Natural gas, composed primarily of methane, has many potential uses as a cleaner and more renewable source of energy than other fossil fuels. However, about 20% of US natural gas reserves contain levels of N2 that are too high for pipeline processing. Using natural gas from renewable sources also encounters this problem. Furthermore, in processing steps to create high-purity methane from its various sources, the removal of N2 remains a significant energetic cost. This separation is typically performed through cryogenic distillation, and improvements in energy efficiency of this separation are necessary to utilize the many available sources of methane. Switching to membrane or adsorbent-based technologies could potentially alleviate this challenge. Size selective molecular sieves and membranes have demonstrated some ability for separating N2 from CH4, but face problems with scalability and selectivity; and current adsorbents need significant improvements in selectivity and capacity for N2 to be commercially viable.
To address this situation, researchers at UC Berkeley have developed a new adsorbent V2Cl2(btdd) with exceptional affinity for nitrogen, such that early experiments already demonstrate a N2/CH4 selectivity of over 10x greater than any reported material. The Berkeley material is a permanently porous vanadium(II)-containing metal-organic framework (MOF). It represents the first example of a MOF with five-coordinate vanadium(II) centers as the primary metal node. The electronic properties of these five-coordinate V(II) centers make this MOF uniquely reactive towards relatively inert and weak electron acceptors, such as nitrogen, creating a stronger M–N2 interaction than any known MOF. Additionally, the high-density of V(II) centers translates to a high gas uptake capacity, qualifying this material as a promising N2/CH4 selective adsorbant. Key performance parameters can be tuned as the building blocks are synthetically modifiable.
Removing dinitrogen from natural gas