Comparison of the catalytic properties of Al‐ZSM‐22 and Fe‐ZSM‐22 in the skeletal isomerization of 1‐butene

The catalytic activities of Al‐ZSM‐22 (Al in the framework) and Fe‐ZSM‐22 (Fe in the framework) were compared in the skeletal isomerization of 1‐butene to isobutene. The catalysts synthesized in the laboratory were characterized by means of XRD, FTIR spectroscopy, SEM and surface area measurements. The activity of the zeolites was investigated using a fixed‐bed microreactor system. Al‐ZSM‐22 demonstrated higher activity in 1‐butene transformation compared to Fe‐ZSM‐22, while the selectivity to isobutene, on the other hand, was higher over Fe‐ZSM‐22. Coke formation was monitored using a microbalance and the results showed that the weight gain of Fe‐ZSM‐22 was slightly higher compared to Al‐ZSM‐22. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stereoselective Formation of 1,4‐Anhydro‐2‐C‐carboxytetritols in the Degradation of 1,5‐Anhydroribitol and 1,5‐Anhydroxylitol with Oxygen in Aqueous Alkali

Abstract

In addition to other acid products, degradation of 1,5‐anhydroribitol (5) and 1,5‐anhydroxylitol (6) with oxygen in 1.25 M NaOH produced diastereomeric 1,4‐anhydro‐2‐C‐carboxy‐D‐erythritol (7) and 1,4‐anhydro‐2‐C‐carboxy‐D‐threitol (8) and their enantiomers as major products. However, the ratio of the diastereomers differed for the two reactants. Thus, their formation could not proceed solely by benzilic acid‐type rearrangements through α‐dicarbonyl intermediates as typically proposed for formation of alkyl C‐carboxyfuranosides from alkyl glycopyranosides in similar reactions. The α‐dicarbonyl species that can form from 5 and 6 are identical. Potential mechanisms to account for stereoselective formation of 7 and 8 are presented.     

Semi‐Empirical Method for Estimating the Combustion‐Wave Transition through the Contact Surface in a Composite Explosive

Through a comparison of experimental data on stable ignition and combustion of initial specimens composing igniter trains of pyrotechnic charges, critical conditions for combustionwave transition through the contact surface between neighboring compoundexplosive compacts are found. The evaluation procedure is tested with twenty explosives representing different combinations of five compacts of pyroxylin (guncotton), black gunpowder, and three types of pyrotechnic mixtures. Minimal dimensions of an interfacial protuberance promoting combustionwave propagation throughout the charge are found.     

Performance of metal‐oxide‐promoted LiCl/sulfated‐zirconia catalysts in the ethane oxidative dehydrogenation into ethene

The effects of some transition‐ and lanthanide‐metal oxides in LiCl/sulfated‐zirconia (SZ) catalysts on catalytic behavior in the oxidative dehydrogenation of ethane were investigated. It is found that modification of LiCl/SZ by metal oxides significantly improves the catalytic activity and ethene yield. Among those additives, Ni and Nd oxides show the best promoting effect in terms of ethane conversion and ethene yield. 93% ethane conversion with 83% selectivity to ethene has been achieved over the Nd₂O₃–LiCl/SZ catalyst at 650°C. In addition, those oxide‐promoted LiCl/SZ catalysts are also found to exhibit a longer stability in catalytic performance. Metal‐oxide additives change the chemical structure and surface redox properties, which accounts for the enhancement of activity. This revised version was published online in July 2006 with corrections to the Cover Date.     

High activity and stability of the Rh‐free Co‐based ex‐hydrotalcite containing Pd in the catalytic decomposition of N2O

The catalytic decomposition of nitrous oxide to nitrogen and oxygen has been studied over calcined hydrotalcite‐like compounds containing different combinations of bivalent (Co, Pd, Mg) and trivalent (Al, La, Rh) cations with carbonate as interlayer anion. The precursors were prepared by co‐precipitation under low supersaturation conditions and characterized by XRD and TG/DSC. The mixed oxides derived after calcination at 723 K were characterized by XRD, N₂ adsorption at 77 K, and XRF. The presence of Rh, La, or Pd in the Co‐based HTlc's improves considerably the catalytic activity. Co–Rh,Al‐HTlc (Co/Rh/Al==3/0.02/1) proved to be a very active catalyst, although the presence of the noble metal Pd in this catalyst ex‐Co,Pd–La,Al‐HT (Co/Pd/La/Al=3/1/1/1) produces a similar catalytic activity to that of Rh‐containing catalyst, both in a N₂O‐containing stream and in one containing also SO₂ and O₂, but with a better performance in stability tests. PdO phase has been identified by XRD as being responsible for the considerable improvement in the activity. The presence of Mg as spinel structure exerts a stabilizing effect in the more active catalysts when mixtures of SO₂ and O₂ are considered. This revised version was published online in July 2006 with corrections to the Cover Date.     

