Reactions of unsaturated oxygenates on metal surfaces Will Medlin Department of Chemical and Biological Engineering Colorado Center for Biorefining and Biofuels (C2B2) University of Colorado Graduate students: Andrea Loh (MS 07), Clay Horiuchi, Steve Marshall Funding: National Science Foundation (collaborative project with John Monnier, Univ. of S. Carolina)

Overall objective: Control selectivity in conversions of multifunctional oxygenates Some probe molecules: The catalyst: metallic and bimetallic surfaces

Reaction pathways for hydrogenation of EpB n-butyraldehyde Schaal et al. 2006

Selectivity to various products is a sensitive function of surface composition Control metal catalyst composition (e.g. PtXAgY) using electroless deposition – Monnier and Williams On supported Pd catalysts, selectivity to C=C products is higher

Surface science approaches used to understand EpB reactivity on metal surfaces ? Methods for studies on Pt(111), Pd(111), and Cu(111) High Resolution Electron Energy Loss Spectroscopy (HREELS) Temperature Programmed Desorption (TPD) Density functional theory (DFT)

Ring-opening of EpB on Ag(110) results in oxametallacycle formation 2.46 2.29 2.27 2.36 Ag O C1 C2 C3 C4 C1 C3 C2 C4 1.67 O 2.81 2.54 2.44 Ag Ag Ag Medlin, Barteau, and Vohs, J. Mol. Catal. (2000) 163, 129. Medlin and Barteau, J. Phys. Chem. B (2001) 105, 10054.

DFT indicates other C4H6O adsorption structures may be stable on Pd(111) and Pt(111) oxametallacycle di-sigma

At low temperatures, EpB adsorbs through olefin function Adsorbed through olefin O Pt(111) (C=C) stretch Di-sigma bonded O Pt(111) (di-s C-H) stretch Loh, Davis, Medlin, JACS 130 (2008) 5507.

EpB undergoes ring-opening on Pt(111) by 230 K Pt(111) C1=O aldehyde C1-H aldehyde C1H2, C4H2 C3H O-C1-C2 C4H2 C3-C4 C2H, C3H 1 2 3 4 Loh, Davis, Medlin, JACS 130 (2008) 5507.

Temperature programmed desorption confirms decarbonylation reaction EpB 1 2 3 4 CrHO 1 2 3 4 At saturation, approximately ¾ of C3 undergoes complete dehydrogenation Loh, Davis, Medlin, JACS 130 (2008) 5507.

Ring-opened EpB intermediate and crotonaldehyde have similar (but not identical) HREEL spectra 1 2 3 4 Crotonaldehyde C4H3 deformations C4-H2 scissoring EpB-derived intermediate C4-H2 stretching 1 2 3 4 Loh, Davis, Medlin, JACS 130 (2008) 5507.

Aldehyde formation predicted in one EpB geometry optimization on Pd(111) Experimental HREELS CO DFT (IR) Loh, Davis, Medlin, JACS 130 (2008) 5507.

HREEL spectra show clear differences between ring-opened EpB on Pt vs HREEL spectra show clear differences between ring-opened EpB on Pt vs. Pd (111) Aldehyde formation and decarbonylation clearly observed on both surfaces…. … but intermediate on Pd(111) is likely to be more highly-coordinated, consistent with catalysis studies Marshall, Horiuchi, Zhang, Medlin, submitted.

EpB and Crotonaldehyde on Pd(111) annealed to 250K have nearly identical HREEL spectra 3 4 EpB 1 2 3 4 CrHO Marshall, Horiuchi, Zhang, Medlin, submitted.

Reaction pathways for hydrogenation of EpB n-butyraldehyde Schaal et al. 2006 15

Computational chemistry studies suggest favorable interaction of Cu/Ag with O end of molecule 2.46 2.29 2.27 2.36 Ag O C1 C2 C3 C4 Pt Pt Pt Schaal, Hyman, Medlin, Monnier, to be submitted.

Parallels with adsorption and reaction of 2(5H)-furanone EpB 2(5H)-furanone Crotonaldehyde

Furanone also undergoes ring-opening and decarbonylation on Pd(111), but at higher temperatures CO formation begins around 300 K Furanone monolayer Multilayers of condensed furanone

DFT helps in assigning vibrational modes, suggests furanone adsorbed in tilted configuration through olefin C=O stretch C1 C4 doop(C-H) C2 C3 C-H stretch

TPD confirms decarbonylation and CO2 formation pathways on Pt(111)

Conclusions: reactions of unsaturated oxygenates on Pt-group metals EpB reacts via ring-opening and C-H scission to form a surface aldehyde intermediate on Pt(111) and Pd(111) Aldehyde formation pathway can be avoided by addition of Ag to surface of Pt Adsorption and reaction of furanone shows numerous parallels with EpB chemistry, though ring-opening reaction is more activated C1 C2 C3 C4

As coverage increases, decomposition is delayed to higher temperatures High-temp. C3H6 Low-temp. Loh, Davis, Medlin, JACS 130 (2008) 5507.