Rational Synthesis of Chabazite (CHA) Zeolites with Controlled Si/Al Ratio and Their CO2 /CH4 /N2 Adsorptive Separation Performances.

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Separation of CO2 from CH4 and N2 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 /N2 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 /N2 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.