Zuzana Cvengrošová

The gas-phase dehydrocyclization of diphenylamine (DPA) to carbazole over alumina-supported 0.4 wt.% Pt catalysts in a fix-bed reactor has been studied. The reaction was carried out at a temperature of 550 °C in the presence of hydrogen. All catalysts became well dispersed Pt after in situ reduction. Pt catalysts prepared in the presence of a competitive adsorbate (citric acid) were reasonably active during first hours on stream (DPA conversion higher than 90%). However, later, their activity decreased rapidly, except the catalyst prepared in the presence of a higher concentration of the competitive agent. The same trend was observed for the Pt catalyst prepared by wet impregnation. The activity of the catalysts prepared by a microemulsion technique was very high and stable during testing, the conversion of DPA being higher than 93%. However, the selectivities of catalysts prepared in the presence of citric acid gradually decreased with time on stream (TOS). The initial selectivityof such prepared catalysts for carbazole was about 60%. The same trend was observed for catalysts prepared by microemulsion technique. The highest selectivities, 73%, were obtained over the catalysts prepared by wet impregnation; it dropped after 6 days of testing to about 62%.

Pyridine and its methyl derivatives were synthesized in vapor phase from ethanol, formaldehyde and ammonia over zeolites H-ZSM-5, Beta, mordenite and over SiO₂. Both the Si/Al molar ratio and modification of zeolite with zinc strongly influenced the selectivity in pyridine and alkylpyridines formation. The highest yield of pyridine was obtained over ZSM-5 modified with zinc and the highest pyridine-to-alkylpyridines ratio was achieved over H-ZSM-5 with high Si content (Si/Al ratio 140). The conversion of ethanol and the selectivity in pyridine formation were correlated with the acidity of used catalyst.

Monoalkylbenzenes, polymethylbenzenes, para-substituted toluenes and monomethylnaphthalenes were oxidized in the vapor phase by oxygen-containing gas in the presence of water over a Sb₂O₃-promoted V₂O₅/TiO₂ catalyst. This type of catalyst yields carboxylic acids with high selectivity. In the oxidation of substituted alkylbenzenes only alkyl groups were oxidized. No products of oxidative dimerization were detected. Only in the oxidation of methylnaphthalenes, also products of aromatic ring oxidation are formed. A correlation between experimental data and results of quantum-chemical calculations of bond dissociation energies is discussed.

Oxidation kinetics of formic acid with aqueous hydrogen peroxide (30-70%) has been studied at 45 °C with 0-0.1 M H₂SO₄ as a catalyst. A kinetic model has been suggested which satisfactorily describes the oxidation process of formic acid to peroxyformic acid.

The oxidation of p-xylene to terephthalic acid has been studied at 130 to 190 °C, using benzoic acid and methyl ester of p-toluic acid as the solvent. It was found that the solvent affects strongly the activity of the cobalt catalyst which effect is dependent on the presence of bromide and pyridine ligands. In the methyl ester of p-toluic acid as the solvent, cobalt-bromide catalysts induce also hydrolysis of the methyl ester which proceeds parallelly to the oxidation of p-xylene. The activating effect of bromide and pyridine ligands results from their effect on the rate of the reaction of peroxy radicals with the catalyst in the propagation step and from their effect on the rate of consecutive oxidation of p-toluic acid to terephthalic acid.