Blocking defects in SAPO-34 membranes with cyclodextrin

The CO2/CH4 separation selectivity of SAPO-34 membranes increased by 20–150% after soaking them in 1–2% aqueous β-cyclodextrin (CD) solution and drying them at 473 K. The percentage increase was higher at higher pressures, and the selectivities increased more for membranes with more defects. The CD decreased the CO2 fluxes at high pressures by about 10–20%, whereas CH4 fluxes decreased by 30–60%. The flux of CF4, which is too large to enter the SAPO-34 pores and thus only permeates through defects, decreased 80% for one membrane (5.7 MPa feed pressure) after CD deposition; this confirmed that defects were effectively blocked. Even before CD exposure, the CF4 permeance at 2.8 MPa was only 0.25% of the CO2 mixture permeance for that membrane. Permporometry also showed that CD blocked defects; the maximum defect size decreased from greater than 30 nm to less than 20 nm. About 91% of the overall defect area in the original membrane was from defects smaller than 8 nm, whereas 99% was for defects smaller than 2 nm after CD deposition. The membrane separation performance did not change during 3 days of continuous measurements, nor did it change after long term storage at 473 K in air.