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A defining characteristic of nearly all catalytically functional MOFs is uniform, molecular-scale porosity. MOF pores, linkers and nodes that define them, help regulate reactant and product transport, catalyst siting, catalyst accessibility, catalyst stability, catalyst activity, co-catalyst proximity, composition of the chemical environment at and beyond the catalytic active site, chemical intermediate and transition-state conformations, thermodynamic affinity of molecular guests for MOF interior sites, framework charge and density of charge-compensating ions, pore hydrophobicity/hydrophilicity, pore and channel rigidity

Well-defined RhMo6O24n− clusters were encapsulated within a hierarchically porous MOF NU-1K and tested for an illustrative gas-phase CO oxidation reaction.

An iron–triazolate metal–organic framework (MOF) is computationally investigated for the catalytic oxidation of strong C–H bonds. The MOF is predicted to form reactive iron-oxo active sites, and design rules to guide future experiments are discussed.

To increase catalytic efficiency, mesoporous supports have been widely applied to immobilize well-defined metal oxide clusters due to their ability to stabilize highly dispersed clusters. Herein, a redox-active heterometallic Ce

Potassium-promoted, alumina-supported molybdenum oxide is investigated for non-oxidative isobutane dehydrogenation, with an emphasis on sub-monolayer coverages. After pre-reduction in H2, initial isobutene turnover frequencies are largely independent of Mo loading, but deactivation rate constants increase by >100-fold as loadings increase to monolayer, leading to a >13-fold difference in reaction rates at extended time on stream. Mo oxidation state by in situ X-ray absorption spectroscopy is stable with time on stream, arguing against continued catalyst restructuring as the origin of deactivation. Across the set of loadings, isobutane dehydrogenation and coke formation are correlated with partially reduced Mo4+ and deeply reduced Moδ+ sites, respectively. Given the stable dehydrogenation activity of low-loaded Mo, in contrast to the rapid deactivation of high-loaded Mo, sub-monolayer metal oxides may warrant further investigation as light alkane dehydrogenation even when their bulk counterparts may not.

Summary The modular nature of metal–organic frameworks (MOFs) enables synthetic control over their physical and chemical properties, but it can be difficult to know which MOFs would be optimal for a given application. High-throughput computational screening and machine learning are promising routes to efficiently navigate the vast chemical space of MOFs but have rarely been used for the prediction of properties that need to be calculated by quantum mechanical methods. Here, we introduce the Quantum MOF (QMOF) database, a publicly available database of computed quantum-chemical properties for more than 14,000 experimentally synthesized MOFs. Throughout this study, we demonstrate how machine learning models trained on the QMOF database can be used to rapidly discover MOFs with targeted electronic structure properties, using the prediction of theoretically computed band gaps as a representative example. We conclude by highlighting several MOFs predicted to have low band gaps, a challenging task given the electronically insulating nature of most MOFs.

Atomically precise cerium oxo clusters offer a platform to investigate structure-property relationships that are much more complex in the ill-defined bulk material cerium dioxide. We investigated the activity of the MCe

Ceria-based materials have been highly desired in photocatalytic reactions due to their redox properties and strong oxygen storage and transfer ability. Herein, we report the structures of one CeCe

Overcoming propylene oxidation Catalysts for the oxidative dehydrogenation of propane to propylene and water become less selective with increasing conversion because propylene itself is more readily oxidized than propane. Yan et al. created nanoscale tandem catalysts by growing an ∼2-nanometer shell of indium oxide, a selective hydrogen combustion catalyst, over a propane dehydrogenation catalyst, platinum nanoparticles supported on alumina spheres (see the Perspective by Pei and Gong). This overcoat exposed the platinum nanoparticles for propane dehydrogenation. Surface hydrogen atoms were oxidized at the indium oxide–platinum interface. This approach boosted yields of propylene to up to 30%. Science , this issue p. 1257 ; see also p. 1203

Catalytically active acid sites associated with the silica-niobia interface were probed with a series of overcoated SiO2 on Nb2O5 (SiO2/Nb2O5) mixed oxide materials prepared by deposition of tetraethyl orthosilicate onto niobic acid (Nb2O5·nH2O) or calcined niobia (Nb2O5). NH3 TPD and pyridine DRIFTS studies indicated that the speciation of acid sites in the materials evolved as a function of SiO2 loading, impacting the quantity and stability of Bronsted sites. Catalyst activity was highly dependent on SiO2 loading in the liquid phase hydroalkoxylation of dihydropyran with n-octanol. At SiO2 surface densities corresponding to approximately 1 Si per 2 surface Nb, the activity of these catalysts passed through a maximum approximately 20 times higher than the activity of calcined Nb2O5. Apparent reaction barriers measured over the most active SiO2/Nb2O5 catalysts were 10 kJ/mol lower than those measured over niobic acid, suggesting that the OH features unique to the SiO2-Nb2O5 interface were slightly more reactive than those on niobic acid.

