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

This study prepared a series of NH 2 -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 CO 2 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 CO 2 , CH 4 , and H 2 O 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 CO 2 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 CO 2 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 H 2 O than CO 2 and CH 4 , indicating that competitive water adsorption would strongly affect their service performance. Henry constant analysis and IAST results showed that CO 2 consistently exhibited higher low-pressure affinity than CH 4 , while the preferential occupation of polar sites and micropores by H 2 O in humid systems significantly weakened the relative CO 2 /CH 4 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 CO 2 capture behavior. Among them, NMT-20% delivered the highest dry-state CO 2 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.