1,721,154 research outputs found
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
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
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
METHODOLOGICAL ASPECTS IN THE DEVELOPMENT OF THE LIFESTYLE SURVEILLANCE TOOLKIT IN THE ASSO PROJECT
Full text linkFollowing the presentation of the ASSO Project, as discussed in the
previous issue of JSSL (Tabacchi G. 2011; 1-3: 267-269 ), one of the first
objectives of the study is to develop helpful and efficient tools to create a webbased
software for the collection of data on adolescent lifestyles. Different
methods have been developed and used throughout Europe to collect information
on adolescent lifestyles. In order to select the best methods to be used in the
ASSO project, a Systematic Literature Review (SLR) on the most valid dietary
and physical activity/fitness assessment methods used in the target population has
been perfomed. After the SLR and a meta-analysis of the retrieved studies,
different tools have been developed within ASSO, in order to create a userfriendly
and cost-effective toolkit for the creation of a web-based software for the
data collection. A Standard Operating Procedure (SOP) for data collection has
been prepared and addressed to all the people involved in the preparation,
administration and management of the questionnaires. The SOP has the purpose
of standardizing methods and procedures to prevent systematic errors in the
collection and reporting of data. Moreover, it is useful to provide reminders and
guidelines of the correct way to deal with the information material related to the
study, to perform the interviews, to manage the compilation of the questionnaire
and to store the collected information. An informative letter on the project and the
informed consent to be signed by the parents have been developed. After
obtaining the parents’ consent, a list of all the students participating in the study
will be prepared and an identification code and a password will be attributed to
each student in order to respect the privacy
Dipolar host/guest interactions and geometrical confinement at the basis of the stability of one dimensional ice in zeolite bikitaite
No full textA LCAO-LDF Study of Brønsted Acids Chemisorption on ZnO(0001)
No full textThe local density functional theory coupled to the molecular cluster approach has been used to study the chemisorption of Br∅nsted acids (H2O, H2S, HCN, CH3OH and CH3SH) on the ZnO(0001) polar surface. Geometrical parameters and vibrational frequencies for selected species molecularly and dissociatively chemisorbed have been computed. The agreement with literature experimental data, when available, has been found to be good. The nature of the interaction between the conjugate base of the examined Br∅nsted acids and the Lewis acid site available on the surface has been elucidated, confirming its leading role in determining the actual relative acidity scale obtained by titration displacement reactions. The strength of this interaction follows the order OH− ≈ CN− > CH3O− > SH− > CH3S−
HP-induced supra-molecular organization of guest molecules in FER-type zeolites
Full text linkThe response to pressure of a synthetic all-silica ferrierite (Si-FER) and of a natural ferrierite from Monastir (Sardinia, Italy) (Mon-FER, Na0.56 K1.19 Mg2.02 Ca0.52 Sr0.14)(Al6.89Si29.04)O72 ·17.86 H2O) is here investigated combining HP synchrotron XRPD experiments and molecular dynamics simulations. The experiments were carried out by using penetrating (methanol:ethanol:water 16:3:1, m.e.w.; ethanol:water 1:3, e.w.) and non-penetrating (silicone oil, s.o.) pressure transmitting media (PTM). In Si-FER compressed in e.w., both water (w.) and ethanol molecules (e.) enter the pore system even at 0.2 GPa. The structural refinement of the data collected at 0.8 GPa reveals 8 w. and 4 e. molecules in the 10- and 6-membered ring channels, in tight agreement with the results of MD simulations. In Si-FER compressed at 0.2 GPa in m.e.w., only water molecules penetrate the 10-membered ring channels (15 per u.c.), organized in chains running along the channel axis. The interactions among the guest species and the framework oxygen atoms are very weak, due to the hydrophobicity of the framework. Upon decompression, the intruded extra-molecules are not completely released, so giving rise to new materials with different extra-framework contents. The results obtained for Si-FER compressed in m.e.w. and s.o. were compared to those obtained for Mon-FER, demonstrating that the zeolite composition and the PTM strongly influence the overall elastic parameters of the investigated samples. Specifically, Mon-FER shows a much higher rigidy than Si-FER in both media, due to the stiffening effect of the numerous extraframework species present in the natural sample. The higher rigidity of Si-FER in m.e.w. with respect to s.o. can be explained by the penetration, in the former case, of the PTM molecules, which contribute to stiffen the framework
Impact of -OH surface defects on the electronic and structural properties of nickel oxide thin films
No full textNickel oxide-based thin films and nanomaterials are a current focus of intense research efforts due to the broad range of end uses in a variety of applications. While the chemico-physical properties of bulk NiO crystals, characterized by a wide band gap (4.0-4.3 eV), antiferromagnetic ordering and p-type character, have been extensively studied, for NiO films/nanomaterials the microscopic-level relationships between the surface defect structure and electronic properties are far from being completely elucidated. In the present work, we show that, by using density functional theory with the Hubbard correction (DFT+U), -OH surface defects, almost ubiquitous on oxide surfaces, can directly influence the electronic structure of NiO(100) model slabs. Depending on the exact defect chemical structure and surface defect density, the energy gap of the -OH bearing NiO(100) system can be engineered, and its behaviour can be modulated from p-type to n-type. The insights provided herein may be of importance for the modulation of NiO nanosystem properties as a function of specific applications, an important issue for their eventual real-world utilization
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