Environmental, green and energy chemistry

Green hydrogen, synthetic fuels, chemical recycling of plastics

How renewable electricity becomes hydrogen, ammonia and synthetic fuels, and how selected plastics can be broken back into monomers or feedstocks, with energy and lifecycle limits.

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

IntuitionHydrogen and circular carbon: connect power to molecules

Green hydrogen is an energy carrier made with low-carbon electricity, not a primary energy source. It can supply hydrogen-dependent chemistry and help store variable renewable power, but every conversion loses energy. Chemical recycling can recover monomers or hydrocarbon feedstocks from selected plastics, yet it complements rather than replaces waste prevention, reuse and mechanical recycling.

Explore renewable electricity to electrolysis and H₂, then ammonia, methanol or Fischer–Tropsch fuels. Switch modes to follow mixed plastic waste through pyrolysis, solvolysis or depolymerisation back to feedstocks.

SchoolElectrolysis: water, electricity and efficiency

Water electrolysis divides water into hydrogen and oxygen. Alkaline systems use an aqueous alkaline electrolyte and often nickel-based electrodes; proton-exchange-membrane (PEM) systems conduct protons through a polymer membrane. Anion-exchange-membrane (AEM) systems aim to combine alkaline chemistry with compact membranes, while solid-oxide electrolysis cells (SOECs) operate hot and can use heat as well as electricity.

\ce{2H2O(l) -> 2H2(g) + O2(g)}

Definition: Reversible and thermoneutral voltage

At 25 °C, splitting liquid water has $\Delta G^\circ\approx237$ kJ mol⁻¹ H₂, corresponding to a reversible voltage near 1.23 V for two electrons per H₂. The enthalpy requirement is about 286 kJ mol⁻¹ H₂, or a thermoneutral voltage near 1.48 V. Actual cells require higher voltage because of activation, ohmic and mass-transfer overpotentials.

\Delta G^\circ=2F E_{\mathrm{rev}}\approx237\ \mathrm{kJ\,mol^{-1}_{H_2}} \qquad \Delta H^\circ=2F E_{\mathrm{tn}}\approx286\ \mathrm{kJ\,mol^{-1}_{H_2}}

Hydrogen pathways and common colour labels
Label (not a formal standard)	Typical route	Key caveat
Grey	Natural gas reforming, no capture	Lifecycle emissions vary; often about 9–12 kg CO₂/kg H₂
Blue	Fossil feed with CO₂ capture	Capture rate, residual emissions and methane leakage matter
Green	Electrolysis with renewable electricity	Additionality and grid carbon intensity matter
Pink / nuclear	Electrolysis powered by nuclear energy	Terminology varies by jurisdiction

Example: CO₂ from steam methane reforming

Steam methane reforming plus water–gas shift gives overall $\ce{CH4 + 2H2O -> CO2 + 4H2}$ . How much CO₂ does the reaction itself produce per kg of H₂?

Solution

One mole CO₂ accompanies four moles H₂: $44.01/(4\times2.016)=5.46$ kg CO₂ per kg H₂ from the chemistry alone. Burning additional natural gas for process heat raises reported totals to roughly 9–12 kg CO₂/kg H₂, depending on plant and system boundary.

Example: Electricity per kilogram of hydrogen

An electrolyser uses 52 kWh/kg H₂. What is its LHV-based efficiency, using 33.3 kWh/kg for hydrogen?

Solution

On an LHV basis, $\eta=33.3/52=0.640$ , or about 64%. On an HHV basis, $39.4/52=75.8$ %. State the basis: typical complete-system consumption is roughly 50–55 kWh/kg H₂, while stack-only values and SOEC systems with supplied heat can differ.

UndergraduateCatalysts, scale and power-to-X products

Hydrogen evolution (HER) commonly uses Pt in acidic PEM cathodes; oxygen evolution (OER) in PEM anodes relies on IrO₂ or RuO₂, while alkaline systems can use Ni-based catalysts and NiFe oxyhydroxides. Iridium is exceptionally scarce, motivating lower loadings, recycling and alternatives. System energy is not just the stack: power electronics, water treatment, gas drying and compression must be counted. Hydrogen stores electricity chemically, but reconversion to power loses much of it; direct electrification is usually more efficient where feasible.