Zirconium-based metal–organic frameworks (Zr-MOFs) have been increasingly studied over the past two decades as heterogeneous catalysts due to their synthetic tunability, well-defined nature, and ch...

Lewis acidic Ti-SiO2 materials are workhorse oxidation catalysts, and they range from microporous substituted zeolites to meso/macroporous materials, often with tradeoffs between steric accessibili...

Through quantum-chemical calculations, we investigate a family of metal-organic frameworks (MOFs) containing triazolate linkers, M2 X2 (BBTA) (M=metal, X=bridging anion, H2 BBTA=1H,5H-benzo(1,2-d:4,5-d')bistriazole), for their ability to form terminal metal-oxo sites and subsequently activate the C-H bond of methane. By varying the metal and bridging anion in the framework, we show how to significantly tune the reactivity of this series of MOFs. The electronic structure of the metal-oxo active site is analyzed for each combination of metal and bridging ligand, and we find that spin density localized on the oxo ligand is not an inherent requirement for low C-H activation barriers. For the Mn- and Fe-containing frameworks, a transition from ferromagnetic to antiferromagnetic coupling between the metal binding site and terminal oxo ligand during the C-H activation process can greatly reduce the kinetic barrier, a unique case of two-state reactivity without a change in the net spin multiplicity.

Solid supports are crucial in heterogeneous catalysis due to their profound effects on catalytic activity and selectivity. However, elucidating the specific effects arising from such supports remains challenging. We selected a series of metal-organic frameworks (MOFs) with 8-connected Zr

Heterometallic Ce

Metal-organic frameworks (MOFs) with coordinatively unsaturated metal sites are appealing as adsorbent materials due to their tunable functionality and ability to selectively bind small molecules. Through the use of computational screening methods based on periodic density functional theory, we investigate O2 and N2 adsorption at the coordinatively unsaturated metal sites of several MOF families. A variety of design handles are identified that can be used to modify the redox activity of the metal centers, including changing the functionalization of the linkers (replacing oxido donors with sulfido donors), anion exchange of bridging ligands (considering μ-Br-, μ-Cl-, μ-F-, μ-SH-, or μ-OH- groups), and altering the formal oxidation state of the metal. As a result, we show that it is possible to tune the O2 affinity at the open metal sites of MOFs for applications involving the strong and/or selective binding of O2. In contrast with O2 adsorption, N2 adsorption at open metal sites is predicted to be relatively weak across the MOF dataset, with the exception of MOFs containing synthetically elusive V2+ open metal sites. As one example from the screening study, we predicted that exchanging the μ-Cl- ligands of M2Cl2(BBTA) (H2BBTA = 1H,5H-benzo(1,2-d:4,5-d')bistriazole) with μ-OH- groups would significantly enhance the strength of O2 adsorption at the open metal sites without a corresponding increase in the N2 affinity. Experimental investigation of Co2Cl2(BBTA) and Co2(OH)2(BBTA) confirms that the former exhibits weak physisorption of both N2 and O2, whereas the latter is capable of chemisorbing O2 at room temperature in a highly selective manner. The O2 chemisorption behavior is attributed to the greater electron-donating character of the μ-OH- ligands and the presence of H-bonding interactions between the μ-OH- bridging ligands and the reduced O2 adsorbate.

Catalytic oxidation of hydrocarbons with hydrogen peroxide (H2O2) has been of the utmost importance for several decades. The vast majority of studies have been performed in the condensed phase, eve...

Thermal treatment of metal nanoparticles at temperatures above 1000 °C leads to the formation of nanopores in amorphous SiO2 planar supports. In this work, we employ Pd/SiO2 as a model system to investigate how the initial size and distribution of nanoparticles on the SiO2 surface affects the behavior of the nanoparticles at high temperatures with respect to the formation of nanopores and related structures. We also examine the role of physical processing parameters such as heating temperature, ramp rate, and heating time in altering the type, size, and number density of features formed. These studies reveal that nanopore formation competes with other surface phenomena, including nanoparticle agglomeration and encapsulation, which also occur at high temperatures. We establish that the dominant behavior, among the many competing phenomena occurring at the metal–oxide interface, depends on the initial surface distribution of the nanoparticles. Using this knowledge, we show that the final nanopore diameter and surface density are highly tunable.

