Plastics are ubiquitous in people’s daily lives, providing a multitude of advantages to the society. They play a vital role in raising living standards and promoting economic growth, from boosting food safety and medical care to advancing technology and infrastructure [
1,
2]. Unfortunately, most of the plastics are disposable, non-biodegradable, and from non-renewable petrochemical sources [
3,
4]. Plastic wastes are estimated to exceed 25 billion tonnes by 2050, posing a significant threat to the global environment and ecology [
5]. The recycling of plastic wastes shows an ultimate solution for turning waste into treasure [
6]. Recently, Han and coworkers reported upcycling of polyethylene (PE), one of the largest produced plastic waste, to gasoline with a remarkable selectivity of 99% and yields of > 80% [
7]. This report represents an important breakthrough in this field due to overcoming the challenge of selectively breaking the inert C(
sp3)−C(
sp3) bonds of polyethylene without using any noble metal catalysts and external hydrogen under mild conditions (Fig.1).
The technological innovation described by Han and coworkers about turning polyethylene into gasoline using a layered self-pillared zeolite (LSP-Z100) and a self-supplied hydrogen (SSH) strategy demonstrates a significant advancement (Fig.1(b)). This approach is distinguished from others by several novel characteristics. The creation of the LSP-Z100 zeolite, which has a special layered self-pillared structure, is the main novelty. Due to their layered structure, LSP zeolites have several strong Lewis acid sites called open framework tri-coordinated Al sites (OFTAl). This gives them a leg up when it comes to activating the inert C−H bonds of PE which allows them to deliver hydrogen internally. The surface area and accessibility of active sites, which are essential to the catalytic process, are increased by this innovative catalyst design. By encouraging the activation of the inert C−H bonds of PE using a novel SSH pathway, the LSP-Z100 facilitates a very efficient conversion of polyethylene into gasoline. This procedure effectively breaks the difficult C(
sp3)−C(
sp3) bonds present in polyethylene, which is frequently a major obstacle to the recycling of plastics. This process comprises hydrogen being transferred from PE or oligomers to iso-alkenes through hydride abstraction, β-scission, and isomerization. Similar catalytic procedures have historically depended on external hydrogen sources and precious metals like palladium or platinum to fuel the reaction [
8‒
13]. The product is commercial-grade gasoline, which contains less than 2 wt.% alkenes and has a high research octane number. Consolidating their tremendous industrial potential, the affordable LSP-Z100 zeolites exhibit an outstanding catalytic stability.
The study on PE depolymerization over LSP-Z100 at 240 °C demonstrates the high efficiency of the catalyst in converting polyethylene into valuable hydrocarbons. Initial reactions show the formation of alkanes and alkenes with alkanes reaching a yield of 75% (Fig.2(a)). Secondary cracking and aromatization occur between 4 to 24.5 h. 13C NMR (nuclear magnetic resonance analysis) of solid residues indicates partial cracking and internal hydrogen transfer (Fig.2(b)). Elemental analysis confirms this by showing shifts in carbon and hydrogen content (Fig.2(c) and Fig.2(d)). Minimal coke formation (0.58%) and efficient cracking are evidenced by thermogravimetric analysis. Small-angle neutron scattering (SANS) studies reveal changes in mass fractal dimensions, reflecting reaction progress (Fig.2(e) and Fig.2(f)). Recycling fresh PE with reaction residues maintains a high gasoline yield (> 70%) across multiple cycles highlighting LSP-Z100s robustness (Fig.2(g)). The catalyst exhibits an excellent structural stability and a minimal degradation after repeated use (Fig.2(i)), making it a promising solution for sustainable fuel production from plastic waste.
In general, the process of plastic wastes recycling may be complicated and expensive because of the use of noble metals and the infrastructure that is necessary to safely manage them. These challenges are eliminated by the LSP-Z100 zeolite which obviates the necessity for these materials. Conversely, the method employs an SSH approach in which the essential hydrogen is internally generated from the polyethylene, substantially streamlining the operation, and diminishing financial expenditures. The catalytic process described not only attains an exceptional selectivity of 99% for gasoline but also accomplishes this with a yield exceeding 80% in a mere 4 h at 240 °C. Considering the superior quality of the gasoline with a research octane number of 88.0 which is comparable to commercial grades, these metrics are particularly remarkable. For industrial processes to be economically viable, high selectivity and yield are indispensable. Therefore, this technology is promising for large-scale applications. This technique is a viable alternative for sustainable plastic waste management and energy production due to its enhanced catalytic efficiency, higher selectivity and yield, robust thermal stability, and significant economic and environmental benefits. Through the integration of these cutting-edge features, this process not only offers an attractive fuel conversion pathway for plastic waste, but also makes contributions to an industrial process that is more cost-effective and sustainable.
To summarize, this technology plays a significant role in waste reduction from an environmental standpoint as it converts plastic waste, a substantial pollutant, into a valuable commodity. Utilizing waste as a feedstock has the potential to result in cost reductions in the procurement of raw materials for fuel production from an economic standpoint. However, it is crucial to emphasize the significance of investigating and incorporating other technologies. The integration of solar-driven processes into the upcycling of plastic waste presents a transformative approach to achieving sustainable and environmentally friendly chemical production. Photocatalysis involves the use of solar energy to activate a catalyst, generating electron-hole pairs that facilitate redox reactions. Photothermal processes convert solar energy into heat, driving thermally activated chemical reactions. Both mechanisms offer significant advantages in terms of sustainability and operational cost reduction. Photo-thermal and solar-driven processes are particularly noteworthy in the field of plastic upcycling. Utilizing solar energy, a renewable and plentiful source, to fuel these transformations improves the sustainability of the upcycling procedure and diminishes the carbon emissions linked to conventional thermal techniques. Ongoing research and collaboration are crucial to overcome these challenges and take full advantage of plastic waste recycling and upcycling and promote a circular economy.
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