Exploration of reaction mechanisms on the plastic waste polyethylene terephthalate (PET) dissolved in phenol steam reforming reaction to produce hydrogen and valuable liquid fuels
Graphical abstract
Introduction
Nowadays, more than 2.0 billion tons of municipal solid waste is produced every year and is estimated to grow to 3.4 billion tones yearly by 2050 [1]. Most of these wastes are plastic and polymer base waste, which is very hard to be decomposed naturally. The present waste management for plastic waste streams is burning and landfill storage. Indiscriminate disposal of plastics on land and open-air burning can lead to the release of toxic chemicals into the air causing public health hazards, potent greenhouse gas, and contributes to global warming. Plastic waste is the mixture of different plastic products that are mainly made from high-density polyethylene (HDPE), low-density polyethylene (LDPE), polyvinyl chloride (PVC), polystyrene (PS), polypropylene (PP), and polyethylene-terephthalate (PET) [2]. In this study, PET as one of the primary plastic wastes used for packaging materials such as bottles and containers, is selected for investigation. In addition to plastic waste, rapid consumption of natural resources, depletion of fossil fuels, and CO2 emissions to the environment concern the well-developed countries. To solve these problems, utilization of PET plastic waste for value‐added liquid fuels and hydrogen gas production is ideal for providing energy sustainably at a low environmental cost. This is mainly ascribed to the environmental advantages of renewable and clean fuels over other fossil fuels such as oil and coal.
To recycle PET plastic waste, only chemical dissolution imitates to the values of sustainable development because it leads to the creation of the raw materials from which PET is made initially. In this study, phenol has been selected as a solvent for PET plastic wastes and a source of hydrogen production. Phenols contain an –OH group bonded to one of the sp2 carbon atoms of a benzene ring and considered as the main liquid waste components of the bio-oil [3]. Phenolic compounds are the main responsible for the catalyst deactivation by coke deposition and are regarded as an unwanted component in the bio-oil. The phenolic component inclines to polymerize into a complex carbonaceous structure, blocking the reactor, especially when it dissolved polymer components, which finally decline system proficiency and increase the operating reaction cost. Therefore a suitable catalyst is needed for the PET-phenol steam reforming reaction.
The catalysts mostly reveal two kinds of responsibilities in the steam reforming reaction. The first one is the division of CC, CH, and OH bond of the phenol and PET molecules. The second one is to alter the path of the phenol steam reforming (Eqs. (1) and (2)) and water gas shift (WGS) (Eq. (3)) reactions.
Many catalysts such as Ni/Al2O3 [4], NiAl2O4 spinel [5], Ni-Cu/SEP [6], Ni-Co/ZrO2 [7], NiCe/ZSM-5 [8], and Ni-Co/La2O3-Al2O3 [9] have been employed for the phenol steam reforming reaction. Moreover, we have used Ni/La-Al [[10], [11], [12]], Ni/Al [13], Ni-Pd/Al-La [14] catalysts for phenol-PET steam reforming reaction for hydrogen and valuable liquid fuel production. Non-noble metals such as Ni could be the right candidate for the PET-phenol steam reforming reaction due to the CC bond rupture capability. Nevertheless, Ni-based catalysts are inclined to deactivation by coke formation, which limits their varied applications. To deal with this issue, impregnating the Ni-based catalyst by another metal could be a great idea in catalytic activity and stability [7]. Among noble metals promoters, stability, high activity in CC bond cleavage, promotion of WGS reaction, and resident against coke deposition [6] marks Pt an appropriate active metal for PET-phenol reforming reactions. Moraes et al. [15] illustrated that the introducing of Pt decreased coke formation by the hydrogenation of adsorbed active carbon species on the catalyst surface at a higher rate than carbon diffusion into bulk nickel. In addition to the metal, supports also play a significant role for supported metal catalysts in taking out the ability of the high, supported metal performance. Alumina is identified as a practical binding component due to its low cost and thermal stability [16]. However, alumina in the reforming reaction might cause unwanted catalyst coking because of the presence of the acid site in its surface. The addition of TiO2 to Al2O3 creates an increase of thermal stability, increase the mechanical strength, enhancing productivity by reducing catalyst consumption [17]. Thus, the bimetallic Ni-Pt catalyst supported on Al2O3 and TiO2 could have an excellent performance in the PET-phenol steam reforming reaction.
However, the catalytic performance and reaction mechanism of the PET-phenol steam reforming reaction is unclear. The reaction mechanism will be very significant to design a catalyst strong against carbon formation that can be used to produce valuable liquid fuel and hydrogen from plastic waste dissolved in phenol steam reforming reaction. The influence of reaction temperature, time on stream, and residence time on catalytic activity and selectivity have been studied. Then the reaction paths and mechanistic studies based on liquid and product compositions from experimental results and reported literature were described and discussed.
Section snippets
Catalyst preparation
Ni-Pt/Al-Ti (ratio: 8:2:45:45) catalyst was synthesized via hydrothermal assisted impregnation method and in according to our previous research [18]. In brief, the specific amount of Ti and then Al was moderately added to the deionized water, followed by gentle adding of 5 M NaOH and stirred for an hour. The solution was then poured into a 100 mL Teflon cylinder and sealed in an autoclave reactor and kept in a furnace at 130 °C for 48 h. After cooling the autoclave at room temperature, the
Effect of Temperature on catalyst activity
The feed conversion and gas volume yields of the three main components (H2, CO, and CO2) over the Ni-Pt/Al-Ti catalyst and without catalyst are shown in Fig. 2 (a) and (b), respectively. As seen in the figure, the columns with circled gray lines belong to the product yield and pink line belongs to the phenol conversion. The reaction in the absence of a catalyst would proceed, but the phenol conversion and product yields are too low to be observed. Therefore, the reaction with Ni-Pt/Al-Ti
Conclusion
In this article, catalytic steam reforming of PET-phenol to produce value-added liquid fuel and hydrogen using Ni-Pt/Al-Ti catalyst with the detailed reaction mechanism was adopted. This research has emphasized the potential approach of solving the threat of PET plastic waste towards the production of valuable fuels as well as hydrogen gas. Phenol conversion hydrogen yield was found to reach 94% and 78%, respectively. Useful liquid fuel product such as benzene, toluene, methylindene, styrene,
Author contributions
Walid NabganFirst Author who carried out most of the writing and drawing parts
Bahador NabganTake care on the mechanism parts, Fig. 8, Fig. 9, Fig. 10
Tuan Amran Tuan AbdullahTook the reaction study part in his laboratory
Norzita NgadiThe supervisor who paid for the experiments and gave the idea of mechanism
Aishah Abdul JalilTook the writing tasks of the experimental part
Nor Aiza Abdul FatahTook the writing tasks of the reaction and stability parts
Hasan AlqaraghuliHelp to draw the Fig. 1, Fig. 2,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The principal author, Walid Nabgan, is thankful for the support from Universiti Teknologi Malaysia in the form of the Post-Doctoral Fellowship Scheme “Development of a New Catalyst from a Waste to Produce Biodiesel” (Grant number: 04E81).
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