”

“…The activation energy for permeation (combination of heat of sorption and activation energy for micropore diffusion) can be obtained by the following modified Fick's law equation [6,13,14,25,26]:…”