SYSTEMATIC APPROACHES IN GREEN CHEMISTRY

SYSTEMATIC APPROACHES IN GREEN CHEMISTRY

1. At most basic level: Pollution Prevention (P2)
- Improved operational practices
(lowering energy consumption, improving yields)
- Batch to continuous processing?
(batch process may increase yield but continuous process may save energy)

2. Development of greener processes to manufacture same chemical products:
(Green Chemistry – Level 1)
- Use intermediates that come from better primary feedstocks
(Ex. Production of ethylene glycol)
- Avoid chlorine compounds if chlorine is not in the final product
(Ex. Production of allyl alcohol)

3. Formulation of alternative products for same use:
(Green Chemistry – Level 2)
- CFC substitutes

4. Avoidance of chemicals:
- Switch from herbicides and pesticides to genetic engineering (trade-offs?)
(Ex. Monsanto’s “NewLeaf Plus” potatoes genetically improved to resist the Colorado
beetle and leaf roll virus)
- Switch to organic farming
- Switch from traditional to digital photography
- Use colored plastics instead of painted metals
- Switch from petroleum fuels to biofuels or hydrogen
ALSO: Use of chemistry for improved environmental performance elsewhere:
- Molecular markers in plastics for easier identification during recycling
- Design of chemical detectors to monitor environmental quality

Production of allyl alcohol (CH₂=CHCH₂OH)
Traditional route: Alkaline hydrolysis of allyl chloride, which generates the product and hydrochloric acid as a by-product:
CH₂=CHCH₂Cl + H₂O → CH₂=CHCH₂OH + HCl
Greener route, to avoid chlorine: Two-step using propylene, acetic acid and oxygen
CH2=CHCH3 + CH3COOH + 1/2 O2 → CH2=CHCH2OCOCH3 + H2O
CH2=CHCH2OCOCH3 + H2O → CH2=CHCH2OH + CH3COOH
Added benefit: The acetic acid (CH3COOH) produced in the second reaction can be recovered and used again for the first reaction, leaving no unwanted by-product.

APPROACHES TO GENERIC CASES

1) If reaction is of the type
A + B → P + W
where A and B are feeds, P is the desired product and W a waste by-product:
- Procure A and B made from renewable sources and clean processes
- Find alternate A and/or B feeds to avoid or decrease amount of W
- Find alternate A and/or B feeds to create a different W, which is a useful by-product
- Find substitute for P that does not entail the co-production of W.

2) If the primary reaction is in competition with a secondary reaction, of the type
A + B → P
A + B → C
where A and B are feeds, P is the desired product and C a competing waste by-product:
- Find alternate A and/or B feeds to avoid the competing reaction.
Example: Chlorination of water by disinfection. Chlorine oxidizes the pathogens to the point of killing them but simultaneously forms harmful chlorinated organic compounds. Remedy is to use another oxidant besides chlorine (Cl2), such as ozone (O3)

3) If the primary reaction is followed by an undesirable secondary reaction, of the type
A + B → P
P → D
where A and B are feeds, P is the desired product and D a decay product from P:
- Adjust temperature and/or pressure to favor the first reaction but impede the second.
- Find catalyst that would speed the first reaction, so that product P can be harvested before much of it has decayed.
- Find a more stable substitute for P.
Example: Production of ethylene oxide, a precursor in the production of ethylene glycol (antifreeze)
CH2=CH2 + 1/2 O2 → H2C-O-CH2 → H2O + CO2
Inhibitors, such as halogenated organics (somewhat problematic, however) can be added to slow down the decay of ethylene oxide.

4) If the reaction requires a harmful catalyst
A + B + C → P + W + C
where A and B are feeds, C a harmful catalyst, P the desired product, W a by-product:
- Find a substitute catalyst.
- Find an alternate path to make P.

Example: Production of the analgesic ibuprofen by the Friedel-Craft alkylation catalyzed by aluminum trichloride (AlCl3). Aluminum trichloride is far from a perfect catalyst and decays significantly in the process (4 kg of catalyst are required to produce 5kg of product), generating acidic gaseous emissions (HCl). Remedy is to use hydrogen fluoride as a substitute catalyst. This catalyst does not decay and can be easily separated from the product mix and recycled back into the process.

BIOCATALYSIS

In biocatalysis, enzymes and antibodies are used to mediate reactions.
Biocatalysis may involve the use of whole living micro-organisms or of enzymes that are separated from the cell and immobilized in a support medium. In order words, entire cells or cell components are used as micro-engines. Reactions that use biocatalysis often proceed with exceptionally high selectivity. In some cases, they have also been shown to increase reaction rates between 9 and 15 orders of magnitude in comparison with uncatalyzed reactions.