The Effect of Cofed Species on the Kinetics of Catalytic Methyl Lactate Dehydration on NaY

Abstract

The catalytic dehydration of biomass-derived methyl lactate to acrylic acid is a renewable route to a high-value, high-volume monomer, but achieving high selectivity and production rates remains challenging. One roadblock is that the role of the species cofed with methyl lactate, typically water, in the dehydration mechanism over alkali-metal-exchanged zeolites remains unclear. In this study, we determined the effects of protic and aprotic cofed species, including water, methanol, and methyl acetate, on the rate and selectivity of methyl lactate dehydration over NaY via kinetic and spectroscopic investigations. We show that water plays multiple competing roles in the mechanism: (1) generating the unselective Brønsted acid sites on NaY leading to acetaldehyde and coke formation; (2) suppressing the activity of the Lewis acid sites responsible for lactate dehydration; and (3) facilitating the displacement of adsorbed sodium acrylate by lactate, leading to the formation of acrylic acid. The first process increases the rate of side product formation, while the second and third processes decrease and increase the dehydration rate, respectively. As a result of these competing roles, the rate-limiting step of methyl lactate dehydration shifts from acrylate product recovery to sodium lactate dehydration as the concentration of water increases.