Article pubs.acs.org/EF
Investigation on Preparation of Mesophase Pitch by the Cocarbonization of Naphthenic Pitch and Polystyrene Dong Liu,*,† Ming Li,† Fengjiao Qu,‡ Ran Yu,† Bin Lou,† Chongchong Wu,§ Jianping Niu,† and Guangkai Chang† †
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, China Exploration and Development Research Institute of Hua Bei Oil Field Company, Renqiu 062550, China § University of Calgary, Calgary, Alberta T3A2L4, Canada ‡
ABSTRACT: Two modified pitches with softening points of 70 °C (LPA) and 110 °C (HPA) produced from modification of naphthenic vacuum residue were cocarbonized with polystyrene (PS), respectively, and the mesophase pitches were obtained. The effects of reaction time, PS addition, pressure, and properties of raw materials on cocarbonization behavior were analyzed by H/C, quinoline insoluble content (QI), carbon residue value, density, polarized optical images, X-ray diffraction pattern (XRD), scanning electron microscopy (SEM) analysis, Fourier transform infrared (FTIR) spectra, and 1H nuclear magnetic resonance (1H NMR) analysis. Comparing with HPA, the cocarbonization process showed more suitable for LPA to prepare the mesophase pitch due to the favorable occurrence of radical chain transfer reactions between LPA and PS components which contributed to the existence of aliphatic side-chains and naphthenic structures in mesogens as well as the formation of a well-developed mesophase pitch. Mesophase pitch with larger domains optical texture and better crystal structure was obtained from cocarbonization with 5 wt % PS under the optimal conditions of 410 °C, 6 h, and 6 MPa.
1. INTRODUCTION Mesophase pitch with low viscosity and high purity is an excellent precursor for carbon materials such as high-performance carbon fiber, C/C composites, carbon foam, lithium ion battery electrodes activated carbon, and mesoporous carbon.1−4 The conditions of the carbonized reactions and the properties of feedstock play an important role in the formation, accumulation, and rearrangement of the planar mesogens and then determine its final properties of the mesophase pitch.5−10 At present coaland petroleum-based pitch is widely used to prepare carbon materials owing to its relatively lower price, sufficient quantity, and higher carbon yield.11,12 However, there are thousands of different types of molecules in petroleum pitch to react distinctively, leading to the poor quality of mesophase pitch which is not suitable for manufacturing advanced carbon materials. As a result, cocarbonization as one of the mostly used modified method has been widely used to improve the performance of the derived mesophase pitch,13−15 and relevant research indicated that the additives such as ferrocene, HF/BF3, carbon black, divinylbenzene, and rosin have a significant influence on the growth, size, and texture of the pitch-based mesophase.16,17 With more and more environmental requirements, the pyrolysis has developed into an important disposal of waste polystyrene (PS) due to the depollution to the environment caused by landfill and incineration of the waste plastic.18−20 Also, the mechanism and products of thermal degradation of a variety of polymers: for example, polyolefins, polystyrenes, poly(vinyl chloride), was described in the many reviews. Recently polymeric additives, for instance, PS, PVC, polycarbonate, and PET, have been extensively used to improve rheological properties of the pitch that is usually applied in building construction, insulatingseal materials, and anticorrosion protective coatings, and strong © XXXX American Chemical Society
interaction between pitch constituents and some polymers can be expected from the distinctly higher carbonization residue yield of blend than that anticipated from the behavior of single components.21−25 Therefore, some interest in copyrolysis of pitch-polymer mixtures has arisen recently from the expectation of both disposing waste plastics and modifying the optical texture of resultant coke. However, in the past, copyrolysis of pitch with polymers has received relatively little attention on the preparation of mesophase pitch.26,27 So in this paper the pitch LPA and modified pitch HPA provided by the CNOOC Company were used as raw materials in this study. Polystyrene (PS) that was always used for pyrolysis treatment was selected as cocarbonizing agents and prepared mesophase pitch by cocarbonization. The effects of reaction temperature, cocarbonization time, reaction pressure, and additive amount of PS on the properties of mesophase formation process were examined. The possible interactions between PS degradation radicals and pitch constituents were also investigated to elucidate the cocarbonization compatibility mechanism of the pitch-PS mixtures.
