CaO-Based CO₂ Sorbents with a Hierarchical Porous Structure Made via Microfluidic Droplet Templating

Abstract

Calcium looping, a CO 2 capture technique based on the cyclic carbonation and calcination of CaO, is a promising short-to midterm solution to reduce CO 2 emissions. However, CaO suffers from sintering under industrially relevant operating conditions, which reduces rapidly its cyclic CO 2 uptake capacity. Here, we report the design and manufacture of a hierarchical, porous (HP) CaO-based CO 2 sorbent. The hierarchically porous sorbent is created through the assembly of calcium carbonate nanoparticles and monodisperse oil droplets generated via microfluidic emulsification. The structure of the sorbent is stabilized by an Al 2 O 3 coating via atomic layer deposition (ALD). The sorbent outperformed the CO 2 uptake of the reference limestone by ca. 140%. The improved CO 2 uptake capacity is attributed to (i) the stabilization of the microand mesoporous structure of the material by the formation of Ca−Al mixed oxides, that is, Ca 12 Al 14 O 33 and Ca 3 Al 2 O 6 , and (ii) an improved mass transport within the sorbent particles owing to the HP structure of the material.

CO2 capture using solid sorbents: Fundamental aspects, mechanistic insights and recent advances

“…The overarching goal in these studies has been to synthesize nano-and micro-structured sorbents that exhibit reduced sintering during repeated carbonation-calcination cycles. First we describe morphologies achieved by template-assisted techniques, which may further be distinguished by the distribution of the stabilizer: (i) A homogenously distributed stabilizer to retard sintering in a nanostructured sorbent [119], [120], [378], [386]- [396] or (ii) a surface layer of the stabilizer, which acts as a physical barrier around the sorbent at the core, referred to as core-shell structures [120], [390], [397]- [400]. Secondly, we discuss the use of so-called polymorphic spacers,…”

“…An overview of the different approaches for morphology stabilization are shown schematically in Figure 5. Sacrificial templates have been used to synthesize nanostructured sorbents with homogenously distributed stabilizers [119], [120], [378], [386]- [388], [390]- [396]. The resulting morphologies have been characterized as microspheres [119], [391], [394], [396], [397], [404], microtubes [393], cage-like structures [392], [405], and nanosheets [387].…”

“…The advantage of the ALD technique is that it enables control of the layer thickness at the atomic scale and can provide a thin conformal coating. The technique has been applied to layer coatings of thickness between 1 nm to 30 nm of stabilizer oxides on a porous CaO support to create core-shell structures for CO2 capture [119], [120], [378], [397], [399], [410]. It is worth mentioning that the core-shell structure is formed in the as-prepared materials but it also tends to collapse during multiple cycle operation [412].…”

“…The left and right column show the carbonated and calcined states, respectively. a) Hierarchical porosity obtained by templateassisted techniques with a homogenous distribution of the stabilizer or a stabilizer coating providing a core-shell morphology, respectively, adapted from[119] and[120]. b) Polymorphic and monomorphic spacer used for a CaO sorbent, adapted from[401].…”

Mechanistic Understanding of CaO‐Based Sorbents for High‐Temperature CO2 Capture: Advanced Characterization and Prospects

“…[9,12,55] It is believed that stabilizers act as physical barriers between otherwise adjacent CaO grains, hence reducing the rate of sintering and stabilizing the pore network and surface area of the sorbent. [23,56,57] Various metal oxides have been explored as stabilizers including Al 2 O 3 [20,23,26,[58][59][60] (and the respective mixed calcium aluminates Ca x Al y O z that form during calcination [23,25,61] ), MgO, [17,18,24,54,62,63] SiO 2 , [64][65][66][67] TiO 2 [68] or ZrO 2 . [57,[69][70][71][72] Stabilizers are commonly added in quantities ranging between 5-20 wt %, with the optimal quantity being a trade-off between the degree of morphological stabilization and the quantity of CO 2 captureinert material added.…”