Aromatic activity

C—S—C) in the main chain. The new polyethers prepared either by new heteroarylene activated or by aromatic activated systems have good melt processability. The thermal stability and glass transition temperature of bisphenol-A based new polymers are shown in Table 10. 

Bursian et al. (66a) suggested metallic platinum sites for dehydrogenation and Pt " sites for ring closure. They studied the effect of several elements added to platinum-on-silica catalyst on the aromatizing activity of n-hexane. Benzene yield increased parallel to the amount of soluble platinum (66b) at the same time, the crystallinity of platinum decreased in the presence of additives promoting aromatization. These are elements (e.g., Ce, Sc, Zr) which do not form an intermetallic compound with platinum (66c). 

The admixture of lead to platinum has a similar effect (Fig. 5). At the same time, the aromatizing activity increases up to about 1 1 Pt Pt atomic ratio 24). With even more lead it scatters aroimd somewhat lower values 66). Electron donation from lead to platinum has been proved by infrared spectroscopy, so one may wonder whether lead is present as metal in the catalyst (75). The additive effect can also be interpreted by its creating hydrogen-deficient surface sites favorable for aromatization. When more lead is present than platinum (i.e., where no more continuous platinum surface is probable), the inverse correlation between hydrogen adsorptivity and activity ceases to exist. 

One carbon atom in a wrong interstice may block the C5 cyclization activity of several surrounding sites. Therefore, C5 cyclic reactions are suppressed first during catalyst deactivation, while aromatization activity lasts much longer 159). This again supports the reactive adsorption mechanism 154). A different type of deactivation was reported as being due to disordered and ordered surface carbonaceous deposits 138,148). 

The effect of crystallite size and shape of K-L zeolite on the dispersion of Pt was examined by a variety of techniques by Resasco and coworkers [143], They obtained multiple overlapping CO bands on these samples and were able to assign them to Pt clusters located inside the zeoHte pores (<2050cm ), near the pore mouth (2050-2075 cm" ) and outside the pores (>2075 cm" ). They were able to correlate high -octane aromatization activity with the K-L zeolite samples with short channels where most of the Pt is inside the pores. 

Aromatization activity of gallium containing MEl and TON zeolite catalysts in n-butane conversion effects of gallium and reaction conditions. Appl. Catal. A, 316, 61-67. 

As a conclusions, we propose the following two-step mechanism for alkyl aromatic activation over vanadia-titania catalysts ... 

The olefinic substitution reaction may also be effected in some instances by using a palladium(II) salt and an aromatic compound instead of an organic halide. Palladium(II) salts are apparently able to metallate some aromatic hydrocarbons directly. The reaction succeeds best with aromatics activated with electron-supplying substituents producing, with certain olefinic compounds, vinylically substituted products. For example, benzene and styrene with palladium acetate in boiling acetic acid produce stilbene in 90% yield (37). 

Fluoromethylbenzoic acids, metallation, 9, 26-27 Fluoro(phenyl) complexes, with platinum(II), 8, 482 Fluorosilanes, elimination in fluorinated alkene activation, 1, 732 in fluorinated aromatic activation, 1, 731 and hydrodefluorination, 1, 748 Fluorosilicate anions, hypercoordinated anions, 3, 484 Fluorotoluenes, metallation, 9, 21 Fluorous alkylstannanes, preparation, 3, 820 Fluorous biphasic system, as green solvent, 12, 844 Fluorous ligands, with supercritical carbon dioxide, 1, 82 Fluorous media... 

CO insertion, Ru and Os clusters, 6, 840 dynamic NMR studies, 1, 421 in fluorinated alkene activation, 1, 732 and fluorinated aromatic activation, 1, 731 to heteroatom ligands, Ru and Os trinuclear clusters,... 

The past 35 years have seen both the asymmetric hydrogenation and asymmetric transfer hydrogenation of imines develop into useful methods for the synthesis of chiral amines. Particularly, focused research over the past 15 years has led to highly enantioselective examples of both reaction types and has added aza aromatics, activated imines, and iminium cations to their purview. In addition, the asymmetric hydrogenation and asymmetric transfer hydrogenation of imines have both been apphed to total syntheses. Because they are necessarily isomeri... 

