Catalytic fast co-pyrolysis of bamboo sawdust and waste plastics for enhanced aromatic hydrocarbons production using synthesized CeO2/γ-Al2O3 and HZSM-5
Introduction
Continuing consumption of waste plastics has become one of the most challenging environmental issues to be addressed [1], [2]. From the perspective of economic sustainability, recovering waste plastics via energy recovery, chemical and/or mechanical recycling are all more beneficial than landfilling [3]. For energy recovery, incineration is the dominated method to process waste plastics which recovers energy in the form of electricity and/or heat. However, more efforts still need to be made to improve the power generation efficiency from waste plastics, and both electricity and heat generated in large amounts are difficult to store. Therefore, given the reduction of fossil dependence, chemical recycling of waste plastics to produce liquid fuels has been shown to be an interesting and promising option.
Among the chemical recovery methods, fast pyrolysis or catalytic fast pyrolysis (CFP) is a prospective option which converts waste plastics into liquid fuels. Studies regarding the catalytic conversion of waste plastics have already been conducted widely [4], [5], [6]. Xue et al. [7] investigated the effect of catalyst contact mode and gas atmosphere in the catalytic conversion of waste plastics (i.e., PE, PS, PP, and PET) over HZSM-5 using a tandem micro-pyrolyzer, and it was found that PS wastes produced the highest yield of aromatic hydrocarbons (up to 85%). Lei et al. [8] studied the production of jet fuel range alkanes from LDPE via catalytic microwave-assisted degradation and followed by hydrogenation process, and it was observed that ∼84.32% selectivity towards cycloalkenes and 8.65% selectivity towards aliphatic alkanes were obtained.
On the other hand, since biomass is a hydrogen deficient material which hinders the production of upgraded bio-oils via CFP, waste plastics such as PE and PP having a higher H/C effective ratio are attractive co-blend materials. Therefore, catalytic fast co-pyrolysis (co-CFP) of waste plastics with biomass to improve the average H/C effective ratio in feedstock blends has been proposed and studied [9], [10], [11]. For instance, Sophonrat et al. [3] explored the synergistic effect of mixed plastics and cellulose using a Py-GCxGC/MS, and their results indicated that co-pyrolysis of PS and cellulose facilitated the formation of aromatic products with a significant increase in the production of ethylbenzene. Ephraim et al. [12] investigated the effect of plastic type and content on product yield and quality during the co-pyrolysis of poplar wood and plastics, they found that PVC illustrated a remarkable positive synergy on pyrolysis oil yield with a maximum value of 11 wt% obtained at 50 wt% PVC content.
Regarding the catalysts employed in the co-pyrolysis of biomass and waste plastics, acid zeolites such as HZSM-5, HY, and SBA were employed with HZSM-5 being the most commonly used [5], [13], [14]. In addition, the effects of alkali metal oxides including MgO and CaO on the production of hydrocarbons during the catalytic conversion of waste plastics and/or biomass were also studied [15], [16], [17]. Due to the different catalytic performance of acid zeolites and metal oxides, a dual catalytic stage combining HZSM-5 and metal oxides was proposed in our previous works and other related studies [18], [19], [20]. For instance, Fan et al. [21] investigated the catalytic co-pyrolysis of lignin and LDPE over HZSM-5 and MgO using a fast microwave assisted reactor, and it was observed that the proportion of aromatics and alkylated phenols decreased with the increasing of HZSM-5/MgO mass ratio. However, the studies regarding the dual catalytic stage during the co-pyrolysis process were focused on the use of commercial metal oxides such as CaO, MgO, and CeO2 [19], [22], [23]. It’s well known that commercial metal oxides usually have a low surface area which might hinder the production of aromatic hydrocarbons, and the structural and textural properties could be improved by using catalyst supports.
In our previous work, synthesized CeO2/γ-Al2O3, ZrO2/γ-Al2O3, CeO2-ZrO2/γ-Al2O3 were shown to be effective in the production of hydrocarbon precursors via CFP of pure bamboo sawdust [24]. However, as mentioned before, biomass is hydrogen deficient which restricts the conversion of hydrocarbon intermediates into aromatic hydrocarbons. In this sense, co-feeding LLDPE with bamboo sawdust will definitely improve the overall H/C effective ratio which might be beneficial to the production of targeted aromatics. Therefore, in this study, synthesized CeO2/γ-Al2O3 and HZSM-5 were used to form a sequential dual catalytic stage to co-pyrolyze bamboo sawdust and waste plastics (LLDPE) via an analytical Py-GC/MS unit. Initially, the effect of CeO2/γ-Al2O3 to bamboo sawdust mass ratio on the production of hydrogen precursors (including ketones and light phenols) and aromatic hydrocarbons was studied. Subsequently, the influence of CeO2/γ-Al2O3 to HZSM-5 mass ratio on the formation of aromatic hydrocarbons, alkanes, and alkenes was investigated. Finally, dual catalytic fast co-pyrolysis of LLDPE and bamboo sawdust with varying LLDPE percentages in the feedstock blends was conducted to explore the optimal reaction condition, and the additive effect of bamboo sawdust and LLDPE on the formation of monocyclic aromatic hydrocarbons during the dual catalytic stage was revealed.
Section snippets
Materials
Powder LLDPE (100 mesh, ∼0.15 mm) was purchased from Hua Chuang Polymer Materials Company in Guangdong province, China. Bamboo sawdust (labeled as BSD) was bought from Huzhou, Zhejiang province, China. The samples were ground and sieved using a 100 screen mesh (∼0.15 mm), and then were dried at 105 °C for 24 h. The ultimate analysis of BSD was carried out by Vario EL II elemental analyzer (Germany), and the experimental results are illustrated in Table 1.
Commercial HZSM-5 (SiO2/Al2O3 mole
Optimization of CeO2/γ-Al2O3 to bamboo sawdust mass ratio
During the CFP of biomass to produce upgraded bio-fuels, it’s well known that catalyst to biomass (C/B) ratio plays a determining role in product distribution. For instance, Huber et al. [30] studied the effect of contact time (by varying C/B ratios) on the CFP of cellulose using ZSM-5 zeolites in a bubbling fluidized bed reactor, and they observed that CO and CH4 were promoted at low C/B ratios (catalyst contact times <1000 s), and higher C/B ratios (catalyst contact times >10,000 s) led the
Conclusion
Catalytic fast co-pyrolysis of BSD and LLDPE employing a dual catalytic stage combining CeO2/γ-Al2O3 and HZSM-5 was carried out. The use of synthesized CeO2/γ-Al2O3 facilitated the production of light phenols when the C/B ratio was elevated from 0 to 1, however, a further increase in C/B led to the decrease of light phenols. The formation of acids was significantly decreased at C/B ratio of 4, which corresponds to ∼93% conversion rate. Simultaneously, the concentration of aromatic hydrocarbons
Declaration of Competing Interest
The authors declare that they have no known competing financial interests.
Acknowledgments
The authors are grateful for the National Key Research and Development Program (2017YFD0600805), and the National Natural Science Foundation of China (No. 31530010, No. 31600590, No. 51776042). Natural Science Foundation of the Jiangsu Higher Education Institutions of China (No. 17KJB610005), and a project funded by Nanjing Xiaozhuang University (No. 2016NXY41).
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