Solid CO2 sorbents move from lab promise to hydrogen-ready carbon capture

A new open-access review argues that the future of medium- and high-temperature carbon capture will be decided not only in the chemistry lab but on the factory floor and inside industrial reactors. Published in the journal Carbon Neutrality, the paper by Faming Liu, Yu Fang, Zhulian Li, Taixiu Liu, Peijing Li, Shuo Gao, Binhui Yu and Qibin Liu synthesizes the state of the art in solid CO2 adsorbents designed to operate where liquid amine scrubbing cannot: at temperatures from 200 up to 800 degrees Celsius. The authors’ central message is blunt. Significant progress has been made in formulation, microstructure, cyclic stability and reaction mechanisms, yet practical evaluation must go beyond laboratory-scale CO2 adsorption tests to encompass formability, wear resistance, regeneration strength, reactor compatibility and application potential.

The review is deliberately deployment-oriented rather than encyclopedic. It does not aim to provide a comprehensive overview of each material type or to propose new concepts for hydrogen production. Instead, it links cross-material comparisons with forming-related trade-offs, reactor operability and the selection of adsorbents within the relevant hydrogen operating window. The materials examined fall into five representative families: hydrotalcite-based layered double hydroxides, MgO-based sorbents, CaO-based sorbents, alkali metal silicates and titanate-based systems. Each occupies a different slice of the temperature spectrum, and each carries a distinct bundle of strengths and liabilities that becomes visible only when a powder is pressed into a pellet and dropped into a fluidized bed.

Medium-temperature sorbents operating between 200 and 400 degrees Celsius fill an awkward gap. The gas stream is too hot for amine scrubbing, which generally works at 40 to 120 degrees Celsius, but not hot enough to benefit fully from CaO-based carbonation. Hydrotalcite compounds, also known as layered double hydroxides, are layered materials with interlayer spacings of 0.3 to 3 nanometers containing hydroxide and carbonate functional groups. In this window they adsorb CO2 preferentially over methane, carbon monoxide and nitrogen, and many show excellent cyclic stability, although capacity retention varies with composition, cycle count and adsorption-regeneration conditions. Their regeneration energy consumption is low and their kinetics are suitable, but their CO2 capacity generally remains below that of calcium-based sorbents.

Engineering details matter enormously at this scale. Organic modifiers such as melamine can introduce porosity and structural support into Mg-Al-CO3 layered double hydroxide derived sorbents, and the resulting hybrid mixed metal oxide pellets reached 1.34 millimoles of CO2 per gram at 200 degrees Celsius and 1 bar, scaling with modifier content and substantially exceeding unmodified and commercial counterparts. Yet the authors caution that organic modifiers may decompose or volatilize during cyclic high-temperature regeneration, limiting long-term durability. Potassium carbonate promotion enhances performance through two distinct mechanisms: in magnesium-rich layered double hydroxides it participates in the bulk reaction to form potassium-magnesium double carbonates as the active capture phase, while in aluminum-rich compositions it mainly generates strong basic surface sites that facilitate chemisorption. A ternary lithium-sodium-potassium nitrate mixture boosted a layered double hydroxide combined with graphene oxide to 4.51 millimoles per gram at 260 degrees Celsius.

MgO-based sorbents are abundant, inexpensive and theoretically attractive, but pure magnesium oxide reacts slowly with CO2 because mass transfer limits the reaction. The review identifies three main remedies: pore structure modification, loading onto porous inert materials, and alkali metal doping. The most striking results come from molten salt promotion. Ternary alkali metal nitrates melt at operating temperature and act as a solvent that dissolves MgO and CO2, bypassing the solid-state diffusion limitation imposed by the magnesium carbonate product layer. An optimized lithium-sodium-potassium ratio of 0.18:0.52:0.30 achieved 5.56 millimoles per gram at 240 degrees Celsius under 15 percent CO2. Using natural magnesite as a low-cost precursor with calcium carbonate as a structural promoter, a potassium-containing ternary nitrate-promoted MgO reached 7.2 millimoles per gram with only 6 percent loss after 50 cycles under 30 percent CO2. Concerns remain, however, about salt volatility, corrosive effects at high temperature and performance fluctuations from complex salt-oxide interactions.

At the high-temperature end, from 500 to 800 degrees Celsius, CaO remains the benchmark. Under 100 percent conversion its theoretical CO2 sorption capacity reaches 0.786 grams of CO2 per gram of CaO, and it can strip CO2 to very low concentrations under sorption-enhanced steam reforming conditions at moderate temperatures of 450 to 750 degrees Celsius and atmospheric pressure. Its Achilles heel is sintering. Calcium carbonate accumulates on the surface, particles agglomerate, pores shrink and collapse, and uptake falls rapidly over repeated cycles. The review classifies anti-sintering dopants into three categories. Completely inert dopants such as MgO physically separate CaO crystals but gradually lose effectiveness as they become encapsulated during cycling. Semi-inert dopants such as alumina, titania, silica and zirconia react with CaO at grain boundaries to form calcium salts that coat the grains, showing the best anti-sintering performance and application prospects. Active dopants rich in oxygen vacancies, such as manganese, iron, yttrium and cerium oxides, accelerate carbonate ion diffusion but work mainly in the slow carbonation regime, which cannot meet the fast cycling demands of fluidized bed systems.

