CO₂ capture using low silica X zeolite synthesized from low-grade coal gangue via a two-step activation method

The capture of carbon dioxide (CO₂) has attracted significant attention to reduce the emissions of greenhouse gases. However, the development of adsorptive materials remains to be a grand challenge. In this work, we successfully synthesized low silica X (LSX) zeolites from low-grade coal gangue (mass ratio of Al₂O₃/SiO₂ < 0.50), a major byproduct in the coal mining industry, without additional Si or Al source via a two-step activation method (TA). The LSX zeolite had a Si/Al mole ratio of 1.05 and a specific surface area of 634 m²/g. Synthesis conditions including calcination temperature, alkali fusion temperature, potassium hydroxide dosage and crystallization time were systematically investigated to understand the reaction mechanism and optimize the synthesis conditions. Results showed that quartz was the main impurity remained in the product and thermal activation at 1200 °C was beneficial to decompose quartz and suppress the formation of sodalite impurity. Zeolite A could be eliminated by optimizing potassium hydroxide dosage and crystallization time. Compared to the conventional alkali fusion method and calcination activation method, the zeolites synthesized via TA method had better crystallinity and higher purity. The LSX product showed a high CO₂ uptake of 4.99 mmol/g at 298 K and 1 bar. The ideal adsorption solution theory (IAST) selectivity of CO₂/N₂ (15/85) and CO₂/CH₄ (50/50) were 376 and 200 at 298 K and 1 bar, respectively. The separation performance was further demonstrated using Aspen Adsorption and results showed a high purity and recovery of 92.6% and 91.3%, respectively, while using 50% CO₂ in N₂ as the feed gas at the adsorption pressure of 211 kPa and desorption pressure of 22 kPa.