Ethanol Dehydration and Dehydrogenation on γ-Al₂O₃: Mechanism of Acetaldehyde Formation

Abstract

Steady state kinetics and measured pyridine inhibition of ethanol dehydration and dehydrogenation rates on γ-alumina above 623 K show that ethanol dehydrogenation can be described with an indirect hydrogen transfer mechanism to form acetaldehyde and ethane and that this mechanism proceeds through a shared surface intermediate with ethylene synthesis from ethanol dehydration. Ethane is produced at a rate within experimental error of acetaldehyde production, demonstrating that ethane is a coproduct of acetaldehyde synthesis from ethanol dehydrogenation. Steady state kinetic measurements indicate that acetaldehyde synthesis rates above 623 K are independent of co-fed water partial pressure up to 1.7 kPa and possess an ethanol partial pressure dependence between 0 and 1 (P ethanol = 1.0− 16.2 kPa), consistent with ethanol dehydrogenation rates being inhibited only by ethanol monomer surface species. The surface density of catalytically active sites for ethylene and diethyl ether production were estimated from in situ pyridine titration experiments to be ∼0.2 and ∼1.8 sites nm −2 , respectively, at 623 K. Primary kinetic isotope effects for ethylene and acetaldehyde are measured only when the C−H bonds of ethanol are deuterated, verifying that C−H bond cleavage is kinetically limiting for both products. The proposed indirect hydrogen transfer model for acetaldehyde synthesis is consistent with experimentally observed reaction rate dependences and kinetic isotope effects and highlights the complementary role of hydrogen adatom removal pathways in the formation of aldehydes on Lewis acidic systems.

Kinetic Modelling at the Basis of Process Simulation for Heterogeneous Catalytic Process Design

“…It may be the desired reaction for the production of this building block from a renewable source, or it may represent a reaction path parallel to the oxidative one, leading to acetaldehyde and then to carbon-carbon scission and conversion into hydrogen/syngas [62]. In this sense, it is an undesired path because it lowers the H2 yield, and it is connected with catalyst deactivation by coking.…”

“…This contribution, added to the theoretical calculations, seems then to indicate a different behavior of alumina and zeolites. Afterwards DeWilde et al [62] re-addressed from an experimental point of view the original discrimination between ethanol dehydration and dehydrogenation, applying also techniques of isotopic labeling and co-feeding pyridine alongside ethanol and water. The main finding of this work, besides the proposed re-hydrogenation of ethylene to ethane, can be summarised as follows: i) at T>600 K water is effectively desorbed from the catalyst, making the conversion rates again insensitive to its partial pressure, ii) unimolecular reaction paths are kinetically determined by C-H breaks, while the DEE production is regulated by -O bonds cleavage; iii) pyridine decreases the yield of both ethylene and acetaldehyde, confirming that the required -H abstraction step takes place on acidic sites.…”

“…Nevertheless, the ethylene production rate constant was given a kinetic pre-factor about 80 times larger than that of acetaldehyde [62], and this poses a substantial quantitative difference between these catalysts and the actually basic ones (e.g. Lanthana or Ceria -see for example the data on ethylene/acetaldehyde selectivity reported by [34].…”

Catalytic Performance and Reaction Pathways of Cu/SiO₂ and ZnO/SiO₂ for Dehydrogenation of Ethanol to Acetaldehyde

“…Many papers have estimated that once ethoxy species formed and followed by dehydrogenation. Density funcitonal theory (DFT) simulation of ethanol dehydrogenation has also been reported for Cu 14) , Pd 18) and Pt 19) catalysts. DFT study on ethanol adsorption and dehydrogenation on Cu/Cr2O3 indicated that ethanol adsorbed dissociatively on Cr 3 as the ethoxy species, and the interface of Cu and Cr2O3 was important for adsorption and dehydrogenation 14) .…”

“…DFT study on ethanol adsorption and dehydrogenation on Cu/Cr2O3 indicated that ethanol adsorbed dissociatively on Cr 3 as the ethoxy species, and the interface of Cu and Cr2O3 was important for adsorption and dehydrogenation 14) . DFT simulation on Pt concluded that the rate determining step was dissociation of C _ H bond of adsorbed ethoxy species 19) . Oxide catalysts were also occasionally studied for the dehydrogenation of ethanol 20), 21) , but metal catalysts are generally recognized as more active for hydrogenation and dehydrogenation reactions.…”