Enhancing Higher Hydrocarbons Capture for Natural Gas Upgrading by Tuning van der Waals Interactions in fcu-Type Zr-MOFs

Higher hydrocarbons in natural gas must be removed for safe storage, transport, and application of natural gas. Considering C3H8 and CH4 are nonpolar molecules, electrostatic interactions between C3 and MOFs are relatively weak, while they could be sensitive to the van der Waals interactions. Thus, it is an effective method to greatly enhance the separation performance by improving the van der Waals interactions through tuning the pore size of MOFs. Herein, we synthesized a series of isostructural Zr-MOFs with different pore sizes, and the separation performances of these materials for C3/C1 were systematically studied. The results indicate that pore size plays an important role in the C3 storage and C3/C1 separation in MOFs. Specifically, Zr-BPDC with large surface area and pore volume has the highest C3H8 and C3H6 adsorption capacity (159.2 cc/g and 161.5 cc/g at 298 K 1 bar, respectively), while Zr-FUM with the smallest surface area and pore volume has the highest adsorption heat for C3 as well as C3/C1 selectivities (292.0 and 242.2 at 298 K and 1 bar for C3H8/CH4 and C3H6/CH4, respectively) among the five Zr-MOFs. In addition, a defective structure in MOFs can largely improve C3 adsorption capacity for its higher surface area and pore volume, while functional groups in Zr-MOF will not obviously affect the C3 adsorption and C3/C1 separation performance. This work shows that van der Waals interactions in MOFs are predominantly for C3 adsorption and C3/C1 separation, and it can be efficiently tuned by changing the surface area and pore volume in MOFs. More importantly, this information could help design and synthesize a novel adsorbent to separate C3/C1 mixtures.