1,721,011 research outputs found
Polarizable atomistic approaches to model electronic properties of complex molecular systems
Full text linkThe UV-Visible Absorption Spectra of Coumarin and Nile Red in Aqueous Solution : A Polarizable QM/MM Study
Full text linkWe present a comprehensive computational study of the UV-visible absorption spectra of 7-methoxycoumarin and Nile red in aqueous solution. Our fully atomistic workflow couples classical molecular dynamics (MD) with polarizable QM/MM based on fluctuating charges (QM/FQ) and dipoles (QM/FQF mu). Ensemble-averaged spectra are constructed from the snapshots extracted from the MD, embedding solvent fluctuations and specific solute-solvent interactions in the electronic response of organic dyes. The spectral profiles, obtained at the various levels, reflect the underlying solute-solvent interactions and dynamics, and we rationalize them in terms of hydrogen bonding and frontier molecular orbitals involved in the main electronic transitions. Finally, the simulated spectra and solvatochromic shifts are compared with the available experimental data, showing an overall good agreement and demonstrating the robustness of the computational protocol
A systematic procedure for the virtual reconstruction of open-cell foams
Full text linkOpen-cell foams are considered a potential candidate as an innovative catalyst support in many processes of the chemical industry. In this respect, a deeper understanding of the transport phenomena in such structures can promote their extensive application. In this contribution, we propose a general procedure to recover a representative open-cell structure starting from some easily obtained information. In particular, we adopt a realistic description of the foam geometry by considering clusters of solid material at nodes and different strut-cross sectional shapes depending on the void fraction. The methodology avoids time-consuming and expensive measuring techniques, such as micro-computed tomography (μCT) or magnetic resonance imaging (MRI). Computational Fluid Dynamics (CFD) could be a powerful instrument to enable accurate analyses of the complex flow field and of the gas-to-solid heat and mass transport. The reconstructed geometry can be easily exploited to generate a suitable computational domain allowing for the detailed investigation of the transport properties on a realistic foam structure by means of CFD simulations. Moreover, the proposed methodology easily allows for parametric sensitivity analysis of the foam performances, thus being an instrument for the advanced design of these structures. The geometrical properties of the reconstructed foams are in good agreement with experimental measurements. The flow field established in complex tridimensional geometries reproduces the real foam behavior as proved by the comparison between numerical simulations and experiments
A 4,4′-bis(2-benzoxazolyl)stilbene luminescent probe: Assessment of aggregate formation through photophysics experiments and quantum-chemical calculations
No full textA combination of experimental and quantum mechanical investigations is applied to the study of the optical features of 4,4′-bis(2-benzoxazolyl)stilbene (BBS) dissolved in solution or in a poly(l-lactic acid) (PLA) thermoplastic matrix at different concentrations. The experimental analyses allow the characterization of BBS solutions and dispersions in terms of absorption and emission features, along with the collection of some key parameters such as fluorescence quantum yield, anisotropy and lifetime, while the computational approach gives a detailed description of the photophysical behavior of BBS in the different environments. For the 10-5 M BBS solution, the fluorescence spectra show the expected peaks at 425 and 455 nm of the non-interacting BBS molecules with a single fluorescence lifetime of 0.85 ns without revealing any aggregation phenomena, prevented by the short lifetime and fast diffusion rate of the monomer. Moreover, the calculated spectra are in excellent agreement with the experiments, thus showing the reliability of the computational approach. In time-resolved emission experiments (TRES) on more concentrated solutions (10-4 M) and on BBS crystals, the presence of an excimer is revealed by the appearance of a broad peak around 540 nm, followed by the disappearance of the two main peaks at 460 nm on a time scale of about 10 ns. The computational analysis attributes this behavior to the formation of aggregates of different geometries. The BBS dispersions in PLA reveal the presence of different BBS architectures depending on the fluorophore content. Even at low concentrations, BBS is mainly dispersed as a monomer in the matrix, spheroid aggregates of about 800-900 nm in diameter are also present and the relevant fluorescence spectra arise from the combination of monomer and aggregate contributions. At higher concentrations, BBS starts forming crystals of a peculiar helicoidal shape, with a diameter of about 2 μm, variable length up to several hundreds of μm and emission spectra similar to those of isolated BBS crystals
Directly electrically heated radial flow reactor for endothermic catalytic processes
No full textAn electrically heated radial flow chemical reactor is described for efficiently providing reaction heat to the endothermic chemical processes of the catalytic type
A fundamental investigation of gas/solid mass transfer in open-cell foams using a combined experimental and CFD approach
