Cracking Of N-alkanes Research Articles

Adsorption and cracking of n-alkanes over ZSM-5: Negative activation energy of reaction

The heat of adsorption of normal-paraffins on ZSM-5 is a linear function of the number of carbon atoms on the paraffin n, and is approximately half of the latent heat of consation. The rate of cracking of n-paraffins over ZSM-5 increases exponentially with n, which is consistent with a linear dependence of Δ G° with n. As temperature is increased, the cracking rate increases when n is less than 16, but decreases when n is greater than 16. This phenomenon is contrary to normal expectations, and is a consequence of the competition between two effects: the increase of intrinsic kinetics with temperature, and the decrease of adsorption strength and the concentration of active intermediates with temperature. The heat of adsorption becomes larger than the activation energy of the intrinsic kinetics when n=16.

Isotopic geochemistry and habitat of the natural gases from the Upper Carboniferous Žacleř coal-bearing formation in the Nowa Ruda coal district (Lower Silesia, Poland)

The Nowa Ruda coal district is located in the SE part of the Lower Silesian Coal Basin. The analysed coals have vitrinite reflectance from 1.12 to 1.56% and δ 13C values from −24.6 to −23.2%. Natural gases accumulated in coal seams reveal considerable variability of molecular and isotopic compositions. Geochemical indices and stable isotope ratios vary in the following ranges: C 2+ wet gas index from 0.6 to 40.7%; carbon dioxide-methane index (CDMI) from 1.9 to 99.9%; δ 13C(methane) from −66.1 to −28.6%; δD(methane) from −256 to −211%; δ 13C(carbon dioxide) from −24.4 to −7.0%; and δ 13C(ethane) from −27.8 to −25.5%. Stable isotope studies allow distinction between thermogenic, endogenic (magmatic and/or upper-mantle derived) and probably even bacterial gases. The intensity of migration, mixing and accumulation of the natural gases of various origins appear to be variable in time and space. The variability of both the molecular and the isotopic compositions of the analysed gases probably results from the specific primary reactions active during the generation of the thermogenic gases in coal seams (decomposition of coal-seam matrix and/or cracking of n-alkanes) and/or in humic dispersed organic matter of coaly mudstones. An isotopic fractionation may also develop during the migration of the natural gases through the ultra-fine pores of coals. The results of this paper suggest that in the case of gases connected with coal seams, the existing gas classifications cannot be directly applied to their genetic interpretation.

A neutron powder diffraction analysis of potassium-exchanged ferrierite

Ferrierite is a versatile catalyst both in hydrocarbon processing and in the petrochemical industry. The H{sup +}-exchanged form of ferrierite has been reported as an efficient catalyst for the hydration of alkenes, the cracking of n-alkanes, and the isomerization of m-xylene to p-xylene. Harrison et al. have noted that its use in methanol conversion gives appreciably lower aromatic selectivity than ZSM-5, probably because the 10-ring windows of ferrierite (0.42 {times} 0.54 nm) are smaller in diameter than those of ZSM-5 (0.53 {times} 0.56 and 0.51 {times} 0.55 nm). Alkali-metal-exchanged zeolites are at present of considerable commercial significance since they are key components of effective catalysts for the aromatization of alkanes, such as n-hexane to benzene. A good example is the K{sup +}-exchange form of zeolite L. The authors considered, therefore, that a sensible test material for a structural investigation of ferrierite would be the K{sup +}-exchanged form of the zeolite. Previously the authors have reported the use of high-temperature X-ray studies of Ni{sup 2+}-exchanged zeolite Y, and a low-temperature neutron diffraction study of La{sup 3+}-exchanged zeolite Y. They have embarked on a study of synthetic powdered samples of this catalyst using the techniques of both neutron and X-ray diffraction, andmore » report in this paper on a neutron Rietveld profile analysis of a K{sup +}-exchanged ferrierite.« less

The nature of the carbon deposits formed on molecular sieves of the CaA type in the cracking of n-alkanes

The nature of the carbon deposits formed on molecular sieves of the CaA type in the cracking of n-alkanes