Preparation Of SAPO-34 Membranes And Their Performances In Separation Of Gas Mixtures

The separation and removal of CO₂ using membrane technology is one of the most important subjects worldwide. The separation of CO₂ from CH₄ is critical for methane transportation and storage because the existence of CO₂ reduces the energy content and causes the corrosion of equipment and pipelines in the presence of water which needs to be prevented. The most widely used technologies for CO₂ removal include the processes of absorption and polymeric membrane separation. But the absorption equipment is complex and not cost-effective. Besides, the high partial pressure of CO₂ can plasticize polymeric membranes and decrease their separation performances. Compared with the traditional polymeric membranes, molecular sieve membranes exhibit a favorable application potential in purification of CH₄ from CO₂/CH₄ gas mixture due to their corrosion-resistant abilities with very good mechanical and thermal stabilities, especially at high pressure of CO₂.The membranes constructed with small, medium and large pore-sized molecular sieves can be used to separate CO₂ from gas mixtures. However, the membranes of medium and large pore-sized molecular sieves are not efficient for separation of gas mixtures with small molecules of similar kinetic diameters such as CO₂ and CH₄, the kinetic diameter of which are 0.33 and 0.38 nm respectively. In contrast, small pore-sized molecular sieve membranes such as zeolite T（0.36×0.51 nm）, DDR（0.36×0.44 nm）, and SAPO-34（0.38 nm） have shown high CO₂/CH₄ selectivities due to the combination of molecular sieving and competitive adsorption because pore sizes are similar in size to CH₄（0.38 nm） but larger than CO₂（0.33 nm）.In the present work, SAPO-34 molecular sieve membranes were fabricated with 10 μm layers on porous α-Al₂O₃ substrates by in-situ hydrothermal synthesis. The effects of pH and water content of the starting reaction solution, and crystallization temperature on SAPO-34 membranes were investigated. As the substrate surface was uneven, the homogeneous nucleation on the supported surface was difficult to achieve. The continuous SAPO-34 membranes were attained by the second-time hydrothermal synthesis. The templates were effectively removed from SAPO-34 crystals by the low-temperature calcination method at 330 °C. The CO₂/CH₄ separation selectivity for the SAPO-34 membrane at 22 °C and a pressure drop of 0.14 MPa was 67, with a CO₂ permeance of 1.38 × 10^-7 mol m^-2·s^-1·Pa^-1.SAPO-34 molecular sieve membranes with high permeance were synthesized as 2³ μm layers on porous α-Al₂O₃ substrates by the seeded growth method. FT-IR showed that templates were effectively removed from SAPO-34 crystals by the low-temperature calcinations method at lower temperatures（330 °C）. The CO₂/CH₄ separation selectivity for the SAPO-34 membrane at 22 °C and a pressure drop of 0.14 MPa was 84, with a CO₂ permeance of 2.17×10^-6 mol m^-2·s^-1·Pa^-1.MeAPSO-34 molecular sieve membranes with high selectivity and high permeance were synthesized as 2³ μm layers on porous α-Al₂O₃ substrates by the seeded growth method with Sr²⁺ and Li+ isomorphously-substituted into SAPO-34 framework. Isomorphous substitution did not decreased the crystallinity with metal ions incorporated into the framework of SAPO-34. The CO₂/CH₄ separation selectivity for the SrAPSO-34 membrane after removed templates at 22 °C and a pressure drop of 0.14 MPa was 196, with a CO₂ permeance of 1.39×10^-6 mol m^-2·s^-1·Pa^-1. The membrane showed a very high selectivity（⁹.5） for N₂/CH₄, with a N₂ permeance of 0.9×10^-7 mol m^-2·s^-1·Pa^-1. The selectivities and permeablities for these SAPO-34 membranes were significantly higher than the upper bound observed for polymeric membranes. The permeation and separation performances of permanent gases through SrAPSO-34 membranes might be explained by the enhanced effect of molecular sieving and competitive adsorption arose from the isomorphous substitution of Sr²⁺ ion.