Removal of diclofenac by adsorption process studied in free-base porphyrin Zr-metal organic frameworks (Zr-MOFs)

As the world population continues to grow, there is also a rising concern regarding water pollution since this condition could negatively impact the supply of clean water. One of the most recent concerns is related to the pollution that comes from various pharmaceuticals, in particular non-steroidal anti-inflammatory drugs (NSAIDs) since they have been industrially produced at large scale and can be easily purchased as an over-the-counter medicine. Diclofenac is one of the most popular NSAIDs because of its high-effectiveness, which leads to its excessive consumption. Consequently, its presence in water bodies is also continuously increasing. An adsorption process could then be employed as a highly effective method to address this issue. In comparison to other conventional adsorbents such as activated carbon, the use of metal–organic frameworks (MOFs) as an alternative adsorbent is very attractive since it can offer various advantages such as tailorability and high adsorption capacity. In this study, the performance of three water-stable, free-base porphyrin MOFs assembled using zirconia-based nodes, namely MOF-525, MOF-545, and NU-902, for diclofenac adsorption was thoroughly investigated. Interestingly, although all three free-base porphyrin MOFs are assembled using the same building block and have a similar specific surface area (based on the experimental argon physisorption and calculation based on non-localized density functional theory), their diclofenac adsorption capacity is substantially different from one another. It is found that the highest diclofenac adsorption capacity is shown by MOF-525, which has maximum capacity around 792 mg g$^{−1}$. This is then followed by MOF-545 and NU-902 that have adsorption capacities around 591 and 486 mg g$^{−1}$, respectively. Some possible adsorption mechanisms are then thoroughly discussed that might contribute to this phenomenon. Lastly, their performance is also compared with other MOFs that are also studied for this purpose to show their performance superiority not only in terms of adsorption capacity but also their affinity towards the diclofenac molecule, which might be useful as an adsorption performance indicator in the real condition where the contaminant concentration is considerably low

Hierarchical assemblies of molecular frameworks—MOF-on-MOF epitaxial heterostructures

Functional, porous metal-organic frameworks (MOFs) have attracted much attention as a very flexible class of crystalline, porous materials. For more advanced applications that exploit photophysical properties, the fabrication of hierarchical assemblies, including the creation of MOF/MOF heterointerfaces, is important. For the manufacturing of superstructures with length scales well beyond that of the MOF pore size, layer-by-layer (lbl) methods are particularly attractive. These allow the isoreticular approach to be extended to superstructures with micrometer length scales, a range that is not accessible using conventional MOF design. The lbl approach further substantially extends the compositional diversity in MOFs. At the same time, the favorable elastic properties of MOFs allow for heteroepitaxial growth, even in the case of lattice misfits as large as 20%. While the MOF-on-MOF approach to designing multicomponent superstructures with synergistic multifunctionality can also be realized with sophisticated solvothermal synthesis schemes, the lbl (or liquid-phase epitaxy) approach carries substantial advantages, in particular when it comes to the integration of such MOF superstructures into optical or electronic devices. While the structure vertical to the substrate can be adjusted using the lbl method, photolithographic methods can be used for lateral structuring. In this review, we will discuss the lbl liquid-phase epitaxy approach to growing surface-anchored MOF thins films (SURMOFs) as well as other relevant one-pot synthesis methods for constructing such hierarchically designed structures and their emerging applications

Chemical Reactions at Isolated Single-Sites Inside Metal–Organic Frameworks

Isolated, coordinatively unsaturated metal sites within metal–organic framework (MOF) materials feature interesting chemical properties and offer applications as single-site catalysts. Here, we report on the recent progress in providing fundamental insight into chemical reactions occurring inside MOFs. In addition to the common form, powders, we discuss the potential of MOF thin films (SURMOFs). The combined spectroscopic and modeling approach applied to selected systems demonstrated that the catalytic activity of MOFs can be precisely tuned. A particular interesting case is the so-called defect-engineering, where structural imperfections are created in a controlled fashion. The chemical properties of MOF materials can be further modified by integration and decoration of linkers or loading with guest species, such as metal or metal–oxide nanoparticles or nanoclusters. A particularly interesting aspect of layer-by-layer approaches for the fabrication of MOF thin films is the prospect to realize tandem catalysts

Electrostatic design of polar metal–organic framework thin films

In recent years, optical and electronic properties of metal–organic frameworks (MOFs) have increasingly shifted into the focus of interest of the scientific community. Here, we discuss a strategy for conveniently tuning these properties through electrostatic design. More specifically, based on quantum-mechanical simulations, we suggest an approach for creating a gradient of the electrostatic potential within a MOF thin film, exploiting collective electrostatic effects. With a suitable orientation of polar apical linkers, the resulting non-centrosymmetric packing results in an energy staircase of the frontier electronic states reminiscent of the situation in a pin-photodiode. The observed one dimensional gradient of the electrostatic potential causes a closure of the global energy gap and also shifts core-level energies by an amount equaling the size of the original band gap. The realization of such assemblies could be based on so-called pillared layer MOFs fabricated in an oriented fashion on a solid substrate employing layer by layer growth techniques. In this context, the simulations provide guidelines regarding the design of the polar apical linker molecules that would allow the realization of MOF thin films with the (vast majority of the) molecular dipole moments pointing in the same direction

Metal‐Organic Framework Thin Films Grown on Functionalized Graphene as Solid‐State Ion‐Gated FETs

The unique properties of 2D-materials like graphene are exploited in various electronic devices. In sensor applications, graphene shows a very high sensitivity, but only a low specificity. This shortcoming can be mastered by using heterostructures, where graphene is combined with materials exhibiting high analyte selectivities. Herein, this study demonstrates the precise deposition of nanoporous metal-organic frameworks (MOFs) on graphene, yielding bilayers with excellent specificity while the sensitivity remains large. The key for the successful layer-by-layer deposition of the MOF films (SURMOFs) is the use of planar polyaromatic anchors. Then, the MOF pores are loaded with ionic liquid (IL). For functioning sensor devices, the IL@MOF films are grown on graphene field-effect transistors (GFETs). Adding a top-gate electrode yields an ion-gated GFET. Analysis of the transistor characteristics reveals a clear Dirac point at low gate voltages, good on-off ratios, and decent charge mobilities and densities in the graphene channel. The GFET-sensor reveals a strong and selective response. Compared to other ion-gated-FET devices, the IL@MOF material is relatively hard, allowing the manufacturing of ultrathin devices. The new MOF-anchoring strategy offers a novel approach generally applicable for the functionalization of 2D-materials, where MOF/2D-material hetero-bilayers carry a huge potential for a wide variety of applications

IR spectroscopy applied to metal oxide surfaces: adsorbate vibrations and beyond

Metal oxides are among the technologically most important materials. Almost all metals are covered by one oxide layer under ambient conditions. The characterization of oxide surface properties, therefore, is still attracting an increasing amount of attention in surface science. A challenge is provided by the fact that these materials cannot be studied with standard techniques in a straightforward fashion. In this review, we summarize recent results obtained by IR-spectroscopy applied in a reflection-geometry to macroscopic oxide monocrystals. These results provide new insights in the adsorption and subsequent reactions and photoreactions of molecules on these highly interesting, very complex class of materials. In addition, the IR-spectroscopy can also be used to probe photophysical properties, e.g. the generation and decay of electron or hole polarons