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

Reijen, Jeroen E. Van Den; Versluis, Caroline; Kanungo, S.; d’Angelo, M.F. Neira; Jong, Krijn P. de; Jongh, Petra E. de

doi:10.1016/j.cattod.2019.04.049

Cited by 29 publications

(35 citation statements)

References 52 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…This is rather remarkable since EO isomerization has been reported to occur over most acidic surface groups, both including Si-OH and Ti-OH. 55 Yet, here, we observe that PO isomerization requires the Lewis acidic Ti-sites of the Ti-containing support as catalytic centers. Similar to ethene oxidation, these acidic sites can be neutralized by an alkali base.…”

Section: ■ Methodsmentioning

confidence: 51%

“…As first suggested by Moulijn et al ., acidic surface groups (Ti-OH or Si-OH) convert small amounts of the propene oxide product into propanal, which is irreversibly adsorbed on acidic Ti-OHs, whereas an excess of base residues (ammonium ions and magnesia) at the surface was shown to enhance selectivity toward acetone. , These suggestions in propene oxidation are similar to observations in ethene oxidation over supported silver catalysts. There, ethene oxide (EO) is converted rapidly over the surface groups of most oxides, such as Si–OH, Al–OH, and Ti–OH. − Strategies to limit the EO isomerization to ethanal (acetaldehyde) are focused on limiting the amount of surface groups. Therefore, low surface area α-alumina is often reported as the superior support for silver catalysts .…”

Section: Resultsmentioning

confidence: 99%

“…Therefore, low surface area α-alumina is often reported as the superior support for silver catalysts. 55 Also, in ethene oxidation, alkali addition is used to neutralize surface acidity. 53,57 However, this is the first time that the role of alkali promotors in the surface acidity is reported for propene epoxidation.…”

Section: ■ Methodsmentioning

confidence: 99%

See 2 more Smart Citations

Steering the Selectivity in Gold–Titanium-Catalyzed Propene Oxidation by Controlling the Surface Acidity

et al. 2021

View full text Add to dashboard Cite

Supported nanoparticulate Au/Ti-SiO2 catalysts are a promising candidate for selective epoxidation of propene with H2/O2 mixtures. Here, we demonstrate that by altering the acidity of the surface titanol groups in Au/Ti-SiO2, the selectivity of these catalysts in propene oxidation can be controlled. That is, Au/Ti-SiO2 prepared using an alkali base during gold deposition shows basic properties due to the formation of Ti-ONa groups. The catalysts that contained Na+ and neutralized acid sites demonstrate high selectivity toward propene oxide. On the contrary, when the acidity of the Ti-OH groups is preserved by using NH4OH as a base during gold deposition, the catalyst is highly selective toward propanal at a similar propene conversion. This difference in selectivity is explained by the isomerization of initially formed propene oxide into propanal over acidic Ti-OH groups as we demonstrated using stacked bed experiments, where the Ti-support was exposed to propene oxide. When Na+ was present, no isomerization was observed, while without Na+ present, propene oxide was isomerized to propanal. In short, we demonstrate the crucial role of Na+ and acidic Ti-sites in steering the selectivity in gold-catalyzed propene epoxidation.

show abstract

Section: ■ Methodsmentioning

confidence: 51%

Section: Resultsmentioning

confidence: 99%

See 1 more Smart Citation

Steering the Selectivity in Gold–Titanium-Catalyzed Propene Oxidation by Controlling the Surface Acidity

et al. 2021

View full text Add to dashboard Cite

show abstract

“…Catalyst supports and silanol groups may also contribute significantly to the isomerization of the epoxide. 71 The comparison of gas and surface conversions and selectivities suggest that gas phase can, in contact with the catalyst, (i) enhance C 2 H 6 activation, and (ii) slightly suppress reactions that occur on peroxo sites. Next, we examine the concentrations of different gaseous and surface reactive species and relative rates of gas and surface reactions to further probe the origins of these effects.…”

Section: Effect Of Gas-phase Reactions On Rate and Selectivitymentioning

confidence: 99%

Effects of surface species and homogeneous reactions on rates and selectivity in ethane oxidation on oxide catalysts

