Synthesis of Template-Free ZSM-5 from Rice Husk Ash at Low Temperatures and Its CO₂ Adsorption Performance

In this paper, a green synthesis method for ZSM-5 zeolite is explored to reduce the synthesis cost, environmental hazard, and reaction temperatures. For the ZSM-5 samples prepared at low temperatures, the influence of factors such as the hydrothermal temperature, crystallization time, and the number of seeds is systematically investigated. The adsorption isotherm of CO₂ is used for fitting analysis of adsorption models and determination of the adsorption selectivity. The results show that the best one among the three samples presents the highest CO₂ adsorption capacity of 2.39 mmol/g at 273 K and 15 bar. It is prepared with a hydrothermal temperature of 393 K, crystallization time of 7 days, and a seed crystal of 1 wt %. The dual-site Langmuir model can well describe the experimental data, indicating that double adsorption sites rather than the simple single-layer adsorption are dominant in samples. As the pressure increases, the adsorption capacity calculated by the model is much lower than the actual value with a deviation index of 12.5%. At a pressure of 1 bar, the optimal selectivity is attained with sample L-20, viz., CO₂/N₂ of 34.3 and CO₂/O₂ of 70.2. The green synthesis method reported in this research can be used to successfully prepare ZSM-5 zeolite, and it shows excellent CO₂ adsorption performance. In addition, the use of low-cost raw materials and template-free synthesis methods will facilitate the large-scale application of green synthesis processes in the future.