Catalytic co-pyrolysis of polycarbonate and polyethylene/polypropylene mixtures: Promotion of oil deoxygenation and aromatic hydrocarbon formation

Highlights

• Deoxygenation of PC pyrolysis oil was achieved by catalytic co-pyrolysis.

• Dehydrogenation of PP and hydrodeoxygenation of PC can be simultaneously catalyzed.

• High value-added aromatics were the target products of all reactants.

• The content of oxygenates in oil reduced to 2.9% when PC: PP was 25 wt%:75 wt%.

• The content of bicyclic aromatic hydrocarbons like naphthalenes could be up to 54.5%.

Abstract

The co-pyrolysis of polycarbonate (PC) and polyethylene (PE)/polypropylene (PP) in the presence of HZSM-5 catalyst was studied in a two-stage tube reactor with the objective of promoting simultaneous deoxygenation and aromatic hydrocarbon formation, at atmospheric pressure and without external hydrogen addition. As the blending ratio of PE/PP increased, the yield of oil phase product, coke, and solid decreased, and the yield of gas phase increased significantly. At the blending ratio of 75% PP, the concentration of oxygenates in the oil was reduced to 2.9% and that of aromatic hydrocarbons increased to 97.1%. Increasing the reaction temperature from 500 °C to 700 °C increased the yield of aromatic hydrocarbons further to 98.1%. Catalyst regeneration decreased the acidity of the HZSM-5 catalyst, resulting in a slight decrease in the deoxygenation and aromatization effects. The results showed that PP is an effective hydrogen donor in the catalytic deoxygenation and aromatization process; the hydrogen provided enhanced the adsorption of phenols produced by the pyrolysis of PC on HZSM-5 and their direct deoxygenation to form new aromatic hydrocarbons.

Introduction

Polycarbonate (PC) is a thermoplastic resin widely used in electrical and electronic appliances, packaging, manufacturing, etc. [1], [2]. The total global PC demand was up to 4.1 million tons in 2017 [3], which made it one of the main oxygen-containing plastics. Most plastic products are non-degradable and readily discarded after use, and usually, their lifetimes are no more than one month [4], [5], which leads to the accumulation of post-recycling plastic wastes. It is neither economical nor environmentally friendly to dispose waste PC products by traditional incineration methods due to its high oxygen content and low calorific value.

“Plastic-to-oil”, as a cost-effective valorization technology, has gained continuously increasing attention in recent years. This technology usually uses pyrolysis to realize the conversion of plastic wastes to fuel oil like jet fuel and diesel-like oil or high-value chemicals, which could alleviate the problem of fossil oil depletion at relatively low cost [6]. Some researchers have summarized the pyrolysis behavior of PC in previous studies. Achilias et al. [7] found that pyrolysis of PC at 550 °C yielded 6.6% pyrolysis gas, 63% oil products, and 30.4% solid residues; the main constituents of the oil were 32% phenols, 16% bisphenol A, and 5.6% p-isopropylphenol. The presence of various oxygenates such as phenol leads to an increase in the viscosity, corrosivity, and thermal instability of the pyrolysis oil and a decrease in its calorific value [8]. This is particularly critical when the pyrolysis oil is used for fuel or chemical production. Therefore, the deoxygenation of oxygenate-enriched oil is one of the grand challenges in oil upgrading at present [9].

Hydrodeoxygenation (HDO) is the main deoxygenation pathway for oxygenates such as phenols, mainly including hydrogenation-dehydration (HYD) route and direct deoxygenation (DDO) routes [10]. Compared with HYD, DDO may convert phenols to aromatic hydrocarbons by direct cleavage of aromatic C-O without hydrogenating the aromatic ring [11]. Thus, DDO is a better deoxygenation route, with low hydrogen consumption and maximized carbon retention in target products [12]. Also, the resulting products, aromatic hydrocarbons like benzenes and naphthalenes, are in high demand, as crucial additives or raw materials in the fuel, chemical, pharmaceutical, printing, and dyeing industries [13], [14]. Especially in the field of fuel production, aromatics could act as important octane enhancers and are significant parts of all the fuels, the content of which in gasoline, diesel, and jet fuel could be up to 35%, 30%, and 25%, respectively [15]. In the traditional commodity chemical industry, the cost of raw materials usually accounts for 60–70% of the total cost [16]. Therefore, recovering aromatic hydrocarbons from wastes would bring remarkable environmental and economic benefits. Nelson et al. [17] investigated the DDO of phenol over Ru/TiO2 catalyst, in the presence of H₂O at 573 K and 550 psig H₂, and the conversion rate reached 30%. Yoosuk et al. [18] compared the DDO and HYD routes of phenol, catalyzed by MoS₂ and CoMoS₂, and a maximum conversion rate of 83.4% was reached at 350 °C and 2.8 MPa H₂. Zhu et al. [19] used bi-functional Pt/HZSM-5 in the DDO of m-Cresol at 400 °C with an H₂/m-cresol molar ratio of 50, and the yield of aromatic hydrocarbons was up to 99%. It should be noted that these studies on catalytic deoxygenation of phenols used external H₂ as the hydrogen source and rare metals as catalysts.

In more recent studies, hydrogen-rich materials instead of H₂ were used as hydrogen donors, and DDO was carried out in catalytic co-pyrolysis, using less costly catalysts. Xue et al. [20] carried out the catalytic co-pyrolysis of lignin and tetralin, using HZSM-5 and HY zeolite catalysts, and the yield of aromatic hydrocarbons was increased from 48.8% to 66.2%. Ding et al. [21] studied the co-pyrolysis of corn stove and polyethylene (PE) catalyzed by CeO₂ and HZSM-5, and the deoxygenation efficiency increased by around 13%. However, there has been no systematic analysis on the low-cost and high-yield catalytic hydrodeoxygenation of PC co-pyrolysis products.

In this study, PE and polypropylene (PP) were used as hydrogen donors, and HZSM-5 was used in the catalytic hydrodeoxygenation of the PC oil produced by the co-pyrolysis of PC with PE and PP, two major components of waste plastics, used as cheap hydrogen sources [22]. HZSM-5 is a commercial catalyst with high catalytic aromatization performance and high deoxygenation activity [23]. The co-pyrolysis behavior and the possible hydrodeoxygenation mechanism of PC and PE/PP were explored in detail. The results of this study can provide new ideas for the upgrading of oils with high oxygen content produced by the pyrolysis of non-recycled plastics.