Abstract:

Polypropylene accounts for about 25% of plastic waste, and its selective conversion into high-value aromatic feedstocks is crucial for promoting a circular economy and achieving carbon neutrality. Although metal-zeolite catalysts have been shown to promote the catalytic conversion of polyolefins to aromatics, the reaction path of polypropylene cracking on the outer surface of molecular sieves and the effect of metals on the cracking reaction are still unclear. Here, metal-zeolite catalysts with selectively exposed external surface sites were designed to investigate the catalytic cracking behavior of the key intermediate 2,4-dimethyl-1-heptene on the catalyst's external surface. The results show that metals can significantly enhance the cracking degree of branched olefin intermediates on the zeolite external surface, leading to the diffusion of small olefin molecules into the zeolite micropores, where further aromatization occurs. This process improves the utilization of aromatization sites within the micropores. Under optimized conditions, the yield of aromatics from the catalytic conversion of polypropylene reaches 73%, with a sum yield of benzene, toluene, and xylene of 65%. This study establishes a relationship between the structure of metal-zeolite catalysts and the cascade polypropylene-to-aromatics process, offering valuable insights for designing highly efficient catalysts for the chemical recycling of polypropylene into aromatics.

Key words: polypropylene, zeolite, catalyst, metal-acid catalysis, aromatics, multiphase reaction

摘要：

废弃塑料中聚丙烯约占25%，将其选择性转化为高值芳烃原料对于推动循环经济和实现碳中和目标具有重要意义。尽管金属-分子筛催化剂已被证明能够促进聚烯烃催化转化为芳烃，但聚丙烯在分子筛外表面裂化的反应路径及金属对该裂化反应的影响仍不明确。本研究通过构筑选择性暴露外表面位点的金属-分子筛催化剂，研究了聚丙烯转化过程中主要中间物种2，4-二甲基-1-庚烯在催化剂外表面的催化裂化行为，揭示金属能够显著增强富含甲基取代基的烯烃中间物种在分子筛外表面的裂化深度，生成的小分子烯烃物种扩散至分子筛微孔中，进一步发生芳构化反应，从而提高微孔内芳构化位点的利用效率。在优化条件下，聚丙烯催化转化制芳烃的总收率为73.5%，其中三苯收率为65.6%。本研究建立了金属-分子筛催化剂结构与多步反应性能之间的关联机制，为化学回收聚丙烯制芳烃的高效催化剂设计提供了理论依据和技术支持。

关键词: 聚丙烯, 分子筛, 催化剂, 金属酸催化, 芳烃, 多相反应