Converting mixed plastics into mesoporous hollow carbon spheres with controllable diameter

Highlights

• Catalytic carbonization of mixed plastics (including PP, PE and PS) into HCSs.

• The diameter of mesoporous HCSs was modulated by the addition of OMMT.

• The yield of uniform HCSs was increased by adding more Co₃O₄ catalyst.

• OMMT promoted the dispersion of Co₃O₄ and the degradation of mixed plastics.

• This method offers a potential way to effectively convert waste plastics into HCSs.

Abstract

Upcycling waste plastics into high value-added carbon nanomaterials has been a hot topic due to ever-increasing generation of waste plastics, however, little attention has been paid to valuable hollow carbon spheres (HCSs). Herein, uniform mesoporous HCSs with controllable diameter and high surface area were efficiently prepared through the carbonization of mixed plastics consisting of polypropylene, polyethylene and polystyrene under the combined catalysis of organically-modified montmorillonite (OMMT)/Co₃O₄ at 700 °C. The morphology, microstructure, phase structure, textural property, surface element composition and thermal stability of HCSs were investigated by scanning electron microscope, transmission electron microscope (TEM), high-resolution TEM, X-ray diffraction, Raman spectroscopy, N₂ sorption, X-ray photoelectron spectroscopy and thermal gravimetric analysis. The effects of OMMT on the dispersion of Co₃O₄ in the mixed plastics and the degradation of mixed plastics were studied. It was found that OMMT not only promoted the dispersion of Co₃O₄ in the mixed plastics, which favored to control the diameter of HCSs, but also promoted the degradation of mixed plastics into light hydrocarbons and aromatics, which facilitated the growth of HCSs. Finally, a possible mechanism was proposed to explain the growth of uniform HCSs with controllable diameter. This facile method provides a novel potential way to convert waste plastics into valuable HCSs, which can be used as catalyst support, adsorbent, storage medium and template for the synthesis of other useful hollow materials, etc.

Introduction

Considerable attention has been paid to the treatment of waste plastics with ever-growing production and consumption of plastics since they are not biodegradation [1], [2], [3]. Landfill and incineration are far from being widely accepted due to their related environmental pollution. Upcycling is the process of converting waste materials into something useful and more valuable. Chemical recycling can recover the petrochemical components of waste plastics, which could be used to produce other synthetic chemicals [4], [5], [6], [7]. Anyhow, great efforts have been made to explore a new technically and economically feasible chemical recycling process for the treatment of waste plastics.

Up to now, many studies [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23] have been conducted on converting virgin or waste plastics including polypropylene (PP), polyethylene (PE) and polystyrene (PS) into high value-added carbon nanomaterials (CNMs) with diverse morphologies and microstructures such as carbon nanotubes (CNTs), cup-stacked CNTs (CS-CNTs) and carbon nanofibers (CNFs). For example, Wu et al. used catalytic gasification to process waste plastics into CNTs and hydrogen-rich synthetic gas by using Ni/Ca-Al or Ni/Zn-Al catalyst [8], [9]. Acomb et al. used pyrolysis–gasification of PP, PE and PS to prepare CNTs and hydrogen by Ni/Al₂O₃ catalyst [10]. Zhuo et al. reported the synthesis of CNTs and CNFs from recycled PE using stainless-steel wire mesh as catalyst by a novel pyrolysis–combustion technique [11], [12], [13]. Pol et al. used autoclave as reactor to convert waste PE into CNTs using Co(Ac)₂ as catalyst under high pressure [14]. Our group put forward a strategy of combined degradation catalyst/carbonization catalyst, including solid acid (such as organically modified montmorillonite (OMMT) or zeolite)/nickel catalyst [15], [16], [17], [18], [19], halogenated compound/Ni₂O₃ [20], [21], [22] and activated carbon/Ni₂O₃ [23], to effectively convert PP and PE into CNTs, CS-CNTs and CNFs under atmospheric condition.

However, most of current studies are limited to focusing on single component plastic, and there are few studies attempting to transform mixed plastics into CNMs. This is essential to effectively convert waste plastics into CNMs because the “real-world” waste plastic is actually mixture which mainly consists of PP, PE and PS [24]. In addition, although CNMs with diverse morphologies and microstructures have been prepared using plastics as carbon sources [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], to the best of our knowledge, little attention has been paid to hollow carbon spheres (HCSs). HCSs with a structure of hollow core and carbon shell have attracted great interests due to their unique physicochemical properties, such as low density, large inner space and specific surface area, and wide applications in catalyst support, adsorbent, storage medium and template for the synthesis of other useful hollow materials [25], [26], [27], [28], [29], [30], [31], [32], [33], [34].

So far, there have been only two reports about synthesizing HCSs using plastics as carbon sources. Chen et al. prepared HCSs using PP as carbon source and the combined OMMT/Co(Ac)₂ as catalyst [35]. But the obtained HCSs showed extreme heterogeneity in the size distribution and the yield of HCSs was just 9 wt%. In our previous report [36], uniform HCSs were prepared through the carbonization of PS under the catalysis of OMMT/cobalt catalyst (including Co₂O₃, Co₃O₄ and Co(AC)₂). It was found that OMMT promoted the degradation of PS into light hydrocarbons and aromatics, while cobalt catalyst acted as template for the growth of HCSs by carbonizing light hydrocarbons and aromatics. However, the yield of HCSs was less than 15 wt%, and the diameter of HCSs could not be controlled. The previous work prompted us to raise further questions: How does the content of cobalt catalyst influence the yield and diameter of HCSs using mixed plastics as carbon sources? How does OMMT affect the degradation products of mixed plastics? Solving these problems will not only provide more insights into the carbonization mechanism of mixed plastics, but also contribute to converting waste plastics into high value-added HCSs.

Herein, firstly, uniform mesoporous HCSs with controllable diameter and high surface area were effectively prepared through the carbonization of mixed plastics consisting of PP, PE and PS under the catalysis of OMMT/Co₃O₄ at 700 °C. The effects of Co₃O₄ content on the yield, morphology, phase structure, textural property, surface element composition and thermal stability of HCSs were investigated. Subsequently, the effects of OMMT on the dispersion of Co₃O₄ in the mixed plastics and the degradation of mixed plastics were studied. Finally, a possible mechanism was proposed to explain the growth of uniform HCSs with controllable diameter using mixed plastics as carbon sources. This simple method provides a novel potential way to convert waste plastics into high value-added HCSs.