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

Low-cost tungsten monometallic catalysts containing 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-Al2O3 (W6+) species on the surface. The effects of the content of W nanoparticles and reaction temperature on activity and selectivity for the partial hydrogenation of 3-hexyne, a non-terminal alkyne, were assessed under moderate conditions of temperature and pressure. The monometallic catalysts prepared were found to be active and stereoselective for the production of (Z )-3-hexene, had the following order: 7.1WN/A > 8.5 WN/A ≥ 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 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 heterolytic bond rupture; the H+ and H- species generated approach a 3-hexyne adsorbate molecule and cause heterolytic rupture of the C≡C bond into C- = C+; and b) steric effects related to the high concentration of WO3 on 8.5WN/A that block the Al2O3 support. Catalyst deactivation was detected, starting at about 50 min of reaction time. Electrodeficient W6+ species are responsible for the formation of green oil at the surface level, blocking pores and active sites of the catalyst, particularly at low reaction temperatures (293 and 303 K). The resulting best catalyst, 7.1WN/A, has low fabrication cost and high selectivity for (Z )-3-hexene (94%) at 323 K. This selectivity is comparable to that of the classical and more expensive industrial Lindlar catalyst (5 wt% Pd). The alumina supported tungsten catalysts are low-cost potential replacements for the Lindlar industrial catalyst. These catalysts could also be used for preparing bimetallic W-Pd catalysts for selective hydrogenation of terminal and non-terminal alkynes.

Selective Journal narrative, 1812-1821

Paper notebook containing select entries copied from "Cambridge Diary No. 1."

Traditions of the Arapaho, collected under the auspices of the Field Columbian museum and of the American museum of natural history, by George A. Dorsey -- and Alfred L. Kroeber --

v.5(1903)

The mechanisms and qualifications of the selective permeability in transport across biological barriers : the example of kyotorphin

Trabalho Final do Curso de Mestrado Integrado em Medicina, Faculdade de Medicina, Universidade de Lisboa, 2014

Magia e medicina in Burkina Faso. Proposta di traduzione del testo scientifico La médecine des traditions africaines.

Il presente elaborato nasce dalla volontà di far conoscere, attraverso una proposta di traduzione del primo capitolo, il libro del farmacista e traditerapeuta Pascal Nadembega, originario del Burkina Faso. Come indica il titolo stesso, il libro tratta della medicina tradizionale africana, nei suoi aspetti magici e scientifici. Il testo è anche pretesto per tracciare una breve panoramica storica della medicina in Africa, dalle origini al giorno d'oggi, con ostacoli e relative soluzioni appartenenti a ogni epoca. Nella proposta di traduzione si cerca di mantenere lo stile oralizzato tipico del francese delle popolazioni africane, con l'aggiunta di note a piè di pagina per facilitare la comprensione di culturemi. L'analisi del testo di partenza e il commento alla proposta di traduzione si propongono di studiare le difficoltà incontrate durante il processo di traduzione, nonché di metterle in relazione con la natura ibrida dell'argomento trattato dall'autore.

The worship of the serpent traced throughout the world, and its traditions referred to the events in Paradise : proving the temptation and fall of man by the instrumentality of a serpent tempter /

Mode of access: Internet.

Ion selective electrodes in water quality analysis /

Mode of access: Internet.

Evaluation of selective erosion control techniques : Piedmont region of S.E. United States /

Mode of access: Internet.

The selective expansion of agricultural production in Latin America;

"United Nations publication. Sales no.: 1957.II.G.4."

The church bells of Bedfordshire : their founders, inscriptions, traditions, and peculiar uses, with a brief history of church bells in that county, chiefly from original and contemporaneous records /

Mode of access: Internet.