Functionalization of the Internal Surface of Pure-Silica MFI Zeolite with Aliphatic Alcohols

Abstract

The functionalization of the internal surface of pure-silica MFI zeolite using aliphatic alcohols (1-butanol and 1-hexanol) is reported. Calcined MFI nanoparticles (50, 100, 200, and 500 nm in size) are treated with neat 1-butanol and 1-hexanol under reflux conditions. The 1-butanol loadings on 200 and 500 nm particles are determined to be 0.7 mmol/g SiO 2 by thermogravimetric analysis (TGA) and are similar to the tetrapropylammonium (TPA) template content of the as-made nanoparticles. 13 C cross-polarization magic-angle spinning (CP-MAS) NMR and TGA data suggest that the observed 1-butanol loading is strongly correlated with the concentration of internal silanol defect sites. In addition, significantly higher 1-butanol loadings on 50 nm (1.1 mmol/g SiO 2 ) and 100 nm (0.93 mmol/g SiO 2 ) MFI nanoparticles reflect the concurrent functionalization of silanols on the external surfaces of the nanoparticles. These results are in systematic agreement with theoretical estimates that consider both internal and external surface functionalization sites. MFI nanoparticles (50, 100, and 300 nm) treated with 1-hexanol result in 1.34, 1.28, and 1.14 mmol/g SiO 2 of 1-hexanol content, respectively, levels that are higher than expected from consideration of the results of butanol treatment. These higher organic loadings of 1-hexanol may imply the existence of additional physisorbed 1-hexanol molecules trapped between other 1-hexanol molecules within the zeolite micropores and/or the formation of dimeric/oligomeric complexes by hydrophobic interactions between the hexyl groups. 13 C/ 29 Si MAS and CP-MAS NMR investigations suggest that the organic groups are covalently bonded to the internal silanol defect sites, consistent with previous work on the chemisorption of methanol in MFI.