(116d) Chemical Upcycling of Polyethylene Using Heterogeneous Alkane Cross-Metathesis

High/low density polyethylene (PE) represents the largest portion of the plastic waste stream and a major challenge in chemical recycling due to stability of the C-C bonds that make up the backbone of these polymers. In the effort of developing effective and economical chemical upcycling processes, catalytic alkane metathesis chemistry comprising tandem (de)hydrogenation–olefin metathesis has been investigated as a novel approach due to two favorable factors for economic viability of the recycling process: its moderate operating temperature and absence of a reactive gas—i.e., H₂. However, long reaction times, poor catalyst stability, a narrow operating temperature range, and the need for expensive metal components (Re) remain important challenges. Here, we demonstrate that WO_x/SiO₂ (WS) catalyst can successfully substitute rhenium oxide for the olefin metathesis stage of the tandem reaction in n-hexadecane (n-C₁₆; a surrogate of PE) alkane-metathesis reaction at 300 °C and shorter reaction time (2 hours) in a 50mL batch reactor.

The high olefin metathesis reactivity of WS catalyst is revealed by the results of the 1-hexadecene (1-C₁₆=) metathesis reaction. The outcome is a broad distribution of olefin products in which selectivity toward olefins of carbon number lower and greater than 16 is 45% and 55%, respectively (Figure 1a). For the alkane metathesis reaction, the simultaneous use of WS and Pt/γ-Al₂O₃ (PA) proved effective for converting a mixture of n-C₁₆ and n-C₇ to alkane compounds of lower carbon number (< C₁₆) with a selectivity of 85% (Figure 1b). A product yield of 165% (n-C₁₆ basis) establishes that n-C₇ participates in the reaction as feedstock through self or cross-metathesis and, consequently, decreases the average molecular weight of overall alkane products. These results validate the premise that the alkane metathesis reaction using tungsten oxide is an attractive approach to PE upcycling.