Tailored nanoporous composite of a bimetallic Zn-based MOF featuring two rare-metal geometries for organic dye sequestration: a mechanistic and kinetic insight into wastewater remediation

In the relentless pursuit of next-generation materials for sustainable water purification, we report the synthesis of a novel metal organic framework i.e., {(Me₂NH₂)₂[Zn₃(tpa)₄](DMF·3H₂O)}_n (ZS-4) synthesized using terephthalic acid (H₂TPA). This framework elegantly integrates two crystallographically distinct zinc coordination environments (tetrahedral and octahedral), leading to an unusual and noteworthy outcome of (3,6)-connected bcg and bct topologies. To further enhance its functional attributes, a hybrid composite, ZS-4@CNT, was engineered by integrating carbon nanotubes (CNTs) into the pristine ZS-4 matrix, resulting in notable improvements in physicochemical performance. Characterization was performed using FTIR, TGA, PXRD, SEM, TEM, BET, and XPS which substantiated the structural integrity, thermal robustness, morphological uniformity, and surface chemistry of both materials. BET analysis revealed surface areas of 923.49 m² g⁻¹ for ZS-4 and 1200.30 m² g⁻¹ for ZS-4@CNT. The BJH pore size distribution displayed narrow peaks at 5.2 nm and 5.5 nm, respectively, confirming enhanced mesoporosity in the composite. XPS data revealed peaks at 284.6 eV (C–C/CC), 286.0 eV (C–O/C–OH), and 288.5 eV (O–CO) in the C 1s spectrum. The O 1s region showed peaks at 531.8 eV (CO) and 533.0 eV (C–OH). Zn 2p peaks at 1022.8 eV and 1042.7 eV confirmed the presence of Zn²⁺. The ZS-4@CNT composite demonstrated a remarkable dye removal performance within the first 20 minutes, achieving 66% removal efficiency for methylene blue (MB). In comparison, under the same pH and temperature conditions, the removal efficiency for methyl orange (MO) was notably lower at 36.3%. The adsorption capacities were evaluated using MB and MO as model pollutants, where ZS-4 demonstrated uptakes of 72.35 mg g⁻¹ for MB and 42.23 mg g⁻¹ for MO, while ZS-4@CNT exhibited significant adsorption capacities of about 114.45 mg g⁻¹ for MB and 43.15 mg g⁻¹ for MO. Kinetic modeling adhered to a pseudo-second order framework, indicating chemisorption as the prevailing mechanism.