OxFA process
The OxFA process is a process to produce formic acid from biomass by catalytic oxidation using molecular oxygen or air. Polyoxometalates of the Keggin-type are used as catalysts.
OxFA-Process
General description
Formic acid is obtained by aqueous catalytic partial oxidation of wet biomass.[1][2][3] A Keggin-type polyoxometalate (H5PV2Mo10O40) is used as the homogeneous catalyst to convert sugars, wood, waste paper or cyanobacteria to formic acid and CO2 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 formic acid and additives are recycled to the reaction. In this step, solids (e.g. soil from dirty biomass) or unreactive inorganics (e.g. inorganic salts from the natural salt content of the biomass) may be separated from the catalyst solution.
The general reaction for a simple sugar like glucose can be summarized as follows:
C6H12O6 + wO2 → xHCOOH + yCO2 + zH2O
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 363 K and 30 bar oxygen
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 are distilled, the concentration of the received formic acid is that of the original reaction mixture.
Extraction of formic acid can be effected with several solvents.
Extracting agent | Partition coefficient | Distribution of catalyst |
---|---|---|
Dibutyl ether | 0.59 | Aqueous phase |
Methylisobutylketone |
0.88 | Aqueous+Organic phase |
Ethyl acetate | 1.07 | Aqueous+Organic phase |
Dibutylformamide | 0.98 | Organic phase |
The solvents listed in the table were found to be stable against oxidation by the catalyst. Only the solvent dibutyl ether leaves the catalyst in the aqueous phase, whereas with dibutylformamide the catalyst is completely extracted into the organic phase.
Catalyst recycling and purification
After most or all of the formic acid is separated from the reaction mixture, 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 allow to use bio-based instead of fossil-based formic acid.