The interfacial chemistry of diborane (B2H6) with hydroxylated silica was investigated via in situ Fourier-transform infrared spectroscopy and temperature-programmed desorption. During exposure of silica to B2H6 under ultrahigh vacuum conditions, a decline in infrared band intensity assigned to excitation of the interfacial silanol O-H vibration at 3750 cm-1 and the associated appearance of a feature at 3687 cm-1 revealed hydrogen-bonding interactions between B2H6 and interfacial silanol groups. The IR spectrum for silica was completely recovered following desorption of the adsorbates, indicating that interactions between B2H6 and clean silica are reversible, in contrast to other reports on this system. During temperature-programmed desorption of diborane from silica, B2H6 was observed to desorb between 80 and 150 K, evidence for weak interactions between B2H6 and the surface. Electronic-structure calculations revealed that these interactions were due to bifurcated dihydrogen bonds between two terminal B-H groups of the adsorbate and interfacial silanol groups.

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.

The high-energy demand of benzene, toluene, and xylene (BTX) separation highlights the need for improved nonthermal separation techniques and materials. Because of their high surface areas, tunable structures, and chemical stabilities, metal organic frameworks (MOFs) are a promising class of materials for use in more energy efficient, adsorption-based separations. In this work, BTX compounds in the pore environment of UiO-66 were systematically examined using in situ infrared (IR) spectroscopy to understand the fundamental interactions that influence molecular transport through the MOF. Isothermal diffusion experiments revealed BTX diffusivities between 10(-8) and 10(-12) cm(2) s(-1), where the rate follows the trend: o-xylene < m-xylene < p-xylene. Corresponding activation energies of diffusion (E-diff) were determined to be 44 kJ mol(-1) for the xylene isomers and 34 kJ mol(-1) for both benzene and toluene, with the diffusion-limiting barrier identified to be molecular passage through the small triangular pore apertures of UiO-66. Furthermore, IR spectroscopy and computational methods showed the formation of two types of hydrogen bonds between BTX molecules and the mu(3)-OH groups located in the tetrahedral cavities of UiO-66, which indicates that BTX molecules are capable of fully accessing the inner pore environment of the MOF. The molecular-level insight into the diffusion mechanism and energetics of BTX transport through UiO-66 presented in this work provides rich insight for the design of next-generation MOFs for cost-effective separation processes.

Zr-based metal organic frameworks (MOFs) have been recently shown to be among the fastest catalysts of nerve-agent hydrolysis in solution. We report a detailed study of the adsorption and decomposition of a nerve-agent simulant, dimethyl methylphosphonate (DMMP), on UiO-66, UiO-67, MOF-808, and NU-1000 using synchrotron-based X-ray powder diffraction, X-ray absorption, and infrared spectroscopy, which reveals key aspects of the reaction mechanism. The diffraction measurements indicate that all four MOFs adsorb DMMP (introduced at atmospheric pressures through a flow of helium or air) within the pore space. In addition, the combination of X-ray absorption and infrared spectra suggests direct coordination of DMMP to the Zr6 cores of all MOFs, which ultimately leads to decomposition to phosphonate products. These experimental probes into the mechanism of adsorption and decomposition of chemical warfare agent simulants on Zr-based MOFs open new opportunities in rational design of new and superior decontamination materials.

Ambient pressure in situ synchrotron-based spectroscopic techniques have been correlated to illuminate atomic-level details of bond breaking and formation during the hydrolysis of a chemical warfare nerve agent simulant over a polyoxometalate catalyst. Specifically, a Cs 8 [Nb 6 O 19 ] polyoxoniobate catalyst has been shown to react readily with dimethyl methylphosphonate (DMMP). The atomic-level transformations of all reactant moieties, the [Nb 6 O 19 ] 8− polyanion, its Cs + counterions, and the DMMP substrate, were tracked under ambient conditions by a combination of X-ray absorption fine structure spectroscopy, Raman spectroscopy, and X-ray diffraction. Results reveal that the reaction mechanism follows general base (in contrast to specific base) hydrolysis. Together with computational results, the work demonstrates that the ultimate fate of DMMP hydrolysis at the Cs 8 [Nb 6 O 19 ] catalyst is strong binding of the (methyl) methylphosphonic acid ((M)MPA) product to the polyanions, which ultimately inhibits catalytic turnover.

Hydrocarbon diffusion and binding within porous molecular networks are critical to catalysis, separations, and purification technologies. Fundamental insight into n-butane uptake and mobility within a new class of materials for separations, metal–organic frameworks (MOFs), has been gained through in situ infrared spectroscopy. These ultrahigh vacuum (UHV) based measurements revealed that adsorption of n-butane within UiO-66 proceeds through the formation of hydrogen bonds between the alkane molecules and hydroxyl groups located at the inorganic node of UiO-66. Modeling the gas transport of n-butane with Fick’s second law yielded diffusion coefficients at several temperatures. The Arrhenius parameter for the activation energy of diffusion was found to be 21.0 ± 1.2 kJ/mol. These studies have further shown that the rate-determining step for diffusion is likely the dissociation of n-butane from a binding site located within the tetrahedral pores of UiO-66.

The mechanism and kinetics of interactions between dimethyl methylphosphonate (DMMP), a key chemical warfare agent (CWA) simulant, and Zr6-based metal organic frameworks (MOFs) have been investigated with in situ infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (PXRD), and DFT calculations. DMMP was found to adsorb molecularly to UiO-66 through the formation of hydrogen bonds between the phosphoryl oxygen and the free hydroxyl groups associated with Zr6 nodes on the surface of crystallites and not within the bulk MOF structure. Unlike UiO-66, the infrared spectra for UiO-67 and MOF-808, recorded during DMMP exposure, suggest that uptake occurs through both physisorption and chemisorption. The XPS spectra of MOF-808 zirconium 3d electrons reveal a charge redistribution following exposure to DMMP. In addition, analysis of the phosphorus 2p electrons following exposure and thermal annealing to 600 K indicates that two types of stable phosphorus-containing species exist within the MOF. DFT calculations, used to guide the IR band assignments and to help interpret the XPS features, suggest that uptake is driven by nucleophilic addition of an OH group to DMMP with subsequent elimination of a methoxy substituent to form strongly bound methyl methylphosphonic acid (MMPA). The rates of product formation indicate that there are likely two distinct uptake processes, requiring rate constants that differ by approximately an order of magnitude. However, the rates of molecular uptake were found to be nearly identical to the rates of reaction, which strongly suggests that the reaction rates are diffusion-limited. The final products were found to inhibit further reactions within the MOFs, and these products could not be thermally driven from the MOFs prior to decomposition of the MOFs themselves.

New materials for the rapid decomposition of chemical warfare agents (CWAs) are in high demand for protecting military and civilian populations from these weapons of mass destruction.The need for novel sorbents and decontamination catalysts has gained great urgency as terrorists groups demonstrate the ability to synthesize and deploy agents in chemical attacks.Although many new materials, such as metalorganic frameworks (MOFs), have been proposed to use as CWA filtration media, their eventual transition requires a detailed understanding of the atomic-scale reaction mechanisms.