Environmental, green and energy chemistry

Green solvents (water, ionic liquids, supercritical CO₂)

A practical framework for choosing reaction media by combining solvent properties, health and environmental hazards, recovery, and process performance.

1 Lower secondary 2 Upper secondary 3 Undergraduate 4 Advanced 5 Research

IntuitionWhy the solvent matters

A reaction flask may contain far more solvent than product. Choosing a medium is therefore a process-design decision: consider hazard, persistence, energy for heating and separation, recovery, and performance—not just whether the liquid is renewable.

A “green” solvent is not a universal label. Compare the full use case, including worker exposure, fire risk, emissions, aqueous waste, solvent recovery, and life-cycle impacts; a safer solvent that requires excessive energy or a difficult work-up may not improve the whole process.

Select temperature and raise pressure; observe the steep density rise near the critical region. At fixed temperature, the pressure control tunes density and thus often solvent strength; the marked critical pressure is 73.8 bar.

SchoolComparing solvent burdens

Definition: Process mass intensity (PMI)

PMI counts all material inputs per mass of isolated product, including reagents, solvents, water, and processing aids. State the boundary and whether recovered material is counted as a fresh input; comparisons are meaningful only on a consistent basis.

\mathrm{PMI}=\frac{m_{\text{all inputs}}}{m_{\text{isolated product}}},\qquad E=\mathrm{PMI}-1

Example: A solvent-heavy work-up

A campaign isolates 10 kg of product using 15 kg of reagents, 120 kg of organic solvent, and 45 kg of water and auxiliaries. Find PMI and the solvent fraction of total input mass.

Solution

Total input is 180 kg, so PMI = 180/10 = 18 kg input per kg product (a dimensionless ratio). Solvent is 120/180 = 0.667, or 66.7% of input mass. Recovering solvent can reduce fresh demand, but energy, losses, and purge treatment must be included.

UndergraduateSolvent properties and molecular interactions

Polarity has several meanings. The Kamlet–Taft solvatochromic descriptors distinguish dipolarity/polarizability (π*), hydrogen-bond donor acidity (α), and hydrogen-bond acceptor basicity (β). They help interpret solvent effects on rates and spectra, but are empirical descriptors—not a single universal polarity number.

\log k = A_0+s\pi^*+a\alpha+b\beta

Hansen solubility parameters resolve cohesive interactions into dispersion (δD), polar (δP), and hydrogen-bonding (δH) contributions. A small distance often suggests compatibility, but swelling, crystallinity, temperature, and specific chemistry can defeat simple “like dissolves like” predictions.

R_a^2=4(\delta_{D,1}-\delta_{D,2})^2+(\delta_{P,1}-\delta_{P,2})^2+(\delta_{H,1}-\delta_{H,2})^2

Illustrative roles of solvent parameters
Descriptor	What it probes
Kamlet–Taft π*	Dipolarity/polarizability
Hansen δP	Polar cohesive interactions
Hansen δH	Hydrogen-bonding contribution

AdvancedWater, designed media, and alternative solvents

Water is inexpensive, non-flammable, and abundant, yet its polarity and hydrogen-bond network can exclude hydrophobic reactants. “In water” reactions have dissolved reactants; “on water” reactions occur at a stirred organic–water interface, where interfacial effects can accelerate some transformations. Hydrophobic effects are context-dependent, not a guaranteed rate boost.

Designer surfactants such as TPGS-750-M form micelles that solubilize hydrophobic substrates in water and create local reaction environments. Micellar catalysis can reduce bulk organic solvent use, but surfactant manufacture, product extraction, aqueous effluent, and reuse belong in the assessment.

Alternative solvent families: benefits and caveats
Medium	Potential benefit / caveat
Ionic liquids	Negligible vapour pressure; toxicity, persistence, synthesis and anion degradation vary widely.
Deep eutectic solvents	Choline chloride:urea (1:2) melts near 12 °C; high viscosity and ecotoxicity must be assessed.
Bio-based liquids	2-MeTHF, Cyrene, ethyl lactate, and γ-valerolactone have useful applications, but renewability alone does not establish safety or sustainability.
Fluorous media	Can enable phase separation and catalyst recovery; perfluorinated compounds may be persistent and environmentally concerning.

Ionic liquids (for example, imidazolium salts such as [C4mim][BF4] or [C4mim][NTf2]) have very low vapour pressure, not zero environmental impact. Their synthesis and purification can be resource-intensive; toxicity and biodegradation depend on both cation and anion. Fluorinated anions may hydrolyse under some conditions, so composition and waste handling matter.

Supercritical CO₂ is above its critical point, Tc = 31.0 °C and Pc = 73.8 bar. Near this region, pressure strongly changes density and solvent power; scCO₂ is used in coffee decaffeination and some cleaning processes. CO₂-based hydrogenation catalysis has also been studied, including work by Noyori, Ikariya, and Jessop on homogeneous catalysis in supercritical fluids.

\rho_{\mathrm{CO_2}}=\frac{pM}{ZRT},\qquad c=\frac{\rho}{M}

The ideal-gas expression is only a rough guide near critical conditions: the compressibility factor Z departs strongly from one, so a real-fluid equation of state or validated property database is needed for design. Density tuning can change solubility and phase behaviour; depressurization also makes product separation convenient.

Bio-based candidates include 2-methyltetrahydrofuran (2-MeTHF), Cyrene (dihydrolevoglucosenone), ethyl lactate, and γ-valerolactone. Their feedstocks and properties differ; compare volatility, viscosity, toxicity, energy for distillation, and end-of-life as well as the fossil feedstock displaced. Solvent-free processing and mechanochemistry can avoid bulk liquid altogether, but milling energy and scale-up must be counted.

ResearchResearch frontier and decision tools

References

Green and Sustainable Solvents in Chemical Processes · C. J. Clarke, W.-C. Tu, O. Levers, A. Bröhl, J. P. Hallett, 2018
CHEM21 selection guide of classical- and less classical-solvents · D. Prat et al., 2016
Homogeneous Catalysis in Supercritical Fluids · P. G. Jessop, T. Ikariya, R. Noyori, 1999
“On water”: unique reactivity of organic compounds in aqueous suspension · S. Narayan et al., 2005