The effect of thermal and non-thermal routes on treatment of the Mg–Al layered double hydroxide catalyst dispersed by titania nanoparticles in products distribution arising from poly(ethylene terephthalate) degradation

Highlights

• A novel LDH catalyst treatment led to achieving the highest PET conversion and BHET yield.

• The effect of thermal-catalyst treatment routes on PET degradation products was studied.

• Highly dispersed titania NPs on the LDH catalyst enhanced the catalytic performance and thermostability.

• LDHs produced by the freeze-drying and plasma routes showed high activity and reusability.

• The synergistic effects of reaction conditions on improved PET glycolysis were studied.

Abstract

The development of sustainable catalysts in the chemical recycling of polyethylene terephthalate (PET) can help to improve the circular economy of this material. In this study, the degradation of PET to the bis(2-hydroxyethyl) terephthalate (BHET) monomer and other main products was investigated by Mg–Al layered double hydroxides (LDHs) dispersed by 20 wt% of titania nanoparticles (NPs). The PET conversion and BHET yield of LDHs treated by thermal and non-thermal treatment routes were compared to assess the thermostability and catalytic activity of LDHs. The LDHs loaded by titania NPs showed higher thermal stability than the unloaded LDHs. A synergistic effect between the brucite sheets and titania was found by using a combination of freeze-drying (FD) and dielectric barrier discharge (DBD) plasma routes, associated with the high dispersion of titania with size ranges of 5–10 nm. The PET conversion and the BHET yield of LDH-FD-DBD were higher than the catalysts annealed by calcination. The high stability of the LDH-FD-DBD allowed it to be reused five times without notable catalytic activity reduction. The proposed technique was an effective approach to manipulate the properties of LDH dispersed by titania NPs that enabled the catalyst to display high activity towards PET degradation.

Introduction

Polyethylene terephthalate (PET) is a thermoplastic polyester that is used in food packaging, fibers, and especially as the main material in the production of mineral water bottles due to its excellent mechanical properties, protection against air and moisture diffusion, and high resistance to chemicals. The lightweight, as well as the low market price, have made it extensively available for commercial applications. Every year, a large number of PET wastes (in tons) are discharged to the environment because of its widespread use [1]. Although PET wastes do not directly generate hazards to the environment, its disposal provides environmental problems because it simply does not decompose in nature. The PET bottle grade has high performance between the different PET grades that provides an opportunity to develop novel techniques in PET recycling. The efficient recycling of PET waste bottles has been considered over the past decade due to environmental considerations, the conservation of raw materials, and save energy [[2], [3], [4]].

The main goal of the chemical recycling of PET wastes is its degradation to initial monomers and reuse of obtained monomers for synthesis of high-added value chemicals. The post-consumer PET bottles are mostly recycled into textiles, carpets, and composite sheets [[4], [5], [6]]. Also, there is the industrial interest to PET recycling to obtaining oligomers that applied in the synthesis of polyurethanes, unsaturated polyester or polyester resins, and softener [5,6]. The methanolysis, glycolysis, hydrolysis, ammonolysis, and aminolysis are accounted for chemical routes of PET recycling. Typically, complicated temperature and pressure conditions, as well as significant amounts of solvent consumption, are mainly required for the de-polymerization process [4,7,8]. The glycolysis is regarded as an esterification reaction of the ester groups of PET and ethylene glycol (EG) to achieve the bis(2-hydroxyethyl) terephthalate (BHET) monomer. It should be noted that the glycolysis route in the absence of a catalyst is a prolonged process from the viewpoints of chemical kinetics of reactants [3]. Furthermore, chemical recycling routes commonly involve separation and purification processes, which impose toxic and environmental hazardous issues.

