Preparation and particle size effects of Ag/α-Al2O3 catalysts for ethylene epoxidation

Abstract

a-AluminaCatalyst design Ethylene oxide Particle size effect a b s t r a c t Currently, for the industrial ethylene epoxidation a-alumina supported silver catalysts are the only catalyst of choice. We demonstrate a novel method to produce these catalysts with different silver particle sizes, but without changing other key parameters that may affect the catalytic performance such as support specific surface area or metal precursor. a-Alumina was impregnated with a silver oxalate solution, and was subsequently dried and treated in different gas atmospheres and at different temperatures to tune the silver particle sizes in the range of 20-500 nm. Particles of 20 nm exhibited a lower turnover frequency than particles of 70 nm and larger, which exhibit a constant turnover frequency, in accordance with results in literature. However, the selectivity, when measured at constant conversion, was particle size independent. This is the first time that the effect of the particle size on the selectivity of ethylene epoxidation is reported at constant conversion. This was made possible by a new method of producing supported silver catalysts, which we expect that is also applicable for silver catalysts with other supports and for the preparation of other supported metal catalysts.

Kinetics of Ethylene Epoxidation on a Promoted Ag/α‐Al2O3 Catalyst—The Effects of Product and Chloride Co‐Feeds on Rates and Selectivity

“…However,v arying the space velocity resulted in ac oncurrent decrease in EO selectivity (0.5-1.6 %) and increase in CO 2 selectivity (0.5-1.6 %) as space velocityw as decreased from % 500 000 cm 3 h À1 cm cat À3 to % 100 000 cm 3 h À1 cm cat À3 (Figure 10). There are two plausible reasons for the observed change in EO and CO 2 selectivity as space time increases:1 )the presenceo fasequential reaction pathway for EO combustion to CO 2 [19,23] under the reactionc onditions investigated, and/or 2) ac hange in the branching ratio of the common reactioni ntermediate with space time similart oa n observation by van den Reijen et al [57] wherein EO selectivity, measured on non-promoted Ag/a-Al 2 O 3 catalysts with silver particles izes between 20-200 nm at 453-543 K, decreases with increasing ethylene conversion.W en ote that only 0.6-1.8 %o fE Of ormed hast ou ndergo combustion to achieve the observed0 .5-1.6 %i ncrease in CO 2 selectivity,w hich is well below % 10 %E Oc ombustionm easured by Cant and Hall [19] on an unsupported silver sponge formulation without chlorine modification at 443 Ka nd ambient pressure.…”

Ethylene epoxidation over supported silver catalysts – influence of catalyst pretreatment on conversion and selectivity

“…An alternative explanation would be that silver particles sinter to larger ones, which could be more active. Recently, van den Reijen et al 32 reported that the turnover frequency increases until the silver particle size is larger than 70 nm and thereafter the turnover frequency is constant. The silver time yield, defined as the moles of ethylene converted per gram of silver per s, had a maximum value over silver particles of approximately 60 nm size.…”

“…The silver time yield, defined as the moles of ethylene converted per gram of silver per s, had a maximum value over silver particles of approximately 60 nm size. 32 The selectivity to ethylene oxide was reported to be independent of silver particle size. However, the selectivity at a 2.8% ethylene conversion was about 50%, which is significantly lower than the selectivity we obtained.…”

Influence of the specific surface area and silver crystallite size of mesoporous Ag/SrTiO3 on the selectivity enhancement of ethylene oxide production

“…Some evidence has been presented, which confirmed an increase of the EO selectivity with increasing Ag crystallite size . However, the effect of this parameter is still open to debate. Van den Reijen et al . reported selectivity values at constant ethylene conversion are independent on the Ag crystallite size.…”

“…Ethylene oxide (EO), industrially produced by the method of direct ethylene oxidation in air, is one of the most important chemical intermediates with the global capacity of 25 million tons per year in 2013. 1 The working catalyst for this reaction is silver (Ag) on an ultrapure aluminum oxide (Al 2 O 3 ) support with low specific surface area, normally below 2 m 2 /g. 2 The EO selectivity for catalysts without any promoter is around 50%, but adding 100-500 mg/kg of alkali or alkaline earth metals improves the selectivity significantly.…”