Organic solvent reverse osmosis characteristics of TiO2-ZrO2-organic chelating ligand (OCL) composite membranes using OCLs with different molecular sizes

In this study, organic chelating ligand (OCL) composite membranes (TiO2-ZrO2-OCL) were fabricated for application to organic solvent reverse osmosis (OSRO). Although it is difficult to control the pore structures of TiO2 and ZrO2 at the RO level, we attempted to control the dense pore structure of the TiO2-ZrO2-OCL membranes by using three different molecular sizes of OCLs: Acetylacetone (ACA), Methyl-2,4-heptanedione (ACAiB), and 2,6-Dimethyl-3,5-heptanedione (ACA2iP). The bonding state, amorphous structure, and pore structure of TiO2-ZrO2-OCL powder samples were investigated. Pair distribution function (PDF) analysis of TiO2-ZrO2-OCL powders suggested that the TiO2-ZrO2 network was changed with the molecular sizes of the OCLs. Nitrogen adsorption isotherms showed that OCLs with a larger molecular size had a larger BET specific surface area. TiO2-ZrO2-ACAiB and TiO2-ZrO2-ACA2iP membranes exhibited excellent methanol selective permeability for methanol (90 wt%) and toluene (10 wt%) at a transmembrane pressure of 5 MPa, and OSRO membranes with total flux that ranged from 0.072 to 0.968 kg m-2h−1 were fabricated with separation factors that ranged from 3.2 to 180. The molecular weight cut-off (MWCO) of TiO2-ZrO2-OCL membranes was evaluated based on the rejection of ethanol or isopropyl alcohol in methanol and methyl orange in methanol, and it was ranged from 80 to 200 g/mol. In addition, TiO2-ZrO2-OCL membranes showed high selectivity at low pressure with relatively low levels of methanol flux.