Combination of chemo and biocatalysis towards the valorization of biomass-derived sugars

MUSCI JJ; TORRES MJ; MERLO AB; CASELLA ML

Madrid

Simposio; 3rd International Symposium on Catalysis for Clean Energy and Sustainable Chemistry; 2016

Instituto de Catálisis y Petroleoquímica ICP-CSIC

From biomass it is possible to derive three general classes of feedstocks: 75% is carbohydrate, mainly in the form of cellulose, starch and saccharose, 20% is lignin and only 5% is other natural compounds, such as fats (oils), triglycerides, proteins and various substances. Cellulose, the most abundant biopolymer on the planet, is made up exclusively of glucose linked unbranched chains (between 38 and 50% of dry weight). The transformation of cellulose into sugars and bio-derived products can be carried out either via fermentation processes or via chemical methods or a combination of both. This last choice, the combination of biocatalytic and chemocatalytic reactions leading to one-pot processes in aqueous medium, represents an economically and ecologically attractive concept due to the potential to avoid time and capacity consuming and waste producing work-up steps of intermediates. Glucose production via cellulose enzymatic hydrolysis requires a pretreatment step to "liberate" the carbohydrates from their structural impediments, so that the material becomes more accessible to the enzymatic attack. In this work, hydrolysis was carried out under mild conditions (pH = 4.8 controlled with citric/citrate buffer, 50°C, gentle stirring) using a commercial Aspergillus sp. cellulase enzyme (Sigma-Aldrich) as catalyst. For the reaction time used (18 h), glucose yields close to 100% were obtained. The enzyme dosage used was 10.9 IU/g cellulose. An excess of enzyme was used to avoid any limitation of this variable. A series of 0.5 mL samples were taken at different reaction times and were subjected to 100°C in a water bath for 5 minutes to inactivate the enzyme. Subsequently, the samples were kept in a refrigerator at -16°C until analysis. Carbohydrates were quantified by liquid chromatography (Dionex UHPLC 3000 instrument), using a RCM-Monosaccharide Phenomenex Rezex Ca+2 (8%) column and the following conditions: eluent: deionized H2O, 0.6 mL/min, 80°C and refractive index detector at 40°C.The chemocatalytic hydrogenation step was performed using a 3 wt.% ruthenium supported catalyst. The catalyst was prepared by classical wet impregnation method using a commercial activated carbon (NORIT, Sg= 1010 m2/g and Vp= 0.53 cm3/g, crushed and sieved to 60-100 mesh). TPR analysis the Ru/C catalyst was performed in a laboratory-built equipment, using a feed of 5 vol.% H2 in Ar at a flow rate of 7.3 cm3.min-1 and a heating rate of 10°C min-1. The TPR profile corresponding to the Ru/C catalyst presented three peaks well differentiated between 80 and 260°C. The low temperature peak can be assigned to the decomposition of surface oxygen groups of the support. The second peak was assigned to the reduction of the salt precursor (Ru+3 → Ru0) and the third one at 250°C is associated to the partial gasification of the support around the metallic particles. The particle size distribution, calculated from TEM results (measured in JEOL 100 CX II instrument) was narrow, with very small ruthenium particles of around 1.12 nm. The dispersion of ruthenium particles (81%) was calculated applying an equation taken from literature using the volume?area average size obtained by TEM. The aqueous phase hydrogenation (APH) of the glucose obtained in the previous step, was carried out in an autoclave-type reactor (Autoclave Engineers) at a pressure of 1.25 MPa H2, 90°C and using 0.25 g of Ru/C catalyst. In each catalytic test, 0.9 g of reagent and 50 mL of water as solvent were used. The temperature used makes the enzyme become denaturalized and therefore does not interfere in this second stage. On the other side, the temperature is not so high as to promote hydrogenolysis reactions toward smaller sugar alcohols (i.e. glycerol) and thus the selectivity toward sorbitol is not sacrificed. In addition, the relatively low H2 pressure used (1.25 MPa compared to 3?8 MPa applied in most studies) makes the process more sustainable. Ru/C was highly active, exhibiting almost 100% conversion after 400 min reaction. The main product form was sorbitol, with a selectivity of 99.1%.The mechanism of sorbitol formation involves the dissociative adsorption of molecular H2 on hydrogenating metals such as Pt, Ru, Ni, and the subsequent addition of the pair of hydrogen atoms to the hemiacetal group of glucose, leading to the cleavage of the C-O bond and the formation of the hydroxyl (-OH) group. This reaction step occurs readily on Ru surface under the reaction conditions employed in our work, explaining the high selectivity obtained.