Recent Advances in Reactions of Alkylbenzenes Over Novel Zeolites: The Effects of Zeolite Structure and Morphology

Alkylbenzenes form an important segment of petrochemical industry for the manufacture of widely used commodities and specialty products. Since the last review on this topic (8), numerous new zeolite-based catalysts have been synthesized, characterized and evaluated in various transformations of aromatic hydrocarbons. This comprehensive review covers major reactions of mono-, di-, and tri-alkylbenzenes such as disproportionation, alkylation, transalkylation, isomerization, etc., over different zeolite-based acid catalysts. During the last decade, significant progress was made in the synthesis and structure determination of novel zeolites, mesoporous single crystals, hierarchic zeolites and two-dimensional zeolites. These developments have enhanced the understanding of the role of zeolites (effects of structural type, morphology, acid sites, accessibility of acid sites, shape selectivity factors) in transformations of aromatics. In this review, the emphasis is on the influence of the type of acid sites, zeolite topology, and reaction conditions on the activity, selectivity and pathways of these reactions. Thermodynamics and reaction kinetics of transformations of aromatic hydrocarbons are also discussed. This article covers mostly literature published during the period of 2002–2013.     

Fuzzy logic control of a reverse flow reactor for catalytic oxidation of ventilation air methane

Fuzzy logic controllers of type-1 and type-2 were implemented to deal with the high nonlinearities and uncertainties in operation of a reverse flow reactor (RFR) for catalytic oxidation of ventilation air methane (VAM). The results indicated that the fuzzy logic controller is distinctly superior to the traditional logic-based controller and works well under the conditions with high nonlinearities and uncertainties. Owing to the robustness of RFR and particular control aim of regulating bed temperature within a relatively broad range, a fuzzy logic controller of type-1 is sufficient to cope with the uncertainty brought by the extensive variation of VAM concentration.     

Low temperature densification by lithium co-doping and its effect on ionic conductivity of samarium doped ceria electrolyte

Low temperature densification and improving the ionic conductivity of doped ceria electrolyte is important for the realization of efficient intermediate temperature solid oxide fuel cell system. Herein, we report the effect of lithium co-doping (1, 3, 5 and 7?mol%) in 20?mol% samarium doped ceria on the low temperature sinterability and conductivity. The synthesized nanoparticles by citrate-nitrate combustion method showed a decrease in lattice parameter and increase in oxygen vacancy with lithium content after calcination due to the substitution of Li⁺ into CeO₂ lattice. Upon sintering at 900?°C, the density improved and reached a maximum value of 98.6% for 5% Li which exhibited a dense microstructure than at 7% Li. 5%Li co-doping exhibited the best conductivity of 3.65?×?10^?04–1.81?×?10^?3 S?cm^?1 in the operative temperature range of IT-SOFC (550–700?°C).Our results demonstrate the significance of lithium as co-dopant for efficient low temperature sintering as well as improving the electrolyte conductivity.     

A numerical approach for a comparative study of laser drilling process under single and repetitive pulse

An axisymmetric model is developed to study laser drilling process under a single pulse as well as repetitive laser pulse. The laser pulse irradiated on the surface of the workpiece is volumetric and Gaussian in nature. The laser irradiated surface is subjected to convective‐radiative boundary condition while rest of the surfaces are insulated. Finite volume method is used to discretize the domain under consideration. The resulting algebraic equations are solved with the help of the tridiagonal matrix algorithm to find temperature distribution throughout the domain. The enthalpy‐porosity method is used to track the solid‐liquid interface generated during the laser melting process. Convective heat transfer occurs inside the generated melt pool. The current model is first used to validate the results of the existing literature and as the results agreed well, further studies are made to find out the advantages of using repetitive laser pulse over single pulse laser source for laser drilling process for the same laser energy and total heating duration. Vaporization has been avoided during the process and metal removal occurs through melting only. The present numerical model can provide some insight for practical laser drilling process.     

Exploring to direct the reaction pathway for hydrogenation of levulinic acid into γ-valerolactone for future Clean-Energy Vehicles over a magnetic Cu-Ni catalyst

A cheap and magnetic Ni/Cu/Al/Fe catalyst for the selective hydrogenation of LA into GVL catalysts was prepared by sol-gel method. The reaction pathway was systematically studied by examining the reaction conditions, such as reaction pressure, catalyst loadings, water content, and reaction temperature. Higher reaction pressure and catalyst loadings were prior to form HA, then MHV, finally GVL; ML was easily to form with a higher reaction temperature. Water, as a key role, which was in favor to form HA, then MHV, finally GVL; more important, a higher LA conversion could be obtained in methanol as the solvent with some certain content of water. And, it can give a reference for future new clean energy vehicles' application.     

