Liquid phase dehydration of d-xylose in the presence of Keggin-type heteropolyacids

The heteropolyacids (HPAs) H3PW12O40 (PW), H4SiW12O40 (SiW) and H3PMo12O40 (PMo) were tested as catalysts for the homogeneous liquid phase dehydration of d-xylose to furfural. With dimethylsulfoxide (DMSO) as the solvent and a reaction temperature of 140 °C, the tungsten-based HPAs are comparable with H2SO4 and p-toluenesulfonic acid catalysts in terms of furfural yield achieved after 4 h (58-67%), whereas PMo yields less than half this amount of furfural. In the presence of PW, the temperature dependence of the initial reaction rate is approximated by the Arrhenius equation with the exponential term exp(−4767 °K/T). The initial rate of xylose conversion exhibits first-order dependence with respect to the initial xylose concentration and a non-linear dependence on initial HPA concentration. Catalytic results also vary according to the HPA composition and the nature of the solvent system (DMSO, water, water/toluene or water/isobutylmethylketone (IBMK)). Xylose conversions are higher in DMSO than in water-containing solvent systems. In DMSO, the turnover numbers (TON, mol/equiv. H+ calculated at 4 h) follow the order PW > SiW > PMo, whereas in the water-containing systems the order changes to PMo > PW > SiW. In IBMK/water SiW is far more selective than PW (57 and 4% selectivity for SiW and PW, respectively, at 83-89% conversion) and yields more furfural (51% compared with <9% for PW and PMo, at 24 h). In DMSO selectivity to furfural is slightly higher with PW (64-69%) than with SiW (52-64%), but much lower with PMo (<27%), above 87% conversion. For PW and SiW selectivity towards furfural production is higher for toluene/water than for DMSO for conversions up to 80%.