Direct Alcoholysis of Carbohydrate Precursors and Real Cellulosic Biomasses to Alkyl Levulinates: A Critical Review

Abstract

Alkyl levulinates (ALs) represent outstanding bio-fuels and strategic bio-products within the context of the marketing of levulinic acid derivatives. However, their synthesis by acid-catalyzed esterification of pure levulinic acid, or by acid-catalyzed alcoholysis of furfuryl alcohol, although relatively simple, is still economically disadvantageous, due to the high costs of the pure precursors. The direct one-pot alcoholysis of model C6 carbohydrates and raw biomass represents an alternative approach for the one-step synthesis of ALs. In order to promote the market for these bio-products and, concurrently, the immediate development of new applications, it is necessary to speed up the intensification of their production processes, and this important achievement is onlypossible by using low-cost or, even better, waste biomasses, as starting feedstocks. This review provides an overview of the most recent and promising advances on the one-pot production of ALs from model C6 carbohydrates and real biomasses, in the presence of homogeneous or heterogeneous acid catalysts. The use of model C6 carbohydrates allows for the identification of the best obtainable ALs yields, resulting in being strategic for the development of new smart catalysts, whose chemical properties must be properly tuned, taking into account the involved reaction mechanism. On the other hand, the transition to the real biomass now represents a necessary choice for allowing the next ALs production on a larger scale. The improvement of the available synthetic strategies, the use of raw materials and the development of new applications for ALs will contribute to develop more intensified, greener, and sustainable processes.

Valorization of cigarette butts for top value-added chemicals: Levulinic Acid

“…In the former case, the increment of concentration of aqueous H 3 PO 4 up to 2.0 M resulted in a neglectable increase of yield in levulinic acid respect to preliminary experiments, thus suggesting that 1.5 M is the best value(Table 1, entries 5, 11-12). In contrast, much prolonged times afforded bene cial effects on reaction yields, allowing to reach a 49.6% of yield in levulinic acid after 6 hours (Table 1, entries[13][14]. As expected, no conversion in LA was observed when H 3 PO 4 was replaced by a weak acid such as CH 3 COOH.…”

“…Formation of Levulinic Acid was surveyed by GC/MS and NMR techniques. Both unsmoked and smoked cigarette butts were tested as source of cellulose biomass, whereas acetic and formic acids were formed as by-products together with HMF as an intermediate ( Figure 3) 13,23 .…”

“…Mass spectra were performed in EI mode (70 eV) and yields of LA were determined via GC-MS by means of a calibration curve. NMR spectra were recorded on a Bruker 500 MHz spectrometer: 1 H NMR (500 MHz) spectra were referenced to residual isotopic impurity of CDCl 3 (7.25 ppm) and 13 C-NMr (125 MHz) spectra were referenced to 77.00 ppm.…”

“…To date, only few companies can produce LA at commercial scale directly from biomass 9,12 . In recent years, much attention has been paid to producing LA by means of milder and more eco-sustainable conditions, 13 for example employing heterogeneous acid catalysts and green solvents such as water or ionic liquids (ILs) 7,14 .…”

Intensification of Processes for the Production of Ethyl Levulinate Using AlCl3·6H2O

“…The two-step process consisting of (i) the consolidate method through sulfuric acids [34], which allows LA to be obtained from biomasses, followed by (ii) the direct esterification of LA with ethanol in the presence of AlCl 3 •6H 2 O could unlock the potential of renewable feedstocks for the industrial production of EL. Not only because the first step is already conducted on an industrial scale [34], but also because the proposed strategy has several advantages if compared with the direct alcoholysis of biomasses [35]. Direct ethanolysis is an alternative route to obtain EL from carbohydrates.…”