Optimizing the CO₂ uptake performance of an anilato-based ultramicroporous 3D MOF through a New Synthetic Protocol

Introduction

Capturing CO₂ from the atmosphere represents a key challenge, since CO₂ has been recognized as the primary anthropogenic greenhouse contributor to the increase of earth’s average temperature. Metal- Organic Frameworks (MOFs) given their porosity and versatility are considered excellent candidates for gas adsorptive separation process. We reported herein a new synthetic protocol to obtain an improvement of the BET surface area of the [Co(trz₂An)]n·3H₂O MOF, where the ultra-microporosity and the presence of a ligand bearing two triazole pendant arms are fundamental in the CO₂ uptake.

Material and Methods

[Co(trz₂An)]n·3H₂O (Co_MOF’) has been synthesized optimizing the synthetic procedure reported in the literature for Co_MOF[1]. A mixture of CoCl₂·6H₂O and trz₂Anhilate ligand in a 1:1 stoichiometric ratio, via a hydrothermal reaction, was heated at 130°C for 48 hours. The dark brown rectangular crystals, suitable for a single X-ray diffraction study, were washed three times by using an aqueous aqueous solution (pH=5) in order to solubilize and remove the Co(OH)₂ obtained during the reaction. FT-IR and BET (Brunauer–Emmett–Teller) measurements were performed to compare the surface area values of Co_MOF and Co_MOF’_.

Results

The BET surface area of Co_MOF, determined with a high-pressure gravimetric analyzer employing CO₂, showed a value of 431 m²/g. Advanced characterization of Co_MOF via FT-IR spectroscopy revealed a peak at 3632 cm^-1, which could be assigned to the presence of Co(OH)₂. The new synthetic protocol allowed us to remove the Co(OH)_2, leading to the presence of Co_MOF’, which exhibited a BET value of 616 m²/g (almost 30% of the pristine value).

Conclusions

The optimized synthetic protocol represents a challenging strategy to obtain novel MOFs with different M^II eco-friendly transition metal ions with different improved sorption properties and selectivities toward CO₂.