Catalyst Deactivation in Pyridine-Assisted Selective Dehydration of Methyl Lactate on NaY

Abstract

Acrylic acid is a major commodity chemical currently produced almost entirely from petroleum-derived sources. The catalytic dehydration of methyl lactate is a promising renewable route to producing this vital chemical feedstock; however, enhancing the selectivity toward the dehydration pathway and catalyst stability remain challenging. We demonstrate a selectivity for dehydration products of ∼90% over a period of 30 h on NaY by introducing a small amount of pyridine in the reactant feed (pyridine/methyl lactate = 1:10). This increase in selectivity is attributed to the inhibition of side pathways, i.e., decarbonylation and coking, both of which are catalyzed by surface Brønsted acid sites generated in situ, rather than the acceleration of the dehydration pathway. Catalyst deactivation is shown to proceed through a drastically different mechanism in the absence and presence of pyridine in the feed via a combination of activity tests, thermogravimetric analysis, N 2 , and transmission FTIR spectroscopic investigations. Coke formation is the primary cause of catalyst deactivation in the pyridine-free feed, whereas when pyridine is used the accumulation of intact bulky and high boiling point acid/base complexes, e.g., pyridinium acrylate and pyridinium lactate, in the zeolite pores limits the access of reactant to the catalytic sites. Regeneration at 330−450 °C in inert atmosphere does not have any effect on the deactivated catalyst in the pyridine-free feed but partially restores the catalytic activity of the spent catalyst in pyridine-spiked feed. Porous catalysts with more open structures that facilitate the desorption of the bulky acid/base complexes are expected to be more resistant to deactivation.

Bio‐Acrylates Production: Recent Catalytic Advances and Perspectives of the Use of Lactic Acid and Their Derivates

“…Murphy et al. suggested that the formation of acetaldehyde over NaY zeolite is taking place over Brønsted acid sites However, recently Sad et al. reported a study showing that protonic forms of zeolites, viz .…”

“…To reveal a possible reaction mechanism of the dehydration of ML on zeolites, Murphy et al. performed an in‐situ FTIR study of ML conversion over NaY (Scheme ) . They suggested that similar to lactic acid, the first step of the dehydration reaction is a dissociative adsorption of ML on zeolite surface with the intermediate formation of a surface sodium lactate species.…”

“…Pyridine adsorption studies were performed to clarify this point. The addition of pyridine to the reaction mixture allowed for a substantial decrease of ML decarbonylation, while an increase of both selectivity and stability of the catalyst due to selective inhibition of in‐situ generated Brønsted acid sites was suggested ,. The concept of the dynamic equilibrium between various cationic (sodium ions and pyridinium) and anionic species (zeolite surface, lactic and acrylic anions) during the reaction was proposed by authors, suggesting the absence of well‐defined ion pairs .…”

“…The addition of pyridine to the reaction mixture allowed for a substantial decrease of ML decarbonylation, while an increase of both selectivity and stability of the catalyst due to selective inhibition of in‐situ generated Brønsted acid sites was suggested ,. The concept of the dynamic equilibrium between various cationic (sodium ions and pyridinium) and anionic species (zeolite surface, lactic and acrylic anions) during the reaction was proposed by authors, suggesting the absence of well‐defined ion pairs . As a consequence of the absence of defined ion pairs, it could be assumed that properties of these in situ generated Brønsted acid sites are different from properties of Brønsted acid sites present initially on the surface of the catalyst before reaction.…”

A FTIR Study of the Acidity of in situ Generated Brønsted Sites on NaY via Displacement Reactions

“…However, the wavenumber of the O−H stretching band of BAS on faujasites is not a reliable descriptor for their acidity . The dissociative adsorption generates a corresponding sodium salt species, which is identified via a unique ν([O−C−O] − ) mode typically located in the range of 1550–1650 cm −1 in FTIR spectroscopy . In a previous study, we demonstrated that molecules with gas‐phase deprotonation enthalpy (GPDE) below ∼1625 kJ/mol generated clear vibrational bands for BAS and corresponding sodium salts when adsorbed on NaY, while molecules with GPDE above this threshold did not.…”