Small Crown-Ether Complexes as Molecular Models for Dihydrogen Adsorption in Undercoordinated Extraframework Cations in Zeolites

1:1 metal complexes of small crown-ethers are structurally similar to extraframework sites in metal-exchanged zeolites. Using <i>ab initio</i> calculations, we show that adsorbed molecular hydrogen follows the same trends in adsorption energies and vibrational frequencies at both types of metal sites. Unlike zeolites, crown-ethers can be characterized in the gas phase, which opens new possibilities for understanding the bonding of dihydrogen at undercoordinated metal sites to help guide the rational design of porous materials for hydrogen isotope separation. Because more strongly binding adsorbates affect the geometry of the hosts, the similarity of crown-ethers and zeolites with regard to the vibrational spectra of the adsorbed molecule seems to be limited to H₂.

Small Crown-Ether Complexes as Molecular Models for Dihydrogen Adsorption in Undercoordinated Extraframework Cations in Zeolites

1:1 metal complexes of small crown-ethers are structurally similar to extraframework sites in metal-exchanged zeolites. Using <i>ab initio</i> calculations, we show that adsorbed molecular hydrogen follows the same trends in adsorption energies and vibrational frequencies at both types of metal sites. Unlike zeolites, crown-ethers can be characterized in the gas phase, which opens new possibilities for understanding the bonding of dihydrogen at undercoordinated metal sites to help guide the rational design of porous materials for hydrogen isotope separation. Because more strongly binding adsorbates affect the geometry of the hosts, the similarity of crown-ethers and zeolites with regard to the vibrational spectra of the adsorbed molecule seems to be limited to H₂.

Dehydration of microporous vanadosilicates: the case of VSH-13Na

Microporous VSH-13Na of composition Na2(VO)(Si4O10)·3H2O was synthesized under mild hydrothermal conditions and studied by single-crystal X-ray diffraction at room temperature and 398 K. Its vanadosilicate framework, consisting of sheets of silicate tetrahedra connected by vanadyl-type square-based pyramids, closely resembles that of the mineral cavansite, Ca(VO)(Si4O10)·4H2O. Due to the disorder in the orientation of the short apical vanadyl groups, the topological symmetry of VSH-13Na was originally described in space group Imma. However, when analysing the systematic absences in our dataset, only the 21 screw axis along b was strictly fulfilled suggesting monoclinic space group P1211. The resulting structure in P21 with a = 14.364 (4), b = 9.134 (2), c = 10.373 (3) Å, β = 90.056 (7)°, V = 1360.9 (7) Å3 was interpreted as a case of allotwinning of two polytypes with topologically idealized orthorhombic symmetry: A (∼62%) with antiparallel orientation of the vanadyl groups in adjacent (100) layers and B (∼38%) with all vanadyl groups in adjacent layers oriented in the same way. At 398 K, the structure of VSH-13Na became fully dehydrated and adopted the unit-cell parameters a = 12.584 (16), b = 9.525 (13), c = 9.696 (14) Å, β = 90.10 (4)°, V = 1162 (3) Å3 (space group P21). Release of H2O caused severe contraction of T—O—T angles and the unit-cell volume decreased by ∼15%. Despite their structural similarity, the VSH-13Na framework seems to be more flexible upon dehydration compared with cavansite, whose structure collapsed before removal of the last H2O molecule. Thus, the presence of monovalent or divalent extraframework cations plays a key role in the dehydration process of natural and synthetic vanadosilicates.

Influences of extraframework cations on features of natrolite group zeolites: the crystal structure of partly dehydrated K-containing paranatrolite

The partly dehydrated paranatrolite from the Khibiny massif, Russia, Na

Self-organization of extraframework cations in zeolites

Structural properties of a series of mordenite and ZSM-5 zeolites with different framework Al distribution modified with oxygenated extraframework Ga, Zn, Al, Cu and Fe complexes were investigated by means of periodic density functional theory calculations. It is demonstrated that mononuclear oxygenated and hydroxylated cationic metal complexes in high-silica zeolites tend to self-organize into binuclear complexes. In the cases of Ga- and Fe-modified zeolites, it is shown that the catalytic activity of the most stable binuclear extraframework cations is much higher than that of the hypothetical very reactive mononuclear counterparts. This is due to a weaker binding of reaction intermediates and easier regeneration of the initial active complexes in the course of the catalytic reaction. The formation of multiple-charged binuclear oxygenated metal species in zeolites is a general phenomenon. It does not require a specific distribution of the equivalent number of negative framework charges that compensate for the positive charge of the cationic complexes. The location and the stability of cationic complexes in zeolite micropores are mainly determined by the coordination properties of the metal centres.