Nanoparticles of Tungsten as Low-Cost Monometallic Catalyst for Selective Hydrogenation of 3-Hexyne

MACCARRONE, M.J.; LEDERHOS, C.; BETTI, C.; CARRARA, N.; YORI, J.; COLOMA-PASCUAL F.; LIPRANDI, D.; VERA, C.; QUIROGA , M.

QUíMICA NOVA

SOC BRASILEIRA QUIMICA

Lugar: San Pablo; Año: 2016 vol. 39 p. 1 - 8

0100-4042

Low cost 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 inexpensiveprecursor. The catalysts were characterized using ICP, XPS, XRD, TPR and hydrogenchemisorption. These techniques revealed mainly WO3-Al2O3 (W6+) species on the surface.The effects of the content of W nanoparticles and the reaction temperature on the activity andselectivity for the partial hydrogenation of 3-hexyne, a non-terminal alkyne, were assessed atmoderate conditions of temperature and pressure.The prepared monometallic catalysts were found to be active and stereoselective for the productionof (Z)-3-hexene, showing the following order: 7.1WN/A > 8.5 WN/A ≥ 4.5 WN/A. Additionally,the performance of the synthesized xWN/A catalysts exhibited a high sensitivity to the variation oftemperature. In all the cases, the maximum 3-hexyne total conversion and selectivity was achievedat 323 K.The performance of the catalysts was considered to be a consequence of two phenomena: a) theelectronic effects, related to the high charge of W (+6), causing an intensive dipole moment in thehydrogen molecule (van der Waals forces) and leading to a heterolytic bond rupture; the generatedH+ and Hspecieswould approach a 3-hexyne adsorbate molecule and would provoke theheterolytic rupture of the CC bond into C-= C+; and b) steric effects related to the highconcentration of WO3 on 8.5WN/A that would block the Al2O3 support.Catalyst deactivation was detected, starting at about 50 min of 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 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 that of the classical and moreexpensive industrial Lindlar catalyst (5 wt% Pd).The alumina supported tungsten catalysts could be low-cost potential replacements of the Lindlarindustrial catalyst. They could also be used for preparing bimetallic W-Pd catalysts for selectivehydrogenation of terminal and non-terminal alkynes