Abstract
In heterogeneous catalysis processes, development of high-performance acid–base sites synergistic catalysis has drawn increasing attention. In this work, we prepared Mg/Zr/Al mixed metal oxides (denoted as Mg2ZrxAl1−x–MMO) derived from Mg–Zr–Al layered double hydroxides (LDHs) precursors. Their catalytic performance toward the synthesis of diethyl carbonate (DEC) from urea and ethanol was studied in detail, and the highest catalytic activity was obtained over the Mg2Zr0.53Al0.47MMO catalyst (DEC yield: 37.6%). By establishing correlation between the catalytic performance and Lewis acid–base sites measured by NH3-TPD and CO2-TPD, it is found that both weak acid site and medium strength base site contribute to the overall yield of DEC, which demonstrates an acid–base synergistic catalysis in this reaction. In addition, in situ Fourier transform infrared spectroscopy (in situ FTIR) measurements reveal that the Lewis base site activates ethanol to give ethoxide species; while Lewis acid site facilitates the activated adsorption of urea and the intermediate ethyl carbamate (EC). Therefore, this work provides an effective method for the preparation of tunable acid–base catalysts based on LDHs precursor approach, which can be potentially used in cooperative acid–base catalysis reaction.
Highlights
Dialkyl carbonates have attracted widespread interest during the last decades due to their extensive industrial applications.[1]
Layered double hydroxides (LDHs) materials have attracted an intensive attention as catalysts and catalyst supports in heterogeneous catalysis, owing to their tunability in chemical composition and phase transformation from metal hydroxides to mixed metal oxides (MMOs) upon calcination treatments.[18]
It is found that the synergistic catalysis between weak Lewis acid and medium strength Lewis base site is responsible for the diethyl carbonate (DEC) production: medium strength base site facilitates the activated adsorption of ethanol while weak acid site activates urea and intermediate ethyl carbamate (EC, the intermediate production from the reaction of ethanol and urea), which is revealed by in situ FTIR measurements
Summary
Layered double hydroxides (LDHs)[16,17] are a class of anionic clay materials with alternating cationic brucite-type layers and charge-balancing anions located in the interlayer region, which can be represented by the formula [M1Àx2+Mx3+(OH)2](AnÀ)x/n$mH2O. LDHs materials have attracted an intensive attention as catalysts and catalyst supports in heterogeneous catalysis, owing to their tunability in chemical composition and phase transformation from metal hydroxides to mixed metal oxides (MMOs) upon calcination treatments.[18] If the chemical composition and element ratio in LDHs host matrix are modulated, the resulting MMOs would possess versatility in concentration and intensity of Lewis acid–base sites. This inspires us to explore the synthesis of LDH precursors and precise control over their. It is found that the synergistic catalysis between weak Lewis acid and medium strength Lewis base site is responsible for the DEC production: medium strength base site facilitates the activated adsorption of ethanol while weak acid site activates urea and intermediate ethyl carbamate (EC, the intermediate production from the reaction of ethanol and urea), which is revealed by in situ FTIR measurements
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