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Oct 11, 2013 - Striking Influence about HZSM-5 Content and Nickel Catalyst on Catalytic Carbonization of Polypropylene and Polyethylene into Carbon ...
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Striking Influence about HZSM‑5 Content and Nickel Catalyst on Catalytic Carbonization of Polypropylene and Polyethylene into Carbon Nanomaterials Jiang Gong,†,‡ Jingdong Feng,† Jie Liu,† Raheel Muhammad,† Xuecheng Chen,*,†,§ Zhiwei Jiang,†,‡ Ewa Mijowska,§ Xin Wen,† 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, Szczecinul. Pulaskiego 10, 70-322 Szczecin, Poland S Supporting Information *

ABSTRACT: A one-pot approach was established to effectively convert polypropylene (PP) and linear low density polyethylene (LLDPE) into carbon nanomaterials (CNMs) including cup-stacked carbon nanotubes and carbon nanofibers under the combined catalysis of HZSM-5/NiO or HZSM-5/Ni2O3 at 700 °C. The effects of HZSM-5 content, a nickel catalyst, and the carbon precursor on the morphology, microstructure, phase structure, and thermal stability of CNMs were investigated with scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, X-ray diffraction, Raman spectroscopy, and thermal gravimetric analysis. The effect of HZSM-5 content on the degradation products of PP and LLDPE was analyzed by gas chromatography and gas chromatography−mass spectrometry. With increasing HZSM-5 content, the yields of light hydrocarbons and aromatics increased due to proton acid sites on the surface of HZSM-5 catalyzing cracking of PP or LLDPE degradation products. This one-pot approach provides a novel potential way to largely transform mixed waste polyolefin into high value-added CNMs. fibers (CNFs), carbon spheres (CSs), etc. Arena et al. proposed a process producing CNTs by pyrolysis of virgin or recycled PP in a fluidized-bed reactor.7 Wu et al. used catalytic gasification to process waste PP into high quality CNTs with high yield hydrogen-rich synthesis gas.8 Kong et al. synthesized straight and helical CNTs and a Fe3O4@C composite through the catalytic decomposition of PE in an autoclave.9,10 Zhuo et al. reported the synthesis of CNTs from recycled PE using a novel pyrolysis−combustion technique.11,12 Pol et al. used an autoclave as a reactor to convert waste PE into CNTs and CSs, which showed high performance in lithium electrochemical cells.13−15 Our group put forward a strategy of “combined catalysts,” that is, degradation catalysts/carbonization catalysts, including solid acids (such as organically modified montmorillonite (OMMT) or HZSM-5)/nickel catalysts,16−19 halogenated compounds/Ni2O3,20,21 and activated carbon/Ni2O322 to convert polyolefin into CNTs, cupstacked CNTs (CS-CNTs), CNFs, and CSs.23 These CNMs can be used in a lot of fields such as catalysis26,27 and composite.28,29 However, although many investigations have been conducted to convert PP or PE into CNMs, there are no studies about the effect of a carbon precursor (that is PP and PE) on the

1. INTRODUCTION The treatment of waste plastics has received more and more attention with ever-increasing plastic production and consumption.1,2 To date, waste plastics are treated predominantly by placement in a landfill, incineration, mechanical recycling, and chemical recycling. The most usual ways are placement in a landfill and incineration, but they are far from being widely accepted due to their related pollution problems. Mechanical recycling of waste plastics is limited by the low quality of the recycled plastic mixture. Chemical recycling can recover the petrochemical components of waste plastics, which could be used to make other synthetic chemicals.3−6 Nevertheless, a lot of efforts have been made to explore a new technically and economically feasible recycling process for waste plastics. Among waste plastics, waste polyolefin is the main component, and the content of carbon in polyolefin is about 85.7 wt %. Hence, from an industrial viewpoint, reutilization of waste polyolefin to largely synthesize high value-added carbon nanomaterials (CNMs) provides a novel potential way to recycle waste plastics. Since complicated degradation reactions generally occur during the conversion of mixed waste polyolefin into CNMs, the conversion of single component polyolefin into CNMs should be investigated first. Recently, many studies7−25 have been conducted to convert virgin or waste polyolefin including polypropylene (PP) and polyethylene (PE) of different types (e.g., linear low density polyethylene (LLDPE)) into CNMs with varied morphologies and microstructures such as carbon nanotubes (CNTs), carbon nano© 2013 American Chemical Society

Received: Revised: Accepted: Published: 15578

July 9, 2013 September 9, 2013 October 11, 2013 October 11, 2013 dx.doi.org/10.1021/ie402178b | Ind. Eng. Chem. Res. 2013, 52, 15578−15588

