Wheat bran biorefinery: Valorization of hemicelluloses to sugar alcohols. Fractionation - Hydrolysis - Purification - Hydrogenation

Abstract

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 ABSTRACT 6 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-1. 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-1 , 43.7 kJ•mol-1 and 92.0 kJ•mol-1 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-1), arabinose (2.8 g•L-1), glucose (0.8 g•L-1), furfural (0.3 g•L-1), proteins (0.9 g•L-1), 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 ABSTRACT 7 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 wi...