Producción catalítica de gamma-valerolactona a partir de sustratos procedentes de biomasa

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2022
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04-10-2022
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This doctoral thesis presents the development of different types of catalysts for the conversion of different biomass-derived compounds into γ-valerolactone, a compound with high added value. Firstly, the study of the production of γ-valerolactone from levulinic acid, one of the most easily isolated compounds from biomass and with a high transformation potential, was carried out. This reaction requires a hydrogenation step and a dehydration step. This process has been studied using different sources of hydrogen and metallic catalysts to carry it out. Different types of Ni-based catalysts were synthesized on natural supports (attapulguite, sepiolite and treated sepiolite) by impregnation. The effect of the preparation method on the catalytic behavior was also studied. Thus, catalysts were synthesized by: simple impregnation with water, precipitation, deposition with urea and using a treatment with APTES (3-aminopropyltriethoxysilane). The transformation of levulinic acid into γ-valerolactone was carried out at different temperatures and reaction times, using different hydrogen sources such as formic acid, molecular hydrogen, 2-propanol and a water + Zn system. When the transformation of levulinic acid was carried out using the water and Zn system, high yields were obtained to γ-valerolactone (greater than 98 % in just 2 hours at 180 ⁰C), while the use of formic acid, 2-propanol and H2 molecular did not produce very high γ-valerolactone yields using Ni catalysts. On the other hand, the use of 2-propanol as solvent and hydrogen source turned out to be very effective to achieve high yields of γ-valerolactone using methyl levulinate and ethyl levulinate. In addition, a comparison of the catalytic results obtained with the Ni catalyst and with ruthenium-based catalysts was made, which is the most widely used element for this reaction in the catalytic literature. It was observed that the use of Ru catalysts was only more effective when molecular hydrogen was used to carry out the hydrogenation, due to Ni was not able to use that pressurized hydrogen to carry out the hydrogenation process. However, using 2-propanol as the hydrogen source, the Ni catalysts were more efficient. It was therefore determined that the optimal catalyst depends on the reaction conditions used and the substrate used. It should be noted that Ni catalysts are capable of produce γ-valerolactone from levulinic acid at temperatures close to room temperature. Thus, using hydrogen generated by reaction of Zn with water (Zn + H2O  ZnO + H2) produced a yield to γ-valerolactone upper than 25 % at only 30 ⁰C. In addition, furfural was also used as a substrate to carry out the production of γ-valerolactone in a single step. Different types of Zr-based catalysts were used using a silica material (silicic spheres) and sepiolite. To carry out the transformation of furfural into γ-valerolactone in a single step, it has been described in the literature that the presence of Lewis acid sites and Brønsted acid sites are necessary, because both are necessary in at least one of the steps that take place in the transformation of furfural into γ-valerolactone: hydrogenation, dehydration/esterification, dehydration/ring opening or cyclization. It was found that when using the support of SiO2 spheres (SiSPH), the Lewis acid sites of ZrO2 interact with the –OH groups on the surface of the support to generate Brønsted acid sites on the supported Zr catalysts, which are absent in pure ZrO2 and in SiSPH. Therefore, if the amount of Zr added to the support is modified, the amount of Lewis and Brønsted acid sites of the catalyst can be controlled, optimizing this ratio to maximize the yield to γ-valerolactone. The catalyst with a Zr load close to 7 % achieved the best yield to γ-valerolactone, being 72.4 % after 8 hours at 180 ⁰C using 2-propanol. Regarding the Zr catalysts supported on sepiolite, the influence of basicity on the catalytic pathway for the production of γ-valerolactone from furfural was also studied. The basicity can be regulated with the amount of Zr added to the support, the greater the amount of Zr added, the greater the basicity of the catalyst and also the greater the acidity. The catalyst that showed the highest yield to γ-valerolactone was the one containing 9 % ZrO2, close to 70 % after 12 hours of reaction at 180 ⁰C. This catalyst presented a very good dispersion and small size of the ZrO2 nanoparticles. With both catalysts, the influence of the reaction time on the yields to different products was observed, being able to establish a reaction mechanism with the different intermediate reaction products. The use of 2-propanol was the most effective for the conversion of furfural to γ-valerolactone using both Zr catalysts. In addition, stability tests were carried out and it was observed that after 3 reaction cycles, the catalyst maintained its catalytic activity with only a slight decrease in the yield to γ-valerolactone. In this work, the role of the Brønsted sites and the basic sites in the reaction to transform furfural into γ-valerolactone is discussed. Finally, bimetallic catalysts based on Sn and Zr were synthesized to carry out the furfural conversion into γ-valerolactone in one-pot and also use it as a catalyst to carry out the conversion of other substrates such as levulinic acid, methyl levulinate, ethyl levulinate, xylose, fructose and glucose. Zeolite Y was processed by a dealuminization treatment and subsequent inclusion of metal oxides was used. A synergistic effect between both metals was detected, since the bimetallic catalyst Sn:Zr with a molar ratio (1:1) was the one that presented the highest yield to γ-valerolactone. Thus, in just 1 hour of reaction at 180 ⁰C in 2-propanol, a yield to γ-valerolactone close to 76 % could be achieved, much higher than that obtained with monometallic Sn or Zr catalysts. When this catalyst was used with levulinic acid, methyl levulinate and ethyl levulinate, high yields to γ-valerolactone were also achieved. However, when using xylose, fructose or glucose as substrates, the yields to γ-valerolactone achieved were very low (less than 10 % in all cases) even after 48 hours of reaction at 180 ⁰C. In this work it has been observed that the direct production of γ-valerolactone from sugars is very inefficient, requiring long reaction times to achieve reasonable conversions and, in all cases, with very low selectivities to γ-valerolactone and with very high formations of humines.
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