Characteristics of ammonia permeation through porous silica membranes

Abstract A sol–gel method was applied for the preparation of silica membranes with different average pore sizes. Ammonia (NH3) permeation/separation characteristics of the silica membranes were examined in a wide temperature range (50–400°C) by measurement of both single and binary component separation. The order of gas permeance through the silica membranes, which was independent of membrane average pore size, was as follows: He > H2 > NH3 > N2. These results suggest that, for permeation through silica membranes, the molecular size of NH3 is larger than that of H2, despite previous reports that the kinetic diameter of NH3 is smaller than that of H2. At high temperatures, there was no effect of NH3 adsorption on H2 permeation characteristics, and silica membranes were highly stable in NH3 at 400°C (i.e., gas permeance remained unchanged). On the other hand, at 50°C NH3 molecules adsorbed on the silica improved NH3‐permselectivity by blocking permeation of H2 molecules without decreasing NH3 permeance. The maximal NH3/H2 permeance ratio obtained during binary component separation was ∼30 with an NH3 permeance of ∼10−7 mol m−2 s−1 Pa−1 at an H2 permeation activation energy of ∼6 kJ mol−1. © 2009 American Institute of Chemical Engineers AIChE J, 2010