1,721,117 research outputs found
Water in zeolite L and its MOF mimic
No full textConfinement of molecules in one dimensional arrays of channel-shaped cavities has led to technologically interesting materials. However, the interactions governing the supramolecular aggregates still remain obscure, even for the most common guest molecule: water. Herein, we use computational chemistry methods (#compchem) to study the water organization inside two different channel-type environments: zeolite L - a widely used matrix for inclusion of dye molecules, and ZLMOF - the closest metal-organic-framework mimic of zeolite L. In ZLMOF, the methyl groups of the ligands protrude inside the channels, creating nearly isolated nanocavities. These cavities host well-separated ring-shaped clusters of water molecules, dominated mainly by water-water hydrogen bonds. ZLMOF provides arrays of "isolated supramolecule" environments, which might be exploited for the individual confinement of small species with interesting optical or catalytic properties. In contrast, the one dimensional channels of zeolite L contain a continuous supramolecular structure, governed by the water interactions with potassium cations and by water-water hydrogen bonds. Water imparts a significant energetic stabilization to both materials, which increases with the water content in ZLMOF and follows the opposite trend in zeolite L. The water network in zeolite L contains an intriguing hypercoordinated structure, where a water molecule is surrounded by five strong hydrogen bonds. Such a structure, here described for the first time in zeolites, can be considered as a water pre-dissociation complex and might explain the experimentally detected high proton activity in zeolite L nanochannels
Direct Solvent-Free Amide Bond Formation Catalyzed by Anatase-TiO2 Surface: Insight from Modeling
Full text linkAmide bond formation processes are of paramount relevance for a broad spectrum of applications. Conventional amidation protocols typically rely on drastic reaction conditions and the use/disposal of large amounts of chemicals. These limitations may be bypassed by heterogeneously catalyzed amidation at dry conditions. However, progress is hindered because the mechanisms of these processes are largely unexplored. By using ab initio metadynamics, a concerted one-step mechanism is proposed for the solvent-free condensation of methylamine and formic acid on TiO2(101)-anatase, leading to methylformamide with concomitant release of molecular water. The activation barrier-14.3 kcal mol-1-is in line with the mild conditions experimentally adopted in amide bond syntheses on TiO2 nanoparticles. The mechanism disclosed herein reveals the key role of Ti4+ sites located on stoichiometric (101) anatase surfaces in promoting amide-bond formation at the TiO2/vapor interface. The acid strength of the adsorbed HCOOH molecules may be tuned by the HCOOH surface coverage, thus influencing the outcome of the amidation reaction. These molecular-level insights may foster further endeavors to improve/upscale TiO2-catalyzed amide syntheses at dry conditions, while raising the interest toward amidation processes at the surface/vapor interface promoted by economically and environmentally sustainable metal oxide nanomaterials
"Enhancing re-use of public sector information in the European Union: overview of practices evolved in the Member States"
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Electronic spectra of Ti(IV) in zeolites: an ab initio approach
No full textCharge transfer in Ti–zeolites: Model titanium–zeolites containing a single Ti(IV) tetrahedral species were built and their electronic excitation spectra were calculated (see picture) via time-dependent density functional theory (TDDFT). A multiple-band profile was obtained in each case, thus highlighting that the features observed in the UV/Vis spectra are not a direct evidence of chemically different Ti(IV) sites
A density functional study of the aluminium dimer
No full textThe ground state of the aluminium dimer was investigated using the density functional method within the self-interaction correction to local spin density. The results confirm that the lowest states are triplets according to the most recent ab initio quantum chemistry calculations, even if the ordering of the symmetries is reversed. The lowest state is interpreted in terms of two spin-parallel one-electron banana bonds. Spin-coupled results are also presented in order to complete the scenario of different computational schemes. © 1992
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