Role of Water on the Activity of Magnesium Silicate for Transesterification Reactions

Abstract

This study aims to reveal how water adsorbed on to a solid can influence its catalytic properties for transesterification in liquid phase. A commercial magnesium silicate was subjected to a range of thermal pretreatments and used for the transesterification of ethyl acetate with methanol. Conversion of ethyl acetate decreased with increasing pretreatment temperature, in direct relation to the release of the water content of the magnesium silicate. Thermogravimetric analysis, diffuse reflectance infrared Fourier transform (DRIFT), and 1H NMR spectroscopies revealed that physically adsorbed water had little influence on the reactivity. The water incorporated within the catalysts, however, which desorbs at higher temperatures, played a key role on the conversion. Calorimetry, in situ DRIFT spectroscopy, and 1H NMR characterization indicate that two kinds of active site exist. These are created from the water coordinated to magnesium located on the edge of the clay‐like particles or in the defects present in the silicate layer, respectively. Their role could be to stabilize methanol deprotonated by basic Mg‐OH groups, activate the ester, or help the departure of the alkoxyl moiety.

Characterization of magnesium silicate hydrate (M-S-H)

“…Similar to the reflection profiles of M-S-H, the XRD patterns of these materials are broad indicating extremely small crystallite size, disorder or both. Lin et al [67] compared M-S-H-like phase to hectorite, swelling clay, and indexed the broads reflections as detailed in Figure 4b.…”

Characterisation and reactivity of oxygen species at the surface of metal oxides

“…Recently, the role of molecular water adsorption was for example evidenced on magnesium silicates for transesterification reaction [184] and demonstrated by DFT calculations in the case of aldol condensation on MgO surfaces [185], showing that hydroxyl groups induce a lowering in the activation barriers for the overall reaction. The authors conclude that this kind of reaction, even if considered to occur on basic surfaces, needs a strong Lewis acid site combined with a medium to week basic site.…”

Nanosized Layered TOT Magnesium Silicates: Equilibrium Morphologies and Surface Speciation, a Computational and Experimental Study

“…1 H shift predictions are compared in Figure to the already published 1 H NMR spectra of MSH com and com-X samples, where X is the temperature of dehydration pretreatment in Kelvin (2 h under 20 mL mn –1 Ar flow). The predicted transition from high coverage (dark and light blue lines for {130} and {100}, respectively) to low coverage (dark and light red lines for {130} and {100}, respectively) by water of TOT magnesium silicate edges as the pretreatment temperature increases is clearly observed experimentally, in good agreement with DFT predictions: disappearance of the strong contribution of molecularly chemisorbed water centered around δ 1 H = 5 ppm, exaltation of edge Si–OH* contributions around δ 1 H = 2 ppm, and shift of the peak assigned to structural Mg 3 –OH* from δ 1 H around 0.3 ppm to δ 1 H around 0.8 ppm.…”

“…The results for this material will be displayed as MSH “com” for commercial. Experimental XRD pattern, scanning electron microscopy (SEM), NMR, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) characterizations of this material have been already published by our group. , The specific surface area was calculated from the Brunauer–Emmett–Teller equation and is 421 m 2 g –1 . The bulk Mg/Si molar ratio measured by X-ray fluorescence for MSH com is 0.290, while it is 0.485 in surface as measured by X-ray photoelectron spectroscopy.…”

“…for commercial. Experimental XRD pattern, SEM, NMR and DRIFTS characterizations of this material have been already published by our group ( 6 ,21 ). The specific surface area was calculated from the BET equation and is 421 m 2 .g -1 .The bulk Mg/Si molar ratio measured by X-Ray Fluorescence (XRF) for MSH com is 0.290, while it is 0.485 in surface as measured by X-ray Photoelectron spectroscopy (XPS).FTIRFTIR spectra of adsorbed CO were collected on a Bruker Vertex 70 spectrometer using a MCT detector (resolution 2 cm -1 , 64 scans per spectrum).…”