On the Spatial Design of Co-Fed Amines for Selective Dehydration of Methyl Lactate to Acrylates

Abstract

Co-feeding an inert and site-selective chemical titrant provides desirable selectivity tuning when titrant adsorption is favored over side reaction pathways on a solid acid catalyst. Here, a selectivity enhancement from 61 to 84 C % was demonstrated for methyl lactate dehydration to methyl acrylate and acrylic acid over a NaY zeolite catalyst using amines as the co-fed titrants to suppress side reactions on in situ-generated Brønsted acid sites (BASs). The effectiveness of BAS titration was evaluated by considering both the basicity and steric properties of the titrant molecule with the goal to maximize the selectivity enhancement. The presence of electron-donating alkyl functional groups not only enhances amine basicity but also introduces additional steric constraints to the molecule with respect to the pore dimensions of the NaY zeolite. While higher basicity of titrant amines favors stronger adsorption on BASs, steric limitations hinder site binding through contributions from internal diffusion limitations and local steric repulsion between the titrant and the zeolite wall around the BAS. Titrant bases with proton affinities above ∼1040 kJ/mol and sizes below 85% of the NaY supercage window or pore diameter are predicted to afford dehydration selectivities above 90 C % to acrylate products.

Multifunctional Amine Modifiers for Selective Dehydration of Methyl Lactate to Acrylates

“…Unconstrained hydroxyl groups, indicating the presence of Brønsted acid sites (BAS) in the supercages of Na-FAU, are indicated by the band at 3633 cm –1 . ,, The shift from 3650 to 3633 cm –1 could be due to weak interactions between the hydroxyl groups and water molecules . Constrained hydroxyl groups, indicating the BAS in the sodalite cages, are assigned to the band at 3540 cm –1 . ,, The presence of BAS in Na-FAU is surprising since no methanol dehydration activity was observed in Na-FAU at 300 °C in our previous work, though a minimal amount of Brønsted acidity (<29 μmol (g cat ) −1 ) was measured on the Na-FAU materials …”

“…Example zeolite catalysts include alkali metal-exchanged FAU ,,,, and BEA , zeolites. One recent study demonstrated a selectivity as high as 80% over a potassium-modified ZSM-5 zeolite . In our previous work, methyl lactate was converted to methyl acrylate and acrylic acid at a dehydration selectivity of 61% and a conversion of 92% at 300 °C over a sodium-exchanged FAU zeolite (Na-FAU, Si:Al = 2.6) . Two major reaction pathways were observed over Na-FAU: (i) dehydration to methyl acrylate and acrylic acid; (ii) decarbonylation to carbon monoxide, methanol, and acetaldehyde (Figure ).…”

“…Cofeeding pyridine as a BAS titrant can suppress decarbonylation and improve dehydration selectivity by 13%. , Our previous work further investigated the effect of titrant basicity and steric hindrance on promoting methyl lactate dehydration beyond the strength and structure of pyridine . High basicity favors stronger BAS titration, while steric constraints limit binding on BAS due to internal diffusion limitations and local steric interactions.…”

“…Based on the predicted criteria, 1,1,3,3-tetramethylguanidine (TMG) was demonstrated by experiment to improve dehydration selectivity to 89% from the baseline performance of 61% over Na-FAU. However, conversion was unstable after 5 h of time on stream, potentially due to the low stability of TMG at 300 °C …”

“…The structure of Na-FAU consists of small, six-membered ring (6-MR) sodalite cages ( sod , diameter of 6.3 Å) connected by double six rings ( d6r ) to form large accessible supercages (diameter of 11.2 Å) having 12-MR openings of 7.4 Å (Figure ). , The Na-FAU catalyst used in our previous work and this work has a Si/Al ratio of 2.6. This high aluminum content correlates to a high density of acid sites in the materials.…”