A metal–organic framework with rich accessible nitrogen sites for rapid dye adsorption and highly efficient dehydrogenation of formic acid

Abstract

MOFs with adequate free nitrogen sites have potential applications in dye adsorption and formic acid dehydrogenation. Here, we successfully synthesized a novel 3-D MOF 1 ([(CH₃)₂NH₂][Cd(L)DMA]·0.5DMA·1.5H₂O) with a special two-fold interpenetrating framework through a simple solvothermal reaction between CdCl₂·1.5H₂O and a nitrogen-rich triangular tricarboxylate-based linker (H₃L, 4,4′,4′′-s-triazine-2,4,6-tribenzoic acid). After removing the guest molecules of dimethylacetamide (DMA) and H₂O, including the coordinated DMA from 1 by vacuum activation at 423 K, a compound named 1′ with a formula of [(CH₃)₂NH₂][Cd(L)] and a similar interpenetrating framework structure was obtained. In comparison with compound 1, the total void volume of 1′ is nearly doubled, and thus may provide higher potential for the adsorption of other guest molecules. Notably, the pyridine N atoms located in the middle of the triangular tricarboxylate-based linker are not involved in the coordination with Cd²⁺, and are all uniformly dispersed throughout the whole framework of the 3-D MOFs. Due to its unique structural features, the 3-D MOF 1′ could effectively adsorb the cationic dye MB⁺ for recycling purposes. The rapid adsorption rate (0.7 × 10⁻² g mg⁻¹ min⁻¹) and the relatively high capacity (900 mg g⁻¹) for MB⁺ demonstrate the potential of 1′ in dye adsorption. In addition, 1′ may also be used as an effective support to immobilize PdAu NPs via the double-solvent method. The resultant catalyst Pd_0.8Au_0.2/1′ exhibits decent catalytic activity for the dehydrogenation of formic acid with a TOF value of 1854 h⁻¹ at 333 K. The existence of a large void volume and accessible pyridine N atoms provide a suitable environment for achieving a high dispersion of PdAu NPs, thereby leading to the formation of a catalytically active and stable supported noble-metal NP catalyst for H₂ generation from formic acid decomposition.