Smectite-catalyzed dehydration of glucose

Abstract

The objective of this study was to determine whether smectites abiotically catalyze transformation of glucose under conditions relevant to soil organic matter (SOM) formation. Four smectites saturated with Na, Ca, Fe and Al were incubated under abiotic conditions with glucose solutions for 21 days at 37°C. After the incubations, soluble organic C recoveries ranged from 95 to 109.3%, relative to the amount of C added as glucose; however, glucose recoveries in the solutions ranged from 18.3 to 98.3%. The results indicate that a significant amount of the added glucose was abiotically transformed to soluble organic compounds other than glucose during the incubations. In general, glucose recoveries decreased with the acidic character of smectites: SWa-1 > Panther > Otay. Also, within clays, glucose recoveries decreased as the exchangeable cation became more acidic: Na > Ca > Al. Higher glucose recoveries were obtained for ‘Fe-rich’ smectites relative to ‘Fe-poor’ smectites, suggesting that Fe-oxyhydroxy coatings on smectite surfaces inhibit the transformation of glucose. High-pressure liquid chromatography analysis of the incubation solutions revealed small peaks for 5-(hydroxymethyl)-2-furaldehyde along with peaks for other unknown compounds. The results suggest that under conditions similar to those found in soils, smectites catalyze glucose dehydration to form furfural compounds. Polymerization of furfural compounds may be a major pathway leading to the formation of new humic materials in soils.