Third Phase Formation in the Extraction of Inorganic Acids by TBP in n‐Octane

Abstract

The extraction of HNO₃, HClO₄, H₂SO₄ and H₃PO₄ by 20% (v/v) TBP (0.73 M) in n‐octane was measured under identical conditions up to and beyond the critical point of third phase formation (Limiting Organic Concentration, or LOC condition). The data, together with those obtained previously for HCl, allowed us to establish the following lyotropic series of effectiveness with respect to third phase formation in the extraction of acids by TBP: HClO₄>H₂SO₄>HCl>H₃PO₄>HNO₃. This series correlates with the amount of water present into the organic phase at the point of phase splitting. This result reinforces the validity of the reverse micellar model developed previously for the extraction of metal salts by TBP. The measurements of LOC values as a function of temperature revealed major differences among the acid‐TBP systems investigated. For HClO₄, the strong increase of the LOC value with increasing temperatures is accompanied by a large favorable entropy change. The opposite is true for HCl, while H₂SO₄ and H₃PO₄ represent intermediate cases. Measurements of the LOC values for the extraction of HClO₄ by TBP dissolved in a series of diluents confirmed that topological parameters, such as the Connectivity Index, CI, are useful for predicting the critical condition for phase splitting in different diluents. Based on the linear correlation between LOC values for HClO₄ and CI values of diluents, the effective Connectivity Index of the French nuclear reprocessing diluent, HTP, a complex mixture of highly branched alkanes, was determined.     

A Striking Effect of Ionic‐Liquid Anions in the Extraction of Sr2+ and Cs+ by Dicyclohexano‐18‐Crown‐6

Abstract

The nature of the ionic‐liquid (IL) anion has been found to have a remarkable effect on the solvent extraction of Sr²⁺ and Cs⁺ by dicyclohexano‐18‐crown‐6 dissolved in ionic liquids. In particular, the extraction efficiency increases with the hydrophobicity of the IL anion as reflected by the solubility in water of ILs having a common cation. Since a cation‐exchange mechanism is operating in these systems, the influence of the IL anion is in large part attributable to an expected Le Chatelier effect in which a greater aqueous concentration of IL cation, obtained when using an IL anion of lower hydrophobicity, opposes cation exchange. This dependence is opposite to that found for IL cations, indicating a significant advantage of using ILs with hydrophobic anions for cation extraction. Furthermore, the extraction selectivity for Sr²⁺ over Na⁺, K⁺, and Cs⁺ can be significantly improved through the use of hydrophobic anions for the ILs containing 1‐ethyl‐3‐methylimidazolium or 1‐butyl‐3‐methylimidazolium cations.     

Prediction of the Elution Profile of Aromatic Compounds in RP‐HPLC

Abstract

An efficient prediction method for the elution profile was used to separate aromatic compounds such as benzene, toluene, chlorobenzene, o‐xylene, and 1,2‐dichlorobenzene by RP‐HPLC. The retention factor and bandwidth were predicted under linear‐gradient condition with the three retention models. The elution profiles were calculated based on the linear and quadratic equations of retention factor, lnk=lnk _w +S?, lnk=L+M?+N?², k=A+B/?, where ? was the vol.% of methanol. The elution profiles were calculated by the Gaussian distribution with obtained retention factor and bandwidth. Two kinds of experiments were performed; one is the isocratic runs to estimate the coefficients of three retention models, and the other is the linear gradient runs that were carried out with same initial mobile phase composition (water/methanol=96/4, vol.%), two final mobile phase compositions (water/methanol=24/76 and 40/60, vol.%), and three gradient times (20, 40, and 60 min). The predicted elution profiles by the three retention models and new prediction method have good agreement with experimental data in the employed gradient conditions. The minimum average errors of calculated and experimental results of aromatic compounds were lower than 3.5% by Bi‐poly equation.     

