Influence of nanocrystalline HZSM-5 external surface on the catalytic cracking of polyolefins

Abstract

Catalytic cracking of both LDPE and HDPE has been studied using three different samples of nanocrystalline HZSM-5 zeolite as catalysts. The zeolite samples were synthesized under different conditions in order to obtain materials with different crystal sizes in the nanometer range (10-60 nm) and high external surface areas (78-242 m 2 /g). The plastic cracking reactions were carried out at 340 8C for 2 h using a plastic/ catalyst mass ratio of 100. Despite these really mild conditions, high activities were observed over the three catalysts. The plastic conversion and the turnover frequency increased strongly with the amount of external surface area of the catalysts, showing the importance of the nonsterically hindered external acid sites for the conversion of the bulky plastic molecules. The main products obtained in the cracking reactions over the three catalysts are light hydrocarbons (C 3 -C 5 fraction) with a considerable amount of olefins, which indicates the possibility of addressing the zeolite selectivity towards hydrocarbons useful as raw chemicals. These products are formed via an end-chain cracking pathway of the polymer molecules over the zeolite acid sites. The occurrence of this mechanism has been confirmed by DSC analysis of the LDPE and HDPE residues remaining within the reactor after the catalytic tests, showing they still consist of plastic molecules with melting temperatures similar to those of the raw polymers. #

Thermal and catalytic pyrolysis of plastic waste

“…The use of zeolites has been shown to be effective in improving the quality of products obtained in the pyrolysis of polyethylene and other addition polymers. The acidity of their active sites and its crystalline microporous structure (textural properties) favor hydrogen transfer reactions and thereby make them suitable for obtaining high conversions of gas at relatively low temperatures, between 350 and 500 °C [11,18,22,[41][42][43][44] . That is, these features allow milder operating conditions (lower temperatures and reaction times) than a thermal pyrolysis [4,25,30,45] .…”

“…This determines the products obtained in these reactions. Solid acid catalysts such as zeolites, favor hydrogen transfer reactions due to the presence of many acid sites [11,18,22,36,42,44] . The acid strength of the solid is characterized by the presence of Lewis or Brønsted acid sites.…”

“…That is, the use of catalysts gives an added value to the pyrolysis and cracking efficiency of these catalysts depends both on its chemical and physical characteristics. These particular properties, promote the breaking of C-C bonds and determine the length of the chains of the products obtained [17,22] . For Brazilian cities, the percentage of high and low density polyethylene (HDPE and LDPE, respectively), polyethylene terephthalate (PET), Poly(vinyl chloride) (PVC) and polypropylene (PP) found in municipal solid wastes are 89% and the other polymers account for the other 11% [13] .…”

Size-Controlled Synthesis of Nano-Zeolites and Their Application to Light Olefin Synthesis

“…Botella et al [42] reported the Beckmann rearrangement reaction over nano-beta zeolite, where the reactions proceed mainly on the external surface of the zeolite. Serrano et al [43] reported catalytic cracking of polyolefins over nano-ZSM-5 zeolite. Because the molecular size of polyolefins is larger than the pore size, cracking of polyolefins occurs mainly over the acid sites on the external surface, followed by the -20-formation of lighter olefins on the internal pore surface.…”

Tailored Mesoporosity Development in Zeolite Crystals by Partial Detemplation and Desilication

“…[38] Acid sites in zeolites (particularly beta) catalyze the pyrolysis of polyethylene, allowing operation at lower temperatures with respect to thermal pyrolysis, as well as a higher selectivity toward the desired products. [39][40][41][42] In plastic degradation, it is known that both Lewis and Brønsted acid sites are involved in the initial step of the LDPE degradation, by hydride abstraction of the polymer or by the addition of a proton to a C-C bond, respectively. [39] The polyethylene molecule suffers successive b-scissions, producing fragments with lower molecular weights.…”