Hybrid quantum-classical simulations of magic angle spinning dynamic nuclear polarization in very large spin systems.

Solid-state nuclear magnetic resonance can be enhanced using unpaired electron spins with a method known as dynamic nuclear polarization (DNP). Fundamentally, DNP involves ensembles of thousands of spins, a scale that is difficult to match computationally. This scale prevents us from gaining a complete understanding of the spin dynamics and applying simulations to design sample formulations. We recently developed an ab initio model capable of calculating DNP enhancements in systems of up to ∼1000 nuclei; however, this scale is insufficient to accurately simulate the dependence of DNP enhancements on radical concentration or magic angle spinning (MAS) frequency. We build on this work by using ab initio simulations to train a hybrid model that makes use of a rate matrix to treat nuclear spin diffusion. We show that this model can reproduce the MAS rate and concentration dependence of DNP enhancements and build-up time constants. We then apply it to predict the DNP enhancements in core-shell metal-organic-framework nanoparticles and reveal new insights into the composition of the particles' shells.

Creating an Aligned Interface between Nanoparticles and MOFs by Concurrent Replacement of Capping Agents.

Applying metal-organic frameworks (MOFs) on the surface of other materials to form multifunctional materials has recently attracted great attention; however, directing the MOF overgrowth is challenging due to the orders of magnitude differences in structural dimensions. In this work, we developed a universal strategy to mediate MOF growth on the surface of metal nanoparticles (NPs), by taking advantage of the dynamic nature of weakly adsorbed capping agents. During this colloidal process, the capping agents gradually dissociate from the metal surface, replaced in situ by the MOF. The MOF grows to generate a well-defined NP-MOF interface without a trapped capping agent, resulting in a uniform core-shell structure of one NP encapsulated in one single-crystalline MOF nanocrystal with specific facet alignment. The concept was demonstrated by coating ZIF-8 and UiO-66-type MOFs on shaped metal NPs capped by cetyltrimethylammonium surfactants, and the formation of the well-defined NP-MOF interface was monitored by spectroscopies. The defined interface outperforms ill-defined ones generated via conventional methods, displaying a high selectivity to unsaturated alcohols for the hydrogenation of an α,ß-unsaturated aldehyde. This strategy opens a new route to create aligned interfaces between materials with vastly different structural dimensions.

Tuning Metal-Organic Framework Nanocrystal Shape through Facet-Dependent Coordination.

We studied coordination-dependent surfactant binding on shaped MOF nanocrystals. Cetyltrimethylammonium bromide (CTAB) on the surface of ZIF-8 was used as a model system. Infrared spectroscopic analysis and molecular dynamics simulations reveal different coordination environments for Zn nodes on {100} and {110} facets, resulting in different CTAB adsorption. We found that we are able to fine-tune the ratio of {100} and {110} facets in the nanocrystals. We also observed that once the MOF nanocrystals are enclosed by pure {110} facets growth along the {100} facets is terminated because the MOF nanocrystal has no surface area for CTAB adsorption. Growth can then be reinitiated through the etching of these rhombic dodecahedral nanocrystals to form a small amount of undercoordinated sites. This work represents the first systematic study of the design principles underpinning the synthesis of shaped MOF nanocrystals.

Probing Interactions between Metal-Organic Frameworks and Freestanding Enzymes in a Hollow Structure.

It has been reported that the biological functions of enzymes could be altered when they are encapsulated in metal-organic frameworks (MOFs) due to the interactions between them. Herein, we probed the interactions of catalase in solid and hollow ZIF-8 microcrystals. The solid sample with confined catalase is prepared through a reported method, and the hollow sample is generated by hollowing the MOF crystals, sealing freestanding enzymes in the central cavities of hollow ZIF-8. During the hollowing process, the samples were monitored by small-angle X-ray scattering (SAXS) spectroscopy, electron microscopy, powder X-ray diffraction (PXRD), and nitrogen sorption. The interfacial interactions of the two samples were studied by infrared (IR) and fluorescence spectroscopy. IR study shows that freestanding catalase has less chemical interaction with ZIF-8 than confined catalase, and a fluorescence study indicates that the freestanding catalase has lower structural confinement. We have then carried out the hydrogen peroxide degradation activities of catalase at different stages and revealed that the freestanding catalase in hollow ZIF-8 has higher activity.

