Spatially resolved gas permeation through SAPO-34 membranes

A technique to measure spatially resolved permeances through tubular membranes with a moveable mass spectrometer probe at the membrane surface is described. This technique was used to determine spatial permeance distributions for SAPO-34 zeolite membranes, alumina membrane supports, and a Vycor glass membrane. The distribution of local permeances provides more detailed information about membrane properties than global permeances. The CO2 and CH4 permeances of SAPO-34 membranes that had high permeances and selectivities at 4.6 MPa were not spatially uniform across the membrane area. Membranes prepared using Al(OH)3 as the aluminum precursor had large regions that were nearly impermeable, whereas membranes prepared with Al(i-C3H7O)3 were more uniform, and thus had higher overall permeances. Scanning electron micrographs indicated that these differences in permeance were not due to differences in membrane thickness, and spatially uniform permeance distributions for alumina supports suggest that they were not due to non-uniformities in the supports. Defect locations in the SAPO-34 layers were detected using i-butane permeation, because i-butane is too large to enter the SAPO-34 pores, and the defect concentrations were low. These spatially resolved measurements indicate the potential to increase both CO2/CH4 selectivity and CO2 permeance, and help understand the factors that affect membranes properties.