Recent developments in the biotechnological production of hydrocarbons: paving the way for bio-based platform chemicals

Lignocellulose Fractionation Using Recyclable Phosphoric Acid: Lignin, Cellulose, and Furfural Production

The conversion of lignocellulose into its building blocks and their further transformation into valuable platform chemicals (e. g., furfural) are key technologies to move towards the use of renewable resources. This paper explored the disentanglement of lignocellulose into hemicellulose-derived sugars, cellulose, and lignin in a biphasic solvent system (water/2-methyltetrahydrofuran) using phosphoric acid as recyclable catalyst. Integrated with the biomass fractionation, in a second step hemicellulose-derived sugars (mainly xylose) were converted to furfural, which was in situ extracted into 2-methyltetrahydrofuran with high selectivity (70 %) and yield (56 wt %). To further increase the economic feasibility of the process, a downstream and recycling strategy enabled recovery of phosphoric acid without loss of process efficiency over four consecutive cycles. This outlines a more efficient and sustainable use of phosphoric acid as catalyst, as its inherent costs can be significantly lowered.

Microbial Hydrocarbon Formation from Biomass

Fossil carbon sources mainly contain hydrocarbons, and these are used on a huge scale as fuel and chemicals. Producing hydrocarbons from biomass instead is receiving increased attention. Achievable yields are modest because oxygen atoms need to be removed from biomass, keeping only the lighter carbon and hydrogen atoms. Microorganisms can perform the required conversions, potentially with high selectivity, using metabolic pathways that often end with decarboxylation. Metabolic and protein engineering are used successfully to achieve hydrocarbon production levels that are relevant in a biorefinery context. This has led to pilot or demo processes for hydrocarbons such as isobutene, isoprene, and farnesene. In addition, some non-hydrocarbon fermentation products are being further converted into hydrocarbons using a final chemical step, for example, ethanol into ethene. The main advantage of direct microbial production of hydrocarbons, however, is their potentially easy recovery because they do not dissolve in fermentation broth.

Lipase-mediated selective oxidation of furfural and 5-hydroxymethylfurfural

Furfural and 5-hydroxymethylfurfural (HMF) are important biomass-derived platform chemicals that can be obtained from the dehydration of lignocellulosic sugars. A possible route for the derivatization of furanics is their oxidation to afford a broad range of chemicals with promising applications (e.g., diacids, hydroxyl acids, aldehyde acids, monomers for novel polymers). Herein we explore the organic peracid-assisted oxidation of furanics under mild reaction conditions. Using lipases as biocatalysts, alkyl esters as acyl donors, and aqueous solutions of hydrogen peroxide (30 % v/v) added stepwise, peracids are formed in situ, which subsequently oxidize the aldehyde groups to afford carboxylic acids with high yields and excellent selectivities. Furthermore, the use of an immobilized silica-based 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) affords the selective oxidation of the hydroxymethyl group of HMF to afford 2,5-diformylfuran. That product can be subsequently oxidized using again lipases for the in situ peracid formation to yield 2,5-furandicarboxylic acid, which is considered to be a key building block for biorefineries. These lipase-mediated reactions proceeded efficiently even with high substrate loadings under still non-optimized conditions. Overall, a proof-of-concept for the oxidation of furanics (based on in situ formed organic peracids as oxidants) is provided.

Lipase-catalyzed (trans)esterification of 5-hydroxy- methylfurfural and separation from HMF esters using deep-eutectic solvents

5-Hydroxymethylfurfural (HMF) is a valuable biomass-derived building block. Among possible HMF valorization products, a broad range of HMF esters can be synthesized. These HMF esters have found some promising applications, such as monomers, fuels, additives, surfactants, and fungicides, and thus several catalytic approaches for HMF (trans)esterifications have been reported. The intrinsic reactivity of HMF is challenging, forcing the use of mild reaction conditions to avoid by-product formation. This paper explores the lipase-catalyzed (trans)esterification of HMF with different acyl donors (carboxylic acids and methyl- and ethyl esters) mostly in solvent-free conditions. The results demonstrate that lipases may be promising alternatives for the synthesis of HMF esters-with high productivities and reactions at high substrate loadings-provided that robust systems for lipase immobilization are applied to assure an adequate reusability of the enzymes. Once (trans)esterifications have been conducted, the separation of unreacted HMF and HMF esters is performed by using deep-eutectic solvents (DES) as separation agents. DES are able to dissolve hydrogen-bond donors (e.g., HMF), whereas non-hydrogen-bond donors (in this case HMF esters) form a second phase. By using this approach, high ester purities (>99 %) and efficiencies (up to >90 % HMF ester recovery) in separations were obtained by using choline chloride-based DES.

Chemo-enzymatic conversion of glucose into 5-hydroxymethylfurfural in seawater

Do you sea water? Water consumption will be a challenge in biorefineries, and the use of non-drinkable sources of water will be preferred. Herein, glucose is converted into 5-hydroxymethylfurfural (HMF) in a chemo-enzymatic one-pot, two-step procedure, involving immobilized glucose isomerase to produce fructose and oxalic acid to dehydrate it to HMF.

Enzymatic hydrolysis of microcrystalline cellulose in concentrated seawater

This communication explores the use of seawater (1X) and concentrated seawater (2X and 4X) as reaction media for the enzyme-catalyzed depolymerization of cellulose. The commercially available Accellerase-1500® - a "cocktail" of different glycosidases - is able to depolymerize several amorphous celluloses and microcrystalline cellulose Avicel® in these reaction media, at slightly lower rates (ca. 90%) than those observed when reactions are performed in pure citrate buffer (control reactions). Remarkably, at concentrated seawater effluents enzymes also display significant rates of cellulose hydrolysis. Considering the expected increasing shortages in accessibility to fresh drinkable water, the herein-reported concept may provide novel inspiring leads for a smart use of resources in an environmentally-friendly and efficient manner, and for the genetic development of cellulases highly active and stable in concentrated seawater solutions.