Isomerization Of Ethylene Oxide Research Articles

Catalytic degradation of ethylene oxide over Ag/α-Al2O3

Ethylene oxide (EO) degradation over Ag/α-Al2O3 catalysts involves bifunctional pathways, whereby α-Al2O3 surfaces catalyze hydration, isomerization, dimerization, and oxidation while Ag surfaces facilitate combustion and isomerization of ethylene oxide. EO degradation reactions run over Ag/α-Al2O3, α-Al2O3, and intrapellet mixtures thereof with co-feeds of O2 and oxygenate products show that Ag surfaces are more reactive than α-Al2O3 and that oxygenate products generated over α-Al2O3 combust over Ag much more (>30x) readily than EO. These observations suggest that additives which mitigate surface hydration of α-Al2O3 and combustion activity of Ag will most effectively mitigate EO degradation.

Synthesis of ZSM-22 and Testing Its Catalytic Properties in the Ethylene Oxide Isomerization Reaction

The influence of the key parameters of the synthesis of a TON-type zeolite (ZSM-22) with or without an organic structure-directing agent on its phase composition has been established. The optimal composition of the reaction mixture and the crystallization time of ZSM-22 in the presence of 1,6-diaminohexane under static conditions have been determined. The dynamics of the formation of an impurity ZSM-5 by cocrystallization during the synthesis with stirring has been revealed. For zeolite ZSM-22 synthesized using the template-free procedure, the most important factors affecting the crystallinity and the crystallization time are the synthesis temperature, the amount of seed, and agitation of the reaction mixture. The zeolites synthesized according to the both procedures have been examined in the reaction of isomerization of ethylene oxide to acetaldehyde. Zeolite ZSM-22 obtained without adding the template is not inferior in activity to the zeolite of the same structure prepared in the presence of 1,6-diaminohexane. Moreover, it exhibits higher selectivity of ethylene oxide conversion to acetaldehyde. The complete conversion of ethylene oxide has been observed on ZSM-22 zeolites of both types at a reaction temperature of 400°C, the selectivity of its conversion to acetaldehyde being at least 93%.

From qualitative to quantitative understanding of support effects on the selectivity in silver catalyzed ethylene epoxidation

Abstract In the epoxidation of ethylene, catalyzed by supported Ag particles, the support plays not only a beneficial role, but can also negatively impact the selectivity to ethylene oxide. Especially high surface area supports, and supports containing acid groups, seem detrimental for the selectivity, which is attributed to secondary reactions on the support surface. We prepared Ag nanoparticles on supports with a wide range of specific surface areas and different chemical nature, but with similar Ag metal loading and particle size. Indeed, a strong dependence of selectivity on both the specific surface area, and the chemical nature of the oxide support was found, with α-Al2O3 giving much more selective catalysts than SiO2. Furthermore, the conversion was an important factor in determining the selectivity, while on the other hand the support had no influence at all on the ethylene conversion. We described our experimental selectivities with a reaction model that used the rate constant for the secondary reaction of oxidation of ethylene oxide to carbon dioxide and water as the only variable parameter. The model was able to adequately describe the experimental results. It gave insight into the dependence of selectivity on conversion, how the secondary reaction depended on the chemical nature of the support, and how its rate scaled linearly with the support’s specific surface area. Building on this insight we modified a SiO2 support to passivate its OH-groups, which indeed yielded a 94% decrease in the rate of the secondary reaction, performing even better than α-Al2O3 with similar specific surface area would. We have shown that the selectivity towards ethylene oxide is dependent on a metal specific, intrinsic selectivity, and the decrease in selectivity with conversion to be support dependent. This decrease is caused by the isomerization of ethylene oxide, which is found to be first order in ethylene oxide and the active sites on the catalyst support.

Isomerization of Ethylene Oxide into Acetaldehyde on Zeolite with MTT (ZSM-23 Type) Structure

It was shown for the example of ZSM-23 zeolite with Si/Al = 50 that medium-porous zeolites with MTT structure are effective catalysts for gas-phase isomerization of ethylene into acetaldehyde. At 300–400°C, a nearly 100% conversion of ethylene oxide is observed, and the selectivity of its conversion to acetaldehyde exceeds 90%. The main by-products of the reaction are crotonic aldehyde and dioxane. ZSM-23 zeolite exhibits a higher selectivity and stability as compared with ZSM-5 zeolite having a 3D MFI structure. This zeolite provides an output of more than 28 g of acetaldehyde per gram of the catalyst per hour.