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PP/NiO, PP/Ni2O3, LLDPE/NiO, LLDPE/Ni2O3 with NiO or Ni2O3 contents of 7.5 (g/100 g PP or LLDPE), PP/HZSM-x, and LLDPE/HZSM-x mixtures were also prepared. 2.3. Preparation of CNMs. CNMs were prepared according to our previous reports.17,20 Briefly, a piece of sample (about 5.0 g) was placed into a crucible, which was heated at 700 °C until the flame from the crucible’s upper brim went out (for about 5 min). Subsequently, the obtained CNMs were cooled down to room temperature and designated as CNM-P1-x, CNM-P2-x, CNM-L1-x, or CNM-L2-x, which represented CNMs from PP/HZSM-NiO-x, PP/HZSM-Ni2O3x, LLDPE/HZSM-NiO-x, or LLDPE/HZSM-Ni2O3-x, respectively. The yield of CNMs was calculated by dividing the amount of the obtained carbon (the amount of the residue after subtracting the amount of the residual catalysts) by that of elemental carbon in the sample. Each measurement was repeated four times for the purpose of reproducibility. The degradation products of PP or LLDPE are carbon feedstock for the formation of CNMs. In order to study the effect of HZSM-5 content on the degradation products of PP and LLDPE, pyrolysis experiments20 for PP, PP/HZSM-x, LLDPE, and LLDPE/HZSM-x were conducted at 700 °C in a fixed bed reactor. The liquid degradation products were collected using a cold trap, and the gas degradation products were collected with a sample bag. 2.4. Characterization. The morphology of CNMs was observed by means of field-emission scanning electron microscopy (SEM, XL30ESEM-FEG). The microstructure of CNMs was investigated using transmission electron microscopy (TEM, JEM-1011) at an accelerating voltage of 100 kV and high-resolution TEM (HRTEM) performed on a FEI Tecnai G2 S-Twin transmission electron microscope operating at 200 kV. The phase structure of CNMs was analyzed by X-ray diffraction (XRD) using a D8 advance X-ray diffractometer with Cu Kα radiation operating at 40 kV and 200 mA. The vibrational property of CNMs was characterized by Raman spectroscopy (T6400, excitation-beam wavelength: 514.5 nm). The thermal stability of CNMs was measured by thermal gravimetric analysis (TGA) under an air flow at a heating rate of 10 °C/min using a TA Instruments SDT Q600. The liquid degradation products from PP, PP/HZSM-x, LLDPE, and LLDPE/HZSM-x at 700 °C were weighed and analyzed by gas chromatography−mass spectrometry (GC-MS, AGILENT 5975MSD). The volume of gas degradation products was determined by the displacement of water. The hydrocarbon gas products were analyzed with 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).

formation of CNMs. This is essential to effectively convert mixed waste polyolefin into CNMs because PP and PE are the main components of mixed waste polyolefin and account entirely for roughly 60% of the whole waste plastics.2,30 In addition, in the combined HZSM-5/Ni2O3 catalysts,18 the carbenium ions from HZSM-5 not only promoted the degradation of PP to predominantly form products with lower carbon numbers that could be easily catalyzed by Ni2O3 for the growth of CNTs but also promoted the growth of CNTs from the degradation products with higher carbon numbers through hydride-transfer reactions. The previous work prompted us to raise a further question: How does the HZSM5 content influence the yield and morphology of CNMs from PP and PE? Furthermore, our group found that the nanosized NiO catalyst, which was prepared with a sol−gel combustion synthesis method, could catalyze the light hydrocarbons and aromatics from PP into long, straight, and smooth-surface CSCNTs.19 In this case, does commercial Ni2O3 also catalyze the formation of CS-CNTs? How does the type of nickel catalyst (commercial Ni2O3 and nanosized NiO) influence the yield and morphology of CS-CNTs? Solving these problems will not only favor effective control of the degradation of mixed waste polyolefins but also help to largely convert mixed waste polyolefin into high value-added CNMs with diverse morphologies using a suitable degradation catalyst and carbonization catalyst. Herein, PP and LLDPE were selected as carbon sources to prepare CNMs including CS-CNTs and CNFs via a one-pot approach under the combined catalysis of HZSM-5/NiO or HZSM-5/Ni2O3 at 700 °C. The effects of HZSM-5 content, a nickel catalyst, and a carbon precursor on the yield, morphology, microstructure, phase structure, and thermal stability of CNMs were fully investigated. In addition, the effect of HZSM-5 content on the degradation products of PP and LLDPE were comparatively discussed. This one-pot approach provides a novel, potential way to largely convert mixed waste polyolefin into high value-added CNMs.

