Zheng-Ming Wang

Graphene oxide membrane (GOM) has recently attracted a great attention because of its exceeding potential for desalination and other water treatment applications. While ionic sieving property of GOMs was so far mainly considered based on geometrical factors, ionicity roles have not been sufficiently addressed. Herein a lab-scale cross-flow type apparatus was used to systematically examine separation behaviors of representative inorganic cations and dye molecules of different ionicity or polarity by GOM. It was found that cation separations through a dried GOM of tens nanometer thickness receives strong influence from electrostatic interactions. As the result, not only ionic permeation rate but also water flux obey a linearly increased relation with the strength of ionic power, but with a different slope for monovalent and bivalent cation groups. On the other hand, accumulation adsorption plays an important role in separation of organic dyes, leading to an increase in rejection but at the sacrifices of water flux. Cation separation mechanism was further understood by analysis of intercalation structure and with molecular dynamic simulations. The results obtained clearly show involvement of ion-exchange mechanism and the important role of ionicity in separation and can provide instructive information in terms of using GOM for various separation applications.

An all silica MEL‐type silicalite‐2 nanocrystal is hydrothermally treated in coexistence with cetyltrimethylammonium bromide (CTAB) and tetrapropylammonium hydroxide (TPAOH) at ordinary and elevated temperatures. From monitoring its structural transition process by various physico‐chemical methodologies, it is found that, instead of simple delamination, silicalite‐2 nanocrystal changes with a dissolution and recrystallization mechanism by treatment at an elevated temperature and for a long time, that is, it is first dissolved and then reorganized to huge 2D platelets via an intermediate step forming periodic mesoporous material. The newly formed nanoplatelets (h‐nanoplatelets) from in‐plane growth along a (or b)‐, c‐directions (normal to b (or a)‐axis) are of silicalite (either MEL or MEL/MFI intergrowth)‐type framework topology with a thickness of 3.3 nm (the equivalent of ≈1.6 unit cell) and interspersed with a minor part of small square‐like MFI‐type nanoplatelets (s‐nanoplatelets). The h‐nanoplatelets have expanded lattice parameters and a larger micropore size compared to the parent silicalite‐2 nanocrystal and s‐nanoplatelets, whose micropore structure are modified so as to benefit from both equilibrium adsorption and desorption‐recovering ability of organic contaminants particularly at a low vapor pressure range. This is the first report regarding the dissolution–recrystallization formation of 2D lamella‐type silicalites mediated by popular types of surfactant and base species.

Silicalite-1-type zeolites with unique intracrystal holes or cracks were successfully prepared using a cellulose nanofiber (CNF) as an additional mediating material, and their vapor phase adsorption properties toward methyl tert-butyl ether (MTBE) and n-nitrosodimethylamine (NDMA) were examined. It was found that the mixing protocol of CNF and structure-directing agents (SDAs), the addition amount of CNF, and the CNF/SDAs amount ratio play important roles in forming the holed silicalite-1. The synthesis route that preliminarily mixes CNF with SDAs in a series of controlled conditions is particularly beneficial for the formation of the holed silicalite-1 with mesoporosity and larger pores because the CNF-SDAs composite structure benefits the zeolite growth closely encompassing CNF inside the crystal structure. It also promotes the preferential formation of the orthorhombic phase vicinal to the CNF surface, namely, the surface of the formed internal holes or cracks, with the twin-type crystal size reduced as compared to the non-CNF-templated sample. On the contrary, the synthesis route that mixes CNF with SDAs–silicate composite ions tends to modify the twin-type crystal shape at the same time to form small but uniform well-crystallized particles with less holes or cracks and a dominative monoclinic phase. It was considered that both the inter-subunit structural defect and silanol defect whose content is increased with CNF addition influence the adsorptivity of MTBE and NDMA. Owing to the small twin-type crystal size, the smaller crystal subunits, and the favored short path from the surface of internal holes or cracks, the holed silicalite-1 derived from the CNF and SDA premixture assures the easiest access of adsorbate molecules to the most energetically favored sites and is most appropriate for the adsorption of both MTBE and NDMA among the examined zeolites.