Conversion of glucose into levulinic acid with solid metal(IV) phosphate catalysts

We have prepared a series of well-characterized solid acid metal(IV) phosphate catalysts and tested them for the two-step dehydration/rehydration reaction to produce levulinic acid from glucose. The catalysts include zirconium (ZrP) and tin (SnP) phosphates with varying ratios of phosphorus to metal(IV). The structural, surface and bulk properties have been investigated using XRD, BET, XPS and P-31 solid-state MAS NMR spectroscopy. ZrP is distinguished by a high concentration of polyphosphate species in the bulk phase, as well as increased hydroxyl groups on the surface. ZrP also shows a higher concentration of total acid sites and Bronsted acid sites compared to SnP, as determined by TPD measurements using gas-phase NH3 and isopropylamine. The catalyst selectivity is a function of the Bronsted to Lewis acid site ratio using either heterogeneous or homogeneous catalysts. Catalytic activity increases with increased Lewis acid sites. The Lewis sites mainly produce fructose via isomerization reactions and undesired degradation products (humins). HMF is produced on both Bronsted and Lewis sites, whereas levulinic acid is exclusively produced on Bronsted sites. Zirconium phosphate with a P/Zr molar ratio of 2 is favorable for levulinic acid production due to its inherently high surface area and enhanced Bronsted acidity. This study lays the grounds for further design of improved solid acid catalysts for aqueous phase production of HMF and levulinic from carbohydrates. (C) 2013 Elsevier Inc. All rights reserved.