Manganese-, cobalt-, and zinc-based mixed-oxide spinels as novel catalysts for the chemical recycling of poly(ethylene terephthalate) via glycolysis
Introduction
Polyesters are the most significant and cost-effective class of polymers. The versatility of the ester bond and its ability to undergo hydrolysis, alcoholysis, ammonolysis, and aminolysis make polyesters one of the preferred materials for recycling. The overall world production of polyesters was 25–30 million tons in 2000; this value increased to 55 million tons in 2012 and mostly consisted of polyethylene terephthalate (PET). Polyester consumption has increased substantially in fibres and moulding resins due to the strong demand for textile applications, as well as in food packaging and bottle markets for glass replacement [1]. Due to the tremendous increase in consumption, the most viable solution for sustainability is economical recycling of polyesters to preserve resources and the environment.
PET has the second highest (second to aluminium) scrap value as a recycled material. There are four distinct approaches (primary, secondary, tertiary and quaternary) by which post-consumer PET can be recycled [2]. Among these approaches, the one that is most suitable from sustainability point of view is tertiary (chemical) recycling because this process generates the monomers from which the polymer was originally fabricated [3]. Chemical recycling of post-consumer PET into useful feedstock is considered to be an axiomatic approach to green sustainability. It involves the chain scission of the polymer with the help of a solvent, and the process is termed ‘solvolysis’ [4]. The solvolysis processes, such as methanolysis [5], [6], hydrolysis [7], [8], [9], glycolysis [10], and aminolysis [11], have been extensively studied and reviewed in the literature [12]. Glycolysis consists of the insertion of ethylene glycol into PET chains to produce BHET along with the dimer and oligomers. The glycolysis reaction products can be used for the manufacturing of unsaturated polyester resins, copolyesters, polyurethanes and hydrophobic textile dyestuffs [13]. Moreover, the BHET produced through glycolysis can be added to fresh BHET, and the mixture can be used in either of the two PET production (DMT-based or TPA-based) lines.
Glycolysis without a catalyst is an extremely sluggish process. There has been a strong interest in the development of highly active transesterification catalysts for the depolymerisation of PET to BHET. A large number of catalysts in the form of metal salts, such as acetates [14], [15], [16], [17], chlorides [18], hydroxides [19], [20], carbonates [21], sulphates [13], [19], and phosphates [22], have been extensively studied over the last two decades. Most of these salts are soluble in ethylene glycol, are difficult to separate after the depolymerisation reaction, and require an additional unit operation (distillation) in the chemical process. It has also been noted that the zinc salts do not increase the glycolysis rate at temperatures above 245 °C, which limits their usage at the moderately high temperatures applied to decrease the overall reaction time [23]. Recently, a new series of glycolysis catalyst, including ionic liquids [23], [24] and metal oxides [25], [26], have been reported. Additionally, a different approach using microwave irradiation in PET glycolysis has also been studied [20]. The synthesis of ionic liquids is cumbersome compared to the synthesis of metal oxides. Our group recently reported the use of metal oxide-doped silica nanoparticles as effective catalysts for PET glycolysis for the first time [25], [26]. The results were promising, with BHET yields above 90 mol%. These catalysts were insoluble in EG and acted heterogeneously when the PET was in the molten state.
Metal oxides exhibit a wide range of applications, from the processing sector (catalysis) to the electronics industry (electrodes and superconductors). Using metal oxides as glycolysis catalysts could be a better option than using conventional catalysts with regard to high monomer yield, high mechanical strength, high melting points, flexibility of usage in fixed and fluidised beds, possibility of regeneration, ease of separation, and long shelf life. Pure metal oxides and their mixed-oxide spinels may possess entirely different characteristics due to their physical, physiochemical, textural, and structural properties, which lead to entirely different catalytic properties [27]. The use of mixed-oxide spinels as glycolysis catalysts has not yet been investigated.
Spinels, which have the general formula AB2O4, represent an important class of mixed metal oxides in which A and B are divalent (A+2) and trivalent (B+3) atoms with tetrahedral and octahedral coordination in the crystal structure, respectively. There are two main classes of spinels: normal and inverse. The spinel structure crystallises in either the cubic or tetragonal structure. In the cubic structure, the A+2 reside on tetrahedral sites, while the B+3 reside on octahedral sites. The tetragonal structure is a deformed form of the cubic structure resulting from deformation in the octahedra because of Jahn–Teller interactions [28]. The mixed metal oxide spinels displayed higher catalytic activity in various applications, such as CO oxidation [29], hydrocarbon combustion [30], and redox reactions of several organic compounds [31]; however, their utilisation as transesterification catalysts for glycolysis reactions has yet to be fully explored.
In the present study, pure oxides (ZnO, Co3O4, and Mn3O4) and mixed-oxide spinel (ZnMn2O4, CoMn2O4, and ZnCo2O4) catalysts were synthesised by a simple precipitation or co-precipitation method. A series of experiments was conducted to investigate the effect of reaction parameters, such as temperature, time, EG/PET mole ratio, and catalyst/PET weight ratio. Attempts were made to effectively separate the BHET from the glycolysis reaction products. The catalysts were characterised by scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), ammonia temperature-programmed desorption (NH3-TPD), and BET surface area analysis, while the monomer BHET and dimer were characterised by high performance liquid chromatography (HPLC), differential scanning calorimeter (DSC), thermogravimetric analysis (TGA), and nuclear magnetic resonance spectroscopy (NMR). The oligomers produced with varying reaction times were characterised by DSC, and a reaction mechanism was also proposed.
Section snippets
Materials
PET waste consisting mostly of post-consumer soft-drink bottles was collected, washed, dried, and cut into small pieces. These pieces were mixed with dry ice and ground to a fine powder with an average particle size of less than 200 μm. The powder PET number-average (MWn = 28,000) and weight-average (MWw = 59,000) molecular weights were determined by gel permeation chromatography (GPC). Ammonium hydroxide (NH4OH), manganese (II) nitrate hydrate (Mn(NO3)2·xH2O), zinc nitrate hexahydrate (Zn(NO3)2
X-ray diffraction (XRD) analysis
The room-temperature XRD was performed on the fabricated catalysts to identify their phase and crystalline structure. The X-ray diffractograms of pure metal oxides (ZnO, Co3O4, and Mn3O4) and mixed metal oxides (ZnMn2O4, CoMn2O4, and ZnCo2O4) are shown in Fig. 1. The hexagonal zincite structure of ZnO corresponds well with the standard JCPDS card 36-1451, as shown in Fig. 1a. Fig. 1b presents the pure cubic spinel phase of Co3O4 (Co+2Co2+3O4), which has good similarity with JCPDS card 42-1467.
Conclusions
The monophasic metal oxides (Mn3O4, ZnO, and Co3O4) and mixed metal oxide spinels (ZnMn2O4, CoMn2O4, and ZnCo2O4) were synthesised via a simple precipitation or co-precipitation method. The fabricated mesoporous materials were used as transesterification catalysts in PET glycolysis. The mixed-oxide spinels exhibited better catalytic performance than the single metal oxides because the former had greater surface areas and higher acid site concentrations on the catalyst surface. Moreover, the
Acknowledgements
This work was supported by the Basic Science Research Program through a National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology (2010-0025671). The authors also wish to acknowledge the Deanship of Scientific Research at King Saud University for funding the work partially through the research group project no; RGP-VPP-107.
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