Geometry and Chemistry: Influence of Pore Functionalization on Molecular Transport and Diffusion in Solvent-Filled Zirconium Metal-Organic Frameworks.

Postsynthetic modification (PSM) of metal-organic frameworks (MOFs) enables incorporation of diverse functionalities in pores for chemical separations, drug delivery, and heterogeneous catalysis. However, the effect of PSM on molecular transport, which is essential for most applications of MOFs, has been rarely studied. In this paper, we used perfluoroalkane-functionalized Zr-MOF NU-1008 as a platform to systematically interrogate transport processes and mechanisms in solvated pores. We anchored perfluoroalkanes onto NU-1008 nodes by solvent-assisted ligand incorporation (SALI-n, with n = 3, 5, 7, and 9 denoting the number of fluorinated carbons). Transport of a luminescent molecule, BODIPY, through individual crystallites of four versions of methanol-filled SALI-n was monitored by confocal fluorescence microscopy as a function of time and location. In comparison with the parent NU-1008, the diffusivity of the probe molecules within SALI-n declined by 2- to 7-fold depending on chain length and loading, presumably due to the reduction in pore diameter or adsorptive interactions with perfluoroalkyl chains. Atomistic simulations were performed to uncover the microscopic behavior of the BODIPY diffusion in SALI-n. The perfluoroalkyl chains are observed to stay close to the pore walls, instead of extending toward the pore center. BODIPY molecules, which preferably interact with linkers, were pushed to the interior of the channels as the chain length increased, resulting in solvated diffusion and minor differences in the short-time mobility of BODIPY in SALI-n. This suggested that the observed decline of transport diffusivity in SALI-n mainly stemmed from the reduction in the pore size when these flexible chains are present. We anticipate that this proof of concept will assist in understanding how pore functionalization can physically and chemically affect mass transport in MOFs and will be useful in further guiding the design of PSM to realize the optimal performance of MOFs for various applications.

Intermetallic alloy structure-activity descriptors derived from inelastic X-ray scattering.

Synchrotron spectroscopy and Density Functional Theory (DFT) are combined to develop a new descriptor for the stability of adsorbed chemical intermediates on metal alloy surfaces. This descriptor probes the separation of occupied and unoccupied d electron density in platinum and is related to shifts in Resonant Inelastic X-ray Scattering (RIXS) signals. Simulated and experimental spectroscopy are directly compared to show that the promoter metal identity controls the orbital shifts in platinum electronic structure. The associated RIXS features are correlated with the differences in the band centers of the occupied and unoccupied d bands, providing chemical intuition for the alloy ligand effect and providing a connection to traditional descriptions of chemisorption. The ready accessibility of this descriptor to both DFT calculations and experimental spectroscopy, and its connection to chemisorption, allow for deeper connections between theory and characterization in the discovery of new catalysts.

Insights and Heuristics for Predicting Diffusion Rates of Chemical Warfare Agents in Zirconium Metal-Organic Frameworks.

Designing nanoporous catalysts to destroy chemical warfare agents (CWAs) and environmental contaminants requires consideration of both intrinsic catalytic activity and the mass transfer of molecules in and out of the pores. Polar adsorbates such as CWAs experience a heterogeneous environment in many metal-organic frameworks (MOFs) due to the arrangement of the metal nodes and organic linkers of the MOF. However, quantitative relationships between the pore architecture and the resulting diffusion properties of polar molecules have not been established. We used molecular dynamics simulations to calculate the diffusion coefficients of the CWA simulant dimethyl methyl phosphonate (DMMP) in a diverse set of 776 MOFs with Zr6 nodes. We developed a 4-parameter machine learning model to predict DMMP diffusivities in Zr6 MOFs and found the model to be transferable to the CWA sarin. We then developed a simplified heuristic based on the machine learning model that the node-node distance and accessible surface area should be maximized to find MOFs with rapid CWA diffusion.

Nanoconfinement and mass transport in metal-organic frameworks.

The ubiquity of metal-organic frameworks in recent scientific literature underscores their highly versatile nature. MOFs have been developed for use in a wide array of applications, including: sensors, catalysis, separations, drug delivery, and electrochemical processes. Often overlooked in the discussion of MOF-based materials is the mass transport of guest molecules within the pores and channels. Given the wide distribution of pore sizes, linker functionalization, and crystal sizes, molecular diffusion within MOFs can be highly dependent on the MOF-guest system. In this review, we discuss the major factors that govern the mass transport of molecules through MOFs at both the intracrystalline and intercrystalline scale; provide an overview of the experimental and computational methods used to measure guest diffusivity within MOFs; and highlight the relevance of mass transfer in the applications of MOFs in electrochemical systems, separations, and heterogeneous catalysis.