Applying a Nanoparticle@MOF Interface To Activate an Unconventional Regioselectivity of an Inert Reaction at Ambient Conditions.

Here we design an interface between a metal nanoparticle (NP) and a metal-organic framework (MOF) to activate an inert CO2 carboxylation reaction and in situ monitor its unconventional regioselectivity at the molecular level. Using a Kolbe-Schmitt reaction as model, our strategy exploits the NP@MOF interface to create a pseudo high-pressure CO2 microenvironment over the phenolic substrate to drive its direct C-H carboxylation at ambient conditions. Conversely, Kolbe-Schmitt reactions usually demand high reaction temperature (>125 °C) and pressure (>80 atm). Notably, we observe an unprecedented CO2 meta-carboxylation of an arene that was previously deemed impossible in traditional Kolbe-Schmitt reactions. While the phenolic substrate in this study is fixed at the NP@MOF interface to facilitate spectroscopic investigations, free reactants could be activated the same way by the local pressurized CO2 microenvironment. These valuable insights create enormous opportunities in diverse applications including synthetic chemistry, gas valorization, and greenhouse gas remediation.

Stabilizing DNAzymes through Encapsulation in a Metal-Organic Framework.

DNAzymes are a promising class of bioinspired catalyst; however, their structural instability limits their potential. Herein, a method to stabilize DNAzymes by encapsulating them in a metal-organic framework (MOF) host is reported. This biomimetic mineralization process makes DNAzymes active under a wider range of conditions. The concept is demonstrated by encapsulating hemin-G-quadruplex (Hemin-G4) into zeolitic imidazolate framework-90 (ZIF-90), which indeed increases the DNAzyme's structural stability. The stabilized DNAzymes show activities in the presence of Exonuclease I, organic solvents, or high temperature. Owing to its elevated stability and heterogeneous nature, it is possible to perform catalysis under continuous-flow conditions, and the DNAzyme can be reactivated in situ by introducing K+ . Moreover, it is found that the encapsulated DNAzyme maintains its high enantiomer selectivity, demonstrated by the sulfoxidation of thioanisole to (S)-methyl phenyl sulfoxide. This concept of stabilizing DNAzymes expands their potential application in chemical industry.

Strain-Enhanced Metallic Intermixing in Shape-Controlled Multilayered Core-Shell Nanostructures: Toward Shaped Intermetallics.

Controlling the surface composition of shaped bimetallic nanoparticles could offer precise tunability of geometric and electronic surface structure for new nanocatalysts. To achieve this goal, a platform for studying the intermixing process in a shaped nanoparticle was designed, using multilayered Pd-Ni-Pt core-shell nanocubes as precursors. Under mild conditions, the intermixing between Ni and Pt could be tuned by changing layer thickness and number, triggering intermixing while preserving nanoparticle shape. Intermixing of the two metals is monitored using transmission electron microscopy. The surface structure evolution is characterized using electrochemical methanol oxidation. DFT calculations suggest that the low-temperature mixing is enhanced by shorter diffusion lengths and strain introduced by the layered structure. The platform and insights presented are an advance toward the realization of shape-controlled multimetallic nanoparticles tailored to each potential application.

Directional Engraving within Single Crystalline Metal-Organic Framework Particles via Oxidative Linker Cleaving.

