Uncharacteristic Adsorption Breakthrough Behavior of a Core-Shell Copper Hydroxysulfate Metal-Organic Framework Against Gaseous Formaldehyde

Abstract

Core-Shell structured microspheres with a packed nano-platelet copper hydroxysulfate (CHS) core and an octahedral crystalized metal-organic framework-199 (CHS@M199) shell are synthesized and tested as adsorbent for gaseous formaldehyde (FA). CHS@M199 outperforms reference materials (e.g., activated carbon (AC), CHS, and M199) with adsorption capacity (Q) of 8.13 mg g-1 and partition coefficient (PC) of 0.263 mol kg-1 Pa-1 against 10 Pa FA at 10% breakthrough (BT) level with the aid of large surface area and copper sites for coordinating with carbonyl group. Contrary to general expectations, CHS@M199 exhibits a unique adsorption behavior in that BT volume increases systematically with the rise in FA inlet partial pressure (e.g., 5-10 Pa). The 10% BT capacity of CHS@M199 decreases noticeably (8.13 to 2.08 mg g-1) with increasing relative humidity (0.016 to 10%), reflecting water-FA competition for hydrophilic adsorption sites. Although FA adsorption on CHS@M199 diminishes with elevated RH levels, such reduction intensifies when simulating ambient conditions (through stepwise addition of competing components: O2, CO2, and H2O). The FA adsorption on CHS@M199 is well described by both pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetics. Polymerization may play a pivotal role in the adsorption of FA on CHS@M199, as Qi-LeVan isotherm best fits the experimental data.

A core-shell structured copper hydroxysulfate (CHS) metal-organic framework-199 (CHS@M199) is synthesized. CHS@M199 exhibits adsorption capacity of 8.1 mg g-1 at 10 Pa formaldehyde (FA) at 10% breakthrough level. Its FA uptake is suppressed by the presence of O2, CO2, and H2O and by elevated temperature, while being promoted at higher inlet concentrations to reflect its unique structural flexibility.image