The highly odd-numbered 15-connected nonanudear [Ln(9)(mu(3)-O)(2)(mu(3)-OH)(12)(O2C-)(12)(HCO2)(3)] and 9-connected trinuclear [Ln(3)(mu(3)-O)(O2C-)(6)(HCO2)(3)] lanthanide-carboxylate clusters with triangular and linear carboxylate bridging ligands were synergistically combined into Ln-MOFs, [(CH3)(2)NH2](3) {[Ln(9)(mu(3)-O)(2)(mu(3)-OH)(12)(H2O)(6)] [Ln(3)(mu(3)-O) (H2O)(3)] (HCO2)(3-) (BTB)(6)}center dot(solvent)(x) (abbreviated as JXNU-3, Ln = Gd, Tb, Er; BTB3- = benzene-1,3,5-tris(4-benzoate)), displaying a (3,9,15)-connected topological net. The JXNU-3(Tb) exhibits highly selective CO2 adsorption capacity over CH4 that resulted from the high localized charge density induced by the presence of the nonanudear and trinudear cluster units. In addition, JXNU-3(Tb) with high chemical stability and characteristic bright green color exhibits fluorescent pH sensing, which is pertinent to the different protonation levels of the carboxylate groups of the benzene-1,3,5-tris(4-benzoate) ligand with varying pH.