Wheat bran biorefineryvalorization of hemicelluloses to sugar alcohols. Fractionation – hydrolysis – purification – hydrogenation

Resumen

In recent years, the conversion of lignocellulosic biomass towards platform chemicals or fuels has received special attention. This fact is a consequence of the exhaustion of fossil resources, the greater concerning about global warming and the more severe environmental laws enacted by the government. Unlike fossil resources, biomass is a sustainable, renewable and abundant feedstock. A biorefinery is “a facility that integrates biomass conversion processes and equipment to produce fuels, power, heat and value-added chemicals from biomass”. The processes of a biorefinery are designed to maximize value products and minimize waste streams. Different processes have been described in literature for the conversion of biomass into high added-value products. In this sense, the transformation of biomass by means of heterogeneous catalysts appears to be a suitable process which allows the development of environmentally-friendly processes working under mild conditions. This PhD Thesis is focused on upgrading the hemicellulosic fraction of wheat bran by its transformation into sugar alcohols. To accomplish this goal, the global process has been divided in several sequential steps, which are described along the five chapters of the Thesis. In Chapter 1, the isolation of the arabinoxylan fraction from wheat bran was studied under hot compressed water conditions combined with heterogeneous catalysts. The aims of this chapter were to obtain high extraction yields of arabinoxylans with low molecular weight to facilitate their subsequent hydrolysis into monosaccharides as well as minimize the co-extraction of the cellulosic fraction. Different mesoporous silica materials (MCM-48 and Al-MCM-48) as well as the corresponding RuCl3-based catalysts were investigated. The effects of temperature (140 – 180 ºC) and time (10 – 30 minutes) were also examined. The arabinoxylans extraction yield was directly related to the catalyst acidity: MCM-48 < Al-MCM-48 < RuCl3/MCM-48 < RuCl3/Al-MCM-48. A high total acidity and the combination of Brönsted and moderate Lewis acid sites of RuCl3/Al-MCM-48 demonstrated to be suitable for arabinoxylan extraction. High yields of arabinoxylans with low molecular weight were achieved combining high temperatures, short times and using solid acid catalysts. Optimum conditions were obtained over RuCl3/Al-MCM-48 at 180 ºC after 10 minutes. Under such conditions, around 78% of total arabinoxylans was extracted with an average molecular weight of ca. 9 kDa. A relation between the experimental conditions (time, temperature, catalyst), the arabinoxylans yield and their molecular weight was also established. Soft operating conditions resulted in a low amount of arabinoxylans extracted with a low molecular weight, which means that only side chains of the polysaccharides were solubilized. Moderate operating conditions led to higher yields and molecular weights. In this case, the backbone of the arabinoxylans was solubilized but not hydrolyzed into short oligomers. Under more severe experimental conditions, the yield increased and the molecular weight decreased drastically. These results exhibited the solubilization of the arabinoxylans backbone as well as its hydrolysis into short chain oligomers. In Chapter 2, the arabinoxylans previously extracted were subjected to a further catalytic hydrolysis process to complete their rupture into monomers. This process was optimized to maximize the production of pentoses (i.e. arabinose and xylose) avoiding their further degradation to furfural. The hydrolysis of poly/oligosaccharides is usually carried out by means of acids or enzymes. In this chapter, different solid acid catalysts were successfully employed for arabinoxylan hydrolysis. Mesoporous silicas (MCM-48 and Al-MCM-48) and RuCl3 catalysts supported on them were tested. As happened in the previous extraction process, RuCl3/Al-MCM-48 resulted to be the most active catalyst due to its high acidity and the combination of Lewis and Brönsted acid sites. Arabinoxylan hydrolysis was then optimized by investigating the influence of catalyst loading and reaction time. Under optimum experimental conditions (180 ºC, 15 min, 4.8 g RuCl3/Al-MCM-48/g C), the yields corresponding to arabinose and xylose were 96% and 94%, respectively, without major degradation to furfural. The effect of different Lewis cations (Ru+3, Fe+3) was also discussed. RuCl3 supported on Al-MCM-48 proved to be more efficient in arabinoxylan hydrolysis than FeCl3/Al-MCM-48, which was attributed to the moderate