Selective transformation of hemicellulose (xylan) into n-pentane, pentanols or xylitol over a rhenium-modified iridium catalyst combined with acids

Abstract

n-Pentane, pentanols and xylitol can be separately produced from hemicellulose (xylan) over an Ir–ReO_x/SiO₂ catalyst combined with acids by simply adjusting the reaction conditions. n-Pentane can be produced by using Ir–ReO_x/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–ReO_x/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–ReO_x/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–ReO_x/SiO₂ and HZSM-5, and they deactivated Ir–ReO_x/SiO₂ in C–O hydrogenolysis performance in part. The mineral ions (such as Na⁺ and K⁺) in xylan decreased the hydrogenolysis activity of Ir–ReO_x/SiO₂ significantly since the mineral ions can make the number of hydroxorhenium sites (Re–OH) smaller, which is the active site of Ir–ReO_x/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–ReO_x/SiO₂ through increasing the number of hydroxorhenium sites by competitive adsorption on the Re site with mineral ions.