Preparation of Millimeter‒Sized Porous Spheres with Hierarchical Pore Structures for Resources and Environmental Materials

Abstract

This research was developed a simple and environmental friendly method to prepare uniform and millimeter‒sized porous inorganic oxide spheres with hierarchical mesoporous‒macroporous structure by combining a foaming and drop‒in‒oil method with added agar solution. Furthermore, prepared the spheres from ion-sieve and modified mesoporous silica particles were investigated to apply practical potential applications as resource and environmental materials for recovery of lithium from natural seawater and CO2 capture. Firstly, the change in size distribution and uniformity (i.e., characterized as coefficient of variation (C.V.)) and pore related properties for the alumina spheres were systematically investigated as functions of agar content. The results showed that uniform‒sized the spheres were successfully fabricated regardless of agar content, that textural properties (i.e., porosity and BET surface area) were improved by added agar solution. The C.V. values for the spheres at 1.00 wt% agar content prepared by drop‒in‒oil method were determined to be 7%. Secondly, millimeter‒sized spherical ion‒sieve foams were prepared from spinel lithium manganese oxide by using drop-in-oil method and agar gelation to recovery of lithium from natural seawater. Increasing agar content during the preparation process led to an increase in the BET surface area and mesoporous volume of the ion‒sieve spheres, however a decrease in the macropore volume. The ion‒sieve spheres with the lowest agar content exhibited greatest lithium adsorption capacity in natural seawater of 3.4 mg g‒1, and the adsorption and desorption efficiency were almost unaffected even after five adsorption‒desorption cycles. Finally, hierarchically structured millimeter‒sized spherical silica foams with high surface area, total pore volume and ordered mesoporous frameworks were successfully prepared using added agar solution and foaming method. Additionally, we evaluated a novel and, highly efficient CO2 spherical sorbent platform based on designed hierarchical mesoporous‒macroporous silica foams impregnated with polyethylenimine (PEI). The specially designed spherical sorbents offer increased amount of amine incorporation and reactive sites for CO2 capture leading to exceptional capturing performance of up to 188.3 mg g‒1 of sorbent, corresponding to 333.3 mg g‒1 of PEI. Our PEI‒impregnated a few mm-sized spherical sorbents show the highest sorption capacities in terms of CO2 per gram of PEI under the experimental conditions reported in the literature to date. At 56.5 wt% loading PEI of the sorbents (SSFs-PEI60) had only a slight decrease in capacity (less than 10 %) after 30 cycles. The novel sorbents or supports are readily and fully regenerated at relatively low temperature (100 oC) and exhibit good stability over repetitive sorption‒desorption cycling. These results have the prepared millimeter-sized porous inorganic oxide spheres with hierarchical pore structure in the present study can be applied to industrial settings as resource and environmental materials for recovery of lithium from seawater and CO2 capture.