Article pubs.acs.org/EF
Influence of Zeolites and Mesoporous Catalysts on Catalytic Pyrolysis of Brominated Acrylonitrile−Butadiene−Styrene (Br-ABS) Chuan Ma, Jie Yu,* Ben Wang, Zijian Song, Fei Zhou, Jun Xiang, Song Hu, and Lushi Sun* State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China ABSTRACT: Three zeolite materials (HY, Hβ, and HZSM-5) and two mesoporous solids (all-silica MCM-41 and active Al2O3) with different textural properties were investigated for their catalytic effects on the pyrolysis of brominated acrylonitrile− butadiene−styrene (Br-ABS). The results indicated that, in the absence of a catalyst, the maximum liquid yield was obtained from the pyrolysis of Br-ABS, including oil and wax. The addition of HY and Hβ zeolites significantly decreased the oil yield from 63.6 to 45 and 44.3 wt %, respectively, with a corresponding increase in the yields of wax and gas. HZSM-5 zeolite slightly changed the product distribution and increased the gas yield at the expense of wax. Noteworthy, the mesoporous catalyst of all-silica MCM-41 retained the oil production while significantly reduced the wax yield to 7.9 wt %. In terms of the composition of the oils, the catalysts could promote the formation of valuable single-ring aromatics in oils. Moreover, the catalysts exhibited pronounced debromination efficiency, especially the zeolites, which could achieve the removal of bromine in oils by over 50% compared to thermal pyrolysis. The results indicated that the product distributions of Br-ABS had interrelationship with the textural properties of catalysts.
1. INTRODUCTION The electronics manufacturing industry has become an emerging and fast growing sector in the world, because of the rapid development of technology innovation and ever-shortening product lifespans. Subsequently, large consumption of electronic products results in a great number of electronic wastes, which have been taken into the agenda for waste management in many countries. It is estimated that the total quantity of waste electrical and electronic equipment (WEEE) in 2014 was 41.8 million tons, and it is expected to hit 50 million tons by 2018 at an annual growth rate of approximately 5 wt %.1 China, as one of the largest electronic manufacturing countries and one of the emerging economies in the world, is rising up as the second biggest entry of WEEE generation.2 To solve this great challenge, WEEE has been legislatively required to be recycled and reused in a proper way in the European Union (EU) according to the WEEE Directive.3 In general, approximately 30% of the total WEEE consists of plastics,4 and thereby, the growing volume of plastics from WEEE has been taken into an imperative recycling treatment to meet these targets. In terms of the plastic fraction of WEEE, acrylonitrile− butadiene−styrene (ABS) and high-impact polystyrene (HIPS) plastics make up a significant proportion of 55% of the plastic fraction in WEEE,5 which generally include toxic brominated flame retardants (BFRs) and antimony trioxide (Sb2O3) synergist for the fire safety performance of polymer materials.6 Nevertheless, the presence of BFRs in WEEE plastics has been the major impediment for recycling treatment because of the potential health risks and environmental impacts.7 For the sake of hazardous plastic wastes, advanced handling methods are essential to eliminating the toxic flame retardant additives during the recycling process of WEEE plastics. Feedstock recycling by means of pyrolysis is one of the promising technologies for WEEE plastics treatment, with the aim of converting the WEEE plastics into fuels or chemical © XXXX American Chemical Society
feedstocks. Considering the high-value chemicals and high energy density of plastics, investigations on feedstock recycling of WEEE plastics have been performed to convert the polymeric materials into valuable chemicals and fuels over recent decades.8−11 However, WEEE plastics containing BFRs are especially prone to produce a great quantity of brominated compounds in the oil, for example, SbBr3, HBr, and organobromine compounds during the pyrolysis process without any additive or catalyst.12−14 Thus, researchers have been attempting to improve the feedstock recycling process and bring back waste plastics for commercial applications by generating high-quality bromine-free chemicals and alternative clean fuels.15−17 In the last few decades, extensive studies on catalytic pyrolysis of plastics were carried out to investigate the effectiveness of the catalyst and the degradation mechanisms over various catalytic systems. A wide range of catalytic materials has been used for plastic degradation: metallic oxides,18−20 zeolites,21−24 fluid catalytic cracking (FCC) catalysts,25 mesoporous materials, and silica−alumina.26−30 Zeolites are the most used catalysts and exhibited outstanding cracking performances in polymer degradation, particularly HZSM-5, as a result of its inherent microporous structure and acid property.22,23 Mesoporous catalysts, e.g., MCM-41, with larger pores and surface areas than zeolites were also applied for polymer degradation, which were beneficial to macromolecules entering the channel of the catalyst and cracking into liquid fuels.26,27 ZSM-5 and Y-zeolite catalysts have been used for the catalytic pyrolysis of flame-retarded plastics.21 It was reported that both zeolite catalysts can achieve a remarkable removal of Received: February 26, 2016 Revised: May 3, 2016
