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The conversion of allylic alcohols into epoxides using the chiral catalyst was discovered by K. Barry Sharpless and is known as Sharpless epoxidation. The use of a chiral catalyst enables the formation of one enantiomer of the product in excess. This chiral catalyst is mainly a chiral complex of titanium tetraisopropoxide and tartrate ester (specific stereoisomer). The stereoisomer used in the chiral catalyst dictates the formation of the enantiomer of the product. In other words, the use of...
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The presence of a catalyst affects the rate of a chemical reaction. A catalyst is a substance that can increase the reaction rate without being consumed during the process. A basic comprehension of a catalysts’ role during chemical reactions can be understood from the concept of reaction mechanisms and energy diagrams.
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Epoxides that are three-membered ring systems are more reactive than other cyclic and acyclic ethers. The high reactivity of epoxides originates from the strain present in the ring. This ring strain acts as a driving force for epoxides to undergo ring-opening reactions either with halogen acids or weak nucleophiles in the presence of mild acid. The acid catalyst converts the epoxide oxygen, a poor leaving group, into an oxonium ion, a better leaving group, making the reaction feasible. The...
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Epoxides result from alkene oxidation, which can be achieved by a) air, b) peroxy acids, c) hypochlorous acids, and d) halohydrin cyclization.
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Due to their highly strained structures, epoxides can readily undergo ring-opening reactions through nucleophilic substitution, either in the presence of an acid or a base. The nucleophilic substitution reactions in the presence of acid are called acid-catalyzed ring-opening reactions, and nucleophilic substitution reactions in the presence of a base are called base-catalyzed ring-opening reactions. Epoxides undergo base-catalyzed ring-opening reactions in the presence of a strong nucleophile...
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Nickel promotes selective ethylene epoxidation on silver.

Anika Jalil1, Elizabeth E Happel2, Laura Cramer2

  • 1Department of Chemical Engineering, University of California, Santa Barbara, CA, USA.

Science (New York, N.Y.)
|February 20, 2025
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Summary

Nickel doping in silver catalysts enhances ethylene epoxidation selectivity by activating oxygen. This approach offers a chlorine-free pathway and improves existing chlorine-promoted reactions, advancing catalyst design for selective oxidation.

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Area of Science:

  • Heterogeneous catalysis
  • Surface science
  • Materials science

Background:

  • Ethylene epoxidation is a crucial industrial process, historically reliant on chlorine (Cl) promotion of silver (Ag) catalysts.
  • Traditional promoters (Cs, Re, Mo) enhance selectivity but exhibit complex codependence on Cl, hindering optimization and mechanistic understanding.
  • Achieving high selectivity (~90%) in ethylene epoxidation remains a challenge due to the intricate interplay of promoters and chlorine.

Purpose of the Study:

  • To identify a novel promoter for silver catalysts in ethylene epoxidation using a theory-guided, single-atom alloy approach.
  • To investigate the potential of nickel (Ni) as a dopant in Ag for facilitating selective oxidation of ethylene.
  • To explore Ni's role in oxygen activation and its impact on catalyst selectivity, both with and without chlorine co-flow.

Main Methods:

  • Theoretical modeling guided the selection of nickel (Ni) as a single-atom dopant in a silver (Ag) catalyst.
  • Surface science experiments were conducted to study oxygen adsorption/desorption on Ni-doped Ag (NiAg) surfaces.
  • Supported Ag catalysts with controlled Ni doping (1:200 Ni:Ag ratio) were synthesized and tested in ethylene epoxidation.

Main Results:

  • Nickel doping in Ag facilitates molecular oxygen (O2) activation without strong oxygen (O) binding, crucial for selective oxidation.
  • NiAg surfaces demonstrated facile O2 adsorption/desorption and stabilization of unselective oxygen species.
  • The NiAg catalyst (1:200 ratio) achieved a ~25% selectivity increase in ethylene epoxidation without Cl co-flow.
  • NiAg acted cooperatively with Cl, yielding an additional ~10% initial selectivity enhancement.

Conclusions:

  • Nickel is identified as an effective dopant for silver catalysts in ethylene epoxidation, promoting selective oxidation via O2 activation.
  • The NiAg catalyst offers a promising route to enhance selectivity, reducing or eliminating the need for chlorine promotion.
  • The synergistic effect between Ni and Cl suggests a new avenue for optimizing ethylene epoxidation catalysts.