Baby steps to closed carbon cycles, production of aromatics by LDPE pyrolysis under a CO2 atmosphere

[NNadir]

This is just a brief note referring to an interesting paper in today's issue of Industrial & Engineering Chemistry Research, this one: Improvement of Aromatics Selectivity from Catalytic Pyrolysis of Low-Density Polyethylene with Metal-Modified HZSM-5 in a CO2 Atmosphere, Shichang Zhou, Peng Li, Helin Pan, and Yayun Zhang Industrial & Engineering Chemistry Research 2022 61 (31), 11407-11416

The supply of aromatic compounds is primarily from petroleum sources, and although synthetic approaches exist via catalytic hydrogenation of carbon dioxide - albeit in multistep processes - and some non-food biomass sources seem viable, notably lignin, this is an important issue for sustainability.

If polymers can be made from carbon dioxide sources, as is suggested by another paper in this issue of IECR, (Rational Design of SAPO-34 Zeolite in Bifunctional Catalysts for Syngas Conversion into Light Olefins Fanhui Meng, Xiaotong Liang, Lina Wang, Guinan Yang, Xiaoyang Huang, and Zhong Li Industrial & Engineering Chemistry Research 2022 61 (31), 11397-11406) then with this and similar approaches it is possible to imagine a circular carbon cycle.

It's a little arcane, but I found it interesting. From the introduction to the paper:

Social development is closely related to energy consumption. (1) With the increasing level of industrialization, the demand for various energy sources is also increasing. In all areas of our lives, we are inseparable from the use of fossil fuels. (2) Plastic products derived from fossil fuels play a key role in improving modern life. In 2015, about 405 million tons of plastic chemicals were put into use, (3) which was expected to continuously grow by 4% in the coming years. (4) Unfortunately, landfill and incineration techniques are the most common methods of disposing discarded plastic due to the cost of disposal, which generates a serious impact on the ecological environment and economy (such as groundwater pollution and large carbon dioxide emissions). Waste plastics are rich in hydrocarbons, so they can be chemically converted into hydrocarbon fuels or chemicals. Moreover, monocyclic aromatic hydrocarbons are not only important organic chemical raw materials but also a key component of jet fuel. (5) Therefore, it is of great importance to develop high-efficiency waste plastic conversion technology to convert it into light aromatic hydrocarbons with high added value, which is of great significance to solve the problem of environmental pollution caused by waste plastic and its efficient resource utilization.

Pyrolysis is an effective technique for nondegradable polymers dumped in landfills for burial or incineration. (6) Waste plastics, such as polyolefin polymers, will randomly break C–C and C–H bonds at a high temperature under an anaerobic atmosphere. Then, pyrolysis oil rich in aliphatic hydrocarbons with different molecular weights (C5–C40) is generated through H transfer, cyclization, and isomerization reactions. In addition, cracked gas containing small molecular products such as hydrogen and methane and solid residues including paraffin-like organic compounds are also formed.

However, the yield of cracked oil obtained by traditional thermal cracking technology is not high, and the selectivity of aromatics is low. (7) In recent years, to improve the quality and yield of pyrolysis oil, researchers have been working on the development of catalysts and their application in the pyrolysis process of waste plastics.
Metal oxides, activated carbon, ZSM-5 zeolites, (8,9) and their metal-modified counterparts (10) are commonly applied in catalytic pyrolysis. However, these catalysts have the disadvantages of low yield of catalyzed cracked oil, low selectivity of aromatics, poor stability, and easy deactivation...

...Based on the aforementioned analysis, herein, the metal-supported hierarchical pore zeolite catalyst was used to improve the aromatics selectivity in catalytic cracking of LDPE under a CO2 atmosphere. Compared with a N2 atmosphere, more light aromatics selectivity can be obtained under a CO2 atmosphere. The introduction of a CO2 active atmosphere reduces the activation energy of C–H and C–C bonds in alkanes and quickly consumes the H radicals formed on the surface of the catalyst during the aromatization of hydrocarbons. The effect of different supported metal types in HZSM-5 on the aromatization of the catalytic reaction is discussed. Ni/HZSM-5 has the best aromatization effect among all supported metals. Synergistic catalysis between a large number of chain hydrocarbon compounds generated in the LDPE cracking process and the conversion of CO2 realizes the efficient conversion of LDPE to aromatics and the utilization of CO2 resources...

Some pictures from the text:



The caption:

Figure 7. (a–c) Different chemical species in liquid products and liquid yield under CO2 atmospheres with HZSM-5 and the metal-modified HZSM-5 catalyst at different temperatures (450, 500, and 550 °C).

The caption:

Figure 8. (a–c) Different chemical species of gas products produced under CO2 atmospheres with HZSM-5 and the metal-modified HZSM-5 catalyst at different temperatures (450, 500, and 550 °C).

The caption:

Figure 9. (a) BTEX (benzene, toluene, ethylbenzene, xylene) selectivity of liquid products produced under CO2 atmospheres with untreated HZSM-5 and metal-modified HZSM-5 and (b) comparison of aromatics and BTEX produced under a N2 and CO2 atmosphere (500 °C).

Of course, what's needed for this sort of thing is energy, clean energy, specifically thermal energy, heat.

And no, tearing the shit out of old desert ecosystems, nor new deserts created by the conversion of granaries into them via climate change - about which we are doing nothing at all - to make huge collections of mirrors for thermal solar plants is not going to do it. By contrast what is needed is reliable clean energy.

And let's be clear on something OK? Energy that is not reliable is not clean.

Have a nice day tomorrow.