Joseph Hupp

Single-site catalysts (SSCs) achieve a high catalytic performance through atomically dispersed active sites. A challenge facing the development of SSCs is aggregation of active catalytic species. Reducing the loading of these sites to very low levels is a common strategy to mitigate aggregation and sintering; however, this limits the tools that can be used to characterize the SSCs. Here we report a sintering-resistant SSC with high loading that is achieved by incorporating Anderson–Evans polyoxometalate clusters (POMs, MMo6O24, M = Rh/Pt) within NU-1000, a Zr-based metal–organic framework (MOF). The dual confinement provided by isolating the active site within the POM, then isolating the POMs within the MOF, facilitates the formation of isolated noble metal sites with low coordination numbers via exsolution from the POM during activation. The high loading (up to 3.2 wt %) that can be achieved without …

Semiconductor nanocrystals (NCs) offer prospective use as active optical elements in photovoltaics, light-emitting diodes, lasers, and photocatalysts due to their tunable optical absorption and emission properties, high stability, and scalable solution processing, as well as compatibility with additive manufacturing routes. Over the course of experiments, during device fabrication, or while in use commercially, these materials are often subjected to intense or prolonged electronic excitation and high carrier densities. The influence of such conditions on ligand integrity and binding remains underexplored. Here, we expose CdSe NCs to laser excitation and monitor changes in oleate that is covalently attached to the NC surface using nuclear magnetic resonance as a function of time and laser intensity. Higher photon doses cause increased rates of ligand loss from the particles, with upward of 50% total ligand desorption …

Non-oxidative ethanol dehydrogenation is a promising route to produce acetaldehyde and hydrogen from sustainable feedstock. Bimetallic NiCu catalysts have shown high efficiency and selectivity for this chemical transformation. In this study, we leverage the high porosity and uniform catalyst deposition sites on a Zr-based metal–organic framework (MOF) catalyst support, NU-1000, to understand how the changes in Cu:Ni ratio affects the reactivity for non-oxidative ethanol dehydrogenation. We found that increasing the Ni2+ concentration significantly reduces the activation energy of the reaction due to the role of Ni2+ in suppressing the onset of Cu reduction. This study illustrates how MOFs can be used as catalyst supports to fine-tune the catalyst compositions and understand their effect on the overall catalytic performances.

Industrialization over the past two centuries has resulted in a continuous rise in global CO2 emissions. These emissions are changing ecosystems and livelihoods. Therefore, methods are needed to capture these emissions from point sources and possibly from our atmosphere. Though the amount of CO2 is rising, it is challenging to capture directly from air because its concentration in air is extremely low, 0.04%. In this study, amines installed inside metal–organic frameworks (MOFs) are investigated for the adsorption of CO2, including at low concentrations. The amines used are polyamidoamine dendrimers that contain many primary amines. Chemically reversible adsorption of CO2 via carbamate formation was observed, as was enhanced uptake of carbon dioxide, likely via dendrimer-amide-based physisorption. Limiting factors in this initial study are comparatively low dendrimer loadings and slow kinetics for …

The high cost of noble metals is a barrier to widespread commercialization of polymer electrolyte membrane fuel cells. Platinum-group-metal-free catalysts are a promising low-cost alternative for catalyzing the oxygen reduction reaction (ORR). Herein, we report a high activity Fe-N-C cathode catalyst derived from a Fe-porphyrinic framework prepared using low-cost precursors and facile one pot synthesis followed by a single heat treatment. The final product has atomically dispersed iron in proximity to nitrogen groups that share transition metal characteristics, as described by electron energy loss spectrometry and x-ray absorption near edge structure results. Electrochemical studies on a rotating ring-disk electrode indicate a four-electron transfer mechanism for the ORR. Membrane electrode assembly testing of the Fe-porphyrin-derived cathode catalyst shows a high kinetic current density of 22 mA cm− 2 at 0.9 V in …

