Fast catalytic conversion of recalcitrant cellulose into alkyl levulinates and levulinic acid in the presence of soluble and recoverable sulfonated hyperbranched poly(arylene oxindole)s

Abstract

Sulfonated hyperbranched polymers were recently reported to efficiently mimic cellulase activity, producing large quantities of glucose from cellulose. The polymer structure allows tuning of the acid properties in terms of active site confinement and acid strength, while being sufficiently flexible to strongly interact with a solid carbohydrate. Whereas previous research focussed on catalysis in water, herein the sulfonated hyperbranched poly(arylene oxindole)s (SHPAOs) were used in alcoholic media, converting cellulose into alkyl glucosides and alkyl levulinates. Interestingly high reaction rates were noticed in the alcoholic solvent, ethanol being the solvent of choice. Unlike most previous reports, low reaction temperature, high cellulose concentrations and no external pressure were employed. A chlorinated SHPAO, denoted as 5-Cl-SHPAO, due to its high acid strength, exhibits the best catalytic efficiency, yielding 79% ethyl glucoside (EG) in 1 h and 60% ethyl levulinate (EL) in 6 h, the latter value being considerably higher than those of the reference sulfuric acid (29%) and 2-naphthalenesulfonic acid (42.5%) under similar reaction conditions. Worth mentioning is a combined ethyl glucosides and ethyl levulinate (levulinic acid) yield of >90% from microcrystalline cellulose at complete conversion. The cellulose reaction runs in a chemical regime in the temperature range of 150 to 190 °C, 160 °C being the most optimal with regard to the reaction speed and product yields. Time profiles and analysis of the product distributions reveal fast formation of alkylglucosides, while their conversion is the slowest step in the cascade to alkyl levulinate. Besides being very fast, reaction rates in an alcoholic solvent appear less affected by the properties of the cellulose. Therefore, even large particles of highly crystalline cellulose are easily converted to high alkyl levulinate yields. Obtaining a high levulinic acid (LA) yield directly from cellulose appears difficult, also in the presence of a hyperbranched polymer. Therefore, a two-stage catalytic strategy that uses the facile formation of alkyl levulinate from cellulose in alcohol in the presence of catalytic amounts of 5-Cl-SHPAO is proposed. After alcoholic evaporation of the alkyl levulinate product solution, aliquots of water are added to hydrolyse the product into LA. As this reaction in the presence of the remaining soluble catalyst is complete, a 60% LA yield from microcrystalline cellulose is demonstrated. Catalyst recovery is demonstrated through nanofiltration. Due to the soluble character of the hyperbranched catalyst in the alcoholic solvent, it is easily separated from the solid humins, and recovered from the solution over a commercial low molecular weight cut-off filter. The recovered catalyst showed comparable catalytic activity (per catalyst weight) and product selectivity.