Engineers at Washington State University (WSU) have found a novel way of recycling old plastics into useable jet fuel hydrocarbons. The researchers have employed a microwave-assisted catalytic process that breaks down the carbon backbone of polymers into alkanes in the jet fuel range.
In a world where the dependence on fuel is increasing and our waste production is increasing, an efficient solution where waste can be transformed into a usable material is a valuable commodity. With the need for jet fuels soon set to rise, this research has provided a potential new avenue for specific alkane production.
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The researchers utilized an activated zeolite to catalyse the reaction. Zeolites are aluminosilicate micropo
rous materials. Zeolites can occur both naturally and synthetically. Synthetic zeolites 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. The pores are also tuneable. The size is governed by the number of units in the lattice around the pore. The charge can also be tuned by tweaking the silicon to aluminium ratio around the pore.
Zeolite Socony Mobile (ZSM) is the most common class of synthetic zeolite produced. ZSM-5 is the most widely used zeolite in industry today. The ZSM-5 unit cell is composed of eight five-membered rings. This is known as a pentasil unit. Each pentasil ring consists of 10 silicon and aluminium atoms which are 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 size selective pores filter out the branched alkanes to leave pure fuel-ready alkanes.
The researchers have created a two-stage process to selectively isolate the hydrocarbon product. The first method is catalytic microwave degradation. Low density poly ethylene pellets were placed in a quartz flask and transferred to 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 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 were JP-5, a navy grade jet fuel. A ratio of 0.2 yielded a mass of 56.32%. This ratio produced RJ-5 and JP-10 fuels, which are high density military jet fuels.
Although these methods are in their infancy, the potential for commercialization is huge. The researchers are confident that their method will provide a “novel and feasible pathway for refineries to produce different grades of jet fuels”.
Aside from fuel production, any new methods designed to remove waste from our lives is a worthwhile endeavour.