Investigation of hydrogen adsorption sites in ferrocene-based hypercrosslinked polymers by DFT calculations

Herein, two types of Fc-based hypercrosslinked polymers (HHCP and DHCP) with high BET surface area (953.9 and 1062.7 m2 g−1) were prepared by direct one-step phenolic polycondensation aiming at high uptake capacity towards H2. As expected, the hydrogen uptake values of HHCP and DHCP were notably higher than those of porous polymers with a higher surface area under the same conditions. The maximum H2 uptake of HCPs at 77 K/1.0 bar are determined to be 2.51 wt % for HHCP and 2.74 wt % for DHCP, respectively. Despite the pore structure playing a determinant role in hydrogen adsorption, the adsorption sites in the polymer framework can be another essential factor to influence the hydrogen adsorption capacity. The interactions between the Fc building block and H2 are revealed by DFT calculation to fully illustrate the role of Fc in H2 adsorption. The results indicate that both iron atoms and benzene rings in Fc-based HCPs contribute to the adsorption of hydrogen. Moreover, it is interesting to find that the most stable adsorption site around the Fc building unit is the position directly above the Cp ring. The results in this work may more reasonably explain the phenomenon that the introduction of Fc units could enhance the adsorption of H2 in many studies, which also provides possible insights into the application of metallocene compounds in H2 adsorption.