Highly-Controlled Soft-Templating Synthesis of Hollow ZIF-8 Nanospheres for Selective CO₂ Separation and Storage

Global warming is an ever-rising environmental concern, and carbon dioxide (CO₂) is among its major causes. Different technologies, including adsorption, cryogenic separation, and sequestration, have been developed for CO₂ separation and storage/utilization. Among these, carbon capture using nano-adsorbents has the advantages of excellent CO₂ separation and storage performance as well as superior heat- and mass-transfer characteristics due to their large surface area and pore volume. In this work, an environmentally friendly, facile, bottom-up synthesis of ZIF-8 hollow nanospheres (with reduced chemical consumption) was developed for selective CO₂ separation and storage. During this soft-templating synthesis, a combined effect of ultra-sonication and low-temperature hydrothermal synthesis showed better control over an oil-in-water microemulsion formation and the subsequent growth of large-surface-area hollow ZIF-8 nanospheres having excellent particle size distribution. Systematic studies on the synthesis parameters were also performed to achieve fine-tuning of the ZIF-8 crystallinity, hollow structures, and sphere size. The optimized hollow ZIF-8 nanosphere sample having uniform size distribution exhibited remarkable CO₂ adsorption capability (∼2.24 mmol g^-1 at 0 °C and 1.75 bar), a CO₂/N₂ separation selectivity of 12.15, a good CO₂ storage capacity (1.5-1.75 wt %), and an excellent cyclic adsorption/desorption performance (up to four CO₂ adsorption/desorption cycles) at 25 °C. In addition, the samples showed exceptional structural stability with only ∼15% of overall weight loss up to 600 °C under a nitrogen environment. Therefore, the hollow ZIF-8 nanospheres as well as their highly controlled soft-templating synthesis method reported in this work are useful in the course of the development of nanomaterials with optimized properties for future CO₂ capture technologies.