[Paper Review] Understanding the Role of Open Metal Sites in MOFs for the Efficient Separation of Benzene/Cyclohexane Mixtures

Carmen González-Galán, Rafael M. Madero‐Castro+3 more|arXiv (Cornell University)|Feb 6, 2024

Metal-Organic Frameworks: Synthesis and ApplicationsChemistry3 citations

TL;DR

This study uses grand canonical Monte Carlo simulations to demonstrate that Ni-MOF-74, Ni-ClBBTA, and Ni-ClBTDD—metal-organic frameworks with open metal sites—achieve exceptional selectivity for benzene over cyclohexane in mixed C6 cyclic hydrocarbon systems. The separation arises from preferential benzene binding to open metal sites followed by strong π–π stacking that fills the pore centers, sterically excluding cyclohexane and enabling high-performance separation beyond conventional methods.

ABSTRACT

Separating C6 cyclic hydrocarbons, specifically benzene and cyclohexane, presents a significant industrial challenge due to their similar physicochemical properties. We conducted Monte Carlo simulations in the Grand-Canonical ensemble to acquire adsorption properties and separation performance data for benzene and cyclohexane in three metal-organic frameworks featuring coordinatively unsaturated metal sites (Ni-MOF-74, Ni-ClBBTA, and Ni-ClBTDD). The separation performance of these MOFs was analyzed and compared with literature data for adsorbents of different natures, demonstrating superior performance. Additionally, we explored the molecular origins of this effective separation, examining the pore-filling mechanism, interaction of guest molecules with metal centers, and mutual interactions of each adsorbate. Our results highlight that the selected adsorbents, with remarkable loading capacity, can efficiently separate both compounds in a mixture with exceptional effectiveness.

Motivation & Objective

• To investigate the molecular mechanisms enabling selective separation of benzene and cyclohexane in metal-organic frameworks (MOFs) with open metal sites (OMS).
• To evaluate the adsorption performance of Ni-MOF-74, Ni-ClBBTA, and Ni-ClBTDD for benzene/cyclohexane mixtures under competitive conditions.
• To identify the role of pore-filling mechanisms, guest–guest interactions, and metal–guest coordination in determining separation efficiency.
• To compare the separation performance of these MOFs with literature data across diverse porous materials, establishing their superiority.

Proposed method

• Grand canonical Monte Carlo (GCMC) simulations were employed to model pure and competitive adsorption isotherms of benzene and cyclohexane in MOFs at 298 K.
• Three MOFs with identical topology but varying ligands—Ni-MOF-74, Ni-ClBBTA, and Ni-ClBTDD—were studied to isolate the effect of functionalization on separation performance.
• Analysis of radial distribution functions and density profiles revealed molecular-level pore-filling behavior and spatial distribution of adsorbates.
• Interaction energy calculations quantified the strength of benzene–metal and benzene–benzene (π–π) interactions, especially at high loading.
• The role of open metal sites in initial benzene binding and subsequent π–π stacking in pore centers was assessed through site-specific adsorption and aggregation analysis.
• Separation performance was evaluated via selectivity coefficients and compared with experimental and simulation data from prior studies.

Experimental results

Research questions

• RQ1 How do open metal sites in MOFs influence the competitive adsorption of benzene and cyclohexane?
• RQ2 What is the molecular mechanism behind the preferential adsorption of benzene over cyclohexane in MOFs with large cavities and OMS?
• RQ3 To what extent do π–π interactions between benzene molecules contribute to pore filling and exclusion of cyclohexane?
• RQ4 How do differences in ligand functionalization (ClBBTA vs. ClBTDD) affect adsorption capacity and selectivity in Ni-based MOFs?
• RQ5 Can MOFs with OMS outperform conventional adsorbents in separating benzene and cyclohexane, especially given their similar boiling points and azeotropic behavior?

Key findings

• Ni-ClBTDD exhibited the highest benzene loading capacity among the three MOFs, reaching up to 1.8 g/g at saturation, due to optimal pore size and strong π–π interactions.
• Benzene showed significantly higher selectivity over cyclohexane in all three MOFs, with separation factors exceeding 100 at low pressures, primarily driven by strong binding to open metal sites.
• At high loading, benzene molecules aggregated in the pore center via π–π stacking, leading to complete pore filling and steric exclusion of cyclohexane.
• The pore-filling mechanism was asymmetric: benzene occupied metal sites first, followed by central cavity filling, while cyclohexane remained largely excluded due to lack of favorable interactions.
• Ni-ClBTDD achieved the highest separation performance, with a selectivity of approximately 150 at low pressure and high loading, surpassing most reported porous materials.
• The absence of aromatic character in cyclohexane prevented favorable interactions with metal centers and π–π stacking, resulting in minimal adsorption even at high pressures.