Earlier kinetic studies have shown that PET glycolysis is a slow process, and complete conversion of PET to BHET monomer is almost impossible without the presence of the catalyst [9]. Metal acetates were the first reported catalysts for PET glycolysis [10,11]. Later on, researchers identified more eco-friendly catalysts, such as sulfates, mild alkalies, metal chlorides, and zeolites for the PET glycolysis. However, the above-mentioned catalysts still had problems such as long reaction time and low BHET monomer yield [12]. Guo et al. [13] showed that the heterogeneous catalysts, including zinc acetate, potassium hydroxide, and phase transfer catalysts, were preferred over homogeneous ones due to the simple way of using, separation, and disposal after the PET glycolysis. Al-Sabagh et al. [14] developed Cu- and Zn-acetate-containing ionic liquids as catalysts for the PET glycolysis. The catalysts consisting of SO₄^2−/ZnO, SO₄^2−/TiO₂, and SO₄²⁻/ZnO–TiO₂ were studied in the PET glycolysis. The results showed that the SO₄²⁻/ZnO–TiO₂ shows high catalytic activity with complete PET conversion after 3 h at 180 °C [13,14]. Pardal and Tersac [17] studied the isothermal kinetics reaction of PET glycolysis through mixtures of diethylene glycol (DEG), dipropylene glycol (DPG) and glycerol solvents through an uncatalyzed route at 220 °C and then catalyzed at 190 °C by titanium (IV) n-butoxide (TBT). They found that the effect of TBT loading on the PET recovery by DPG solvent was much more than that of DEG. Chen et al. [18] reported that Mg–Al hydrotalcites and their corresponding mixed oxides act as efficient catalysts in PET glycolysis process. Sharma et al. [19] proposed the possible mechanism of the PET glycolysis reaction catalyzed by hydrotalcite catalyst. They found that the metallic group of the hydrotalcite catalyst interacts with the oxygen atom of the carbonyl CO group in the ester molecule. Simultaneously, the –OH group attacks the carbon atom of the ester group to form a tetrahedral intermediate. These two interactions result in the breakage of the CO bond and also the decrease in the PET amount. Using the hydrotalcite catalysts can be a great opportunity to explore a new set of catalysts, which opens new horizons for the PET recycling. According to the literature review, little research has yet been done on the treatment methods of hydrotalcite, and more studies are needed for the development of novel routes to improve its catalytic properties for use in the PET glycolysis reaction.

The lamellar layered double hydroxides (LDHs) are anionic synthetic clays with the general formula of [M_1-x^II M_x^III (OH)₂]^a+ [A^n–]_a/_n. mH₂O (where M ^II and M ^III represent divalent and trivalent cations, respectively, and A^n– is the charge balancing the anion in the interlayer zone, and 0 < x < 1). The earliest LDH catalyst was named as hydrotalcite, which was originated in a Norwegian geological specimen. The LDHs enhance the reaction rate of the glycolysis route due to the large surface area, week Lewis acidity, and high adsorption capacity [2,[18], [19], [20], [21]]. The reusability and nontoxicity have known as other advantages of hydrotalcite [22]. Commonly, the LDH was developed by a mixture of Mg²⁺ (or Zn²⁺) and Al³⁺ cations to form Mg–Al LDH (or Zn–Al LDH). One of the most LDH catalysts is [Mg_0.75Al_0.25(OH)₁₆CO₃]_0.125 that introduced as (Mg₃Al–CO₃-LDH). In this structure, Mg²⁺ cation can be completely replaced by Al³⁺ to form a brucite layer with a positive charge. The carbonate ions inside the layered zone compensated for this negative charge. The interlayer zone contains water molecules, which bond to the hydroxyl groups and the present anions [21]. Li et al. [23] showed that the calcination product of LDH, called LDO, contains small highly-dispersed crystals, which exhibited interesting catalytic properties.

It is generally difficult to achieve distribution above 20 wt% of NPs within the LDH catalyst structure due to their inability to mix homogeneously. The activity of LDHs can be modified through the practical approaches based on the preparation methods, reaction conditions, and surface treatment techniques. In the first case, the effective parameters include the molar ratio and composition of hydrotalcite cations, type of interlayer anions, the size and distance of the interlayer region, aspect ratio, substitution of the carbonate ions by new nucleophilic species [22,24], while the last two ones depend on the operational conditions [25,26]. The thermal treatment of the LDHs is done by the conventional calcination process [27,28], while the freeze-drying (FD), dielectric barrier discharge (DBD) plasma technique, and microwave treatments introduced as the non-thermal routes [[29], [30], [31], [32]]. The results showed that calcination at high temperatures, T (T ≥ 500 °C), causes the agglomeration of particles. Meanwhile, the low-temperature operations (T ≤ 200 °C) led to incomplete decomposition of catalyst precursors with weak crystal growth [28]. Chen et al. [18] found that the LDH with Mg: Al molar ratio of 3, and calcinated at 500 °C showed the highest catalytic activity in the PET glycolysis by using EG in the presence of Mg–Al LDH. Sharma et al. [19] showed that the catalytic activity of LDH decrease during the calcination process, while the re-hydration of calcined LDHs can lead to an increase in catalytic activity over the untreated type.

