Influence of Trace Metal in Plastics on Catalyst Activity\nand Reusability during Polyolefins Upcycling with Polyoxometalates

The\ninfluence of trace metals in waste plastics on catalyst activity\nand reusability has received scant attention in the plastics upcycling\nliterature. Catalytic cracking of high-density polyethylene (HDPE)\nwas demonstrated over unsupported acidic W-based polyoxometalates\nin an up-flow fixed-bed reactor using flowing nitrogen to recover\nthe cracked hydrocarbon products. The product distribution depends\non the catalyst prereduction method, the reactor temperature and the\ntemperature in the reactor effluent reflux zone. For recycling the\ncatalyst, we harnessed the aqueous solubility of polyoxometalates\nto dissolve and separate the spent catalyst from coke. Prereducing\nthe catalyst mitigates coke formation by substrate (HDPE) dehydrogenation\nand eases catalyst recovery. On a freshly H<sub>2</sub>-prereduced\ncatalyst, we observe cracking activity at temperatures as low as 573\nK. At reactor and reflux temperatures of 623 and 313 K respectively,\nwe observe ∼80% yield of predominantly paraffinic hydrocarbons\nwith an average formula of C<sub>21.9</sub>H<sub>45.9</sub>. We discovered\nthat Cr traces present in the HDPE substrate progressively accumulate\nin the recycled catalyst and increase cracking activity. After five\ncatalyst recycles, a Cr accumulation of ∼137 ppm in the catalyst\nshifts the product distribution to lower hydrocarbons with an average\nformula of C<sub>15.1</sub>H<sub>31.9</sub>. Higher Cr content reduces\nthe stability of the Keggin structure in the strongly reducing environment\nof H<sub>2</sub>. In sharp contrast, catalyst prereduction by olefins\ninstead of H<sub>2</sub> can preserve the structural integrity. Electron\nparamagnetic resonance (EPR) spectroscopy studies and computational\nresults support the hypothesis that both the incorporation of Cr,\nwhich is much harder than W and introduces an odd number of <i>d</i> electrons (as either olefin or PE reduction does), as\nwell as the extent of reduction of the catalysts may affect the atomic\nand electronic structures of the heteropolytungstates, influencing\ntheir acidity and activity. These fundamental insights highlight challenges\nposed by trace metals in plastics upcycling.