Selective ketonization of acetic acid over HZSM-5: The importance of acyl species and the influence of water

Abstract

The ketonization of acetic acid is investigated over HZSM-5 via a combination of temperature programmed techniques (TP), IR, and reaction kinetics. TP experiments demonstrate the generation of stable acyl intermediates that require higher temperatures to facilitate CC coupling with a second acetic acid molecule. Near exclusive selectivities for the ketonization reaction devoid of significant sequential aldol condensation are observed at temperatures ranging from 270-330°C. The ratedetermining step is proposed to involve the interaction of an acyl group with a second acid, with an apparent reaction order of 1.6. This order can be explained by a second order rate-determining step that is reduced as a result of acid-derived surface species. Isotope labeling experiments reveal that the rate-determining step is the formation of the CC bond rather than the activation of the second acid. The rate of reaction per gram scales with the concentration of Brønsted sites present, with the exchange of Brønsted sites with Na resulting in a corresponding drop in activity. The ketonization reaction exhibits a-0.46 order with respect to water, but this loss in activity is accompanied by a significant increase in catalyst stability.

Kinetics of Valeric Acid Ketonization and Ketenization in Catalytic Pyrolysis on Nanosized SiO2, γ‐Al2O3, CeO2/SiO2, Al2O3/SiO2 and TiO2/SiO2

“…Whereas the majority of current experimental results support the β‐keto acid mechanism, we think that it is still too early to completely disregard the concerted mechanism in favor of the β‐keto acid route because we believe the current results still do not conclusively confirm it either. In particular, a recently published isotope labeling study failed to confirm the β‐keto acid mechanism . Specifically, the article reported by Gumidyala et al.…”

“…Specifically, the article reported by Gumidyala et al. dealt with the ketonization of acetic acid over HZSM‐5 . Presented isotope labeling experiments revealed that the rate‐determining step in the ketonization reaction is the formation of the C−C bond rather than the activation of the second acid.…”

Enhancing the Acylation Activity of Acetic Acid by Formation of an Intermediate Aromatic Ester

“…Different acylating agents, such as carboxylic acids, acyl halides, or acyl anhydrides, have been investigated with acid catalysts, such as AlCl 3 , HF, zeolites, or heteropolyacids . It has been shown recently that over H‐ZSM5 at 220 °C the vapor‐phase acylation proceeds by dehydration of acetic acid to form an acyl intermediate, in which acetic acid may well act as an acylating agent . These intermediates can desorb as ketenes at elevated temperatures, which results in catalyst deactivation.…”

“…The other physical characteristics of these catalysts have been studied by using XRD, SEM, and diffuse reflectance Fourier infrared transform spectroscopy (DRIFTS). These characteristics, along with detailed experimental procedures, have been reported elsewhere …”

Mechanism study of the conversion of esters to high‐octane‐number aromatics over HZSM‐5

“…They concluded that at low temperature acetate ions tightly attached to the hydroxyl group of the zeolite, and above 297°C ions began to decompose to ethane and carbon oxide molecules . Gumidyala and co‐workers published details about ketonization of AA to acetone by the formation of acyl species over HZSM‐5 zeolite . Gayubo and co‐workers observed the poor conversion of AA over HZSM‐5 above 340°C.…”