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Synthesis of solvents from organic carbonates
Dimethyl carbonate is a solvent with dielectric properties, which is interesting in lithium batteries. Its synthesis can be carried out in the gas phase by transesterification of ethylene carbonate with methanol, as shown in Figure 1.4[25]. This reaction is carried out on basic surfaces, such as hydrotalcites or magnesium oxide.
It is also possible to obtain other solvents from dimethyl carbonate with particular dielectric properties by transesterification in the gas phase. This will make it possible, in mixtures of organic carbonates, to adjust the overall dielectric constant of the liquid. For example, by transesterifying dimethyl carbonate with 2-propanol[26] (Figure 1.5), with phenol[27] or with ethanol[28], asymmetric organic carbonates are obtained. For this reaction, the catalysts claimed are supported titanium oxide[27], supported potassium carbonate[29], or magnesium oxide[28].
Catalysts for transesterification
Transesterification reaction is known to be accelerated by acid, base, and enzyme catalysts. Homogeneous catalyst shows a higher activity than heterogeneous catalysts for transesterification reaction. For instance, alkali metal hydroxides and alkoxides, that exhibit high activity, are currently used industrially for biodiesel production. However, due to the presence of free fatty acids (FFAs) and water in crude oils, soap is produced under alkaline conditions, which not only consumes the catalyst, but also causes separation challenges[11]. Accordingly, the feedstock has to be purified that increases the cost of process. In addition, homogeneous catalysts drawback is that they are hardly separated from products. In order to overcome these disadvantages, solid acid and base catalysts for transesterification reaction are widely studied.
Catalysts preparation and activities tests
Magnesol® catasorb and polysorb were kindly provided by the Dallas group of America©. The first one contains Cl (<0.5%) and Na (<2%) impurities, the second one, sulfate (<1.5%) impurities but they have the same specific surface area. Both have the same reactivity therefore the influence of these impurities on the catalysis can be neglected. The results for Magnesol® catasorb will be displayed with the label “Mg silicate (com)”. Two other magnesium silicate samples were synthesized using sol-gel and coprecipitation methods. The molar ratio Mg/Si is chosen to be similar to the one of Mg silicate (com), i.e. 0.29.
Sol-gel magnesium silicate, labelled as “Mg silicate (sg)”, is obtained with a procedure adapted from Kalampounias et al [39]. It consists in an hydrolysis and polycondensation of tetraethyl orthosilicate (TEOS) (20 mL, Sigma-Aldrich, 99.999%) with magnesium nitrate (Mg(NO3)2.6H2O) (9.9 g, Sigma-Aldrich, 98%) and nitric acid (HNO3), (2N, 13.44 mL, VWR AnalaR Normapur). Nitric acid was used to catalyze the TEOS hydrolysis, using a molar ratio ÇÉmÑÇÖmÜ Vnmá = 8. After the addition of all reactants in a Teflon bottle, the solution was stirred for 1 h at room temperature to allow the hydrolysis and polycondensation reactions, until the formation of a viscous gel. The gel was aged, stored in the sealed Teflon bottles and kept at 60 oC for 3 days, then it was dried in three stages at 60 oC, 90 oC and 130 oC for 20 h, 24 h and 40 h, respectively and calcined by steps at 100 oC, 400 oC and 700 oC for 1 h, 2 h and 5 h, respectively. The temperature increase rate was 0.1 oC min-1 for all heating procedures. The Mg/Si mole ratio determined by X-ray fluorescence is 0.32.
Coprecipitated magnesium silicate, labelled as “Mg silicate (cp)”, was obtained through a method adapted from Ozgul et al.[40]. A solution of magnesium chloride (MgCl2·6H2O, 50 mL, 0.5 mol.L-1, Sigma-Aldrich, ≥ 99%) was added dropwise into 10 ml of sodium silicate solution (10% NaOH and 27% SiO2 wt.%, Sigma-Aldrich, reagent grade). A white precipitate was formed immediately. After 1 h, the precipitate was filtered, washed with 3 x 200 mL of distilled water and dried overnight at 60 oC. X-ray fluorescence of the sample indicate that the Cl concentration in the solid is lower than 0.1% and lower than 1% for Na. The Mg/Si mole ratio determined by X-ray fluorescence is 0.27.
Catalytic performance of magnesium silicates
First of all, the potential occurrence of homogeneous catalysis by dissolution of the material in the reaction mixture was tested by removing the solid catalyst by means of centrifugation. The separated liquid phase was again used under the same reaction conditions overnight without addition of catalyst. It is confirmed that the catalytic process is purely heterogeneous on the three magnesium silicates as no additional conversion was observed in these conditions.
Catalytic tests with these three silicates are performed and the results are given in Table 2.6. It can be observed that the two more efficient are coprecipitated and commercial Mg silicates while a much lower activity is observed for the sol-gel Mg silicate. Although silicate (cp) and (com) exhibit high specific surface area, they still show high activity if the conversions are divided by specific surface area (SSA).
