Catalizadores metálicos soportados para la obtención de glicoles a partir de lignocelulosa y derivados

Resumen

This memory is developed in a worldwide stage where the demand of chemicals from renewable sources by consumers is increasing. Besides, many of them are obtained from petrochemistry industry with environmental, supply and price fluctuations problems, so a change is required regarding the current model. The only renewable source that contains carbon is biomass and thus, its valorization is very attractive to produce chemicals as glycols. Glycols have a multitude of applications such as the synthesis of polymers, their use as coolant, antifreeze and hydraulic fluids, in the manufacture of detergents and cosmetics and as additives in the pharmaceutical industry. Among all types of biomass, lignocellulose and its derivatives are the most interested due to their abundance, their low cost and because they do not compete directly with the agri-food industry. Therefore, their chemical valorization by heterogeneous catalysis is presented as a viable and an important route to compete with the existing pathways minimizing the environmental and socioeconomic problems associated with these sources. In this context the work performed in this Doctoral Thesis is included whose main objective is the identification and development of solid active, selective and stable catalysts in the lignocellulose biomass and its derivatives transformation into glycols. The studies conducted in this work can be divided into two groups depending on the starting material selected to produce glycols. Firstly, sorbitol is selected as starting reagent as it can be obtained from lignocellulose through cellulose (its principal constituent) hydrolysis/hydrogenation reactions. A series of Ru based catalysts (5 wt. %) are prepared by wet impregnation on different supports (Al2O3, SiO2, TiO2 and ZrO2) to study the role of the same in the physicochemical properties and in the catalytic behavior in sorbitol hydrogenolysis to glycols. The reactions were conducted under moderate H2 pressures and in the absence of basic promoters since despite these parameters are very important for achieving high yields to glycols, the role of the support in these conditions will be more visible. RuAl catalyst showed the best catalytic activity in the hydrogenolysis of sorbitol due to the presence of a higher acid sites concentration on its surface and Ru¿+ species observed by the physicochemical characterization, favoring the dehydrogenation/hydrogenation reactions to obtain glycols. With the aim of improving the glycols yield, the hydrogenolysis of sorbitol was studied in an alkaline medium by using RuAl catalyst by adding a basic promoter to the reaction medium or in the presence of Ru catalysts directly impregnated in a basic support as Ca(OH)2. This second approach presents a greater glycols yield suggesting that a close contact between the metal and the basic promoter is required. In addition, the promoter effect of Ni incorporation to RuCa-HT catalyst was analyzed and this effect was observed due to a charge transfer between both metals which induces the ruthenium surface reducibility favoring the hydrogenation of unsaturated intermediates formed by retroaldol condensation to produce finally glycols. NiRuCa-HT catalyst which showed the best catalytic behavior, is not stable due to the partial solubility of Ca(OH)2 support, however the addition of small amounts of this base in each cycle allows it to be reused for at least five runs maintaining practically the same activity in all of them and slowing down its deactivation. In the second part of this work, sorbitol is replaced by cellulose or lignocellulose as starting reagent to study their conversion to glycols. In particular, for cellulose conversion to glycols, catalysts which combine a hydrogenating metal and species that provide a slight acidity to the catalyst are required. Therefore, the addition of W, Mo and Nb oxides to Ni based catalysts supported on SiO2 and prepared by incipient wetness impregnation was evaluated. The best results are achieved with NiWSi catalyst because an exclusive effect between Ni and W which leads to the formation of the mixed oxide NiWO4, but no mixed phases with the other oxides promoters were detected. Subsequently, a study of the activation procedure is performed evaluating the influence of the reduction and calcination temperatures in the NiWO4 phase formation. It is confirmed by these experiments that the activation conditions which favor to a greater extent the NiWO4 formation are those that also allow obtaining higher yields to glycols. However, NiWSi-800-873 catalyst, the most active of the series, is not stable and reusable in more than two reaction cycles as it is mainly deactivated by its components (Ni, W and Si) leaching in the reaction medium. Considering that NiW based catalysts are active in the conversion of cellulose to glycols but unstable under the conditions used, another series of catalysts were synthesized by changing certain parameters such as (i) the atomic Ni/W ratio, (ii) the precursors used, (iii) the preparation methodology, (iv) the support and (v) the hydrogenating metal, to increase its performance and mainly to improve the stability of these systems. The 0.63NiWSi-co and 0.05PtWAl catalysts showed the highest yields to glycols, so a stability study was conducted with these samples. 0.63NiWSi-co catalyst despite being slightly more active is not stable because it is deactivated by leaching of its constituents. However, 0.05PtWAl is an active and stable catalyst in this reaction and it can be reused at least during four cycles. Finally, some experiments were done in the presence of these catalysts but using different types of lignocellulose biomass either untreated with which lower glycols yields were obtained as by exsitu (with high temperatures and with solvents as H2O and mixtures of H2O/GVL) and insitu processes to achieve high yields to glycols in a single step. Despite the efforts done in the biomass pretreatment methodology, the glycols yield was not improved. However, it has been found that the direct addition of GVL as solvent to the reaction medium (insitu) allows achieving a 63.6 % glycols yield in the cardoon biomass reaction with 0.05PtWAl catalyst.