Holistic synthesis of iron-based metal-organic frameworks using unconventional metal feedstock and polyethylene terephthalate derived organic linkers

Abstract

The high costs associated with the synthesis of Metal-organic frameworks (MOFs) owing to the expensive raw materials have significantly attributed to the lack of large-scale production of MOFs regardless of their appealing qualities. The synthesis of MOFs from polyethylene terephthalate (PET) waste source for organic linker and other metal sources has recently been reported as an alternative way to lower the costs of the synthesis process of MOFs thus enabling their industrialization. Though other metal sources used as metal precursors for MOF synthesis had been reported, this study focuses on the use of AMD as well as PET waste precursors to successfully synthesize MOFs of two different morphologies but in the same class. In this work, Fe-MIL-101 and Fe-MIL-88B were successfully synthesized using acid mine drainage (AMD) wastewater as the source for the Iron (Fe) metal as well as BDC derived from waste PET bottles. Two extraction methods (A. oxidation & precipitation with air; B. oxidation with Hydrogen peroxide & precipitation with NaOH) were explored for the extraction of Fe in the AMD thus yielding Fe of different phases / species. The synthesis procedure follows a widely reported solvothermal route and the waste-derived MOFs are compared to MOFs synthesized from commercial precursors. Fe-MIL-101 synthesized via the solvothermal route from waste precursor sources displayed similar properties to those synthesized from commercial grade precursors. Octahedral morphologies were observed on SEM imaging however, materials prepared from AMD-derived Fe dissolved in HCl displayed mixed morphologies of Fe-MIL-88B and Fe-MIL-101 as well as broken Fe-MIL-88 structures. XRD and FTIR exhibited characteristic diffraction peaks as well as vibrations and stretching bends that confirmed the coordination of BDC to the metal centre, respectively. OHoAMD-Fe-MIL-101(Cl)pet exhibited the lowest textural properties of 52.5 m2/g and 0.0623 cm3/g, however, all Fe-MIL-101 materials displayed type I N2 isotherms with surface areas within the range of 501-2052 m2/g and pore size distribution in the micro- and mesoporous range. Fe-MIL-101 samples prepared from waste Fe displayed decreased surface areas and pore volumes compared to Fe-MIL-101 prepared from commercial precursors. All prepared materials proved to be thermally stable up to temperatures above 300 °C, typical of Fe-MIL-101 and further confirming the successful preparations of Fe-MIL-101 from both commercial and waste-derived precursors. Hydrogen adsorption studies on the Fe-MIL-101 material yielded 1.18 wt% at 1 bar and 77 K which is comparable to the values previously reported in literature relative to the surface area obtained. Waste derived Fe-MIL-88B displayed similar morphologies, crystal phases as well as vibrations and stretching bands when compared to those prepared from commercial precursors. Most Fe-MIL-88B MOFs showed a type I N2 isotherm however, waste-derived Fe-MIL-88B had enhanced surface areas when compared to those prepared from commercial grade precursors with surface areas significantly below 100 m2/g. The pore size distribution mostly lied in the meso-porous range with some in the macro-porous range for most Fe-MIL-88B MOFs. The use of AMD as a source for Fe yielded Fe-MIL-88B MOFs with high thermal stabilities surpassing Fe-MIL-88B prepared from commercial precursors. Hydrogen adsorption studies conducted at 77 K and 1 bar showed that waste derived Fe-MIL-88B MOFs were good candidates since a value of 0.81 wt% found to be consistent with the previously reported data for M-MIL-88B MOFs to date. This study has therefore, successfully demonstrated that a holistic strategy that utilizes two waste starting materials (AMD and waste PET) can be used to produce Fe-based MOFs of comparable properties as those obtained when commercial feedstock is used. This strategy contributes to lowering of costs associated with the production of MOFs and thus advancing them towards commercialization.