Low-Temperature Hydrocracking of LDPE to Narrow-Distribution C₃H₈-C₅H₁₂ Alkanes over Precision-Impregnated Ni/HZSM-5

In this work, a 5 wt.% Ni/HZSM-5 catalyst is prepared via precision impregnation and applied in the low-temperature hydrocracking of low-density polyethylene (LDPE), realizing the directional conversion of LDPE into narrow-distributed C₃ (propane)-C₅ (pentane) alkanes using a simple catalytic system. Experimental results show that the LDPE conversion reaches 99.0%, the C₃-C₅ yield is 85.4 wt.%, and the selectivity toward the target products exceeds 99.0%. The catalyst exhibits excellent stability with no obvious deactivation after 5 recycling runs. The catalyst preparation parameters and reaction conditions are experimentally optimized, and the optimal preparation protocol is determined: HZSM-5 with a Si/Al ratio of 25 is used as the support with a Ni loading of 5 wt.%, followed by calcination at 550 ℃ for 4 h and reduction in a hydrogen atmosphere at 450 ℃ for 4 h. Hydrogen radical regulation experiments using C₆H₁₄ and CH₃OH as model compounds confirmed that the polymerization of methyl radicals (CH₃·) contributed to the high selectivity, and hydrogen radicals (H·) mainly promoted the formation of small-molecule intermediates and stabilized the formation of alkanes from intermediates with carbon chain lengths of 3-5. This catalytic mechanism is proven to be universal for both LDPE and polypropylene. This work provides an efficient non-noble metal catalytic system and theoretical support for the directional conversion of waste polyethylene.