1,721,049 research outputs found
Magnesium-based systems for carbon dioxide capture, storage and recycling: from leaves to synthetic nanostructured materials
No full textA steep rise of carbon dioxide level in the atmosphere is one of the main causes of global warming. This increase is ascribed to the fact that, since the beginning of the industrial revolution, natural processes for CO2 sequestration are no longer able to cope with the excess of CO2 produced by anthropogenic activities. In recent years, research has been focused on defining artificial CO2 cycles to support the natural one. The element magnesium is used in this review as leitmotif to explore the majority of systems involved in each step of the natural and artificial CO2 cycles (separation, storage, sequestration or recycling). Magnesium is in fact ubiquitous, being present in the mesosphere as global layers, on the Earth's surface in the most important enzyme for carbon fixation (Rubisco) and in silicates that constitute the major part of rocks, where CO2 is sequestered through natural weathering. For what concerns synthetic materials, zeolites, metal-supported particles and metal-organic frameworks are only a few of the systems considered in the literature. The intent of this review is to connect different fields of study to create an interdisciplinary review in the chemistry domain. Research outlooks are suggested for the different fields. In the end, a qualitative analysis of the advantages and limits of different processes and a rough estimate of their potential are given in terms of the time needed to reduce the atmospheric CO2 level. Although economical, political and health evaluations would be also necessary, this analysis indicates that forestation could be a possible winning solution in the short-middle term for lowering the atmospheric CO2 concentration
Thermal Treatment Effect on CO and NO Adsorption on Fe(II) and Fe(III) Species in Fe3O-Based MIL-Type Metal-Organic Frameworks: A Density Functional Theory Study
Get PDFThe properties of metal-organic frameworks (MOFs) based on triiron oxo-centered (Fe3O) metal nodes are often related to the efficiency of the removal of the solvent molecules and the counteranion chemisorbed on the Fe3O unit by postsynthetic thermal treatment. Temperature, time, and the reaction environment play a significant role in modifying key features of the materials, that is, the number of open metal sites and the reduction of Fe(III) centers to Fe(II). IR spectroscopy allows the inspection of these postsynthetic modifications by using carbon monoxide (CO) and nitric oxide (NO) as probe molecules. However, the reference data sets are based on spectra recorded for iron zeolites and oxides, whose structures are different from the Fe3O one. We used density functional theory to study how the adsorption enthalpy and the vibrational bands of CO and NO are modified upon dehydration and reduction of Fe3O metal nodes. We obtained a set of theoretical spectra that can model the modification observed in previously reported experimental spectra. Several CO and NO bands were previously assigned to heterogeneous Fe(II) and Fe(III) sites, suggesting a large defectivity of the materials. On the basis of the calculations, we propose an alternative assignment of these bands by considering only crystallographic iron sites. These findings affect the common description of Fe3O-based MOFs as highly defective materials. We expect these results to be of interest to the large community of scientists working on Fe(II)- and Fe(III)-based MOFs and related materials
Increasing the stability of Mg2(dobpdc) metal-organic framework in air through solvent removal
No full textMg2(dopbdc) (H4-dobpdc = 4,4′-dihydroxy-(1,1′-biphenyl)-3,3′-dicarboxylic acid) is an important MOF for its astonishing properties in gas separation and storage, in particular for applications related to CO2 separation. Its technological relevance is limited by its reputation for fast decomposition in air. In this article, this reputation is denied by showing that, if fully desolvated, the MOF is stable in air for 24 h (25 °C and 28-35 RH%), a time sufficiently long to allow its processing in a non-inert environment and its use as a CO2 scrubber in a wet post-combustion flow. In contrast, the material solvated with methanol showed a 93% decrease in surface area under the same conditions. Material activation is proposed here as a simple way to increase the stability of Mg2(dobpdc) in air, by the removal of preadsorbed polar molecules that can act as seeds for water condensation. This work allows us to make some important general considerations which can be taken into account in studies on material sensitivity to air. The importance of quantifing the aging time and testing the materials at different relative humidity is stressed. These parameters are important to determine, when possible, the conditions of use and storage, suitable for large scale processing and applications. Among the adopted characterization techniques (XRD, IR spectroscopy, N2 and CO2 volumetry), N2 volumetry at 77 K turned out to be the most suitable to evidence small variations in the structure due to decomposition. In contrast, CO2 uptake at RT and 1 bar was not a reliable quantity to evaluate MOF stability, because of the significant change in the (apparent) MOF affinity toward CO2 upon damaging
