Perspective on Intensification of Haber−Bosch to Enable Ammonia Production under Milder Conditions

Ammonia is one of the most widely produced chemicals in the world. It is synthesized by reacting hydrogen and nitrogen gases at high temperature and high pressure. Over 180 million metric tonnes of ammonia produced annually accounts for 1%–2% of all global greenhouse gas (GHG) emissions, mainly due to the production of hydrogen via fossil fuel feedstocks. As such, making this process cheaper, more efficient, and less energy intensive has been a major focus of research for scientists and engineers throughout the past century. Modern research on ammonia production largely focuses on decarbonizing this process through a myriad of techniques, such as improving energy efficiency via intensification or electrification. This perspective provides insights into the progress made toward the intensification of thermocatalytic processes for ammonia synthesis under milder conditions, including alternative ammonia separation methods and better catalysts. We first review ammonia separation methods that can enable the transition from high-pressure to low-pressure ammonia manufacturing, including ammonia-selective membranes and sorbents. The performance of membranes for ammonia separation is discussed in terms of ammonia selectivity and permeance for a wide range of temperatures, with the focus on the strengths and limitations of both organic and inorganic materials. Recently developed sorbents that selectively uptake ammonia from gas mixtures are also discussed, with a special emphasis on the performance of absorbents at various experimental conditions. Since one of the potential prospects for decarbonizing ammonia synthesis lies in creating a catalyst that can operate at a lower temperature, catalytic materials that have been reported in the last two decades and can sustain production rates at temperatures below 400 °C are reviewed.