(131c) Bilayer MFI Zeolite Membranes: Hydrogen Separation and Long Term Stability

Metallic or amorphous microporous inorganic membranes perm-selective for hydrogen separation suffer from thermal or chemical stability problems.  MFI type zeolite membrane offers good stable and high hydrogen performance, making it attractive for application in membrane reactors for water gas shift (WGS) reactionfor hydrogen production. The hydrogen permselectivity of MFI zeolite membranes can be improved by intracrystalline pore modification. However, more than one order of magnitude reduction in H₂permeance usually accompanies with the significant improvement in H₂/CO₂ separation factor of MFI zeolite membranes modified by catalytic cracking deposition (CCD) of methyldiethoxysilane. In this work, a thin, high quality ZSM-5 top layer was synthesized on a thick silicalite bottom layer to form a ZSM-5/silicalite bilayer membrane. The ZSM-5/silicalite bilayer zeolite membrane was supported on a porous a-alumina support coated with an yttria stabilized zirconia (YSZ) intermediate barrier layer for membrane stability improvement.  The CCD modification of the bilayer zeolite membrane resulted in an improvement of H₂/CO₂ separationfactor of the membrane by over five-fold from 5  to25 with only about 31% reduction in H₂permeance from 1.9x10^-7 to 1.3x10^-7mol/m².s.Pa at 450^oC. Long-term hydrothermal and chemical stability of the modified bilayer zeolite membrane were studied under water gas shift (WGS) reaction conditions at high temperatures. The stability testing results showed that the H₂ recovery and the H₂permeance of the CCD modified membrane increased by 2.1% and 2.7%, respectively, and the H₂/CO₂ separation factor decreased from 22.4 to 21.7, afterthe membrane reactor was operated for WGS reaction for 28 days (500^oC, H₂O/CO=3, GHSV=60,000 h^-1). This membrane was further tested under WGS reaction conditions (25% H₂: 25% CO: 25% H₂O: 25% CO₂, 500^oC, no WGS catalyst was packed) in the presence of 400 ppm H₂S for 24 days, and no significantchange in H₂permeance and H₂/CO₂ separation factor was observed. The results demonstrated that the modified bilayer MFI zeolite membranes with an yttria stabilized zirconia (YSZ) intermediate barrier layer have excellent long-term hydrothermal and chemical stability under WGS reaction conditions.