Na2HPO4-modified NaY nanocrystallites: efficient catalyst for acrylic acid production through lactic acid dehydration

Abstract

By regulating the H₂O/SiO₂ molar ratio (n = 13.8, 16.9, 20.1, 26.3), NaY zeolites of different particle sizes (NaY-n, 50–400 nm) were synthesized, and NaY-n was modified with disodium hydrogen phosphate, Na₂HPO₄. The as-prepared catalysts are highly effective for the dehydration of sustainable lactic acid (LA) to produce acrylic acid (AA). Through the optimization of NaY particle size and Na₂HPO₄ loading, AA yields as high as 74.3% can be achieved under mild conditions (340 °C). In terms of employed feed rate of lactic acid and amount of catalyst, the AA formation rate is 12.0 mmol g_cat⁻¹ h⁻¹, the highest ever reported for LA dehydration to AA. Employing slightly low Si/Al ratios in NaY-n can modify catalyst acidity, while pre-applied Na⁺ ion-exchange and following Na₂HPO₄ loading can effectively reduce the number of acid sites, particularly the Brønsted ones on NaY-n. Equally important structural features of NaY-n are their shorter pore channels (or even incompletely developed zeolite texture) and relatively larger S_external/S_BET ratio, which favors quick product departure from the catalyst surface, and reduces the possibility of side reactions occurring inside the long channels. The appropriate surface acidity together with unique structural features of NaY-n ensure efficient LA conversion with high dehydration selectivity. Higher surface concentrations of Na⁺ on Na₂HPO₄/NaY-n also favor sodium lactate meanwhile suppressing poly-lactate as well as preventing carbon formation, corresponding to superior performance as well as improved durability of Na₂HPO₄/NaY-n.