Kinetics and mechanism of cubic boron nitride formation in the AlN–BN system at 6 GPa

The kinetics of hBN-into-cBN transformation at 6 GPa and 1770, 1880 and 1990 K in the presence of AlN have been studied. The transformation proceeds without liquid phase. It has been established that a limiting stage of the transformation is diffusion of boron and nitrogen atoms in wurtzitic aluminum nitride. Activation energy of the transformation is 170±40 kJ/mol. On the basis of our results we conclude that hexagonal BN dissolves in aluminum nitride solid solution and supersaturates it with respect to cBN, and then cubic boron nitride precipitates from the supersaturated solution of BN in AlN. An AlN–BN system phase diagram is proposed.     

Improving Raney® Catalysts Through Surface Chemistry

The unique value of the Raney^® catalyst is revealed through surface modifications that have extended this familiar platform into novel materials with enhanced productivity in practical hydrogenations. The illustrative examples each yielded new compositions through use of non-nickel metals: as efficient dopants (Pd, Pt), as an inexpensive alloy diluent (Fe) and substrate for plating, or as the primary catalyst metal (Raney^® Cu) in a recently identified process upgrading a renewable feedstock. A perspective on the surface chemistry of metals links the various approaches.     

Kinetics and mechanism of ethylene homopolymerization and copolymerization reactions with heterogeneous Ti-based Ziegler–Natta catalysts

A detailed kinetic analysis of ethylene homopolymerization reactions and its copolymerization reactions with 1-hexene with a supported Ti-based Ziegler–Natta catalyst (reactions in the absence and the presence of hydrogen) shows a number of distinct kinetic features which are interpreted as a manifestation of multi-site catalysis; the catalyst contains several types of polymerization centers which differ in stability and formation rates, the molecular weight of polymers they produce, and in their response to the presence of α-olefins and hydrogen. All these effects require introduction of a special kinetic mechanism which postulates an unusually low activity of growing polymer chains containing one ethylene unit, the Ti–C₂H₅ group, in the ethylene insertion reaction into the Ti–C bond. This peculiarity of the Ti–C₂H₅ group, which is probably caused by its β-agostic stabilization, predicts two kinetic/chemical features of ethylene polymerization reactions which have not been described yet, the deuterium effect on the homopolymer structure and the activation effect of α-olefins on chain initiation. Both effects were confirmed experimentally. This revised version was published online in June 2006 with corrections to the Cover Date.     

Self-propagating high-temperature synthesis of advanced ceramics in the Mo–Si–B system: Kinetics and mechanism of combustion and structure formation

The goal of this work is to investigate the combustion mechanisms of reactions in the Mo–Si–B system and to obtain ceramic materials using the SHS method. It is concluded that the following processes are defined by the SHS for Si-rich Mo–Si–B compositions: silicon melting, its spreading over the surfaces of the solid Mo and B particles, followed by B dissolution in the melt, and formation of intermediate Mo₃Si-phase film. The subsequent diffusion of silicon into molybdenum results in the formation of MoSi₂ grains and molybdenum boride phase forms due to the diffusion of molybdenum into B-rich melt. The formation of MoB phase for B-rich compositions may occur via gas-phase mass transfer of MoO₃ gaseous species to boron particles. The stages of chemical interaction in the combustion wave are also investigated. The obtained results indicate the possibility of both parallel and consecutive reactions to form molybdenum silicide and molybdenum boride phases. Thus the progression of combustion process may occur through the merging reaction fronts regime and splitting reaction fronts regime. Molybdenum silicide formation leads the combustion wave propagation during the splitting regime, while the molybdenum boride phase appears later. Finally, targets for magnetron sputtering of promising multi-phase Mo–Si–B coatings are synthesised by forced SHS compaction method.     

Anodic Oxidation of Nitric Oxide on Au/Nafion®: Kinetics and Mass Transfer

The rate determining step for the anodic oxidation of nitric oxide on Au/Nafion^® was experimentally and theoretically found to beNO + Au Au–NO_ads The anodic oxidation of nitric oxide was first order with respect to nitric oxide. The reaction rate constant increased from 3.3×10^–5 to 9.6×10^–5cm s^–1 as the applied potential increased from 0.74 to 0.77V. The anodic oxidation of nitric oxide was controlled by the electrochemical kinetics when the anodic potential was less than 0.8 V. When the potential was greater than 1.0 V, it was located in the mass transfer region. The limiting current increased from 1184 to 1589A with increase in gas flow rate from 250 to 750ml min^–1 when the potential was set at 1.05 V and the concentration of nitric oxide was 100 ppm. The diffusion resistance in the gas diffusion layer can be neglected for gas flow rates greater than 750 ml min^–1. The diffusivity of nitric oxide and the equivalent diffusion layer thickness within the porous electrode were evaluated to be 3.43×10^–4cm²s^–1 and 0.051 cm, respectively.     

