Metal–organic frameworks to satisfy gas upgrading demands: fine-tuning the soc-MOF platform for the operative removal of H2S

Abstract

Structure/properties relationships for a series of isostructural (MOFs) with the soc topology were explored for various gas separation applications.

Abstract

A cooperative experimental/modeling strategy was used to unveil the structure/gas separation performance relationship for a series of isostructural metal–organic frameworks (MOFs) with soc-topology (square-octahedral) hosting different extra-framework counter ions (NO ₃ ⁻, Cl ⁻ and Br ⁻). In ³⁺-, Fe ³⁺-, Ga ³⁺- and the newly isolated Al( iii)-based isostructural soc-MOF were extensively studied and evaluated for the separation-based production of high-quality fuels ( i.e., CH ₄, C ₃H ₈ and n-C ₄H ₁₀) and olefins. The structural/chemical fine-tuning of the soc-MOF platform promoted equilibrium-based selectivity toward C ₂₊ (C ₂H ₆, C ₂H ₄, C ₃H ₆ C ₃H ₈ and n-C ₄H ₁₀) and conferred the desired chemical stability toward H ₂S. The noted dual chemical stability and gas/vapor selectivity, which have rarely been reported for equilibrium-based separation agents, are essential for the production of high-purity H ₂, CH ₄ and C ₂₊ fractions in high yields. Interestingly, the evaluated soc-MOF analogues exhibited high selectivity for C ₂H ₄, C ₃H ₆ and n-C ₄H ₁₀. In particular, the Fe, Ga and Al analogues presented relatively enhanced C ₂₊/CH ₄ adsorption selectivities. Notably, the Ga and Al analogues were found to be technically preferable because their structural integrities and separation performances were maintained upon exposure to H ₂S, indicating that these materials are highly tolerant to H ₂S. Therefore, the Ga- soc-MOF was further examined for the selective adsorption of H ₂S in the presence of CO ₂- and CH ₄-containing streams, such as refinery-off gases (ROG) and natural gas (NG). Grand canonical Monte Carlo (GCMC) simulations based on a specific force field describing the interactions between the guest molecules and the Ga sites supported and confirmed the considerably higher affinity of the Ga- soc-MOF for C ₂₊ (as exemplified by n-C ₄H ₁₀) than for CH ₄. The careful selection of an appropriate metal for the trinuclear inorganic molecular building block (MBB), i.e., a Ga metal center, imbues the soc-MOF platform with the requisite hydrolytic stability, H ₂S stability, and exceptional gas selectivity for ROG and NG upgrading. Finally, the soc-MOF was deployed as a continuous film on a porous support, and its gas permeation properties as a membrane were evaluated.