Organic solvent nanofiltration (OSN) membrane incorporated with silica nanoparticles: fabrication and molecular dynamics simulation

Abstract

The separation process accomplished by physical and chemical processes such as chromatography, filtration, distillation, evaporation, crystallization, etc. is the most critical industrial process during the purification and recovery of commercial products. Currently, distillation is the most conventional method for the separation of products mixed with organic solvents, however, consumes a tremendous amount of energy and causes severe environmental pollution. Organic solvent nanofiltration (OSN) membrane, which could be used to separate compounds according to different molecular sizes in harsh organic solvents, is being explored as a possible alternative. However, the research of OSN membrane is still in its infancy due to the poor stability, and the tradeoff between permeability and selectivity. In the past decade, mixed matrix membranes (MMM) composed of polymer and inorganic materials have been proposed to overcome these limitations. In this study, APTES ((3-aminopropyl)triethoxysilane)-modified silica nanoparticles (NPs) are prepared and added for the preparation of a novel OSN membrane with high permeability and selectivity. At the optimal fabrication condition (polyimide P84 concentration of 22wt%, dimethylformamide (DMF) to 1,4-dioxane ratio of 3 to 1, and silica NPs concentration of 0.2wt%), the fabricated OSN membrane showed 3.9 times higher methanol permeability than that of the commercial membrane. Moreover, the surface modification of silica NPs with APTES further improved the permeability of methanol up to 4.5 times with comparable MWCO. However, due to the low degree of crosslinking, the prepared OSN membrane showed more swelling than the commercial OSN membrane. To understand the mechanisms of silica NPs in improving the membrane, molecular dynamics (MD) simulations are performed with Materials Studio software and COMPASS force field (FF). Models of MMMs with silica NPs contents from 0 to 20wt% have been constructed step by step and relaxed to the optimum state by several simulation processes before further calculation and analysis. At the low contents of silica NPs, the hydrogen bonding between silica NPs and polymers decreased the polymer chain motion, thus, hindered the passage of large molecules. However, as the increase of silica NPs contents, the agglomeration of silica NPs happens and enlarges the fractional free volume, which resulted the increase of both methanol permeability and MWCO. Finally, the fabricated OSN membranes are tested for the separation of Congo red (CR) and alizarin (AL), which have been widely used as dyes in the textile industry. After the 4-staged OSN separation system, CR on the retentate side was enriched up to 4.2 times (CR to AL ratio), and at the permeate side, AL to CR ratio was reached 19.9. In conclusion, the addition of modified silica NPs improved the permeability, selectivity, and mechanical strength compared to the OSN membrane without silica NPs. However, the chemical stability of the newly fabricated membranes still has a room to be improved, which will be retained for the further study.