Selective c-o hydrogenolysis of sugar polyols to deoxygenated products over heterogeneous catalysts
Résumé
Hydrogenolysis reactions of C6-C4 sugar alcohols such as sorbitol, xylitol and erythritol in aqueous medium are important pathways for the production of high valuable chemicals.[1] The selectivity obtained during the conversion of these highly functionalized molecules is a major issue because both C-C and C-O bonds cleavage can occur. The most challenging is the selective C-O bond cleavage without C-C bond cleavage in order to produce partially deoxygenated chemicals used for polymer applications. Taking sorbitol as a model molecule, deoxy-hexitols are obtained through C-O bond cleavage, whereas ethyleneglycol (EG), propyleneglycol (PG) and glycerol (GLY) are produced by C-C bond rupture. In addition to C-O and C-C bond cleavage, sorbitol may be dehydrated to sorbitan and isosorbide or undergoes isomerization to mannitol and iditol.In this contribution, catalytic systems consisting of supported Rh and Ir catalysts promoted with rhenium oxide [2,3] were compared in the hydrogenolysis reactions of erythritol and sorbitol in neutral and acidic medium. Erythritol was mainly converted to butane-triols and diols with relatively high selectivity (65% at 80% conversion) together with a moderated amount of its isomer (threitol).During sorbitol reaction, in addition to C-O bond hydrogenolysis which yielded the valued deoxypolyols (triols and diols) with selectivity of only 8%, isomerization and dehydration reactions were significative; the cyclic ethers 1,4-sorbitan and some isosorbide were partially converted to deoxygenated products (Fig .2b). The catalytic results were not affected by the addition of an acid (H2SO4 , nH+/nRh=1). Rh-ReOx/TiO2 generated more cyclisation and less isomerization than Ir-ReOx/TiO2 .