The role of Mg-Al hydrotalcite derived mixed oxides as catalytic support materials : Applications in the transesterification of vegetable oils for biodiesel production and in the steam reforming of glycerol for hydrogen production

Abstract : This work aims to study the transesterification of sunflower oil for biodesel production and the steam reforming of glycerol for renewable hydrogen production. Mg-Al hydrotalcite derived mixed oxides were chosen as catalytic support materials given their known basicity, thermal stability and low cost. Only one active phase was studied for biodiesel production (Ca) and two different active phases (Ru and Ni) for glycerol steam reforming. The various prepared catalysts were characterized by different techniques such as X-Ray Diffraction (XRD), specific surface area determination by BET method, Fourier Transform Infrared Spectroscopy (FTIR), H₂-Temperature Programmed Reduction (H₂-TPR), CO₂-Temperature Programmed Desorption (CO₂-TPD) and Simultaneous Thermogravimetric-Differential Scanning Calorimetry (TG-DSC). For biodiesel production, the effects of thermal treatment and the preparation method for Ca incorporation were both studied. The catalyst that showed the best catalytic performance was an uncalcined Mg₄Al₂ support impregnated with 40 wt% Ca followed by a calcination at 600°C (Ca/600Mg₄Al₂HT) due to its enhanced basic properties. After several optimization steps over this catalyst, the optimum conditions for biodiesel synthesis were: a methanol to oil molar ratio of 15:1, a catalyst to oil ratio of 2.5 wt% and a reaction time of 6 hours which gave a FAME yield of 95%. The properties of the produced biodiesel were studied and found to be in good agreement with ASTM (American Society for Testing and Materials) requirements. For the catalytic steam reforming of glycerol, the activity of NI-based pure metal oxides (Ni/CeO₂, Ni/Y₂O₃, Ni/ZrO₂) was first studied as pure metal oxides are commonly used as support materials. The effect of the support was evaluated and the Ni/ZrO₂ catalyst was found to be the most efficient for hydrogen production. To study the effect of the zirconia phase, a tetragonal Ni/ZrO₂ was also prepared. The tetragonal catalyst was less active for hydrogen production compared to the monoclinic catalyst. A stability test at 600°C also showed the desactivation of the tetragonal Ni/ZrO₂ after 6 hours on stream. For the catalytic steam reforming of glycerol using Mg-Al hydrotalcite derived mixed oxides, Ru-based Mg-Al catalysts were first studied. The effect of the preparation method (impregnation vs grafting) on the catalytic activity of Ru-Mg-Al catalysts was studied. The catalyst prepared by the impregnation method resulted in a better catalytic activity than the catalyst prepared by the grafting method as it allowed a higher accessibility of the active phase. For the Ni-based Mg-Al catalysts, the hydrotalcite supports were modified with La to study the effects of promoter addition on catalytic properties and activity. The bimetallic effect (1%Ru-5%Ni) and effect of a higher Ni loading (15 wt%) were also studied. The 5 wt% Ni impregnated on a La modified Mg-Al support (Ni/Mg₆Al₁.₆La₀.₄) catalyst was the most efficient for hydrogen production due to its enhanced basicity and metal-support interaction. The activities of the most efficient catalysts, Ni/ZrO₂ and Ni/Mg₆Al₁.₆La₀.₄, were compared. Both catalysts produced similar hydrogen yields. Differences in glycerol conversion to gaseous products were attributed to a higher formation of liquid by-products over the hydrotalcite support compared to the zirconia support. Nevertheless, less coke was produced over the hydrotalcite catalyst given its higher basicity. A stability test at 600°C showed the desactivation of the Ni/Mg₆Al₁.₆La₀.₄ catalyst after 6 hours on stream. Upon optimization of the reaction conditions on Ni/Mg₆Al₁.₆La₀.₄, a significant improvement in the stability was observed as the catalyst lasted for 24 hours on steam.Therefore, Ni/Mg₆Al₁.₆La₀.₄ could be a promising candidate for industrial application.
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Eliane Dahdah. The role of Mg-Al hydrotalcite derived mixed oxides as catalytic support materials : Applications in the transesterification of vegetable oils for biodiesel production and in the steam reforming of glycerol for hydrogen production. Catalysis. Université du Littoral Côte d'Opale; Université de Balamand (Tripoli, Liban), 2018. English. ⟨NNT : 2018DUNK0498⟩. ⟨tel-02137402⟩

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