Developing supported single-site catalysts is an important goal in heterogeneous catalysis since the well-defined active sites afford opportunities for detailed mechanistic studies, thereby facilitating the design of improved catalysts. We present herein a method for installing Ni ions uniformly and precisely on the node of a Zr-based metal-organic framework (MOF), NU-1000, in high density and large quantity (denoted as Ni-AIM) using atomic layer deposition (ALD) in a MOF (AIM). Ni-AIM is demonstrated to be an efficient gas-phase hydrogenation catalyst upon activation. The structure of the active sites in Ni-AIM is proposed, revealing its single-site nature. More importantly, due to the organic linker used to construct the MOF support, the Ni ions stay isolated throughout the hydrogenation catalysis, in accord with its long-term stability. A quantum chemical characterization of the catalyst and the catalytic process complements the experimental results. With validation of computational modeling protocols, we further targeted ethylene oligomerization catalysis by Ni-AIM guided by theoretical prediction. Given the generality of the AIM methodology, this emerging class of materials should prove ripe for the discovery of new catalysts for the transformation of volatile substrates.
Some metal-organic frameworks (MOFs) incorporate nodes that are metal oxide clusters such as ZrO. Vacancies on the node surfaces, accidental or by design, act as catalytic sites. Here, we report elucidation of the chemistry of ZrO nodes in the MOFs UiO-66 and UiO-67 having used infrared and nuclear magnetic resonance spectroscopies to determine the ligands on the node surfaces originating from the solvents and modifiers used in the syntheses and having elucidated the catalytic properties of the nodes for ethanol dehydration, which takes place selectively to make diethyl ether but not ethylene at 473-523 K. Density functional theory calculations show that the key to the selective catalysis is the breaking of node-linker bonds (or the accidental adjacency of open/defect sites) that allows catalytically fruitful bonding of the reactant ethanol to neighboring sites on the nodes, facilitating the bimolecular ether formation through an S2 mechanism.
frameworks (MOFs) have been shown
to be excellent catalyst supports in heterogeneous catalysis due to
their exceptional stability. Additionally, their crystalline nature
affords the opportunity for molecular level characterization of both
the support and the catalytically active site, facilitating mechanistic
investigations of the catalytic process. We describe herein the installation
of Co(II) ions to the Zr6 nodes of the mesoporous MOF,
NU-1000, via two distinct routes, namely, solvothermal deposition
in a MOF (SIM) and atomic layer deposition in a MOF (AIM), denoted
as Co-SIM+NU-1000 and Co-AIM+NU-1000, respectively. The location of
the deposited Co species in the two materials is determined via difference
envelope density (DED) analysis. Upon activation in a flow of O2 at 230 °C, both materials catalyze the oxidative dehydrogenation
(ODH) of propane to propene under mild conditions. Catalytic activity
as well as propene selectivity of these two catalysts, however, is
different under the same experimental conditions due to differences
in the Co species generated in these two materials upon activation
as observed by in situ X-ray absorption spectroscopy.
A potential reaction mechanism for the propane ODH process catalyzed
by Co-SIM+NU-1000 is proposed, yielding a low activation energy barrier
which is in accord with the observed catalytic activity at low temperature.
Some metal organic frameworks (MOFs) incorporate nodes that are nanoscale metal oxides, and the hydroxy-containing functional groups on them provide opportunities for introducing catalytic sites with precisely defined structures. Investigations have been done to understand the structures of these groups on nodes and node vacancies, because, in prospect, atomic-scale modulation of the composition, areal density, and/or siting of the groups would open up possibilities for exquisite tuning of the siting and performance of subsequently anchored catalytic units (e.g., single metal ions, pairs of metal ions, or well defined metal-ion-containing clusters). We have combined infrared (IR) spectroscopy and density functional theory (DFT) to demonstrate tuning of these sites, namely, hydrogen-bonded OH/OH2 groups on the Zr6 nodes of the MOFs UiO-66 and NU-1000 via the intermediacy of node methoxy (or ethoxy) groups formed from methanol (or ethanol). Methoxy (or ethoxy) groups on node vacancy sites are converted to a structure incorporating one vacant Zr site and one terminal OH group per face by reaction with water. Our results highlight how the combination of DFT and IR spectroscopy facilitate the determination of the identity and chemistry of the functional groups on MOF node vacancies and defect sites.
INTRODUCTIONThe nodes of some metal-organic frameworks (MOFs) are essentially small, uniform metal oxide clusters, exemplified by the Zr6 nodes of
The quantum mechanical SMD continuum universal solvation model can be applied to predict the free energy of solvation of any solute in any solvent following specification of various macroscopic solvent parameters. For three ionic liquids where these descriptors are readily available, the SMD solvation model exhibits a mean unsigned error of 0.48 kcal/mol for 93 solvation free energies of neutral solutes and a mean unsigned error of 1.10 kcal/mol for 148 water-to-IL transfer free energies. Because the necessary solvent parameters are not always available for a given ionic liquid, we determine average values for a set of ionic liquids over which measurements have been made in order to define a generic ionic liquid solvation model, SMD-GIL. Considering 11 different ionic liquids, the SMD-GIL solvation model exhibits a mean unsigned error of 0.43 kcal/mol for 344 solvation free energies of neutral solutes and a mean unsigned error of 0.61 kcal/mol for 431 water-to-IL transfer free energies. As these errors are similar in magnitude to those typically observed when applying continuum solvation models to ordinary liquids, we conclude that the SMD universal solvation model may be applied to ionic liquids as well as ordinary liquids.
A dicobalt complex catalyzes N2 silylation with Me3SiCl and KC8 under 1 atm N2 at ambient temperature. Tris(trimethylsilyl)amine is formed with an initial turnover rate of 1 N(TMS)3/min, ultimately reaching a turnover number of ∼200. The dicobalt species features a metal-metal interaction, which we postulate is important to its function. Although N2 functionalization occurs at a single cobalt site, the second cobalt center modifies the electronics at the active site. Density functional calculations reveal that the Co-Co interaction evolves during the catalytic cycle: weakening upon N2 binding, breaking with silylation of the metal-bound N2 and reforming with expulsion of [N2(SiMe3)3](-).
progress in the synthesis and characterization of metal–organic
frameworks (MOFs) has opened the door to an increasing number of possible
catalytic applications. The great versatility of MOFs creates a large
chemical space, whose thorough experimental examination becomes practically
impossible. Therefore, computational modeling is a key tool to support,
rationalize, and guide experimental efforts. In this outlook we survey
the main methodologies employed to model MOFs for catalysis, and we
review selected recent studies on the functionalization of their nodes.
We pay special attention to catalytic applications involving natural
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