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Stimuli-responsive behaviors of flexible metal–organic frameworks (MOFs) make these materials promising in a wide variety of applications such as gas separation, drug delivery, and molecular sensing. Considerable efforts have been made over the last decade to understand the structural changes of flexible MOFs in response to external stimuli. Uniform pore deformation has been used as the general description. However, recent advances in synthesizing MOFs with non-uniform porous structures, i.e. with multiple types of pores which vary in size, shape, and environment, challenge the adequacy of this description. Here, we demonstrate that the CO 2 -adsorption-stimulated structural change of a flexible MOF, ZIF-7, is induced by CO 2 migration in its non-uniform porous structure rather than by the proactive opening of one type of its guest-hosting pores. Structural dynamics induced by guest migration in non-uniform porous structures is rare among the enormous number of MOFs discovered and detailed characterization is very limited in the literature. The concept presented in this work provides new insights into MOF flexibility.

Understanding the role of diffusion in catalysis is essential in the design of highly active, selective, and stable industrial heterogeneous catalysts. By using a combination of advanced in situ spectroscopic characterization tools, particularly quasi-elastic and inelastic neutron scattering, we outline the crucial differences in diffusion modes and molecular interactions of active sites within solid-acid catalysts. This, coupled with 2D solid-state NMR and probe-based FTIR spectroscopy, reveals the nature of the active site in our SAPO-37 catalyst and affords detailed information on the evolution of solid-acid catalysts that can operate at temperatures as low as 130 °C, for the Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam (precursor for Nylon-6). The versatility of this approach leads to structure−property correlations that contrast the dynamics of the diffusion process in the different materials studied. Our results illustrate the power of these techniques in unravelling the interplay between active site and molecular diffusion in single-site heterogeneous catalysts, which can play a vital role in designing low-temperature, sustainable catalytic processes.

We briefly review some of the approaches which have been used to study the distributions of defect properties in amorphous silica and focus mainly on the implementation of the embedded cluster method. We illustrate some of the results obtained using this method and discuss the remaining problems using the example of oxygen vacancy defects in amorphous SiO 2 . The neutral vacancies are characterized by a wide distribution of formation energies and structural parameters. Our modelling predicts the two major structural kinds of positively charged vacancies (E centres): dimers and dangling bond centres. The local structure for both kinds of centres depends on the medium range structure of the surrounding amorphous network. We found that the majority of the dangling bond centres are unpuckered. The optical spectra and electron paramagnetic resonance parameters calculated for all defects are in good agreement with experimental data. The structural criteria which favour the formation of different kinds of centres in the original amorphous structure are formulated in terms of the average Si-O distance of oxygen ion with its two neighbouring silicon ions.

Single-site Au species supported on carbon have been shown to be the active sites for acetylene hydrochlorination. The evolution of these single-site species has been monitored by Au L 3 X-ray absorption spectroscopy (XAS). Alternating between a standard reaction mixture of HCl/C 2 H 2 and the single reactants has provided insights into the reaction mechanism and catalyst deactivation processes. We demonstrate that oxidative addition of HCl across an Au(I) chloride species requires concerted addition with C 2 H 2 , in accordance with both the XAS measurements of Au oxidation state and the reaction kinetics being first order with respect to each reactant. Excess C 2 H 2 changes the Au speciation and results in the formation of oligomeric acetylene species which were detected by inelastic neutron scattering. Catalyst deactivation at extended reaction times can be correlated with the formation of metallic Au particles. These Au(0) species generated during the sequential gas experiments, or after prolonged reaction times, results in the analysis of the normalized near-edge white line intensity becoming an unsuitable guide for identifying the active Au species, affecting the strong correlation between normalized white line height and VCM productivity usually observed in the active catalyst. Thus, a combination of scanning transmission electron continued...

Inelastic neutron scattering (INS) spectroscopy, contrary to other vibrational spectroscopic techniques such as infrared or Raman spectroscopies, provides much richer microscopic insight into a material due to the absence of selection rules induced by the system's symmetry and via its dependence on both energy (E) and momentum (Q) transfer. Firstprinciples density functional theory (DFT) based calculations are now routinely used to interpret infrared and Raman spectra. These calculations can also be used to interpret INS spectra, however, the need to include the neutron scattering cross sections, overtones and combination modes, together with instrument specific E-Q windows make the data analysis challenging. Here we present AbINS: a new generation of software to interpret INS spectra using ab initio phonon data.AbINS is an open-source package implemented as a plugin to the neutron data analysis software, Mantid and offers the facility to plot the full (Q, E) map for powder samples, with the option to extract individual atomic contributions. This option is then applied to analyse the vibrational spectrum of non-hydrogenous K 2 SiF 6 to extract atom-type contributions identifying T 1g librational mode of the [SiF 6 ] 2− ion together with the T 2u F-Si-F bending mode.

An understanding of the phase stability of AlF3 surfaces as a function of their environment is an important prerequisite in the development of, and an ability to control, their catalytic properties. In this study, all electron hybrid-exchange density functional theory is used to calculate the structure and corresponding energies of several α-AlF3 surfaces. It is shown that the surfaces expose under-coordinated Al ions that are potential Lewis acid sites. The binding energy of NH3 to these sites is calculated and used to quantify their relative acidities. The Lewis acid sites are significantly weaker than the strongest sites predicted to occur on β-AlF3 surfaces. The equilibrium morphology of α-AlF3 crystallites is predicted from the construction of an approximate Wulff plot. The stabilities of two representative terminations of α-AlF3, as a function of HF and H2O chemical potentials are computed using ab initio thermodynamics. The geometries of their stable surfaces are found to be strongly dependent on the environmental conditions.