(224e) Metallic Membranes for N2 Separation and NH3 Production

The availability of nitrogen, phosphorus, and water are the three main factors that limit our ability to produce enough food to feed the growing population of the planet. The nitrogen cycle is one of the most significant biogeochemical cycles on Earth, as nitrogen is an essential nutrient for all forms of life. Although freely available in the atmosphere as dinitrogen (N₂), access to fixed forms of nitrogen constitutes, in many cases, the most limiting factor for plant growth. The industrial production of ammonia for fertilizers via the current Haber-Bosch process is an energy-intensive process that consumes 1-2% of the world’s annual energy supply. For these reasons, the need for advanced catalytic methods for the reduction of N₂ to ammonia remains a requirement for sustainability in the food, energy and water systems cycle.

The aim of this work is to explore the potential of metallic membranes for N₂ separation with the final intent to produce NH₃. Based on a preliminary theoretical investigation using density functional theory, the Group V transition metals (e.g., vanadium (V), niobium (Nb) and tantalum (Ta)) show strong affinity toward N₂. Moreover, from solubility and diffusivity values taken from the literature, iron (Fe) is a suitable fit for this application. Therefore, V, Nb, Ta and Fe foils with a thickness of 40 μm have been chosen as membrane materials in this study.

Permeation tests with pure gases (He, N_2, CO₂ and CH₄) have been performed to characterize the membranes in terms of the N₂ permeating flux and ideal selectivity at different temperature and pressure conditions, varying from 400 to 600 °C and from 2.0 to 60 bar, respectively. Moreover, scanning electron microscopy (SEM) and an electron microprobe analyzer (EMPA) have been used to investigate the effect of the different operating conditions on the membrane surface. From the experimental results, all of the membranes show infinite selectivity towards N₂ permeation at each operating condition investigated and V showed better performance with respect to Nb, Ta, and Fe.

The V membrane is housed in the membrane reactor where a H₂ stream is used as the sweep gas to promote the NH₃ reaction. Lower pressure than the conventional Haber-Bosch process is used. Specifically, an operating pressure of 60 bar, instead of 200 bar (Haber-Bosch process), and a temperature of 500 °C are used as operating conditions to produce NH₃. The performance of the membrane reactor in terms of NH₃ conversion is, hence, investigated.