Have we solved the plastic recycling problem?

A new nickel-based catalyst could revolutionize the plastic recycling process

Recycling seems easy enough. Dump it in the recyling bin, take it out to the curb, and all of it dissapears the next morning when the garbage collectors come through. However, at the recycling facility, things are not so simple. One of the largest impediments to achieving efficient and cheap plastic recycling is the sorting process. Most plastic recycling methods today involve some form of heating or melting, which means that the plastics need to be sorted before as different types have different melting points, properties, etc. Even if a little food residue or a plastic of another type gets mixed in, the entire batch can be compromised and sent to the landfill. Luckily, scientists at Northwestern University have recently discovered a new nickel-based catalyst that has the potential to eliminate the entire sorting process, which would revolutionize the recycling industry.

Polyolefin plastics consisting of polyethylenes and polypropylenes make up almost two-thirds of global plastic consumption today. Look in your fridge, and it's almost certain you'll have something made of polyolefin in there: from squeeze bottles to plastic wrap to juice cartons. Most notably, this plastic is made up of molecules linked together by carbon-carbon bonds, which are very strong and hard to break. Traditional methods to recycle polyolefins either require tedious sorting beforehand or huge amounts of energy to melt the plastics.

To combat this problem, the team of scientists turned to hydrogenolysis, a process that utilizes hydrogen gas and a catalyst to break down platics. With polyolefins, this process converts these low-value wastes into liquid oils and waxes (more generally, hydrocarbons), which can then be converted into higher-value products such as fuels and candles. What really makes using this nickel-based catalyst unique is that it specifically targets carbon-carbon bonds, which makes it effective at separating mixed polyolefins--such as branched polyolefins (like isotactic polypropylene) and unbranched polyolefins (like polyethylene)-- that would otherwise be difficult to separate mechanically.

Another important benefit of this new process is the chemical stability of the nickel-based catalyst. Normally, when PVC, a type of plastic, is heated, it releases hydrogen chloride gas that significantly disrupts the hydrogenolysis process. The new catalyst, however, is so chemically and thermally stable that the release of these gases from PVC actually speeds up the decomposition process. What's more, this procedure involving a nickel-based catalyst allows this process to be done at 100 degrees lower, half the hydrogen gas pressure, and using ten times less catalyst, saving lots of energy. The catalyst itself can also be recycled and prepared for use again simply by treating it with inexpensive alkylaluminium.