Ammonia (NH3), a primary feedstock for fertilizer, has recently been proposed as an energy vector for transportation and storage. The major challenge with the latter aspect is that NH3 production largely relies on the Haber-Bosch (HB) process, which burns fossil fuel for raw material. HB synthesis loop maintains high pressure (≥ 10 MPa) and temperature (≥ 400°C) in the reactor for cost-effective conversion, along with energy-intensive phase-changing condensation to separate NH3. Such harsh conditions make the HB still a energy- and cost-intensive process. Therefore, conventional manufacturing is taking place in large, centralized facilities. Such large-scale plants make integration with renewable resources almost impossible. It is proposed that the HB process can be improved by replacing the condensation with absorption. The previous reports suggest the promise of this approach by using metal halide (MH) salts as absorbents. The major improvement is resulted from the instant and selective separation of NH3 at higher temperatures. The proposed reaction-absorption process provides the flexibility to operate at more moderate conditions that better serves inherently safer distributed manufacturing. The major shortcoming of MHs is that they are not mechanically stable in their pure form. We have been working on designing and developing stable MH absorbents that can effectively and reproducibly separate and release NH3 at moderate conditions. We have investigated various chemistries to design stable supported MH absorbents. Such absorbents have shown the promise to achieve +95% NH3 purity when the absorber is operated at its optimal conditions. To synthesize a stable absorbent, MHs were impregnated into highly ordered mesoporous silica (HMS) through various methods. We found that slight modifications in impregnation conditions by adjusting the electrostatic charges can lead to the formation of highly robust supported absorbents, in which MHs are effectively distributed within the porous structure of the support. It was found that the supported absorbent synthesized using the charge-enhanced impregnation possesses a larger surface area. In this presentation, I will present our recent findings on the design and performance of several classes of impregnated absorbents for facile NH3 separation and release.

Robust supported metal halide absorbents for ammonia separation and storage

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