Core-shell material based on ion-exchange-assisted growth of ZIFs on chamfered-edged zeolite crystals for N2/CO₂ adsorption: Modeling and mechanism
Cyrille Ghislain Fotsop, Alexandra Lieb, Franziska Scheffler
Abstract
The hybrid core-shell materials Zeo-4A/ZIF-8, Zeo-4A@ZIF-8, Zeo-4A/ZIF-67, Zeo-4A@ZIF-67, ZIF-8 and ZIF-67 were synthesized from kaolin and Zeolitic imidazolate frameworks (ZIFs) for N 2 and CO 2 adsorption followed by isotherm modelling and error function analysis. The growth of ZIF-8 and ZIF-67 on LTA zeolite (Zeo-4A) crystals was assisted by ion exchange and solvothermal methods. X-ray diffraction (XRD) patterns showed the simultaneous co-existence of LTA zeolite, ZIF-8 and ZIF-67 diffraction peaks on the hybrid core-shell materials. Field emission scanning electron microscopy (FE-SEM) images showed a cubic morphology and the presence of core-shell structure due to the coordination of Zn-Im-Zn and Co-Im-Co. N 2 sorption showed the presence of micro- and mesopores. 29 Si and 27 Al solid-state magic angle spinning (MAS)-nuclear magnetic resonance (NMR) showed the formation of Q 2 (2Al) and tetrahedral aluminum groups AlO 4 . The CO 2 adsorption capacities obtained were 3.4, 1.3, 1.5, 1.8, 1.7, 2.4 and 1.6 mmol/g at 0°C and the average Q st values were 58.23, 54. 26, 50.39, 34.24, 23.15, 21.29 and 19.62 kJ.mol -1 for Zeo-4A-Na, ZIF-8, ZIF-67, Zeo-4A/ZIF-8, Zeo-4A/ZIF-67, Zeo-4A@ZIF-8 and Zeo-4A@ZIF-67, respectively. The DSLF model showed a good fit with R 2 > 0.999, relatively low χ 2 (1.81E -4 -4.91E -4 ) and HYBRID error (8.55E -5 and 1.86E -4 ). Langmuir's isotherm revealed continuous coverage of the pore surface. The main adsorption mechanisms were physical interactions, such as Van der Waals forces and polarity interactions, and chemical interactions. XRD analysis after CO 2 adsorption showed no significant changes. CO 2 adsorption on zeolites preferentially binds to framework oxygen’s and Na + ions in the LTA zeolite structure, the results highlight the effectiveness of Zeo-4A@ZIF-8 and Zeo-4A@ZIF-67 as suitable core-shell materials for the adsorption and separation of N 2 /CO 2 . • The ex situ ion-exchange-assisted hydrothermal method was found to be suitable for growing ZIF-8 and ZIF-67 on LTA zeolite (Zeo-4A) crystals. • SEM images revealed cubic morphology and a core-shell/satellite structure due to Zn-Im-Zn and Co-Im-Co coordination. • NMR-MAS revealed the formation of Q2(2Al) and tetrahedral aluminum groups (AlO₄). • The hybrid core-shell materials Zeo-4A/ZIF-8, Zeo-4A@ZIF-8, Zeo-4A/ZIF-67, Zeo-4A@ZIF-67, ZIF-8, and ZIF-67 were found to be suitable for N₂ and CO₂ adsorption.