FTIR Study of Adsorption and Reactions of Ethylene Oxide on Powdered TiO2

Adsorption, thermal decomposition, and photoreactions of ethylene oxide (EO) on powdered TiO 2 have been studied by Fourier-transformed infrared spectroscopy. Most of the adsorbed EO molecules at the saturated coverage on 35 °C TiO 2 remain intact. As the temperature is increased (≥100 °C), the surface EO molecules dissociate into ethylene glycol-like species (-OCH 2 CH 2 OH or -OCH 2 CH 2 O-) by ring rupture and crotonaldehyde (CH 3 CH=CHCHO) possibly via acetaldehyde from EO isomerization. The ring-opening process is enhanced in the presence of coadsorbed H 2 O molecules. EO with the highly strained three-membered ring is thermally more reactive than 1,4-dioxane on TiO 2 . EO molecules adsorbed on TiO 2 readily undergo photodegradation in O 2 , forming surface H 2 O, CO 2 , HCO 3 , CO 3 , and HCOO. The comparative studies of 16 O 2 and 18 O 2 indicate that both O 2 and lattice oxygen take part in the EO photodecomposition. The reaction pathways involving the two oxygen species are discussed.

High surface area α-aluminas III. Oxidation of ethylene, ethylene oxide, and acetaldehyde over silver dispersed on high surface area α-alumina

Silver nanocrystallites dispersed on high surface area (HSA) α-alumina were prepared, and the activity and selectivity to ethylene oxide (EO) during ethylene oxidation over these catalysts were determined in the absence of Cl promoters. Cesium was also added to some of the samples to neutralize any surface acidity. Over all the Ag/ (HSA) α-Al 2O 3 catalysts, the selectivity to EO was negligible regardless of the addition of cesium while the apparent activation energy for carbon dioxide formation of 110 kJ mol −1 over the unpromoted samples was higher than that of 80 kJ mol −1 over a low surface area (LSA) α-alumina-supported silver catalyst. When this latter catalyst with selectivity to EO was mixed with (HSA) α-Al 2O 3, only carbon dioxide and water were obtained as a final product. EO isomerization and oxidation as well as acetaldehyde oxidation were also studied, and both silver and the support were found to be active in these reactions; hence these (HSA) α-aluminas are poor supports for ethylene epoxidation. In contrast to a promoting effect obtained at low concentrations in commercial catalysts, the higher loadings of cesium on these Ag/ (HSA) α-Al 2O 3 catalysts increased the turnover frequency of ethylene oxidation as well as the rates of EO and acetaldehyde oxidation, while the apparent activation energy for ethylene combustion decreased to 80 kJ mol −1 in the presence of cesium.

Support participation in chemistry of ethylene oxidation on silver catalysts

Activity and selectivity of supported silver catalysts in ethylene epoxidation and combustion is shown to be a strong function of the carrier employed. All supports used in this study, except for α-alumina, were found to exhibit significant activity in ethylene oxide isomerization and oxidation. Their activity was found to be proportional to their surface acidity. Poisoning of surface acid sites by alkali and alkaline earth metal impregnation resulted in suppression of isomerization activity and reduction of oxidation activity. These phenomena are explained evoking the concept of acid-base bifunctional catalysis.

Supported silver catalysts for the oxidation of ethylene: The effects of thermal treatment of alumina as catalyst support

Ethylene oxidation was investigated in a flow-type reactor, using silver catalysts supported on a series of aluminas which were pretreated at different temperatures between 300 and 1800 °C. Steady activity and selectivity at a set of reaction conditions were dependent upon the surface area of the catalyst which was a function of the thermal treatment of alumina. The specific rates of ethylene oxidation, the oxidation of ethylene oxide, ethylene oxide isomerization to acetaldehyde, and acetaldehyde oxidation were investigated in a pulse reactor, using three kinds of catalysts supported on aluminas pretreated at 1150, 1200, and 1800 °C. By comparing the activities of these catalysts for the above four reactions, the following were deduced: (1) A direct oxidation path of ethylene to carbon dioxide and water could be assumed for each catalyst; (2) the consecutive oxidation of ethylene via ethylene oxide was assumed to occur on each catalyst tested, and it predominated on catalysts supported on aluminas pretreated at lower temperatures, which was interpreted in terms of the complete oxidation of acetaldehyde formed in the pores of alumina by the isomerization of ethylene oxide.

