Capturing CO2 Emissions with Plastic Waste

Rising CO₂ emissions since the industrial revolution have prompted action by lawmakers and corporations to set optimistic zero carbon emission goals. Improvement of CO₂ capture adsorbents is clearly a large roadblock to deployment of carbon capture, utilization, and sequestration (CCUS). Innovation in both the sorbent materials and processes space is of utmost importance for CO₂ emissions curtailment. While reducing CO₂ emissions is a challenge, another significant concern is plastic pollution. Annually, millions of tons of plastics are disposed improperly each year, with only a small fraction fully recycled by any type of recycling method due to the current limitation in plastic waste recycling technologies. For example, physical recycling is high in cost as it requires human sorting, detergent washing, melting, and reshaping. Furthermore, the plastic produced via physical recycling is lower in grade than virgin plastic and has limited applications. Chemical recycling, on the other hand, is conducted via the thermal cracking of plastic to produce polymer building blocks that are used to synthesize virgin plastic, while also producing lubricating oils and waxes. Thermal cracking is economically viable and produces usable chemicals, but it results on the production of large quantities of carbon char that has no useful applications, making the overall process low in circularity. Clearly, both CO₂ emissions and plastic pollution issues must be addressed to mitigate their environmental damage. The question remains, how can these problems be lessened? 

At Rice University, we have developed a solution to reducing CO₂ emissions while also alleviating the plastic waste burden. By pyrolyzing waste commodity thermoplastics, such as high-density polyethylene (HDPE), low-density polyethylene (LDPE), and polypropylene (PP), in the presence of potassium acetate (KOAc) at 600 °C for 45 minutes, a high surface area porous carbon CO₂ adsorbent can be synthesized. Typically, plastic pyrolysis methods yield an unusable char in addition to distilled monomers, oils, and waxes; however, when thermally treating HDPE, LDPE, and PP with mild KOAc, the unusable char is upgraded to a CO₂ adsorbent. This adsorbent has many properties like other activated carbons, namely good CO₂ uptake capacity, good CO₂ to nitrogen (N2) selectivity, resilience under elevated temperatures, and excellent cycling stability. These properties lend itself to flue gas carbon capture implementation. The optimized reaction temperature and time produce a CO₂-capture material that can bind up to 18% of its weight in flue gas CO₂ at room temperature. Pores of 0.7 nanometers are produced by this process, the perfect size for selective CO₂ capture. This waste plastic-derived sorbent is a highly stable solid carbon material. Our tests demonstrate CO₂ capture stability up to 100 adsorption and desorption cycles when using pressure swings. Temperatures up to 275 °C under air will not cause any significant structural or performance deterioration. Therefore, any issues regarding the traditionally used liquid amine CCUS degradation, volatilization, and corrosivity are avoided. Additionally, the estimated capital and operational expenditures of such solid systems is much lower than when using traditional liquid amines systems. A proposed capture system contains multiple sorbent beds that are either in a capture state or regeneration state. The sorbent actively captures flue gas CO₂ during the capture stage, and this captured CO₂ is driven off by heat, vacuum, or a combination of the two during regeneration. Owing to the low CO₂ desorption temperature of the adsorbent (75 to

110 °C under pure CO₂), hot flue gas could be used to provide some heat to release captured CO₂ during the regeneration mode.

This process, the first of its kind in converting HDPE, LDPE, and PP into a CO₂ adsorbent, contains a high degree of circularity. With minimal modifications to current chemical recycling approaches, plastic waste recycling could be greener than ever. The sorbent allows chemical recycling to be carbon negative by capturing emissions associated with the pyrolysis process if a sorbent capture vessel is attached. When pyrolyzing HDPE with KOAc, any polymer not converted to porous carbon decomposes and distills out, forming the monomers, oils, and waxy byproducts. We demonstrated the distilled waxes can be mixed with KOAc again to form a comparable CO₂ adsorbent. These waxes could also be further recycled into lubricants, detergents, or virgin PE via closed or open loop chemical recycling. Virgin PE could be fed back into the sorbent synthesis process at the end of its product life to make more adsorbent. Another circular feature of this process is the use of nonhazardous chemical activating agents.

Chemical activation of carbon materials often use hazardous phosphoric acid (H3PO4) or potassium hydroxide (KOH). Both chemicals are corrosive and difficult to handle. In the specific case of KOH, the full benefits of using KOH as an activating agent are typically gleaned above 700 °C; however, flammable potassium (K) metal also forms above 700 °C. KOAc, which thermally decomposes into environmentally friendly potassium carbonate (K2CO3) and acetone byproducts at 430 °C, acts as a plastic carbonization substrate at 600 °C and greatly reduces the risk of flammable K metal formation. Rather than forming K metal, K2CO3 remains stable until 730 °C. Typically, 12% of the plastic waste converts into an activated carbon. The environmentally friendly salt KOAc, which finds use as a deicer, has no known human toxicity or corrosive behavior. Further, the K2CO3 can be used to form KOAc and re-used as a chemical activator after it is rinsed from the activated carbon.

Through our process described here, the upcycling of HDPE, LDPE, and PP waste into a CO₂ sorbent with good uptake capacity, CO₂/N2 selectivity, cycling, and thermal stability is now attainable. Minimal waste can be produced from this process since non-carbonized plastic waste could be further recycled into useful chemicals. Used virgin plastic and distilled waxes could eventually be reincorporated back into sorbent production. Additionally, the CO₂ footprint of pyrolysis processes could be lowered by the CO₂ sorbent. Incorporation of KOAc into thermal cracking processes of commodity thermoplastics will lead to greener and more circular chemical recycling.

Author: Paul E. Savas1,
Wala A. Algozeeb, PhD.1,2, and
Professor James M. Tour1,3,4


AFFLICATION(S)
1. Department of Chemistry, Rice University, 6100 Main Street, Houston,
Texas 77005, United States
2. Research and Development Center, Technical Services Division, Saudi Aramco, Dhahran, Saudi Arabia
3. Department of Materials Science and NanoEngineering, Rice University,
4. Smalley-Curl Institute, The NanoCarbon Center and the Welch Institute for Advanced Materials, Rice University, 6100 Main Street, Houston, Texas 77005, United States

REFERENCE
1. Algozeeb, W.A.; Savas, P.E.; Yuan, Z.; Wang, Z.; Kittrell, C.; Hall, J.N.; Chen, W.; Bollini, P.; Tour, J.M. Plastic Waste Product Captures Carbon Dioxide in
Nanometer Pores. ACS Nano. 2022, 16, 7284–7290.

Printed in Issue 2 of Carbon Capture Magazine