In the hunt for cost-effective, high-yield ways to convert biomass into renewable liquid hydrocarbons, one attractive approach is the controlled reduction of the oxygen content in the feedstock to produce platform chemicals that retain sufficient functionality for upgrading to a variety of useful end products (biorefining). Levulinic acid (LA) is an attractive platform molecule that can be converted to, among others, GVL.

In turn, a promising, cost-effective approach for the production of LA from biomass is via hydrolysis with dilute sulfuric acid. However, Dumesic and his colleagues note in their paper, the production of GVL by catalytic reduction of LA is complicated by the need to separate LA from sulfuric acid, as residual sulfur leads to low catalytic activity and deactivation with time-on-stream.

Therefore, the motivation of the present work is to demonstrate improved sulfuric acid management in levulinic acid-centered biorefining. In the present state of the art, H₂SO₄ is recovered from LA in an energy-intensive process that involves solvent extraction combined with distillation. Herein, we report an improved, synergistic biorefining strategy that does not require the use of external solvents or energy-intensive distillation steps to separate the levulinic and formic acids from H₂SO₄, and instead employs reactive extraction, using butene, to produce hydrophobic esters of levulinic and formic acids.

Moreover, we show that these esters spontaneously separate from H₂SO₄ and can be converted to GVL over a dual-catalyst- bed system. As we have shown previously, GVL can be converted to butene and CO₂ by catalytic decarboxylation over an acid catalyst, thereby providing the source of butene required for the reactive extraction step.

—Gürbüz et al.

The new process comprises the following steps:

• Production of an aqueous solution containing equimolar concentrations of LA and formic acid (FA) by hydrolysis of cellulose at 423 K (150 °C) using sulfuric acid (0.5m);
• Some of the water and the FA co-product are then removed by an evaporation step to obtain a more concentrated solution of LA and sulfuric acid, containing residual amounts of water and FA.
• The FA product in water is retained for downstream hydrogen production in a dual-catalyst-bed system, and the concentrated LA product is contacted with butene, generating sec-butyl levulinate (BL) and sec-butyl formate (BF) esters as major and minor products, respectively, using H₂SO₄ as a catalyst. The team has demonstrated that high yields, 85%, of the levulinate ester can be attained at moderate temperatures (<373 K, 100 °C) and short contact times (<120 min).
• Excess butene is recovered by vaporization, and the concentrated ester product is then contacted with water, during which the hydrophobic ester separates spontaneously from the aqueous phase, the latter of which retains 99% of the sulfuric acid to be recycled for use in biomass deconstruction.
• The BL and BF esters are subsequently processed in combination with the aqueous FA product stream (obtained from the evaporation step) using a dual-catalyst-bed in a single reactor to produce GVL in nearly quantitative yields with 2-butanol and CO₂ as coproducts.
• The liquid effluent from this reactor, consisting of an aqueous solution of GVL and 2-butanol, can undergo decarboxylation and dehydration (of GVL and butanol, respectively) over a catalyst to obtain butene and CO₂.

...we have described an integrated biorefining strategy for the production of butyl esters and GVL starting from cellulose and utilizing reactive extraction of levulinic and formic acids with butene. This strategy simplifies the recovery and recycle of sulfuric acid for cellulose deconstruction and enables downstream catalytic processing in the absence of sulfur. The mixture of levulinic and formic esters, along with residual levulinic and formic acids, can be converted to an aqueous solution of GVL and 2-butanol in a single step over a dual-catalyst- bed consisting of Pd/C followed by Ru/C, in which H₂ generated from FA and its ester over Pd/C is used for the reduction of LA and its ester to GVL over Ru/C.

—Gürbüz et al.

Resources

• E. I. Gürbüz, D. M. Alonso, J. Q. Bond, J. A. Dumesic (2011) Reactive Extraction of Levulinate Esters and Conversion to γ-Valerolactone for Production of Liquid Fuels. ChemSusChem doi: DOI: 10.1002/cssc.201000396