**1. Introduction**

Depleting resources and waste management are the major driving factors for environmental conservation practices, such as recycling and development of alternative energy. Plastics recycling and generation of renewable energy, in particular, are among the direct approaches that aim to curb the consumption of oil-derived commodities. Research efforts on these areas can contribute to the realization of long-term environmental sustainability, aiming for increased productivities and minimal environmental impact. In these efforts, the development of effective catalysts to improve their performance remains a major goal in optimizing processes for resource conservation. Taking both catalytic activity and practical synthesis into account, we present catalyst design strategies that truly conform to principles of green catalysis.

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## **1.1. Chemical recycling of poly(ethylene terephthalate)**

Poly(ethylene terephthalate) or PET is currently the world's most widely recycled plastic. Due to its extensive use in various applications, largely in packaging, its worldwide consumption has increased tremendously, adding to the generation of plastic solid wastes. The global market for PET packaging was worth \$48.1 billion in 2014, equivalent to about 16 million tons of PET. Furthermore, the projected demand for PET packaging material is estimated to increase further by 4.6% annually, reaching up to 20 million tons by 2019 [1]. The dramatic increase in PET consumption over the past decades has led to significant efforts to recycle the thermoplastic polyester.

The case of PET recycling is currently a logistically well-established, large-scale recycling in‐ dustry. Bottle recollection rates, for example, have shown a steady increase worldwide, which accounted for at least 2.8 million tons of collected bottles in 2008 (Figure 1). Most of recycled PET flakes produced worldwide are utilized for fiber applications. PET recycling, at present, is largely dominated by physical methods or melt-phase recycling strategies, al‐ though various process routes have been studied and remain open to major advancements for better sustainability [2–4].

**Figure 1.** a) Increase in PET bottle recollection rates in various regions worldwide from 1995 to 2008. b) Use distribu‐ tion of recycled PET flakes.

Significant efforts and renewed interest in chemical recycling routes have resurfaced recently, as stronger calls for long-term sustainability and new process developments in chemical recycling appeared. Extensive reviews on the progress and various aspects of PET chemical recycling are available elsewhere [2–7]. It has been regarded in the recycling literature that chemical recycling offers the most viable approach of sustainable recycling, allowing the recovery of raw materials and other useful chemicals while dealing with long-term waste disposal issues [5]. Figure 2 shows the monomer recovery routes in PET synthesis and chemical recycling. Among them, glycolysis is the simplest and well established, requiring low to moderate process conditions and capital investment (Table 1) [2].

**Figure 2.** PET monomer recovery routes.

**1.1. Chemical recycling of poly(ethylene terephthalate)**

142 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

polyester.

for better sustainability [2–4].

tion of recycled PET flakes.

Poly(ethylene terephthalate) or PET is currently the world's most widely recycled plastic. Due to its extensive use in various applications, largely in packaging, its worldwide consumption has increased tremendously, adding to the generation of plastic solid wastes. The global market for PET packaging was worth \$48.1 billion in 2014, equivalent to about 16 million tons of PET. Furthermore, the projected demand for PET packaging material is estimated to increase further by 4.6% annually, reaching up to 20 million tons by 2019 [1]. The dramatic increase in PET consumption over the past decades has led to significant efforts to recycle the thermoplastic

The case of PET recycling is currently a logistically well-established, large-scale recycling in‐ dustry. Bottle recollection rates, for example, have shown a steady increase worldwide, which accounted for at least 2.8 million tons of collected bottles in 2008 (Figure 1). Most of recycled PET flakes produced worldwide are utilized for fiber applications. PET recycling, at present, is largely dominated by physical methods or melt-phase recycling strategies, al‐ though various process routes have been studied and remain open to major advancements

**Figure 1.** a) Increase in PET bottle recollection rates in various regions worldwide from 1995 to 2008. b) Use distribu‐

Significant efforts and renewed interest in chemical recycling routes have resurfaced recently, as stronger calls for long-term sustainability and new process developments in chemical recycling appeared. Extensive reviews on the progress and various aspects of PET chemical recycling are available elsewhere [2–7]. It has been regarded in the recycling literature that chemical recycling offers the most viable approach of sustainable recycling, allowing the recovery of raw materials and other useful chemicals while dealing with long-term waste disposal issues [5]. Figure 2 shows the monomer recovery routes in PET synthesis and chemical recycling. Among them, glycolysis is the simplest and well established, requiring low to

moderate process conditions and capital investment (Table 1) [2].


**Table 1.** Process and economic considerations for PET monomer recovery routes [2]

The products obtained from glycolysis are the polycondensation monomer of PET, bis(2 hydroxyethyl terephthalate) or BHET (Figure 3) and low molecular weight oligomers, which are purified for subsequent use in PET manufacture or conversion into other chemicals. There have been significant issues, however, in process efficiency and product purification that led to recent efforts in the development of novel catalysts.

**Figure 3.** Glycolysis of PET chain releasing the monomer BHET.

Catalyst use is crucial in glycolysis and those used in earlier studies still required long reaction times despite catalyst application. These include metal acetates, alkalis, sulfates, and chlorides, some of which are those of toxic heavy metals [5]. Aside from efficiency, catalyst separation has been a major drawback in these catalysts together with environmental concerns. Despite chemical recycling being touted as the superior viable recycling route in the long-term, significant process developments are needed before it can truly be considered as a practical and competitive alternative to mechanical recycling. Results of life cycle assessments (LCA) show (Figure 4) that not only chemical recycling incurs greater process costs, but it also contributes to an overall negative impact in terms of a larger net consumption of nonrenewable energy source and environmental effects [4]. Fortunately, recent developments in novel catalyst design may offer effective solutions to achieve the goal of a truly green and viable PET chemical recycling.

**Figure 4.** Comparison of life cycle assessment (LCA) results for PET manufacture and recycling in terms of non-renew‐ able energy use and equivalent CO2 generated per ton of PET fiber [4].

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

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.
