Stef Smeets

A two-dimensional zeolitic imidazolate framework with a cushion-shaped cavity for CO2 adsorption

A new two-dimensional zeolitic imidazolate framework (named as ZIF-L) was synthesized in zinc salt and 2-methylimidazole (Hmim) aqueous solution at room temperature. ZIF-L (Zn(mim)2·(Hmim)1/2·(H2O)3/2 or C10H16N5O3/2Zn) has unique cushion-shaped cavities and leaf-like crystal morphology, and exhibits excellent CO2 adsorption properties.

SSZ-87: A Borosilicate Zeolite with Unusually Flexible 10-Ring Pore Openings

The structure of the as-synthesized borosilicate zeolite SSZ-87 has been solved by combining high-resolution X-ray powder diffraction (XPD) and rotation electron diffraction (RED) techniques. The unit cell and space group symmetry were found from the XPD data, and were essential for the initial analysis of the RED data. Although the RED data were only 15% complete, this proved to be enough for structure solution with the program Focus. The framework topology is the same as that of ITQ-52 (IFW), but for SSZ-87 the locations of the structure directing agent (SDA) and the B atoms could also be determined. SSZ-87 has large cages interconnected by 8- and 10-rings. However, results of hydroisomerization and Al insertion experiments are much more in line with those found for 12-ring zeolites. This prompted the structure analyses of SSZ-87 after calcination, and Al insertion. During calcination, the material is also partially deboronated, and the location of the resulting vacancies is consistent with those of the B atoms in the as-synthesized material. After Al insertion, SSZ-87 was found to contain almost no B and to be defect free. In its calcined and deboronated form, the pore system of SSZ-87 is more flexible than those of other 10-ring zeolites. This can be explained by the fact that the large cages in SSZ-87 are connected via single rather than double 10-ring windows and that there are vacancies in some of these 10-rings.

Synthesis, structural elucidation, and catalytic properties in olefin epoxidation of the polymeric hybrid material [Mo3O9(2-[3(5)-pyrazolyl]pyridine)]n.

The reaction of [MoO2Cl2(pzpy)] (1) (pzpy = 2-[3(5)-pyrazolyl]pyridine) with water in an open reflux system (16 h), in a microwave synthesis system (120 °C, 2 h), or in a Teflon-lined stainless steel digestion bomb (100 °C, 19 h) gave the molybdenum oxide/pyrazolylpyridine polymeric hybrid material [Mo3O9(pzpy)]n (2) as a microcrystalline powder in yields of 72–79%. Compound 2 can also be obtained by the hydrothermal reaction of MoO3, pzpy, and H2O at 160 °C for 3 d. Secondary products isolated from the reaction solutions included the salt (pzpyH)2(MoCl4) (3) (pzpyH = 2-[3(5)-pyrazolyl]pyridinium), containing a very rare example of the tetrahedral MoCl4(2–) anion, and the tetranuclear compound [Mo4O12(pzpy)4] (4). Reaction of 2 with excess tert-butylhydroperoxide (TBHP) led to the isolation of the oxodiperoxo complex [MoO(O2)2(pzpy)] (5). Single-crystal X-ray structures of 3 and 5 are described. Fourier transform (FT)-IR and FT Raman spectra for 1, 4, and 5 were assigned based on density functional theory calculations. The structure of 2 was determined from synchrotron powder X-ray diffraction data in combination with other physicochemical information. In 2, a hybrid organic–inorganic one-dimensional (1D) polymer, ∞(1)[Mo3O9(pzpy)], is formed by the connection of two very distinct components: a double ladder-type inorganic core reminiscent of the crystal structure of MoO3 and 1D chains of corner-sharing distorted {MoO4N2} octahedra. Compound 2 exhibits moderate activity and high selectivity when used as a (pre)catalyst for the epoxidation of cis-cyclooctene with TBHP. Under the reaction conditions used, 2 is poorly soluble and is gradually converted into 5, which is at least partly responsible for the catalytic reaction.

Using FOCUS to solve zeolite structures from three‐dimensional electron diffraction data

The program FOCUS [Grosse-Kunstleve, McCusker & Baerlocher (1997). J. Appl. Cryst. 30, 985–995] was originally developed to solve zeolite structures from X-ray powder diffraction data. It uses zeolite-specific chemical information (three-dimensional 4-connected framework structure with known bond distances and angles) to supplement the diffraction data. In this way, it is possible to compensate, at least in part, for the ambiguity of the reflection intensities resulting from reflection overlap, and the program has proven to be quite successful. Recently, advances in electron microscopy have led to the development of automated diffraction tomography (ADT) and rotation electron diffraction (RED) techniques for collecting three-dimensional electron diffraction data on very small crystallites. Reasoning that such data are also less than ideal (dynamical scattering, low completeness, beam damage) and that this can lead to failure of structure solution by conventional direct methods for very complex zeolite frameworks, FOCUS was modified to accommodate electron diffraction data. The modified program was applied successfully to five different data sets (four ADT and one RED) collected on zeolites of different complexities. One of these could not be solved completely by direct methods but emerged easily in the FOCUS trials.

