Synthesis of a modified HF-free MIL-101(Cr) nanoadsorbent with enhanced H2S/CH4, CO2/CH4, and CO2/N-2 selectivity

Despite worldwide attention toward MIL-101(Cr) as a superior adsorbent during past decade, it is still failed to be commercialized. The main barrier can be addressed to the poisonous and expensive synthesis procedure of MIL-101(Cr). In this study, nitric acid (HNO3) was introduced as a premier, inexpensive and non-poisonous substitute for typical hydrofluoric acid (HF) in the hydrothermal synthesis of MIL-101(Cr). A series of MIL-101-HNO3 nanoadsorbents, with different amounts of HNO3, were synthesized and characterized by different techniques. For the first time, the H2S, CO2, CH4, and N2 isotherms on the MIL-101-HNO3 samples were measured in a wide pressure region (1–35 bar) and different temperatures (273, 283, and 293 K) using an in-house made volumetric setup. The results indicated that the MIL-101-HNO3-1 possessed the maximum surface area (3841 m2/g), pore volume (1.72 cm3/g), and as a consequence, the highest H2S (8.80 mmol/g at 1 bar and 27.16 mmol/g at 35 bar) and CO2 uptake (5.86 mmol/g at 1 bar and 21.98 mmol/g at 35 bar) among all synthesized samples. More importantly, ideal adsorbed solution theory (IAST) indicated that H2S/CH4, CO2/CH4, and CO2/N2 adsorption selectivity elevated up to 166.9%, 21.8%, and 13.6% at 1 bar compared to the conventional MIL-101-HF-1. These advantageous were all ascribed to the additional open Cr3+ metal sites and more electrostatic adsorptive behavior of MIL-101-HNO3-1. The impressive preferential gas adsorption, along with excellent cyclic performance, thermal and water stability, made the MIL-101-HNO3-1 a promising replacement for hazardous MIL-101-HF-1 and other commercial adsorbents.