The Effects of Water and CO2 on the Reaction Kinetics in the Iron‐Based Low‐Temperature Fischer‐Tropsch Synthesis: A Literature Review

A review of kinetic models for the iron‐based Fischer‐Tropsch synthesis revealed some important areas of progress, namely (a) a transition from rate equations with a first order denominator (consistent with an Eley‐Rideal type mechanism) to Langmuir‐Hinshelwood expressions with a second order inhibition term; (b) whereas kinetic models originally specified full coverage of the catalytic surface, later models found the influence of vacant sites to be important; (c) whereas water or CO₂ was traditionally always included in the FT rate equations, the most recent lumped kinetic models do not account for any influence of water. It was further concluded that the perceived inhibiting influence of water on the FT rate may have been an indirect effect via the water‐gas‐shift reaction that changed the hydrogen and CO partial pressures.     

Propagation of the Chain Explosive‐Decomposition Reaction in Silver Azide Crystals

The velocity of the chain explosivedecomposition reaction in silver azide whiskes has been measured (1500 m/sec). The measured velocity is identified as the propagation velocity of the diffusion front of holes generated in the course of explosive decomposition.     

Methane dehydro‐aromatization over a Mo/phosphoric rare earth‐containing penta‐sil type zeolite in the absence of oxygen

The dehydro‐aromatization of methane over a Mo‐modified penta‐sil type high‐silica zeolite containing phosphoric and rare earth oxide (abbreviated as Mo/HZRP‐1) was investigated. As a modification of HZSM‐5, HZRP‐1 is also a good support for the preparation of Mo‐based zeolite catalysts, and is active for methane dehydro‐aromatization. Mo/HZRP‐1 catalysts are more active at high Mo loadings compared with Mo/HZSM‐5 catalysts. ²⁷Al MAS NMR spectra of Mo/HZRP‐1 reveal that there are two kinds of framework Al in HZRP‐1. It is suggested that only the tetrahedral coordinated Al atoms in the form of Al–O–Si species in the zeolite, in the proton forms, are responsible for the formation of aromatics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Studies on the Sorption of Palladium using Cross‐Linked Poly (4‐Vinylpyridine‐Divinylbenzene) Resins in Nitric Acid Medium

Various cross‐linked (4, 8, and 12%) gel‐type weak‐base poly(4‐vinylpyridine) (PVP) resins were studied for palladium recovery from nitric acid medium. The sorption of palladium was found to decrease with an increase in cross‐linkage of the resin. 8 and 12% PVP resins exhibited maximum D _Pd(II) values at 2–6 M HNO₃, whereas 4% PVP resin showed maximum D _Pd(II) values at lower acidities (0.1 M HNO₃). FT‐IR, SEM, and XPS techniques were used for the characterization of palladium‐loaded resins. Detailed studies were carried out with the resin of modest cross‐linkage i.e., 8% PVP resin. The sorption isotherm studies revealed that the maximum palladium loading approaches the theoretical capacity of the resin, presuming the sorption of palladium as divalent anion at 4 M HNO₃. The pseudo‐second order kinetics model yielded the best fit for the experimental data of sorption kinetics. An increase in temperature accelerates the rate of palladium extraction and also the addition of chloride ions increases the palladium uptake. Column studies were performed using 4 and 8% PVP resins in 2 and 4 M nitric acid concentrations. The loaded palladium could be eluted efficiently with acidic thiourea solution.     

Dependence of the Time‐Space Structure of the Chemical‐Reaction Zone on the Initial Density of the Explosive

The results of a study of the chemicalreaction zone structure for a number of high explosives (HEs) are discussed. It is found that an increase in the initial density of the HEs leads to a structural change in the chemicalreaction zone involving elimination of the Von Neumann spike at the critical density point. Conversely, as the initial density of the explosive decreases, the ratios of the gasdynamic parameters (in particular, mass velocities) at the Von Neumann point to the same parameters at the Jouguet point increase. It is shown that to explain the indicated regularities, one does not need to invoke the assumption of an increasing contribution of the exothermic decomposition of the HE the shock. The results can be explained within the framework of classical Zeldovich–Von Neumann–Döring theory with an ordinary shock at the detonationwave front and, hence, with a nearly zero contribution to the total energy release in the chemical reaction zone.     

