Synthesis of thin SAPO-34 zeolite membranes in concentrated gel

Abstract

Synthesis of thin zeolite membranes in the sub-micron range is desirable but a great challenge. Despite the successful synthesis of ultrathin Silicalite-1 and CHA membranes, the pursuit of SAPO-34 membranes thinner than 1 μm remains difficult. Herein, thin SAPO-34 membranes were prepared in concentrated gel by conventional hydrothermal synthesis. Concentrated membrane gel (TEAOH/Al2O3 = 4 and H2O/Al2O3 = 10) enhanced nucleation and facilitated the formation of small SAPO-34 crystals and therefore thin membranes. High TEAOH/Al2O3 ratio and special mixing sequence resulted into a homogeneous membrane gel with low viscosity which is beneficial for membrane synthesis with high reproducibility. Al(OH)3 as Al source led to formation of cubic SAPO-34 crystals which facilitated crystal intergrowth and reduced defects. These membranes (∼1.2 μm thick) exhibited high CO2 permeance of 3.9 × 10−6 mol/(m2 s Pa) and CO2–CH4 selectivity of 152 for equimolar CO2–CH4 mixture at room temperature and 0.14 MPa pressure drop. Further optimization of aging treatment or limiting the excessive membrane growth by a two-step hydrothermal synthesis + dry gel conversion method, the membrane thickness was successfully reduced to ∼0.9 μm, which increased the CO2 permeance to ∼5.0 × 10−6 mol/(m2 s Pa) without sacrificing CO2–CH4 selectivity.