2. EXPERIMENTAL SECTION 2.1. Materials. Polypropylene (PP, trademark T30S) was supplied by Yanan Petrochemical Company. Linear low density polyethylene (LLDPE, trademark DFDA-7042) was obtained from Sinopec Yangzi Petrochemical Company Ltd. Commercial H-form ZSM-5 (HZSM-5, a kind of commercial zeolite, Si/Al = 25) was provided from The Catalyst Plant of Nankai University. Commercial Ni2O3 with analytical grade quality was purchased from Lingfeng Chemical Company of Shanghai and used without further purification. Nanosized NiO was prepared using the sol−gel combustion synthesis method. A precursor solution was prepared as follows: 29.08 g of Ni(NO3)2·6H2O and 5.76 g of citric acid were dissolved in 200 mL of deionized water. The resultant solution was evaporated at 80 °C to gel, and the gel was further heated in the air up to the temperature of self-ignition. Upon ignition, the obtained NiO was used as a carbonization catalyst to synthesize CNMs. All of the chemicals were of analytical-grade quality. 2.2. Preparation of Samples. PP powder or LLDPE pellets (40.00 g) were mixed with NiO or Ni2O3 (3.00 g) and a designed amount of HZSM-5 in a Brabender mixer at 100 rpm and 180 °C (or 160 °C) for 10 min. The resultant sample was denoted as PP/HZSM-NiO-x, PP/HZSM-Ni2O3-x, LLDPE/ HZSM-NiO-x, or LLDPE/HZSM-Ni2O3-x, where x represented the content of HZSM-5 in the mixture. For comparison,

3. RESULTS AND DISCUSSION 3.1. Effect of HZSM-5 Content on the Yield of CNMs. Figure 1 shows the effect of HZSM-5 content on the yield of CNMs from catalytic carbonization of PP or LLDPE by NiO or Ni2O3 at 700 °C. Without HZSM-5 or a nickel catalyst (including NiO and Ni2O3), the yield of CNMs was less than 8.5 or 1.0 wt %, respectively, suggesting that HZSM-5 or a nickel catalyst alone could not effectively catalyze carbonization of PP or LLDPE. In contrast, in the presence of both HZSM-5 and a nickel catalyst, the yield of CNMs dramatically increased at first but stagnated as the content of HZSM-5 was enhanced, regardless of the type of carbon precursor or nickel catalyst. 15579

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Furthermore, the yield of CNM-P2-x was slightly higher than that of CNM-P1-x, indicating that commercial Ni2O3 showed little higher efficiency than nanosized NiO in the catalytic carbonization of PP into CNMs. This was probably attributed to Ni2O3 containing more lattice oxygen than NiO, which promoted the carbonization of degradation products.31 Besides, the yield of CNM-P1-x or CNM-P2-x showed no obvious differences with that of CNM-L1-x or CNM-L2-x, respectively, indicating the type of carbon source did not remarkably affect the yield of CNMs. Moreover, the compositions of CNMs are listed in Table S1 in the Supporting Information. The contents of CNMs, nickel catalyst, and HZSM-5 were in the ranges of 34.5−70.9 wt %, 0−17.0 wt %, and 13.3−65.5 wt %, respectively, which depended on the yield of CNMs (or the carbon conversion of PP or LLDPE). Certainly, the resultant CNMs can be purified by acid treatment.18 3.2. Effects of HZSM-5 Content on the Morphology and Microstructure of CNMs. To investigate whether the HZSM-5 content affected the morphology and microstructure of CNMs, SEM and TEM observations were performed on the CNMs from PP/HZSM-NiO-x at 700 °C. Without HZSM-5, most of the carbon product from PP/NiO was amorphous carbon.19 Figure 2 presents the typical SEM images of CNMP1-x. When 1 g/100 g PP of HZSM-5 was added into PP/NiO, some twisted filamentous carbon was found in the resultant CNM-P1-1 (Figure 2a and b). Strikingly, after increasing the

Figure 1. Effect of HZSM-5 content on the yield of CNMs from catalytic carbonization of PP or LLDPE by NiO or Ni2O3 at 700 °C.

This indicated that the combination of a nickel catalyst with HZSM-5 showed a synergetic effect on the carbonization of PP or LLDPE into CNMs. Taking PP as an example, the yield of CNMs increased from 7.0 to 48.2 wt % when 7.5 g/100 g PP of HZSM-5 was added into PP/NiO, but when further increasing HZSM-5 content to 10 g/100 g PP, it was increased by only 3.3 wt % and reached 51.5 wt %, suggesting the HZSM-5 content played a significant influence on the yield of CNMs.

Figure 2. Typical SEM images of CNM-P1-1 from PP/HZSM-NiO-1 (a and b), CNM-P1-5 from PP/HZSM-NiO-5 (c and d), and CNM-P1-10 from PP/HZSM-NiO-10 (e and f). 15580

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Figure 3. Typical TEM images of CNM-P1-1 from PP/HZSM-NiO-1 (a and b), CNM-P1-5 from PP/HZSM-NiO-5 (c and d), and CNM-P1-10 from PP/HZSM-NiO-10 (e and f).

