Particulate Matter Capturing via Naturally Dried ZIF-8/Graphene Aerogels under Harsh Conditions

摘要

class="head no_bottom_margin" id="sec1title">IntroductionHaze problem, mainly caused by particulate matter (PM)2.5 defined by an aerodynamic diameter of 2.5 μm or less, strongly influences human health in our daily life (, ). In 2013, 87% of the world population was living in regions that have surpassed the PM2.5 limit of 10 μg m⁻³, an air quality limit set by the World Health Organization (). Especially in countries like China, the average concentrations of PM2.5 reached 54.5 μg m⁻³, and 92% of the population has experienced over 120 h of exposure to unhealthy air during a 4-month period (). An analysis of the sources of PM2.5 pollution in China indicates that the primary particulate emission comes from traffic, coal or biomass burning, and dust, whereas the secondary origin is mainly from aerosol precursors from SOx, NOx, and volatile organic compounds (). PM2.5 seriously threatens human health because of its toxic component and the potential menace caused by its small size of penetration of human bronchi and lungs. Based on a report by the Global Burden of Disease, PM2.5 was a major predisposing factor for 2.9 million deaths in the 2013 (). Therefore highly efficient technology for rapid PM2.5 capture, particularly at the source of emission, is urgently needed.Previous strategies have been explored to tackle PM2.5 pollution particles, such as using commercially available air filters (thick fabric, active carbon, etc.), polar polymer nanofiber filters (, ), conductive silver nanowire filters (), and porous metal-organic frameworks (MOFs) as filters (, ). However, the filtration of conventional air filters is usually performed at a low initial concentration (<1,000 μg cm⁻³) and ambient conditions. For some specific applications, such as the filtration of vehicle exhaust and chimney exhaust, the flow rate and pressure of pollution gas are high under harsh conditions (e.g., high temperature, rapid flow rate, and large humidity). Some particles may not be effectively captured if only these conventional membranes are used. The robust air filters for the realization of both high removal efficiency and low pressure drop for highly concentrated PM under harsh conditions is still a big challenge (). Therefore it is desirable to design a 3D porous framework with evenly distributed and well-connected pores for efficient particle capture (, ). An ideal structure should be highly porous to minimize resistance to a high-rate flow gas and possess a large surface area to capture the particles.Graphene aerogels (GAs), which combine the chemical nature of the material with porous networks, meet the requirements mentioned above. They also exhibit high mass efficiency for separation and adsorption for oils, metal ions, and organic solvents (, href="#bib5" rid="bib5" class=" bibr popnode">Cong et al., 2012). Self-assembly, cross-linking, chemical vapor deposition (CVD), and 3D printing are common methods for the synthesis of GAs (href="#bib3" rid="bib3" class=" bibr popnode">Cao et al., 2011, href="#bib31" rid="bib31" class=" bibr popnode">Wei et al., 2013, href="#bib34" rid="bib34" class=" bibr popnode">Xu et al., 2010, href="#bib42" rid="bib42" class=" bibr popnode">Zhu et al., 2015). Most of these approaches with the exception of CVD involve freeze- or supercritical drying, leading to high cost and low production yield. Compared with these approaches, a natural drying technique is more practical owing to its productivity and good scalability (href="#bib22" rid="bib22" class=" bibr popnode">Li et al., 2016, href="#bib33" rid="bib33" class=" bibr popnode">Xu et al., 2016, href="#bib35" rid="bib35" class=" bibr popnode">Yang et al., 2015). To maximize the removal efficiency using GAs, high-specific-surface-area adsorbents with micro- or nanoscale porosity could be decorated on the GA networks. In this regard, MOFs are ideal candidates as they are ultraporous materials with secondary building units (metal clusters, or known as metal-containing nodes) and organic linkers (href="#bib8" rid="bib8" class=" bibr popnode">Furukawa et al., 2013). They have attracted a great deal of interest in energy and environmental fields, such as energy storage, separation, and pollutant control (href="#bib29" rid="bib29" class=" bibr popnode">Sumida et al., 2012, href="#bib30" rid="bib30" class=" bibr popnode">Wang et al., 2014, href="#bib38" rid="bib38" class=" bibr popnode">Zhang et al., 2016).Here we demonstrate a novel strategy to uniformly decorate MOFs on reduced graphene oxide aerogel (rGA) by the combination of in situ crystallization of MOFs and naturally drying the resultant composited hydrogel. The microporous structures of zeolite imidazole framework-8 (ZIF-8) contribute to the high special surface area, whereas the macropores of rGAs provide accessibility to the active surfaces. Free metal sites, functional groups, and electrostatic interaction of ZIF-8/rGAs play the roles of ensuring good filtration efficiency of PM2.5. Benefited from these merits, the capture efficiencies for both PM2.5 and PM10 are over 99.3%, and >99.6%, respectively. The capture efficiencies remain high (>98.6% and 98.9%) after 7-h use. For practical application, we also demonstrated the post-adsorption separation of PM2.5 and filter reactivation for reuse, which has been largely neglected in the previous research. This study opens a new avenue for the next-generation filters with 3D advanced networks for fast, efficient, and sustainable treatment of air pollution under harsh working conditions.