Metal–organic frameworks (MOFs) have been reported to effectively detoxify chemical warfare agents (CWAs) and their simulants. Early studies of Zr-MOF-catalyzed hydrolytic degradation of an organophosphoester type CWA, Sarin, and its simulant, dimethyl-4-nitrophenylphosphate (DMNP), suggested that the activity originates from ZrIV–OH–ZrIV moieties that resemble the structure of the active-site phosphotriesterase enzyme. Measurements of pKa values for hexa-zirconium-node-sited bridging hydroxo, terminal hydroxo, and aqua ligands reveal essentially complete conversion of mu-hydroxo ligands to substitution-inert mu-oxo ligands at alkaline pHs, ruling out a primary role for ZrIV–OH–ZrIV moieties, despite the resemblance to the phosphotriesterase enzyme active-site. The measurements also rule out a secondary role as a hydrogen bond donor/stabilizer of bound DMNP. Additionally, the measurements …

The mechanism of photochemical CO2 reduction to formate by PCN-136, a Zr-based metal–organic framework (MOF) that incorporates light-harvesting nanographene ligands, has been investigated using steady-state and time-resolved spectroscopy and density functional theory (DFT) calculations. The catalysis was found to proceed via a “photoreactive capture” mechanism, where Zr-based nodes serve to capture CO2 in the form of Zr–bicarbonates, while the nanographene ligands have a dual role of absorbing light and storing one-electron equivalents for catalysis. We also find that the process occurs via a “two-for-one” route, where a single photon initiates a cascade of electron/hydrogen atom transfers from the sacrificial donor to the CO2-bound MOF. The mechanistic findings obtained here illustrate several advantages of MOF-based architectures in molecular photocatalyst engineering and provide insights on …

Magnesium based composites MgH2 + X (X=Ti, Co) were synthesized by ball milling in an argon atmosphere using stainless steel vial and balls. The crystallographic behavior of the resulting powders was examined by XRD. Thermal stability and hydrogen desorption properties were investigated by thermal analysis methods. In order to obtain a deeper insight into bonding mechanisms of the transition metal in MgH2 relaxed structure, ab initio electronic structure calculation of MgH2 + X (X=Ti, Co) was performed using Full Potential Linearized Augmented Plane Wave method, implemented in WIEN2K code. DOS analysis, confirmed by DTA measurements, resulted in the conclusion that, in the composite, in comparison to MgH2, the bonding Mg-H was weakened, on account of the shortening of interatomic distances hydrogentransition metal.

Purifying crude ethylene streams from an acetylene contaminant using state-of-the-art thermal hydrogenation requires high temperature, an external feed of H2 gas, and noble metal catalysts and is not only expensive and energy-intensive but also prone to overhydrogenation to ethane. This Letter reports the photocatalytic semihydrogenation of acetylene to ethylene using the metal–organic framework (MOF) Co-PCN-222. Under pure acetylene atmosphere the system achieves an overall conversion of 1.6 mol ethylene per gram of Co and remains catalytically active for 1 week. Under a mixed acetylene/ethylene atmosphere (the industrially relevant conditions), the system achieves nearly 100% conversion of acetylene after 87 h with >99.9% ethylene selectivity over ethane. The heterogeneous nature of the MOF lends advantages over the homogeneous catalyst, namely, facile recyclability and increased longevity …

The sulfidation of metal oxides is a critical process to create catalysts for industrially relevant reactions. However, the exact sulfidation mechanism remains unclear. To investigate the sulfidation process through the heterolytic splitting of sulfur-containing molecules, we chose a structurally well-defined material, Zr-based metal-organic framework (MOF) NU-1000, as the model system. Sulfidation was achieved through thiol binding on thermally distorted Zr6 clusters. Key to the binding chemistry heterolytic S-H bond cleavage enabled by the presentation of inorganic frustrated Lewis-acid/Lewis-base pairs (FLPs) in the form of under-coordinated Zr(IV) sites and a terminal O(-II) site. In situ synchrotron based structural analysis, spectroscopic characterization, and computational studies showed that thiols bearing different functional groups interact with the distorted Zr6 nodes through dissociative adsorption, forming bridging Zr-S-Zr bonds while converting a terminal oxo to a terminal hydroxo. This study provides insights into the surface sites responsible for the sulfidation of metal oxide catalysts and offers a promising strategy for introducing S-bearing moieties to MOF scaffolds.

