Pyrolyzed chitosan-based materials for CO2/CH4 separation

Abstract

Chitosan is a biopolymer obtained by deacetylation of chitin extracted from sub-products of the food industry and it is rich in nitrogen content. Pyrolyzed chitosan– and chitosan-periodic mesoporous organosilica (PMO)– based porous materials with different pore structures and chemical features are prepared using different dry methods and ensuing pyrolysis at 800 °C, for application in the CO2/CH4 adsorption/separation. The highest CO2 adsorption capacity (1.37 mol·kg−1 at 100 kPa; 1.9 mol·kg−1 at 500 kPa) and the best selectivity for CO2/CH4 separation (95 at 500 kPa) is obtained using 1.5% (m/v) of chitosan solution dried under supercritical CO2. This material combines a good CO2 adsorption capacity with one of the highest selectivities for CO2/CH4 separation of the literature, arising as a promising alternative adsorbent for natural gas or biogas upgrading at reduced cost. The presence of high nitrogen content together with pores of diameter around 2 nm leads to an increase of the CO2 adsorption capacity. In the case of chitosan-PMO-based materials, the activation step using both acid and crushing methods is crucial to increase the CO2 adsorbed amount. Here, the highest CO2 adsorption capacity and the highest selectivity are obtained by the chitosan-PMO crushed adsorbent and the chitosan-PMO material activated with sulfuric acid, respectively. These observations indicate the importance of the controlled attack of the material surface to enhance the diffusion of the target gases within the adsorbent, avoiding the adsorption of other species.