Waste plastics turned into hydrogen and CNTs

A MULTINATIONAL team of chemical engineers has developed a pyrolysis process to transform waste plastics into hydrogen and carbon nanotubes.

Plastic makes up around 10% of municipal solid waste and in Europe, around 38% of this is landfilled. Many researchers are seeking alternative uses for waste plastics. Paul Williams at the University of Hull, UK led the team which developed the pyrolysis-catalytic reforming process, which effectively makes carbon nanotubes as a byproduct of hydrogen production, and “significantly” increases the economic feasibility of the process. They note that carbon nanotubes are presently worth around US$20,000/t.

The team used waste polypropylene, ground into particles around 1 mm in diameter, and tested two different catalysts, based on nickel, manganese and aluminium, which they prepared themselves. NiMnAl 444 contained equal molar ratios (4:4:4) of the three metals, while NiMnAl 424, as the numbers suggest, contained nickel, manganese and aluminium in a 4:2:4 ratio.

The pyrolysis-catalytic steam reforming reaction of the polypropylene was carried out in a two-stage reaction system. They placed 1 g of the waste plastic in the first reactor and 3 g of catalyst in the second reactor. Nitrogen gas provided an inert atmosphere. The reactor containing the polypropylene was heated to 500?C to pyrolyse the plastic. The products of this passed into the reactor containing the catalyst, at 800?C, into which water was injected to provide steam. The reaction time was 40 minutes. The carbon nanotubes deposited onto the catalyst, while the gaseous products were collected.

The two catalysts gave slightly different results. NiMnAl 444 gave a yield of 57.7 wt% of carbon and 34.6 wt% gaseous products, of which 75.62% was hydrogen, while NiMnAl 424 gave a yield of 46.6 wt% carbon and 71.2 wt% gaseous products, of which 70.79% was hydrogen.

The carbon was removed from the catalyst particles using aqua regia (nitro-hydrochloric acid), before being washed with deionised water and dried overnight. Around 90% of the mass of carbon with both catalysts was carbon nanotubes, although NiMnAl 444 was slightly higher at 91.2%, compared to 89.7% with NiMnAl 424. However, the carbon nanotubes produced by NiMnAl 424 were slightly more uniform.

The researchers tested the efficacy of the carbon nanotubes produced in strengthening virgin low density polyethylene (LDPE). The composite plastics produced had significantly better mechanical properties than the virgin LDPE. The NiMnAl 424 nanotubes were the most effective due to their uniformity. The tensile strength of the composite, containing just 2% carbon nanotubes, was 15% higher than the virgin LDPE and the flexural strength 19% higher.

“Cost-effective CNTs could be produced from waste plastics as by-products of the production of hydrogen, enhancing the potential applications of CNTs in the composite industry,” say the researchers.

Process Safety and Environmental Protection DOI: 10.1016/j.psep.2016.07.001