Adsorption of CO2 and CO on H-zeolites with different framework topologies and chemical compositions and a correlation to probing protonic sites using NH3 adsorption

Abstract

Adsorption of CO2 and CO at 25 °C has been conducted using commercially-available (Y, ZSM-5) and laboratory-synthesized (SSZ-13, SAPO-34) H-zeolites with different framework topologies and chemical compositions, and their textual and surface properties have been characterized by N2 sorption and NH3 adsorption techniques. All the zeolites were microporous, although ZSM-5 and SSZ-13 apparently showed a mesoporous sorption behavior due to the interparticle spaces. The zeolites had Si/Al values in the order of SSZ-13 (16.44) > ZSM-5 (16.08) ≫ Y (2.82) ≫ SAPO-34 (0.19). Regardless, high CO2 adsorption capacity was obtained for SSZ-13 and SAPO-34 with a CHA framework. The FAU zeolite Y with the highest micropore volume showed less CO2 adsorption than the CHA zeolites and the MFI-type ZSM-5 yielded the poorest performance. Probing acid sites in the H-form zeolites using NH3 disclosed that these all contain both weak and strong acid sites with significant dependence of their strengths and amounts on the topology. The acid strength of the weak acid sites in the CHA zeolites was the weakest, which might allow a stronger interaction with CO2. The H-zeolites gave CO2/CO selectivity factors that were in the range of 4.61–11.0, depending on the framework topology.