Nanoparticles of Tungsten as Low-Cost Monometallic

MAR¨ªA J. MACCARRONE; CECILIA R. LEDHEROS; CAROLINA BETTI; NICOL¨¢S CARRARA; JUAN C. YORI; FERNANDO COLOMA-PASCUAL; CARLOS R. VERA; M¨®NICA E. QUIROGA

QUíMICA NOVA

SOC BRASILEIRA QUIMICA

Lugar: San Pablo; Año: 2015 vol. 39 p. 1 - 8

0100-4042

Tungsten monometallic catalysts with variable amounts of metal (4.5, 7.1 and 8.5 %W) were prepared by impregnating alumina with ammonium metatungstate as an inexpensive precursor. The catalysts were characterized using ICP, XPS, XRD, TPR and hydrogen chemisorption. These techniques revealed mainly WO3 (W6+) species on the surface.The effects of the content of W nanoparticles and the temperature on the activity and selectivity for the partial hydrogenation of 3-hexyne, a non-terminal alkyne, were assessed at moderate conditions of temperature and pressure. The prepared monometallic catalysts were found to be active, showing the following order for production of (Z)-3-hexene: 7.1WN/A > 8.5 WN/A ¡Ý 4.5 WN/A. The resulting best catalyst, 7.1WN/A, has both a low cost of fabrication and a high selectivity to (Z)-3-hexene (94%) at 323 K. This selectivity is comparable to the one shown by the classical and more expensive Lindlar catalyst (5 wt% Pd) used in the industry for this kind of reactions. The performance of the catalysts was considered to be a consequence of two phenomena: a) the electronic effects, related to the high charge of W(+6), causing an intensive dipole moment in the hydrogen molecule (van der Waals forces) and leading to a heterolytic bond rupture; the generated H+ and H- species would approach a 3-hexyne adsorbate molecule and would provoke theheterolytic rupture of the C¡ÔC bond into C- = C+; b) steric effects related to the high concentrationof WO3 that would block the Al2O3 support.Catalyst deactivation was detected, starting at about 50 min reaction time. Electrodeficient W6+species would be responsible for the formation of green oil at the surface level, blocking pores andactive sites of the catalyst, particularly at low reaction temperatures (293 and 303 K).The alumina supported tungsten catalysts could be low-cost potential replacements of Lindlarindustrial catalyst. They could also be used for preparing bimetallic W-Pd catalysts for selectivehydrogenation of terminal and non-terminal alkynes.