Insights to the hydrothermal synthesis of highly crystalline aluminum-free Na[Co]ZSM-5 zeolites and their CO2 adsorption performance

Abstract

In this work, MFI-type cobaltosilicates as inorganic polymers consisting of tetrahedral TO4 (T = Si and Co) building units were synthesized by utilization of a novel pretreatment method aimed for the silicon precursor activation. Crystallization of zeolites was carried out using a conventional hydrothermal method. The effect of various parameters such as Co/Si molar ratio, crystallization time and pH of initial gel on the crystallization process was studied to provide the optimal conditions for crystallization. The prepared zeolites were characterized using XRD, FT-IR, UV–vis, SEM, TEM, N2 adsorption, and TGA techniques. The XRD patterns confirmed the formation of pure crystalline ZSM-5 zeolites without any cobalt oxide impurities. A small shift towards lower diffraction angles in the XRD patterns of the samples was detected by increasing cobalt content in the initial gel, which validated the incorporation of Co2+ ions into the zeolites’ framework. The presence of Co2+ ions in the tetrahedral positions of zeolites’ framework was also proved by FT-IR and UV–vis spectra. EDX-mapping images exhibited a more uniform dispersion of Co2+ ions in the structure of ZSM-5 zeolite synthesized at pH = 9.5 compared to its analogous counterpart synthesized at higher pH. The results of N2 adsorption analysis revealed that the specific surface area of Na[Co]ZSM-5 zeolites was comparable to aluminosilicate ZSM-5 zeolites. The synthesized zeolites were examined for CO2 adsorption. The adsorption capacities measured at 298 K and 1 bar were approximately 2.19 and 5.33 mmol g−1 for samples prepared at pH = 9.5 and 10.5, respectively.