Ionic Liquids for CO2 Capture
The project indicates that ionic liquids can be used in CO2 capture both in their neat form and – more commercially relevant – as supported ionic liquid phase (SILP) materials.
The need for reductions in atmospheric CO2 levels in order to mitigate climate change is widely accepted. For instance, the EU has a goal to emit 40 % less CO2 in year 2030 compared to year 1990. It is possible to remove CO2 from flue gases, but the cost is currently high. The thesis investigates innovative capture techniques based on ionic liquids.
Ionic liquids are salts that are liquid under 100°C. Unlike molecular solvents, ionic liquids consist of ions – just like more common salts – but they are thermodynamically more stable in the liquid phase due to a high degree of disorder, low ion density and high degree of asymmetry. This is practical since these conditions are unfavourable to crystal packing.
In the project, ionic liquids were tested as CO2 absorbents both in their neat form and as so called supported ionic liquid phase (SILP) materials. A SILP material consists of an ionic liquid deposited in the pores of an inorganic or organic porous support material.
Firstly, CO2 absorption in amino acid-based ionic liquids was studied. The amino acids glycine and proline yielded the best ionic liquids for reversible CO2 capture. Water in ionic liquid significantly enhanced both the total CO2 uptake and the rate of uptake. Both ammonium and phosphonium cations were investigated. The tetrahexyl phosphonium yielded the highest CO2 uptake: 1.55 mole CO2 per mole of ionic liquid.
SILPs were found to absorb slightly less than the corresponding neat ionic liquids. This was attributed to passivation of a small portion of the ionic liquid by the support surface. Supports with high surface area were preferred, and of the studied materials ordinary SiO2-60 gave the highest CO2 uptake. The SILP materials were selective towards CO2 uptake at low partial pressure.
In another part of the project, methyltrioxirhenium was tested as a catalyst for the commercially interesting propene to propene oxide reaction. The catalyst was very selective towards the desired oxide and could be recycled with moderate success.
Overall, the project has shown that ionic liquids are versatile both as reactant and as solvent, and that both neat ionic liquids and SILP materials can be used in CO2 capture. Only the SILP materials have commercial interest. Further engineering development is necessary after this proof-of-concept.