The OxFA process is a process to produce
formic acid
Formic acid (), systematically named methanoic acid, is the simplest carboxylic acid, and has the chemical formula HCOOH and structure . It is an important intermediate in chemical synthesis and occurs naturally, most notably in some ants. Es ...
from
biomass
Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...
by
catalytic oxidation
Catalytic oxidation are processes that rely on catalysts to introduce oxygen into organic and inorganic compounds. Many applications, including the focus of this article, involve oxidation by oxygen. Such processes are conducted on a large scale ...
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. A
Keggin-type polyoxometalate (H
5PV
2Mo
10O
40) is used as the homogeneous catalyst to convert sugars, wood, waste paper or
cyanobacteria
Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...
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 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:
C
6H
12O
6 + wO
2 → xHCOOH + yCO
2 + zH
2O
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
partial pressure
In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas ...
, either as pure oxygen or air. Since hot formic acid is corrosive, suitable autoclaves (e.g.
Hastelloy) must be used. Reaction times depend on the reactivity of the feed. For example, microcrystalline
cellulose is converted to 15 and 22% after 24 h and 66 h, respectively, whereas the more reactive
xylan is 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 the additive, conversion of cellulose rises from 22% to 68% after 66 h.
Under the reaction conditions, overoxidation of the produced formic acid does not occur. In another recent example, formic acid was produced from biomass-derived sugars using
hydrogen peroxide as the oxidant over heterogeneous catalysts.
Product separation
Formic acid can be separated from the reaction mixture by means of
distillation or
extraction Extraction may refer to:
Science and technology
Biology and medicine
* Comedo extraction, a method of acne treatment
* Dental extraction, the surgical removal of a tooth from the mouth
Computing and information science
* Data extraction, the pro ...
.
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.
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.
[{{cite journal , last1 = Joó , first1 = Ferenc , title = Breakthroughs in Hydrogen Storage - Formic Acid as a Sustainable Storage Material for Hydrogen , journal = ChemSusChem , volume = 1 , issue = 10 , year = 2008 , pmid = 18781551 , doi = 10.1002/cssc.200800133 , pages = 805–8] This process would allow to use bio-based instead of fossil-based formic acid.
References
Chemical processes
Biomass