Direct Determination of Enthalpies of Sorption Using Pressure-Gradient Differential Scanning Calorimetry: CO2 Sorption by Cu-HKUST

Enthalpy of sorption (ΔH) is an important parameter for the design of separation processes using adsorptive materials. A pressure-ramped calorimetric method is described and tested for the direct determination of ΔH values. Combining a heatflow thermogram with a single sorption isotherm enables the determination of ΔH as a function of loading. The method is validated by studying CO₂ sorption by the well-studied metal-organic framework Cu-HKUST over a temperature range of 288-318 K. The measured ΔH values compare well with previously reported data determined by using isosteric and calorimetric methods. The pressure-gradient differential scanning calorimetry (PGDSC) method produces reliable high-resolution results by direct measurement of the enthalpy changes during the sorption processes. Additionally, PGDSC is less labor-intensive and time-consuming than the isosteric method and offers detailed insight into how ΔH changes over a given loading range.

CO2-induced single-crystal to single-crystal transformations of an interpenetrated flexible MOF explained by in situ crystallographic analysis and molecular modeling

A molecular-level investigation is reported on breathing behaviour of a metal-organic framework (1) in response to CO2 gas pressure. High-pressure gas adsorption shows a pronounced step corresponding to a gate-opening phase transformation from a closed (1cp ) to a large-pore (1lp ) form. A plateau is observed upon desorption corresponding to narrow-pore intermediate form 1np which does not occur during adsorption. These events are corroborated by pressure-gradient differential scanning calorimetry and in situ single-crystal X-ray diffraction analysis under controlled CO2 gas pressure. Complete crystallographic characterisation facilitated a rationalisation of each phase transformation in the series 1cp → 1lp → 1np → 1cp during adsorption and subsequent desorption. Metropolis grand-canonical Monte Carlo simulations and DFT-PBE-D3 interaction energy calculations strongly underpin this first detailed structural investigation of an intermediate phase encountered upon desorption.

Accordion and layer-sliding motion to produce anomalous thermal expansion behaviour in 2D-coordination polymers

Solvent-free (1) and solvated (2) 2D-coordination polymers have been synthesised by varying the amount of solvent during crystallisation. 1 undergoes a unique accordion motion of 2D zig-zag interwoven layers whereas 2 experiences layer-sliding within 2D layers to produce anomalous thermal expansion behaviour.

A new dynamic framework with direct in situ visualisation of breathing under CO2 gas pressure

Structural evidence from in situ single-crystal X-ray diffraction analysis reveals flexibility in a new non-interpenetrated pillared-layer MOF that switches between a wide-pore and a narrow-pore form.

EwaldSphere: an interactive approach to teaching the Ewald sphere construction

EwaldSphere is a Microsoft Windows computer program that superimposes the Ewald sphere construction onto a small-molecule single-crystal X-ray diffractometer. The main objective of the software is to facilitate teaching of the Ewald sphere construction by depicting our classical description of the X-ray diffraction process as a three-dimensional model that can be explored interactively. Several features of the program are also useful for introducing students to the operation of a diffractometer. EwaldSphere creates a virtual reciprocal lattice based on user-defined unit-cell parameters. The Ewald sphere construction is then rendered visible, and the user can explore the effects of changing various diffractometer parameters (e.g. X-ray wavelength and intensity, goniometer angles, and detector distance) on the resulting diffraction pattern as captured by a virtual area detector. Additional digital resources are provided, including a simple but comprehensive program manual, a PowerPoint presentation that introduces the essential concepts, and an Excel file to facilitate calculation of lattice dhk spacings (required for the presentation). The program and accompanying resources are provided free of charge, and there are no restrictions on their use.

