Regeneration of hierarchically porous silica monoliths by acid and alkali treatments

Abstract

Durability of hierarchically porous silica monolith columns against strong acid and alkali aqueous solutions was examined under flow-through conditions relevant to cleaning-in-place (CIP) as a cleaning method. Silica monoliths were exposed to strong acids, including aqua regia, and strong alkaline solutions of sodium hydroxide, followed by controlled rinse/neutralization protocols. Under flow-through washing, the monoliths showed no significant degradation in pore structural parameters after strong acid and alkaline treatments over a short time period. Alkaline CIP in 0.01 M or 0.1 M NaOH treatment induced systematic pore-parameter changes consistent with silica leaching during flow, while the retained bulk structure of silica monolith was confirmed. The observed pH breakthrough curves of a two-step neutralization behavior were attributed to the sequential deprotonation of surface silanol groups. In contrast, high-concentration alkaline exposure without post-treatment neutralization caused residual Na species and surface coarsening. These results show that the pore structure of bare silica monolith during alkaline CIP is altered depending on alkali concentration, effective contact time, and post-treatment neutralization, which serves as a reaction-quenching mechanism. These findings are useful in the regeneration of the silica monoliths for repeated use.