Topology-Dependent Alkane Diffusion in Zirconium Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) can be designed for chemical applications by modulating the size and shape of intracrystalline pores through selection of their nodes and linkers. Zirconium nodes with variable connectivity to organic linkers allow for a broad range of topological nets that have diverse pore structures even for a consistent set of linkers. Identifying an optimal pore structure for a given application, however, is complicated by the large material space of possible MOFs. In this work, molecular dynamics simulations were used to determine how a MOF's topology affects the diffusion of propane and isobutane over the full range of loadings and to understand how MOFs can be tuned to reduce transport limitations for applications in separations and catalysis. High-throughput simulation techniques were employed to efficiently calculate loading-dependent diffusivities in 38 MOFs. The results show that topologies with higher node connectivity have reduced alkane diffusivities compared to topologies with lower node connectivity. Molecular siting techniques were used to elucidate how the pore structures in different topologies affect adsorbate diffusivities.

Structure and solvation of confined water and water-ethanol clusters within microporous Brønsted acids and their effects on ethanol dehydration catalysis.

Aqueous-phase reactions within microporous Brønsted acids occur at active centers comprised of water-reactant-clustered hydronium ions, solvated within extended hydrogen-bonded water networks that tend to stabilize reactive intermediates and transition states differently. The effects of these diverse clustered and networked structures were disentangled here by measuring turnover rates of gas-phase ethanol dehydration to diethyl ether (DEE) on H-form zeolites as water pressure was increased to the point of intrapore condensation, causing protons to become solvated in larger clusters that subsequently become solvated by extended hydrogen-bonded water networks, according to in situ IR spectra. Measured first-order rate constants in ethanol quantify the stability of SN2 transition states that eliminate DEE relative to (C2H5OH)(H+)(H2O) n clusters of increasing molecularity, whose structures were respectively determined using metadynamics and ab initio molecular dynamics simulations. At low water pressures (2-10 kPa H2O), rate inhibition by water (-1 reaction order) reflects the need to displace one water by ethanol in the cluster en route to the DEE-formation transition state, which resides at the periphery of water-ethanol clusters. At higher water pressures (10-75 kPa H2O), water-ethanol clusters reach their maximum stable size ((C2H5OH)(H+)(H2O)4-5), and water begins to form extended hydrogen-bonded networks; concomitantly, rate inhibition by water (up to -3 reaction order) becomes stronger than expected from the molecularity of the reaction, reflecting the more extensive disruption of hydrogen bonds at DEE-formation transition states that contain an additional solvated non-polar ethyl group compared to the relevant reactant cluster, as described by non-ideal thermodynamic formalisms of reaction rates. Microporous voids of different hydrophilic binding site density (Beta; varying H+ and Si-OH density) and different size and shape (Beta, MFI, TON, CHA, AEI, FAU), influence the relative extents to which intermediates and transition states disrupt their confined water networks, which manifest as different kinetic orders of inhibition at high water pressures. The confinement of water within sub-nanometer spaces influences the structures and dynamics of the complexes and extended networks formed, and in turn their ability to accommodate the evolution in polarity and hydrogen-bonding capacity as reactive intermediates become transition states in Brønsted acid-catalyzed reactions.

Investigating the Process and Mechanism of Molecular Transport within a Representative Solvent-Filled Metal-Organic Framework.

Effective permeation into, and diffusive mass transport within, solvent-filled metal-organic frameworks (MOFs) is critical in applications such as MOF-based chemical catalysis of condensed-phase reactions. In this work, we studied the entry from solution of a luminescent probe molecule, 1,3,5,7-tetramethyl-4,4-difluoroboradiazaindacene (BODIPY), into the 1D channel-type, zirconium-based MOF NU-1008 and subsequent transport of the probe through the MOF. Measurements were accomplished via in situ confocal fluorescence microscopy of individual crystallites, where the evolution of the fluorescence response from the crystallite was followed as functions of both time and location within the crystallite. From the confocal data, intracrystalline transport of BODIPY is well-described by one-dimensional diffusion along the channel direction. Varying the chemical identity of the solvent revealed an inverse dependence of probe-molecule diffusivity on bulk-solvent viscosity, qualitatively consistent with expectations from the Stokes-Einstein equation for molecular diffusion. At a more quantitative level, however, measured diffusion coefficients are about 100-fold smaller than expected from Stokes-Einstein, pointing to substantial channel-confinement effects. Evaluation of the confocal data also reveals a non-negligible mass transport resistance, i.e., surface barrier, associated with the probe molecule leaving the solution and permeating the exterior surface of the MOF. Permeation by the probe entails displacement of solvent from the MOF channels. The magnitude of the resistance increases with the size of the solvent molecule. This work draws attention to the importance of MOF structure, external-surface barriers, and solvent molecule identity to the overall transport process in MOFs, which should assist in understanding the performance of MOFs in applications such as condensed-phase heterogeneous catalysis.