Visualizing Structural Transformation and Guest Binding in a Flexible Metal-Organic Framework under High Pressure and Room Temperature

Understanding the effect of gas molecules on the framework structures upon gas sorption in porous materials is highly desirable for the development of gas storage and separation technologies. However, this remains challenging for flexible metal-organic frameworks (MOFs) which feature "gate-opening/gate-closing" or "breathing" sorption behaviors under external stimuli. Herein, we report such a flexible Cd-MOF that exhibits "gating effect" upon CO₂ sorption. The ability of the desolvated flexible Cd-MOF to retain crystal singularity under high pressure enables the direct visualization of the reversible closed-/open-pore states before and after the structural transformation as induced by CO₂ adsorption/desorption through in situ single-crystal X-ray diffraction experiments. The binding sites of CO₂ molecules within the flexible MOF under high pressure and room temperature have also been identified via combined in situ single-crystal X-ray diffraction and powder X-ray diffraction studies, facilitating the elucidation of the states observed during gate-opening/gate-closing behaviors. Our work therefore lays a foundation to understand the high-pressure gas sorption within flexible MOFs at ambient temperature, which will help to improve the design efforts of new flexible MOFs for applications in responsive gas sorption and separation.

Giant Hysteretic Sorption of CO2 : In Situ Crystallographic Visualization of Guest Binding within a Breathing Framework at 298 K

A dynamic Zn^II -MOF has been shown to exhibit extreme breathing behavior under gas pressure. The very narrow pore form of the activated framework opens up in the presence of carbon dioxide, thus making it a suitable material for CO₂ capture. Sorption of CO₂ at 298 K and relatively high pressure clearly shows a two-step isotherm with giant hysteresis for the second step. In-situ single-crystal diffraction analysis was carried out under CO₂ gas pressure at 298 K using an environmental gas cell in order to visualize the interaction between CO₂ and the host framework. The results are well supported by pressure-gradient differential scanning calorimetry (P-DSC) and variable-pressure powder X-ray analysis. Theoretical calculations have been carried out in order to further back up the crystallographic data.

A thermo-responsive structural switch and colossal anisotropic thermal expansion in a chiral organic solid

Trianglimine, a chiral triangular-shaped hexaimine, exists in at least three apohost polymorphic forms. Form I had been previously obtained by crystallisation from a mixture of dichloromethane and acetonitrile and we have now crystallised Form II from acetone. Both forms possess similar packing arrangements, but Form II undergoes a reversible phase transition to Form III, as well as colossal anisotropic positive thermal expansion. Form I does not exhibit any remarkable thermal properties.

Hand-twistable plastically deformable crystals of a rigid small organic molecule

The crystals of the small rigid molecule 4-bromobenzonitrile exhibit highly flexible plastic bending behaviour that occurs on two perpendicular faces of the crystal, a rare situation, leading to the formation of helical/twisted and curled crystals.

Correction: A combined stretching–tilting mechanism produces negative, zero and positive linear thermal expansion in a semi-flexible Cd(ii)-MOF

Correction for ‘A combined stretching–tilting mechanism produces negative, zero and positive linear thermal expansion in a semi-flexible Cd(ii)-MOF’ by Prem Lama et al., Chem. Commun., 2014, 50, 6464–6467.

Large volumetric thermal expansion of a novel organic cocrystal over a wide temperature range

A novel cocrystal ABN·2DMABN shows the largest volumetric thermal expansion over a wide temperature range of 100–300 K for an organic cocrystal.

Uniaxial negative thermal expansion induced by moiety twisting in an organic crystal

Anomalous thermal expansion of a new diyn-diol molecule was studied by means of variable-temperature single-crystal X-ray diffraction. Analysis of the unit cell axes as a function of temperature shows that the material experiences uniaxial negative thermal expansion. Packing analysis of the crystal structures reveals twisting of the cyclopentyl moiety relative to the diyne spine with increasing temperature.

Supramolecular solvatochromism: mechanistic insight from crystallography, spectroscopy and theory

A solvatochromic dinuclear copper(ii) metallocycle effectively traps tetrahydrofuran, diethyl ether and pentane significantly above their boiling points.