Selectivity Control in the Catalytic Dehydration of Methyl Lactate: The Effect of Pyridine

Catalytic dehydration of biomass -derived methyl lactate to produce acrylic acid and its esters promises a renewable route to produce a major commodity chemical. Alkali metal cation exchanged zeolites are capable of catalyzing this reaction; however, selectivity control toward the dehydration pathway remains a challenge. Through combined kinetic and transmission spectroscopic investigations, we demonstrate that introducing pyridine, a base, to the reaction feed increases selectivity to acrylates while inhibiting the formation of side products (i.e., acetaldehyde and coking). The ratio of the turnover frequencies for the desired dehydration and undesired decarbonylation pathways (TOFDH/TOFAD) increases by a factor of similar to 20 when pyridine is included in the feed (pyridine/methyl lactate 1/10), in comparison to the case where pyridine is absent. Transmission FT1R investigations show that the reduced decarbonylation activity can be directly related to pyridine quenching Brensted acid sites generated during the reaction, which are identified as the active sites for the decarbonylation pathway. Exposing NaY to pyridine prior to the reaction does not affect the product distribution, because Brensted acid sites are produced by ion exchange between NaY and reactants. In contrast, exposing NaY to a pyridine -containing feed for I h increases TOFDH/TOFAD after switching to a pyridine -free feed by a factor of similar to 4, in comparison to the case where the catalyst is never exposed to pyridine.