Electrolyser families
Type	Electrolyte / strengths	Challenges
Alkaline	Liquid KOH; mature, non-noble catalysts	Gas crossover and dynamic operation
PEM	Polymer proton membrane; compact, responsive	Ir/Pt use and acidic environment
AEM	Anion-conducting membrane; alkaline catalysts	Membrane durability and carbonate management
SOEC	High temperature; can use process heat	Thermal cycling and materials lifetime

Example: Iridium for gigawatt-scale PEM electrolysis

An illustrative PEM stack runs at 2.0 A cm⁻² and 1.8 V with an anode loading of 0.5 mg Ir cm⁻². Estimate the iridium needed per GW of electrical input.

Solution

Active area $=P/(jV)=10^{9}/(2.0\times1.8)=2.8\times10^{8}$ cm². Ir mass $=2.8\times10^{8}\times0.5$ mg $=1.4\times10^{8}$ mg $\approx139$ kg per GW. A build-out of 100 GW would need about 14 t, roughly two years of world primary supply if that is taken as of order 7–8 t/yr, which explains the push for lower loadings, recycling and Ir-free anodes.

UndergraduatePower-to-X: ammonia and carbon-based fuels

Green ammonia combines renewable H₂ with nitrogen separated from air in the exothermic Haber–Bosch reaction. E-methanol hydrogenates captured CO₂ over Cu/ZnO/Al₂O₃ or In₂O₃-based catalysts. Fischer–Tropsch synthesis can turn CO and H₂ into hydrocarbons; chain-growth probability α in an Anderson–Schulz–Flory distribution controls the broad product slate, not a single pure fuel. These routes can decarbonise feedstocks or aviation/shipping fuels where direct electrification is difficult, but they require substantial clean power and carbon accounting.

\ce{N2 + 3H2 <=> 2NH3} \qquad \Delta H^\circ \approx -92\ \mathrm{kJ\,mol^{-1}_{N2}}

n\,\ce{CO} + (2n+1)\,\ce{H2} \longrightarrow \mathrm{C}_n\mathrm{H}_{2n+2} + n\,\ce{H2O}

w_n=n\,(1-\alpha)^2\,\alpha^{\,n-1}

Example: Anderson–Schulz–Flory product cuts

For chain-growth probability $\alpha=0.85$ , estimate the mass fractions in C₅–C₁₁ (naphtha/gasoline range) and C₁₂–C₁₈ (diesel/jet range).

Solution

Summing $w_n$ numerically for $\alpha=0.85$ gives about 0.39 for C₅–C₁₁, 0.25 for C₁₂–C₁₈, 0.20 for C₁₉₊ waxes and 0.17 for C₁–C₄. No single cut can exceed roughly 0.48 (the C₅–C₁₁ maximum, near $\alpha\approx0.77$ ) and C₁₂–C₁₈ peaks near 0.25 (at $\alpha\approx0.875$ ). Hence practical plants add wax hydrocracking to raise the yield of the desired fuel range.

Hydrogen’s lower heating value (LHV) is about 33.3 kWh/kg and higher heating value (HHV) about 39.4 kWh/kg. A 52 kWh/kg electrolyser followed by synthesis cannot deliver more fuel energy than supplied electricity; electrolysis alone is roughly 64% LHV-efficient at that consumption. For methanol made from CO₂, stoichiometry requires three H₂ per CO₂; for ammonia, three H₂ per N₂. Compression, nitrogen separation, CO₂ capture, synthesis recycle and product upgrading add energy.

Example: Electricity for one tonne of e-methanol

Estimate the H₂ and electrolyser electricity needed for 1.00 t methanol, using $M(\ce{CH3OH})=32.04$ g/mol and 52 kWh/kg H₂. Ignore CO₂ capture and synthesis auxiliaries.

Solution

The reaction uses 3 mol H₂ per mol methanol: $m_{H2}=1000(3\times2.016/32.04)=188.8$ kg. Electrolysis then needs $188.8\times52=9.82$ MWh. The methanol LHV is only about 5.53 MWh/t, so the fuel holds at most $5.53/9.82\approx56\%$ of the electricity spent on H₂ alone, before capture and synthesis; this is why e-fuels are energy-intensive.

UndergraduateChemical recycling: target polymer chemistry

Mechanical recycling usually retains polymer chains and is often less energy-intensive when feed is clean and well sorted. Chemical routes break chains or alter them: pyrolysis of PE/PP at roughly 400–600 °C yields a variable hydrocarbon oil, gas and wax that need upgrading; gasification produces syngas. PET can undergo glycolysis, methanolysis or hydrolysis to recover monomers or intermediates. Enzymatic PET depolymerisation with engineered cutinase-like enzymes is a lower-temperature route under development. Nylon-6 can depolymerise to caprolactam; PMMA can return to methyl methacrylate, and polystyrene can yield styrene-rich streams.

