Experimental investigations into the irregular synthesis of iron(iii) terephthalate metal-organic frameworks MOF-235 and MIL-101

MOF-235(Fe) and MIL-101(Fe) are two well-studied metal-organic frameworks (MOFs) with dissimilar crystal structures and topologies. Previously reported syntheses of the former show that it has greatly varying surface areas, indicating a lack of phase purity of the products, i.e. the possible presence of both MOFs in the same sample. To find the reason for this, we have tested and modified the commonly used synthesis protocol of MOF-235(Fe), where a 3 : 5 molar ratio of iron(iii) ions and a terephthalic acid linker is heated in a 1 : 1 DMF : ethanol solvent at 80 degrees C for 24 h. Using XRD and BET surface area (SA(BET)) measurements, we found that it is difficult to obtain a pure phase of MOF-235, as MIL-101 also appears to form during the solvothermal treatment. Comparison of the XRD peak height ratios of the synthesis products revealed a direct correlation between the MOF-235/MIL-101 content and surface area; more MOF-235 yields a lower surface area and vice versa. In general, using a larger (3 : 1) DMF : ethanol ratio than that reported in the literature and a stoichiometric (4 : 3) Fe(iii) : TPA ratio yields a nearly pure MOF-235 product (SA(BET) = 295 m(2) g(-1), 67% yield). An optimized synthesis procedure was developed to obtain high-surface area MIL-101(Fe) (SA(BET) > 2400 m(2) g(-1)) in a large yield and at a previously unreported temperature (80 degrees C vs. previously used 110-150 degrees C). In situ X-ray scattering was utilized to investigate the crystallization of MOF-235 and MIL-101. At 80 degrees C, only MOF-235 formed and at 85 and 90 degrees C, only MIL-101 formed.

Automated summary

The study found that impurities commonly seen in the synthesis of MOF-235(Fe) may be due to the simultaneous formation of MIL-101(Fe) during the solvothermal treatment. The content of MOF-235 and MIL-101 directly influences their surface area, and an optimized synthesis method can yield pure MOF-235 product and high-surface area MIL-101.