Influence of heat flow direction on pore development in activation of corn stalk pith under microwave or furnace heating

Abstract

Heat transfer determines sequence for activation of outer/inner organics in a bulky biomass particle. This affects evolution of reaction intermediates and reaction network, probably also pore characteristics of resulting activated carbon (AC). Herein, this was investigated by activation of corn stalk pith (CSP) with varied activators under microwave heating and furnace heating, two heating method with distinct heat transfer direction. The results showed that microwave heating enhanced cracking reactions with K₂C₂O₄ as an activator, decreasing yield of AC. The enhanced cracking resulted in merge of micropores and higher percentage of mesopores/macropores (13.3 % versus 6.4 %). Nonetheless, simultaneous heating with microwave generated high density of reaction intermediates during activation with ZnCl₂, enhancing condensation to form AC of higher yield. More abundant polymeric intermediates led to blockage of large pores, resulting in lower specific surface area (1274.0 m²·g⁻¹ versus 1428.1 m²·g⁻¹) but higher percentage of micropores (97.1 % versus 83.2 %), facilitating adsorption of phenol. In-situ IR characterization of reaction intermediates showed that H₃PO₄ could not form strong cross-linking bonds (C-O-P) with CSP. Dominant cracking with K₂C₂O₄ diminished formation of oxygen-containing species like -OH and CO below 300°C, while ZnCl₂ could remove these oxygen-containing intermediates through condensation, leading to reconstruction of morphology of resulting AC.