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

The influence of trace metals in waste plastics on catalyst activity and reusability has received scant attention in the plastics upcycling literature. Catalytic cracking of high-density polyethylene (HDPE) was demonstrated over unsupported acidic W-based polyoxometalates in an up-flow fixed-bed reactor using flowing nitrogen to recover the cracked hydrocarbon products. The product distribution depends on the catalyst prereduction method, the reactor temperature and the temperature in the reactor effluent reflux zone. For recycling the catalyst, we harnessed the aqueous solubility of polyoxometalates to dissolve and separate the spent catalyst from coke. Prereducing the catalyst mitigates coke formation by substrate (HDPE) dehydrogenation and eases catalyst recovery. On a freshly H2-prereduced catalyst, we observe cracking activity at temperatures as low as 573 K. At reactor and reflux temperatures of 623 and 313 K respectively, we observe ∼80% yield of predominantly paraffinic hydrocarbons with an average formula of C21.9H45.9. We discovered that Cr traces present in the HDPE substrate progressively accumulate in the recycled catalyst and increase cracking activity. After five catalyst recycles, a Cr accumulation of ∼137 ppm in the catalyst shifts the product distribution to lower hydrocarbons with an average formula of C15.1H31.9. Higher Cr content reduces the stability of the Keggin structure in the strongly reducing environment of H2. In sharp contrast, catalyst prereduction by olefins instead of H2 can preserve the structural integrity. Electron paramagnetic resonance (EPR) spectroscopy studies and computational results support the hypothesis that both the incorporation of Cr, which is much harder than W and introduces an odd number of d electrons (as either olefin or PE reduction does), as well as the extent of reduction of the catalysts may affect the atomic and electronic structures of the heteropolytungstates, influencing their acidity and activity. These fundamental insights highlight challenges posed by trace metals in plastics upcycling.