Editor's choice paperIonic liquids/ZnO nanoparticles as recyclable catalyst for polycarbonate depolymerization
Graphical abstract
Introduction
Growing of plastic wastes, together with CO2 emission, is another plague of modernity. The increasing level of plastic wastes in the environment is an urgent problem that needs fast responses. Actually, politics and researchers are pursuing two different strategies to reduce plastic dispersal: i) recycling of plastic items, and ii) incineration of plastic wastes [1]. However, in this latter case, a further drawback has to be considered, connected to the identification of suitable storage sites and to the minimization of the risk of the industrial plants. For these reasons, in the last years, the chemical valorisation of plastic wastes is regaining more and more interest as a complementary way to the thermovalorisation. In fact, some plastics can be considered as a source of CO2 that can be introduced, as chemical synthon, in important and valuable products such as carbamates, urethanes and heterocyclic structures, just to cite a few [2]. For example, carbon dioxide is also present into an important and widespread polymer such as bis-phenol A polycarbonates (BPA-PC), a thermoplastic material with significant properties as good stability, excellent flexibility and good transparency [3]. BPA-PC recycling is often difficult, then incineration becomes an easy way to produce electricity and to avoid landfills disposal. However, pyrolysis of BPA-PC results in the release of a large amount of greenhouse gases released and low-value carbonaceous products.
Alternatively, chemical depolymerization has to be considered the most promising technique for recycling waste polymers and plastics. Chemical polymer degradation can represent a potential option for converting plastics into their monomers or other valuable products (depolymerization) [4]. Alcoholysis, aminolysis and hydrolysis [5] are conventional procedures typically performed at high pressure and temperatures as well as in the presence of large quantities of concentrated acids or bases during long time processing [6].
Among these methods, depolymerization of BPA-PC has been done by using hot compressed water [7], alkaline solutions [8], supercritical ethanol [9], microwave irradiation [10], and metal catalysts such as Mn(OAc)2 [7] and a variety of metal oxides and salts [11], [12].
Recently, besides the chemical recovery of the starting monomer (BPA), particularly attractive became the use of polycarbonate as carbonylating agent in an economic and environmentally benign non-phosgene alternative for preparation of many carbonyl derivatives (carbonates, carbamates, etc.). As an example, methanolysis of polycarbonate can lead simultaneously to the recovery of the starting monomer BPA and to the synthesis of dimethylcarbonate (DMC), an industrially important solvent and methylating agent.
During the past decades, we have been focusing our efforts in developing powerful catalytic methods using ionic liquid (ILs) as both catalysts and reaction media [13]. In particular, we paid attention to tetraalkylammonium salts that are capable of stabilizing metal nanoparticles catalysts providing them a protecting shell that impedes aggregation, thus prolonging catalyst lifetime and facilitating its prompt recycling. In addition, due to the shielded positive charge on cationic nitrogen, tetraalkylammonium salts anions are good nucleophiles and can behave as basic catalysts themselves .
Based on our skills in handling metal nanocatalysts [14], and being PC transesterification also promoted by a variety of Lewis acids, we decided to conjugate the nucleophilic/stabilizing ability of quaternary tetraalkylammonium ILs with the Lewis acid properties of ZnO, a catalyst successful employed in depolymerization processes (e.g. on polyethylene terepththalate) [15], developing an efficient and recyclable bifunctional NBu4Cl/ZnO-NPs catalyst useful for depolymerization of polycarbonate waste material. In this study, ZnO nanoparticles (ZnO-NPs) were synthesized by a hybrid electrochemical-thermal process [16] and the influence of calcination temperature on catalytic performance was evaluated.
Section snippets
Materials and general procedure
Unless otherwise stated, all manipulations were carried out in air. Solvents, metal oxides, nucleophilic reactants (alcohols, amines, phenols, diols, glycerol, diamines and aminols) and additives (BuNH2, pyridine, ILs), were Aldrich or Fluka products and were used as received. Bisphenol A-Polycarbonate (BPA-PC) with an average molecular mass of 45,000 was purchased by Sigma-Aldrich.
Reaction products were identified by IR, GLC–MS and 1H NMR, by comparison with the literature data or with
Synthesis and characterization of ZnO nanocatalyst
ZnO-NPs were prepared according to a two-step process developed in our laboratory [16]. In the first step, jellified nanocolloids consisting of zinc hydroxide-carbonate were electrochemically synthesized under mild and green conditions, in the presence of an anionic stabilizer PSS in sodium bicarbonate aqueous solution. In place of zinc(II) salts used in hydrothermal methods, a sacrificial anode made of pure Zn (see experimental part) was employed as a source of Zn2+ ions. This process proved
Conclusions
Chemical depolymerization has to be considered the most promising technique for recycling waste polymers and plastics, the sole allowing the conversion of plastics into their monomers or other valuable products. Alcoholysis and aminolysis are conventional procedures for chemical recycling of polycarbonates. With them it is possible not only the recovery of the starting monomer (BPA), but also the use of the polymer as carbonylating agent for the synthesis of valuable compounds, without wasting
Acknowledgment
We thank University of Bari, Ministry of University of Italy (MIUR) and Regione Puglia (“PON Ricerca e Competitività” 2007–2013–Avv. 254/Ric. del 18/05/2011, Project PONa3 00369 “Laboratorio SISTEMA”) for the financial supports.
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