An oxidative linker cleaving (OLC) process was developed for surgical manipulation of the engraving process within single crystalline MOFs particles. The strategy relies on selective degradation of 2,5-dihydroxyterephthalic acid linker into small molecular fragments by oxidative ring-opening reactions, resulting in controllable scissoring of framework. By regulation of the generation and diffusion of oxidative species, the core MOFs will undergo divergent etching routes, producing a series of single crystalline hollow and yolk-shell MOF structures. In addition, the OLC process can be initiated and localized around the pre-embedded Pd NPs through on-site catalytic generation of oxidative species, leading to solitary confinement of multiple NPs within one single crystalline MOF particle, namely, a multi-yolk-shell structure. This unique architecture can effectively protect NPs from agglomeration while realizing size selective catalysis at the same time.

A Metal-Organic Framework Thin Film for Selective Mg2+ Transport.

The incompatibility between the anode and the cathode chemistry limits the used of Mg as an anode. This issue may be addressed by separating the anolyte and the catholyte with a membrane that only allows for Mg2+ transport. Mg-MOF-74 thin films were used as the separator for this purpose. It was shown to meet the needs of low-resistance, selective Mg2+ transport. The uniform MOF thin films supported on Au substrate with thicknesses down to ca. 202 nm showed an intrinsic resistance as low as 6.4â€…Ω cm2 , with the normalized room-temperature ionic conductivity of ca. 3.17×10-6  S cm-1 . When synthesized directly onto a porous anodized aluminum oxide (AAO) support, the resulting films were used as a standalone membrane to permit stable, low-overpotential Mg striping and plating for over 100 cycles at a current density of 0.05 mA cm-2 . The film was effective in blocking solvent molecules and counterions from crossing over for extended period of time.

Shielding against Unfolding by Embedding Enzymes in Metal-Organic Frameworks via a de Novo Approach.

We show that an enzyme maintains its biological function under a wider range of conditions after being embedded in metal-organic framework (MOF) microcrystals via a de novo approach. This enhanced stability arises from confinement of the enzyme molecules in the mesoporous cavities in the MOFs, which reduces the structural mobility of enzyme molecules. We embedded catalase (CAT) into zeolitic imidazolate frameworks (ZIF-90 and ZIF-8), and then exposed both embedded CAT and free CAT to a denature reagent (i.e., urea) and high temperatures (i.e., 80 °C). The embedded CAT maintains its biological function in the decomposition of hydrogen peroxide even when exposed to 6 M urea and 80 °C, with apparent rate constants kobs (s-1) of 1.30 × 10-3 and 1.05 × 10-3, respectively, while free CAT shows undetectable activity. A fluorescence spectroscopy study shows that the structural conformation of the embedded CAT changes less under these denaturing conditions than free CAT.

Cytotoxicity of Postmodified Zeolitic Imidazolate Framework-90 (ZIF-90) Nanocrystals: Correlation between Functionality and Toxicity.

Using a simple method, the aldehyde groups of zeolitic imidazolate framework-90 (ZIF-90) nanocrystals were converted into carboxyl, amino, and thiol groups, without affecting the integrity of the framework. Notably, for the first time, correlations between functionality and cytotoxicity are also demonstrated via in vitro cytotoxicity assays. The positive charged aminated-ZIF-90 presumably results in either perturbation of cell membrane, more efficient cell uptake, or both. Therefore, the half-maximal effective (EC50 ) concentration of aminated-ZIF-90 has a higher cytotoxicity of about 30 µg mL(-1) .

Hydrogen bonding to oxygen in siloxane bonds drives liquid phase adsorption of primary alcohols in high-silica zeolites.

Upon liquid phase adsorption of C1-C5 primary alcohols on high silica MFI zeolites (Si/Al = 11.5-140), the concentration of adsorbed molecules largely exceeds the concentration of traditional adsorption sites: Brønsted acid and defect sites. Combining quantitative in situ1H MAS NMR, qualitative multinuclear NMR and IR spectroscopy, hydrogen bonding of the alcohol function to oxygen atoms of the zeolite siloxane bridges (Si-O-Si) was shown to drive the additional adsorption. This mechanism co-exists with chemi- and physi-sorption on Brønsted acid and defect sites and does not exclude cooperative effects from dispersive interactions.

