Abstract:

Formic acid (FA) is an important chemical for the production of leathers, medicines, preservatives, rubbers, textiles, and other materials. FA is also used as H₂ and CO carriers, and as a fuel in fuel cells. Although most commonly synthesized from fossil fuels, FA can also be obtained from more sustainable sources, such as biomass (e.g., straw, husk, and sawdust). Oxygen and air are affordable and easily available oxidants used for the oxidation of biomass to FA. Because solid biomass is not soluble in water or organic solvents, homogeneous catalysts are preferred for the catalytic oxidation of biomass to FA by O₂ in water. It has been demonstrated that homogeneous catalysts, such as vanadium-containing heteropoly acids (HPA), HPA+H₂SO₄, NaVO₃+H₂SO₄, HPA-containing ionic liquids, VOSO₄, NaVO₃-FeCl₃+H₂SO₄, and FeCl₃+H₂SO₄, can convert complex biomass substrates to FA with high atom economy using O₂ as the oxidant. The reported biomass substrates include model compounds, cellulose, wood, straw, and corncobs. The reaction conditions were summarized to compare the biomass conversions and FA yields. Vanadium-containing catalysts had the highest FA yield at mild conditions (T ≤ 170 ºC and P(O₂) ≤ 3 MPa). Both the reaction rate and FA yield were improved by adding H₂SO₄. This high conversion can be explained by an electron transfer and oxygen transfer (ET-OT) mechanism, where high-valence transition metals (V⁵⁺ or Fe³⁺) oxidize biomass to FA and are reduced to low-valence species (V⁴⁺ or Fe²⁺). The catalysts are then regenerated by O₂. This reaction occurs through C2―C3 and/or C3―C4 bond cleavages via retro-aldol condensation, followed by continued C―C bond cleavages to form FA. Using isotope-labeled D-glucose as substrate, we determined that oxidation occurs via successive C1―C2 bond cleavages; a V⁵⁺ catalyst reacts with C1―C2 to form a five-membered ring complex, without C―H bond cleavages, followed by oxidation from another V⁵⁺ species to form FA. The oxidation of solid cellulose occurs through hydrolysis (hydrolysis of cellulose to monosaccharides, and deep hydrolysis of monosaccharides to levulinic acid) and oxidation (monosaccharides to FA and levulinic acid to acetic acid) reactions. The catalytic oxidation of monosaccharides and deep hydrolysis steps are competitive, and the reaction rate of the latter increases faster with increasing temperature. However, catalytic oxidation was favored by higher P(O₂). The addition of methanol, ethanol and DMSO to the reaction system, and in situ extraction of FA were performed to inhibit CO₂ formation. FA was separated by extraction and the catalyst system was reused. A continuous process for producing FA from molasses was established using a three-phase liquid-liquid-gas system with a reaction volume of 2 L. Finally, the limitations and future requirements of this oxidation reaction are discussed: (1) improving separation or in situ conversion of FA; (2) improving homogeneous catalysts for both biomass hydrolysis and catalytic oxidation to FA; (3) studying the impact of ash in biomass, particularly after catalyst reuse; and (4) understanding the mechanism through which organic solvents such as methanol inhibit CO₂ formation.

Key words: Formic acid, Biomass, Oxygen, Catalytic oxidation, Homogeneous catalyst