Chr. Dimitrov

Dependence of para-xylene selectivity of pentasils on the Si/Al-ratio

Abstract Using a series of ZSM-5 zeolites with different Si:Al ratios (11, 50, 140, 250 and 500), the disproportionation and alkylation of toluene with methanol was investigated in the temperature range 593–743 K. It was found that with similar values of toluene conversion (between 23 and 30%) the yield of para-xylene increased with increasing Si:Al-ratio in HZSM-5 zeolites.

Structure-retention correlations of hydrocarbons in GLC and GSC. Alkenylbenzenes

SummaryFor the study of relationships between retention behaviour and molecular structure in GLC and GSC the retention indices of alkenylbenzenes with 2–6 carbon atoms in a straight or branched side chain and of isotopological alkylbenzenes were determined on capillary columns coated with OV-1 and Ucon LB and on a micropacked column filled with graphitized thermal carbon black. From the values obtained index differences corresponding to the increments of the double bond in the side chain were derived. These differences depend significantly on the structure of the side chain and of the position of the double bond.In the case of 1-phenyl-1-alkenes the double bond close to the aromatic ring gives rise to an increase of retention values, especially for the trans-isomers, causing remarkable retention differences between the cis and transisomers. The alkenylbenzenes which exhibit a terminal double bond are less strongly retained than the isotopological alkylbenzenes.

Infrared study on the mechanism of olefins and alcohols conversions over H-ZSM-5 zeolite

Abstract The IR spectra of 1-hexene, 1-heptene, cyclohexene, n-propanol and tert.-butanol, adsorbed on H-ZSM-5 zeolite, evacuated at 675 K, revealed a band at 1520 cm −1 after contacting with the zeolite at 373 J. The band was absent in the case of adsorbed benzene and cyclohexane and of very low intensity by methanol and ethanol adsorption. Considerations are given in favour of very strong interaction either of the type of X-complex or of carbenium ion. The second assumption was confirmed by the subsequent disappearance of the zeolitic OH bands.

Isomerization of 1-methylnaphthalene on magnesium modified NaY zeolites

The conversions of 1-methylnaphthalene on NaY zeolite and its magnesium modifications with different degrees of ion exchange and various Si/Al ratios have been studied at 310 and 350 °C. The investigated catalysts are highly active for positional isomerization. The observed changes in their catalytic activity are explained by the promotion of proton formation and by the absorption properties of magnesium cations as well as by their localization in cations as well as by their localization in the crystal lattice of the zeolite.AbstractИсследованы превращения 1-метилнафталина на магниевых формах цеолита NaY с разной степенью йонного обмена и разного соотношения Si/Al при температурах 310 и 350°C. Все образцы проявляют высокуй активность в реакции позционной изоперизации. Изменения наблюдаемые в каталитической активности объясняются протонирующими и адсорбционными свойствами магниевых катионов, а также их локализацией в кристаллической решетке цеолита.

Influences of copper on physico-chemical and catalytic properties of ZSM-5 zeolites in the reaction of ethene aromatization

Abstract Copper-containing ZSM-5 zeolites were used as catalysts for ethene aromatization. Physico-chemical properties of the catalysts were studied by means of electron paramagnetic resonance (EPR), electron spectroscopy for chemical analysis (ESCA), and electron microscopy. ESCA spectra show that the surface layer of fresh catalysts is enriched in aluminium whereas copper distribution is nearly uniform. The addition of copper enhances activity and yield in the aromatization reaction. This fact and the changes in the chemical state and distribution of copper after catalytic experiments, regeneration and reduction lead to the conclusion that copper plays an important role in the studied reaction.

Modification of supports with liquid phases by thermal treatment in a closed system

Diatomaceous earth type supports are modified with a liquid phase, by high-temperature thermal treatment in a closed system. It is shown that the retention characteristics of the support changes by this treatment. The main advantage of this method, as compared with modification in a flow-through system is that liquid phase evaporation and possibility of oxydation during the process can be avoided. This ensures a better uniformity of the modified support.

Dehydrocyclization of α-amylnaphthalene on silica-alumina catalyst

It has been demonstrated in the course of studying the catalytic conversions of α-n-amylnaphthalene on synthetic silica-alumina catalyst that parallel with the reactions of isomerization, fragmentation, and dealkylation there is also a reaction of dehydrocyclization with formation of 2-methylanthracene and anthracene. An analogy has been established between the silica-alumina catalyst and the platinum-covered charcoal in the practically selective course of cyclization in the series of monoalkylnaphthalenes to anthracene structures. The assumption is expressed that the above reactions on a silica-alumina catalyst possess an ionic mechanism: the reactions of dealkylation and isomerization are determined by the formation of a carbonium ion with positive charge at the nucleus, the reactions of fragmentation and dehydrocyclization being determined by the formation of a carbonium ion with positive charge at a carbon atom from the side chain.

Plasma-modified thermal carbon black as a packing material for gas chromatography

Abstract The chromatographic properties of thermal carbon black (TCB) TG-10 could be improved by treating it with a high-frequency plasma, in the presence of benzene vapour. In comparison with the untreated TCB, chromatographic peaks are more symmetrical and retention volumes are lower. The data obtained show that the surface of TCB after modification becomes more uniform and non-specific, and the properties of modified TCB approach those of graphitized carbon black. TCB modified in this manner is good sorbent for selective separation of structural and geometrical isomers.

Gas chromatographic analysis of organic compounds on non-polar adsorbents

Abstract The applicability of new non-polar adsorbents in gas chromatography has been investigated. Molybdenum and tungsten sulphides are characterized by a lamellar structure, which resembles the structure of graphite, and hence are expected to show chromatographic properties similar to those of graphitized thermal carbon black (GTCB). This assumption was verified chromatographically by comparing the properties of argon plasma-treated thermal carbon black (TCB) with the properties of MoS 2 and WS 2 and MoSe 2 . The adsorption characteristics (relative retentions, heats of differential adsorption) were determined at low levels of adsorbent surface coating with reference compounds. The results indicate that the treatment of TCB in a hydrogen-containing plasma (oxygen-free flow) results in lower retentions. The retention of n -alkanes by MoS 2 and WS 2 is higher than that of monoalkylbenzenes with the same number of carbon atoms. Di- n -butyl ether is adsorbed by MoS 2 and WS 2 more weakly than n -nonane, which indicates a homogenous sorbent surface. The adsorption of di- n -butyl ether on MoSe 2 is stronger than that of n -nonane, which indicates an non-homogeneous adsorbent surface. The results made it possible to derive several practically applicable conclusions. MoS 2 and WS 2 , for instance, can be used for analytical purposes at lower temperatures than thermal carbon black. The adsorbents under consideration are suitable for the separation of structural and geometrical isomers.

Correlation between the structure of the stationary phase and activity coefficients in gas—liquid chromatography: esters of phthalic acid with aliphatic alchohols

Abstract In an investigation of the correlation between the structure of the stationary phase and activity coefficients in gas—liquid chromatography, the aliphatic esters of phthalic acid were chosen for study as the liquid stationary phase and aromatic hydrocarbons were used as solutes. The dependence of the activity coefficients of the aromatic hydrocarbons on the structure of the stationary phase was demonstrated, and the effect of the length of the chain of the esterifying alcohol in the phthalic esters on the chromatographic behaviour was also shown. It is thought that the data obtained in this work might serve as a reasonable basis for liquid phase selection.