Hierarchically porous monoliths based on low-valence transition metal (Cu, Co, Mn) oxides: gelation and phase separation

Abstract

Hierarchically porous monoliths based on copper (Cu), cobalt (Co) and manganese (Mn) oxides with three-dimensionally (3D) interconnected macropores and open nanopores were prepared using metal bromides as precursors via a sol-gel process accompanied by phase separation. The difficulty of gelation for low-valence metal cation was overcome by introducing a highly electronegative Br atom near to the metal atom to control the rates of hydrolysis and polycondensation. The 3D interconnected macropores were obtained using appropriate polymers to induce phase separation. The domain sizes of macropores and skeletons can be controlled by reaction parameters such as concentration and/or average molecular weight of polymers, and the amount of hydrochloric acid. The crystalline metal oxide monoliths with their 3D interconnected macroporous structure preserved were obtained after heat treatment in air.

Keywords: 3D interconnected macropore; hierarchically porous monolith; low-valence transition metal oxide; phase separation; sol–gel process.

Scheme 1.

Synthetic strategy based on sol–gel process accompanied by phase separation. From left to right, first row: SiO₂ [2] (Copyright 2008 Elsevier), Al₂O₃ [18] (Copyright 2007 American Chemical Society), TiO₂ [3] (Copyright 2011 John Wiley and Sons), ZrO₂ [19] (Copyright 2008 American Chemical Society) and Fe₂O₃ [12] (Copyright 2018 Royal Society of Chemistry); second row: carbon [7] (Copyright 2016 Elsevier), LiFePO₄ [5] (Copyright 2011 American Chemical Society), ZrP₂O₇ [10] (Copyright 2014 Royal Society of Chemistry), MgO/niobium phosphate [13] (Copyright 2019 American Chemical Society) and titanium phosphate [23] (Copyright 2015 Royal Society of Chemistry).

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Figure 1.

FT-IR spectra of (a) control solution with the stoichiometric ratio of ECH to CuBr₂ equal to 2:1 over time; (b) solution with practical stoichiometric ratio of ECH to CuBr₂ over time.

Figure 2.

(a–g) SEM images of Cu-based as-dried samples prepared with various concentrations of PVP-40k and PEO-100k; the inset in (f) is the appearance of the Cu-based as-dried sample. Scale bars: 10 μm. (h) Dependence of resultant samples morphology on starting polymer composition. (i) Pore size distributions of Cu-based as-dried samples prepared with varying concentration of PVP-40k. The macropore size distribution was obtained by mercury intrusion porosimetry, and the mesopore size distribution was obtained by nitrogen adsorption–desorption measurement. The adsorption–desorption isotherms are shown in Fig. S3.

Figure 3.

SEM images of Co-based as-dried samples prepared with various amounts and average molecule weights of PEO. PEO-100k: (a) 60 mg, (b) 70 mg, (c) 80 mg; PEO-200k: (d) 40 mg, (e) 45 mg, (f) 50 mg; PEO-600k: (g) 20 mg, (h) 25 mg, (i) 30 mg; PVP-40k: 50 mg for all samples. The inset in (h) is the appearance of the Co-based as-dried sample. The brown color in the surface is because the sample was oxidized into CoO(OH) in air. Scale bars: 10 μm. (j) Nitrogen adsorption–desorption isotherms and (k) mesopore size distributions of the Co-based as-dried samples prepared with different PEO.

Figure 4.

SEM images of Mn-based as-dried samples prepared with various amounts of HCl aq.: (a) 75 μL, (b) 90 μL, (c) 105 μL, (d) 120 μL, (e) 135 μL, (f) 150 μL, (g) 165 μL, (h) 180 μL; the inset in (e) is the appearance of the Mn-based as-dried sample. Scale bars: 10 μm. (i) Nitrogen adsorption–desorption isotherms and (j) mesopore size distributions of the Mn-based as-dried samples prepared with varied amounts of HCl aq. PEO-100k: 40 mg, PVP-40k: 60 mg for all samples.

Figure 5.

SEM images of the as-dried and heat-treated samples (insets are the appearance of heat-treated samples), and XRD patterns of heat-treated samples in Cu-system (a–c), Co-system (d–f) and Mn-system (g–i), respectively. Scale bars: 10 μm.