Synthesis of long-chain alkyl levulinates from furfuryl alcohol: the perspective of sulfonated hydrochar as renewable catalyst

The increasing environmental and economic issues due to the employment of fossil resources for the production of chemicals and fuels have prompted significant efforts toward their replacement with renewable chemicals and fuels. In this context, alkyl levulinates (ALs) are attracting significantly growing interest because they show an extremely broad potential of applications, including as versatile biofuels. Many studies have focused on the production of short-chain levulinates, such as methyl and ethyl levulinates, but these esters suffer from some issues, such as a high oxygen content, low energy density and water solubility. Thus, long-chain ALs are becoming more attractive, providing higher carbon content and energy density and water insolubility. Generally, the ALs synthesis involves the conversion of lignocellulosic C6 fraction, through the one-pot alcoholysis reaction, but they can also be obtained from the C5 fraction. This path is worthy of further investigation because it allows the increase of the potential productivity of ALs from lignocellulosic biomass. However, it involves a multi-step process, made up of a first step of xylose hydrolysis to furfural, followed by its hydrogenation to furfuryl alcohol, which is the direct precursor of ALs through the acid-catalyzed alcoholysis reaction. In this work, the synthesis of long-chain ALs, as butyl levulinate (BL) and n-hexyl levulinate (HL), starting from furfuryl alcohol has been investigated in the presence of heterogeneous acid catalysts adopting microwaves as an efficient and sustainable heating system and the high gravity approach (furfuryl alcohol loading of 10 wt%), which allows the production of concentrated ALs streams, thus increasing productivity and making downstream operations easier and cheaper. Among all the tested commercial heterogeneous catalysts, Aquivion PW98S led to the best performances in terms of conversion and BL selectivity, thus a deeper investigation on the influence of the reaction conditions has been carried out, reaching the highest BL yield of 68 mol%. The same catalyst was also employed for the synthesis of HL and the optimization of the reaction conditions was performed, reaching the highest HL yield of 52 mol%. To increase the sustainability of the process, a sulfonated hydrochar obtained by the hydrothermal carbonization of the leaves of the perennial herbaceous plant Arundo donax L. was adopted as catalyst. It was able to catalyze the conversion of furfuryl alcohol to both BL and HL, resulting a promising renewable catalyst. In conclusion, this work contributes to the increase of long-chain ALs sustainable production within a biorefinery scheme, as also demonstrated by the preliminary LCA analysis carried out at the laboratory scale.