The reaction of 3,5-diamino-4,4′-bis(1H-pyrazole) (3,5-H2L) with copper(II) and nickel(II) acetates under solvothermal conditions led to the four mixed-metal metal–organic frameworks (MIXMOFs) [CuxNi1–x(3,5-L)] (CuxNi1–x, x = 0.05, 0.1, 0.2, 0.5), which were thoroughly characterized in the solid state. The textural analysis unveiled their macroporous nature, with BET specific surface areas falling in the 140–240 m2/g range. Despite the low specific surface areas, their CO2 adsorption capacity at ambient temperature and pressure (highest: Cu0.05Ni0.95 and Cu0.2Ni0.8; 5.6 wt % CO2) and isosteric heat of adsorption (highest: Cu0.2Ni0.8; Qst = 26.2 kJ/mol) are reasonably high. All of the MIXMOFs were tested as heterogeneous catalysts in carbon dioxide electrochemical reduction (CO2RR) in acetonitrile solution at variable potential. The best results were obtained at E = −1.5 V vs Ag/AgCl/KClsat: besides H2 …

Chemically stable metal–organic frameworks (MOFs) featuring interconnected hierarchical pores have proven to be promising for a remarkable variety of applications. Nevertheless, the framework’s susceptibility to capillary-force-induced pore collapse, especially during water evacuation, has often limited practical applications. Methodologies capable of predicting the relative magnitudes of these forces as functions of the pore size, chemical composition of the pore walls, and fluid loading would be valuable for resolution of the pore collapse problem. Here, we report that a molecular simulation approach centered on evacuation-induced nanocavitation within fluids occupying MOF pores can yield the desired physical-force information. The computations can spatially pinpoint evacuation elements responsible for collapse and the chemical basis for mitigation of the collapse of modified pores. Experimental isotherms …

Activating the C–H bonds of alkanes without further oxidation to more thermodynamically stable products, CO and CO2, is a long-sought goal of catalytic chemistry. Inspired by the monocopper active site of methane monooxygenase, we synthesized a Cu-doped ZIF-8 metal–organic framework with 25% Cu and 75% Zn in the nodes and activated it by heating to 200 °C and dosing in a stepwise fashion with O2, methane, and steam. We found that it does oxidize methane to methanol and formaldehyde. The catalysis persists through at least five cycles, and beyond the third cycle, the selectivity improves to the extent that no CO2 can be detected. Experimental characterization and analysis were carried out by PXRD, DRUV−vis, SEM, and XAS (XANES and EXAFS). The reaction is postulated to proceed at open-coordination copper sites generated by defects, and the mechanism of methanol production was explicated …

Starting with ferrocyanide ions in acidic aqueous solution, cyano-ferrate(II) species are post-synthetically grafted to the nodes of a mesoporous zirconium-based MOF, NU-1000. As indicated by single-crystal X-ray crystallography, grafting occurs by substitution of cyanide ligands by node-based hydroxo and oxo ligands rather than by substitution of node aqua ligands by cyanide ligands as bridges between Fe(II) and Zr(IV). The installed moieties yield a broad absorption band that is tentatively ascribed to iron-to-zirconium charge transfer. Consistent with Fe(III/II) redox activity, a modest fraction of the installed iron complexes are directly electrochemically addressable.

In article number 2300673, Dongmok Whang, Sung Jong Yoo, Namgee Jung, Joseph T. Hupp, and co-workers present a strategy to enhance the efficiency and durability of sub-nanometer catalysts for electrochemical reactions. The cover art depicts the strong interaction between single-atom site Ru catalysts and chemically stable ZrO 2-x nanoparticles, enabling superior stability and improved activity of the catalysts for hydrogen evolution reactions.

