Green Chemistry: Using green solvents and green methods for synthesis

Alternative solvents

·         water and water-based systems

·         supercritical fluids

·         liquid organic salts

·         derivatized and immobilized solvents

·         solvent free systems 

Water as solvent - pros and cons

·         Cheap -but purification/destruction costs high

·         Nontoxic, inflammable, odourless

·         Decreases emissions: VOC and CO2 (combustion of solvents )

·         Poor solubility into organic phases: easy separation

·         Unique solvent properties

·         Observing leaks in a process? Recovery of leaks?

·         High boiling point and high heat of vaporization

Targets of use: separations, coatings and syntheses (homogeneous and two phase processes)

Maturity of technology: in industrial use and in R&D stage 

Water as solvent: 2-phase hydroformylation

·         separation of catalyst and products is easy

·         a more feasible process

·         lower investment- and operation costs

·         environmental aspects

·         nontoxic, inflammable

·         Observation of leaks: water smells like butanal!

·         Ligands that react with heavy metals/water soluble ligands that contain heavy metals: transportation risks!

·         not suitable for longer chain alkenes

Supercritical solvents

Definition:Supercritical fluids are substances that are heated over the critical temperature (Tc) of the compound and pressurized over the critical pressure (pc) of the compound. The resulting solvents possess the density of liquids and the viscosity of gases. Solvent properties can be altered with minor changes of temperature and pressure  Most common in use:scCO2 and scH2O

Supercritical solvents - pros and cons

·         hazardous solvents can be replaced

·         solvent properties can be tuned

·         no mass transfer limitationsscCO2

·         Target of use for CO2!

·         cheap, inflammable, nontoxic

·         easy separation of catalyst, products and solvent

·         no corrosion

·         dissolves well nonpolar and even polar substances when surface active additives are applied

·         temperature and pressure requirements reasonable: 31 °C and 73 bar

scH2O

·         Quite nonpolar, but acidic

·         Extremely corrosive

·         Requires demanding conditions: 374 °C and 218 bar

Areas of use: separations/extractions, dispersions, syntheses, chromatography, waste water treatment

Maturity of technology: extractors and analytical equipment in industrial use, use as solvent in reactions under R&D

Liquid organic salts

Definition:Salts that possess low melting point and are liquids under room temperature. Typically contain quaternary ammonium cations and inorganic anions.

·         Low volatility

·         Easy recycling

·         Some salts can function as solvent and catalyst

·         Generally mild reaction conditions

·         Easy separation of products

·         Toxicity yet unclear

·         Costs yet unclear

Areas of use: dimerization of alkenes, Friedel-Crafts

Maturity of technology: R&D stage, high expectations: revolution of synthetic chemistry methods!