Infrared spectroscopy of some chemisorbed molecules on tungsten oxide-silica

Abstract

Transparent plates of a tungsten oxide-silica metathesis catalyst were prepared by hydrolyzing a WCl6-Si(OC2H6)4 mixture. From the infrared spectra of adsorbed pyridine and ammonia, it is concluded that the chemisorption sites on the catalyst surface are of the Lewis type. Strong Bronsted acid sites could not be observed: the surface hydroxyls appear to resemble the weakly Bronsted-acidic hydroxyls on dry silica gel. Temperature-programmed reduction showed that ~95% of the tungsten on the calcined catalyst is present as surface compounds and less than 5% as metathesis-inactive "free" oxide. The Lewis acidity is ascribed to these surface compounds, which are thought to be coordinatively unsaturated species resulting from the dehydration of W06 octahedra sharing edges or planes with the silica lattice.Stoichiometric reactions between superoxide ions on MgO and simple hydrocarbons were observed at 175 °C, although the 02" ions were much less reactive than either 0~or Oa~i ons. Several types of oxygen-containing products were formed, as well as other hydrocarbons and C02. The reaction of propylene with 02~gave no gaseous product at 175 °C; however, at elevated temperatures acetaldehyde and methanol were obtained. Following the reaction of propane with 02~, acetone was detected at 175 °C in addition to acetaldehyde and methanol which were observed at higher temperatures. With 1-butene as the reactant, 2-butanol was formed together with methanol, acetaldehyde, and acrolein above 300 °C. Infrared spectra of surface intermediates indicate that the reaction of 02~w ith propylene at 175 °C resulted in the simultaneous formation of formate and acetate ions which are consecutively converted to carbonate ions at elevated temperatures. It is proposed that hydrogen atom abstraction is the initial step in the reaction of Of with simple hydrocarbons. The resulting radicals react with lattice oxygen ions forming carboxylate ions or with H02~i ons forming alkoxy or epoxide intermediates.

Effects of calcination and pretreatment temperatures on the catalytic activity and stability of H2-treated WO3/SiO2 catalysts in metathesis of ethylene and 2-butene

“…8,[10][11][12][13][14][15] Among them, it was reported that the tungsten dispersion, tetrahedral tungsten oxide species, intermediate WO 3Àx , or W 5+ species are the active sites for metathesis, [16][17][18][19] and that WO 3 crystals are not active in metathesis and catalyst sites should be contained in the amorphous surface. 8,20 At present, the activation by pretreatment conditions to improve performance of supported WO x /SiO 2 catalysts in metathesis reaction has received much attention because of pre-activating the active sites of catalysts to form the activated state before running the reaction.…”

Olefin Metathesis by Group VI (Mo, W) Metal Compounds

“…The irst studies were made by Raman and led to the conclusion that the active site was an isolated surface complex of tungsten but of unknown structure [43]. The irst postulated surface species was a pentacoordinated tungsten complex but no experimental justiication was given [44].…”

Metathesis of 1‐Hexene over Heterogeneous Tungsten‐Based Catalysts

“…The proton comes from the interaction of the reactant alkene with the non-reduced surface species. Adsorption of 1-hexene and hydrogen addition yield an intermediate which through back donation of the proton gives a carbenoid chain initiator [8]. The key intermediate in the metathesis reaction is believed to be a metallocyclobutane complex that forms around this carbene center.…”

“…The methyl alkylidene is reused in another metathesis cycle. A limited number of metatheses proceed through the initiator before the chain of reactions terminates [8].…”