The choice of reaction conditions, such as temperature, pressure, solvent, and catalyst, can significantly impact the yield and selectivity of diethyl ether synthesis. For example, in the dehydration of ethanol over HZSM-5 zeolite, a higher Si/Al ratio and lower concentration of ammonium sulfate modifier were found to give the best performance[1]. Optimal conditions were determined to be a 1:4 molar ratio of ethylene glycol to ethanol, 7 MPa pressure, and 180°C for 4 h, resulting in 69.17% glycol conversion and 67.81% yield of glycol ethers[1].
Mechanistic studies have shown that the dehydration of ethanol to diethyl ether and ethylene on γ-Al2O3 proceeds through a common surface intermediate, which can be inhibited by the formation of ethanol-water dimers at lower temperatures (<500 K). At higher temperatures (>600 K), ethanol dehydrogenation to acetaldehyde and ethane also becomes significant, proceeding via an indirect hydrogen transfer mechanism.
Diethyl Ether Synthesis Mechanism:
The synthesis of diethyl ether from ethanol typically proceeds through an acid-catalyzed dehydration mechanism. The detailed mechanism is as follows:
1. Protonation of Ethanol:
– The first step involves the protonation of the ethanol molecule by the acidic catalyst (e.g., Brønsted acid like H+ from water or a solid acid catalyst like HZSM-5 zeolite). [2]
– This forms a resonance-stabilized ethanol-proton complex
[CH3CH2OH2]+.
2. Elimination of Water (Dehydration):
– The protonated ethanol complex undergoes intramolecular rearrangement, leading to the elimination of a water molecule.[3]
– This forms a carbocation intermediate, [CH3CH2]+, which is stabilized by resonance.
3. Nucleophilic Attack by Ethanol:
– The carbocation intermediate is susceptible to nucleophilic attack by another ethanol molecule.
– The oxygen atom of the ethanol molecule attacks the carbocation, forming a tetrahedral intermediate.
4. Proton Transfer and Elimination:
– The tetrahedral intermediate undergoes a proton transfer, with the proton moving from the oxygen atom to the hydroxyl group.
– This results in the elimination of a water molecule, forming the diethyl ether product.
The overall reaction can be summarized as follows:
2 CH3CH2OH ⇌ CH3CH2OCH2CH3 + H2O
The reaction conditions, such as temperature, pressure, and the nature of the catalyst, can significantly influence the yield and selectivity of the diethyl ether synthesis. Higher temperatures (>600 K) can also promote the dehydrogenation of ethanol to acetaldehyde and ethane via an indirect hydrogen transfer mechanism.
The choice of catalyst, such as the HZSM-5 zeolite with a higher Si/Al ratio and lower concentration of ammonium sulfate modifier, can enhance the performance and selectivity towards diethyl ether production.
References
1. Gang-se, L., Synthesis of Glycol Monoethyl Ether and Glycol Diethyl Ether Using Ethylene Glycol and Ethanol with Heterogeneous Catalysts. Journal of Chemical Engineering of Chinese Universities, 2016. and Synthesis of Glycol Monoethyl Ether and Glycol Diethyl Ether Using Ethylene Glycol and Ethanol with Heterogeneous Catalysts. 2016.
2. Xin, H., Li, X., Fang, Y., Yi, X., Hu, W., Chu, Y., … & Li, X. (2014). Catalytic dehydration of ethanol over post-treated ZSM-5 zeolites. Journal of catalysis, 312, 204-215., Catalytic dehydration of ethanol over post-treated ZSM-5 zeolites. (2014).
3. Kingston, D.G., Hobrock, B. W., Bursey, M. M., & Bursey, J. T. (1975). Intramolecular hydrogen transfer in mass spectra. III. Rearrangements involving the loss of small neutral molecules. Chemical Reviews, 75(6), 693-730., Intramolecular hydrogen transfer in mass spectra. III. Rearrangements involving the loss of small neutral molecules. (1975).