Characteristics of Ammonia Permeation Through Porous Silica Membranes

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 degrees 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 > H-2 > NH3 > N-2. These results suggest that, for permeation through silica membranes, the molecular size of NH3 is larger than that of H-2, despite previous reports that the kinetic diameter of NH3 is smaller than that of H-2. At high temperatures, there was no effect of NH3 adsorption on H-2 permeation characteristics, and silica membranes were highly stable in NH3 at 400 degrees C (i.e., gas permeance remained unchanged). On the other hand, at 50 degrees C NH3 molecules adsorbed on the silica improved NH3-permselectivity by blocking permeation of H-2 molecules without decreasing NH3 permeance. The maximal NH3/H-2 permeance ratio obtained during binary component separation was similar to 30 with an NH3 permeance of similar to 10(-7) mol m(-2) s(-1) Pa-1 at an H-2 permeation activation energy of similar to 6 kJ mol(-1). (C) 2009 American Institute of Chemical Engineers AIChE J, 56: 1204-1212, 2010