Selective transformation of hemicellulose (xylan) into n-pentane, pentanols or xylitol over a rhenium-modified iridium catalyst combined with acids†
n-Pentane, pentanols and xylitol can be separately produced from hemicellulose (xylan) over an Ir–ReOx/SiO2 catalyst combined with acids by simply adjusting the reaction conditions. n-Pentane can be produced by using Ir–ReOx/SiO2 combined with HZSM-5 + H2SO4 in a biphasic solvent system (4 ml n-dodecane + 9.5 ml H2O) with a reaction temperature of 463 K for 24 h. Pentanols can be produced by using Ir–ReOx/SiO2 combined with H2SO4 in a biphasic solvent system (20 ml n-dodecane + 9.5 ml H2O) with a reaction temperature of 413 K for 144 h. Xylitol can be produced by using Ir–ReOx/SiO2 combined with H2SO4 in the aqueous phase with a reaction temperature of 413 K for 12 h. The highest yields of n-pentane, pentanols and xylitol could reach 70%, 32% and 79%, respectively. The reuse of the catalyst was feasible when the catalyst was regenerated by calcination at 773 K for 3 h. The calcination step is for removing the humins which were formed at the hydrolysis + hydrogenation step during conversion of xylan. The humins covered the active site of Ir–ReOx/SiO2 and HZSM-5, and they deactivated Ir–ReOx/SiO2 in C–O hydrogenolysis performance in part. The mineral ions (such as Na+ and K+) in xylan decreased the hydrogenolysis activity of Ir–ReOx/SiO2 significantly since the mineral ions can make the number of hydroxorhenium sites (Re–OH) smaller, which is the active site of Ir–ReOx/SiO2 for C–O hydrogenolysis, by ion exchange. The appropriate amount of H2SO4 addition is very crucial for the production of target products in high yield. The addition of H2SO4 not only neutralized the residual alkali of xylan after being isolated from lignocellulose to make the reaction solution acidic, but also improved the C–O hydrogenolysis activity of Ir–ReOx/SiO2 through increasing the number of hydroxorhenium sites by competitive adsorption on the Re site with mineral ions.
- This article is part of the themed collection: 3rd International Symposium on Green Chemistry