The effect of copper valence on catalytic combustion of styrene over the copper based catalysts in the absence and presence of water vapor☆

Catalysts Cu Ox/γ-Al_2O_3-IH and Cu Ox/γ-Al_2O_3-IM were prepared, characterized, and tested for styrene combustion in the absence and presence of water vapor. The effect of copper valence of the catalysts on the catalytic activity for styrene combustion was discussed using the theory of hard soft acids and bases(HSAB).The results showed that the existence of water vapor in feed stream inhibited the catalytic activity for styrene combustion due to the competition adsorption of water molecule. HSAB theory confirmed that the local soft acidity of the catalyst Cu Ox/γ-Al_2O_3-IH was much stronger than that of the catalyst Cu Ox/γ-Al_2O_3-IM because of the higher content of soft acid Cu+on its surface, which increased the adsorption ability toward soft base of styrene and reduced the adsorption toward hard base of water vapor, and thus increased the catalytic activity for styrene combustion and weakened the negative influence of water vapor.     

Selective Conversion of Glycerol to Propylene Glycol Over Fixed Bed Raney® Cu Catalysts

Net propylene glycol (1,2 propanediol) yields of up to 94% at 100% glycerol conversion have been achieved over a fixed bed Raney^® Cu catalyst in trickle bed mode, at relatively low total pressure, 14 bar (200 psig), and minimal feedstock dilution (20 wt% water). The main identified byproducts are ethylene glycol and ethanol (each <2%), with methanol and 1,3 diol both <1%. The other key operating parameters for high yields are a narrow optimum in temperature (near 205 °C), and a high H₂/liquid flow ratio, about 375/0.05 by volume. The effects of chromium promotion have also been studied for effects on side reactions and rates. Our evidence points to initial dehydrogenation as the rate-limiting step in a likely three step mechanism.     

Investigation of the effect of minor ions on the stability of β-C2S and the mechanism of stabilization

In this paper the effect of minor ions on the stability of β-C₂S has been studied. It is discovered that the effect of minor ions on the stability of β-C₂S is closely related to the polarization ability of the ions. According to this the authors propose a new judgement, the ion polarization ability judgement, by which we can determine if minor ions have the stabilizing affect on β-C₂S. The judgement is that those ions, either having smaller polarization ability than that of calcium ion or greater than that of silicon ion, all have the ability to effect the stability of β-C₂S. This judgement is more practical than those made by other authors. In this paper a new mechanism about the stabilization has been described on the basis of applying the principles of thermodynamics and dynamics of the transformation of solid phases in accordance with the above judgement.     

Kinetics,mechanism, and simulation of hydrogen transfer reaction of α, β-unsaturated aldehydes to allylic alcohols

Homogeneous hydrogen transfer reactions of methacrolein (MAL) and isopropanol to methallyl alcohol (MAA) were investigated in batch reactor (Conv. 89%, Select 93.1%) and tubular reactor (Conv. 88.1%, Select 95%) using aluminum isopropoxide (Al(OPrⁱ)₃) as catalyst. Kinetic experiments on hydrogen transfer reactions and reaction order were investigated in batch reactor and tubular reactor. Response surface methodology was applied to optimize the optimum reaction conditions of hydrogen transfer reaction. Purification process of MAA from product mixture after hydrogen transfer reaction was simulated with Aspen Plus software; theoretical stages, reflux ratio, and feed stage of distillation tower were optimized. Density functional theory was used to investigate viable reaction pathway and to probe the catalytic mechanism between reactants and catalyst, including dehydrogenation, coupling, and hydrogenation reaction. Microscopic mechanisms of hydrogen transfer reaction from MAA to MAL were acquired in detail and could be easily extended to other series of hydrogen transfer reaction.     

