OxFA process

OxFA-Process

General description

Formic acid is obtained by aqueous catalytic partial oxidation of wet biomass. ^{[1] [2] [3]} A Keggin deviation polyoxometalate (H ₅ PV ₂ Mo ₁₀ O ₄₀ ) is used as the homogeneous catalyst to convert sugars, wood, waste paper or cyanobacteria to formic acid and CO ₂ as the sole byproduct . Yields of up to 53% formic acid can be achieved.

It is possible to convert water-insoluble biomass in a suspension with the catalyst. After the reaction, formic acid is separated from the reaction mixture. Depending on the separation process, the formic acid can be further purified or used as it is. The remaining solution of the catalyst, residual acid and acid additives are recycled to the reaction. In this step, solids (eg soil from dirty biomass) or unreactive inorganics (eg inorganic salts from the natural salt content of biomass) can be separated from the catalyst solution.

The general reaction for a simple glucose sugar can be summarized as follows:

C ₆ H ₁₂ O ₆ + wO ₂ → xHCOOH + yCO ₂ + zH ₂ O

Biomass pretreatment

Water-insoluble biomass must be shredded to an appropriate size to enhance the surface area at which the reaction takes place. Water-soluble biomass needs no special pretreatment.

Biomass oxidation

The reaction is carried out at 363K and 30bar oxygen partial pressure, or pure oxygen or air. Since hot formic acid is corrosive, suitable autoclaves (eg Hastelloy ) must be used. Reaction times depend on the reactivity of the feed. for example, microcrystalline cellulose is converted to 15 and 22% after 24h and 66h, respectively, while the most reactive xylanis converted to 88% and 94% respectively. The use of additives more than doubles the reaction rates, especially for the refractory compound cellulose. If p-toluenesulfonic acid is used as additive, conversion of cellulose rises from 22% to 68% after 66h. Under the reaction conditions, over-oxidation of the acidic product does not occur. In another recent example, formic acid has been produced from biomass-derived sugars using hydrogen peroxide and the oxidant over heterogeneous catalysts. ^[4] [5]

Product separation

Formic acid can be separated from the reaction mixture by means of distillation or extraction .

Distillation of the reaction mixture yields a condensate that is high in water content, since water and formic acid form a high boiling azeotrope . Formic acid is concentrated in the still bottom. If all the volatile compound is distilled, the concentration of the acidic compound is that of the original reaction mixture.

Extraction of formal acid can be effected with several solvents.

The solvents listet in the table were found to be stable against oxidation by the catalyst. Dibutyl ether in the aqueous phase Dibutyl etherides the catalyst in the aqueous phase.

Catalyst recycling and purification

Afterwards, the remaining solution contains water, the catalyst, additives and residual formic acid. This solution can be directly recycled to the reaction without loss of performance.

Potential uses of biobased formic acid

Formic acid has been considered as a material for hydrogen storage . ^[6] This process would be allowed to use bio-based instead of fossil-based formic acid.

References