Dendritic tip selection during solidification of alloys:Insights from phase-field simulations

The effect of undercooling DT and the interface energy anisotropy parameter e4 on the shape of the equiaxed dendritic tip has been investigated by using a quantitative phase-field model for solidification of binary alloys.It was found that the tip radius r increases and the tip shape amplitude coefficient A4 decreases with the increase of the fitting range for all cases.The dendrite tip shape selection parameter sdecreases and then stabilizes with the increase of the fitting range,and sincreases with the increase of e4.The relationship between sand e4 follows a power-law function sµea 4,and a is independent of DT but dependent on the fitting range.Numerical results demonstrate that the predicted sis consistent with the curve of microscopic solvability theory(MST)for e4<0.02,and sobtained from our phase-field simulations is sensitive to the undercooling when e4 is fixed.

Shape-selective alkylation of benzene with ethylene over a core–shell ZSM-5@MCM-41 composite material

A series of ZSM-5@MCM-41 core-shell composite materials prepared via a multi-cycle-sol-gel coating strategy is investigated as the catalyst for benzene alkylation with ethylene,in which both ethylbenzene and para-diethylbenzene(p-DEB)are aimed as the target products.With multi-cycle-sol-gel coating,the external acid sites on the samples are gradually passivated by the inert MCM-41 shell.As a result,the shape selectivity to p-DEB is greatly enhanced.Nevertheless,the coating of mesoporous MCM-41 shell on ZSM-5 accelerates deactivation of the catalyst only due to the dilution effect of ZSM-5 content in the catalyst at the same space velocity,which is a reason that core-shell ZSM-5@MCM-41 will potentially be a practical catalyst in shape selective alkylation of benzene.In order to enhance the yield of p-DEB on ZSM-5@MCM-41,the reaction conditions at the fixed bed reactor including temperature,the molar rate of benzene to ethylene and GHSV,are also optimized.

Local Radial Basis Function Methods: Comparison, Improvements, and Implementation

Radial Basis Function methods for scattered data interpolation and for the numerical solution of PDEs were originally implemented in a global manner. Subsequently, it was realized that the methods could be implemented more efficiently in a local manner and that the local approaches could match or even surpass the accuracy of the global implementations. In this work, three localization approaches are compared: a local RBF method, a partition of unity method, and a recently introduced modified partition of unity method. A simple shape parameter selection method is introduced and the application of artificial viscosity to stabilize each of the local methods when approximating time-dependent PDEs is reviewed. Additionally, a new type of quasi-random center is introduced which may be better choices than other quasi-random points that are commonly used with RBF methods. All the results within the manuscript are reproducible as they are included as examples in the freely available Python Radial Basis Function Toolbox.

Phosphite Ligand Modified Supported Rhodium Catalyst for Hydroformylation of Internal Olefins to Linear Aldehydes

A phosphite ligand modified heterogeneous catalyst was developed for the hydroformylation of internal olefins to linear aldehydes, which showed a high activity and high regioselectivity and could be separated easily by filtration after reaction in an autoclave. Three nanoporous silica sieves were used to investigate the influence of pore structure and shape selective performance of support on the regioselectivity to the linear products.

Plasmonic coupling-enhanced in situ photothermal nanoreactor with shape selective catalysis for C–C coupling reaction

Carbon-carbon(C–C)coupling reactions represent one of the most powerful tools for the synthesis of complex natural products,bioactive molecules developed as drugs and agrochemicals.In this work,a multifunctional nanoreactor for C–C coupling reaction was successfully fabricated via encapsulating the core-shell Cu@Ni nanocubes into ZIF-8(Cu@Ni@ZIF-8).In this nanoreactor,Ni shell of the core-shell Cu@Ni nanocubes was the catalytical active center,and Cu core was in situ heating source for the catalyst by absorbing the visible light.Moreover,benefiting from the plasmonic resonance effect between Cu@Ni nanocubes encapsulated in ZIF-8,the absorption range of nanoreactor was widened and the utilization rate of visible light was enhanced.Most importantly,the microporous structure of ZIF-8 provided shape-selective of reactant.This composite was used for the highly shape-selective and stable photocatalysed C–C coupling reaction of boric acid under visible light irradiation.After five cycles,the nanoreactor still remained high catalytical activity.This Cu@Ni@ZIF-8 nanoreactor opens a way for photocatalytic C–C coupling reactions with shape-selectivity.

Shape selective catalysis in methylation of toluene： Development, challenges and perspectives

Toluene methylation with methanol offers an alternative method to produce p-xylene by gathering methyl group directly from C1 chemical sources. It supplies a ＂molecular engineering＂ process to realize directional conversion of toluene/methanol molecules by selective catalysis in complicated methylation system. In this review, we introduce the synthesis method ofp-xylene, the development history of methylation catalysts and reaction mechanism, and the effect of reaction condition in para-selective technical process. If constructing p-xylene as the single target product, the major challenge to develop para-selective toluene methylation is to improve the p-xylene selectivity without, or as little as possible, losing the fraction of methanol for methylation. To reach higher yield ofp-xylene and more methanol usage in methylation, zeolite catalyst design should consider improving mass transfer and afterwards coveting external acid sites by surface modification to get short ＂micro-tunnels＂ with shape selectivity. A solid understanding of mass transfer will benefit realizing the aim of converting more methanol feedstock into para-methyl group.

Effect of Powder Particle Shape on the Properties of In Situ Ti–TiB Composite Materials Produced by Selective Laser Melting

This work studied the preparation of starting powder mixture influenced by milling time and its effect on the particle morphology （especially the shape） and, consequently, density and compression properties of in situ Ti-TiB composite materials produced by selective laser melting （SLM） technology. Starting powder composite system was prepared by mixing 95 wt% commercially pure titanium （CP-Ti） and 5 wt% titanium diboride （TiB2） powders and subsequently milled for two different times （i.e. 2 h and 4 h）. The milled powder mixtures after 2 h and 4 h show nearly spherical and irregular shape, respectively. Subsequently, the resultant Ti-5 wt% TiB2 powder mixtures were used for SLM processing. Scanning electron microscopy image of the SLM-processed Ti-TiB composite samples show needle-shape TiB phase distributed across the Ti matrix, which is the product of an in-situ chemical reaction between Ti and TiB2 during SLM. The Ti-TiB composite samples prepared from 2 h and 4 h milled Ti-TiB2 powders show different relative densities of 99.5% and 95.1%, respectively. Also, the compression properties such as ultimate strength and compression strain for the 99.5% dense composite samples is 1421 MPa and 17.8%, respectively, which are superior to those （883 MPa and 5.5%, respectively） for the 95.1% dense sample. The results indicate that once Ti and TiB2 powders are connected firmly to each other and powder mixture of nearly spherical shape is obtained, there is no additional benefit in increasing the milling time and, instead, it has a negative effect on the density （i.e. increasing porosity level） of the Ti-TiB composite materials and their mechanical properties.

Shape Selectivity of a Metallo Cavitand Host Allows Separation of n-Alkanes from Isooctane

The use of a metallo cavitand for the selective sequestration of n-alkanes from their mixtures with isooctane is described.Competition experiments and NMR spectroscopy are used to show the preference of a water-soluble cavitand host for guests that are hydro-phobic and have narrow or flat shapes.The host showed selectivity for trans 1,4-dimethylcyclohexane over the cis-isomers.The sep-arations of n-alkanes from isooctane by gas/liquid adsorption and a liquid/liquid extraction are detailed.