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

Abstract

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 °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 m² g⁻¹, 67% yield). An optimized synthesis procedure was developed to obtain high-surface area MIL-101(Fe) (SA_BET > 2400 m² g⁻¹) in a large yield and at a previously unreported temperature (80 °C vs. previously used 110–150 °C). In situ X-ray scattering was utilized to investigate the crystallization of MOF-235 and MIL-101. At 80 °C, only MOF-235 formed and at 85 and 90 °C, only MIL-101 formed.