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42402-B5
Using Model Systems to Enhance Understanding of Silica Surface Reactions
PROJECT 1
The reaction kinetics of silica surfaces with organosilanes, useful to impart functionality for a wide range of technologically useful materials, is fraught with difficulties. This work uses a soluble, incompletely condensed silsesquioxane as a model for kinetics studies.
One undergraduate student supported by this grant has studied the reaction of 3-aminopropyldimethylmethoxysilane reaction with a model silsesquioxane silanol. The kinetics of these reactions in hexane and THF solutions were studied using transmission FTIR spectroscopy. The silanol OH stretching band position of the silsesquioxane in hexane is close to that of non-hydrogen-bonded silanols on silica (i.e. above 3700 cm-1); while in THF the silanol band position is similar to that of hydrogen-bonded silanols on silica (i.e. approximately 3300 cm-1). Thus a comparison of the kinetics of the loss of silsesquioxane silanol with the aminosilane provides a comparison of the reactivity of these silanols in the two different chemical environments. It was found that these reactions exhibit very well behaved overall second order kinetics (R2 > .99), affording the possibility to compare rate constants. The reaction rate constant at room temperature was found to be 5.86 x 10-4 mM-1 s-1 in hexane, and 1.06 x 10-5 mM-1 s-1 in THF. The aminosilane reaction with nanoparticulate fumed silica showed a similar solvent dependence on reaction rate. Results suggest that hydrogen bonded silanols are inherently less reactive than non-hydrogen-bonded silanols.
The reaction rate constant as a function of temperature exhibited a maximum rate constant at -20oC in hexane. At higher temperatures the reaction rate decreased. At temperatures lower than 20oC an Arrhenius plot was made to experimentally determine the activation energy for this reaction, which is approximately 40 kJ/mol. This reaction is now being modeled using ab initio computational techniques to help explain its behavior.
The undergraduate student who worked on this project is now in his first year as a Ph.D. candidate at the University of Wisconsin, and presented this work at a regional ACS meeting supported by PRF funds. In addition he is first author on one submitted paper, and will be first author on a second when the computational work is complete.
PROJECT 2
This project studies the role of various structural and textural factors on the use of surface modified high surface area silicas as adsorbents for heavy metals in aqueous solutions. A second undergraduate student studied the use of modified silicas as adsorbents for heavy metals Cu(II) and Pb(II). Atomic absorption spectroscopy was used to obtain adsorption isotherms of the metals on silicas with varying functional groups as well as structural and textural properties. It was found that mesoporous silicas with relatively large pores exhibit the largest adsorption capacity, whereas narrower pore sizes result in a larger negative ΔGadsorption. Organosilanes that can crosslink result in a larger reduction in pore size compared to monofunctional non-crosslinkable surface modifiers. This work was presented by the undergraduate student at a regional ACS meeting supported by PRF funds. This work has also now been published in the peer reviewed literature with the student as first author. This student is currently a senior and is planning to attend a Ph.D. school in chemistry in the next academic year.
Both of these projects have afforded me the opportunity to give seminars at various regional institutions in the past year equipped with new data as an extension of my previous work. The ability to publish results made possible by PRF funding allows me to continue on a trajectory where I can remain an active researcher with the internal and external benefits associated with this endeavor, as well as to contribute positively to the nascent careers of undergraduate science students.
