Conversion of cellulosic biomass towards added value products over bifunctional

Resumo

In view of the current problems such as global warming, high oil prices, more severe environmental laws and other geopolitical consequences surrounding the use of fossil feedstocks, the efficient conversion of cellulosic biomass towards chemicals, energy and fuels, has achieved a good piece of attention. In this sense, the use of heterogeneous catalysts could allow to develop environmentally clean processes working under relatively mild conditions, for the selective production of high added value products from cellulosic biomass. In the present thesis, Ru and Ni deposited on mesoporous silica materials (MSM) were evaluated in order to produce sugar alcohols, ethylene glycol and straight-chain alkanes. Differing from other catalytic supports such as activated carbon, which have been highly used as support material for biomass conversion, not many works have reported the use of MSM for this purpose. The thermal and mechanical stability and the interesting pore structure of catalysts based on MSM, make them promising candidates for their application in conversion of biomass, being an interesting alternative toovercome diffusional problems in this kind of applications. This PhD thesis has been structured in five chapters. In each of them, a literature review was done in order to know the main achievements and challenges previously reported about the selected topic. In addition, the partial objectives and the obtained results were presented and discussed. The main content of each chapter is described below. In Chapter 1, the synthesis and characterization of a bifunctional solid catalyst using MCM-48 as the support and ruthenium as the metal phase, is presented. Hydrolytic and hydrogenation capacities were the desired features of this catalyst to perform the synthesis of sorbitol from cellulose. MCM-48 demonstrated a good catalytic response for the hydrolysis of cellulose into D-Glucose attributable to surface acidity, with a yield of 14,1% of glucose in 30 min at 230ºC, much more higher than that reported in the literature. Catalytic activity of Ru/MCM-48 (4 wt. % Ru) prepared by wet impregnation method, was compared to that obtained by commercial Ru/C and a Ru deposited on a commercial TiO2 in D-Glucose hydrogenation towards sorbitol. Ru/MCM-48 showed a high activity in the hydrogenation of D-Glucose, being 100 % selective to sorbitol working at 80 – 120 ºC and 5 MPa H2. Activation energy for Ru/MCM-48 was around 45 KJ·mol-1. Moreover, Ru/MCM-48 demonstrated a good stability after three reaction cycles, both in terms of specific catalytic activity and ruthenium crystallite size.According to the previous results, Ru/MCM-48 stands as a promising candidate for the conversion of cellulose into hexitols (sorbitol and mannitol). Thus, in Chapter 2, one – pot hydrolytic hydrogenation of cellulose was studied using Ru/MCM-48 (4 wt.% Ru). Alternatively, Supercritical water (SCW) hydrolysis in ultrafast reactors was performed at 400 ºC – 25 MPa and extremely low reaction times ~ 0.20 s to partially depolymerize cellulose with high selectivity to sugars (70 % w·w-1) avoiding degradation reactions. From this SCW hydrolysis step, although the 100% of cellulose was converted, a high yield of the hydrolysed products were detected as oligosaccharides. The hydrogenation of the liquid product from SCW hydrolysis of cellulose over Ru/MCM-48 allowed to complete the hydrolysis of oligosaccharides to monomeric sugars and maximize the yield to hexitols by changing experimental parameters such as temperature and reaction time. A greater yield to hexitols was obtained by the subsequent hydrogenation of the liquid product from cellulose hydrolysis in SCW (49 %) than using the one – pot catalytic hydrogenation from untreated cellulose (21 %). Ru/MCM-48 showed better behaviour than standard Ru/C during the hydrogenation processes, under comparable experimental conditions in all the cases. Thus, Ru/MCM-48 was also tested in the hydrogenation of the product from sugar beet pulp (SBP) hydrolysis in SCW. SBP was used in order to evaluate the behaviour of a real biomass in the proposed two-steps process. A yield to hexitols of 15 % was achieved by hydrogenation of SBP hydrolysates, ethylene glycol was the main compound in the liquid product with a yield of 21 % and glycerol was also obtained as by-product. These results have shown the potential of coupling the SCW hydrolysis with the hydrogenation using Ru/MCM-48 as catalyst for the production of biomass derived alcohols. The high price of noble metals such as Ru, could represent an important drawback for the development of these catalytic systems. Thus, the development of novel bimetallic nickel-based catalytic systems with comparable high activity to noble metal