et al. 2021

View full text Add to dashboard Cite

Selective alkane oxidations on metal oxide catalysts involve complex mechanisms with multiple reactions in series and parallel, different types of reduced and oxidized surface species, and potential contributions from gas‐phase reactions. Here, kinetics and thermodynamics of elementary steps involved in C2H6‐O2 reactions on SiO2‐supported small vanadium oxide domains are determined using density functional theory. These surface reactions together with gas‐phase mechanisms are incorporated in kinetic simulations to determine how surface and gaseous reactions interact and contribute to rates and selectivity. The results show that gas‐phase reactions within pore volumes in contact with the catalyst contribute significantly to C2H6 activation rates, even at conditions where gas‐phase reactions in empty volumes without catalyst are negligible. The majority of C2H6 activations occur on the surface, via H abstraction by vanadium oxo species present at terminal lattice oxygens. The gas‐phase activations via H‐abstraction by OH radicals also exhibit significant contributions. The reduced centers formed by reactions at vanadium oxo species are re‐oxidized rapidly and, therefore, are present in very small concentrations at reaction conditions. The re‐oxidation steps lead to the formation of HO2 radicals and surface peroxo species that are also rapidly consumed and are present in small concentrations. The peroxo species preferentially convert C2H4 to its epoxide product and influence selectivity even at low concentrations. The gas‐phase reactions decrease the concentrations of peroxo species and improve selectivity slightly. The effects of reaction conditions and catalyst site density provide further insights into how factors beyond conversions at lattice oxygens influence rates and selectivity in alkane oxidation reactions of significant industrial importance.

show abstract

“…corresponding to an oxygen conversion of 100% or more. 345 The materials with the lowest support specific surface area for each support type produce the highest selectivity towards ethylene oxide for a given conversion. Striking is that, although the selectivities vary widely, at the lowest possible conversions measured all selectivities converge to a similar value (around 76%).…”

Section: 33mentioning

confidence: 99%

Supported Silver Catalysts for Ethylene Epoxidation

Reijen¹,

Elias²

View full text Add to dashboard Cite

12or to increase the stability of the catalyst. [74][75][76] The activity of the catalyst is less important as high conversions decrease the ethylene oxide selectivity due to the secondary reaction (r 3 in Figure 1.2, the full oxidation of ethylene oxide).The presence of chlorine on the surface suppresses the full oxidation of ethylene, 74,77,78 as the presence of chlorine on the catalyst surface prevents the formation of certain surface species responsible for the unselective pathway, leading to an increase in the intrinsic selectivity. 75 The competitive adsorption of chlorine on the silver surface indiscriminately suppresses oxygen adsorption on the silver surface and therefore decreases the overall activity by suppressing both the selective (r 1 ) and unselective (r 2 ) pathways. 47,77,79,80 A secondary effect of chlorine promotion leads to a suppression of the secondary, unselective reaction (r 3 ); the decomposition of ethylene oxide to form carbon dioxide and water. 79,81 In a typical epoxidation reactor a feed containing oxygen, ethylene and ppm-levels of the chlorine moderator in an inert diluent gas is fed over the reactor at 200 -280 °C at 15 bar. 55,72 The reactor is designed to have good control over the temperature as to prevent hot-spots. 82 The oxygen content is important to maintain an oxide overlayer on the silver, which is required to obtain high selectivities. However, the oxygen content is kept below 10 % per volume to prevent the formation of explosive mixtures. 55,72,83 When the oxygen content is below this value, the ethylene concentration can be maintained at a relatively high value of 30 -40 vol% safely. 55,72,83 As selectivity decreases with conversion, the (ethylene) conversion is kept low to maintain a high yield. Commercially, a yield of 1 -3 vol% EO is viable. 55 Modern industrial catalysts obtain selectivities toward ethylene oxide of 90 % 77,84 compared to about 50-70 % for early epoxidation catalysts. 85,86 Besides (promoted) silver catalysts, also other catalytic systems are being investigated for this reaction. [87][88][89][90] High selectivities towards ethylene oxide are reported by Jankowiak and Barteau for CuAg bimetallic catalysts, 89,91 and later also for AgPd bimetallic catalysts. 87 Due to the bimetallic nature of these catalysts, the environment during reaction can greatly influence composition of the catalyst surface, as for example in the CuAg catalysts segregation of the metals occurs under reaction conditions. 92,93 Depending on the metal ratios and reaction conditions, alloying, but also promotion, additional metals can therefore be either beneficial or unfavorable for the catalytic performance of the silver catalyst. 88,92,93 As silver is capable of selectively oxidizing ethylene, the simplest olefin, it seems straightforward that this system is also an interesting candidate for the epoxidation of other olefins. However, silver appears to be unable to selectively oxidize higher olefins in gas phase, due to activation of the β-hydrogen (one of the hydrogen atoms b...

show abstract

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

Cited by 29 publications

References 52 publications

Steering the Selectivity in Gold–Titanium-Catalyzed Propene Oxidation by Controlling the Surface Acidity

Steering the Selectivity in Gold–Titanium-Catalyzed Propene Oxidation by Controlling the Surface Acidity

Effects of surface species and homogeneous reactions on rates and selectivity in ethane oxidation on oxide catalysts

Supported Silver Catalysts for Ethylene Epoxidation

Contact Info

Product

Resources

About