Alkyltrimethoxysilanes with different chain lengths (trimethoxypropylsilane, trimethoxyoctylsilane, and dodecyltrimethoxysilane) were utilized as mesopore-generating agents to synthesize hierarchical ZSM-5 samples with different amounts of mesoporous volume. The samples were characterized by X-ray powder diffraction, nitrogen adsorption–desorption, scanning electron microscopy, transmission electron microscopy, and CO2-temperature-programmed desorption. With the growth of chain length, the alkyl­trimethoxy­silanes showed low reactivity and affinity for the surfaces of zeolite precursors due to the increase of hydrophobic character of the alkyl moiety, which resulted in the decrease of mesoporous volume. CO2 adsorption behaviors of the samples including adsorption capacity, adsorption kinetics, adsorption selectivity, adsorption thermodynamics, and adsorbent stability were studied. The experimental results indicated that hierarchical ZSM-5 modified by trimethoxy­propyl­silane exhibited the highest mesopores volume (0.12 cm3·g–1), corresponding to the fastest capture rate (about 2.5 times of conventional ZSM-5) and the highest capture capacity (2.15 mmol·g–1 at 25 °C and 100 kPa). Therefore, hierarchical ZSM-5 synthesized by the alkyl­trimethoxy­silanes with short chain length can generate extra mesopores and active adsorption sites, which provided a new strategy to regulate the structure of ZSM-5 for rapid CO2 adsorption.

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The adsorbent showed a high capacity for CO2 with 3.60 mmol g−1 under flue gas conditions over 90 consecutive adsorption cycles.