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Influence of the reaction atmosphere
Generally, depending on the catalyst used, the DCX/DCN reactions will be favored when no H2 or low concentrations of H2 are used in the gas phase [6, 9, 10].
Under inert atmosphere
As illustrated by several studies in the literature, the catalytic deoxygenation pathway depends on the atmosphere of the reaction. Actually, it was observed that some routes are more dominant than others depending on the atmosphere of the reaction. Murzin et al. [6] determined the reaction routes of stearic acid over several heterogeneous catalysts at 300 °C under inert atmosphere. Apart from the decarboxylation and decarbonylation reactions, ketonization and cracking and other consecutive reactions, such as isomerization, dehydrogenation, hydrogenation, cyclisation and dimerization occured (Figure 4). The cracking produces shorter fatty acids and hydrocarbons, whereas the ketonization produces symmetrical ketones. Isomers of n-alkane and n-olefins can also be formed. Finally, cyclic and aromatic products can also be generated, but at very low amounts [6, 11]. Under inert atmosphere, the DO of unsaturated fatty acid occurred firstly via the hydrogenation of the double bonds (C=C) to form the corresponding saturated fatty acid (for example, hydrogenation of oleic acid to stearic acid). Generally, the hydrogen needed to carry out this step originates either from residual H2 present on the catalyst surface, which was adsorbed during the reductive pretreatment [11, 12] or was formed in situ via the simultaneous dehydrogenation reactions of the feed to unsaturated by-products such as diunsaturated acids, triunsaturated acids as well as aromatics [13]. Furthermore, compared to DO of saturated fatty acid, the DO of unsaturated fatty acid under inert gas leads to low conversion to hydrocarbons and to an increase of deactivation due to the presence of highly unsaturated coumpounds [11, 13, 14].
Under H2-containing atmosphere
The reaction pathways of fatty acids deoxygenation over Pd/C catalyst under H2 was investigated by Gosselink et al. [11] and the schematic representation is given on Figure 5. DCN of fatty acids under H2 atmosphere occurs via an aldehyde and a subsequent ketene intermediate whereas DCX occurs via direct decarboxylation or via the release of formic acid and a subsequent hydrogenation of the olefin formed. Figure 5: Deoxygenation through the DCX/DCN under H2 atmosphere. Adapted from [11, 15] The deoxygenation pathway of unsaturated fatty acids over Pd/C under H2 occurs firstly via the hydrogenation of the double bonds to form a saturated fatty acid. This is due to the high hydrogenation activity of the Pd catalysts [4]. Then, through deoxygenation of the saturated acid, the final hydrocarbon is obtained [13, 16, 17]. Consequently, the deoxygenation pathway of unsaturated fatty acid is similar than those of a saturated fatty acid in its second step. Pd-based catalysts for the decarboxylation of fatty acids Motivated by the high performances obtained in the first studies on catalytic deoxygenation of fatty acids over Pd/C catalyst, several studies were carried out to better understand the influence of the metal nanoparticles size, of their dispersion, of the support, of the metal loading and of the FFA/catalyst ratio. Various reaction conditions, in particular pressure, temperature, reaction time and nature of the carrier gas were also studied.
Preparation of the Pd-based catalysts
Several techniques are described in the literature to prepare supported Pd-based catalysts as it strongly influences the physicochemical properties of the solid and, therefore, its catalytic performances. Among the various methods, the most commonly used are incipient wetness impregnation and wet impregnation [10]. But other techniques such as deposition of palladium hydroxide obtained by hydrolysis of PdCl2 at different pH followed by adsorption on carbon were also used [12, 18, 19] . The variation of the pH of the palladium hydroxide solution was showed to lead to different metal particles dispersion [20]. It was observed that, to improve the total wetting of the carbon pores, the pre-treatment of its surface by HNO3 during several hours is necessary [18]. However, it is also possible to use carbon directly for wet impregnation without prior treatment of the surface.
Influence of the catalysts properties and loading on the performances
Simakova et al. [20] studied the effect of the Pd particles size and of their dispersion on the catalytic properties in the decarboxylation of stearic acid using four different Pd/C (Sibunit) catalysts all containing 1 wt.% of Pd but with different dispersions obtained by changing the pH of the palladium hydroxide solution during the synthesis [21]. The reaction was carried out 34 at 300 °C under 17.5 bar of 5 vol.% of H2 in Ar. The properties and performances of each catalyst are shown in Table 3. It was found that large particles (catalyst A) and highly dispersed Pd species (catalyst D) are not very active in the DO of the fatty acids. In fact, the authors found that it could be due to their small surface area and their strong interaction with the support, which lead to a modification of the metal structure of the Pd responsible for the cleavage of the C-C bonds and the effective decarboxylation of the fatty acid. Catalysts B and C showed the highest performance in stearic acid decarboxylation because of an optimal metal dispersion (with 47 and 65% respectively). The selectivity to n-heptadecane at fatty acids isoconversion shows clearly that the change in the metal dispersion affects much more the deoxygenation rate than the distribution of products.