Tailoring Heterogeneous Catalysts at the Atomic Level: In Memoriam, Prof. Chia-Kuang (Frank) Tsung.

Professor Chia-Kuang (Frank) Tsung made his scientific impact primarily through the atomic-level design of nanoscale materials for application in heterogeneous catalysis. He approached this challenge from two directions: above and below the material surface. Below the surface, Prof. Tsung synthesized finely controlled nanoparticles, primarily of noble metals and metal oxides, tailoring their composition and surface structure for efficient catalysis. Above the surface, he was among the first to leverage the tunability and stability of metal-organic frameworks (MOFs) to improve heterogeneous, molecular, and biocatalysts. This article, written by his former students, seeks first to commemorate Prof. Tsung's scientific accomplishments in three parts: (1) rationally designing nanocrystal surfaces to promote catalytic activity; (2) encapsulating nanocrystals in MOFs to improve catalyst selectivity; and (3) tuning the host-guest interaction between MOFs and guest molecules to inhibit catalyst degradation. The subsequent discussion focuses on building on the foundation laid by Prof. Tsung and on his considerable influence on his former group members and collaborators, both inside and outside of the lab.

Enhancing Four-Carbon Olefin Production from Acetylene over Copper Nanoparticles in Metal-Organic Frameworks.

Four-carbon olefins, such as 1-butene and 1,3-butadiene, are important chemical feedstocks for the production of adhesives and synthetic rubber. These compounds are found in the C4 fraction of "green oil" products that can arise during the hydrogenation of acetylene. Here, we demonstrate that control of the catalyst structure increases the yield and productivity of these important olefins with a family of catalyst materials comprising Cu nanoparticles (CuNPs) bound within the pores of Zr-based metal-organic frameworks. Using carbon monoxide as a probe molecule, we characterize the surfaces of these catalytic CuNPs with diffuse reflectance infrared Fourier transform spectroscopy, revealing that the electronic structure of the CuNP surfaces is size-dependent. Furthermore, we find that as the CuNP diameter decreases, the selectivity for C4 products increases and that lowering the stoichiometric ratio of H2/acetylene improves the selectivity and productivity of the catalyst.

Investigating lattice strain impact on the alloyed surface of small Au@PdPt core-shell nanoparticles.

We investigated lattice strain on alloyed surfaces using ∼10 nm core-shell nanoparticles with controlled size, shape, and composition. We developed a wet-chemistry method for synthesizing small octahedral PdPt alloy nanoparticles and Au@PdPt core-shell nanoparticles with Pd-Pt alloy shells and Au cores. Upon introduction of the Au core, the size and shape of the overall nanostructure and the composition of the alloyed PdPt were maintained, enabling the use of the electrooxidation of formic acid as a probe to compare the surface structures with different lattice strain. We have found that the structure of the alloyed surface is indeed impacted by the lattice strain generated by the Au core. To further reveal the impact of lattice strain, we fine-tuned the shell thickness. Then, we used synchrotron-based X-ray diffraction to investigate the degree of lattice strain and compared the observations with the results of the formic acid electrooxidation, suggesting that there is an optimal intermediate shell thickness for high catalytic activity.

Structural Control of Uniform MOF-74 Microcrystals for the Study of Adsorption Kinetics.

Metal-organic frameworks (MOFs) is a promising class of sorbent materials for swing adsorption gas separation. However, although sorption kinetics plays a major role in column breakthrough experiments, it is rarely studied with MOF materials. This is largely because the synthesis of uniform yet separation-relevant MOFs is a challenging task. Here, we report a dual-modulation approach for the synthesis of well-defined Mg-MOF-74 hexagonal rods with an extremely uniform size distribution (polydispersity index = 1.02). Through epitaxial growth and wet chemical etching, uniform hollow Ni-MOF-74 with plate-shaped caps were obtained. CO2 adsorption kinetic study shows that hollow Ni-MOF-74 exhibits 54% faster diffusion rate compared to solid Ni-MOF-74 due to a shortened diffusion length, despite their identical CO2 uptake capacity. This has led to a 21% extension of column breakthrough time during CO2/N2 separation under identical conditions.