Shaping and granulation emerge as decisive and underappreciated variables. Limestone is inherently fragile, and within a fluidized bed fine calcium oxide powder is readily entrained by the gas stream, increasing sorbent consumption and contaminating particles. Reported forming methods include compression molding, agar-assisted forming, spray-water pelletization and extrusion-spheronization. Extrusion granulation produces dense, rod-shaped pellets with high wear resistance but poor adsorption performance, while rotary granulation yields less dense spheres with lower mechanical strength. Two-step rotary granulation encapsulates small sorbent spheres in a hard inert shell to improve mechanical performance, but the shell cannot capture CO2 and interferes with molecular diffusion, reducing conversion. Pore-forming agents such as cellulose and lignin help restore microstructure. Crucially, the review notes that the CO2 sorption capacity of CaO derived from inexpensive precursors is severely reduced after pelletization, a fact that laboratory powder studies routinely obscure.

Lithium silicate sorbents, particularly Li4SiO4, offer a different bargain. They react with CO2 above 400 degrees Celsius to form lithium carbonate and lithium silicate, regenerate at a suitable temperature of around 700 degrees Celsius, and show excellent cycling stability at a lower synthesis cost than other lithium ceramics, though still more expensive than calcium-based sorbents. The accepted mechanism is a double-shell model in which a dense carbonate outer shell forms rapidly and then throttles the reaction as CO2 and lithium ions must diffuse through it. Molten salt doping attacks exactly this bottleneck: potassium carbonate forms a low-melting eutectic melt with the carbonate product at around 500 degrees Celsius, dramatically enhancing ion mobility through the product layer. Potassium-titanium co-doped pellets made with cellulose fiber pore formers retained more than 0.25 grams of CO2 per gram over 340 cycles under 15 percent CO2 at 600 degrees Celsius while maintaining a crush strength of 15.9 newtons. Additive manufacturing via digital light processing built cellular architectures reaching 0.329 grams per gram and retaining about 0.274 grams per gram over 15 cycles.

The deployment chapters of the review are where it diverges most sharply from earlier surveys. On mechanical robustness, shaped Li4SiO4 sorbents have reported attrition weight losses typically below 5.5 percent in standardized tests, within the acceptable range for circulating fluidized-bed processes, although these attributes were measured largely under simplified laboratory conditions. On regeneration, raising the calcination temperature by just 25 degrees Celsius cuts desorption time for CaO by roughly 40 percent, and adding 2.5 percent steam accelerates regeneration further while stabilizing morphology. Li4SiO4 regenerates at 600 to 700 degrees Celsius, closely matching steam reforming temperatures, while MgO regenerates at 400 to 500 degrees Celsius, enabling the use of low-grade waste heat. Reactor compatibility differs too: fixed beds suit shaped particles and 3D-printed monoliths, moving beds enable countercurrent contact and heat recovery, and circulating fluidized beds demand attrition resistance above all else.

Technology readiness separates the families dramatically. Calcium looping is the most mature solid sorbent route, demonstrated at technology readiness level 7 in pilots treating more than 2000 normal cubic meters of flue gas per hour with CO2 capture approaching 99 percent. Lithium silicate and MgO systems sit at levels 3 to 5. A techno-economic assessment using real industrial flue gas data estimated capture costs of roughly 60 dollars per tonne of CO2 for calcium looping with limestone-derived CaO and about 80 dollars per tonne for magnesium looping, the premium reflecting slow MgO carbonation kinetics and a costlier raw material chain. In hydrogen service, a multifunctional nickel-CaO material produced about 80 percent hydrogen by volume during sorption-enhanced reforming at 650 degrees Celsius, and a coupled chemical looping water splitting and sorption-enhanced reforming scheme was projected to reach 79.3 percent net efficiency. The authors close with a call for standardized benchmarking matrices specifying temperature, CO2 partial pressure, steam level, cycle time and impurity tolerance, warning that performance measured under pure CO2 often overestimates what mixed industrial streams will deliver, and that only reactor-relevant, deployment-conscious testing will translate laboratory promise into scalable carbon-neutral technology.

Subject of Research: A review of medium- and high-temperature solid CO2 sorbents for carbon capture in hydrogen-related thermochemical processes

Article Title: Solid CO2 sorbents for medium- and high-temperature carbon capture: material design, shaping, and deployment for hydrogen-related processes

Article References: Liu, F., Fang, Y., Li, Z., Liu, T., Li, P., Gao, S., Yu, B., & Liu, Q. (2026). Solid CO2 sorbents for medium- and high-temperature carbon capture: material design, shaping, and deployment for hydrogen-related processes. Carbon Neutrality, 5(1), Article 34. https://doi.org/10.1007/s43979-026-00189-4

Image Credits: AI Generated

DOI: 10.1007/s43979-026-00189-4

Keywords: carbon capture, solid sorbents, hydrogen production, sorption-enhanced reforming, CaO sorbents, Li4SiO4, MgO sorbents, hydrotalcite, titanates, fluidized bed, technology readiness, regeneration energy