The molecular state is crucial for precise gas separation using a zeolite membrane, yet the state control remains a big challenge. Herein, we report a NO2 dimerization facilitated high performance SO2/NO2 separation on a SSZ-13 zeolite membrane. The NO2 dimerization is triggered by temperature and pressure to form N2O4 with big molecular size, and N2O4 diffusion into the zeolite pore is inhibited on the basis of size exclusion, leading to high separation selectivity. Consequently, SO2 rather than NO2 preferentially permeates through the SSZ-13 membrane with a high SO2 permeance of 2 × 10–7 mol m–2 s–1 Pa–1 and high SO2/NO2 separation factor of 22, ∼50-fold of that measured without dimerization. The dimerization effect for SO2/NO2 separation prevails in other small-pore zeolites such as NaA. This advanced function is revealed through membrane separation using single and mixture gases.

NO2 and SO2, as valuable chemical feedstock, are worth being recycled from flue gases. The separation of NO2 and SO2 is a key process step to enable practical deployment. This work proposes SO2 separation from NO2 using chabazite zeolite (SSZ-13) membranes and provides insights into the feasibility and advantages of this process using molecular simulation. Grand canonical ensemble Monte Carlo and equilibrium molecular dynamics methods were respectively adopted to simulate the adsorption equilibria and diffusion of SO2, NO2, and N2O4 on SSZ-13 at varying Si/Al (1, 5, 11, 71, +∞), temperatures (248–348 K), and pressures (0–100 kPa). The adsorption capacity and affinity (SO2 > N2O4 > NO2) demonstrated strong competitive adsorption of SO2 based on dual-site interactions and significant reduction in NO2 adsorption due to dimerization in the ternary gas mixture. The simulated order of diffusivity (NO2 > SO2 > N2O4) on SSZ-13 demonstrated rapid transport of NO2, strong temperature dependence of SO2 diffusion, and the impermeability of SSZ-13 to N2O4. The membrane permeability of each component was simulated, rendering a SO2/NO2 membrane separation factor of 26.34 which is much higher than adsorption equilibrium (6.9) and kinetic (2.2) counterparts. The key role of NO2–N2O4 dimerization in molecular sieving of SO2 from NO2 was addressed, providing a facile membrane separation strategy at room temperature.

igh-silica SSZ-13 zeolite membranes are promising in industrial separations of light gases and continuous membranes are highly demanded for better separation performances. Herein, pure-phase, continuous and thin SSZ-13 zeolite membranes were synthesized using dual templates of N,N,Ntrimethtyl-1-adamantammonium hydroxide(TMAdaOH) and tetraethylammonium hydroxide(TEAOH). Systematical investigations of TMAdaOH/TEAOH ratios and their concentrations show that TMAdaOH acts as the main structure-directing agent in the formation of the SSZ-13 zeolite. TMAdaOH cooperatively plays with TEAOH in promoting the SSZ-13 crystal intergrowth to form a continuous polycrystalline membrane. Additionally, appropriate introduction of TEAOH is able to adjust the membrane thickness to the crystal-comparable size of ca. 2.0 μm. The SSZ-13 membranes are further applied for N2/NO2 separation, which is firstly reported on zeolite membranes. The gas permeation results show that the SSZ-13 membrane synthesized by the dual-template approach exhibits selective separation of N 2 over NO 2 with N 2 /NO 2 separation factor of 7.6 and N 2 permeance of 1.66×10 -8 mol•m -2 •s -1 •Pa -1 .

Chabazite (CHA) zeolite membranes exhibit superior performances in light gas separations owing to the eight-membered ring channel structure with small pore size (0.38 nm), adjustable surface characteristics, and high material stability and preparation reproducibility, and have gradually become one of the hot spots of zeolite membrane research in recent years. This review article first introduces the basic characteristics, the two typical CHA zeolite membranes (SAPO-34 and SSZ-13 membranes), then compares the synthesis and preparation methods of CHA zeolite membranes (in-situ synthesis, secondary growth synthesis, microwave heating methods) and analyzes their advantages and disadvantages in application status. The influences of their key synthesis conditions of the secondary growth as themainstream synthesis method on the qualities of SSZ-13 and SAPO-34 membranes have been elaborated in detail, mainly including 1) the seeding conditions, such as carrier type, seeding crystal and seeding approach; 2) the hydrothermal synthesis conditions, such as crystallization time and temperature, water content, silica-to-alumina ratio, structure directing agent, and cation type; 3) the calcination approaches, such as conventional calcination, staged calcination, and rapid heating treatments. After comparative analysis, the preferred synthesis of the above two typical CHA zeolite membranes are proposed. Furthermore, the modulation of membrane surface chemistry is discussed for the enhancement in gas separation, such as silica-to-alumina ratio adjustment, ion exchange, heteroatom substitution, amino-group functionalization, and surface modification. The detailed characteristics of gas separation in various gas mixture systems and the permeation properties of different single gases on CHA zeolite membranes are analyzed and summarized as well. Finally, the future development of CHA zeolite membranes is prospected.

No abstract