Recycling route and realistic target
Polymer / route	Product target	Key condition or caveat
PET glycolysis / methanolysis	BHET or DMT plus ethylene glycol	Purify monomers; solvent and catalyst recovery
PET enzymatic hydrolysis	Terephthalic acid and ethylene glycol	Enzyme access, crystallinity and pretreatment
PE/PP pyrolysis	Naphtha-like oil, wax and gas	Mixed feed and upgrading demand
Nylon-6 depolymerisation	Caprolactam	Purity and energy for separation

Polyethylene and polypropylene pyrolysis oil is not automatically “circular plastic”: chlorine, additives, mixed polymers and contaminants affect yields and downstream cracking. Hydrogenolysis can upgrade polyolefins to shorter hydrocarbons, but its hydrogen source, catalyst and product use determine emissions. Durable closed-loop polymers and designs for depolymerisation can reduce sorting and quality losses; lifecycle analysis must compare them with mechanical recycling and virgin resin on equivalent service.

Example: Energy retained in pyrolysis oil (assumed yields)

Illustration: 1.00 t of polyethylene (LHV 43 GJ/t) is pyrolysed with assumed mass yields of 70% oil (43 GJ/t), 20% gas (45 GJ/t) and 10% wax/char (40 GJ/t). Estimate the energy in the products and the share in the oil.

Solution

Oil: $0.70\times43=30.1$ GJ; gas: $0.20\times45=9.0$ GJ; wax/char: $0.10\times40=4.0$ GJ; total $\approx43.1$ GJ, equal to the feed energy within the rounding of the assumed heating values. The oil holds $30.1/43\approx70\%$ of the feed energy. Process heat (of order 1–2 GJ/t, often supplied by burning the gas) and upgrading come on top. The yields are assumed for illustration; real values depend strongly on feed, temperature and catalyst.

Definition: Polymers designed for depolymerisation

Closed-loop recyclable polymers are designed with a low ceiling temperature or a labile linkage so that heat or a catalyst returns the monomer selectively. Polyesters based on γ-butyrolactone reported by Eugene Chen’s group are examples. The usual trade-off is between easy depolymerisation and the thermal and mechanical stability needed in use.

AdvancedSystems analysis and mass–energy balances

\eta_{\mathrm{LHV}}=\frac{m_{\mathrm{H_2}}\times 33.3\ \mathrm{kWh\,kg^{-1}}}{E_{\mathrm{electricity}}}

A power-to-X chain multiplies efficiencies: renewable electricity to H₂, then synthesis and upgrading. Use it where molecules are needed, especially for feedstocks and hard-to-electrify transport; avoid comparing a fuel’s energy only with electrolyser stack efficiency. Green ammonia production stoichiometrically uses 3 mol H₂ per mol N₂, or about 0.178 t H₂ per tonne NH₃, requiring roughly 9.2 MWh at 52 kWh/kg H₂ before air separation and synthesis. Net climate benefit depends on additional clean electricity and what product is displaced.

For a feed-basis PET methanolysis example, the ideal repeat-unit reaction is $\ce{C10H8O4 + 2CH3OH -> C10H10O4 + C2H6O2}$ (PET unit to dimethyl terephthalate plus ethylene glycol). One tonne of repeat units (192.17 g/mol) corresponds to about 5.20 kmol, theoretically yielding 1.01 t dimethyl terephthalate and 0.323 t ethylene glycol while consuming 0.333 t methanol. These rounded masses close to the repeat-unit balance; actual isolated product is lower after incomplete conversion and purification.

ResearchResearch frontier

References

Sustainable Hydrogen Production · J. A. Turner, 2004
Chemical recycling of waste plastics for new materials production · A. Rahimi, J. M. García, 2017
An engineered PET depolymerase to break down and recycle plastic bottles · V. Tournier et al., 2020
Net-zero emissions energy systems · S. J. Davis et al., 2018
Production, use, and fate of all plastics ever made · R. Geyer, J. R. Jambeck, K. L. Law, 2017
A synthetic polymer system with repeatable chemical recyclability · J.-B. Zhu, E. M. Watson, J. Tang, E. Y.-X. Chen, 2018