(298c) Hydro Cracking of Waste Plastics to Naphthenes over Hierarchical Y Zeolites: From Catalyst Synthesis to Process Development

Polyolefins represent 36% of global plastic production and are widely used daily due to their durability, non-toxicity and chemical inertness ¹. However, the same characteristics make them the most challenging class of plastics to recycle. Globally, polyolefins account for 53% of plastic waste and even a small percentage of polyethylene (PE) gets recovered through recycling (~14% in 2015) ^2,3. Thus, the non-biodegradable and petroleum-derived unrecycled plastics result in significant economic loss and raise serious environmental concerns ⁴. Also, current repurposing approach via recycling and/or downcycling has a detrimental effect on the properties of recycled materials ⁵. Therefore, governments and global organisations are focusing on more effective ways to simultaneously address the ecological and economic challenges of plastics.

Photo reforming ⁶, depolymerization ⁷, deconstruction ¹, gasification ⁸, thermal ⁹ or catalytic pyrolysis ^9,10 and hydrocracking ¹¹ have been exploited. However, low evolution rate (typically in mmol g^−1) during photo-reforming and low-value products (i.e., waxy products) during depolymerization make both processes inefficient and somewhat limited in the scope of waste plastics management. Similarly, high temperature requirements during gasification (600-700 ℃) and pyrolysis (500-800 ℃) and high content of olefins with products instability and prone to low-value gas products during thermal and catalytic pyrolysis lead these scenarios to some industrial issues. Contrary to that, upcycling of plastic through hydrocracking into high quality fuel significantly accumulates plastic wastes problem ^12,13. Hydrocracking enables a circular economy by utilizing plastics as a feedstock to produce fuels that mitigate the requirement for ongoing feedstock procurement and could transform significant economic losses into benefits with energy savings and reduced greenhouse gas emissions.

Hydrocracking over a bifunctional metal/acid catalysts under a reductive atmosphere is appealing since the acid support can crack carbon–carbon bonds together with the hydrogenation of intermediates over metal loading and prevent catalytic coking ¹⁴. However, current methods and catalysts are not yet practicable for implementation due to their relatively poor yields, less acidity, low porosity and external surface area, high energy and prolonged time requirements. Therefore, it’s the need of the hour to develop a catalytic technique which creates a balance between acid and metal sites for high product yields, requires low energy and tunes product distribution for various applications.

Here, commercial zeolite supports i.e., faujasite (FAU) type Y is utilized as a base catalyst. Various novel, energy-efficient, time-saving and environmentally friendly techniques have been investigated to improve the accessibility of the active sites within zeolite frameworks through hierarchical modification of zeolites. Although efficient, steaming at high temperatures is generally an energy-intensive and time-consuming process whereas acid leaching significantly produces a large quantity of acidic and toxic effluent, which limits the sustainable properties of the resulting hierarchical zeolites ¹⁵. Also, these processes cause some noticeable changes in the acidic properties of hierarchical zeolite depending on the applied routine of dealumination, because of the adjustments in Si/Al proportion and crystallinity ^16,17. Therefore, to enhance the porosity of zeolite framework, two novel, energy-efficient and time-saving techniques are studied and results are compared with the steaming and acid-leaching approaches. The fabricated hierarchical zeolite framework retained the crystallinity, acidity and showed enhanced selectivity of liquid products in the gasoline range fuels with better affinity towards iso-paraffins and naphthalene.

Therefore, the present study highlights the catalytic hydrocracking route for the management of virgin as well as post-consumed HDPE which potentially produces renewable liquid fuels. Ni loaded zeolite Y and its respective hierarchical zeolites with enhanced porosity and external surface area were synthesized and characterized by low and wide angle XRDs, TEM, FTIR-py, DRS UV-vis, N₂-physiorption, and H₂-TPR. Hydrocracking experiments were performed in a 300 ml stirred batch Parr reactor (series 4560 mini reactors) at 10-30 bar initial cold H₂ pressure at a temperature of 350-400 ^oC for a reaction time of 15-60 min and with a feed to catalyst ratio of 20:1.

Compared to the pristine Y zeolite, Ni incorporated Y zeolite showed better conversion and selectivity to oils due to hydrogenation ability, and additional Lewis acidity of Ni loaded samples. Similarly, hierarchical modification of the Y zeolite led to a considerable improvement of the conversion and selectivity to liquid products, which were further boosted to >99% by the addition of Ni. The increase in the external surface area and mesoporous volume that improved the bulk polymer molecules diffusion, combined with the uniformly dispersed small Ni particles, were at the origin of the observed behaviour. Based on the GC-analysis, all Ni loaded hierarchical Y zeolite samples showed remarkable selectivity towards lighter hydrocarbons (C₅-C₁₂) with a little affinity towards diesel range fuels which was shifted to 100 % gasoline range at 30 bar H₂ pressure. Similarly, the Ni loaded composite samples showed enhanced production of aromatics and naphthenes with reduced selectivity of n-paraffins. Compared to virgin HDPE, all the Ni loaded hierarchical Y zeolites showed similar conversion and selectivity over post-consumed HDPE milk bottles. Although, there was some decline in activity due to coke deposition and Ni sintering after multiple reaction runs, however, the zeolites performance could be almost completely recovered to their initial activity through thermal regeneration.

Moreover, the advantages and challenges of hydrocracking of real waste plastics streams over other linear end of life (i.e., incineration) and circular end of life (i.e., pyrolysis) were quantified by conducting techno economic and life cycle assessment. The study was focused on the process simulation which identify the key pathways to enhance the impact of each process for a better plastics circular economy. From the sustainable perspective, life cycle assessment and techno economic analysis showed the benefits of hydrocracking over pyrolysis and incineration.

Therefore, comparative analyses showed that produced hierarchical zeolites through novel techniques have dramatically increased catalytic activity for catalysis involving large substrates, like catalytic cracking and hydrocracking of polymers into narrower gasoline range fuels with minimum environmental impacts. Overall, the present study proposed a promising route to government organizations, industries and stakeholders to achieve ‘Net Zero’ plan and to execute better plastic waste management option in the long run.

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