diameter and length were 56 nm and 8.9 μm, respectively. However, CNM-P1-10 contained many short and winding CNTs (Figure 3e and f), which had a mean outer diameter and length of 54 nm and 4.7 μm, respectively. Hence, the content of HZSM-5 remarkably affected the morphology and microstructure of CNMs from the carbonization of PP under the catalysis of NiO. XRD, Raman, and TGA measurements were conducted to further investigate the effect of HZSM-5 content on the phase structure and thermal stability of CNMs from PP/HZSM-NiOx at 700 °C. Figure 4a shows XRD patterns of the resultant CNMs. A characteristic diffraction peak at 2θ = 26.2° was observed, which corresponded to the (002) reflection of graphite. The full width at half-maximum (fwhm) of the

content of HZSM-5 to 5 g/100 g PP, a great amount of relatively straight and long filamentous carbon appeared in the obtained CNM-P1-5 (Figure 2c and d). However, when the content of HZSM-5 further increased to 10 g/100 g PP, the resultant CNM-P1-10 mainly consisted of short and winding filamentous carbon (Figure 2e and f). Figure 3 displays the typical TEM images of the resultant CNMs from PP/HZSM-NiO-x at 700 °C. The obtained CNMP1-1 was composed of many amorphous carbon covering nickel catalysts and some CNTs (Figure 3a and b). In contrast, the resultant straight and long filamentous carbon from CNM-P1-5 had a clear tubular-like form (Figure 3c and d), which is characteristic of CNTs. The CNTs from CNM-P1-5 had narrow diameter and length distributions. The mean outer 15581

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Figure 4. XRD patterns (a), Raman spectra (b), and DTG curves (c) of CNM-P1-1, CNM-P1-5, and CNM-P1-10.

Figure 5. Typical SEM and TEM images of CNM-P2-1 from PP/HZSM-Ni2O3-1 (a and b), CNM-P2-5 from PP/HZSM-Ni2O3-5 (c and d), and CNM-P2-10 from PP/HZSM-Ni2O3-10 (e and f).

at about 1580 cm−1 (G band) corresponds to an E2g mode of hexagonal graphite and is related to the vibration of sp2-bonded carbon atoms in a graphite layer, and the D band at about 1350 cm−1 is associated with vibration of the carbon atoms with dangling bonds in the plane terminations of disordered graphite

graphite (002) peak decreased from 0.841 for CNM-P1-1 to 0.805 for CNM-P1-5, but then it increased to 0.831 for CNMP1-10. This result indicated that CNM-P1-5 contained fewer lattice disorders than CNM-P1-1 or CNM-P1-10.32 Figure 4b displays Raman spectra of the corresponding CNMs. The peak 15582

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or glassy carbons.33,34 A larger IG/ID ratio indicates a higher degree of structural ordering for CNMs.35 Evidently, the IG/ID ratio from CNM-P1-5 was larger than that from CNM-P1-1 or CNM-P1-10, suggesting CNM-P1-5 had relatively fewer defects than CNM-P1-1 or CNM-P1-10. TGA was used to measure thermal stability of CNMs, which gave an overall quality of CNMs. Higher oxidation temperature is always associated with purer, less defective CNMs. Figure 4c displays the derivative TGA (DTG) curves of CNMs under an air flow at a heating rate of 10 °C/min. The weight loss from 500 to 650 °C was due to the oxidation of CNTs. CNM-P1-5 showed the maximum oxidation temperature at 583.7 °C, higher than CNM-P1-1 (476.5 °C) or CNM-P1-10 (575.2 °C), demonstrating that CNM-P1-5 contains more graphitized CNTs and/ or less amorphous carbon, which was consistent with the results of SEM, TEM, XRD, and Raman. As a result, the content of HZSM-5 also notably influenced the phase structure and thermal stability of CNMs. 3.3. Effects of the Type of Nickel Catalyst on the Morphology and Microstructure of CNMs. To investigate the effect of the type of nickel catalyst on the morphology and microstructure of CNMs, SEM and TEM observations were carried out on the CNM-P2-x from PP/HZSM-Ni2O3-x at 700 °C. Clearly, CNM-P2-1 was mainly composed of amorphous carbon with few CNTs or CNFs (Figure 5a and b), but CNMP2-5 contained many relatively long and straight CNTs with some amorphous carbon (Figure 5c and d), while CNM-P2-10 consisted of short and twisted CNTs and CNFs (Figure 5e and f). Consequently, the variations for morphology and microstructure of CNMs from catalytic carbonization of PP using HZSM-5/Ni2O3 and HZSM-5/NiO showed similar trends. However, when the content of HZSM-5 was identical, the IG/ID ratio or the maximum oxidation temperature from CNM-P2-x (except for CNM-P2−1) was smaller or lower than that from CNM-P1-x (Figure S1). That is to say, CNM-P2-x had less graphitic structure and/or more amorphous carbon than CNMP1-x. To further clarify the effect of the type of nickel catalyst on the microstructure of CNMs, HRTEM observations were conducted on CNM-P1-5 and CNM-P2-5 (Figure 6). The graphene layers in the long and straight CNTs from CNM-P1-5 (Figure 3d) were oblique to the CNT axis at the angle of 19− 26° (Figure 6a). Hence, the obtained CNTs from CNM-P1-5 are identified as CS-CNTs, which are different from conventional CNTs made up of multiseamless cylinders of hexagonal carbon networks. As a result, a large portion of exposed and reactive edges with abundant dangling bonds exist on the outer surface and in the inner channel of CS-CNTs,36 which makes them excellent candidates in the applications of nanoelectronics,37 absorbents,38 nancomposites,39 energy,40 electrochemical biosensors,41 heterogeneous catalysis,42 etc. Moreover, the interlayer spacing between graphitic layers was about 0.34 nm, which was consistent with the ideal graphitic interlayer spacing. The CNTs from CNM-P2-5 (Figure 5d) were also CS-CNTs with a slight larger angle of 23−29° between graphene layers and the CNT axis (Figure 6b). But the graphene layers were discontinuous and had more defects, suggesting the lower graphitization of CS-CNTs using Ni2O3 as a carbonization catalyst, agreeing with the results of Raman and TGA measurements (Figure S1). This difference was caused by the different morphologies of nickel catalysts in CNM-P1-5 and CNM-P2-5. It was observed that most of the metallic nickel catalysts in the long and straight

