Interfacial Charge Induction and Surface Olefin Adsorption on Activated Ag Nanoparticles for Facilitated Transport Membranes

Abstract

The extremely high selectivity of propylene/propane mixture was obtained due to strong carrier activity of Ag NPs by generating partial positive charge on their surface with organic polarizers through the composite membranes where the surface-activated NPs were dispersed in poly(vinyl pyrrolidone). It was also found that the partial positive charge induced by the organic polarizer can give rise to the interactions with propylene for carrier activity. Finally, the strong linear correlation between the surface charge density and the mixed-gas selectivity was found through facilitated olefin transport membranes comprising polymer/Ag NPs/polarizer. The positive charge on the surface of Ag NPs, representing the carrier activity, was modulated by an organic polarizer 7,7,8,8-tetracyanoquinodimethane (TCNQ) adsorbed on their surface. Then, the carrier activity of the surface was quantified by measuring the amount of adsorbed propylene on the surface as a function of the amount of adsorbed TCNQ. The strong linear correlation between the charge density of Ag NPs surface and the amount of propylene adsorption was found for the first time. Therefore it was concluded that the interactions of propylene with the positively polarized Ag NPs surface for olefin carrier characteristic are mainly decided by the positive charge density of Ag NPs. The excellent mixed-gas selectivity, using the positively charged surface of Ag NPs as an olefin carrier, has been obtained in composite membranes of PVP/Ag NPs/polarizer. The surface charge density of the Ag NPs and the mixed-gas selectivity exhibited a strong linear correlation. Moreover, the linear correlation was also attained with various polarizers such as dinitrobenzene (DNB), dinitrotoluene (DNT) having two strong electron-withdrawing groups in addition to TCNQ. Even tetrathiofulvalene (TTF), which is known as an electron donor, also leaded to charge transfer from Ag NPs to TTF molecules, then positive surface charge of Ag NPs, and finally high selectivity. Therefore, it can be generalized that the carrier activity for facilitated olefin transport is mainly determined by the extent of the positive charge of Ag NPs, but not the kind of polarizer used. However, the correlations are not completely overlapped between the semiconductor polarizer such as TCNQ and TTF and the insulator polarizer DNB and DNT, which is not completely understood at this moment and should be investigated more rigorously in the future.