Upcycling Waste Polypropylene into Graphene Flakes on Organically ...

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Upcycling Waste Polypropylene into Graphene Flakes on Organically Modified Montmorillonite Jiang Gong,†,‡ Jie Liu,*,† Xin Wen,† Zhiwei Jiang,†,‡ Xuecheng Chen,†,§ Ewa Mijowska,§ and Tao Tang*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § Institute of Chemical and Environment Engineering, West Pomeranian University of Technology, Szczecin, ulica Pulaskiego 10, 70-322 Szczecin, Poland S Supporting Information *

ABSTRACT: Recently, upcycling waste plastics into high-value-added carbon nanomaterials has attracted much attention; however, few studies have focused on the conversion of waste plastics into graphene with high yield. Herein, we report a simple novel method to synthesize graphene flakes (GFs) with high yield through catalytic carbonization of waste polypropylene (PP) using organically modified montmorillonite (OMMT) as degradation catalyst and template at 700 °C. The yield, morphology, microstructure, phase structure, thermal stability, and surface element composition of GFs were investigated. In addition, it was found that OMMT not only promoted the degradation of waste PP into light hydrocarbons and aromatics but also acted as template and catalyzed carbonization of the light hydrocarbons and aromatics into GFs. At last, a possible mechanism for the formation of GFs was put forward. This simple approach provides a novel way to effectively prepare high-value-added GFs using waste plastics as carbon sources. CNTs with high-yield hydrogen-rich synthesis gas.10,11 Acomb et al. used pyrolysis−gasification of PP, PE, and PS to prepare CNTs and hydrogen.12 Zhuo et al. reported the synthesis of CNTs and CNFs from recycled PE using a novel pyrolysis− combustion technique.13−15 Pol et al. used an autoclave as a reactor to convert waste PE into CNTs and CSs under high pressure.16 Our group found that the combination of solid acid (or halogenated compound or activated carbon) with nickel catalyst was efficient for high-yielded conversion of plastics into CNTs, CS−CNTs, CNFs, and CSs under atmospheric conditions.17−26 However, although many investigations have been carried on to transform virgin or waste plastics into CNMs with diverse morphologies and microstructures,8−27 to the best of our knowledge, there are few reports focusing on the conversion of waste plastics into graphene, which has received significant attention for its fascinating electrical, mechanical, thermal, and chemical properties since 2004.28,29 Recently, Ruan et al. transformed waste PS into single layer, bilayer, and few layer graphene on Cu foil,30 but the yield of graphene was not provided. High conversion of waste plastics into graphene is challenging because the degradation products of waste plastics are complicated. The degradation products include light hydrocarbons, aromatics, and long-chain olefins. The light hydrocarbons and aromatics are effective in the growth of graphene,31−33 but the conversion of olefins with long chains into graphene is difficult. Thereby, the key question of

1. INTRODUCTION The world production of plastics increased from 1.7 million tons in 1950 to 280 million tons in 2011.1 More than 31 million tons of waste plastics in 2011 were generated in United States.2 The ever-increasing production and consumption of plastics have aroused more and more attention to the treatment of waste plastics, since they are not biodegradable.3,4 The typical methods are landfill and incineration, but they 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. Mechanical recycling of waste plastics is limited by the low quality of the recycled plastic mixture. Chemical recycling can recover the petrochemical components from waste plastics, which could be used to produce other synthetic chemicals.5−7 Nevertheless, development of a new technically and economically feasible chemical recycling process is of great importance for the treatment of waste plastics. Plastics mainly consist of carbon and other (such as oxygen and hydrogen) elements. Hence, from an industrial viewpoint, reutilization of waste plastics to synthesize high-value-added carbon nanomaterials (CNMs) not only shows advantages with cheap and abundant sources but also provides a potential way to largely recycle waste plastics. To date, many studies8−27 have been conducted to convert virgin or waste plastics including polypropylene (PP), polyethylene (PE), and polystyrene (PS) into CNMs such as carbon nanotubes (CNTs), cup-stacked CNTs (CS-CNTs), carbon nanofibers (CNFs), and carbon spheres (CSs). For example, Kong et al. synthesized straight and helical CNTs and Fe3O4@C composite through catalytic decomposition of PE in an autoclave.8,9 Wu et al. used catalytic gasification to process real-world waste plastics into high-quality © 2014 American Chemical Society

Received: Revised: Accepted: Published: 4173

December 20, 2013 February 17, 2014 February 20, 2014 February 20, 2014 dx.doi.org/10.1021/ie4043246 | Ind. Eng. Chem. Res. 2014, 53, 4173−4181

