Discrimination of Xylene Isomers by Precisely Tuning the Interlayer Spacing of Reduced Graphene Oxide Membrane.

Separating xylene isomers is a challenging task due to their similar physical and chemical properties. In this study, we developed a molecular sieve incorporating a reduced graphene oxide (rGO) membrane for the precise differentiation of xylene isomers. We fabricated GO membranes using a vacuum filtration technique followed by thermal-induced reduction to produce rGO membranes with precisely controllable interlayer spacing. Notably, we could finely tune the interlayer spacing of the rGO membrane from 8.0 to 5.0 Å by simply varying the thermal reduction temperature. We investigated the reverse osmosis separation ability of the rGO membranes for xylene isomers and found that the rGO membrane with an interlayer spacing of 6.1 Å showed a high single component permeance of 0.17 and 0.04 L m-2 h-1 bar-1 for para- and ortho-xylene, respectively, exhibiting clear permselectivity. The separation factor reached 3.4 and 2.8 when 90:10 and 50:50 feed mixtures were used, respectively, with permeance 1 order of magnitude higher than that of current state-of-the-art reverse osmosis membranes. Additionally, the membrane showed negligible permeance and selectivity decay even after continuous operation for more than 5 days, suggesting commendable membrane resistance to solvent swelling and operating pressure.

Highly stable graphene oxide/nylon membrane for molecular separation.

Graphene oxide (GO), due to its one-atom-thick structure and enriched oxygenated functionalities, is a promising candidate material to develop nanofiltration membranes to tackle the current worldwide water shortage. However, the stability of the GO membrane in an aqueous environment and its long-term operation remains unresolved. These issues greatly affect the mass transfer in the GO membrane. Here, we fabricate an ultrathin GO membrane on a nylon substrate within 5 min with the help of vacuum filtration for molecular separation. Thus, GO/nylon membranes dried in an oven at temperatures of 70 °C show greater aqueous solution stability than those dried at room temperature. To validate the stability, both GO membranes were immersed in DI water for 20 d. As a result, the GO/nylon membrane dried at room temperature was completely detached from the substrate within 12 h, whereas the GO/nylon membrane that dried at 70 °C remained stable for more than 20 d without any physical damage. We suppose the enhanced stability is due to the thermally induced balance in electrostatic repulsion resulting in stabilizing of the GO membrane. This method improves the GO membrane's operating time, selectivity, and permeability. Therefore, the optimized GO/nylon membrane shows higher rejection of organic dyes (∼100%) and good selectivity for sulfate salts such as Na2SO4and MgSO4(>80%). The membrane continuously operates for more than 60 h with only a 30% water permeability decline and 100% rejection of dyes. We believe that the drying of GO/nylon membranes at a moderate temperature is important for enhanced separation performance and stability. This drying technique can be applied to other applications.

Essence of the Enhanced Osmotic Energy Conversion in a Covalent Organic Framework Monolayer.

Low membrane conductivity originated from a high membrane thickness has long been the "Achilles heel" of the conventional polymeric membrane, greatly hampering the improvement of the output power density in osmotic power generation. Herein, we demonstrate a molecularly-thin two-dimensional (2D) covalent organic framework (COF) monolayer membrane, featured with ultimate thickness, high pore density, and tight pore size distribution, which performs as a highly efficient osmotic power generator. Despite the large pore size up to 3.8 nm and relatively low surface charge density of 2.2 mC m-2, the monolayer COF membrane exhibits a high osmotic current density of 16.7 kA m-2 and an output power density of 102 W m-2 under 50 times the NaCl salinity gradient (0.5 M/0.01 M). This superior power density could be further improved to 170 W m-2 in the real seawater/river water gradient system. When the large pore size and low surface charge density are considered, this superior performance is not expected. Computational studies further reveal that the ultimate membrane permeability originated from the high membrane porosity, rather than ion selectivity, plays a dominant role in the production of high current density, especially under high salinity. This work provides an alternative strategy to realize improved output power density in ultrapermeable membranes.