Abstract Linker functionalization is a common route used to affect the electronic and catalytic properties of metal-organic frameworks. By either pre- or post-synthetically installing linkages with differing linker moieties the band gap, workfunction, and exciton lifetimes have been shown to be affected. One overlooked aspect of linker functionalization, however, has been the impact on the metal d -orbital energies to which they are bound. The ligand field differences should result in substantial changes in d -splitting. In this study we use density functional theory (DFT) to study the energetics of d -orbital energy tuning as a function of linker chemistry. We offer a general descriptor, linker pK a , as a tool to predict resultant band energies in metal-organic frameworks (MOFs). Our calculations reveal that simple functionalizations can affect the band energies, of primarily metal d lineage, by up to 2 eV and illustrate the significance of this band modularity using four archetypal MOFs: UiO-66, MIL-125, ZIF-8, and MOF-5. Together, we show that linker functionalization dramatically affects d -energies in MOF clusters and highlight that linker functionalization is a useful route for fine-tuning band edges centered on the metals, rather than linkers themselves.
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Electronic effects due to organic linker-metal surface interactions: implications on screening of MOF-encapsulated catalysts
MOF-encapsulated nanoparticles (NP@MOFs) are hybrid, heterogeneous catalysts, where the MOF could boost the activity and selectivity of the encapsulated NP for the reaction of choice by controlling reactant orientation. However, due to overwhelming combinatorics, methods to rapidly identify promising NP + MOF combinations for a given application are needed. Earlier work used a “surrogate” inert pore on top of NP-representative surfaces to generically capture MOF steric effects, hence enabling computational screening to focus on NP composition. However, the surrogate pore method neglects electronic effects of the MOF on the NP. Here, we use density functional theory to study how paradigmatic MOF linkers (imidazolate, carboxylate, and thiolate) impact the electronic structure of representative metal surfaces, and in turn the binding of small species, whose formation energies are commonly used in descriptor-based catalyst screening. We find that the coordinating moiety and functionalization of the linker modulates the shift in the metal d-band center and the electron transfer, which is correlated to experimentally measurable quantities such as C–O vibration frequencies. However, in the majority of cases, the effect of the linker on binding energies (for C*, O*, N*, H*, OH*, CH 3 * and CO*) was less than 10 kJ mol −1 . Furthermore, scaling relationships between binding energies were only slightly affected by linker-originated electronic effects. Therefore, activity/selectivity “heat maps” derived from calculations under “generic” steric constrains could remain useful to screen the optimal NP composition of an NP@MOF catalyst. On the other hand, the placement of a given NP composition on the aforementioned heat maps is affected by the MOF. For an n -butane oxidation case study, we estimated that Ag 3 Pd—a promising NP composition for selective 1-butanol formation according to previous screenings using the surrogate pore method—has a ∼85% probability of retaining a selectivity for 1-butanol above 75% when encapsulated in a carboxylic MOF of suitable pore size.
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- Award ID(s):
- 1846707
- NSF-PAR ID:
- 10136380
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 22
- Issue:
- 4
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 2475 to 2487
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract 2D metal–organic frameworks (2D‐MOFs) have recently emerged as promising materials for gas separations, sensing, conduction, and catalysis. However, the stability of these 2D‐MOF catalysts and the tunability over catalytic environments are limited. Herein, it is demonstrated that 2D‐MOFs can act as stable and highly accessible catalyst supports by introducing more firmly anchored photosensitizers as bridging ligands. An ultrathin MOF nanosheet‐based material, Zr‐BTB (BTB = 1,3,5‐tris(4‐carboxyphenyl)benzene), is initially constructed by connecting Zr6‐clusters with the tritopic carboxylate linker. Surface modification of the Zr‐BTB structure was realized through the attachment of porphyrin‐based carboxylate ligands on the coordinatively unsaturated Zr metal sites in the MOF through strong Zr‐carboxylate bond formation. The functionalized MOF nanosheet, namely PCN‐134‐2D, acts as an efficient photocatalyst for1O2generation and artemisinin production. Compared to the 3D analogue (PCN‐134‐3D), PCN‐134‐2D allows for fast reaction kinetics due to the enhanced accessibility of the catalytic sites within the structure and facile substrate diffusion. Additionally, PCN‐134(Ni)‐2D exhibits an exceptional yield of artemisinin, surpassing all reported homo‐ or heterogeneous photocatalysts for the artemisinin production.
