While metal−organic frameworks (MOFs) are promising gas adsorbents, their tortuous microporous structures cause additional resistance for gas diffusion, thus hindering the accessibility of interior active sites. Here, we present a practical strategy to incorporate missing cluster defects into a representative low-coordinated MOFs structure, Mg-MOF-74, while maintaining the stability of a defect-rich structure. In this proposed method, graphene oxide (GO) is employed as modulator, and crystallization time is varied to promote defect formation by altering the nucleation and crystal growth processes. The best performing GOmodified Mg-MOF-74 sample (MOF@GO 40 h) achieved 18% and 15% improvement in surface area and total pore volume, respectively, over pristine Mg-MOF-74. The reduced diffusion resistance to gas flow translates to increased accessibility for gas molecules to active Mg adsorption sites inside the MOFs, leading to enhanced CO 2 capture performance; the CO 2 uptake quantity of MOF@GO 40 h arrives at 6.06 mmol/g at 0.1 bar and at 9.17 mmol/g at 1 bar and 25 °C, 19.29% and 16.37% higher, respectively, than that of the pristine Mg-MOF-74, with a CO 2 /N 2 selectivity around 17.36% greater than that of pristine Mg-MOF-74. Our study demonstrates a facile approach for incorporating defects into MOFs systems with low coordination environments, thus expanding the library of defect-rich MOFs beyond the current highly coordinated MOF systems.
The performance of oxygen carriers contributes significantly to the efficiency of chemical looping combustion (CLC), an emerging carbon capture technology. Despite their low cost, Fe 2 O 3 -based oxygen carriers suffer from sintering-induced deactivation and low oxygen-carrying capacity (OCC) during CLC operations. Here, we report the development of a sinteringresistant MgO-doped Fe 2 O 3 oxygen carrier with an optimal composition of 5MgO•MgFe 2 O 4 , which exhibits superior cyclic stability and an OCC of 0.45 mol O/mol Fe (2.25 mmol O/g solid ), exceeding the widely accepted OCC limit of 0.167 mol O/mol Fe (2.08 mmol O/g solid ) of unmodified commercial Fe 2 O 3 . This result distinguishes this report from all past studies, in which efforts to enhance the cyclic stability of Fe-based oxygen carriers would always result in dilution of the OCC. The capacity enhancement by MgO is attributed to the unique mixtures of Mg x Fe 1−x O (halite) and Mg 1−y Fe 2+y O 4 (spinel) solid solutions, which effectively reduce the exergonicity for the reduction from Fe 3+ to Fe 2+ , while preventing any irreversible structural transformations during the redox process. This hypothesis-driven oxygen carrier design approach provides a new avenue for tailoring the lattice oxygen activities of oxygen carriers for chemical looping applications.
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