Modeling metal influence on the gate opening in ZIF-8 materials
Get PDFZeolitic imidazolate frameworks (ZIFs) undergo pressure-induced phase transitions that are peculiar to each ZIF. The phase transition is associated with the rotation of the imidazolate, and it is accompanied by an increase of their pore openings, affecting ZIFs performance in separation processes. This phenomenon is known as the gate opening or the swing effect. Here we report the metal dependence of the ambient pressure and high pressure (HP) phases of ZIF-8(M) with M = Mg, Fe, and Zn, determined by using periodic Kohn-Sham density functional models. We show that the substitution of Zn with Mg or Fe has a big influence on the gate opening energy, which significantly decreases, an opposite trend than what was previously reported upon functionalization of the linker. The lowest energy phase of ZIF-8(Fe) is different than for ZIF-8(Mg) and ZIF-8(Zn), and its structure is significantly closer to the HP phase. Multireference wave function methods have been used to study the electronic structure of ZIF-8(Fe), confirming the metal center to be high spin (S = 2) divalent iron in antiferromagnetic coupling. The high-spin nature of the iron species coupled with a band gap in the visible light range makes ZIF-8(Fe) an interesting material for catalysis and photocatalysis
Monolithic Aerogels Based on Poly(2,6-diphenyl-1,4-phenylene oxide) and Syndiotactic Polystyrene
No full textMolecular sorption behavior of amorphous and semicrystalline samples based on poly(2,6-diphenyl-1,4-phenylene oxide) (PPPO) has been compared. Fully amorphous PPPO powders, as obtained by supercritical carbon dioxide (scCO(2)) extraction of concentrated solutions, present uptake of pollutants much higher than for commercial sorbent materials based on semicrystalline PPPO (Tenax TA). Robust monolithic aerogels with good handling characteristics can be easily obtained by solvent extraction by scCO(2) from gels including PPPO blends with syndiotactic polystyrene.(s-PS). These monolithic PPPO/s-PS aerogels present many advantages as sorbent materials with respect to both amorphous and semicrystalline PPPO powders. In fact, besides the obvious advantages in terms of easier and safer handling, the new monolithic aerogels present higher surface areas and equilibrium guest uptakes
Water-driven structural transformation in cobalt trimesate metal-organic frameworks
Get PDFWe report on the synthesis and the characterization of a novel cobalt trimesate metal-organic framework, designated as KCL-102. Powder X-ray diffraction pattern of KCL-102 is dominated by a reflection at 10.2◦ (d-spacing = 8.7 Å), while diffuse reflectance UV-Vis spectroscopy indicates that the divalent cobalt centers are in two different coordination geometries: tetrahedral and octahedral. Further, the material shows low stability in humid air, and it transforms into the well-known phase of hydrous cobalt trimesate, Co3 (BTC)2·12H2O. We associated this transition with the conversion of the tetrahedral cobalt to octahedral cobalt
Comparing the reaction profiles of single iron catalytic sites in enzymes and in reticular frameworks for methane-to-methanol oxidation
Get PDFThe design of synthetic inorganic catalysts mimicking the first coordination spheres of enzymatic cofactors often results in lower yields and selectivity than their biological counterparts. In this study, we exploit Kohn-Sham density functional methods to compare the reaction profiles of four single iron-based catalysts for the direct oxidation of methane to methanol: two biomimetic models based on two enzymes (cytochrome P450 and taurine dioxygenase [TauD]) and two synthetic reticular frameworks (iron-BEA zeolite and tri-iron oxo-center-based metal-organic framework). Both the biomimetic and inorganic catalysts show almost zero selectivity toward methanol for methane conversions >1% at ambient temperature. This study highlights that iron's first coordination shell can influence selectivity toward methanol but to a limited extent. In the absence of methanol protection strategies, high selectivity can be reached only by mimicking the reaction microenvironment of enzymes beyond the first coordination shell of iron
- …