Oxidative aromatization. Activated manganese dioxide (excess) oxidatively dehydrogenates certain cyclohexene aldehydes, ketones, and Schiff bases, but not esters, to the corresponding aromatic derivative. For example, 2-methyl-cyclohexene-3-carboxaldehyde (3 and 4), obtained by the Diels-Alder reaction of pentadiene-1,3(1) with acrolein (2) is oxidized to o-tolualdehyde (5) in 69% yield. 4-Acetylcyclohexene is oxidized to acetophenone in 71 % yield.3... 

The liquid phase catalytic chlorination of aromatics was performed at atmospheric pressure in a glass reactor equipped with a magnetic stirrer, a reflux condenser, CI2 or N2 gas feed line and a septum. Aromatic, activated cateilyst eind monochloroacetic acid were introduced in the reactor, in the order mentioned. The mixture was then stirred and heated to attain the reaction temperature in the presence of nitrogen gas. The nitrogen gas was disconnected and chlorine gas was supplied at a rate of 0.08 mol/h to conduct the reaction. The reaction products were analyzed by a Hewlett-Packard model 5890 series II gas chromatograph. 

Influence of the addition of silica, as a binder at a concentration of 10 or 50 wt%, to H-gallosilicate (MFI) zeolite on its inter- and intracrystalline acidity, initial activity, product selectivity and distribution of aromatics formed in the propane amortization (at 550°C) and also on its deactivation due to coking in the aromatization process has been thoroughly investigated. Silica binder caused an appreciable decrease in the zeolitic acidity (both external and intracrystalline acid sites) and also in the propane conversion/aromatization activity. Because of it, the deactivation due to coking of the zeolite in the propane aromatization is, however, decreased. The deactivation rate constant for the initial fast deactivation is decreased but that for the later slow deactivation is increased because of the binder. The aromatics selectivity for aromatics and para shape selectivity of the zeolite, particularly at lower conversions, are increased but the propylene selectivity and dehydrogenation/cracking activity ratio are decreased due to the presence of binder in the zeolite catalyst. 

Both the intracrystalline and intercrystalline (or external) acid sites of the zeolite are decreased by the silica binder. The changes in the intracrystalline acidity of the zeolite are reflected in its propane aromatization activity the activity is reduced significantly by the silica binder. The aromatics selectivity and the dehydrogenation / cracking and aromatization / cracking activity ratios and aromatization/(methane + ethane) mass ratio are also affected appreciably by the silica binder. The shape selectivity of the zeolite is increased markedly by the silica binder. Also because of the binder, the deactivation rate constant for initial fast deactivation is decreased but for the later slow deactivation is increased. 

To further understand the role of strong acid sites in the aromatization activity of the catalysts, time-on-stream (TOS) activity studies were carried out over these three catalysts, and the deactivation patterns of acid sites were studied. It can be seen from fig.4 that, the as-synthesized catalysts T and WT show a constant aromatization activity, where as the hydrothermally treated sample (HT) shows rapid deactivation with in 12 hrs. The highest initial activity in aromatization with the steep fall within 12 hrs TOS observed in case of the catalyst HT, can be understood by considering the high turn over number of super acid sites created during hydrothermal treatment, and their propensity to rapid deactivation (8). 

Cs-ETS-10 shows better aromatization selectivity among the alkali metal exchanged ETS-10 samples. An increase in basicity of ETS-10 by Cs cation exchange appears to be responsible for the enhanced aromatization activity. The aromatization selectivity is maximum at 0.4 wt.% Pt in the catalyst. Pt-Cs-ETS-10 has a greater aromatization selectivity when compared to Pt-Al203 which possesses greater isomerization and cracking activities. 

A V-ZSM-5 sample with a Si/V o 42 was synthetized outgoing from V0(C00)2 and Q-brand sodium silicate using TPA-Br as template. ESR spectroscopy proved that vanadium(IV) ions in the zeolitic framework exhibit a distorted square planar symmetry. Upon heat treatment a part of the framework vanadium ions migrate to extra-framework positions. After dehydration no Bronsted acidity was found. Treatment in oxygen and hydrogen above 570 K revealed the redox character of the V-ZSM-5 sample. In oxidation of n-bu-tane (as catalytic test reaction) the V-ZSM-5 zeolite exhibits selective dehydrogenation and aromatization activity. 

---