(663f) Strontium-Based Bimetallic Metal Organic Frameworks (MOFs) for CO2 Separation

Metal organic frameworks (MOFs) are promising candidates for separation and purification applications due to their tunable chemistry, high surface area, and porosity [1]. An interesting class of nanoporous bimetallic compounds known as strontium-based metal-organic frameworks have been synthesized on the organic basis of malonate. Cobalt and copper were employed to form MOFs with the general formula of [SrM(C₃H₂O₄)_{2 (}H₂O)_n], (M=Co, n=7 and M=Cu, n=4). These MOFs are designated with the refcodes of FOHQUO and VAMQOP in the Cambridge Structural Database, respectively.

Haldoupis and co-workers [1] examined a new category of MOFs using atomic simulation for gas sorption including FOHQUO. According to their results, FOHQUO with pore limiting diameter of 3 Ã? has a binary adsorption selectivity above 190 with water-free molecular lattice [1]. Having removed all the guest water molecules between 70°C to 190°C, they report that FOHQUOâ??s framework integrity remain stable up to 350°C [1]. A molecular simulation study on mixed matrix membranes with FOHQUO as the dispersive phase was conducted by Yilmaz and Keskin [2] for CO₂/N₂ separation. According to their predictions, FOHQUO has a CO₂ permeability of about 500000 Barrer and CO₂/CH₄ selectivity of about 5000.

One of the challenges in MOFs nature is their thermal, chemical and physical stability under heat treatment and in presence of acids and moistures. The crystalline lattice of as-synthesized MOFs are typically filled with guest molecules of solvents [3]. Solvent removal can pose obstacles in crystallinity of MOFs and collapse the framework integrity and convert to amorphous materials [4].

In this study, we have synthesized FOHQUO and VAMQOP to investigate their potential for use in CO₂ separation and mixed matrix membrane formation. We examined their thermal and structural stability, surface area, and gas sorption capacity. Interestingly, X-ray diffraction (XRD) analysis proved that FOHQUO framework was not stable above 80°C. However, TGA analysis reflected the dehydration step at 80°C. On the other hand, VAMQOP has a stable structure up to 140°C. XRD analysis revealed that, after an amorphous period above 170°C, phase transformation occurs.

This work is supported by TUBITAK (Turkish Scientific and Technological Research Council) through Project No. 113M776.

References:

1. Haldoupis, E., Nair, S., Sholl, D. S. (2012), Finding MOFs for Highly Selective CO2/N2 Adsorption Using Materials Screening Based on Efficient Assignment of Atomic Point Charges, J Am Chem Soc, 134, 4313-4323.
2. Yilmaz, G., Keskin, S. (2014). Molecular Modeling of MOF and ZIF-Filled MMMs for CO2/N2 Separations, Journal of Membrane Science, 454, 407-417.
3. Qiu, S., Xue, M., Zhu, G. (2014), Metal-Organic Framework Membranes: From Synthesis to Separation Applicatio,. Chem Soc Rev, 43, 6116-6140.
4. Mondloch, J. E., Karagiaridi, O., Farha, O. K., Hup, J. T. (2013,. Activation of Metalâ??Organic Framework Materials, CrystEngComm, 15, 9258.