Surface-modified siliceous monoliths for CO2 capture produced from low cost and green bio-based substances as multifunctional additive

Industrialization and accelerated population growth have been creating tremendous air pollution. Carbon dioxide (CO2) from fossil fuels combustion is a concerning waste gas, responsible of greenhouse effect: it increases atmospheric temperature contributing to more than 60% of global warming. Hence, reduction of CO2 emissions is one of the biggest challenges for environmental safety, but in the meantime CO2 levels need to be decreased. Several approaches should be adopted for this purpose, for instance physical absorption using solvents, membrane separation or capture by materials with adsorptive properties (i.e. zeolites and activated carbons). [1] In this work, some low cost silica-based materials were prepared as CO2 adsorbents. Silica monoliths were synthesized according to a reported procedure [2], from a concentrated dispersion of a commercial silica powder in a solution of bio-based substances (BBS) isolated from composted urban green wastes, which act as templates and binders. After the formation of a wet and workable paste, pellets were yielded in the desired shape (spheres of about 0.5 cm diameter) and calcined in order to remove all organics. The obtained siliceous monoliths were maintained as prepared (M-sil) or further modified by a covalent functionalization of the surface with (3-Aminopropyl)triethoxysilane (M-NH2) and BBS (M-BBS), in order to improve the adsorption features thanks to the introduction of CO2-philic functional groups. Moreover, a sample of SBA-BBS was produced immobilizing BBS on powdery mesoporous SBA silica to check the influence of an ordered porous structure on CO2 capture [3]. All the samples were characterized by N2 adsorption at 77K (for specific surface area and porosity determination) and thermogravimetric analysis (for evaluating the functionalization extent on silica surface and the thermal stability of the functionalized materials). CO2 uptake and the related molar heats of adsorption were measured at 30°C by an adsorption microcalorimeter Tian-Calvet type equipped with a lab-made gas-volumetric apparatus. The use of BBS as multifunctional additive was successful: on one hand, they resulted convenient and eco-friendly reaction intermediate; on the other hand they represented an active phase towards CO2 uptake. In general, the functionalized siliceous monoliths showed good performances in CO2 adsorption in terms of uptake and low interaction energy, suggesting an easy regeneration of the adsorbing material after a simple outgassing in vacuo at 30°C. Finally, the shape of the monoliths can be advantageous for a possible scale-up of the process.