Silica Nanopowder Supported Frustrated Lewis Pairs for CO2Capture and Conversion to Formic Acid

Treatment of hydroxylated silica nanopowders S1and allyl-functionalized silica nanopowders S2with 3-(diphenylborano)- or 3-bis(pentafluorophenylborano)propyltrimethoxysilane or 2-(diphenylphosphino)- or 2-(dicyclohexylphosphino)ethyltriethoxysilane generates silica nanopowder supported Lewis acids S3and 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 S3is 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 S4and solution-phase Lewis acids BR3with R = C6F5, C6H5, mesityl produced six more associated supported FLPs. The capture of CO2by these FLPs producing FLP-CO2Lewis pair adducts S5and S6were highlighted by ATR FTIR, and it was found that FLP S5ewith R = C6H5on both the supported Lewis acid and solution-phase Lewis base trapped the largest quantities of CO2on the silica nanopowder supports. Conversion of CO2to HCOOH was achieved by first activating H2to generate activated FLP-H2surfaces S7and S9. Addition of CO2then generated HCOOH via the silica nanopowder supported FLP-HCOOH adducts S8and S10. Qualitative identification of HCOOH generation was achieved by ATR FTIR measurements, and surface 10bwith R = C6H5proved to be the most successful silica nanopowder surface bound FLP in HCOOH generation. In some cases, diborano formates (−BO(CH)OB−) S11and S12were also identified as side products during HCOOH formation. Spectroscopic characterization of purposefully synthesized S11and S12included 11B and 31P CP MAS NMR.