Defect-Mediated Ordering of Condensed Water Structures in Microporous Zeolites.

Ab-initio molecular dynamics simulations and transmission infrared spectroscopy are employed to characterize the structure of water networks in defect-functionalized microporous zeolites. Thermodynamically stable phases of clustered water molecules are localized at some of the defects in zeolite Beta, which include catalytic sites such as framework Lewis acidic Sn atoms in closed and hydrolyzed-open forms, as well as silanol nests. These water clusters compete with ideal gas-like structures at low water densities and pore-filling phases at higher water densities, with the equilibrium phase determined by the water chemical potential. The physical characteristics of these phases are determined by the defect identity, with the local binding and orientation of hydroxyl moieties around the defects playing a central role. The results suggest general principles for how the structure of polar solvents in microporous solid acids is influenced by local defect functionalization, and the thermodynamic stability of the condensed phases surrounding such sites, in turn, implies that the catalysis of Lewis acids will be influenced by local water ordering.

Cooperative Effects between Hydrophilic Pores and Solvents: Catalytic Consequences of Hydrogen Bonding on Alkene Epoxidation in Zeolites.

Hydrophobic voids within titanium silicates have long been considered necessary to achieve high rates and selectivities for alkene epoxidations with H2O2. The catalytic consequences of silanol groups and their stabilization of hydrogen-bonded networks of water (H2O), however, have not been demonstrated in ways that lead to a clear understanding of their importance. We compare turnover rates for 1-octene epoxidation and H2O2 decomposition over a series of Ti-substituted zeolite *BEA (Ti-BEA) that encompasses a wide range of densities of silanol nests ((SiOH)4). The most hydrophilic Ti-BEA gives epoxidation turnover rates that are 100 times larger than those in defect-free Ti-BEA, yet rates of H2O2 decomposition are similar for all (SiOH)4 densities. These differences cause the most hydrophilic Ti-BEA to also give the highest selectivities, which defies conventional wisdom. Spectroscopic, thermodynamic, and kinetic evidence indicate that these catalytic differences are not due to changes in the electronic affinity of the active site, the electronic structure of Ti-OOH intermediates, or the mechanism for epoxidation. Comparisons of apparent activation enthalpies and entropies show that differences in epoxidation rates and selectivities reflect favorable entropy gains produced when epoxidation transition states disrupt hydrogen-bonded H2O clusters anchored to (SiOH)4 near active sites. Transition states for H2O2 decomposition hydrogen bond with H2O in ways similar to Ti-OOH reactive species, such that decomposition becomes insensitive to the presence of (SiOH)4. Collectively, these findings clarify how molecular interactions between reactive species, hydrogen-bonded solvent networks, and polar surfaces can influence rates and selectivities for epoxidation (and other reactions) in zeolite catalysts.

Changes in Catalytic and Adsorptive Properties of 2 nm Pt3Mn Nanoparticles by Subsurface Atoms.

Supported multimetallic nanoparticles (NPs) are widely used in industrial catalytic processes, where the relation between surface structure and function is well-known. However, the effect of subsurface layers on such catalysts remains mostly unstudied. Here, we demonstrate a clear subsurface effect on supported 2 nm core-shell NPs with atomically precise and high temperature stable Pt3Mn intermetallic surface measured by in situ synchrotron X-ray Diffraction, difference X-ray Absorption Spectroscopy, and Energy Dispersive X-ray Spectroscopy. The NPs with a Pt3Mn subsurface have 98% selectivity to C-H over C-C bond activation during propane dehydrogenation at 550 °C compared with 82% for core-shell NPs with a Pt subsurface. The difference is correlated with significant reduction in the heats of reactant adsorption due to the Pt3Mn intermetallic subsurface as discerned by theory as well as experiment. The findings of this work highlight the importance of subsurface for supported NP catalysts, which can be tuned via controlled intermetallic formation. Such approach is generally applicable to modifying multimetallic NPs, adding another dimension to the tunability of their catalytic performance.