(552f) Adsorption of Carbon Dioxide on Mg-Al Based Hydrotalcites and Z-13X Zeolites for Industrial Sorption-Enhanced Hydrogen Production

Pre-combustion CO₂ capture is well suited for hydrogen (H₂) production via integrated coal gasification combined cycle (IGCC), and by steam reforming of methane (SRM). In particular, the so-called sorption-enhanced H₂ (SE-H₂) processes are one of the most promising low-carbon strategies to boost the amount of H₂ produced. However, the selection of suitable materials and the relatively low maturity of the existing SE-H₂ configurations are hindering their commercial deployment.[1,2]

Hydrotalcites (HTs) are the most important group of CO₂ adsorbents for SE-H₂ production since they show their highest CO₂ sorption performance in the temperature (473–773 K) and pressure range of interest (1–30 bar). Also, they can be used as individual particles with simultaneous adsorptive and catalytic properties, exhibiting fast kinetics.[3] The main challenge to use HTs commercially, is yet to improve their overall performance in terms of capacity, multicycle stability, and energy efficiency. In addition, few studies have investigated the sorption characteristics of HTs under relevant operating conditions for SE-H₂.

In this work, a thorough CO₂ adsorption study on HTs at temperatures of 473–673 K and high pressures, up to 10 bar is presented.[4] A unique combination of pressure- and temperature swing adsorption-desorption (PSA-TSA) cycles and detailed characterisation analyses are provided. The description of the adsorption isotherms by the Freundlich and Langmuir models is also given. In addition, the heat of adsorption is calculated. Analogous analyses of performance and characterisation under CO₂ exposure are presented for a zeolite material (Z-13X), which is also a promising candidate to be used in CO₂ capture related processes (e.g. CO₂ removal downstream of the water gas shift reactor in an integrated gasification cycle). Zeolites can also be regenerated at temperatures such as 623 K, which favour their use as adsorptive-catalytic materials. In this work, studies using nitrogen as atmosphere were also performed for further understanding of the adsorption behaviour of the materials.

Commercial HTs and zeolites are used in the tests, aiding the scalability of the process. Furthermore, novel graphene oxide based HTs are synthesised using a co-precipitation technique. Earlier studies at atmospheric pressure have shown that high surface area supports such as graphene oxide could significantly enhance the CO₂ performance of HTs.[3] Alkali promotion of the materials is also addressed. An intelligent gravimetric analyser operating up to 20 bar is used to carry out the adsorption measurements. The CO₂ performance is correlated with a wide range of physicochemical characterisation techniques including N₂ physisorption, TGA, TPD, XRD, ICP, TEM and FTIR. Full details of the experimental methodology used are given elsewhere.[4]

The results of this work revealed that the HTs exhibited higher adsorption capacities than zeolites in terms of molCO₂ kg ads^{− 1} up to 2 bar and in terms of µmolCO₂ m^-2 up to 10 bar. The HTs showed a decrease in adsorption capacity upon cycling whereas the zeolites showed reversible performance. Consistency between characterisation and CO₂ adsorption performance for HTs and zeolites was found. For instance, ATR-FTIR showed the same surface chemistry for fresh and spent zeolites samples, and this correlates with the CO₂ reversible process observed. On the other hand, different functional groups to those of the fresh HTs are present in the samples exposed to CO₂. This is consistent with the presence of a hysteresis loop in the isotherms of the HTs and also correlates with the decrease in adsorption capacity upon cycling observed. The fresh and cycled adsorption isotherms were fitted better by the Freundlich model for the HTs, whereas a similar fit to Langmuir and Freundlich was found for the zeolites. The isosteric heats of adsorption determined for HTs have a magnitude that indicates chemisorption of CO₂ on the material. Graphene oxide appears to enhance the multicycle-stability of HTs whereas alkali promotion increases their capacity under the operating conditions investigated. The data presented in this work are novel and useful for a wide range of H₂–CCS applications carried out at relatively high pressures, and in specific the data provided here are significantly useful for the design of SE-H₂ commercial units.

Acknowledgements

This work is supported by the Imperial College London Research Fellowship Grant. Complementary funding from PAIP is acknowledged.

References

[1] D.P. Harrison, Sorption-enhanced hydrogen production: A review, Industrial and Engineering Chemistry Research, 47 (2008), 6486.

[2] D. Iruretagoyena, K. Hellgardt, D. Chadwick, Towards autothermal hydrogen production by sorption-enhanced water gas shift and methanol reforming: A thermodynamic analysis, International Journal of Hydrogen Energy, 43 (2018), 4211.

[3] D. Iruretagoyena, Milo S. P. Shaffer, and David Chadwick, Layered Double Oxides Supported on Graphene Oxide for CO₂ Adsorption: Effect of Support and Residual Sodium, Industrial and Engineering Chemistry Research, 54 (2015), 6781.

[4] D. Iruretagoyena, P. Fennell, R. Pini, Adsorption of CO₂ and N₂ on bimetallic Mg-Al hydrotalcites and Z-13X zeolites under high pressure and moderate temperatures, Chemical Engineering Journal Advances, 13 (2023), 100437.