(638c) Functionalization of the Internal Surface of Pure Silica-Mfi with N-Butanol
AIChE Annual Meeting
Thursday, November 8, 2007 - 4:06pm to 4:24pm
Organic-inorganic hybrid materials made by incorporating organic moeities on the surface of inorganic host materials have attracted significant research efforts due to the possibility of obtaining novel functionalities not possible with the individual component materials. Hybrid materials can often be prepared via grafting procedures that employ silane coupling agents (R-Si(R')3, R'=OMe, OEt, or Cl). This method works well for mesoporous materials but is considerably more difficult for microporous zeolites when attempting to tune the functionality of their interior (rather than the exterior) surface. The capability to tune the interior pore spaces of zeolite materials by attaching organic groups at specific locations could have important implications in separations, catalysis and other applications of zeolite materials. In particular, it would provide a new handle (in addition to the zeolite structure) to influence host-guest interactions in ways that are not possible with a purely inorganic framework material.
Here we describe an approach to tune the functionality of the internal surface of pure-silica MFI that employs the silanol nest defects commonly found in MFI crystals. Specfically, we report esterification of n-butanol on pure-silica MFI nanoparticles (100 nm, 200 nm and 500 nm in size). 13C CP MAS NMR spectra indicate the appearance of butyl groups on the n-butanol treated materials. The organic loadings of the n-butanol treated materials were characterized by TGA, showing that the n-butanol loadings are strongly correlated with the density of internal silanol defect sites. The contribution of external silanol groups on the n-butanol loading becomes more prominent as the particle size decreases, and are separated from the contribution of internal silanol nest defects by combining experiments on different nanoparticle sizes with theoretical estimates of internal and external silanol defect concentration. Micropore volume measurements (via N2 physisorption), X-ray diffraction, as well as p-xylene adsorption isotherms, confirm that the n-butanol molecules are chemically grafted to the MFI structure within the micropores. The experimental evidence, taken together, supports the interpretation that Si-O-Bu linkages are formed via an esterification reaction. This is corroborated by the finding that the n-butanol content remains stable after exposure to vacuum (10 mtorr) and elevated temperature (200 °C) for 2 days. 29Si CP MAS NMR spectra show that the Q3/Q4 population ratio decreases after n-butanol treatment, further supporting the hypothesis of formation of Si-O-C bonds. However, the Si-O-Bu linkages appear to be susceptible to re-hydrolysis upon exposure to water vapor, as indicated by the nature of hysteresis seen in water adsorption-desorption isotherm measurements. Overall, our results demonstrate a promising method for functionalizing the internal surface of zeolites, such as pure-silica MFI, with relatively mild reaction conditions, that should be generalizable to the creation of a wide range of functional zeolite-organic hybrids.