2. EXPERIMENTAL SECTION 2.1. Materials. The modified pitches LPA and HPA were provided by CNOOC Company. The general properties of raw materials are shown in Table 1. The oxygen content, H/C, softening point (SP), carbon residue rate, and QI of HPA were higher than those of LPA. It indicates that the condensation degree of HPA is higher than that of LPA. The nitrogen contents of the two kinds of pitches were similar. The sulfur is deprived through oxidation, so the sulfur content of HPA is Received: December 7, 2015 Revised: January 27, 2016
A
DOI: 10.1021/acs.energyfuels.5b02859 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 1. General Properties of the Raw Materialsa sample
LPA
HPA
C (wt %) H (wt %) N (wt %) S (wt %) O (wt %) H/C (−) SP (°C) CR (%) QI (%) density M ash content
87.07 10.15 0.98 0.96 0.84 1.36 70 20.9 3.9 1.0139 949.3 0
86.76 9.94 1.00 0.91 1.39 1.39 110 23.5 5.7 1.0717 1681.3 < 0.1
polarized microscope (Shanghai Milite Precise Instrument Co. Ltd., China) equipped with an adjusted ocular (10×), an oil immersion objective (50×), and a 1-λ retarder plate. The softing point was analyzed by penetration method and the specific procedures were as follows. Mesophase pitch powder (1.5 g) was put in the sample cell and the cell was recharged with nitrogen. The sample cell was soaked in the salt bath and heated up with 4−6 °C/min. The sample was touched by a fine steel needle (diameter 1 mm) which had been inserted in the sample. When the temperature reached the softing point, the pitch was soft to the touch and melted to flakiness.
3. RESULTS AND DISCUSSION 3.1. Characterization of Raw Materials. 3.1.1. FTIR Analysis of Raw Materials. Figure 1 presented the FTIR spectra
a
SP, softing point; CR, carbon residue rate; QI, quinoline insoluble; M, average molecular weight.
lower than that of LPA. Additionally, the average molecular weight of HPA is higher than that of LPA. The analysis results of six components analysis of raw material are shown in Table 2. Compared with LPA, HPA contained more asphaltene and aromatics but less resin and saturates. Therefore, comparing with LPA, HPA had higher condensation degree.
Table 2. Six Component Analysis of Raw Materiala sample
LPA
HPA
SH (%) MAH (%) DAH (%) PAH (%) resin (%) asphaltene (%)
13.56 3.54 1.31 4.32 71.13 6.14
12.68 2.83 17.30 2.09 31.23 33.87
Figure 1. FTIR spectra of LPA and HPA.
a
SH, saturated hydrocarbon rate; MAH, mononuclear aromatic hydrocarbons; DAH, diaromatics hydrocarbons; PAH, polycyclic aromatic hydrocarbons.
of LPA and HPA. Both the spectra included a strong C−H stretching of aliphatic groups attached to aromatic compounds at 2856−2954 cm−1, ring vibration of aromatic hydrocarbons at 1604 cm−1, a strong peak of aliphatic hydrocarbon chains at 1462 cm−1, methylbenzene derivatives at 1377 cm−1, and several peaks of aromatic C−H out-of-plane bending between 710 and 890 cm−1. These peaks suggest that LPA and HPA are mainly composed of aromatic compounds substituted by aliphatic sidechains. Whereas the stretching vibration peak of aromatic hydrogen (3048.2 cm−1) in FTIR spectrum of the HPA was relatively distinct. This difference suggests that the HPA shows a high aromaticity, which is consistent with the H/C value. Only HPA had CO stretching vibrational absorption at 1695 cm−1, while LPA did not. The results showed that HPA contained oxidative cross-linking compounds. 3.1.2. 1H NMR Analysis of Raw Materials. The 1H NMR spectra of raw materials are shown in Figure 2. The distributions of the constituent hydrogens and the suggestive structural parameters calculated by the modified Brown−Ladner method14 were summarized in Table 3. HPA possessed a larger percent of Har and higher aromaticity than those of LPA, which were consistent with the FTIR analysis. The f N of HPA was higher than that of LPA, implying that HPA contained more naphthenic structures which contribute to smooth development of mesophase during the carbonization process. Additionally, HPA contained less percent of aliphatic carbons (f P) than those of LPA, despite the discrepancy of being small. However, the side-chains’ average length (L)14 of HPA was longer than that of LPA.
2.2. Experimental Methods. The cocarbonization was conducted in a 300 mL autoclave. The raw material and polystyrene (PS) were premixed and placed in a reactor and heated in an electric furnace to 400−440 °C with a rate of less than 5 °C/min. During the experiment, the pressure was maintained at about 6 MPa. When the reactor was heated just to reaction temperature, the soaking time is regarded as 0 h. 2.3. Analysis. The elemental compositions were determined on a PE-2400 series HCSN elementary analyzer. The average molecular weight was measured by a KNAUER K-700 vapor pressure osmometer. The quinoline insoluble (QI) content of mesophase pitches was measured according to GB/T 2293-2008. The specific procedures were as follows. All mesophase pitches were ground to
