Seven Plastics in the Bin, No Universal Recycler

No catalytic process can selectively depolymerize mixed post-consumer plastic waste streams into monomers or useful chemical feedstocks. Current chemical recycling approaches require pre-sorted, clean feedstocks — but municipal plastic waste is a heterogeneous mixture of polyethylene, polypropylene, PET, polystyrene, PVC, and nylons contaminated with additives, dyes, fillers, and food residue. Pyrolysis of mixed plastics produces low-value tar and char rather than defined chemical products. Achieving selective depolymerization of one polymer backbone in the presence of others — without costly pre-sorting — is an unsolved catalysis problem.

Global plastic production is ~380 million tonnes/year and growing at 3–4% annually. Approximately 80% of all plastic ever produced has ended up in landfills or the environment. Current mechanical recycling handles <10% of waste plastic and degrades material quality with each cycle. A catalytic process that could selectively upcycle mixed plastic waste would capture an estimated $120–150 billion in material value annually and address one of the most visible environmental crises. NSF CHE identifies plastic upcycling as a priority research direction.

Mechanical recycling (shredding, washing, remelting) works for clean single-stream plastics but produces progressively lower-quality material and cannot handle mixed waste. Thermal pyrolysis converts mixed plastics to fuel-grade hydrocarbons but without selectivity — the product distribution is broad and low-value. Solvent-based dissolution/precipitation can separate some polymer types but uses large volumes of hazardous solvents and fails when polymers have similar solubility parameters. Enzymatic depolymerization (PETase variants) works specifically for PET but not for polyolefins, which lack the hydrolyzable bonds enzymes target. Heterogeneous catalytic hydrogenolysis has shown promise for polyethylene alone but is poisoned by chlorine from PVC, nitrogen from nylons, and oxygen from PET — exactly the contaminants present in mixed waste.

Catalysts that can tolerate or selectively process mixed feeds — analogous to how petroleum refining catalysts handle complex crude oil mixtures. Tandem catalytic systems where sequential reactions selectively depolymerize different polymers under progressively different conditions. Alternatively, a low-cost pre-treatment step that converts mixed plastics into a uniform intermediate (e.g., syngas or methanol) that can then be upcycled, bypassing the selectivity challenge entirely.

A student team could characterize the chemical composition of actual municipal plastic waste from a local recycling facility, mapping the distribution of polymer types, additives, and contaminants — generating the kind of realistic feedstock data that lab studies rarely use. Alternatively, a team could test existing catalytic depolymerization conditions (published for pure polymers) on systematically contaminated feedstocks to identify which contaminants are the most damaging and at what concentrations. Relevant skills: polymer chemistry, catalysis, analytical chemistry, chemical engineering.