A metal–organic framework, MIL-47(V) containing coordinatively saturated V+IV sites linked together by terephthalic linkers, was prepared by a solvothermal method and evaluated as a catalyst in the epoxidation of cyclohexene. We have compared the catalytic activity in the condensed and gas phase oxidation of cyclohexene to discuss the effect of temperature and reaction phase in cyclohexene epoxidation over MIL-47(V). The catalysts were examined for the epoxidation of cyclohexene with H2O2 at 50, 65, 120, and 150 °C. We observed significant differences in product selectivity between liquid-phase and gas-phase operations and confirmed that the active sites are tightly incorporated into the MOF as node channels and thus resistant to leaching.

The speciation and catalytic activity of ceria supported transition metal oxides synthesized via incipient wetness impregnation are highly dependent on synthesis parameters such as the choice of metal precursor and the concentration of defects such as oxygen vacancies in the support. Here, cobalt oxide domains were synthesized on cerium oxide nanorods and commercial nanopowder via incipient wetness impregnation of cobalt (II) nitrate hexahydrate, cobalt (II) acetate, cobalt (III) acetylacetonate, and disodium cobalt (II) ethylenediaminetetraacetic acid at surface densities below monolayer coverage. Their reactivity was tested in the catalytic reduction of NO by CO. The steady-state deNOx activity at 250 °C of the catalyst prepared with cobalt (III) acetylacetonate and ceria nanorods was nearly three times greater than that of the control catalyst. From X-ray absorption spectroscopy it was seen that as-deposited cobalt precursors on a nanorod support were more highly coordinated than were cobalt precursors on comparable commercial ceria nanoparticles, possibly indicating anchoring at defects on the nanorods. A relationship was found between catalyst activity and the cobalt oxidation state of the fully calcined catalysts that suggests that deNOx activity is separately dependent on the cobalt oxide distribution and the presence of defects in the support. Overall, here it is shown that the two critical synthesis parameters of ligand choice and defected supports combine additively to improve reactivity of the supported cobalt oxide.

Bulk and supported MoSx materials have gained interest as alternative catalysts for light alkane dehydrogenation, but there is little kinetic data published with which rigorous comparisons can be m...

The modular structure of metal–organic frameworks (MOFs) makes them promising platforms for catalyst design and for elucidating structure/performance relationships in catalysis. In this work, we sy...

Density functional theory (DFT) calculations, experimental data, and microkinetic modeling are used to extend a triple-pathway (Lewis acid, water-mediated, and alcohol-mediated) mechanism for tris(...

Highly dispersed, supported oxides are ubiquitous solid catalysts but can be challenging to characterize with atomic precision. Here, it is shown that crystalline anatase TiO2 nanosheets (∼5 nm thi...

Highly dispersed silica-supported CuOx/SiO2 catalysts were synthesized via solution-phase deposition and studied for their activity, selectivity, and stability in catalyzing the selective oxidation of propylene to acrolein. Strategies for ensuring high metal dispersion included controlling the surface density of silanols (via covalent silanol-capping) or by pre-installing different “promoter” transition metals at submonolayer coverages. A comparison of the effect of first row transition metal promoters showed that V and Cr significantly boost catalyst performance and stabilize CuOx sites against aggregation.

Epoxide alcoholysis is extensively employed in the synthesis of polymers and chemical intermediates, and it generally requires an acid catalyst for high rates and selectivity. Tris(pentafluoropheny...

A novel tri-layer approach for immobilizing metal nanoparticles in SiO2 supports is presented. In this work, we show that under rapid heating to temperatures of approximately 1,000 °C, metal nanoparticles less than 15 nm in size will entrench in the SiO2 layer on a silicon wafer to create pores as deep as 250 nm. We studied and characterized this entrenching behavior and subsequent nanopore formation for a wide variety of metal nanoparticles, including Au, Ag, Pt, Pd, and Cu. We also demonstrate that an Al2O3 layer acts as a barrier to such pore formation. Thus, by creating a tri-layer architecture consisting of SiO2 on Al2O3 on silicon wafers, we can control the depth to which nanoparticles entrench between 3–5 nm. This small range allows one to entrench particles for the purpose of immobilization but still present them above the surface. The two advances of moving into the sub-15 nm size regime and of controlled particle immobilization through entrenchment have important implications in studying site-isolated and stabilized metal nanoparticles for applications in sensing, separations, and catalysis.

