Hybrid Membranes based on Functionalized Metal-Organic Molecule Nanocages for Efficient Aromatic Hydrocarbons Recovery
Cui Zhao, Naixin Wang, Lin Wang, Shunan Sheng, Shulan Ji*, Jian-Rong Li*
Beijing Key Laboratory for Green Catalysis and Separation and Department of Chemistry and Chemical Engineering, Beijing University of Technology, Beijing 100124, P. R. China.
The development of efficient and economic technology to separate aromatic hydrocarbons from their aliphatic mixtures has been of great concerns in chemical industry. Among them, the membrane-based separation has been recognized as one of the most promising approaches in aromatic hydrocarbons gathering.[1,2] However, the challenge for this membrane separation technique is the selection/preparation of high quality membranes with good selectivity and permeability.
Recent years, inorganic-organic hybrid membranes have gained urgent attention in the membrane separations.[3-6] Where, the unique nature of the inorganic fillers and their good dispersion and compatibility in/with the polymer are crucial for the separation performances of this type of membranes. Therefore, in order to solve the dispersion and uniformity problems, and at the same time to efficaciously control their loading in polymer, herein, our strategy is to use soluble metal-organic polyhedra (MOPs) molecules as fillers to prepare hybrid membranes, exemplified as below.
A nano-hybrid membrane with soluble MOP-tBu, [Cu24(5-tBu-1,3-BDC)24(S)24] as fillers was initially fabricated by adopting the typical co-blending method, and used in the pervaporation separation of aromatic/aliphatic hydrocarbons.[7,8] It was found that this membrane is interfacial defect-free and homogeneous, and shows excellent performance for the toluene/n-heptane separation with a high total flux of 229.5 g/m2 h and a separation factor of 19.0; while for the benzene/cyclohexane separation with a high total flux of 392.3 g/m2 h and a separation factor of 15.39 at 40 °C. More remarkable, this MOP-based hybrid membrane had long-term stability in its performance in separating aromatic/aliphatic mixtures (see Figure. 1).
Furthermore, in order to improve the separation of aromatic/aliphatic hydrocarbons, a series of isostructural functionalized MOPs/hyperbranched polymer Boltorn W3000 membranes are molecularly designed and fabricated by a new method, which helps plugging the pores within the substrate. Pervaporation tests employing these MOP-X (X = SO3Na, OH, and tBu) filled hybrid membranes demonstrated an significant improvement for the aromatic hydrocarbons separations. Particularly, the membrane with MOP-SO3Na showed the highest permselectivity for a toluene/n-heptane mixture. Simultaneously, adsorption experiments and molecular simulations also confirmed that stronger polar functionalized groups have higher adsorption capacity towards the aromatic hydrocarbons, being in agreement with pervaparation experiments results.
Overall, in light of the versatile structures and customizable chemical functionalities of MOPs materials, a lot of hybrid membranes can thus be fabricated for different separation applications, after paving the road in this work.
Keywords: hybrid membrane, metal-organic polyhedra, pervaporation, aromatic/aliphatic hydrocarbons separation
 C. P. Ribeiro, B. D. Freeman, D. S. Kalika and S. Kalakkunnath, Aromatic polyimide and polybenzoxazole membranes for the fractionation of aromatic/aliphatic hydrocarbons by pervaporation, J. Membr. Sci., 390-391 (2012), 182-193.
 N. Wang, S. Ji, J. Li, R. Zhang and G. Zhang, Poly(vinylalcohol)-graphene oxide nanohybrid "pore-filling" membrane for pervaporation of toluene/n-heptane mixtures, J. Membr. Sci., 455 (2014), 113-120.
 T. Yang, G.-M. Shi and T. S. Chung, Symmetric and Asymmetric Zeolitic Imidazolate Frameworks (ZIFs)/Polybenzimidazole (PBI) Nanocomposite Membranes for Hydrogen Purification at High Temperatures, Adv. Energy. Mater., 2 (2012), 1358-1367.
 R. Zhang, S. Ji, N. Wang, L. Wang, G. Zhang and J.-R. Li, Coordination-Driven In Situ Self-Assembly Strategy for the Preparation of Metal-Organic Framework Hybrid Membranes, Angew. Chem. Int. Ed., 53 (2014), 9775-9779.
 B. Zornoza, C. Tellez, J. Coronas, J. Gascon and F. Kapteijn, Metal organic framework based mixed matrix membranes: An increasingly important field of research with a large application potential, Micropor Mesopor Mat., 166 (2013), 67-78.
 J. Campbell, G. Sz¨¦kely, R. P. Davies, D. C. Braddock and A. G. Livingston, Fabrication of Hybrid Polymer/Metal Organic Framework Membranes: Mixed Matrix Membranes versus In-Situ Growth, J. Mater. Chem. A, 2 (2014), 9260-9271.
 J.-R Li and H.-C. Zhou, Bridging-ligand-substitution strategy for the preparation of metal-organic polyhedral, Nat. Chem., 2 (2010), 893-898.
 C. Zhao, N. Wang, L. Wang, H. Huang, R. Zhang, F. Yang, Y. Xie, S. Ji and J.-R. Li, Hybrid membranes of metal-organic molecule nanocages for aromatic/aliphatic hydrocarbon separation by pervaporation, Chem. Commun., 50 (2014), 13921-13923.
 C. Zhao, N. Wang, L. Wang, S. Sheng, S. Ji and J.-R. Li, Functionalized Metal-Organic Molecule Nanocages Filled Hybrid Membranes for Effective Aromatic Recovery: Further Analysis by combining Adsorption Experiment and DFT Calculations, submitted.
Figure 1. Pervaporation performances of MOP-tBu/W3000 nanohybrid membrane and a comparison of pervaporation performances of some reported hybrid membranes towards the separation of toluene/n-heptane mixture.