Treatment of hydroxylated silica
nanopowders S1 and
allyl-functionalized silica nanopowders S2 with 3-(diphenylborano)-
or 3-bis(pentafluorophenylborano)propyltrimethoxysilane or 2-(diphenylphosphino)-
or 2-(dicyclohexylphosphino)ethyltriethoxysilane generates silica
nanopowder supported Lewis acids S3 and silica nanopowder
supported Lewis bases S4. These surfaces were characterized
by 13C, 11B, and 31P cross-polarization
magic angle spinning nuclear magnetic resonance (CP MAS NMR), X-ray
photoelectron spectroscopy (XPS), and attenuated total reflection
Fourier transform infrared (ATR FTIR). When S3 is combined
with solution-phase Lewis bases PR3 (R = C6F5, C6H5, mesityl), six associated silica
nanopowder supported frustrated Lewis pairs (FLPs) are formed. In
another set of six reactions, the interactions between the supported
Lewis bases S4 and solution-phase Lewis acids BR3 with R = C6F5, C6H5, mesityl produced six more associated supported FLPs. The capture
of CO2 by these FLPs producing FLP-CO2 Lewis
pair adducts S5 and S6 were highlighted
by ATR FTIR, and it was found that FLP S5e with R = C6H5 on both the supported Lewis acid and solution-phase
Lewis base trapped the largest quantities of CO2 on the
silica nanopowder supports. Conversion of CO2 to HCOOH
was achieved by first activating H2 to generate activated
FLP-H2 surfaces S7 and S9. Addition
of CO2 then generated HCOOH via the silica nanopowder supported
FLP-HCOOH adducts S8 and S10. Qualitative
identification of HCOOH generation was achieved by ATR FTIR measurements,
and surface 10b with R = C6H5 proved
to be the most successful silica nanopowder surface bound FLP in HCOOH
generation. In some cases, diborano formates (−BO(CH)OB−) S11 and S12 were also identified as side products
during HCOOH formation. Spectroscopic characterization of purposefully
synthesized S11 and S12 included 11B and 31P CP MAS NMR.
The influence of a sterically large ligand (CH3COC(C10H21OC6H4COO)COCH3)−, substituted on complex [Rh(CH3COC(C10H21OC6H4COO)COCH3)(CO)(PPh3)], on the ease of rhodium oxidation, is described.
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