Abstract
Heteropolyacids (HPAs), super strong Brønsted acid solids, were recently reported to achieve cellulose conversion with high efficiency. To achieve higher levulinic acid (LA) yield, a series of Lewis metal-substituent phosphotungstic acids HnPW11LO39 (HPWL, L = CuII, SnIV, CrIII, ZnII, FeIII; L = TiIV, ZrIV, oxygen is 40) were used as catalysts in polysaccharides conversion. Identification of the reaction intermediates and products provided some information for cellulose conversion pathway. H5PW11TiO40 (Brønsted/Lewis acid = 2.84:1) was found to be most active with highest cellulose conversion giving highest glucose yield in water and highest LA yield in methyl isobutyl ketone (MIBK)/water. In addition, for overcome the drawbacks in mass transferring and separation, the temperature-responsive catalyst of HOCH2CH2N(CH3)3H4PW11TiO40 (ChH4PWTi) was prepared giving as high as 76.1% LA yield at 93.8% cellulose conversion more than those by HPWTi (65.6%, 90.5%). Also, ChH4PWTi was obtained 93.7% conversion and 70.9% yield at microwave. ChH4PWTi also presented higher feedstocks - tolerance to mono-, di- or polysaccharides and even lignocellulose (corn straw, pinewood, husk of xanthoceras, coconut shell) to LA. As highest as 33.0% LA yield at 68.0% conversion was obtained by ChH4PWTi from husk of xanthoceras under mild conditions. ChH4PWTi could be recycled 12 times without significant loss of activity, which showed longer duration.
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