Table of contents :
Chapter 1: Bibliography synthesis
INTRODUCTION
I.1. DEFINITION
I.2. DECARBOXYLATION/DECARBONYLATION PATHWAYS OF FATTY ACIDS
I.2.1. Influence of the nature of the catalyst
I.2.2. Influence of the reaction atmosphere
I.2.2.1. Under inert atmosphere
I.2.2.2. Under H2-containing atmosphere
II.1. PREPARATION OF THE PD-BASED CATALYSTS
II.2. INFLUENCE OF THE CATALYSTS PROPERTIES AND LOADING ON THE PERFORMANCES
II.2.1. Effect of the Pd metal particle size and dispersion
II.2.2. Effect of the Pd metal loading
II.2.3. Effect of the FFA/catalyst ratio
II.2.4. Deactivation of the Pd/C catalysts
III.1. DECARBOXYLATION OF FATTY ACIDS OVER MONOMETALLIC CATALYSTS
III.2. INFLUENCE OF THE PREPARATION METHODS OF NI-BASED CATALYSTS
III.3. DECARBOXYLATION OF FATTY ACIDS OVER BIMETALLIC CATALYSTS
III.4. DECARBOXYLATION OF FATTY ACIDS OVER TRIMETALLIC CATALYSTS
IV.1. INFLUENCE OF THE NATURE OF THE ATMOSPHERE
IV.2. INFLUENCE OF THE REACTION TEMPERATURE
IV.3. INFLUENCE OF THE REACTION PRESSURE
IV.4. INFLUENCE OF THE REACTION TIME
IV.5. INFLUENCE OF CHAIN LENGTH OF THE SATURATED FATTY ACID
IV.6. SUMMARY OF THE REPORTED PERFORMANCES ON NON-NOBLE METAL-BASED CATALYSTS
CONCLUSION
REFERENCES
Chapter 2: Experimental procedures
INTRODUCTION
I.1. MATERIALS
I.2. PREPARATION OF MONO- AND BIMETALLIC CATALYSTS
I.2.1. Bench-scale manual preparation
I.2.2. High-throughput catalysts preparation
I.2.2.1. Preparation by deposition-precipitation method
I.2.2.2. Preparation by wet impregnation method
I.3. CHARACTERIZATION METHODS
I.3.1. Morphological and textural properties
I.3.1.1. Nitrogen adsorption/desorption
I.3.1.2. Transmission Electron Microscopy (TEM)
I.3.2. Surface properties
I.3.2.1. X-ray Photoelectron Spectroscopy (XPS)
I.3.3. Bulk properties
I.3.3.1. X-ray fluorescence (XRF)
I.3.3.2. X-ray diffraction (XRD)
I.3.3.3. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES)
I.3.3.4. Temperature-programmed reduction (TPR)
II.1. MATERIALS
II.2. EXPERIMENTAL SETUP
II.2.1. SPR system presentation
II.2.2. Description of the experimental protocol of catalyst reactivation
II.2.3. Description of the experimental protocol
III.1. ANALYTICAL METHOD
III.1.1. GC analysis
III.1.2. Calibration of the GC
III.1.3. Sample preparation
III.2. DATA TREATMENT
Chapter 3: Characterization study
INTRODUCTION
I.1. NITROGEN ADSORPTION/DESORPTION
I.1.1. Textural properties of the bare carbon support and of the mono- and bimetallic catalysts
I.1.2. Textural properties of the bimetallic 10%Ni10%Cu/C
I.1.3. Impact of the preparation method of the catalyst on their textural proprieties
I.2. TEM AND EDX
II.1. XPS
II.1.1. XPS Analysis of the 10%Ni10%Cu/C catalyst
II.1.2. Analysis of the Ni-Cu catalysts
III.1. ICP-OES ANALYSIS
III.2. XRF
III.3. XRD
III.3.1. XRD analysis of the 10%Ni /C catalyst
III.3.2. XRD analysis of the 10%Ni10%Cu/C catalyst
III.3.3. XRD analysis of the 10%Ni10%Cu/C-calc catalyst
III.3.4. Determination of the crystallite size and of the d-spacings
III.3.5. XRD analysis of other monometallic and bimetallic catalysts
III.4. TPR
Chapter 4: Reactivity of monometallic catalysts in the decarboxylation of palmitic acid
INTRODUCTION
I.1. BLANK TEST
I.2. EFFECT OF THE REACTION TEMPERATURE
I.3. EFFECT OF THE CONCENTRATION OF THE SUBSTRATE
I.4. EFFECT OF THE REDUCTION OF THE CATALYST WITH H2
I.5. EFFECT OF THE MASS OF CATALYST LOADED IN THE REACTOR
II.1. EFFECT OF THE NATURE OF THE METAL
II.2. NICKEL-BASED CATALYSTS
II.2.1. Effect of the activation of the catalyst under H2
II.2.2. Effect of the nickel loading
II.2.3. Influence of the mass of catalyst loaded in the reactor
II.2.4. Recyclability of the catalyst
DISCUSSION
Chapter 5: Reactivity of bimetallic catalysts in the decarboxylation of palmitic acid
INTRODUCTION
I.1. NI-AG-BASED CATALYSTS
I.1.1. Effect of Ag addition
I.1.2. Effect of the Ni-Ag loading
I.2. NI-FE-BASED CATALYSTS
I.2.1. Effect of the Fe addition
I.2.2. Effect of the Ni-Fe loading
I.3. NI-CU-BASED CATALYSTS
I.3.1. Effect of the Cu addition
I.3.2. Effect of the Ni-Cu loading
I.4. OPTIMIZATION OF THE REACTION CONDITIONS OVER THE 10%NI10%CU/C CATALYST
I.4.1. Effect of the amount of catalyst
I.4.2. Effect of the reaction temperature
I.4.3. Effect of the reaction time
I.4.4. Effect of the reaction atmosphere
I.4.5. Effect of the solvent
I.4.6. Manual and robotized preparations of the catalyst
I.4.7. Effect of the preparation method of the catalyst
REFERENCES
