Concurrent first- and second-order photodegradation of azo dyes using TMU-16 pillared-layer microporous metal organic framework under visible light

One of the important research fields is the efficient elimination of pollutants from wastewater due to its ecological and environmental importance. Metal organic frameworks (MOFs) have emerged as a significant class of porous crystalline materials that various studies have shown the promising applications of MOFs in catalytic and photocatalytic areas. In this work a mesoporous MOF known as TMU-16, [Zn2(BDC)2(4-bpdh)].3DMF (BDC ​= ​1,4-benzendicarboxylate, 4-bpdh: 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene, DMF: N,N′-dimethylformamide), was employed as a heterogeneous catalyst to photodegradation of azo dyes from aqueous solution under sunlight. Complete degradation occurred just by 40 ​mg of TMU-16 for solution of each dyes with 20 ​ppm concentration without auxiliary oxidants such as H2O2. In the case of Congo red 96% degradation occurred after 1 ​h. Eriochrome Black T and Methyl orange after 3 ​h and 5 ​h show 97% and 82% degradation, respectively. The kinetic of dye degradation follows the first- and second-order models. For Eriochrome Black T, the rate is completely second-order, while for Congo red and methyl orange, as long as the dye concentration is high, it is a second-order, but with decreasing concentration, the rate model deviates from the second-order and fits by first-order model. Using 2-hydroxyterephthalic acid as a fluorescent probe confirmed the reaction mechanism is performed by mediated hydroxyl radicals. Excellent reusability and high degradation efficiency recommended that, TMU-16 could be efficient visible heterogeneous photocatalyst for degradation of other organic pollutants.