One of the largest sources of carbon, CO2, is emitted as a pollutant and is attributing to the rising climate temperatures. So then why are we not using CO2 as a feedstock? Well, for one thing is it very very difficult. CO2 is incredibly stable as it is a thermodynamic sink. Therefore, It requires even more energy to convert it into something else, and where does all of energy come from? Fossil fuels. Thereby generating even more CO2 to convert CO2 into something useful. But if we can find a means for converting or reducing CO2 into a commodity through a less energy intensive pathway, then there is potential to generate revenue from “waste” and reduce emissions.
Currently, the US generates 5,500 million metric tons/yr of CO2. Industries are currently capturing and using CO2, approximately 200 million metric tons/yr in the food industry and oil and gas industry, but a majority of this is released back into the atmosphere. Only about 0.5% of the CO2 that is captured is sequestered and not released.
We can think about carbon/CO2 utilization in two categories, 1.) carbon sequestration (burying in deep geological formations) and 2.) carbon as a useful feedstock. I am focusing on the second categories because investigations into into using CO2 as a freely available and abundant feedstock to develop commercial chemicals, plastics, and building materials has the potential to be an economically viable industry. Additionally, carbon sequestration has its limitations, and although it is being heavily federally funded, it’s large scale deployment has estimates in the range of $30-70/ton attributed to the new CO2 transmission lines that will need to be built. There are instances where CO2 cannot be transported to sequestration sites.
One of my favorite examples being investigated for CO2 utilization to generate commercial chemicals is the oxidative coupling of CO2 with ethylene to generate acrylic acid with molybdenum catalysts. This is work done at Brown University in the lab of Dr. Bernskoetter.
Acrylic acid is used heavily as the raw material for polymers, coatings and adhesives. Global production of acrylic acid is 3.4 million metric tons/yr and with 60% by weight CO2, that is over 2 million metric tons/yr of CO2 that could be resold and prevented from entering the atmosphere. The production of acrylic acid through a more economically viable method would be advantageous, so much so that Dow has begun similar efforts to generate acrylic acid through the generation of 3-hydroxypropionic acid with the use of a biocatalyst. They claim that their process is 25% cheaper and 75% less greenhouse gas intensive.
The current process for acrylic acid production is the oxidation of propene and is incredibly energy intensive because it not only requires reaction temperatures of 200 – 300 C but also multiple distillations to remove impurities. Dr. Bernskoetter’s catalysts can oxidatively add CO2 and ethylene slightly above, if not close to, room temperature. However, at the moment, the biggest challenge is the reductive elimination of the hydroxide to release acrylic acid from the metal. But once that can be done, unlike the use of catalytic microbes, organometallic catalysts can more easily (and usually cheaply) be modified to improve upon turnover rates and efficiency. I am especially excited and looking forward to Dr. Bernskoetter’s next publication on this catalyst.
One of my many interests is to read about alternative processes that turn waste into meaningful/reusable materials. For example: dog poop into methane to light a lamp. or turning plastic into fuel. In an earlier post I outlined a few attributes as to why perhaps we cannot completely do away with plastic. But what we can do is to find a means to have it not sit in landfills forever. Therefore, naturally, this article in the NYTimes about Reaping Oil From Discarded Plastic caught my attention. An Oregon-based start-up, Agilyx, has a system that can turn plastic into crude oil. They only have a small prototype system at the moment and are looking to start selling commercial systems soon. From the descriptions of the article, we figured that polyethylene was undergoing an incomplete combustion, then the resulting vapor (H2 and CO) is converted into hydrocarbons that can then be converted to diesel, jet fuel or other forms of oil. Pretty cool, right?! Well, we did some calculations with the numbers that were in the article and although a very good idea, it only about 64% efficient by weight. Which is comparable to other processes and actually might work, considering the input source is, in essence, free, if it is in fact coming from the local waste management. This process could perhaps be a way for municipal waste to fuel their own fleet. If they had the $5 million upfront to invest in the system.
* Special thanks to my labmate for the helpful discussion and unit conversion reminder. (thanks to article for not having easily interchangeable units.)
The polymers that can be synthesized through ROMP have a lot of industrial applications. One of the most famous is Norsorex, or polynorbornene, which is a shock-absorbing materialused in protective equipment, sound insulation, and vibrational damping. Through a single monomer, ROMP can access structures normally difficult copolymerize with individual monomers. One example is a perfectly alternating copolymer of 1,4-butadiene and isoprene.