Polyoxometalates (POMs) featuring 7, 12, 18, or more redox-accessible transition metal ions are ubiquitous as selective catalysts, especially for oxidation reactions. The corresponding synthetic and catalytic chemistry of stable, discrete, capping-ligand-free polythiometalates (PTMs), which could be especially attractive for reduction reactions, is much less well developed. Among the challenges are the propensity of PTMs to agglomerate and the tendency for agglomeration to block reactant access of catalyst active sites. Nevertheless, the pervasive presence of transition metal sulfur clusters metalloenzymes or cofactors that catalyze reduction reactions and the justifiable proliferation of studies of two-dimensional (2D) metal-chalcogenides as reduction catalysts point to the promise of well-defined and controllable PTMs as reduction catalysts. Here, we report the fabrication of agglomeration-immune, reactant …

In line with the longstanding physical electrochemistry interests and numerous innovative contributions of Grahame Award recipient Dr. Keith Stevenson, this presentation will focus on a physical electrochemistry problem: how to separate redox-hopping based hole-transport within porous, electrocatalytic metal-organic framework coatings from transport of charge-compensating ions through the same coatings. In principle, either process, or both, could define transport rates. For electrocatalysis, the general goal is to achieve the fastest transport possible. Wall-jet electrochemistry – the approach used here – offers a useful way of assessing redox-hopping rates without complications from ion ingress or egress. The wall-jet approach turns out to be especially useful when samples cannot be made compatible with RDE or IDE assemblies.

Zr6(µ3-O)4(µ3-OH)4 node cores are indispensable building blocks for almost all zirconium-based metal–organic frameworks. Consistent with the insulating nature of zirconia, they are generally considered electronically inert. Contrasting this viewpoint, we present spectral measurements and calculations indicating that emission from photoexcited NU-601, a six-connected Zr-based MOF, comes from both linker-centric locally-excited and linker-to-node charge-transfer (CT) states. The CT state originates from a hole transfer process enabled by favorable energy alignment of the HOMOs of the node and linker. This alignment can be manipulated by changing the pH of the medium, which alters the protonation state of multiple oxy groups on the Zr-node. Thus, the acid-base chemistry of the node has a direct effect on the photophysics of the MOF following linker-localized electronic excitation. These new findings open opportunities to understand and exploit, for energy conversion, unconventional mechanisms of exciton formation and transport in MOFs.

Chemically and hydrolytically stable metal–organic frameworks (MOFs) have shown great potential for many water-adsorption-related applications. However, MOFs with large pores that show high water-uptake capacity and high hydrolytic and mechanical cycle stability are rare. Through a deliberate adjustment of the linker of a typical zirconium-based MOF (Zr-MOF) (NU-1000), a new isomer of NU-1000 with blocked c-pores, but large mesopores was successfully synthesized. This new isomer, ISO-NU-1000, exhibits excellent water stability, one of the highest water vapor uptake capacities, and excellent cycle stability, making it a promising candidate for water-vapor-sorption-based applications such as water-adsorption-driven heat transfer. We find that the high water-cycling stability of ISO-NU-1000 is traceable to its blocking c-pore that hinders the hydrolysis of node-coordinating formate in the c-pore area and …

Zirconium-based metal–organic frameworks (MOFs) are candidate materials for effective nerve agent detoxification due to their thermo- and water stability as well as high density of catalytic Zr sites. However, as high-porosity materials, most of the active sites of Zr-MOFs can only be accessed by diffusion into the crystal interior. Therefore, the transport of nerve agents in nanopores is an important factor in the catalytic performance of Zr-MOFs. Here, we investigated the transport process and mechanism of a vapor-phase nerve agent simulant, dimethyl methyl phosphonate (DMMP), through a representative Zr-MOF, NU-1008, under practical conditions of varying humidity. Confocal Raman microscopy was used to monitor the transport of DMMP vapor through individual NU-1008 crystallites, where the relative humidity (RH) of the environment was tuned to understand the impact of water. Counterintuitively, water in the …