Heterogeneous catalysts supported on metal-organic frameworks (MOFs), which possess uniform porosity and crystallinity, have attracted significant interest for recent years due to the ease of active-site characterization via X-ray diffraction and the subsequent relation of the active site structure to the catalytic activity. We report the syntheses, structures, and oxidation catalytic activities of single-ion iron catalysts incorporated into the zirconium MOF NU-1000. Single-ion iron catalysts with different counteranions were anchored onto the Zr node through postsynthetic solvothermal deposition. Crystallographic characterization of the resulting MOFs (NU-1000-Fe-Cl and NU-1000-Fe-NO3) revealed that, while both frameworks have similar Fe coordination, the distance between Fe and the Zr6 node differs significantly between the two. The product rate profiles of the two catalysts for vapor-phase cyclohexene epoxidation demonstrate different initial rates and product formations, likely originating from the different Fe-O distances.

Metal–organic frameworks (MOFs) are a widely investigated class of crystalline solids with tunable structures that make it possible to impart specific chemical functionality tailored for a given application. However, the enormous number of possible MOFs that can be synthesized makes it difficult to determine which materials would be the most promising candidates, especially for applications governed by electronic structure properties that are often computationally demanding to simulate and time-consuming to probe experimentally. Here, we have developed the first publicly available quantum-chemical database for MOFs (the “QMOF database”), which consists of properties derived from density functional theory (DFT) for over 14,000 experimentally synthesized MOFs. Throughout this study, we demonstrate how this new database can be used to identify MOFs with targeted electronic structure properties. As a proof-of-concept, we use the QMOF database to evaluate the performance of several machine learning models for the prediction of DFT-computed band gaps and find that crystal graph convolutional neural networks are capable of achieving superior predictive performance, making it possible to circumvent computationally expensive quantum-chemical calculations. We also show how unsupervised learning methods can aid the discovery of otherwise subtle structure–property relationships using the computational findings in this work. We conclude by highlighting several MOFs with low band gaps, a challenging task given the electronically insulating nature of most MOF structures. The data and predictive models generated in this work, as well as the database of MOF structures, should be highly useful to other researchers interested in the predictive design and discovery of MOFs for the many applications dictated by quantum-chemical phenomena.

In this work, we leverage advances in computational screening based on periodic density functional theory (DFT) to study a diverse set of experimentally derived metal–organic frameworks (MOFs) with...

Tandem catalysis couples multiple reactions and promises to improve chemical processing, but precise spatiotemporal control over reactive intermediates remains elusive. We used atomic layer deposition to grow In

A nonconventional, water-mediated catalytic mechanism was proposed to explain the effects of residual water on the reactivity and regioselectivity of tris(pentafluorophenyl)borane catalyst in the ring-opening reaction of 1,2-epoxyoctane by 2-propanol. This nonconventional mechanism was proposed to operate in parallel with conventional Lewis acid-catalyzed ring-opening. Microkinetic modeling was conducted to validate the proposed reaction mechanism, with all kinetic and thermodynamic parameters derived from density functional theory (DFT) calculations. Experimental data at a variety of temperatures and water contents were captured by the model after adjustments within reasonable limits set by experimental benchmarking and accuracy of theory of a small subset of parameters. In addition, the microkinetic model was able to generate accurate predictions at reaction conditions that were not used for parameter estimation. Detailed analysis of the net reaction rates showed that >95% of the reaction flux passed th...

Gas phase acceptorless dehydrogenative coupling (ADC) of ethanol to ethyl acetate is investigated over bulk MoS2. Yields of up to 56% are achieved at gas hourly space velocity of 1700 h−1 and 260 °C using a high surface area co-precipitated MoS2 material. Addition of water to the ethanol feed decreases overall conversion but increases selectivity to acetic acid. After a short activation period, the catalyst is stable over reaction times exceeding 13 h. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with CO as a probe molecule and X-ray absorption spectroscopy (XAS) are used to measure structural disorder within MoS2 samples. Samples with larger amounts of structural disorder are demonstrated to have enhanced dehydrogenation activity per total surface area.

