Abstract
In this study, we conduct a density functional theory investigation to study the mechanical stability of a titanium-based metal organic framework (MOF-901), which was hypothetically assumed to possess 2D characteristics. It is systematically found that the encapsulation of methanol enhances the mechanical stability of MOF-901 as the elastic tensors Cij of MOF-901∙nMeOH are higher than the corresponding Cij quantities reported for solvent-free MOF-901. Moreover, the 2D characteristics of MOF-901 is confirmed by verifying the negative values of C33. At the same time, the band gap of MOF-901 is observed to be solvent-dependent. In its pure form, MOF-901 possesses a direct gap (Eg) of 2.07 eV, with the valence and conduction bands mainly constituted by electrons of 4-aminobenzoate linkers. Introducing methanol into MOF-901 causes distortion to the 4-aminobenzoate geometry, thereby induces electronic degeneracy to the conduction bands. Consequently, Eg is narrowed to 1.84 eV with 5.7 wt% MeOH or 1.63 eV with 11.4 wt% MeOH. Hence, it is possible to tailor the band gap of MOF-901 by controlling methanol guest, which only acquires van der Waals interaction to the framework. In addition, our theoretical prediction shows a Ti(IV) site can undergo electronic hopping to become Ti(III) under the effect of visible light (~440–443 nm). Then, Ti(III) is capable of breaking the C-Br bond in ethyl α-bromophenylacetate spontaneously, which in turn activates the polymerization of methyl methacrylate with an energy barrier of 0.30 eV.
Highlights
For those purposes, MOFs based on tetravalent metal–oxo building units (i.e. Ti(IV), Zr(IV), and Hf(IV)) have been especially realized as promising candidates due to its high stability under harsh-working conditions[9]
MOFs based on so-mentioned secondary building units (SBU) possess suitable band gaps for photocatalytic transformation under UV-vis irradiation which can be tailored due to the nature of MOFs structures
The modification of electronic structures and band gaps of MOFs were further demonstrated by Pham et al11. by employing state-of-the-art density functional theory[12,13,14] (DFT) calculations to investigate the effect of functional groups to the isoreticular structures of MOF-5
Summary
MOFs based on tetravalent metal–oxo building units (i.e. Ti(IV), Zr(IV), and Hf(IV)) have been especially realized as promising candidates due to its high stability under harsh-working conditions[9]. We utilize molecular modeling through DFT calculations to investigate the mechanical stability of MOF-901 and present a novel approach to tailor the highest-occupied molecular orbital (HOMO)-lowest-unoccupied molecular orbital (LUMO) gap energy of this material by controlling guest molecules in the pores. We attempt to resolve such a hypothesis by investigating the contribution of guest molecules on the elastic tensors of the framework. In another aspect, we seek for an energetic profile of mechanism activation under visible-light irradiation of a polymerization reaction promoted by MOF-901 in the presence of ethyl α-bromophenylacetate co-initiator, which has been mentioned in the previous experimental report[15]
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