Silica Nanopowder Supported Frustrated Lewis Pairs for CO₂ Capture and Conversion to Formic Acid

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.