Scanning electron microscopy
The images in Figure 2.7 confirm the distinction observed by the X-ray techniques. Indeed, Mg silicate (sg) surface is smooth whereas the ones for the (cp) and the (com) are very corrugated. It is in line with a very homogeneous composition of the sol-gel particle and the presence of poorly crystalline phases over the surface of Mg silicate (com) and (cp). Chiang and co-workers[46] as well as Tonelli and co-workers[47] observe the same kind of structure. According to Chiang et al., the MSH forms globules of a few nanometers (the mean value of their size being 3 nm) which corresponds to what is shown for Mg silicate (com) and (cp).
Diffuse reflectance infrared Fourier transform spectroscopy characterization
Figure 2.10 shows the Diffuse reflectance infrared Fourier transform (DRIFT) spectra of the silicates, both taken at 140 °C, before and after a 350 °C pretreatment, respectively. After pretreatment, the contribution of adsorbed molecular water (1630 cm-1)[50] disappeared but a broad absorbance in the zone associated with O-H elongations (3000-3800 cm-1) remains for all the samples, similar to the contribution usually found on pure silica. However, a sharp band emerges at 3737 cm-1 for all the samples and a second one at 3672 cm-1 for the Mg silicate (com) and, in a lower extend, for the Mg silicate (cp). Those two bands can also be seen on the samples pretreated only at 140 °C but were more difficult to recognize because of the presence (C) of residual molecular water adsorbed in these conditions. The band located at 3737 cm-1 is in the range observed for silanols over silica[47] and should be attributed to OH stretching of silanols species that can be in close proximity to magnesium cations. The contribution at 3672 cm-1 was previously assigned to OH stretching in the talc structure[58], or to similar structures[50] and is therefore compatible with the presence of MSH structure. A band at 1720 cm-1 appears then, that is attributed to Si-OH vibrations[47].
Table of contents :
Table of Contents
General introduction
Chapter 1. Transesterification reactions
1.1. Application of transesterification
1.1.1. Application of transesterification in liquid phase
1.1.2. Application of transesterification in gas phase
1.2. Catalysts for transesterification
1.2.1. Solid acid catalysts
1.2.2. Solid base catalysts
1.3. Preliminary work
1.4. Magnesium silicates
1.5. Aims and objectives
Chapter 2. Role of magnesium silicate hydrate formation
2.1. Introduction
2.2. Choice of reaction conditions
2.2.1. Data from literature
2.2.2. Thermodynamic study
2.2.3. Test with the chosen conditions
2.2.4. Experimental setup
2.3. Catalysts preparation and activities tests
2.3.1. Catalysts preparation
2.3.2. Catalytic performance of magnesium silicates
2.4. Characterisation of acido-basic properties
2.5. Structural characterization of magnesium silicates samples
2.5.1. X-ray spectroscopies
2.5.2. Scanning electron microscopy
2.5.3. 29Si and 25Mg NMR study
2.5.4. Diffuse reflectance infrared Fourier transform spectroscopy characterization
2.5.5. Conclusion on the characterization of the magnesium silicates samples
2.6. Discussion: a bifunctional catalyst
2.7. Kinetic study
2.7.1. Deactivation behaviour of catalyst
2.7.2. Calculation of the order of reaction in ethyl acetate
2.7.3. Interpretation of reaction order in ethyl acetate
2.8. Influence of the nature of the reactants
2.9. Conclusion
Chapter 3. Role of water on reactivity of commercial magnesium silicate
3.1. Introduction
3.2. Role of thermal pretreatment
3.2.1. Experimental procedure
3.2.2. Catalytic performances and TGA analysis
3.3. Characterizations of the pretreated magnesium silicate
3.3.1. DRIFTS analysis
3.3.2. The transmission FTIR spectra of adsorbed CO
3.3.3. 1H NMR analysis
3.3.4. Calorimetry analysis
3.4. Discussion of the structural characterizations and role of water
3.5. Conclusions
Chapter 4. Phyllosilicates
4.1. Introduction
4.1.1. Classification of phyllosilicates
4.1.2. Structure of phyllosilicates
4.2. Preparation of catalysts
4.3. Characterisation of the phyllosilicates
4.3.1. X-ray spectroscopy analysis
4.3.2. Nitrogen sorption analysis
4.4. Catalytic test of phyllosilicates
4.5. Kinetic study
4.5.1. Deactivation behaviour of catalyst
4.5.2. Calculation of the order of reaction in ethyl acetate
4.6. Role of thermal treatment
4.6.1. Catalytic performance and TGA measurement
4.6.2. Characterisation of laponite after pretreatment at different temperatures
4.7. Conclusion
General Conclusion and Outlook
Appendices
Appendix I. Transesterification reaction
Appendix II. Acido-basic properties study: MBOH conversion
Appendix III. X-ray Diffraction (XRD)
Appendix IV. X-ray spectroscopies
i. X-ray Fluorescence spectroscopy (XRF)
ii. X-ray Photoelectron Spectroscopy (XPS)
Appendix V. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) ..130
i. Infrared and diffuse reflection
ii. Experimental implementation
Appendix VI. Fourier Transform Infrared spectroscopy (FTIR) measurement of adsorbed CO
Appendix VII. Measurement of specific surfaces
i. Principle
ii. Procedure
Appendix VIII. Thermogravimetric analysis
Appendix IX. Calorimetry
Appendix X. Nuclear Magnetic Resonance (NMR) study
Appendix XI. Scanning Electron Microscopy (SEM)
