Green chemistry judges a process not just by yield but by how much
waste it takes to make one kilogram of product. The flask holds the
catalyst (orbiting spheres) turning reagent (blue stream) into product
(growing gold block); everything that doesn't end up in the product
exits as waste (the red stream into the drum) — mostly spent solvent
and excess reagent.
E-factor = mass(waste) / mass(product)
PMI = mass(total inputs) / mass(product) = E-factor + 1
- Catalyst type — homogeneous catalysts dissolve with the product and are hard to recover (raises waste); heterogeneous and enzyme catalysts can be filtered out and reused.
- Solvent greenness — water/ethanol-like solvents (high) vs. chlorinated/aprotic ones (low); low greenness inflates PMI even at constant yield.
- Temperature — higher temperature speeds the reaction but raises energy use per kg of product.
- Catalyst loading — more catalyst per mole of substrate raises conversion rate but adds to the waste stream if it's not recovered.
Real processes report E-factor and PMI alongside yield because a
98%-yield reaction run in a huge excess of hazardous solvent can still
be far worse for the environment than an 85%-yield reaction in water.