Full text linkIn this work, we combine numerical (CFD) simulations and experimental measurements in a fundamental investigation of the fluid-solid mass transfer properties of open-cell foams, which are promising support for catalytic applications limited by external heat and mass transfer. CFD simulations are exploited to gain insight into the complex transport mechanisms and to enable a parametric analysis of the geometrical features by means of virtually-generated structures. Catalytic activity experiments under diffusion control are used to validate the CFD results and to extend the range of conditions and foam morphologies investigated. Analysis of the flow field by CFD simulations provides a rational basis for the choice of the average strut size as a physically sound characteristic length for mass transfer correlations. Results from both numerical simulations and experimental tests are interpreted according to a fully-theoretically based geometrical model for the prediction of the specific surface area, which accounts for the detailed node-strut geometry. The effects of cell size and strut shape are properly included in the functional dependence of the Sherwood number on the Reynolds number. The effect of porosity requires one additional dependence, wherein the Sherwood number is inversely proportional to the square of the void fraction. The resulting Sherwood–Reynolds correlation is in excellent agreement with experimental data and CFD simulations. It enables accurate (±15%) estimation of the external mass transfer coefficients for open-cell foams when coupled with the proposed geometrical model from two readily accessible pieces of geometrical information, i.e. the void fraction and either the cell size or the pore diameter of the foam. The derived correlation can be applied to the design of novel enhanced open-cell foam catalyst substrates and structured reactors
Reactor design via scan line patterning: an implicit approach to create scalable microstructured parts in selective laser melting
No full textThe adaptation of additive manufacturing for chemical flow reactors has recently gained momentum as the methods become more advanced and equipment is increasingly affordable. With the design of periodic open cellular structures (POCS) from metal selective laser melting (SLM), structured catalyst substrates can be realized in a much more ordered fashion than well-established metal foams with a random structure. Hence, tailored transport properties are achieved and flow profiles are homogenized, eliminating flow channeling and stagnant flow zones. However, with current SLM methods, the minimal achievable feature size and therewith the specific surface area is still limited. Moreover, the size of finely resolved STL files that define the structure grows exponentially with decreasing feature sizes, further limiting the scalability of methods with direct structure definition. In this work, we present a method for implicit structure design by metal SLM with specific surface areas and porosities that compete with established catalyst substrates. The definition of the microstructure is realized by control of the laser scan lines, where single scan lines create fine struts. The general applicability of our method to various metal materials is demonstrated by production from stainless steel and copper. In a numerical study for a model steam reforming application, we show that the POCS provide mass transfer coefficients and volumetric reaction rates that compete with benchmark substrates. Scan line patterning therefore provides a promising method to create highly ordered, well-scalable substrates for heterogeneous catalysis from a wide range of materials, e.g., for the application in highly efficient small-scale reactors
Joule-Heated Catalytic Reactors toward Decarbonization and Process Intensification: A Review
Full text linkThe supply of the heat required for chemical processes via renewable electricity, i.e., process electrification, provides an alternative strategy for replacing conventional fossil fuel combustion. This approach enables fast, selective, and uniform heating, offers great potential for utilizing the excess renewable electric energy, and brings about an important chance for mitigating CO2 emissions. In this work, we provide an overview of the state-of-the-art electricity-to-heat driven catalytic processes. The principle and fundamentals of Joule heating are provided and briefly compared to induction and microwave heating in view of electrifying catalytic processes. By this comparison, we assess that Joule heating can be regarded as the most promising method for process electrification, and its applications to methane reforming, cracking reactions, CO2 valorization, and transient process operation are then reviewed. Advantages and disadvantages are critically addressed in terms of efficiency, potential for scale-up and possibility of retrofitting. The current challenges in the development of advanced electrified processes as well as the opportunities of next generation electrification techniques are discussed
Close-Up Look at Electronic Spectroscopic Signatures of Common Pharmaceuticals in Solution
Full text linkSimulating electronic properties and spectral signals requires robust computational approaches that need tuning with the system’s peculiarities. In this paper, we test implicit and fully atomistic solvation models for the calculation of UV-vis and electronic circular dichroism (ECD) spectra of two pharmaceutically relevant molecules, namely, (2S)-captopril and (S)-naproxen, dissolved in aqueous solution. Room temperature molecular dynamics simulations reveal that these two drugs establish strong contacts with the surrounding solvent molecules via hydrogen bonds. Such specific interactions, which play a major role in the spectral response and are neglected in implicit approaches, are further characterized and quantified with natural bond orbital methods. Our calculations show that simulated spectra, and especially ECD, are in good agreement with experiments solely when conformational and configurational dynamics, mutual polarization, and solute-solvent repulsion effects are considered
Analytical Geometrical Model of Open Cell Foams with Detailed Description of Strut-Node Intersection
No full textOpen cell foams are regarded as potential enhanced catalyst carriers due to their high specific surface areas, low pressure drops, and good heat and mass transfer rates. However, it is difficult to evaluate their surface area. An analytical model is proposed, which is derived entirely from geometrical hypotheses and can be applied using different easily accessible input parameters. It can accurately estimate the specific surface and other features of foams for a wide range of porosities. The application of the new model to the reevaluation of published gas/solid mass transfer data is demonstrated
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