Effect of Liquid-phase Silane Modification on Competitive Adsorption Behavior of NO_x and Water Vapor on ZSM-5 Zeolite

To address the critical issue of rapid degradation in the denitrification performance of molecular sieves caused by competitive adsorption of water vapor in the high-humidity, high-concentration NO_x exhaust gas generated during the reprocessing of spent nuclear fuel, in this study ZSM-5 zeolite was selected as the substrate and modified to be hydrophobic using a liquid-phase grafting method. Initially, six typical zeolites (ZSM-5, ZSM-22, MOR, SSZ-13, MCM-22, and β) were evaluated under 90% relative humidity. Despite SSZ-13 and β showing higher equilibrium capacities, ZSM-5 exhibits the steepest breakthrough curve, indicating the shortest mass transfer zone and lowest diffusion resistance. Furthermore, ZSM-5 demonstrates excellent cyclic stability with nearly 100% capacity retention after six adsorption-desorption cycles, and the lowest NO_x desorption peak temperature (270 ℃), making it the optimal substrate for subsequent hydrophobic modification. Subsequently, the ZSM-5 was modified using five different silanes: TMCS, OTS, HMDS, PTMS, and BTS. Among these, butyltrichlorosilane (BTS) modification achieves the best balance between hydrophobicity and NO_x adsorption capacity. Through dual-component NO_x/water vapor breakthrough experiments and temperature-programmed desorption (TPD) tests, the adsorption separation performance, micro-mechanisms, and influence of carbon chain length on adsorption properties were systematically investigated. The results indicate that after BTS modification, the static water vapor adsorption capacity of ZSM-5 at atmospheric pressure decreases from 7.28 mmol/g to 5.22 mmol/g, a reduction of 28.3%. During high-humidity NO_x adsorption breakthrough, the roll-up peak concentration ratio significantly decreases from 2.84 to 1.08, and the amount of NO_x displaced by water vapor drops from 6.0 μmol/g to 0.5 μmol/g, demonstrating a substantial improvement in hydrophobicity. However, due to the steric hindrance introduced by surface-grafted silane groups, partial pore blockage occurs, and the catalytic oxidation of NO to NO₂ is inhibited. Consequently, the NO_x adsorption capacity of the modified sample slightly decreases from 39 μmol/g to 34 μmol/g, and the desorption product shifts from NO₂ to NO. Despite this reduction in capacity, the modified material significantly lowers the NO_x desorption temperature, with the peak temperature decreasing from 270 ℃ to 85 ℃. Furthermore, within the carbon chain length range of C₁ to C₄, the hydrophobic gain brought by longer carbon chains gradually outweighs the steric hindrance loss, thereby improving the overall adsorption performance. This study provides new insights and a solid theoretical foundation for the development of low-temperature regenerative hydrophobic adsorbents suitable for the advanced purification of high-humidity NO_x exhaust gases from nuclear facilities, offering a promising strategy to balance water resistance and adsorption capacity.