Synthesis of Composite Lithium Orthosilicate Based Materials for High Temperature CO2 Capture

Lithium orthosilicate (LOS) is a highly effective, reversible sorbent for CO₂ capture at high temperatures (600-650°C), with a theoretical working capacity of 36.7%. Suggested applications are reforming operations to produce hydrogen, water-gas shift operations, blast-furnace gas upgrading, and IGCC energy production. The material undergoes the following reaction during CO₂ sorption:

Li_4­SiO₄ (s) + CO₂ (g) ↔ Li₂CO₃ (s) + Li₂SiO₃ (s) (1)

Bulk LOS phases form a shell of lithium carbonate and lithium metasilicate, which the CO₂ needs to diffuse through for further sorption with the unreacted core. This has generally been modelled with a double-exponential curve fit, but we have designed a model based on physical properties of the materials. In our lab, we prepared a nanostructured LOS composite material synthesized with a surfactant that shows rapid uptake kinetics and cyclic stability up to 80 cycles. We hypothesize the surfactant results in grain boundaries that facilitate diffusion of reacting species.

Our goals are to reduce production costs as well as synthesis duration. It would also be ideal to slightly reduce the optimal temperature for carbon capture in the material for use in reactions with a water gas shift, to allow for an increase in H₂ production. Studies indicate that the inclusion of alkali dopants can lower the temperature for carbon capture in the material, making it more suited for inclusion in water gas shift processes. Other studies indicate that mesoporous silica sources such as SBA-15 and KIT-6 can be converted into LOS. This is typically done by a solid-state conversion of silica with a lithium salt such as lithium nitrate. We hypothesize that through pyrolysis rather than calcination, the carbon-carbon bonds can be preserved in the silica sources which will lead to measurable porosity in the LOS product. This would therefore increase the diffusivity of CO₂ in the material, as well as provide a cheaper and faster production method.