Plastic waste is expected to reach 25 billion tons in 2050, so it is urgent to develop diversified waste plastic recycling technologies. As the main component of plastics, polyolefins have inert C- C bonds, and it is difficult to control their conversion to target products. Existing technologies rely on organometallic, noble metal catalysts and complex treatment processes, facing industrialization challenges. Emerging technologies show great industrial potential through emerging methods for converting polyolefins to fuel-grade hydrocarbons. However, the H/C ratio of alkanes in fuels is higher than that of polyolefins (-2.2-2.3 and 2.0, respectively). Thus, this process requires an external source of H2 or a hydrogen-rich co-reactant to facilitate the conversion. For example, high pressure hydrogen is used to convert polyolefins to liquid fuels over noble metal catalysts. Systems that do not require an external source of hydrogen are particularly attractive to industry, but are currently only capable of producing long-chain aromatics or are limited in terms of low conversion and production of volatile mixtures. Therefore, there is an urgent need to develop efficient, robust and economical processes for the conversion of polyolefins to fuels.

Based on the above problems, Academician Han Buxing of the Institute of Chemistry of the Chinese Academy of Sciences, Researcher Lin Longfei, Professor Han Xue of Beijing Normal University, Professor Yang Sihai of Peking University and others published a paper entitled "Upcycling of polyethylene to gasoline through a self-supplied hydrogen strategy in a layered self-pillared Chemistry" in Nature zeolite, A LSP-Z100 self-column molecular sieve (self-pillared zeolite) is reported to catalyze the conversion of polyethylene to gasoline at 240°C and 4 h with a selectivity of 99% and a yield of 80%. The main products in the liquid phase of the synthesis are branched alkanes (72% selectivity) with an octane number of 88.0, which is very close to that of commercial gasoline (86.6). Through inelastic neutron scattering spectroscopy, small angle neutron scattering spectroscopy, solid state NMR, XAS, isotope labeling and other characterization techniques, it is shown that the tri-coordinated Al site of the molecular sieve contributes to the activation of polyethylene molecules, followed by the cleavage of β-chemical bonds and isomerization at the Brönsted acid site, and hydride transfer occurs at the Al site by providing hydrogen by itself to selectively generate branched alkanes. This study shows that the layered self-column molecular sieve catalyst has great application prospect in the field of plastic waste recycling. The first author of the article is Cen Ziyu, a doctoral student in the Institute of Chemistry.

The researchers successfully synthesized a LSP-Z100 molecular sieve with a self-supporting layer and MFI/MEL intergrowth structure by hydrothermal method using tetrabutylaminohydroxide as a structure directing agent, which has a large external surface area and mesoporous structure.

Fig.1 Structure characterization of catalyst©2024 Springer Nature

The high density polyethylene was mixed with the catalyst at a mass ratio of 5:1, and then heated at 240 ℃ for 4 h in N2 atmosphere. The researchers studied LSP-Z100, HZSM-5, HY, USY and other microporous molecular sieves and LSP, meso-HY, MCM-41, SBA-15 and other mesoporous molecular sieve catalysts, and found that only LSP-Z100 had catalytic activity.

2 Catalytic activity and stability of LSP-Z100©2024 Springer Nature

3 Study on catalytic active sites©2024 Springer Nature

Based on in-situ INS characterization and DFT theoretical calculations, the researchers proposed a LSP-Z100 catalytic reaction mechanism.

Fig.4 Reaction mechanism©2024 Springer Nature

In summary, the LSP structure provides the material with oFTAl sites and accessible Brönsted acid sites, which promotes the realization of the SSH mode. Through the combined action of hydrogen extraction on oFTAl sites, β-cleavage/isomerization on Brönsted acid sites and hydrogen transfer on oFTAl sites, gasoline is generated with high selectivity and yield. In addition, LSP molecular sieves exhibit excellent catalytic performance for the production of commercial-grade gasoline from low-density and high-density PE waste (about 25% of today's plastic waste). The potential profits of this approach make it economically attractive. Compared with the traditional oil-based route, the PE-based route to produce gasoline reduces carbon emissions. The combination of low energy inputs, inexpensive, precious metal-free and highly stable catalysts, no need for an external hydrogen source, and the direct use of the product as a transportation fuel, minimizing environmental impact, provides a promising solution for mitigating future plastic pollution through a "waste to chemical" strategy.

原文详情：Upcycling of polyethylene to gasoline through a self-supplied hydrogen strategy in a layered self-pillared zeolite (Nat. Chem. 2024, DOI: 10.1038/s41557-024-01506-z)

This article is contributed by the elder brother of the soldier.