Multicomponent isoreticular metal-organic frameworks: Principles, current status and challenges

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.

Nanoconfinement and mass transport in metal–organic frameworks

Summary Hydrogen has a remarkably flexible chemistry in oxides. We show that the introduction of H into simple TiO2 leads to greatly enhanced photocatalytic properties, and specifically yields a 60 times enhancement of photocatalytic hydrogen evolution activity over commercial rutile. The hydrogenated TiO2 (H-TiO2) is synthesized by a new one-step wet-chemistry approach yielding switchable defect via controlled annealing. As-prepared H-TiO2 has Ti-H bonds in the lattice from replacement of oxygen by hydrogen atoms. The Ti-H bonds are converted to oxygen vacancies by loss of H2O with Ar annealing. Oppositely, Ti-H defects are healed by Ti-O with O2 annealing. The strongly enhanced photocatalytic activity is associated with increased visible light absorption and effective separation of photogenerated carriers. This work provides a new and powerful approach for the preparation of hydrogenated titanium dioxide with switchable defect control, and striking improvement of photocatalytic activity.

Wet-chemistry hydrogen doped TiO2 with switchable defects control for photocatalytic hydrogen evolution

High-yield solar-driven atmospheric water harvesting of metal–organic-framework-derived nanoporous carbon with fast-diffusion water channels

Summary Hierarchical nanoporous carbons (HNC) have been proven to be an effective adsorbent for the adsorption of volatile organic compounds (VOCs) and CO2. However, questions remain regarding the hierarchical structure regulation, the adsorption mechanisms of adsorbate uptake, and interactions within the HNC. We synthesize HNC from wood, using a microwave-induced heating method incorporating K2CO3 activation. Our HNC exhibit Murray's law multiscale structures, prompting a molecular-scale study of adsorbate adsorption via nuclear magnetic resonance (NMR). NMR chemical shifts are consistent with ring-current effects from the adsorbent. Our NMR technique provides a convenient way to quantitate adsorption of adsorbate in HNC. VOC vapor adsorption results show NMR chemical-shift changes with time, suggesting initial adsorption into mesopores, followed by diffusion into micropores. Schroeder's paradox is demonstrated by differences in observed shifts for adsorbed liquid vis-a-vis vapor phase in these HNC. These HNC show high CO2 adsorption capacity, portending applications to carbon capture.

/pdf/revealing-molecular-mechanisms-in-hierarchical-nanoporous-2wglam852p.pdf

Revealing Molecular Mechanisms in Hierarchical Nanoporous Carbon via Nuclear Magnetic Resonance

Understanding the factors that affect self-diffusion in isoreticular and multivariate (MTV) MOFs is key to their application in drug delivery, separations, and heterogeneous catalysis. Here, we measure the apparent self-diffusion of solvents saturated within the pores of large single crystals of MOF-5, IRMOF-3 (amino-functionalized MOF-5), and 17 MTV-MOF-5/IRMOF-3 materials at various mole fractions. We find that the apparent self-diffusion coefficient of N,N-dimethylformamide (DMF) may be tuned linearly between the diffusion coefficients of MOF-5 and IRMOF-3 as a function of the linker mole fraction. We compare a series of solvents at saturation in MOF-5 and IRMOF-3 to elucidate the mechanism by which the linker amino groups tune molecular diffusion. The ratio of the self-diffusion coefficients for solvents in MOF-5 to those in IRMOF-3 is similar across all solvents tested, regardless of solvent polarity. We conclude that average pore aperture, not solvent-linker chemical interactions, is the primary factor responsible for the different diffusion dynamics upon introduction of an amino group to the linker.

/pdf/precise-control-of-molecular-self-diffusion-in-isoreticular-12td8ermxe.pdf

Ariel Z. Plantz

Papers

Revealing Molecular Mechanisms in Hierarchical Nanoporous Carbon via Nuclear Magnetic Resonance

Precise Control of Molecular Self-Diffusion in Isoreticular and Multivariate Metal-Organic Frameworks.