catalysts still remains a technological challenge. In Chapter 3, the preparation of three different bimetallic Ru:Ni catalysts supported on a mesoporous silica MCM-48 by consecutive wet impregnations is proposed. These catalysts were compared with the corresponding monometallic Ni/MCM-48. All the catalytic materials were characterized by means of X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), adsorption / desorption of N2, Temperature Programmed Reduction (H2-TPR),desorption at programmed temperature of ammonia (NH3– TPD) and Atomic Absorption (AA) and tested in the liquid phase hydrogenation of D-Glucose into sorbitol in the temperature range 120 – 140 ºC under 2.5 MPa of H2 pressure. The catalyst so prepared presented a total metal loading of ca. 3 % (w·w-1), where Ru:Ni ratios spanned the range of 0.15 – 1.39 (w·w-1). Ru:Ni/MCM-48 catalysts with Ru:Ni ratios higher than 0.45 enhanced the specific activity of the monometallic Ni/MCM-48 in the hydrogenation of D-Glucose to sorbitol, increasing the reaction rate and showing complete selectivity to sorbitol. Activation energy values of 36 KJ·mol-1 and 70 KJ·mol-1 were obtained for Ni/MCM-48 and Ru:Ni/MCM-48 (0.45). In addition, Ru:Ni/MCM-48 (0.45) demonstrated a good stability after three reaction cycles, standing as a good option for the effective hydrogenation of carbohydrate sugars into sugar alcohols. The experimental work corresponding to the first three chapters were carried out at the University of Valladolid, within the High Pressure Processes Group. However, next two chapters were developed in the Institut de recherches sur la catalyse et l'environement de Lyon (Lyon, France) within a three-month stay, where different mesoporous catalyst were prepared and tested for biomass conversion purposes. In Chapter 4, bifunctional Ru/Al-MCM-48 was prepared by wet impregnation method and characterized by means of adsorption-desorption of N2, small angle X-Ray scattering (SAXS), H2-TPR, XRD, TEM, NH3-TPD and AA. The so prepared Ru/Al-MCM-48 (3.5 wt. % Ru) was tested in the catalytic hydrogenolysis of cellobiose towards hexitols. The introduction of Aluminum into MCM-48 structure increased the number of acid sites compared to MCM-48 and these features played an important role for hydrolysis steps involved in the global reaction mechanism. Moreover, hydrogenation steps took place on metallic ruthenium sites. A kinetic study of the hydrogenolysis of cellobiose to hexitols over Ru/Al-MCM-48 was developed, where different parameters such as pressure of hydrogen, temperature and reaction time were evaluated. Cellobitol was the main intermediate of the conversion of cellobiose to hexitols over Ru/Al-MCM48. Temperatures in the range of 140 – 180 ºC and pressures between 30 and 50 MPa H2 were studied and it was noticed a positive influence of both parameters in order to maximize the production of hexitols. A maximum yield of 91 % of hexitols was achieved by catalytic hydrogenolysis of cellobiose over Ru/Al-MCM48 at 180 ºC, 5 MPa H2 and 7 min, where sorbitol was the main compound in the final product with a yield of 82 %. A kinetic model covering different reaction temperatures was also developed, which predicted well the concentration of the different compounds involved in the proposed reaction pathway. Specific reaction rates and activation energy values were also obtained for the different steps of the catalytic process. In addition, mass transfer effects were evaluated in detailed. Besides conversion of biomass into sugar alcohols, further transformation into liquid alkanes has currently attracted a great deal of attention in order to reduce the dependence of fossil fuels. In Chapter 5, conversion of D-Glucose into short-chain alkanes (mainly n-butane, n-pentane, n-hexane) in a one-pot biphasic catalytic system is reported. Catalytic tests were performed at 190 ºC under 5 MPa of H2 in a biphasic ndecane – water reaction medium, where different catalytic materials were used for the production of straight-chain alkanes. Catalytic behaviour of supported Ru/Al-MCM-48 and acidified Ru/MCM-48 with tungstophosphoric acid (TPA) was compared with that obtained by a standard Ru/C. Mesoporous catalytic materials were characterized by means of adsorption / desorption of N2, XRD, TEM, H2-TPR, NH3 – TPD and AA. A higher yield to alkanes around 57 % was obtained over Ru/Al-MCM-48, while a yield to alkanes of 29 % was achieved by commercial Ru/C. Moreover, the addition of Al- MCM-48 combined with Ru/C resulted in an important improvement in the production of alkanes compared to that achieved by Ru/C. The influence of TPA as homogeneous catalyst was also studied with Ru/Al-MCM-48 and commercial Ru/C. The highest yield of alkanes around 81.9 % (n-butane 7.1 %, n-pentane 29.3 % and n-hexane 45.4 %) was obtained over Ru/C + Al-MCM-48 system.