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Mechanism of reaction involved in the reactor
The reaction involved in the reactor is epoxidation where ethylene is oxide to produce ethylene oxide with the silver (Ag) as a catalyst in a packed bed reactor. Other than ethylene oxide, several products such as vinyl alcohol, acetaldehyde, and vinyl radical also can be formed by oxidation of ethylene with oxygen. Scheme 1 shows these possible paths.
Possible reaction paths for partial oxidation of ethylene
Two parallel paths have been found for the ethylene oxide mechanism, both catalyzed with silver.
As shown in Scheme 2, parallel reactions of selective (k1) and non-selective (k2) are occurring on an Ag particle, while consecutive burning of ethylene oxide (k3) has been found to be sensitive to catalyst support acidity.
Initially proposed molecular mechanism for ethylene oxide
The interesting consequence of the present proposal was that the ethylene oxide selection should be limited to a maximum value of 6/7 (85.7 percent) that indicates the ethylene oxide selection according to Eq. 1 of a particular atomic oxygen atom.
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The selectiveness value for the promoted catalyst was typically observed where the silver surface adsorbed oxygen.
A weakly bound surface oxygen (electrophilic, Oδ+) has been found to allow selective oxidation and strongly bound surface oxygen (nucleophilic, Oδ-) for combustion after contact with ethylene. Selective ethylene oxidation was proposed in the mechanism Eley–Rideal (E–R), while non selective combustion was performed by Langmuir–Hinshelwood (L–H).
A double bond of the C = C of ethylene oxide (k1) and the activated C – H path (k2) forms the COC ring by insertion O with the electrons deficient oxygen attack (Oδ+).
On the contrary, the C–H bond attack of electron-rich oxygen (Oδ-) removes H from ethylene to form a radical molecule which ends up in total combustion. Although there was no intermediate stable acetaldehyde (AA) present in the suggested path of ethylene combustion(k2), AA (k3) was known as ethylene oxide isomerization.
Chlorine is supplied as an inhibitor to reactors. The inhibitor is a product used in catalytic processes to enhance catalyst performance and to reduce the reaction rate by competing with catalyst activity reactants. The chlorine prevents the adsorption of atomic oxygen on the silver surface so that only molecular oxygen as shown in Eq. 2 is inhibited optimally in the silver surface. Adsorbed molecular oxygen reacts to ethylene formed ethylene oxide and after desorption leaves an oxygen atom, as shown in Eq. 3. Ethylene is burned to CO2 and H2O by the atomic oxygen, as shown in Eq. 4.
Since six oxygen atoms are required to fully oxidize an ethylene molecule, six ethylene oxide molecules have to be formed in advance of the total oxidation of one ethylene molecule. Therefore, if atomic oxygen adsorption inhibition is optimal, the maximum selectivity is 6/7 (85.7 percent), as discussed earlier. This mechanism clearly explains how the inhibitor works. A variation of this mechanism is supposed to cause the formation of both ethylene oxide and total oxidation through the common intermediate molecular oxygen. The effect of the inhibitor is interpreted differently. In Scheme 3, reaction 2 needs more space than Reaction 1. Reaction 1 is promoted because of the reduction of space on the surface of the catalyst. Reaction 2 is completely suppressed on a surface optimally inhibited, so that complete oxidation takes place only through reaction 3. This again leads to a selectivity maximum of 85.7 %.
Initially proposed molecular mechanism for ethylene oxide with silver
Ethylene oxidation is generally referred to as alkene oxidation or alkeneepoxidation. The alkeneepoxidation is a reaction in which both carbons of the two bonds become connected to the same oxygen atom by alkene forming cyclic ethers. Epoxides or oxiranes are the products of this reaction. The RCO3H reaction of epoxidation of alkene with peracids is one example of the peroxide production. The oxygen-oxygen bond of these peroxide derivatives is not only weak, but also polarized to negative acyl groups and positive hydroxyls. If we assume that the OH movement has electrophilic character, the equation can be written as shown in Scheme 4.
The mechanism of alkeneepoxidation
The scheme 4 shows the mechanism of alkeneepoxidation. There are four steps in the mechanism of alkene. Step 1, the electrons in the C=C attack the electrophilic, Oδ+ atom. This forms a C-O bond and leaves a positive charge on the other C atom of the alkene. Step 2, the O-O bond is broken and the electrons are moved into the new C = O group. Step 3, the electrons of the old group C=O form an O-H bond with the old group OH atom and the last step is in the old O-H bond the electrons form a C-O bond with the C atom of the alkene that is positively charged.
A dipolar intermediate is unlikely to form, as above shown. The epoxy reaction is supposed to occur in one step with a transition state that includes all the bonds shown in the equation. Peracidepoxidation therefore always has the stereoslectivity synthesized and seldom rearranges structural.
Insights into the Mechanism of Ethylene Oxide Production in Packed Bed Reactors. (2024, Feb 22). Retrieved from https://studymoose.com/document/insights-into-the-mechanism-of-ethylene-oxide-production-in-packed-bed-reactors
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