Casenave et al. [33] showed that the base properties on the LDH surface could be attributed to the presence of hydroxide ions formed at temperatures of below 450 °C. They also showed that these ions were removed by thermal treatment at 650 °C. Arena et al. [34] claimed that the thermal treatment of catalysts led to strengthening the interaction inside Mg(Ni, Al)O structure, which, in turn, led to nickel atom diffusing from the surface into its bulk and reduction of catalytic activity [35]. Tichit et al. [35] showed that the thermal stability of Ni–Mg–Al catalyst increases with increasing the weight percent of Mg atoms, while the reducibility of the catalyst decreases with increasing Mg/Al molar ratio and calcination temperature (T ≥ 700 °C). Fornasari et al. [36] found that the surface area and the catalytic activity of the Ni–Mg–Al catalyst decreased considerably with an increase in the calcination temperature. Khare et al. [37] found that the replacement of the carbonate anions by hydroxide and alkoxide groups in the LDH resulted in a faster polycondensation reaction. However, they found that the re-hydration of the calcinated LDH showed higher catalytic activity than that of the untreated catalyst. Moriyama et al. [29] used the freeze-drying technique in the synthesis of Mg–Al LDH nanocrystalline and bimetallic oxide for fluoride sorption. The structural properties of the freeze-dried LDHs were compared with those of LDHs dried at 100 °C. They found that the freeze-dried LDHs and resulting bimetallic oxides had a higher ability for fluoride removal than the LDHs dried at 100 °C, while the freeze-dried LDHs had low degrees of crystallinity with small particle sizes.

In the plasma technique, high-energy electrons, free radicals, and active ions dissociate from the carrier gas to form chemically reactive species [29]. A variety of plasma techniques, such as radiofrequency discharge, microwave discharge, and DBD treatment have been used for calcination and reduction of the catalyst that could lead to high dispersion of metal oxide NPs into the catalyst structure [30]. Typically, the surface of the catalyst treated by the plasma route becomes rough associated with the formation of new chemical species and removal of contaminants after plasma treatment [30,31]. Wang et al. [38] and Di et al. [31] summarized the advances in the treatment of catalysts with plasma, and the mechanism of preparation was discussed as well. Besides the advantages that can be expected for the plasma technique, the plasma technique facilitates some thermodynamically unfavorable chemical reactions to proceed at moderate conditions, especially for inert molecule conversions, such as CO₂, CH₄, and N₂. Tan et al. [39] used the plasma technique to remove protective ligands of the Al NPs loaded on the ZnAl-HT catalyst. They found that the plasma technique could not separate interlayer ions of LDH. Subsequently, the obtained Ni–Mg–Al-HT/γ-Al₂O₃ was reduced and calcinated by cold plasma technique to achieve better catalytic performance. Xu et al. [40] were compared the catalytic activity of Ni(NO₃)₂/MgO/Al₂O₃ in two routes. In the first route, they applied the consecutive processes of calcination and reduction by atmospheric cold plasma jet, while in the second route they used the conventional calcination at 550 °C for 4 h and reduction of catalyst at 750 °C for 2 h. Their results showed that the LDH treated by the calcination process and the plasma technique had better catalytic performance than the latter method in methane reforming [40].

In this study, we developed novel treatment techniques for the LDH dispersed by titania NPs to degrade the PET bottle waste through glycolysis reaction to attain optimum conditions. The following questions were discussed to find the feasible routes to fabricate novel LDHs with enhanced properties through attaining maximize PET conversion and BHET yield. (1) What is the effect of LDH catalyst treatment on the PET conversion and BHET yield by each of the thermal and non-thermal treatment routes? (2) What is the synergistic effect between the treated or untreated LDHs in the presence or absence of titania NPs on the PET conversion and BHET yield? (3) What is the effect of using the calcination process and FD and DBD plasma techniques or a combination of these methods on improving the catalytic properties of the obtained LDHs? (4) What is the impact of titania NPs dispersion with high loading on improving the mechanical properties and thermal stability of the prepared LDHs? To answer the above questions, the catalyst treatment routes and related results are presented in the following.