Highly Active Silver ion-Exchanged Silicotungstic Acid Catalysts for Selective Esterification of Glycerol with Lauric Ac
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Highly Active Silver ion‑Exchanged Silicotungstic Acid Catalysts for Selective Esterification of Glycerol with Lauric Acid Chao Xu1 · Jiacheng Gan1 · Xingyu Mei1 · Yousheng Zhou1 · Jiahui Duanmu1 · Guangqi Zhu1 · Haijiang Zhang2 · Xiaoxiang Han1 · Yanbo Wang1 · Shang‑Bin Liu3 Received: 9 January 2020 / Accepted: 16 May 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract A series of highly active silver ion-exchanged silicotungstic acid ( H4SiW12O40; STA) catalysts, A gxH4−xSiW12O40 (x = 1–4), were successfully synthesized and applied for the production of glycerol monolaurate (GML) via esterification of lauric acid (LA) with glycerol (GL). The physicochemical properties of these Ag-promoted STA were characterized by TGA, FE-SEM, XRD, and FT-IR, and their acidic properties were studied by a solid-state 31P NMR probe molecule method using trimethylphosphine oxide (TMPO) as the probe. Among various catalysts, the Ag2H2SiW12O40 catalyst showed extraordinary activity owing to the strong Brønsted acidity, Brønsted–Lewis acid synergy, as well as the unique Keggin structure possessed by the catalyst. A Box–Behnken design (BBD) based on response surface methodology (RSM) was exploited for the optimization of process variables. A maximum GML yield of 80.2% was achieved for the Ag2H2SiW12O40 catalyst under a GL/LA molar ratio of 4.4, catalyst amount of 4.6 wt%, reaction temperature of 429 K, and reaction time of 153 min, which was in conformity with those predicted by the mathematical model. Further recyclability and kinetic studies confirmed that the A g2H2SiW12O40 catalyst was reusable, robust, and highly efficient for the synthesis of GML with an activation energy (Ea) of 38.14 kJ/mol.
* Xiaoxiang Han [email protected] * Yanbo Wang [email protected] 1
Department of Applied Chemistry, Zhejiang Gongshang University, Hangzhou 310018, China
2
Jiangsu Key Laboratory of Regional Resource Exploitation and Medicinal Research, Huaiyin Institute of Technology, Huaian 223003, China
3
Institute of Atom and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan
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Graphic Abstract Due to the strong acidity and synergy of B/L acid sites, Ag ion-exchanged silicotungstic acid catalysts show superior activity and durability for the synthesis of glycerol monolaurate.
Keywords Esterification · Metal-promoted silicotungstic acid · Glycerol utilization · Glycerol monolaurate · Response surface methodology Abbreviations GML Glycerol monolaurate LA Lauric acid GL Glycerol TMPO Trimethylphosphine oxide BBD Box–Behnken design RSM Response surface methodology HPA Heteropolyacid POMs Polyoxometalates PTA Phosphotungstic acid STA Silicotungstic acid FT-IR Fourier-transform infrared spectra XRD X-ray diffraction TG-DTG Thermogravimetric and differential thermogravimetric FE-SEM Field-emission scanning electron microscopy ICP-OES Inductively coupled plasma optical emission spectrometry CID Charge injection device SSNMR Solid-state nuclear mag
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