Catalytic microwave-assisted pyrolysis of waste plastics for fuels and chemicals

Abstract

Discarded plastics can be converted to various fuels and chemicals to generate economic value instead of polluting the environment. In the past few years, pyrolysis has attracted much attention in the industrial and scientific communities as a promising versatile platform to convert plastic wastes into valuable resources. However, it is still difficult to develop an efficient pyrolysis process with an improved heat and mass transfer, desirable product selectivity, and minimum environmental impacts. This thesis aims to develop and optimize a novel technology that has great potentials to be scaled up for cost effective conversion of waste plastics to commercial fuels or chemicals.First, catalytic fast pyrolysis of low density polyethylene (LDPE) for fuel production by a relay catalysis process (Al2O3 followed by ZSM-5 zeolite) was investigated. Effects of different catalysts, pyrolysis temperatures, catalyst loading, and Al2O3 to ZSM-5 ratio, on product distribution and selectivity were studied. Al2O3 showed an excellent catalytic performance for LDPE pyrolysis vapors reforming, mainly producing C5-C23 olefins that are the important precursors to form aromatics via Diels-Alder, aromatization, and polymerization reactions in the pores of ZSM-5 catalyst. Experimental results also showed that the selectivity of monoaromatics and C5-C12 alkanes/olefins can be up to 100% over Al2O3 followed by ZSM-5 relay catalysis at the temperature of 550 ºC, the catalyst to plastic ratio of 4:1, and Al2O3 to ZSM-5 ratio of 1:1. Considering that zeolite is considered a promising catalyst for plastic cracking because of its well-defined acid sites, high product selectivity, and outstanding stability, the relationship between ZSM-5 structure and catalytic cracking performance was investigated in detail. For this purpose, we developed a series of ZSM-5 zeolites with differential surface acid density and pore structure. The results were presented in this systematic investigation on the effects of these two parameters on catalytic performance. Our results demonstrate that Brønsted acid site density had a profound impact on catalyst lifetime and aromatic selectivity. The relationship between Brønsted acid site density and catalyst lifetime displays a tendency that catalyst lifetime declines with acid site density at the beginning, then rises up later, instead of a linear correlation. Also, the increase of mesoporosity extends the catalyst lifetime to some extent. In order to design higher-performance catalysts for chemical upcycling of waste plastics with the goal of maximizing catalyst lifetime, the Brønsted acid site density should be controlled within a proper range and the catalyst mesoporosity can be improved as much as possible. However, although catalytic pyrolysis of plastics for fuels production can alleviate the plastic solid waste pollution, the fuels are still burned and not able to offset the demand for virgin plastics, making no contribution to a circular economy. It seems like another expensive and complicated way to burn fossil fuels instead of recycling. Therefore, we developed a new catalytic pyrolysis process to convert waste plastic into low-carbon synthetic naphtha which can be used as a feedstock for new plastic production. First, a one-step in-situ catalytic pyrolysis over Al2O3 pillared montmorillonite clay catalyst was developed to convert waste plastics into low aromatic naphtha. Experimental results show that Al2O3 pillared montmorillonite clay produces up to 60.3% C5-C12 alkanes, while ZSM-5 gives high contents of aromatics (46%) and olefins (35%). One challenge related to in-situ catalysis mode is that the impurities in the PSW could potentially deactivate the catalyst materials and the catalyst separation after reaction is much more difficult. In contrast, ex-situ catalytic process shows clear advantages, including independent temperature optimization, easy catalyst recycling, and no contact between the reforming catalysts and feedstock. Based on this, we developed a tandemly ex-situ catalytic microwave-assisted pyrolysis process, involving microwave-assisted pyrolysis, heterogeneous catalytic cracking, and subsequent catalytic reforming. This tandem ex-situ catalysis system was also applicable to catalytic cracking of various waste polyolefins including high density polyethylene (HDPE), LDPE, and polypropylene (PP), producing high quality naphtha. Furthermore, both pure and real-world plastic mixtures can also be used to produce high quality naphtha, with a C5-C12 paraffin selectivity of 60.39 % and 57.16 %, respectively. In order to improve the cracking catalyst stability, extremely high silica ZSM-5 zeolites were tested. The high selectivity of C5-C12 hydrocarbons (98.9%) with low selectivity of C5-C12 aromatics (28.5%) was obtained over a high silica ZSM-5 zeolite at a pyrolysis temperature of 500 °C, catalytic cracking temperature of 460 °C, and a weight hourly space velocity (WHSV) of 7 h-1. Stability testing demonstrated that the high silica zeolite was mostly deactivated after a time on stream of 6 h. 8 cycles of regeneration-reaction cycles were carried out successfully with little change on the product distribution, showing the great potential for continuous production of low-aromatic liquid oil. Catalyst characterization showed that the catalyst deactivation was primarily caused by coke deposition (approximately 16.0 wt.%) on the surface of the catalysts, and oxidative regeneration was able to recover most of the pore structure and acidity of the zeolite by effectively removing coke. Furthermore, we demonstrate that catalyst lifetime during catalytic cracking of HDPE can be remarkably improved (4.3× to 12.3×) without significant effects on C5-C12 aromatics selectivity by using hierarchically micro-meso-macropore high silica ZSM-5 compared with the conventional analogue. The lifetime improvement by using the well-developed hierarchical ZSM-5 can be rationalized based on the more open channels that promote the diffusion of reaction intermediates and the increase of Brønsted acid sites that catalyze the cracking reactions. The economic and life cycle assessments showed that the plastic-to-naphtha route can improve the environmental benefits of plastic recycling, with a great economic potential. These outcomes highlight the potential of creating a plastic circular economy and will move the technology closer to commercial implementation.