Impact of biogenic impurities on catalytic synthesis of multi-walled carbon nanotubes and hydrogen from polyolefin resins

Abstract

Biogenic impurities are common contaminants of plastic waste. This study investigated the effects of biogenic impurities on the conversion of a polyolefin mixture (6 wt% low-density polyethylene, 24 wt% high-density polyethylene, and 70 wt% polypropylene) into multi-walled carbon nanotubes (MWCNTs) and hydrogen via pyrolysis-chemical vapor deposition (CVD). Biogenic components from diverse sources were added to the polyolefin mixture, including lignocellulosic materials (pine sawdust, coconut coir pith), biosolids from wastewater treatment (municipal sewage sludge), animal-based waste (crustaceans), and bio-based plastic (polylactic acid). At contamination levels up to 9.1 wt%, 3 out of 5 impurities (coconut coir pith, sewage sludge and crustaceans) caused a decline in the conversion efficiency of feedstock into MWCNTs (33 ± 7% for polyolefins only compared with 20 ± 6%, 16 ± 1% and 16 ± 2% for polyolefins with 9.1 wt% coconut coir pith, sewage sludge and crustaceans, respectively). According to gas analysis, impurities led to a loss of catalyst activity, resulting in a decreased concentration of H2 and increased concentrations of methane and ethylene in the CVD gas. Furthermore, for polyolefins containing 9.1 wt% coconut coir pith, sewage sludge, crustaceans and polylactic acid, microscopy data revealed changes in the growth of filamentous carbon, leading to the appearance of carbon nanofibers in MWCNT samples. On the contrary, biogenic impurities barely changed outer diameters, ID/IG ratios, interlayer spacing, crystallite sizes of graphite-like sites, and the surface chemistry of MWCNT samples. The results suggest the development of quality management system for contaminated plastic feedstock could be advantageous to ensure consistent outcomes of pyrolysis-CVD process.