Tracking Lattice Distortion Induced by Defects and Framework Tin in Beta Zeotypes.

The use of powder X-ray diffraction (PXRD) coupled with lattice parameter refinement is used to investigate the crystal structure of Sn-Beta materials. A newly developed semiempirical PXRD model with a reduced tetragonal unit cell is applied to obtain the characteristic crystallographic features. There is a robust correlation between lattice parameters and the concentration of tin and defects for materials prepared via hydrothermal (HT) and postsynthetic (PT) methods. With tin incorporation, PT Sn-Beta samples, which possess a more defective structure, exhibit an extended interlayer distance in the stacking sequence and expansion of the translation symmetry within the layers, leading to larger unit cell dimensions. In contrast, HT Sn-Beta samples, having fewer defects, show a minimal effect of tin site density on the unit cell volume, whereas lattice distortion is directly correlated to the framework tin density. Furthermore, density functional theory (DFT) studies support an identical trend of lattice distortion following the monoisomorphous substitution of T sites from silicon to tin. These findings highlight that PXRD can serve as a rapid and straightforward characterization method to evaluate both framework defects and heteroatom density, offering a novel approach to monitor structural changes and the possibility to evaluate the catalytic properties of heteroatom-incorporated zeotypes.

Observation of an O8 molecular lattice in the epsilon phase of solid oxygen.

Of the simple diatomic molecules, oxygen is the only one to carry a magnetic moment. This makes solid oxygen particularly interesting: it is considered a 'spin-controlled' crystal that displays unusual magnetic order. At very high pressures, solid oxygen changes from an insulating to a metallic state; at very low temperatures, it even transforms to a superconducting state. Structural investigations of solid oxygen began in the 1920s and at present, six distinct crystallographic phases are established unambiguously. Of these, the epsilon phase of solid oxygen is particularly intriguing: it exhibits a dark-red colour, very strong infrared absorption, and a magnetic collapse. It is also stable over a very large pressure domain and has been the subject of numerous X-ray diffraction, spectroscopic and theoretical studies. But although epsilon-oxygen has been shown to have a monoclinic C2/m symmetry and its infrared absorption behaviour attributed to the association of oxygen molecules into larger units, its exact structure remains unknown. Here we use single-crystal X-ray diffraction data collected between 13 and 18 GPa to determine the structure of epsilon-oxygen. We find that epsilon-oxygen is characterized by the association of four O2 molecules into a rhombohedral molecular unit, held together by what are probably weak chemical bonds. This structure is consistent with existing spectroscopic data, and further validated by the observation of a newly predicted Raman stretching mode.

Real-time regeneration of a working zeolite monitored via operando X-ray diffraction and crystallographic imaging: how coke flees the MFI framework.

We have monitored the regeneration of H-ZSM-5 via operando time-resolved powder X-Ray diffraction (PXRD) coupled with mass spectroscopy (MS). Parametric Rietveld refinements and calculation of the extra-framework electronic density by differential Fourier maps analysis provide details on the mode of coke removal combined with the corresponding sub-unit cell changes of the zeolite structure. It is clear that the coke removal is a complex process that occurs in at least two steps; a thermal decomposition followed by oxidation. In a coked zeolite, the straight 10-ring channel circumference is warped to an oval shape due to structural distortion induced by rigid aromatic coke species. The data presented explain why the difference in length between the a-vector and the b-vector of the MFI unit cell is a robust descriptor for bulky coke, as opposed to the unit cell volume, which is affected also by adsorbed species and thermal effects. Our approach holds the promise to quantify and identify coke removal (and formation) in structurally distinct locations within the zeolite framework.

Pyrolysis of a metal-organic framework followed by in situ X-ray absorption spectroscopy, powder diffraction and pair distribution function analysis.

Metal-organic frameworks (MOFs) can serve as precursors for new nanomaterials via thermal decomposition. Such MOF-derived nanomaterials (MDNs) are often comprised of metal and/or metal oxide particles embedded on porous carbon. The morphology of MDNs is similar to that of the precursor MOF, and improved stability and catalytic properties have been demonstrated. However, the pathway from MOF to MDN is only well understood for a few systems, and in situ studies are needed to elucidate the full phase behaviour and time/temperature dependency. In this work, we follow the MOF-to-MDN transformation in situ by using three complementary techniques: X-ray absorption spectroscopy (XAS), powder X-ray diffraction (PXRD), and X-ray total scattering/pair distribution function (TS/PDF) analysis. The thermal decomposition of HKUST-1, i.e. the archetypical MOF Cu3(btc = 1,3,5-benzenetricarboxylate)2, is followed from room temperature to 500 °C by applying different heating ramps. Real space correlations are followed by PDF and extended X-ray absorption fine structure (EXAFS) analysis, and quantitative phase fractions are obtained by refinement of PXRD and PDF data, and by linear combination analysis (LCA) of X-ray absorption near edge Structure (XANES) data. We find that HKUST-1 decomposes at 300-325 °C into copper(I) oxide and metallic copper. Above 350-470 °C, metal particles remain as the only copper species. There is an overall good agreement between all three techniques with respect to the phase evolution, and the study paves the road towards rational synthesis of a Cu2O/Cu/carbon material with the desired metal/metal oxide composition. More importantly, our investigations serve as a benchmark study demonstrating that this methodology is generally applicable for studying the thermal decomposition of MOFs.

