Stability of ZSM-11 and BETA zeolites during the catalytic cracking of low-density polyethylene

The stability of H-ZSM-11 (H-Z) and H-BETA (H-B) zeolites during the catalytic degradation of low-density polyethylene (LDPE) was studied using the same sample of catalyst in eleven consecutive cycles. The gaseous hydrocarbons, liquid hydrocarbons and waxes generated in each cycle were analyzed as well as the used catalyst. The zeolites were characterized by XRD, FTIR of adsorbed pyridine and N₂ adsorption, while the physical mixtures of LDPE/zeolites were subjected to TG-DTG analysis.The H-Z zeolite exhibited an important stability during the successive cycles of LDPE conversion. On the contrary, the behavior H-B zeolite was completely different; from the sixth cycle the yields of products changed progressively, approaching to that obtained in a purely thermal process.The yields of accumulated coke increased steadily throughout the cycles up to maximum values in the eleventh cycle of ∼6 and ∼15 wt% for H-Z and H-B, respectively. These results were confirmed by TG under air flow.     

H-ZSM-11 and Zn-ZSM-11 zeolites and their applications in the catalytic transformation of LDPE

Low density polyethylene was converted into hydrocarbons over Zn- and H-ZSM-11 zeolite catalysts in a fixed-bed reactor during 20 and 60 min reaction time, 0.5 and 2.0 polymer to catalyst mass ratio at 500 °C. The zeolites were synthesized by conventional techniques and characterized by XRD, pyridine FTIR and N₂ adsorption. The adsorbed pyridine spectra demonstrated that new Lewis sites were formed after Zn exchange, and that the relationship between Lewis and Brönsted sites in the Zn-ZSM-11 zeolite (3.53) was much higher than that in the H-ZSM-11 zeolite (0.09). Thermal analyses confirmed that the temperature of decomposition of the polymer can be decreased in as much as about 145 °C when the catalysts were added. As compared to the thermal degradation, the catalytic conversion produced less solid residues and much higher amounts of gas and liquid hydrocarbons. The catalysts showed different yield profiles: the H-ZSM-11 zeolite yielded more gases, while the Zn-ZSM-11 zeolite yielded more liquid products. Notably over Zn-ZSM-11 zeolite, these liquid products were mainly aromatic, and depending on experimental conditions (higher temperature, longer reaction time, smaller polymer/catalyst relationship), aromatic selectivity could be increased to almost 100%.     

Single star evolution I. Massive stars and early evolution of low and intermediate mass stars

We survey the results of those model calculations of the early evolution of single stars that have been obtained over approximately the last decade, and compare some of these results with the observations, concentrating particularly on the comparison between theoretical predictions regarding surface abundances of red giants and Cepheids and abundance estimates obtained by an analysis of spectral data.For massive stars, we discuss the ramifications of the fact that the time scale for mass loss (via stellar winds) during main sequence and red supergiant evolution can be comparable to the nuclear burning timescale, noting in particular the unusual distribution of stars in the Hertzsprung-Russell diagram, and the probability that significant mass loss is responsible for the chemically highly evolved spectra of Wolf-Rayet stars. finally, we sketch (1) recent progresses in following the evolution of massive stars to the presupernova stage, which is described by a configuration consisting of a core of near-Chandrasekhar mass made up of iron peak elements and a series of “onion”-skin layers of less highly thermonuclearly processed matter and (2) recent progress in understanding the nature of the type II supernova phenomenon.For low and intermediate mass stars, we discuss postulated “extramixing” (beyond convective) processes, which may occur on the main sequence and on the first red giant branch, and continue on to a discussion of asymptotic giant branch evolution, placing considerable emphasis on the character of the thermal pulses that occur in such stars and, in particular, on the nucleosynthesis that occurs in the helium burning convective shells during these pulses and on the dredge-up phenomenon that brings fresh carbon and neutron-rich isotopes to the surface following pulse peak.