Catalytic isomerization of olefins on BeO, I

The catalytic activity of BeO for the double bond shift of 1-pentene has been investigated. The maximum activity was exhibited by samples calcined at 1173 K. The effect of pyridine, CO₂ and CCl₄ on the activity was also studied. Lewis acid-base site pairs appear to be active for the isomerization.

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BeO 1- . , 1173 . , CO₂ CCl₄ . - .

     

Thermisches Verhalten von Cyclopropa[c]chromenen

The Cyclopropa[c]chromenes 14 , trans-and cis- 15 , trans-and cis- 16 and 17 rearrange on heating > 200° in N, N-diethylaniline to give 2-alkyl-2H-chromenes 7, 8, 21, 22. The rate determining step of this rearrangement is the ‘homoelectrocyclic’ ring opening of the cyclopro-pa[c]chromenes to give ω-allyl-quinomethanes of type 4. These intermediates show fast [1,5s] and [1,7a] H-shifts, followed by electrocyclic ring closure. Deuterium labelling experiments are in agreement with this mechanism. The remarkable dependence of the rates of rearrangement with respect to the stereochemistry of the cyclopropa[c]chromenes (cf. table 2) suggests that in the first step only one of the two possible disrotatory modes of ring opening is involved.     

Hydration of acetylene on Cd2+ exchanged zeolites

The reaction takes place on Cd-containing Lewis sites, whereas the irreversible adsorption and condensation of product acetaldehyde on acidic OH groups of Cd-X, FAU catalyst results in deactivation. Preadsorbed ammonia moderates the deactivation, because it reacts with acetylene, and pyridine homologues are formed which block the accessible OH groups. Since in divalent cationic forms of clinoptilolite no OH groups are generated, Cd-clinoptilolite proved to be a catalyst of permanent activity.

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, Cd, — OH FAU, Cd-X. , .. , OH . OH, Cd- .

     

Experiences and practical hints on using the DDRP method,illustrated by the example of the H2 + H reaction

By using the dynamically defined reaction path (DDRP) method and starting from various initial polygons, the intrinsic reaction coordinate (IRC) of the H₂ + H → H + H₂ reaction has been calculated. The numerical stability of the method is illustrated by the evolution phases of the reaction path. Techniques and experiences on the parameter choice and effects of the parameter values on the stability and computer time consumption are discussed. © John Wiley & Sons, Inc.     

Bestimmung von Calcium-Ionen durch flammenphotometrische Titration

Zusammenfassung Es wird eine maßanalytische Methode zur Bestimmung von Calcium-Ionen beschrieben, bei der die zu bestimmende Lösung mit einer Phosphorsäure-maßlösung titriert wird. Man photometriert von Zeit zu Zeit die Flamme der zu bestimmenden Lösung in dem Zeiss-Flammenphotometer. Die Herstellung der Aerosole erfolgt durch einen modifizierten Zerstäuber. Mit Hilfe der einzelnen Versuchsdaten zeichnet man eine Titrationskurve, aus deren Knickpunkt der Äquivalenzpunkt ermittelt wird. Die Ergebnisse sind reproduzierbar. Die Emissionserniedrigung von Calcium in Anwesenheit von Phosphaten wird durch die Bildung von Ca₃ (PO₄)₂ erklärt.     

Capillary isoelectric focusing of proteins utilizing poly(vinylpyrrolidone)- and plexiglas-coated columns

Fused-silica capillaries chemically derivatized with silane/poly(vinylpyrrolidone) (PVP) or dynamically modified with plexiglas [poly(methyl methacrylate)] were prepared and evaluated with regard to column stability and separation performance for capillary isoelectric focusing of standard proteins. The PVP coating showed the better stability and was good for at least 100 runs while the plexiglas coating started to deteriorate after about 30 runs. The time spent for the plexiglas coating is about 40 minutes while the PVP coating requires two days. The migration time reproducibility was better with the PVP capillary (RSD 0.7-1.6%, n = 5) compared to the plexiglas-coated column (RSD 1.2-2.9%, n = 5) while peak area and height varied over a similar interval (RSD 2-28.1% area; 0.9-22.7% height, n = 5). The two most consistent proteins in this evaluation, viz. myoglobin A and carbonic anhydrase II, showed linear dynamic ranges between 5-150 and 5-50 microg/mL, and limits of detections at 2 and 1 microg/mL, respectively, employing UV detection at 280 nm.     

Effect of adsorbed water on the activity of MoO3 catalysts in the double bond isomerization of olefins

Adsorbed water on molybdena catalysts calcined at different temperatures, may both increase or decrease the catalytic activity, depending upon the mode of adsorption and the number of adsorbed molecules. Our results prove that Brönsted acidic hydroxy groups are principal parts of the active sites, and Lewis acidic and/or basic centers are also required for catalytic activity in the double bond isomerization of olefins.

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, , , , . , , / .

     

Semiautomatic and automatic catalytic amperometric and catalytic constant-current potentiometric titrations of gold(III)

Semiautomatic and automatic amperometry and constant-current potentiometry were used to follow the course of catalytic titration of gold(III) with potassium iodide. The Ce(IV)-As(III) system in the presence of sulfuric acid was used as the indicator reaction. The possibilities of the application of two types of graphite electrodes were investigated. The effect of concentration of the indicator reaction components, and the presence of organic solvents and acids on the shape of the catalytic titration curves was studied. Amounts of 80–900 μg of gold(III) were determined with a relative standard deviation less than 1.1%. The results obtained are in good agreement with those of comparable methods.     

Can a rate constant independent of sample size be obtained from the rate equation generally used in thermal analysis?

Some consequences of using the rate equation d/dt=k _t f() are shown on a simple example. It is demonstrated thatk _t usually depends on the sample size, and that it is usually incorrect to assume that if mass and heat transport effects are avoided both and d/dt — as functions of time — are independent of the initial mass. Rate equations should be based on real local quantities.