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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).
Characterisation of acido-basic properties
In order to understand the key parameters influencing the activity of catalysts, the acidobasic properties of the different solids used are checked. It should be noted that “acidity” or “basicity”, usually defined by the position of an equilibrium, are thermodynamic concepts. Nevertheless, in catalysis, thermodynamic equilibria are rarely achieved for each of the elementary steps, even the proton transfer steps. The ability to react rapidly with a base or acid site is therefore a fundamental parameter, that can be evaluated by studying the reactivity of solids in model reactions.
We employ 2-methylbut-3-yne-2-ol (MBOH) conversion test to characterize acidobasic properties of magnesium silicate catalysts (see Figure 1.10 in chapter 1). Magnesium silicates have a very interesting reactivity since products of acidic route (Mbyne) and those of basic route (acetone and acetylene) are detected together (Figure 2.4). However, the ratio of the products resulting from acidic and basic routes is not the same for all the silicates. For similar conversion, the coprecipitated silicate gives 78% selectivity on the basic route compared to 19% for the sol-gel. Moreover, the commercial magnesium silicate is slightly more active than the coprecipitated one but quite similar in terms of selectivity. Thus, among these three catalysts, the two most active in transesterification are those exhibiting the best basic selectivity in MBOH conversion, while, the sol-gel magnesium silicate, that gives 81% selectivity in Mbyne, is much less active in transesterification.
In conclusion, it should be first reminded that the results of the preliminary work given in chapter 1 show that there is no strict correlation between the catalytic activity in liquid phase and the acido-basic properties as the strongest base, MgO, is less active than magnesium silicate. Nevertheless, it can be noted that magnesium silicates exhibit both acid and base sites and that a first distinction of the less active catalyst (Mg silicate (sg)) is that it is more acidic than basic. We cannot conclude here on the mechanism of the reaction and on the required nature of the active sites but, obviously, Mg silicate (sg) exhibits different surface properties. Structural characterization, i.e. X-ray spectroscopies, scanning electron microscopy, 29Si and 25Mg NMR studies and DRIFT spectroscopy will be thus used in order to identify the differences among these materials.
29Si and 25Mg NMR study
To go further, {1H – 29Si} direct polarization (DP) experiments, in Figure 2.8 were performed. As reported in the literature[28,47–52], the 29Si MAS NMR spectra (Figure 2.8) can be Mg silicate (com) Mg silicate (com) Mg silicate (cp) Mg silicate (cp) Mg silicate (sg) Mg silicate (sg) decomposed in two ranges. The resonances from -112 ppm to -99 ppm and -98 to -70 ppm are attributed respectively to silicates without direct connection to magnesium (labelled SiOSi), and silicate with at least one oxygen connected to magnesium (labelled SiOMg). In addition to this first classification, the signals recorded by 29Si MAS NMR experiments can be classified into Qn species (where Q represents the Si atom which is bonded to four oxygen atoms and n is the number of Si neighbors)[53]. For the SiOSi, the two broad peaks centered to around -110 ppm and -101 ppm are assigned to completely condensed species Qáqmáq â (Si(OSi)4) and Qáqmáq ä silica species (Si(OSi)3(OH)), respectively[47,50,52,53]. For the SiOMg silicates, due to structural deformations, the decomposition is more difficult because each type of Qn may consist of different contributions[28,47,49–51,54]. According to the literature[28,47,50,51], it is possible to define three zones for the SiOMg phase: from -89 to -98 ppm for QáqmÅã ä species (i.e. Si(OSi)3(OMg)), from -82 to -88 ppm for QáqmÅã g species (i.e. Si(OSi)2(OMg)(OH)) and from -70 to -81 ppm for QáqmÅã F species (i.e. Si(OSi)(OMg)(OH)2). 29Si spectra of Mg silicates (com) and (cp) (Figure 2.8A-B), show clearly overlapping signals at around -94 ppm, which are ascribed to different QáqmÅã ä sites. Moreover, one peak was identified at -85 ppm, due to QáqmÅã g site[49,50,52,54] and two peaks were found at around -80 ppm which can be attributed to different QáqmÅã F species.[47] The Mg silicate (sg) sample (Figure 2.8C) appears to have the same contributions but the two peaks at -101 ppm (Qáqmáq ä ) and -110 ppm (Qáqmáq â ) account for larger proportion which is consistent with the XRD result that shows a broad peak of silica in Mg silicate (sg).
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 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].
Deactivation behaviour of catalyst
In order to get insights on the reaction mechanism on bifunctional catalyst magnesium silicates, we will focus on Mg silicate (com) to study the kinetics of the reaction.
The deactivation of the catalyst, linked to the disappearance of active sites or to their poisoning by chemical species which adsorb on the surface of the catalyst, usually happens. It is thus necessary to check whether deactivation accurs in the operating conditions. The deactivation behavior of catalyst is then studied. As shown in Figure 2.11, the conversion of AcOEt raises with the increase in the reaction time. It is interesting to note that, during the first 5 hours, the conversion increases rapidly and linearly with the reaction time showing that the reaction rate is constant so that no significant deactivation occurs.
Influence of the nature of the reactants
In last section, we propose that the transesterificaction of ethyl acetate with methanol on magnesium silicate follows the Langmuir-Hinshelwood mechanism, based on the kinetic study. It would be interesting to further investigate the effect of carbon chain length of ester and alcohol on the reactivity. For this purpose, we will study the transesterification of ester such as ethyl acetate (AcOEt), ethyl butyrate (BuOEt), ethyl hexanoate (HeOEt) and ethyl laurate (LaOEt) with methanol. As shown in Table 2.12, it can be found that, similar to transesterification of AcOEt with MeOH, the activities of these three magnesium silicate catlyatsts show the same order for transesterification of BuOEt and HeOEt: Mg silicate (sg) <Mg silicate (com) < Mg silicate (cp). Moreover, for all the three catalysts, the conversions of esters decrease as the order: AcOEt > BuOEt ≈ HeOEt > LaOEt. These phenomena may confirm that the esters need to be adsorbed on the surface and react with adsorbed methanol as, if the esters in liquid phase directly react with methanol like in an Eley-Rideal mechanism, the reaction rate of different esters would show no significant distinction at the same initial concentration.
Table of contents :
Acknowledgements
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
References
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
References
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
References
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
References
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)
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)
List of Figures
List of Tables
