Mechanism investigation of acid-tailored USY zeolites via ion exchange in polypropylene cracking

Precisely tailoring the location and strength of Brønsted acid is crucial for the catalytic cracking of polyolefins. Here, by simple sodium cation exchange (xNa-USY, where x is the Na/Al ratio), it was unexpectedly discovered that xNa-USY (x < 0.5) displayed a lower cracking temperature of polypropylene (PP) with optimum performance and excellent reproducibility at x = 0.38, rather than pure H-USY, as generally believed. Combining spectroscopic and microstructural characterizations, a putative mechanism was proposed from the perspective of proton location and acid strength. When lacking strong Brønsted acid near site II, PP tended to cleave into longer alkane fractions at low temperatures and volatilized directly, exhibiting significant apparent cracking capacity. Meanwhile, the lower distribution of olefins and aromatics remarkably reduced coke generation at high temperatures. These findings not only shed light on the optimization of plastic degradation catalysts but also expand our knowledge of the zeolitic acid-catalyzed cracking process.