Acrylic Acid Production by Gas-Phase Dehydration of Lactic Acid over K+-Exchanged ZSM-5: Reaction Variable Effects, Kinetics, and New Evidence for Cooperative Acid–Base Bifunctional Catalysis

Abstract

This study investigates the effects of reaction variables (including space velocity, temperature, and partial pressures of LA and H2O) and acidic and basic additives on the gas-phase dehydration of lactic acid (LA) for acrylic acid (AA) production, using an aluminum-rich K+-exchanged ZSM-5 zeolite (molar SiO2/Al2O3 = 27, K/Al = 1.0, K/Na = 8) identified among many high-silica zeolites as the most efficient catalyst in our prior work (Yan et al. ACS Catal. 2017, 7, 538−550). Under widely varied reaction conditions (340–380 °C, feed GHSV: 9,500–38,000 h–1, 1.8–11.7 kPa LA, 52.0–73.0 kPa H2O), the catalyst offers not only high selectivity for AA (>75 mol %) but also long-term catalytic stability. Rate equations and their associated kinetic parameters for LA consumption and AA production are obtained by measuring the kinetically relevant catalytic rates under high feed space velocity (GHSV ≥ 57,000 h–1). Acidic (CO2, acetic acid, and trifluoroacetic acid) and basic (NH3) additives are added, respectively, to the reaction feed to investigate their effects on performance of the working catalyst. While addition of either CO2 or acetic acid shows no effect on the catalyst performance, the addition of either NH3 or trifluoroacetic acid causes significant decline in both the catalyst activity and selectivity for AA production, due to poisoning of the surface acidic sites by NH3 or basic sites by trifluoroacetic acid. These data well highlight the excellent catalytic performance of the investigated catalyst and offer a new piece of evidence in support of the cooperative acid–base bifunctional catalysis for the LTA reaction.