Titanium Silicalite-1 (TS-1), because of its crystalline structure and its well-defined Ti sites, represents the prototype of a single site catalyst. According to this fundamental aspect and to the relevant role of TS-1 as selective catalyst in important industrial partial oxidation reactions, TS-1 has been widely characterized through both experimental and computational techniques. Still, several fundamental aspects of its structural and catalytic properties have to be addressed. Among these, an intriguing topic is the Ti location in the various sites of the MFI framework. The independent sites are generally considered to be 12, following the Pnma space group of TS-1 at high Ti loading. However, when Ti loading is lower than 2 atoms per unit cell, diffraction showed that the system must be described by the P21/n space group, thereby allowing 24 independent sites. With respect to previous studies, this work aims to exploit this datum to give a more accurate description of the TS-1 system at low Ti loadings, adopting a state of the art methodology (all electron periodic B3LYP-D2 calculations). The relative stabilities of the 24 Ti sites have been evaluated, showing a good agreement with previous studies. The simulation of adsorption energies for ammonia (present as reactants in some of the most important industrial reactions catalyzed by TS-1) over the most stable sites have been computed as well, in order to validate the obtained models. Additionally to binding energies, adsorption enthalpies and Gibbs free energies have been obtained through an approximate reduced Hessian scheme. The improved local description of the Ti sites (in combination with the adducts stabilities given by the energetic data) allowed the deep understanding of subtle effects, such as the number of molecular ligands each Ti atom can actually host upon adsorption. These results, showing only few sites can efficiently host two ligands in the neighborhoods of STP conditions, allowed for the first time the heterogeneity in the experimental outcomes reported over the last two decades to be rationalized.
The peculiar adsorption-induced phase transitions in an ultramicroporous copper phosphonate framework were investigated with a combination of experimental and computational methods.
This contribution aims at analysing the current understanding about the influence of Al distribution, zeolite topology, ligands/reagents and oxidation state on ions mobility in Cu-zeolites, and its relevance toward reactivity of the metal sites. The concept of Cu mobilization has been originally observed in the presence of ammonia, favouring the activation of oxygen by formation of NH
ITQ-1 layered zeolites, with MWW framework, were prepared using different synthesis and calcination procedures and fully characterized by means of X-ray diffraction, scanning electron microscopy, N2 sorption and vibrational and nuclear magnetic resonance spectroscopies. Exploring different compositions of the precursor gel, the role of the Organic Structure Directing Agent (OSDA) was definitely disclosed. We proved that the concentration of OSDA in the synthesis gel affects the short-range crystalline order of zeolite crystals. In particular, diluted precursor gels led to the formation of low-density materials in the form of hollow spheres, with partially disordered layers, thinner crystals and high defectivity. In general, all ITQ-1 samples had the same crystalline structure but different morphology, either rose-like or hollow spheres. Moreover, the calcination procedure had an impact on the structural defects of the ITQ-1 zeolite: the higher the removal rate of the OSDA, the higher the defectivity degree of the zeolite.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
Formamide has been recognized in the literature as a key species in the formation of the complex molecules of life, such as nucleobases. Furthermore, several studies reported the impact of mineral phases as catalysts for its decomposition/polymerization processes, increasing the conversion and also favoring the formation of specific products. Despite the progresses in the field, in situ studies on these mineral-catalyzed processes are missing. In situ UV-Raman characterization of the chemical evolution of formamide over amorphous SiO2 samples, selected as a prototype of silicate minerals, was performed. The experiments were carried out after reaction of formamide at 160 °C on amorphous SiO2 (Aerosil OX50) either pristine or pre-calcined at 450 °C, to remove a large fraction of surface silanol groups. Our measurements, interpreted on the basis of density functional B3LYP-D3 calculations (Figure 1), allow to assign the spectra bands in terms of specific complex organic molecules, namely, diaminomaleonitrile (DAMN), 5-aminoimidazole (AI), and purine, showing the role of the mineral surface on the formation of relevant prebiotic molecules.
Data type: Experimental spectroscopic measurements and related elaboration from Figure 7 Files are with filename extensions: txt Information on origin of the data: Normalized Cu K-edge XANES of a Cu-CHA sample (Si/Al = 5, Cu/Al = 0.3), collected during heating from RT to 350 °C under a He flow. In situ data were collected at the BM23 beamline of the European Synchrotron Radiation Facility (ESRF, Grenoble, France) in a Microtomo reactor cell Cu L3-edge TEY NEXAFS spectra of of a Cu-CHA sample (Si/Al = 5, Cu/Al = 0.3), collected during heating from RT to 350 °C under a He flow. In situ data were collected at the APE-HE beamline of Elettra Sincrotrone Trieste (Basovizza, Italy) in a dedicated ambient pressure cell. Information on: specialized abbreviations: XANES – X-ray absorption near edge structure, TEY - Total Electron Yield, NEXAFS – near edge X-ray absorption fine structure, CHA - chabazite