Equation-based SPYRO® model and solver for the simulation of the steam cracking process

The explicit SPYRO® model is transformed into the residual form using orthogonal collocation on finite elements. An effective and flexible system of sub-models is developed for the Open Spyro model. The Open Spyro model is solved using a fast and globally converging quasi-Newton method based on the update proposed by Broyden. The simulation gives the same reliable results as the original SPYRO® model.     

Kinetics and Selectivity of the Fischer–Tropsch Synthesis: A Literature Review

A critical review of the kinetics and selectivity of the Fischer–Tropsch synthesis (FTS) is given. The focus is on reaction mechanisms and kinetics of the water–gas shift and Fischer–Tropsch (FT) reactions. New developments in the product selectivity as well as the overall kinetics are reviewed. It is concluded that the development of rate equations for the FTS should be based on realistic mechanistic schemes. Qualitatively, there is agreement that the product distribution is affected by the occurrence of secondary reactions (hydrogenation, isomerization, reinsertion, and hydrogenolysis). At high CO and H₂O pressures, the most important secondary reaction is readsorption of olefins, resulting in initiation of chain growth processes. Secondary hydrogenation of α-olefins may occur and depends on the catalytic system and the process conditions. The rates of the secondary reactions increase exponentially with chain length. Much controversy exists about whether these chain-length dependencies stem from differences in physisorption, solubility, or diffusivity. Preferential physisorption of longer hydrocarbons and increase of the solubility with chain length influences the product distribution and results in a decreasing olefin-to-paraffin ratio with increasing chain length. Process development and reactor design should be based on reliable kinetic expressions and detailed selectivity models.     

Thermal Stabilities and the Thermal Degradation Kinetics of Poly(ε-Caprolactone)

The thermal degradation behavior of poly(ε-caprolactone) (PCL) in nitrogen atmosphere was investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) techniques and Fourier Transform Infrared (FTIR) spectroscopy. TGA/DTG/DSC curves display only one main degradation step. TGA/FTIR results indicate PCL decomposes into CO₂ and hexanoic acid before 500°C. The activation energies were estimated by iterative isoconversional procedure. Friedman plot indicates a change in degradation mechanism. The differential method and integral method were used to confirm the exact kinetic mechanism. The best models are Dn (n = 1, 2, 3, and 4) when 0.08≤a≤0.2 and R2 when 0.25≤a≤0.8.     

Importance of the rheological properties of resorcinol–formaldehyde sols in the preparation of Cu-doped organic and carbon xerogel microspheres

The rheological properties of resorcinol–formaldehyde sols at the very beginning of the emulsion polymerization process have a profound impact on the final shape of the microparticles obtained. This is especially the case with Cu-doped organic hydrogels at reasonably large temperatures (40 °C), when an abrupt increase in viscoelastic properties is observed within only a few minutes.     

Proteins of the castor bean—Their preparation,properties, and utilization

Summary This article is a review of technical papers and information that are now available on the proteins of the castor bean. Included among the various aspects of the subject are the following: The chemical composition of castor bean seeds and castor cake; the preparation and properties of their proteins, including the toxic albumin riein, its properties, physiological reactions and amino acid content; the allergenic properties of castor bean protein; the utilization of castor bean cake or pomace for feeding, fertilizer and industrial purposes. An extensive bibliography is appended. A part of this paper was prepared while the author was previously connected with the Bureau of Agricultural and Industrial Chemistry of this Department.     

Effect of H+ and N+ Irradiation on Structure and Permeability of the Polyimide Matrimid®

Abstract

This paper presents a comparison of the impact of H⁺ and N⁺ ion irradiation on the chemical structure, microstructure, and gas permeation properites of the polyimide, Matrimid®. While irradiation with both ions resulted in evolution of chemical structure with loss of functional groups and crosslink formation, there was greater modification of polyimide structure following N⁺ irradiation at similar total deposited energy. Irradiation with N⁺ resulted in simultaneous large increases in permeance and permselectivity at comparatively low ion fluences or irradiation time. For example, irradiation at 4 × 10¹⁴ N⁺/cm² resulted in a 2.5 fold increase in He permeance with a selectivity of He/CH₄ of 340. Much higher H⁺ fluences were required to achieve similar total deposited energy and combined increases in permeance and permselectivity. The larger modification in chemical structure and gas permeation properties following N⁺ irradiation was attributed to the relatively large energy loss and damage from the nuclear energy relative to electronic energy loss.     