When using nanocrystalline oxides as catalytic supports, different nanoshapes have different numbers of oxygen vacancy defects, surface energies, and surface terminations; these properties may affect the dispersion, oxidation state, and therefore catalytic activity of a second supported metal oxide. Here, cobalt oxide domains were synthesized on cerium oxide nanorods, nanocubes, and commercial nanopowder via incipient wetness impregnation of cobalt (II) nitrate hexahydrate at several surface coverages. The resulting materials were examined for their reducibility under CO and H2 and their reactivity in the catalytic reduction of NO by CO from 200 to 450 °C. Activity per total mass of catalyst is highest at intermediate loadings, while activity per Co atom is maximized at the lowest loadings, implicating highly dispersed Co-O-Ce sites as the active species. Across all Co loadings, catalysts with nanorod supports have higher activity than catalysts made with ceria nanoparticles or nanocubes. From X-ray absorption spectroscopy, it was found that the more-active nanorod supports are better able to maintain Co in a partially reduced state under reaction conditions after initial calcination. This stabilization appears to be further correlated with the high number of defects on the ceria nanorods observed via Raman spectroscopy, rather than with properties of the exposed crystal plane, as has been traditionally reported for these materials. These insights may lead to new generations of selective catalysts based on nanocrystal-supported oxides for emissions abatement and other important reactions.

Alkane oxidative dehydrogenation (ODH) over supported redox active metal oxides is highly sensitive to support identity, but the underlying cause of support effects has not been well-established. Here, we provide evidence that charge transfer between the support and active oxide phase impacts the rates of C–H bond abstraction and COX formation pathways in the oxidative dehydrogenation of cyclohexane over supported copper oxide catalysts. The surface Lewis acid strength of nine metal oxide supports is quantified by alizarin dye intramolecular charge transfer shifts and compared with supported copper oxide d–d transition energies to determine the relationship between support Lewis acid strength and copper oxide electronic properties. Model cyclohexane ODH reaction studies show that selectivity to C6 products increases with increasing support Lewis acid strength, with selectivities to benzene and cyclohexene over combustion products at zero conversion increasing from 20% over nucleophilic Cu/MgO to over 90% ...

The redox properties of CeO2 are crucial in its applications in a wide range of catalytic processes. In the present research, in-situ IR spectroscopy is shown to be a viable and convenient method for the characterization of the oxidation state of Ce by monitoring the spin-orbit transition in Ce3+ (2F5/2 → 2F7/2) at ∼2147 cm−1. By monitoring this transition in CeO2 nanorods, the apparent activation energy for the production of oxygen vacancies that accompany the formation of Ce3+ has been determined and is shown to be lower for reduction with cyclohexene than with hydrogen. The bi-exponential kinetics for the formation of oxygen vacancies in CeO2 nanorods is discussed. An application of this method to real time monitoring of the oxidation state of Ce in the oxidation of cyclohexene on vanadia supported on ceria is presented as an example of how this method can be used as an operando probe of reaction mechanisms.

Acid-catalyzed skeletal C–C bond isomerizations are important benchmark reactions for the petrochemical industries. Among those, o-xylene isomerization/disproportionation is a probe reaction for strong Bronsted acid catalysis, and it is also sensitive to the local acid site density and pore topology. Here, we report on the use of phosphotungstic acid (PTA) encapsulated within NU-1000, a Zr-based metal–organic framework (MOF), as a catalyst for o-xylene isomerization at 523 K. Extended X-ray absorption fine structure (EXAFS), 31P NMR, N2 physisorption, and X-ray diffraction (XRD) show that the catalyst is structurally stable with time-on-stream and that WOx clusters are necessary for detectable rates, consistent with conventional catalysts for the reaction. PTA and framework stability under these aggressive conditions requires maximal loading of PTA within the NU-1000 framework; materials with lower PTA loading lost structural integrity under the reaction conditions. Initial reaction rates over the NU-1000...

A comprehensive set of surface phase diagrams addressing the catalytically relevant edges of the (100) surface of MoS2 catalysts is developed using dispersion-corrected density functional theory and ab initio thermodynamic modeling. The results of the temperature-dependent, free energy-based thermodynamic model are presented over the full range of catalytically relevant temperatures and pressures, in addition to S- and H-coverages ranging from 0 to 100%. The results of this work allow for a full thermodynamic analysis to be performed at the conditions relevant to any promising reaction involving MoS2, ranging from hydrodesulfurization to dehydrogenation to electrocatalysis. Several methodological recommendations are discussed and implemented with the goal of improving the accuracy of the surface phase diagrams at minimal computational expense. A library of the most stable S- and H-adsorption modes is also developed so that linear scaling relationships can be used to correlate thermodynamic stability with ...