Triply‐promoted ethene epoxidation: NOx promotion of the Ag‐catalysed reaction in the presence of alkali and chlorine under electrochemical control

Potassium, electrochemically supplied from K β"‐alumina to a silver thin film catalyst in the presence of ppm levels of NO_x, strongly promotes the selectivity of ethene epoxidation. However, in the absence of gaseous NO_x, alkali catastrophically suppresses both activity and selectivity. Addition of surface chlorine via ppm levels of ethylene dichloride further enhances the promotional effect of alkali + NO yielding the highest selectivity of all. The minimum necessary and sufficient conditions for the appearance of NO_x promotion are submonolayer quantities of alkali on the metal surface, and ppm levels of gaseous NO_x. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of decrease in number of acid sites located on the external surface of Ni‐SAPO‐34 crystalline catalyst by the mechanochemical method

In order to improve shape selectivity of the methanol to ethylene conversion and mitigate coke formation, the acid sites located on the external surface of Ni‐SAPO‐34 crystals were neutralized by the intrinsic mechanochemical method. Ni‐SAPO‐34 crystals were mixed in an agate mortar with basic alkaline or alkaline earth metal oxides supported on microspherical non‐porous silica. Their catalytic performances in methanol conversion were enhanced, especially in the case of BaO‐modified catalyst. The reason was verified by adopting the cracking of t‐butylbenzene, which could not access into the pore channel due to its bulky molecular size. These changes in the reaction performance consistently could be ascribed to the decrease of the acid sites on the external surfaces. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cobalt‐catalyzed amination of 1,3‐cyclohexanediol and 2,4‐pentanediol in supercritical ammonia

The onestep procedure of amination of bifunctional secondary alcohols to diamines has been investigated in a continuous fixedbed reactor. Application of supercritical NH₃ as a solvent and reactant suppressed catalyst deactivation and improved selectivities to amino alcohol intermediates, whereas selectivities to diamines remained poor (8–10%). The main reason for the low diamine selectivity of 1,3dihydroxy compounds is water elimination leading to undesired monofunctional products via ,unsaturated alcohol, ketone or amine intermediates. This side reaction does not occur with 1,4dihydroxy compounds which afford high aminol and diamine selectivities under similar conditions. Amination of secondary diols with ammonia was found to be faster, but less selective than that of the corresponding primary 1,3propanediol.     

Surfactant‐Enhanced Carbon Regeneration in a Vapor‐Phase Application

Abstract

Coal‐based granular activated carbon (GAC) is saturated with trichloroethylene (TCE) by passing air through a fix bed adsorber. In surfactant‐enhanced carbon regeneration, an aqueous solution of anionic surfactant, sodium dodecyl sulfate (SDS), is passed through the bed to induce desorption of TCE. More than 95% of the sorbed TCE was removed in the desorption operation with a 0.1 M SDS solution at a superficial flow rate of 1 cm/min. The desorption rate of TCE from pores of GAC is limited by pore diffusion and not significantly affected by either the concentration of SDS in the regenerant (when well above the critical micelle concentration) or its flow rate. From the breakthrough curve of a subsequent adsorption cycle without a flushing step following the desorption, only 7% of the virgin carbon effective adsorption capacity is observed for the regenerated carbon. With a water flushing step following the regeneration step, the effective adsorption capacity is significantly improved to about 15% of that of virgin carbon. Increased temperature of the flushing water also enhances the effective adsorption capacity of the regenerated GAC. Separate batch adsorption‐desorption isotherms of SDS on GAC support the enhanced desorption of SDS at elevated temperatures. The drastic reduction in the effective adsorption capacity of regenerated GAC results from the residual SDS remaining in the pores of GAC as confirmed by thermal gravimetric analysis. Both the regeneration and water flush steps are rate limited under conditionsused here.     

Recent Advances and Future Aspects in the Selective Isomerization of High n‐Alkanes

This article provides an overview of the advances that have been marked during the last decades in the field of hydroconversion of high n‐alkanes (C₇ ⁺) with particular stress on the promising ways to meet the requirements for improved quality of motor fuels and oils. Particular attention is given to a catalyst formulation for selective conversion of high n‐alkanes to branched hydrocarbons. The challenge for successfully solving this problem is to find an effective catalyst favoring the isomerization of n‐alkanes without too much cracking. The regulation of active sites and adsorption properties, as well as the topology of support surfaces, allows a more predictive design of novel catalysts for selective conversion of high n‐alkanes into their branched isomers.