**2. Synthesis and performance of metal oxide-based catalysts for PET glycolysis**

Clearly, there is a need to consider various factors in designing PET glycolysis catalysts such as catalytic activity, ease of process applicability, and environmental impact. The coordination between a metal cation and the PET carbonyl oxygen is the widely exploited basis of catalytic activity in glycolysis, although other catalytic pathways do exist. Numerous metal compounds have been used as glycolysis catalysts with varying success [6]. Some of these made its way to commercial application through patented technologies [7]. Recently, a significant amount of work in the development of solid catalysts has appeared, potentially addressing the problem of product/catalyst separation.

The first solid catalysts used for glycolysis were zeolites, which showed catalytic performance of only moderate monomer yields (<70%). We have developed various glycolysis catalysts based on metal oxides that were thermally stable and recoverable and provided excellent monomer recovery yields. These are the ideal characteristics of industrially applicable heterogeneous catalysts that could solve issues in existing conventional glycolysis catalysts. We discuss in the following sections several methods used to synthesize them, their properties, and performance as stable and effective glycolysis catalysts. It was possible to obtain free and supported versions of these catalysts using flexible synthesis strategies [10–13] tailored to specific process applications.

## **2.1. Glycolysis reaction set-up and product analysis**

novel catalyst design may offer effective solutions to achieve the goal of a truly green and

**Figure 4.** Comparison of life cycle assessment (LCA) results for PET manufacture and recycling in terms of non-renew‐

The development of hybrid catalysts for polymer electrolyte membrane fuel cells (PEMFCs) has received attention recently due to advantageous qualities in its interaction characteristics and physical durability. The combination of conductivity enhancement and water manage‐ ment becomes widely used to improve PEMFC performance without external humidifying system. In this respect, requirements for next promising materials for PEMFCs include such categories as high proton conductivity, membrane electrode assemblies (MEA), and non-toxic and easy fabrication method. The humidifying ability of MEA in fuel cells is crucial for proton conductivity. As such, the approach to enhance conductivity and allow self-humidification at the same time has been considered recently to improve the performance of PEMFCs [8]. Given these requirements, the potential solution is platinum doping for conductivity and using silica as support for its hygroscopic properties. It is a challenge, however, to increase the platinum doping and enhance catalyst conductivity. With our bio-coating strategy for metal doping, we demonstrate a successful implementation of N-doped carbon composite through a silica-based templating technique [9]. In contrast to other reported methods that use chemical vapor deposition (CVD) and plasma treatment of various nitrogen precursors, our method remains on the practical side requiring no expensive equipment and complex treatment processes.

**2. Synthesis and performance of metal oxide-based catalysts for PET**

Clearly, there is a need to consider various factors in designing PET glycolysis catalysts such as catalytic activity, ease of process applicability, and environmental impact. The coordination between a metal cation and the PET carbonyl oxygen is the widely exploited basis of catalytic

able energy use and equivalent CO2 generated per ton of PET fiber [4].

**1.2. Hybrid catalysts for polymer electrolyte membrane fuel cells**

viable PET chemical recycling.

144 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**glycolysis**

Virgin PET pellets were obtained, mixed with dry ice in a grinder, and reduced to a fine powder with a particle size of <200 μm. The glycolysis reactions were carried out in a 10-mL stainless steel batch-type pressure reactor at 300°C and 1.1 MPa, unless otherwise specified. The reactant mixture of 0.3 g of PET, 1.1 g of EG, and a set amount of catalyst was loaded into the reactor. The reactor was placed in a furnace preheated at the reaction temperature, which, after the given reaction time, was taken out and quenched in cold water to stop the reaction. For quantitative determination of the monomer yield, the reaction products were dissolved in tetrahydrofuran (THF) and analyzed via HPLC. A reverse-phase Zorbax-C8 column and an ultraviolet (UV) detector set at 254 nm were used. The mobile phase used was a 50:50 (v/v) THF/H2O solution at a flow rate of 1.0 ml/min. The molar yield of BHET was calculated based on the following equation:

$$\text{BHET Molar yield, } \% = \frac{\text{moles of BHET produced}}{\text{moles of PET units}} \times 100\%$$

The monomer BHET was separated from the glycolysis products and purified for qualitative analysis. Boiling water was added to the reaction mixture to dissolve BHET and the hot solution was immediately filtered. Repeated washing was done to extract residual BHET in the filter cake. The filtrate was stored in a refrigerator at 4°C for 24 h, after which white needlelike crystals formed. These were filtered and dried at 70°C for 12 h. The dried crystals were then used for various characterization steps, such as FT-IR, DSC, TGA, and NMR to verify the structure and properties of the recovered monomer, properly distinguishing BHET from its dimer and/or oligomers. Its chemical structure was analyzed by 1 H NMR and 13C NMR using DMSO-d6 as the solvent [10].