High‐Silica Zeolite SSZ‐61 with Dumbbell‐Shaped Extra‐Large‐Pore Channels

The synthesis of the high-silica zeolite SSZ-61 using a particularly bulky polycyclic structure-directing agent and the subsequent elucidation of its unusual framework structure with extra-large dumbbell-shaped pore openings are described. By using information derived from a variety of X-ray powder diffraction and electron microscopy techniques, the complex framework structure, with 20 Si atoms in the asymmetric unit, could be determined and the full structure refined. The Si atoms at the waist of the dumbbell are only three-connected and are bonded to terminal O atoms pointing into the channel. Unlike the six previously reported extra-large-pore zeolites, SSZ-61 contains no heteroatoms in the framework and can be calcined easily. This, coupled with the possibility of inserting a catalytically active center in the channel between the terminal O atoms in place of H(+), afford SSZ-61 intriguing potential for catalytic applications.

Locating Organic Guests in Inorganic Host Materials from X-ray Powder Diffraction Data

Can the location of the organic structure-directing agent (SDA) inside the channel system of a zeolite be determined experimentally in a systematic manner? In an attempt to answer this question, we investigated six borosilicate zeolites of known framework structure (SSZ-53, SSZ-55, SSZ-56, SSZ-58, SSZ-59, and SSZ-60), where the location of the SDA had only been simulated using molecular modeling techniques in previous studies. From synchrotron powder diffraction data, we were able to retrieve reliable experimental positions for the SDA by using a combination of simulated annealing (global optimization) and Rietveld refinement. In this way, problems arising from data quality and only partially compatible framework and SDA symmetries, which can lead to indecipherable electron density maps, can be overcome. Rietveld refinement using geometric restraints were then performed to optimize the positions and conformations of the SDAs. With these improved models, it was possible to go on to determine the location of the B atoms in the framework structure. That is, two pieces of information that are key to the understanding of zeolite synthesis-the location of the organic SDA in the channel system and of the positions adopted by heteroatoms in the silicate framework-can be extracted from experimental data using a systematic strategy. In most cases, the locations of the SDAs determined experimentally compare well with those simulated with molecular modeling, but there are also some clear differences, and the reason for these differences can be understood. The approach is generally applicable, and has also been used to locate organic guests, linkers, and ligands in metal-organic compounds.

Structure analysis of a BEC-type germanosilicate zeolite including the location of the flexible organic cations in the channels

A BEC-type zeolite (polymorph C of zeolite beta) with low germanium content (Si:Ge = 5.1) has been synthesised using the flexible linear diquaternary cationic form of pentamethyldiethylenetriamine (2,2′-(methylazanediyl)bis(N,N,N-trimethylethanammonium) as the organic structure-directing agent (SDA). The distribution of germanium within the framework and the location of the SDA within the pores have been determined by analysing synchrotron X-ray powder diffraction data collected on the as-synthesised form. The findings, which are corroborated by 13C and 19F NMR data, indicate that the structure of the SDA is retained during the synthesis and that the Ge atoms occupy only sites in the double 4-ring. Although the BEC framework structure has a three-dimensional 12-ring channel system, the SDA molecules are found to orient themselves in two dimensions along the channels running parallel to the a and b axes.

Temperature-dependent analysis of thermal motion, disorder and structures of tris(ethylenediamine)zinc(II) sulfate and tris(ethylenediamine)copper(II) sulfate

The crystal structures of the title compounds have been determined in the temperature range 140-290 K for the zinc complex, and 190-270 K for the copper complex. The two structures are isostructural in the trigonal space group P31c with the sulfate anion severely disordered on a site with 32 (D(3)) symmetry. This sulfate disorder leads to a disordered three-dimensional hydrogen-bond network, with the N-H atoms acting as donors and the sulfate O atoms as acceptors. The displacement parameters of the N and C atoms in both compounds contain disorder contributions in the out-of-ligand plane direction owing to ring puckering and/or disorder in hydrogen bonding. In the Zn compound the vibrational amplitudes in the bond directions are closely similar. Their differences show no significant deviations from rigid-bond behaviour. In the Cu compound, a (presumably) dynamic Jahn-Teller effect is identified from a temperature-independent contribution to the displacement ellipsoids of the N atom along the N-Cu bond. These conclusions derive from analyses of the atomic displacement parameters with the Hirshfeld test, with rigid-body models at different temperatures, and with a normal coordinate analysis. This analysis considers the atomic displacement parameters (ADPs) from all different temperatures simultaneously and provides a detailed description of both the thermal motion and the disorder in the cation. The Jahn-Teller radii of the Cu compound derived on the basis of the ADP analysis and from the bond distances in the statically distorted low-temperature phase [Lutz (2010). Acta Cryst. C66, m330-m335] are found to be the same.

Ionothermal Synthesis and Structure of a New Layered Zirconium Phosphate

A new layered zirconium phosphate material has been synthesized ionothermally using N-ethylpyridinium (Epy) bromide as both the solvent and the template, and its structure has been solved from synchrotron X-ray powder diffraction data using the charge-flipping routine implemented in Superflip. Rietveld refinement coupled with difference electron density map analysis was used to locate the organic cations between the layers. In the final stages of refinement, it became clear that not only ethylpyridinium but also pyridinium ions were present between the zirconium phosphate layers. These findings were then corroborated using elemental analysis, TGA, and solid-state (13)C CP/MAS NMR data.

Serial electron crystallography for structure determination and phase analysis of nanocrystalline materials

Serial crystallography using electron diffraction has been developed as a fully automated method for collecting electron diffraction data from a large number of crystals. It is demonstrated how the data can be used for structure determination and phase analysis.