A
DOI: 10.1021/acs.energyfuels.6b00460 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
mm). The catalysts were characterized by different techniques and presented in Table 2. The details of the characterization of the catalysts have been illustrated previously.10 The physical properties of the catalysts were characterized by N2 adsorption−desorption measurement (ASAP 2010, Micromeritics). The Brunauer−Emmett−Teller (BET) surface area of the catalysts followed the trend: all-silica MCM-41 > HY > Hβ > HZSM-5 > Al2O3. The acid properties of the catalysts were obtained using temperatureprogrammed desorption of NH3 (NH3-TPD) performed on the ChemiSorb 2720 (Micromeritics). The total acidity of the catalysts followed the trend: HZSM-5 > HY > Hβ > Al2O3 > all-silica MCM-41. 2.2. Experimental Procedures. The fixed-bed reactor composed of a quartz tube (height, 46 cm; inner diameter, 41 mm) was used in this work, as shown in Figure 1. Before the catalytic pyrolysis experiments, the Br-ABS sample and the catalyst were mixed thoroughly (the catalyst and the Br-ABS sample at a mass ratio of 1:9) and then a quartz crucible (height, 50 mm; inner diameter, 30 mm) containing 10 g samples was placed inside the reactor. For comparison, the thermal degradation of Br-ABS without a catalyst was carried out under the same experimental conditions. Prior to the experiment, high-purity nitrogen (100 mL min−1) was continuously fed into the system for 30 min to form an inert atmosphere. Experiments were performed from ambient to the target temperature (410 °C) at 10 °C min−1 with high-purity nitrogen (30 mL min−1) as a carrier gas. For the isothermal stage, the target temperature stayed for 90 min. The parameters used in the paper are optimized operating conditions and were well-established by the authors centered on this subject. The products from the pyrolysis of Br-ABS were classified as oil, wax, gas, and char residue. The volatile products were first cooled by an ice/salt bath condenser. The mass of oil products collected in the tar trap and the mass of heavy waxy products coated on the walls of the reactor (namely, wax) were measured separately. A small amount of trapped product coated on the U-tube was dissolved in chloroform and filtered to distill at a reduced pressure to separate the oil and wax, which was weighed accurately.33 A fiber filter was used to remove any pyrolysis oil from the gas stream, and the alkali solution flask was used to trap HBr or Br2. After each experiment, the solid residue that remained in the crucible was weighed and defined as char residue. The yield of char derived from the pyrolysis of raw materials was calculated as follows:
bromine in the oil, and Y-zeolite exhibited a better debromination performance than ZSM-5. However, the yield of oil and the composition of the pyrolysis products was significantly influenced by the catalysts. In fact, the authors also claimed that the composition of the pyrolysis oil was not greatly altered and the organobromine compounds were reduced in the presence of the FCC catalyst.25 Fe-modified zeolite catalysts were investigated in the catalytic pyrolysis of brominated HIPS, and it was found that modified zeolite catalysts could enhance the debromination performance during the pyrolysis−catalysis process.9 Marcilla et al.30 investigated the particular pyrolysis performance of HIPS with the Al-MCM-41 catalyst and found that the catalyst had a significant influence on the decomposition of each polymeric constituent in the HIPS copolymer, including the polystyrene (PS) and polybutadiene (PB) phases. However, the ABS copolymer is a composite material consisting of a styrene−acrylonitrile (SAN) continuous phase, partially grafted to a dispersed PB rubber phase, which usually contains BFRs and Sb2O3 synergist. The catalytic effects of different solid acid catalysts on the pyrolysis of brominated ABS (Br-ABS) have scarcely been investigated as a result of its specific thermal degradation characteristics and the formation of brominated products.21,25,31 However, these solid acid catalysts with different catalytic activities can be closely associated with their textural properties. Acidity, particularly the strength and number of active acid sites, is also demonstrated to play an vital role in the catalytic cracking performance of polymers.26,32 Understanding the behavior of catalytic pyrolysis of Br-ABS is crucial to promote the potential commercial application of catalytic treatment for feedstock recycling of plastics from WEEE. In this work, catalytic pyrolysis of Br-ABS over various solid acid catalysts was conducted in a fixed-bed reactor at 410 °C. Three zeolite materials (HY, Hβ, and HZSM-5) and two mesoporous solids (all-silica MCM-41 and active Al2O3) with different textural and acid properties were applied to investigate the catalytic effects of solid acid catalysts on the pyrolysis of BrABS. Meanwhile, the migration and transformation behaviors of bromine during the pyrolysis process of Br-ABS were demonstrated to enhance the potential benefits of feedstock recycling for high-quality bromine-free fuel oils.
yield of char (wt%) =
2.1. Materials. The Br-ABS plastics (PA-765A) were supplied by Qimei Co., Ltd., Zhenjiang, China. Br-ABS contained the flame retardant additive of tetrabromobisphenol A (TBBPA) and synergist Sb2O3. The particle size of the plastic sample was ground to
