Quasi-Elastic Neutron Scattering to probe the mechanism of catalytic plastic recycling

Currently over 350 million metric tonnes of plastic are produced globally each year, with 55 % being discarded, leading to urban, and aquatic pollution. Low-Density Polyethylene (LDPE), one of the most common polymers, takes centuries to decompose, meaning there is a sprawling glut of polymer waste in the world. In the last 10 years processes such as polymer cracking with solid acid catalysts have been explored to form olefins, and aromatics,[2] which would normally be synthesised from fossil fuels, thus establishing a “circular economy”, limiting waste, and lowering fossil fuel demand. Zeolites (microporous solid-acid catalysts) have been widely used for this process, as many different frameworks exist, allowing delicate control over catalytic reaction pathways. HZSM-5 (MFI framework, pore sizes 5.1 x 5.5 Å, and 5.3 x 5.6 Å) is widely used for LDPE cracking, due to its powerful acid sites, lowering the reaction temperature (Figure 1). However, little is known about the catalytic mechanism of this process, particularly whether the reaction occurs on the surface, or within the pores. In this work we will perform QENS measurements on both pure LDPE, and a physically mixed LDPE-zeolite mixture, to observe changes in the polymers motion, linked with pore infiltration and surface interactions. Measurements will be performed above the melting point of LDPE, but below reaction temperatures, allowing the motions to be captured by the spectrometers.

目前全球每年塑料总产量超过3.5亿公吨，其中55%被直接丢弃，由此引发城市与水生环境污染。低密度聚乙烯（Low-Density Polyethylene, LDPE）作为最常见的聚合物之一，需数百年才能完全降解，致使全球聚合物废弃物堆积成灾。近十年来，学界已探索出以固体酸催化剂催化聚合物裂解制备烯烃与芳烃的工艺[2]——这类产物通常由化石燃料合成，该工艺可构建“循环经济”模式，减少废弃物产生并降低化石燃料依赖。沸石（Zeolites，微孔固体酸催化剂）因具备多种不同骨架结构，可精准调控催化反应路径，因而被广泛应用于该裂解工艺。氢型ZSM-5（HZSM-5，具有MFI骨架结构，孔径分别为5.1×5.5 Å与5.3×5.6 Å）因拥有强酸性位点，可降低反应温度（图1），因而被广泛用于LDPE裂解反应。然而，目前学界对该过程的催化机理仍知之甚少，尤其是反应究竟发生在催化剂表面还是孔道内部。本研究将分别对纯LDPE样品以及物理混合的LDPE-沸石复合物开展准弹性中子散射（Quasi-Elastic Neutron Scattering, QENS）测试，以观测与孔道浸润、表面相互作用相关的聚合物分子运动变化。测试将在LDPE熔点以上、反应温度以下的条件下开展，确保光谱仪可捕捉到聚合物分子的运动信号。