Catalytic dehydration of methyl lactate: Reaction mechanism and selectivity control

Abstract

Catalytic dehydration of lactic acid and its esters is a promising approach to renewably produce acrylic acid and its esters. Molecular level understanding of the dehydration reaction mechanism on NaY has been achieved via a combination of reactivity and in-situ transmission Fourier Transform infrared (FTIR) spectroscopic investigations. Brønsted acid sites generated insitu with the assistance of water have been identified as the primary active sites for the dehydration pathway. The key branching point between the desired dehydration and undesired decarbonylation pathways is the dissociation of methyl lactate on NaY to form adsorbed sodium lactate and methyl groups. Brønsted and Lewis acid sites mainly catalyze the dehydration of adsorbed sodium lactate, whereas the decarbonylation pathway to acetaldehyde dominates when methyl lactate is not dissociated. Similar mechanistic steps are likely followed in the catalytic dehydration of lactic acid to acrylic acid. The mechanistic understanding gained will enable rational design of catalysts for selective dehydration of methyl lactate.

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

“…The good match of the rates of ML and IPA dehydration on Lewis acid sites suggests that ML dehydration occurs primarily on Lewis rather than Brønsted acid sites. In addition, a band at 1575 cm -1 , attributable to sodium acrylate, 41 is observed when ML is introduced to NaY in the absence of water ( Figure 3), which confirms that the dehydrated intermediate forms in the presence of only Lewis acid sites (no Brønsted acid is present in the absence of water). Another potential reason for the observed decoupling of the Brønsted acid sites density and dehydration rate could be that the acid-catalyzed dehydration of sodium lactate is not rate-limiting in ML dehydration.…”

“…We propose that the Brønsted acid sites are produced via the hydrolysis of the surface O-CH 3 bond formed by the dissociation of ML on NaY (step 2, Scheme 1). 41 Previous studies from our laboratory show that introducing water directly to NaY does not produce Brønsted acid sites in any appreciable quantities, as no peaks related to Brønsted acidity were observed even at elevated temperatures. 48 Therefore, the dissociative adsorption of ML 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 20 200 ºC confirms the presence of in-situ generated Brønsted acid sites via the weak but distinct 1541 cm -1 band corresponding to pyridinium (Figure 4b(iv-v)).…”

“…[13][14][15]41 We have also demonstrated that Brønsted acid sites can be produced when ML dissociatively adsorbs on NaY in the presence of water (step 1 in Scheme 1). 41 Given that water is a byproduct of the desired reaction, it is reasonable that the in situ generated Brønsted acid sites will be present regardless of whether water is used as a solvent. Further, solvent, e.g., alcohols, could have a significant effect on the reactivity and product distribution, however, it is outside the scope of this work.…”

“…[20][21][22][23][24][25][26][27][28]41 Moreover, Na + in NaY has been shown 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 25 to be active in the dehyration of IPA, which is a useful surrogate molecule for ML because both have a secondary hydroxyl group and IPA cannot generate Brønsted acid sites on NaY via ionexchange. 41 To further explore the hypothesis that Lewis acid sites are the active sites for ML dehydration, IPA dehydration is used as a model reaction. TOF of IPA (TOF IPA ) dehydration over NaY at 330 °C is on the order of ~10 h -1 (on a per Al basis), which is comparable to the value obtained for ML dehydration (5-12 h -1 ).…”

Catalytic Gas‐Phase Cyclization of Glycolate Esters: A Novel Route Toward Glycolide‐Based Bioplastics

“…Surprisingly, no degradation of GD occurred at higher reaction temperatures. This is in sharp contrast to the conversion of MLA, where the decarbonylation of LD or MLA itself with formation of acetaldehyde was observed at reaction temperatures above 220 °C . Because additional methyl groups are lacking on the 6‐membered ring of GD, or on the α‐carbon of MGA, no decarbonylation to acetaldehyde can take place.…”