Solvent-regulated pore structure of NH2-MIL-125(Ti) and its CO2 capture performance in cement surface coatings

This study prepared a series of NH2-MIL-125(Ti) samples (denoted as NMT-x%) by tuning the methanol fraction (20%, 30%, 40%, and 50%) in a mixed methanol/N,N-dimethylformamide (MeOH/DMF) solvent system, and investigated their pore structure and morphological evolution as well as the pore structure-performance relationship in cement-based CO2 capture coatings. An anhydrous organic-solvent system with trace silane was employed to construct NMT coatings on cement surfaces. The crystal structure, particle morphology, and pore characteristics of NMT were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM), combined with quantitative pore analysis based on BET surface area, t-plot micropore volume/micropore volume fraction, and DFT pore size distribution. Single-component adsorption isotherms of CO2, CH4, and H2O were further measured at 298 K, and the multicomponent partitioning trends of different samples under ideal equilibrium conditions were analyzed using Langmuir-Freundlich fitting combined with ideal adsorbed solution theory (IAST). On this basis, the service-related performance of the coatings was evaluated by dry-state CO2 uptake, humidity tolerance factor (HTF), and retention behavior over three adsorption-desorption cycles. The results showed that variation in the MeOH/DMF ratio did not alter the fundamental framework structure or the main pore size of NMT, but significantly regulated crystal packing and pore accessibility. NMT-20% exhibited the highest specific surface area, micropore volume, and CO2 uptake, whereas NMT-50% showed markedly reduced pore-structure parameters, indicating that agglomeration and densification under high-MeOH conditions weakened accessible micropore volume. All samples adsorbed substantially more H2O than CO2 and CH4, indicating that competitive water adsorption would strongly affect their service performance. Henry constant analysis and IAST results showed that CO2 consistently exhibited higher low-pressure affinity than CH4, while the preferential occupation of polar sites and micropores by H2O in humid systems significantly weakened the relative CO2/CH4 partitioning advantage. Coating tests further demonstrated that the structural differences formed at the powder stage could be partially retained on cement surfaces and translated into distinct CO2 capture behavior. Among them, NMT-20% delivered the highest dry-state CO2 capture gain, NMT-50% exhibited relatively higher humidity tolerance, and NMT-30% showed better overall engineering suitability when dry capture capacity, humid-state response, and short-term cyclic retention were considered together. Further characterization indicated that coating composite and short-term alkaline/humid exposure mainly altered the effective utilization of pore structure rather than framework integrity. Overall, solvent regulation influenced the performance differentiation of NMT from powder to cement-based coatings by affecting crystal growth, particle packing, and the practical utilization of pore structure.