Studies on Improvement of Surface Characteristics and Stability of Cr, Cu and Zn Based Metal Organic Frameworks (MOFs)

Abstract

Metal Organic Frameworks” or MOFs is a term being associated to represent a class of novel adsorbents that has caught the attention of researchers owing to their great diversity in structures resulting from co-ordination bonds between inorganic metal atoms as nodes and organic ligands as linkers. They have high specific surface area (ca. 1000-5000 m2/g), large pore volume (ca. 0.7-2.5 cc/g) and low to moderate heat of adsorption (ca. 12 to 30 kJ/mol at moderate coverage).
Proper selection of metal atoms and organic linkers leads to innumerable possibilities in the coordination geometry with wide variation in topology and functional groups. The tunable matrices or so-called tailor made structures has made it possible to design and synthesize materials meeting specific applications. Porosity, crystallinity and product purity are some of the key features of MOFs. Adsorptive gas separation/purification, gas storage and catalysis are some of the potential areas of study.
Although MOFs have shown tremendous potential, however there remain certain challenges which are typical of this class of adsorbents. Before being explored effectively at the industrial level, it is important to study some of their key features. MOFs are reported to show highest surface area to the tune of ca. 1000-5000 m2/g. A careful review of literature has shown a major anomaly though. It is been observed that surface area and pore volume for the same MOF synthesized and reported by different research groups across different laboratories varied considerably. This is even true for different batches of the same MOF synthesized in the same lab. The reasoning behind this can be attributed to various synthesis conditions and post-treatment thereafter. It is been observed that percentages of impurities formed during synthesis requires a thorough post synthesis treatment to remove all the residual impurities which eventually dictates the “effective surface area” for any gas adsorption study. Thermal and chemical stability of MOFs is another area which needs to be studied carefully. The effectiveness of any MOF adsorbent can be measured from its robustness at high temperature and structural immunity against various organic, inorganic and atmospheric conditions.
We conceived our idea on present research work based on the discussions in the preceding paragraph. Our research objectives can be broadly divided into following sub-sections:
(I) Synthesizing MOFs of various topologies and tuning the synthesis route for better reproducibility
(II) Studying thermal and chemical stability of the synthesized materials
(III) Studying the surface characteristics of the synthesized materials
It is noteworthy to mention here that a careful review of the literature reveals more than 2,000 different MOF structures being synthesized and characterized. Although the number speaks volumes about their variation in structural configuration but the most important in that series include IRMOFs, Cu-BTC and MIL series. For our present research work we have opted for
(I) Zn-BDC also known as MOF-5 or IRMOF-1
(II) Cu-BTC more familiarly HKUST-1 or MOF-199
(III) Cr-BDC popularly known as MIL-101
Synthesis and characterization of Cu-BTC, Cr-BDC and Zn-BDC samples were successfully carried out in our present study. SEM images confirmed the authenticity of each of the MOFs. Cr-BDC and Cu-BTC were found octahedral in shape whereas Zn-BDC crystals appeared to be cubic in nature. The presence of impurities was not uncommon in MOF synthesis and here as well we had seen reactions leading to the formation of impure by-products. Post synthesis treatments in different organic environments were particularly important in removal of these impurities. Powder XRD patterns validated the crystalline nature of the products formed and thereby degree of purity in various batches owing to different solvent treatments. TGA patterns were typical of specific MOFs synthesized. However, a shift towards low thermal stability had been observed for cases where solvents eliminated impurities. Although it appeared that elimination of impurities decreased the thermal stability of the products but if not, eventually it would lead to reducing the effective surface area which is most crucial in any gas adsorption study. The packing density data nicely corroborated the TGA analysis.