Trimming Nanostructured Walls While Fluorinating their Surfaces: A Route to Making and Widening Pores of Nanoporous Materials and Efficient Catalysts

Abstract

We report on the synthesis and characterization of highly ordered mesoporous fluorosilicas (OMFs) and nanoporous and corrugated fluorosilica nanospheres which contain 2.4−7.0 wt % F, corresponding to a loading of 1.3−3.7 mmol/g. Synthesis of these materials is carried out from parent mesoporous silicas (MCM-41 and SBA-15) and silica nanospheres under ambient conditions using dilute nonaqueous solutions of triethyloxonium tetrafluoroborate (Et3OBF4). As evidenced by nitrogen physisorption measurements, small angle powder X-ray diffraction (XRD), and transmission electron microscopy (TEM), fluorination of the mesostructures is accomplished with only minor alteration of the materials’ overall order. Detailed compositional analyses before and after fluorination are carried out with the aid of FTIR spectroscopy and X-ray photoelectron spectroscopy (XPS) as well as elemental analysis proving the existence of silicon oxyfluoride species. FTIR studies show the appearance of new absorptions (730−750 cm−1) upon fluorination corresponding to [F6−nSi(OH)n]2−, O4/2SiF species or Si−O−Si stretches altered by the presence of nearby Si−F species as well as general broadening of the Si−O−Si asymmetric stretching region (1000−1200 cm−1). XPS analyses exhibit two distinct F species observed at 690.5 and 687.6 eV corresponding to silicon oxyfluoride (SiOF) and silicon fluoride (SiF), respectively. Surface silanol content is evaluated by 29Si HP-MAS NMR spectroscopy, which shows significant reductions in (Q2+Q3)/Q4 ratios from 1.26, 0.61, and 1.07 to 0.22, 0.38, and 0.61 for SBA-15, trimethylsilyl-capped SBA-15, and MCM-41 mesoporous silicas, respectively, because of the formation of (SiO3)SiF or pentacoordinated (SiO4)SiF species. The as-described synthetic procedure is also applied to sulfonic acid (−SO3H)-functionalized SBA-15. Enhancement of the acid-catalyzed ring opening of styrene oxide by aniline to produce the corresponding β-aminoalcohol by the fluorinated materials is presented. Ring opening of styrene oxide by aniline at room temperature after 4 h using −SO3H functionalized SBA-15 result in 77% conversion, whereas the as-synthesized OMF and OMF-SO3H materials exhibit 79% and 87% conversions, respectively. Fluorination of silica nanospheres with Et3OBF4 under similar conditions also produces etched nanoporous fluorosilica nanospheres with different degrees of exfoliation depending on the fluorination temperature.