Definition of Green Chemistry

Designing Chemical Processes to Reduce Hazards

Green chemistry means designing chemical products and processes to reduce the use or formation of hazardous substances from the start. The word "green" describes that goal. It does not refer to a material's color or require plant-based ingredients.

Green chemistry applies across the life cycle of a chemical product, from design and manufacture to use and final fate.

Green chemistry begins with process design. Waste treatment remains important after waste forms. Before the process runs, green chemistry evaluates whether the starting materials, solvent, energy use, catalyst, and final product can be made safer.

Preventing Waste during Process Design

Waste treatment reduces harm after waste has formed. Green chemistry changes the materials or process before production so less hazardous waste forms in the first place. EPA calls this source reduction. Green chemistry therefore evaluates the starting materials, solvent, energy use, catalyst, product, and end of use.

In a chemical reaction, the product is not the only part that matters.

Process part	Green chemistry question
Starting materials	Are these materials hazardous, scarce, or replaceable with safer choices?
Solvents and auxiliaries	Is the solvent truly needed, and is there a safer option?
Energy	Does the reaction need very high temperature or pressure, or can it run with less energy?
Product

Here, a hazardous substance includes flammable, explosive, corrosive, toxic, persistent, and pollution-forming substances, even when they are not immediately deadly.

Design Reactions to Reduce Hazards

Chemical reactions can be designed to perform a useful function while reducing hazards, waste, and energy use.

Water electrolysis shows why a green chemistry assessment must include the energy source. The process splits water into hydrogen and oxygen.

\begin{aligned} &2\mathrm{H_2O(l)} \\ &\xrightarrow{\text{electrical energy}} 2\mathrm{H_2(g)} + \mathrm{O_2(g)} \end{aligned}

This reaction does not directly produce carbon dioxide. Its life-cycle emissions depend on how the electricity is generated. Electrolysis can have very low emissions when powered by renewable or nuclear electricity. Electricity from fossil-fuel power plants gives the process higher life-cycle emissions.

Methanol from Carbon Dioxide

Methanol is used as an industrial feedstock and fuel. Conventional production often starts from fossil feedstocks. One process under development reacts carbon dioxide with hydrogen over a catalyst.

\begin{aligned} &\mathrm{CO_2(g)} + 3\mathrm{H_2(g)} \\ &\xrightarrow{\text{catalyst}} \mathrm{CH_3OH(l)} + \mathrm{H_2O(l)} \end{aligned}

$\mathrm{CO_2}$ is relatively stable, so methanol synthesis from $\mathrm{CO_2}$ needs energy input and an efficient catalyst.

Using $\mathrm{CO_2}$ does not by itself make the methanol process green. Evaluate the other inputs and outputs as well:

Where does the $\mathrm{H_2}$ come from?
Where does the reaction energy come from?
Is the catalyst safe and durable?
Are by-products and waste handled safely?

A green chemistry assessment covers the materials, energy, hazards, products, and waste from the start of the process to the end of use.

Checking Green Claims

When a product or process is described as "greener", check its materials, energy source, hazards, waste, function, and end of use before judging the claim.

Common claim	How to check it
The material is natural	Natural does not always mean safe. Check toxicity, dose, and waste.
The waste can be treated	Treating waste reduces harm after it forms. Preventing waste reduces the amount that must be handled.
It uses renewable energy	Check whether the whole process uses that energy or only a small part.

The twelve green chemistry principles include waste prevention, atom economy, safer solvents, catalysts, energy efficiency, and design for degradation.

Use those principles to check both the reaction design and the final product.