Olefin metathesis - as a degradation route for PE?

Normally when I hear of olefin metathesis in connection with polymers, it is regarding polymerization (such as ring-opening metathesis polymerization (ROMP)). So I was surprised the other day to read a paper where metathesis was used to depolymerize a polymer, and not just any polymer, but a polymer lacking in olefin groups - polyethylene.

The report appears in Science Advances (open access) and shows off some clever tricks. The polyethylene is dissolved in a light alkane (naptha or similar) and then a dehydrogenation is carried out on both the polyethylene and the solvent. After that, the metathesis can happen.

A metathesis reaction is one where there a cross-exchange between two different chemicals. An example would be A-X + B-Y → A-Y + B-X. For olefin metathesis, the A and Y are on either side of a double bond, as are the B and X. And the same is true for the A-Y and the B-X, so the reaction is A=X + B=Y → A=Y + B=X. where A=X is the partially dehydrogenated polyethylene and B=Y is the partially hydrogenated light alkane. If the double bond is near the center of the PE molecule, you are able to pretty much cut the molecular weight in half in just one reaction. Since the molecules are dehydrogenated in multiple locations, the PE can quickly be reduced to very short chains by allowing the reaction to repeatedly occur. And all the while, the degradation products are completely soluble in the solvent.

The technique works for the whole spectrum of PE, from M_w = 3350 daltons to ultra-high molecular weight (M_v = 1.7 x 10⁶ daltons), as well as LDPE and LLDPE (no surprise there, but glad they checked) as well as on PE that had antioxidants compounded into it. Would it work for polypropylene? Polystyrene? PVC? Inquiring minds want to know!

The statistics of this reaction are intriguing to think about. This degradation reaction is actually more akin to a condensation reaction (run in reverse, of course) than the addition reaction that created the PE, but there is so much more. Is there an optimal level of dehydrogenation (as a function of MW, MWD, branching...)? Is there is an optimal light alkane mix? Would having alkenes already in it help or hurt? Modeling this could be quite a bit of fun.

The use of the end product as a fuel is suggested by the authors, who strongly believe that it is an economically feasible route, much better than anaerobic pyrolysis. No numbers are provided however. I won't get into criticizing them at this point, since this is just a first discovery and with just 56% yield, there is a lot of work ahead for someone. But this process and its simplicity seems promising and I would encourage the researchers to push on.

Previous Years

June 23, 2011 - Older workers

June 23, 2010 - Skewing the results - heavily

June 23, 2009 - If anybody dares quote Paul Simon...

Polyethylene Production coming to North Dakota

While my homestate of Minnesota remains a frack-free zone (due to Mother Nature's choice to stock us with iron, copper and other minerals rather than even a drop of petroleum), our neighboring state of North Dakota is pretty much ground zero for the effort. And we hear about it a lot since many people have taken the day's drive out there for the good paying jobs. Western North Dakota is not highly populated, so manpower is short. If you can pass a drug test and supply your own housing, you can get a job. (And many hiring managers will say under their breath that they might let one of those conditions slide). Minnesota is also affected by the transportation of the flammable, petroleum liquids along the rail lines in our state. And the sudden increase in demand for the trains causes other logistic nightmares for anyone shipping anything else by rail.

Now comes a report from Plastemart that North Dakota will soon have their own world class polyethylene production facility - a $4 billion dollar investment. $1.5 million metric tonnes a year of HDPE - from North Dakota. That is pretty impressive.

In my mind, it was just a matter of time before this happened, although I've not heard anything previous about it unlike the plant being built in West Virginia to take advantage of the Marcellus shale production. At the same time, I bet that this plant will be far more expensive than the original estimate for many of the reasons I already discussed. The labor to build the plant will need to be imported - there aren't too many pipefitters out there and they are already kept busy with the existing fracking operations. And the housing shortage will only increase. While transporting polyethylene by train is much less risky than transporting petroleum liquids, it is not as efficient. The bulk density of polyethylene is about 0.5 g/cm³, a good fraction less than any hydrocarbon liquids. So that means more strain on the train network.

Looking at the very-long-term picture, at some point fracking production will dry up, and so the question be what happens to the facility. Will it be abandoned or will it continue to operate, albeit with a biobased source of ethylene, such as that produced by dehydration of ethanol? North Dakota isn't a very large corn producer, but over the coming decades, alternative biofeedstocks for ethanol will be developed, including some that could be raised in the dry regions of North Dakota. Either way, I'm prety sure that I will not see that future. It's too many decades down the road. (Yes, fracking will go on that long.)

Previous Years

October 15, 2013 - Turning Plastic Bags into Carbon Nanotubes