The activity and selectivity of oxygen atoms adsorbed on a Ag/?-Al2O3 catalyst in ethene epoxidation

Abstract

The bonding of the oxygen species held on a Ag/a-Al 2 O 3 catalyst has been studied by temperature programmed desorption and their reactivity in ethene epoxidation by temperature programmed reduction using ethene as the reductant. The Ag/a-Al 2 O 3 catalyst was produced by the thermal decomposition of a Ag oxalate/a-Al 2 O 3 precursor. Oxygen desorbs from this Ag/a-Al 2 O 3 catalyst in two states, one (peak maximum temperature 520 K) having a desorption activation energy of 140 kJ mol À1 -oxygen desorbing from Ag(111), and one (peak maximum temperature 573 K) having a desorption activation energy of 155 kJ mol À1 -oxygen desorbing from a highly stepped or defected Ag surface. Temperature programmed reduction of the two oxygen states existing on the surface of the Ag/a-Al 2 O 3 catalyst using ethene as the reductant produced two peaks at 373 and 473 K in which ethene epoxide and CO 2 evolved coincidently. The peak at 373 K derives from the reduction of oxygen atoms adsorbed on Ag(111). The higher temperature peak (473 K) corresponds to the reduction of oxygen atoms adsorbed on highly stepped or defected Ag surface. The selectivity to ethene epoxide for the 373 K peak is $57%, while that of the 473 K peak is 34%. The coincident evolution of ethene epoxide and CO 2 shows that the selective and unselective reaction pathways have a common surface intermediate-probably an oxametallacycle. The higher selectivity of the oxametallacycle formed by the bonding of ethene to the weaker AgAO bond is considered to result from its having a lower activation energy to cyclisation than that produced by ethene bonding to the higher AgAO bond.

Oxidation of Ethylene on Oxygen Reconstructed Silver Surfaces

“…The measured desorption temperatures of m/z = 44 amu are in line with those seen during similar TPR experiments on high surface area catalysts [23]. However, unlike results on high surface area catalysts we saw no m/z = 29 amu that would indicate partial oxidation products, in support of our theoretical simulations.…”

“…Though the finding that the oxygen-induced p(2x1) reconstruction on silver strongly favors AcH production through an OMC mechanism, and ultimately combustion, is interesting, the analysis of temperature programed desorption (TPD) data suggests much of the oxygen on high surface area catalysts is adsorbed on close-packed faces [23]. And while the oxygen-induced reconstructions on the Ag(110) surface appear oxidic due to their linear Ag-O-Ag bonding motif, the oxygen-induced reconstructions on the Ag(111) surface are unlike any known bulk phases [16][17][18][19].…”

An investigation on the role of Re as a promoter in Ag Cs Re/α-Al 2 O 3 high-selectivity, ethylene epoxidation catalysts

“…Thus, the average particle size of 0.12 lm determined by O 2 chemisorption at 170°C for the unpromoted Ag catalyst was assumed to be representative for all catalysts evaluated in this investigation. We conclude that because no apparent morphological changes in the Ag particles caused by addition of promoters were observed, the mechanism for higher EO selectivity must occur either through a site-blocking process, as stated by Waugh et al [6,[22][23][24], or through an electronic interaction, as proposed by Monnier and others [12,[25][26][27][28][29][30][31]33]. However, the results in Table 3 show that when 350 ppm of Cs is added to the AgARe catalyst, the required shell temperature to achieve 2.0 mol% EO drops from 262°C to 218°C, while the selectivity to EO increases from 49.85% to 82.35%.…”

“…A chlorine-containing moderator such as ethyl chloride (CH 3 CH 2 Cl) is also continuously fed to the reactor at ppm levels because it similarly increases the selectivity to EO. The use of Cs as a promoter has been discussed at length, but its specific role in the selective oxidation of ethylene is still the subject of considerable debate [22][23][24][25][26][27][28][29][30][31]. For example, Waugh and coworkers [6,[22][23][24] recently argued that the role of Cs is mainly geometric in nature and claimed that Cs is preferentially bound to Ag on unselective, stepped silver sites.…”

Epoxidation of Propylene by Molecular Oxygen Over the Ag–Y2O3–K2O/α-Al2O3 Catalyst

“…If the modified silver catalyst could produce the mild electrophilic oxygen species to attack the C=C bond of the alkenes, the catalyst would be highly effective for the epoxidation of propylene. By the kinetics studies of adsorption and desorption of O 2 over the modified Ag/aAl 2 O 3 catalyst, Atkins et al [10,11] found that Cl or Cs could weaken the strength of Ag-O bond and block the adsorption of oxygen onto the stepped Ag surface, resulting in the increase of the selectivity to ethylene oxide from 75% to 85%. Some patents claimed that the conversion of propylene of 1.5-12.6% and the selectivity to PO of 32-60% were achieved over the K, W, Re and Cl modified Ag/CaCO 3 , CaF 2 , CaHPO 4 , CaMoO 4 and BaTiO 3 catalysts [12][13][14][15][16][17].…”