The promotion of metal catalysts with partially oxidized oxophilic MO x species, such as ReOx-promoted Rh, has been demonstrated to produce Brønsted acid sites that can promote hydrogenolysis of oxygenate intermediates such as those found in biomass-derived species. A wide variety of alloy compositions and structures are examined in this work to investigate strongly acidic promoters by using DFT-calculated deprotonation energies (DPE) as a measure of acid strength. Sites with the highest acid strength had DPE less than 1100 kJ mol-1, similar to DPE values of heteropolyacids or acid-containing zeolites, and were found on alloys composed of an oxophilic metal (such as Re or W) with a noble metal (such as Rh or Pt). NH3 adsorbs more strongly to sites with increasing acid strength and the activation barriers for acid-catalyzed ring opening of a furan ring decrease with increasing acid strength, which was also shown to be stronger for OH acid sites bound to multiple oxophilic metal atoms in a three-fold configuration rather than OH sites adsorbed in an atop configuration on one oxophilic metal, indicating that small MOx clusters may yield sites with the highest acid strength.
Bibliographical noteFunding Information:
The authors are grateful to the National Science Foundation for the financial support of this work under the NSF Award EEC-0813570 as part of the Center for Biorenewable Chemicals (CBiRC) at Iowa State University and the NSF award OISE 0730277 for the Partnership in International Research and Education (PIRE) and also kindly acknowledge access to the Molecular Science Computing Facility (MSCF) in the William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. Department of Energy, Office of Biological and Environmental Research at the Pacific Northwest National Laboratory. The authors also kindly acknowledge the very helpful discussions with Robert J. Davis (University of Virginia), Craig Plaisance (University of Virginia), James A. Dumesic (University of Wisconsin) and Mei Chia (University of Wisconsin).
- DFT, bifunctional catalysis
- Deprotonation energies
- Hydroxymethyl tetrahydrofuran
- Oxophilic metal promoters
- Ring opening cyclic ethers