The impact of Ti and temperature on the stability of Nb5Si3 phases: a first-principles study

Abstract

Nb-silicide based alloys could be used at T > 1423 K in future aero-engines. Titanium is an important additive to these new alloys where it improves oxidation, fracture toughness and reduces density. The microstructures of the new alloys consist of an Nb solid solution, and silicides and other intermetallics can be present. Three Nb₅Si₃ polymorphs are known, namely αNb₅Si₃ (tI32 Cr₅B₃-type, D8_l), βNb₅Si₃ (tI32 W₅Si₃-type, D8_m) and γNb₅Si₃ (hP16 Mn₅Si₃-type, D8₈). In these 5–3 silicides Nb atoms can be substituted by Ti atoms. The type of stable Nb₅Si₃ depends on temperature and concentration of Ti addition and is important for the stability and properties of the alloys. The effect of increasing concentration of Ti on the transition temperature between the polymorphs has not been studied. In this work first-principles calculations were used to predict the stability and physical properties of the various Nb₅Si₃ silicides alloyed with Ti. Temperature-dependent enthalpies of formation were computed, and the transition temperature between the low (α) and high (β) temperature polymorphs of Nb₅Si₃ was found to decrease significantly with increasing Ti content. The γNb₅Si₃ was found to be stable only at high Ti concentrations, above approximately 50 at. % Ti. Calculation of physical properties and the Cauchy pressures, Pugh’s index of ductility and Poisson ratio showed that as the Ti content increased, the bulk moduli of all silicides decreased, while the shear and elastic moduli and the Debye temperature increased for the αNb₅Si₃ and γNb₅Si₃ and decreased for βNb₅Si₃. With the addition of Ti the αNb₅Si₃ and γNb₅Si₃ became less ductile, whereas the βNb₅Si₃ became more ductile. When Ti was added in the αNb₅Si₃ and βNb₅Si₃ the linear thermal expansion coefficients of the silicides decreased, but the anisotropy of coefficient of thermal expansion did not change significantly.     

Investigating the influence of Ti powder purity on phase evolution during NiTi sintering using in-situ neutron diffraction

The influence of Ti powder purity on phase evolution during the reactive sintering of elemental Ni and Ti powders to form NiTi was studied using differential scanning calorimetry (DSC) and in-situ neutron diffraction. Reaction between the Ni and Ti is not significant until 600 °C. From 600 to 700 °C, Ti₂Ni forms in mixtures made from high (HP) and low purity (LP) Ti powder. The Ni₃Ti phase also grows in this temperature range in the LP mixture. The most significant phase evolution takes place between 700 and 920 °C. The α to β phase transformation in (Ti) begins at the eutectoid temperature (765 °C) and ends at 820 °C. The highest growth rates for all three intermetallic phases, including NiTi, and the decay rate of the elemental Ni occur in this temperature range. At approximately 1000 °C, all reactants are consumed and homogenization occurs, with NiTi continuing to grow at the expense of the other intermetallic phases. The Ti rich intermetallic phase persists above its melting point, due to the formation of a solid-solution with oxygen (i.e. Ti₂Ni(O)). From 1100 to 1200 °C, the microstructure becomes a stable mixture of NiTi with a small fraction of Ti₂Ni(O). The phase evolution is similar in the LP and HP mixtures. However, the rate of reaction is higher in the LP mixture due to the influence of impurities (O, Fe and Ni) on the diffusivities in the many phases involved.     

Properties of alkali activated slag–fly ash blends with limestone addition

In this article, the effects of raw materials’ composition on fresh behavior, reaction kinetics, mechanical properties and microstructure of alkali activated slag–fly ash–limestone blends are investigated. The results indicate that, with the increasing content of fly ash and limestone, the slump flow increases. The setting times are shortened when increasing the slag content, while both fly ash and limestone show a negligible influence. The reaction process is slightly accelerated by the presence of limestone due to the extra provided nucleation sites, but the reaction process is mainly governed by the slag. The slag content exhibits a dominant role on strength in this ternary system, while for a constant slag content, the compressive strength increases with the increasing limestone content up to 30%. The microstructure analysis shows that the gel characteristics are independent of the limestone powder content. The presence of limestone in initially high Ca and Al conditions does not lead to the formation of additional crystalline phases, which is different from Portland cement systems. Both physically and chemically bound water contents are slightly increased when limestone powder is incorporated.     

羟基(—OH)对煤自燃侧链活性基团氧化反应特性的影响

针对煤结构及其氧化反应机理不明等问题,以羟基(—OH)和煤自燃侧链活性基团—OCH,—CHOHCH 3和—OCH 3为研究对象,采用密度泛函理论(DFT)的B3LYP-311G(d,p)方法,构建出了羟基(—OH)处于侧链活性结构邻位的小分子结构模型。基于前线轨道理论和量子化学理论,采用Gaussian 16软件对小分子模型的静电势、前线轨道的能级和电荷分布及煤氧复合反应过程中的热力学参数进行了模拟计算,探究了侧链活性基团的低温氧化特性和羟基(—OH)对其的影响。计算结果显示,在侧链活性结构中,氢原子周边呈强正电势,为亲核反应活性位点,而氧原子附近呈强负电势,为亲电反应活性位点。当羟基(—OH)处于侧链活性基团邻位时,会削弱侧链活性基团的亲电反应能力,增加—CHO和—CHOHCH 3的亲核反应能力,而使—OCH 3的亲核反应能力消失;通过分析各活性基团最高占据轨道(HOMO:Highest Occupied Molecular Orbital)和最低未占轨道(LUMO:Lowest Unoccupied Molecular Orbital)可知,侧链活性基团的稳定性与其前线轨道的成键能力并不一致,而活性基团与氧气发生复合反应的难易程度主要取决于该基团前线轨道中LUMO的成键能力,成键能力越强,复合反应越容易发生;由于羟基(—OH)改变了侧链活性基团前线轨道上的电子特性,故当其与侧链基团共存时,会使—CHO和—CHOHCH 3与氧气的复合反应更容易发生,而使—OCH 3与氧气的复合由自发的放热反应转变为非自发的吸热反应。该研究成果可为揭示煤自燃微观作用机理和研发煤自燃新型高效阻化材料提供参考。