Silanization of the silica gel surface in the synthesis of silica gel polyamine composites uses (chloropropyl)trichlorosilane (CPTCS). It is possible to substitute a molar fraction of reagent CPTCS with methyltrichlorosilane (MTCS), creating a mixed silane surface layer. Two types of silica gels were modified with a series of MTCS:CPTCS molar ratios. Solid-state CP/MAS 29 Si and 13 C NMR spectroscopies were used to evaluate the surface silane composition. Surface silane coverage was markedly improved for the resulting gels. When polyamines were grafted to the resultant MTCS:CPTCS silane layers, it was shown that the decrease in the number of propyl attachments to the polyamine resulted in increased quantities of "free amines". Optimum MTCS:CPTCS ratios were determined for three polyamines grafted onto one silica gel. A substantial free amine increase was observed for poly(allylamine) (PAA). Metal uptake studies show increases in Cu(II) capacity and/or an improvement in Cu(II) mass-transfer kinetics. The effect of polymer molecular weight upon Cu(II) capacity was investigated for each polyamine. Substantial differences in Cu(II) capacity between 50 000 MW poly(vinylamine) (PVA) and >1000 MW PVA were evident. Similar differences between 25 000 MW poly(ethyleneimine) (PEI) and 1200 MW PEI were found. The mass-transfer kinetics was shown to be improved for composites prepared using a large fraction of MTCS in the reagent silane mixture. This resulted in substantial improvements in the 10% breakthrough Cu(II) capacity for PVA (50 000 MW). PEI composites were further modified to form an amino-acetate ligand. The impact of the MTCS:CPTCS silane ratio on the acetate ligand loading and ultimately on the Cu(II) capacity at pH ) 2 was investigated. A ratio of 12.5:1 was shown to result in an acetate modified PEI composite with a Cu(II) capacity 140% of the Cu(II) capacity of the same composite prepared with "CPTCS only".
The surface coverage of amorphous silica gels used in the synthesis of silica polyamine composites has been investigated by 29 Si NMR. By diluting the polyamine anchor silane, chloropropyl trichlorosilane, with methyl trichlorosilane it was found that surface coverage could be markedly improved for a range of amine polymers after grafting to the silica surface. The commensurate decrease in the number of anchor points and increase in the number of free amines results in an increase in metal capacity and/or an improvement in capture kinetics. Solid state CPMAS-13 C NMR has been employed to investigate the structure and metal ion binding of a series of these composite materials. It is reported that the highly branched polymer, poly(ethyleneimine) (PEI) exhibits much broader 13 C NMR resonances than the linear polymers poly(allylamine) (PAA) and poly(vinylamine) (PVA). These results are understood in terms of the low energy conformations calculated from molecular modeling studies. Three new applications of the technology are also presented: 1) separation of lanthanides as a group from ferric ion and all other divalent ions; 2) a multi step process for recovering and concentrating the valuable metals in acid mine drainage; 3) a process for removing low level arsenic and selenium in the presence of sulfate using immobilized cations on the composite materials.
Selective ion transport across membranes
is critical to the performance
of many electrochemical energy storage devices. While design strategies
enabling ion-selective transport are well-established, enhancements
in membrane selectivity are made at the expense of ionic conductivity.
To design membranes with both high selectivity and high ionic conductivity,
there are cues to follow from biological systems, where regulated
transport of ions across membranes is achieved by transmembrane proteins.
The transport functions of these proteins are sensitive to their environment:
physical or chemical perturbations to that environment are met with
an adaptive response. Here we advance an analogous strategy for achieving
adaptive ion transport in microporous polymer membranes. Along the
polymer backbone are placed redox-active switches that are activated
in situ, at a prescribed electrochemical potential, by the device’s
active materials when they enter the membrane’s pore. This
transformation has little influence on the membrane’s ionic
conductivity; however, the active-material blocking ability of the
membrane is enhanced. We show that when used in lithium–sulfur
batteries, these membranes offer markedly improved capacity, efficiency,
and cycle-life by sequestering polysulfides in the cathode. The origins
and implications of this behavior are explored in detail and point
to new opportunities for responsive membranes in battery technology
development.
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