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

Abstract

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.

Activation of Small Molecules and Hydrogenation of CO2 Catalyzed by Frustrated Lewis Pairs

“…Both the catalytic cycles are shown in Figures 6a,b. Heterogenous CO2 reduction to HCOOH is also reported [89] by silica nanopowder supported FLP where the said system forms an FLP-CO2 adduct on the silica surface. The activation of H2 is followed by the conversion of the captured CO2 to HCOOH.…”

Photocatalytic Decomposition of an Azo Dye Using Transition-Metal-Doped Tungsten and Molybdenum Carbides

“…Although not normally used as a quantitative technique, XPS accurately detected the ratios between atomic %, − thus giving a good indication (estimation) of the composition. A summary of the atomic ratios between the metals as well as the ratios between different compounds (e.g., WC and WO) is given in Table .…”

Diverse Uses of the Reaction of Frustrated Lewis Pair (FLP) with Hydrogen

“…Combination with Lewis base t -Bu 3 P generated a biphasic FLP which was shown to activate H 2 at 65 °C (Scheme ). Another approach to the deposition of one component of an FLP on silica was derived from the hydroboration or hydrophosphination of allyl-functionalized silica nanopowders . The resulting particles together with the complementary FLP partners were shown to activate H 2 and subsequently react with CO 2 to give HCOOH.…”

“…Another approach to the deposition of one component of an FLP on silica was derived from the hydroboration or hydrophosphination of allyl-functionalized silica nanopowders. 90 The resulting particles together with the complementary FLP partners were shown to activate H 2 and subsequently react with CO 2 to give HCOOH. Taoufik et al 91 used an alternative strategy, employing HOC 6 H 4 PPh 2 in the presence of Al(i-Bu) 3 , to deposit a phosphine unit on silica.…”