Benzothiazolium-functionalizedNU-1000: a versatile material for carbon dioxide adsorption and cyanide luminescence sensing

Abstract

© The Royal Society of Chemistry 2020. A tailor-made benzothiazolium bromide salt functionality (BzTz) is introducedviasolvent-assisted ligand incorporation (SALI) into the mesoporous Zr-based metal-organic frameworkNU-1000. The resultingNU-1000-BzTzcomposite has been thoroughly characterized in the solid state. The functional group loading has been determined through1H NMR analysis of the digested sample (5% HF-DMSO-d6): a maximum value of 1.7BzTzligand per [Zr6] node is achieved. The material preserves its pristine crystallinity after SALI, as witnessed by powder X-ray diffraction. The functionalized MOF has a slightly lower thermal stability than its parent material (Tdec= 780vs.800 K, respectively). The N2adsorption isotherm collected at 77 K disclosed that its BET specific surface area (1530 m2g−1) is lower than that of prisitineNU-1000(2140 m2g−1), because of the space taken and weight added by the dangling benzothiazolium groups inside the pores. A total CO2uptake of 2.0 mmol g−1(8.7 wt% CO2) has been calculated from the CO2adsoprtion isotherm collected atT= 298 K andpCO2= 1 bar. Despite the lower BET area,NU-1000-BzTzshows an increased thermodynamic affinity for CO2(isosteric heat of adsorptionQst= 25 kJ mol−1) if compared withNU-1000(Qst= 17 kJ mol−1), confirming that the presence of a polar functional group in the MOF pores improves the interaction with carbon dioxide. Finally,NU-1000-BzTzhas been exploited as a luminescent sensor for polluting anions (CN−, SCN−, OCN−, and SeCN−as sodium or potassium salts) in aqueous solutions, after bromide exchange. A marked reversible blue shift of its emission band from 490 to 450 nm is observed in all cases, with the associated emission color change from light green to blue under a UV lamp. The detection limit of CN−(1.08 × 10−6M) is much lower than that measured for the other “stick-like” anions considered in this study. The process occurs efficiently even in the presence of other competing ions (i.e.in ordinary tap water), opening promising application perspectives in cyanide luminescence sensing in drinking water.