Acyclic diene metathesis (ADMET) depolymerization of functionalized furan-based polymers

Acyclic dience metathesis (ADMET) depolymerization of functionalized furan-based polymers prepared via aqueous ring-opening polymerization of 7-oxanobornenes has been investigated. Results indicate that while very high molecular weight poly [exo-N-methyl-7-oxabicyclo [2.2.1] hept-2,5-diene-2,3-dicarboximide] can be depolymerized to oligomers with ease, poly [2,3-dicarbomethoxy-7-oxabicyclo [2.2.1] hept-2,5-diene] is more resistant to depolymerization under similar conditions. This difference may be due to differential interaction of the carbonyls in the side chains with the metal atom of the catalyst in the proposed metallacyclobutane intermediate. ADMET depolymerization of poly [2,3-bis (trifluoromethyl)-7-oxabicyclo [2.2.1] hept-2,5-diene] was feasible, however, the extent of depolymerization was decreased due to the use of a coordinating solvent (THF) used during the depolymerization process. © 1994 John Wiley & Sons, Inc.     

Platinum-palladium catalysts in hydrogenation of methylbenzenes

Hydrogenation of benzene, toluene, o,m and p-xylene over Pt, Pd and Pt–Pd/Al₂O₃ catalysts has been studied. Experimental data show that when Pd is added to Pt catalysts, a decrease in activity and changes in selectivity occur. This is explained by the formation of electron-deficient and electrorich metallic species.

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, , -, - - Pt, Pd Pt–Pd/Al₂O₃. , Pd Pt . .

     

Modification of the methylene blue method for spectrophotometric selenium determination

The spectrophotometric method for Se(IV) determination based on its catalytic effect in the reduction of Methylene Blue (MB) by sulphide is modified. The variables that affect the decolorization of MB were taken into account: reagent concentrations, order of addition, mixing and standing times, pH, ionic strength, temperature, solution volume, wavelength, etc. The results of this study allowed a decrease of the determination limit and, by selection of the appropriate analytical conditions, choice of optimum linear range according to the selenium content in the sample. The lower limit range is 15-75 mug/1., with 3% relative standard deviation and no systematic errors. Procedures for overcoming several potential interferences were studied. The proposed method was applied to several environmental samples and the results were compared with those obtained by other standard methods.     

A method for the characterization of emulsions, thermogranulometry: application to water-in-crude oil emulsion

Emulsions are used in a wide range of applications and industries. Their size distribution is an important parameter because it influences most of the emulsion properties of emulsions. Several techniques of characterization are used to determine the granulometric distribution of emulsions, but they are generally limited to dilute samples and are based on complex algorithms. We describe a method that allows characterization of the droplet size distribution of emulsions using thermal analysis (thermogranulometry). This method permits the use of very concentrated samples without any dilution or perturbation of the system. We first define our method by a thermodynamic and kinetic approach. We studied a real system, i.e., crude oil emulsions, which form very concentrated, viscous, and opaque emulsions with water. We present a correlation between the size of droplets and their freezing temperature, corresponding to our system. Then we compare the size distributions obtained by our method with those derived by direct microscopy observations. The results obtained show that thermogranulometry may be an interesting method of characterization of emulsions, even for concentrated systems.