Metal chloride functionalized MOF-253(Al) for high-efficiency selective separation of ammonia from H2 and N2

Abstract

Ammonia (NH 3 ) is produced from nitrogen (N 2 ) and hydrogen (H 2 ) by the Harber-Bosch process at high pressure and high temperature. Due to the reversibility and low conversion efficiency of the reaction, the product contains a large amount of raw gas, which seriously affects the production efficiency of ammonia. Therefore, it is essential to exploit high-performance adsorbents for separating NH 3 from NH 3 /N 2 /H 2 mixtures. Herein, we designed and synthesized a kind of metal–organic framework (MOF) composites by anchoring metal chloride (NiCl 2 , CoCl 2 , or SnCl 2 ) on the bipyridinium groups in the MOF-253(Al) pores. It is found that the optimal composite shows an NH 3 uptake capacity of 18.0 mmol/g at 25.0 °C and 1.0 bar, which is 3.33 times that of the pristine MOF-253(Al), and the composite exhibits a much faster adsorption kinetics of NH 3 than the pristine MOF-253(Al). Significantly, the excellent NH 3 uptake capacity at low pressure allows selective trapping of NH 3 from NH 3 /N 2 /H 2 at 25.0 °C with the selectivity coefficients of 5708 to N 2 and 2320 to H 2 . Spectral measurements reveal that the synergistic action of NH 3 coordination to the metal in the MOF and the hydrogen bonding of NH 3 with both O atoms of the carboxyl groups and Cl − of the guest molecules account for such an excellent capture and selectivity. In general, the strategy developed here is simple and effective for improving the selectivity of NH 3 separation, which opens a new way for the design of high-performance solid adsorbents.