Topics in Catalysis

, Volume 23, Issue 1, pp 5–22

Multifunctionality of Active Centers in (Amm)oxidation Catalysts: From Bi–Mo–Ox to Mo–V–Nb–(Te, Sb)–Ox

  • Robert K. Grasselli
  • James D. Burrington
  • Douglas J. Buttrey
  • Peter DeSantoJr.
  • Claus G. Lugmair
  • Anthony F. VolpeJr.
  • Thomas Weingand
Article

DOI: 10.1023/A:1024859917786

Cite this article as:
Grasselli, R.K., Burrington, J.D., Buttrey, D.J. et al. Topics in Catalysis (2003) 23: 5. doi:10.1023/A:1024859917786

Abstract

Catalytic centers in selective (allylic) oxidation and ammoxidation catalysts are multimetallic and multifunctional. In the historically important bismuth molybdates, used for propylene (amm)oxidation, they are composed of (Bi3+)(Mo6+)2 complexes in which the Bi3+ site is associated with the α-H abstraction and the (Mo6+)2 site with the propylene chemisorption and O or NH insertion. An updated reaction mechanism is presented. In the Mo–V–Nb–Te–Ox systems, three crystalline phases (orthorhombic Mo7.5V1.5NbTeO29, pseudohexagonal Mo6Te2VO20, and monoclinic TeMo5O16) were identified, with the orthorhombic phase being the most important one for propane (amm)oxidation. Its active centers contain all necessary key catalytic elements (2V5+/Mo6+, 1V4+/Mo5+, 2Mo6+/Mo5+, 2Te4+) for this reaction wherein a V5+ surface site (V5+ = O ↔ 4+V–O) is associated with paraffin activation, a Te4+ site with α-H abstraction once the olefin has formed, and a (Mo6+)2 site with the NH insertion. Four Nb5+ centers, each surrounded by five molybdenum octahedra, stabilize and structurally isolate the catalytically active centers from each other (site isolation), thereby leading to high selectivity of the desired acrylonitrile product. A detailed reaction mechanism of propane ammoxidation to acrylonitrile is proposed. Combinatorial methodology identified the nominal composition Mo0.6V0.187Te0.14Nb0.085Ox for maximum acrylonitrile yield from propane, 61.8% (86% conversion, 72% selectivity at 420 °C). We propose that this system, composed of 60% Mo7.5V1.5NbTeO29, 40% Mo6Te2VO20, and trace TeMo5O16, functions with a combination of compositional pinning of the optimum orthorhombic Mo7.5V1.5±xNbyTezO29±δ phase and symbiotic mop-up of olefin intermediates through phase cooperation. Under mild reaction conditions, a single optimum orthorhombic composition might suffice as the catalyst; under demanding conditions this symbiosis is additionally required. Improvements in catalyst performance could be attained by further optimization of the elemental distributions at the active catalytic center of Mo7.5V1.5NbTeO29, by promoter/modifier substitutions, and incorporation of compatible cocatalytic phases (preferably epitaxially matched). High-throughput methods will greatly accelerate the rational catalyst design processes.

multifunctionalityactive catalytic centerssite isolationphase cooperationsymbiosisselective oxidationammoxidationpropylenepropaneacroleinacrylic acidacrylonitrileBi-molybdatesMo–V–Nb–(Te,Sb)–Ox

Copyright information

© Plenum Publishing Corporation 2003

Authors and Affiliations

  • Robert K. Grasselli
    • 1
    • 2
  • James D. Burrington
    • 3
  • Douglas J. Buttrey
    • 1
  • Peter DeSantoJr.
    • 1
  • Claus G. Lugmair
    • 4
  • Anthony F. VolpeJr.
    • 4
  • Thomas Weingand
    • 2
  1. 1.Departments of Chemical Engineering and Materials Science, Center for Catalytic Science and TechnologyUniversity of DelawareNewark
  2. 2.Institute of Physical ChemistryUniversity of MunichMunichGermany
  3. 3.The Lubrizol CorporationWickliffe
  4. 4.Central ExpresswaySymyx Technologies Inc.Santa Clara