Insights into structure-composition-capacity relationship of ultrastable metal halide absorbents for ammonia separation and storage
S. M. Tanveer Mahtab, Mahdi Malmali
Abstract
Metal halide (MH) salts have emerged as promising absorbents for ammonia (NH 3 ) separation and storage due to their high NH 3 affinity in a wide range of temperatures and concentrations. This study explores the performance and stability of NH 3 sorbents synthesized using various impregnation techniques under separation conditions closely similar to NH 3 synthesis temperature in a dynamic breakthrough system. Various chemistries were explored with the goal to enhance the structure of supported MH absorbents, including incipient wetness impregnation (IWI), wet impregnation (WI), and in-situ impregnation (ISI). MHs were impregnated into highly ordered mesoporous SBA-15 via different methods to create an ultrastable sorbent, which outperformed the baseline sorbent supported on silica gel. Optimizing structure-composition through pH adjustment led to highly robust absorbents with precisely dispersed MHs within the support. Among various absorbents evaluated for NH 3 synthesis, IWI-SBA-15-MgCl 2[Mg-OH] exhibited an exceptional capacity of 80 mg NH3 /g sorbent (≈293 mg NH3 /g salt ) at 45 kPa (NH 3 ) and 300 °C. This sorbent maintained reproducible working capacity for over 50 cycles without showing any capacity deterioration and pressure build up. Notably, minimal in-situ impregnation of MgCl 2 in SBA-15 (ISI-SBA-15-MgCl 2[Mg-OH] ) sample led to the highest capacity (48 mg NH3 /g sorbent ≈ 380 mg NH3 /g salt ) at 45 kPa and 300 °C, thus obviating the need for direct salt impregnation step. These findings shed light on the structure-composition-capacity relationship needed for designing ultrastable MH absorbents.