On the structure of high-pressure high-temperature eta-O2.

In situ high-pressure high-temperature x-ray diffraction and optical studies have been conducted on solid oxygen between 10 and 20 GPa and up to 700 K. Optical observations and Raman spectroscopic studies have been utilized to confirm the existence of eta-O(2) and to identify phase behavior and phase boundaries of beta-, epsilon- and eta-O(2) at elevated temperatures. Subsequent single-crystal synchrotron x-ray diffraction studies yielded the structure of the eta-O(2) phase at 15.9 GPa and 625 K.

K-paracelsian (KAlSi3O8·H2O) and identification of a simple building scheme of dense double-crankshaft zeolite topologies.

During screening of the phase space using KOH and 1-methyl-4-aza-1-azoniabicyclo-[2.2.2]octane hydroxide (1-methyl-DABCO) under hydrothermal zeolite synthesis conditions, K-paracelsian was synthesized. Scanning electron microscopy, energy dispersive X-ray spectroscopy and ex situ powder X-ray diffraction analysis revealed a material that is compositionally closely related to the mineral microcline and structurally closely related to the mineral paracelsian, both of which are feldspars. In contrast to the feldspars, K-paracelsian contains intrazeolitic water corresponding to one molecule per cage. In the case of K-paracelsian it might be useful to consider it a link between feldspars and zeolites. It was also shown that K-paracelsian can be described as the simplest endmember of a family of dense double-crankshaft zeolite topologies. By applying the identified building principle, a number of known zeolite topologies can be constructed. Furthermore, it facilitates the construction of a range of hypothetical small-pore structures that are crystallo-chemically healthy, but which have not yet been realized experimentally.

Deactivation of Zeolite Catalyst H-ZSM-5 during Conversion of Methanol to Gasoline: Operando Time- and Space-Resolved X-ray Diffraction.

The deactivation of zeolite catalyst H-ZSM-5 by coking during the conversion of methanol to hydrocarbons was monitored by high-energy space- and time-resolved operando X-ray diffraction (XRD) . Space resolution was achieved by continuous scanning along the axial length of a capillary fixed bed reactor with a time resolution of 10 s per scan. Using real structural parameters obtained from XRD, we can track the development of coke at different points in the reactor and link this to a kinetic model to correlate catalyst deactivation with structural changes occurring in the material. The "burning cigar" model of catalyst bed deactivation is directly observed in real time.

Characterization of Cu-exchanged SSZ-13: a comparative FTIR, UV-Vis, and EPR study with Cu-ZSM-5 and Cu-ß with similar Si/Al and Cu/Al ratios.

Cu-SSZ-13 has been characterized by different spectroscopic techniques and compared with Cu-ZSM-5 and Cu-ß with similar Si/Al and Cu/Al ratios and prepared by the same ion exchange procedure. On vacuum activated samples, low temperature FTIR spectroscopy allowed us to appreciate a high concentration of reduced copper centres, i.e. isolated Cu(+) ions located in different environments, able to form Cu(+)(N2), Cu(+)(CO)n (n = 1, 2, 3), and Cu(+)(NO)n (n = 1, 2) upon interaction with N2, CO and NO probe molecules, respectively. Low temperature FTIR, DRUV-Vis and EPR analysis on O2 activated samples revealed the presence of different Cu(2+) species. New data and discussion are devoted to (i) [Cu-OH](+) species likely balanced by one framework Al atom; (ii) mono(µ-oxo)dicopper [Cu2(µ-O)](2+) dimers observed in Cu-ZSM-5 and Cu-ß, but not in Cu-SSZ-13. UV-Vis-NIR spectra of O2 activated samples reveal an intense and finely structured d-d quadruplet, unique to Cu-SSZ-13, which is persistent under SCR conditions. This differs from the 22,700 cm(-1) band of the mono(µ-oxo)dicopper species of the O2 activated Cu-ZSM-5, which disappears under SCR conditions. The EPR signal intensity sets Cu-ß apart from the others.

Structural diversity of sodium.

Sodium exhibits a pronounced minimum of the melting temperature at approximately 118 gigapascals and 300 kelvin. Using single-crystal high-pressure diffraction techniques, we found that the minimum of the sodium melting curve is associated with a concentration of seven different crystalline phases. Slight changes in pressure and/or temperature induce transitions between numerous structural modifications, several of which are highly complex. The complexity of the phase behavior above 100 gigapascals suggests extraordinary liquid and solid states of sodium at extreme conditions and has implications for other seemingly simple metals.