Abstract
An assortment of hydrothermal reactions of chromic(III) nitrate and 2-aminoterephthalic acid was systematically studied to yield high-quality amine-functionalized MIL-101(Cr) nanoparticles (MIL-101(Cr)-NH2). A comprehensive understanding of the role that synthesis conditions and basic modulators have on the formation of MIL-101(Cr)-NH2 in aqueous media was extracted and reported herein. With the aid of a NaOH modulator at optimized concentration, sub-20 nm MIL-101(Cr)-NH2 nanoparticles were prepared with good yield, minimized particle agglomeration, and a high Brunauer–Emmett–Teller (BET) surface area of 2800 ± 200 m2/g. To the best of our knowledge, these are the smallest particle sizes and the highest surface areas reported for directly synthesized MIL-101(Cr)-NH2. Owing to their superior surface area and Lewis basic amine functionality, the MIL-101(Cr)-NH2 nanoparticles exhibit a high CO2 adsorption of up to 3.4 mmol/g at 288 K and 1 bar and a superior CO2/N2 selectivity of 26.5 at 308 K and 0.1 bar. A high isosteric heat of −54.6 kJ/mol for CO2 adsorption implies the strong interaction between CO2 and the amine-functionalized framework. The successful synthesis of sub-20 nm amine-functionalized MIL-101(Cr) nanoparticles offers a great opportunity to engineer advanced MIL-101(Cr)-based functional adsorbents and membranes for CO2 capture and separation.
Highlights
The physical and chemical properties of the as-prepared nanoparticles were thoroughly characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and BET measurements. Using this extensive set of characterization tests, we summarize a comprehensive study pinpointing the role of incremental variations in composition and other process parameters like basic modulators on the direct synthesis of amine-functionalized MIL-101(Cr)
Reacted at 140 °C for 24 h, large aggregates with the same amorphous structure and low BET surface area were obtained as previously presented in Figures 9 and 10. This finding demonstrates the sensitive nature of using basic modulators to form Metal−organic frameworks (MOFs) from trivalent transition metals
It was observed that the reaction temperature and time play an important role in MOF crystal formation, while the influence of reactant concentration is of marginal importance because of the low solubility of the ligand in water, which appears to be the ratelimiting step for particle growth
Summary
MIL-101(Cr) (MIL: Mateŕ ial Institut Lavoisier) is a terephthalate-based mesoporous MOF composed of trimeric chromium(III) octahedral clusters interconnected by 1,4-. Benzenedicarboxylates, resulting in an augmented MTN zeotype crystal structure.[14−16] The framework contains two types of mesoporous quasi-spherical cages with diameters of ∼29 and 34 Å, respectively. MIL-101(Cr) is thermally stable in air of up to 275 °C before decomposition and can withstand various organic solvents.[15,16] These remarkable properties make MIL-101(Cr) attractive for potential applications in heterogeneous catalysis, Received: February 1, 2020 Revised: March 12, 2020 Accepted: March 22, 2020 Published: March 23, 2020
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