Molecular insight into the structure of heterometallic metal-organic frameworks MIL-53-M (M = Al and Ga) and their intermolecular interaction with pyridine: A periodic density functional theory

Heterometallic metal-organic frameworks (MOFs) as part of functional materials possess interesting functionalities which may benefits for many potential applications. In this study, quantum mechanical approach was used to investigate the effect of multi metal ion nodes of Al3+ and Ga3+ in a famous breathing MOF of MIL-53. The ratio of Al3+ to Ga3+ was modulated to give bimetallic MIL-53(Al/Ga). The presence of Al3+ and Ga3+ in MIL-53(Al/Ga) induces the structural changes as evidence from the calculated unit cell parameters, where increasing the concentration of Al3+ generally reduces the size of the unit cell dimension. The electronic properties have been identified to give small fine tuning of the band gap energy where the presence of Al3+ shows a dominant factor to shift the edge of the conductance band according to the density of states plots. The band gap energy of MIL-53(Ga) was calculated to be 3.47 eV and it decreases when Ga3+ was substituted by Al3+ to give band gap energy of 3.33 eV for MIL-53(Al). Therefore, the bandgap energy can be fine-tuned by modulating the ratio of Al3+ to Ga3+ in bimetallic MIL-53(Al/Ga). The nature of the bonding within various MIL-53(Al/Ga) was also identified using electron localization maps indicating a possible ionic characteristic for metal ions to ligand interaction. In addition, the adsorption of pyridine within MIL-53(Al/Ga) provides an insight into the Bronsted acidity of MIL-53(Al/Ga) due to the presence of bridging -µOH. In this case, adsorbed pyridine in MIL-53-2Al2Ga shows the lowest adsorption energy compared to the other configurations indicating a stronger Bronsted acidity of the -µOH site. This is also in correlation with the more positively charge of pyridine molecules inside MIL-53-2Al2Ga due to the hydrogen bonding interaction with hydrogen of -µOH. This study gives an insight into the functional material design in term of the electronic properties as well as the behavior towards guest molecules.