Conversion of Carbohydrates to Organic Acids in Aqueous Medium Using Aluminum Nitrate as the Catalyst Precursor

Abstract

Fructose, glucose, and sucrose were converted to organic acids in the presence of aqueous aluminum nitrate (Al(NO₃)₃) to develop a technically viable route for upgrading sugarcane molasses. Reactions were carried out in a microwave reactor and a muffle oven with conventional heating (convective heat transfer) using a sealed glass tubes and a hydrothermal stainless-steel autoclave as reaction vessels, respectively. Conversion was evaluated for different reaction times and temperatures. Lactic acid predominated as the product from the retro-aldol chain splitting of fructose, reaching a 67.5 % molar yield using 2.67 mmol·L^-1 Al(NO₃)₃ (4 wt% based on the carbohydrate dry mass) in a stainless-steel reactor with conventional heating. Sucrose required hydrolysis, glucose isomerization, retro-aldol chain splitting, dehydration, tautomerization, and 1,2-H migration to produce lactic acid in molar yields approaching those obtained from fructose (65.5 %). Besides lactic acid, formic and levulinic acids were produced in variable amounts through a fructose dehydration pathway, having 5-(hydroxymethyl)-furfural (HMF) as reaction intermediate. The use of a stepwise heating regime was a critical parameter to achieve high product yields and good lactic acid selectivity in these reaction systems.