Rapid mechanochemical encapsulation of biocatalysts into robust metal-organic frameworks.

Metal-organic frameworks (MOFs) have recently garnered consideration as an attractive solid substrate because the highly tunable MOF framework can not only serve as an inert host but also enhance the selectivity, stability, and/or activity of the enzymes. Herein, we demonstrate the advantages of using a mechanochemical strategy to encapsulate enzymes into robust MOFs. A range of enzymes, namely ß-glucosidase, invertase, ß-galactosidase, and catalase, are encapsulated in ZIF-8, UiO-66-NH2, or Zn-MOF-74 via a ball milling process. The solid-state mechanochemical strategy is rapid and minimizes the use of organic solvents and strong acids during synthesis, allowing the encapsulation of enzymes into three prototypical robust MOFs while maintaining enzymatic biological activity. The activity of encapsulated enzyme is demonstrated and shows increased resistance to proteases, even under acidic conditions. This work represents a step toward the creation of a suite of biomolecule-in-MOF composites for application in a variety of industrial processes.

Green and rapid synthesis of zirconium metal-organic frameworks via mechanochemistry: UiO-66 analog nanocrystals obtained in one hundred seconds.

A UiO-66 analog was synthesized in 100 s using water-assisted grinding. The linker solubility suggested that tetrafluorobenzene-1,4-dicarboxylic acid was the best linker. Zr-metal-organic framework nanocrystals displayed good topologies and hydrophobicities, and high water/thermal stabilities. The less amorphous complex led to higher porosities and pore volumes with a 60 min grinding time.

Long-Range Olefin Isomerization Catalyzed by Palladium(0) Nanoparticles.

Long-range olefin isomerization of 2-alkenylbenzoic acid derivatives going through two to five sp3-carbon atoms to give (E)-alkenes was achieved with palladium(0) nanoparticles. The substrate scope of this reaction includes carboxylic acid, ester, and primary to tertiary amides and tolerates various substituents on the benzene ring. This isomerization reaction was catalyzed by recyclable Pd(0) nanoparticles, prepared in situ from PdCl2 and characterized by X-ray powder diffraction and scanning electron microscopy analyses. 1H NMR studies and kinetic modeling supported a stepwise process. This new process was applied to synthesize a natural dihydroisocoumarin with good efficiency.

Mechanism-based inhibition of CYP1A1 and CYP3A4 by the furanocoumarin chalepensin.

The cytochrome P450 (P450, CYP) 2A6 inhibitor chalepensin was found to inhibit human CYP1A1, CYP1A2, CYP2A13, CYP2C9, CYP2D6, CYP2E1, and CYP3A4 to different extents. CYP1A1 and CYP3A4 underwent pronounced mechanism-based inactivation by chalepensin and had the smallest IC50 ratios of inhibition with NADPH-fortified pre-incubation (IC50(+)) to that without pre-incubation (IC50(-)). CYP2E1 had the least susceptibility to mechanism-based inactivation. This inactivation of CYP1A1 and CYP3A4 exhibited time-dependence, led to a loss of spectrophotometrically detected P450, and could not be fully recovered by dialysis. Pre-incubation with chalepensin did not affect NADPH-P450 reductase activity. Cytosol-supported glutathione conjugation protected CYP3A4 but not CYP1A1 against the inactivation by chalepensin. Cytosolic decomposition of chalepensin may contribute partially to the protection. The high epoxidation activities of CYP1A1, CYP2A6, and CYP3A4 were in agreement with their pronounced susceptibilities to mechanism-based inactivation by chalepensin. Considering both the IC50 values and inactivation kinetic parameters, the threshold concentrations of chalepensin for potential drug interactions through inhibition of CYP2A6 and CYP3A4 were estimated to be consistently low. These results demonstrate that chalepensin inhibits multiple P450s and that epoxidation activity is crucial for the potential drug interaction through mechanism-based inhibition.