Direct Production of o-Xylene from Six-Component BTEXs Using a Channel-Pore Interconnected Metal-Organic Framework

Organic Chemistry Chemistry Chemical Engineering Materials Chemistry
o-Xylene Metal-Organic Framework Adsorption Separation BTEXs Molecular Sieve

Academic Background

In the chemical industry, the separation of benzene derivatives is a critically important and challenging process. Benzene, toluene, ethylbenzene, and the xylene isomers (o-xylene, m-xylene, p-xylene) typically exist as mixtures in the petroleum industry, collectively referred to as BTEXs. Among them, o-xylene (OX) is a key feedstock for the production of phthalic anhydride, with global demand expected to exceed $4.3 billion by 2025. However, the main industrial method for isolating OX is still distillation, a process that is not only highly energy-consuming but also environmentally unfriendly. Due to the extremely close boiling points between OX and other BTEXs, the distillation process requires a large number of theoretical plates and a high reflux ratio to achieve high-purity OX.

To address this challenge, scientists have been seeking more efficient and environmentally friendly separation methods. Metal–organic frameworks (MOFs), owing to their tunable pore sizes and surface chemistries, have been considered ideal materials to replace distillation. However, existing MOFs often lack sufficient adsorption selectivity when separating OX from other BTEXs, especially OX and ethylbenzene (EB), due to their nearly identical quadrupole moments and polarizabilities.

Source of the Research

This study is reported in a paper titled “Direct Production of o-Xylene from Six-Component BTEXs Using a Channel-Pore Interconnected Metal-Organic Framework,” co-authored by Xiao-Jing Xie, Heng Zeng, Yong-Liang Huang, Ying Wang, Qi-Yun Cao, Weigang Lu, and Dan Li. The research team is from the College of Chemistry and Materials Science at Jinan University, the Department of Chemistry at Shantou University Medical College, and other institutions in China. The paper was published in Chem on March 13, 2025, with DOI 10.1016/j.chempr.2024.10.006.

Research Process and Results

1. Material Synthesis and Characterization

The research team first synthesized a metal–organic framework material named JNU-2. JNU-2 is a channel–pore interconnected MOF, with precisely tuned pore dimensions that can completely exclude OX molecules while adsorbing large amounts of other BTEXs. Using powder X-ray diffraction (PXRD) and nitrogen adsorption experiments, the team confirmed the phase purity and porosity of JNU-2. Single-crystal X-ray diffraction (SCXRD) analysis further showed that JNU-2’s framework remains stable before and after activation, indicating its excellent chemical stability.

2. Adsorption Performance Testing

The team conducted vapor-phase adsorption experiments, individually measuring JNU-2’s adsorption capacities for benzene, toluene, ethylbenzene, o-xylene, m-xylene, and p-xylene. The results showed that JNU-2’s adsorption of OX is negligible, while its adsorption of other BTEXs is significantly higher. For example, under conditions of 353 K and 0.8 kPa, JNU-2’s adsorption amounts for m-xylene (MX), p-xylene (PX), ethylbenzene (EB), toluene (TOL), and benzene (BZ) reached 341, 344, 319, 307, and 232 mg/g, respectively.

3. Competitive Adsorption Experiments

To assess the potential of JNU-2 in separating OX from other BTEXs, the research team conducted competitive adsorption experiments. The results showed that JNU-2’s adsorption selectivity for PX/OX, EB/OX, MX/OX, TOL/OX, and BZ/OX reached 261, 272, 100, 83, and 27, respectively. These results indicate that JNU-2 can efficiently separate OX from other BTEXs under vapor-phase conditions.

4. Liquid-Phase Extraction Experiments

To further verify the potential of JNU-2 for practical application, the team carried out liquid-phase extraction experiments. 10 grams of JNU-2 were soaked in 18 milliliters of a BTEX mixture containing 90% OX, and gently shaken at room temperature for 24 hours, after which the liquid was collected under vacuum. The results showed that JNU-2 can directly extract high-purity OX from the BTEX mixture, obtaining an average of 15.2 milliliters of OX per cycle, with purity exceeding 99.5% and a recovery rate of 94%. In addition, JNU-2 maintained its structural integrity after a 30-day reflux experiment in BTEXs, further demonstrating its potential for real industrial environments.

5. Diffusion Kinetics Study

To quantify the adsorption kinetics of JNU-2, the research team used the linear driving force (LDF) model to fit the experimental data and estimated the diffusion rate constants. Results showed that the diffusion rate constants of PX, EB, MX, TOL, and BZ on JNU-2 were significantly higher than those of other adsorbent materials such as ZSM-5 zeolite and Co-MOF-74. This further demonstrates JNU-2’s high efficiency in industrial adsorption separations.

Conclusion and Significance

This research reports a novel channel–pore interconnected MOF material, JNU-2, whose precisely tuned pore size can efficiently produce high-purity OX directly from six-component BTEXs mixtures. JNU-2 not only exhibits excellent adsorption selectivity and capacity, but also shows outstanding OX purification performance in liquid-phase extraction experiments. Moreover, JNU-2 maintains its structural integrity after prolonged reflux, suggesting strong potential for industrial application.

This study provides a new design approach for efficient, environmentally friendly molecular sieve materials, promising more energy-saving chemical separation processes for the chemical industry. The successful application of JNU-2 may significantly reduce the energy consumption for OX separation, decrease environmental impact, and has important scientific value and practical significance.

Research Highlights

  1. Direct Production of High-Purity OX: JNU-2 can directly produce high-purity OX from six-component BTEXs mixtures, with purity exceeding 99.5%.
  2. Record-High Adsorption Selectivity: JNU-2 exhibits significantly higher adsorption selectivity for other BTEXs over OX compared to existing materials, especially in PX/OX and MX/OX separation.
  3. Outstanding Liquid-Phase Extraction Performance: JNU-2 demonstrates excellent performance in liquid-phase extraction, achieving OX recovery rates as high as 94%.
  4. Excellent Structural Stability: JNU-2 maintains its structural integrity after 30 days of BTEXs reflux, indicating strong potential for industrial application.

Other Valuable Information

The research team also employed density functional theory (DFT) simulations to calculate the binding energies of JNU-2 with BTEX molecules, further explaining the origin of its adsorption selectivity. In addition, JNU-2 can be synthesized simply and on a large scale, facilitating its industrial application.

This research not only provides a new solution for efficient OX separation, but also offers an important reference for developing other similar molecular sieve materials.