Advances in the Catalytic Production of Valuable Levulinic Acid Derivatives

Currently, the conversion of biomass to levulinic acid and its derivatives has been one of the hottest topics in the field of energy and resources, because it opens up a new avenue for achieving sustainable energy supply and chemicals production. Levulinic acid can be used as a platform chemical for the production of a wide range of value-added compounds, such as levulinate esters and g-valerolactone. This review mainly discusses the catalytic routes for synthesis of valuable levulinate esters and g-valerolactone under different efficient reaction systems. Meanwhile, some promising and valuable researching directions and effective catalysts are suggested based on the major challenges emerged in recent studies. With the gradual depletion of fossil resources and further deterioration of the environment and its ecosystem, abundant renewable biomass are regarded as a promising alternative to non-renewable natural resources for the sustainable production of biofuels and biochemicals in the future (Figure 1). Recently, extensive research has been performed worldwide to identify and study chemical or biological transformations for converting biomass into fuels and raw chemicals. Among these explorations, one attractive approach is the preparation and conversion of levulinic acid (LA), owing to its importance as one of the 12 important target chemicals that US-DOE selected in their biomass program. It is widely used in food, agriculture, drugs, cosmetics, spice industries, as well as its derivatives. With regard to sources of LA, the LA preparation method can be classified into two categories according to the raw material resources, based on previous studies, namely, furfuralcohol hydrolysis and biomass conversion. Both of these two methods were conducted under acidic conditions or with acid catalysts, while the materials including starch and cellulose were used as the reactant for method of biomass conversion. Besides, high purity, LA can be obtained by furfuralcohol hydrolysis with a suitable solvent and catalyst. Considerable research over the past years has focused on the catalytic conversion of biomass into LA by both batch processes and continuous technology. As for transformation pathways, if agricultural and forest wastes can be rationally utilized, many valuable chemicals can be created at the same time. Most importantly of all, it opens a new way for the production of biochemicals in an economical manner. Many kinds of raw materials such as lignocellulose, corn starch, and discarded cellulose are used extensively in this route. However, there still are some clear shortcomings, especially as the apparent LA yield is not high and the final reaction products are complicated. In subsequent research, glucose has also been adopted as a raw material to yield LA, and a desired yield of LA can be obtained with solid acids as catalyst. As the studies develop in depth, some valuable levulinic acid derivatives have been discovered by researchers in recent years, especially levulinate esters and g-valerolactone. As these compounds are widely applied in food and chemical industry, it is of great significance for us to do more research on the preparation of the above high value-added chemicals by means of heterogeneous or homogeneous reaction. The levulinate esters were produced by the esterification of LA between alcohols, such as methanol, ethanol, and butanol under acidic conditions. Meanwhile, g-valerolactone (GVL) is a selective hydrogenation and dehydration product of LA catalyzed by metal catalysts. Clearly, it is a research direction with great significance and many researchers are now working on this new aspect. Currently, there is no review of the catalytic preparation of levulinate esters and GVL to date. Given that the rapid progress on the catalytic conversion of biomass, this minireview mainly concentrates on describing the work reported over the last few years. Basic information about levulinic acid is given,