DDR-type zeolite membrane was synthesized on porous α-alumina substrate by hydrothermal treatment of a ball-milled Sigmal-1 crystal seed layer in an aluminum-free precursor solution containing 1-Adamantylamine as the structure directing agent. The as-synthesized DDR zeolite membranes were defect-free but the supported zeolite layers were susceptible to crack development during the subsequent high-temperature SDA removal process. The cracks were effectively eliminated by the liquid phase chemical deposition method using tetramethoxysilane as the precursor for silica deposits. The modified membrane was extensively studied for H 2 , He, O 2 , N 2 , CO 2 , CH 4 , and i-C 4 H 10 pure gas permeation and CO 2 /CH 4 mixture separation. At 297 K and 2-bar feed gas pressure, the membrane achieved a CO 2 /CH 4 separation factor of ~92 for a feed containing 90% CO 2 , which decreased to 62 for a feed containing 10% CO 2 with the CO 2 permeance virtually unchanged at ~1.8 × 10-7 mol/m 2 •s•Pa regardless of the feed composition. It also exhibited an O 2 /N 2 permselectivity of 1.8 at 297 K. The gas permeation behaviors of the current aluminum-containing DDR type zeolite membrane are generally in good agreement with the findings in both experimental and theoretical studies on the pure-silica DDR membranes in recent literature.

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A submicrometer-thick molecular sieve zeolite nanosheet laminated (ZNL) membrane has been synthesized on macroporous polyvinylidene fluoride (PVDF) substrate via the simplistic vacuum filtration-coating method. The membrane has been demonstrated with extraordinary water flux (∼11 kg/m2·h) and salt rejection (∼99.9%) in pervaporation (PV) desalination of a multicomponent brine with 22 wt % of total dissolved salts (TDS) including Li+, Na+, K+, Mg2+, Cl–, and SO4 2–, etc. The desalination of high-salinity brines is critical to treatment and beneficial use of many industrial produced waters. The large aspect ratios (>100) and preferable b-orientation of the densely layered nanosheets make the solution dewatering rate surpass the ion diffusion rates in the nanoscale internanosheet spaces. This unique transport behavior effectively prevents the dissolved salts from migrating through the ZNL layer and stabilizes the water flux and ion rejection rates. The supported multilayered ZNL membrane, which is interlocked by the substrate-type PVDF binder, exhibits structure and performance stability in PV desalination of high-TDS brines.

Abstract:The separation of small molecule gases by membrane technologies can help performance enhancement and process intensification for emerging advanced fossil energy systems with CO 2 capture capacity. This paper reports the demonstration of controlled modification of zeolitic channel size for the MFI-type zeolite membranes to enhance the separation of small molecule gases such as O 2 and N 2 . Pure-silica MFI-type zeolite membranes were synthesized on porous α-alumina disc substrates with and without an aluminum-containing thin skin on the outer surface of zeolite membrane. The membranes were subsequently modified by on-stream catalytic cracking deposition (CCD) of molecular silica to reduce the effective openings of the zeolitic channels. Such a pore modification caused the transition of gas permeation from the N 2 -selective gaseous diffusion mechanism in the pristine membrane to the O 2 -selective activated diffusion mechanism in the modified membrane. The experimental results indicated that the pore modification could be effectively limited within the aluminum-containing surface of the MFI zeolite membrane to minimize the mass transport resistance for O 2 permeation while maintaining its selectivity. The implications of pore modification on the size-exclusion-enabled gas selectivity were discussed based on the kinetic molecular theory. In light of the theoretical analysis, experimental investigation was performed to further enhance the membrane separation selectivity by chemical liquid deposition of silica into the undesirable intercrystalline spaces.

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