Figure 6. Typical HRTEM micrographs for CNM-P1-5 from PP/ HZSM-NiO-5 (a) and CNM-P2-5 from PP/HZSM-Ni2O3-5 (b).

CS-CNTs from CNM-P1-5 were rhombic in shape and existed in the middle of CS-CNTs, while all of the metallic nickel catalysts in CS-CNTs from CNM-P2-5 were fusiformis in shape and located at the tip of CS-CNTs. Figure 7 presents typical HRTEM micrographs of a rhombic-shaped Ni catalyst in the middle of CS-CNTs from CNM-P1-5 and a fusiformis-shaped Ni catalyst on the tip of CS-CNTs from CNM-P2-5. In the case of CNM-P1-5, the graphene layers near the surface of the rhombic-shaped Ni catalyst were almost parallel to the surface of the rhombic-shaped Ni catalyst (Figure 7a). However, graphene layers in CNM-P2-5 covered most of the Ni catalyst and became distorted near the Ni catalyst (Figure 7b), which was not favorable to the formation of CS-CNTs with a high graphitic structure. Hence, the above results indicated that the coalescence and reconstruction of NiO and Ni2O3 catalysts during the growth of CS-CNTs should be different, which was most likely ascribed to the different morphologies of NiO and Ni2O3 catalysts since the carbon source was actually same. According to morphology characterization (Figure S2), the 15583

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Figure 8. Typical SEM and TEM images of CNM-L1-1 from LLDPE/ HZSM-NiO-1 (a and b), CNM-L1-5 from LLDPE/HZSM-NiO-5 (c and d), and CNM-L1-10 from LLDPE/HZSM-NiO-10 (e and f).

demonstrated by the results of Raman and TGA measurements (Figure S3). For example, the IG/ID ratio of CNM-L1-5 was 0.68, smaller than that of CNM-P1-5 (0.81); in addition, CNML1-5 had a lower maximum oxidation temperature (563.1 °C) than CNM-P1-5 (583.7 °C). Therefore, the carbon precursor was also an important factor influencing the morphology and microstructure of CNMs. More explanation is provided in section 3.5. 3.5. Discussion about the Effects of HZSM-5 Content and Carbon Precursors on the Formation of CNMs. The degradation product of PP or LLDPE is carbon feedstock for the formation of CS-CNTs and CNFs. In order to study the effect of HZSM-5 content on the degradation products of PP and LLDPE, PP, PP/HZSM-x, LLDPE, and LLDPE/HZSM-x were pyrolyzed at 700 °C, and subsequently GC and GC−MS measurements were conducted to analyze the composition of the degradation products. Table 1 presents the mass balance of the degradation products. Compared to PP, the quantity of gas degradation products increased from 36.9 to 40.6 g/100 g PP after adding 1 g/100 g PP of HZSM-5 and then increased to 51.6 g/100 g PP when the content of HZSM-5 increased to 10 g/100 g PP. Meanwhile, the liquid degradation products decreased from 63.1 to 59.3 g/100 g PP after adding 1 g/100 g PP of HZSM-5,and gradually decreased to 47.9 g/100 g PP when the content of HZSM-5 increased to 10 g/100 g PP. Similar results were observed for LLDPE/HZSM-x. The above results demonstrated that HZSM-5 could promote the dehydrogenation of PP or LLDPE degradation products into light hydrocarbons. It was because the proton acid sites on the surface of HZSM-5 could catalyze the cracking of PP or LLDPE

Figure 7. Typical HRTEM micrographs of a rhombic-shaped Ni catalyst in the middle of CS-CNTs from CNM-P1-5 (a) and a fusiformis-shaped Ni catalyst on the tip of CS-CNTs from CNM-P2-5 (b).

