Article pubs.acs.org/EF
Catalytic Degradation of High-Density Polyethylene over a Clay Catalyst Compared with Other Catalysts Mi Liu, Jian K. Zhuo,* Si J. Xiong, and Qiang Yao Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: The catalytic activity of a clay catalyst was studied through the degradation of high-density polyethylene (HDPE) using a thermogravimetric (TG) and fixed bed batch reactor and comparison of that with other catalysts (HZSM-5, all-silica MCM-41, Al2O3, and CaO). The results of TG experiments showed that clay had the same catalytic effects on degradation temperature as Al2O3 and CaO, while HZSM-5 and MCM-41 were able to shift the degradation reaction to lower temperatures. The catalytic degradation results of HDPE with a fixed bed batch reactor showed that the major product over HZSM-5 was fuel gases, all-silica MCM-41 produced the highest fuel oil yield, and the clay catalyst produced the highest yield of liquid products, including wax and oil. Compared with the composition of the gaseous products and fuel oil, that of the clay catalyst was favorable to the formation of alkanes, which indicated that the intermolecular hydrogen transfer reaction was enhanced while the β-scission reaction of radicals was inhibited over a clay catalyst. The results further verified that the catalytic cracking performance of HDPE and the product distributions were related to the textural properties of clay.
1. INTRODUCTION The invention of plastics has brought convenience to people’s lives. However, the consumption of plastics has caused great damage to environment because of their low biodegradability. Feedstock recycling, which turns plastic wastes into chemical raw materials or fuels by means of chemical reactions, has been considered the most economically viable and environmentally friendly method for mitigating the consequences of plastics.1,2 High-density polyethylene (HDPE) is the third most widely used commercial plastic material in the world, which is mainly utilized in the packaging industry and the construction industry.3,4 Polyethylene degradation has attracted interest in recent years. Compared to thermal degradation, catalytic degradation of polyethylene requires a lower temperature. More importantly, catalytic degradation can improve product selectivity and yield high-quality liquid fuels. So far, the most extensively used catalysts for HDPE degradation are solid acids, including zeolite, molecular sieves, and silica−alumina.5−10 Zeolite, especially HZSM-5 because of its specific acidity and microporous structure that favors the production of light olefins or low-aromatic content gasoline, was the most frequently studied catalyst for polyethylene degradation.1 The relationship between the textural properties of zeolite and the catalytic activity and product selectivity during the degradation of HDPE has been studied.5−7 Mesoporous materials such as MCM-41 were also used for the degradation of HDPE because the pores of MCM-41 are larger than those of zeolite, which was beneficial to macromolecules or branched chain molecules entering and cracking into liquid fuels.8,9 Other catalysts have also been used for the catalytic degradation of HDPE, such as spent FCC, superacid soild (ZrO2/SO42−), basic carbonate, activated carbon, BaTiO3-based catalyst, etc.11−17 Although the HZSM-5 zeolite and ordered mesoporous MCM-41 because of their unique acidity and porous structure showed excellent catalytic performance in the degradation of HDPE, the deactivation problem of the catalyst due to carbonaceous © 2014 American Chemical Society
deposits and the expensive cost of the catalyst became obstacles in commercial processes.18 Clay catalysts, a convenient, cheap, and widely available alternative to expensively manufactured catalysts, attract little attention in terms of the catalytic degradation of HDPE. Breen et al.19 studied the catalytic degradation of HDPE over two acid-activated clay catalysts (SEP and K10) and two pillared smectite catalysts (AZA and FAZA) under dynamic (35 to 650 °C at a rate of 10 °C min−1) and isothermal (60 min at 420 °C) conditions using a Synergic chemical analysis system that consisted of a thermo-balance fitted with two outlets. Compared to the products of thermal degradation, the catalytic cracking products over clay catalysts contained little alk-1-enes, alk-x-enes, and α-ω-dienes, which were converted into light gases and aromatic species, including toluene, xylenes, and triand tetramethylbenzenes. AZA and FAZA produced the largest yield of aromatics, whereas quantities of low-molecular weight gases were produced by sepiolite in the isothermal process. In addition, Breen et al.20 studied the degradation of HDPE over acid-activated smectites (JP and ST). The results showed that the amount of aromatics was related to the acid treatment conditions of the catalyst and the process temperature. The greatest proportion of aromatics was produced using the catalysts prepared by a short acid treatment time at a low process temperature. Manos et al.21,22 conducted catalytic cracking of polyethylene over two natural clays (C-27saponite and Zenith-N) and their pillared derivatives (AZA and ATOS) in a semibatch reactor. The clay-based catalysts did not easily produce overcracking to small molecules because of the mild acid sites, which contributed to liquid product yields of ∼70%, which were higher than that of US-Y zeolite (
