Editing Ethylene oxide (section)

====Chemistry and kinetics of the direct oxidation process====
Formally, the direct oxidation process is expressed by the following equation:

: <chem>2CH_2=CH2 + O2 ->[\ce{Ag}] 2(CH2CH2)O</chem>, ΔH=−105 kJ/mol

However, significant yield of carbon dioxide and water is observed in practice, which can be explained by the complete oxidation of ethylene or ethylene oxide:

: CH<sub>2</sub>=CH<sub>2</sub> + 3 O<sub>2</sub> → 2 CO<sub>2</sub> + 2 H<sub>2</sub>O, ΔH=−1327{{nbsp}}kJ/mol
: (CH<sub>2</sub>CH<sub>2</sub>)O + 2.5 O<sub>2</sub> → 2 CO<sub>2</sub> + 2 H<sub>2</sub>O, ΔH=−1223{{nbsp}}kJ/mol

According to a kinetic analysis by Kilty and Sachtler, the following reactions describe the pathway leading to EO. In the first step, a [[superoxide]] (O<sub>2</sub><sup>−</sup>) species is formed:<ref name="kilty">{{cite journal
|author=Kilty P. A. |author2=Sachtler W. M. H.
|title=The Mechanism of the Selective Oxidation of Ethylene to Ethylene Oxide
|journal=Catalysis Reviews: Science and Engineering
|year=1974
|volume=10
|pages=1–16
|doi=10.1080/01614947408079624
}}</ref>
: O<sub>2</sub> + Ag → Ag<sup>+</sup>O<sub>2</sub><sup>−</sup>
This species reacts with ethylene
: Ag<sup>+</sup>O<sub>2</sub><sup>−</sup> + H<sub>2</sub>C=CH<sub>2</sub> → (CH<sub>2</sub>CH<sub>2</sub>)O + AgO
The resulting silver oxide then oxidizes ethylene or ethylene oxide to CO<sub>2</sub> and water. This reaction replenishes the silver catalyst. Thus the overall reaction is expressed as

: 7 CH<sub>2</sub>=CH<sub>2</sub> + 6 O<sub>2</sub> → 6 (CH<sub>2</sub>CH<sub>2</sub>)O + 2 CO<sub>2</sub> + 2 H<sub>2</sub>O

and the maximum degree of conversion of ethylene to ethylene oxide is theoretically predicted to be 6/7 or 85.7%,<ref name="kilty"/> although higher yields are achieved in practice.<ref>{{cite journal |last1=Özbek |first1=M. O. |last2=van Santen |first2=R. A. |date=2013 |title=The Mechanism of Ethylene Epoxidation Catalysis |journal=Catalysis Letters |volume=143 |issue=2 |pages=131–141 |doi=10.1007/s10562-012-0957-3 |s2cid=95354803}}</ref>

The catalyst for the reaction is metallic silver deposited on various matrixes, including [[pumice]], [[silica gel]], various [[silicate]]s and [[aluminosilicate]]s, [[alumina]], and [[silicon carbide]], and activated by certain additives ([[antimony]], [[bismuth]], [[barium peroxide]], etc.).<ref name="lebedev">{{cite book
|author=Lebedev, N.N.
|title=Chemistry and Technology of Basic Organic and Petrochemical Synthesis
|year=1988
|edition=4
|publisher=Khimiya
|pages=420–424
|isbn=5-7245-0008-6
}}</ref> The process temperature was optimized as {{convert|220–280|C||sigfig=2}}. Lower temperatures reduce the activity of the catalyst, and higher temperatures promote the complete oxidation of ethylene thereby reducing the yield of ethylene oxide. Elevated pressure of {{convert|1-3|MPa||abbr=on}} increases the productivity of the catalyst and facilitates absorption of ethylene oxide from the reacting gases.<ref name="lebedev"/>

Whereas oxidation by air is still being used, oxygen (> 95% purity) is preferred for several reasons, such as higher molar yield of ethylene oxide (75–82% for oxygen vs. 63–75% for air), higher reaction rate (no gas dilution) and no need of separating nitrogen in the reaction products.<ref name="ect"/><ref>{{cite book
|author=Gunardson H.
|title=Industrial gases in petrochemical processing
|location=New York
|publisher=Marcel Dekker, Inc.
|year=1998
|pages=131–132
|isbn=0-8247-9908-9
}}</ref>