In order to adapt hydrotalcite based sorbents (LDO) to high temperature CO2 sorption and suitable temperature for tar steam reforming, the addition of CaO was investigated, maintaining their structure to keep porosity and accessibility but mostly assure the CO2 sorption stability during sorption/desorption cycles. In co-precipitation synthesis, various interlayer anions with different sizes and valences (carbonate, oxalate, and stearate) and various pH values were studied for different Mg/Ca ratios. The characterization of these sorbents (LDO) by TGA, XRD, N2 adsorption, SEM, sorption capacity, and sorption/desorption stability (cyclic TGA) permitted to understand the effect of the various synthesis conditions and to highlight the interest of oxalate use as interlayer anion. Typical LDH sand roses were formed with carbonate and oxalate anions after calcination until Mg/Ca/Al ratio = 1/2/1. For carbonate, this optimal ratio reached the highest sorption capacity and CaO sites accessibility at 600°C, better than pure CaO. However, the best stability during cycles was obtained with the sorbent from oxalate and Mg/Ca/Al ratio = 1.5/1.5/1 at pH 10 for which sorption results were almost the best values observed. For these two samples, the observed macroporosity were associated to the highest specific surface area and pore volume.
Statement of Novelty
In hydrotalcite structure, a width interlayer space permits to integrate various anions (CO32-, SO4-2, or NO3-) in order to modify CO2 sorption kinetics and capacity. The spheroidal “sand rose” morphology observed with CO32- anions explained the large BET surface area and the better CO2 sorption capacity. Stearate (and longer molecules) exchange was extensively studied as long-carbon-chain organic anion and allowed a better CO2 capture performance.
This work focuses on another interlayer anion (oxalate) with the same valence of carbonate (2), a shape and valence different to stearate (1), and not yet reported in the literature. The novelty is to demonstrate that oxalate can replace carbonate as interlayer anion and moreover stearate by a less toxic one.