NiO catalyst was composed of uniform NiO nanoparticles with a mean diameter of 13.5 ± 2.1 nm, while the commercial Ni2O3 catalyst was laminas with a size range of 50−400 nm. 3.4. Effects of Carbon Precursor on the Morphology and Microstructure of CNMs. Figure 8 shows the typical SEM and TEM images of CNM-L1-x from LLDPE/HZSMNiO-x at 700 °C. Obviously, the variations for morphology and microstructure of CNMs from catalytic carbonization of LLDPE and PP by HZSM-5/NiO were similar. When the HZSM-5 content was 1 g/100 g LLDPE, the obtained CNML1-1 was mainly amorphous carbon (Figure 8a and b); after increasing the HZSM-5 content to 5 g/100 g LLDPE, many relatively long and straight CNTs were yielded (Figure 8c and d), but further increasing HZSM-5 content resulted in the formation of short CNTs and CNFs (Figure 8e and f). Compared to CNM-P1-x, CNM-L1-x had a less graphitic structure and/or more amorphous carbon, which could be 15584

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Figure 10 displays GC−MS profiles of liquid degradation products from PP, PP/HZSM-x, LLDPE, and LLDPE/HZSM-x at 700 °C. Tables S4 and S5 show the main composition of liquid degradation products according to the results in Figure 10. The main liquid degradation products from PP or LLDPE were olefins with long chains, and the content of aromatics (such as benzene, toluene and xylene) was scarce. It was observed that the content of aromatics increased with increasing HZSM-5 content. For example, it increased to 22.8 (area %) for PP/HZSM-1, 30.4 (area %) for PP/HZSM-5, and 53.7 (area %) for PP/HZSM-10. In the case of LLDPE, it increased from 10.6 to 44.2 (area %) for LLDPE/HZSM-1, 45.9 (area %) for LLDPE/HZSM-5, and 55.5 (area %) for LLDPE/HZSM-10. As is well-known, PP or LLDPE is first degraded into the degradation products under the catalysis of HZSM-5, in which these products form carbenium ions (step 1 in Figure 11).2 On the basis of the above results, it was demonstrated that HZSM5 could promote the dehydrogenation and aromatization of PP and LLDPE degradation products into light hydrocarbons and aromatics via carbenium ions (step 2). With the increasing content of HZSM-5, the yield of light hydrocarbons particularly for gaseous olefins increased; meanwhile the content of aromatics also increased. Consequently, after adding a low content of HZSM-5 (e.g., 1 g/100 g PP), the yield of light hydrocarbons was still low, leading to the low yield of CNMs and the formation of amorphous carbon (step 3a).19 However, when more HZSM-5 (e.g., 5 g/100 g PP) was added, HZSM-5 could effectively catalyze cracking of PP degradation products to form a lot of light hydrocarbons particularly for gaseous olefins with a certain amount of aromatics, which favored the formation of long and straight CS-CNTs (step 3b).19,21 After further increasing the content of HZSM-5 (e.g., 10 g/100 g PP), the yield of light hydrocarbons increased slightly, but the content of aromatics increased significantly, resulting in the formation of short CNTs or CNTs, and amorphous carbon (step 3c).45 In addition, since aromatics could be formed either by means of secondary reactions of oligomerization, cyclization, and aromatization of the primary olefins or by direct catalytic cracking of LLDPE, LLDPE yielded more aromatics than PP (Figure 10). This was the reason for the formation of lower graphitic structure and/or more amorphous carbon in CNML1-5 than in CNM-P1-5 (Figures 8 and S3).

Table 1. Yields of the Different Fractions from the Degradation of PP, PP/HZSM-x, LLDPE, and LLDPE/ HZSM-x at 700 °C

sample

carbon (g/100 g PP or LLDPE)

liquid (g/100 g PP or LLDPE)

gasa (g/ 100 g PP or LLDPE)

gasb (g/ 100 g PP or LLDPE)

gasc (mL/g PP or LLDPE)

PP PP/HZSM-1 PP/HZSM-5 PP/HZSM-10 LLDPE LLDPE/HZSM-1 LLDPE/HZSM-5 LLDPE/HZSM-10

0.0 0.1 0.3 0.5 0.0 0.2 0.4 0.4

63.1 59.3 52.1 47.9 73.5 53.6 46.3 45.4

36.9 40.6 47.6 51.6 26.5 46.2 53.3 54.2

33.0 38.7 49.8 53.9 22.9 46.2 51.8 53.6

193.8 262.7 275.7 345.6 185.7 261.9 309.3 336.9

a Calculated by the mass balance. bCalculated by the volume of one gas being first divided by 22.4 L/mol, multiplying the molar number of the gas by its molar mass, and then obtaining the yield of one gas (g/100 g PP or LLDPE) by dividing the mass of the gas by the mass of PP or LLDPE in the mixture, and finally obtaining the yield of total gas products by adding the yield of one gas together. cCalculated by the displacement of water.

degradation products to form more molecules with a lower carbon number via a cationic mechanism.18 Figure 9 and Tables S2 and S3 show the composition of gas degradation products from PP, PP/HZSM-x, LLDPE, and

Figure 9. Effect of HZSM-5 content on the yield of gas degradation products from PP, PP/HZSM-x, LLDPE, and LLDPE/HZSM-x at 700 °C.

