Multifunctional Metal-Organic Frameworks (MOFs)

Metal organic frameworks are a new class of advanced porous material. They attracted many scientists’ attention due to their larger surface area (Langmuir, i.e. from ca. 100 to over 6000 m2 g-1) and pore size (i.e. 0.2 to 3.8 nm) and lower densities (i.e. 1.00 to 0.09 g cm-3). In the addition, their possession of inorganic-organic properties make them more flexible for some industrial applications in comparison to other porous materials such as zeolites. However, their physical and chemical stabilities are still relatively lower than those of carbon materials (carbon–carbon bonds) and inorganic zeolites (metal-oxide bonds). MOFs can be assembled in regular orders from a diverse set of organic linkers and metal ions (or clusters) via coordination into two-dimensional (2D) or three-dimensional (3D) porous networks which makes them suitable for different applications such as gas sorption, gas separation, and many others.

B. Li, H. Wang, B. Chen, Chemistry-An Asian Journal, June 2014, 9, 1474

Separation of small gases such as hydrogen, methane, acetylene, oxygen, carbon dioxide, and nitrogen is very important for industrial processes because these small gases can be utilized as energy sources and raw materials for the production of a range of chemical product compounds. Below is some of our work for using specific MOFs as:

CO2 Capture

Here we show that a metal-organic framework (UTSA-16) displays high uptake (160 cm3 cm−3) of CO2 at ambient conditions, making it a potentially useful adsorbent material for post-combustion carbon dioxide capture and biogas stream purification. This has been further confirmed by simulated breakthrough experiments. The high storage capacities and selectivities of UTSA-16 for carbon dioxide capture are attributed to the optimal pore cages and the strong binding sites to carbon dioxide, which have been demonstrated by neutron diffraction studies.

S. Xiang, Y. He, Z. Zhang, H. Wu, W. Zhou, R. Krishna and B. Chen, Nat. Commun. 2012, 3, 954.

Methane Storage

MOFs with high methane uptake per unit mass, the key component of natural gas, can significantly facilitate the application of natural gas fuelled vehicles. We have realized a new porous metal–organic framework UTSA-76a with pyrimidine groups on the linker, exhibiting high volumetric methane uptake of ∼260 cm3 (STP) cm–3 at 298 K and 65 bar and record high working capacity of ∼200 cm3 (STP) cm-3 (between 5 and 65 bar). Such exceptionally high working capacity is attributed to the central “dynamic” pyrimidine groups within UTSA-76a, which are capable of adjusting their orientations to optimize the methane packing at high pressure, as revealed by computational studies and neutron scattering experiments.

B.Li, H-M. Wen, H.Wang, H.Wu, M. Tyagi, T.Yildirim, W. Zhou, B. Chen, J. Am. Chem. Soc., April 2014, 136, 6207.

C2H2 / C2H4 Separation

One of the most promising alternative energy and cost-efficient separation methods is to use microporous adsorbents that can selectively separate C2H4 from C2 hydrocarbons at room temperature. Below are four porous isostructural mixed-metal–organic frameworks (M′MOFs) that have been synthesized and structurally characterized. The pores within these M′MOFs are systematically tuned by the interplay of both the metalloligands and organic ligands which have enabled us not only to direct their highly selective separation of chiral alcohols 1-phenylethanol (PEA), 2-butanol (BUT), and 2-pentanol (2-PEN) with the highest ee up to 82.4% but also to lead highly selective separation of achiral C2H2/C2H4 separation. The potential application of these M′MOFs for the fixed bed pressure swing adsorption (PSA) separation of C2H2/C2H4 has been further examined and compared by the transient breakthrough simulations in which the purity requirement of 40 ppm in the outlet gas can be readily fulfilled by the fixed bed M′MOF-4a adsorber at ambient conditions.

M.C. Das, Q. Guo, Y. He, J. Kim, C.-G. Zhao, K. Hong, S. Xiang, Z. Zhang, K. M. Thomas, R. Krishna, B. Chen, J. Am. Chem. Soc., 2012, 134, 8703. 
(http://pubs.acs.org/doi/abs/10.1021/ja302380x)

Heterogeneous Catalysis

The enantioselective ring-opening of meso-epoxides by aromatic amines was achieved by using a new chiral metal–organic framework UTSA-32a. The corresponding α-hydroxyamines were obtained with good yields and ee values (up to 89% ee).

S.Regati, Y. He, M. Thimmaiah, P. Li, S. Xiang, B. Chen and C.-G. Zhao ,Chem. Commun., Aug 2013, 49, 9836.

Also, two new Zn(II) and Cd(II) MOFs have been synthesized. These MOFs have been applied as heterogeneous catalysts for the green synthesis of a variety of dihydropyrimidinone derivatives through the Biginelli reaction and the desired products were obtained in high yields with short reaction time under mild solvent-free conditions. Moreover, the MOF catalysts may be readily recovered after the reaction and reused for many cycles.

P. Li, S. Regati, R. J. Butcher, H. D. Arman, Z. Chen, S. Xiang, B. Chen, C.-G. Zhao, Tetrahedron Letters, 2011, 52, 6220.

Luminescent Materials

A luminescent mixed lanthanide metal–organic framework approach has been realized to explore luminescent thermometers. The targeted self-referencing luminescent thermometer Eu0.0069Tb0.9931-DMBDC (DMBDC = 2, 5-dimethoxy-1, 4-benzenedicarboxylate) based on two emissions of Tb3+at 545 nm and Eu3+ at 613 nm is not only more robust, reliable, and instantaneous, but also has higher sensitivity than the parent MOF Tb-DMBDC based on one emission at a wide range from 10 to 300 K.

Y. Cui, H. Xu, Y. Yue, Z. Guo, J. Yu, Z. Chen, J. Gao, Y. Yang, G. Qian, B. Chen, J. Am. Chem. Soc., 2012, 134, 3979.