Deactivation Behaviors of Zeolite and Silica−Alumina Catalysts in the Degradation of Polyethylene

Abstract

For chemical recycling of waste plastics, HZSM-5, HY, and H-mordenite zeolites and silica−alumina were examined as catalysts for the degradation of polyethylene in a fixed-bed flow reactor system, and their activities and deactivation behaviors caused by coke deposition were studied. HZSM-5 catalyst was found to be very effective for the production of gasoline-range fuel oils mainly consisting of isoparaffins and aromatics and showed no deactivation due to a very low yield of coke deposited on the catalyst surface, whereas in the degradation of polystyrene a marked deactivation was observed (Uemichi et al. Kobunshi Ronbunshu 1993, 50, 887). Silica−alumina gradually deactivated as time on stream increased, but the degree of deactivation was less than expected from the deposition of a significant amount of coke, probably because the coke deposition in the large pores of the catalyst caused no marked influence on the diffusion of the decomposed fragments involved in the reaction. On the other hand, deactivations of HY and H-mordenite were striking; the latter was most abruptly deactivated, resulting in a marked decrease in the liquid yield. From the surface area measurements of the used catalysts, it was suggested that the pores of HY were sufficiently filled out with coke, while pore blocking by coke occurred in the unidimensional channels of H-mordenite.

Co-Production of Liquid and Gaseous Fuels from Polyethylene and Polystyrene in a Continuous Sequential Pyrolysis and Catalytic Reforming System

“…Furthermore, when compared with that of HDPE, the higher concentraion of aromatic hydrocarbons in the pyrolysis gas of PS also led to more formation of coke on the internal and external surfaces of the HY-zeolite, which led to the increase of the proportion of the solid products. It was mainly due to the fact that, as mentioned previously, the aromatic hydrocarbons especially unsaturated and polyaromatic compounds such as styrene monomer and indan and naphthalene derivatives were formed as major products in the degradation of polystyrene and then they were readily converted into coke (Uemichi et al, 1998).…”

“…2; consequence of the coking on the catalyst surfaces at a higher temperature. The formation of coke influenced catalyst activity by covering some of the active sites and blocking the channels which could make the inner active sites inaccessible for the reactant molecules (Miskolczi, Bartha, Deak, Jover, & Kallo, 2004 ;Uemichi, Hattori, Itoh, Nakamura, & Sugioka, 1998).…”

“…The effect of WHSV on the composition of gaseous products from HDPE pyrolysis was also investigated at the pyrolysis temperature of 450 °C and the reforming temperature of 450 °C. It is well known that the pore size is important for determining the size selectivity of reactants and products, which can enter and leave the active sites of the catalyst (Uemichi et al, 1998). When compared with other zeolite catalyst, such as HZSM-5, the HY-zeolite catalyst has a relatively large pore size, which may allow a little larger molecular gaseous products to leave the HY-zeolite catalyst.…”

Materials and Methods for the Chemical Catalytic Cracking of Plastic Waste

“…For the catalytic cracking reaction we have built a semiautomatic reaction system (Figure 9), initially based on a design reported by Uemichi (Uemichi et al, 1998). The reaction system has (1) a loading section, where the polymer melts, (2) a capillary tube for controlling the polymer feed, employing nitrogen as carrier gas, (3) a fixed bed stainless steel reactor, (4) a condenser and (5), recipients for liquid and gas products.…”

Environmentally Harmful Low Density Waste Plastic Conversion into Kerosene Grade Fuel

“…However, material recycling will reach its limits, while chemical recycling will be gradually expanded. Thermal or catalytic cracking of waste plastics is one of the possible methods of their utilization [4][5][6][7][8][9][10][11].…”