4. CONCLUSIONS We reported a one-pot approach to effectively convert PP and LLDPE into CNMs including CS-CNTs and CNFs under the combined catalysis of a HZSM-5/nickel catalyst (nanosized NiO and commercial Ni2O3) at 700 °C. The yield of CNMs increased at first and then stagnated as the HZSM-5 content increased. The type of carbon precursor or nickel catalyst did not remarkably affect the yield of CNMs but significantly influenced the morphology of CNMs. HZSM-5 promoted the dehydrogenation and aromatization of PP or LLDPE degradation products into light hydrocarbons and aromatics due to proton acid sites on the surface of HZSM-5 catalyzing cracking of PP or LLDPE degradation products. When a low content of HZSM-5 (e.g., 1 g/100 g PP) was added, the yield of light hydrocarbons was low, resulting in the low yield of CNMs and the formation of amorphous carbon. However, when more HZSM-5 (e.g., 5 g/100 g PP) was introduced, more light hydrocarbons particularly for gaseous olefins with a certain amount of aromatics were yielded, which promoted the

LLDPE/HZSM-x in detail. The gas degradation products mainly consisted of hydrogen, methane, ethane, ethylene, propane, propylene, and i-butene. Compared to PP, the yield of gaseous olefins increased from 20.9 to 28.5 g/100 g PP for PP/ HZSM-1, 32.7 g/100 g PP for PP/HZSM-5, and 34.8 g/100 g PP for PP/HZSM-10, meanwhile the yield of gaseous alkanes increased from 12.1 to 18.8 g/100 g PP. Likewise, the yield of gaseous olefins from LLDPE increased from 14.4 to 36.3 g/100 g LLDPE for LLDPE/HZSM-1, 39.3 g/100 g LLDPE for LLDPE/HZSM-5, and 39.5 g/100 g LLDPE for LLDPE/ HZSM-10; meanwhile the yield of gaseous alkanes increased from 8.4 to 13.8 g/100 g LLDPE. Since the gaseous olefins are good carbon sources for the growth of CNTs,43,44 the relatively high yield of gaseous olefins from the degradation of PP or LLDPE favored the formation of CNTs. 15585

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Figure 10. GC−MS profiles of liquid degradation products from PP, PP/HZSM-x, LLDPE, and LLDPE/HZSM-x at 700 °C.

formation of relatively long and straight CS-CNTs. With further increasing HZSM-5 content (e.g., 10 g/100 g PP), the yield of light hydrocarbons increased slightly but the content of aromatics increased significantly, leading to the formation of short CNTs or CNFs and amorphous carbon. Furthermore,

owing to the formation of more aromatics from LLDPE than PP, less graphitic structure and/or more amorphous carbon were formed in the CNMs from LLDPE. This one-pot approach provides a novel, potential way to largely transform mixed waste polyolefin into high value-added CNMs. Further 15586