Heterogeneous Isomerization of Ethylene Oxide to Acetaldehyde

Surface catalyzed isomerization of (CH2)2O to CH3CHO occurs in a small, Pyrex reactor containing 15 or 230 Torr (CH2)2O at temperatures in the range 339.2 °C to 461.4 °C. Ro(CH3CHO), the initial rate of formation of CH3CHO, depends on the number of successive pyrolyses and on the evacuation time between pyrolyses, which is not true of the initial rates of formation of H2, CO, CH4, CH2CO, and C2H6. Ro(CH3CHO) also depends on the reactor S/V and on pretreatment of the reactor with product gases, particularly H2. The reproducible and limiting values of Ro(CH3CHO) are independent of the (CH2)2O pressures used. Above 385 °C these initial rates exhibit an Arrhenius temperature dependence while below 385 °C they pass through a maximum.Analysis of the Langmuir–Hinshelwood, low surface coverage mechanism shows that the T > 385 °C isomerization is partially or totally controlled by CH3CHO desorption and that −ΔHads < (55 ± 10) kcal mol−1, for CH3CHO on this surface.Ea = 57 kcal mol−1 for the homogeneous first order decomposition of (CH2)2O, in the 15 Torr pyrolysis at temperatures in the range 374.0 to 406.7 °C, and any vibrationally excited CH3CHO, formed in the gas phase at this pressure (and possibly even at 230 Torr), dissociates into chain initiating radicals.

Reactions of α-oxides: Acid- and auto-catalyses in reactions of ethylene oxide with amines

Silver nanocrystallites dispersed on high surface area (HSA) α-alumina were prepared, and the activity and selectivity to ethylene oxide (EO) during ethylene oxidation over these catalysts were determined in the absence of Cl promoters. Cesium was also added to some of the samples to neutralize any surface acidity. Over all the Ag/ (HSA) α-Al2O3 catalysts, the selectivity to EO was negligible regardless of the addition of cesium while the apparent activation energy for carbon dioxide formation of 110 kJ mol−1 over the unpromoted samples was higher than that of 80 kJ mol−1 over a low surface area (LSA) α-alumina-supported silver catalyst. When this latter catalyst with selectivity to EO was mixed with (HSA) α-Al2O3, only carbon dioxide and water were obtained as a final product. EO isomerization and oxidation as well as acetaldehyde oxidation were also studied, and both silver and the support were found to be active in these reactions; hence these (HSA) α-aluminas are poor supports for ethylene epoxidation. In contrast to a promoting effect obtained at low concentrations in commercial catalysts, the higher loadings of cesium on these Ag/ (HSA) α-Al2O3 catalysts increased the turnover frequency of ethylene oxidation as well as the rates of EO and acetaldehyde oxidation, while the apparent activation energy for ethylene combustion decreased to 80 kJ mol−1 in the presence of cesium.

THE SILVER CATALYZED OXIDATION OF ETHYLENE: III. KINETICS OF ETHYLENE OXIDE OXIDATION

A flow type apparatus was used for kinetic studies of the silver catalyzed oxidation of ethylene oxide (EtO) by oxygen at 274 °C. Using N2 as diluent the concentrations of O2 and ethylene oxide were varied independently from 9.9 to 79% and 2.35 to 9.4% respectively while a total pressure of 1 atmosphere was maintained. Flow rates were varied to give a range of contact times varying from 0.06 to 0.25 sec. It was shown that EtO is oxidized without previous dissociation into C2H4 and O2. The dependence of the initial rate of oxidation of EtO on reactant concentrations excludes isomerization of EtO (to acetalde hyde) as a main step in its oxidation, and a direct oxidation mechanism is suggested. The results of a few experiments to determine the extent of isomerization of EtO to acetaldehyde in the absence of oxygen are presented. No steady state could be achieved but the results may be used semiquantitatively to support the belief that isomerization is not the rate determining step in the oxidation of ethylene oxide.