Dehydration of biomass-derived methyl lactate over sodium-exchange zeolite Y (NaY) provides a sustainable route to acrylates. Improving dehydration selectivity is achieved by understanding the fundamental mechanisms of NaY zeolite catalyst. The reaction scheme in Figure 1a describes the two major pathways: dehydration to acrylates over sodium acid sites, and the major side reaction, decarbonylation to byproducts including acetaldehyde, over in situ generated Brønsted acid sites (BAS). Co-feeding an inert and site-specific titrant, including amines, affords desirable selectivity control when titrant adsorption is favored over side reactions on the BAS. Selectivity enhancement from 60 to 80% was demonstrated for methyl lactate dehydration over NaY using amine as the co-fed titrant (Figure 1b). The maximum in selectivity with varying size of amine indicates that the effectiveness of BAS titration should be evaluated by considering both the basicity and steric properties of the titrant. The presence of electron-donating alkyl groups enhances amine basicity but also introduces steric constraints to the molecule with respect to the pore dimensions of NaY. While higher basicity of the amines favors stronger adsorption on the BAS, steric limitations hinder BAS binding through contributions from internal diffusion limitations and local steric repulsion between the amine and the zeolite wall around the BAS. Internal diffusion limitations can be overcome if amine is allowed sufficient time to diffuse. However, local steric repulsion weakens amine binding on the BAS by reducing the stabilization effect, including electrostatic stabilization and van der Waals interactions, of the confined pore environment on the adsorbed amine. Titrants with proton affinity above 1040 kJ/mol and molecular size below 85% of the NaY pore diameter are predicted to enhance dehydration selectivity above 90% to acrylate products.

On the spatial design of co-fed amines for selective dehydration of methyl lactate to acrylates

ACS Spring Meeting 2021

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