Nanoconfinement of metal chlorides in MIL-101(Cr) toward high ammonia adsorption and selectivity separation

A series of MCl 2 @MIL-101(Cr) composites were fabricated via vacuum wet impregnation. • CaCl 2 @MIL-101(Cr)-0.35 exhibits ultrahigh NH 3 uptake of 28.9 mmol/g at 298 K and 100 kPa. • Composite shows excellent NH 3 /N 2 selectivity and cycling stability. • Adsorption rate is extremely fast and follows pseudo-second-order kinetic. • Synergistic effect between chemical and physical adsorption was proved. Ammonia (NH 3 ) is a vital chemical raw material and promising carbon-free energy carrier, its efficient capture is essential for both industrial production and energy development. However, achieving high capacity, high selectivity and high stability in NH 3 capture adsorbents still remains a challenge. Herein, a series of metal chloride@MIL-101(Cr) composite adsorbents (M = Ca, Co, Ni) were fabricated via a vacuum wet impregnation method. Structural characterization confirmed that metal chlorides were highly dispersed within the MIL-101(Cr) pores as an amorphous phase without damaging the MOF framework. The optimal CaCl 2 @MIL-101(Cr)-0.35 achieved an ultrahigh NH 3 adsorption capacity of 28.9 mmol/g at 298 K and 100 kPa, which is 3.6 times that of pure MIL-101(Cr). The extremely high NH 3 adsorption capacity is attributed to strong coordination bonds between Ca 2+ and NH 3 , and synergistically enhanced by N–H···Cl − and N–H···O hydrogen bonds involving the Cl − as well as the framework carboxylate oxygen atoms, respectively. It also demonstrated estimated high NH 3 /N 2 selectivity (>6000 at low pressure) and good short-term cycling stability, retaining over 95% of its initial capacity after 10 adsorption-desorption cycles. The adsorption kinetics were rapid, and the process was best described by the pseudo-second-order model, indicating a chemisorption-dominated mechanism. This “confinement loading” strategy effectively combines the physisorption advantages of MOFs host and chemisorption merits of metal chlorides guests, achieving synergistic enhancement of NH 3 adsorption performance. The prepared composite material exhibits shows potential for further evaluation in industrial ammonia separation and capture.