Rational Synthesis Of Chabazite (Cha) Zeolites With Controlled Si/Al Ratio And Their Co2/Ch4/N-2 Adsorptive Separation Performances

Separation of CO2 from CH4 and N-2 is of great significance from the perspectives of energy production and environment protection. In this work, we report the rational synthesis of chabazite (CHA) zeolites with controlled Si/Al ratio by using N,N,N-trimethyl-1-adamantammonium hydroxide (TMAdaOH) as an organic structure-directing agent, wherein the dependence of TMAdaOH consumption on the initial Si/Al ratio was investigated systematically. More TMAdaOH is required to direct the crystallization of CHA with higher Si/Al ratio. Once the product Si/Al ratio is larger than 24, the amount of TMAdaOH consumption remains nearly constant. CHA zeolites with different Si/Al ratios and charge-compensating cations were then applied for the separation of CO2/CH4/N-2 mixtures. The equilibrium selectivities predicted by ideal adsorbed solution theory (IAST) and ideal selectivities calculated from the ratio of Henry's constants for both CO2/CH4 and CO2/N-2 decrease with the zeolite Si/Al ratio increasing, whereas the percentage regenerability of CO2 presents the opposite trend. Therefore, there is a trade-off between adsorption selectivity and regenerability for the adsorbents. There is a weaker interaction between CO2 molecules and the H-type zeolites than that on the Na-type ones, thus a higher regenerability can be achieved. This study indicates that it is possible to design CHA zeolites with different physicochemical properties to meet various adsorptive separation requirements.