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null (Ed.)Computational methods can provide first-principles insights into the thermochemistry and kinetics of reactions at interfaces, but this capability has not been widely leveraged to design soft materials that respond selectively to chemical species. Here we address this opportunity by demonstrating the design of micrometer-thick liquid crystalline films supported on metal-perchlorate surfaces that exhibit selective orientational responses to targeted oxidizing gases. Initial electronic structure calculations predicted Mn 2+ , Co 2+ , and Ni 2+ to be promising candidate surface binding sites that (1) coordinate with nitrile-containing mesogens to orient liquid crystal (LC) phases and (2) undergo redox-triggered reactions upon exposure to humid O 3 leading to a change in the strength of binding of the nitrile group to the surface. These initial predictions were validated by experimental observations of orientational transitions of nitrile-containing LCs upon exposure to air containing parts-per-billion concentrations of O 3 . Additional first-principles calculations of reaction free energies of metal salts and oxidizing gases predicted that the same set of metal cations, if patterned on surfaces at distinct spatial locations, would provide LC responses that allow Cl 2 and O 3 to be distinguished while not responding to environmental oxidants such as O 2 and NO 2 . Experimental results are provided to support this prediction, and X-ray diffraction measurements confirmed that the experimentally observed LC responses can be understood in terms of the relative thermodynamic driving force for formation of MnO 2 , CoOOH, or NiOOH from the corresponding metal cation binding sites in the presence of humid O 3 and Cl 2 .more » « less
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null (Ed.)Silica-encapsulated gold core@shell nanoparticles (Au@SiO 2 CSNPs) were synthesized via a tunable bottom-up procedure to catalyze the aerobic oxidation of benzyl alcohol. The nanoparticles exhibit a mesoporous shell which enhances selectivity by inhibiting the formation of larger species. Adding potassium carbonate to the reaction increased conversion from 17.3 to 60.4% while decreasing selectivity from 98.4 to 75.0%. A gold nanoparticle control catalyst with a similar gold surface area took 6 times as long to reach the same conversion, achieving only 49.4% selectivity. These results suggest that the pore size distribution within the inert silica shell of Au@SiO 2 CSNPs inhibits the formation of undesired products to facilitate the selective oxidation of benzaldehyde despite a basic environment. A smaller activation energy, mass transport analysis, and mesopore distribution together suggest the Au@SiO 2 CSNP catalyst demonstrates higher activity through beneficial in-pore orientation, promoting a lower activation energy mechanistic pathway. Taken together, this is a promising catalytic structure to optimize oxidation chemistries, without leveraging surface-interacting factors like chelating agents or active support surfaces.more » « less
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The synthesis of metal–organic frameworks (MOFs) by using traditional wet-chemistry methods generally requires very long durations and still suffers from non-uniform heat and mass transfer within the bulk precursor solutions. Towards addressing these issues, a microdroplet-based spray method has been developed. In a typical spray process, an MOF's precursor solution is first atomized into microdroplets. These droplets serve as microreactors to ensure homogeneous mixing, fast evaporation, and rapid nucleation and crystal growth to form MOF particles. However, the fundamental MOF formation mechanisms by using this strategy have not been fully understood. In this work, the role of the operating pressure in the synthesis of a representative MOF ( i.e. , Cu(TPA)·(DMF); TPA: terephthalic acid, DMF: dimethylformamide) was systematically investigated. Detailed characterization showed that the pressure variations significantly affected both the morphologies and crystalline structures of Cu(TPA)·(DMF). Numerical simulations revealed that the morphology changes are mainly attributed to the variations in supersaturation ratios, which are caused by different microdroplet evaporation rates due to the regulation of operating pressure, while the crystalline structure variations are closely related to the dissociation of DMF molecules at lower operating pressures. Besides, the dissociation of DMF molecules decreased the surface area of the MOF crystals, but gave rise to massive coordinatively unsaturated metal sites, which greatly enhanced the interaction of CO 2 with the MOF crystal and thus led to improved CO 2 adsorption capacity and selectivity. The outcome of this work would contribute to the fundamental understanding of MOF synthesis using the microdroplet-based spray method.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C9CP05380H
