Ligand Geometry-Driven Structural and Morphological Evolution in Erbium-Based Metal-Organic Frameworks

Erbium-based metal-organic frameworks (Er-MOFs) were synthesized via a solvothermal route using three structural isomers of benzene dicarboxylic acid-phthalic acid (PA), isophthalic acid (IPA), and terephthalic acid (TPA) to systematically investigate the role of ligand geometry on framework formation, crystallinity, and morphology under identical synthesis conditions. The materials were characterized by powder X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), particle size analysis (PSA), and nitrogen adsorption-desorption measurements (BET and BJH). XRD analysis revealed a clear crystallinity trend of Er-TPA > Er-IPA > Er-PA, attributed to increasing ligand symmetry from ortho to para substitution, which promotes enhanced long-range ordering. FTIR spectra confirmed successful coordination between Er3+ ions and carboxylate groups through characteristic symmetric and asymmetric −COO− stretching vibrations. SEM images demonstrated pronounced ligand-dependent morphological evolution, with Er-PA forming irregular plate-like aggregates, Er-IPA exhibiting layered microcrystals, and Er-TPA producing relatively uniform spherical crystallites. PSA indicated right-skewed particle size distributions with mean diameters in the submicron to low-micron range. Nitrogen sorption studies displayed Type-IV isotherms with H3/H4 hysteresis loops, revealing mesoporosity primarily arising from interparticle voids with dominant pore sizes in the 10–100 nm range. This comparative study highlights ligand geometry as a key parameter governing nucleation behavior, crystallinity, morphology, and textural properties in Er-based MOFs, providing valuable guidance for the rational design of rare-earth MOFs with tailored structural characteristics.