Kinetics and mechanism of Horner–Wadsworth–Emmons reaction of weakly acidic phosphonate in solid–liquid phase-transfer catalysis system

Homer–Wadsworth–Emmons (HWE) reaction of diethyl benzylphosphonate (DEBP, pKa = 27.55) with aldehyde was performed in a solid–liquid phase-transfer catalysis (SL–PTC) system using sodium hydroxide as the solid phase. Various parameters that influenced the pseudo-first-order rate constant including stirring speed, catalysts, salt, water and temperature were investigated to explore the process of the generation and transfer of the active intermediate. A reasonable interfacial mechanism of the PTC reaction of the weakly acidic substrate was proved for the first time. HWE reactions under SL–PTC conditions showed high activity and geometric selectivity. It is anticipated that this simple and controllable synthesis method should provide a new idea for HWE reaction in chemical industry.     

Selective Hydrogenation of 2-Butyne-1,4-diol to 1,4-Butanediol Over Particulate Raney® Nickel Catalysts

The current study describes the results on the selective hydrogenation of the 2-butyne-1,4-diol to 1,4-butanediol over Raney^® nickel catalysts both in batch and in CSTR mode. The detailed kinetic analysis of the reaction in batch mode revealed the existence of three characteristic regions. In the first region, A, the starting 2-butyne-1,4-diol produces primarily cis-2-butene-1,4-diol. In the second region, B, the dominant species is cis-2-butene-1,4-diol, which is either hydrogenated to 1,4-butanediol or isomerizes to trans-2-butene-1,4-diol. In the third region, C, the accumulated 4-hydroxybutanal is slowly hydrogenated to 1,4-butanediol. When the same reaction was carried out in a CSTR mode, the only products detected initially are the 1,4-butanediol and n-butanol. The first by-product detected immediately after the end of the first stage is the linear hemiacetal between the 4-hydroxybutanal with 1,4-butanediol. This species has been used as convenient tracer for determining the length of the selective region of the catalyst performance.     

Matrimid® 5218 in preparation of membranes for gas separation: Current state-of-the-art

Over the last decades, different polymers have been used as continuous phase for preparing selective membranes for gas separation. Today, some of these materials have been consolidated commercially; however, the necessity to improve the performance (in terms of permeability/selectivity) of polymeric membranes above Robeson’s upper bound has been conducted by blending polymers, use of additives, implementation new methods, development of new materials, coating films, development of mixed matrix membranes, and so on. One of the most recent approaches is the use of polymers such as polyimides, i.e., Matrimid® 5218, which has demonstrated, to provide remarkable gas separation performance using the attempts aforementioned. The aim of this work is to present the current state-of-the-art of the use of Matrimid® 5218 in preparation of membrane for gas separation. The progress in this field is summarized and discussed chronologically in two periods, decade (from 1998 to 2008) and current (from 2009 up to now) frameworks. This contribution leads to take a complete and compelling overview of the state-of-the-art based on Matrimid. Furthermore, the main approaches, aim of study, gas separation evaluated, main techniques used for membrane characterization, main supplier of the polymer, main secondary materials for blending, fillers incorporated into the matrix, and remark of the study are summarized in detail. Finally, it denotes the prospects and future trends on use of Matrimid® 5218 for membrane applications.     

Kinetics and mechanisms for the densification and grain growth of the α-alumina fibers isothermally sintered at elevated temperatures

Understanding the densification and grain growth processes is essential for preparing dense alumina fibers with nanograins. In this study, the alumina fibers were prepared via isothermal sintering at 1200, 1300, 1400, and 1500 °C for 1–30 min. The phase, microstructure, and density of the sintered fibers were investigated using XRD, SEM, and Archimedes methods. It was found that the phase transformation during the isothermal sintering enhances the densification of Al₂O₃ fibers in the initial stage, while the pores generated during the phase transformation retard the densification in the later period. The kinetics and mechanisms for the densification and grain growth of the fibers were discussed based on the sintering and grain growth models. It was revealed that the densification process of the fibers sintered at 1500 °C is dominated by the lattice diffusion mechanism, while the samples sintered at 1200–1400 °C are dominated by the grain boundary diffusion mechanism. The grain growth of the Al₂O₃ fibers sintered at 1200–1300 °C is governed by surface-diffusion-controlled pore drag, and that sintered at 1400 °C is dominated by lattice-diffusion-controlled pore drag.