Carbon Dioxide as a Feedstock for Acrylic Acid Production
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.