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Flame Retardant: Effect of Decabromo Diphenylethane (DDE). Polym. Degrad. Stab. 2007, 92, 211−221. (4) Wu, C. F.; Williams, P. T. Investigation of Ni-Al, Ni-Mg-Al and Ni-Cu-Al Catalyst for Hydrogen Production from Pyrolysis−Gasification of Polypropylene. Appl. Catal., B: Environ. 2009, 90, 147−156. (5) Escola, J. M.; Aguado, J.; Serrano, D. P.; García, A.; Peral, A.; Briones, L.; Calvo, R.; Fernandez, E. Catalytic Hydroreforming of the Polyethylene Thermal Cracking Oil over Ni Supported Hierarchical Zeolites and Mesostructured Aluminosilicates. Appl. Catal., B: Environ. 2011, 106, 405−415. (6) Aguado, J.; Serrano, D. P.; Escola, J. M. Fuels from Waste Plastics by Thermal and Catalytic Processes: A Review. Ind. Eng. Chem. Res. 2008, 47, 7982−7992. (7) Arena, U.; Mastellone, M. L.; Camino, G.; Boccaleri, E. An Innovative Process for Mass Production of Multi-Wall Carbon Nanotubes by Means of Low-Cost Pyrolysis of Polyolefins. Polym. Degrad. Stab. 2006, 91, 763−768. (8) Wu, C. F.; Wang, Z. C.; Wang, L. Z.; Williams, P. T.; Huang, J. Sustainable Processing of Waste Plastics to Produce High Yield Hydrogen Rich Synthesis Gas and High Quality Carbon Nanotubes. RSC Adv. 2012, 2, 4045−4047. (9) Kong, Q. H.; Zhang, J. H. Synthesis of Straight and Helical Carbon Nanotubes from Catalytic Pyrolysis of Polyethylene. Polym. Degrad. Stab. 2007, 92, 2005−2010. (10) Zhang, J. H.; Yan, B.; Wan, S.; Kong, Q. H. Converting Polyethylene Waste into Large Scale One-Dimensional Fe3O4@C Composites by a Facile One-Pot Process. Ind. Eng. Chem. Res. 2013, 52, 5708−5712. (11) Zhuo, C. W.; Hall, B.; Richter, H.; Levendis, Y. Synthesis of Carbon Nanotubes by Sequential Pyrolysis and Combustion of Polyethylene. Carbon 2010, 48, 4024−4034. (12) Zhuo, C. W.; Alves, J. O.; Tenorio, J. A. S.; Levendis, Y. A. Synthesis of Carbon Nanomaterials through Up-Cycling Agricultural and Municipal Solid Wastes. Ind. Eng. Chem. Res. 2012, 51, 2922− 2930. (13) Pol, V. G.; Thackeray, M. M. Spherical Carbon Particles and Carbon Nanotubes Prepared by Autogenic Reactions: Evaluation as Anodes in Lithium Electrochemical Cells. Energy Environ. Sci. 2011, 4, 1904−1912. (14) Pol, V. G. Upcycling: Converting Waste Plastics into Paramagnetic, Conducting, Solid, Pure Carbon Microspheres. Environ. Sci. Technol. 2010, 44, 4753−4759. (15) Pol, V. G.; Thiyagarajan, P. Remediating Plastic Waste into Carbon Nanotubes. J. Environ. Monit. 2010, 12, 455−459. (16) Tang, T.; Chen, X. C.; Meng, X. Y.; Chen, H.; Ding, Y. P. Synthesis of Multiwalled Carbon Nanotubes by Catalytic Combustion of Polypropylene. Angew. Chem., Int. Ed. 2005, 44, 1517−1520. (17) Jiang, Z. W.; Song, R. J.; Bi, W. G.; Lu, J.; Tang, T. Polypropylene as a Carbon Source for the Synthesis of Multi-Walled Carbon Nanotubes via Catalytic Combustion. Carbon 2007, 45, 449− 458. (18) Song, R. J.; Jiang, Z. W.; Bi, W. G.; Cheng, W. X.; Lu, J.; Huang, B. T.; Tang, T. The Combined Catalytic Action of Solid Acids with Nickel for the Transformation of Polypropylene into Carbon Nanotubes by Pyrolysis. Chem.Eur. J. 2007, 13, 3234−3240. (19) Gong, J.; Liu, J.; Jiang, Z. W.; Wen, X.; Chen, X. C.; Mijowska, E.; Wang, Y. H.; Tang, T. Effect of the Added Amount of OrganicallyModified Montmorillonite on the Catalytic Carbonization of Polypropylene into Cup-Stacked Carbon Nanotubes. Chem. Eng. J. 2013, 225, 798−808. (20) Gong, J.; Liu, J.; Ma, L.; Wen, X.; Chen, X. C.; Wan, D.; Yu, H. O.; Jiang, Z. W.; Borowiak-Palen, E.; Tang, T. Effect of Cl/Ni Molar Ratio on the Catalytic Conversion of Polypropylene into Cu−Ni/C Composites and Their Application in Catalyzing “Click” Reaction. Appl. Catal., B: Environ. 2012, 117−118, 185−193. (21) Gong, J.; Yao, K.; Liu, J.; Wen, X.; Chen, X. C.; Jiang, Z. W.; Mijowska, E.; Tang, T. Catalytic Conversion of Linear Low Density Polyethylene into Carbon Nanomaterials under the Combined

Figure 11. Schematic drawing for the degradation mechanism of PP and LLDPE under the catalysis of HZSM-5 and the formation of CNMs catalyzed by NiO.

exploration to effectively convert mixed polyolefin into high quality CNMs using the strategy of “combined catalysis” is conducted in our laboratory. Also, the potential applications of CS-CNTs in the adsorption, catalysis and energy storage are being explored in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

The yield of CNMs and their compositions (Table S1), main composition of gas and liquid degradation products from PP/ HZSM-x and LLDPE/HZSM-x mixtures (Tables S2−S5), Raman patterns and DTG curves of CNM-P2 (Figure S1), TEM images of NiO and Ni2O3 catalysts (Figure S2), and Raman patterns and DTG curves of CNM-L1 (Figure S3). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 (0) 431 85262004. Fax: +86 (0) 431 85262827. Email: [email protected]. *Tel.: +86 (0) 431 85262004. Fax: +86 (0) 431 85262827. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51373171, 2124079, 50873099, and 20804045) and Polish Foundation (No. 2011/03/D/ST5/ 06119).

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