As with any other chemical reaction, there are limitations. ROMP’s limitation is that the monomers must be cyclic and possess sufficient ring strain. Although it might be difficult to synthesize momomers with these desired attributes, it is possible to derivatize naturally existing materials. Last year, the Larock group developed biorenewable-based thermosets from the ring-opening metathesis polymerization (ROMP) of fatty alcohols derived from soybean oil and castor oil.# Of course, this work illustrated the robustness and versatility of Grubbs’ catalyst (like so many papers), but what is key is that it also highlighted the substantial role that metathesis catalysts can have in the development of polymers from biorenewable feedstocks as the pressures of traditional petroleum-derived feedstocks grows.
Well, that is a great place to end. The thought about using renewable feedstocks not just for energy but also for carrying out chemistry is an important direction to consider. Actually, a reaction very similar to ROMP that I have not expanded is, ROP (Ring-Opening Polymerization) and that polymerization is responsible for the material in biodegradable “corn pens”(polylactic acid).
1. Polymers have a narrow mass distribution. Meaning that the Gaussian distribution curve of polymer lengths is narrow indicating that a majority of the polymers are similar in length.
2. No premature chain termination. This helps to achieve the first point. If the growth of the polymer chains are cut off at different times then there will be a wider distribution of polymer lengths. This point also allows us to build copolymers. Once all of the first monomer is consumed and a second monomer is added, the polymerization continues adding in the second monomer.
3. All the chains start fast and at the same time.
We have all seen the pictures of coastlines littered with plastics bottles and bags and we also cringe at the thought that these materials will persist for many years after they are disposed. However, it is also not reasonable to do away with plastics completely as their inexpensive production and ease of disposal are a staple to sterile environments such as hospitals. Yet, these fleeting applications for polymers are not appropriate for their persistence in our environment. In the last decade the chemical community has been working on the development of biodegradable polymers that can degrade through chemical hydrolysis or enzymatic processes. The applications for such polymers are when they are soiled by food or other biological substances and cannot be recycled. The benefits of these polymers are that they are generated from renewable resources and then degrade into CO2, CH4, water, biomass, and other natural substances in a short amount of time.
Last week, a study, published in Environmental Science and Technology, carefully modeled the methane release of rapidly degrading polymers in landfills to gauge the benefits of biodegradable polymers. The researchers conclude that the polymer that they looked at (poly(3-hydroxybutyrate-co-3-hydroxyoctanoate (PHBO)) degraded and released a significant amount of methane too fast to be sufficiently captured by landfills with gas collection systems. Methane is a concern because like CO2 it is a greenhouse gas but is over 20 times more effective in trapping heat in the atmosphere. Their suggestion therefore were to develop biodegradable polymers that degraded slower to account for the delay in the gas collection systems to reach a critical concentration of GHG emissions to operate.
There are a couple of questions concerning this conclusion, as the authors only analyzed a single biodegradable polymer. There are other more common biodegradable polymers like poly(-e-caprolactone) PCL or poly(lactic acid) (PLA). PCL, in landfill reactors, degrades at a much slower rate than PHBO. Also there have been studies that have shown that biodegradable characteristics depend on the type of polymer and degradation conditions.
Although the authors of the paper do have a good point to have manufacturers of these types of polymers to consider the environmental impact of their products even after disposal. They put it best at the conclusion of their paper, I am paraphrasing: If the emissions for producing a biodegradable material is comparable to that of producing a material from petroleum-based feedstocks, and yet the disposal emissions is higher for the biodegradable materials, is it then a viable alternative?
But also this paper is a reminder for the need to create an infrastructure for biodegradable polymer disposal. Now that worldwide consumption of these types of polymers perhaps exceeds 68 million kg, we should worry about its disposal emissions. A possibility that was not considered in the paper was to analyze composting conditions for biodegradable polymers. A recent study in Korea compared the aerobic and anaerobic environments of poly(caprolactone) and poly(butylene succinate). They concluded that pretreatment technologies coupled with composting would be a means to decrease the waste of biodegradable polymers. This is important for them as the Korean government recently encouraged substituting biodegradable polymers for non-biodegradable polymers.
In addition to considering the disposal emissions of our materials, we should consider an infrastructure for disposal of biodegradable materials that would reduce or mitigate the emissions from these materials as they are an advantage because that do not persist in our environment and are made from non-petroleum-based (renewable) feedstocks.