Selective adsorption and separation of salicylic acid and 4-hydroxyisophthalic acid from industry-grade 4-hydroxybenzoic acid on UiO-66

In this study, UiO-66 was employed for the first time as an adsorbent to separate phenolic acid analogues, specifically 4-hydroxyisophthalic acid and salicylic acid, from impurities. Synthesized in-house, UiO-66 was shown to exhibit high selectivity towards 4-HIPA/4-HBA and SA/4-HBA when a molar equivalent of acetic acid modulator to terephthalic acid was set at 44. The adsorption capacities for 4-HBA, 4-HIPA, and SA were determined to be 56.34, 55.02, and 60.34 mg/g, respectively. Furthermore, it was observed that after six regeneration cycles, the adsorption capacity for 4-HBA remained nearly unchanged, whereas those for 4-HIPA and SA decreased by 5.6% and 2.6%, respectively. FTIR and XPS analyses revealed that all three compounds were adsorbed at the same dominant Zr cluster site on UiO-66, primarily through hydrogen bonding and electrostatic interaction. Dynamic adsorption experiments revealed that 4-HBA was the first to elute, maintaining the residual contents of 4-HIPA and SA below 0.1 wt%. Compared to traditional separation techniques, this paper provided a simple and effective method to purify industrial grade 4-hydroxybenzoic acid.