(467a) High-Performance Magnetic Activated Carbon from Solid Waste from Lignin Conversion Processes. 1. Their Use As Adsorbents for CO2

Wood is a major source of raw materials for renewable fuels, chemicals and materials. Depending on its type, lignin can account for roughly one-third of the mass. It is considered a waste product in most of the processes that convert wood into valuable products. In the paper and pulp industry, where most of the lignin is available, 98 % is combusted to supply the processes with energy and low-grade heat[1]. This combustion is wasting potential value of lignin, both in an economical aspect and in the potential replacement of fuels, materials and chemicals based on fossil resources. Lignin can be used to produce value-added and renewable products in lignin-to-liquid processes. In these processes, lignin is depolymerized to form bio-oil containing various useful organic molecules for use in chemicals, fuels and materials[2]. Integrating such processes can be important in future biorefineries, where renewable resources are converted in to useful products. However, these processes produce large amounts solid by-products which, so far, has not found any constructive use. In the biorefinery concept, it is imperative to maximize the use of the renewable raw materials, both for the sustainability of the process and to generate enough added values to make the sustainable processes economically viable. In this work, we used the solid by-products of lignin-to-liquid processes to produce highly porous and magnetic activated carbons for use in carbon dioxide separation processes. Activated carbons have many commercial applications within separation, deodorization, purification and catalysis[3]. However, the majority of activated carbons are produced from fossil resources[4] and there is a need to replace these fossil-based materials with renewable alternatives. In carbon dioxide separation processes, activated carbons can potentially lower the carbon dioxide capture cost, which is a problem for CCS (carbon capture and storage) systems[5]. The general advantages of activated carbons compared to other solid sorbents are high adsorption capacities, stable adsorption/desorption cycling and low production costs. To produce the magnetic activated carbons in this work, Spruce and Eucalyptus lignin was heated at 380 ^oC with water, formic acid and a catalyst, the solid by-product of this bio-oil producing process was mixed with an aqueous solution of potassium hydroxide, heated at 200 ^oC for 5 hours and activated under nitrogen gas flow for 4 hours at 700-800 ^oC. The carbons activated at 700 ^oC exhibited carbon dioxide adsorption capacity of up to 6.0 mmol/g at 273K and 1 atm and very high ultramicropore volumes. The carbons activated at 800 ^oC exhibited BET specific surface areas, measured by nitrogen adsorption at 77K, of up to 2875 m²/g. The activated carbon based on Eucalyptus lignin and activated at 700^oC showed an isosteric heat of CO₂ adsorption was 27-30 kJ/mol, a simplified description indicated a CO₂-over-N₂selectivity of 14, while a full IAST prediction indicated a selectivity of between 3 and 4, and very good cyclic performance. All the activated carbons exhibited soft magnetic behaviour due to embedded nanoparticles of iron and magnetite. We show that the activation temperature does not only alter the porosity of the carbons but also the textual properties. The combination of the highly porous nature, the magnetic properties and the option to optimize the porosity through the activation temperature makes these materials highly interesting for many other potential applications outside CCS as well. This work has been published elsewhere[6].

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[3] R.C. Bansal, M. Goyal, Activated carbon adsorption, CRC Press, 2005.

[4] A. Ahmadpour, D.D. Do, The preparation of active carbons from coal by chemical and physical activation, Carbon N. Y. 34 (1996) 471–479. doi:10.1016/0008-6223(95)00204-9.

[5] V. Presser, J. McDonough, S.-H. Yeon, Y. Gogotsi, Effect of pore size on carbon dioxide sorption by carbide derived carbon, Energy Environ. Sci. 4 (2011) 3059. doi:10.1039/c1ee01176f.

[6] W. Hao, F. Björnerbäck, Y. Trushkina, Mikel O. Bengoechea, G. Salazar-Alvarez, T. Tarth, N. Hedin, High-Performance Magnetic Activated Carbon from Solid Waste from Lignin Conversion processes. 1. Their Use As Adsorbents for CO2, ACS Sustainable Chem. Eng., 2017, 5 (4), pp 3087–3095, doi: 10.1021/acssuschemeng.6b02795