Lewis acidity of Ru+3 cations compared to the strong Lewis acid sites of Fe+3. In Chapter 3, the behavior of different mesoporous silicas (MCM-48 and Al-MCM-48) and microporous aluminosilicates (commercial zeolites; H-Y (12), H-ZSM-5 (23) and H-ZSM-5 (80) from Zeolyst International Inc.) was compared in the hydrolysis of wheat bran arabinoxylans. Not only the number of acid sites, but also their strength and nature played an important role. Al-MCM-48 was more active than MCM-48. MCM-48 has a low acidity which corresponds mainly to weak Lewis acid sites. However, Al-MCM-48 has a higher acidity and a combination of Lewis and Brönsted acid sites, which improved the arabinoxylan hydrolysis compared to MCM-48. H-ZSM-5 (23) showed the highest catalytic activity, which was attributed to its high acidity and strong Brönsted acid sites. The process was then optimized over H-ZSM-5 (23) at 180 ºC. Optimum reaction time and catalyst loading were 15 minutes and 9.2 g catalyst/g C, respectively. The yields corresponding to arabinose and xylose were 96% and 76%, respectively. Arabino-oligosaccharides were therefore more readily hydrolyzed than xylo-oligosaccharides. This is related to the type of bond existing between arabinose (linked by α-glycosidic bonds, weak) and xylose (linked by β-glycosidic bonds, strong) molecules. The easier access to the side chains (composed of arabinose) than to the backbone (composed of xylose) also explains the faster release of arabinose than xylose. Prior to the hydrogenation of sugars from wheat bran, a kinetic study with sugar model mixtures was carried out in Chapter 4 using ruthenium catalysts supported on H-ZSM-5 zeolites with different SiO2/Al2O3 ratio (23 and 80). Reaction temperature was varied from 80 to 120 ºC and time between 5 and 30 minutes. Likewise, the influence of catalyst loading was analyzed in the range 0 – 0.060 g Ru/g C. The acidity of the support (SiO2/Al2O3 ratio) played a crucial role in the reaction mechanism. Ru/H-ZSM-5 (80) resulted in higher conversion and selectivity than Ru/H-ZSM-5 (23), since low acidic supports promoted hydrogenation over secondary reaction pathways, such as isomerization. Experimental results showed that C5 sugars were faster hydrogenated than C6 sugars. Indeed, the optimum temperature for arabinose and xylose hydrogenation was 100 ºC, whereas 120 ºC was the most suitable temperature for glucose hydrogenation. In this sense, operating conditions could be tuned to maximize the yield of pentitols and/or hexitols. Additionally, experimental data was successfully reproduced by a pseudo-first order kinetic model with relatively low absolute deviations (< 11%) and high regression coefficients (> 0.950). The activation energy values were 47.9 kJ/mol, 43.7 kJ/mol and 92.0 kJ/mol for the hydrogenation of arabinose, xylose and glucose, respectively, using Ru/H-ZSM-5 (80) as catalyst. In Chapter 5, the purification of wheat bran hydrolysates and the subsequent catalytic production of sugar alcohols was investigated. This hydrolysate was composed of xylose (5.6 g/L), arabinose (2.8 g/L), glucose (0.8 g/L), furfural (0.3 g/L), proteins (0.9 g/L), different inorganic elements (Mg, Ca, K, S) and some lignin derivatives. A purification strategy was defined to maximize the sugar content in this hydrolysate. The process was based on the selective recovery of sugars by anionic extraction with a boronic acid (hydroxymethyl phenylboronic acid) which was dissolved in an organic phase composed by a quaternary ammonium salt (Aliquat® 336) and 1-octanol. The sugars were then back-extracted in an acidic solution which was further purified by means of ion exchange resins (Amberlyst® 15 and Amberlite® IRA-96). After this process, an aqueous phase with a purity in sugars of 90% (based on carbon balance) was obtained. It was free of inorganic salts and proteins and it had a lower content of sugar degradation products and lignin derivatives than the initial hydrolysate. Importantly, the organic phase was successfully recycled. Purified sugars were then hydrogenated over Ru/H-ZSM-5 (80). A high pentitols yield of ~70% with 100% selectivity was achieved at 100 ºC after 10 minutes with a catalyst loading of 0.060 g Ru/g C. An attempt to hydrogenate the sugars in the hydrolysate prior to purification was performed. However, neither sugar conversion nor sugar alcohol production were observed. It was determined that proteins deactivated the ruthenium catalyst and consequently the production of sugar alcohols was inhibited. Therefore, a purification step to remove proteins from wheat bran hydrolysates was crucial for the successful catalytic hydrogenation of sugars.