Synthesis of zeolite NaX based on fly ash: Mechanism of metal impurity retention and utilisation to promote CO2 capture efficiency.

The steel industry urgently requires cost-effective and efficient adsorbents for carbon capture from blast furnace gas. This study explored the use of fly ash to synthesize zeolite NaX (ZZ-NaX) through an impurity retention strategy, with the aim of enhancing CO2 adsorption and selectivity. The results indicate that major metal impurities (Fe, Ca, and Ti) successfully integrate into the counter cation sites of zeolite NaX, with Fe3+ and Ti4+ preferentially occupying the I/I' and III/III' positions. Their high charge density significantly strengthens the electrostatic field within the pores. Ca2+, which preferentially occupies the II/II' positions, serves as both an additional basic site and a modifier of the Brønsted acid sites, increasing the zeolite's alkaline sites through neutralisation. Compared to hydrothermally synthesized zeolite NaX, ZZ-NaX zeolite exhibits a significantly increased specific surface area of 784.94 m2/g, a higher micropore proportion of 83.58%, and a moderate increase in average pore size to 1.78 nm. At standard temperature and pressure, ZZ-NaX zeolite demonstrates a dynamic CO2 adsorption capacity of 4.63 mmol/g, outperforming acid-leached zeolite. In the CO2/CO/N2 atmosphere of blast furnace gas, ZZ-NaX zeolite preferentially adsorbs CO2 due to its strong electrostatic field and abundant alkaline sites. Under actual blast furnace gas pressure of 3 bar, the selectivity coefficient for CO2/CO reaches 103.82, with no adsorption of N2. Therefore, fly ash-based zeolites synthesized via metal impurity retention offer a promising solution for the efficient recycling of fly ash and provide a low-cost, high-performance adsorbent material for carbon capture in the steel industry.