Industrial & Engineering Chemistry Research

Article

HNO3 solution and refluxed at 110 °C for 3 h. Then, the solution was centrifuged, the supernatant liquid was decanted off, and the black sediment was resuspended in water. This rinse procedure was performed three times. The yield of GFs was calculated by dividing the amount of purified carbon product by the amount of carbon in the PP from OMMT/ waste PP or MMT/waste PP. Each measurement was repeated four times for reproducibility purposes. The degradation products of waste bumper or panel are carbon feedstock for the formation of GFs. To study the effect of OMMT on the degradation products of waste PP, pyrolysis experiments22 for waste bumper and OMMT/waste bumper0.5 were conducted at 700 °C in a fixed bed reactor. The liquid pyrolyzed products were collected using a cold trap, and the gas pyrolyzed products were collected using a sample bag. 2.4. Characterization. The morphology of GFs was observed by means of a field-emission scanning electron microscope (SEM, XL30ESEM-FEG). The samples were sputter-coated with gold before SEM observation. The microstructure of GFs was investigated using a transmission electron microscope (TEM, JEM-1011) at an accelerating voltage of 100 kV and a high-resolution TEM (HRTEM) on a FEI Tecnai G2 S-Twin transmission electron microscope operating at 200 kV. X-ray diffraction (XRD) was conducted using a D8 advance X-ray diffractometer with Cu Kα radiation operating at 40 kV and 200 mA. Raman spectroscopy (T6400, excitation-beam wavelength: 514.5 nm) was used to characterize the vibrational property of GFs. Thermal gravimetric analysis (TGA) was performed using TA Instruments SDT Q600 under air flow at a heating rate of 10 °C/min. The surface element composition of GFs was characterized by means of X-ray photoelectron spectroscopy (XPS) carried out on a VG ESCALAB MK II spectrometer using Al Kα exciting radiation from an X-ray source operated at 10.0 kV and 10 mA. The surface element distributions of the residue from OMMT/ waste bumper-1 before purification and GFs-B1 were analyzed by an energy dispersive X-ray spectrometer (EDX, Genesis 2000). The collected liquid products from waste bumper and OMMT/waste bumper-0.5 at 700 °C were weighed and analyzed by gas chromatography−mass spectrometry (GC− MS, AGILENT 5975MSD). The volume of collected gas products was determined by the displacement of water. The hydrocarbon gas products were analyzed by a GC (Kechuang, GC 9800) equipped with a FID, using a KB-Al2O3/Na2SO4 column (50 m × 0.53 mm i.d.). H2, CO, and CH4 were analyzed by a GC (Kechuang, GC 9800) equipped with a TCD, using a packed TDX-01 (1 m) and molecular sieve 5A column (1.5 m).

efficiently converting waste plastics into graphene is how to transform waste plastics into light hydrocarbons and aromatics and subsequently catalyze carbonization of these degradation products into graphene. Herein, we put up a simple novel method to convert waste PP (selected as an example of waste plastics) into graphene flakes (GFs) with high yield on organically modified montmorillonite (OMMT) at 700 °C. OMMT can be prepared by exchanging the cations initially present in the montmorillonite (MMT) interlayer with cationic surfactants such as alkylammoniums or alkylphosphoniums (oniums), while MMT exists widely in the world as the main constituent of the volcanic ash weathering product and belongs to the family of 2:1 phyllosilicates. MMT crystal structure consists of stacked layers made of two silica tetrahedrons fused to an edge-shared octahedral sheet of alumina.34 Until now, there are no reports about using OMMT to synthesize graphene material. In this work, the yield, morphology, microstructure, phase structure, thermal stability, and surface element composition of GFs were investigated. Subsequently, the effect of OMMT on the degradation products of waste PP was studied, and the role of OMMT on the growth of GFs was explored. Finally, a possible mechanism was proposed to explain the formation of GFs. This simple approach provides a new potential way to transform waste plastics into high-value-added GFs, which have potential applications in the fields of catalysis, environment, energy, composites, etc.28

2. EXPERIMENTAL SECTION 2.1. Materials. Waste bumper and panel were supplied by a local commercial company and used as-received with granular diameter of about 2 mm. The main components were polypropylene (PP, about 89 wt %) and additives (about 11 wt %) such as talcum. MMT and OMMT (Closite 15A, organic modifier, dimethyl-dihydrogenated tallow quarternary ammonium; modifier concentration, 125 mequiv per 100 g clay; high purity; moisture content