Nanoparticles of tungsten as low-cost monometallic catalyst for selective hydrogenation of 3-hexyne

MARÍA JULIANA MACCARRONE; CECILIA R. LEDERHOS; CAROLINA BETTI; NICOLÁS CARRARA; JUAN C. YORI; FERNANDO COLOMA-PASCUAL; DOMINGO LIPRANDI; CARLOS R. VERA; MÓNICA E. QUIROGA

QUíMICA NOVA

SOC BRASILEIRA QUIMICA

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

0100-4042

Low-cost tungsten monometallic catalysts containing variable amounts of metal (4.5, 7.1 and 8.5%W) were prepared by impregnatingalumina with ammonium metatungstate as an inexpensive precursor. The catalysts were characterized using ICP, XPS, XRD, TPRand hydrogen chemisorption. These techniques revealed mainly WO3-Al2O3 (W6+) species on the surface. The effects of the contentof W nanoparticles and reaction temperature on activity and selectivity for the partial hydrogenation of 3-hexyne, a non-terminalalkyne, were assessed under moderate conditions of temperature and pressure. The monometallic catalysts prepared were foundto be active and stereoselective for the production of (Z)-3-hexene, had the following order: 7.1WN/A > 8.5 WN/A ¡Ý 4.5 WN/A.Additionally, the performance of the synthesized xWN/A catalysts exhibited high sensitivity to temperature variation. In all cases,the maximum 3-hexyne total conversion and selectivity was achieved at 323 K. The performance of the catalysts was consideredto be a consequence of two phenomena: a) the electronic effects, related to the high charge of W (+6), causing an intensive dipolemoment in the hydrogen molecule (van der Waals forces) and leading to heterolytic bond rupture; the H+ and H- species generatedapproach a 3-hexyne adsorbate molecule and cause heterolytic rupture of the C¡ÔC bond into C- = C+; and b) steric effects related tothe high concentration of WO3 on 8.5WN/A that block the Al2O3 support. Catalyst deactivation was detected, starting at about 50 minof reaction time. Electrodeficient W6+ species are 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 lowfabrication cost and high selectivity for (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 are low-cost potential replacements forthe Lindlar industrial catalyst. These catalysts could also be used for preparing bimetallic W-Pd catalysts for selective hydrogenationof terminal and non-terminal alkynes.Keywords: tungsten; monometallic catalyst; selective hydrogenation; non-terminal alkyne.