Scientists use Zeolites to Recycle Plastics into Jet Fuel

Stack of plastic bottles for recycling. Image credit: iStock

In a world where the dependence on fuel is increasing and so as our waste production, an efficient solution where waste can be transformed into a usable material is a valuable commodity.

Recently, engineers from Washington State University (WSU) have found a novel way to recycle plastics, specifically polyethylene, into useable jet fuel hydrocarbons.

The researchers have employed a microwave-assisted catalytic method which breaks down the carbon backbone of polymers into useable alkanes found within the jet-fuel range.

With jet fuels set to rise by up to 27% soon, this research has provided a novel avenue for specific alkane production. The team utilized an activated zeolite to catalyse the reaction.

Stack of plastic bottles for recycling
Stack of plastic bottles for recycling. Image credits: iStock
ZEOLITES

Zeolites are aluminosilicate microporous materials, which can occur both naturally and synthetically. Synthetic zeolites produced in the laboratory generally hold better separation properties than their natural counterparts.

Zeolites are a fantastic material for the separation of materials, because the pores can be synthesized with a defined size and charge by simply tuning the molecular structure around the pore.

The size of the pores in a zeolite is governed by the number of units in the lattice, and the charge can be tuned by tweaking the silicon to aluminium ratio around the pore- A higher aluminium content produces a polar pore, and an apolar pore for a higher silicon content.

ZSM-5

Zeolite Socony Mobile (ZSM) is the most common class of synthetically produced zeolites. ZSM-5 has found the most widespread use in industry. The ZSM-5 unit cell is composed of eight five-membered rings. This is known as a pentasil unit.

The microporous molecular structure of a zeolite, ZSM-5. Image credits: Wikipedia

Each pentasil ring consist of 10 silicon and aluminium atoms bridged by oxygen species. The pores act as ideal channels for the separation of branched and unbranched alkanes.

In fuels, branching can lower the octane number, making the fuel less effective. The pores filter out the branched alkanes by size exclusion to leave pure fuel-ready linear alkanes.

TWO-STAGE PROCESS TO ISOLATE HYDROCARBONS FROM PLASTIC

The researchers created a two-stage process to selectively isolate the specific hydrocarbon products. The first one is a catalytic microwave degradation process. The low density polyethylene pellets were placed into a quartz flask and transferred the contents into a microwave oven.

The reaction was performed at 350 ˚C for 20 minutes, until all the polymeric material had vaporized. The vaporized polymer gas was then passed over a packed-bed reactor containing ZSM-5 at 375 ˚C.

The second stage utilized a nickel-catalysed hydrogenation step to breakdown the unsaturated hydrocarbons. The hydrocarbons were mixed with n-heptane (the linear structural isomer of heptane) and placed in a sealed reactor with the catalyst, at 200 ˚C.

The researchers found that different catalyst-to-feed mass ratios of the catalyst produced hydrocarbons of different fuel grades. A ratio of 0.1 gave a yield of 66.18%.

After the second stage, the fuels produced a navy grade fuel known as JP-5. A ratio of 0.2 yielded a mass of 56.32%. This ratio was found to produce two types of high density military jet-fuel known as RJ-5 and JP-10.

POTENTIAL FOR COMMERCIALIZATION

Although these methods are in their infancy, there is a huge potential for commercialization specially seeing the increasing jet fuel price graph. The team is confident that their method will provide a “novel and feasible pathway for refineries to produce different grades of jet-fuels”.

Apart from alternate and cheap source of fuel production, any new method designed to remove waste from our surroundings is a worthwhile endeavour- especially for materials that don’t readily degrade in nature.

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Source Rsc.org

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